PTQ Q2 2024 Issue

Residue gas compressor

Fractionator

Air cooler

To sales

Dryout gas supply

Inlet gas

Liquid product

Cold plant

Low - point drain

Figure 3 Fractionator dryout connection

Dust lters

Dehydrators

Liquid product

the suction side of the reflux pumps inside the pump iso - lation 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 pres - ent 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 desig - nated 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 nor - mal 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

Dryout ow path

O-line

Figure 4 Recommended location to monitor dryout water content

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 tempera - ture 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.

PTQ Q2 2024