Pressure Swing Distillation for Separation of Ethyl Acetate and Ethanol in Sub-p

Wang Keliang; Li Jing; Lian Minglei; Li Zhi; Du Tingzhao

(1. College of Chemistry and Materials Engineering, Liupanshui Normal University, Liupanshui 553004;2. North China Company, China Petroleum Engineering Co., Ltd., Renqiu 062552)

Abstract: The pressure swing distillation (PSD) process for separation of ethyl acetate and ethanol mixture in subplateau region was simulated. The pressure of low-pressure column was set at 0.086 MPa, which is in accordance with the atmospheric pressure in the Yunnan-Guizhou region, while the pressure of high-pressure column was determined as 0.304 MPa. Various design parameters, including the plate number, the re flux ratio, and the feeding positions, were optimized,while taking into consideration the total annual cost (TAC). Furthermore, based on the general PSD, the partially heatintegrated pressure swing distillation (PHIPSD) process and the fully heat-integrated pressure swing distillation (FHIPSD)process were also studied. The processes with heat integration showed lower capital cost and lower energy cost, and TACs of the PHIPSD and FHIPSD decreased to 377.21×103 $/a and 371.66×103 $/a, respectively. Compared with the non-heat integrated process, TACs of the PHIPSD and FHIPSD could be reduced by 27.82% and 28.89%, respectively. The results showed that the FHIPSD process could effectively separate the ethyl acetate-ethanol mixture, and it was more economical and reasonable. This work can provide some technical references for the separation of such azeotropes in the sub-plateau region.

Key words: pressure swing distillation; ethyl acetate; ethanol; azeotrope; heat-integrated

1 Introduction

In the chemical and oil industry, it is impossible to separate an azeotrope or a mixture consisting of components with close boiling points by a conventional distillation technology[1-3]. In these cases, some special distillation processes are currently applied, such as the extractive distillation (ED), the catalytic distillation (CD),the reactive distillation (RD), and the pressure swing distillation (PSD)[4-7]. Generally, the energy consumption of the distillation accounts for approximately 49% of the whole separation process[8]. Therefore, it is of great significance to develop the energy-saving distillation techniques. In recent years, Cong, et al.[9-11]have developed a series of novel configurations based on the heat integrated distillation column (HIDiC) and the vapor recompression distillation column (VRC) to improve the energy efficiency, such as the intensified HIDiC (int-HIDiC), the multi-tube type HIDiC, and the middle vapor recompression distillation column (MVRC).

Due to the good solubility and volatility, ethyl acetate(EtAc) is widely used as an organic solvent in paint,coating, artificial flavors, and other manufacturing industries[12-13]. At present, the industrial production of EtAc is mainly realized through the esterification of acetic acid and ethanol (EtOH). Excessive EtOH is generally used to improve the productivity[14]. Therefore, the crude ester produced by esterification contains EtOH, and the two components form a minimum-boiling azeotrope under normal pressure.

Compared with extractive distillation and azeotropic distillation, PSD has been widely studied and applied in industry because it does not introduce entrainers and can protect the environment[15-17]. By using the heat integration technology, the energy consumption in distillation process could be effectively reduced[18-19].Zhang, et al.[20]conducted a study on the separation of EtAc and EtOH azeotrope by PSD. The pressure in the low-pressure column (LPC) and the high-pressure column(HPC) was set at 0.051 MPa and 0.608 MPa, respectively.The results showed that the PSD with heat integration could effectively reduce the energy consumption, CO2emissions, and TAC. On this basis, Wang, et al.[21]further improved the con figuration of the PSD by adding a side stream between LPC and HPC. By optimizing the flow rate, the feed location, and the outlet location of the sidestream, the energy consumption and the TAC could be reduced by 18.32% and 15.21%, respectively.

The Yunnan-Guizhou region belongs to the sub-plateau region in China. It is characterized by high altitude and low pressure, which is normally 0.086 MPa on average. The higher altitude needs a Specifically designed separation of azeotropic mixtures as compared with that adopted in the plain regions. As far as we know, there is no specific study on azeotrope separation in the subplateau region. Therefore, this research aims to study the unique characteristics of azeotrope separation by the PSD process in sub-plateau region, while taking the separation of EtAc and EtOH mixture as an example. The characteristics of the low pressure and low boiling point of corresponding substances in this region were utilized.In this work, the operating pressure of LPC was set at an atmospheric pressure of 0.086 MPa in the Yunnan-Guizhou region. Upon focusing on the total annual cost (TAC), the design parameters were optimized and compared with those of the plain region. On this basis,the heat-integrated techniques were adopted in the PSD process to reduce energy consumption and equipment investment. The performance of heat integration and nonheat integration processes was compared.

2 Separation Scheme

2.1 Phase diagram analysis

In this work, the NRTL equation was adopted to simulate and optimize the PSD process. The T-x/y phase diagram of the EtAc-EtOH mixture under various pressures is shown in Figure 1. It indicates that as the pressure increased from 0.086 MPa to 0.304 MPa, the azeotropic temperature changed from 340.50 K to 378.84 K, while the azeotropic composition changed from 0.7161 to 0.5745 for the mass fraction of EtAc. Furthermore, more than 5% of the composition variation of the azeotrope was observed, which could meet the conditions for separating azeotropes by the PSD process.

Figure 1 T-x/y phase diagram of the EtAc-EtOH mixture under 0.086 MPa and 0.304 MPa

2.2 Process description

The feeding rate of EtAc and EtOH mixture at the normal temperature was 3000 kg/h, and the mass fraction of EtAc and EtOH was 20% and 80%, respectively. The desired mass purity of the final product was 99.9%. The PSD process is illustrated in Figure 2. The mixture was fed into the middle of LPC, and a high-purity EtOH was yielded from the bottom of LPC. The azeotrope, formed at the top of LPC under a pressure of 0.086 MPa, was transferred to HPC for further separation. EtAc with a high purity was produced from the bottom of HPC. And the azeotrope formed at the top of HPC under a pressure of 0.304 MPa was sent back to LPC.

2.3 Total annual cost

Total annual cost (TAC) is used to evaluate the economic plausibility and feasibility of a proposed process[22-23],which, including the total capital cost and the total energy cost, is calculated by Eq. (1) with a payback period of 3 years. Based on the calculation basis proposed by Luben,et al.[24], various design parameters were optimized to achieve a minimum TAC. The calculation details are presented in Table 1. Total capital cost includes the columns, the reboiler at the bottom of column and the condenser at the column overhead. The total energy cost mainly comes from the low pressure (LP) steam used in the reboiler. Q and ΔTmare the heat duty (kW) and the heat transfer temperature difference (K), respectively.The tool of “tray sizing” with sieve tray in Aspen Plus software was used to calculate the diameter of the column.

Figure 2 Flowsheet of the PSD process for separating EtAc-EtOH mixture

Table 1 Basis of economic calculation

3 Process Simulation and Optimization

3.1 Process optimization sequence

Seven design parameters were iteratively optimized in sequence, with focus being aimed at attaining a minimum TAC. These parameters included the number of the plates in LPC and HPC (NT1and NT2), the three feeding locations of the LPC and HPC (NF1，NRECand NF2), and the reflux ratios (RR1 and RR2). The details of the sequential iterative optimized sequence can be found in our previous work[16].

3.2 Optimization of feeding positions

Three feeding locations were subjected to optimization.These locations included the feeding positions of LPC(NF1), the recycle stream (NREC), and the HPC (NF2). The optimization results are shown in Figure 3. For all feeding locations, TAC at first increased, and then decreased to dispaly a downward movement. For a minimum TAC, the feeding locations of the LPC, the recycle stream, and the HPC were identified at the 28thplate, the 17thplate, and the 18thplate, respectively.

Figure 3 Effect of three feeding positions on TAC

3.3 Optimization of the re flux ratio

The reflux ratio of two columns, RR1 and RR2, was then optimized. At first, the energy cost increased with an increasing reflux ratio, whereas the capital cost drastically decreased, resulting in a decreasing TAC.However, when the reflux ratio increased to beyond a kind of threshold, both the capital cost and the energy cost increased with a continuously increasing reflux ratio. As indicated in Figure 4, a minimum TAC was obtained with an RR1 of 1.4 and an RR2 of 1.5 for the LPC and HPC, respectively.

Figure 4 Effect of the re flux ratio of two columns on TAC

3.4 Optimization of the plate number

Finally, the outermost plate number was optimized. The optimization was divided into two parts: the plate number of the LPC and the HPC (NT1and NT2). The plate number of LPC was optimized at first, with the results provided in Table 2. It is found that the plate number directly affected the flow rate of the recycle stream. High numbers of plates were associated with low recycle stream flow rate and energy consumption. A minimum TAC was achieved with a plate number of 45 for LPC.

Table 2 Results for optimization of the plate number for LPC

Table 3 Results for optimization of the plate number for HPC

The plate number of HPC was optimized after completing the optimization of the plate number of LPC. The results are given in Table 3, which shows that a minimum TAC was reached with a plate number of 32 for HPC. Therefore, the plate number for HPC was conclusively determined to be 32.The overall optimization results for separation of EtAc and EtOH mixture by the PSD process are presented in Figure 2.In addition, the separation process of EtAc and EtOH mixture by PSD in the plain region was optimized, where the operating pressure of LPC and HPC was 0.101 MPa and 0.304 MPa, respectively. The optimization results are listed in Table 4. The flow rate of the recycle stream was higher than that of the sub-plateau region due to the smaller pressure difference between the two columns. This characteristic would increase the energy consumption and the TAC.

3.5 The heat-integrated PSD processes

As shown in Figure 2, the top temperature of HPC was approximately equal to an azeotropic temperature of 378.8 K,and the bottom temperature of LPC was approximately 347.4 K, which was close to the boiling point of EtOH under a pressure of 0.086 MPa. Obviously, there is a temperature difference around 31 K between the top of HPC and the bottom of LPC. Therefore, heat integration between the condenser of HPC and the reboiler of LPC can be performed, implying that the top stream of HPC can be used as a heating medium for the bottom stream of LPC, thereby reducing the energy consumption and the equipment investment. Furthermore, the partially heatintegrated pressure swing distillation (PHIPSD) process and the fully heat-integrated pressure swing distillation(FHIPSD) process are discussed next.

For the PHIPSD, given that the heat duty of the condenser at the top of HPC was 537.48 kW and that of the reboiler at LPC was 766.42 kW, they were not equal. Therefore, an auxiliary reboiler was needed at the bottom of LPC. The flowsheet of the PHIPSD process is shown in Figure 5,with the Specific results presented in Table 4.

For the FHIPSD process, it can be achieved by adjusting the re flux ratio of the two columns to balance the heat duty of the condenser at top of HPC and the reboiler at bottom of LPC. In other words, the energy consumption and equipment investment can be reduced by utilizing the top stream of HPC as a heating medium for the bottom stream of LPC. As indicated in Figure 6, a minimum TAC was acquired with a re flux ratio of 1.53 and 2.90 for the LPC and HPC, respectively. The results for optimization of the FHIPSD process for treating the EtAc-EtOH mixture are shown in Figure 7, with the Specific results listed in Table 4.

Figure 5 Flowsheet of the PHIPSD process for treating EtAc-EtOH mixture

Figure 6 Effect of the re flux ratios on TAC

3.6 Comparison of the results of the flowsheets

After optimizing the design variables, such as the plate number, the feeding locations, and the reflux ratio of the two columns, the optimum process parameters were obtained. Moreover, based on the general PSD, the PHIPSD and FHIPSD processes were performed. The results on comparison of the three processes are listed in Table 4. The processes with heat integration showed apparently lower capital cost and energy cost, and TACs of the PHIPSD and FHIPSD were reduced to 377.21×103$/a and 371.66×103$/a,respectively. Compared with the non-heat integration process, TACs of the PHIPSD and FHIPSD could reach 27.82% of savings and 28.89% of savings, respectively.

Table 4 Comparison of the results of the flowsheets

Figure 7 Flowsheet of the FHIPSD process for treating EtAc-EtOH mixture

4 Conclusions

In this study, the PSD process for separating the EtAc-EtOH mixture was simulated by the Aspen Plus software.Given the high altitude and low atmospheric pressure in the Yunnan-Guizhou region, the operating pressure of LPC was set at a local atmospheric pressure of 0.086 MPa, and the operating pressure of HPC was set at 0.304 MPa. Various design parameters, including the plate number, the re flux ratio, and the feeding locations, were optimized to obtain a minimum TAC of 522.62×103$/a.Compared with the total annual cost of the PSD process in the plain region, the TAC in sub-plateau region could reach a 9.26% of savings.

Moreover, based on the general PSD, the PHIPSD and FHIPSD processes were also studied. The process with heat integration showed an apparently lower capital cost and energy cost, and TAC of the PHIPSD and FHIPSD was reduced to 377.21×103$/a and 371.66×103$/a, respectively. Compared with TAC of the non-heat integration process, TAC of the PHIPSD and FHIPSD could reach a 27.82% of savings and 28.89% of savings, respectively. The results showed that the FHIPSD process could effectively separate the EtAc and EtOH mixture, and it was more economical and reasonable. This work can provide some technical references for the separation of such azeotropes in subplateau region.

Acknowledgements: This work is financially supported by the Guizhou Province United Fund (Qiankehe J zi LKLS[2013]27),the Guizhou Province Education Department (Qianjiaohe KY zi[2019]137), the Guizhou Province United Fund (Qiankehe LH zi[2015]7608), the Guizhou Solid Waste Recycling Laboratory of Coal Utilization ([2011]278), the Guizhou Provincial Education Department's Scientific and Technological Innovation Team([2017]054), and the Academician Workstation of Liupanshui Normal University (Qiankehepingtairencai [2019]5604).