(71a) Depressurization of Process Equipment during Emergency or Planned Shutdown on a High Potential CO2-Solid-Forming Stream Utilizing the Latest Dynamic Simulation and Blowdown Models

Natural gas produced from many major reservoirs can contain significant amounts of carbon dioxide (CO2) and must be treated to meet typical specifications for pipelines or liquefaction plant feed. These treatment processes often require high-pressure operations forming highly concentrated CO2-rich streams. Pressure protection for these systems has been challenging to date because of the potential for CO2 solids generation upon pressure let down and the consequent potential for plugging.

Blowdown, the emergency or planned depressuring of process equipment, is a critical process safety operation. It may be necessary, in the event of a fire, leak, pipe rupture or other hazardous situation, as well as for a planned shutdown. Devices such as control valves, relief valves, restriction orifices, rupture disks, and safety valves transfer the potentially dangerous contents of process equipment to a safe lower-pressure location, or to the flare system for controlled combustion.

To ensure blowdown can be executed safety and effectively, a number of design concerns must be addressed, such as solid CO2 identification, low temperature (for both process and equipment material). Rapid depressuring and gas expansion can potentially putting equipment at risk of brittle fracture and if the construction material goes below its ductile-brittle transition temperature as well as potential plugging, due to CO2 solid formation. In addition, the entire pressure relief system, including safety valves, relief orifices, flare piping and knockout drums, must be sufficiently sized to handle the flowrates that occur during blowdown, in addition to the piping and capacity of the flare system.

For new installations, accurately predicting the minimum vessel wall temperature during blowdown is important for selecting the appropriate construction material, for helping reduce overdesign and consequently for lowering project cost. Similarly, having an accurate prediction of the maximum flow rate during blowdown reduces overdesign associated with the relief valve/network, without compromising on safety.

The paper will address the potential of solid CO2 formation based on commercially available software for blowdown and propose some mitigation plan with respect to solid CO2 formation.