Conventional pressure-swing adsorption (PSA) has been used to separate and purify industrial gases for more than 35 years. The technology, first developed by ExxonMobil and Air Liquide in the late 1950s, is based upon the capacity of adsorbents to selectively adsorb and desorb particular gases as gas pressure is raised and lowered. PSA is currently used in applications ranging from the production of nitrogen and oxygen from air to dehydration and hydrocarbon recovery.
New fast-cycle Pressure Swing Adsorption technology is currently being developed, offering more-compact, less-expensive and more-energy-efficient gas separation equipment. This article looks at this new technology and its potential in areas such as hydrogen purification and recovery.
A cyclic process
The PSA process involves a cyclic repetition of four basic steps: production, depressurizing, purging and repressurizing (Fig. 1). First, a gas mixture is fed under high pressure into a vessel containing a bed of 2-6-mm diameter adsorbent beads, typically alumina, silica gel, activated carbon or molecular sieves. Impurities in the feed gas adsorb onto the internal surfaces of the adsorbent, leaving purified product gas in the void spaces of the vessel. Product gas is then withdrawn from the top of the vessel under pressure.
Figure 1. Cyclic Repetition
The four basic steps of pressure-swing adsorption technology selectively adsorb and desorb gases as pressure is raised and lowered.
The pressure in the adsorption vessel is then reduced, and product gas remaining in the void spaces of the vessel is removed. The adsorbed impurities are released back into the gas phase, regenerating the adsorbent bed. The vessel is then purged with a small amount of purified product gas, to complete regeneration of the adsorbent bed. Impurities exit the PSA process in a low-pressure exhaust stream.
Finally, the vessel is repressurized with a mixture of product gas from the depressurization step, feed gas and high-purity product gas. This cycle is repeated every 2-20 minutes in conventional PSA systems.
Since each cycle is essentially a batch process, multiple pressure vessels are used together in sequence to provide a semicontinuous flow of product gas. In addition, large surge tanks are used to dampen variations in flows of feed, product and exhaust streams.
To improve product gas yields, more-complex PSA cycles can be used, involving more intermediate steps such as cocurrent and countercurrent depressurization. A larger number of adsorbent beds are needed for these complex PSA cycles. When determining the optimum process cycle for a given gas purification requirement, an economic tradeoff must be made between increased efficiency and higher capital cost.
In most PSA installations, a complex network of valves is used to switch gases between the adsorbent beds. The valves are actuated by programmable logic controller (PLC)-controlled solenoids. For an eight-bed hydrogen PSA process, 42 valves are typically needed to switch gases, and the number of valves required increases significantly in more complex PSA cycles using larger numbers of adsorbent beds.
Disadvantages of PSA
Despite PSA's widespread use in industry, traditional processes using beaded adsorbents and multiple switching valves have the following inherent disadvantages:
Fluidization: Gas flow through the adsorbent beds, and, by association, the PSA cycle speed, are limited by the fluidization velocity of the adsorbent beads. Fast cycle speeds and rapid gas flows result in fluidization, which leads to abrasion, dusting and, ultimately, destruction of the adsorbent beads. Consequently, slow cycle speeds (2-20 minutes per cycle) and correspondingly large pressure vessels with large adsorbent inventories have to be used, resulting in very large systems, with high materials and vessel costs. The large vessel sizes mean that most conventional PSA systems are field erected, with the associated costs and onsite construction requirements.
Kinetic limitations: The fluidization constraint limits the minimum adsorbent bead size that can practically be used in commercial PSA processes to about 2 mm (1/16 inches) in diameter. This particle size means the adsorbed gas must travel a relatively long diffusion path between the bulk gas stream and the adsorption sites on the internal surfaces of adsorbent macrostructure. This long path results in relatively slow mass transfer between the bulk gas and adsorption sites. High adsorbent inventories and slow cycle speeds are required to compensate for this.
Drawbacks of conventional valves: The networks of individual switching valves, with associated instrumentation, control systems and process piping, add complexity and cost to conventional PSA systems. In addition, the complex piping network required to link the array of valves adds dead volume to the overall PSA process, reducing product yield and overall process efficiency. Solenoid-actuated valves also have high lifecycle costs due to high replacement rates of valve internals.