Simulation and multi-objective optimization of the dimethyl carbonate production process

Greenhouse gases such as CO2 are considered effective materials in global warming due to their high absorptivity. So, lowering atmospheric CO2 is one of the most practical strategies. Utilizing carbon dioxide in chemical processes is an applicable method. In this study, Aspen HYSYS 10 was used to investigate how carbon dioxide can be added to the process of dimethyl carbonate production, and the affective parameters of the process, including temperature, residence time, feed ratio, and recycle ratio, were evaluated. It was observed that the production of DMC grew as temperature rose. The simulation results also revealed that a maximum conversion of roughly 8% was attained in the MeOH/EC. Additionally, boosting the recycle ratio is detrimental, and the impact of temperature and MeOH/EC has been enhanced by increasing the residence time. The interactions of the above parameters have been studied by Design Expert 12. The optimum value of effective parameters for the production of dimethyl carbonate has been obtained as follows: temperature of 164.7° C, recycle ratio of 0.2, residence time of 139.45 min, and feed ratio of 5.9%, leading to the conversion of 70%.


Simulation of DMC production process
The DMC production process (the transesterification of ethylene oxide with methanol) has been simulated based on a kinetic study carried out by Cui et al. 2004 by Aspen HYSYS 10.The NRTL equation of state has been implemented to predict the thermodynamic behavior of compounds.The reactions were carried out in the presence of KI and K 2 CO 3 as catalysts.
Simulation of the DMC production process has been performed in two stages.In the first stage, EO and CO2 reach to the reaction temperature and the first instantaneous reaction has occurred in a conversion reactor as follows 16 : The produced ethylene carbonate along with methanol injected into a CSTR and DMC is produced as follows (Cui et al. 2004) [23]: In addition; 2-Methoxyethanol (ME) is produced as a by-product as follows 16 : The simulation of the DMC production process in Aspen HYSYS has been shown in Fig. 1.The properties and conditions of feed have been presented in Table S1.

Verifying the validity of results
Figure 2 shows the effect of residence time on DMC production at 160 °C and 180 °C for experimental data (Cui et al. 2004)  16 and simulation results.Comparison of the simulation results with experimental data (Fig. 2) illustrates that the simulation results are in good agreement with experimental data.The average error 160 °C and 180 °C are 3.56% and 3.99%, respectively.So we can extend the simulation for investigating the effective parameters and optimizing the process. (1)

Developing the process
According to the accuracy of the simulation results, it has been used for developing the process for evaluating the effects of parameters and optimizing the system.For this purpose, a part of the unreacted reactants has been separated using a flash drum and recycled into CSTR.Figure 3 shows the developed process containing the flash drum and recycle stream.

Process optimization
In this study, optimization of EC conversion has been performed by response surface methodology (RSM).In statics, RSM examines the interactions between a number of explanatory factors and one or more response variables.Sequential statistics of four-level Box-Behnken design using Design Expert 12 have been implemented to optimize the production of dimethyl carbonate.The significant parameters of the process including Temperature, recycle ratio, residence time, and MeOH/EO have been examined on EC conversion.Total of thirty runs of experiments have been carried out in a standard manner (Table S2).The values for the factor's level for RSM analysis are as follows:   where Y is the predicted response, X i and X j represent the independent variables, β 0 , Β ii , β ij are constant coefficients.k indicates the number of studied factors and e is the error value.Analysis of variance (ANOVA) has been implemented for model verification.

Results and discussion
In this section, effective parameters in the DMC production process using carbon dioxide and ethylene oxide contains including temperature, feed ratio, and recycle ratio have been investigated.

Effects of temperature on DMC production
Figure 5 shows the effects of temperature on EC conversion in different residence times.DMC production process using carbon dioxide and ethylene oxide contains three reactions of Eqs. ( 1), (2), and (4).It is evident that increasing temperature enhances the conversion of ethylene carbonate.The effect of temperature on increasing the EC conversion in lower residence times is more significant compared to higher residence times.i.e., increasing the temperature from 120 ͦ C to 140 ͦ C increases the EC conversion from 43.22% to 55.74% in residence time of 100 min, while this value for residence time of 300 min varies from 69.25% to 76.16%.The mentioned trends have been also reported by Song et al. 2011 18 .

The effect of feed ratio on EC conversion
The feed ratio is one of the most influential factors in ethylene carbonate conversion.As mentioned, the first reaction (converting ethylene oxide to ethylene carbonate) is an instantaneous reaction, hence; the ratio of methanol (MeOH) to ethylene carbonate has been considered as an operating parameter.Figure 6 shows the effect of the MeOH/EC mass ratio on EC conversion.It is shown that a mild maximum point has occurred in the range of MeOH/EC = 6.This trend has been also observed by Cui et al. 16 .

Effects of recycle ratio on EC conversion
Figure 7 illustrates the effects of recycle ratio on EC conversion in different residence times.Recycling the unreacted reactants into the rector avoids wasting the feed and decreases the cost of the separation unit, however; reduces the residence time of reactants in constant reactor volume and disturbs the optimum feed\ ratio.A similar trend is shown at higher residence time, but the impact of recycle ratio on EC conversion is delayed up to 0.75-0.8.

Analyses of variance (ANOVA)
In order to assess the interaction between effective parameters for obtaining an optimized range of EC conversion, Design Expert 12 has been used.The process has been studied using the quadratic VS 2FI model and square root transformation.The developed regression model's sufficiently and validity have been evaluated using the ANOVA approach (Table S3).The analysis showed that linear, reciprocal, and square for the production of DMC ( 6) www.nature.com/scientificreports/are significant, with the exception of the interactions between recycle ratio-Feed ratio (BD) and residence timefeed ratio (CD).The Model F-value of 52.51 and P-values of less than 0.0001 imply the model is significant.The adjusted R 2 of 0.9464 and the predicted R 2 of 0.9167 are reasonably in agreement.An ideal signal-to-noise ratio is more than 4, as measured by the Adeq Precision.The ratio of 29.525 indicates a sufficient signal, and the following parameter combinations can be used with this model to predict the responses: Figure 8 shows the response surface diagram for temperature-recycle ratio and temperature-residence time interactions on the EC conversion.According to the ANOVA analysis, the interactions between temperaturerecycle ratio and temperature-residence time have significant impacts on EC conversion.It was shown in Fig. 7 that increasing temperature and residence time simultaneously enhances the conversion of EC, However; recycle ratio had mild effects compared to the temperature.

Optimization
In order to optimize the process and specify the best conditions to achieve proper DMC production, constraints have been applied to parameters according to the allowed range.Table S4 shows Process effective parameters and their applied constrains.Several combinations of parameters reported by Design Expert as optimum values have been shown in Table 1.According to Fig. 5 to Fig. 8 along with Table 1, the recycle ratio < 0.7 has not significant effects on EC conversion, however; significantly increases the cost of separation unit due to the existence of azeotrope between methanol and DMC and challenges of separation of homogeneous catalyst.Increasing MeOH/ EC increases the conversion of reversible reaction of DMC production (Eq.2).In contrast, increasing the recycle ratio and MeOH/EC increase reactor volume.The reaction temperature is low and does not significantly affect the cost of the process.So, the best set of the reported optimum conditions is selected as raw 1.

Conclusion
In this study, CO 2 utilization for DMC production as a green and environmental friend process has been simulated and optimized.For this purpose simulation of the process has been performed by Aspen HYSYS 10 and operating conditions of temperature, residence time, feed ratio, and recycle ratio have been evaluated.It is shown that increasing the temperature and residence time increases the conversion of EC.Ethylene carbonate conversion versus MEOH/EC has a mild maximum.In addition; optimization of the process has provided several combinations of parameters as optimum points and temperature of 164.7 ͦ C, Recycle ratio of 0.2, residence time of 139.45 min and MeOH/EC of 5.9 have been selected as optimum operating condition of process which results in 70.1% for EC conversion.

Figure 4 Figure 2 .
Figure 4 shows the algorithm of optimization for DMC production process.The quadratic VS 2FI model has been implemented to predict the response function and estimate the coefficients by multiple regression analysis 17 :

Figure 3 .
Figure 3. Simulation of modified DMC production process.

Figure 5 .Figure 6 .
Figure 5. Effects of temperature on EC conversion in different residence times.

Figure 7 .
Figure 7.The effect of recycle ratio on EC conversion in residence times of 100, 200 and 300 min.

Figure 8 .
Figure 8.The effect of (a) temperature and recycle ratio interaction on EC conversion (residence time = 99 min and MeOH/EO = 8.5)(b) temperature and residence time interaction on EC conversion (Recycle ratio = 0.2 and MeOH/EO = 8.5).