Optimum Economic Design and Control of a Gas Permeation Membrane Coupled with the Hydrodealkylation (HDA) Process

Abstract

This paper studies the design and control of a modified hydrodealkylation (HDA) process that uses a membrane to reduce hydrogen losses in the methane purge stream. A dynamic, counter-current membrane module is written using Aspen Custom Modeler to capture both the steady-state and dynamic performance of the membrane. The feed to the membrane is the purge stream from the conventional HDA process without a membrane. The hydrogen-rich permeate is compressed and recycled back into the process to reduce the demand for fresh hydrogen feed. The methane-rich retentate is the new purge stream from the system. A steady-state economic analysis is performed to determine the optimum membrane area and permeate pressure. Increasing the area and decreasing the pressure result in lower hydrogen consumption; however, the capital investment and energy costs increase. The return on the incremental investment is used to select the best design. The final design reduces the required flow rate of fresh hydrogen feed by approximately one-third. This results in an estimated annual savings of almost $400 000 in hydrogen feed by investing approximately $836 000 in capital. A plantwide control structure is developed for the new process. A composition controller manipulates the flow rate of the membrane retentate, which acts as the new purge stream, to control the composition of methane in the large total gas recycle stream. A second composition controller is used to control the composition of the hydrogen being lost in the retentate purge by manipulating the power input to the permeate compressors. Dynamic simulations performed in the Aspen Dynamics environment show that the control structure is effective in rejecting disturbances in throughput and hydrogen fresh feed composition.