Fast Cycle PSA Process

Features

Comparison of PSA

Application

Limitations

To sort out the deficiencies associated with the conventional PSA process new fast-cycle Pressure Swing Adsorption technology is used now a days. This Fast cycle PSA is more-compact, less costly and energy- wise efficient gas separation equipment.

Operating at speeds of 100 cycles per minute or more, this process is significantly faster than conventional PSA. This added advantages over the conventional PSA process has opened the doors for newer applications in a wide range of industries. For example, typically a fast-cycle PSA system for purifying 100 Nmch (3,500 scfh) of hydrogen offers the follwing advantage over conventional PSA cycle:

• Size-Up to one-fiftieth the size of conventional PSA systems
• Costs- 50% lesser

A simple layout of fast cycle PSA

As can be seen from the figure the elements of a rotary fast-cycle PSA system consists of a set of structured adsorbent beds arranged between the two rotary valves. The beds are made to rotate around a central shaft. Feed gas enters the PSA module from one end and product gas is withdrawn from the opposite end of the module.

Features of Fast Cycle PSA Process:

• Structured Adsorbent: It replaces the conventional beaded adsorbent.
• Improvised Valve: Integrated rotary valves that replace solenoid-actuated valves.
• Scalability: Fast-cycle PSA technology can be scaled from small, portable-style equipment to large scale industrial use.
• Increased valve switching speed: Rotary valves are operated at switching speeds of 30 milliseconds (approx) compared to the around 10 seconds switching time needed for conventional solenoid actuated valves. As a result, the rotary valve design can switch gas flows quick enough to allow PSA cycle speeds of 100 cycles per minute or greater.
• Flexibility: Fast cycle PSA systems can be modified to purify a diverse range of gases and by changing the type of structured adsorbent material and the design of the rotary valve.
• Compact size: The increased cycle speed and improved rotary mechanical design considerably reduce the size of systems compared to conventional PSA systems. This in turn results in reduced capital and installation costs.
• Enhancing Durability: Low friction between the rotating valve surfaces makes the rotary valve design durable compared to more mechanically complex valve designs.

The following table gives a comparison between a Fast Cycle PSA and Conventional PSA Process:

Points of distinction	Points of distinction	Points of distinction
Adsorbents	Structured	Beaded
Valves	Two numbers Rotary	Multiple switching
Cycle Speed	Rotary valves	Valve networks
Comparative Capital Cost	0. 5	1

Application of Fast Cycle PSA Process:

• Hydrogen recovery: Refineries and chemical processes generate bulk amounts of hydrogen-containing offgas, that is flared or burned for low-grade heat recovery. Fast-cycle PSA gives a viable option for the recovery of hydrogen up to 10 MM scfd of these off-gas streams, for on-site or over-the-fence use.
• Modification of Synthesis gas: Fast-cycle PSA are used to tailor the Hydrogen/Carbon Monoxide ratio of crude synthesis gas for applications that includes ammonia production.
• Methane separations: Fast-cycle PSA technology is used profitably for recovering methane from CO₂ contaminated sources such as poor-grade natural gas, coal bed methane, biogas etc.

Limitations of Fast Cycle PSA Process:

• Vulnerability factor: The advantage of the reduced adsorbent inventory can make fast-cycle PSA systems more vulnerable to process upsets like contamination by liquid water or other condensible liquids.
• Cyclic loading stress: Another limiting factor is that fast cycle speeds increase the cyclic loading stresses on the adsorbent pressure vessels.