Previously, only the older J Unit provided hydrogen for external customers. Switching to cleaner mercury-free hydrogen produced by the MCP's advanced-membrane-cell electrolysis method enabled us to furnish higher-quality product — and use the J Unit's hydrogen to power our onsite boiler (Figure 1).
Because the MCP hadn't supplied hydrogen externally before, we had to design and implement a new control strategy to enable flexible use of compressors to service variable demand from external customers. The new strategy had to prevent extreme differential pressures — for example, from a compressor startup — that could cause the MCP to trip or damage its membranes.
Once again we set about designing, modeling and testing various control strategies until we found one that allowed linking the compression plant to the MCP without unacceptable disturbances. The dynamic simulation enabled us to accurately predict system performance before commissioning took place. This gave everyone confidence that investing in the compression project would pay off.
The control system now carefully monitors and regulates differential-pressure variations potentially affecting the compressor, burners and MCP membrane. An auto-isolation limit vents off hydrogen temporarily to prevent any damage to the membrane.
Previously, boilers received hydrogen from only a single production stream (either MCP1 or MCP2) at any one time because pressure variations between the two streams increased risk of a boiler trip. Because the new control strategy minimized these variations, we immediately could use a dual stream, which boosted the amount of hydrogen we could send to the boilers.
When a production electrolyzer in the MCP is offline, it's purged with nitrogen gas. The new system enables this nitrogen to be removed without disturbing hydrogen pressure — ensuring customers receive pure hydrogen and avoiding MCP trips.
Maintaining a Solid Foundation
The new control strategies depend upon both instrumentation and control valves working precisely; otherwise we wouldn't be able to reach optimum plant performance levels.
Maintaining this high level of instrument and valve performance has been simplified by the Rosemount pressure and flow devices and Fisher intelligent valves used throughout the compressor, boiler and chlorine plants. Like the control system, they are part of Emerson's PlantWeb digital plant architecture and communicate instrument health as well as process information using Foundation Fieldbus.
We use Emerson's AMS Suite predictive-maintenance software and AMS ValveLink software tool to gather health data, check that valves are working at their optimum levels, and ensure there's no stiction that could affect control performance.
Improving Step by Step
Design and installation took 18 months. While this might seem a long time, a cautious approach was essential to avoid any disruption to chlorine production. Commissioning occurred in August 2008.
We built confidence and acceptance within the production team by gradually introducing elements of the new control strategy, which allowed us to verify that performance fully matched modeled predictions. We did this in two major phases, each time increasing the amount of hydrogen that could be burned in the boilers.
We also implemented a new monitoring system to automatically calculate venting totals and costs based on actual gas prices. Since we started the project, the site has reduced hydrogen venting by 90%. The resulting increase in hydrogen available to fuel the boilers has saved us several million pounds/year in natural gas. In addition, the improved control has prevented many boiler trips and events that would have exposed MCP membranes to unfavorable pressures.
Not wanting to rest on our laurels, we already have designed further improvements to the MCP differential-control system that provide an instant response to load changes and better response to boiler starts and trips below the auto-isolation limit.
Philip Masding is process control manager for INEOS Chlor, Runcorn, U.K. E-mail him at Philip.email@example.com.INEOS Chlor is a sister company to INEOS NOVA, a CP 50 company.