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Ind. Eng. Chem. Res. 2009, 48, 6358–6362
Influence of p-Xylene (PX) Accumulation on the Operation of Pure Terephthalic Acid (PTA) Solvent Dehydratic Distillation Column Shaojun Li Institute of Automation, East China UniVersity of Science and Technology, Shanghai 200237, People’s Republic of China
In the production of pure terephthalic acid, a tiny amount of reactant p-xylene may enter into the acetic acid dehydration distillation column through the feed stream. In normal operation, p-xylene will not leave the column from the top or bottom product. It will be accumulated in the top of dehydration distillation column and decanter. With the accumulation of p-xylene, the operation cost will be increased gradually, to ensure the purity specifications of the top and bottom products. To reduce the operation cost, it is necessary to cut down the concentration of p-xylene by draining some organic reflux and make up the pure entrainer intermittent. In this paper, reboiler duties at different concentrations of p-xylene in the reflux were analyzed. The optimal operation conditionsthat the concentration of p-xylene is decreased by draining some organic reflux when the mass fraction of p-xylene accumulation exceeds 0.15shad been suggested. Finally, the residue curves diagram of three key components in the top of the column was used to interpret the draining place of p-xylene. 1. Introduction Acetic acid (HAc) dehydration is an important operation in the production of aromatic acids, such as pure terephthalic acid (PTA). Although acetic acid and water do not form an azeotrope at atmospheric pressure, using conventional distillation to separate these two components would require more equilibrium stages and a large reflux ratio. Such a process requires high investment and operating costs. Because the system has a tangent pinch on the pure-water end, it is customary to use an entrainer via a heterogeneous azeotropic distillation column for the separation, to reduce the investment and operating costs. The designs of acetic acid dehydration systems that utilize an entrainer have been addressed in the literature. Before 1932, ethylene dichloride was used as an entrainer; now, the most generally used entrainers are acetic esters, such as normal propyl acetate (NPA), normal butyl acetate (NBA), isobutyl acetate (IBA), and ethyl acetate (EA). In a review, Othmer1 discussed an azeotropic distillation system that contained a dehydration column, a decanter, and a water column for the separation of acetic acid and water. Chien et al.2 discussed the design and control of acetic acid dehydration system using three candidate entrainers (ethyl acetate, isobutyl acetate, and normal butyl acetate), and optimal column designs and operating conditions were obtained for these three candidate systems through rigorous process simulation. Later, Chien et al.3 investigated the influence of feed impurity on the design and operation of an industrial acetic acid dehydration column in which isobutyl acetate was used as the entrainer. Huang et al.4 and Lee et al.5 also addressed the design and control of acetic acid dehydration column with p-xylene (PX) or m-xylene (MX) feed impurity in which IBA was used as the entrainer. Wasylkiewicz et al.6 proposed a geometric method for the optimal process design of an acetic acid dehydration column using NBA as an entrainer. Huang and Chien7 investigated batch distillation system for acetic acid dehydration. By comparing the performances of the batch operation of vinyl acetate and ethyl acetate by dynamic simulation, they showed that the acetic acid dehydration system that used vinyl acetate as an entrainer resulted in less batch time and better recovery of the water and the entrainer.
Most of the mentioned works about the acetic acid dehydration system are focused on the issues of process synthesis and design and control; however, little attention is given to the influence of impurities on the industrial azeotropic distillation column, especially on the system using NBA as an entrainer. In this paper, the design and optimal operation of an industrial column for acetic acid dehydration via heterogeneous azeotropic distillation is investigated. As known, heterogeneous azeotropic distillation is commonly used in industry to separate azeotropes or mixtures with a relative volatility of ∼1. Some investigations on azeotropic distillation were comprehensively reviewed by Widagdo and Seider.8 In this work, NBA is used to aid the acetic acid and water separation. This entrainer is recycling inside the column through an organic reflux stream from a decanter. One feed from the upstream process is fed into the dehydration distillation column. The feed contains not only acetic acid and water, but also small amounts of methyl acetate (MA) and PX. Although the amount of PX is very small, it can have a great impact on the separation of acetic acid and water. In this paper, the influence of PX on separating performance will be investigated. This paper is organized as follows: In Section 2, the thermodynamic properties of the acetic acid dehydration distillation system with PX and MX as impurities is described, and the validation of thermodynamic models with boiling points of components and azeotropes are illustrated for the system. In Section 3, the acetic acid dehydration distillation column is simulated using Aspen Plus software. The temperature and component profile curves for the azeotropic distillation column are listed to illustrate the simulating result. In Section 4, the influence of PX on the separation of dehydration distillation column is discussed. Particularly, a detailed investigation is addressed for the mass fraction profiles of PX at different PX reflux concentrations. Some concluding remarks are given in Section 5. 2. Thermodynamic Model The acetic acid dehydration azeotropic distillation using NBA as an entrainer includes five key components: acetic acid, water, NBA, PX, and MA. To represent the system accurately, the
10.1021/ie900526n CCC: $40.75 2009 American Chemical Society Published on Web 05/28/2009
Ind. Eng. Chem. Res., Vol. 48, No. 13, 2009
3. Simulation of Acetic Acid Dehydration Column
Table 1. UNIQUAC Binary Parameters for the Azeotropic Distillation Systema component i component j PX PX HAc HAc HAc water water NBA
HAc water water NBA MA NBA MA MA
aij
aji
bij
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bji
-338.67 54.63 -965.70 -325.61 0.7446 0.0042 -615.26 196.90 140.77 -336.68 180.32 -300.57 -232.22 -345.06 2.54 -18.28 -880.03 5656.49 -5.17 4.07 1614.67 -1298.84
a Legend: PX ) p-xylene; HAc ) acetic acid; NBA ) normal butyl acetate; and MA ) methyl acetate.
selection of the thermodynamic model and appropriate parameters is very important. In this paper, a liquid activity coefficient model is used to describe the phase equilibrium. An extended universal quasichemical (UNIQUAC) model parameter set has been selected from the Aspen Plus9 built-in association parameters. Second virial coefficients of Hayden and O’Connell10 are used to account for the vapor-phase association of acetic acid due to dimerization and trimerization. The Aspen Plus built-in binary association parameters are used to compute fugacity coefficients. The complete parameter sets of UNIQUAC model are listed in Table 1. According to the computation of Aspen Split, five azeotropes, including two homogeneous azeotropes (HAc-PX and MA-water) and three heterogeneous azeotropes (PX-water, NBA-water, and NBA-PX-water), can be obtained. The azeotropic compositions and temperatures that are predicted using the UNIQ-HOC model, in comparison with the experimental data of azeotrope from Wang et al.11 and Chien et al.,2 are listed in Table 2. The ternary azeotropic data have not been found in the literature. By comparing the binary azeotropes data, we can determine that small differences exist between these two sets of data. Thus, we can use the model and parameters to describe the dehydration system. With the entrainer added into the system, the difficult tangent pinch of the pure-water side can be avoided at the top of the column. Because the entrainer (NBA)-water azeotrope is heterogeneous, the top vapor stream will be coagulated and must be separated into two liquid phases in a decanter. The organic phase will be refluxed back to the heterogeneous azeotropic distillation column, to provide enough entrainer to the column. The aqueous phase that contained mostly water will be drawn off from the system for further treatment or discharge. Some of the aqueous phase can be refluxed back into the azeotropic column if the concentration of acetic acid in water is higher than the specifications. For the HAc-water-NBA system with PX as an impurity (not considering MA), the temperatures of the normal boiling points of the pure components and azeotropes point can be ranked as follows: PX-NBA-water (90.88 °C) < NBA-water (90.94 °C) < water (100.00 °C) < HAc-PX (115.3 °C) < HAc (117.90 °C) < NBA (126.11 °C) < PX (138.36 °C) PX has the maximum boiling temperature and PX-NBA-water has the minimum azeotropic boiling temperature of the system, if we do not consider the MA component. From the design principle of this separation system, the column top vapor components should be near the PX-NBA-water and NBA-water azeotropes, whereas the bottom is almost-pure HAc corner. PX can form an azeotrope with water or HAc, and it can also form a ternary azeotrope with water and NBA. Its appearance changes the relative volatility of water and HAc. PX will have a great effect on the separation of water and acetic acid.
The azeotropic temperature determines the temperature difference between the top and bottom of the column. A large temperature difference implies good separability and less column stages or a lower reflux ratio. The quantity of entrainer should be strictly controlled to decrease the consumption of energy. The dosage of entrainer should be strictly controlled, especially when the boiling temperature of the entrainer is higher than that of acetic acid in the system, to prevent it from falling to the bottom and bringing forward the loss of entrainer. Figure 1 shows the flowsheet of a PTA plant in China that been designed according to Mitsui technology. T501 is the dehydration distillation column; it has 61 stages (including a partial reboiler, numbering from the top of the column to the bottom), and it is operated at 1 atm. Its pressure drop is 25 kPa. It includes one feed stream (34) and three refluxes streams (77, 78, and 79). A four-component mixture including HAc, water, MA, and PX at 65 °C and 1.03 atm is fed to the 35th stage. One organic reflux (stream 78) and one aqueous reflux (stream 77) return to the first stage. To control the column quickly, there is one organic (stream 79) reflux back to the 27th stage. The feed composition considered for the Aspen Plus simulation is 26.8 mass % acetic acid, 72.96 mass % water, 0.24 mass % MA, and 18 ppm (mass) PX, according to the plant data. The flow rate of feed (stream 34) is 15710 kg/h at 65 °C. The decanter (D501) temperature is 40 °C. In the rigorous simulation, the bottom HAc composition is set at 92.5 mass %. It is unnecessary to have a high-purity HAc because acetic acid will return back to the oxidation reactor, in which water will be produced as a byproduct. Usually, the composition of acetic acid in the bottom of the column is set at 5-8 mass %, for the sake of saving energy. It is unlikely that the acetic acid dehydration distillation column in the production of vinyl acetate can control the water in the bottom of the column to 0.15, it is better to drain some organic reflux and make up NBA to reduce the concentration of PX in the reflux. Figure 6 gives the PX mass fraction profiles in the azeotropic distillation column at different concentrations of PX in the reflux. In the figure, the abscissa (“0”) denotes the organic reflux drum. The place of maximum concentration varies with the mass fraction of PX in the reflux. When the mass fraction of PX in the reflux drum is >0.15, the fraction of PX decreases gradually as the tray number increases. The bigger fraction is located in the reflux drum. When mass fraction of PX is >0.18, the maximum mass fraction of PX is no longer located in the reflux drum. The mass fraction of PX at the first tray is less than that in the reflux drum. As the number of trays increases, the fraction of PX increases gradually to the maximum at a tray between the first tray and the feed tray. After that tray, the fraction of PX decreases gradually near to zero. When the mass fraction of PX is >0.15, the bigger the mass fraction of PX, the nearer the maximum concentration appears to the feed tray. The simulation result shows that, when the mass fraction of PX in reflux is
