Gas Dehydration Regeneration Options for Natural Gas

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Various Possibility of Regeneration of a Gas Dryer for Natural Gas
Regeneration is the most important aspect for a
dehydration system. Proper regeneration helps
maintenance of the right outlet dew point through
out the cycle without the moisture breakthrough.
The quality of Regeneration gas determines the
dew point achievable and pressure determines
the flow required so that the required regeneration
velocity can be maintained to avoid channeling.
Channeling restrict the movement of the heat front
through the bed and effect the desorption
process.
So to design a good regeneration system , we
need to analyze the following: The moisture content of the regeneration gas.
.Higher moisture content , in spite of high
temperature of heating , can not achieve lower
dew point . Higher moisture content gas is
know as wet gas and mostly this is the same
gas as we see at the inlet of the dehydration
package. Dry gas is termed when a slip stream
form the outlet of the dryer or a separate dry
stream is used to regenerate the bed . Here
the dew point can even go down to 0.1 ppm v.
 The regeneration pressure – Same pressure
regeneration as well as reduced pressure
regeneration are used extensively . But in case
of a high pressure adsorption , a reduced
pressure regeneration is preferred to reduce
the regeneration gas flow.
We can even go down to 30 ppm v moisture at
the outlet. The moisture load of the same gas at
lower temperature will be as under:



400C – 4530 ppm v
300C - 2630 ppm v
200C – 1470 ppm v
100C – 790 ppm v
Means we save lot of energy and incidentally ,
cooling requires less power than heating a large
bed.
Wet Gas Regeneration
SEPARATOR
COOLING
REGENERATING
At this high moisture load , the adsorbent bed
becomes bigger , regeneration gas flow goes up
to remove higher moisture load and we have
difficulty to reach even 100 ppm v at the outlet.
The unit becomes unknowingly a power guzzler
which goes un noticed.
Some inlet cooling and low temperature
regeneration cooling can make the dryer smaller
and the power cost can be reduced .Sometime
the cold gas form dew point depression
unit helps us to reduce the moisture level.
Figure-2 is a three bed wet gas regeneration unit
where the use of three beds shortens the heating
and cooling time and gas over two bed design.
Hence the bed can be made smaller.
ADSORBING
This type is used when the same inlet gas is
used for regeneration. Normally the aftercoolers
of the compressors are air cooled and the
temperature to the dryer comes to as 550C .
At this temperature , the moisture content at 16.6
bar g for a 0.735 specific gravity gas will be 9540
ppm v. With this inlet , we split the stream , and
the split stream goes for regeneration. The split
stream again get mixed up at the inlet with the
same ppm level due to air cooling of the
regeneration stream.
The Figure-1 on the right will explain the same.
Figure-3 is same as above but this recovers the heat
form the cooling bed through a recuperative cooler.
Gas Processing Solutions
Wet Gas Heating & Dry Gas Cooling
Figure-4 uses wet gas heating and dry gas
cooling. This is a better method to obtain
deeper dew point than the schematics
shown earlier but needs three adsorber.
Cooling heat recovery can be part of this wit
a recuperative exchanger..
Figure-7 shows the pipe line gas is boosted and
the dry gas used for regeneration is pumped
back to mail pipe line
Dry Gas Heating & Cooling
This is a must for lower dew point but
even for higher dew point , this approach will
also reduce the power cost
Figure-6 above is used when residual
gas is not available. A slip stream assisted by
a booster recycles the dry gas for heating and
cooling.
REGEN ERATING
S EP ARAT OR
The Figure-5 uses dry residual gas for
regeneration at lower pressure than the
adsorption pressure . The common application
is feed gas drying in LPG extraction plants.
COOLING
ADSORBING
Sometimes we use the last stage of the feed gas
compressor to help the recycle and dry gas does
the regeneration. Figure-8 depicts the same.
Sometimes we use the last stage of the feed gas
compressor to help the recycle and dry gas does
the regeneration. Figure-8 depicts the same.
Figure-9 depicts a combined unit for moisture
removal and dew point depression. By shifting the gas
to gas exchanger in the front of the dryer , we can
reduce the moisture load means smaller dryer.
Gas Processing Solutions
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