improve mixing and reduce composition variations. Pressure surge may be managed with control valves (in and/or out) of the surge drum.
Control System
The PSA cycles are controlled by a programmable logic controller (PLC). In some
cases, this logic has been incorporated into the plant’s distributed control system
(DCS), but more often the logic is housed in a dedicated PLC, with interfaces to
the DCS.
The control system will normally match the capacity of the PSA to the plant rate
and handle rate changes. The system can manage some types of valve failures by
switching between operation modes (number of beds on line), with minimal upset.
Many systems have built-in diagnostics to help analyze problems. The systems can
also be combined with online gas analyzers to optimize recoveries.
Operations
Start-Up
From a routine standpoint, the start-up of a PSA is relatively simple:
• The PSA system is air-freed.
• Feed gas is lined up to the unit. Off-gas is lined up to the surge drum and, usually,
flare. The vessel block valves are all opened, with the control valves still closed.
• The adsorber pressures are set to the required starting pressure profile for a
desired start-up step using available gases. This may also be automatic.
• The PLC is started at the desired step and the PSA valves begin cycling. All the
incoming feed gas goes out through the surge drum to flare initially for a few
cycles to establish the correct impurity profiles.
• Once the system is operating reliably and smoothly, hydrogen production to
users can begin.
• Once the system is stable, the off-gas can be routed to either the SMR or fuel gas.
Shutdown
Shutdown of a PSA unit is accomplished by stopping the PSA cycle. The control
logic in the PLC will automatically block in the vessels and freeze the cycle and
pressures in the current step. Incoming feed gas will go to flare or its alternative
destination. The off-gas/tail gas will stop immediately.
Adjustments will need to be made to downstream, and possibly upstream, units
that feed the PSA or use the off-gas.
Routine Operation
Once in service, the PLC controlling the PSA will operate the unit and continuously
monitor for problems. The operator may need to adjust the adsorption cycle time
periodically to fine-tune the cycle or for major rate changes. Generally, the PSA
system will take care of itself.
630
S.A. Treese
Control System
The PSA cycles are controlled by a programmable logic controller (PLC). In some
cases, this logic has been incorporated into the plant’s distributed control system
(DCS), but more often the logic is housed in a dedicated PLC, with interfaces to
the DCS.
The control system will normally match the capacity of the PSA to the plant rate
and handle rate changes. The system can manage some types of valve failures by
switching between operation modes (number of beds on line), with minimal upset.
Many systems have built-in diagnostics to help analyze problems. The systems can
also be combined with online gas analyzers to optimize recoveries.
Operations
Start-Up
From a routine standpoint, the start-up of a PSA is relatively simple:
• The PSA system is air-freed.
• Feed gas is lined up to the unit. Off-gas is lined up to the surge drum and, usually,
flare. The vessel block valves are all opened, with the control valves still closed.
• The adsorber pressures are set to the required starting pressure profile for a
desired start-up step using available gases. This may also be automatic.
• The PLC is started at the desired step and the PSA valves begin cycling. All the
incoming feed gas goes out through the surge drum to flare initially for a few
cycles to establish the correct impurity profiles.
• Once the system is operating reliably and smoothly, hydrogen production to
users can begin.
• Once the system is stable, the off-gas can be routed to either the SMR or fuel gas.
Shutdown
Shutdown of a PSA unit is accomplished by stopping the PSA cycle. The control
logic in the PLC will automatically block in the vessels and freeze the cycle and
pressures in the current step. Incoming feed gas will go to flare or its alternative
destination. The off-gas/tail gas will stop immediately.
Adjustments will need to be made to downstream, and possibly upstream, units
that feed the PSA or use the off-gas.
Routine Operation
Once in service, the PLC controlling the PSA will operate the unit and continuously
monitor for problems. The operator may need to adjust the adsorption cycle time
periodically to fine-tune the cycle or for major rate changes. Generally, the PSA
system will take care of itself.
630
S.A. Treese