Pressure-swing adsorption adsorbent productivity

Figure 2. Combined pressure-swing, non-adsorbed-product-purge cycle (pressure-swing adsorption).

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). 

Processes involving total-pressure reduction to remove the adsorbed species, called pressure-swing adsorption (PSA) or heatless adsorption, are mechanically complex, since they must include separate adsorption, depressurization, desorption, and repressurization steps. To accommodate a steady flow of feed and products, several beds - usually three or more in parallel -are used. A typical four-bed process flowsheet is shown in... 

The objectives of the present study were to decrease the basic complexity of PSA and to increase its productivity - the amount of product produced per unit time per unit weight of adsorbent. Success in both of these areas should further enhance process economics and increase the potential application for pressure-swing-adsorption-based processes. 

FIGURE 11 Sequence of steps in a two-bed pressure swing adsorption system, (a) Skarstrom cycle, (b) modified cycle for production of nitrogen using a carbon molecular sieve adsorbent. 

The third process used in the production of carbon dioxide is pressure swing adsorption. The feed gas usually contains approximately 20 percent carbon dioxide, 70 percent hydrogen, and the remainder methane, carbon monoxide, nitrogen, and water. The feed gas is typically under a pressure of 125 100 psig at temperatures of 80-120°F. The carbon dioxide and water are strongly adsorbed in the adsorb beds and the residual gas stream is depressurized for further recovery. The adsorber vessel is then evacuated through vacuum blowers where the carbon dioxide, which has been adsorbed by the bed, is released at purities of essentially 99+ percent pure. 

Pressure swing adsorption is a commercial process for separating fluids based on their different affinities for an adsorbent. A sequence of steps involves more than one vessel ("bed") packed with adsorbent. Bed No. 1 receives the feed at the high supply pressure while bed No. 2 is opened to the low exhaust pressure. When Bed No. 1 becomes saturated with desired product (the undesired product leaves the bed at the exit), the supply is switched to Bed No. 2, and Bed No. 1 is opened to the low pressure so that the desired product is recovered. In the meantime desired product collects in Bed No. 2. After Bed No. 2 is saturated, the supply is again shifted back to Bed No. 1, and so on. More than two beds can be employed, and the specific design of the system and operating conditions are based on economical operation to conserve compression energy. 

Early work of Barrer (3), McKee (4) and Domine and Hay (5) showed that calcium A, calcium X, mordenite and several types of natural zeolites could be used to enrich air by a selective adsorption of nitrogen. Several pressure-swing-adsorption processes utilizing zeolite adsorbents have been developed which yield a product containing up to 95% oxygen at rates of 20 tons per day (6,7). 

Pressure swing adsorption processes are also designed to produce high-purity (99.95+ %) H2 products from refinery-off gases containing H2 (65-90%) and C1-C5 hydrocarbon impurities with high H2 recoveries ( 86+ %). Silica gel and activated carbons are used as adsorbents. 

When producing hydrogen as the final product, impurities such as CO, sulfur compounds, and other trace contaminants must be removed, particularly for application in fuel cells. Currently, pressure swing adsorption (PSA) is commonly used for the separation and purification of hydrogen from mixed gas streams. PSA systems are based on selective adsorbent beds. The gas mixture is introduced to the bed at an elevated pressure and the solid adsorbent selectively adsorbs certain components of the gas mixture, allowing the unadsorbed components, in this case hydrogen, to pass through the bed as purified gas. 

Milton and Breck discovered that these engineered materials could be produced with pore structures capable of selectively adsorbing nitrogen from air. Exploitation of this phenomenon soon led to pressure swing adsorption (PSA) air separation plants for the production of oxygen at purities between 90 and 94 percent. 

Some chemical production processes use the municipal gas supply as chemical feed stock. In some cases the gas has to be treated to meet the production specifications. Pressure swing adsorption is often used to remove higher hydrocarbons. One drawback of this application is the Umited lifetime of the adsorber in the presence of higher hydrocarbons and the costs for replacement and trans-... 

The pilot unit consists of six reactors, each six meters tall. The reactors contain beds of pellets that simultaneously catalyze the water gas shift reaction and adsorb CO2. The unit is operated as a reactive pressure swing adsorption installation. By switching valves, the reactors are operated in cycles of C02-adsorption and desorption. Since at any time, at least one reactor is in adsorption mode and at least one reactor in desorption mode, a continuous production of purified hydrogen (and CO2) is obtained. Design, engineering, construction, and commissioning of the unit took less than one year. 

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