Ion transport membrane heat engine integration with autothermal reforming-based methanol production

Abstract

This study focused on improving the energy demand of conventional large-scale natural gas to methanol flow sheet where even a small efficiency improvement has significant economics and carbon emissions benefits. A conventional flow sheet, case A, was based on best available literature information. A case B where the conventional cryogenic air separation unit (ASU) is replaced with the novel ion transport membrane (ITM) oxygen unit was developed. The ITM oxygen is also heat integrated into the autothermal reformer (ATR) process. A case C where the methanol synthesis process is configured into a gas turbine cycle was developed from further modifying case B. The flow sheets were constructed and modelled in Aspen Plus V10 and, heat and material balance results are reported. To our knowledge, the integration of ITM oxygen membranes into the ATR-based methanol process has not been assessed previously.Energy and exergy results are generated and analysed. It was found that is it possible to replace the cryogenic ASU with the ITM oxygen for oxygen production in the ATR process. There is enough process syngas heat available to provide the ITM oxygen unit heating requirement and to generate process steam feed to the ATR. Case A was found to consume only 12 % natural gas as utility fuel which is lower than typical SMR-based methanol processes. This reduced to about 5 % in case B and C. Power production improved by 47 % in case B and 68 % in case C compared to case A.A thermal efficiency definition suitable for combined process steam, power and oxygen production was proposed. This showed that integration of a high temperature gas power cycle enables combined steam (heat) and power configuration which has a higher efficiency compared to single cycles, such as the steam cycles. The overall plant exergy losses decreased by up to 21 %.