Performance Assessment of Shell and Tube Heat Exchangers in an Ammonia Plant

Abstract

The main objective of this work is to assess the performance of two heat exchanger units (Stripper/Gas Overhead Condenser and Methanator Effluent Cooler) operating under steady state conditions in two stages. Two different methods are employed in monitoring the heat exchanger fouling, namely dirt factor trend method and a statistical control technique where a Cumulative Sum (CuSum) chart is used to check the stability of the process. Data were obtained through steady state monitoring and direct measurements from the plant. The data were analyzed using various energy equations and a computer program to determine the overall heat transfer coefficient, heat duty, capacity ratio, corrected log-mean-temperature difference, fouling factor, temperature range of both fluids and effectiveness. The result shows that for the Stripper/Gas Overhead Condenser, the overall heat transfer coefficient was 63.13% less than the design value in stage1 and 12.59% less in stage2. For the Methanator Effluent Cooler the value of heat duty and the overall heat transfer coefficient were 51.76% and 59.62% less respectively than the design value in stage1 and 30.72% and 30.16% less respectively in stage2. This was traceable to increase in heat transfer rate as a result of injecting the tubes of the exchangers with NALCO fluid. In order to detect small changes in the heat exchanger operation and to know the actual time fouling starts to build up, a Cusum chart is used. This work made use of QI Macros software to check the stability of the heat exchanger units and to know if processes are on-target.