Synthesis of Diethyl Carbonate by the Combined Process of

2 Jan 2012 - A new process that partially integrates reaction and distillation was developed to enhance the transesterification of ethylene carbonate ...
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Synthesis of Diethyl Carbonate by the Combined Process of Transesterification with Distillation Peng Qiu, Lu Wang, Xuedong Jiang, and Bolun Yang* Department of Chemical Engineering, State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, People’s Republic of China ABSTRACT: A new process that partially integrates reaction and distillation was developed to enhance the transesterification of ethylene carbonate (EC) with ethanol for the production of diethyl carbonate (DEC). Sodium ethoxide was used as a homogeneous catalyst. The top of a three-necked flask reactor was connected to a distillation apparatus equipped with an independent reboiler and θ ring packing to achieve the separation of DEC and ethanol that was returned back to the reactor. The effects of process variables, such as reactant ratio, reflux ratio, reboiler duty, and catalyst concentration, on the DEC production were investigated to obtain the optimum operating conditions. Experimental results indicate that the DEC product can be removed from the reactor and purified by distillation operation during the course of reaction using the proposed process. Under the optimum operating conditions, which are ethanol/EC mole ratio of 4.5, reflux ratio of 1.5, reboiler duty of 124 W, and catalyst concentration of 0.4 wt %, the yield of DEC can reach 91% and the DEC purity can reach 97 wt %.

1. INTRODUCTION Diethyl carbonate (DEC), an environmentally friendly and biodegradable chemical, has been considered as a promising fuel additive to replace methyl tert-butyl ether (MTBE).1 Its oxygen content is higher than that of MTBE (40.6 versus 18.2%). Engine tests show that 5 wt % DEC in diesel fuel can reduce particulate emissions by up to 50%.1 In comparison to two other potential fuel additives, dimethyl carbonate and ethanol, DEC has a higher energy content, lower vapor pressure, and more favorable gasoline/water distribution coefficients.2 Furthermore, when released into the environment, DEC could slowly hydrolyze into carbon dioxide and ethanol and, thus, has little impact on the environment.3 In addition, DEC can be used as an ethylating reagent in organic synthesis4 and used as an electrolyte for lithium ion batteries.5 Thus, much attention has been paid to the synthesis of DEC. Several chemical routes have been reported for the preparation of DEC: the phosgene route,6 oxidative carbonylation of ethanol,1,7 reaction between ethyl nitrite and carbon monoxide,8 and alcoholysis of urea.9 The major drawback of these routes is either the use of poisonous gases (phosgene, ethyl nitrite, and carbon monoxide) or the low yield of DEC. One promising method to produce DEC is the transesterification of ethylene carbonate (EC) with ethanol. The major advantage of this reaction is that both EC and ethanol are low toxic substances, which means a safe operation. The reaction equation is shown as follows:

ethanol/EC mole ratio is 8 and 10, the equilibrium yield of DEC is about 58 and 65%, respectively.10,11 Therefore, it would be desirable to develop process intensification technologies to promote the formation of DEC. Reactive distillation (RD), an excellent example of process intensification, has been successfully applied to several esterification12−14 and transesterification reactions.15 Unfortunately, RD is not feasible for every reversible reaction. Its application is limited by several constraints.16 A major limitation for the application of RD is the relative volatilities of the components.17 RD requires that the reactants and products must have suitable volatilities, so that the reactants can be maintained in the reaction zone and the products can be easily removed from the column. This means that the products are the lightest or heaviest components in an ideal reactive distillation system. However, the normal boiling points of each component in the current reaction system are in the order: ethanol (351 K) < DEC (399 K) < EG (471 K) < EC (521 K). One can note that the two reactants, ethanol and EC, are the lightest and heaviest components, respectively, and the normal boiling point differences between reactants and products are large (48 and 50 K). This fact means that the reactants of ethanol and EC are much more easily removed from the reaction zone in a reactive distillation column than the products. Therefore, it is unfavorable for the transesterification of EC with ethanol if a common reactive distillation system is used. In this study, we developed a new combined process, in which a tank reactor is connected to the middle of a distillation apparatus that was packed with θ ring and equipped with an independent reboiler and condenser. In this process, ethanol and DEC, the lightest components of the reaction system, are preferentially vaporized from the reactor and enter the distillation

Thermodynamic analysis indicates that this is a reversible exothermic reaction. The standard enthalpy change of this reaction is −0.7 kJ/mol, and the reaction equilibrium constant is only about 0.23 at 353 K. Thus, the formation of DEC is severely constrained by equilibrium. For instance, when the © 2012 American Chemical Society

Received: November 7, 2011 Revised: December 30, 2011 Published: January 2, 2012 1254

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heated to boil. The reaction mixture vapor, which mainly contains ethanol and DEC, continuously entered the distillation column and condensed into liquid. Once the liquid started to drop into the reboiler, the heating mantle was turned on. At the same time, the reflux ratio was adjusted to the desired setting. During the course of the experiment, liquid samples were periodically taken from the reactor, reboiler, and distillate stream. The samples were analyzed with a gas chromatograph (GC9790, Fuli Analytical Instrument Co. Ltd., Zhejiang, China) equipped with a flame-ionization detector (FID) and a KB-5 capillary column (30 m × 0.32 mm × 0.25 μm). The injector and detector were maintained at temperatures of 543 and 573 K, respectively. To obtain good separation, the column oven temperature was controlled using two different temperature programs depending upon the samples. For samples taken from the reboiler and distillate stream, the oven temperature was maintained constantly at 403 K. For samples taken from the reactor, the oven temperature was held at 403 K for 2 min and then ramped at 30 K/min to 453 K. After the experiment was finished, the reactor and reboiler were cooled to room temperature. The liquids in the reactor and reboiler were weighed and analyzed by gas chromatography to make sure the mass balance. The DEC yield was calculated by the following equation:

apparatus. Then, ethanol is separated from DEC by distillation and returned into the reactor to participate in the reaction, while DEC is concentrated and collected in the reboiler of the distillation apparatus. Eventually, only the product DEC is continuously removed and purified from the reaction system, which shifts the reaction in the forward direction.

2. EXPERIMENTAL SECTION 2.1. Materials. Sodium ethoxide and diethyl carbonate were purchased from Sinopharm Chemical Reagent Co., Ltd. Ethanol 99.7% anhydrous was obtained from Fuyu Chemical Reagent Co., Ltd. (Tianjing, China). Ethylene carbonate was purchased from Taixing Taipeng Chemical Reagent Co., Ltd. (Jiangsu, China). All chemicals were of analytical grade, except sodium ethoxide, which was chemically pure grade. 2.2. Experimental Setup. The schematic diagram of the experimental setup is shown in Figure 1. The reactor employed was a

DEC yield = mole number of DEC formed/ initial mole number of EC

(1)

2.4. Batch Reactor Experiment. Batch reactor experiments were performed to compare to the proposed process in terms of the DEC yield. The reactor used was a 250 mL round-bottom flask equipped with a thermometer, a magnetic stirrer, and a water-cooled condenser. Typically, the reactor was charged with 0.4 mol of EC and a certain mount of ethanol. The charged reactor was then placed in a thermostatic bath and heated to the desired temperature with stirring at 800 rpm. Once the reaction mixture reached the desired temperature, a certain amount of catalyst was added and the reaction started. The reaction mixture was sampled periodically and analyzed by gas chromatography.

3. RESULTS AND DISCUSSION 3.1. Batch Reactor Experiment Results. Figure 2 shows typical composition profiles in a batch reactor. As seen, the

Figure 1. Schematic diagram of the experimental setup for DEC synthesis. 250 mL round-bottom flask equipped with a thermometer and a magnetic stirrer. The top of the reactor was connected to the middle of a distillation column via a glass tube. The distillation column was made of glass and equipped with a total condenser, a reboiler, and a reflux splitter. The column has an inner diameter of 20 mm and a total height of 500 mm. To reduce heat loss, the column was insulated with foam material. The rectification section and stripping section were packed with θ ring (diameter, 2 mm; height, 3 mm; made of wire mesh), and each has a packing height of 200 mm. The reflux ratio was controlled using an electromagnet with a time relay. The distillate, which mainly contains ethanol, was returned into the reactor via a glass tube. The reboiler was externally heated with a heating mantle, and the reboiler duty was controlled with a transformer by adjusting the voltage applied to the heating mantle. 2.3. Experimental Procedure. A certain amount of ethanol, EC, and sodium ethoxide was charged into the reactor. The charged reactor was then immersed in an oil bath, and the reaction mixture was

Figure 2. Composition profiles in a batch reactor. Reaction conditions: EC, 0.4 mol; ethanol, 1.8 mol; reaction temperature, 362 K; and catalyst concentration, 0.4 wt %.

compositions of reactants and products do not change with time after 15 min because of the limitation of reaction equilibrium. Table 1 summarizes the equilibrium yields of DEC and equilibrium compositions at different initial ethanol/EC mole ratios under the boiling point of the reaction mixture. It can be seen from this table that the equilibrium yield of DEC increases with the ethanol/EC mole ratio. However, even in the presence of a large excess of ethanol, the DEC yield obtained is only moderate (less than 65%). In addition, a high ethanol/EC mole ratio leads to a relatively low DEC 1255

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product EG is a heavy component and is accumulated in the reactor, so that the EG composition in the reactor increases monotonically with time. In addition, it should be noted that the reactor composition virtually does not change with time after 5 h for the reason that the process has reached a steady state. The final mixture in the reboiler consists of DEC and a small amount of ethanol, EG, and EC, as shown in Figure 4. Because ethanol and DEC are much more volatile than EC and EG (the normal boiling point difference between DEC and EG is 72 K), the reaction mixture vapor mainly contains ethanol and DEC. As the process proceeds, the ethanol in the reboiler is continuously distillated and returned into the reactor and DEC is maintained in the reboiler. Therefore, the composition of ethanol in the reboiler declines with time, and the DEC composition increases to a constant value. 3.3. Effect of the Ethanol/EC Mole Ratio. Figure 5 shows the effect of the ethanol/EC mole ratio on the DEC yield and

Table 1. Equilibrium Yield of DEC and Composition at Different Ethanol/EC Mole Ratios equilibrium composition (wt %) ethanol/EC mole ratio

equilibrium yield of DEC (%)

ethanol

DEC

EC

EG

10 6 4 3

63 49 41 35

73.5 63.5 53.8 46.8

13.5 15.9 17.8 18.3

5.9 12.3 19.1 25.3

7.1 8.3 9.3 9.6

composition in the reaction mixture. This will increase the energy consumption in the subsequent separation process. 3.2. Typical Results for the Combined Process. Figures 3 and 4 show the reactor and reboiler composition profiles for a

Figure 3. Reactor composition profiles for a typical experiment. Figure 5. Effect of the ethanol/EC mole ratio on the DEC yield and reboiler duty. Reaction conditions: EC, 0.4 mol; catalyst concentration, 0.2 wt %; and reflux ratio, 1.

reboiler duty. As expected, a high ethanol/EC mole ratio favors the conversion of EC to DEC. The DEC yield increases 15% from the value of 75% when the ethanol/EC mole ratio increases from 3 to 4.5. However, a further increase in the ethanol/EC mole ratio from 4.5 to 6 leads to a slow increase in the DEC yield from 90 to 92%. The reboiler duty increases monotonically with the ethanol/EC mole ratio as shown in the same figure. The explanation for this result is as follows: because more ethanol is distilled from the reactor into the distillation apparatus when the ethanol/EC mole ratio increases, a higher reboiler duty is required to recover the ethanol. From the viewpoint of energy saving for distillation operation, a high ethanol/EC mole ratio is unfavorable. Balancing the results of the reaction and separation, the optimum ethanol/ EC mole ratio in this experimental system is selected as 4.5. 3.4. Effect of the Reflux Ratio. The reflux ratio is an important parameter for this process, because it affects both reaction and separation performances. A high reflux ratio increases the separation between DEC and ethanol; thus, a high purity of ethanol can be obtained in the distillate. The returning of high-purity ethanol into the reactor favors the conversion of EC to DEC. In contrast, a low reflux ratio causes insufficient separation between DEC and ethanol. As a result, a part of DEC in the distillation apparatus is returned into the reactor, which will increase the backward reaction rate and, consequently, reduce the DEC yield. Figure 6 shows the effect of the reflux ratio on the DEC composition in the distillate. As seen, the composition of DEC

Figure 4. Reboiler composition profiles for a typical experiment.

typical experiment conducted with the combined process, respectively. The operation conditions were the same as those for the batch reactor experiment, which were EC of 0.4 mol, ethanol of 1.6 mol, and catalyst concentration of 0.2 wt %. In addition, the reflux ratio of 1 and reboiler duty of 114 W were selected. As seen from Figure 3, the composition of ethanol in the reactor decreases initially, then increases to around 49 wt %, and is finally maintained at this value, which indicates that the amount of ethanol returned into the reactor is nearly equal to that consumed by the reaction and removed from the reactor when a steady state has been reached. Unlike ethanol, the DEC composition in the reactor reaches a maximum at around 1 h and then decreases. This is due to the combined effect of DEC formation and removal. The removal of the DEC product enhances the conversion of EC; thus, the EC composition decreases significantly. Another reaction 1256

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3.5. Effect of the Reboiler Duty. The effect of the reboiler duty on the DEC yield and DEC purity was investigated. The experiments were performed with an ethanol/EC mole ratio of 4.5 and a reflux ratio of 1.5. The results are shown in Figure 8.

Figure 6. Effect of the reflux ratio (R) on the DEC composition in the distillate. Reaction conditions: EC, 0.4 mol; ethanol, 1.8 mol; catalyst concentration, 0.2 wt %; and reboiler duty, 124 W.

in the distillate decreases with an increase in the reflux ratio. When the reflux ratio is 0.4, the DEC composition in the distillate can reach as high as 5.6 wt % because of insufficient separation. As the reflux ratio increases to 1, the composition of DEC in the distillate can be maintained below 0.8 wt %. This means that the composition of ethanol in the distillate can reach 99.2 wt %. However, with a sufficiently high reflux ratio, the separation of ethanol with DEC is mainly limited by the packing height in the distillation column. Thus, the reflux ratio shows little effect on the distillate composition when the reflux ratio exceeds 1. However, excessive reflux leads to the fact that a significant amount of ethanol is returned and kept in the reboiler. This implies a low ethanol composition in the reactor, which is unfavorable for the reaction. Therefore, there should be an optimum reflux ratio at which ethanol can be effectively separated and returned into the reactor. The optimum reflux ratio was determined experimentally. Figure 7 shows the effect of the reflux ratio on the DEC yield. The DEC yield is only 66% at the reflux ratio of 0.4, because a part

Figure 8. Effect of the reboiler duty on the DEC yield and purity. Reaction conditions: EC, 0.4 mol; ethanol, 1.8 mol; catalyst concentration, 0.2 wt %; and reflux ratio, 1.5.

As seen, the DEC yield increases to 91% as the reboiler duty increases from 70 to 124 W. Meanwhile, the purity of DEC in the reboiler increases from 58 to 97 wt %. However, with a reboiler duty higher than 124 W, both the DEC yield and purity decrease. Therefore, the optimum reboiler duty is 124 W in terms of the DEC yield and purity. The reason for the above results can be considered as follows: with a low reboiler duty, the ethanol in the reboiler cannot be efficiently vaporized and returned into the reactor; thus, much of the ethanol is kept in the reboiler, which results in a low DEC yield. However, an excessively high reboiler duty leads to the vaporization of more DEC in the reboiler and will make the DEC return to the reactor via the distillate together with ethanol. This will increase the DEC concentration in the reactor, causing a significant amount of backward reaction to take place. As a consequence, the DEC yield is reduced. 3.6. Effect of the Catalyst Concentration. The effect of the catalyst concentration on the DEC production was investigated. In these experiments, the ethanol/EC mole ratio, reflux ratio, and reboiler duty were fixed at 4.5, 1.5, and 124 W, respectively. The catalyst concentration was varied between 0.1 and 0.4 wt %. Because the catalyst concentration only affects the reaction rate and not the equilibrium state of the reaction, it has little effect on the final DEC yield. It was found that the DEC yield is maintained at around 90%, regardless of the catalyst concentration. However, the catalyst concentration shows a significant influence on the DEC composition profile in the reboiler. As shown in Figure 9, the initial DEC composition in the reboiler increases with the catalyst concentration; however, the DEC composition is finally maintained at about 97 wt % over the catalyst concentration range investigated. This can be explained by the fact that the reaction rate increases with the catalyst concentration. Therefore, with a high catalyst concentration, DEC can be quickly formed in the reactor and removed into the reboiler, leading to a relatively higher DEC composition in the reboiler. 3.7. Optimum Operating Conditions. From the above experimental studies, the optimum operating conditions of the combined process for the production of DEC are obtained and shown as follows: ethanol/EC mole ratio, 4.5; catalyst concentration, 0.4 wt %; reflux ratio, 1.5; and reboiler duty, 124 W.

Figure 7. Effect of the reflux ratio on the DEC yield. Reaction conditions: EC, 0.4 mol; ethanol, 1.8 mol; catalyst concentration, 0.2 wt %; and reboiler duty, 124 W.

of DEC is returned into the reactor via the distillate. As the reflux ratio increases, the amount of DEC returned into the reactor becomes less, which enhances the forward reaction and, consequently, increases the DEC yield. The DEC yield reaches a maximum when the reflux ratio is 1.5. With a further increase in the reflux ratio, the DEC yield decreases gradually. Therefore, the optimum reflux ratio is at around 1.5. 1257

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ACKNOWLEDGMENTS Financial support for this work from the National Basic Research Program of China (973 Program, 2009CB219906), the National Natural Science Foundation of China (20976144), and the Specialized Research Fund for the Doctoral Program of Higher Education of China (20070698037) are gratefully acknowledged.

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Figure 9. Effect of the catalyst concentraton on the DEC composition profile in the reboiler.

An additional experiment was carried out under the optimum operating conditions, and the results are compared to those obtained with a batch reactor. As shown in Table 2, the DEC yield Table 2. Comparison between the Batch Reactor and Combined Process example

batch reactor

combined process

ethanol/EC mole ratio catalyst concentration (wt %) reactor temperature (K) DEC yield (%) DEC purity (wt %)

4.5 0.4 362 46 18.4

4.5 0.4 ∼364 91 97

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can reach 91% using the combined process, which is nearly 2 times higher than the equilibrium yield of 46% obtained in a batch reactor. Furthermore, the DEC purity obtained can reach 97 wt %. The DEC purity is limited to 97 wt % because a small amount of heavy components (EC and EG) are distilled into the distillation apparatus and accumulated in the reboiler. One way to increase the DEC purity is to partially condense the reaction mixture vapor to reduce the volatilization of the heavy components (EC and EG) and ensure that only the light components (ethanol and DEC) are introduced into the distillation apparatus. This can be achieved by adding several condensers on the top of the reactor. As the ethanol is returned into the reactor, a high purity of DEC can be obtained in the reboiler of the distillation column. Further studies are underway in our laboratory to improve the DEC purity.

4. CONCLUSION A new process in which transesterification and distillation are partially integrated has been developed to synthesize DEC from EC and ethanol. In this process, the product DEC can be removed from the reaction mixture during the course of the reaction, which overcomes the reaction equilibrium limitations. It was found that the reflux ratio and reboiler duty are key parameters for the production of DEC because they affect both reaction and separation performances. Under the optimum operating conditions, the DEC yield (on the basis of EC) obtained can reach 91%, which is nearly twice the equilibrium yield. The DEC purity can reach 97 wt %.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-29-82663189. Fax: +86-29-82668789. E-mail: [email protected]. 1258

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