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,"
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
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
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 26 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.
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 productsparaffins and olefinsand discussed how product is
transported to facilities via pipeline and rail car.
Mr. Danilewicz cited two NGL products as being ideal for transport
overseasHD-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 isobutaneexport 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.