PSA process

Feedstocks are natural gas, refinery fuel gas, LPG and paraffinic naphthas. After elimination of CO2, the last traces of contaminants are converted to methane (methanation) or eliminated by adsorption on molecular sieves (PSA process). 

CO-PSA process flow Copynght Copyrightability Copyright license Copyright protection... 

The flow directions in a PSA process are fixed by the composition of the stream. The most common configuration is for adsorption to take place up-flow. AH gases with compositions rich in adsorbate are introduced into the adsorption inlet end, and so effluent streams from the inlet end are rich in adsorbate. Similarly, adsorbate-lean streams to be used for purging or repressurizing must flow into the product end. 

Relatively new methods for separating helium from natural gas use pressure swing adsorption (PSA) processes to recover helium at better than 99.99% purity. This type of process is probably less costiy for the production of gaseous helium but might be uneconomical for liquefied helium production. The PSA process is widely used to produce specification pure helium from 85+% cmde helium in conjunction with cryogenic enrichment of the ca 50% helium raffinate. 

The impurities usually found in raw hydrogen are CO2, CO, N2, H2O, CH, and higher hydrocarbons. Removal of these impurities by shift catalysis, H2S and CO2 removal, and the pressure-swing adsorption (PSA) process have been described (vide supra). Traces of oxygen in electrolytic hydrogen are usually removed on a palladium or platinum catalyst at room temperature. 

Pressure Swing Adsorption. A number of processes based on Pressure Swing Adsorption (PSA) technology have been used in the production of carbon dioxide. In one version of the PSA process, CO2 is separated from CH using a multibed adsorption process (41). In this process both CH4 and CO2 are produced. The process requires the use of five adsorber vessels. Processes of this type can be used for producing CO2 from natural gas weUs, landfiU gas, or from oil weUs undergoing CO2 flooding for enhanced oil recovery (see Adsorption, gas separation). 

Adsorption Processes. More recendy, pressure swing adsorption (PSA) processes utilizing a high selectivity copper adsorbent have been utilized to effectively separate carbon monoxide from blast furnace gas and coke oven gas (97—101). 

In these processes, a carbon monoxide containing gas is fed to an adsorber bed containing copper, typically dispersed on a high surface area support such as alumina or carbon. The copper is present predominately as Cu", which selectively adsorbs carbon monoxide. The remainder of the gas stream passes through the adsorbent bed. The carbon monoxide is then removed from the adsorbent by lowering the pressure. Figure 6 shows a typical process for a CO-PSA process. Process conditions are typically adsorption pressures of 0.68—204 MPa (6.8—20.4 atm) and temperatures of 313—373 K. Regeneration occurs at reduced pressure or by vacuum. 

Fig. 4.5-2 General incorporation of HRA into the PSA process (from EPRINP3583). Reprinted with pcrmis uin from the Electric Power Research Instii< ic l - S A.

This section reflects on the limitations of the PSA process and draws extensively from NUREG-1050. These subjects are discussed as plant modeling and evaluation, data, human errors, accident processes, containment, fission product transport, consequence analysis, external events, and a perspective on the meaning of risk. 

Although PSA is a batchwise process, by using multiple beds in a sequential manner the overall process is operated in a continuous fashion. Each bed may contain layers of different adsorbent materials selective for specific contaminants in the hydrogen gas stream to be purified. Each bed undergoes a sequence of four basic steps in a PSA cycle adsorption, depressurization, purge at low pressure, and repressurization. This sequence of cyclic operations for each bed is shown schematically for a four-bed PSA process in Figure 8.4 (Yang, 1987 Cassidy, 1980 Miller and Stocker, 1999). 

A pressure-swing adsorption (PSA) process cycle is one in which desorption takes place at a pressure much lower than adsorption. Reduction of pressure is used to shift the adsorption equilibrium and affect regeneration of the adsorbent. Figure 16-39 depicts a simplified pressure-swing cycle. Feed fluid containing an adsorbate at a molar concentration of y i = pi /Pi is passed through an adsorbent at conditions... 

KURASEP [Kuraray Separation] A process for separating nitrogen from air by a variant of the PSA process, using carbon molecular sieve as the adsorbent. Developed by Kuraray Chemical Company. 

LO-FIN [Last out-first in] A version of the PSA process for separating hydrogen from other gases. It includes a unique gas-retaining vessel which preserves the concentration gradient in one stream before using it to repressurize another bed. Developed jointly by Toyo Engineering Corporation and Essex Corporation. 

MOLPSA-nitrogen [Molecular sieve pressure swing adsorption] A version of the PSA process for separating nitrogen from air, developed by Kobe Steel. Most PSA processes for nitrogen use molecular sieve carbon as the adsorbent, but this one uses zeolite X. Water and carbon dioxide are first removed in a two-bed PSA system, and then the nitrogen is concentrated and purified in a three-bed system. 

NOVOX Not a process, but a trademark used by BOC to designate its PSA process for separating oxygen from air. 

RPSA [Rapid pressure swing adsorption] A version of the PSA process which uses fast pressure-cycles known as parametric pumping. The molecular sieve adsorbent for this duty has to be of a smaller grain size than that for PSA. Developed by the Union Carbide Corporation. 

As with air separation the purification of hydrogen via a PSA process is also a well developed field. The market place is highly competitive and UOP competes with... 

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