Hydrocarbon Processing - December 2020 - GP-41

BACK TO BASICS

Adsorption on solid sorbent. This family of technologies is based on the van der
Waals interactions between water and the
surface of a solid desiccant, typically accommodated in fixed-bed columns. Due
to these interactions, it is possible to dry
natural gas to less than 0.01 ppm. The
van der Waals bonding is not as strong
as chemical bonding. Upon heating, the
increased kinetic energy of the adsorbed
molecules loosens these bonds, enabling
the separation of water from the sorbent.
This phenomenon is used to regenerate
the sorbent when it has become exhausted.
As the top layer of fresh desiccant
comes into contact with the wet natural
gas, it soon becomes water-saturated, and
a mass transfer zone (saturation front)
forms. Above the mass transfer zone the
bed is saturated-i.e., the " wet " sorbent is
in equilibrium with the wet inlet gas, and
the water concentration in the gas flowing
through the upper side of the mass transfer
zone remains unchanged. No further adsorption takes place in this zone. Below the
mass transfer zone the desiccant is in contact with dry gas, and so it remains fresh.
Within the mass transfer zone, the water content is reduced from the inlet concentration, Cin , to the equilibrium value of
the fresh adsorbent, as represented by the
green line in FIG. 4. It is highlighted that
the mass transfer zone can be seen as the
length of the bed involved in the absorber
process at time τ.
As time passes, the mass transfer zone
moves downward through the bed until it reaches the bottom of the column.
At this point the entire bed has become
water-saturated, and the adsorption process comes to an end. In time, the water
dewpoint at the tower outlet draws an " S "

curve, known as the breakthrough curve,
represented by the blue line in FIG. 4.
When the water concentration at the
bed outlet touches the (Cb , τb) point (the
water dewpoint corresponding to the gas
specification) on the breakthrough curve,
the fixed bed is considered exhausted.
It is then excluded from the production
cycle and regenerated. In industrial practice, the end of the production cycle is
not set by the analytical measurement of
water dewpoint in the treated gas; rather,
it is set by a predetermined time τ (estimated to be close to τb).
Activated alumina, silica gel and mo-

lecular sieves are the most-used sorbents
for drying natural gas. Activated alumina
is the least-expensive sorbent material for
drying natural gas to -70°C. The major
disadvantages of activated alumina are
the coadsorption of hydrocarbons, which
results in a reduced water load and loss
of production, and the rehydration by
contact with wet gas at high temperature
during the regeneration stage. Silica gel
features a high water adsorption capacity and can also adsorb C5+ components,
making it available for the simultaneous
removal of water and gasoline from natural gas. The water dewpoint attainable

MTZ

Cin
Concentration at
bed outlet
Concentration

separator as stripping gas. In this case, the
gas from the separator is directed to the
stripping gas heating coil of the reboiler.
In this arrangement, it is possible to use
any flow of stripping gas without having
to import fuel gas.
Both the secondb and thirdc processes
are able to produce a lean glycol with a purity greater than 99.98%, and ultimately
achieve a dewpoint depression of 100°C
against the 45°C of a standard gas dehydration unit when the contact temperature is approximately 25°C. However, it
must be said that these advanced processes entail additional equipment relative to
standard glycol units.

MTZ
concentration
proﬁle

Breakthrough

Cb
Cout

␶b

Time

FIG. 4. Adsorption process principles.

Raw gas

Filter

Filter
Air cooler
Knockout
drum
Hydrocarbons
Wastewater
Adsorption

Hot regeneration

Fire heater

Dry gas
Filter

FIG. 5. A typical adsorption process.
Gas Processing & LNG | NOVEMBER/DECEMBER 2020

Hydrocarbon Processing - December 2020

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