GPA ’14: Engineers examine gas plant designs for cryogenic, NGL exports

DALLAS -- The GPA Annual Convention featured technical sessions on Monday afternoon that were devoted to a variety of topics, including gas processing facility design. At the design forum, companies discussed gas plant designs, export considerations and shale gas processing operations, among other subjects.

Dryout designs for cryogenic gas plants. Process engineer Scott Miller of Ortloff Engineers spoke about dryout practices and design considerations for cryogenic gas plants. "Completing a successful dryout begins during the detailed design stage of a project," he noted.

However, proper attention to dryout is not always achieved at the design stage, which can lead to blockage of process flow in equipment. For this reason, all water must be removed prior to cooldown. Any dryout-related design work that can be done beforehand shortens the time needed for dryout.

Dryout system types. Effective dryouts accomplish several goals. They move free water to low points, absorb the remaining water in a dryout gas stream, finish in a shorted time period and allow monitoring progress of the dryout operation. Design options for dryout systems include pressure cycling with nitrogen, once-through dryout, and closed-loop recirculation using a residue gas compressor.

Nitrogen pressure cycling is costly, Mr. Miller explained, and it does a poor job of removing free water. Similarly, once-through dryout drawbacks include the requirement of a dryout feature to be added during design, the loss of hydrocarbon product, dictation of the dryout rate by flaring limitations, and the requirement of an outlet that can handle wet inlet gas.

With the closed-loop recirculation option, warm, dry gas is recirculated through the loop using a residue gas compressor. This loop includes an air cooler, a dehydrator, a dust filter, and the coal plant. Any water is removed by dehydrators.

Mr. Miller also outlined basic criteria for dryout design and used examples of closed-loop recirculation systems at propane recovery plants to show attendees how these criteria can be met to ensure the shortest dryout time. The installation of a temporary inlet valve, low-point drains, a moisture analyzer, a recirculation line and a secondary source of dryout gas are all features of the cold plant that must be considered when examining dryout strategies.

Optimizing the dryout process. Before beginning dryout, the expander/booster must be isolated, the deethanizer bottoms must be isolated, the orifice plate must be installed, the dryout moisture analyzer must be put in place, the cold plant must be pressurized to 450 psig, the dehydration beds must be regenerated, and all parties must agree on a maximum water content (with a recommended target level of less than 10 ppm).

To initiate dryout flow, the Joule-Thomson valve first must be opened. Recirculation should continue through the main flow path. Cold plant temperatures must be supervised, all low points must be drained and water content at the cold plant must be monitored. Then, stagnant areas remaining after the main flow path is clear can undergo dryout.

Post-dryout considerations. However, Mr. Miller warned to "Expect water content spikes during the course of dryout." Completion of the dryout of the main flow path and stagnant areas should be followed by an extended dryout as a "due diligence" measure, "just to make sure no water was missed," he said. If the water content remains below 10 ppm, then the dryout is finished.

The total length of time of the dryout depends on the water content in the cold plant; Mr. Miller pegged the average time at 2–6 days. With a well-executed dryout, startup and cooldown are smoother, and the time needed to reach normal operation is minimized. The closed-loop recirculation dryout removes water more efficiently, monitors dryout progress more easily and allows for a quicker transition to plant cooldown, he said.

Refrigeration and compression for NGL exports. Paul Danilewicz from Enerflex discussed refrigeration and boiloff compression system considerations for natural gas liquid (NGL) products export facilities. He presented an overview of the sources and types of products—paraffins and olefins—and discussed how product is transported to facilities via pipeline and rail car.

Mr. Danilewicz cited two NGL products as being ideal for transport overseas—HD-5 propane (with a propane content of 90%–95%) and purity propane (with a minimum propane content of 98%). He also noted that there has been some discussion of transporting ethane (as a purity product above 98% ethane) overseas.

NGL export facility design. The components of a typical export facility for NGL products include optional inlet separation, filtration and storage; an optional dehydration system (if the product arrives over-saturated); refrigeration and boiloff systems; product storage and loading systems; and product metering facilities.

Typically, throughput rates differ from loadout rates, Mr. Danilewicz said. Feed can be continuous if the product is supplied via pipeline or from storage, or it can be processed in batches if brought in by rail car. Loadout rates, on the other hand, are determined by vessel sizes.

Refrigeration and boiloff systems. These systems convert warm, high-pressure liquid feed into a cold, low-pressure product stream. For purity products, direct refrigeration systems are preferred. Indirect systems are preferred for HD-5 and products having various compositions; these systems utilize closed-loop refrigeration independently of the product. Separate compression equipment is required to handle boiloff gas and ship vapors.

Cascading systems, on the other hand, are typically a mix of direct and indirect refrigeration systems in a cascading arrangement. They use a propane system to chill the incoming feed, and are complex compared to direct and indirect systems.

Mr. Danilewicz gave an overview of the typical system components required for all refrigeration systems, including pressure requirements, compression requirements and other design details. He also considered the benefits of air coolers vs. evaporative coolers. For compression systems, oil-flooded screw compressors are the most popular design. These compressors include positive displacement machinery and off-the-shelf designs, among other features.

Modular LPG facilities. Lastly, Mr. Danilewicz discussed the modularization of liquefied petroleum gas (LPG)—i.e., propane, butane and isobutane—export systems. Modular systems can be built in controlled manufacturing environments, where the modules are fitted, piped, tubed, wired and painted. This process minimizes field installation costs and time while providing a fully integrated design for an LPG export facility.