Separation of carbon dioxide from biogas - Department of Chemical ...

Removal of carbon dioxide from biogas
Introduction
Mirsada Nozic
Department of Chemical Engineering,, Lund University, P. O. Box 124, SE-221 00 Lund, Sweden
The aim of this degree thesis is to simulate the absorption with water part of one
combined technique for upgrading of biogas with the absorption by water and PSA
(Pressure Swing Adsorption). The method of the simulation of process is partly to bring
up the necessary relations for each unit that is included in the technique (scrubber, flash,
and stripper) and to connect together the units in one common Matlab-code. The two
Matlab-codes are brought up, one for counter-current absorption and one for co-current
absorption in the scrubber. The methane losses, a recirculation of gas and design K-
parameter for the scrubber respective the stripper has been studied. The different
parameters of process can have an effect on these variables and the best result is the
combination of parameter value. It means that is impossible to indicate the smallest
value of each one of these parameters because they affect each other.
The usage of biogas as vehicle fuel has
significantly increased in the last years.
Consequently the demand for a calculations model
for one technique for upgrading of biogas to vehicle
fuel is increasing as well. An interesting technique
is absorption with water which is the most common
technique in Sweden.
The aim of this study is to create a Matlab code
for simulation of absorptions part of one combined
technique with the absorption by water and PSA.
The target with the simulation is to make the image
that show how different process’s parameters affect
for the process important design’s parameters and
to find the conditions witch give the smaller
methane losses.
The thesis has been carried out in cooperation
with BioMil AB, a company with the long
experience of production and upgrading of biogas.
Upgrading of biogas
The biogas is a produced by the anaerobic
decomposition of organic matter. It is primarily
composed of methane (CH4) and carbon dioxide
(CO2) with smaller amounts of hydrogen sulphide
(H2S), ammonia (NH3) and nitrogen (N2). Usually,
the mixed gas is saturated with water vapour [1].
Biogas can be used for all applications designed
for natural gas. Not all gas appliances require the
same gas standards. The usage of biogas as vehicle
fuel has significantly increased in the last years. For
an effective use of biogas as vehicle fuel it has to be
enriched in methane. This is primarily achieved by
carbon dioxide removal which then enhances the
energy value of the gas to give longer driving
distances with a fixed gas storage volume [1].
At present four different techniques for
upgrading of biogas are used commercially in
Sweden:
o Absorption with water
o PSA (Pressure Swing Adsorption)
o Absorption with Selexol TM
o Chemical absorption with amines
The absorption with water or water scrubbing is
the most common technique. The technique is used
in such a way that the carbon dioxide absorbs better
in water due to better solubility than methane.
Because the solubility of carbon dioxide increases
with pressure so the separation occurs at high
pressure [2].
The simplified schema for process is shown in
figure below (figure1).
Figure 1. Absorption with water
Usually the biogas is pressurized and fed to the
bottom of the absorption column where water is fed
on the top and so the absorption process is operated
counter-currently. The co-current flow is also
possible but it is seldom used. In the column,
carbon dioxide is absorbed by water and gas out of
the column is enriched in methane. The water

which exits the column wits absorbed carbon
dioxide and a smaller amount of methane which is
partly soluble in water leads to the flash tank there
the gas is regenerated by de-pressuring and returned
to the absorption column. The regeneration of water
Figur 2. Process schema of the upgrading plant with Absorption with water and PSA
Water scrubbing can be used for the removal of
hydrogen sulphide since hydrogen sulphide is also
soluble in water.
Modeling of ‘Absorption with water’ - process
Absorption with water is purely physical
process. It means that it is the absorption without
chemical reaction. The mass transfer from the gas
to the liquid phase can be described by the two film
theory. It is the approximated model which always
assume the steady state, but because the simply
mathematics expressions it is relatively easy to
understand and it give the good accuracy.
According to the two film theory the resistance to
the mass transfer can describes with one or two
stagnant films, the gas and the liquid film. Because
the solubility of the gas follows the Henry’s law
and Henry’s constant of carbon dioxide is large, it
means that the solubility of carbon dioxide is small
and concentration gradient in the liquid phase is
large. According to the two films theory it result in
that the significant resistance for the mass transfer
is in the liquid phase and the gas film resistance and
gas film itself can be neglected. If the process is
controlled by the rate of mean transfer through the
liquid film, such system is called for liquid phase
controlled system.
With these conditions, the total transfer rate of
the component A (methane) respective component
is made by stripping with air in the desorption
column, the stripper. Apart from carbon dioxide,
the gas which exits the stripper contains methane
losses [2].
B (carbon dioxide) from the gas to the liquid phase
in the differential volume at the absorption column
(scrubber) is described by equation (1) and (2).
⎛ FA
⎜ ⋅ p
dFA 0 ⎜ FA
+ FB
= k AL ⋅ a ⋅
dV ⎜ H A
⎜
⎝
⎛ FB
⎜ ⋅ p
dFB 0 ⎜ FA
+ FB
= k BL ⋅ a ⋅
dV ⎜ H B
⎜
⎝
tot
tot
⎞
⎟
FA
− FAt
+ C At ⋅ QL
−
⎟
Q ⎟
L
⎟
⎠
⎞
⎟
FB
− FBt
+ CBt
⋅Q
L
−
⎟
Q ⎟
L
⎟
⎠
(1)
(2)
In order the modeling of flash tank requires
following equations (for the indexes see the
nomenclature and the process schema in figure 2):
y ⋅
A ⋅ pT
= C A H
(3)
2 A
y ⋅
B ⋅ pT
= CB
H
(4)
2 B
( C A − C A2
) ⋅ ( − y A ) = ( CB1
− CB
2 ) ⋅ y A
1 1 (5)
The stripper works as the convert scrubber and
the total transfer rate of component A respective
component B from the liquid to the gas phase in the
differential volume at the desorption column is
described by equation (6) and (7).

⎛
⎜
dFA 0
= − ⋅ ⋅
⎜ C
k AL a
dV
⎜
⎜
⎝
⎛
⎜
dFB 0
= − ⋅ ⋅
⎜ C
k BL a
dV ⎜
⎜
⎝
ALt
BLt
FA
⎞
⋅ ptot
⎟
⋅Q
L − FAt
+ FA
FA
+ FB
+ FC
−
⎟
Q
⎟
L
H A
⎟
⎠
FB
⎞
⋅ ptot
⎟
⋅Q
L − FBt
+ FB
FA
+ FB
+ FC
−
⎟
Q
⎟
L
H B
⎟
⎠
(6)
(7)
It is assumed that the raw gas content only
methane and carbon dioxide.
Design parameter K
Parameter K is introduced in the material
balances for the scrubber and the stripper because
the easily dimensioning of the columns. K is
defined as
dK = k AL ⋅ a ⋅ dV
0 (8)
Because the same parameter should be used for
both methane and carbon dioxide, the rewriting of
the mass transfer coefficient of carbon dioxide is
introduced. According to the turbulence model
which is the most reliable empirical relation, the
mass transfer coefficient for carbon dioxide can be
written as
k
0
BL
= k
0
AL
2
3
⎛ D ⎞ B ⋅
⎜
⎟
⎝ DA
⎠
(9)
With the value of diffusivities at 20°C the
following relation is received
0
0
k = . 09⋅
k
(10)
BL
1 AL
The material balances for methane and carbon
dioxide in the counter-currently scrubber can be
written as
Figure 3. The block schema for the simulations input and received parameters
⎛ FA
⎜ ⋅ p
dFA
⎜ FA
+ FB
=
dK ⎜ H A
⎜
⎝
tot
⎛ FB
⎜ ⋅ p
dFB
⎜ FA
+ FB
= 1.
09 ⋅
dK ⎜ H B
⎜
⎝
⎞
⎟
FA
− FAt
+ C At ⋅ QL
−
⎟
Q ⎟
L
⎟
⎠
tot
⎞
⎟
FB
− FBt
+ CBt
⋅Q
L
−
⎟
Q ⎟
L
⎟
⎠
(11)
(12)
With the same reasoning, parameter K is
introduced in the material balances of the stripper.
Numerical solution of the model
In the numerical solution or the simulation of
the process model, the each unit that is included in
the technique (scrubber, flash, and stripper), is
connected together in one common Matlab-code.
The two Matlab-codes are brought up, one for
counter current absorption and one for cocurrent
absorption in the scrubber. The gas recirculation
from the PSA and the dryer is added to the code as
the constant percent of the gas which enter the unit.
The Matlab-code is iterated until the system is
converged.
The simulation begins with input of the different
process parameters which the user inputs from the
keyboard. The block schema that shows which
parameters has been inputted and which parameters
are received, is presented in the figure 3.

Result
The important variables to study are the loss of
methane, a recirculation of gas and design
parameter K for the scrubber respective the stripper.
The loss of methane is important both from the
economic and the environment point of view. For
these reasons it is necessary to keep it as low as
possible. The different parameters of process can
have an effect on these variables. The base case is
chosen and one parameter at a time is varied and its
effect is studied. The process’s parameters that can
be varied are the liquid (water) flow, the pressure in
the flash tank, the air flow in the stripper, the
amount of stripped carbon dioxide, and the amount
of absorbed methane for counter current absorption,
and the methane fraction in the gas out of the
scrubber for cocurrent absorption.
The influence of the pressure in the flash tank
on the methane losses and the recirculation of the
gas for the counter current absorption are shown in
diagram below (figure 4 and 5).
Methane losses (%)
6
5
4
3
2
1
0
3 4 5 6 7 8
P flash (bar)
Figure 4. The influence of the pressure in flash tank on
the methane losses
Recirculation of gas (%)
60
55
50
45
40
35
30
25
3 4 5 6 7
P flash (bar)
Figure 5. The influence of the pressure in flash tank on
the recirculation of gas
It is important to have the low water flow, the
low flash pressure, and to strip the smaller amount
of carbon dioxide to reduce the loss of methane. It
is impossible to indicate the smallest value of each
one of these parameters because they affect each
other, e.g. extremely low flash pressure demands
larger water flow. In other words, it is important to
find a combination of the parameter values that will
give the best answer. The best result with regard to
the methane losses is 0.5 % for the counter current
absorption and 0.9 % for the cocurrent absorption
for the process that has been the basis of this thesis
with the raw gas flow of 360 Nm 3 /h.
Conclusion
The choice of the process’s parameter values
has some limitations. The minimum liquid flow is
controlled by the conditions which should been
filled, the amount of absorbed methane for the
counter current absorption and the methane fraction
in the gas out of the scrubber for cocurrent
absorption. If the liquid flow is too small, the
condition can not been filled and no response for
the Kscrubber is determined.
The pressure in the flash tank is controlled by
the liquid flow and recirculation of gas. The
extremely low pressure demands larger liquid flow
and larger recirculation of gas.
The methane losses increase with the amount of
striped carbon dioxide because the Kstripper is
increased and with it, the amount of striped
methane is also increased.
Over- respective under dimensioned scrubber
can be simple regulated by the change of the
amount of absorbed methane for the counter current
absorption and the methane fraction in the gas out
of the scrubber for cocurrent absorption. The
regulation of the stripper is preformed by the
amount of striped carbon dioxide.
In other words, it is important to find the
combination of the process’s parameter values
which will give the best answer for the variable
which is more interesting to keep as low as
possible.
Nomenclature
A
B
Methane (CH4)
Carbon dioxide (CO2)
C Air
G Gas phase
L Liquid phase
0
System
reaction
without the chemical
t Top of column
b Bottom of column
0
k L Mass transfer coefficient, m/s
F Molar rate of gas, mol/s
QL Liquid rate, m 3 /s
V Volume of the column, m 3
a Specific surface area , m 2 /m 3
Ci Molar concentration of component
i, mol/m 3

D Diffusion coefficient, m 2 /s
HA Henry’s constant, Pa m 3 /mol
K Design variabel, s/m 3
ptot, pT Total pressure, Pa
yi Mol fraction of component i in the
gas phase, dimensionless
References
[1] Jarvis, Å. (2004) Biogas – renewable energy from
organic waste, The Swedish Biogas Association,
Stockholm
[2] Dahl, A. (2003) Quality fuse of biogas as the vehicle
fuel, Swedish Gas Centre AB (SGC), Rapport 138,
Malmö
Received for review February 08, 2006