May-2024

Dryout design considerations for cryogenic gas plants: Part 1

A well-conducted cold plant dryout will help plant commissioning and ensure start-up goes smoothly and does not last any longer than necessary.

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

Article Summary

A cryogenic plant dryout is a critical step during start-up, but it often does not receive proper attention early in the project that would allow successful execution. For this discussion, ‘cold plant’ refers to a cryogenic turboexpander plant for natural gas liquids (NGLs) recovery, which recovers liquids comprised of either ethane and heavier components (often referred to as C₂+ liquids or NGLs) or propane and heavier components (often referred to as C₃+ liquids or liquefied petroleum gas [LPG]).

A cold plant dryout can be executed correctly the first time. The owner/operator can be confident that the cold plant is dry prior to cooling down when proper design features are implemented, and guidelines are followed. All gas processors know that water must be removed from the cold plant. However, knowing the best method for removing water; how much water has been removed during the dryout period; and when dryout is complete, can be challenging. Far too often, a dryout is stopped before the cold plant is completely dry.

Hydrate problems
The solid that forms when water is present in a hydrocarbon stream is not ‘ice’, but a crystalline structure known as a hydrate. Hydrates can form at conditions where solids would not be expected and will form above the freezing point of water. They are a physical combination of water and other chemical constituents, like those found in natural gas processing, which have an ‘ice-like’ appearance.¹

Hydrates form when enough water is present at the right combination of temperature and pressure and tend to favour systems with low temperature and high pressure.2 For gas plant owners/operators, this means that when the plant begins to cool down, hydrates will form in process areas where sufficient water is present, restricting or completely obstructing process flow.

It is not uncommon to see hydrates obstructing flow through heat exchanger passes or the strainers that protect the heat exchanger from construction dirt and debris. Hydrates can cause enough pressure drop to rip apart strainers, allowing dirt and debris to enter and damage the downstream heat exchanger. In the case of thermosyphon reboilers and side heaters, hydrate formation may restrict the flow through the exchanger and reduce the amount of heat input to the column enough to prevent achieving the bottoms liquid product purity specification.

Hydrates may also form on the cold plant fractionation column trays and packing. The result is a decrease in efficiency, causing low product recovery and potentially off-specification liquid product. Hydrates are also known to plug control valves and plant instrumentation.

Water can enter the plant equipment through rain or condensation from open piping during construction and through water left after hydrostatic testing. The single most important step that can be taken prior to the start-up of a cold process plant is to drain and blow out as much free water as possible from the piping and equipment. Eventually, all the water must be removed from the cold plant to the parts per million (ppm) level for the cold plant to operate safely and efficiently. Not all of the water can be removed simply by draining low points. The remaining water must be removed by a combination of moving the water to a low point where it can be drained and absorbing the water in a vapour stream so it can be removed from the cold plant equipment and piping. Several options for eliminating this remaining water are presented in the next section.  

Dryout options
The following options are common approaches to drying out a cold plant prior to cooldown. A description of each option is given, discussing its advantages and disadvantages, in order from the least cost-effective to the most cost-effective option.

Option 1 involves pressure cycling with nitrogen. In this approach, sections of the cold plant are isolated to be pressurised and depressurised multiple times using nitrogen. This method requires no piping design considerations other than properly locating low-point drains and ensuring the drains are of sufficient number and size to remove water from the system.

This dryout approach requires a large quantity of nitrogen to be available and can be very expensive because of the amount of nitrogen consumed. This dryout option is less likely to be successful if large quantities of free water (puddles of water) are still present in the system. It is more difficult to determine the amount of water remaining in the cold plant after nitrogen purging and whether all water has been removed from the system compared to the other dryout approaches. Water content readings must be taken at many more locations to get an accurate assessment of the amount of water remaining in the system.

Option 2, the once-through dryout approach flows warm, dehydrated inlet gas through the cold plant, equipment, and then to the flare stack, reinjection, or a sales gas pipeline. The dryout path is operated at as low a pressure as possible. The pressure drop through the cold plant is minimised to prevent any Joule-Thomson (J-T) expansion that would cool down the process while drying the plant. The flow rate should be maintained to move any free water to the low-point drains, or to absorb the water in the vapour stream and remove it from the process. A pressure-reducing device (such as a temporary flow orifice or valve) must be included to take the pressure drop upstream of the cold section of the plant. Figure 1 illustrates the main process flow path for this dryout method.

Keep in mind that the dryout flow rate may be limited by the flare system’s tolerance for flaring or reinjection system capacity. If the wet gas is sent to a sales gas pipeline, the gas water content should be monitored to ensure it remains below the maximum amount specified. The once-through dryout is an effective approach and has been used on many projects. However, the dryout flow rate can be limited by the flare system or reinjection capacity. For this scenario, the dryout period will most likely be longer in order to remove all the water in the cold plant.

Option 3, the closed-loop re-circulation approach, recirculates warm, dehydrated gas in a closed loop through the cold plant back to the dehydration system inlet using a residue gas compressor. The dryout loop is operated at as low a pressure as possible without shutting down the residue compressor. The pressure drop is minimised through the cold plant to prevent any J-T expansion that would cool down the process while drying the plant. Again, the goal is to minimise pressure drop through the cold plant but maintain a high enough flow rate to ‘sweep’ free water to low-point drains or carry the water away in the gas to be removed by the front-end dehydrators.

A recirculation line is required that connects the residue gas line downstream of the residue gas compressors to the inlet gas piping upstream of the dehydrators to make the ‘closed loop’. A pressure-reducing device (such as a temporary orifice or valve) must be included in the dryout design to take the pressure drop upstream of the cold plant. Figure 2 illustrates the main process flow path for this dryout method.