INTEGRATION OF DISTILLATION COLUMN USING PINCH TECHNOLOGY

Abstract

In chemical industries, the task of separation such as distillation is an energy intensive process, and it is still the most widely used technique for fluid separations. Distillation columns are used for about 95% of liquid separations and the energy used for this process accounts for an estimated 3% of the world energy consumption. With rising energy awareness and growing environmental concerns there is a need to reduce the energy use in industry. For the distillation process any energy savings should have an impact on the plant energy consumption. The use of heat integration and more complex configurations for distillation columns hold great promise for energy savings. The above fact makes it imperative to have a technique similar to pinch analysis for improving distillation column designs. Integration of the distillation column using pinch technology is one such technique based on thermodynnmics for identifying_appropriate_ design modifications with respect to energy conservation. These design modifications makes it possible to save substantial amount of energy resulting in optimum or near-optimum column design. Integration of the column can be within the column or with the background process. A lot of work is carried out in energy savings in distillation column in which much of the work is related to integration of column with the background process and little work is done on internal integration of column. The present study is related to internal integration of distillation column using pinch technology by considering the column modification using Column Grand Composite Curve (CGCC). CGCC is a profile of net heat surplus or deficit over various trays corresponding to minimum thermodynamic condition (MTC) of the column, on the temperature-enthalpy axis or stage-enthalpy axis. The MTC for a distillation column represent a reversible operation or zero thermodynamic loss in the column. iii Aspen Plus software is used to simulate the column and to generate column targeting results, to plot required CGCC for the problem under study and to test different strategies such as feed location, reflux ratio modification, feed conditioning and side-reboiler/ side-condenser to decrease the heat load of the column so that savings in operating cost could be made. By the addition of pre-heater the load on distillation column can be curtailed to some extent. An optimally designed pre-heater will obviously do the job in a better way. It is a well known fact that optimal design of heat exchanger is a tedious work and takes substantial time. In the present work a heat exchanger optimization technique is refined and tested using an equation based technique based on Bell's method in which constraints are plotted on pressure drop diagram to obtain minimum —area and — total annual cost. A case study of the kerosene pre-fractionation unit of a refinery is considered and Column Targeting technique is applied. Two options are considered; Option A: Without capital investment and Option B: With capital investment The economic analysis for Option A indicates the annual saving of Rs. 25.25 Crores whereas for Option B the annual saving is Rs. 48.14 Crores with an additional investment in preheater and side reboiler. The payback period Of the additional equipment i.e. preheater and reboiler comes out to be 7.4 days. This payback period can be further reduced if preheater and reboiler are designed optimally. By adopting the optimization techniques developed in the present work the payback period of preheater can be reduced from 7.4 days to 3.8 days. Similarly the overall payback period for additional equipments can be reduced from 7.4 days to 6.5 days.