Jun-2024

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

Article Summary

Part 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 recirculation 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).

Part 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 recirculation 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 strainers to have the capability for removing a plugged strainer without depressuring and clearing a much larger plant section (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 leaving 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.
Reflux system (propane recovery plant)

Many technologies for propane recovery have a more complex 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 connection 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 the suction side of the reflux pumps inside the pump isolation valves, as shown in Figure 2. The pump isolation valves can then direct dryout gas flow downstream of the pumps through the reflux piping to the de-ethaniser column.

Lower fractionator section (below expander outlet feed)
The lower section of the fractionator column below the expander outlet feed requires an external flow source to remove water from the mass transfer equipment inside the column. A pipe connecting dryout gas to the column bottoms outlet piping should be designed and installed, as shown in Figure 3. The piping and connection size must be large enough for this dryout connection to be effective. For a 200 MMSCFD (5.4 x 106 Nm3/hr) cold plant, a 4in connection is recommended. As the cold plant nameplate capacity decreases, so can the dryout connection size, but use at least a 2in connection.

For propane recovery plants, it is important to note that any water remaining inside the de-ethaniser column after cooldown will become trapped, eventually freezing because the column overhead is colder than the freezing point of water, and the column bottom is warmer than the boiling point of water.

Location to monitor cold plant water content
A sample location for monitoring the wet gas water content at the outlet of the cold plant upstream of the residue gas compressor must be available (see Figure 4). The wet gas at this location is representative of the amount of water present in the main dryout circulation loop. Any water carried away by the warm dryout gas that will then be removed by the dehydrators can be measured at this point.

Before starting dryout
Ensure the following before beginning a cold plant dryout:
•    The pressure-reducing device is installed at the designated location.
•    The residue gas compressor is operational and running in total recycle before starting dryout gas flow through the train.
•    All dehydrator beds are available for service in their normal cycle. Each bed should have already had at least one successful regeneration cycle, and the water content of the dry gas exiting each bed should be less than 1 part per million by volume (ppmv). At least one bed should be online with one bed in regeneration (assuming a two-bed design).
•    The dehydrator dust filters must be available for service. A typical design includes two full-flow filters (one in service and one off-line).
•    All control valves and manual valves which would restrict flow through the main flow path are open, except the J-T valve.
•    The fractionator bottom is isolated from its downstream equipment.
•    The expander/booster compressor is completely isolated, with flow through the J-T valve and around the booster compressor through its bypass line.
•    The moisture analyser used to monitor the cold plant water content is in service.

System design limitations
The following design limits must be adhered to keep from damaging process equipment during dryout:
•    The recommended maximum dryout gas feed temperature to the cold plant is 140°F (60°C), limited by the brazed aluminum heat exchangers (BAHE) design temperature of 150°F (65°C). Warmer dryout gas is better. Adjust the louvres on the residue gas compressor discharge cooler to provide a temperature near the maximum.
•    Understand the flow rate limitations, which would result in reaching the maximum velocity limitations across major process equipment (dehydrators, filters, and mass transfer equipment) during dryout because of system flow at lower pressure. However, while it is prudent to know the plant process design limits to keep from damaging equipment, the maximum flow calculated to limit J-T expansion through the cold plant will most likely be less than the process equipment maximum velocity limits.

Dryout method
At this point, all process requirements should be met so dryout can begin.
Summary of steps
υ Initiate dryout gas flow through equipment
ϖ Circulate dryout gas through the main flow path
ω Periodically drain system low points
ξ Establish flow through stagnant piping loops.