METHODOLOGY AND OPTIMIZATION APPROACH ON LIQUEFACTION PROCESSES FOR BIOMETHANE ORIGINATED FROM BIOGAS UNITS
Résumé
Due to the increasing demand of natural gas, reserves are depleting. Hence, a considerable interest is given to the use of biogas as substitute for natural gas. However for certain applications, biogas must be liquefied. Such applications include: exploiting a stranded gas reserves, exploiting customer sites located remotely from pipeline, storing gas in case of peak production and developing new generation for biofuel [1]. Many studies have focused on large and mid scale natural gas liquefaction processes with a lack of literature studies on biogas micro-liquefaction. Due to the similarities between the two gases, mixed refrigerant (MR) cycles for natural gas liquefaction was studied.
In this paper, a novel mixed refrigerant cycle for biogas micro-liquefaction process is proposed. The cycle is first simulated in Aspen HYSYS and analyzed using exergy analysis method to evaluate process performance and to locate exergy destruction. Based on exergy analysis results, two configurations of MR cycle were developed.
The architecture and the operating variables play an important role in improving the efficiency as well as the cost of the cycle [2]. In order to find the optimum operating parameters and the best design, a link between Aspen HYSYS, PIAKAI genetic algorithm tools and Microsoft visual studio was developed. The objective function of the optimization problem is the maximization of the amount of the liquefied biomethane per unit of the required work. The manipulated operating variables for the optimization include: the molar flow rate of MR components, the outlet temperatures of exchanger hot streams and the outlet pressures of compressor and valves.
The analysis of the cycle optimization results showed that the exergy loss and compressor required work are reduced by 46% and 50 % respectively compared to the initial simulation of the first architecture and by 32.32% and 35% for the second configuration.