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Gas-liquid mass transfer intensification for bubble generation and breakup in micronozzles

Gas-liquid mass transfer intensification for bubble generation and breakup in micronozzles The local gas-liquid mass transfer was characterized during bubble generation in T-contactors and in an adjacent micronozzle. A colorimetric technique with the oxygen sensitive dye resazurin was investigated to visualize gas-liquid mass transfer during slug flow, bubble deformation, as well as laminar and turbulent bubble breakup in the wake of a micronozzle. Two optimized nozzle geometries from previous studies were evaluated concerning volumetric mass transfer coefficients for low pressure loss, narrow −1 residence time distribution, or high dispersion rates. Highest values in k a up to 60 s were found for turbulent bubble breakup and an optimized micronozzle design in respect to pressure drop and dispersion rate. The achieved mass transfer coefficients were correlated with the energy dissipation rate within the micronozzles and with the inverse Kolmogorov time scale in vortex dissipation in good agreement for laminar and turbulent breakup regimes. . . . Keywords Gas-liquid mass transfer Micronozzle Bubble breakup Resazurin oxidation Introduction mass transport limitations can prevent the intrinsic reaction kinetics from evolving, resulting in extended reaction times, Gas-liquid reactions are highly relevant in chemistry such as poor reactor performance, and often low product quality [9]. for oxidation, halogenation, or hydrogenation [1–6], which Here, microstructured reactors offer increased surface-to- play an important role for pharmaceutical and fine chemical volume ratio and benefit from large interfacial area enabling industry [7]. In these applications, mass transfer of a gaseous fast mixing and reduced transfer resistances [10]. component into the liquid phase is the crucial and rate-limiting Consequently, reaction rates are enhanced and gas-liquid step. In conventional equipment, gaseous reactants are often mass transfer can be intensified applying continuous flow re- used in large excess due to poor interfacial mixing [8]. In case actors [7]. Gas-liquid reactions in microreactors therefore of insufficient mixing in processes including rapid reactions, have been subject to a variety of academic studies regarding hydrodynamics and mass transfer [10–18]. In gas-liquid microreactors the contacting of the two phases Scientific highlights: can be realized by either keeping both phases continuous with mass transfer during bubble generation nearly constant for various flow a stabilized interface (e.g. falling film or membrane reactor) or rates by dispersing one phase into the other using appropriate inlets energy dissipation rate is important parameter for dispersion and mass transfer or micromixers (e.g. T-junction, Y-contactor, flow focusing) rapid expansion after micronozzle leads to small bubble with high mass [19]. Particularly, micromixers are employed for intense transfer mixing as high specific interfacial area can be attained [20]. inverse Kolmogorov time scale determines the mass transfer coefficient Often, the gas phase is dispersed into the liquid phase using a in bubble breakup T-contactor and is then refined in a downstream micronozzle for increased interfacial area. The mass transfer is closely as- * Norbert Kockmann norbert.kockmann@tu-dortmund.de sociated with two-phase flow patterns, which rely on channel characteristics, fluid properties, and process parameters. Department of Biochemical and Chemical Engineering, Laboratory Typical flow regimes encountered in microchannel gas– of Equipment Design, TU Dortmund University, liquid flow include parallel flow, slug flow, and bubbly flow 44227 Dortmund, Germany 430 J Flow Chem (2021) 11:429–444 with respective transition regimes [21]. The interfacial area The concluded volumetric mass transfer coefficients for per unit volume increases from parallel flow to bubbly flow bubble breakup correspond to the obtained k a value range [22]; however, due to prevailing surface forces in capillaries, found by Yang et al. [34] for bubble formation in a T- slug flow is encountered for the most part [23]. contactor, which is also a very dynamic process justifying a Converging–diverging micronozzles are used to disperse comparison. gas-liquid flow with low pressure loss, breaking up bubbles In this work, mass transfer processes related to micronozzle into significantly smaller ones; hence, larger interfacial area is induced bubble breakup are examined and quantified for the created [24–28]. External forces, originating from the liquid, known bubble breakup regimes. Moreover, two optimized act on the phase boundary. Once the bubble preserving nozzle geometries developed by Tollkötter [27]and Laplace pressure is surpassed, bubble breakup is induced. Reichmann et al. [28] are analyzed concerning their gas- The degree of bubble breakup is determined by energy dissi- liquid mass transfer characteristics. Measurements are based pation rates [27]. on a colorimetric method introduced by Dietrich et al. [37]. In a previous fundamental study [28], various bubble This non-invasive technique uses a colorless, reduced form of breakup regimes have been characterized and breakup mech- the oxygen-sensitive dye resazurin, which is oxidized to pink anisms have been projected. In laminar bubble breakup, bina- resorufin in the presence of oxygen, and enables the quantifi- ry bubble breakup or shearing off of satellite bubbles are ex- cation of local mass transfer in the microchannel. The pro- amined at moderate flow rates. The daughter bubble size dis- duced resorufin is directly proportional the oxygen uptake into tribution has a bimodal shape and is rather broad with a sig- the liquid phase. Finally, volumetric mass transfer coefficients nificantly larger mean daughter bubble diameter than for tur- k a are determined for the refinement of two-phase flow. bulent bubble breakup. Turbulent breakup is reached at higher flow rates and mother bubbles are broken up into many small daughter bubbles of similar size. Hence, the daughter bubble Theoretical background size distribution is rather narrow and features a unimodal shape. Consequently, larger interfacial area is created [29]. Bubble generation and gas-liquid dispersion Moreover, internal jet flow within mother bubbles were ex- amined, which can also be used for bubble breakup [30]. Bubble generation is a dynamic process containing interac- Other studies were dedicated towards optimized nozzle de- tions of gas and liquid flow on short time and length scales signs regarding residence time distribution [27] and bubble [38]. In this work, primary bubble generation is carried out via breakup efficacy concerning pressure drop [28]. a T-contactor and refinement of the two-phase flow is realized The range of obtained k a values in a straight reference by a downstream micronozzle in order to create large interfa- channel is small and values are in the known range of cial area for enhanced mass transport as shown in Fig. 1. Taylor and bubbly flow in microchannels as shown in The hydrodynamics of the continuous liquid phase are de- Table 1. The volumetric mass transfer coefficients obtained termining primary bubble generation and the refinement of with the nozzles are in the upper range for micro reactors [12, two-phase flow at low void fractions [39]. In general, laminar 35, 36]. flow and liquid Reynolds numbers Re < 2300 prevail in Table 1 Literature values for −1 author experimental characteristics k a [s ] overall volumetric mass transfer l coefficient k a on gas-liquid mass Yue et al. [31] � slug flow, slμg-annular, churn flow transfer in microchannels −1 −1 � u =0–2m·s , u =0.09–1m·s 0.3–21 g l � straight microchannel, d =0.67 mm Yang et al. [32] � slug flow −1 −1 � u =0.04–0.08 m·s , u =0.16–0.27 m·s 4.1–8.9 g l � straight microchannel, d =0.50 mm � bubble formation stage (Taylor bubble) 6.3–17.1 Zhu et al. [33] � bubbly flow, slμg flow, annular flow −1 −1 � u =0.017–0.556 m·s , u =0.017–0.139 m·s 0.5–15 g l � straight microchannel, d =0.40 mm Yang et al. [34] � bubble formation stage (bubbly flow) −1 −1 � μg=0.0035–0.0046 m·s , u =0.01–0.03 m·s 15–77 � T-contactor and co-flowing device J Flow Chem (2021) 11:429–444 431 a) ρ u d l l We ¼ ð3Þ The Weber number is important for the bubble generation b) regime. Fig. 1 Primary bubble generation in a T-contactor and refinement of gas- Gas-liquid mass transfer and test reaction liquid flow in a micronozzle optimized regarding pressure drop [28]with turbulent bubble breakup (a) and in a micronozzle optimized regarding residence time distribution [27] with laminar bubble breakup (b) Liquid side volumetric mass transfer coefficient is used for quantification of the transport of a solute from the gas phase to the liquid phase as the resistance is mainly in the liquid straight microchannels and turbulent flow can only be phase [11]. Applying film theory, the concentration change achieved with comparatively high energy input [40]. In ducts, over time depends on the liquid side mass transfer coefficient Re is defined by mean flow velocity u , the channel’shydrau- l l k , the interfacial area a, the difference of equilibrium solubil- lic diameter d , and kinematic viscosity of the liquid ν . h l ity concentration c* at the interface, and the concentration in the liquid of for a given time c(t). u d 2wh l h Re ¼ with d ¼ ð1Þ l h ν wþh dc ¼ k ac −ctðÞ ð4Þ dt The hydraulic diameter is described by channel width w and channel height h. Within layered flow, radial mass trans- The mass transfer within the liquid phase is determined fer is controlled by molecular diffusion only, which is a rather with the help of an oxidation reaction originating from the slow process in the order of seconds and minutes [41]. For redox reaction network of resazurin [37]. The completely re- Re > 100, convection contributes to the mixing process [42]. duced form (colorless dihydroresorufin) is oxidized by pure In this work, Re numbers in transient regime are reached by oxygen to resorufin (pink color) and finally to resazurin with combining micro- and millichannels, keeping the pressure blue color. This reaction is sufficiently fast with an enhance- drop moderate at the same time. Fully turbulent flow can only ment factor of E = 1.03 ± 0.01 and a related Hatta number with be achieved with comparatively high energy input. Therefore, Ha = 6.66 for microchannels [32, 48]. The chemical reac- min the mean energy dissipation rate ε is an influencing parameter tion does not significantly enhance the oxygen mass transfer for mixing [7]. It is defined in Eq. (2) with total volumetric into the liquid phase. flow rate V ,pressureloss Δp, the density ρ , of the liquid tot l For the instantaneous reactions, the concentration of oxy- phase, and dissipation volume V . diss gen in the bulk liquid phase can be assumed as c(t) = 0 = const [48]. Consequently, the driving force at the gas-liquid inter- V Δp d þ 3:84d face is also constant with (c − 0), and k a can be determined tot 0 0 l ε ¼ with V ¼ d l h þ 16d h ðÞ x ð2Þ diss 0 0 0 0 1 ρ V 2 according to Eq. diss Δc k a ¼ ð5Þ The dissipation volume relies on the geometry of the tur- c ⋅Δt bulence generator (hydraulic diameter d ,length l , and height 0 0 h of smallest cross section) and the downstream channel The mass transfer coefficient is proportional to the depth h [43]. The index “−1” relates to the converging nozzle resazurin concentration at a certain location Δc divided by region, index “0” to the smallest cross section, and index “1” the oxygen saturation concentration c and the time of the to the diverging outlet nozzle part, see also Fig. 2. For bubble fluid elements after the first gas-liquid phase contact. This formation in T-contactors, three mechanisms of bubble forma- time is the mean residence time up to the location of concen- tration measurement. tion were proposed: dripping, squeezing, and jetting. These depend on contactor geometry, flow rates, and the fluids’ properties and have an impact on mass transfer [13, 44]. Here, slμg flow resulted in the T-contactor at low flow rates Experimental setup and methods and bubbly flow was obtained at higher flow rates. The breakup of these bubbles into smaller daμghter bub- Experimental setup bles in the wake of the nozzle depends on the interactions between the bubble’s surface force and the liquid’s inertia The employed microreactor setup is shown in Fig. 2a and is force [45, 46]. The liquid Weber number We puts these forces l adapted from previous works [28, 29, 49]. The microreactor into relation [47] with surface tension σ. consists of a reaction plate featuring a milled in flow channel 432 J Flow Chem (2021) 11:429–444 Fig. 2 a) 3D-modell of the microreactor setup in explosion view. b)Reactionplate with T- contactor and nozzle inlay. Arrows indicate inlet of gas and liquid phase and exit of the mix- ture. c) Close-up of the ex- changeable nozzle inlay. d) Close-up of the micronozzle with geometrical parameters d , d , −2 0 and d for hydraulic diameter of the inlet channel, the nozzle and the outlet channel, respectively, -1 1 together with the inlet and outlet angle α and α , respectively −1 1 d-2 d2 (rectangular cross section, w = 5 mm, h = 1 mm) on its upper Both nozzles N1 and N2 represent optimized geometries concerning low pressure loss N1 and dispersion intensity N2 side (cf. Figure 2b). A material recess within the reaction plate allows for the quick exchange of nozzle inlays (cf. Figure 2c) and will be investigated in the following study. The straight and thus the simple variation of nozzle geometries. The im- reference channel R serves as a reference. portant geometrical parameters are labeled in Fig. 2d. The microchannel is sealed with a view glass and two outer Experimental parameters flanges, made from stainless steel, clamp the view glass and the reaction plate together. The highly transparent The solution for mass transfer experiments is prepared with a −1 polymethylmethacrylate (PMMA) reaction plate and view concentration of 0.1 g L resazurin (Thermo Fisher Scientific glass, in combination with a light-emitting diode (LED) panel Inc., USA), 0.1 M glucose (D(+)-glucose anhydrous, AnalaR that is placed below the microreactor, enable optical observa- NORMAPUR® for analysis, VWR Chemicals, Belgium) and tion of bubble breakup and mass transfer characterization 0.3 M NaOH (pellets, VWR Chemicals, Belgium). These con- using a high-speed camera from above. centrations are adopted from Dietrich et al. [37]. The colorless Fluidic connections for liquid and gas supply and outlet flow dihydroresorufin solution is conveyed into the microreactor at are laterally attached to the reaction plate (cf. Figure 2b). The varying volumetric flow rates. However, volumetric flow rate entire experimental setup is described in an earlier contribution of oxygen (Messer Group GmbH, Germany) is held constant [28]. Themicroreactorand thenozzleinlaysaremanufactured for all presented experiments. Thus, gas content was variable by high precision drilling in the mechanical workshop of TU despite a broad spectrum of flow and bubble breakup regimes. −1 Dortmund University. A reference element is manufactured Total volumetric flow rates in the range of 20 mL min and −1 without the micronozzle. Therefore, the pressure drop induced 140 mL min are employed at resulting gas contents of GC = by adjacent channels can be determined and subtracted from the 0.07–0.5. Experiments were carried out at room temperature. measurements using nozzle inlays to obtain solely the pressure drop caused by the micronozzle. Mass transfer within the Image acquisition and processing straight reference channel is investigated and respective mass transfer coefficients serve as a benchmark for the nozzle in- Images of the bubbles moving in the microchannel are record- duced mass transfer intensification. Table 2 gives an overview ed with a monochromatic high-speed camera (Xtra Motion of different nozzle geometries. NR4, Imaging Solutions GmbH, Germany). The different Nozzle element N1 represents a compromise between levels of pink coloration, which depend on the reaction prog- grade of fine bubble dispersion and narrow residence time ress, are represented by 256 grey values in the acquired im- distribution within the channel developed by Tollkötter [27]. ages. The gray values correlate with the concentrations of Large outlet angles induce distinct recirculation zones in the resorufin or oxygen transferred into the solution. wake of the nozzle. These countercurrent flows trap bubbles The recorded images had to be digitally processed to ex- so that residence time distribution is rather broad. Nozzle inlay tract an accurate quantification of the resorufin concentration, N2 features an optimized nozzle geometry regarding bubble which then is converted into an equivalent oxygen concentra- size and pressure drop developed by Reichmann et al. [28]. tion taking stoichiometry into account. The image processing J Flow Chem (2021) 11:429–444 433 Table 2 Manufactured micronozzle designs for mass transfer characterization in micronozzles. is carried out with the Image Processing Toolbox within For high flow rates and turbulent bubble breakup, the can- Matlab (R2012a). The method from Dietrich et al. [37]is ny edge algorithm reaches its limits where strongly deformed modified for this study and described in the following. In a bubbles are not detected. Here, manual bubble detection and first step, an averaged background image from 20 images is masking is carried out. subtracted from the raw images to eliminate the effect irregu- lar backlight distribution. Pictures are inverted prior to sub- Correlation of grey values and equivalent oxygen traction to assure increasing grey values correlate with in- concentration creasing concentrations. Images are cropped to reduce neces- sary computing power. The interfaces of the bubbles lead to A calibration curve was created in order to convert the grey refraction and reflection of the transmitted light. The areas of values into equivalent oxygen concentrations. The de-facto the bubbles that appear dark due to light refraction and reflec- oxygen concentration in the solution is zero due to the instan- tion are detected with the canny edge algorithm and masked in taneous reaction of the dissolved oxygen and dihydroresorufin order to exclude them from analysis. Finally, a heat map is in the liquid phase, hence, the term “equivalent” is used. The created from grey values, indicating the local concentrations equivalent oxygen concentration can be deduced from the along the channel. The steps are visualized in Fig. 3. measured resorufin concentrations, according to Eq. (6). Fig. 3 Image processing procedure for quantification of resorufin and equivalent oxygen concentration 434 J Flow Chem (2021) 11:429–444 −1 Therefore, a grey value can be processed to an equivalent wake of the micronozzle at 20 and 25 mL min . Bubble −1 oxygen concentration. deformation occurs for 35 mL min and laminar breakup −1 for 50 and 60 mL min . Turbulent breakup regime starts at n n dihydroresorufin resorufin −1 n ¼ n ¼ ¼ 70 mL min in N2. O ;transferred O ;reacted 2 2 2 2 resazurin ¼ ð6Þ Mass transfer measurement For calibration, resorufin solutions were fed into the The reference channel serves as a benchmark for mass transfer microreactor with different concentrations and the related im- using the micronozzles. Investigated flow regimes are limited ages were analyzed. A corresponding calibration curve, which to slug and bubbly flow due to their highest interfacial areas correlates grey values and equivalent oxygen concentrations, per unit volume. These regimes are also employed in mass is shown in Fig. 4 and validates the linear trend from Dietrich transfer experiments using the micronozzle inlays. et al. [37]. Monochromatic pictures from resorufin solutions and respective equivalent oxygen concentration are displayed Reference channel mass transfer in Fig. 4b. The flow regime and related heat maps are displayed exem- plarily in Fig. 6 for the reference channel with the respective Results and discussion original images, the corrected images, and the heat maps fea- turing O -equivalent species concentration profiles. The slight Flow regime maps asymmetric concentration profile origins from the 90° gas inlet channel. Since all nozzles were investigated with this Various liquid flow rates are investigated for dispersion at generation setup there is no difference observed. constant gas flow rate. Figure 5a shows the flow regimes in The heat map images in Fig. 6 show slug flow configura- the reference channel, which are also valid for experiments tion over a total time of 9.6 ms in steps of 2.4 ms. Oxygen using micronozzles N1 and N2 for the section upstream of bubbles have been numbered for identification purposes, and the orifice with the only exception that slμg/bubbly flow is flow direction is from left to right. During bubble generation −1 observed at 35 mL min for N2. At high gas contents and in the T-contactor first O -equivalent species was already lower total flow rates, slμg flow is noticed. The Taylor bub- found between bubbles 1 and 2 at t . However, mass transfer bles always stay in contact with the channel walls and are has started, and distinct fields of higher O -equivalent species merely stretched due to the narrower channel cross section. concentration in the upper and lower channel half, with an Bubble breakup regimes are shown in Fig. 5b and c) for noz- area of lower concentration in the channel center, have already zle inlay N1 and N2, respectively. formed between bubbles 2 and 3. Here, the formation of In case of bubbly flow in front of the nozzle, a variety of Taylor vortices in the liquid compartment can be seen, which bubble behaviors were observed behind the micronozzles N1 is well known for slug flow in straight microchannels [49–54]. and N2. At moderate flow rates, a deformation of bubbles was However, since Re is relatively high (Re = 153) for slug flow l l noted. Thus, external forces are not strong enough to cause a in microchannels, the vortices show wavy movements and the breakup. For increased total volumetric flow rates, laminar diffusion process is supported by convection. The closed bubble breakup is noticed. Finally, turbulent bubble breakup Taylor vortex structure is maintained over the observed chan- regime is reached behind the micronozzle for total flow rates nel length, which can be seen in the liquid slugs between −1 greater than 100 mL min . For N2, slug flow adjusts in the bubbles 4 and 5, and bubbles 5 and 6. At t , the lower part Fig. 4 a) Calibration curve for the correlation between grey values and equivalent oxygen concentrations. b)Variation in grey values for various resorufin concentrations in the microchannel J Flow Chem (2021) 11:429–444 435 Fig. 5 a) Flow regime map for gas-liquid contacting in the T-contactor Refinement regimes of gas-liquid flow downstream of the micronozzle −1 for the reference channel and upstream of nozzle inlay N1. b) Refinement N2. V =10 mL min = const. For all experiments gas regimes of gas-liquid flow downstream of the micronozzle N1. c) −1 of the Taylor vortex has been completely formed between volumetric flow rates (V =20 mL min ,Re =153) and tot l,2 bubbles 2 and 3. The asymmetrical development of hydrody- high oxygen content (GC = 0.5). In order to allow comparison namics and mass transfer is attributed to the eccentric bubble between the flow regimes and among the reference channel generation in the T-contactor. Further, a thin center line in the and the micronozzle, a global scale 0–10 [mg O -equivalent −1 liquid compartment between bubbles 5 and 6 indicates high L ] is used and applied to all flow regimes. O -equivalent species concentration, which is accounted for Bubbles are formed in the T-contactor and once they enter by lateral film flow between bubbles and channel walls [55]. the micronozzle, they are stretched in length before returning The evolution of the thin center line can be followed, tracking to their previous shape eventually behind the diverging chan- the liquid slug between bubbles 3 and 4 over time (t -t ). The nel. The concentration increase of O -equivalent species can 0 4 2 obtained concentration profiles correlate well with experimen- already be noticed in the original and the corrected image in tal results [32] and CFD simulations [56]. Fig. 7. The image series is subdivided into the upstream and downstream region of the micronozzle and covers a total time Micronozzle mass transfer in Taylor bubble flow span of 9.6 ms in 2.4 ms steps. Even though slug flow sets in upstream of the nozzle, a fully developed Taylor vortex can- Mass transport for two-phase flow through micronozzles is not be observed in contrast to the straight reference channel. qualitatively examined for the observed flow regimes up- Here, the channel length leading to the orifice seems to be too and downstream of the nozzle in the following and exemplar- short. Merely a slight increase in concentration can be detect- ily shown for micronozzle N1. The flow combination slug ed in the liquid slugs between bubbles 1 and 2 and bubbles 2 and 3 (Fig. 7, t ), respectively. However, the evolution of flow - slug flow (“flow configuration upstream of the nozzle” - “bubble behavior downstream of the nozzle”)is shown in Taylor vortices can be sensed, since areas of higher concen- trations form at the channel walls with the characteristic Fig. 7 with the original and the corrected images for low total Fig. 6 Image series of heat maps for mass transfer in slug flow in the reference channel R with respective local concentration profiles 436 J Flow Chem (2021) 11:429–444 Fig. 7 Image series of heat maps for Taylor flow and bubble deformation through micronozzle N1 with respective local concentration profiles upstream (left hand side) and downstream (right hand side) of the nozzle, global scale 0–10 [mg O -equiv- −1 alent L ] poorly-mixed zone in the channel center between bubbles 3 concentrations are generally higher downstream of the orifice and4(Fig. 7, t ). The asymmetric flow development is linked despite an increased flow rate and thus shorter time for mass to the eccentric bubble generation again. transport. −1 Once a liquid slug enters the nozzle, it is stretched in length For increased flow rates of V =80 mL min and GC = tot and the concentration profile seems to be homogenized across 0.13 bubbly flow is still observed upstream of the micronozzle the channel width (Fig. 7, liquid slug between bubble 4 and (Fig. 8b). However, bubbles break up in laminar regime 5at t ). In the diverging nozzle outlet, a well-mixed zone can downstream of the orifice. No significant alternations are de- be found in the channel center in the form of a thin jet, which tected in respect to bubbly flow configuration or concentration seems to evolve from bubble 6 to bubble 7 (Fig. 7, t ). profiles upstream of the nozzle compared to the previously −1 However, this jet emerges and disappears rapidly and the con- observed phenomena for V = 60 mL min . The bubbles tot centration profile in between the bubbles homogenizes quick- break up in laminar regime; hence, greater mass transport area ly and; in the end, the liquid slugs seem evenly mixed (Fig. 7, is created and an increase in mass transfer is expected. Both liquid slug between bubbles 9 and 10 at t ). laminar breakup regimes are observed: binary breakup and shearing off of small satellite bubbles. The external forces exceeded a critical value, so that the Micronozzle mass transfer in laminar bubble break up surface tension is no longer sufficiently high to keep the bub- When the total volumetric flow rate is increased (V = bles intact (We = 25) and; finally, the bubbles burst. Binary tot l,2 −1 breakup is observed, tracking bubble 8 in the downstream 50 mL min , GC = 0.2, Re = 613), flow transitions to bub- l,2 bly flow upstream and bubble deformation downstream of the images over time in Fig. 8 b). At t , bubble 8 is stretched in length due to the smaller cross section and accelerated core micronozzle (Fig. 8a). Bubble train formation is observed ahead of the orifice. Downstream of the nozzle, external forces flow. The bubble is then decelerated, compressed (t and t ), 1 2 and takes a characteristic dumbbell shape at t . Once a critical act on the bubble and actually surpass the surface tension to a multi-fold extent (We = 8.12). Nonetheless, a critical value neck diameter is exceeded [27], the bubble splits into two l,2 daughter bubbles (t ). The little, unnumbered bubbles over is obviously not exceeded and the bubbles are merely de- formed but not broken up, regaining their former shape the course of the channel result from shearing off mechanism. A clear increase in concentration is found across the complete eventually. The concentration profiles upstream of the nozzle coincide channel further downstream. Nonetheless, a slight gradient is with those from the bubble train formation in the reference present across the channel width with higher concentrations at the lower channel wall, which can be explained by the flow channel. A distinct O -equivalent species trail can be observed between the bubbles in flow direction, representing well- around the bubbles and the bubbles’ path through the channel. A rise in concentration is found at the location of bubble mixed areas. Flow disturbances in the form of velocity fluc- tuation in the wake of the micronozzle seem to affect the breakage hinting to an intensification of mass transfer. ordered shear flow and the vortex structures in between the bubbles (Re = 1226). Hence, turbulent motions in the liquid Micronozzle mass transfer in turbulent bubble break up l,0 are promoted, resulting into enhanced mass transfer from the gas into the liquid phase. Comparing bubbly flow in the Turbulent bubble breakup is observed in the micronozzle N1 −1 straight microchannel and the one containing the nozzle, for a flow rate of V = 120 mL min (Re =3370, Re = tot l,0 l,2 J Flow Chem (2021) 11:429–444 437 Fig. 8 Picture series of heat maps for bubbly flow through the micronozzle N1 and laminar bubble breakup at Re =613 (a)) l,2 and 1071 (b)) with respective local concentration profiles at global scale 0–10 [mg O -equiv- −1 alent L ]. The flow regime in b) can also be treated as transition to turbulent regime, since smaller daughter bubble appear 1685) and an oxygen content of GC = 0.08. Corresponding hence, the focus is only on the region of bubble breakup. heat map images are shown in Fig. 9 for O -equivalent species Bubble movement and deformation indicate strong dynamics concentration. Upstream of the nozzle, bubbly flow is ob- in the wake of the smallest cross section and the external served; however, bubble diameter is smaller compared to pre- forces tear the mother bubbles apart (We =61). Numerous, l,2 vious flow rates and gas contents. Nonetheless, two O -equiv- significantly smaller daughter bubbles are generated and an alent species trails are still detected between the bubbles, increased phase boundary is obtained. Fig. 9 Picture series of heat maps for bubbly flow through the micronozzle N1 and turbulent bubble breakup behind it with respective local concentration profiles at global scale 0–10 [mg −1 O -equivalent L ] 2 438 J Flow Chem (2021) 11:429–444 Figure 9 depicts heat map images of O -equivalent species In general, the equivalent O -equivalent species concentrations 2 2 concentration for bubbly flow through the micronozzle at glob- increase over the observed channel section for slug and bubbly al scale for a time span of 0.4 ms in 0.1 ms steps. No significant flow within the straight reference microchannel, which corre- increase in concentration can be seen ahead of the smallest sponds well with studies by Krieger et al. [51] and Yang et al. cross section and in the smallest cross section. However, shortly [32]. The filled symbols originate from linear regressions fitting ahead of the end of the divergent nozzle outlet there is a distinct the respective experimental data in positions from −2to2, which rise in concentration, particularly, next to the lower channel have been introduced for the micronozzles. All coefficients of wall. This location marks the starting point of bubble breakup, determination are R > 0.97 for the linear trends and indicate a too. The increase in concentration occurs mainly at the lower good linear fit. Light grey filled symbols resemble slug flow and channel wall, but with some distance to the nozzle. Higher black ones portray bubbly flow configuration in the reference concentrations are found across the entire channel width. channel. Higher flow rates show decreasing slopes of the graphs. Despite a significantly shorter residence time in the observed Therefore, largest concentration increase is found for slug flow at −1 area due to higher flow rates, the obtained concentrations are V = 20 and 25 mL min with Re = 153 and 230. The regime tot l,2 actually slightly higher in Fig. 9 than for laminar breakup in transitions to bubbly flow for higher flow rates (V =35– tot −1 Fig. 8 b) indicating an intensified mass transfer). 140 mL min ) and a smaller increase in concentration is found −1 with the smallest gain for 140 mL min . Reynolds numbers are in the range of 383 < Re < 1991 indicating mainly laminar flow. Quantitative mass transfer evaluation l,2 A distinct drop to lower concentrations at position −2, which is right behind the T-contactor, is evident between 50 Reference channel - concentration profiles −1 and 60 mL min and can be explained by the transition in bubble formation mechanism from dripping to jetting. In order to evaluate the mass transfer in the investigated chan- The time a bubble remains in the considered area must be nels, O -equivalent species concentrations for various flow considered, when this data is evaluated regarding mass transfer rates are determined along the channel length and displayed coefficients. The residence time within the considered area de- in Fig. 10 over the respective time. The concentration is drawn creases with increasing volume flows. In Fig. 10b, concentra- over the mean flow time connected with the respective posi- tions are plotted over the mean flow time until positions from −2 tion in the channel. Total superficial velocities are used for this to 2 are reached. As the mean velocity in the reference channel purpose, which are in very good agreement with velocities remains constant for a specific flow rate, the linear interrelation- determined by bubble tracking over a defined amount of time ship between concentration and channel length transfers to and channel length in image analysis. Fig. 10 a) Concentrations are plotted over the course of the reference scale. Light grey filled symbols resemble Taylor flow and black symbols channel for various flow rates. Unfilled symbols resemble display bubbly flow regime. Highlighted, thicker lines are representative concentrations obtained from experiments. Standard deviations for for the observed regimes. b) Concentrations between positions −2and2 every measured concentration are exemplarily shown for four for the investigated volumetric flow rates plotted over time until the experiments. Linear trend lines are added and used to extrapolate respective position is reached by the flow. For higher clarity the time concentrations between the positions −2 and 2. The channel positions in scale is plotted on a logarithmic scale the diagram a) can be roughly transferred to the top image by the length J Flow Chem (2021) 11:429–444 439 concentration and flow time. From the slope m of the graphs At higher flow rates, bubbly flow sets in upstream and −1 and taking c*(c*= 8 g L ) into account, volumetric mass bubble deformation downstream of the nozzle (V = 35, 50, tot −1 transfer coefficient k a can be obtained using Eq. (5). A loga- 60 mL min , dark grey filled symbols). Compared to slug rithmic scaling of the time axis is selected here for a clearer data flow, the overall mean gradient declines, which is also ob- presentation; however, the linear rise of concentration over time served in the straight reference channel for increasing flow is obscured and compressed due to the logarithmic display. The rates. The bend in the graphs’ courses across the nozzle section graphs’ slopes are very similar, indicating a narrow span of mass gets more and more pronounced. Here, mass transport inten- transfer coefficient values for flow in the reference channel. sification is not strong enough to compensate for the shorter time spent in the nozzle section. Reynolds numbers behind the nozzle take values of Re = 383, 613, and 766, respectively. Micronozzle N1 and N2 - concentration profiles l,2 Even higher values are reached within the smallest cross sec- tion with Re = 766, 1225, and 1531 while laminar flow is the The O -equivalent species concentrations from experiments l,0 superordinate flow type. Weber numbers take values of using micronozzle N1 and N2 are displayed in Fig. 11. We = 3.17, 8.12, and 12.68. However, external forces are Highlighted data (thicker lines) are characteristic for the respec- l,2 not sufficiently high in order to break the bubbles. tive flow regimes. The concentration increase across the ob- ̇ Once the bubbles begin to break up behind the micronozzle served area is the largest for slug flow (V =20, tot −1 −1 in laminar regime (V = 70, 80, and 90 mL min , black sym- 25 mL min , light grey filled symbols). In this regime, the tot bols containing crosses), a turning point is reached. The gra- graphs’ courses are similar to the ones from the reference chan- dient between positions −2and − 1 continues to decline with nel as the concentration increase takes an approximate linear increased volume flow. However, the steady drop in concen- progression over the channel length. For micronozzle N1 in tration in position 1 with increasing flow rates comes to an Fig. 11a, the gradient between position −1and1isreduced −1 end; hence, the concentration in position 1 for 90 mL min is slightly employing the nozzle. Due to the narrower channel −1 above the one for 80 mL min despite a shorter residence cross section and accelerated flow, thenozzlesectionispassed time in the channel up to that location. Mass transport process- in shorter time. Hence, there is less time for the mass transfer to es seem to be strongly enhanced, significantly outweighing progress as the superordinate flow regime is maintained across shorter residence times. Furthermore, it is characteristic for the smallest cross section, and mass transport is not significant- this regime that the slope of the graph behind the nozzle is ly enhanced, which explains the bend in the graphs. Reynolds steeper than the slope of the graph ahead of the nozzle. This is numbers within the smallest cross section take values of Re = l,0 probably due to the larger mass transfer area resulting from 306 and 460, respectively. Downstream of the nozzle the flow bubble breakup and the multi-directional motions intensifying is gradually decelerated, and Reynolds numbers are Re =153 l,2 the flow around the bubbles and consequently the gas-liquid and 230. Consequently, the influence of the nozzle in terms of mass transfer. Reynolds numbers equal Re = 919, 1072, and mass transfer enhancement seems to be rather small in slug flow l,2 1225 behind the nozzle and Re = 1838, 2144, and 2450 regime. Weber numbers are We =0.507 and We =1.141; l,0 l,2 l,2 within the smallest cross section. The increased inertial forces yet, bubbles do not break up despite inertia force exceeding −1 of the fluid acting on the bubbles and finally lead to their surface tension for V =25 mL min . tot Fig. 11 Concentrations in positions from −2 to 2 are plotted over time until the respective position is reached by the flow for nozzle N1 in a)and forN2 in b) 440 J Flow Chem (2021) 11:429–444 binary breakup or shearing off of satellite bubbles with mass transfer coefficient and they are plotted over flow rate We = 18.26, 24.85, and 32.46 for the respective flow rates. in Fig. 12a. The time difference steadily declines with increas- l,2 For turbulent bubble breakup regime (V = 100, 120, and ing flow rates for the reference channel, nozzle N1, and nozzle tot −1 140 mL min , black symbols), the concentration increases in N2. Time is reciprocally proportional to volume flow rate position 1 continues as recirculation zones evolve in the wake of explaining the hyperbolic course of the graphs. The mean flow the nozzle. The concentration gradients across the nozzle section time from −2 and 2 is slightly lower for the nozzles due to the are clearly steeper than the mean gradients across the complete narrower cross section and resulting flow acceleration. The channel length for this regime. The slopes downstream of the dissimilarity between the nozzle inlays is marginal. nozzle are significantly increased compared to the one ahead of The concentration difference between positions −2and2 the nozzle, presumably due to the increased interfacial area and steadily declines for the reference channel R, too. The flow is the turbulent motions, which lead to intensified flow around the not disturbed along the channel length, where laminar flow bubbles. Reynolds number rise to Re = 1379, 1685, and 1991 regimes prevail as residence time decreases simultaneously, l,2 behind the micronozzle and Re = 2758, 3370, and 3982 in the resulting in less time for mass transfer. All three concentration l,0 −1 smallest cross section indicating transition to turbulent flows. graphs take a similar progression up to 50 mL min . For N2, Weber numbers take values of We = 41.09, 61.38, and 85.72. a minimum in concentration difference is reached here and the l,2 The data from experiments using micronozzle N2 is present- concentration difference increases again when reaching bub- ed in Fig. 11b. Highlighted data (thicker lines) are characteristic ble breakup regimes. The transition to bubble breakup occurs −1 −1 for the respective flow regime. The concentration along the at 70 mL min for N1 with a minimum at 60 mL min . microchannel length are following in general the observed Bubble breakup leads to an increase in concentrations despite trends from nozzle N1. An approximate linear increase in con- shorter residence times in the observed region. centration over channel length is noticed for slug flow (V = 20, Finally, volumetric mass transfer coefficients between po- tot −1 25 mL min ) while the trend for bubbly flow in the upstream sitions −2 and 2 are determined with Δt, Δc, and the satura- −1 region and laminar bubble breakup behind the nozzle (V =50 tion concentration c*=8gL according to Eq. (5). The re- tot −1 and60mLmin , dark grey filled symbols) shows a reverse sults for all inlays are displayed in Fig. 12b. In the reference trend with increasing concentrations in position 1. The flow is channel R, k a values increase with higher flow rates initially. decelerated more abrupt due to a larger outlet angle for N2 and The flow regime in the reference channel transitions from slug −1 velocity fluctuations occur at lower flow rates compared to N1. to bubbly flow between 25 and 35 mL min , where bubbly The predominant laminar breakup mechanism in nozzle N2 is flow features larger interfacial area per unit volume leading to bubble shearing at We = 6.09 and 9.51, respectively. the initial rise. Then, the slope declines, and a maximum is l,2 −1 The gradients across the nozzle section grow steeper for reached at 60 mL min . Subsequently, the k a values decrease turbulent bubble breakup (V = 70, 80, 90, 100, 115, 120, in asymptotic behavior with larger volume flows. tot −1 and 140 mL min , black squared symbols containing crosses For nozzle N1, the graph’s course is similar to the one of −1 and black symbols). Within a few milliseconds the concentra- the reference channel R up to 50 mL min . Bubble deforma- −1 tions in position 1 jump to higher levels. The Weber numbers tion is observed in N1 at 35, 50, and 60 mL min and laminar −1 behind the micronozzle range from 13.7 < We <64.3. bubble breakup is observed for 70, 80, and 90 mL min . l,2 Mean flow times have to be considered once again regard- Apparently, the influence of the nozzle is already existent −1 ing mass transfer evaluation and; consequently, Fig. 11b for V ≤ 50 mL min as k a values are slightly higher than tot l shows concentrations applied over time for the employed flow the ones in the reference channel for a given flow rate. −1 rates. In general, the same applies to N2 as to N1; the higher Pressure drop for 50 mL min is 19 mbar and, therefore, only the flow velocity, the shorter the average residence time in the slightly higher than for the reference channel. Thus, the impact area under consideration. In addition, the concentrations in of the nozzle on mass transfer rises significantly for V ≥ tot −1 positions −2and − 1 decrease continuously the higher the flow 60 mL min and bubble breakup. Particularly, once bubble rate is and the slopes of the graphs within the micronozzle breakup is reached, the two graphs diverge greatly with con- section grow steeper and steeper. The jump to higher concen- siderably higher mass transfer coefficients for nozzle N1. A trations between positions −1and1in bubblebreakup regimes maximum pressure drop of 150 mbar is measured at −1 −1 is even more precipitous for N2 than for N1. The trend reversal 140 mL min with a k a =46 s . in terms of rising concentrations in positions 1 and 2 for higher In micronozzle N2, even higher mass transfer coefficients −1 flow rates is already detectable at 50 mL min in Fig. 11b. are reached; however, they come at the cost of higher pressure drop. The graph’s course is similar to the one of N1 for V ≤ tot −1 Mass transfer coefficients for R, N1, and N2 50 mL min . For higher flow rates and bubble breakup, the twographs diverge andnozzle N2provides highest k a values. The concentration and time difference between positions −2 Pressure drop is in the range of 16–520 mbar. Highest k a −1 −1 and 2 are the important parameters in terms of volumetric value is obtained for 140 mL min with k a =60 s .The l J Flow Chem (2021) 11:429–444 441 Fig. 12 a) Concentration and time differences between positions −2 and 2 over flow rate for the reference channel R, micronozzle N1, and micronozzle N2. b) Mass transfer coefficients for the region between positions −2 and 2 over flow rate for the reference channel R, micronozzle N1, and micronozzle N2 volumetric mass transfer coefficients obtained with the nozzle Figure 13 shows k a values (channel region −2 - 2) plotted are in the upper range of microreactors. over experimental mean energy dissipation rates obtained for the micronozzle region (pos. -1 - 1) and for calculated mean energy dissipation rates resembling the sum of experimental Pressure drop and mass transfer rate values for the micronozzle region and theoretical values for the adjacent channels (pos. -2 to 2). The data are separately fit In order to evaluate a cost/benefit ratio, the pressure drops are with power law trend lines (dashed lines in grey) for bubble presented and the mean energy dissipation rates are linked to breakup and no bubble breakup. Coefficients of determination the overall mass transfer coefficients. As the reference channel 2 are R ≥ 0.93 indicating good fits. Deviations of −15% and + pressure drop is subtracted to exclude pressure drops from the 15% are indicated with dotted lines in black. Hence, an in- inlets or the downstream meandering channel, only the pres- creasing mass transfer coefficient comes at the cost of higher sure loss induced by the micronozzle is accessed (between energy input into the system. However, these correlations are positions −1 and 1). However, pressure drop and mass transfer not dimensionally accurate. Thus, in the next step, mean en- have to be evaluated for the identical channel region and vol- ergy dissipation rate is related to the kinematic viscosity ν and umetric mass transfer coefficients between positions −2and2 the square root of the quotient is taken, matching the unit from have been assessed before. Therefore, the theoretical pressure −1 k a with s . This quotient is the inverse Kolmogorov time drop of the straight channel sections −2to −1 and1to 2is scale in vortex dissipation [33]. calculated and added to the experimentally determined value The fitting with linear trend lines of data in Fig. 13 reveals from the micronozzle, represented by the unfilled symbols. high accuracy and allows for predicting mass transfer coeffi- Nevertheless, the graphs do not differ much to those excluding cients based on energy input and kinematic viscosity. A the adjacent channel sections. In general, higher pressure drop trendline is assigned to each of the flow regimes: no bubble is induced by N2 compared to N1, which is most probably breakup, laminar bubble breakup, and turbulent bubble break- based on flow detachment and more intensive recirculation up. An overview of the corresponding equations is presented zones in the wake of the nozzle [57]. in Table 3. A fit encompassing laminar and turbulent breakup The pressure loss coefficients obtained in the experiments is additionally given, also displaying high accuracy. over mean Reynolds number for nozzle inlays N1 and N2 For low flow rates and energy input, the k a values remain decrease with increasing Reynolds number; however, they at a constant level. Once bubble breakup begins at approxi- take constant values for Re ≥ 1500. Larger pressure loss co- 0.5 −1 mately (ε / ν ) ≈ 28.800 s (for both nozzle inlays), k a l l efficients are obtained for nozzle N2, which is based on the increases. The corresponding trend lines for the individual increased pressure drop despite similar flow velocities. Fig. 13 Overall mass transfer coefficient determined between positions −2 and 2 plotted over mean energy dissipation rate 442 J Flow Chem (2021) 11:429–444 Table 3 Overview of trend lines from Fig. 13. Also, trend line equations encompassing laminar and turbulent regime are shown, where the bold data are valid over the entire investigated range 0.5 Data regime trend line from k a=A (ε / ν ) +B coefficient of determination l l −4 −1 2 N1 & N2 exp. no breakup A: 2.794⋅10 B: 7.3164 [s]R =0.95 −4 2 laminar breakup 4.092⋅10 4.3088 R =0.86 −4 2 turbulent breakup 3.012⋅10 14.604 R =0.96 −4 2 breakup (lam. & turb.) 3.682⋅10 7.5264 R =0.94 −4 −1 2 N1 & N2 exp. + adj. Channels calc. no breakup A: 4.699⋅10 B: 7.2086 [s]R =0.91 −4 2 laminar breakup 8.948⋅10 1.1339 R =0.98 −4 2 turbulent breakup 6.679⋅10 10.595 R =0.96 −4 2 breakup (lam. & turb.) 7.782⋅10 4.7290 R =0.97 Abbreviations GC, gas content; LED, light emitting diode; N1, nozzle regimes are presented for no bubble breakup and bubble design 1; N2, nozzle design 2; O , oxygen; pos, position; PMMA, breakup. polymethylmethacrylate; R, reference channel Subscripts and superscripts *, solubility concentration.; -2, position −2, upstream channel.; -1, position −1, nozzle inlet.; 0, position 0, smallest Conclusion cross section.; 1, position 1, nozzle outlet.; 2, position 2, downstream channel.; diss, dissipation.; l, liquid.; tot, total. Since gas-liquid mass transfer is often limiting chemical −1 Latin letters A, coefficient, −.; B, coefficient, s .; c, concentration, kg reactions in flow reactors, a colorimetric method based on −1 L .; d , hydraulic diameter, m.; E, enhancement factor, −.; h , channel the oxygen sensitive dye resazurin is used in order to height, m.; Ha , Hatta number, −.; k a , volumetric mass transfer coeffi- −1 visualize and quantify mass transfer during the refinement cient, s .; l , length, m.; n , amount of substance, mol.; p , pressure, Pa.; Re , Reynolds number, −.; Re , Reynolds number in the nozzle, −.; Re of two-phase flow using micronozzles. Mass transfer phe- l,0 l,2 , Reynolds number in the outlet channel, −.; Re , Reynolds number of the nomena are shown for slug flow through micronozzles, −1 liquid phase, −.; t , time, s.; u , mean flow velocity, m s .; V , volumetric −1 3 bubble deformation, laminar bubble breakup, and turbu- flow rate, L s .; V ,volume,m .; w, channel width, m.; We, Weber lent bubble breakup. The gas-liquid mass transfer is quan- number, −.; We , Weber number in the nozzle, −.; We , Weber number l,0 l,2 in the outlet channel, −.; We , Weber number of the liquid phase, −.; x, tified for these regimes concerning volumetric mass trans- channel length, mm. fer coefficient k a and benchmarked against mass trans- port in a straight reference microchannel. Moreover, two Greek letter Α,angle,°.; Δ, difference.; ε , mean energy dissipation 2 −3 2 −1 −3 nozzle designs are evaluated. The reference channel yields rate, m s .; ν, kinematic viscosity, m s .; ρ, density, kg m .; σ, −1 −1 surface tension, N m . mass transfer coefficients in the range of 7–12 s for slug and bubbly flow with higher values for the latter as more Acknowledgments The authors would like to thank the Mechanical interfacial area per unit volume is evident. A nozzle de- Workshop of TU Dortmund University for excellent manufacturing of the micronozzle equipment. Additionally, we would like to acknowledge sign optimized in respect to pressure loss and residence C. Schrömges (Laboratory of Equipment Design) for technical assistance. time distribution of the gaseous phase and bubble diame- ter yields mass transfer coefficients in the range of 9– Availability of data and material Not applicable. −1 46 s . Mass transfer is greatly enhanced for bubble breakup in the wake of the nozzle and highest values Code availability Not applicable. are reached for turbulent breakup at the costs of slightly Funding Open Access funding enabled and organized by Projekt DEAL. elevated pressure drop. The optimized nozzle design, in respect to pressure drop and bubble diameter, yields even −1 higher k a values in the range of 9–60 s .For slugflow Declarations and bubble deformation the micronozzles produce similar Conflicts of interest/competing interests The authors declare that they results; yet, diverge for higher flow rates with improved have no known conflicts of interest/competing interests. results for the optimized pressure drop design. The achieved mass transfer coefficients are correlated with Open Access This article is licensed under a Creative Commons the energy dissipation rate within the micronozzles. A Attribution 4.0 International License, which permits use, sharing, adap- correlation is derived based on the inverse Kolmogorov tation, distribution and reproduction in any medium or format, as long as time scale in vortex dissipation with good agreement. you give appropriate credit to the original author(s) and the source, pro- Micronozzles in combination with millichannels present vide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included a great opportunity to enhance gas-liquid mass transfer. J Flow Chem (2021) 11:429–444 443 in the article's Creative Commons licence, unless indicated otherwise in a segmented flow in microchannels. Chem Eng Sci 60(22):5895– credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by 19. 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Chem Eng Sci 169:151–163 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Journal of Flow Chemistry Springer Journals

Gas-liquid mass transfer intensification for bubble generation and breakup in micronozzles

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Springer Journals
Copyright
Copyright © The Author(s) 2021
ISSN
2062-249X
eISSN
2063-0212
DOI
10.1007/s41981-021-00180-3
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Abstract

The local gas-liquid mass transfer was characterized during bubble generation in T-contactors and in an adjacent micronozzle. A colorimetric technique with the oxygen sensitive dye resazurin was investigated to visualize gas-liquid mass transfer during slug flow, bubble deformation, as well as laminar and turbulent bubble breakup in the wake of a micronozzle. Two optimized nozzle geometries from previous studies were evaluated concerning volumetric mass transfer coefficients for low pressure loss, narrow −1 residence time distribution, or high dispersion rates. Highest values in k a up to 60 s were found for turbulent bubble breakup and an optimized micronozzle design in respect to pressure drop and dispersion rate. The achieved mass transfer coefficients were correlated with the energy dissipation rate within the micronozzles and with the inverse Kolmogorov time scale in vortex dissipation in good agreement for laminar and turbulent breakup regimes. . . . Keywords Gas-liquid mass transfer Micronozzle Bubble breakup Resazurin oxidation Introduction mass transport limitations can prevent the intrinsic reaction kinetics from evolving, resulting in extended reaction times, Gas-liquid reactions are highly relevant in chemistry such as poor reactor performance, and often low product quality [9]. for oxidation, halogenation, or hydrogenation [1–6], which Here, microstructured reactors offer increased surface-to- play an important role for pharmaceutical and fine chemical volume ratio and benefit from large interfacial area enabling industry [7]. In these applications, mass transfer of a gaseous fast mixing and reduced transfer resistances [10]. component into the liquid phase is the crucial and rate-limiting Consequently, reaction rates are enhanced and gas-liquid step. In conventional equipment, gaseous reactants are often mass transfer can be intensified applying continuous flow re- used in large excess due to poor interfacial mixing [8]. In case actors [7]. Gas-liquid reactions in microreactors therefore of insufficient mixing in processes including rapid reactions, have been subject to a variety of academic studies regarding hydrodynamics and mass transfer [10–18]. In gas-liquid microreactors the contacting of the two phases Scientific highlights: can be realized by either keeping both phases continuous with mass transfer during bubble generation nearly constant for various flow a stabilized interface (e.g. falling film or membrane reactor) or rates by dispersing one phase into the other using appropriate inlets energy dissipation rate is important parameter for dispersion and mass transfer or micromixers (e.g. T-junction, Y-contactor, flow focusing) rapid expansion after micronozzle leads to small bubble with high mass [19]. Particularly, micromixers are employed for intense transfer mixing as high specific interfacial area can be attained [20]. inverse Kolmogorov time scale determines the mass transfer coefficient Often, the gas phase is dispersed into the liquid phase using a in bubble breakup T-contactor and is then refined in a downstream micronozzle for increased interfacial area. The mass transfer is closely as- * Norbert Kockmann norbert.kockmann@tu-dortmund.de sociated with two-phase flow patterns, which rely on channel characteristics, fluid properties, and process parameters. Department of Biochemical and Chemical Engineering, Laboratory Typical flow regimes encountered in microchannel gas– of Equipment Design, TU Dortmund University, liquid flow include parallel flow, slug flow, and bubbly flow 44227 Dortmund, Germany 430 J Flow Chem (2021) 11:429–444 with respective transition regimes [21]. The interfacial area The concluded volumetric mass transfer coefficients for per unit volume increases from parallel flow to bubbly flow bubble breakup correspond to the obtained k a value range [22]; however, due to prevailing surface forces in capillaries, found by Yang et al. [34] for bubble formation in a T- slug flow is encountered for the most part [23]. contactor, which is also a very dynamic process justifying a Converging–diverging micronozzles are used to disperse comparison. gas-liquid flow with low pressure loss, breaking up bubbles In this work, mass transfer processes related to micronozzle into significantly smaller ones; hence, larger interfacial area is induced bubble breakup are examined and quantified for the created [24–28]. External forces, originating from the liquid, known bubble breakup regimes. Moreover, two optimized act on the phase boundary. Once the bubble preserving nozzle geometries developed by Tollkötter [27]and Laplace pressure is surpassed, bubble breakup is induced. Reichmann et al. [28] are analyzed concerning their gas- The degree of bubble breakup is determined by energy dissi- liquid mass transfer characteristics. Measurements are based pation rates [27]. on a colorimetric method introduced by Dietrich et al. [37]. In a previous fundamental study [28], various bubble This non-invasive technique uses a colorless, reduced form of breakup regimes have been characterized and breakup mech- the oxygen-sensitive dye resazurin, which is oxidized to pink anisms have been projected. In laminar bubble breakup, bina- resorufin in the presence of oxygen, and enables the quantifi- ry bubble breakup or shearing off of satellite bubbles are ex- cation of local mass transfer in the microchannel. The pro- amined at moderate flow rates. The daughter bubble size dis- duced resorufin is directly proportional the oxygen uptake into tribution has a bimodal shape and is rather broad with a sig- the liquid phase. Finally, volumetric mass transfer coefficients nificantly larger mean daughter bubble diameter than for tur- k a are determined for the refinement of two-phase flow. bulent bubble breakup. Turbulent breakup is reached at higher flow rates and mother bubbles are broken up into many small daughter bubbles of similar size. Hence, the daughter bubble Theoretical background size distribution is rather narrow and features a unimodal shape. Consequently, larger interfacial area is created [29]. Bubble generation and gas-liquid dispersion Moreover, internal jet flow within mother bubbles were ex- amined, which can also be used for bubble breakup [30]. Bubble generation is a dynamic process containing interac- Other studies were dedicated towards optimized nozzle de- tions of gas and liquid flow on short time and length scales signs regarding residence time distribution [27] and bubble [38]. In this work, primary bubble generation is carried out via breakup efficacy concerning pressure drop [28]. a T-contactor and refinement of the two-phase flow is realized The range of obtained k a values in a straight reference by a downstream micronozzle in order to create large interfa- channel is small and values are in the known range of cial area for enhanced mass transport as shown in Fig. 1. Taylor and bubbly flow in microchannels as shown in The hydrodynamics of the continuous liquid phase are de- Table 1. The volumetric mass transfer coefficients obtained termining primary bubble generation and the refinement of with the nozzles are in the upper range for micro reactors [12, two-phase flow at low void fractions [39]. In general, laminar 35, 36]. flow and liquid Reynolds numbers Re < 2300 prevail in Table 1 Literature values for −1 author experimental characteristics k a [s ] overall volumetric mass transfer l coefficient k a on gas-liquid mass Yue et al. [31] � slug flow, slμg-annular, churn flow transfer in microchannels −1 −1 � u =0–2m·s , u =0.09–1m·s 0.3–21 g l � straight microchannel, d =0.67 mm Yang et al. [32] � slug flow −1 −1 � u =0.04–0.08 m·s , u =0.16–0.27 m·s 4.1–8.9 g l � straight microchannel, d =0.50 mm � bubble formation stage (Taylor bubble) 6.3–17.1 Zhu et al. [33] � bubbly flow, slμg flow, annular flow −1 −1 � u =0.017–0.556 m·s , u =0.017–0.139 m·s 0.5–15 g l � straight microchannel, d =0.40 mm Yang et al. [34] � bubble formation stage (bubbly flow) −1 −1 � μg=0.0035–0.0046 m·s , u =0.01–0.03 m·s 15–77 � T-contactor and co-flowing device J Flow Chem (2021) 11:429–444 431 a) ρ u d l l We ¼ ð3Þ The Weber number is important for the bubble generation b) regime. Fig. 1 Primary bubble generation in a T-contactor and refinement of gas- Gas-liquid mass transfer and test reaction liquid flow in a micronozzle optimized regarding pressure drop [28]with turbulent bubble breakup (a) and in a micronozzle optimized regarding residence time distribution [27] with laminar bubble breakup (b) Liquid side volumetric mass transfer coefficient is used for quantification of the transport of a solute from the gas phase to the liquid phase as the resistance is mainly in the liquid straight microchannels and turbulent flow can only be phase [11]. Applying film theory, the concentration change achieved with comparatively high energy input [40]. In ducts, over time depends on the liquid side mass transfer coefficient Re is defined by mean flow velocity u , the channel’shydrau- l l k , the interfacial area a, the difference of equilibrium solubil- lic diameter d , and kinematic viscosity of the liquid ν . h l ity concentration c* at the interface, and the concentration in the liquid of for a given time c(t). u d 2wh l h Re ¼ with d ¼ ð1Þ l h ν wþh dc ¼ k ac −ctðÞ ð4Þ dt The hydraulic diameter is described by channel width w and channel height h. Within layered flow, radial mass trans- The mass transfer within the liquid phase is determined fer is controlled by molecular diffusion only, which is a rather with the help of an oxidation reaction originating from the slow process in the order of seconds and minutes [41]. For redox reaction network of resazurin [37]. The completely re- Re > 100, convection contributes to the mixing process [42]. duced form (colorless dihydroresorufin) is oxidized by pure In this work, Re numbers in transient regime are reached by oxygen to resorufin (pink color) and finally to resazurin with combining micro- and millichannels, keeping the pressure blue color. This reaction is sufficiently fast with an enhance- drop moderate at the same time. Fully turbulent flow can only ment factor of E = 1.03 ± 0.01 and a related Hatta number with be achieved with comparatively high energy input. Therefore, Ha = 6.66 for microchannels [32, 48]. The chemical reac- min the mean energy dissipation rate ε is an influencing parameter tion does not significantly enhance the oxygen mass transfer for mixing [7]. It is defined in Eq. (2) with total volumetric into the liquid phase. flow rate V ,pressureloss Δp, the density ρ , of the liquid tot l For the instantaneous reactions, the concentration of oxy- phase, and dissipation volume V . diss gen in the bulk liquid phase can be assumed as c(t) = 0 = const [48]. Consequently, the driving force at the gas-liquid inter- V Δp d þ 3:84d face is also constant with (c − 0), and k a can be determined tot 0 0 l ε ¼ with V ¼ d l h þ 16d h ðÞ x ð2Þ diss 0 0 0 0 1 ρ V 2 according to Eq. diss Δc k a ¼ ð5Þ The dissipation volume relies on the geometry of the tur- c ⋅Δt bulence generator (hydraulic diameter d ,length l , and height 0 0 h of smallest cross section) and the downstream channel The mass transfer coefficient is proportional to the depth h [43]. The index “−1” relates to the converging nozzle resazurin concentration at a certain location Δc divided by region, index “0” to the smallest cross section, and index “1” the oxygen saturation concentration c and the time of the to the diverging outlet nozzle part, see also Fig. 2. For bubble fluid elements after the first gas-liquid phase contact. This formation in T-contactors, three mechanisms of bubble forma- time is the mean residence time up to the location of concen- tration measurement. tion were proposed: dripping, squeezing, and jetting. These depend on contactor geometry, flow rates, and the fluids’ properties and have an impact on mass transfer [13, 44]. Here, slμg flow resulted in the T-contactor at low flow rates Experimental setup and methods and bubbly flow was obtained at higher flow rates. The breakup of these bubbles into smaller daμghter bub- Experimental setup bles in the wake of the nozzle depends on the interactions between the bubble’s surface force and the liquid’s inertia The employed microreactor setup is shown in Fig. 2a and is force [45, 46]. The liquid Weber number We puts these forces l adapted from previous works [28, 29, 49]. The microreactor into relation [47] with surface tension σ. consists of a reaction plate featuring a milled in flow channel 432 J Flow Chem (2021) 11:429–444 Fig. 2 a) 3D-modell of the microreactor setup in explosion view. b)Reactionplate with T- contactor and nozzle inlay. Arrows indicate inlet of gas and liquid phase and exit of the mix- ture. c) Close-up of the ex- changeable nozzle inlay. d) Close-up of the micronozzle with geometrical parameters d , d , −2 0 and d for hydraulic diameter of the inlet channel, the nozzle and the outlet channel, respectively, -1 1 together with the inlet and outlet angle α and α , respectively −1 1 d-2 d2 (rectangular cross section, w = 5 mm, h = 1 mm) on its upper Both nozzles N1 and N2 represent optimized geometries concerning low pressure loss N1 and dispersion intensity N2 side (cf. Figure 2b). A material recess within the reaction plate allows for the quick exchange of nozzle inlays (cf. Figure 2c) and will be investigated in the following study. The straight and thus the simple variation of nozzle geometries. The im- reference channel R serves as a reference. portant geometrical parameters are labeled in Fig. 2d. The microchannel is sealed with a view glass and two outer Experimental parameters flanges, made from stainless steel, clamp the view glass and the reaction plate together. The highly transparent The solution for mass transfer experiments is prepared with a −1 polymethylmethacrylate (PMMA) reaction plate and view concentration of 0.1 g L resazurin (Thermo Fisher Scientific glass, in combination with a light-emitting diode (LED) panel Inc., USA), 0.1 M glucose (D(+)-glucose anhydrous, AnalaR that is placed below the microreactor, enable optical observa- NORMAPUR® for analysis, VWR Chemicals, Belgium) and tion of bubble breakup and mass transfer characterization 0.3 M NaOH (pellets, VWR Chemicals, Belgium). These con- using a high-speed camera from above. centrations are adopted from Dietrich et al. [37]. The colorless Fluidic connections for liquid and gas supply and outlet flow dihydroresorufin solution is conveyed into the microreactor at are laterally attached to the reaction plate (cf. Figure 2b). The varying volumetric flow rates. However, volumetric flow rate entire experimental setup is described in an earlier contribution of oxygen (Messer Group GmbH, Germany) is held constant [28]. Themicroreactorand thenozzleinlaysaremanufactured for all presented experiments. Thus, gas content was variable by high precision drilling in the mechanical workshop of TU despite a broad spectrum of flow and bubble breakup regimes. −1 Dortmund University. A reference element is manufactured Total volumetric flow rates in the range of 20 mL min and −1 without the micronozzle. Therefore, the pressure drop induced 140 mL min are employed at resulting gas contents of GC = by adjacent channels can be determined and subtracted from the 0.07–0.5. Experiments were carried out at room temperature. measurements using nozzle inlays to obtain solely the pressure drop caused by the micronozzle. Mass transfer within the Image acquisition and processing straight reference channel is investigated and respective mass transfer coefficients serve as a benchmark for the nozzle in- Images of the bubbles moving in the microchannel are record- duced mass transfer intensification. Table 2 gives an overview ed with a monochromatic high-speed camera (Xtra Motion of different nozzle geometries. NR4, Imaging Solutions GmbH, Germany). The different Nozzle element N1 represents a compromise between levels of pink coloration, which depend on the reaction prog- grade of fine bubble dispersion and narrow residence time ress, are represented by 256 grey values in the acquired im- distribution within the channel developed by Tollkötter [27]. ages. The gray values correlate with the concentrations of Large outlet angles induce distinct recirculation zones in the resorufin or oxygen transferred into the solution. wake of the nozzle. These countercurrent flows trap bubbles The recorded images had to be digitally processed to ex- so that residence time distribution is rather broad. Nozzle inlay tract an accurate quantification of the resorufin concentration, N2 features an optimized nozzle geometry regarding bubble which then is converted into an equivalent oxygen concentra- size and pressure drop developed by Reichmann et al. [28]. tion taking stoichiometry into account. The image processing J Flow Chem (2021) 11:429–444 433 Table 2 Manufactured micronozzle designs for mass transfer characterization in micronozzles. is carried out with the Image Processing Toolbox within For high flow rates and turbulent bubble breakup, the can- Matlab (R2012a). The method from Dietrich et al. [37]is ny edge algorithm reaches its limits where strongly deformed modified for this study and described in the following. In a bubbles are not detected. Here, manual bubble detection and first step, an averaged background image from 20 images is masking is carried out. subtracted from the raw images to eliminate the effect irregu- lar backlight distribution. Pictures are inverted prior to sub- Correlation of grey values and equivalent oxygen traction to assure increasing grey values correlate with in- concentration creasing concentrations. Images are cropped to reduce neces- sary computing power. The interfaces of the bubbles lead to A calibration curve was created in order to convert the grey refraction and reflection of the transmitted light. The areas of values into equivalent oxygen concentrations. The de-facto the bubbles that appear dark due to light refraction and reflec- oxygen concentration in the solution is zero due to the instan- tion are detected with the canny edge algorithm and masked in taneous reaction of the dissolved oxygen and dihydroresorufin order to exclude them from analysis. Finally, a heat map is in the liquid phase, hence, the term “equivalent” is used. The created from grey values, indicating the local concentrations equivalent oxygen concentration can be deduced from the along the channel. The steps are visualized in Fig. 3. measured resorufin concentrations, according to Eq. (6). Fig. 3 Image processing procedure for quantification of resorufin and equivalent oxygen concentration 434 J Flow Chem (2021) 11:429–444 −1 Therefore, a grey value can be processed to an equivalent wake of the micronozzle at 20 and 25 mL min . Bubble −1 oxygen concentration. deformation occurs for 35 mL min and laminar breakup −1 for 50 and 60 mL min . Turbulent breakup regime starts at n n dihydroresorufin resorufin −1 n ¼ n ¼ ¼ 70 mL min in N2. O ;transferred O ;reacted 2 2 2 2 resazurin ¼ ð6Þ Mass transfer measurement For calibration, resorufin solutions were fed into the The reference channel serves as a benchmark for mass transfer microreactor with different concentrations and the related im- using the micronozzles. Investigated flow regimes are limited ages were analyzed. A corresponding calibration curve, which to slug and bubbly flow due to their highest interfacial areas correlates grey values and equivalent oxygen concentrations, per unit volume. These regimes are also employed in mass is shown in Fig. 4 and validates the linear trend from Dietrich transfer experiments using the micronozzle inlays. et al. [37]. Monochromatic pictures from resorufin solutions and respective equivalent oxygen concentration are displayed Reference channel mass transfer in Fig. 4b. The flow regime and related heat maps are displayed exem- plarily in Fig. 6 for the reference channel with the respective Results and discussion original images, the corrected images, and the heat maps fea- turing O -equivalent species concentration profiles. The slight Flow regime maps asymmetric concentration profile origins from the 90° gas inlet channel. Since all nozzles were investigated with this Various liquid flow rates are investigated for dispersion at generation setup there is no difference observed. constant gas flow rate. Figure 5a shows the flow regimes in The heat map images in Fig. 6 show slug flow configura- the reference channel, which are also valid for experiments tion over a total time of 9.6 ms in steps of 2.4 ms. Oxygen using micronozzles N1 and N2 for the section upstream of bubbles have been numbered for identification purposes, and the orifice with the only exception that slμg/bubbly flow is flow direction is from left to right. During bubble generation −1 observed at 35 mL min for N2. At high gas contents and in the T-contactor first O -equivalent species was already lower total flow rates, slμg flow is noticed. The Taylor bub- found between bubbles 1 and 2 at t . However, mass transfer bles always stay in contact with the channel walls and are has started, and distinct fields of higher O -equivalent species merely stretched due to the narrower channel cross section. concentration in the upper and lower channel half, with an Bubble breakup regimes are shown in Fig. 5b and c) for noz- area of lower concentration in the channel center, have already zle inlay N1 and N2, respectively. formed between bubbles 2 and 3. Here, the formation of In case of bubbly flow in front of the nozzle, a variety of Taylor vortices in the liquid compartment can be seen, which bubble behaviors were observed behind the micronozzles N1 is well known for slug flow in straight microchannels [49–54]. and N2. At moderate flow rates, a deformation of bubbles was However, since Re is relatively high (Re = 153) for slug flow l l noted. Thus, external forces are not strong enough to cause a in microchannels, the vortices show wavy movements and the breakup. For increased total volumetric flow rates, laminar diffusion process is supported by convection. The closed bubble breakup is noticed. Finally, turbulent bubble breakup Taylor vortex structure is maintained over the observed chan- regime is reached behind the micronozzle for total flow rates nel length, which can be seen in the liquid slugs between −1 greater than 100 mL min . For N2, slug flow adjusts in the bubbles 4 and 5, and bubbles 5 and 6. At t , the lower part Fig. 4 a) Calibration curve for the correlation between grey values and equivalent oxygen concentrations. b)Variation in grey values for various resorufin concentrations in the microchannel J Flow Chem (2021) 11:429–444 435 Fig. 5 a) Flow regime map for gas-liquid contacting in the T-contactor Refinement regimes of gas-liquid flow downstream of the micronozzle −1 for the reference channel and upstream of nozzle inlay N1. b) Refinement N2. V =10 mL min = const. For all experiments gas regimes of gas-liquid flow downstream of the micronozzle N1. c) −1 of the Taylor vortex has been completely formed between volumetric flow rates (V =20 mL min ,Re =153) and tot l,2 bubbles 2 and 3. The asymmetrical development of hydrody- high oxygen content (GC = 0.5). In order to allow comparison namics and mass transfer is attributed to the eccentric bubble between the flow regimes and among the reference channel generation in the T-contactor. Further, a thin center line in the and the micronozzle, a global scale 0–10 [mg O -equivalent −1 liquid compartment between bubbles 5 and 6 indicates high L ] is used and applied to all flow regimes. O -equivalent species concentration, which is accounted for Bubbles are formed in the T-contactor and once they enter by lateral film flow between bubbles and channel walls [55]. the micronozzle, they are stretched in length before returning The evolution of the thin center line can be followed, tracking to their previous shape eventually behind the diverging chan- the liquid slug between bubbles 3 and 4 over time (t -t ). The nel. The concentration increase of O -equivalent species can 0 4 2 obtained concentration profiles correlate well with experimen- already be noticed in the original and the corrected image in tal results [32] and CFD simulations [56]. Fig. 7. The image series is subdivided into the upstream and downstream region of the micronozzle and covers a total time Micronozzle mass transfer in Taylor bubble flow span of 9.6 ms in 2.4 ms steps. Even though slug flow sets in upstream of the nozzle, a fully developed Taylor vortex can- Mass transport for two-phase flow through micronozzles is not be observed in contrast to the straight reference channel. qualitatively examined for the observed flow regimes up- Here, the channel length leading to the orifice seems to be too and downstream of the nozzle in the following and exemplar- short. Merely a slight increase in concentration can be detect- ily shown for micronozzle N1. The flow combination slug ed in the liquid slugs between bubbles 1 and 2 and bubbles 2 and 3 (Fig. 7, t ), respectively. However, the evolution of flow - slug flow (“flow configuration upstream of the nozzle” - “bubble behavior downstream of the nozzle”)is shown in Taylor vortices can be sensed, since areas of higher concen- trations form at the channel walls with the characteristic Fig. 7 with the original and the corrected images for low total Fig. 6 Image series of heat maps for mass transfer in slug flow in the reference channel R with respective local concentration profiles 436 J Flow Chem (2021) 11:429–444 Fig. 7 Image series of heat maps for Taylor flow and bubble deformation through micronozzle N1 with respective local concentration profiles upstream (left hand side) and downstream (right hand side) of the nozzle, global scale 0–10 [mg O -equiv- −1 alent L ] poorly-mixed zone in the channel center between bubbles 3 concentrations are generally higher downstream of the orifice and4(Fig. 7, t ). The asymmetric flow development is linked despite an increased flow rate and thus shorter time for mass to the eccentric bubble generation again. transport. −1 Once a liquid slug enters the nozzle, it is stretched in length For increased flow rates of V =80 mL min and GC = tot and the concentration profile seems to be homogenized across 0.13 bubbly flow is still observed upstream of the micronozzle the channel width (Fig. 7, liquid slug between bubble 4 and (Fig. 8b). However, bubbles break up in laminar regime 5at t ). In the diverging nozzle outlet, a well-mixed zone can downstream of the orifice. No significant alternations are de- be found in the channel center in the form of a thin jet, which tected in respect to bubbly flow configuration or concentration seems to evolve from bubble 6 to bubble 7 (Fig. 7, t ). profiles upstream of the nozzle compared to the previously −1 However, this jet emerges and disappears rapidly and the con- observed phenomena for V = 60 mL min . The bubbles tot centration profile in between the bubbles homogenizes quick- break up in laminar regime; hence, greater mass transport area ly and; in the end, the liquid slugs seem evenly mixed (Fig. 7, is created and an increase in mass transfer is expected. Both liquid slug between bubbles 9 and 10 at t ). laminar breakup regimes are observed: binary breakup and shearing off of small satellite bubbles. The external forces exceeded a critical value, so that the Micronozzle mass transfer in laminar bubble break up surface tension is no longer sufficiently high to keep the bub- When the total volumetric flow rate is increased (V = bles intact (We = 25) and; finally, the bubbles burst. Binary tot l,2 −1 breakup is observed, tracking bubble 8 in the downstream 50 mL min , GC = 0.2, Re = 613), flow transitions to bub- l,2 bly flow upstream and bubble deformation downstream of the images over time in Fig. 8 b). At t , bubble 8 is stretched in length due to the smaller cross section and accelerated core micronozzle (Fig. 8a). Bubble train formation is observed ahead of the orifice. Downstream of the nozzle, external forces flow. The bubble is then decelerated, compressed (t and t ), 1 2 and takes a characteristic dumbbell shape at t . Once a critical act on the bubble and actually surpass the surface tension to a multi-fold extent (We = 8.12). Nonetheless, a critical value neck diameter is exceeded [27], the bubble splits into two l,2 daughter bubbles (t ). The little, unnumbered bubbles over is obviously not exceeded and the bubbles are merely de- formed but not broken up, regaining their former shape the course of the channel result from shearing off mechanism. A clear increase in concentration is found across the complete eventually. The concentration profiles upstream of the nozzle coincide channel further downstream. Nonetheless, a slight gradient is with those from the bubble train formation in the reference present across the channel width with higher concentrations at the lower channel wall, which can be explained by the flow channel. A distinct O -equivalent species trail can be observed between the bubbles in flow direction, representing well- around the bubbles and the bubbles’ path through the channel. A rise in concentration is found at the location of bubble mixed areas. Flow disturbances in the form of velocity fluc- tuation in the wake of the micronozzle seem to affect the breakage hinting to an intensification of mass transfer. ordered shear flow and the vortex structures in between the bubbles (Re = 1226). Hence, turbulent motions in the liquid Micronozzle mass transfer in turbulent bubble break up l,0 are promoted, resulting into enhanced mass transfer from the gas into the liquid phase. Comparing bubbly flow in the Turbulent bubble breakup is observed in the micronozzle N1 −1 straight microchannel and the one containing the nozzle, for a flow rate of V = 120 mL min (Re =3370, Re = tot l,0 l,2 J Flow Chem (2021) 11:429–444 437 Fig. 8 Picture series of heat maps for bubbly flow through the micronozzle N1 and laminar bubble breakup at Re =613 (a)) l,2 and 1071 (b)) with respective local concentration profiles at global scale 0–10 [mg O -equiv- −1 alent L ]. The flow regime in b) can also be treated as transition to turbulent regime, since smaller daughter bubble appear 1685) and an oxygen content of GC = 0.08. Corresponding hence, the focus is only on the region of bubble breakup. heat map images are shown in Fig. 9 for O -equivalent species Bubble movement and deformation indicate strong dynamics concentration. Upstream of the nozzle, bubbly flow is ob- in the wake of the smallest cross section and the external served; however, bubble diameter is smaller compared to pre- forces tear the mother bubbles apart (We =61). Numerous, l,2 vious flow rates and gas contents. Nonetheless, two O -equiv- significantly smaller daughter bubbles are generated and an alent species trails are still detected between the bubbles, increased phase boundary is obtained. Fig. 9 Picture series of heat maps for bubbly flow through the micronozzle N1 and turbulent bubble breakup behind it with respective local concentration profiles at global scale 0–10 [mg −1 O -equivalent L ] 2 438 J Flow Chem (2021) 11:429–444 Figure 9 depicts heat map images of O -equivalent species In general, the equivalent O -equivalent species concentrations 2 2 concentration for bubbly flow through the micronozzle at glob- increase over the observed channel section for slug and bubbly al scale for a time span of 0.4 ms in 0.1 ms steps. No significant flow within the straight reference microchannel, which corre- increase in concentration can be seen ahead of the smallest sponds well with studies by Krieger et al. [51] and Yang et al. cross section and in the smallest cross section. However, shortly [32]. The filled symbols originate from linear regressions fitting ahead of the end of the divergent nozzle outlet there is a distinct the respective experimental data in positions from −2to2, which rise in concentration, particularly, next to the lower channel have been introduced for the micronozzles. All coefficients of wall. This location marks the starting point of bubble breakup, determination are R > 0.97 for the linear trends and indicate a too. The increase in concentration occurs mainly at the lower good linear fit. Light grey filled symbols resemble slug flow and channel wall, but with some distance to the nozzle. Higher black ones portray bubbly flow configuration in the reference concentrations are found across the entire channel width. channel. Higher flow rates show decreasing slopes of the graphs. Despite a significantly shorter residence time in the observed Therefore, largest concentration increase is found for slug flow at −1 area due to higher flow rates, the obtained concentrations are V = 20 and 25 mL min with Re = 153 and 230. The regime tot l,2 actually slightly higher in Fig. 9 than for laminar breakup in transitions to bubbly flow for higher flow rates (V =35– tot −1 Fig. 8 b) indicating an intensified mass transfer). 140 mL min ) and a smaller increase in concentration is found −1 with the smallest gain for 140 mL min . Reynolds numbers are in the range of 383 < Re < 1991 indicating mainly laminar flow. Quantitative mass transfer evaluation l,2 A distinct drop to lower concentrations at position −2, which is right behind the T-contactor, is evident between 50 Reference channel - concentration profiles −1 and 60 mL min and can be explained by the transition in bubble formation mechanism from dripping to jetting. In order to evaluate the mass transfer in the investigated chan- The time a bubble remains in the considered area must be nels, O -equivalent species concentrations for various flow considered, when this data is evaluated regarding mass transfer rates are determined along the channel length and displayed coefficients. The residence time within the considered area de- in Fig. 10 over the respective time. The concentration is drawn creases with increasing volume flows. In Fig. 10b, concentra- over the mean flow time connected with the respective posi- tions are plotted over the mean flow time until positions from −2 tion in the channel. Total superficial velocities are used for this to 2 are reached. As the mean velocity in the reference channel purpose, which are in very good agreement with velocities remains constant for a specific flow rate, the linear interrelation- determined by bubble tracking over a defined amount of time ship between concentration and channel length transfers to and channel length in image analysis. Fig. 10 a) Concentrations are plotted over the course of the reference scale. Light grey filled symbols resemble Taylor flow and black symbols channel for various flow rates. Unfilled symbols resemble display bubbly flow regime. Highlighted, thicker lines are representative concentrations obtained from experiments. Standard deviations for for the observed regimes. b) Concentrations between positions −2and2 every measured concentration are exemplarily shown for four for the investigated volumetric flow rates plotted over time until the experiments. Linear trend lines are added and used to extrapolate respective position is reached by the flow. For higher clarity the time concentrations between the positions −2 and 2. The channel positions in scale is plotted on a logarithmic scale the diagram a) can be roughly transferred to the top image by the length J Flow Chem (2021) 11:429–444 439 concentration and flow time. From the slope m of the graphs At higher flow rates, bubbly flow sets in upstream and −1 and taking c*(c*= 8 g L ) into account, volumetric mass bubble deformation downstream of the nozzle (V = 35, 50, tot −1 transfer coefficient k a can be obtained using Eq. (5). A loga- 60 mL min , dark grey filled symbols). Compared to slug rithmic scaling of the time axis is selected here for a clearer data flow, the overall mean gradient declines, which is also ob- presentation; however, the linear rise of concentration over time served in the straight reference channel for increasing flow is obscured and compressed due to the logarithmic display. The rates. The bend in the graphs’ courses across the nozzle section graphs’ slopes are very similar, indicating a narrow span of mass gets more and more pronounced. Here, mass transport inten- transfer coefficient values for flow in the reference channel. sification is not strong enough to compensate for the shorter time spent in the nozzle section. Reynolds numbers behind the nozzle take values of Re = 383, 613, and 766, respectively. Micronozzle N1 and N2 - concentration profiles l,2 Even higher values are reached within the smallest cross sec- tion with Re = 766, 1225, and 1531 while laminar flow is the The O -equivalent species concentrations from experiments l,0 superordinate flow type. Weber numbers take values of using micronozzle N1 and N2 are displayed in Fig. 11. We = 3.17, 8.12, and 12.68. However, external forces are Highlighted data (thicker lines) are characteristic for the respec- l,2 not sufficiently high in order to break the bubbles. tive flow regimes. The concentration increase across the ob- ̇ Once the bubbles begin to break up behind the micronozzle served area is the largest for slug flow (V =20, tot −1 −1 in laminar regime (V = 70, 80, and 90 mL min , black sym- 25 mL min , light grey filled symbols). In this regime, the tot bols containing crosses), a turning point is reached. The gra- graphs’ courses are similar to the ones from the reference chan- dient between positions −2and − 1 continues to decline with nel as the concentration increase takes an approximate linear increased volume flow. However, the steady drop in concen- progression over the channel length. For micronozzle N1 in tration in position 1 with increasing flow rates comes to an Fig. 11a, the gradient between position −1and1isreduced −1 end; hence, the concentration in position 1 for 90 mL min is slightly employing the nozzle. Due to the narrower channel −1 above the one for 80 mL min despite a shorter residence cross section and accelerated flow, thenozzlesectionispassed time in the channel up to that location. Mass transport process- in shorter time. Hence, there is less time for the mass transfer to es seem to be strongly enhanced, significantly outweighing progress as the superordinate flow regime is maintained across shorter residence times. Furthermore, it is characteristic for the smallest cross section, and mass transport is not significant- this regime that the slope of the graph behind the nozzle is ly enhanced, which explains the bend in the graphs. Reynolds steeper than the slope of the graph ahead of the nozzle. This is numbers within the smallest cross section take values of Re = l,0 probably due to the larger mass transfer area resulting from 306 and 460, respectively. Downstream of the nozzle the flow bubble breakup and the multi-directional motions intensifying is gradually decelerated, and Reynolds numbers are Re =153 l,2 the flow around the bubbles and consequently the gas-liquid and 230. Consequently, the influence of the nozzle in terms of mass transfer. Reynolds numbers equal Re = 919, 1072, and mass transfer enhancement seems to be rather small in slug flow l,2 1225 behind the nozzle and Re = 1838, 2144, and 2450 regime. Weber numbers are We =0.507 and We =1.141; l,0 l,2 l,2 within the smallest cross section. The increased inertial forces yet, bubbles do not break up despite inertia force exceeding −1 of the fluid acting on the bubbles and finally lead to their surface tension for V =25 mL min . tot Fig. 11 Concentrations in positions from −2 to 2 are plotted over time until the respective position is reached by the flow for nozzle N1 in a)and forN2 in b) 440 J Flow Chem (2021) 11:429–444 binary breakup or shearing off of satellite bubbles with mass transfer coefficient and they are plotted over flow rate We = 18.26, 24.85, and 32.46 for the respective flow rates. in Fig. 12a. The time difference steadily declines with increas- l,2 For turbulent bubble breakup regime (V = 100, 120, and ing flow rates for the reference channel, nozzle N1, and nozzle tot −1 140 mL min , black symbols), the concentration increases in N2. Time is reciprocally proportional to volume flow rate position 1 continues as recirculation zones evolve in the wake of explaining the hyperbolic course of the graphs. The mean flow the nozzle. The concentration gradients across the nozzle section time from −2 and 2 is slightly lower for the nozzles due to the are clearly steeper than the mean gradients across the complete narrower cross section and resulting flow acceleration. The channel length for this regime. The slopes downstream of the dissimilarity between the nozzle inlays is marginal. nozzle are significantly increased compared to the one ahead of The concentration difference between positions −2and2 the nozzle, presumably due to the increased interfacial area and steadily declines for the reference channel R, too. The flow is the turbulent motions, which lead to intensified flow around the not disturbed along the channel length, where laminar flow bubbles. Reynolds number rise to Re = 1379, 1685, and 1991 regimes prevail as residence time decreases simultaneously, l,2 behind the micronozzle and Re = 2758, 3370, and 3982 in the resulting in less time for mass transfer. All three concentration l,0 −1 smallest cross section indicating transition to turbulent flows. graphs take a similar progression up to 50 mL min . For N2, Weber numbers take values of We = 41.09, 61.38, and 85.72. a minimum in concentration difference is reached here and the l,2 The data from experiments using micronozzle N2 is present- concentration difference increases again when reaching bub- ed in Fig. 11b. Highlighted data (thicker lines) are characteristic ble breakup regimes. The transition to bubble breakup occurs −1 −1 for the respective flow regime. The concentration along the at 70 mL min for N1 with a minimum at 60 mL min . microchannel length are following in general the observed Bubble breakup leads to an increase in concentrations despite trends from nozzle N1. An approximate linear increase in con- shorter residence times in the observed region. centration over channel length is noticed for slug flow (V = 20, Finally, volumetric mass transfer coefficients between po- tot −1 25 mL min ) while the trend for bubbly flow in the upstream sitions −2 and 2 are determined with Δt, Δc, and the satura- −1 region and laminar bubble breakup behind the nozzle (V =50 tion concentration c*=8gL according to Eq. (5). The re- tot −1 and60mLmin , dark grey filled symbols) shows a reverse sults for all inlays are displayed in Fig. 12b. In the reference trend with increasing concentrations in position 1. The flow is channel R, k a values increase with higher flow rates initially. decelerated more abrupt due to a larger outlet angle for N2 and The flow regime in the reference channel transitions from slug −1 velocity fluctuations occur at lower flow rates compared to N1. to bubbly flow between 25 and 35 mL min , where bubbly The predominant laminar breakup mechanism in nozzle N2 is flow features larger interfacial area per unit volume leading to bubble shearing at We = 6.09 and 9.51, respectively. the initial rise. Then, the slope declines, and a maximum is l,2 −1 The gradients across the nozzle section grow steeper for reached at 60 mL min . Subsequently, the k a values decrease turbulent bubble breakup (V = 70, 80, 90, 100, 115, 120, in asymptotic behavior with larger volume flows. tot −1 and 140 mL min , black squared symbols containing crosses For nozzle N1, the graph’s course is similar to the one of −1 and black symbols). Within a few milliseconds the concentra- the reference channel R up to 50 mL min . Bubble deforma- −1 tions in position 1 jump to higher levels. The Weber numbers tion is observed in N1 at 35, 50, and 60 mL min and laminar −1 behind the micronozzle range from 13.7 < We <64.3. bubble breakup is observed for 70, 80, and 90 mL min . l,2 Mean flow times have to be considered once again regard- Apparently, the influence of the nozzle is already existent −1 ing mass transfer evaluation and; consequently, Fig. 11b for V ≤ 50 mL min as k a values are slightly higher than tot l shows concentrations applied over time for the employed flow the ones in the reference channel for a given flow rate. −1 rates. In general, the same applies to N2 as to N1; the higher Pressure drop for 50 mL min is 19 mbar and, therefore, only the flow velocity, the shorter the average residence time in the slightly higher than for the reference channel. Thus, the impact area under consideration. In addition, the concentrations in of the nozzle on mass transfer rises significantly for V ≥ tot −1 positions −2and − 1 decrease continuously the higher the flow 60 mL min and bubble breakup. Particularly, once bubble rate is and the slopes of the graphs within the micronozzle breakup is reached, the two graphs diverge greatly with con- section grow steeper and steeper. The jump to higher concen- siderably higher mass transfer coefficients for nozzle N1. A trations between positions −1and1in bubblebreakup regimes maximum pressure drop of 150 mbar is measured at −1 −1 is even more precipitous for N2 than for N1. The trend reversal 140 mL min with a k a =46 s . in terms of rising concentrations in positions 1 and 2 for higher In micronozzle N2, even higher mass transfer coefficients −1 flow rates is already detectable at 50 mL min in Fig. 11b. are reached; however, they come at the cost of higher pressure drop. The graph’s course is similar to the one of N1 for V ≤ tot −1 Mass transfer coefficients for R, N1, and N2 50 mL min . For higher flow rates and bubble breakup, the twographs diverge andnozzle N2provides highest k a values. The concentration and time difference between positions −2 Pressure drop is in the range of 16–520 mbar. Highest k a −1 −1 and 2 are the important parameters in terms of volumetric value is obtained for 140 mL min with k a =60 s .The l J Flow Chem (2021) 11:429–444 441 Fig. 12 a) Concentration and time differences between positions −2 and 2 over flow rate for the reference channel R, micronozzle N1, and micronozzle N2. b) Mass transfer coefficients for the region between positions −2 and 2 over flow rate for the reference channel R, micronozzle N1, and micronozzle N2 volumetric mass transfer coefficients obtained with the nozzle Figure 13 shows k a values (channel region −2 - 2) plotted are in the upper range of microreactors. over experimental mean energy dissipation rates obtained for the micronozzle region (pos. -1 - 1) and for calculated mean energy dissipation rates resembling the sum of experimental Pressure drop and mass transfer rate values for the micronozzle region and theoretical values for the adjacent channels (pos. -2 to 2). The data are separately fit In order to evaluate a cost/benefit ratio, the pressure drops are with power law trend lines (dashed lines in grey) for bubble presented and the mean energy dissipation rates are linked to breakup and no bubble breakup. Coefficients of determination the overall mass transfer coefficients. As the reference channel 2 are R ≥ 0.93 indicating good fits. Deviations of −15% and + pressure drop is subtracted to exclude pressure drops from the 15% are indicated with dotted lines in black. Hence, an in- inlets or the downstream meandering channel, only the pres- creasing mass transfer coefficient comes at the cost of higher sure loss induced by the micronozzle is accessed (between energy input into the system. However, these correlations are positions −1 and 1). However, pressure drop and mass transfer not dimensionally accurate. Thus, in the next step, mean en- have to be evaluated for the identical channel region and vol- ergy dissipation rate is related to the kinematic viscosity ν and umetric mass transfer coefficients between positions −2and2 the square root of the quotient is taken, matching the unit from have been assessed before. Therefore, the theoretical pressure −1 k a with s . This quotient is the inverse Kolmogorov time drop of the straight channel sections −2to −1 and1to 2is scale in vortex dissipation [33]. calculated and added to the experimentally determined value The fitting with linear trend lines of data in Fig. 13 reveals from the micronozzle, represented by the unfilled symbols. high accuracy and allows for predicting mass transfer coeffi- Nevertheless, the graphs do not differ much to those excluding cients based on energy input and kinematic viscosity. A the adjacent channel sections. In general, higher pressure drop trendline is assigned to each of the flow regimes: no bubble is induced by N2 compared to N1, which is most probably breakup, laminar bubble breakup, and turbulent bubble break- based on flow detachment and more intensive recirculation up. An overview of the corresponding equations is presented zones in the wake of the nozzle [57]. in Table 3. A fit encompassing laminar and turbulent breakup The pressure loss coefficients obtained in the experiments is additionally given, also displaying high accuracy. over mean Reynolds number for nozzle inlays N1 and N2 For low flow rates and energy input, the k a values remain decrease with increasing Reynolds number; however, they at a constant level. Once bubble breakup begins at approxi- take constant values for Re ≥ 1500. Larger pressure loss co- 0.5 −1 mately (ε / ν ) ≈ 28.800 s (for both nozzle inlays), k a l l efficients are obtained for nozzle N2, which is based on the increases. The corresponding trend lines for the individual increased pressure drop despite similar flow velocities. Fig. 13 Overall mass transfer coefficient determined between positions −2 and 2 plotted over mean energy dissipation rate 442 J Flow Chem (2021) 11:429–444 Table 3 Overview of trend lines from Fig. 13. Also, trend line equations encompassing laminar and turbulent regime are shown, where the bold data are valid over the entire investigated range 0.5 Data regime trend line from k a=A (ε / ν ) +B coefficient of determination l l −4 −1 2 N1 & N2 exp. no breakup A: 2.794⋅10 B: 7.3164 [s]R =0.95 −4 2 laminar breakup 4.092⋅10 4.3088 R =0.86 −4 2 turbulent breakup 3.012⋅10 14.604 R =0.96 −4 2 breakup (lam. & turb.) 3.682⋅10 7.5264 R =0.94 −4 −1 2 N1 & N2 exp. + adj. Channels calc. no breakup A: 4.699⋅10 B: 7.2086 [s]R =0.91 −4 2 laminar breakup 8.948⋅10 1.1339 R =0.98 −4 2 turbulent breakup 6.679⋅10 10.595 R =0.96 −4 2 breakup (lam. & turb.) 7.782⋅10 4.7290 R =0.97 Abbreviations GC, gas content; LED, light emitting diode; N1, nozzle regimes are presented for no bubble breakup and bubble design 1; N2, nozzle design 2; O , oxygen; pos, position; PMMA, breakup. polymethylmethacrylate; R, reference channel Subscripts and superscripts *, solubility concentration.; -2, position −2, upstream channel.; -1, position −1, nozzle inlet.; 0, position 0, smallest Conclusion cross section.; 1, position 1, nozzle outlet.; 2, position 2, downstream channel.; diss, dissipation.; l, liquid.; tot, total. Since gas-liquid mass transfer is often limiting chemical −1 Latin letters A, coefficient, −.; B, coefficient, s .; c, concentration, kg reactions in flow reactors, a colorimetric method based on −1 L .; d , hydraulic diameter, m.; E, enhancement factor, −.; h , channel the oxygen sensitive dye resazurin is used in order to height, m.; Ha , Hatta number, −.; k a , volumetric mass transfer coeffi- −1 visualize and quantify mass transfer during the refinement cient, s .; l , length, m.; n , amount of substance, mol.; p , pressure, Pa.; Re , Reynolds number, −.; Re , Reynolds number in the nozzle, −.; Re of two-phase flow using micronozzles. Mass transfer phe- l,0 l,2 , Reynolds number in the outlet channel, −.; Re , Reynolds number of the nomena are shown for slug flow through micronozzles, −1 liquid phase, −.; t , time, s.; u , mean flow velocity, m s .; V , volumetric −1 3 bubble deformation, laminar bubble breakup, and turbu- flow rate, L s .; V ,volume,m .; w, channel width, m.; We, Weber lent bubble breakup. The gas-liquid mass transfer is quan- number, −.; We , Weber number in the nozzle, −.; We , Weber number l,0 l,2 in the outlet channel, −.; We , Weber number of the liquid phase, −.; x, tified for these regimes concerning volumetric mass trans- channel length, mm. fer coefficient k a and benchmarked against mass trans- port in a straight reference microchannel. Moreover, two Greek letter Α,angle,°.; Δ, difference.; ε , mean energy dissipation 2 −3 2 −1 −3 nozzle designs are evaluated. The reference channel yields rate, m s .; ν, kinematic viscosity, m s .; ρ, density, kg m .; σ, −1 −1 surface tension, N m . mass transfer coefficients in the range of 7–12 s for slug and bubbly flow with higher values for the latter as more Acknowledgments The authors would like to thank the Mechanical interfacial area per unit volume is evident. A nozzle de- Workshop of TU Dortmund University for excellent manufacturing of the micronozzle equipment. Additionally, we would like to acknowledge sign optimized in respect to pressure loss and residence C. Schrömges (Laboratory of Equipment Design) for technical assistance. time distribution of the gaseous phase and bubble diame- ter yields mass transfer coefficients in the range of 9– Availability of data and material Not applicable. −1 46 s . Mass transfer is greatly enhanced for bubble breakup in the wake of the nozzle and highest values Code availability Not applicable. are reached for turbulent breakup at the costs of slightly Funding Open Access funding enabled and organized by Projekt DEAL. elevated pressure drop. The optimized nozzle design, in respect to pressure drop and bubble diameter, yields even −1 higher k a values in the range of 9–60 s .For slugflow Declarations and bubble deformation the micronozzles produce similar Conflicts of interest/competing interests The authors declare that they results; yet, diverge for higher flow rates with improved have no known conflicts of interest/competing interests. results for the optimized pressure drop design. The achieved mass transfer coefficients are correlated with Open Access This article is licensed under a Creative Commons the energy dissipation rate within the micronozzles. A Attribution 4.0 International License, which permits use, sharing, adap- correlation is derived based on the inverse Kolmogorov tation, distribution and reproduction in any medium or format, as long as time scale in vortex dissipation with good agreement. you give appropriate credit to the original author(s) and the source, pro- Micronozzles in combination with millichannels present vide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included a great opportunity to enhance gas-liquid mass transfer. J Flow Chem (2021) 11:429–444 443 in the article's Creative Commons licence, unless indicated otherwise in a segmented flow in microchannels. Chem Eng Sci 60(22):5895– credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by 19. 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Journal

Journal of Flow ChemistrySpringer Journals

Published: Sep 1, 2021

Keywords: Gas-liquid mass transfer; Micronozzle; Bubble breakup; Resazurin oxidation

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