Full length articleDesign and control of entrainer-assisted reactive distillation for N-propyl propionate production
Introduction
N-Propyl propionate (Propro) is a widely used chemical solvent. It has many applications in paints, coatings, and other industrial products. Propro can be produced by the liquid-phase esterification of N-propanol (POH) and propionic acid (ProAc) (Altman et al., 2012, Buchaly et al., 2012, Gooch, 2007). The esterification reaction is a kind of reversible reaction and it is limited by chemical equilibrium. Propro is produced in a batch or continuous reactor in a homogeneous or heterogeneous system catalyzed by acids followed by several distillation columns (Cruz-Díaz et al., 2012, Duarte et al., 2006). It has many problems: it has low conversion rate; it consumes large capital and energy cost and the corrosion of the equipment is serious. Hence, reactive distillation (RD) is an effective method to solve the problems. RD is an integration of reaction and separation into a single column. It allows the simultaneous production and removal of the products, it can also improve the productivity and reduce capital costs and energy consumption (Kiss, 2013). It has been applied to the industrial production of MTBE, ETBE and TAME (Al-Arfaj and Luyben, 2004, Domingues et al., 2014, Huang et al., 2008).
Several studies reported the production of Propro in a reactive distillation column (RDC). Kotora et al. (2008) studied the experiment for the Propro production in a pilot-scale RDC, the purity of Propro in the process is 0.698; Keller et al. (2011) studied the experiment of the production of Propro in a pilot-scale RDC assisted with a liquid−liquid phase separator. The purity of Propro in the process is 0.521. Based on the processes, the experimental purities of Propro are below 0.7, this is because POH and water form a minimum-boiling azeotrope so that most of POH is removed from the reaction zone of the RDC. Since POH is not totally reacted in the reactive section, and the chemical equilibrium of the esterification is limited, an extra separation unit is necessary to recover and recycle the unreacted alcohol. Xu et al. (2014) proposed a two-column process to synthesize Propro, they concluded that it is hard to produce high-purity Propro in a single RDC because it could not operate ‘neat’. The two-column process feathers a RDC coupled with a decanter and a recovery column (RC). Consequently, the purity of Propro in the process is 0.9975.
Though high-purity Propro can be obtained in the two-column process, the two-column process needs larger capital cost and energy consumption. To reduce the capital cost and energy consumption, Dimian et al. (2002) introduced a novel entrainer-assisted RD process by adding an entrainer to the reactive distillation system. The entrainer is able to form a minimum ternary heterogeneous azeotrope with alcohol and H2O. The formation of azeotrope can enhance water removal and improve the concentration of reactants. A second column is unnecessary for alcohol recovery. Recently, some researchers studied the entrainer-assisted reactive distillation process. Wang and Huang (2011) concluded that the entrainer assisted reactive distillation process combines both the advantages of heterogeneous azeotropic distillation (HAD) and RD. De Jong et al. (2008) studied the entrainer assisted RD process and the conventional RD process for the fatty acid esters production. They concluded that the E-RD process is able to reach the required conversion of 99%, and the E-RD process needs fewer reactive stages and energy consumption than that of the conventional RD process. Hu et al. (2011) investigated an entrainer assisted RD process for the ethyl acetate production, in the process, N-butyl acetate is selected as the entrainer to carry out H2O from the RD column. The entrainer assisted reactive distillation process can save 32% of energy consumption compared with the conventional RD process. As is concluded in the literature (De Jong et al., 2008, Dimian et al., 2002, Hu et al., 2011, Wang and Huang, 2011), the entrainer-assisted reactive distillation process can not only produce high purity products, but also reduce capital investment and operation costs. To reduce capital cost and energy consumption of the two-column process, the entrainer-assisted reactive distillation (E-RD) process can be used to produce Propro.
Dynamic control is another important aspect in the E-RD process. The control structure of E-RD process is more complex than the control structures of the RD systems and the azeotropic distillation systems. The control of RDC and the azeotropic distillation column have been deeply investigated by many researchers. But the control structure of the entrainer-assisted reactive distillation column has attracted less attention. Chen et al. (2016) proposed a temperature control structure to control a RD process for the Methyl Valerate production. Based on the control structure, the reboiler duty is utilized to control the tray temperature of the column. In terms of dynamic performance, the control structure was able to deal with disturbances, maintain the methyl valerate purity, and get to steady state very fast. Huang et al. (2004) proposed a temperature control structure to control the heterogeneous reactive distillation process. The ratio scheme between the two reactants is used to keep the balance of reactants and the reboiler heat duty is used to control the tray temperature of the column. The dynamic results show that the control structure can reject throughput disturbances very fast and maintain the product purity. Wang and Wong (2006) investigated a temperature control structure to control the entrainer-added RD process for the fatty ester production. In the process, the tray temperature of the entrainer-added RD is controlled by manipulating the reboiler duty. The dynamic results show that the temperature control scheme has good dynamic performance. Hung et al. (2014) investigated the tray-temperature control strategy to control the triacetin reactive distillation process for the utilization of glycerol. The dynamic results show that the proposed tray-temperature control strategy is able to maintain product purity despite disturbances from throughput and feed composition changes. Since temperature control structure has good dynamic performance of reactive distillation, heterogeneous azeotropic distillation and entrainer-assisted reactive distillation. Thus the temperature control structure can be used to the control the E-RD process for the production of Propro.
Though many researches illustrate the advantages of the E-RD process, the E-RD process hasn’t been studied for Propro production, so far. The aim of the research is to investigate the synthesis of Propro by the esterification of POH and ProAc in the E-RD process. A proper entrainer is selected for the E-RD process. And the E-RD process is optimized through calculating the minimum total annual cost (TAC). Moreover, two control structures are proposed to evaluate the stability and controllability of the E-RD process.
Section snippets
Kinetics and thermodynamics
Propro is synthetized by the reversible liquid-phase esterification reaction of ProAc and POH, the reaction equation is shown as:
The esterification reaction is a reversible reaction. It requires to be catalyzed by acidic cation exchange resin (Amberlyst 46™). Amberlyst 46™ has the maximum operating temperature of 120 °C (Ilgen, 2014). We assume the catalyst occupies 50% of the tray holdup volume and the density of the catalyst is 770 kg/m3 (Huang et al., 2004).
The kinetic equation provided by
Two-column process
Xu et al. (2014) investigated a two-column process for the Propro production. Fig. 1 shows the two-column flowsheet. The process consisted of a RDC coupled with a decanter and a recovery column (RC). The RDC was divided into rectifying section, reaction zone and stripping section. ProAc and POH were fed to the top tray and bottom tray of the reaction zone, respectively. The vapor of RDC was mainly the ternary and binary azeotrope formed by POH, water and Propro. Obviously, the azeotrope carried
Design of entrainer-assisted reactive distillation (E-RD) process
In this section, an entrainer-assisted reactive distillation (E-RD) process was designed to reduce TAC compared to the two-column process. E-RD process is a RD process by adding an external entrainer stream to the system. The entrainer can break the water-alcohol azeotrope and enhance the removal of water, thus the separation efficiency of the system can be increased, and the RC for alcohol recovery is unnecessary.
Comparisons of the two-column process and E-RDC process
Table 4 summarizes the optimal design and operating parameters of the two-column process and the E-RD process. The two-column process has two columns with a total of 79 trays while the E-RD process has only one column with a total of 41 trays. E-RD process has fewer reactive stages than that of the two-column process. It can be observed that the E-RD process can reduce the total reboiler duty greatly than that of the two-column process. The E-RD process can reduce the total heat transfer area
Control structure
In this part, the control structure of the E-RD process is developed. Aspen Dynamics can be utilized for the purpose of control study. The tray sizing tool can be applied to calculate the diameter of the E-RD. For the reboiler of the E-RD, it is sized to have the holdup time of 10 min with 50% liquid level. For the decanter, it is sized to have 20 min hold time.
The main control issue is to maintain the propro purity at 99.7% and maintain the water purity at 99.6%. The disturbances considered
Conclusion
In this paper, an entrainer-assisted reactive distillation (E-RD) process is investigated in order to produce high-purity Propro from the esterification of N-Propanol and propionic acid. Cyclohexane is selected as the entrainer in the E-RD process. The optimal design and operating parameters of the E-RD process is conducted by calculating the minimum of total annual cost (TAC). Compared with the two-column process proposed by Xu et al., the E-RD process can save 46.11% of TAC. Furthermore, two
Acknowledgments
Comments and suggestions from two anonymous reviewers are gratefully acknowledged. We are thankful for the assistance from the staff at the Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology from the School of Petrochemical Engineering (Changzhou University).
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