Using turbine expanders to recover exothermic reaction heat for the combined production of power and chemicals

Abstract

Many reactions carried out in the chemical industry are exothermic. The heat liberated by the reaction is often transferred to another medium such as steam by heat exchange. This heat can then be used elsewhere or be used to generate power via a steam cycle. In this work the focus is on another method of reaction heat recovery. When an exothermic reaction is conducted at elevated pressures, a turbine expander can be placed directly behind the reactor. The hot, high-pressure product gas from the reactor can then be expanded in the turbine. During the expansion process the physical energy of the product gas is converted to kinetic energy (or electricity if the turbine is connected to a generator). Three chemical processes were studied to determine the feasibility of turbine integration into the processes. They are ethylene oxide production, phthalic anhydride production and the hydrodealkylation of alkylaromatic compounds. The chosen processes differ in terms of reactor operation, reactant conversion as well as the presence or absence of recycle loops. Simulation models were developed for the mentioned processes with the process simulator Aspen Plus®. Results from the simulations show that, without the turbine, the processes require power from external sources. They can however operate independently from external power sources when a turbine is present. Excess power can be exported or used for electricity generation. It is therefore feasible• to incorporate turbine expansion units in all the processes considered. The operating conditions of some unit operations have to be changed to accommodate the turbine expander. With the additional product namely power, a re-evaluation of all the operating conditions and tradeoffs in the process is necessary. Further investigation into the impact of turbine integration on the optimal operating conditions of the process is therefore recommended. Traditional definitions used to evaluate the performance of a process generating or consuming power, were found to be inadequate for use in processes where power and chemicals are produced together. New performance parameters are required for the evaluation of processes where power and chemicals are produced simultaneously. An exergy analysis was performed for one of the cases. This analysis method provides insight as to where thermodynamic losses occur in a process. The exergy analysis was useful to quantify the losses occurring in an isenthalpic expansion valve, and the savings obtained by replacing such a valve with an expansion turbine.