Optimizing electricity generation through the utilization of wasted heat from process streams in a LURGI methanol plant

Abstract

In the realm of petrochemical operations, persistent efforts have been made to curtail specific energy consumption; however, certain heat sources continue to be underutilized. This paper investigates the condition of a Lurgi methanol plant to identify process streams with enough wasted heat that have the potential to produce electricity. After a thorough analysis, one particular zone is identified where an air cooler package and seawater exchanger are used to decrease the temperature of crude methanol vapor. To harness this untapped heat for electricity generation, an organic Rankine cycle (ORC) is proposed. In the quest for efficient electricity production, three distinct working fluids-dry (R600a), wet (R134a), and isentropic (R11)- are scrutinized to compare their performance in generating electricity within the ORC system. In addition, the maximum amount of electricity that can be generated from this waste heat recovery (WHR) project is determined and optimized using a multi-objective optimization method. By leveraging genetic algorithms, the system's exergy is enhanced while minimizing costs. A comprehensive economic comparison is conducted using probability analysis to evaluate each system's financial viability. The results show that dry and isentropic working fluids yield the best results for electricity production, generating approximately 9–10 MW. The return on investment (ROI) for these working fluids is also nearly similar, at 16 %. Among the chosen working fluids, R600a is selected as the superior option due to its lower outlet temperature for crude methanol. Additionally, because the cooling water flow for the condenser of the ORC is identified as a limitation in the current petrochemical plant, a new optimization is conducted with a constraint on the flow of cooling water, aiming for approximately 3000 tons/h. The results show that the proposed WHR-ORC system can generate a maximum of approximately 7.5 MW net electricity in the studied petrochemical plant, while also reducing CO2 emissions by up to 40,000 tons per year. The innovative methodology showcased in this research underscores its economic viability, paving the way for heightened energy efficiency and environmentally conscious methanol production practices.