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Simultaneous determination of enthalpy of mixing and reaction using milli-scale continuous flow calorimetry

Simultaneous determination of enthalpy of mixing and reaction using milli-scale continuous flow... A simultaneous determination of the enthalpy of mixing and reaction in a scalable continuous milli-scale flow calorimeter is investigated. As obtained calorimetric data is pivotal for the safety assessment of chemical reactions and processes. The acid- catalysed selective, homogeneous hydrolysis of acetic anhydride with half-lives from a few seconds to a few minutes is investigated as a model reaction. For the enthalpy of mixing 7.2 ± 2.8 kJ/mol and for the enthalpy of reaction −60.8 ± 2.5 kJ/mol were determined. For reactions that show complete conversion in the continuous reactor, a technique is introduced to further improve the accuracy of the reaction enthalpy determination. Thereby, the resolution of the observed temperature profile is increased by measuring the profile at different flow rates. Applying this procedure, the reaction enthalpy of −62.5 kJ/ mol was determined which is in good agreement with literature values for this model reaction. . . . . Keywords Continuous flow calorimetry Polytropic reaction calorimetry Heat of reaction Hydrolysis of acetic anhydride Process development Scale-up Introduction safety assessment of a process, the determination of the enthalpy of reaction is an elementary component [5, 6]. The closer this Today, mainly batch reactors are used to produce fine chemicals in determination is to the industrial process, the more robust safety the range of up to 100 t/y [1]. As the industry is striving for process data are obtained. For example, Mortzfeld et al. [7] found devia- intensification, the changeover from a multifunctional batch plant tions in non-selective reactions when they were investigated using to a smaller continuous monoplant can make economic sense, as batch calorimeters or continuous flow calorimeters. Several flow there is no need for cleaning, charge and discharge [2]. In addition, calorimeters have been developed measuring the enthalpy of re- continuous reactors are smaller for the same production capacity action using thermoelectric heat flow [8–10], infrared thermogra- since higher temperatures and pressures allow a shorter residence phy [11], or segmental temperature sensors [7]. time than batch reactors [3]. The heat transfer is better in a con- Recently, we have shown that the reaction enthalpy of a tinuous flow reactor and there is less potential chemical energy, so fast and selective reaction can be measured in the continuous the consequences of an incident are drastically reduced [4]. For the flow calorimeter without extensive calibration of the heat transfer coefficient [12]. However, organometallic reactions, nitrations or polymerisations have half-lives in the range of Highlights � Simultaneous determination of enthalpy of mixing and heat of reaction seconds or minutes [13]. If full conversion is not achieved, the using a flow calorimeter. analysis of the exiting product is indispensable. Therefore, � Facile and fast flow screening procedure for accurate calorimetric data. infrared spectroscopy is a suitable method of analysis due to � Safe and scalable setup combined with online reaction monitoring. its sensitivity, economic viability, and broad application pos- sibilities [14, 15]. Furthermore, the enthalpy of mixing must * Daniel M. Meier meid@zhaw.ch be quantified since the conversion of the reaction is not com- plete and influences the calorimetric data. To determine the School of Engineering, Institute of Materials and Process enthalpy of mixing, the heat of reaction in relation to the Engineering, ZHAW Zurich University of Applied Sciences, conversion must be known. In a batch calorimeter, the con- Technikumstrasse 9, 8401 Winterthur, Switzerland version can be determined over time and known kinetics [16] Fluitec mixing + reaction solutions AG, Seuzachstrasse 40, or inline measurements [17]. Ładosz et al. [18]have shown 8413 Neftenbach, Switzerland 390 Journal of Flow Chemistry (2022) 12:389–396 that this determination of the enthalpy of mixing is possible in provided in the supplementary information. Briefly, two pis- a continuous micro reactor at steady state under isothermal ton pumps, dosing system 1 and 2 (DZRP-200, 63,639, conditions. Fluitec mixing + reaction solutions AG, Switzerland), were In this work we aim to simultaneously investigate the heat used to convey the feed streams. Both liquid feeds were con- of reaction and the enthalpy of mixing for the hydrolysis of ditionedto20°Cin separate lines usinga thermostat, acetic anhydride in a scalable continuous flow calorimeter. Thermostat 1 (Ministat 230, Peter Huber Kältemaschinenbau This model reaction has served for several continuous [18] AG, Germany). Before entering the flow calorimeter, the tem- and batch [16, 17, 19–21] calorimetric studies and is particu- perature was measured inline with a temperature probe im- larly suitable because the reaction rate can simply be tuned by merged into the feed (Class A, Pt-100, 53,286, Fluitec). The the help of the acid concentration, which catalyses the reac- reaction started immediately after mixing the two feeds in the tion. A milli-scale continuous flow calorimeter, scalable to flow calorimeter (L = 500 mm, D = 12.3 mm, V = 44.3 ml production scale, as used by us [12] is applied. Compared to Contiplant PFR-50-SS, Fluitec), which was equipped with a other reactor setups to determine the enthalpy of reaction, for pre-mixer, static CSE-X mixers and 10 axial temperature sen- example Ładosz et al. [18], who used three temperature sen- sors (Class 1, type K, Fluitec). Prior the experiments, the axial sors, this calorimeter measures the temperature profile using temperature sensors were calibrated at 20, 30, 40, 50, 60, 70 ten temperature sensors. Even though the temperature profile and 80 °C. In addition, the equilibrated temperatures recorded is not indicated gapless, we report that a continuous indication by the sensors were compared to the heat transfer medium is not necessary since the resolution of the temperature profile (HTM) temperature, measured with a class A temperature can be increased with different flow rates and the obtained sensor before each experiment. No temperature drift was de- data are highly reliable. tected during the measurement period. At the outlet of the flow calorimeter, the reaction solution was analysed in-line with a Fourier transform infrared spec- trometer (FTIR) equipped with a liquid cell and a hold up Materials and methods volume of 0.3 ml (Alpha II, Bruker Corporation, USA). After passing through the backpressure regulator (Contiplant Experimental setup valve, 64,144, Fluitec), the liquid was collected and neutralised. Water was used as HTM, which was tempered The experimental setup is shown in Fig. 1 and described in a and circulated by a thermostat, Thermostat 2 (CC304, Peter recent publication [12], a picture of the experimental setup is Huber). A Coriolis mass flow meter (Promass 80F15, Endress Fig. 1 Schematic of the setup of the continuous flow calorimeter. The feeds contain acetic anhydride (Ac O) and acetic acid (AcOH), Water (H O) and 2 2 nitric acid (HNO ), respectively 3 Journal of Flow Chemistry (2022) 12:389–396 391 For the determination of conversion, five Ac Ostandard solutions with concentrations between 0.3 and 5 mol/l in AcOH were measured by FTIR and served as calibration. Fig. 2 Reaction equation of the hydrolysis of acetic acid anhydride, The conversion was then determined by analysing the baseline HNO is used as catalyst corrected FTIR signal of the reaction solution where the signal at 1125 1/cm was identified as a fingerprint signal for Ac O + Hauser (Schweiz) AG, Switzerland) recorded the mass flow (supplementary information). of the HTM. To ensure a constant temperature in the calorimeter, the Temperatures, flow rates and pressures were recorded ev- thermostats were started at least 30 min before the pumps. ery second using a Siemens S7 control system. The reaction The total flow rates of the experiments are listed in Table 2, solution at the outlet was analysed by FTIR every 6.2 seconds. where the flow rate ratio between Ac O and AS is 8:10. The HTM was circulated at approximately 517 kg/h and set to Hydrolysis of acetic anhydride 20 °C, the backpressure valve was set to 1 bar. The selective acid catalysed hydrolysis of acetic anhydride Determination of the heat of reaction (Ac O) to acetic acid (AcOH), shown in Fig. 2, served as the model reaction for this study. Importantly, the reaction rate The heat of reaction was determined as explained in [12]and can be easily adjusted by varying the catalyst concentration. is therefore only summarised here. The temperature profile is As the reaction has been studied as a model reaction by several linearly interpolated and divided into 1000 segments, research groups, [16–22], the results can be well compared. allowing a precise calculation of the exchanged power. The In order to estimate the operating range of continuous flow sum of the exchanged power Q and non-exchanged power ex calorimetry, we introduced a flow factor β in a recent publi- ˙ ˙ Q is equal to the chemically generated power Q . nex r cation for the present reactor [12]. The product of the adiabatic temperature increase ΔT and the volume flowV should be at dQ ad ˙ ˙ ˙ 0 ¼ ¼ Q −Q −Q ð1Þ r ex nex least 200 K ml/min for an accurate determination of heat of dt reaction. The flow factor β was estimated to a range between If the product of the mass flow m, the heat capacity c ,and 1700 and 8400 K ml/min for this reaction, fulfilling well the the temperature difference ΔT between the initial and final requirement. temperature of the segment is summed over each segment, the non-exchanged power can be calculated Experimental procedure Q ¼ ∑ m⋅c ⋅ΔT ð2Þ nex p j j¼1 The flow calorimetry experiments were performed using Ac O and an aqueous solution (AS) containing AcOH and Using the same summation, the exchanged power can be nitric acid (HNO ) at different concentrations (Table 1). In calculated from the heat transfer coefficient k, the segment the AS solution, acetic acid (AcOH, Honeywell International shell area A and the temperature difference ΔT between Inc., puriss) was added because acetic anhydride (Ac O, HTM and reaction solution. Honeywell International Inc., puriss) no longer dissolved in Q ¼ ∑ k ⋅ A ⋅ΔT ð3Þ water at the concentrations used. The reaction rate was adjust- j ex j¼1 ed by the concentration of HNO (65%, Roth AG, purum). Two experimental series were conducted with a lower catalyst concentration (E1 and E2) and two with a higher catalyst Table 2 Total volumetric flow V of the conducted experiments. E2, concentration (E3 and E4). 18 ml/min was performed in duplicates ˙ ˙ ˙ ˙ V E1 [ml/min] V E2 [ml/min] V E3 [ml/min] V E4 [ml/min] Table 1 List of the components and their concentration in the feeds. 18 18 18 44 Acetic anhydride was used undiluted, which corresponds to a concentration of 10.52 mol/l 36 36 36 48 54 54 54 52 Compound E1 E2 E3 E4 72 72 72 56 aqueous solution (AS) HNO [mol/l] 2.172.575.325.32 90 60 H O [mol/l] 35.5 33.6 28.5 28.6 AcOH[mol/l] 5.465.735.565.54 68 acetic anhydride (Ac O) Ac O [mol/l] 10.52 10.52 10.52 10.52 72 2 2 392 Journal of Flow Chemistry (2022) 12:389–396 E1 50 20.5 18 ml/min 1.2 mol/L 20.0 36 ml/min 54 ml/min 19.5 72 ml/min 90 ml/min 19.0 18.5 18 ml/min 36 ml/min 18.0 54 ml/min 72 ml/min 17.5 90 ml/min HTM 17.0 0 100 200 300 400 500 0 20 40 60 80 100 L [mm] E2 L [mm] 50 20.5 18 ml/min 1.4 mol/L 20.0 18 ml/min 36 ml/min 40 19.5 54 ml/min 72 m 7 7 l// /m min 19.0 30 18.5 18 ml/min 18.0 36 ml/min 54 ml/min 20 17.5 72 ml/min HTM 17.0 0 100 200 300 400 500 0 20 40 60 80 100 L [mm] L [mm] E3 18 ml/min 3.0 mol/L 36 ml/min 54 ml/min 70 72 ml/min 18 ml/min 50 36 ml/min 54 ml/min 72 ml/min HTM 0 100 200 300 400 500 0 20 40 60 80 100 L [mm] L [mm] Fig. 3 Left: Temperature profiles obtained from experimental series E1- the temperature drops compared to the HTM-temperature at the begin- E3. The maximum temperature reached is strongly dependent on the ning of the slower reactions due to the enthalpy of mixing amount of catalyst and thus on the reaction rate. Right: Highlighted are ˙ ˙ The exo- or endothermicity of a reaction is given by the Since Q and Q are known from eqs. 2 and 3, the reac- nex ex enthalpy of reaction ΔH . This can be calculated with eq. 5 tion power can now be determined using eq. 1. With the mass from the heat of reaction Q , the density ρ and the molar flowm and the reaction power, the specific heat of reaction Q concentration of the limiting acetic anhydride c at the can now be calculated. Ac O;0 beginning of the reaction. ˙ ˙ Q þ Q ex nex Q ¼ ð4Þ r −Q ⋅ρ ΔH ¼ ð5Þ Ac O;0 T [°C] T [°C] T [°C] T [°C] T [°C] T [°C] Journal of Flow Chemistry (2022) 12:389–396 393 The values of the heat capacity and the density used are ∆H Mix + React Enthalpy of mixing listed in the supplementary information. Fit of ∆H Mix + React ∆H React 95 % CI Results and discussion ∆H = 60.79 X - 7.18 -20 R = 0.9915 In Fig. 3, the temperature profiles of the experimental series E1-E3 are plotted against the reactor length. In the experimen- tal series E3 and a total flow rate of 18 ml/min, the reaction is -40 ∆H = 60.79 X already completed, as the outlet temperature is close to the inlet temperature. Hence, the heat of reaction is almost iden- tical to the dissipated heat. In contrary, all reactions in series -60 Reaction enthalpy E1 were not complete due to the lower acid concentration and the last temperature sensor revealed the highest value except 0.0 0.2 0.4 0.6 0.8 1.0 with 18 ml/min. The dissipated and non-dissipated heat is Conversion X [-] therefore relevant for the heat of reaction. Experiment E2, Fig. 4 The measured enthalpies are plotted against the conversion. The 18 ml/min was carried out twice resulting in a deviation in intercept of the regression shows the enthalpy of mixing since there is no heat of reaction of only 0.64%, demonstrating the reproduc- reaction. If the regression curve is shifted by the intercept as indicated, the ibility of the experiments and analysis (Fig. 3,middle left). enthalpy of reaction can be read at 100% conversion. The grey shaded areas show the 95% confidence interval (CI) of the regression curves Furthermore, Fig. 3 shows the influence of the nitric acid concentration, which increases from 1.2 mol/l (E1) over 1.4 mol/l (E2) to 3.0 mol/l (E3). The differences in reaction might depend on the mixing ratio of the substances [23]. We rates are reflected in the temperature rise, which increases speculate that the range of reported values are explained by accordingly from E1 to E3. At low catalyst concentration both the reactor type and various mole fractions. Batch exper- and thus slow reaction rates, a temperature dip at the begin- iments were conducted at a low mole fraction ning of the reactor is clearly visible (Fig. 3, top and middle), (χ =0.006) [16, 17], whereas continuous flow Ac O=ðÞ H OþAc O 2 2 2 which is presumably due to the enthalpy of mixing. At higher calorimeter results are recorded at χ = 0.02 Ac O=ðÞ H OþAc O catalyst concentration and thus fast reaction rates as in E3 the 2 2 2 [18]and χ = 0.2 (this work), respectively. dip cannot be seen in the profile (Fig. 3, bottom) as the en- Ac O=ðÞ H OþAc O 2 2 2 thalpy of mixing is overcompensated by the heat of reaction. We conclude that mixing enthalpy determinations at condi- tions close to production are important for a safe scale-up. The enthalpy of mixing can be determined when plotting the conversion against the enthalpy, as shown in Fig. 4. For this Compared to the setup used by Ładosz et al. [18], where the analysis, only the results of E1 and E2 are used, which have a temperature was measured at the inlet zone and at two loca- conversion <100%, so that the measuring points are well dis- tions in the reaction zone in which isothermal conditions were tributed over the conversion range. assumed, the milli-calorimeter used in this study shows the Since the heat of reaction is 0 kJ/kg at 0 conversion, the temperature profile in the reactor. The temperature drop at the intercept is equal to the enthalpy of mixing and can be iden- beginning of the profile at low acid concentrations confirms tified in Fig. 4. The enthalpy of reaction is calculated from eqs. the endothermic enthalpy of mixing in continuous reactors. As 2–5. If the regression curve is shifted by the enthalpy of the kinetics are included in the temperature profiles and since the heat transfer is precisely known for this reactor [12], the mixing, the enthalpy of reaction can be simply read at com- plete conversion. The value obtained for the heat of reaction here presented results allow a clear prediction of the maximum temperature in an industrial plant scaled-up from the used correlates with the slope of the regression curve in Fig. 4. For the model reaction applied in this study, Ładosz et al. milli-reactor. [18] have determined a mixing enthalpy of 8.8 ± 2.1 kJ/mol The E3 experiment series resulted in a mean enthalpy of and a reaction enthalpy of −63 ± 3.0 kJ/mol in a continuous reaction of −54.5 kJ/mol. Although we had expected a con- microreactor. These values agree with the measured values of stant enthalpy of reaction, the individual determinations 7.2 ± 2.8 kJ/mol and − 60.8 ± 2.5 kJ/mol. Since no catalyst ranged from −61.5 to −46.1 kJ/mol. The origin of the ob- was used in Ładosz et al. [18] and we used 1.2 or 1.4 mol/l served deviation can be rationalised by the scheme shown in HNO , the catalyst plays only a minor role in the determina- Fig. 5, where three arbitrary temperature profiles represent tion of the enthalpy of mixing. Interestingly, an exothermic three cases with different flow rates. In case 1, the maximum peak is reached before the first temperature sensor (T1) and enthalpy of mixing at 25 °C was determined in two studies for the same reaction but using batch reactors instead of a flow the reaction solution has cooled down at the first temperature probe already. Due to the mismatch of the position of the calorimeter [16, 17]. Furthermore, the enthalpy of mixing Enthalpy ∆H [kJ/mol] 394 Journal of Flow Chemistry (2022) 12:389–396 T0 T1 T2 T3 Obviously, the flow step size allows to adjust the uncertainty Case 1 to the desired level. In Fig. 6A, the temperature of the first two Case 2 thermocouples and the temperature of the inlet are plotted Case 3 against the residence time. Apparently, at a higher flow rate, a particular thermocouple is reached faster, so that the resi- dence time is shorter. It is demonstrated that the highest tem- perature is not detected at a high or low flow rate, so that the exothermicity of the reaction would be measured too low. At a flow rate of 48 ml/min, the second thermocouple detects ex- actly the highest temperature, which then leads to the most accurate calorimetric data. According to the proposed model in Fig. 5, the reaction enthalpy calculated for the different flow rates shows a mini- mum at 48 ml/min (Fig. 6B). The observed enthalpy of reac- tion at 48 ml/min equals −62.5 kJ/mol, which is in the litera- ture range of −57 [16]to −63 kJ/mol [17]. Length of the reactor Each flow rate was maintained for 4 minutes, so that the Fig. 5 The figure shows three arbitrary temperature profiles. The shaded complete experiment with the 10 flow rates was completed areas are lost in each case due to the interpolation. This area is the smallest after 40 minutes. With this method, the expensive installation when the temperature sensor is located at the maximum of the temperature profile of a fiber Bragg temperature sensor can be dispensed without compromising the quality of the result. Due to the fast and temperature probe and the position of the maximum tempera- industry-oriented determination, the upscale can be carried out ture in the reactor, the shaded area between the temperature precisely, safely, and cost-effectively. profile and the linear interpolation, which is used for the eval- uation is lost for the calculation of the exchanged power. This area is the smallest for case 2, where the highest temperature is Conclusion found at the same position as the first thermocouple. In case 3, where the maximum peak lies between sensor one and two, When processes are scaled up, it is pivotal to know in advance the reaction enthalpy is also underestimated. how much energy a reaction will release. This data is prefer- To obtain an overlap of the maximum temperature and the ably tested in a system that is already set up similarly to the thermocouple position, in E4 the total flow rates were in- production plant. In this way, predictions can be made more creased stepwise by 4 ml/min between 36 and 72 ml/min. confidently. For example, the enthalpy of mixing was 36 ml/min A B -54 40 ml/min 44 ml/min 48 ml/min 52 ml/min -56 56 ml/min peak of the 60 ml/min temperature profile 64 ml/min -58 68 ml/min 72 ml/min -60 -62 Inlet Thermocouple 1 Thermocouple 2 -64 05 10 15 35 40 45 50 55 60 65 70 75 Residence time [s] V [ml/min] Fig. 6 While the temperature measurements of E4 at the inlet are all taken measured at the position of thermocouple 2 is 48 ml/min (Diagram A). at residence time 0, the residence times vary at different flow rates in the Diagram B shows that the highest exothermicity is also measured at reactor. The shaded area indicates which thermocouple measured the 48 ml/min temperature. The flow rate at which the highest temperature is Temperature [°C] Temperature H [kJ/mol] r Journal of Flow Chemistry (2022) 12:389–396 395 determined as exothermic by two research groups in a batch Declarations reactor, while we measured the enthalpy of mixing as endo- Conflicts of interest/Competing interests The authors have no conflicts thermic at 7.2 kJ/mol. This fact was explained by the different of interest to declare that are relevant to the content of this article. The mole fractions of Ac O and different reactors used by the authors Marlies Moser and Alain Georg work in the research department research groups. Since the batch reactors were operated as of the company that produces the flow calorimeter used in this study. In isothermally as possible, only small concentrations were used, detail, Marlies Moser supported the literature search and the reaction planning. Alain Georg assisted with the evaluation and the analysis of whereas we can use significantly higher and industrially rele- the measured data. vant concentrations with a polytropic reaction control. Less nitric acid was used in the E1 and E2 series of exper- iments, so that the reaction proceeded relatively slow, and the Consent to participate Yes linear interpolation depicted the effective temperature profile well. On the other hand, the determined reaction enthalpies of Consent for publication Yes the fastest reaction with 3.0 mol/l HNO showed a standard deviation of 5.5 kJ/mol. This uncertainty results from the fact that the highest point of a temperature profile was not recorded Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adap- with a temperature probe. Therefore, a facile and fast experi- tation, distribution and reproduction in any medium or format, as long as ment scanning of different flow rates was performed, where you give appropriate credit to the original author(s) and the source, pro- the highest temperature was recorded. The enthalpy of reac- vide a link to the Creative Commons licence, and indicate if changes were tion could then be determined to be −62.5 kJ/mol, which is in made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a the literature range of −57 [16]to −63 [17] kJ/mol. With the credit line to the material. If material is not included in the article's variation of the flow rate and the inline analysis, calorimetric Creative Commons licence and your intended use is not permitted by data can be analysed accurately for reactions with a half-life of statutory regulation or exceeds the permitted use, you will need to obtain a few seconds to minutes. permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/ . In this work we presented a procedure to determine the heat of mixing and reaction accurately, simultaneously and contin- uously, such that predictions can be made more confidently. References 2 −1 Symbols A, heat transfer surface [m ]; β,flow factor [Kml min ]; 1. Pashkova A, Greiner L (2011) Towards small-scale continuous −3 c , initial concentration of the limiting reactant Ac O[mol m ]; Ac O;0 2 2 chemical production: technology gaps and challenges. 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Simultaneous determination of enthalpy of mixing and reaction using milli-scale continuous flow calorimetry

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Springer Journals
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Copyright © The Author(s) 2022
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2062-249X
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2063-0212
DOI
10.1007/s41981-022-00237-x
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Abstract

A simultaneous determination of the enthalpy of mixing and reaction in a scalable continuous milli-scale flow calorimeter is investigated. As obtained calorimetric data is pivotal for the safety assessment of chemical reactions and processes. The acid- catalysed selective, homogeneous hydrolysis of acetic anhydride with half-lives from a few seconds to a few minutes is investigated as a model reaction. For the enthalpy of mixing 7.2 ± 2.8 kJ/mol and for the enthalpy of reaction −60.8 ± 2.5 kJ/mol were determined. For reactions that show complete conversion in the continuous reactor, a technique is introduced to further improve the accuracy of the reaction enthalpy determination. Thereby, the resolution of the observed temperature profile is increased by measuring the profile at different flow rates. Applying this procedure, the reaction enthalpy of −62.5 kJ/ mol was determined which is in good agreement with literature values for this model reaction. . . . . Keywords Continuous flow calorimetry Polytropic reaction calorimetry Heat of reaction Hydrolysis of acetic anhydride Process development Scale-up Introduction safety assessment of a process, the determination of the enthalpy of reaction is an elementary component [5, 6]. The closer this Today, mainly batch reactors are used to produce fine chemicals in determination is to the industrial process, the more robust safety the range of up to 100 t/y [1]. As the industry is striving for process data are obtained. For example, Mortzfeld et al. [7] found devia- intensification, the changeover from a multifunctional batch plant tions in non-selective reactions when they were investigated using to a smaller continuous monoplant can make economic sense, as batch calorimeters or continuous flow calorimeters. Several flow there is no need for cleaning, charge and discharge [2]. In addition, calorimeters have been developed measuring the enthalpy of re- continuous reactors are smaller for the same production capacity action using thermoelectric heat flow [8–10], infrared thermogra- since higher temperatures and pressures allow a shorter residence phy [11], or segmental temperature sensors [7]. time than batch reactors [3]. The heat transfer is better in a con- Recently, we have shown that the reaction enthalpy of a tinuous flow reactor and there is less potential chemical energy, so fast and selective reaction can be measured in the continuous the consequences of an incident are drastically reduced [4]. For the flow calorimeter without extensive calibration of the heat transfer coefficient [12]. However, organometallic reactions, nitrations or polymerisations have half-lives in the range of Highlights � Simultaneous determination of enthalpy of mixing and heat of reaction seconds or minutes [13]. If full conversion is not achieved, the using a flow calorimeter. analysis of the exiting product is indispensable. Therefore, � Facile and fast flow screening procedure for accurate calorimetric data. infrared spectroscopy is a suitable method of analysis due to � Safe and scalable setup combined with online reaction monitoring. its sensitivity, economic viability, and broad application pos- sibilities [14, 15]. Furthermore, the enthalpy of mixing must * Daniel M. Meier meid@zhaw.ch be quantified since the conversion of the reaction is not com- plete and influences the calorimetric data. To determine the School of Engineering, Institute of Materials and Process enthalpy of mixing, the heat of reaction in relation to the Engineering, ZHAW Zurich University of Applied Sciences, conversion must be known. In a batch calorimeter, the con- Technikumstrasse 9, 8401 Winterthur, Switzerland version can be determined over time and known kinetics [16] Fluitec mixing + reaction solutions AG, Seuzachstrasse 40, or inline measurements [17]. Ładosz et al. [18]have shown 8413 Neftenbach, Switzerland 390 Journal of Flow Chemistry (2022) 12:389–396 that this determination of the enthalpy of mixing is possible in provided in the supplementary information. Briefly, two pis- a continuous micro reactor at steady state under isothermal ton pumps, dosing system 1 and 2 (DZRP-200, 63,639, conditions. Fluitec mixing + reaction solutions AG, Switzerland), were In this work we aim to simultaneously investigate the heat used to convey the feed streams. Both liquid feeds were con- of reaction and the enthalpy of mixing for the hydrolysis of ditionedto20°Cin separate lines usinga thermostat, acetic anhydride in a scalable continuous flow calorimeter. Thermostat 1 (Ministat 230, Peter Huber Kältemaschinenbau This model reaction has served for several continuous [18] AG, Germany). Before entering the flow calorimeter, the tem- and batch [16, 17, 19–21] calorimetric studies and is particu- perature was measured inline with a temperature probe im- larly suitable because the reaction rate can simply be tuned by merged into the feed (Class A, Pt-100, 53,286, Fluitec). The the help of the acid concentration, which catalyses the reac- reaction started immediately after mixing the two feeds in the tion. A milli-scale continuous flow calorimeter, scalable to flow calorimeter (L = 500 mm, D = 12.3 mm, V = 44.3 ml production scale, as used by us [12] is applied. Compared to Contiplant PFR-50-SS, Fluitec), which was equipped with a other reactor setups to determine the enthalpy of reaction, for pre-mixer, static CSE-X mixers and 10 axial temperature sen- example Ładosz et al. [18], who used three temperature sen- sors (Class 1, type K, Fluitec). Prior the experiments, the axial sors, this calorimeter measures the temperature profile using temperature sensors were calibrated at 20, 30, 40, 50, 60, 70 ten temperature sensors. Even though the temperature profile and 80 °C. In addition, the equilibrated temperatures recorded is not indicated gapless, we report that a continuous indication by the sensors were compared to the heat transfer medium is not necessary since the resolution of the temperature profile (HTM) temperature, measured with a class A temperature can be increased with different flow rates and the obtained sensor before each experiment. No temperature drift was de- data are highly reliable. tected during the measurement period. At the outlet of the flow calorimeter, the reaction solution was analysed in-line with a Fourier transform infrared spec- trometer (FTIR) equipped with a liquid cell and a hold up Materials and methods volume of 0.3 ml (Alpha II, Bruker Corporation, USA). After passing through the backpressure regulator (Contiplant Experimental setup valve, 64,144, Fluitec), the liquid was collected and neutralised. Water was used as HTM, which was tempered The experimental setup is shown in Fig. 1 and described in a and circulated by a thermostat, Thermostat 2 (CC304, Peter recent publication [12], a picture of the experimental setup is Huber). A Coriolis mass flow meter (Promass 80F15, Endress Fig. 1 Schematic of the setup of the continuous flow calorimeter. The feeds contain acetic anhydride (Ac O) and acetic acid (AcOH), Water (H O) and 2 2 nitric acid (HNO ), respectively 3 Journal of Flow Chemistry (2022) 12:389–396 391 For the determination of conversion, five Ac Ostandard solutions with concentrations between 0.3 and 5 mol/l in AcOH were measured by FTIR and served as calibration. Fig. 2 Reaction equation of the hydrolysis of acetic acid anhydride, The conversion was then determined by analysing the baseline HNO is used as catalyst corrected FTIR signal of the reaction solution where the signal at 1125 1/cm was identified as a fingerprint signal for Ac O + Hauser (Schweiz) AG, Switzerland) recorded the mass flow (supplementary information). of the HTM. To ensure a constant temperature in the calorimeter, the Temperatures, flow rates and pressures were recorded ev- thermostats were started at least 30 min before the pumps. ery second using a Siemens S7 control system. The reaction The total flow rates of the experiments are listed in Table 2, solution at the outlet was analysed by FTIR every 6.2 seconds. where the flow rate ratio between Ac O and AS is 8:10. The HTM was circulated at approximately 517 kg/h and set to Hydrolysis of acetic anhydride 20 °C, the backpressure valve was set to 1 bar. The selective acid catalysed hydrolysis of acetic anhydride Determination of the heat of reaction (Ac O) to acetic acid (AcOH), shown in Fig. 2, served as the model reaction for this study. Importantly, the reaction rate The heat of reaction was determined as explained in [12]and can be easily adjusted by varying the catalyst concentration. is therefore only summarised here. The temperature profile is As the reaction has been studied as a model reaction by several linearly interpolated and divided into 1000 segments, research groups, [16–22], the results can be well compared. allowing a precise calculation of the exchanged power. The In order to estimate the operating range of continuous flow sum of the exchanged power Q and non-exchanged power ex calorimetry, we introduced a flow factor β in a recent publi- ˙ ˙ Q is equal to the chemically generated power Q . nex r cation for the present reactor [12]. The product of the adiabatic temperature increase ΔT and the volume flowV should be at dQ ad ˙ ˙ ˙ 0 ¼ ¼ Q −Q −Q ð1Þ r ex nex least 200 K ml/min for an accurate determination of heat of dt reaction. The flow factor β was estimated to a range between If the product of the mass flow m, the heat capacity c ,and 1700 and 8400 K ml/min for this reaction, fulfilling well the the temperature difference ΔT between the initial and final requirement. temperature of the segment is summed over each segment, the non-exchanged power can be calculated Experimental procedure Q ¼ ∑ m⋅c ⋅ΔT ð2Þ nex p j j¼1 The flow calorimetry experiments were performed using Ac O and an aqueous solution (AS) containing AcOH and Using the same summation, the exchanged power can be nitric acid (HNO ) at different concentrations (Table 1). In calculated from the heat transfer coefficient k, the segment the AS solution, acetic acid (AcOH, Honeywell International shell area A and the temperature difference ΔT between Inc., puriss) was added because acetic anhydride (Ac O, HTM and reaction solution. Honeywell International Inc., puriss) no longer dissolved in Q ¼ ∑ k ⋅ A ⋅ΔT ð3Þ water at the concentrations used. The reaction rate was adjust- j ex j¼1 ed by the concentration of HNO (65%, Roth AG, purum). Two experimental series were conducted with a lower catalyst concentration (E1 and E2) and two with a higher catalyst Table 2 Total volumetric flow V of the conducted experiments. E2, concentration (E3 and E4). 18 ml/min was performed in duplicates ˙ ˙ ˙ ˙ V E1 [ml/min] V E2 [ml/min] V E3 [ml/min] V E4 [ml/min] Table 1 List of the components and their concentration in the feeds. 18 18 18 44 Acetic anhydride was used undiluted, which corresponds to a concentration of 10.52 mol/l 36 36 36 48 54 54 54 52 Compound E1 E2 E3 E4 72 72 72 56 aqueous solution (AS) HNO [mol/l] 2.172.575.325.32 90 60 H O [mol/l] 35.5 33.6 28.5 28.6 AcOH[mol/l] 5.465.735.565.54 68 acetic anhydride (Ac O) Ac O [mol/l] 10.52 10.52 10.52 10.52 72 2 2 392 Journal of Flow Chemistry (2022) 12:389–396 E1 50 20.5 18 ml/min 1.2 mol/L 20.0 36 ml/min 54 ml/min 19.5 72 ml/min 90 ml/min 19.0 18.5 18 ml/min 36 ml/min 18.0 54 ml/min 72 ml/min 17.5 90 ml/min HTM 17.0 0 100 200 300 400 500 0 20 40 60 80 100 L [mm] E2 L [mm] 50 20.5 18 ml/min 1.4 mol/L 20.0 18 ml/min 36 ml/min 40 19.5 54 ml/min 72 m 7 7 l// /m min 19.0 30 18.5 18 ml/min 18.0 36 ml/min 54 ml/min 20 17.5 72 ml/min HTM 17.0 0 100 200 300 400 500 0 20 40 60 80 100 L [mm] L [mm] E3 18 ml/min 3.0 mol/L 36 ml/min 54 ml/min 70 72 ml/min 18 ml/min 50 36 ml/min 54 ml/min 72 ml/min HTM 0 100 200 300 400 500 0 20 40 60 80 100 L [mm] L [mm] Fig. 3 Left: Temperature profiles obtained from experimental series E1- the temperature drops compared to the HTM-temperature at the begin- E3. The maximum temperature reached is strongly dependent on the ning of the slower reactions due to the enthalpy of mixing amount of catalyst and thus on the reaction rate. Right: Highlighted are ˙ ˙ The exo- or endothermicity of a reaction is given by the Since Q and Q are known from eqs. 2 and 3, the reac- nex ex enthalpy of reaction ΔH . This can be calculated with eq. 5 tion power can now be determined using eq. 1. With the mass from the heat of reaction Q , the density ρ and the molar flowm and the reaction power, the specific heat of reaction Q concentration of the limiting acetic anhydride c at the can now be calculated. Ac O;0 beginning of the reaction. ˙ ˙ Q þ Q ex nex Q ¼ ð4Þ r −Q ⋅ρ ΔH ¼ ð5Þ Ac O;0 T [°C] T [°C] T [°C] T [°C] T [°C] T [°C] Journal of Flow Chemistry (2022) 12:389–396 393 The values of the heat capacity and the density used are ∆H Mix + React Enthalpy of mixing listed in the supplementary information. Fit of ∆H Mix + React ∆H React 95 % CI Results and discussion ∆H = 60.79 X - 7.18 -20 R = 0.9915 In Fig. 3, the temperature profiles of the experimental series E1-E3 are plotted against the reactor length. In the experimen- tal series E3 and a total flow rate of 18 ml/min, the reaction is -40 ∆H = 60.79 X already completed, as the outlet temperature is close to the inlet temperature. Hence, the heat of reaction is almost iden- tical to the dissipated heat. In contrary, all reactions in series -60 Reaction enthalpy E1 were not complete due to the lower acid concentration and the last temperature sensor revealed the highest value except 0.0 0.2 0.4 0.6 0.8 1.0 with 18 ml/min. The dissipated and non-dissipated heat is Conversion X [-] therefore relevant for the heat of reaction. Experiment E2, Fig. 4 The measured enthalpies are plotted against the conversion. The 18 ml/min was carried out twice resulting in a deviation in intercept of the regression shows the enthalpy of mixing since there is no heat of reaction of only 0.64%, demonstrating the reproduc- reaction. If the regression curve is shifted by the intercept as indicated, the ibility of the experiments and analysis (Fig. 3,middle left). enthalpy of reaction can be read at 100% conversion. The grey shaded areas show the 95% confidence interval (CI) of the regression curves Furthermore, Fig. 3 shows the influence of the nitric acid concentration, which increases from 1.2 mol/l (E1) over 1.4 mol/l (E2) to 3.0 mol/l (E3). The differences in reaction might depend on the mixing ratio of the substances [23]. We rates are reflected in the temperature rise, which increases speculate that the range of reported values are explained by accordingly from E1 to E3. At low catalyst concentration both the reactor type and various mole fractions. Batch exper- and thus slow reaction rates, a temperature dip at the begin- iments were conducted at a low mole fraction ning of the reactor is clearly visible (Fig. 3, top and middle), (χ =0.006) [16, 17], whereas continuous flow Ac O=ðÞ H OþAc O 2 2 2 which is presumably due to the enthalpy of mixing. At higher calorimeter results are recorded at χ = 0.02 Ac O=ðÞ H OþAc O catalyst concentration and thus fast reaction rates as in E3 the 2 2 2 [18]and χ = 0.2 (this work), respectively. dip cannot be seen in the profile (Fig. 3, bottom) as the en- Ac O=ðÞ H OþAc O 2 2 2 thalpy of mixing is overcompensated by the heat of reaction. We conclude that mixing enthalpy determinations at condi- tions close to production are important for a safe scale-up. The enthalpy of mixing can be determined when plotting the conversion against the enthalpy, as shown in Fig. 4. For this Compared to the setup used by Ładosz et al. [18], where the analysis, only the results of E1 and E2 are used, which have a temperature was measured at the inlet zone and at two loca- conversion <100%, so that the measuring points are well dis- tions in the reaction zone in which isothermal conditions were tributed over the conversion range. assumed, the milli-calorimeter used in this study shows the Since the heat of reaction is 0 kJ/kg at 0 conversion, the temperature profile in the reactor. The temperature drop at the intercept is equal to the enthalpy of mixing and can be iden- beginning of the profile at low acid concentrations confirms tified in Fig. 4. The enthalpy of reaction is calculated from eqs. the endothermic enthalpy of mixing in continuous reactors. As 2–5. If the regression curve is shifted by the enthalpy of the kinetics are included in the temperature profiles and since the heat transfer is precisely known for this reactor [12], the mixing, the enthalpy of reaction can be simply read at com- plete conversion. The value obtained for the heat of reaction here presented results allow a clear prediction of the maximum temperature in an industrial plant scaled-up from the used correlates with the slope of the regression curve in Fig. 4. For the model reaction applied in this study, Ładosz et al. milli-reactor. [18] have determined a mixing enthalpy of 8.8 ± 2.1 kJ/mol The E3 experiment series resulted in a mean enthalpy of and a reaction enthalpy of −63 ± 3.0 kJ/mol in a continuous reaction of −54.5 kJ/mol. Although we had expected a con- microreactor. These values agree with the measured values of stant enthalpy of reaction, the individual determinations 7.2 ± 2.8 kJ/mol and − 60.8 ± 2.5 kJ/mol. Since no catalyst ranged from −61.5 to −46.1 kJ/mol. The origin of the ob- was used in Ładosz et al. [18] and we used 1.2 or 1.4 mol/l served deviation can be rationalised by the scheme shown in HNO , the catalyst plays only a minor role in the determina- Fig. 5, where three arbitrary temperature profiles represent tion of the enthalpy of mixing. Interestingly, an exothermic three cases with different flow rates. In case 1, the maximum peak is reached before the first temperature sensor (T1) and enthalpy of mixing at 25 °C was determined in two studies for the same reaction but using batch reactors instead of a flow the reaction solution has cooled down at the first temperature probe already. Due to the mismatch of the position of the calorimeter [16, 17]. Furthermore, the enthalpy of mixing Enthalpy ∆H [kJ/mol] 394 Journal of Flow Chemistry (2022) 12:389–396 T0 T1 T2 T3 Obviously, the flow step size allows to adjust the uncertainty Case 1 to the desired level. In Fig. 6A, the temperature of the first two Case 2 thermocouples and the temperature of the inlet are plotted Case 3 against the residence time. Apparently, at a higher flow rate, a particular thermocouple is reached faster, so that the resi- dence time is shorter. It is demonstrated that the highest tem- perature is not detected at a high or low flow rate, so that the exothermicity of the reaction would be measured too low. At a flow rate of 48 ml/min, the second thermocouple detects ex- actly the highest temperature, which then leads to the most accurate calorimetric data. According to the proposed model in Fig. 5, the reaction enthalpy calculated for the different flow rates shows a mini- mum at 48 ml/min (Fig. 6B). The observed enthalpy of reac- tion at 48 ml/min equals −62.5 kJ/mol, which is in the litera- ture range of −57 [16]to −63 kJ/mol [17]. Length of the reactor Each flow rate was maintained for 4 minutes, so that the Fig. 5 The figure shows three arbitrary temperature profiles. The shaded complete experiment with the 10 flow rates was completed areas are lost in each case due to the interpolation. This area is the smallest after 40 minutes. With this method, the expensive installation when the temperature sensor is located at the maximum of the temperature profile of a fiber Bragg temperature sensor can be dispensed without compromising the quality of the result. Due to the fast and temperature probe and the position of the maximum tempera- industry-oriented determination, the upscale can be carried out ture in the reactor, the shaded area between the temperature precisely, safely, and cost-effectively. profile and the linear interpolation, which is used for the eval- uation is lost for the calculation of the exchanged power. This area is the smallest for case 2, where the highest temperature is Conclusion found at the same position as the first thermocouple. In case 3, where the maximum peak lies between sensor one and two, When processes are scaled up, it is pivotal to know in advance the reaction enthalpy is also underestimated. how much energy a reaction will release. This data is prefer- To obtain an overlap of the maximum temperature and the ably tested in a system that is already set up similarly to the thermocouple position, in E4 the total flow rates were in- production plant. In this way, predictions can be made more creased stepwise by 4 ml/min between 36 and 72 ml/min. confidently. For example, the enthalpy of mixing was 36 ml/min A B -54 40 ml/min 44 ml/min 48 ml/min 52 ml/min -56 56 ml/min peak of the 60 ml/min temperature profile 64 ml/min -58 68 ml/min 72 ml/min -60 -62 Inlet Thermocouple 1 Thermocouple 2 -64 05 10 15 35 40 45 50 55 60 65 70 75 Residence time [s] V [ml/min] Fig. 6 While the temperature measurements of E4 at the inlet are all taken measured at the position of thermocouple 2 is 48 ml/min (Diagram A). at residence time 0, the residence times vary at different flow rates in the Diagram B shows that the highest exothermicity is also measured at reactor. The shaded area indicates which thermocouple measured the 48 ml/min temperature. The flow rate at which the highest temperature is Temperature [°C] Temperature H [kJ/mol] r Journal of Flow Chemistry (2022) 12:389–396 395 determined as exothermic by two research groups in a batch Declarations reactor, while we measured the enthalpy of mixing as endo- Conflicts of interest/Competing interests The authors have no conflicts thermic at 7.2 kJ/mol. This fact was explained by the different of interest to declare that are relevant to the content of this article. The mole fractions of Ac O and different reactors used by the authors Marlies Moser and Alain Georg work in the research department research groups. Since the batch reactors were operated as of the company that produces the flow calorimeter used in this study. In isothermally as possible, only small concentrations were used, detail, Marlies Moser supported the literature search and the reaction planning. Alain Georg assisted with the evaluation and the analysis of whereas we can use significantly higher and industrially rele- the measured data. vant concentrations with a polytropic reaction control. Less nitric acid was used in the E1 and E2 series of exper- iments, so that the reaction proceeded relatively slow, and the Consent to participate Yes linear interpolation depicted the effective temperature profile well. On the other hand, the determined reaction enthalpies of Consent for publication Yes the fastest reaction with 3.0 mol/l HNO showed a standard deviation of 5.5 kJ/mol. This uncertainty results from the fact that the highest point of a temperature profile was not recorded Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adap- with a temperature probe. Therefore, a facile and fast experi- tation, distribution and reproduction in any medium or format, as long as ment scanning of different flow rates was performed, where you give appropriate credit to the original author(s) and the source, pro- the highest temperature was recorded. The enthalpy of reac- vide a link to the Creative Commons licence, and indicate if changes were tion could then be determined to be −62.5 kJ/mol, which is in made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a the literature range of −57 [16]to −63 [17] kJ/mol. With the credit line to the material. If material is not included in the article's variation of the flow rate and the inline analysis, calorimetric Creative Commons licence and your intended use is not permitted by data can be analysed accurately for reactions with a half-life of statutory regulation or exceeds the permitted use, you will need to obtain a few seconds to minutes. permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/ . In this work we presented a procedure to determine the heat of mixing and reaction accurately, simultaneously and contin- uously, such that predictions can be made more confidently. References 2 −1 Symbols A, heat transfer surface [m ]; β,flow factor [Kml min ]; 1. Pashkova A, Greiner L (2011) Towards small-scale continuous −3 c , initial concentration of the limiting reactant Ac O[mol m ]; Ac O;0 2 2 chemical production: technology gaps and challenges. 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Journal

Journal of Flow ChemistrySpringer Journals

Published: Dec 1, 2022

Keywords: Continuous flow calorimetry; Polytropic reaction calorimetry; Heat of reaction; Hydrolysis of acetic anhydride; Process development; Scale-up

References