By ADRIENNE BLUME
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
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.
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.
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 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
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
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
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.