Modeling, Simulation and Optimization of Internal Heat Recovery Using Process Integration Technique of Pinch Analysis

Abstract

Efficient use of energy in processing plants is one of the ways of reducing production costs. Among the existing tools, pinch analysis presents opportunities for process design engineers to have an integrative approach to reducing energy consumption through internal heat recovery. Even though pinch analysis has been used before to guide computation of energy targets and design of heat exchangers to meet the targets, the tool has not been enhanced to take into consideration stream specific temperature dependent properties of process materials and heat exchanger geometry. In order to improve the tool, it is important to incorporate these design considerations into a model that can be used for design of internal heat recovery systems. The objective of this study was to design, validate and test internal heat recovery models based on pinch analysis technique. Three models were developed to obtain an optimum model based on heat balance, energy targeting and heat exchanger sizing. The heat balance and energy targeting models were coded and integrated into a single hypertext preprocessor (PHP) platform. This is a general-purpose server-based coding language. The model for design and optimization of heat exchanger size used mathematical programming method. The method used equations from Kern design guidelines and was used using visual basic for analysis (VBA) Solver. The models were validated using secondary data from a Dairy Specialty Plant. Performance of the proposed heat balance and energy targeting models were tested using data from Plant A, B, C and a Dairy Specialty Plant. Plants A, B and C produced linear alkyl benzanoic acid, dairy products and alcohol compounds, respectively. Optimization of heat exchanger size was tested on 19 heat exchangers from Plants A, B and C. Simulations were performed on some streams and some heat exchanger for energy balance and thermal performance, respectively. The results from the proposed models were compared with results from the traditional models. A comparison of the performance of the proposed model to the traditional model revealed mixed results for heating and cooling. The heating loads computed using the proposed model for Plants A, B and C were higher by an average of 0.58%. The heating load computed for the Dairy Specialty Plant were lesser by 2.77%. For cooling loads, there was no observed difference in Plant C. The loads for Dairy Specialty Plant and Plant A were less an average of 17.38%. Plant B’s cooling computed under this model were more 0.64 %. What-if simulation results further explained these findings. The results revealed that gaseous streams had low heating duties while liquid and steam streams had high duties. The proposed model targeted internally recoverable heat savings of 2.2%, 10.56% and 20.88% for Plants A, B and C respectively. The conventional model computed savings of 1.5%, 4.5% and 2.2%, in a similar order. The proposed heat exchanger model came up with area requirements that cost less by average of 13%. A simulation of the effects of tube side fouling factors on thermal performance of heat exchangers for internal heat recovery revealed varied ranges of effects for the 19 heat exchangers tested. Average drop of 0.3% in performance of the exchangers was noted. These deviations are attributed to increase in fouling, which increases resistance to heat transfer. The findings of this study are applicable in the design of heating and cooling systems in thermo-chemical processing plants. The accuracy of estimation of required sizes of heating utilities (furnaces, heat pumps and boilers) and cooling utilities (chillers and cooling towers), through heat balance, can be improved if the proposed model is used. The amount of heat that can be recovered internally in a process plant can be predicted accurately using the proposed heat targeting model.