ANALYSIS OF OPTIMAL HEAT INTEGRATED DISTILLATION COLUMN

Abstract

The present investigation pertains to a theoretical study of analysis of an optimal sequence, of heat integrated distillation columns. Basically it deals with the development of mathematical formulation of optimal sequence of heat integrated distillation columns. It includes an algorithm for the synthesis of optimal sequence of distillation columns. It also describe the effect of operating variables viz. column pressure and the fraction of feed flow rate (the ratio of feed flow rate to the maximum possible feed flow rate to a column) on the optimal sequence. In the last it describes feasibility of the optimal sequence under alteration of feed flow rate and feed composition. Using the list slitting techniques along with heuristics the superstructure of three component feed (Hexane, Benzene, Heptane) has been developed in this investigation. Further feasible heat matches basedonATmm.hna ve been identified in. the superstructure. Pressure in a column of sequence has been considered to be a continuous variable. Based on shortcut simulation cost coefficients and coefficients of linear relationship of flow rate and heat duty of condeftser and reboiler have been evaluated. The synthesis problem of distillation columns has been formulated as MILP (mixed integer linear programming) model. The model developed has been solved as the transportation problem. From this analysis it has been inferred that the optimal sequence of the - heat integrated distillation columns matches with the sequence obtained by Floudas and Paules. The MILP represented as transportation formulation consists of an objective function in the form of total annual cost comprising of investment as well as operating costs and constraints. The heat integration options have been incorporated by assigning potential reboiler-condenser match an integer variable. Solution of the MILP has been obtained through a commercial solution code. The effect of operating variables viz. column pressure, and the flow rate of feed on individual distillation tasks have been studied. Increase in column pressure and flow rate of feed increases the total annual cost of the column sequence. Feasibility of the optimal sequence of the distillation column under variation of feed composition and feed flow rate has been studied by calculating the flexibility index which is a major of structural flexibility of the column. In this study the value of flexibility index is .3896, implying that the maximum limit of changes in the feed flow rate to individual columns is 38.96% of the base feed flow rate otherwise the sequence will become inoperable. This is an important index which provides the information to plant personnel to change their feed flow rate to individual columns and monitor them in the feasible ranges of operation.