PSA unit

Simplified schematics of hydrogen production by SMR. (a) SMR with solvent removal of C02 and a methanation unit, (b) SMR with a PSA unit. HT- and LT-WGS high- and low-temperature WGS reactors, respectively. 

Modern SMR plants (Figure 2.5b) incorporate a PSA unit for purifying hydrogen from C02, CO, and CH4 impurities (moisture is preliminarily removed from the process gas). The PSA unit consists of multiple (parallel) adsorption beds, most commonly filled with molecular sieves of suitable pore size it operates at the pressure of about 20 atm. The PSA off-gas is composed of (mol%) C02—55, H2—27, CH4—14, CO—3, N2—0.4, and some water vapor [11] and is burned as a fuel in the primary reformer furnace. Generally, SMR plants with PSA need only a HT-WGS stage, which may somewhat simplify the process. 

COPISA [CO pressure induced selective adsorption] A process for separating carbon monoxide from the effluent gases from steel mills by a two-stage PSA unit. Developed jointly by Kawasaki Steel Corporation and Osaka Oxygen Industry. In the first stage, carbon dioxide is removed by activated carbon. In the second stage, carbon monoxide is removed by sodium mordenite. 

To reach a better CO conversion, it is possible to add a low-temperature shift reactor, which increases the CO2 capture rate (see also Fig. 10.3). If both clean CO2 for storage and clean hydrogen for fuel cell applications are required, a combination of a C02-capture plant (e.g., absorption with Rectisol) and a PSA plant is necessary. If only pure hydrogen is required, a PSA unit would be sufficient (and is standard practice), but the C02 stream would be contaminated by impurities, such as H2, N2 or CO, which have to be removed for geological storage. 

The earlier PSA units typically utilized 2,3, or 4 adsorber beds in a cyclic sequence in which one of the beds was always on an adsorption step while the other bed or beds were being regenerated. However, these systems were inherently inefficient for two reasons. First, the blowdown from adsorption pressure to the low waste pressure caused large losses of the desired product gas which was stored in the bed at high pressure. Second, some pure product gas was cycled to waste since it was used to purge a regenerating bed. 

Traditionally PSA processes have found particularly widespread application in hydrogen production systems wherein the PSA unit is used to produce ultra-high purity hydrogen (99.999%+) from steam-reformed hydrocarbons and other hydrogen sources. However, several limitations have existed with these processes. 

It was recognized that conventional PSA units were not broadly applicable in these areas because of their... 

Heck, J. L., "A Report on the Performance of the First POLYBED PSA Unit Operating in the United States", Oil and Gas Journal,... 

Since Texaco syngas process was not being used to make ammonia, some of the steps would have to be modified to apply it to ammonia production. A CO Shift step might be needed upstream of the C02 Removal unit the membrane unit might be replaced by a Pressure-Swing Absorption (PSA) unit and a methanation unit might be needed to remove residual CO and C02. 

The SELEXOL unit removes H2S and most of the CO2 in the syngas stream. The final hydrogen purification is accomplished in a downstream PSA unit. The purified hydrogen stream is combined with nitrogen from the air separation unit (ASU) and converted to ammonia. A unique CO2 purification scheme was developed to remove traces of H2S and COS from the CO2 that is fed to the UAN plant185. 

The HYSEC Process was developed by Mitsubishi Kakoki K. Ltd. and The Kansai Coke Chemicals Company. It has basically the same PSA unit as the UCC Process. It has prefilter beds with activated carbon that remove dirty components. After the main PSA beds, trace amounts of remaining oxygen are removed by a deoxo catalytic converter followed by a zeolitic dehumidifier. A Ni-LaaOj-Rh catalyst, supported on silica, could lower the reaction temperature to about 30°( a. 

After shift conversion, the gas is cooled by direct contact with circulating process condensate and then fed to a pressure swing adsorption (PSA) unit to remove excess nitrogen, CO2 and inerts. The C02 can be recovered from the pressure swing adsorption waste gas by using an aqueous solution of tertiary... 

The gas that leaves the PSA unit is methanated, cooled and dried. The dried gas enters the ammonia synthesis loop at the circulator suction. In the synthesis loop, gas from the circulator is heated and passed over low pressure ammonia synthesis catalyst to produce ammonia17. 

In modern plants, a Pressure Swing Adsorption (PSA) unit replaces the complete LTS, the CO2 stripping section and the final purification. 

A schematic diagram for the processing of synthesis gas to meet these three objectives using a combination upgrader/PSA unit is presented in Figure 9. A portion of the synthesis gas is diverted to the cold box for partial condensation and separation of the bulk of the methane and carbon monoxide from the hydrogen. The enriched (94-98%) H2 vapor fraction is reheated... 

In ammonia and hydrogen plants, part of the carbon dioxide is removed in the condensate in the knockout pots, which are made of type 304L SS or type 304L SS clad (Figure 4.6). The overhead lines of the knockout pots may be made of carbon steel if no condensation occurs. This is particularly true in those ammonia plants in which some of the ammonia formed in the shift converter is present in the stream. In general, the overhead lines of the knockout pots used in hydrogen plants are made of type 304L SS. The remainder of the carbon dioxide is removed by absorption in a potassium carbonate solution, an amine solution, or a low-temperature PSA unit. 

After further heat recovery, final cooling and condensate separation (6), the gas is sent to the pressure swing adsorption (PSA) unit (7). Loaded adsorbers are regenerated isothermally using a controlled sequence of depressurization and purging steps. 

Low manufacturing costs The unlimited catalyst service is combined with the low losses via the highly efficient cyclone system (less than 20g catalyst per ton of produced EDC). High raw-material yields (98.5 % ethylene, 99 % anhydrous hydrochloride and 94 % oxygen) and the possibility to use low-cost oxygen from PSA units ensure a highly competitive process with low production costs. 

PSA unit—Used to purify a H2 syngas stream of specific desired impurities through the use of absorbents. The gas purity of the H2 produced is determined by factors such as which absorbents are used. 

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