Reactive Distillation Column for Disproportionation ... - ACS Publications

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Reactive Distillation Column for Disproportionation of Trichlorosilane to Silane: Reducing Refrigeration Load with Intermediate Condensers Xun Huang, Wei-Jie Ding, Jian-Min Yan, and Wen-De Xiao* School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China ABSTRACT: This paper presents a reactive distillation column for the catalytic disproportionation of trichlorosilane to silane which includes three consecutive reversible reactions with a thermodynamic conversion to silane as low as 0.2% and is of no practical significance using the conventional reactors. This reaction system is however characterized by a large distinction in the boiling points of the components, which makes the reactive distillation extremely favored. Nevertheless, the normal reactive distillation column possesses the shortage of high refrigeration requirement as the standard boiling point of the overhead product silane is −112 °C. A novel reactive distillation column with intercondensers placed within has been simulated using Aspen Plus package for a feasible alternative to alleviate this drawback. The calculated results show that a reduction in the refrigeration load of more than 97% can be obtained when one intermediate condenser is inserted between the rectifying and reaction zones, and that another normal intermediate condenser equipped within the reaction section can further decrease the condensation requirement of the first intermediate condenser by 50%. Effects of the configuration parameters and operational conditions have also been investigated for optimal design and operation of the proposed reactor.

1. INTRODUCTION With the increasing energy crisis and global warming, clean and sustainable solar power is attracting more and more attention.1,2 The photovoltaic (PV) solar cells based on monocrystalline and polycrystalline silicon wafers are presently dominant and will probably continue to dominate for a long time since the raw material availability is not a problem. Therefore silicon, the starting material for the crystalline wafer, is the most important material in PV industry today. To widely commercialize the solar technology, one challenge faced by the PV industry is to decrease the manufacturing costs, especially the cost of solar-grade Si feedstock material. Currently, the most well-known chemical route to produce solar-grade silicon is the modified Siemens process, which performs decomposition of trichlorosilane (TCS) by chemical vapor deposition (CVD) on an inverse U-shape hot filament in a batchwise bell jar reactor. However, the extremely high consumption of electrical energy as well as the corrosion caused by the byproduct of hydrochloric acid has restrained the Siemens process from application to low-cost polysilicon production.3−5 One alternative method is the silane pyrolysis process operated in a continuous fluidized bed reactor6,7 for its low consumption of energy. The starting material silane can be produced through various methods, such as the reduction of silicon tetrafluoride by lithium aluminum hydride.8 But the more practical way used in the polysilicon industry is the disproportionation of trichlorosilane9,10 by exchanging chlorine with hydrogen via dichlorosilane (DCS) and monochlorosilane (MCS), since trichlorosilane is a mass-produced intermediate to produce polysilicon and can be obtained from the hydrogenation of metallurgical silicon and silicon tetrachloride (STC). The disproportionation of trichlorosilane can be catalyzed by several kinds of catalysts11−16 such as aluminum trichloride and © 2013 American Chemical Society

acyclic nitriles. Among these catalysts anion amine ionexchange resin is the best choice because of its immobilized solid state and relatively high activity. For instance, Amberlyst A-21 by Rohm Hass Company, the selected catalyst in our study in which dimethylamino groups are bonded to a polystyrene structure, shows that reasonable reaction rates are achieved at temperatures between 30 and 80 °C,14 with a suggested thermal resistance operation temperature of 100 °C. The conventional process10 for highly pure silane production from trichlorosilane disproportionation is through two reactors and several separation units. However, due to unfavorable chemical equilibrium, this reaction and separation process requires an extremely large recycle ratio and thus the cost of energy as well as investment is very high. Therefore, reactive distillation (RD), a technology that combines reaction and distillation in one column and is particularly attractive for equilibrium limited reactions, can be suitable for the disproportionation reactions. It eliminates conversion and phase equilibrium limitations by continuous removal of products from the reaction zone and has already been used for the commercial production of chemicals like MTBE.17 One RD column can replace several downstream units used in the conventional process. The design procedure of RD technology like MINLP has been established18 and some process intensification strategies such as internal heat integration,19 thermally coupled design,20 and vapor recompression column21 have also been proposed by researchers to further reduce the energy consumption. Bakay 22 proposed a process for manufacturing silane in a bed of anion exchange resin by Received: Revised: Accepted: Published: 6211

November 27, 2012 April 10, 2013 April 16, 2013 April 16, 2013 dx.doi.org/10.1021/ie3032636 | Ind. Eng. Chem. Res. 2013, 52, 6211−6220

Industrial & Engineering Chemistry Research

Article

experimental results14 showed that the reactions in the vapor phase are faster than those in the liquid phase. While the density of liquid is much larger than that of vapor, the former can be ignored if most of the catalyst is surrounded by liquid. For example, if held on trays or downcomers in the column, the packed catalyst-containing envelopes are essentially vapor free.27 Figure 1 shows how k and K vary with temperature between 0 and 100 °C. In this temperature range, the relationships of k and K are k1 < k2 < k3 and K1 < K2 < K3, which means the dismutation of MCS is the easiest to take place while that of TCS is the most difficult under the same operation conditions. The dismutation of MCS is exothermal while the other two reactions are slightly endothermic. As the boiling point of MCS is the lowest and it reaches high concentration mainly in the colder section of the column, the temperature and composition profiles along the column are beneficial to the chemical equilibrium. Figure 2 shows the equilibrium yields changing with temperature from the redistribution of TCS. Because of the rather low chemical equilibrium constant, when pure TCS is fed, the yield of silane is extremely low, with the order of about 0.2%, and it is impractical to produce silane through one conventional reactor. 2.2. Phase Equilibrium Model. The atmospheric boiling points of STC, TCS, DCS, MCS, and silane are 56.85 °C, 31.85 °C, 8.3 °C, −30 °C, and −112.15 °C, respectively, which reflects a very different relative volatility. The lightest component silane can be obtained as overhead product while the heaviest component STC can be obtained as bottom product. The reactant of disproportionation, including TCS, DCS, MCS, can be maintained in the reaction zone because of their medium boiling points. A proper VLE model is as important as the kinetic model in RD simulation since this process is based on the simultaneous reaction and separation on stages. In this work, as the mixture is nonpolar, the Aspen built-in Peng−Robinson equation of state was selected to perform thermodynamic calculation. The binary interaction coefficients of chlorosilanes are blank in the Aspen Database, and some of them were regressed with the experimental VLE data exported from NIST ThermoData Engine, and displayed in Table 2, where kij stands for the binary interaction coefficient. The values of kij are small and calculation shows that there is no azeotropy in this system. There are totally 10 binary interaction coefficients between the five components in the disproportionation system, and Table 2 lists only half of them. The rest were set to zero in our study, which is the default value in Aspen Plus. This simplification will not result in intolerable error. First, the available coefficients are all close to zero and other coefficients may have comparable values. Second, other component pairs do not have adjacent boiling points and therefore do not show relatively high concentration on the same stage as shown in later section. 2.3. Model of RD Column. For a heterogeneous catalytic reaction, the typical RD column is a distillation column with some zones packed with the solid catalyst, which functions as mass transfer internals as well, and the coupled reaction and separation by distillation takes place at the same sites. In this paper, a typical reactive distillation column as shown in Figure 3 was investigated first which composed of three sections: the rectifying, catalytic reaction and stripping sections, located at the upper, middle, and lower part, respectively, with the solid catalyst packed in the middle of the column. Practically, the

controlling the top temperature between the boiling points of silane and trichlorosilane. Müller et al.23 and Block et al.24 with Solarworld AG disclosed some reactive distillation schemes with intercondensers, and Sonnenschein et al.25 with Envonik Degussa presented a distillation column with a side reactor. Although the idea of applying RD to silane production has been presented in several patents, there are few published papers studying its performance and features. In this work, we investigate the feasibility of applying RD to the production of silane through the disproportionation of trichlorosilane. The available kinetic and thermodynamic models are discussed and applied to RD simulation using Aspen Plus. Different RD schemes are discussed and how the pressure, reflux ratio, and intercondensers influence the performance of RD column is also elucidated.

2. MODEL DEVELOPMENT 2.1. Chemical Reaction Characteristics. The trichlorosilane disproportionation process consists of three reversible reactions and five components shown as follows: cat

2SiHCl3 ↔ SiCl4 + SiH 2Cl 2

(1)

cat

2SiH 2Cl 2 ↔ SiHCl3 + SiH3Cl

(2)

cat

2SiH3Cl ↔ SiH 2Cl 2 + SiH4

(3)

A plausible mechanism involving an intermediate of ionic amine-chlorosilane26 indicates that the dismutation reactions are second order, and the rate equations of each step can be described as follows:14,15 r1 = k1(x12 − x0x 2/K1)

(4)

r2 = k 2(x 2 2 − x1x3/K 2)

(5)

r3 = k 3(x32 − x 2x4 /K3)

(6)

where x0, x1, x2, x3, and x4 are the mole fraction of STC, TCS, DCS, MCS, and silane in liquid, respectively; r1, r2, and r3 are the reaction rates of TCS, DCS, and MCS disproportionation, respectively. k and K are the rate constant of forward reaction and chemical equilibrium constant, which are expressed in following formulas:

k = k 0e−E /(RT )

(7)

K = K 0e−ΔH /(RT )

(8)

In these formulas, k0 and K0 are pre-exponential factors, respectively; E is activation energy of forward reaction, and ΔH the heat of reaction. For cases where reactants are in the liquid phase and catalyzed by resin Amberlyst A-21, their values are regressed from the experimental data14 and the results are summarized in Table 1. It should be emphasized that variations may occur when other kinds of catalyst are used. Besides, Table 1. Kinetic Parameters of Trichlorosilane Disproportionation in the Liquid Phase

r1 r2 r3

k0 (s−1)

E (J/mol)

K0

ΔH (J/mol)

73.5 949466.4 1176.9

30045 51083 26320

0.1856 0.7669 0.6890

6402 2226 −2548 6212

dx.doi.org/10.1021/ie3032636 | Ind. Eng. Chem. Res. 2013, 52, 6211−6220


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Figure 1. The effect of temperature on rate and equilibrium constants.

Figure 2. The effect of temperature on equilibrium yields.

Table 2. Binary Interaction Coefficients of PR-EOS for Chlorosilane System component i

component j

kij

experimental reference

SiCl4 SiCl4 SiHCl3 SiH2Cl2 SiH3Cl

SiHCl3 SiH2Cl2 SiH2Cl2 SiH3Cl SiH4

0.01603 0.02108 0.05183 −0.00538 0.000953

Zanta and Laskafeld28 Zhurenko et al.29 Olson30 Zhurenko et al.29 Zhurenko et al.29

Figure 3. The typical RD process.

feed TCS may flow down to the stripping section but back upward to the reaction section with the aid of the vapor from the bottom reboiler. This operational mode thus effectively make the overall reaction approach complete, and the most desired product silane is able to be withdrawn from the top of the column while the coproduct STC can be withdrawn from the bottom. In practice, STC is usually recycled to an upstream hydrocholorination reactor to regenerate TCS again, as described in ref 3 according to the following reaction:

anion amine ion-exchange resin beads, with particle sizes of 0.3−1.2 mm, are enveloped by the stainless steel wire mesh for a well fixation and low pressure drop. The rectifying and stripping sections can be filled with any type of the packing commonly used in the general distillation applications, such as the kinds of structured and random packing. Trichlorosilane is fed into the column at the bottom of reaction section or the top of the stripping section. Then disproportionation takes place in the reaction section and vaporous product mixture containing light components such as silane ascend to the rectifying section while the heavy components like STC descend to the stripping section. The intermediate products such as DCS and MCS, occasionally TCS, rise into the rectifying section but must return to the reaction section with the help of the top reflux flow, and the

cat

3SiCl4 + 2H 2 + Si ↔ 4SiHCl3

(9)

Equilibrium stage (EQ) model and nonequilibrium stage model (NEQ) are commonly used to simulate the typical as well as the reactive distillation.27 The equations that model EQ stages are known as MESH, which consist of material balance, vapor−liquid equilibrium relations, summation equations, and heat balance. The nonequilibrium stage, also known as the ratebased model, considers rate equations for mass and heat transfer, and phase equilibrium is assumed only at the phase interface rather than on the whole stage. The nonequilibrium 6213



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because STC is produced in the first step of the three TCS disproportion reactions while silane is produced in the third step. Besides, since the top silane and the bottom STC can reach the purity over 99% when the reflux ratio is more than 63, the total conversion of TCS to silane and STC is nearly completed (99.83%). The product silane is desired and more important than the coproduct STC, therefore in this work the purity of silane is chosen to characterize the performance of RD process. Figure 5(A) demonstrates the liquid composition profiles along the column. It is obvious that each component reaches the highest concentration at different stage in accordance with its boiling point. TCS maintains a high concentration in a wide stage interval because its consumption by disproportionation is relatively slow and the generation from disproportionation of DCS is fast as depicted in Figure 1. Because of the relatively high volatility, the concentration of silane is close to zero below stage 4 and increases sharply at the top stages of the column. The component generation rates along the reaction section are shown in Figure 5B. As the disproportionation reactions are consecutive and reversible, the reactants TCS, DCS, and MCS are generally consumed on some stages and generated on other stages. Below stage 25 the dominant reaction is TCS disproportionation, represented by the generation of STC and DCS, while between stage 25 and 17 the disproportionation of DCS to TCS and MCS is obvious. Silane is generated mainly on the last three stages (stage 18−16) with DCS by the MCS dismutation where the concentration of MCS is high. On stages 26−28, two reactions of TCS and DCS disproportionation take place simultaneously, and on stages 17 and 18, those for DCS and MCS do the same way. The temperature in the reaction section varies from 37.5 to 95.8 °C, and is below the thermal resistance temperature level of the anion resin catalyst, i.e., 100 °C, which can be realized by adjusting the column pressure, in order for a reasonable catalytic activity and longevity as well. However, the operational temperature at the top stage of the column is as low as −78 °C, which means that the overhead condenser must be run at a very low temperature level, and a particular cascade refrigerator is required with a small coefficient of performance (COP) and a high electric power expenditure.33 For the calculated case, the refrigerating capacity of the top condenser is as high as 535 kW, corresponding to a value of 6.69 kWh per kilogram of silane produced. The refrigerator is generally operated at the temperature level of the order of −90 °C, with a theoretical Carnot cycle COP of about 1.4, and a practical COP less than 0.5. Thus the electric power consumption used for the vapor compression of the coolant recycling in the cascade refrigerator must be high up to 13 kWh/kg silane. 3.2. RD Column with One Intercondenser. To reduce the refrigeration load and the electric power consumption of the top condenser, one intermediate condenser is introduced between the reaction section and rectifying section to remove most of the heat of condensation at a higher temperature level, as proposed by Müller et al.23 and Block et al.,24 and shown in Figure 6. Owing to the intercondenser, the ascending heavy components (TCS, DCS) are condensed and return to the reaction section before accessing the rectifying section, and the mixture is enriched with silane and ascend into the rectifying section for further concentration. As the function of the rectifying section is to purify the lightest component silane, and a sharp difference exsists of boiling points between the silane (−112 °C) and the other components (−30 °C for MCS, 8.3

stage models should be more precise for its possibility to eliminate the phenomena of multiple steady states and its ability to take the influence of hardware design into account.31,32 Good agreement between the two kinds of models is achieved when correct Murphree stage efficiencies are assigned, yet they are difficult to predict for a multicomponent system. Despite its fine accuracy, the nonequilibrium stage model is relatively complex in mathematics and requires detailed design of column internals. As this work was a preliminary study to find the feasibility and the guideline for the reactive distillation process, the EQ model was preferred. With the existing algorithms robust and flexible enough to handle RD simulation, the RadFrac module of Aspen Plus was chosen to implement the computations of steady states involving trichlorosilane disproportionation with the components, kinetics, and phase equilibrium model mentioned above. For simplification, the reactions were specified to be pseudohomogeneous in liquid phase. The objective of our simulation was to obtain silane with a purity over 99% (mol) and to cut down the energy cost in the RD mode. The typical RD scheme was simulated with a 60-stages RD column, where the top condenser was defined as stage 1 while the reboiler was as stage 60. The reaction section was from stage 16 to stage 45, with a liquid residence time of 2.5 s on each stage which is defined as hold-up/liquid-flow-rate. The column is operated at a top pressure of 5 atm with a pressure drop through each stage of 0.5 kPa. The distillate to feed ratio is defined as 0.25 based on mole. TCS at 10 kmol/h was fed into the column on stage 46 at 50 °C and 5.5 atm.

3. RESULT AND DISCUSSION 3.1. Conventional RD Column. Initially, the reflux ratio was determined by a sensitivity analysis that investigated the influence of the reflux ratio on the purity of product. The results are shown in Figure 4. Under the conditions mentioned

Figure 4. Effect of reflux ratio on purity of product.

above, when the reflux ratio is smaller than 25, silane is barely produced because of the very low reaction rates for the three consecutive reversible reactions. If the reflux ratio is increased further from 25, one can obtain silane from the reactive distillation column, and a 99.30% (mol) purity of silane can be extracted from the overhead when thereflux ratio is up to 63. As shown in Figure 4, the purity of STC, which can be discharged as the bottom product, is always higher than that of silane, 6214



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Figure 5. Profiles of preliminary scheme. (A) liquid compositions; (B) reaction rates.

rectifying section to purify silane, and only a small reflux ratio is required. For the studied case, the reflux ratio is set to unity. Simulation shows that the purity of silane can reach as high as 99.30%, equal to that of the preliminary case when the duty of intercondenser is defined as 508.5 kW by a trial and error method. The load of the top condenser, which is proportional to the reflux ratio, is reduced to 16.3 kW, about only 3% of the typical column without intercondenser as above-mentioned. Although the total refrigeration capacities of the RD columns are almost the same, the temperature of intercondenser is just about −13 °C, much higher than that of the top condenser, −78 °C, which will make the refrigerator for the intercondenser show a much higher COP than the top one. The Carnot cycle COP will increase to 3.8 at a condensation temperature of −25 °C and a room temperature of 40 °C. Consequently, the column with an intercondenser can save a lot of electric power, compared with the typical column. As shown in Figure 7, the concentration of silane on stages 5−15 is about 12%, much larger than that of the preliminary case without the intercondenser, in which near zero concentration of silane can be noticed. But the composition profiles in the reaction section and stripping section of both cases are almost the same, which reveals that the intercondenser does not affect the reaction but concentrates the silane in rectifying section.

Figure 6. RD process with one intercondenser.

°C for DCS, 31.85 °C for TCS, and 56.85 °C for STC at the atomerspheric pressure), it is therefore very easy for the

Figure 7. Profiles of scheme with one intercondenser. (A) liquid compositions; (B) generation rates. 6215



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3.3. RD Column with Two Intercondensers. Although the use of the intercondenser has significantly improved the energy efficiency, the refrigeration capacity is still large at a relatively low temperature level. By analogy, another intercondenser is added to reduce the duty of the first intercondenser, as shown in Figure 8, which is placed on stage

a little because the temperature of the reboiler is as low as 121 °C and the cheap low-pressure steam is competent as the heating medium. In a word, the energy cost can be reduced furthermore by adding the second intercondenser. Since the second intercondenser is placed inside the reaction section, the profiles in the reaction sections are a little different from those of column with one intercondenser. The peaks of the composition curves of DCS and TCS shift to lower stages. MCS begins to show apparent concentration on stage 26, while without the second intercondenser it appears from stage 22. But the composition distributions in the rectifying section are almost the same, which indicates that the second intercondenser mainly affects the reaction and stripping sections, and has little impact on rectifying section. As shown in Figure 9, TCS disproportionation takes place mainly below the second intercondenser, although the concentration of TCS is still large above this stage. The disproportionation reactions of DCS and MCS are slower and thus occur on more stages. The abrupt change of reaction rate profiles between stages 24 and 25 is a result of the different liquid flow rate caused by the intermediate condensation. 3.4. Comparison of the Three Schemes. As discussed before, the disproportionation of TCS can be carried out through any one of the three processes and the high purity of silane is achievable. Using the first intercondenser between the rectifying and reaction sections can significantly reduce the operation cost by increase cooling temperature from −78 °C to about −15 °C, thus avoiding very-low-temperature refrigeration. The cooling temperature of second intercondenser, which is within the reaction section, is high enough to use a normal heat exchanger with a further reduction in the energy cost. More intercondensers could be arranged, but it will increase the cost of installation. Generally one or two intercondensers are most rational. Figure 10 presents the temperature profile comparison along the column for the three schemes. Obviously the reaction temperature profiles are reasonable from the viewpoint of both reaction activity and catalyst lifetime. The use of the first intercondenser did not change the temperature distribution of reaction and stripping sections, but lowered the temperature in the rectifying section as the concentration of silane, the most volatile component, is increased. On the contrary, the use of the second intercondenser did not change the temperature in rectifying section compared to the case with the first intercondenser, but lowered the temperature between stages 16 and 30 of the reaction section slightly.

Figure 8. RD process with two intercondensers.

25 and divides the reaction section into two parts. The duty of first intercondenser is arbitrarily set to 250 kW, as half as the one intercondenser case. The mole fraction of silane in the overhead stream is also as high as 99.30%, approximating the results of the above studied cases without and with one intercondenser, when the calculated condensation duty of the second intercondenser is 320.5 kW. The corresponding condensation temperature is 65 °C, implying that the normal water is adequate without an extra refrigeration, thus expending little additional electricity. The duties and the temperatures of the top condenser and the first intercondenser are almost the same as the above cases. Though the reboiler duty increases from 568 kW for the former two cases to 628 kW for this case, the heating cost will increase only

Figure 9. Profiles of the scheme with two intercondensers. 6216



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locations and duties affect the performance of the RD column. Based on the RD column with one intercondenser, four cases have been conducted to study the effect of the intercondenser location. In case 1, the intercondenser is in the middle of the rectifying section at stage 10, and in case 2 it is between the rectifying section and the reaction section at stage 15, as discussed above. In cases 3 and 4, the intercondensers are located at stage 20 with different condensation duties. Cases 1− 3 have the same duty. The calculated results are displayed in Table 3. It can be seen that the higher the location of the condenser is the higher the Table 3. Effect of Intermediate Condenser Location case no. 1 2 3 4

Figure 10. Comparison of temperature distribution.

Figure 11 shows the liquid and vapor flow rate along the column. As shown in this figure, the flow rate of column without intercondenser varies within a narrow range and the corresponding column diameter will be constant. However, owing to the condensation of first intercondenser, the flow rate of the rectifying section is very small, while that in the other sections is significantly large. When adding a second intercondenser, the profile of flow rate in the rectifying section is the same, but in the reaction section it is divided into two intervals. So the diameter of the column will no longer be uniform and is divided into different segments by intercondensers. For a structured packing column packed with the MELLAPAK 500X, a trademark by Sulzer, the diameters of the three sections estimated by the Packing Sizing function of Aspen Plus are 0.11, 0.50, and 0.82 m, respectively. From stage 25 to stage 60, the vapor flow rate of the column with two intercondensers is larger than that of other two cases due to the increased total condensation. As the flow rates are decreased between the two intercondensers, those below the second intercondenser are required to increase to guarantee the same reaction conversion, resulting in an increase in the total condensation and reboiler duties. 3.5. Parametric Studies. Effect of Intercondensers. It has been expounded that the intercondensers can significantly lower the energy cost, but one question arises of how their

condenser location stage stage stage stage

10 15 20 20

cooling duty (kW)

refrigeration temperature (°C)

purity of silane

508.5 508.5 508.5 577.0

−19.46 −12.48 13.58 −8.71

99.98% 99.31% 77.94% 99.31%

silane purity and the lower the condensation temperature will be. Generally, if the intercondenser is placed within the rectifying section, the purity of silane can be improved slightly. In addition, when the intercondenser is placed within the reaction section, the cooling temperature is elevated, and more heat needs to be removed to acquire the same purity of silane as demonstrated in cases 3 and 4. Besides, as shown in Figure 12, the temperature above the intercondenser is below 0 °C, and the activity of the catalyst on stage 16−20 will be rather low in cases 3 and 4. In a word, the best location of this intercondenser should be between the reaction section and the rectifying section. For the scheme with two intercondensers, the location effect of the second intercondenser was studied by placing the first intercondenser at its best position, on stage 25. From Figure 13, one notices how the location of the second intercondenser affects its cooling duty and temperature under the precondition that the purity of silane is fixed to 99.3%. When its location is lower, more heat needs to be removed by the intercondenser, but the temperature of the cooling stage will be higher, which makes the removal of heat easier. Stages 22−26 are rational locations for the second intercondenser for both the comparatively low duty and the relatively high cooling

Figure 11. Comparison of liquid and vapor flow rate: (A) liquid phase, (B) vapor phase. 6217



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because more unreacted chlorosilanes is refluxed into reaction zones when more heat is removed. It is found that the contour lines, that is, the intersecting lines of horizontal plane and the surface, are straight and parallel, which shows a linear relationship between the duties of two intercondensers. It is possible to further reduce the duty of the first intercondenser by increasing the duty of second one. Effect of Pressure. In a reactive distillation column, operation pressure cannot only influence the separation performance but also affect the reaction rate and chemical equilibrium. Increasing pressure will raise the temperature of reactive stages and fasten the reactions. Besides, the cooling temperature of the first intercondenser will also be increased at a higher pressure. However, the deactivation of the catalyst will be accelerated under excessively high temperature when it surpasses the thermal resistance level. As it is difficult to remove and replace the catalyst in RD column, attention should be paid to the deactivition of the catalyst when selecting operation pressure. Figure 15 shows the effect of pressure on the purity of silane and reaction temperature. As pressure increases, the reaction

Figure 12. Effect of intercondenser location on temperature profile.

Figure 13. Effect of the location of second intercondenser on its cooling temperature and duty. Figure 15. Effects of pressure on silane purity and reaction temperature.

temperature. When altering the location of second intercondenser, the cooling temperatures of the first one is varied from −12.50 °C to −15.73 °C, which is a negligible variation. Figure 14 is a three-dimensional surface that exhibits how the purity of silane changes with the duties of both intercondensers. Within the scope considered here, increasing the duty of either intercondenser is always positively effective to improve silane purity, especially when the purity is less than 90%. This is

temperature is elevated. The highest reaction temperature at the lower end of the reaction section will reach 140 °C, and the corresponding lowest reaction temperature at the upper end is 60 °C when the column is operated under 10.0 atm at the top. The purity of silane is improved when pressure varies from 5.0 to 6.5 atm as the reaction rate is sped up by elevated reaction temperature. But it decreases slightly when pressure exceeds 7.0 atm, as the increase in reaction temperature will decrease the equilibrium conversion of the exothermic and reversible MCS disproportionation reaction. Effect of Reflux Ratio. In a reactive distillation column, reflux flow can enhance not only the separation but also the reaction by recycling unconverted reactants back into the reaction section. Figure 16 displays the effect of the reflux ratio on the purity of silane and the condensation temperature of the first intercondenser . As expected, when increasing reflux ratio, the purity of silane will be improved, especially when the reflux ratio is below 1.0. However, as the silane is a much higher volatile compared with the rest of the components and the most condensation heat has been taken off by the intercondensers, the purity of silane is still as high as 97.4% (mol) even if the reflux ratio is as small as 0.1. A smaller reflux ratio will result in smaller cooling duty of the top condenser,

Figure 14. Effects of the intercondensers’ duties on silane purity. 6218



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stage number, other measures can also be taken to ensure the purity of silane when residence time is lowered. For instance, increasing the duties of the intercondensers and the reboiler.

4. CONCLUSION This work has probed into the RD column applied to a unique system of trichlorosilane disproportionation to silane and silicon tetrachloride with unfavorable reaction equilibria but favorable phase equilibria by using Aspen Plus. Hereby we conclude as follows. Simulation results show that it is feasible to obtain a high purity of silane over 99% and a complete conversion of trichlorosilane to silane and silicon tetrachloride by a typical reactive distillation column. Because of the exorbitantly low condensation temperature (nearly −80 °C) and the excessively high reflux ratio (more than 60) at the column top, an intercondenser was introduced between the rectifying and reaction sections to reduce the refrigeration requirement. By this means the reflux ratio was decreased to be lower than unity, resulting in a 97% reduction of the overhead refrigeration load. A second intercondenser was added within the reaction section with a further 50% reduction of the condensation load for the first intercondenser. As a result, the overall process economy can be improved significantly. Parametric studies display that the optimal location of the two intercondensers is between the rectifying and the reaction sections and within stages 22−26, respectively. The temperature, especially in the catalytic reaction section, can be adjusted by the pressure, which is of the order of 5 atm. Moreover, an optimal reflux ratio can be found by taking the total energy cost into account, and to a great extent, the optimal stage number is defined by residence time, rationally between 2.0 and 4.0 s.

Figure 16. Effect of reflux ratio on silane purity and refrigeration temperature.

but it will also lead to lower condensation temperature of the intercondenser as shown in Figure 16, and thus increase the corresponding electric power consumption. The reflux ratio should be optimized by taking the total energy cost into account for the further study. Effect of Residence Time. The above-mentioned calculations are based on the hypothesis that the residence time on each stage in the reaction section is conformably 2.5 s, but actually the liquid hold up and HETP (height equivalent to theoretical plate) of a packed tower depend on the internals and operation conditions, and this assumption may not be the practical situation. Figure 17 shows the relationship between residence

■

AUTHOR INFORMATION

Corresponding Author

*Tel/Fax: +86 02134203788. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■ Figure 17. The effect of reaction stage number on required residence time.

■

time and total reaction stage number to obtain 99.3% silane, where the ratio of the two parts of the reaction section divided by the second intercondenser is 1:2. It is seen that if residence time is decreased, more reaction stages are demanded to meet the requirement of purity. Especially when residence time varies between 2 and 4 s, a rational range, the required stage number changes rapidly, which reveals a large influence. Since the disproportionation reactions are consecutive and reversible, the maximal purity of silane on stages can be only obtained when chemical equilibrium is reached. However, less residence time counts against reaching chemical equilibrium on stages and as a consequence lowers the purity of silane. Besides increasing

NOTATION RD = reactive distillation STC = silicon tetrachloride TCS = trichlorosilane DCS = dichlorosilane MCS = monochlorosilane ri = reaction rate of reaction i ki = rate constant of reaction i Ki = equilibrium of reaction i xj = mole fraction of component j kjl = binary coefficient of components j and l REFERENCES

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