Reactive Distillation Columns with Multiple Reactive Sections: A Case

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Reactive Distillation Columns with Multiple Reactive Sections: A Case Study on the Disproportionation of Trichlorosilane to Silane Xinxiang Zang, Kejin Huang, Yang Yuan, Haisheng Chen, Liang Zhang, Shaofeng Wang, and Kun Wang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03715 • Publication Date (Web): 03 Jan 2017 Downloaded from http://pubs.acs.org on January 4, 2017

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Industrial & Engineering Chemistry Research

Research paper submitted to Industrial & Engineering Chemistry Research

Reactive Distillation Columns with Multiple Reactive Sections: A Case Study on the Disproportionation of Trichlorosilane to Silane

Xinxiang Zang, Kejin Huang,∗ Yang Yuan, Haisheng Chen, Liang Zhang, Shaofeng Wang, and Kun Wang College of Information Science and Technology, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China

1

ACS Paragon Plus Environment


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ABSTRACT With reference to the disproportionation of trichlorosilane to silane, a three-stage consecutive reversible reaction with rather unfavorable reaction kinetics of a near zero thermodynamic conversion, in-depth comparison in steady-state performance is performed between the reactive distillation column with a single reactive section (RDC-SRS) and those with multiple reactive sections (RDC-MRS), under the assumptions of the same total number of stages and the same total amount of catalyst employed. With the incremental arrangement of reactive sections, the RDC-MRS shows a steady improvement in steady-state performance with considerably reduced operating cost and capital investment as compared with the RDC-SRS. The great advantages originate essentially from the additional degrees of freedom resulted from the arrangement of multiple reactive sections in process synthesis and design. Apart from the coordination effect to the three-stage consecutive reversible reactions processed, the additional degrees of freedom serve also to reinforce internal mass integration and internal energy interaction between the reaction operations and the separation operations involved. Arrangement of side-condensers is also examined towards the RDC-MRS 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 considered to be of general significance for the synthesis and design of reactive distillation columns separating complicated reacting mixtures involving multiple reversible reactions.

Keywords:

Reactive distillation column, Multiple reactive sections, Internal mass integration, Internal energy integration, Process synthesis, Process design

2


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1.


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 single reactive section in between their rectifying section and stripping section.

1, 2

For the

separations of reacting mixtures with favorable thermodynamics and reaction kinetics (e.g., A + B ↔ C + D with αC > αA > αB > αD), such kind of process intensification facilitates generally the simultaneous occurrence of the reaction operations and separation operations involved and can bring about significant benefits in the aspect of capital investment and operating cost as compared with its conventional counterparts, e.g., a continuous stirred tank reactor followed by a number of distillation columns. 3−5 For the separations of reacting mixtures with unfavorable thermodynamics and reaction kinetics (e.g., A + B ↔ C + D with αA > αC > αD > αB), such kind of process intensification may, however, not be well suitable for the simultaneous occurrence of the reaction operations and separation operations involved and is likely to lose its competition with its conventional counterparts in the aspect of capital investment and operating cost. 6−8 The realities, in fact, reveal a serious drawback of the common configuration of the RDC-SRS, i.e., the low flexibility to cope with the unfavorable thermodynamics and reaction kinetics, and for the purpose of extending its applications, careful modifications must be made during process synthesis and design. 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 columns with multiple reactive sections (RDC-MRS), as shown in Figure 1b. The arrangement of multiple reactive sections renders apparently more flexibilities than in the RDC-SRS for not only counteracting the negative effects of the unfavorable thermodynamics and reaction kinetics but also reinforcing internal mass integration and/or internal energy interaction between the reaction operations and the separation operations involved. In the case of separating the unfavorable quaternary reacting mixtures (i.e., A + B ↔ C + D with αA > αC > αD > αB), Tung and Yu firstly advocated employing two reactive sections at the top and the bottom ends (including condenser and reboiler as well), respectively, to take into account the unfavorable relative volatilities and this lead to the derivation of the reactive distillation columns with double reactive sections (RDC-DRS). 9 Zhang et al. demonstrated further the advantages of the RDC-DRS over the RDC-SRS in the separations of 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 integration and/or internal energy interaction between the reaction operations and the separation operations involved.

∗

10, 11

Chen et al. recently found that an

Phone: +86 10 64437805. Fax: +86 10 64437805. E-mail: [email protected]. 3



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external recycle between the top and bottom ends could also serve to enhance the performance of the RDC-DRS due to its favorable impact to internal mass integration and/or internal energy interaction between the reaction operations and the separation operations involved. 12, 13 In the case of separating two-stage consecutive reversible reactions, such as A + B ↔ C + D, C + B ↔ E + D with αD > αB > αC > αA > αE and Dimethyl Carbonate + Ethanol ↔ Ethyl Methyl Carbonate + Methanol, Ethyl Methyl Carbonate + Ethanol ↔ Diethyl Carbonate + Methanol, Yu et al. showed that the two reactive sections could also be arranged in the middle of the RDC-DRS and worked effectively to coordinate the interactions between the two consecutive reversible reactions involved.

14

Mueller and Kenig once indicated the possibility of employing dividing-wall distillation

columns to construct the RDC-DRS. 15 In order to achieve the reactive separations of cyclohexene/cyclohexane binary mixtures with water as reactive entrainer, Yu devised a novel and effective RDC-DRS with the aid of a dividing wall, in which the two reactive sections were located at the both sides of the wall.

16

Note that the above researches focused exclusively on the

synthesis and design of the RDC-DRS, i.e., the simplest structure of the RDC-MRS. In the case of separating reacting mixtures with much more unfavorable thermodynamics and much more complicated reaction kinetics (e.g., the three-stage consecutive reversible reaction to be studied in the current work), the RDC-MRS should certainly be reinforced with more than two reactive sections during process synthesis and design. For such kind of process designs, they are quite likely to be more thermodynamically efficient than the RDC-SRS and worth a detailed evaluation, especially regarding the relationship between the number of reactive sections and steady-state performance of the RDC-MRS. Unfortunately, so far no systematic studies have been conducted and reported, yet. In this paper, a case study on the disproportionation of trichlorosilane to silane is chosen to evaluate the steady-state performance of the RDC-MRS. The reaction system features 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. In terms of in-depth comparison with the RDC-SRS, the favorable effect of 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 summarized in the last section of this article.

2.

A CASE STUDY ON THE DISPROPORTIONATION OF TRICHLOROSILANE TO SILANE 4


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2.1. Problem Description. With resin Amberlyst A-21 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. 17, 18 2TCS ↔ STC + DCS

∆Ηr = 6402kJ/kmol

(1)

2DCS ↔ TCS + MCS

∆Ηr = 2226 kJ/kmol

(2)

2MCS ↔ DCS + Silane

∆Ηr =–248 kJ/kmol

(3)

The reaction rates for the first-, second-, and third-stage reactions are expressed with the following equations. r1=k1(x2TCS−xSTCxDCS/K1)

(4)

r2=k2(x2DCS−xTCSxMCS/K2)

(5)

r3=k3(x2MCS−xDCSxSIL/K3)

(6)

where xSTC, xTCS, xDCS, xMCS, and xSIL represent the mole fractions of STC, TCS, DCS, MCS, and silane, respectively; r is the reaction rate for each stage reaction involved. The rate constants of the forward reactions (k) and chemical equilibrium constant (K) are described as follows. k=k0exp(−E/RT)

(7)

K=K0exp(−∆H/RT)

(8)

where k0 and K0 denote the pre-exponential factors, respectively; E is the activation energy of the forward reactions, and ∆H, the thermal heat of reactions. Table 1 lists the physicochemical properties and operating conditions of the RDC-SRS and RDC-MRS to be developed. Note that the five reacting components exhibit an ascending order of boiling-points as shown in eq. 9. It seems then to be 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. Huang et al. showed firstly the feasibility of obtaining a high purity of silane over 99 % (mol) and approaching a complete conversion of TCS in a RDC-SRS.

18

They indicated further the great potential to employ

side-condensers to improve steady-state economics. Through an iterative simulation-optimization procedure, Alcántara-Ávila et al. proposed a multitask RDC-SRS with intermediate heat exchangers that was flexible to switch between the production of 5



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silane, MCS, and DCS and found dramatic improvement in steady-state economics.19, 20 Ramírez-Márquez et al. examined the dynamics and control of the multitask RDC-SRS and demonstrated that the process could be operated smoothly with a decentralized control structure comprised of either temperature, composition, or temperature/composition cascade loops.

21

Owing to the fairly unfavorable reaction kinetics of a near zero thermodynamic conversion 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. Steady-state simulation is performed in the environment of Aspen Plus, and the module of RacFrac is adopted to represent the behaviors of the RDC-SRS and RDC-MRS. 19 As the reacting mixture is nonpolar, the Peng-Robinson equation of state is chosen to calculate the required thermodynamic properties. Huang et al. regressed the binary interaction coefficients and yielded an acceptable estimation of the vapor-liquid equilibrium relationship.18 The model was also frequently adopted by others in the modeling and simulation of the RDC-SRS for the disproportionation of trichlorosilane to silane. silane (161K) < MCS (243.15K) < DCS (281.45K) < TCS (305K) < STC (330K)

(9)

2.2. Synthesis and Design of the RDC-SRS and RDC-MRS. In view of the fact that the disproportionation of trichlorosilane to silane is a three-stage consecutive reversible reaction, it is then 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 reactive distillation columns with triple reactive sections (RDC-TRS). In order to establish a fair basis for the comparison of the RDC-SRS, RDC-DRS, and RDC-TRS, we specify here the total number of stages and the total number of reactive stages to be 60 (including condenser and reboiler as well) and 31 (each reactive stage is assumed to contain 60.25 mole of resin Amberlyst A-21), respectively, for the three process designs to be examined. In fact, these values are slightly different from those of reference 18. Although the process designs and their operating conditions are designated somewhat arbitrarily, they do not influence the reasonability and conclusions of the current work. Under these assumptions, the condenser heat duty can simply be adopted as an effective metric for the comparison of their steady-state performance (because the normal boiling-point of silane is quite low (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 capital investment and operating cost in case that total annual cost is chosen as a performance index. A simple grid-search method is adopted to synthesize and design the RDC-SRS, RDC-DRS, and RDC-TRS. Although it is not a globally convergent method, the obtained result can frequently approach to the optimum 6


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process design and this is why so many researchers have adopted it in process development. 2, 9, 14 For the RDC-SRS, it contains two separating sections (i.e., the rectifying section and stripping section) and one reactive section and the structural decision variables for process synthesis and design include the number of stages in the two separating sections and the feed location of TCS (Throughout the current work, the reflux flow rate and condenser heat duty are employed, respectively, to keep the top and bottom products on their specifications and are not addressed here as operating decision variables). Figure 2 shows the relationship between the condenser heat duty and those structural decision variables. Note that the first separating section should contain 13 stages and moving away from this value elicits increases in the heat duty of condenser (c.f., Figure 2a). The influence of the number of reactive stages in the reactive section is also examined here and the optimum value is confirmed to be 31, i.e., the exact value as we assumed in the preceding paragraph (c.f., Figure 2b). The feed of TCS should be introduced onto stage 23, above which an enlarged heat duty of condenser is aroused (c.f., Figure 2c). The resultant optimum design of the RDC-SRS is depicted in Figure 5a, with the heat duties of condenser and reboiler as 412.6 kW and 453.8 kW, respectively. For the RDC-DRS, it involves three separating sections (i.e., the rectifying section, one intermediate separating section, and stripping section) and two reactive sections and the structural decision variables comprise the number of stages in the three separating sections, the number of reactive stages in the two reactive sections, and the feed location of TCS. Figure 3 shows the relationship between the condenser heat duty and those structural decision variables. While the three separating sections accommodate, respectively, 4, 10, and 15 stages (c.f., Figures 3a and 3c), the two reactive sections 7 and 24 stages, respectively (c.f., Figure 3b). In particular, a high degree of sensitivity is noticed between the number of stages in the upper reactive section and the heat duty of condenser, implying the great impact by the arrangement of two reactive sections. The feed of TCS enters now onto stage 24 due to the adoption of two reactive sections in process configuration (c.f., Figure 3d). The optimum design of the RDC-DRS is sketched in Figure 5b, with the heat duties of condenser and reboiler shrunk to be 376.4 kW and 417.7 kW, respectively. For the RDC-TRS, it consists of 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. Figure 4 shows the relationship between the condenser heat duty and those structural decision variables. While the four separating sections 7



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accommodate, respectively, 4, 3, 8, and 14 stages (c.f., Figures 4a, 4c, and 4e), the three reactive sections 1, 5, and 25 stages, respectively (c.f., Figures 4b and 4d). Again, a relatively high degree of sensitivity is observed between the number of stages in the upper and middle reactive sections and the heat duty of condenser, implying the great impact by the arrangement of three reactive sections. The feed of TCS still enters onto stage 24 in spite of the adoption of three reactive sections (c.f., Figure 4f). The optimum design of the RDC-TRS is depicted in Figure 5c, with the heat duties of condenser and reboiler dropped further to be 367.6 kW and 408.8 kW, respectively. 2.3. RDC-SRS versus RDC-MRS. Table 2 details the comparison of steady-state performance between the RDC-SRS, RDC-DRS, and RDC-TRS. With the adoption of two reactive sections, the RDC-DRS diminishes the heat duties of condenser and reboiler by 8.77 % and 7.96 %, respectively, as compared with the RDC-SRS. Likewise, with the adoption of three reactive sections, the RDC-TRS abates the heat duties of condenser and reboiler by 10.91 % and 9.92 %, 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 worth noting here that the great improvement in steady-state performance implies also a great reduction of capital investment by the arrangement of multiple reactive sections in process synthesis and design. 2.4. Roles of Multiple Reactive Sections in the RDC-MRS. In Figure 6, the steady-state profiles of liquid compositions are shown for the RDC-SRS, RDC-DRS, and RDC-TRS. 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 6a), the steady-state profile of MCS exhibits a fairly high and wide plateau with a 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 synthesis and design that makes the RDC-SRS 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.

18

Regarding

the RDC-DRS (c.f., Figure 6b), due to the arrangement of two reactive sections, the MCS composition has been considerably 8


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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 6c), owing to the 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. In Figure 7, the steady-state profiles of net reaction rates for the three consecutive reversible reactions involved in the disproportionation of trichlorosilane to silane are shown, respectively, for the RDC-SRS, RDC-DRS, and RDC-TRS. In the case of the RDC-SRS, because all of the three consecutive reversible reactions compete within the single reactive section, the three curves intersect each other and this signifies rather strong couplings between them (c.f., Figure 7a). With regard to the RDC-DRS, while the first-stage reaction occurs almost completely and independently in the lower reactive section, the secondand third-stage reactions take place almost completely in the upper reactive section (c.f., Figure 7b). In particular, in the upper reactive section the second-stage reaction proceeds primarily at the bottom end and the third-stage reaction at the top end, sharing a common area between stages 6 and 10. The intersection of the two curves still implies the existence of rather strong coupling between the two reactions and the need to adopt an additional reactive section to further alleviate it. In the case of the RDC-TRS, the three consecutive reversible reactions involved now proceed almost independently (c.f., Figure 7c), with the first-stage reaction occurring almost completely in the lower reactive section, the second-stage reaction occurring almost completely in the middle reactive section, and the third-stage reaction occurring almost completely in the upper reactive section. The changing pattern of the profiles of net reaction rates from the RDC-SRS to the RDC-DRS and RDC-TRS highlights the pivotal role of adopting multiple reactive sections in coordinating the three consecutive reversible reactions involved in the disproportionation of trichlorosilane to silane.

3.

DISCUSSION Apart from the rather unfavorable reaction kinetics of a near zero thermodynamic conversion, the disproportionation of

trichlorosilane to silane is also characterized by a fairly interactive nature because TCS, DCS, and MCS function as both 9



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reactants (or intermediate reactants) and intermediate products 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 reactive distillation columns. It should be stressed here that it also represents a key issue that is closely related to the reinforcement of internal mass integration and internal energy integration between the reaction operations and the separation operations involved.

10, 11, 13, 14

It is, therefore, of great importance to

effectively tackle this issue during the stage of process synthesis and design. The RDC-SRS fails to tackle that issue effectively due to the shortage of structural design variables and this is why it requires the highest heat duties in condenser and reboiler. With the arrangement of 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 the heat duties of condenser and reboiler. With the arrangement of 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 heat duties of condenser and reboiler. The varying tendency indicates clearly the favorable effect by the incremental adoption of reactive sections in the synthesis and design of reactive distillation columns 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 example system studied, it should be considered to be of general significance to the synthesis and design of any other reactive distillation columns because all of our research results obtained so far have been in excellent accordance with this deduction.

10, 11, 13, 14

It is worthy to mention here the fact that the RDC-MRS (i.e., the RDC-DRS and

RDC-TRS) has been derived based on exactly the same operating conditions as the RDC-SRS. This signifies that no prerequisites are needed at all for the application of the novel scheme as compared with its conventional analogues, thereby permitting wide applicability and great advantages in process synthesis and design. The disproportionation of trichlorosilane to silane by various reactive distillation columns features a common drawback of high refrigeration requirement because the normal boiling-point of silane is quite low (i.e., only 161 K) and this makes the application of side-condensers a frequently adopted strategy to improve the thermodynamic efficiency of these processes. 18 For the RDC-MRS, the unique cascade structure of separating sections and reactive sections 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 10


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relatively easier in the RDC-MRS than in the RDC-SRS. Figure 8 shows the arrangement of two side-condensers to the RDC-DRS in case of their heat duties being assumed uniformly to be 100 kW. For the top side-condenser (where the bottom side-condenser has been fixed arbitrarily at stage 18), it should be located at stage 4 since further descending its location introduces relatively small increase of temperature (The higher the temperature is, the cheaper the coolant of the side-condenser becomes) and relatively great increase of condenser heat duty (c.f., Figure 8a). For the bottom side-condenser (where the top side-condenser has been fixed at stage 4), it should be located at stage 22 because moving away from the stage leads to either relatively small increase of temperature or relatively great increase in condenser heat duty (c.f., Figure 8b). The resultant design of the RDC-DRS with two side-condensers is sketched in Figure 8c. Note the fact that the two side-condensers are connected, respectively, to two specific stages of the top two separating sections. Figure 9 shows the arrangement of three side-condensers to the RDC-TRS in case of their heat duties being assumed uniformly to be 100 kW. Extremely similar to the situation of Figure 8a, the top side-condenser should again be located at stage 4 when the intermediate and bottom side-condensers have been fixed arbitrarily at stages 8 and 18, respectively (c.f., Figure 9a). In terms of Figure 9b (where the top and bottom side-condensers have been fixed at stages 4 and 18, respectively), the intermediate side-condenser should be located at stage 7 because further lowering its position arouses relatively great augment of condenser heat duty. With reference to Figure 9c (where the top and intermediate side-condensers have been fixed, respectively, at stages 4 and 8), the bottom side-condenser should be located at stage 21 because moving away from this stage results in either relatively small or relatively great increase in temperature and condenser heat duty (c.f., Figure 9c). The resultant design of the RDC-TRS with three side-condensers is delineated in Figure 9d. 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 synthesis and design of reactive distillation columns separating complicated reacting mixtures involving multiple reversible reactions. Although not shown explicitly in the current work, the stage temperatures in the reactive sections of the RDC-SRS, RDC-DRS, and RDC-TRS are all below the thermal resistance temperature of the resin Amberlyst A-21 (i.e., 373.15 K). With the arrangement of two and three side-condensers to the RDC-DRS and RDC-TRS, respectively, the resultant temperatures and compositions of silane in the reactive sections display only slight changes as compared with their original values.

11



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4.

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CONCLUSIONS Adopting multiple reactive sections allows the RDC-MRS to have more degrees of freedom for process synthesis and

design than the RDC-SRS. These additional degrees of freedom can be employed to reinforce internal mass integration and/or internal energy interaction between the reaction operations and the separation operations involved, leading frequently to great improvement in system performance as compared with the RDC-SRS. In the case of the separations of multiple-stage consecutive reversible reactions, these additional degrees of freedom can also be used to coordinate the potentially intricate couplings between the multiple consecutive reversible reactions involved, yielding again a favorable effect to the steady-state performance of the RDC-MRS. 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 the incremental arrangement of reactive sections, the heat duties of condenser and rebolier display a downward trend from the RDC-SRS to the RDC-DRS and then the RDC-TRS. Arrangements of side-condensers have also been attempted to the RDC-DRS and RDC-TRS and they all have been found to locate in the separating sections above the feed of TCS. 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 for the development of reactive distillation columns separating complicated reacting mixtures involving multiple reversible reactions.

ACKNOWLEDGEMENTS The current work is financially supported by The National Natural Science Foundation of China under the grant numbers of 21076015, 21376018, 21576014, and 21676011 and The Fundamental Research Funds for the Central Universities under the grant number of ZY1503.

NOTATION A, B, C, D = hypothetical reacting components DCS = dichlorosilane F = feed flow rate, kmol/s 12


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∆Ηr =reaction heat, kJ/kmol k = rate constant of forward reactions K = chemical equilibrium constant MCS = monochlorosilane NF = feed stage NR, I = number of stages in the ith reactive section NS, I = number of stages in the ith separating section P = pressure, Pa Q = heat duty, kW r = reaction rate, kmol/s R = reactive section RDC-DRS = reactive distillation column with double reactive sections RDC-MRS = reactive distillation column with multiple reactive sections RDC-SRS = reactive distillation column with a single reactive section RDC-TRS = reactive distillation column with triple reactive sections RR = reflux ratio S = separating section SIL = silane STC = silicon tetrachloride T = temperature, K TCS = trichlorosilane x = liquid composition α = relative volatility Subscripts COND = condenser REB = reboiler 13



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LITERATURE CITED (1)

Taylor, R.; Krishna, R. Modeling Reactive Distillation. Chem. Eng. Sci. 2000, 55, 5183.

(2)

Luyben, W. L.; Yu, C. C. Reactive Distillation Design and Control. New Jersey: John Wiley & Sons, Inc., 2008.

(3)

Malone, M. F.; Doherty, M. F. Reactive Distillation. Ind. Eng. Chem. Res. 2000, 39, 3953.

(4)

Kaymak, D. B.; Luyben, W. L. Quantitative Comparison of Reactive Distillation with Conventional Multiunit

Reactor/Column/Recycle Systems for Different Chemical Equilibrium Constants. Ind. Eng. Chem. Res. 2004, 43, 2493. (5)

Huang, K.; Nakaiwa, M.; Wang, S. J.; Tsutsumi, A. Reactive Distillation Design with Considerations of Heats of

Reaction. AIChE J. 2006, 52, 2518. (6)

Kaymak, D. B.; Luyben, W. L. Effect of Relative Volatility on the Quantitative Comparison of Reactive Distillation

and Conventional Multi-unit Systems. Ind. Eng. Chem. Res. 2004, 43, 3151. (7)

Chen, C. S.; Yu, C. C. Effects of Relative Volatility Ranking on Design and Control of Reactive Distillation

Systems with Ternary Decomposition Reactions. Ind. Eng. Chem. Res. 2008, 47, 4830. (8)

Thotla, S.; Mahajani, S. Reactive Distillation with Side Draw. Chem. Eng. Proc. 2009, 48, 927.

(9)

Tung, S.T.; Yu, C.C. Effects of Relative Volatility Ranking to the Design of Reactive Distillation. AIChE J. 2007, 53,

1278. (10) Zhang, L.; Chen, H.; Yuan, Y.; Yu, J.; Wang, S.; Huang, K. Synthesis and Design of Reactive Distillation Columns with Two Reactive Sections. Chem. Eng. Res. Des. 2015, 100, 311. (11) Zhang, L.; Chen, H.; Yuan, Y.; Wang, S.; Huang, K. Adopting Feed Splitting in Design of Reactive Distillation Columns with Two Reactive Sections. Chem. Eng. Proc. 2015, 89, 9. (12) Chen, H.; Huang, K.; Liu, W.; Zhang, L.; Wang, S.; Wang, S. J. Enhancing Mass and Energy Integration by External Recycle in Reactive Distillation Columns. AIChE J. 2013, 59, 2015. (13) Chen, H.; Zhang, L.; Huang, K.; Yuan Y.; Zong, X.; Wang, S.; Liu, L. Reactive Distillation Columns with Two Reactive Sections: Feed Splitting plus External Recycle. Chem. Eng. Proc. 2016, 108, 189. (14) Yu, C.; Yao, X.; Huang, K.; Zhang, L.; Wang, S.; Chen, H. A Reactive Distillation Column with Double Reactive Sections for the Separations of Two-Stage Consecutive Reversible Reactions. Chem. Eng. Proc. 2014, 79, 56. 14


Page 15 of 29

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(15) Mueller, I.; Kenig, E. V. Reactive Distillation in a Dividing Wall Column: Rate-Based Modeling and Simulation. Ind. Eng. Chem. Res. 2007, 46, 3709. (16) Yu, J.; Shi, L.; Yuan, Y.; Chen, H.; Wang, S.; Huang, K. Thermally Coupled Reactive Distillation System for the Separations of Cyclohexene/Cyclohexane Mixtures. Ind. Eng. Chem. Res. 2016, 55, 311. (17) Union Carbide. Feasibility of the Silane Process for Producing Semiconductor Grade Silicon. Final Report (Phases I and II), JPL Contract 954334; U.S. DOE, 1979. (18) Huang, X.; Ding, W.; Yan, J.; Xiao, W. Reactive Distillation Column for Disproportionation of Trichlorosilane to Silane: Reducing Refrigeration Load with Intermediate Condensers. Ind. Eng. Chem. Res. 2013, 52, 6211. (19) Alcántara-Ávila, J. R.; Sillas-Delgado, H. A.; Segovia-Hernández, J. G.; Gómez-Castro, F. I.; Cervantes-Jauregui, J. A. Optimization of a Reactive Distillation Process with Intermediate Condensers for Silane Production. Comput. Chem. Eng. 2015, 78, 85. (20) Alcántara-Ávila, J. R.; Tanaka, M.; Ramírez-Márquez, C.; Gómez-Castro, F. I.; Segovia-Hernández, J. G.; Sotowa, K. I.; Horikawa, T. Design of a Multitask Reactive Distillation with Intermediate Heat Exchangers for the Production of Silane and Chlorosilane Derivates. Ind. Eng. Chem. Res. 2016, 55, 10968. (21) Ramírez-Márquez, C.; Sánchez-Ramírez, E.; S.; Quiroz-Ramírez, J. J.; Gómez-Castro, F. I.; Ramíre-Corona, N. Cervantes-Jauregui, J. A.; Segovia-Hernández, J. G. Dynamic Behavior of a Multi-tasking Reactive Distillation Column for Production of Silane, Dichlorosilane and Monochlorosilane. Chem. Eng. Proc. 2016, 108, 125. (22) Wang, K. Synthesis and Design of Reactive Distillation Columns with Multiple Reactive Sections. Master Thesis of Beijing University of Chemical Technology. 2015.

15



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Page 16 of 29

Figure Captions Figure 1.

Schemes of the RDC-SRS and RDC-MRS: (a) RDC-SRS, (b) RDC-MRS.

Figure 2.

Relationship between condenser heat duty and relevant decision variables for the RDC-SRS.

Figure 3.

Relationship between condenser heat duty and relevant decision variables for the RDC-DRS.

Figure 4.

Relationship between condenser heat duty and relevant decision variables for the RDC-TRS.

Figure 5.

Optimum process designs for the disproportionation of trichlorosilane to silane: (a) RDC-SRS, (b) RDC-DRS, (c) RDC-TRS.

Figure 6.

Profiles of liquid compositions: (a) RDC-SRS, (b) RDC-DRS, (c) RDC-TRS.

Figure 7.

Profiles of reaction rates: (a) RDC-SRS, (b) RDC-DRS, (c) RDC-TRS.

Figure 8.

Arrangement of two side-condensers to the RDC-DRS: (a) impact of the top side-condenser, (b) impact of the bottom side-condenser, (c) RDC-DRS with two side-condensers.

Figure 9.

Arrangement of three side-condensers to the RDC-TRS: (a) impact of the top side-condenser, (b) impact of the intermediate side-condenser, (c) impact of the bottom side-condenser, (d) RDC-TRS with three side-condensers.

16


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(a)

(b)

Figure 1 Xinxiang Zang, Kejin Huang, Yang Yuan, Haisheng Chen, Liang Zhang, Shaofeng Wang, and Kun Wang Reactive Distillation Columns with Multiple Reactive Sections: A Case Study for the Disproportionation of Trichlorosilane to Silane

17



QCOND (kW)

427

426

425

424 11

12

13

14

15

31

32

33

23

24

25

NS, 1

(a)

QCOND (kW)

440

430

420

410 29

30 NR, 1

(b) 418

QCOND (kW)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 29

416

414

412 21

22 NF

(c)

Figure 2 Xinxiang Zang, Kejin Huang, Yang Yuan, Haisheng Chen, Liang Zhang, Shaofeng Wang, and Kun Wang Reactive Distillation Columns with Multiple Reactive Sections: A Case Study on the Disproportionation of Trichlorosilane to Silane

18


Page 19 of 29

392

394

386

QCOND (kW)

QCOND (kW)

389

380

384

379

374

374 2

3

4

5

5

6

6

7

8

9

25

26

NR, 1

NS, 1

(a)

(b)

380

377.3

QCOND (kW)

379

QCOND (kW)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


378

377.0

376.7

377

376

376.4 8

9

10

11

12

22

NS, 2

23

24 NF

(b)

(d)


19


382

385

379 QCOND (kW)

390

380

375

Page 20 of 29

376

373

370

370

367

365 2

3

4

5

1

6

2

3

4

5

5

6

7

24

25

26

NR, 1

NS, 1

(a)

(b)

370

382

369

QCOND (kW)

QCOND (kW)

378

374

368 370

366

367 1

2

3

4

3

5

4 NR, 2

NS, 2

(c)

(d)

371

368.6

QCOND (kW)

370

QCOND (kW)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

QCOND (kW)


369

368.2

367.8

368

367

367.4

6

7

8

9

10

22

23

NS, 3

NF

(e)

(f)


20


Page 21 of 29

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(a)

(b)

(c)

Figure 5 Xinxiang Zang, Kejin Huang, Yang Yuan, Haisheng Chen, Liang Zhang, Shaofeng Wang, and Kun Wang Reactive Distillation Columns with Multiple Reactive Sections: A Case Study for the Disproportionation of Trichlorosilane to Silane

21



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(a)

(b)

(c)


22


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(a)

(b)

(c)


23



193

323

200

356

Condenser heat duty Temperature

Condenser heat duty Temperature 305 192

269

196

350

191

192

344

T (K)

T (K)

287

QCOND (kW)

QCOND (kW)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 29

251

190

233 3

4

5

188

6

338 16

Stage number

19

22

25

Stage number

(a)

(b)

(c)


24


Page 25 of 29

94

91

313


333


323

298

92

89

T (K)

268 88

313 87

303

T (K)

283 90

QCOND (kW)

QCOND (kW)

293

253

85 86

283

238

84

83

223 3

4

5 Stage number

6

273 6

7

7

(a)

8 Stage number

9

10

(b)

91

352 Condenser heat duty Temperature

89

349

87

346

85

343

83

340

T (K)

QCOND (kW)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


81

337 15

17

19 Stage number

21

23

(c)

(d)


25



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Table Captions Table 1.

Physicochemical Properties and Operating Conditions for the Disproportionation of Trichlorosilane to Silane

Table 2.

Comparison between the RDC-SRS, RDC-DRS, and RDC-TRS

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Table 1. Physicochemical Properties and Operating Conditions for the Disproportionation of Trichlorosilane to Silane Parameter

Value

Top stage pressure (atm)

5.0

Stage pressure drop (kPa)

0.5

Top product specification (Silane, mol %)

99

Bottom product specification (STC, mol %)

99

Reaction activation energy, EA (kJ/kmol)

Reaction frequency factor, K0 (1/s)

Reaction equilibrium constant

Boiling temperatures at atmospheric pressure (K)

First-stage reaction

30045

Second-stage reaction

51083

Third-stage reaction

26320


73.5


949466.4


1176.9


0.1856


0.7669


0.6890

STC

330

TCS

305

DCS

281.45

MCS

243.15

Silane

161

Xinxiang Zang, Kejin Huang, Yang Yuan, Haisheng Chen, Liang Zhang, Shaofeng Wang, and Kun Wang Reactive Distillation Columns with Multiple Reactive Sections: A Case Study on the Disproportionation of Trichlorosilane to Silane

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Table 2. Comparison between the RDC-SRS, RDC-DRS, and RDC-TRS Process design

RDC-SRS

RDC-DRS

RDC-TRS

QCOND (kW)

412.6

376.4

367.6

Comparison

100 %

91.23 %

89.09 %

QREB (kW)

453.8

417.7

408.8

Comparison

100 %

92.04 %

90.08 %

Xinxiang Zang, Kejin Huang, Yang Yuan, Haisheng Chen, Liang Zhang, Shaofeng Wang, and Kun Wang Reactive Distillation Columns with Multiple Reactive Sections: A Case Study on the Disproportionation of Trichlorosilane to Silane

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For Table of Contents Only (a)

(b)

(c)

29


Reactive Distillation Columns with Multiple Reactive Sections: A Case

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