PTQ Q2 2024 Issue

Dryout design considerations for cryogenic gas plants: Part 2

Closed-loop recirculation dryout removes water more efficiently, monitors dryout progress more easily, and allows for a quicker transition to plant cooldown

Scott A Miller, David A Jelf, J A Anguiano and Joe T Lynch Honeywell UOP

P art 1 in PTQ Gas 2024 discussed the importance of a properly conducted dryout for cryogenic gas plants during start-up, the most common dryout options, and the general design features needed for a closed-loop recircu- lation dryout. Closed-loop recirculation dryout performs best at effectively removing water from the system while easily monitoring dryout progress This allows for a quick transition to plant cooldown after dryout is completed. Part 2 will cover some of the design challenges, the method for implementing a closed-loop recirculation dryout, and monitoring progress. Cold plant dryout challenges Typical cold plant stagnant areas include: • Thermosyphon reboiler loops • Reflux system (propane recovery plant) • Lower fractionator section (below expander outlet feed). Thermosyphon reboiler loops There is no obvious location to introduce dryout gas flow into a thermosyphon reboiler loop unless isolation valves are installed around the strainers for the exchanger. Some cold plant designs include isolation valves for the strain- ers to have the capability for removing a plugged strainer without depressuring and clearing a much larger plant sec- tion (if not the entire cold plant) of hydrocarbons. This is the easiest location to introduce dryout flow because the isolation valves can be used to direct flow where needed. For this case, connect the dryout gas source between the exchanger start-up screen isolation valves (see Figure 1 ). A 2in minimum dryout connection is recommended.

If strainer isolation valves are not installed, or another means to direct flow through the stagnant pipe loop is not provided, portions of the loop will likely see smaller amounts of dryout flow (or no flow at all), potentially leav - ing unknown water in the piping and equipment. Any flow path that leaves the fractionator and then returns to the fractionator will be stagnant. Therefore, consider the same dryout approach as for a thermosyphon reboiler loop. If strainer isolation valves are not installed, or another means to direct flow through the stagnant pipe loop is not provided, portions of the loop will likely see smaller amounts of dryout flow (or no flow at all) Reflux system (propane recovery plant) Many technologies for propane recovery have a more com- plex reflux system and can be difficult to dry out without proper dryout connections. A section of the reflux system from Figure 7 in Part 1, which typically is stagnant during dryout, is shown in Figure 2 . A 2in minimum dryout con - nection is recommended. The lowest point in the reflux system is the reflux pumps. The reflux system should be designed to free-drain to the lowest point. The dryout gas source can be connected to

Fractionator

Reux accum.

Dryout gas supply

Reux to fractionator

Reboiler pass

Strainer

Reux pump

Low - point drain

Liquid product

Low - point drain

Figure 1 Possible dryout connection for reboiler loops

Figure 2 Dryout connection for reflux system

PTQ Q2 2024