Exergy Analysis of Nitric Acid, Ethylene Oxide/Ethylene Glycol Processes and Methanol Reactor

Abstract

Exergy analysis provides insight for improvement in the process or equipment to enhance its energy efficiency by reducing the loss of exergy. Exergy analysis can also be helpful in improving an existing processes or design and developing new environmental friendly processes. To investigate possible causes of exergy losses in actual process in the chemical industry, Nitric acid and Ethylene oxide/ethylene glycol production plants are selected. Overall energy and exergy analysis are carried for these processes. Apart from this, exergy analysis is also used as a tool to select life of catalyst based on an economic evaluation of the process. Initially, energy balance and exergy balance of both the processes are carried out. Exergy destruction in equipments is listed for each plant. Based on the exergy destructions analysis, key parameters are identified and various options to reduce the exergy destruction through various practically feasible solutions are proposed. An attempt has been made to improve exergy efficiency and thus energy efficiency of the nitric acid process by changing inlet temperature of oxidation air and recovering heat in the cooler condenser. Energy and exergy balance of ethylene oxide/ethylene glycol process depends upon catalyst selectivity. The performance of plant on exergy basis is evaluated at start run and end run of the plant. Raw material cost and catalyst cost are the factors affecting plant economy; hence exergoeconomic analysis based on fixed cost is evaluated. EXCEM method is used for this analysis. This analysis will be helpful to decide catalyst selectivity for the end run. In present work, exergy analysis is also used as a tool to evaluate the performance of thermally coupled reactors for the production of methanol. Exergy destruction per ton of methanol production and per ton hydrogen production is also calculated. Ammonia oxidation is a major source of energy in the nitric acid plant. Energy efficiency of the nitric acid plant is 31% and exergy efficiency is 20.83% based on the data collected for selected nitric acid plant. Reduction in the inlet air temperature in compressor increases exergy efficiency up to 21.33%. It is also proposed to use organic Rankine cycle to recover the heat available in the cooler condenser. Suggested transformation enhances exergy efficiency up to 24.15%. Exergy efficiency of the ethylene oxide/ethylene glycol process is higher (74.59%) at start run and lower (73.29%) at end run. Ethylene recovery from purge gas will improve exergy efficiency of the entire plant up to 75.17%. A new process developed by Mitsubishi Chemicals Ltd. for ethylene glycol is compared with the conventional process on the basis of exergy analysis. Statistical data for thermodynamic loss rate to capital cost ratio values shows that relative spread in Mean value (R) for different devices are large when it is based on energy and small when it is based on exergy. Based on exergy loss rate in ethylene oxide reactor, catalyst usage for a longer time is proposed by reducing exergy destruction. Such analysis can also be extended for any other type of catalyst or any other process plant by selecting suitable process parameters, operating cost and fixed cost of the process. Almost 18% of input exergy is lost in the reformer in steam methane reforming process. Exergy efficiency of the reformer is 87.3% and exergy destruction in reformer per ton of hydrogen is 392.47 kW. Exergy destruction per ton of methanol is 1.48 kW in membrane coupled reactor and 1.73 kW in the thermally coupled reactor. But for hydrogen production, thermally coupled reactor is a better candidate than membrane coupled reactor.