Insight into controllability and operation of extractive dividing-wall column

Highlights

• There is loss of one control freedom in extractive divided-wall column (E-DWC).

• The thermal coupling in E-DWC enhances interaction behavior.

• Intermediate heating is used to add one control freedom for E-DWC.

• Intermediate heating directly increases vapor flowrate to change vapor split.

• The control performance of E-DWC is improved through intermediate heating.

Abstract

The extractive dividing-wall column (E-DWC) as process intensification can achieve more sustainable chemical process with economic and safe performance. However, the operation and controllability are in question and are biggest obstacle to the industrialization of extractive dividing-wall column. The coupling effect of E-DWC improves the thermal efficiency significantly but loses one control freedom and enhance interaction behavior compared with the conventional process. In this paper, we investigate the interaction behavior in E-DWC and illuminate the necessary of the extra control freedom to address the interaction behavior although the number of control freedom degrees are just enough for holding purity specification of three products.

To solve the coupling effect and interaction behavior, in this paper, the intermediate heating is used to add one control freedom to change the vapor split through directly increasing the flowrate of vapor. We use two systems-acetone and methanol, and bioethanol dehydration as cases to explore the controllability and operation of E-DWC. The results of dynamic simulation prove that the interaction behavior and coupling effect indeed cause poor controllability of E-DWC. Moreover, the intermediate heating strategy indeed is valid and can improve controllability of E-DWC significantly. The integral of squared error and deviation of product purity are reduced significantly both for two cases. Especially for acetone and methanol, the setting time and integral of squared error in the presence of +20% composition disturbance is reduced significantly.

Introduction

Process intensification is one of the major strategies to create the safe and energy effective and economic technology and processes for a more sustainable chemical industry. Economic and environmental considerations have encouraged industry to focus on technologies based on process intensification.

For azeotrope or close-boiling mixtures, simple distillation is difficult to accomplish the separation task. Many distillation processes have been devised to address this problem. One method of these complex distillations is introduction of third compound called solvent or entrainer such as azeotropic distillation [1], [2], extractive distillation [2], [3], [4], [5], [6] or reactive distillation [7], [8]. Pressure swing distillation achieves the separation task through azeotrope composition and boiling temperature with the pressure. Typical azeotropic distillation uses a light solvent to form a heterogeneous minimum-boiling azeotrope, which achieves the separation of azeotrope mixtures. Compared with azeotropic distillation, typical extractive distillation process introduces a heavy entrainer to alter the relative volatility of compounds. Several studies state that extractive distillation is more profitable than heterogeneous azeotropic distillation for many systems, in particular for the dehydration of aliphatic alcohols [2], [9], [10], [11]. Several studies compare pressure swing distillation and extractive distillation in economic and dynamic issues [12], [13], [14], [15], [16], [17]. Several results show that the extractive distillation is more attractive from the standpoint of both capital investment and energy consumption [12], [13], [14]. Several results show that pressure swing distillation is superior to extractive distillation in economic performance [15], [16], [17]. Two papers use the fully heat integration and varied-diameter column (VDC) in the pressure swing distillation, respectively [16], [17]. The dynamic performances of both processes are equivalent [12], [13], [14], [15], [16] while pressure swing distillation with VDC is superior to extractive distillation with VDC in dynamic performance [17]. The typical conventional-two-extractive distillation process is shown in the Fig. 1. The heavy entrainer is fed to the extractive distillation and extracts one key component to distillation bottom. Then, the mixture including solvent is fed to recover column to recover entrainer, which is recycled back to extractive distillation.

For pursing energy saving, thermally coupled technology such as dividing wall column (DWC) can be applied to conventional extractive distillation (ED) process, which is called extractive dividing wall column (E-DWC). Several configurations as shown in the Fig. 2. are proposed in the open literatures [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31].

The configuration called a split shell column with divided overhead section and common bottoms section [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31] in Fig. 2(a) is common in E-DWC, which is different from the normal DWC configuration. The other E-DWC configuration in Fig. 2(b) is same with the normal DWC configuration [18]. In this article, configuration of E-DWC with a split shell column with divided overhead section and common bottoms section most often used is studied.

For economic aspect of E-DWC, many researches point out that E-DWC has better performance compared with conventional ED process [2], [18], [19], [20], [21], [22], [23], [24], [25]. However, Wu et al. [20] also demonstrates that E-DWC isn’t always preferred in the economic aspect since high bottom temperature due to the high-boiling solvents requires more expensive heat sources with higher-temperature.

As far as the dynamic operation of E-DWC, there are extensive literatures [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31]. Xia et al. [26] proposed temperature control for E-DWC with an adjustable vapor split to eliminate composition controller proposed in their previous work [21]. Wu et al. also explored the controllability of E-DWC further and compared with conventional ED process [19]. The results demonstrate that E-DWC has worse controllability due to the loss of control freedom. In the conventional process, four quality control loops are used while in the E-DWC, only three quality control loops are left. In addition, Zhang et.al [22] explored control of extractive dividing-wall column for separating ethyl acetate-isopropyl alcohol mixture. They proposed and compared control structure with a fixed vapor split and control structure with manipulation of the vapor split. They find the latter control structure has better performance than the former especially for composition disturbance. They also point out that the former control structure is more likely to be used in industrial practice since the device to realize control of vapor split will make the inner of the EDWC more complicated. Tututi-Avila et al. [27] explore the controllability for ethanol dehydration E-DWC process. They compared the dynamic controllability of the E-DWC with and without manipulation of the vapor split. They demonstrate that the control structure with variable vapor split ratio can handle feed and composition disturbance while control structure with fix vapor split failed. Sun et al. [23] propose the basic control strategy using four composition controllers, and two improved control strategies with and without vapor split ratio use temperature controllers, which can handle feed composition disturbance. However, control structures with variable vapor split ratio show better performance in products purity than that with constant vapor spilt in the presence of feed disturbance. They also state the impossibility of adjusting vapor split ratio in the chemical industry at present. Dai et al. [24] explored the design and controllability of EDWC for separating benzene/cyclohexane azeotrope using mixed entrainer. They find the large step of feed and composition disturbances can be handled with fixed vapor split when using two composition controllers. Furthermore, Li et al. [25] proposed the control structure with fixed vapor split ratio using composition control loops to adjust the solvent flowrate for E-DWC. They find the control structure is better than that manipulating variable vapor split ratio for 2-methoxyethanol/toluene system with DMSO as entrainer. To achieve manipulation of the vapor split, a recent paper by Luyben [28] proposed pressure-swing method that the operating pressures of the two condensers on the two sides of the wall in the E-DWC is manipulated. The research described above are based on proportional-integral control (PI). Feng et al. [29] applied temperature differences in the temperature PI and model predictive control (MPC). The result show control performance of single temperature is inferior to that of temperature differences both for PI and MPC. In addition, MPC scheme can achieve much better control performance than PI control. Later, Feng et al. [30] applied system identification technologies with closed-loop identification to model predictive control (MPC) to replace linear time-invariant state space model used in their previous work. Note that the manipulation of vapor split in Feng's two works are variable vapor split and pressure-swing method proposed by Luyben [28], respectively. In our recent work [31], we propose novel control structure integrating pressure-swing and pressure compensation for separation of dichloromethane-methanol (DCM-MeOH) mixture using E-DWC. The control structure proposed is to address the problem that a constant temperature of the sensitive tray is not indicative of a constant composition if the pressure varies.

In addition, some researchers achieve manipulation of the vapor split in experiment work on laboratory column by vapor split control devices [32], [33], [34]. However, application of vapor split control devices in the commercial-size column remains difficult. The practical commercial application has not yet been reported.

The extractive dividing-wall column as process intensification technology is a promising configuration for industrial applications. The lack of robust control system is the biggest obstacle to the industrialization of extractive dividing-wall column, so it is urgent to design a reliable and stable control scheme. In addition, there is need to consider both steady-state economics and dynamic controllability through all stages of process development should be considered following the theology simultaneous design [35]. Therefore, this paper tends to investigate the controllability and operation of E-DWC.