The Effect of Fuel Mass Fraction on the Combustion and Fluid Flow in a Sulfur Recovery Unit Thermal Reactor
The Effect of Fuel Mass Fraction on the Combustion and Fluid Flow in a Sulfur Recovery Unit...
applied sciences Article The Effect of Fuel Mass Fraction on the Combustion and Fluid Flow in a Sulfur Recovery Unit Thermal Reactor Chun-Lang Yeh Department of Aeronautical Engineering, National Formosa University, Huwei, Yunlin 632, Taiwan; email@example.com; Tel.: +886-5-6315527 Academic Editor: Yuyuan Zhao Received: 7 September 2016; Accepted: 26 October 2016; Published: 2 November 2016 Abstract: Sulfur recovery unit (SRU) thermal reactors are negatively affected by high temperature operation. In this paper, the effect of the fuel mass fraction on the combustion and ﬂuid ﬂow in a SRU thermal reactor is investigated numerically. Practical operating conditions for a petrochemical corporation in Taiwan are used as the design conditions for the discussion. The simulation results show that the present design condition is a fuel-rich (or air-lean) condition and gives acceptable sulfur recovery, hydrogen sulﬁde (H S) destruction, sulfur dioxide (SO ) emissions and thermal 2 2 reactor temperature for an oxygen-normal operation. However, for an oxygen-rich operation, the local maximum temperature exceeds the suggested maximum service temperature, although the average temperature is acceptable. The high temperature region must be inspected very carefully during the annual maintenance period if there are oxygen-rich operations. If the fuel mass fraction to the zone ahead of the choke ring (zone 1) is 0.0625 or 0.125, the average temperature in the zone behind the choke ring (zone 2) is higher than the zone 1 average temperature, which can damage the downstream heat exchanger tubes. If the zone 1 fuel mass fraction is reduced to ensure a lower zone 1 temperature, the temperature in zone 2 and the heat exchanger section must be monitored closely and the zone 2 wall and heat exchanger tubes must be inspected very carefully during the annual maintenance period. To determine a suitable fuel mass fraction for operation, a detailed numerical simulation should be performed ﬁrst to ﬁnd the stoichiometric fuel mass fraction which produces the most complete combustion and the highest temperature. This stoichiometric fuel mass fraction should be avoided because the high temperature could damage the zone 1 corner or the choke ring. A higher fuel mass fraction (i.e., fuel-rich or air-lean condition) is more suitable because it can avoid deteriorations of both zone 1 and heat exchanger tubes. Although a lower fuel mass fraction (i.e., fuel-lean or air-rich condition) can avoid deterioration of zone 1, the heat exchanger tubes may be damaged. This paper provides a guideline for adjusting the fuel mass fraction to reduce the high temperature inside the thermal reactor and to ensure an acceptable sulfur recovery. Keywords: SRU thermal reactor; fuel mass fraction; sulfur recovery; H S destruction; SO emission; 2 2 thermal reactor temperature 1. Introduction Desulfurization is very important in the petroleum reﬁning process because oxysulﬁdes from the petroleum reﬁning process are one of the major sources of air pollution. The most frequently used desulfurization process is the Claus process, which converts the H S in natural gas or crude oil into sulfur elements, which reduces the formation of oxysulﬁdes. A sulfur recovery unit (SRU) thermal reactor is the most important equipment in a sulfur plant. It converts the NH , H S and hydrocarbons 3 2 in the reactants into sulfur. Most of the sulfur elements are recovered from the SRU thermal reactor. Appl. Sci. 2016, 6, 331; doi:10.3390/app6110331 www.mdpi.com/journal/applsci Appl. Sci. 2016, 6, 331 2 of 13 The ﬁrst section of a SRU that uses the Claus process is composed of a burner, a thermal reactor and a waste heat exchanger. The conﬁguration and dimensions of the ﬁrst section of a SRU for a typical petroleum reﬁnery are shown in Figure 1. The main chemical reactions in a SRU thermal reactor include a thermal step and a catalytic step. Appl. Sci. 2016, 6, 331 2 of 12 2H S + 3O ! 2SO + 2H O 2 2 2 2 hydrocarbons in the reactants into sulfur. Most of the sulfur elements are recovered from the SRU thermal reactor. The first section of a SRU that uses the Claus process is composed of a burner, a 2H S + SO ! 3S + 2H O 2 2 2 thermal reactor and a waste heat exchanger. The configuration and dimensions of the first section of a SRU for a typical petroleum refinery are shown in Figure 1. In addition to the above reactions and the combustion of hydrocarbon fuels, other chemical The main chemical reactions in a SRU thermal reactor include a thermal step and a catalytic reactions taking place in a SRU thermal reactor are step. 2H2S + 3O2 → 2SO2 + 2H2O 2NH + 1.5O ! N + 3H O 3 2 2 2 2H2S + SO2 → 3S + 2H2O In addition to the abov 2NH e reacti+ ons SO and ! the N combustion + H S + of 2H hydro Ocarbon fuels, other chemical 3 2 2 2 2 reactions taking place in a SRU thermal reactor are The interior of a SRU thermal reactor is divided into two zones by a choke ring, in order to increase 2NH3 + 1.5O2 → N2 + 3H2O the residence time and enhance the chemical reaction. The zone ahead of the choke ring is called zone 2NH3 + SO2 → N2 + H2S + 2H2O 1 and the zone behind the choke ring is called zone 2. The inner surface of a SRU thermal reactor The interior of a SRU thermal reactor is divided into two zones by a choke ring, in order to is fabricated using a refractory to protect its walls because a SRU thermal reactor operates at very increase the residence time and enhance the chemical reaction. The zone ahead of the choke ring is high temperatures. SRU thermal reactors are negatively affected by high temperature operations called zone 1 and the zone behind the choke ring is called zone 2. The inner surface of a SRU thermal reactor is fabricated using a refractory to protect its walls because a SRU thermal reactor because high temperature can damage the refractory and the heat exchanger tubes. Therefore, operates at very high temperatures. SRU thermal reactors are negatively affected by high the operating temperature range that is suggested by the manufacturers for the operation of a SRU temperature operations because high temperature can damage the refractory and the heat thermal reactor exchanger must be tube strictly s. Thereadher fore, the ed to. operating temperature range that is suggested by the manufacturers for the operation of a SRU thermal reactor must be strictly adhered to. (a) (b) Figure 1. The conﬁguration and dimensions of the ﬁrst section of the SRU for a typical petroleum reﬁnery: (a) The overall view; (b) Enlarged view for the burner section. There have been theoretical and experimental studies of SRU thermal reactors. Adewale et al.  studied the thermal decomposition of H S into hydrogen and sulfur using a process simulator. Using 2 Appl. Sci. 2016, 6, 331 3 of 13 the net fraction of the acid gas feed to the cracking coils as the controlling parameter, its effect on hydrogen production, the thermal reactor ’s energy requirement, the stability of the burner ﬂame, steam production, the temperature of a Claus reactor and sulfur recovery of the primary SRU was studied. Chardonneaua et al.  presented experimental and simulation results for the addition of various amounts of toluene or carbon dioxide/toluene mixtures into the H S gas stream. The results show that there is a decrease in the conversion efﬁciency when the amount of toluene or carbon dioxide/toluene added to the H S gas stream increases. The role of the reactor ’s operating temperature was also studied. The addition of toluene increases the optimum reactor temperature for enhanced sulfur recovery, but the presence of CO reduces the optimum operating temperature. Selim et al.  examined the quality of sulfur deposits that were collected from H S combustion. Sulfur deposits from H S combustion under various conditions were captured and analyzed using X-ray powder diffraction and laser-induced breakdown spectroscopy diagnostics. Monnery et al.  experimentally studied the reaction between H S and SO using practical Claus thermal reactor temperatures between 850 and 2 2 1150 C and residence times between 0.05 and 1.2 s. The kinetic data obtained were used to develop a new reaction rate expression. Our experience of operating a practical SRU thermal reactor in Taiwan shows that the refractories at the zone 1 corner and the choke ring are the parts of the thermal reactor that experience the greatest deterioration. The zone 1 corner has a suddenly expanded geometry and a recirculation zone forms behind it. The temperature at the zone 1 corner can exceed the maximum service temperature of the refractory and cause collapse or deformation. The choke ring is subjected to a bending moment from the rapid combustion gas stream and can collapse or deform. Several methods to alleviate this problem have been assessed, including (1) changing the choke ring dimensions ; (2) changing the choke ring position ; (3) modifying the geometry of the zone 1 corner to a streamlined geometry [7,8]; and (4) replacing the choke ring by a vector wall . However, these methods are either expensive or require an overhaul of the SRU thermal reactor. This paper presents a new, easier and more economical method. The effect of the fuel mass fraction on the combustion and ﬂuid ﬂow in a SRU thermal reactor is investigated numerically. This paper provides a guideline for adjusting the fuel mass fraction to reduce the high temperature inside a thermal reactor and to ensure an acceptable sulfur recovery. Practical operating conditions from a petrochemical corporation in Taiwan were used as the design conditions for the discussion. 2. Numerical Methods and Physical Models In this study, the FLUENT commercial code is used to simulate the reaction and ﬂuid ﬂow in a SRU thermal reactor. The SIMPLE algorithm by Patankar  is used to solve the governing equations. The discretizations of convection terms and diffusion terms are respectively performed using the power-law scheme and the central difference scheme. In terms of physical models, considering the accuracy and stability of the models and the evaluations of other researchers, the standard k-" model , the P-1 radiation model  and the non-premixed combustion model with a -type probability density function  are respectively used for the turbulence, radiation and combustion simulations. The standard wall functions  are used to resolve the ﬂow quantities (velocity, the temperature, and the turbulence quantities) at the near-wall regions. For the steady-state three-dimensional ﬂow ﬁeld with the chemical reaction in this study, the governing equations for the continuity equation, momentum equation and turbulence equation (k-" model) are described below. The governing equations for the energy equation, radiation equation, combustion equation and convergence criterion are described in a previous study by the author . Continuity equation: r v = 0 (1) Appl. Sci. 2016, 6, 331 4 of 13 Momentum equation: In an inertial reference frame, the momentum equation  may be expressed as !! ! r ($ v v ) = rp +r + $ g + F (2) where F is the possible source term of the momentum equation which may include the body force caused by the interaction with dispersed phases or the extra force terms caused by physical models or numerical schemes. In Equation (2), the stress tensor t may be expressed as ! ! 2 ! = r v +r v r v I (3) where I is the unit tensor, the superscript T represents the transpose operator, and the third term on the right-hand side of the equal sign represents the volume dilation effect. By the long-time averaged method, Equation (2) may be converted to Reynolds-averaged Navier-Stokes (RANS) equations as follows: " !# ¶u ¶ ¶p ¶ ¶u j 2 ¶u ¶ i l $u u = + + + $u 0u 0 (4) i j ij i j ¶x ¶x ¶x ¶x ¶x 3 ¶x ¶x j i j j i l j In Equation (4), the turbulent stresses (Reynolds stresses) may be related to the mean velocity gradient by the Boussinesq hypothesis as follows: ¶u ¶u j 2 ¶u i k $u 0u 0 = + $k + (5) i j ij t t ¶x ¶x 3 ¶x j i k In Equation (5), the turbulent (or eddy) viscosity may be expressed in terms of the turbulence kinetic energy (k) and turbulence dissipation rate (") as: = C $ (6) As mentioned above, k and # are modeled by the k-" model, as described below. k-" model equation: The turbulence kinetic energy (k) and turbulence dissipation rate (") may be solved from their transport equations  as follows: " # ¶ ¶ ¶k ($u k) = + + G + G $"