CORE Metadata, citation and similar papers at core.ac.uk Provided by CONICET Digital EFFECT OF VAPOR-LIQUID EQUILIBRIUM DATA ON THE DESIGN OF SEPARATION SEQUENCES BY DISTILLATION C. A. PARODI† and E. A. CAMPANELLA‡ † Facultad de Ingeniería Química, Univ. Nac. del Litoral, 3000 Santa Fe, Argentina [email protected] ‡ INTEC (CONICET- UNL), Güemes 3450, 3000 Santa Fe, Argentina [email protected] Abstract−− An evaluation of the effect of vapor- design of the process. The case studies belong to two dis- liquid equilibrium experimental data on the design of tillation sequences to obtain benzene, acetone, and chlo- separation sequences by distillation was done using roform under certain specifications from a ternary mix- computer simulation. Separation of a mixture of ace- ture of all three components. tone-chloroform-benzene was chosen as an example II. PROCESSES STUDIED AND CALCULATION problem. Two sequences were compared. To quantify DESCRIPTION the thermodynamic data uncertainties for each se- We consider the problem of separation of a mixture of quence two sets of binary vapor-liquid equilibrium acetone, chloroform, and benzene as it was posed in the data were chosen. These two sets of data were used to literature (Westerberg and Wahnschafft, 1996). A 100 generate simulation cases as in classical two-level fac- kmol/h feed of 36 % of acetone (1), 24 % of chloroform torial design of experiments. A third set of binary va- (2), and 40 % of benzene (3) is to be separated into por-liquid experimental data allows comparing phase three products with the following specification: one liquid models. For the two-column sequence, analysis stream with a acetone molar concentration better than done to each column alone or to the whole sequence 99.9%, one stream with a benzene molar concentration gave the same results. In the three-column sequence, better than 99.9 % and one stream rich in chloroform (> results were different and simulation of the whole se- 99 %) with no more than 0.0001 % of benzene. quence gave a complete different account that simula- To accomplish the proposed separation we have tion of each column alone. used the two alternatives of Fig. 1 and Fig. 2 and specifi- Keywords−− Azeotropic distillation, sensitivity cations set in Table 1. In the alternative of Fig. 1 ben- analysis, phase equilibrium, azeotrope. zene is mixed with the feed and introduced to column 1, yielding a pure acetone top product and a bottom of al- I. INTRODUCTION most all the benzene and chloroform in its feed plus a The design of chemical processes involves computer trace of acetone. This bottom product is fed to column simulations. The results of these simulations are 2, where it is separated into benzene and chloroform. strongly dependent on the thermodynamic models for Part of the benzene is then recycled to mix with the the phase equilibrium. While proper choice of the ther- original feed. In the alternative of Fig. 2, we start with modynamic model is important, the uncertainties in ex- the direct split, where we take pure acetone off as a perimental data that are used to regress the model pa- product from the first column. The first column pro- rameters are also significant. Techniques for assessing duces nearly pure acetone from the top and a mixture of these effects are required for improving thermodynamic all three species in the bottom. This mixture is near to modeling, designing experiments and selecting data. the distillation boundary. We feed the bottom product to The effect of property inaccuracies on process design is a second column, which separates benzene (bottom therefore of great importance in the chemical industry. product) from the acetone and chloroform (top product). Studies of this problem have been reported earlier This top product from the second column is separated in (Streich and Kistenmacher, 1979; Nelson et al., 1983; a third column into pure chloroform (top product) and a Hernandez et al., 1984). More recent studies have ana- mixture of acetone and chloroform very near to the lyzed the effect of model (Mandagarán et al., 1999) and azeotrope (bottom product). Finally, the azeotropic mix- of thermodynamic data (Whiting et al., 1999) on calcu- ture is recycled to feed column 1. lated process performance. On the other hand new de- Simulations of the processes were performed with sign methods have been introduced that allows screen- the program HYSYS (Hyprotech software) version ing of distillation sequences (Thong and Jobson, 2001; 2.4.1. Distillation columns have a partial reboiler and a Brüggemann and Marquardt, 2004; Liu et al., 2005). total condenser. Pressure at the top and bottom of the The purpose of the present work is to study the ef- columns was 1 atm. Table 1 presents specification and fect of the uncertainties of the vapor-liquid equilibrium design parameters for each column in the two sequences data on the design of the separation process of a mixture under analysis. Our aim was to compare the results of of acetone, chloroform, and benzene. Through a series separation in each case study obtained on the basis of of case studies we present how thermodynamic models different data for the phase equilibrium. For representing and uncertainties of the thermodynamic data impact the the liquid-vapor equilibrium, we assumed ideal gas be- the auxiliary data used in the simulation are from its da havior for the gas phase and we modeled the liquid tabase. To quantify vapor-liquid equilibrium data uncer- phase with the Wilson equation (Prausnitz et al., 1999). tainties we used a factorial design of experiment Also, results using UNIQUAC and NRTL equations (Gunter, 1993) type of approach. We used three differ- (Prausnitz et al., 1999) for the liquid phase are included ent sets (called levels) of binary vapor liquid equilib in the study. As we did the calculation with HYSYS all Table 1. Summary of column design parameters and specifications. PARAMETER Two - column Three - column Col. 1 Col. 2 Col. 1 Col. 2 Col. 3 Number of stages 68 60 40 54 36 Feed stage (from top) 26 30 18 27 21 Acetone recovery 99.5 % - - - - Distillate rate (kmol/h) - - 35.7 - 24.0 Distillate acetone molar fraction 0.999 - 0.999 - - Distillate chloroform molar fraction - 0.990 - - 0.990 Bottom rate (kmol/h) - - - 40.0 - Bottom benzene molar fraction - 0.999 - 0.999 - Table 2. Wilson binary parameters a) for the system acetone (1) – chloroform (2) – benzene (3) i j aij (cal/mol) aji (cal/mol) Reference Level 0 1 2 116.1171 -506.8518 Gmehling and Onken, 1979 a 1 3 682.4061 -243.9651 Gmehling and Onken, 1979 b 2 3 -71.81089 -11.821 Gmehling and Onken, 1997 Level + 1 2 -107.2916 -394.53953 Fredenslund et al., 1977 1 3 120.68724 203.32064 Fredenslund et al., 1977 2 3 -217.86487 34.59343 Fredenslund et al., 1977 Level – 1 2 37.087174 -453.83994 Kojima et al., 1991 1 3 468.67944 -155.58156 Kojima et al., 1991 2 3 -296.68263 225.73953 Kojima et al., 1991 a) ln γi = 1 – ln ∑i xj Aij – ∑k ( xk Aki / ∑j xj Akj ) ; Aij = ( vj / vi ) exp (–aij / RT ); i,j,k = 1,..., n Table 3. Comparison of data and Mc Cabe-Thiele results at P = 1 atm. FACTOR DESCRIPTION EFFECT a) 1 b) 2 3 4 5 A Acetone – chloroform binary -46.91 -37.40 -0.82 17.87 34.39 B Chloroform – benzene binary -11.72 -20.35 -15.01 -8.95 -1.75 C Acetone – benzene binary -12.14 -10.07 -14.09 -16.7 -21.50 a) % variation in reflux ratio of column i (Ri); [(Ri (level – ) – Ri (level + )) / Ri (level +)] * 100 b) Molar fraction of more volatile component: 1) x = 0.2, 2) x = 0.35, 3) x = 0.5, 4) x = 0.65, 5) x = 0.8. Figure 1. Two – column flowsheet for the acetone (1) – chlo- Figure 2. Three – column flowsheet for the acetone (1) – chlo- roform (2) – benzene (3) separation roform (2) – benzene (3) separation. rium data to make the analysis. The experimental data references for the binary vapor-liquid equilibrium are presented in Table 2 that also shows the values of Wil- son binary parameters. Three levels of data were used: "0" level to compare phase liquid models, and "+" and "–" levels in the factorial approach. Figure 3 shows the setting in three factor two level design involving 9 simulations. Figures 4, 5 and 6 display the difference between the level + and – of the experimental data. The level + is obtained when UNIFAC (Fredenslund et al., 1977) is used to get the Wilson parameters. UNIFAC represents an "average" set of experimental data, as UNIFAC is a Figure 5. Comparison of x–y and t–x diagrams of isobaric data Figure 3. Points describing the settings in three factor design. level + and level – for the system chloroform (1) – benzene Points at the center and corners of the cube are the cases of (2). Binary parameters values in Table Table 4. 2. Figure 4. Comparison of x–y and t–x diagrams of isobaric data Figure 6. Comparison of x–y and t–x diagrams of isobaric data level + and level – for the system acetone (1) –chloroform (2). level + and level – for the system acetone (1) – benzene (2). Binary parameters values in Table 2. Binary parameters values in Table 2. predictive method that summarized a big number of ex- por-liquid equilibrium data and considering the effect perimental data. The level – is a set of data reported by on column reflux ratio. For each binary system (factor), Kojima et al. (1991). Then, the difference between the we considered two levels of experimental data as men- "average" data and the Kojima's data is used to repre- tioned.