Hydrocarbon Processing - December 2021 - GP-33

PLANT OPTIMIZATION
Customize selection of hydrocarbon
dewpoint control technology
A. A. LODHI, Zishan Engineers Pvt. Ltd., Karachi, Pakistan
The purpose of a hydrocarbon dewpoint
control unit is to remove hydrocarbon
liquids (NGL) from natural gas so
that the hydrocarbon dewpoint specifications
of the sales gas (local requirement is
0°C at any pressure) can be met. A challenge
for plant designers is to choose the
most suitable technology for hydrocarbon
dewpoint control. The technology
must be technically and economically
capable of continuous, trouble-free operation,
with optimum outcome. It must
have the flexibility to adjust the water
and hydrocarbon dewpoints of the sales
gas to the required pipeline specification
on maximum and turndown conditions
without affecting the performance of the
plant, even in the worst-case scenario.
The selection of appropriate technology
for hydrocarbon dewpoint reduction
must consider several factors. It is
imperative for the plant designer to understand
the key factors that contribute
to the choice of the best technology for
the plant, particularly at the conceptual
design stage.
Three hydrocarbon dewpoint control
technologies are commonly used:
Joule-Thomson (JT) valve technology,
turboexpander technology and mechanical
refrigeration technology. Significant
research is available on each type of technology
independently, but a comparative
analysis must be performed for each
technology in relation to the others based
on key impacting factors, as the optimum
performance of the plant relies on the
proper selection of technology.
This comparative process analysis will
ascertain the application and pros and
cons of each dewpoint control process
and technology with the objective of selecting
the most suitable process for the
specified gas treatment plant. The ultimate
selection is based on the financial
viability of the process schemes and the
technical comparison where the impact
of the contributing factors is identified.
Process simulations are prepared for
all three technologies with results to show
the optimum scheme, along with other
contributing factors in the selection of
technology. The key contributing factors
considered for comparison are complexity
of operation, turndown, reliability and
availability, pressure drop, sales gas specifications
and others.
Mechanical refrigeration technology.
In the mechanical refrigeration scheme,
gas from the upstream processing unit
is routed to the gas/gas heat exchanger
that provides a means of heat exchange
between the raw gas and the low-temperature
separator (LTS) gas; therefore, the
precooling of the raw gas is achieved in
this heat exchanger. As the temperature
drops, some condensation occurs downstream
of the gas/gas exchanger. The gas
is then passed through the chiller to further
reduce its temperature, which promotes
additional condensation.
The
combined stream [gas,
condensed
hydrocarbon and the ethylene
glycol (EG)/methanol solution] is then
introduced in the LTS to segregate the
gas, liquid hydrocarbons and the EG/
methanol solution. After the separation
of NGL, gas from the LTS will have dewpoint,
as per pipeline specifications. This
dewpoint-specified gas is then routed
through the gas/gas heat exchanger, as
previously discussed, to exchange heat
with the raw gas and exit as sales gas.
Cooling of the raw gas in the chiller is
achieved by using propane as refrigerant
for heat exchange. Propane gains energy
and vaporizes in the chiller after heat exchange.
Vaporized gas is routed to the
refrigeration unit, where it is condensed
after being recompressed in the refrigerant
compressor and routed to the refrigerant
accumulator. Propane fill/makeup in the
refrigerant loop is carried out at the refrigerant
accumulator. Compressed propane is
finally cooled/chilled at the required temperature
by pressure letdown across the
valve. Chilled propane is then introduced
back into the chiller for heat exchange.
NGL from the LTS is first passed
through the multipass exchanger to raise
its temperature above the hydrate formation
temperature before being routed to
NGL storage.
Glycol solution (EG) or methanol is
injected in raw gas at the inlet of the gas/
gas exchanger to avoid possible hydrate
formation at low operating temperature.
Rich EG/methanol solution collected
from the LTS is routed to the glycol/
methanol regeneration system, which,
after processing, recycles back lean EG/
methanol solution to the dewpoint reduction
unit (FIG. 1).
Phase envelope. Proprietary simulation
steady-state softwarea
is utilized to
simulate the hydrocarbon dewpoint control
unit. FIG. 2 shows the phase envelope
for lean sales gas after processing through
a mechanical refrigeration-based hydrocarbon
dewpoint control unit.
Application pros and cons. The application
of the mechanical refrigeration
processing route has a number of benefits
and challenges:
* Although this technology is
conventional and can produce the
required sales gas hydrocarbon
dewpoint specification, it is
not suitable at higher operating
pressures, as more cooling is
required at high operating pressures
to extract the same amount of
heavier component as compared to
relatively low or moderate pressures
Gas Processing & LNG | NOVEMBER/DECEMBER 2021 33
Hydrocarbon Processing - December 2021

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