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. The treatment process selected was low temperature CO2 distillation which involve high pressure operation and formation of highly concentrated CO2 streams. Pressure protection for high pressure, low temperature and high CO2 systems have been challenging to date because of potential solid CO2 formation during pressure let down and the consequent plugging.
Blowdown or depressuring of process equipment during an emergency or planned shutdown is a critical process safety operation. It may be necessary in the event of fire, leak, pipe rupture or other hazardous situations, as well as for planned shutdown. Devices such as blowdown valves, relief valves, restriction orifices, rupture disks, and safety valves transfer the potentially hazardous content of process equipment to a safe lower-pressure location or to the flare/vent system for controlled combustion or safe venting.
To ensure blowdown will be executed safety and effectively, a number of design concerns must be addressed such as low temperature solid CO2 identification and Minimum Design Metal Temperature (MDMT) for piping and equipment material selection. Rapid depressurizing and gas expansion can potentially put equipment at risk of brittle fracture if the temperature goes below its ductile-brittle transition temperature of the selected material and potential plugging due to solid CO2 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/vent system.
Accurate prediction on the minimum vessel wall temperature during blowdown is important for selecting the appropriate construction material, for reducing overdesign and consequently 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 proprietary software for blowdown and proposed some mitigation plan with respect to solid CO2 formation within the process piping and equipment.