Integrated design of a gas separation system for the upgrade of crude SNG with membranes

This paper investigates different design strategies and process layouts for upgrading crude synthetic natural gas (SNG) produced from lignocellulosic biomass to grid quality. The design problem is outlined by studying the involved key aspects with respect to the crude composition, purity requirements and process integration. A discussion of candidate technology identifies multistage membrane processes as a promising option, for which a multicomponent thermo-economic design model is coupled to a process model for SNG production. In a design study using multi-objective optimisation, the most promising membrane configurations are identified and optimised with respect to cost and efficiency. Comparing design strategies that consider different levels of process integration, it is shown that coupling the reactive and separation sections improves the process design. If process integration is not considered, the separation system is oversized by up to 60% and its investment cost is up to 46% too high. In the last part of the paper, the by-production of biogenic CO2 at sufficient purity for storage is investigated, which would turn the process into a CO2-sink for the atmosphere. The cost of captured CO2 is assessed between 15 and 40 €/ton, which makes it potentially more advantageous than the capture at a fossil fuel power plant.