NUMERICAL ANALYSIS OF THE EFFECTS OF STREAMLINING GEOMETRY AND A VECTOR WALL ON THE THERMAL AND FLUID FLOW IN A SRU THERMAL REACTOR Chun-Lang Yeh Department of Aeronautical Engineering, National Formosa University, Yunlin, Taiwan, R.O.C. E-mail: clyeh@nfu.edu.tw IMETI 2015 J5042_SCI No. 16-CSME-47, E.I.C. Accession 3933 ABSTRACT To resolve the abnormality of a SRU thermal reactor under high temperature operation and to improve the recovery of sulfur, the effects of streamlining geometry and a vector wall on the thermal and fluid flow in a SRU thermal reactor are investigated numerically. It is found that the compression effect caused by a streamlined zone 1 corner leads to an increase in the average temperature. However, the corner recirculation zone using a streamlined zone 1 corner becomes smaller and this yields a reduction in temperature. The combined effect of compression and a smaller corner recirculation zone leads to an optimal radius of curvature at the zone 1 corner. The lowest peak temperature is obtained using a radius of curvature 1m at the zone 1 corner. With larger radii of curvature at the zone 1 corner, the compression effect overwhelms the effect of a smaller corner recirculation zone and the peak temperature is higher. The specific arrangement of the vector wall holes results in a spiral motion behind the vector wall. The average temperature increases and becomes more uniform across a vector wall. The peak temperature and the exit sulfur concentration are higher using a vector wall. Finally, the skin friction coefficient increases abruptly across a vector wall but becomes lower downstream, compared with using a choke ring. The results of this paper are helpful in improving the performance and safety of a SRU thermal reactor. Keywords: SRU thermal reactor; choke ring; streamlined corner; vector wall; sulfur recovery. ANALYSE NUMÉRIQUE DES EFFETS DE LA RATIONALISATION GÉOMÉTRIQUE ET DU MUR VECTEUR SUR LE FLUX THERMIQUE DANS UN RÉACTEUR THERMIQUE SRU RÉSUMÉ Pour résoudre les anomalies dans un réacteur thermique SRU en fonctionnement à haute température et pour améliorer la récupération du soufre, les effets de la rationalisation géométrique du mur vecteur sur le flux thermique et sur l écoulement du fluide dans le réacteur thermique SRU sont investigués d un point de vue numérique. Il a été constaté que les effets combinés de la compression et d un plus petit angle de la zone de récupération conduit à un rayon de courbure optimal à l angle de la zone 1. La température de pointe la plus basse est obtenue en utilisant le rayon de la courbure 1m à l angle de la zone 1. La température moyenne augmente et devient plus uniforme pour traverser la paroi du vecteur. La température de pointe à la sortie de la concentration de soufre est plus élevée si on utilise le mur vecteur. Les résultats obtenus sont utiles dans l amélioration de la performance et de la sécurité du vecteur thermique SRU. Mots-clés : réacteur thermique SRU; anneau d étranglement; angle simplifié; récupération de soufre. Transactions of the Canadian Society for Mechanical Engineering, Vol. 40, No. 5, 2016 811
NOMENCLATURE C f skin friction coefficient C µ turbulence model constant (= 0.09) k turbulence kinetic energy (m 2 /s 2 ) L hydraulic diameter (m) l characteristic length (m) P pressure (N/m 2 ) T temperature (K) XY Z Cartesian coordinates with origin at the centroid of the burner inlet (m) Greek symbol ε turbulence dissipation rate (m 2 /s 3 ) 1. INTRODUCTION The most frequently used desulfurization process is the Claus process which converts the hydrogen sulfide in natural gas or crude oil into sulfur elements and thereby reduces the formation of oxysulfide. The sulfur recovery unit (SRU) thermal reactor is perhaps the most important equipment in a sulfur plant. It converts the ammonia, hydrogen sulfide and hydrocarbons in the reactants into sulfur. Most of the sulfur elements are recovered from the SRU thermal reactors. The SRU first section based on the Claus process is composed of a burner, a thermal reactor and a waste heat exchanger. Since the operating temperature of a SRU thermal reactor is very high and the hydrogen sulfide in the reactants is a highly acid gas, the refractory and the heat exchanging tubes may be damaged and the sulfur recovery efficiency may be negatively influenced. Owing to the extremely high temperature, the experimental measurements inside a SRU thermal reactor are difficult. Therefore, existent researches of the SRU thermal reactors are concentrated on the discussions of practical operation problems [1]. On the other hand, numerical simulations can give more detailed information inside a SRU thermal reactor. In the author s previous research [1], the effects of choke ring dimensions on the thermal and fluid flow in a SRU thermal reactor were investigated. In this paper, two other methods of alleviating the abnormality of a SRU thermal reactor under high temperature operations are investigated numerically, including using a streamlined zone 1 corner and using a vector wall. The thermal and fluid flow in a SRU thermal reactor are analyzed. The purpose of this study is to improve the performance and safety of a SRU thermal reactor. The remaining sections of this paper are organized as follows: in Section 2 the numerical methods and physical models are discussed, Section 3 contains the results and discussion and in Section 4 we present the conclusions. 2. NUMERICAL METHODS AND PHYSICAL MODELS In this study, the FLUENT commercial code [2] is employed to simulate the reacting and fluid flow in a SRU thermal reactor. The SIMPLE algorithm by Patankar [3] is used to solve the governing equations. The discretizations of convection terms and diffusion terms are carried out by the power-law scheme and the central difference scheme, respectively. In respect of physical models, considering the accuracy and stability of the models and referring to the evaluation of other researchers, the standard k ε model [4], P-1 radiation model [5] and non-premixed combustion model with β-type probability density function [6] are adopted for turbulence, radiation and combustion simulations, respectively. The standard wall functions [7] are used to resolve the flow quantities (velocity, temperature, and turbulence quantities) at the near-wall regions. Detailed governing equations and convergence criterion were described in the author s previous study [8]. 812 Transactions of the Canadian Society for Mechanical Engineering, Vol. 40, No. 5, 2016
Fig. 1. Numerical models of the SRU thermal reactors using different radii of curvature at the zone 1 corner. The configuration and dimensions of the SRU first section for a typical petroleum refinery were shown in [1]. The numerical model of the SRU thermal reactor is constructed using an unstructured grid described in [1]. Two cases of oxygen supplies are investigated: the normal oxygen supply and the rich oxygen supply [1]. The rich oxygen supply is designed to increase the sulfur recovery. Practical operating conditions from a petrochemical corporation in Taiwan were used as the design conditions for the discussion. Boundary conditions (including species compositions, temperature, pressure, velocity, turbulence kinetic energy and the turbulence dissipation rate) were described in [1]. 3. RESULTS AND DISCUSSION To validate the numerical methods used in this study, the simulation results were compared with available practical data. The exit S 2 mole fractions from the SRU thermal reactor of a petrochemical corporation in Taiwan were measured at the design conditions [1]. They are 0.078 for an oxygen-normal supply and 0.084 for an oxygen-rich supply. However, the simulation results are 0.079 and 0.091, respectively. The respective deviations are 1.3% for an oxygen-normal supply and 8.3% for an oxygen-rich supply, which are acceptable from a viewpoint of engineering applications. 3.1. The Effects of Streamlining Geometry Five different radii of curvature at the zone 1 corner, 0 (i.e. without streamlining), 1, 2, 3 and 4 m, are calculated to investigate the geometric effects of the zone 1 corner. Figure 1 shows the numerical models of the SRU thermal reactors using different radii of curvature at the zone 1 corner. Figure 2 shows the comparison of cross-sectional average temperatures for the SRU thermal reactors using different radii of curvature at the zone 1 corner. Figures 3 and 4 show the temperature profiles on the symmetric planes of the SRU thermal reactors. It is observed that zone 1 is a higher temperature region while zone 2 is a lower temperature region. The average temperature for a rich oxygen supply is higher than that of a normal oxygen supply. Temperature decreases abruptly across the choke ring because of the flow acceleration and the conversion of thermal energy into kinetic energy across the choke ring. With a Transactions of the Canadian Society for Mechanical Engineering, Vol. 40, No. 5, 2016 813
Fig. 2. Comparison of the cross-sectional average temperatures for the SRU thermal reactors using different radii of curvature at the zone 1 corner. Fig. 3. Temperature profiles for the SRU thermal reactors using different radii of curvature at the zone 1 corner (normal oxygen supply). streamlined zone 1 corner, the average temperature may be increased due to the compression effect caused by the decreased volume in zone 1. However, the corner recirculation zone in zone 1 becomes smaller due to the streamlining effect, which may yield a reduction in temperature. These two effects (compression effect and a smaller corner recirculation zone) lead to an optimal radius of curvature at the zone 1 corner. 814 Transactions of the Canadian Society for Mechanical Engineering, Vol. 40, No. 5, 2016
Fig. 4. Temperature profiles for the SRU thermal reactors using different radii of curvature at the zone 1 corner (rich oxygen supply). Table 1. Peak temperatures in the SRU thermal reactors and sulfur concentrations at the exits. (a) normal oxygen supply Peak temperature (K) Sulfur concentration at exit (mole fraction) without streamlining 1902 0.0791 using radius of curvature 1 m 1890 0.0793 using radius of curvature 2 m 1942 0.0798 using radius of curvature 3 m 1926 0.0803 using radius of curvature 4 m 1935 0.0797 without streamlining using radius of curvature 1 m using radius of curvature 2 m using radius of curvature 3 m using radius of curvature 4 m (b) rich oxygen supply Peak temperature (K) Sulfur concentration at exit (mole fraction) 2103 0.0902 2071 0.0915 2111 0.0915 2110 0.0912 2117 0.0907 In a practical SRU thermal reactor, the refractory might be ruptured due to an excessively high temperature, e.g., near the zone 1 corner. Table 1 shows the peak temperatures in the SRU thermal reactors and the sulfur concentrations at the exits. It can be seen that the lowest peak temperature is obtained using a radius of curvature 1 m at the zone 1 corner. On the other hand, if larger radii of curvature at the zone 1 corner Transactions of the Canadian Society for Mechanical Engineering, Vol. 40, No. 5, 2016 815
Fig. 5. Numerical models of the SRU thermal reactors using a choke ring or a vector wall. Table 2. Peak temperatures in the SRU thermal reactors and sulfur concentrations at the exits. (a) normal oxygen supply Peak temperature (K) Sulfur concentration at exit (mole fraction) using a choke ring 1902 0.0791 using a vector wall 1913 0.0796 (b) rich oxygen supply Peak temperature (K) Sulfur concentration at exit (mole fraction) using a choke ring 2103 0.0902 using a vector wall 2120 0.0925 are used, the compression effect overwhelms the effect of a smaller corner recirculation zone and the peak temperature is higher. From Table 1 it is also observed that the sulfur concentration at the exit for a radius of curvature 1m at the zone 1 corner is not the lowest. The sulfur concentration depends not only on the peak temperature but also on the average temperature which is not the lowest for a radius of curvature 1m at the zone 1 corner. 3.2. The Effects of a Vector Wall Figure 5 shows the numerical models of the SRU thermal reactors using a choke ring or a vector wall. The choke ring (or vector wall) is located at 6m away from the zone 1 corner. Figure 6 shows the comparison of cross-sectional average temperatures for the SRU thermal reactors using a choke ring or a vector wall. Figures 7 and 8 show the temperature profiles on the symmetric planes of the SRU thermal reactors. It is observed that the specific arrangement of the vector wall holes results in a spiral motion behind the vector wall and thereby increases the residence time. The average temperature in zone 2 is therefore increased due to the better mixing. The spiral motion caused by a vector wall can be 816 Transactions of the Canadian Society for Mechanical Engineering, Vol. 40, No. 5, 2016
Fig. 6. Comparison of cross-sectional average temperatures for the SRU thermal reactors using a choke ring or a vector wall. Fig. 7. Temperature profiles for the SRU thermal reactors using a choke ring or a vector wall (normal oxygen supply). Fig. 8. Temperature profiles for the SRU thermal reactors using a choke ring or a vector wall (rich oxygen supply). seen from Fig. 9. The larger solid surface area of a vector wall results in a larger blockage effect and hence the average temperature in zone 1 is also increased. It can be observed that the average temperature across a vector wall becomes more uniform due to the better mixing. Table 2 shows the peak temperatures in the SRU thermal reactors and the sulfur concentrations at the exits. It is seen that the peak temperature using a vector wall is higher than that using a choke ring. With a vector wall, the peak temperatures are increased by 0.6% for the normal oxygen operation and 0.8% for the rich oxygen operation, compared with those using a choke ring. In addition, the sulfur concentration at the exit is higher using a vector wall. The exit sulfur concentrations are increased by 0.6 and 2.5%, respectively, for the normal oxygen operation and the rich oxygen operation. Figure 10 shows the comparison of cross-sectional Transactions of the Canadian Society for Mechanical Engineering, Vol. 40, No. 5, 2016 817
Fig. 9. Stream traces for the SRU thermal reactors using a choke ring or a vector wall. average skin friction coefficients for the SRU thermal reactors using a choke ring or a vector wall. Because of the larger solid surface area, the skin friction coefficient increases abruptly across a vector wall. However, because the thermal and flow field is more uniform behind a vector wall, the skin friction coefficient becomes lower downstream a vector wall, compared with using a choke ring. 4. CONCLUSIONS In this paper, the effects of streamlining geometry and a vector wall on the thermal and fluid flow in a SRU thermal reactor are investigated numerically. It is found that the compression effect caused by a streamlined 818 Transactions of the Canadian Society for Mechanical Engineering, Vol. 40, No. 5, 2016
Fig. 10. Comparison of cross-sectional average skin friction coefficients for the SRU thermal reactors using a choke ring or a vector wall. zone 1 corner leads to an increase in the average temperature. However, the corner recirculation zone using a streamlined zone 1 corner becomes smaller and this yields a reduction in temperature. The combined effect of compression and a smaller corner recirculation zone leads to an optimal radius of curvature at the zone 1 corner. The lowest peak temperature is obtained using a radius of curvature 1 m at the zone 1 corner. With larger radii of curvature at the zone 1 corner, the compression effect overwhelms the effect of a smaller corner recirculation zone and the peak temperature is higher. The specific arrangement of the vector wall holes results in a spiral motion behind the vector wall. The average temperature increases and becomes more uniform across a vector wall. The exit sulfur concentration and the peak temperature are higher using a vector wall. Finally, the skin friction coefficient increases abruptly across a vector wall but becomes lower downstream, compared with using a choke ring. ACKNOWLEDGEMENTS The author gratefully acknowledges the grant support from the Ministry of Science and Technology, Taiwan, R.O.C., under contract MOST 104-2221-E-150-032. The author would also like to express his gratitude for the useful data and constructive suggestions provided by the Formosa Petrochemical Corporation in Taiwan. REFERENCES 1. Yeh, C.L., Effects of choke ring dimension on thermal and fluid flow in a SRU thermal reactor, Transactions of the Canadian Society for Mechanical Engineering, Vol. 40, No. 4, pp. 511 520, 2016. 2. ANSYS FLUENT 12 Theory Guide, ANSYS, Inc., April 2009. 3. Patankar, S.V., Numerical Heat Transfer and Fluid Flows, McGraw-Hill, New York, 1980. 4. Launder, B.E. and Spalding, D.B., Lectures in Mathematical Models of Turbulence, Academic Press, London, UK, 1972. 5. Siegel, R. and Howell, J.R., Thermal Radiation Heat Transfer, Hemisphere Publishing Corporation, Washington DC, 1992. Transactions of the Canadian Society for Mechanical Engineering, Vol. 40, No. 5, 2016 819
6. Sivathanu, Y.R. and Faeth, G.M., Generalized state relationships for scalar properties in non-premixed hydrocarbon/air flames, Combustion and Flame, Vol. 82, No. 2, pp. 211 230, 1990. 7. Launder, B.E. and Spalding, D.B., The numerical computation of turbulent flows, Computer Methods in Applied Mechanics and Engineering, Vol. 3, No. 2, pp. 269 289, 1974. 8. Yeh, C.L., Numerical analysis of the combustion and fluid flow in a carbon monoxide boiler, International Journal of Heat and Mass Transfer, Vol. 59, pp. 172 190, 2013. 820 Transactions of the Canadian Society for Mechanical Engineering, Vol. 40, No. 5, 2016