Chemical Engineering July 2021 - 43

the preferred temperature range for
TSA adsorbents. At the outlet of the
moisture separator, the metallurgy
can be downgraded from stainless
steel to carbon steel [5].
CH4 and CO2 are separated
in the PSA by adsorption of CO2
from high-pressure LFG. The PSA
vendor specifies the minimum required
pressure (typically 120
psig) to achieve >99% CH4 in the
pipeline gas.
Blowcase operation. As discussed,
the LFG from the blower is chilled
to 40°F upstream of the compressor.
This condenses the majority
of the humidity (roughly 2 gal/h per
100 std. ft3/min), which collects in
the compressor knockout drum.
One control
strategy
for condensate
disposal is to pump the condensate
to the landfill or storage via
on-off level control at high or low
liquid levels.
Another strategy is to collect the
liquid in a small vessel called a blowcase
and blow it down using on-off
level control (Figure 7). During normal
operation, the knockout-drum drain
valve to the blowcase is open and the
blowdown valves are closed. At the
blowcase high-level signal, the inlet
valve is closed and the blowdown
valves are opened. The condensate
is blown down with compressed
LFG. At low liquid-level signal, the
blowdown valves are closed and
the drain valve is opened, which
depressurizes the blowcase to the
knockout drum. The process engineer
should consider slow-opening
valves to limit the blowdown rate and
the impact of pressurization. The engineer
must design the blowcase
for cyclic service, because of the
repeated operation of pressurization
and depressurization.
Solvent absorption. In the acidgas
LFG purification process, highpressure
LFG contacts a low-temperature
amine (or other solvent) in
an absorber column. Lean solvent
absorbs moisture, CO2, sulfur and
inert gases from the LFG. Rich solvent
passes to a low-pressure,
high-temperature regenerator, which
separates the absorbed CO2 and
sulfur. The lean solvent is circulated
through the absorber, and the
acid gas passes to a sulfur treatment
process. Note that siloxane is
not absorbed.
Methane recovery with PSA. A
PSA system (Figure 5) operates on
the same principle of adsorption
and desorption as the TSA. However,
the PSA adsorbs at high pressure
and releases at low pressure.
In a PSA, the smaller molecules are
adsorbed and the larger molecules
pass through the beds. Methane is
a larger molecule (3.8 Å) compared
with nitrogen (3.6 Å), O2 (3.5 Å), CO2
(3.4 Å) and water (3.0 Å). Zeolite 13X
(a crystalline polymer) with a pore
size of 3.7 Å is usually selected by
the PSA vendor to adsorb the inert
substances and pass the methane to
the pipeline.
The PSA unit is a complex vendor
package with a sequence of switching
valves that control the pressurization,
depressurization and adsorption
steps (Figure 5). The PSA beds'
valves open and close to control the
steps at each bed. In the depressurization
step, the vessel inlet valve
and the CH4 compressor's suction
valve close. Then the suction valve
to the CO2 vacuum compressor
opens. The very low pressure regenerates
the bed by releasing all inerts
adsorbed on the media. In the pressurization
step, the inlet valve and
compressor suction valve close and
the outlet valve opens. This floods
the bed with high-purity CH4. Once
pressured, the inlet valve opens and
the adsorption step begins.
The process engineer should
specify the PSA system for at least
97% CH4 purity (based on local utility
requirements)
and 85% recovery.
PSA technology can achieve
greater than >99% purity and
95% recovery. The natural gas from
the PSA should be compressed to
pipeline requirements, typically 200
psig or greater, also based on local
utility requirements.
CO2 destruction by thermal oxidation.
PSA depressurization releases
inert gases and 4 to 10% CH4. Environmental
regulations typically do not
allow release of these inert gases and
CH4 to atmosphere. This low-BTU
gas is then destroyed in a very hightemperature
thermal oxidizer (TOX),
which is also complex and typically
supplied as a vendor package. The
TOX operates under a similar principle
as a high-temperature enclosed
flare. The TOX burns the CH4 at very
high DRE to meet environmental
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM JULY 2021
regulations. A very high combustion
temperature is required because the
high concentration of CO2 significantly
decreases the DRE of CH4.
In poorly designed or poorly operating
TSA systems, some liquid may
remain in the LFG and thus may be
adsorbed in the PSA. To capture
the liquid droplets and prevent fireballs
in the TOX, a pulsation dampener
(as shown in Figure 5) should
be considered. Any remaining water
is adsorbed in the PSA. Liquid droplets
are knocked out in the pulsation
dampener, which should be sized
for droplets in the range of 300 to
600 μm in diameter [11]. Because
the PSA depressurization steps are
not continuous, a pulsation dampener
reduces the high surge of
CO2 to the TOX at the start of the
depressurization step.
n
Edited by Mary Page Bailey
References
1. Agency for Toxic Substances & Disease Registry, Landfill
Gas Primer, U.S. Department of Health and Human Services,
Nov. 2001.
2. U.S. Code of Federal Regulations, Title 40 - Protection
of Environment, Part 258 - Criteria for Municipal Solid
Waste Landfills, July 2012.
3. Arnold, M., Reduction and monitoring of biogas trace
compounds, VTT Technical Research Centre of Finland,
Aug. 2009.
4. U.S. Army Corps of Engineers, Landfill Off-Gas Collection
and Treatment Systems, Department Of The Army, EM
1110-1-4016, May 2008.
5. Hansen, D.A., Puyear, R.B., " Materials Selection for Hydrocarbon
and Chemical Plants, " CRC Press, 1st edition,
Aug. 1996.
6. Gas Processors Suppliers Association, Engineering Data
Book, 12th edition, 2004.
7. Scottish Environment Protection Agency (SEPA), Guidance
on Landfill Gas Flaring, Nov. 2002.
8. Schweigkofler, M., Niessner, R., Removal of siloxanes in
biogases, Journal of Hazardous Materials., 2001.
9. Wang, L.K., Pereira, N.C., Hung, Y.T., " Handbook of Environmental
Engineering, Volume 2: Advanced Air and
Noise Pollution Control, " Humana Press, Nov. 2004.
11.
American
Author
Babak Firoozi is the vice
president of HVAC Engineering at
A/C Control, Inc. (2355 Westwood
Blvd #145,
Los
Firoozi's
Angeles, CA
90064, Phone: (310) 909-4814;
Email:
He previously served as a process
director at Oilcor,
babak@ac-control.com).
Inc.
process engineering experience
includes landfill gas processes,
downstream refining and process simulation. He earned
his B.S.Ch.E. from University of Baja California, Mexico,
and he earned his M.S.Ch.E. from California State University.
He also is a registered professional engineer
in California.
43
Petroleum
Institute,
Pressure-Relieving
and Depressuring Systems, API STD 521, 6th edition,
Jan. 2014.
http://WWW.CHEMENGONLINE.COM
Chemical Engineering July 2021

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