Chemical Engineering July 2021 - 42

Refrigeration
CS SS
LFG from
blower
Sulfur
removal
40ºF
LFG
compressor
Refrigerant
@ 30ºF
TC
75-85ºF
Condensate
to landfi ll or
storage
FIGURE 6. LFG is chilled to help remove any remaining condensate, and also to relieve the power load on
the compressor
sures that the adsorbent particles are
not crushed.
The minimum mass flux
across
the bed should exhibit a particle
Reynolds number (ReP) greater than
25. The high ReP ensures adequate
turbulence for best adsorbent effectiveness.
The total quantity of each
adsorbent required is calculated
based on the total mass of water and
siloxane to be removed against the
adsorption capacity and saturationzone
efficiency of each media. TSA
dryers are designed to lower the dew
point to -40°F. Industry experience
indicates that about 8 lb of water are
adsorbed per 100 lb of fresh activated
alumina, and about 4 lb/100 lb
after 4 years. About 1 lb of siloxane is
adsorbed per 100 lb of silica gel.
Typically, siloxane concentration
can be estimated at a conservative
2,000 to 3,000 ppmw. However, the
actual concentration should be determined
by gas analysis. Note that
while there are techniques to analyze
the siloxane concentration, there is
no standard testing method.
Theoretically, adsorbent layers are
divided into a saturation zone at the
top of the bed and a mass transfer
zone below. The mass transfer zone
is the layer of the bed where the
water and siloxane are adsorbed,
and the saturation zone is the layer
where the adsorbent is fully loaded.
As water and siloxane are adsorbed,
the saturation zone increases and the
mass transfer zone " moves " down
the bed. The repeated cycle of adsorption
and regeneration throughout
the life of the bed reduces the
adsorbent efficiency.
LFG to pipeline-grade natural gas
Processes that convert LFG to pipeline-grade
natural gas are complex
and require high-purity product. Purification
requires compression of the
42
LFG, then removal of moisture, siloxanes,
CO2 and inert substances.
Pipeline-grade natural gas characteristics
are specified by local utilities.
These specifications typically
require that LFG be compressed to
greater than 200 psig, and the CH4
concentration must meet the heating
value of about 1,000 Btu/std. ft3. The
total inert gases should not exceed
4 mol%. The gas must be free of all
siloxanes, solids and impurities. H2S
must be limited to 4 ppmv and total
sulfur (including H2S) must be limited
to 12 ppmv. A TSA removes siloxane
and moisture. Acid-gas treating and
pressure swing adsorption (PSA) are
two common processes for CO2 and
sulfur removal.
In the acid-gas LFG purification
process, a solvent is continuously
circulated (Figure 4) through an absorber
column and a regenerator
column. Low-temperature lean solvent
treats high-pressure acid gas
in the absorber, and high-temperature,
low-pressure rich solvent releases
the concentrated acid gas to
sulfur treatment.
The PSA LFG purification process
(Figure 5) is a batch-wise, high-pressure
adsorption of CO2 on a PSA bed.
The pressure swings low in the PSA
bed to release
the CO2.
Sulfur removal.
One common
technology
used
for sulfur
removal is the
adsorption of
sulfur species on
iron oxide, activated
alumina
or another porous
metal catalyst.
Adsorption
units typically
consist
of
two
LFG at
100%
relative
humidity
LFG chiller
TSA
Dried LFG
to PSA
40ºF
Moisture
separator
LC
or more beds operated in parallel.
Once sulfur nears its breakthrough
point, the adsorption media is replaced.
The media is then treated as
hazardous waste.
Wet scrubbing, another sulfur-removal
technology, is typically vendorsupplied
and proprietary. Wet scrubbing
chemically transforms the H2S
and mercaptans into elemental sulfur.
The LFG stream is contacted with an
aqueous iron chelate solution, where
the sulfur is ionized into hydrogen and
sulfide ions [9].
The chelate complex is regenerated
by oxidation and is returned to
the absorber. The elemental sulfur
can be sold to the fertilizer industry.
LFG compression and dewpoint
depression. Sulfur-free LFG at 100%
relative humidity should be chilled to
40°F (Figure 6). The chilled LFG is then
compressed by a vendor-supplied
compressor. Chilling the LFG to 40°F
upstream of the compressor serves
a dual purpose. First, any remaining
aggressive condensate is knocked
out for increased reliability of the compressor.
Second, lower temperatures
decrease the horsepower required by
the compressor.
Hot, high-pressure LFG from the
compressor should be chilled and
dried through a coalescing moisture
separator (or similar) to 40°F
dewpoint. Typically, these separators
experience a pressure drop
of less than 20 in. H2O. The condensed
moisture is blown down by
on-off level control at high or low
liquid levels.
The cold, dried LFG is warmed
against hot LFG from the compressor.
A temperature-control bypass
valve is sized for about 30 to 40% of
the total flow. About 75°F (or lower) is
LFG
40ºF
LC
Knockout
drum
PC
LC
Pump
Condensate
to
landfi ll
or storage
Condensate
Option 1
Condensate
Option 2
FIGURE 7. Larger LFG plants may use a blowcase configuration to help
collect liquid
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
JULY 2021
Compressed
LFG from
dryer
Blowcase
compressor
Compressed
LFG to dryer

http://WWW.CHEMENGONLINE.COM

Chemical Engineering July 2021

Table of Contents for the Digital Edition of Chemical Engineering July 2021