Hydrocarbon Processing - December 2021 - GP-34

PLANT OPTIMIZATION
* If adequate pressures are not
available at the feed of the
hydrocarbon dewpoint control
unit, then mechanical refrigeration
can be used for the elimination
of hydrocarbons from raw gas
* The availability and reliability
of the unit is lower as compared
to other units because of
rotating equipment like the
refrigerant compressor and the
refrigerant cooler (air cooler);
however, this can be mitigated
with scheduled maintenance
and proper monitoring
* Pressure drop in the gas path of
the chiller and gas/gas exchanger
is low; therefore, sales gas
compression is not required
* A refrigerant is required for first fill
and makeup due to losses from the
system, which adds to OPEX; the
costs of other components of the
refrigeration unit add to CAPEX.
Turboexpander technology. A turboexpander
scheme is used where a substantial
amount of C3
+ components are
available in the feed gas stream to be
extracted. In the turboexpander scheme,
gas from the upstream processing unit
is routed to the gas/gas heat exchanger,
which provides a means of heat exchange
between the raw gas and the 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, and the combined stream
(gas, condensed hydrocarbon and the
EG/methanol solution) is then introduced
in the cold separator to segregate
the gas, liquid hydrocarbons and the
EG/methanol solution. Outlet gas from
the cold separator is then isentropically
expanded in the expander section of the
turboexpander.
At the suction nozzle of the turboexpander,
the ethylene glycol/methanol
solution is injected as a fine mist to
avoid possible hydrate formation in the
expander and at the downstream system.
The decrease in temperature due to expansion
promotes further condensation.
The combined stream (gas, condensed
hydrocarbon and the ethylene glycol/
methanol solution) is then introduced in
the LTS to segregate gas, liquid hydrocarbons
and the ethylene glycol solution. After
separation of NGL, gas from the LTS
will have dewpoint as per pipeline specifications;
however, it still requires compression.
This dewpoint-specified gas
is then routed through the gas/gas heat
exchanger to exchange heat with raw gas,
followed by compression through the
turboexpander-driven sales gas compressor
up to the required sales gas pressure.
NGL from the LTS and cold separator
is first passed through the multipass
exchanger to raise its temperature above
the hydrate formation temperature before
being routed to NGL storage.
EG or methanol is injected in the raw
FIG. 1. Process scheme for mechanical refrigeration technology.
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 and the cold separator is routed to
the glycol/methanol regeneration system,
which, after processing, recycles
back lean ethylene glycol solution to the
dewpoint reduction unit (FIG. 3).
Phase envelope. Proprietary simulation
steady-state softwarea
is utilized to
simulate the hydrocarbon dewpoint control
unit. FIG. 4 shows the phase envelope
for lean sales gas after processing through
the turboexpander-based hydrocarbon
dewpoint control unit.
Application pros and cons. The apFIG.
2. Phase envelope for lean sales gas after processing through a mechanical refrigerationbased
hydrocarbon dewpoint control unit.
34 NOVEMBER/DECEMBER 2021 | GasProcessingNews.com
plication of the turboexpander technology
processing route has a number of
benefits and challenges:
* Turboexpander-based dewpoint
reduction technology is mostly used
in cryogenic process-i.e., where

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Hydrocarbon Processing - December 2021

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