Design and Control of a Middle Vessel Batch Distillation Process for

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Article pubs.acs.org/IECR

Design and Control of a Middle Vessel Batch Distillation Process for Separating the Methyl Formate/Methanol/Water Ternary System Zhaoyou Zhu, Xin Li, Yujuan Cao, Xingzhen Liu, and Yinglong Wang* College of Chemical Engineering, Qingdao University of Science and Technology, Qingdao 266042, China S Supporting Information *

ABSTRACT: The middle vessel batch distillation process for separating the methyl formate/ methanol/water ternary system was simulated using Aspen Plus and Aspen Plus Dynamics. The composition control structure (CCS) and temperature control structure (TCS) were studied in a dynamic simulation based on the results of a steady-state simulation. Two temperature control structures, one the traditional TCS and the other TCS with high selectors, show better control performances than CCS. The two control structures can stably control the product purities and liquid holdups in the product vessels at the end of the batch. The results show that TCS with high selectors is better than traditional TCS in terms of the liquid holdups. The performance of the TCS with high selectors is assessed, and controllability is demonstrated.

1. INTRODUCTION Methyl formate/methanol/water is one of the most common ternary systems in chemical processes such as pharmaceutical intermediate production and methyl formate and its downstream production processes. The efficient separation of this ternary mixture plays a key role in boosting the production efficiency and decreasing the production cost. Researchers have studied different methods for separating ternary mixtures. Huang et al.1 studied an ideal heat-integrated distillation column (HIDiC) system to separate a close-boiling ternary mixture, and they found that the ideal HIDiC system is much more thermodynamically efficient than its conventional counterpart and can be a competitive alternative for the separation of a close-boiling multicomponent mixture. Raeva and Sazonova2 separated ternary mixtures by extractive distillation and presented a thermodynamic criterion for entrainer selection to achieve the goal of high purity. Meidanshahi and Adams3 explored a novel process called semicontinuous distillation without a middle vessel for ternary separation. The new configuration can not only reduce the total direct costs of the system but also facilitate the retrofitting of available distillation columns for ternary purification. Theoretically, methyl formate, methanol, and water can be separated by common distillation according to the differences between their boiling points because there are no azeotropes among the three components. However, many manufacturing enterprises tend to use batch distillation processes to separate mixtures for reasons of flexibility and convenience. Batch distillation is widely used in fine chemical production such as in the pharmaceutical industry.4−7 Diwekar8 divided batch distillation processes into six types, one of which is called batch distillation with a middle vessel. The middle vessel batch distillation process has attracted much scholarly attention in recent years.9−13 Barolo et al.14 found that the feed rate to the © XXXX American Chemical Society

column affects the column performance, and operation at infinite reflux and reboil ratios may be more profitable. The dynamic behavior of the process was studied by a detailed mathematical model that was used to investigate the effect of different restrictions on the choice of the column operating variables.15 Espinosa16 studied the integration of reaction and separation in a batch distillation column with a middle vessel and demonstrated that the process has the potential to promote the complete conversion of reactants. Arellano-Garcia et al.17 paid more attention to the start-up operation and proposed a new operation mode. Warter et al.18 proved that middle vessel batch distillation offers many practical advantages such as a reduction of the temperature in the feed vessel. Tang et al.19 proposed a new operation mode of multivessel batch distillation and found that it has great advantages in time savings and operational flexibility. The results of previous works show that the middle vessel batch distillation process has great prospects to be applied in the chemical industry. However, the control structures need to be explored precisely to make the process work efficiently in practice because good control structures can not only simplify the separation process but also guarantee the high purity of the products. Accurate control structures for the middle vessel batch distillation process are essential, and simulation software can assist in the study. Gruetzmann et al.20 theoretically analyzed and experimentally studied the process of middle vessel batch distillation with cyclic operation. They presented a combination control structure of mass control and temperature control and Received: October 28, 2015 Revised: January 24, 2016 Accepted: February 24, 2016

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DOI: 10.1021/acs.iecr.5b04067 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

atmospheric pressure. The LS column distillate rate was set at 100.0 kmol/h, and it contained more than 99.0 mol % methyl formate, with the remainder mostly methanol (stream LS-OUT) after running the simulation. The stream VUPPER was added to the simulation and specified to be 100.0 kmol/h, with a composition of 99.0 mol % methyl formate and 1 mol % methanol. In the dynamic simulation, the valve VD that connected with the US column distillate stream (stream D) would be closed so that the mole rate of stream D was small and set at 1.0 kmol/h in the steady-state simulation. Stream FEED, stream VUPPER, and stream D, which are used to obtain the desired equipment sizes, specify the steady-state simulation because there are three design degrees of freedom when the pressure and stage number have been fixed. However, the flowsheet cannot be defined completely as a batch distillation process because there are feed and product streams in the process. Some detailed modifications would enable the realization of the conversion of the batch process in the dynamic simulation section. Between the pumps and valves, the pressure drops should be large enough so that the valve can adjust the flow rate efficiently. The outlet pressures of the valves that connect with the two columns and the middle vessel should be equal to the stage pressures and middle vessel pressure, respectively. Three vessels, which are the reflux drum of the US column, the middle vessel, and the sump of the LS column, were used to store the products, so that their volume must be large enough. The reflux drum, which holds most of the methyl formate, was set at 20.09 m3, the middle vessel, which holds most of the methanol, was set at 18.84 m3, and the sump, which initially holds almost the entire ternary system, was set at 37.68 m3. The diameter of the column was calculated by Tray Sizing, which is a built-in function of Aspen Plus. Table 1 shows the results of the steady state simulation. The compositions of streams D, RMID, and B can reflect the composition in the three product vessels. Unlike steady-state continuous distillation, batch distillation cannot separate the products efficiently in the steady-state simulation, as shown in Table 1. This is one of the biggest differences between the simulation of the middle vessel batch distillation and continuous distillation. To realize high-purity separation in a middle vessel batch distillation, accurate control strategies in the dynamic simulation are essential.

found that the temperature set point has a great impact on purity, especially that of the light component. Gruetzmann and Fieg21 simulated the separation process of hexanol/octanol/ decanol by the Aspen Custom Modeler. Using the temperature controller, the accumulation of the middle-boiling component in the distillate receiver can be avoided, and the process can achieve the goal of high purity for each component. Luyben22 explored the application of Aspen Plus and Aspen Plus Dynamics in a middle vessel batch distillation simulation and studied the control structures for the separation of benzene/ toluene/xylene. The control structure achieves 99.0 mol % purity for benzene, toluene, and xylene through adjusting two reflux flow rates (one from the reflux drum to the top of the upper column and the other from the middle vessel to the top of the lower column). The paper promoted the study of batch distillation control structure simulation. Nevertheless, compared with continuous distillation control structure simulation, the studies on batch distillation control are not sufficient and more studies are needed. In this paper, we simulated the process of middle vessel batch distillation to separate the methyl formate/methanol/water ternary system by Aspen Plus and Aspen Plus Dynamics. The composition control structure (CCS) with high selectors was studied, and two temperature control structures (TCS) for the batch process were investigated to achieve good control.

2. STEADY-STATE MIDDLE VESSEL BATCH DISTILLATION The middle vessel batch distillation was simulated in the manner of continuous distillation in the steady-state simulation. According to the different boiling points of the ternary mixture, the light component and a certain amount of the intermediate component are boiled up from the sump of the lower section (LS) column and separated in the upper section (US) column, while the rest of the intermediate component and the heavy component are separated in the LS column. The NRTL physical properties were selected. Two Radf rac models were used as the US column that purified the light component and the LS column that held back the heavy component, and a Flash 2 model was set as the middle vessel. Several valves and pumps were set to change the pressure of the streams to make the control of the process more efficient. The flowsheet of the middle vessel batch distillation is shown in Supporting Information, Figure S1. The feed stream that was fed to the sump of the LS column was set at 1000.0 kmol/h, and its composition was 30.0 mol % methyl formate, 30.0 mol % methanol, and 40.0 mol % water. The stage numbers of the US column and LS column were 30 and 20, respectively, and the column as well as the middle vessel were operated at

3. CONTROL STRUCTURES OF THE BATCH DISTILLATION 3.1. Initial Conditions of the Batch Distillation. The start-up and heat-up procedures of the middle vessel batch distillation are important for the control system. The ternary mixture is fed into the sump of the column, and the liquid level

Table 1. Streams Results of Steady-State Simulation D

RMID

B

1.00

98.99

998.99