PSA Technology Hits the Fast Lane

Fast-cycle technology promises to reduce the size and costs of PSA gas-separation equipment

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Enter fast-cycle PSA, a technology that operates at cycle speeds of 100 cycles per minute or greater, significantly faster than conventional PSA.

Now in precommercial development, fast-cycle PSA promises to significantly reduce the size and capital cost of PSA equipment, opening up new applications in a wide range of industries. As an example, prototype fast-cycle PSA systems for purifying 100 Nmch (3,500 scfh) of hydrogen are up to one-fiftieth the size of conventional PSA systems, and cost less than 50 percent as much.

Adsorbents, rotary valves
Two proprietary technologies lie at the heart of fast-cycle PSA: structured adsorbents that replace conventional beaded adsorbents, and integrated rotary valves that replace solenoid-actuated valves.

In fast cycle PSA, the beds of adsorbent beads are replaced with a three-dimensional adsorbent structure featuring a large number of uniform gas channels. The structure's design shortens the diffusion path from the bulk gas phase to the internal surface area of the adsorbent, increasing the mass-transfer rate. The structures also increase the contact surface area of the adsorbent by an order of magnitude, and remove the fluidization limit associated with beaded adsorbents by immobilizing the adsorbent material, allowing for significantly higher bulk gas velocities (Table 1). Both commercially available PSA adsorbents and proprietary, custom-manufactured materials are used in fast-cycle adsorbent structures.


Fast-cycle PSA also simplifies the valving used in traditional PSA, replacing complex valve networks with two low friction, multiport rotary valves (Fig. 2). The rotary valve design has the following advantages:

Figure 2. Simpler Valve Layout with Fast-Cycle PSA

The elements of a rotary fast-cycle hydrogen PSA consist of set of structured adsorbent beds arranged between two rotary valves. The beds rotate around a central shaft. Feed gas enters the PSA module at one end and product gas is withdrawn from the opposite end of the module.


Increased valve switching speed: Rotary valves can be operated reliably at switching speeds of approximately 30 milliseconds, compared to the roughly 10 seconds switching time needed for conventional solenoid actuated valves. Consequently, the rotary valve design can switch gas flows fast enough to allow for PSA cycle speeds of 100 cycles per minute or greater (Table 2).

Complex, efficient PSA cycles: The rotary valve design simultaneously controls multiple gas flows within a single integrated valve assembly, eliminating the piping between valves and beds, and allowing for simple control of complex multistage PSA processes. The complexity, cost and dead-volume issues associated with networks of individual switching valves are avoided.

Improved Durability: Low friction between the rotating valve surfaces means that the rotary valve design is inherently durable compared to more mechanically complex valve designs incorporating impact surfaces and multiple moving parts.

Not perfect
However, like any technology, fast-cycle PSA is not without potential weaknesses. First, while operation at fast cycle speeds results in a significant reduction in the size and cost of the adsorbent beds, the reduction in adsorbent inventory can make fast-cycle PSA systems more vulnerable to process upsets such as contamination by liquid water or other condensable liquids. In such cases, careful design of feed-processing equipment such as liquid knock-out drums placed upstream of the PSA system would prevent any contamination. Alternatively, advanced structured adsorbent designs, using proprietary materials, could be used to prevent contamination.

Another issue is that fast cycle speeds increase the cyclic loading stresses on the adsorbent pressure vessels. This fact must be considered when selecting materials and designing the vessels that house the structured adsorbent.

More-compact units
Fast cycle speed, together with the high mass transfer and high surface area of the structured adsorbent results in a significant increase in the productivity of a unit volume of PSA adsorbent bed. Increased productivity, in turn, results in a direct reduction in the size of adsorbent beds required to produce a fixed flow of purified product gas (Fig. 3).

Figure 3. Super-Size Reduction Cycle PSA

 

Fast-cycle PSA modules can be up to one-fiftieth the size of traditional PSA modules.

Switching matches cycling

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