Design, control and comparison of fixed-bed methanation reactor systems for the production of substitute natural gas

• Design alternatives for fixed bed methanation reaction systems are compared.
• A process with a non-adiabatic reactor and catalyst dilution has the lowest cost.
• All processes respond well dynamically to feed flow rate disturbances.
• Processes cannot accommodate feed composition disturbances because of stoichiometry.

The design and control of processes for the methanation of synthesis gas to produce substitute natural gas (SNG) using fixed-bed reactors is investigated. Three different strategies for controlling the reactor temperature rise are considered: recycle of a portion of the reactor effluent, introduction of additional water into the reactor feed, and non-adiabatic reactor operation with catalyst dilution. The results show that the process with a non-adiabatic reactor has the lowest cost and produces the greatest amount of high-pressure steam. However the efficacy of catalyst dilution for preventing reaction run-away has not been tested experimentally. Among the remaining options, partial recycle of reactor effluent is preferred because it can produce a greater amount of high-pressure steam than the process with additional water. Control studies indicate that all processes can be controlled and can tolerate production rate changes, however none can tolerate a large change in the feed composition due to inherent stoichiometric limitations.