HUNGARIAN JOURNAL
OF INDUS1RIAL CHEMISTRY
VESZPREM
Vol. 30. pp. 103- 105 (2002)
ON THE PURIFICATION OF GASES CONTAINING IMPURITIES OF SMALL
CONCENTRATION
C. BOY ADJIEV
(Bulgarian Academy of Sciences, Institute of Chemical Engineering, "Acad. G.Bontchev" str.,Bl.l03, 1113 Sofia,
BULGARIA)
Received: November 05, 2001
A theoretical analysis was made of the methods used for the purification of gas, containing impurities of small
concentrations. A comparison is proposed between absorption and adsorption methods. It is shown that adsorption
processes are more advantageous in these cases.
Keywords: gas purification, absorption, adsorption, comparative analysis
Introduction
Numerous ecology problems are related to the
purification of air from its impurities.~ The main problem
occurred in the case, when the concentration of the
impurity is really small. In these cases absorption in a
packed column is frequently used. A high absorption
rate should be realized by a suitable chemical reaction
between the gas impurity and a liquid absorbent. In
reality, the absorption rate is usually very small and as a
result, a very high absorption column has to be applied,
to reach the expected efficiency.
Absorption
If a gas of Q(m3 Is) flow rate has to be purified from its
impurity with concentration of C (kg I m3 gas) in a
packed column (diameter d (m) and a height of the
active zone h (m), height of the packing).
The concentration of the impurity (an active
component of the gas) is equal to zero as result of the
chemical reaction between this component and the
absorbent. In this case the concentration difference in
the equation of the mass transfer rate is equal to C . The
concentration difference is changing along the active
zone height. Thus, it should be used the average value
of the concentration difference ilC(kg / m3 gas}:
Contact information: E-mail: chboyadj @bas.bg
.tlC (1)
where
C1 and C2 (kg I m3 gas) are the inlet and exit
concentrations of the active component in two ends of
the active zone.
The chemisorption rate in the column is J (kg/ s)
and the volumetric chemisorption rate is
j (kg I s.m3 active zone).
If u (m/ s), V {m3 ) are the gas velocity and active
zone volume, then:
7Td2
V = S.h, Q = S.u, S == --. (2)
4
If k (m3 gas/ s.m3 actve zone} is the volumetric mass
transfer coefficient and
ks (m3 gas/ s.m2 mass transferswface) is the surface
mass transfer coefficient, the chemisorption rate
equation has the following form:
(3)
where
s(m2 masstransfersuiface/m3 actvezone) is the
specific mass transfer surface in the active zone.
Eq.( 3) shows that if the volume concentration of the
active component is small and the diameter of the
104
Table 1 Comparison between the absorption and the adsorption:
Q=0.4m%ec, C=0.01kg/m3, 1J.==99%
.N2 Parameters
1 The gas velocity, u {Jnjs)
2The column section, s (m~)
3The column diameter, d (m)
4The volume mass transfer coefficient, k (s-1 )
5The active zone volume, v (m1 )
6The active zone height, h (m)
7The relative decreasing of the volume, A V Vn 3 )
8The relative increasing of the height, M (m)
9The stoichiometric quantity of a liquid phase, l (m 3 Is)
1 OThe real quantity a liquid phase, L (m 3 j s)
11 The relative increasing of the liquid phase .AL (m 3 Is)
12The relative increasing of the consummations for the liquid phase, I1E
13The pressure drops, !J.rp mm H
2
0
column is constant, the increasing rate of chemisorption
is a result of the active zone height increase or the
increase of the volume mass transfer coefficient:
(4)
Under the chemisorption conditions, when the gas
velocity and chemical reaction rate are constant, the
mass transfer coefficient does not change significantly.
Thus, from Eq.(4) follows that the increase of the input
concentration C1 (respectively AC) needs the increase
of the specific mass transfer surface, s .
The specific active surface in packed columns is
limited by the specific surface of the packing. As a
result. it is not possible to i'ncrease the volumetric mass
transfer coefficient, i.e. for increasing of the column
height. This shows that it is better to use adsorption
process for gas purification because its specific mass
transfer surface is significantly greater compared to
packed columns.
The chemisorption in the active zone may be
realized by means of absorption in a packed column
(s=l02 m2/m 3 ) or by the adsorption of a synthetic
adsorbent (s= 108 m2 /m 3 ) .. because the ratio of the
volumetric mass transfer coefficients of the adsorption
and absorption is 10
4
•
The experimental data for volumetric mass transfer
coefficient in the case the sa}. adsorption by synthetic
anionite shows that
k = 12.1 u m3 gas/ s.m3 active zone. {S)
The experimental data for the gas absorption in the
packed column [4] show that the volumetric mass
transfer coefficient in gas phase for Raschig rings
(502t50x2)mm is
k = l.4kg/m 3 .bar.s. {6}
Adsorption
0.7
0.57
0.85
8.5
0.046
0.081
25
3.3 w-s
3.3 w-s
1
80
0.1
4
2.25
1.2
0.32
0.081
1
25
3.3 to·5
3.3 w·s
1
1
24
Absorption
1
0.4
0.71
0.5
0.79
2
25
3.3 w-s
6710"5
20
500
24
if the gas velocity is 1 m/ s and the liquid flow rate is
6m3/m
2
h.
From (6) it is immediately obtained, that:
k = 0.5 m 3 gas/ s.m3 active zone. (7)
These data allow possibility to make a comparative
analysis on the absorption and adsorption, on the basis
of the next example (Table 1.).
Let to purify airflow of Q = 0.4m3 /s with
concentration of 802 C =0.0lkgjm
3 with 11 =99%.
Efficiency. The values of the process parameters of
chemisorption under the absorption and adsorption
conditions are shown in the Table 1. The results in the
Table show that an increase of the volume AV and M
in the case of absorption, compared to the adsorption.
In the Table 1 the stoichiometric quantity of the
liquid phase l (m 3 Is) 200 g /l Na2 co3 is obtained
immediately. The real quantities of the liquid phase
L~n3 / s) are obtained immediately under the adsorption
condition L = l from the liquid flow rate under an .
absorption condition. The relative increase of the liquid
flow rate in the case of absorption compared to the
absorption condition M can be obtained immediately.
The consummations of the liquid phase movement are
linear function of the liquid flow rate L and the active
zone height. h. As a result. the consummation of the
relative increase AE is /J.L : M = AL.Ah .
Raschig rings (50x50x5) mm are used for the
pressure drops comparison. The second adsorption data
are valid in the case of lower gas velocity (u = 0.1 m/ s).
Conclusions
The obtained results show that the adsorption process is
significantly more effective for gas purification
compared to the absorption, when the concentration
impurities are small.
In these conditions the saturation of the adsorbent is
very slow and this is another cause for a real use of
adsorption processes for solving ecology problems.
SYMBOLS
C concentration, (kg/ m3 gas)
C1, C2 input and output concentration of the active
component, (kg/ m3 gas)
A.C average value of the concentration difference,
(kg/m 3 gas)
d column diameter, (m)
DE relative increasing of the consummations for
liquid phase,
h column height, (m)
A.h increasing of the column height, (m)
1 chemisorption rate, (kg/ s)
j
k
volume chemisorption rate,
(kg/ s.m3 active zone)
volume mass transfer coefficient,
(m3 gas/ s.m3 active zone)
surface mass transfer
coefficient,
(m3 gas/ s.m2 mass transfer surface)
L
u
Q
s
u
v
A.V
1]
8.cp
105
stoichiometric quantity of the liquid phase,
(m 3 /s)
real quantity of the liquid phase, (m 3 /s)
relative increasing of the liquid flow rate,
(m3 /s)
gas flow rate, (m 3 / s)
specific mass transfer surface,
(m 2 mass transfer surface/ m 3 active zone)
gas velocity, (m/ s)
active zone volume, (m3 )
increasing active zone volume, (m3 )
purification degree, (%)
pressure drops, mm H 20
REFERENCES
1. L. PANTOVCHIEV A and CHR. BOY ADJIEV: Bulg.
Chern. Commun., 1995, 28, .N2 %, 780-794
2. CHR. BOYADJIEV, L. PANTOVCHIEVA and J.
CHRISTOV: Theor. Found. Chern. Eng., 2000, 34, 2,
141-144
3. J. CHRISTOV, CHR. BOYADJIEV and L.
PANTOFCHIEVA; Theor. Found. Chern. Eng., 2000,
34, 5, 439-443
4. V. M. RAMM: Gas Absorption, Chemistry, Moscow,
1976
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