Effect of Two-Liquid Phases on the Dynamic and the Control of Trayed Distillation Columns Gardênia Cordeiro, Karoline Brito, Brenda Guedes, Luis Vasconcelos, Romildo Brito Federal University of Campina Grande Chemical Engineering Department Av.AprígioVeloso, 882, Bodocongó, Campina Grande, PB, 58109-970, Brasil romildo.brito@deq.ufcg.edu.br In a previous work, we show that the energy saving associated with an azeotropic distillation column is closely related to the formation of two liquid phases in some plates. At this time, defining a formal objective function and considering the restriction imposed on the process, this paper quantifies the reduction (optimization) of energy consumption and presents the dynamic and the control of the process for two optimized operational conditions: with and without two-liquid phases on stages. For both conditions, the results indicate that it is possible to maintain the specification of the product using temperature as controlled variable; however, for operating the column with two liquid phases that one should avoid controlling the temperature of plates with two liquid phases. 1. Introduction The term azeotropic distillation is applied to the class of distillation techniques based on fractional separation in which azeotropic behavior is explored in order to achieve the separation. The agent that causes the specific azeotropic behavior, often called the separation agent (entrainer), may already be present in the feed (self-entraining) or can be added as an agent of mass separating. Widagdo and Seider (1996) published one of the most complete reviews on azeotropic distillation, where they related the scant information from the literature to a real understanding of the processes involved, as well as the difficulties involved in the control of these processes. This review intends on the two liquid phases inside the column; according to some authors, the formation of these two liquid phases drastically reduces the efficiency of the stages, a position contested by other authors. Although the number of papers involving azeotropic distillation is large, there is a lack of published studies concerning the dynamic and the control of azeotropic distillation column with two liquid phases on plates, since, studies have considered two liquid phases only in the external decanter (Herron Jr, 1998; Bekiaris et al., 2000; Alliet- Gaubert, 2001; and Luyben, 2008). In Ciric et al. (2000), the azeotropic distillation column involving an internal decanter. According to the consulted literature, just Guedes et al. (2007) studied the problem of the dynamic and the control of azeotropic distillation column with two liquid phases on plates. However, the initial motivation was to evaluate the possibility of energy saving for an azeotropic distillation column.
2. Problem Statement The distillation column considered in this article is part of the purification train of 1.2- dichloroethane (EDC) of a commercial plant producing vinyl monochloride (MVC). Figure 1 shows the flowsheet of the dehydration process of EDC. In the reflux vessel there are two liquid phases: an organic one, saturated in H 2 O, and an aqueous one, saturated in organic matter. The stream of the organic phase, comprising mainly of chlorinated organic substances saturated with H 2 O, returns to the reflux of the column, while the stream of the aqueous phase is discarded. Figure 1: Flowsheet of the dehydration process of EDC. Although less volatile than the EDC, the H 2 O leaves from the top of the column due to the reversal in the value of the K-value, which is due to the fact that H 2 O forms a minimal azeotrope with almost all organic compounds present in the process. A close analysis of Figure 1 leads to the conclusion that the system as a whole can be seen as a conventional column - with reboiler, condenser and reflux vessel; with the feed (FROMR1 and WATER streams) in the reflux vessel. However, considering the perfect level control in the decanter (reflux vessel), there is only one degree of freedom; unlike what is observed for a conventional column, which has two. In industry, the variable used for manipulation is the reboiler heat duty. According to Guedes et al. (2007), the reboiler heat duty was gradually reduced when, after a certain amount, it was noted there was a significant change in the profiles of concentration and temperature along the column, especially when there was the formation of a 2 nd liquid phase on the plates of the column. However, since the reduction of heat duty was carried out without formally defining the objective function and constraints (optimization) and since no control study for the new operating conditions (with and without formation of two liquid phases on stages) was performed, the work of Guedes et al. (2007) only partially answered the two questions above; this work is a complement of the work published by Guedes et al. (2007).
3. Modeling and Simulation In the steady-state, the simulation was performed using the commercial simulator Aspen Plus, version 7.1 as a tool. In the article by Guedes et al. (2007), the data obtained from simulations were compared (validated) with the data from the plant. To model the column in question, the RadFrac routine was used. The system was modeled using reboiled absorption, followed by a condenser (Heater) and a decanter (Decanter). In this study, a Murphree efficiency equal to 64% for all plates was used. In the industrial plant, the column has 25 stages (numbered from top to bottom) and a reboiler type thermosyphon. For the representation of the equilibrium between liquid-liquid-vapor phases (LLVE), a γ-φ procedure was used; Redlich-Kwong Equation of State (EOS) for vapor phase and NRTL model for the activity coefficient. In order to determine the optimal energy consumption, the objective function (J) to be minimized was defined as the reboliler heat duty (Qr). Optimization was performed considering two situations: with and without the formation of two liquid phases on the plates of the column. When the two liquid phases on the plates were allowed to form, the restriction was the mass fraction of H 2 O in the bottom of the column (x B H2O ); for which value was set at 10 ppm. When the formation of the two liquid phases was not allowed, the restriction was 2,500 ppm of H 2 O in the liquid phase of the 1st stage (counted from top to bottom) of column (x #1 H2O ). The choice for the 1 st plate occurred after verifying that the formation of the 2 nd liquid phase starts on this plate. The optimization procedure considered the distillate flowrate (stream OCSUM1) as the manipulated variable. The objective function was inserted through the Model Analysis / Optimization of the Aspen Plus TM tool, which uses the Sequential Quadratic Programming (SQP) search method for the optimum. The restrictions were inserted through the Model Analysis / Constraint tool. 4. Steady-state Results Table 1 illustrates the current conditions from the reactor (FROMR1). Table 2 presents the results for the two optimized situations. With the formation of two liquid phases (Case II), the reduction in energy consumption comparable to the situation of a single liquid phase (Case I) is close to 19%. Given that the problem contains only one constraint, at the end of optimization, this restriction was active for both cases and determined the final value of the reboiler heat duty. In Figure 2, the large distinction between the temperature profiles for the two situations should be noted. For Case I the initial variation is significant, increasing almost linearly from stage 5. For Case II, the initial variation is almost linear until stage 17, and increases significantly after that stage. Although Case II (two liquid phases) presents a reduction of energy consumption, it is necessary to evaluate the control process in this condition.
Temperature, K According to the SVD analysis (Luyben, 2005), for Case I the temperature to be controlled is in stage 1, while for Case II, it is in Stage 16. These results were used in closed loop simulations for temperature, which are presented in the following item. Table 1: Characteristics of the feed stream. Value Temperature, (K) 313.15 Flowrate, (kg/)s 16.46 Mass fraction 1,1 dichloroethane 9e-05 Carbon-tetrachloride 2e-05 1,2 dichloroethane (EDC) 0.99398 Water 0 1, 1, 2 Trichloroethane 0.0013 1, 2, 3 Trichlorobenzene 0.00461 Table 2: Main results of the optimizations. Case I Case II Reflux flowrate (kg/s) 17.71 16.84 Distillate flowrate (kg/s) 1.28 0.41 Reboiler heat duty (J/s) 1.7427E6 1.4047E6 368 363 Case I Case II 358 353 348 343 0 4 8 12 16 20 24 28 Stage Figure 2: Temperature profile for the two optimized situations. 5. Dynamic Results Figure 3 shows the transition from Case I (a liquid phase) to Case II (two liquid phases) and vice versa. For both situations, the disturbance was realized after 2 h of steady-state operation. For Case I the reboiler heat duty was reduced (with a view to provoking the 2 nd liquid phase) from 1.7427e6 J/s to 1.3723E6 J/s, whereas for Case II an increase (for the purpose of having the 2 nd liquid phase go away) from 1.3258E6 J/s to 1.4047E6 J/s was brought about.
Split fraction Split fraction 1.005 1.005 1.000 1.000 0.995 0.990 0.985 Stage 1 Stage 11 Stage 20 0.995 0.990 0.985 Stage 1 Stage 11 Stage 20 0.980 0.980 0.975 0.975 0.970 (a) 0.970 (b) Figure 3: Transition between the existence of one and two liquid phases to change in the reboiler heat duty: (a) Case I. (b) Case II. Figure 3b shows that the disappearance of the 2 nd liquid phase, in the stages where this 2 nd phase is present, begins at the bottom region of the column, it being at this point where the effect of disturbance will be felt earliest, i.e. when the H 2 O is removed. Figure 3a also shows that in stage 11, the 2 nd liquid phase appears only about 3 h after the disturbance and that stage 20 did not show the formation of the 2 nd liquid phase. The open loop results indicate that the transition to the situation with a liquid phase occurs much faster.. The results in closed loop for temperature are shown in Figure 4. As can be seen in Figure 4a, the temperature of the plate 16 remained at the set-point and the variation was even lower (solid curve). However, the base product was totally out of specification. While plate 16 is the penultimate to present two liquid phases, it was decided to control the temperature of the next plate presenting a single liquid phase (plate 18). The result is represented by the dotted curve in Figure 4b, where it can once more be observed that the temperature is suitably controlled and the offset of the mass fraction of H2O in the bottom stream was less than 2 ppm. 6. Conclusions The operation of the column with two liquid phases over a number of stages satisfies the specification of the mass fraction of H 2 O at the bottom stream and consumes less energy. For the two cases the most distinct mass transfer occurs in different regions of the column: on the upper region for Case I and, for Case II, at the bottom region.
Temperature, K Mass fraction, ppm 356 1600 355 1200 354 353 TC = 16 TC = 18 800 352 351 400 TC = 16 TC = 18 350 (a) 0 (b) Figure 4: Closed loop transient behavior for temperature (a) and mass fraction of H 2 O at bottom stream (b) to change in the feed flowrate to the process. Case II: 10% increase in the valve opening. For some plates, the temperature can be used as an indicator of the appearance or disappearance of a 2 nd liquid phase. For both situations, the control system maintains the controlled variable (temperature) close to the set-point. However, the results indicate that one should avoid controlling the temperature of plates with two liquid phases. References Alliet-Gaubert M. A., Gerbaud V., Joulia X., Sere Peyrigain P. and Pons M., 2001, Analysis and Multiple steady states of an industrial heterogeneous azeotropic distillation, Ind. Eng. Chem. Res., 40, 2914-2924. Bekiaris N., Guttinger E. G. and Morari M., 2000, Multiple steady states in distillation: effect of VL(L)E inaccuracies, AIChE J., 46, 5. Ciric A. R., Mumtaz H. S., Corbett G., Reagan M., Seider W. D., Fabiano L. A., Kolesar D.M. and Widagdo S., 2000, Azeotropic distillation with an internal decanter, Comput. Chem. Eng., 24, 2435-2446. Guedes B. P., Feitosa M. F., Vasconcelos L. G. S., Araújo A. C. B. and Brito R. P., 2007, Sensitivity and dynamic behavior analysis of an industrial azeotropic distillation column, Sep. Purif. Technol., 56, 270-277. Herron Jr C. C., Kruelskie B. K. and Fair J. R., 1998, Hydrodynamics and mass transfer on three-phase distillation trays, AIChE J., 34. Luyben W. L., 2008, Control of the heterogeneous azeotropic n-butanol/water distillation system, Energy & Fuels, 22, 4249-4258. Widagdo S. and Seider W. D., 1996, Azeotropic distillation, AIChE J., 42, 1, 96-130.