Novel Procedure for the Synthesis of Dimethyl Carbonate by Reactive

Jan 29, 2014 - The energy integration in a distillation column provides a large energy savings. With the narrower temperature difference between the t...
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Novel Procedure for the Synthesis of Dimethyl Carbonate by Reactive Distillation Zhixian Huang, Junlan Li, Lieyun Wang, Haiming Jiang, and Ting Qiu* College of Chemistry and Chemical Engineering, Fuzhou University, Fuzhou 350108, China ABSTRACT: A novel energy saving procedure for the synthesis and separation of dimethyl carbonate by reactive distillation is proposed, and its principle technical feasibility is demonstrated based on the physical properties of a dimethyl carbonate synthesis system. The steady-state simulation is performed with Aspen Plus, and the UNIQUAC-RK model is used in this work. The simulation result shows that the novel procedure can simplify the process and reduce energy consumption. The new procedure can save energy costs by 29.50% compared with the traditional processes. For further energy consumption reduction, heat integrations between the low-pressure column and the high-pressure column are adopted. It is found that energy savings of 53.80% are achieved for this novel process. The total annual costs of the four procedures are estimated and compared. It is shown that costs can be reduced by more than 31.54% by using the new processes. To sum up, the new procedure has a better economic investment than the original.

1. INTRODUCTION Dimethyl carbonate (DMC) is considered to be an environmental friendly chemical product because of its negligible ecotoxicity and its biodegradability. It is widely used in the chemistry industry due to its versatile chemical properties. Because the molecular structure of DMC contains several active groups, such as methyl, carbonyl, and methoxyl, it is usually employed as a methylation reagent to substitute extremely toxic dimethyl sulfate or methyl halides and as a “green” alternative to acylating agents.1 It also can be used as a solvent for the production of electrolytes for lithium-ion batteries.2 In addition, its utilization as an oxygenated fuel additive in gasoline or diesel oil replacing methyl tert-butyl ether has been discussed.3 Several methods available for the synthesis of DMC4−6 have been reported, such as phosgenation of methanol, oxidative carbonylation of methanol, the transesterification method, and the esterification of carbon dioxide with methanol. However, these routes suffer from the obvious drawbacks of, for example, being poisonous, being an explosive complex, and having extremely low conversion.7 In previous work, the production of DMC by urea methanolysis was found to be a green method with the advantage of its byproduct ammonia being recycled to produce urea.4 The traditional process for the synthesis of DMC is by the transesterification of propylene carbonate (PC) or ethylene carbonate with methanol (MeOH).8 Similar to the esterification reaction, the transesterification reaction is limited by thermodynamic equilibrium, which leads to a low conversion. Reactive distillation (RD) is a good choice for equilibrium limited reactions because it can enhance the conversion by continually removing the product from the reaction system.9 Recently, the attractive method of the synthesis of DMC by the transesterification of PC with MeOH has been conducted in a reactive distillation. However, the most abundant product of the reactive distillation column is a mixture of DMC and MeOH, which forms a minimum-boiling homogeneous azeotrope. To obtain pure DMC product, this azeotropic mixture requires further separation. The processes of azeotrope separation for this system © 2014 American Chemical Society

have been extensively studied including extractive distillation, membrane pervaporation,10 low-temperature crystallization, and pressure-swing distillation.11 Of these methods, the extractive distillation method is effective if a suitable solvent can be found. It consumes less energy but may cause environmental concerns. Conversely, the pressure-swing distillation method is an environmentally friendly process, without adding a separating agent but leading to high energy cost. With an overview of the previous research, most of the studies focus on the modeling, design, control, and optimization of the separation process of the DMC/MeOH azeotrope.11,12 Li et al. and Wang et al. have proposed a pressured-atmospheric series separation process based on the UNIQUAC and NRTL equations, respectively.12,13 Zhang et al. has also investigated the process model for atmospheric-pressured rectification to separate the azeotrope (DMC/MeOH).14 Wei et al. explored the design and control of pressure-swing distillation systems for the separation of DMC/MeOH and proposed an optimized separation configuration based on the global economic analysis.15 However, the integration of the reactive distillation and the separation of the DMC/MeOH azeotrope, though achieving energy savings, are seldom researched. In this work, the process of the synthesis of DMC by the transesterification of PC with MeOH is studied. The UNIQUAC-RK physical properties are selected in the Aspen Plus simulations, and a simulation of the DMC synthesis process is performed. The RADFRAC module, based on the rigorous equilibrium stage model, is used to describe the multistage vapor−liquid transfer in the columns. For the purpose of saving energy, a novel process is put forward based on the traditional pressure-swing distillation. The heat integrations of both the traditional and novel processes are performed, and the economic Received: Revised: Accepted: Published: 3321

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Figure 1. Flowsheet of case 1the traditional process of the DMC synthesis system (operating pressure: RD, 101.3 kPa; HP, 744.3 kPa; LP, 101.3 kPa).

pressures.17 Under atmospheric pressure, the azeotropic contains 70 wt % MeOH and 30 wt % DMC. Table 1 shows that the content of DMC in the azeotrope gradually decreases as the pressure increases. The content of MeOH in the azeotrope increases from 70 to 93 wt % as the pressure varies from 101.3 to 1500 kPa. In the traditional process, the distillate product from the RD column is fed into the HP column to permit the production of DMC from the bottom and the high-pressure azeotrope from the top of this HP column. Taking these findings into account, a method for directly recycling the distillation product from the HP column back to the RD column is assumed. The stream contains the product of the transesterification, DMC, which may have an adverse effect on the performance of the RD column. The transesterification reaction is reversible; therefore, the rate of the forward reaction decreases as the production of the product increases. However, the amount of recycled DMC is very small compared to the DMC yield, and the effect of the recycled stream on the performance of the RD column may be neglected, especially because the HP column is operated at higher pressure (≥1100 kPa). Based on this, the LP column in the pressure-swing distillation system could be removed, and the energy of the system would decrease accordingly. Furthermore, there is the possibility of heat integration between the HP column and the LP column. In this case, the heat of the condenser of the HP column is used for heating the RD column. Therefore, to save energy, a novel process for the production of DMC is proposed based on pressure-swing distillation. The process only contains the RD column and the HP column, as shown in Figure 2. Compared with the traditional process, does the existence of the product (DMC) in the feed of the RD column decrease the conversion of PC? Can the novel process save energy? Therefore, the simulations of the process, including the traditional and the novel process, will be performed in later sections.

performances of the processes are evaluated through the comparison of their total annualized costs (TACs).

2. FEASIBILITY ANALYSIS The traditional process of DMC synthesis system includes two parts: reactive distillation and pressure-swing distillation, as shown in Figure 1. PC reactant and a homogeneous catalyst (sodium methoxide solution) are fed into the RD column, operated at atmospheric pressure, from the top of the reactive zone. MeOH reactant is fed into the bottom of the reactive zone. The distillate of the RD column is an azeotropic mixture of DMC and unreacted methanol, and the bottom of the RD column is mainly a mixture of propylene glycol (PG) and MeOH, which is further separated by a simple column to obtain the desired PG product. The azeotrope is then introduced into a pressure-swing distillation system consisting of a high-pressure (HP) column and a low-pressure (LP) column. In the HP column, DMC is obtained at the column base with a purity of >99.7 mol %. The mixture of MeOH and DMC, which is rich in MeOH, is distillated from the top and fed into the LP column. High-purity methanol is withdrawn from the bottom of the LP column and recycled back to the fresh methanol feed location in the RD column. The distillate of the LP column, containing MeOH and DMC, is recycled back to the HP column. For convenience, we named this process case 1. Figure 1 shows the material balance obtained from Feiyang Chemical Co., Ltd., Shandong, China, based on this process. The mixture of DMC and MeOH exhibits a minimum-boiling azeotrope. It has the same compositions for both the vapor and liquid phases. Table 1 shows the experimental methanol composition of the azeotrope MeOH/DMC at different Table 1. Experimental Methanol Composition of the Azeotrope MeOH/DMC at Different Pressures P (kPa)

T (K)

xMeOH

wMeOH

101.3 405.2 607.8 1013.0 1519.5

337.35 377.15 391.15 411.15 428.15

0.8677 0.9150 0.9298 0.9521 0.9739

0.7000 0.7929 0.8249 0.8761 0.9300

3. MODELING AND SIMULATION Distillation is an extremely complex process; it consists of the mass transfer process and the separation process. Reactive distillation also depends on the reaction process. In this work, the steady-state simulation of the synthesis process of DMC in an RD column was conducted by Aspen Plus. 3322

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rPC = k+c PCc MeOH − k −

3.1. Thermodynamic Model and Data Regression. There are four components (MeOH, PC, DMC, and PG) in the system, among which DMC/MeOH form an azeotrope. Allowing for the highly nonideal nature of the quaternary system, the selection of the physical property is particularly important. The activity coefficients model is necessary for the liquid-phase nonidealities. In this work, the vapor−liquid equilibrium (VLE) is described by the use of a UNIQUAC model for the liquid phase and an RK model for the vapor phase. To obtain the UNIQUAC model parameters of the system, regression has been performed based on the experimental vapor−liquid equilibrium data. The experimental data for the MeOH/DMC pair were achieved from Shi et al.17 and Luo et al.18 The VLE data of the two pairs of DMC/PC and DMC/PG were obtained from Luo et al.19 The PG/PC pair experimental data were obtained from Mathuni et al.20 By utilizing the UNIFAC group contribution method, the estimation of the binary interaction parameters of MeOH/PC was committed. The model parameters of MeOH/ PG and the other physical properties were obtained from the Aspen Plus databank. The resulting parameter set can be observed in Table 2. The comparison of the regression result of Table 2. Binary Interaction Parameters aij and bij of the UNIQUAC Model MeOH

DMC

DMC

PG

MeOH

j

DMC

PC

PG

PC

PG

PC

aij aji bij bij

−0.201 0.273 14.87 306.55

0 0 172.470 −216.199

0 0 −209.529 −9.027

0 0 −290.492 59.416

0 0 −301.778 142.266

0 0 49.94 −244.595

(1)

⎛ 41373.5 ⎞ ⎟ k+ = 16551.8 exp⎜ ⎝ RT ⎠

(2)

⎛ 28285.5 ⎞ ⎟ k − = 19254.75 exp⎜ ⎝ RT ⎠

(3)

where rPC is the reaction rate of PC (L·mol−1·min−1) and ci is the concentration of the ith component (mol·L−1). The kinetic expression was obtained by the experiment catalyzed by sodium methylate as its concentration varied from 0.15 to 0.3 wt %. Due to the very low concentration, the catalyst component was ignored in the simulation for simplification. 3.3. Simulation. First, the appropriate module should be chosen. The RADFRAC module is based on the rigorous equilibrium stage model and contains the mass balance, phase equilibrium, summation and energy balance equations. Due to the complexity of distillation and reactive distillation, the RADFRAC module was chosen to describe the multistage vapor−liquid separation. The equilibrium stage model, used in this work, has often been applied with great success for the simulation of RD columns. To simplify the calculation, the data of the pressure drop in the columns achieved from the plant test were employed. 3.3.1. Detailed Description of the Process. The traditional process (Figure 1) consists of three columns: RD column, HP column, and LP column. With the RD column, there is no side reaction in the RD system. The initial data used were obtained from Feiyang Chemical Co., Ld. The mixture of the PC and MeOH reactants with a flow rate of 5700 kg/h and a homogeneous catalyst (sodium methoxide, soluble in MeOH) is fed into a 35-stage reactive distillation column at stage 10. For the stream, almost all MeOH from the bottom of the LP column with a flow rate of 5055 kg/h and fresh MeOH with a flow rate of 1645 kg/h were fed into stage 30 of the RD column. The RD column has 10 stages (including a total condenser) in the rectifying section. The reactive zone of the column is from 10 to 30 stages and is followed by a stripping section with five stages (including a partial reboiler). In order to ensure that there is a high liquid holdup and liquid-phase residence time in the reactive section, the weir for each tray is 0.055 m high in agreement with the suggestion made by Krishna.22 So a reaction volume of 150 L is assumed in every stage of the reactive zone. The column is operated at 101.3 kPa. In the study, some parameters of the column geometry, such as the diameter of the column, were calculated using the sizing function provided by the Aspen Plus simulator. For the HP column, the stage number of the HP column is 20 (including a total condenser and a partial reboiler). A mixture of DMC and MeOH with a flow rate of 15080 kg/h is fed into the HP column at stage 6. The rectifying section is from 2 to 6 stages, and the stripping section from 6 to 19 stages. The column is operated at 744.3 kPa. For the LP column, the LP column constitutes 27 theoretical stages. A mixture of DMC and MeOH from the top of the HP column is fed into the LP column at stage 8 with a flow rate of 12935.3 kg/h. In the simulation of the novel process (case 2), the specifications of the reactive distillation column are designed according to the reactive distillation column of case 1, except the reaction volume in every stage of the reactive zone is assumed to be 200 L. The HP column contains 23 theoretical stages and consists of a total condenser at stage one and a partial reboiler at

Figure 2. Flowsheet of case 2novel procedure for the synthesis and separation of DMC (operating pressure: RD, 101.3 kPa; HP, 1100 kPa).

i

c DMCc PG c MeOH

MeOH

four pairs with the experimental data is shown in Figure 3. The UNIQUAC model is suitable for the azeotrope of the system and for predicting the VLE behavior. 3.2. Kinetic Model. The transesterification of propylene carbonate with methanol has been studied using sodium methoxide as a catalyst.21 The reversible reaction and the kinetics equation of this reaction can be expressed as follows:

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Figure 3. Experimental and estimated y−x and T−x−y data at 101.3 kPa: (▲,■) experimental data from the literature; (−) estimated data by the UNIQUAC correlated model. (a) MeOH and DMC; (b) DMC and PC; (c) DMC and PG; (d) PG and PC.

the last stage. Meanwhile, the rectifying section included stages 2−10, and the stripping section includes stages10−22. The mixture of DMC and MeOH, from the top of the RD column with a flow rate of 9300 kg/h, is fed into the HP column at stage

10. The column is operated at 1100 kPa. The column specifications of case 1 and case 2 are presented in Table 3. 3.3.2. Simulation Result. The diameters of all of the columns in Table 3 were determined by using the “traying sizing” function 3324

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Table 3. Column Specifications for Case 1 and Case 2 case 1 total stage rectifying section reactive section stripping section feed stage feed temp (K) reflux ratio diameter (m)

Table 5. Simulation Results of Case 2

case 2

parameters

RD

HP

LP

RD

HP

35 2−10 10−30 30−34 10 30 333 2.34 1.84

20 2−6

27 2−8

23 2−10

6−19 6

8−26 8

363 1.5 2.13

333 2.5 2.44

35 2−10 10−30 30−34 10 30 333 2.34 2.13

condense duty (MW) reboiler duty (MW) distillate flow (kg/h) distillate composition (wt %) MeOH DMC PG PC bottoms flow (kg/h) bottoms composition (wt %) MeOH DMC PG PC conversion of PC (wt %)

10−22 10 383 2.5 2.13

of Aspen Plus. Table 4 shows the comparison of the plant experiments and the simulation data of the traditional process. Table 4. Comparison of the Plant Experiments and the Simulation Data of Case 1 RD

HP

parameters

expt

sim

sim

expt

sim

5.91

5.84

7.76

expt

7.67

6.85

6.80

6.12

6.07

8.22

8.10

6.89

6.85

7200

7200

12935

12935

7880

7880

70.35 29.65 0 0 5200

69.78 30.22 0 0 5200

82.37 17.63 0 0 2145

82.68 17.32 0 0 2145

71.13 28.87 0 0 5055

71.60 28.40 0 0 5055

62.64 0.88 35.04 1.44 97.01

63.35 1.99 35.13 1.32 97.26

0.23 99.77 0 0

0 100 0 0

99.90 0.10 0 0

99.94 0.056 0 0

HP 5.99 6.30 7150

71.11 28.89 0 0 4750

92.20 7.80 0 0 2150

59.95 0.26 38.10 1.69 96.80

0.95 99.05 0 0

the LP column. Energy integration is a simple and effective way to produce an energy saving process. In this work, therefore, heat integrations of the two processes are investigated to reduce energy consumption. 4.1. Heat Integration. In case1, the temperature of the overhead vapor from the HP column is 399 K, higher than that of the reboiler of the LP bottom. In addition, the condensing ability of HP is 7.76 MW, which is higher than that of the LP column reboiler, which is 6.89 MW. Therefore, heat can be exchanged between the reboiler of one column and the condenser of the other column. Figure 4a illustrates the heat transfer from the condenser in the HP column to the reboiler of the LP column and to the preheater of the HP column without violating the minimum approach temperature of 20 K used in this system. This heat-integrated process is named case 3. The heatintegrated flowsheet of the new process, case 4, is similar to case 3; that is, the HP column is placed overhead of the condenser and reboils the RD column. The flowsheets of the two heat integration processes are displayed in Figure 4. 4.2. Economic Evaluation. To evaluate the complete economic investment, the total annualized costs (TAC) of the four processes are calculated in this work. The TAC contains the utility cost and the annualized capital cost. The utility cost includes the steam cost for the reboiler and the cost of cooling the water. The capital cost includes the column cost and the heat exchanger (preheater, reboiler, and condenser) cost. Because the heat capacities of the streams vary radically over their temperature range, a uniform heat capacity assumption is not appropriate.24 Therefore, the temperature change and the heat duty of the streams were separately calculated by the HEATER module in Aspen Plus. Available utilities for this process are two heating utilities and one cooling utility. Heating utilities are high-pressure steam (1800 kPa), medium-pressure steam (1500 kPa), and low-pressure steam (400 kPa). The LP steam cost is $2.848/(1000 lb), the MP steam cost $3.616/(1000 lb), and the HP steam cost $3.745/(1000 lb) according to Douglas.25 A capital charge factor of 3 years is assumed in the calculation. The depreciation rate of the column, reboiler, condenser, and packing is 10%, and the annual plant operating time is 7200 h. The cost calculation method was referenced in the work by Douglas.25 Tables 6 and 7 show estimates of the economical efficiency measured by TAC. The comparison of the energy costs and the capital costs of the four cases is presented in Table 8. The energy

LP

condense duty (MW) reboiler duty (MW) distillate flow (kg/h) distillate composition (wt %) MeOH DMC PG PC bottoms flow (kg/h) bottoms composition (wt %) MeOH DMC PG PC conversion of PC (wt %)

RD 8.03 8.27 9300

From Table 4, it can be observed that the simulation results are consistent with the experimental data in the plant, which demonstrates that the simulation model presented is capable of describing the process quantitatively and can be used for further process evaluation. Table 5 shows the simulation result of the new process. The conversion of PC is deceased from 97.01% in case 1 to 96.80% in case 2 due to the existence of the DMC product in the fed stream. However, the compositions of the distillation and the bottoms of the RD column are similar in both case 1 and case 2. Furthermore, the reboiler duty of the new process (14.57 MW) is lower than that of the traditional process (21.24 MW). Therefore, the proposed process is feasible and preferable for the synthesis of DMC.

4. ENERGY SAVINGS In view of increasing energy costs, designing a distillation system that consumes less energy is very important.23 Generally, there is the possibility of heat integration between the HP column and 3325

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Figure 4. Flowsheet of case 3 and case 4: (a) the traditional synthesis process of DMC with heat integration; (b) the novel process of DMC synthesis system with heat integration.

Table 6. Annualized Cost of Case 1 and Case 2 case 1

case 2

TAC (USD/year)

RD

HP

LP

steam cost cooling water cost total utility cost case energy cost column cost preheater cost reboiler cost condenser cost capital cost total capital cost TAC

47275 3484 50759

89927 4569 94496 201619 134822 13438 201750 95665 445676 1238113 1439733

52329 4035 56364

138492 5553 94168 152367 390580

Table 7. Annualized Costs of Case 3 and Case 4

138087 96096 167673 401857

RD

case 3 HP

62524 70279 4732 3527 67256 73806 142134 163226 130993 5553 15158 114515 228624 185915 75111 469209 400307 919095 1061230

TAC (USD/year)

RD

HP

steam cost cooling water cost utility cost case energy cost column cost preheater cost reboiler cost condenser cost capital cost total capital cost TAC

47275 3484 50759

86903 341 87244 142038 134822 18074 201750 17706 372353 1190819 1332858

138492 5553 94168 152367 390580

case 4 LP 0 4035 4035 138087 122126 167673 427886

RD

HP

18128 70279 4732 0 22860 70279 93139 163226 130993 5553 15158 56787 228624 185915 106302 411481 431498 892558 985697

investment for the individual columns increased, for instance, in the LP column of case 1. Nevertheless, the capital costs of the heat-integrated processes are both lower than the base-case processes for the traditional and the novel method. In addition,

integration in a distillation column provides a large energy savings. With the narrower temperature difference between the top of the HP column and the base of the LP column, the heat exchanger needs a larger heat transfer area, and the capital 3326

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Table 8. Comparison of the Four Cases

case 1 case 2 case 3 case 4

energy cost (USD/year)

energy saving (%)

TAC (USD/year)

TAC saving (%)

201619 142134 142038 93139

0 29.50 29.55 53.80

1439733 1061230 1332858 985697

0 29.29 7.42 31.54

REFERENCES

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the energy cost is reduced by saving on the utilities consumption. On the whole, in the heat-integration processes, the TAC is actually lower than the base-case process. From the result of the calculation, it is found that the new procedure (case 2) can save 29.67% of energy costs compared with the traditional processes. Using heat integration reduces energy costs by 29.03% and 53.48% for case 3 and case 4, respectively. The data in Table 8 show that close to 34.95% savings in the total annual cost is achieved by using the new process with heat integration (case 4).

5. CONCLUSION In this work, the process of synthesis of DMC by reactive distillation and pressure-swing distillation is studied, and the physical properties of the system are analyzed. Based on these results, a novel energy saving procedure is put forward. Using Aspen Plus, the steady-state simulation of both the traditional and the proposed processes are performed. The UNIQUAC-RK model is chosen in this work. The binary interaction parameters of the physical model are obtained based on the experiment data in previous literature. The result of the steady-state simulation shows that the proposed new process is feasible and preferable for the synthesis of DMC. Through investigating the feasibility of the proposed process, this study presents a systematic analysis of energy consumption and cost. The potential energy integration leads to achieving energy savings of approximately 53.80% and savings of 31.54% of the total annual cost.

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support for this work from the National Natural Science Foundation of China (Grants 21306025 and 21176045) and the Natural Science of Fujian Province (Grant 2012J01040).

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NOMENCLATURE DMC = dimethyl carbonate TAC = total annualized costs (U.S. dollars per year (USD/ year)) PC = propylene carbonate MeOH = methanol RD = reactive distillation HP = high pressure LP = low pressure PG = propylene glycol r = reaction rate (L·mol−1·min−1) c = concentration (mol·L−1) k = kinetic constant (L·mol−1·min−1·g−1) R = gas constant (J·mol−1·K−1) 3327

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(24) Yoon, S.; Lee, J.; Park, S. Heat integration analysis for an industrial ethylbenzene plant using pinch analysis. Appl. Therm. Eng. 2007, 27, 886−893. (25) Douglas, J. M., Conceptual design of chemical processes; McGrawHill: New York, 1988.

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