Gas 2024 Issue

Residue gas compressor

Air cooler

To sales

To sales , reinjection, or are

Cold plant

Inlet gas

Cold plant

Inlet gas

Dust lters

Dehydrators Dryout f low path

Liquid product

Dehydrators

O-line

Liquid product

Dryout fl ow path

O-line

Figure 1 Once-through dryout

Figure 2 Closed-loop recirculation dryout

been removed from the system compared to the other dry- out 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, equip- ment, 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-reduc- ing 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, recir- culates 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 res- idue 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. The closed-loop recirculation approach is our recom- mended approach, as it achieves a proper dryout in the shortest time. This dryout option removes both free water and ambient condensation. It does so without excessive amounts of nitrogen, as with nitrogen pressure cycling, or excess flaring, as with a once-through dryout with dehy - drated inlet gas. Nitrogen cycling is only effective at remov- ing ambient condensation within the cold plant and does a poor job of moving free water to the low points where it can be removed from the system. The once-through dryout option must have a location to discharge the wet dryout gas after passing through the cold plant. Monitoring dryout progress using closed-loop recircula- tion is relatively straightforward. Once dryout is complete, transitioning to cooldown can be done quickly with min- imal effort simply by shifting the pressure drop from the dryout pressure-reducing device to the J-T valve. If desired, cooldown can commence using the dryout recirculation gas prior to introducing fresh inlet gas. A well-executed dryout begins during the detailed design phase by determining the dryout features required for a spe- cific cold plant design. A dryout procedure should be prepared prior to dryout, including any cold plant design limitations as well as the specific steps to follow for removing water and monitoring the cold plant water content during dryout. Cold plant design features: Executing dryout Several design features should be considered during the detailed design phase for the cold plant to be included as part of the cold plant design package. Although some of the features listed in this section are required specifically for dry - out, many will be used for alternative purposes as well (such as depressurisation and venting hydrocarbons from process equipment prior to maintenance or repair, isolating equip- ment, and monitoring the dehydrator outlet water content). These features are: • Recirculation piping • Pressure-reducing device and location • Drain valves • Provisions for stagnant process areas • Location to monitor cold plant water content.

Gas 2024