Brief Description of Proposed Paths
Path I
Waste syngas comes in from the PSA unit with residual hydrogen, rejected components and is stored in a buffer tank as shown. The amount of residual hydrogen will depend on the % recovery performance of the PSA unit. However, 65-70 % recovery is typical for most units. This means there is 30-35 % of initial hydrogen that will be available for methanol production. Methanol production depends on the total hydrogen to CO and CO2 ratio, the stoichiometric number (SN).
Upon leaving the storage tank, the rejected PSA syngas (tail gas) undergoes sequential condensation first to remove water. The dry residual gas is condensed in the next separation step to recover liquid hydrocarbons at the bottom. Off-gas rich in CO, H2 and CH4 is released at the top. The CH4 from the first off-gas is knocked off in the next condensation step generating yet an off-gas mixture containing mostly CO and H2. The recovered liquid methane from this step is mixed with the previously recovered liquid hydrocarbon mixture. This mixture is revaporised before it is sent to the reformer. The reformer uses either CO2 or steam to produce a second generation synthesis gas that is sent to the methanol reactor for the production of methanol. Unreacted hydrocarbons coming out of either the methanol reactor or the reformer are recycled to improve yields and process efficiency.
Path II
The same process is followed except this time, only two condensation steps are involved. The second condensation generates syngas contaminated with CH4, which is fed into methanol reactor together with the syngas coming from the reformer. The expectation is that methane with not be converted into methanol but will be recovered as part of methanol reactor off-gas, when it is cooled to condense water with the CH4 and any unconverted hydrocarbons that are recycled into the reformer. Path II is proposed to reduce energy demands from CH4 liquefaction. Instead a condenser will be introduced which might have a smaller duty than CH4 cooler used in methane liquefaction It also eliminates the need for a compressor and a third liquid-gas separator.
The development of these schematics represents phase I of the methanol production process. The challenge will be to run modelling trials to see if the proposed flow sheets work and select the one with the better heat and material balance and process economics. That will be phase II work.
Note that both the reforming and methanol production processes are catalytic processes. Hence a vendor with the right set of catalyst should be identified.
Eliasu A. Teiseh
Research and Development Manager, Ph.D