Fischer-Tropsch's high purity hydrogen required for the Unionfining Unit and Unicracking Unit is supplied by a Pressure Swing Adsorption (PSA) system. The hydrogen requirement is met by separating other gases from the sweet syngas produced in the gasifier with subsequent cleaning and sulfur removal. Separated tail gases from the PSA unit are routed to the fuel gas system.
Pressure Swing Adsorption technology works by alternating the flow through two pressure vessels which are packed with an adsorbent material which acts as a molecular sieve. While the hydrogen is being passed through one vessel, a small bleed taken from the dry gas is passed down the other vessel. No further adsorption capacity is available; at this point the adsorbent material is forced to regenerate. This action completely regenerates the adsorbent material in the column so that no replacement of the material is required. The vessel is ready for another production cycle after a small amount of the product hydrogen flushes away the waste. The hydrogen produced has a drastically removed moisture content of just 1ppm.
The following process configuration and operating parameters are selected to illustrate maximizing diesel production: A renewable energy plant converting 1000 TPD (dry) urban green waste can generate 600 BPD of Fischer-Tropsch fuels and 16.5 MW of export electricity to yield 466 bbl/day of "green diesel" or syngas and 166 bbl/day of Naphtha.
Pressure Swing Adsorption Unit
Cleaned and desulfurized syngas flows to the Pressure Swing Adsorption (PSA) hydrogen purification system. Gas from the Sulfur Recovery Absorber is routed to the feed knockout drum to remove entrained liquids prior to entering the PSA beds. Process syngas will contain primarily hydrogen, carbon monoxide, carbon dioxide, nitrogen and smaller quantities of water, methane and other impurities. Syngas is passed through the adsorbent where 61 percent of the hydrogen is recovered at a purity of 99.9 mol%. The purified hydrogen is routed to the Makeup Gas Compressor for use in the Unionfining and Unicracking units.
The PSA system consists of four beds of adsorbent operating in a staggered batch sequence arrangement to continuously produce the purified hydrogen. One bed is operated under pressure to purify the hydrogen by retaining other components within the adsorbent. The other beds are in various sequences of the depressurization, purge and repressurization cycles to release the tail gas and prepare the beds for continued processing.
Low-pressure desorbed gas from the PSA system flows to the Tail Gas Compressor. After compressing to 40 psig, the tail gas is routed to the fuel gas Mix Drum for use in burners and power block generation equipment.