1. Introduction
Several binary azeotropic systems containing low carbon alcohols, such as benzene and methanol, are widely used in the chemical and petrol chemical industries [
1,
2,
3]. To separate and recycle the azeotropic mixture, several enhanced distillations including pressure swing distillation (PSD) [
4,
5,
6], azeotropic distillation (AD) [
7,
8], and extractive distillation (ED) [
9,
10,
11] have been applied to separate the azeotrope successfully. For example, Shen et al. put emphasis on the separation of azeotropic systems for the ED method [
12]. Shirsat and Wang et al. optimized the ED and PSD method of separating the isobutyl alcohol/isobutyl acetate and tetrahydrofuran/ethanol azeotropic system, and indicated that the cost and product contamination should be concerned when selecting the separation method [
13,
14]. Compared with AD and ED methods, PSD has drawn more and more attention for the benefit of no third component added into the azeotropic system. Fulgueras and Maloney [
15] pointed out that PSD method was economically feasible when the mass fraction of azeotropic composition changed more than 5% (10% is better) in the range of pressure 10 bar. The PSD process has two columns which includes low pressure column (LPC) and high pressure column (HPC). For the minimum boiling azeotropes, the high-purity component can be obtained at the bottom of LPC and HPC. For the maximum boiling azeotropes, the opposite is true.
To reduce heating and cooling loads of thermal utilities and improve heat recovery capacity of the system, some methods such as heat integration are proposed. In general, two modes of heat integration are usually applied in the internal system [
6,
16,
17]. One mode is the heat integration between condenser and reboiler, which includes partial and fully heat integration. The heat integration between the rectifying and stripping section is another mode. In the past years, the economy of PSD processes was compared with different heat integrated modes [
1,
18,
19]. Luyben [
1] explored the separation of tetrahydrofuran/water azeotropic system by PSD method with two different heat integrated modes. Wang et al. [
19] made a comparison of partial and fully heat integrated pressure swing distillation (HIPSD) modes for the separation of methanol/tetrahydrofuran azeotropic system. These results show that the economy of the PSD process depends on the different azeotropic systems and heat-integrated modes.
Not only the economy but also the controllability of the separation process should be considered in the determination of a suitable separation method. Many researchers pointed out that the economy of the fully HIPSD mode was better but the controllability was weaker than that of the partial HIPSD mode [
1,
17,
19]. The main reason is that the distillate stream of the HPC is used to provide the heat source of the reboiler in the LPC and no auxiliary reboiler is needed, leading to a degree of freedom missed in the full HIPSD process. Luyben [
1] studied the energy consumption and controllability of partial and fully HIPSD modes for the separating water/tetrahydrofuran azeotropic system. It was revealed that the energy consumption for the fully HIPSD mode was lowest among the three heat integration modes, but its dynamic controllability was worse compared with the other two heat integration modes. As a result, it is necessary to weigh the energy consumption and controllability for different heat integrated modes. Zhang et al. explored the separation of a methyl acetate/methanol azeotropic system by different heat-integrated modes. It can be seen that the fully HIPSD mode has a little economic benefit compared with partial HIPSD mode, but its controllable performance is also poor [
6]. Zhu et al. contrasted the economics and controllability for separating of ethanol and toluene azeotropic system by PSD process with three modes of heat integration [
3]. The results indicated that the total annual cost (TAC) for the fully heat integration is lowest, but this kind of heat integration mode can handle only small disturbances very well. Similar studies were done for separating an n-butyl alcohol and toluene system [
20], a methanol and tetrahydrofuran system [
2], and an ethanol and chloroform system [
21].
The selection of the temperature control stage (TCS) is very important to achieve the excellent control performance because the signal of TCS is detected by the temperature controller and transmitted to the adjusted variable. In general, five criteria including product fluctuation minimum criterion, constant temperature criterion, singular value decomposition, sensitivity criterion, and slope criterion are usually used to select the suitable TCS. Zhang et al. used singular value decomposition and sensitivity criterion to select the TCS [
6]. Hosgor et al. explored the separation of the chloroform/methanol azeotropic system by fully HIPSD process and used slope criterion to select the TCS [
22]. Zhang et al. presented a derivative slope criterion to choose the TCS when the temperature distribution is quite close to linear [
23].
So far, there has been no article published that compares PSD with different heat integrated modes for separating benzene/methanol azeotrope through economy and controllability angles. In this work, the feasibility of the PSD process with two energy-saving modes for the separation of the benzene/methanol azeotrope were proved, and a comparison of the economy and controllability for the partial and fully HIPSD process were made in detail. Firstly, the partial and fully HIPSD processes were optimized based on the minimization of the TAC. Secondly, a composition slope criterion was presented to assist the selection of the suitable TCS for the HPC, when the temperature distribution in the column is rising rapidly near the bottom and the maximum of temperature slope value occurs in the bottom of the column. The different degrees of the disturbance in the feed flowrate and composition were introduced to verify the rationality of the selection of TCS and evaluate the industrial application of two processes.
4. Conclusions
The PSD with two energy-saving modes for the separation of the benzene/methanol azeotropic system was studied by Aspen Plus and Aspen Dynamics. We contrasted the economy and controllability of the PSD with two heat integration modes. The optimization results show that the fully HIPSD process with a TAC of $187,189 has some economic benefit compared with the partial HIPSD process with a TAC of $198,891.
The temperature profile in the HPC rose rapidly near the bottom of this column and the maximum of temperature slope value occurred in the last stage, which is the bottom of the HPC. The composition slope profile was proposed to assist with the selection of the temperature sensitive plate. According to the two criterions, stages 13 and stage 20 in the LPC and HPC were chosen as the TCS. Several control structures were developed to evaluate the industrial application and check the rationality of the selection of the temperature sensitive plate. The different degrees of disturbances in the feed flowrate and composition were introduced in two processes at 2 h. The results illustrate that the selection of the TCS is reasonable and the composition/temperature control structure can provide rust control performance for the PSD with two heat integrated modes. Compared with the partial HIPSD mode, the fully HIPSD mode can handle much smaller composition disturbance despite little economic benefit.
These works can provide a reference for selecting TCS, especially for azeotropic systems whose maximum value of temperature slope profile occurs in the bottom of the column. Meanwhile, the findings can also help engineers to weigh the economy against controllability for selection of the suitable separating process.