Performance of Reactive Distillation Columns with Multiple Reactive Sections for the Disproportionation of Trichlorosilane to Silane

Transcription

1 6th International Symposium on Advanced Control of Industrial Processes (AdCONIP) May 8-31, 017. Taipei, Taiwan Performance of Reactive Distillation Columns with Multiple Reactive Sections for the Disproportionation of Trichlorosilane to Silane Xinxiang Zang, Yang Yuan, Haisheng Chen, Liang Zhang, Shaofeng Wang, and Kejin Huang Abstract With reference to the disproportionation of trichlorosilane to silane, a three-stage consecutive reversible reaction with a rather unfavorable reaction kinetics, in-depth comparison in steady-state performance is performed between the column with a single reactive section and those with multiple reactive sections, adopting the equal number of trays and the equal amount of catalyst. The reactive distillation column with multiple reactive sections appears to be considerably superior in the aspect of economic advantages to the reactive distillation columns with single reactive section and these strengths originate essentially from the additional degrees of freedom resulted from the arrangement of multiple reactive sections in process synthesis and design. Arrangement of side-condensers is also examined towards the columns with multiple reactive sections and the outcomes reveal the thermodynamic rational to adopt multiple reactive sections in process development. Although these findings are derived from the specific case study chosen, it should be regarded as the significant potential for analyzing the reactive distillation columns with intricate multiple reversible reactions and separating intricate multiple components. I. INTRODUCTION As shown in Figure 1a, reactive distillation columns with a single reactive section (RDC-SRS) are generally characterized by a common structure with a rectifying section, a single reactive section and a stripping section. 1, With regard to separating intricate mixtures of reactants and products under the circumstance of favorable thermodynamics and reaction kinetics (e.g., A + B C + D with C > A > B > D), such a process intensification facilitates generally the simultaneous occurrence of the reaction operations and separation operations involved and can bring about significant benefits in the respect of economic efficiency and convenient operation as compared with its conventional counterparts, e.g., a continuous stirred tank reactor connected to numerous distillation columns. 3 However, for the unfavorable thermodynamics and reaction kinetics, this configuration reveals a serious drawback and loses its competition, i.e., the low flexibility to cope with it. 6-8 For the purpose of extending its applications careful modifications must be made at process analysis and simulation. One of the potential strategies is to adopt multiple reactive sections in process development and this gives rise to a novel configuration of reactive distillation *Research funded by National Natural Science Foundation of China. Kejin Huang is with the College of Information Science and Technology, Beijing University of Chemical Technology, Beijing 10009, People s Republic of China (corresponding author to provide Phone: Fax: huangkj@mail.buct.edu.cn). columns with multiple reactive sections, as described in Figure 1b. B A FB, 1 FA, 1 FB, FA, FB, i FA, i FB, n FA, n NS, 1 C NR, 1 A + B C + D NS, NS, 1 NR, 1 NS, NR, NR, i NR, n NS, n+1 C, D C > A > B > D A + B D C + D A > C > D > B Figure 1. Schemes of the Single reactive section and Multiple reactive sections: Single reactive section, Multiple reactive sections. Under the circumstance of separating the unfavorable quaternary reacting components (i.e., A + B C + D with A > C > D > B), Tung and Yu firstly advocated employing double reactive sections at both ends of the column (containing condenser and reboiler), respectively, to put the unfavorable relative volatilities into account and this lead to the derivation of columns which have double reactive sections (RDC-DRS). 9 Huang et al. demonstrated further the advantages of the RDC-DRS over the RDC-SRS in separating these kinds of reacting mixtures and pointed out that the locations of the two reactive sections as well as feed splitting could play pivot roles in strengthening internal mass /17/$ IEEE 63

2 Q COND = 41.6 kw Q COND = kw F TCS = 10 kmol/h RR = 0.9 N R, 1 = 31 Silane: 99 %.487 kmol/hr FTCS = 10 kmol/h RR = 44.8 N R, 1 = 1 N R, = N R, 3 = Silane: 99.3 %.487 kmol/hr 9 Q REB = 43.8 kw 46 9 Q REB = kw T: 393. K STC: 99 % 7.13 kmol/hr T: 393. K STC: 99.1 % 7.13 kmol/hr Figure. Optimum process designs of the RDC-SRS for the disproportionation of trichlorosilane to silane. and energy interaction among the relevant reaction operations and the separation operations. 10, 11 Chen et al. recently found that an external circulation could also serve to strengthen the capability of the RDC-DRS due to its favorable impact to internal mass and energy interaction among the relevant operations in the whole reaction system. 1, 13 Note that the above researches focused exclusively on the analysis and simulation of the RDC-DRS, i.e., the simplest structure of the RDC-MRS. Under the circumstance of separating mixtures of reactants and products with much more unfavorable thermodynamics and much more complicated reaction kinetics, the RDC-MRS should certainly be reinforced with more than two reactive sections at process analysis and simulation. For such a procedure designs, they are quite likely to be more thermodynamically efficient than the RDC-SRS and worth a detailed evaluation. Unfortunately, so far no systematic studies have been conducted and reported, yet. F TCS = 10 kmol/h Q COND = kw RR = 46.1 N R, 1 = 7 N R, = 4 Q REB = kw Silane: 99 %.487 kmol/hr T: 393. K STC: 99 % 7.13 kmol/hr Figure 3. Optimum process designs of the RDC-DRS for the disproportionation of trichlorosilane to silane. Figure 4. Optimum process designs of the RDC-TRS for the disproportionation of trichlorosilane to silane. In this paper, a case study on the disproportionation of trichlorosilane to silane is chosen to assess the steady-state superiority of the reactive distillation columns with multiple reactive sections. The reaction system exhibits a fairly unfavorable reaction kinetics involving a three-stage consecutive reversible reaction with a near zero thermodynamic conversion, thereby representing an extremely challenging example for the applications of reactive distillation columns. By means of detailed comparison between the RDC-DRS and RDC-MRS, the favorable effect resulting from arranging multiple reactive sections in the RDC-MRS is clearly demonstrated. Arrangements of side-condensers are also conducted to reveal the thermodynamic reasonability of adopting multiple reactive sections in process synthesis and design. Some concluding remarks are finally given in the last part of the paper. II. A CASE STUDY ON THE DISPROPORTIONATION OF TRICHLOROSILANE TO SILANE.1. Problem description With resin Amberlyst A-1 as the heterogeneous catalyst, the disproportionation of trichlorosilane to silane proceeds through three consecutive reversible reactions with the involvement of five components, including silicon tetrachloride (STC), trichlorosilane (TCS), dichlorosilane (DCS), monochlorosilane (MCS), and silane (SIL). The detailed reaction kinetics can be described as below. 1 TCS STC + DCS ΔHr = kj/kmol (1) DCS TCS + MCS ΔHr = + 6 kj/kmol () MCS DCS + Silane ΔHr = 48 kj/kmol (3) The reaction rates for the first-, second-, and third-stage reactions are expressed with the following equations. r 1=k 1(x TCS x STCx DCS/K 1) (4) r =k (x DCS x TCSx MCS/K ) () 64

3 r 3=k 3(x MCS x DCSx SIL/K 3) (6) where x STC, x TCS, x DCS, x MCS, and x SIL represent the mole fractions of STC, TCS, DCS, MCS, and silane, respectively; r i is the reaction rate for each stage reaction involved. Note that the five reacting components exhibit an ascending order of boiling points as shown blew. silane (161K) < MCS (43.1K) < DCS (81.4K) < TCS (30K) < STC (330K) (7) Steady-state simulation is conducted in the environment of Aspen and the module of RacFrac is adopted to simulate the features of the RDC-SRS and RDC-MRS. 17 The Peng-Robinson equation of state is chosen to calculate the required thermodynamic properties with the binary interaction coefficients taken from reference 16 and the top stage pressure of three configurations are fixed at atm at the same time the stage pressure drop is set to 0. kpa. It seems then reasonable to carry out the three-stage consecutive reversible reactions by means of a reactive distillation column and extract the lightest component silane and the heaviest component STC as top and bottom products, respectively, while keeping the rest components within its reactive section. Owing to the fairly unfavorable reaction kinetics and the strong interactive nature of the three consecutive reversible reactions, the RDC-MRS is considered to be potentially more attractive than the RDC-SRS and this remains to be the primary purpose of the current study... Analysis and simulation of the RDC-SRS and RDC-MRS. Considering that the disproportionation of trichlorosilane to silane is a three-stage consecutive reversible reaction, it is then to be reasonable and sufficient to assume here that the RDC-MRS accommodates up to three reactive sections, namely, including the options for the RDC-DRS and the RDC-TRS. For setting up a basic premise for the comparison of the three different configurations, we specify here the total number of stages and the total number of reactive stages as 60 (containing condenser and reboiler) and 31 (each reactive stage is assumed to contain 60. mole of resin Amberlyst A-1), respectively, for the three process designs to be examined. Under these assumptions, the condenser heat duty can simply be adopted as an effective measure for the contrast of these steady-state capability because of low boiling point of silane (i.e., only 161 K) and the required coolant in condenser is much more expensive than the heating medium in reboiler), thereby avoiding the complicated estimations of economic benefit in case that the consumption over a period is chosen as evaluation criterion. A simple grid-search method is adopted to synthesize and design the RDC-DRS and RDC-TRS. For the RDC-SRS, as shown in Figure, it has the characteristics of traditional reactive distillation column with single reactive distillation and the feed location of TCS (Throughout the current work, the reflux flow rate and condenser heat duty are employed to keep the top and bottom products on their specifications and are not addressed here). The result of corresponding optimization of columns with single reactive section is described in Figure, with the heat duties of refrigeration and re-boiling as 41.6 kw and Table 1. Comparison among the three different configurations Process design RDC-SRS RDC-DRS RDC-TRS QCOND (kw) Comparison 100 % 91.3 % % QREB (kw) Comparison 100 % 9.04 % % 43.8 kw, respectively. For the RDC-DRS, it involves the rectifying section, intermediate separating section, stripping section and two reactive sections and the structural decision variables include the number of stages in the separating sections and two reactive sections and the location of feed. The optimal result of the columns with double reactive sections is sketched in Figure 3, with the duties of refrigeration and re-boiling shrunk to be kw and kw, respectively. For the RDC-TRS, it possesses four separating sections (i.e., the rectifying section, two intermediate separating sections, and stripping section) and three reactive sections and the structural decision variables include the number of stages in the four separating sections, the number of reactive stages in the three reactive sections, and the feed location of TCS. The optimal result of the columns with three reactive sections is described in Figure 4, with the duties of refrigeration and re-boiling further dropped to be kw and kw, respectively..3. RDC-SRS versus RDC-MRS. Table 1 details the result analysis of steady-state capability among the three different configurations. Through adopting two sections, the RDC-DRS diminishes the duties of refrigeration and re-boiling by 8.77 % and 7.96 %, respectively, as compared with the RDC-SRS. Likewise, with the adoption of three reactive sections, the RDC-TRS depresses the heat duties of cooling and re-boiling by % and 9.9 %, respectively, as compared with the RDC-SRS. The dramatic declines in operating cost highlight the favorable effect by the arrangement of multiple reactive sections in the RDC-DRS and RDC-TRS. It is emphasized that the substantial promotion of system capability also implies the great economic benefit by means of arranging multiple reactive sections in process analysis and simulation..4. Roles of Multiple Reactive Sections in the RDC-MRS. In Figure, the liquid compositions distributions of the steady state are described for the three different configurations. Note that the separation between STC and TCS is mainly conducted near the bottom end and there exists almost no differences between the three process designs examined there. As for the top end, sharp differences can, however, be readily observed. In the case of the RDC-SRS (c.f., Figure a), the steady-state profile of MCS exhibits a fairly high and wide plateau with a 6

4 Mole fraction Mole fraction Mole fraction S 1 R 1 S STC TCS DCS MCS SILANE S 1 R 1 S R S 3 STC TCS DCS MCS SILANE S 1 R 1 S R S 3 R 3 S 4 arrangement of three reactive sections, the peaks of the steady-state profiles of MCS and DCS are slightly enhanced as compared with those of the RDC-DRS and this adjustment helps to generate more silane and MCS at the top ends of the intermediate and upper reactive sections, thereby presenting a favorable impact to the purification of silane in the rectifying section. For the three reactions involved in the whole reaction system, with the increase of the separating sections providing the benefits of separations of products in time, certainly the complicated couplings among the different reactions progressively decreased and the net reaction rates certainly increase from the RDC-SRS to the RDC-TRS. This changing tendency highlights the pivotal role of adopting multiple reactive sections in coordinating the three consecutive reversible reactions involved in the disproportionation of trichlorosilane to silane STC TCS DCS MCS SILANE (c) Figure. Profiles of liquid compositions: Single reactive section, Double reactive sections, (c) Three reactive sections. value nearly to one from stage 3 to stage 11 (which forms essentially a pinch zone there) and this may incur great difficulties in the purification of silane because their relative volatilities are adjacent to each other within the five components involved (The stages are counted from the top down to the bottom, with the condenser as stage 1 and the reboiler as stage 60). The drawback is apparently related to the adoption of a single reactive section in process analysis and simulation that makes reactive distillation columns containing single reactive reaction unable to effectively coordinate the three consecutive reversible reactions involved. Huang et al. also reported an extremely similar phenomenon for the RDC-SRS and attempted to rely on the applications of side-condensers to counteract its negative effect, however, only slight improvement was gained. 16 As for the RDC-DRS (c.f., Figure b), due to the arrangement of two reactive sections, the MCS composition has been considerably suppressed in that area as compared with that of the RDC-SRS and this results in the ascending of the peak of the DCS composition. The difficulty in the purification of silane is thus greatly alleviated because the separation now occurs mainly between silane, MCS, and DCS in this situation. As far as the RDC-TRS is concerned (c.f., Figure c), owing to the F TCS = 10 kmol/h Q COND = 190. kw 4 Q REB = kw (c) Silane: 99 %.487 kmol/hr 344 Q COND-1 = 100 kw Q COND- = 100 kw T: K STC: 99 % 7.13 kmol/hr Figure 6. Arrangement of two side-condensers to the RDC-DRS: impact of the top side-condenser, impact of the bottom side-condenser, (c) RDC-DRS with two side-condensers. 66

5 FTCS = 10 kmol/h (c) Q COND = 84. kw Q REB = 4.7 kw (d) T: 19.0 K Silane: 99.3 %.487 kmol/hr Q COND-1 = 100 kw Q COND- = 100 kw Q COND-3 = 100 kw T: K STC: 99.1 % 7.13 kmol/hr Figure 7. Arrangement of three side-condensers to the RDC-TRS: impact of the top side-condenser, impact of the intermediate side-condenser, (c) impact of the bottom side-condenser, (d) RDC-TRS with three side-condensers. III. DISCUSSION Apart from the near zero thermodynamic conversion, the disproportionation of trichlorosilane to silane is also characterized by a rather complicated reaction kinetics in the three-stage consecutive reversible reaction processed. This feature implies strong coupling between the three consecutive reversible reactions involved and a careful coordination must be made between them in order to seek the potentials of conventional columns. It is worth emphasizing it also represents a key issue that is closely related to the strengthening in mass and energy integration in the whole system. 10, 11, 13, 14 It is, therefore, of great importance to effectively tackle this issue during process analysis and simulation. Through arranging two reactive sections, the RDC-DRS receives two degrees of freedom more than the RDC-SRS in coordinating the three consecutive reversible reactions involved and this gives rise consequently to great reductions in heating and refrigeration of the column. By the arranging three reactive sections, the RDC-TRS gains further two degrees of freedom more than the RDC-DRS and the additional flexibility permits further reductions in the duties of refrigeration and re-boiling. The varying tendency indicates clearly the favorable effect by the incremental adoption of reactive sections in the process analysis and simulation and confirms definitely the number of reactive sections can work as important and effective decision variables to enhance their steady-state performance. Although the interpretation has been derived from the current case discussed, it is greatly significant to the synthesis and design of any other reactive distillation columns because all of our research results obtained so far 10, 11, 13, have been in excellent accordance with this deduction. 14 The process of this disproportionation by the columns features a common drawback because the normal boiling point of target product is quite low (i.e., only 161 K) and this process is strict demand for refrigeration and this makes the application of side-condensers a frequently adopted strategy to enhance the efficiency of thermodynamics under these circumstances. 16 For the RDC-MRS, the unique cascade structure of separating section and reactive section may even simplify the arrangement of side-condensers to its rectifying section (which behaves essentially as a heat source in the light of the second law of thermodynamics). Because the side-condensers usually locate in the separating sections, the search of their locations becomes relatively easier in the RDC-MRS than in the RDC-SRS. Figures 6, 7 show the relationship among the top condenser heat duty, the locations and the operation temperature of the side-condensers in case of their heat duties being assumed uniformly to be 100 kw and the higher temperature is, the cheaper the coolant of the side-condenser becomes. Finally, the optimal arrangement of side-condensers is shown in the same Figures. Note again the fact that the three side-condensers are connected, respectively, to three specific stages of the top three separating sections. These interesting results help also to reveal the thermodynamic rational of adopting multiple reactive sections in the analysis and simulation involving multiple reversible reactions. 67

6 IV. CONCLUSION Adopting multiple reactive sections allows the reactive distillation columns with multiple reactive sections to have more degrees of freedom at process analysis and simulation than the reactive distillation columns with single reactive section. Compared with the reactive distillation columns with single reactive section, these additional degrees of freedom can be employed to strengthen the internal mass and energy integration among the relevant operations in the whole reaction system, leading frequently to great enhancement at the system capability. With reference to the disproportionation of trichlorosilane to silane, the potential advantages of the RDC-MRS has been clearly demonstrated through the in-depth comparison of steady-state performance between the RDC-SRS, RDC-DRS, and RDC-TRS. With arranging incremental reactive sections, the duties of refrigeration and re-boiling display a downward trend gradually. Arrangements of side-condensers have also been attempted to the RDC-MRS and they all have been found to locate in the separating sections above the feed. Since these separating sections are essentially heat sources, the outcomes have shown the thermodynamic rational of adopting multiple reactive section in process synthesis and design. Although these findings have been derived from the specific case study chosen, it should be viewed to be of general significance to separate intricate mixtures of reactants and products involving multiple reversible reactions. ACKNOWLEDGMENT The current research is funded by The National Natural Science Foundation of China under Grants of , , , and and The Fundamental Research Funds for the Central Universities under the Grant of ZY103. NOTATION DCS = dichlorosilane Hr =reaction heat, kj/kmol MCS = monochlorosilane k = rate constant of the forward reactions K = chemical equilibrium constant N F = feed stage N R, I = number of stages in the ith reactive section N S, I = number of stages in the ith separating section RR = reflux ratio S = separating section STC = silicon tetrachloride T = temperature, K TCS = trichlorosilane x = liquid composition = relative volatility Subscripts COND = condenser REB = reboiler [] W. L. Luyben and C. C. Yu, Reactive Distillation Design and Control, New Jersey: John Wiley & Sons, Inc., 008. [3] M. F. Malone and M. F. Doherty, Reactive Distillation, Ind. Eng. Chem. Res, vol. 39, pp , Nov [4] D. B. Kaymak and W. L. Luyben, Quantitative Comparison of Reactive Distillation with Conventional Multiunit Reactor/Column/Recycle Systems for Different Chemical Equilibrium Constants, Ind. Eng. Chem. Res, vol. 43, , Apr [] K. Huang, M. Nakaiwa, S. J. Wang, and A. Tsutsumi, Reactive Distillation Design with Considerations of Heats of Reaction, AIChE J, vol., pp 18 34, Apr [6] D. B. Kaymak and W. L. Luyben, Effect of Relative Volatility on the Quantitative Comparison of Reactive Distillation and Conventional Multi-unit Systems, Ind. Eng. Chem. Res, vol. 43, pp , May [7] C. S. Chen and C. C. Yu, Effects of Relative Volatility Ranking on Design and Control of Reactive Distillation Systems with Ternary Decomposition Reactions, Ind. Eng. Chem. Res, vol. 47, pp , Jun [8] S. Thotla and S. Mahajani, Reactive Distillation with Side Draw, Chem. Eng. Proc, vol. 48, pp , Apr [9] S. T. Tung and C.C. Yu, Effects of Relative Volatility Ranking to the Design of Reactive Distillation, AIChE J, vol. 3, pp , May [10] L. Zhang, H. Chen, Y. Yuan, J. Yu, S. Wang, and K. Huang, Synthesis and Design of Reactive Distillation Columns with Two Reactive Sections, Chem. Eng. Res, vol. 100, pp , Aug. 01. [11] L. Zhang, H. Chen, Y. Yuan, S. Wang, and K. Huang, Adopting Feed Splitting in Design of Reactive Distillation Columns with Two Reactive Sections, Chem. Eng. Proc, vol. 89, pp. 9 18, Mar. 01. [1] H. Chen, K. Huang, W. Liu, L. Zhang, S. Wang, and S. J. Wang, Enhancing Mass and Energy Integration by External Recycle in Reactive Distillation Columns, AIChE J, vol. 9, pp , Jun [13] H. Chen, L. Zhang, K. Huang, Y. Yuan, X. Zong, S. Wang, and L. Liu, Reactive Distillation Columns with Two Reactive Sections: Feed Splitting plus External Recycle, Chem. Eng. Proc, vol. 108, pp , Oct [14] C. Yu, X. Yao, K. Huang, L. Zhang, S. Wang, and H. Chen, A Reactive Distillation Column with Double Reactive Sections for the Separations of Two-Stage Consecutive Reversible Reactions, Chem. Eng. Proc, vol. 79, pp. 6 68, May [1] Union Carbide. Feasibility of the Silane Process for Producing Semiconductor Grade Silicon. Final Report (Phases I and II), JPL. Contract 94334; U.S. DOE, [16] X. Huang, W. Ding, J. Yan, and W. Xiao, Reactive Distillation Column for Disproportionation of Trichlorosilane to Silane: Reducing Refrigeration Load with Intermediate Condensers, Ind. Eng. Chem. Res, vol., pp , Apr [17] K. Wang, Synthesis and Design of Reactive Distillation Columns with Multiple Reactive Sections, M. S. thesis, Dept. Electron. Eng., Beijing Univ. of Chem Tech., Beijing, China, 01. REFERENCES [1] R. Taylor and R. Krishna, Modeling Reactive Distillation, Chem. Eng. Sci, vol., pp , Nov