MODELING, SIMULATION AND OPTIMIZATION OF INDUSTRIAL HEAT EXCHANGER NETWORK FOR OPTIMAL CLEANING SCHEDULE

Sustaining the thermal and hydraulic performances of heat exchanger network (HEN) for crude oil preheating is one of the major concerns in refining industry. Virtually, the overall economy of the refineries revolves around the performance of crude preheat train (CPT). Fouling in the heat exchangers deteriorates the thermal performance of the CPT leading to an increase in energy consumption and hence giving rise to economic losses. Normally the energy consumption is compensated by additional fuel gas in the fired heater. Thus, increase of energy consumption causes an increase in carbon dioxide emission and contributes to green house effect. Due to these factors, heat exchanger cleaning is performed on a regular basis either by chemical or mechanical cleanings. The disadvantage of these cleanings is the potential environmental problem through the application, handling, storage and disposal of cleaning effluents. Nevertheless, the loss of production caused by plant downtime for cleaning is often more significant than the cost of cleaning itself, particularly in refineries. Thus, it is essential to optimize the cleaning schedule of heat exchangers in the HEN of CPT.
The present research focuses on the analysis of the effects of fouling on heat transfer performance and optimization of the cleaning schedule for the CPT. The study involves collection and analysis of plant historical operating data from a Malaysian refinery processing sweet crude oils. A simulation model of the CPT comprising 7 shell and tube heat exchangers post desalter with different mechanical designs and physical arrangements was developed under Petro-SIM™ environment to perform the studies.
In the analysis of effects of fouling on heat transfer performance in CPT, the simulation model was integrated with threshold fouling models that are unique to each heat exchanger. The fouling model parameters are estimated from the historical data. The simulation study was performed for 300 days and the analysis indicated that the position of heat exchangers has a dominant role in the heat transfer performance of CPT under fouled conditions. It is observed from this simulation study, fouling of upstream heat exchangers of the CPT will have higher impact to overall heat transfer performance of the CPT. For the downstream heat exchangers, the decline in their heat transfer performances due to fouling can be compensated by the log-mean temperature difference (LMTD) effect, which will reduce or even increase the heat transfer performance of these heat exchangers.
An optimization problem for the cleaning schedule of the CPT was formulated and solved. The optimization problem considered un-recovered energy cost and cleaning cost of the heat exchangers in the objective function. Optimization of the cleaning schedule was illustrated with a case study of simulation over a period of two years. Constant fouling rates that are extracted from the historical data are used to estimate the fouling characteristics of each heat exchanger in the CPT. For the purpose of comparison, a base case was developed based on the assumption that the heat exchangers will be cleaned when the maximum allowable fouling resistance was reached. mixed integer programming approach was used to optimize the cleaning schedule of heat exchangers. An optimized cleaning schedule with significant cost savings has been determined and reported over a period of two years.