Simulation of Batch Distillation

Prof. Dr. Nada B. Nakkash

      The importance of batch distillation process in chemical, petroleum and pharmaceutical industries to separating the liquid mixture into their constituent components. The present work is concerned with developing a computer program using short-cut model and a simulation computer program using rigorous model to analyze multi-components non ideal mixture, multistage batch distillation column using MATLAB version 6.5. The short-cut model has been used to calculate minimum number of stage using Fenske equation and minimum reflux ratio using Underwood equation. The Theoretical number of stages required to distillate the mixture of Benzene-Toluene-Ethylebenzene is calculated by Gilliland correlation that related minimum number of stage with the actual number  , and that have been used in rigorous method to calculate the concentration profile, temperature profile, vapor and liquid flow rate on each stage.  Rigorous model composed of the MESH (Material balance, Equilibrium , Sum of mole fractions ,Heat balance)equations.The results obtained for both short-cut and a rigorous model gave good agreement with previous work [1,2] and with experimental work [3]. Different liquid phase activity coefficient models have been studied such as (Wilson,NRTL, UNIQUAC, UNIFAC) for Benzene-Toluene-Ethylebenzene system and the appropriate model was selected according to the purity of the benzene that is separated as a top product. The results showed that UNIFAC and NRTL models give more acceptable results. In the present work the UNIFAC model has been used because it is modern and the parameters used in this model are easy to obtaine.  The distillation column was operated under constant operating pressure of 1 atm, 8 number of theoretical stages and initial feed temperature at 298.15 K.The influence of various parameters such as reflux ratio (1 , 2 , 4), batch time (0.5 - 2 hr), liquid hold up in condenser and on each tray ((1.25 , 0.5) , (5 , 1) , (20 , 5)mole), respectively and initial feed composition of benzene in the still (0.2 , 0.3 ,0.5 mole) upon the performance of the batch non-ideal distillation have been studied through vapor and liquid flow rate profiles, temperature profile, liquid mole fraction profile ,heat input to the still and heat out put from the condenser.Increasing the reflux ratio the composition of the desired component (Benzene)decreases in the distillate at 0.5 hr comparison with the composition of benzene in the distillate when decreasing the reflux ratio.Increasing the batch time the distillate composition of benzene decreased .Changing the liquid hold up in plates and in condenser  give different results, increasing  the liquid hold up in plates and condenser cause decreasing in distillate composition. Increasing the initial feed composition of light component in the still give high quantity in distillate. The best design condition for operating batch distillation to obtain purity of the desired product was reflux ratio equal to 1 ,  batch time less than 1 hr, liquid hold up in condenser and in each tray  is 1.25 and 0.25 mole, respectively and the initial feed  
composition of benzene is 0.5 mole in the still.

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