Pressure swing adsorptions

Pressure swing adsorption (PSA) is a gas purification process, which consists of the removal of impurities on adsorbent beds. The usual adsorbents and gases adsorbed thereon are molecular sieves for carbon monoxide, activated carbon for carbon dioxide, activated alumina or silica gel. lyuke et td. reported that the addition of tin onto activated carbon improved the efficiency of the adsorptive removal of carbon monoxide from hydrogen [420]. 

The adsorbents usually have a long lifetime. The impurities are adsorbed at high partial pressure. Then the adsorber is depressurised in two steps firstly, co-current to the adsorption process down to a medium pressure level, followed by counter-current depressurisation to the low pressure level of the desorption step. While a minimum of two adsorbers is required to run the process, industrial pressure swing adsorption plants consist of up to 12 adsorbers and along with the number of valves required, this makes the systems complicated [421]. 

counter-current depressurisation to (sub-)atmospheric pressure usually starting at 10% to 70% of the feed pressure 

counter-current purge with hydrogen enriched or product gas at ambient pressure 

co-current pressurisation with feed or secondary process gas (see below). 

A multitude of sophisticated PSA processes and designs have been implemented for various separations. However, there are not many basic ideas on which these designs are made. These ideas can be combined and synthesized, with minor modifications, to accomplish the desired separations. 

To facilitate the discussions that follow, some terminologies will first be defined. The performance of the separation process is measured by three parameters (1) product purity, (2) product recovery, and (3) adsorbent productivity. If more than one of the constituents are to be recovered, (1) and (2) apply to all products. The product purity is a volume-averaged quantity, as the effluent 

Source-. Basmadjian et af. (1975b). Reprinted with permission. Values are accurate within 10%, because of uncertainties in K, Cps, and Cpif, 

The basic PSA cycle was described by Skarstrom in 1960 (Skarstrom, 1960 1972). A similar cycle was the Guerin-Domine Cycle, invented at about the same time (Guerin-Domine, 1964 see a detailed account of these inventions in Yang, 1987). The latter was the basis for the modem vacuum swing cycle. The major additions to these cycles are co-current depressurization (CD) and pressure equalization (PE) steps. The CD step was added to increase product recovery, whereas the pressure equalization step was added to conserve the mechanical energy. 

The Skarstrom cycle has been widely used for air-drying. For oxygen production from air and other bulk separations, however, the product recovery was too low, and hence the energy requirement too high, to be economical. 

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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. 

As an alternative to scmbbing out the CO2 followed by methanation, the shifted gas can be purified by pressure-swing adsorption (PSA) when high purity hydrogen is desirable. 

Fig. 2. Hydrogen production flow sheet showing steam reforming, shift, and pressure swing adsorption (PSA). BFW = boiling feed water.

The second CO2 removal is conducted using the same solvent employed in the first step. This allows a common regeneration stripper to be used for the two absorbers. The gases leaving the second absorption step stiU contain some 0.25—0.4% CO and 0.01—0.1% CO2 and so must be methanated as discussed earlier. The CO, CO2, and possibly small amounts of CH, N2, and Ar can also be removed by pressure-swing adsorption if desired. 

Fig. 4. Coal gasification process. PSA = pressure-swing adsorption.

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. 

H. A. Stewart and J. L. Heck, Hydrogen Purification By Pressure Swing Adsorption, Union Carbide Corp., Linde Division, New York. 

Fig. 3. Pressure swing adsorption nitrogen generation system. CMS = carbon molecular sieve.

Pressure Swing Adsorption. Carbon dioxide can be removed by pressure adsorption on molecular sieves. However, the molecular sieves are not selective to CO2, and the gases must be further processed to achieve the high purity required for "over the fence" use as in the urea process. Use of pressure swing adsorption for CO2 removal appears most appHcable to small, stand-alone plants (29). 

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). 

Hydrogen Liquefaction. Hydrogen can be produced from caustic—chlorine electrolytic cells, by decomposition of ammonia or methanol, or by steam—methane reforming. Hydrogen recovered by these methods must be further purified prior to Hquefaction. This is generally achieved by utilizing pressure swing adsorption methods whereby impurities are adsorbed on a soHd adsorbent. 

Many different forms of the energy balance have been used in fixed-bed adsorption studies. The form chosen for a particular study depends on the process considered (e.g., temperature swing adsorption or pressure swing adsorption) and on the degree of approximation that is appropriate. 

Competing Processes Membranes are not the only way to make these separations, neither are they generally the dominant way. In many apphcations, membranes compete with ciyogenic distillation and with pressure-swing adsorption in others, physical absorption is the dominant method. The growth rate for membrane capacity is higher than that for any competitor. 

Pressure swing adsorption using molecular sieves High purity, 99 to 99.9% Modei ate High purity high-pressure storage maybe required simple process economical... 

Do Not Require Process Modifications iv. Thermal swing adsorption V. Pressure swing adsorption... 

Figure 4.1. A process for producing hydrogen by steam reforming of hydrocarbons (1) reforming furnace (2,3) purification section, (4) shift converter, (5) pressure swing adsorption.

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