Hydrocarbon Processing - February 2021 - GP-33

BACK TO BASICS
The cold two-phase stream from the expander heads to the
de(m)ethanizer, where the light components dissolved in the
condensate are stripped out according to the operating mode
(C2 extraction or rejection mode). The Y-grade NGL are extracted from the bottom of the de(m)ethanizer and routed to
the fractionation section of the facility, while the residue gas is
compressed in the compression end of the turboexpander.
Since only a fraction of the mechanical work produced by
the expander can be transferred to the compressor, the pressure
of the residue gas is significantly lower than that of the raw gas.
To feed the downstream pipeline, a booster compressor must be
installed to restore the pipeline pressure. The booster compressor is a major consumer of electric power in the process plant.
Some processing technology (e.g., Fluor's TCHAP) has been
specifically developed to minimize this recompression cost.
It is worth mentioning that in cryogenic service, the most
widely used heat transfer equipment is the brazed aluminum
heat exchanger (BAHE) or the more recent printed circuit heat
exchanger (PCHE). A BAHE is a stack of alternating layers
of parting sheets and heat transfer fins. Each layer is provided
with inlet and outlet ports for the streams and is sealed along
the edge by means of a side bar, as shown in FIG. 2. The block
of layers is bounded by cap sheets at the top and bottom and is
provided with headers and nozzles for the entrances and exits
of process flows.
BAHEs feature a large surface area per unit volume, high
heat transfer, a minimum temperature approach as low as 1°C,
and the ability to handle multiple process streams. Compared
with a shell-and-tube heat exchanger, a BAHE can be 80%-85%
smaller and lighter. BAHEs are generally made of aluminum.
Since aluminum is prone to mercury corrosion, the use of BAHEs require a mercury removal unit in the pretreatment section
of the central processing facility. Moreover, aluminum is a weak
material, making BAHEs vulnerable to thermal shocks during
startup and shutdown operations.
PCHEs are compact heat exchangers without gaskets, joints
or welds. The core of the heat transfer is obtained by means of
diffusional bonding, which is a solid state joining process. A
number of platelets are stacked in a pile, pressed together, heated at 50%-70% of the material melting point and then allowed
to rest so that the material grains can grow and fill the gaps between the surfaces. In doing so, a monolithic block of core heat
exchange is obtained. The channels for fluid flow are obtained
by chemical etching or pressing.
Designs to accommodate market volatility. When designing central processing facilities for NGL production, processing
facilities must be made to accommodate both the ethane extraction and ethane rejection modes to tune the plant operation to
market volatility. Rejecting ethane entails a partial loss of propane, which is generally more valuable as a liquid than as natural
gas. Therefore, a design goal would be to maximize the propane
recovery while ensuring the maximum ethane rejection.
The different ways in which these objectives have been
pursued has given rise to different distinctive features of the
various processing technologies. The fundamental concepts
underlying these different ways can be traced to the basic concepts outlined in the following sections.
Dual column. A dual column consists of the installation of an

Residue gas
Raw gas

C3 chiller or
J-T valve

Gas/gas
exchanger

Fuel gas

Booster*

LTS separator
LTS
separator

Flash
drum

NGL
Rich-lean
heat exchanger

Lean MEG
*Only for JT process

Glycol
regeneration

Rich MEG

FIG. 4. Simplified PFD of a mechanical refrigeration plant.

Fired heater

Coalescer
Raw gas

Knockout drum

Filter

NGL
Adsorbers

Air cooler

Normal operation
Regeneration circuit
Cooling circuit

Final filter

Water
Residue gas

FIG. 5. Process scheme for adsorption on silica gel.

absorber tower between the turboexpander and the deethanizer
(FIG. 3A). A cold, ethane-rich stream coming from the deethanizer
is pumped to the top of the absorber, and the bulk of the C3+ rich
stream is extracted from the bottom. Generally, the dual column
also entails the dual reflux concept for the bulk removal of NGL
in the absorber. In this arrangement, the deethanizer overhead
gas is partially condensed, and the liquid is used to reflux both the
deethanizer and the absorber towers. In the dual-column, dual-reflux configuration, as much as 99% of propane can be recovered.
Split-feed gas subcooled process. A slipstream (FIG.3B),
typically 15%-30% of the main stream exiting the separators of
the cooling train, is subcooled against the cold residue gas and
refluxed to the de(m)ethanizer tower (in the case of a single-column arrangement) or to the absorber tower.
Tailgate gas recycle. A slipstream of compressed tailgate gas
is cooled in the feed multi-stream plate-fin exchanger, expanded
and fed to the top of the de(m)ethanizer. This arrangement improves the C2+ extraction.
Once the Y-grade NGL have been extracted from natural gas,
each purity NGL is obtained by separation of the Y-grade in the
fractionation end of the production chain. The fractionation is
accomplished by means of a direct sequencing operation of distillation, in which the lightest component is taken overhead in
each column. Ethane is first separated from the liquid blend, and
then propane, n-butane, iso-butane and natural gasoline are separated in sequence.
Gas Processing & LNG | JANUARY/FEBRUARY 2021 33

Hydrocarbon Processing - February 2021

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