Mike Stinn, Global Refining Manager, for the flow controls products of Emerson in Marshalltown, Iowa, wrote an article for the Q4 2018 issue of Petroleum Technology Quarterly magazine titled Selecting Valves for Pressure Swing Adsorption. Here’s a summary, please click on the link above for the full text.
Pressure swing adsorption (PSA) is a process that involves separating impurities from
a feed gas mixture to produce a single product gas such as hydrogen, oxygen or nitrogen. PSA applications have stringent requirements for stroking frequency and shutoff.
A PSA process operates at ambient temperature on a cyclical basis. The PSA process is a semi-batch-type process that uses multiple adsorbers to provide constant feed, product and off-gas flows. For high-purity hydrogen production, the product leaves the system close to the feed gas pressure. The off-gas (impurities and the hydrogen losses) is available at low pressure as fuel.
A PSA unit is a demanding process for control valves. The constant production and operating scheme of the PSA unit requires an extremely high number of cycles, which can lead to damaging effects on PSA process equipment, specifically production and maintenance concerns with mechanical equipment.
Control valve shutoff is a major concern because it affects PSA unit efficiency. If valve leakage causes contamination from one PSA bed to another, gas purity can be compromised and process inefficiencies will be realized.
Emerson performed a detailed analysis of control valves at a large refinery in Oklahoma. The original control valve manufacturer at the refinery recommended routine maintenance on PSA control valves every 100,000 cycles. The cost to replace soft parts on the refinery’s 30 PSA valves was $100,000, labor costs included. Thus, the cost to rebuild each OEM valve every 100,000 cycles was $3,333 annually.
In this large refinery, as each bed went down due to a valve failure, the entire PSA unit was forced to shut down. The unit had to be modified to operate on five beds instead of six in preparation for valve failures. This reduction in hydrogen throughput led to cutbacks in refinery production to keep available hydrogen in balance.
Poor reliability and the resulting high cost of maintenance and reduced profitability led the refinery to seek a better solution. Emerson analyzed the application and recommended Fisher GX globe valves for throttling, and Fisher high performance butterfly valves for isolation on the PSA units.
To cure PSA problems at an Oklahoma refinery, Fisher(TM) GX globe valves and high performance butterfly valves were installed.
During a 24-month test with 200,000 cycles of service, there were no critical maintenance issues and no observable leakage. Based on this experience, all 30 existing valves on the PSA units were replaced with Fisher valves and actuators.
Revamping a Refinery
At a refinery in Texas, a PSA skid was recovering only about 65 to 70% hydrogen. Plant maintenance personnel worked with Emerson to identify application requirements for the PSA skid. They used diagnostics to analyze the performance of the PSA skid's valves and to justify the unit's revamp.
This PSA skid was producing only 65-70% hydrogen. Replacing the valves improved production by 20%.
Initially, more than two dozen PSA valves, especially those with coupling wear issues, were repaired. Others were replaced with new Fisher rotary valves specified for high-cycle operation, including chromium carbide-coated shafts, PEEK bearings and/or durable soft seals. The assemblies included rack and pinion actuators and FIELDVUE digital valve controllers.
After the installation of 40 new valves in the PSA skid, the PSA unit was recovering H2 at an increased rate of 80 to 84%. In addition, the tail gas compressor was operating with 26% hydrogen compared to the 45-50% required before the valve revamp. Increased density of the process flow also improved compressor performance.
The 20% improvement in H2 recovery equated to an additional 3.75 MMSCFD. With a cost of $1500/MMSCFD, this valve project saved about $5600 per day, or over $2 million per year.
Selecting Control Valves
Two types of control valves are suitable for PSA applications: globe and rotary (butterfly) valves. Each should have features specifically designed for stringent leakage requirements and rapid cycling.
Globe valves meet tight shutoff requirements with the use of durable soft seats that enable long-lasting Class VI shutoff per ANSI/FCI 70-2. To further ensure tight shutoff, contours of the unbalanced plug design should only contact the PTFE soft seat when shutoff is required. The seat ring centers the plug as it enters the seat, so the valve establishes a concentric seal, ensuring prolonged valve trim over the life of the valve.
Rotary control valves meet critical shutoff requirements via the use of seal rings with a pressure-assisted sealing action, and with spring-loaded shafts that center the disk in the seal.
Rotary control valve assemblies can also achieve 1,000,000 cycles under load conditions with a spring-and-diaphragm actuator similar to that of the sliding stem spring-and-diaphragm design. The benefits of these actuator designs include no O-rings to wear, a defined position on air failure, low actuator pressures for operation and double-sided diaphragms.
Although both rotary and globe valves can be used for the various PSA applications, globe valves are better suited for installations requiring operation at intermediate travels.
Control valves are critical to the operation and efficiency of a PSA system. The wrong valves can cause expensive maintenance problems, and under-performing valves can adversely affect process efficiency, costing a refinery millions of dollars annually.
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