RETROFITTING OF HEN

Abstract

The present study undertakes a theoretical investigation for the retrofitting of heat exchanger network operating as crude preheat train. The problem of retrofitting a HEN is generally related with generation of near optimal HEN, which can then he screened out and selected by the user based on ground realities existing in the industry. Process of retrofitting generally starts from the MER design and then moves down the hill to develop different suboptimal topologies of HEN. In the present study an algorithm for retrofitting of HEN as proposed by Tjoe and Linnhoff is used as the base algorithm and then it was improved upon. The deficiency of above algorithm has been removed by considering the stream pressure drops as a parameter fcr operating cost during the targeting stage. In the present investigation it has been found that a saving of 20% in the total heat exchange area required can be realized when the stream pressure drops are considered. The area efficiency of the existing network is found to be 0.74 and hence any proposed near optimal topology for retrcfitting should have an area efficiency of more than 0.74. The existing heat exchangers operating in the crude preheat train are tried to he adjusted in the best possible way wherever possible and with this two topologies were suggested for retrofitting. The first topology proposes installation of 8 new exchangers and the second topology proposes installation of 5 new exchangers. In both the topologies, the existing area is used completely and the retrofitted network shows an area efficiency of 0.92. A saving of 30717 kW in hot utility are achieved, which is equivalent to a saving of Rs1.125x108/yr. Payback period is found to be very attractive and the money invested can be recovered back in 2.07 years.