Wet process for removing specific component from gas

Disclosed is a wet process for efficiently and economically removing a specific gas component and/or solid component from a gas containing the same wherein the gas is countercurrently contacted with a scrubbing liquid in a perforated or grid plate column without weir and downcomer and having a free-space ratio of 0.30 to 0.60 under the conditions of a superficial gas velocity of from 1.5 to 8 m/sec and a liquid flow rate of from more than 110,000 to 250,000 kg/m.sup.2 .multidot. hr.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a wet process for removing a specific 
component from a gas containing the same. More specifically, it relates to 
a wet process for removing a specific component such as a specific gas 
component or a specific solid particle from a gas containing the same by 
using a scrubbing column provided with at least one Moredana plate. 
The term "Moredana plate" as used in this specification means a perforated 
plate or grid plate without weir and downcomer. 
2. Description of the Prior Art 
Typical wet processes for the removal of a specific component from a gas 
containing the same include, for example, those of the type wherein the 
gas to be treated is brought into countercurrent contact with a scrubbing 
liquid by means of a conventional perforated plate column provided with 
perforated plates having a free-space ratio of less than 0.30, a packed 
column, a spray scrubber, a bubble-cap tray column or the like. 
However, the process employing a packed column has the following 
disadvantages: occurrences of channeling of liquid and gas streams in the 
packed column and occurrences of plugging or blocking in the packed column 
during operation when the gas or liquid contains solid materials, dust 
particles or the like. The process employing a spray scrubber has the 
following disadvantages: requirement of a large amount of power to spray 
the liquid, likely occurrence of liquid entrainment and an unsatisfactory 
absorption capacity. 
The processes employing a plate column such as, for example, a bubble-cap 
tray column, a conventional perforated plate column and the like also have 
some disadvantages in that the pressure drop of the column is relatively 
high and the plate efficiency of the plate column is usually low. In 
addition, the superficial gas velocity in such plate column is generally 
limited to the range of from approximately 0.3 m/sec to approximately 2 
m/sec in conventional scrubbing columns. Accordingly, in order to treat a 
large flow rate of gases, a large column is required. Therefore, the 
development of gas scrubbing processes having a high gas capacity has been 
eagerly desired in the industry. 
In order to obviate the above-mentioned problems in the conventional gas 
scrubbing processes, two of the three inventors of the present invention 
have developed and proposed a process for removing a specific gas 
component and/or fine dust from gas comprising passing the gas containing 
the specific gas component and/or fine dust upwardly through a plate 
column comprising at least one perforated or grid plate without weir and 
downcomer and having a free-space ratio (Fc) of 0.25 to 0.60 at a 
superficial gas velocity falling within an undulation region, while 
passing a liquid absorbent downwardly through the plate column in a 
countercurrent flow relationship to the upflowing gas under a liquid-gas 
ratio (L/G) of 0.5 or more. This process is disclosed in Japanese Patent 
Publication No. 51-31036(1976) (published on Sept. 4, 1976) and U.S. Pat. 
No. 3,941,572 (issued on Mar. 2, 1976). The term undulation region 
mentioned above is also defined in the above publications. Stated in these 
publications are the following six equations for calculating Ugm (i.e., 
the minimum superficial gas velocity of the undulation region) and Ugc 
(i.e., the maximum superficial gas velocity of the undulation region) 
under a liquid flow rate of from 9,000 to 110,000 kg/m.sup.2 .multidot.hr. 
Four of the six equations are as follows: 
##EQU1## 
wherein g=gravitational acceleration (m/sec.sup.2) 
Fc=free-space ratio of perforated plate and grid plate (-) 
L=liquid flow rate (kg/m.sup.2 sec) 
G=gas flow rate (kg/m.sup.2 sec) 
.rho.l=liquid density (kg/m.sup.3) 
.rho.g=gas density (kg/m.sup.3) 
l=.sqroot.2.sigma./g.rho.l=capillary constant (m) 
.sigma.=surface tension (kg/sec.sup.2) 
The above equation (1) is applicable to the perforated plate in the case of 
EQU Fc.gtoreq.0.16 and .rho.g/.rho.l .times.10.sup.3 .gtoreq.0.838, 
and the equation (2) is applicable to the perforated plate in the case of 
EQU Fc.gtoreq.0.16 and .rho.g/.rho.l.times.10.sup.3 .ltoreq.0.838, 
the equations (3) and (4) are applicable to perforated plate 
(Fc.ltoreq.0.16) and grid plate, when 
EQU .rho.g/.rho.l.times.10.sup.3 .gtoreq.1.20 and .rho.g/.rho.l.times.10.sup.3 
.ltoreq.1.20, 
respectively. 
The remaining two equations are as follows: 
EQU Ugc/Ugm=7.509.times.10.sup.2 .times.L.sup.-0.5704 ( 5) 
EQU Ugc/Ugm=3.434.times.L.sup.-0.0807 ( 6) 
wherein L is the same in equations (1) through (4). 
The above equations (5) and (6) are applicable to the perforated or grid 
plate, when L=6.times.10.sup.4 .about.11.times.10.sup.4 kg/m.sup.2 
.multidot.hr and L=10.sup.4 .about.6.times.10.sup.4 kg/m.sup.2 
.multidot.hr, respectively. 
The above-mentioned problems of the conventional gas scrubbing processes 
can be obviated to some extent by contacting a gas with a scrubbing liquid 
under the conditions of a superficial gas velocity being within the range 
of from Ugm to Ugc and a liquid flow rate being within the range of from 
9,000 to 110,000 kg/m.sup.2 .multidot.hr according to the process proposed 
above. However, this process is still insufficient in terms of being used 
as practical industrial processes, especially in the case where a large 
amount of a scrubbing liquid, for example, 110,000 kg/m.sup.2 .multidot.hr 
or more is used. For instance, 110,000 kg/m.sup.2 .multidot.hr or more of 
a scrubbing liquid are required for industrial processes in the case where 
a high content (e.g. 1000 ppm or more) of sulfur oxides present in a waste 
gas is treated with a scrubbing liquid containing calcium carbonate. 
SUMMARY OF THE INVENTION 
Accordingly, an object of the present invention is to obviate the 
above-mentioned problems of the conventional processes and to provide a 
process which is capable of removing a specific gas component and/or solid 
component from a gas with a high degree of efficiency and which is capable 
of treating such gas with a scrubbing liquid at very increased gas flow 
rate and at a liquid flow rate of more than 110,000 kg/m.sup.2 
.multidot.hr to 250,000 kg/m.sup.2 .multidot.hr. 
Other objects and advantages of the present invention will be apparent from 
the following description. 
In accordance with the present invention, there is provided a process for 
removing a specific component from a gas containing the same which 
comprises passing the gas upwardly through a scrubbing column provided 
with at least one perforated or grid plate without weir and downcomer, 
such plate having a free-space ratio (Fc) in the range of from 0.30 to 
0.60, and passing the gas through the column at a superficial gas velocity 
within a range of from 1.5 to 8.0 m/sec, and simultaneously passing a 
scrubbing liquid downwardly through the column at a liquid flow rate of 
more than 110,000 kg/m.sup.2 .multidot.hr to 250,000 kg/m.sup.2 
.multidot.hr. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The term "free-space ratio" as used herein is defined as the ratio of the 
total hole or slit area (m.sup.2) of a plate to the cross-sectional area 
(m.sup.2) of the column. The term "superficial gas velocity" as used 
herein is defined as the ratio of the actual gas flow rate (m.sup.3 /sec) 
to the column cross-sectional area (m.sup.2). The unit of the superficial 
gas velocity is "m/sec".

As shown in FIGS. 1 and 2, zone A, i.e., the operating zone of the present 
invention, is clearly distinguishablefrom zone B, i.e., the operating zone 
of the former process. That is to say, the operating zone of the former 
process is limited to a liquid flow rate range of from 9000 to 110,000 
kg/m.sup.2 .multidot.hr, whereas the liquid flow rate of the process of 
the present invention is limited to the range of from more than 110,000 to 
250,000 kg/m.sup.2 .multidot.hr, and more preferably, within the range of 
from more than 110,000 to 230,000 kg/m.sup.2 .multidot.hr, which range is 
larger than that of the former process. The operating zone of the present 
invention (i.e., zone A) is heretofore not known. It has now been found 
that, when a gas to be treated is countercurrently contacted with a 
scrubbing liquid, under the conditions falling within the zone A, in a 
Moredana plate column having a free-space ratio of from 0.30 to 0.60, a 
specific gas component and/or solid component present in the gas can be 
effectively and efficiently removed from the gas without causing any rapid 
increase in the pressure drop of the plate. 
The liquid-gas contact apparatus to be employed in the present invention 
includes a scrubbing column comprising at least one perforated plate or 
grid plate having no weir and no downcomer and having a free-space ratio 
of from 0.30 to 0.60, and more preferably, of from 0.32 to 0.52. The plate 
number of the scrubbing column is generally from 1 to 7, and more 
preferably, from 3 to 5, and the space of the plates in the column is 
generally 0.3 to 1.5 m, and more preferably, from 0.5 to 1.2 m. Although 
no particular dimension of the hole or slit in the plate is required, the 
hole diameter or slit width is generally selected from the range of from 4 
to 30 mm. The diameter of the scrubbing column to be employed in the 
present invention is preferably 300 mm or more, and more preferable 500 mm 
or more. Furthermore, there is no critical upper limit to the diameter of 
the scrubbing column. In fact, a scrubbing column having a diameter of 
approximately 10.3 m has been satisfactorily utilized in a Practical 
situation. 
When the free-space ratio of the plate is less than 0.30, the superficial 
gas velocity in the column is unpreferably limited to less than 3 m/sec 
due to the fact that the pressure drop of gas through the plate becomes 
high. This limitation of the superficial gas velocity causes the scrubbing 
column to be undesirably enlarged for any practical use. Contrary to this, 
when the free-space ratio of the plate is more than 0.60, the plate 
efficiency unpreferably decreases due to the reduction of the amount of 
liquid hold up on the plate. It is also difficult from an industrial point 
of view to manufacture a plate having a free-space ratio of more than 0.6. 
According to the present invention, the liquid flow rate of a scrubbing 
liquid is generally within the range of from more than 110,000 kg/m.sup.2 
.multidot.hr to 250,000 kg/m.sup.2 .multidot.hr, and more preferably, 
within the range of from more than 110,000 to 230,000 kg/m.sup.2 
.multidot.hr. When the liquid flow rate of the scrubbing liquid is more 
than 250,000 kg/m.sup.2 .multidot.hr, the gas pressure drop in a column 
becomes relatively high and a substantial amount of the scrubbing liquid 
is also unpreferably entrained in the treated gas. 
According to the present invention, the superficial gas velocity Ug is 
generally selected from within the range of from 1.5 to 8.0 m/sec, as 
shown in FIGS. 1 and 2, and more preferably, within the range of from 2.4 
to 6.0 m/sec. When the superficial gas velocity is less than 1.5 m/sec, 
the plate efficiency decreases due to the reduction of the amount of 
liquid hold up caused by the low gas velocity. Contrary to this, when the 
superficial gas velocity is greater than 8.0 m/sec, not only is the 
pressure drop of the plate unpreferably increased, but also the scrubling 
liquid tends to be entrained in the treated gas. Under these conditions, a 
stable continuous operation cannot be effected. 
Since the gas flow rate and the liquid flow rate are clearly defined in the 
present invention, it will be appreciated by those skilled in the art that 
a liquid to gas flow ration (L/G) can be determined from the given 
superficial gas velocity and the given liquid flow rate, depending on the 
density of the gas to be treated. The liquid to gas flow ratio (L/G) of 
the present invention is usually 3.0 or more, and more preferably, in the 
range of from 4 to 17. 
According to the present invention, waste gases containing at least one 
harmful gas and/or solid component selected from the group consisting of 
sulfur oxides, nitrogen oxides and/or dust particles can be treated. Waste 
gases containing various odors or smelly components as well as coke oven 
gases containing acidic components and/or ammonia gas can also be treated 
according to the present invention. The present invention can be further 
applied to the case where other types of gas components and/or solid 
components contained in a gas must be removed. When gases are heated or 
cooled, the present invention can be utilized due to the fact that the 
efficiency of the gas-liquid contact of the present invention is very 
high. 
The scrubbing or treating liquid to be used in the present invention can 
include any conventional scrubbing solutions or suspensions, any 
conventional absorbing solutions or suspensions and any aqueous solutions 
or emulsions. For instance, when a gas containing sulfur oxides and/or 
nitrogen oxides is treated, an aqueous solution or suspension containing, 
as an absorbing agent, the hyroxide of alkali metals, alkaline earth 
metals or ammonia such as sodium hydroxide, potassium hydroxide, calcium 
hydroxide, magnesium hydroxide or ammonium hydroxide; the carbonate of 
alkali metals, alkaline earth metals or ammonia such as sodium carbonate, 
potassium carbonate, calcium carbonate, magnesium carbonate or ammonium 
carbonate; the sulfite of alkali metals, alkaline earth metals or ammonia 
such as sodium sulfite, potassium sulfite, calcium sulfite, magnesium 
sulfite or ammonium sulfite; or the like can be used as the scrubbing 
liquid. An aqueous ammonia solution can be used for removing acidic gas 
components such as hydrogen sulfide from, for example, a coke oven gas. In 
addition, in the case where ammonia contained in a gas is removed, an 
aqueous solution containing sulfuric acid, phosphoric acid, carbolic acid, 
acetic acid, oxalic acid, ammonium hydrogenphosphate or the like can be 
used. When a gas containing solid particles such as fine dust or soot is 
treated, water or water containing any conventional surface active agent 
can be used for physically removing the solid particles. When the solid 
particles are removed from a gas simultaneously with, for example, sulfur 
oxides and/or nitrogen oxides, the above-mentioned scrubbing or absorbing 
liquid for removing sulfur oxides and/or nitrogen oxides can also act as a 
scrubbing liquid for the solid particles. 
The present invention will be further illustrated by the following 
Examples. However, it is noted that the present invention is by no means 
limited to such Examples. 
EXAMPLE 1 
Air was brought into a countercurrent contact with water under the various 
conditions listed in Table 1 below by using a "Moredana" scrubbing column 
having a diameter of 5600 mm and provided with four perforated plates 
without weir and downcomer. The free-space ratio (Fc) of the plates used 
was 0.32 or 0.52. The water content of the exhaust air from the column, 
the total pressure drop of the four plates and the superficial gas 
velocity in the column were measured under stable operating conditions. 
From the measurement of the water content of the exhaust air, a ratio of 
the amount of the water contained in the exhaust air to the amount of the 
water charged into the column was calculated. The results are shown in 
Table 1. 
Table 1 
______________________________________ 
Run No. 1 2 3 4 5 6 
______________________________________ 
Flow rate of water 
charged into column 
(kg/cm.sup.2 .multidot. hr) 
12.2 .times. 10.sup.4 
1.6 .times. 10.sup.4 
18.0 .times. 10.sup.4 
Free-space ratio (Fc) of 
plates 0.32 0.52 0.32 0.52 0.32 0.52 
Ratio of amount of water 
contained in exhaust air to 
amount of water charged 
into column (kg/kg) 
0.07 0.08 0.10 0.11 0.24 0.25 
Total pressure drop of 
four plates (mmH.sub.2 O) 
190 203 230 234 290 300 
Superficial gas velocity 
(m/sec) 4.5 8.0 4.5 8.0 4.5 8.0 
______________________________________ 
In addition, it was observed from further operations that gas is preferably 
treated with liquid under the conditions wherein the superficial gas 
velocity is from 2 to 8 m/sec and the liquid flow rate is from 11,000 to 
170,000 kg/m.sup.2 .multidot.hr for effecting more stable operations. 
EXAMPLE 2 
95000 m.sup.3 /hr of exhaust gas from a boiler containing 1700 ppm of 
SO.sub.2 were continuously introduced into the bottom portion of a 
Moredana scrubbing column having a diameter of 2900 mm and provided with 
four perforated plates without weir and downcomer. The free-space ratio 
(Fc) of the perforated plates was 0.52 and the diameter of holes of each 
of the plates was about 10 mm. 230,000 kg/m.sup.2 .multidot.hr of an 
aqueous absorbing liquid containing 0.15 g/liter of calcium carbonate 
(CaCO.sub.3) were simultaneously fed into the top portion of the scrubbing 
column, whereby the gas containing SO.sub.2 was brought into 
countercurrent contact with the aqueous absorbing liquid containing 
CaCO.sub.3. The superficial gas velocity in the column was 4.0 m/sec and 
the ratio of the liquid flow rate L to the gas flow rate G (L/G) was 16 
kg/kg. 
From the measurement of the SO.sub.2 content of the treated off-gas, the 
desulfurization efficiency was determined to be 98%. The total pressure 
drop of the four plates was 180 mm H.sub.2 O. 
EXAMPLE 3 
600,000 m.sup.3 /hr of exhaust gas from a furnace for sintering iron ore 
were continuously fed into the bottom portion of a Moredana scrubbing 
column having a diameter of 10.3 m and provided with two perforated plates 
without weir and downcomer, while 115,000 kg/m.sup.2 .multidot.hr of an 
aqueous absorbing liquid having a pH of 6.0 and containing calcium 
carbonate (CaCO.sub.3) were also continuously fed into the top portion of 
the scrubbing column. The free-space ratio (Fc) of the perforated plates 
was 0.31 and the hole diameter of each of the perforated plates was about 
8.5 mm. The gas containing SO.sub.2 was brought into countercurrent 
contact with the downflowing aqueous absorbing liquid containing 
CaCO.sub.3, whereby SO.sub.2 containe in the gas was removed from the gas. 
The superficial gas velocity in the column was 2.0 m/sec and the ratio of 
the liquid flow rate L to the gas flow rate G (L/G) was 16 (kg/kg). 
From the measurement of the SO.sub.2 content of the treated off-gas, the 
desulfurization efficiency was determined to be 93%. The total pressure 
drop of two plates was 65 mm H.sub.2 O. 
EXAMPLE 4 
Simulated gas containing approximately 2.4 g/Nm.sup.3 of dust having an 
average diameter of 5.6 microns and a true specific gravity of 3.3 
g/cm.sup.3 was continuously introduced into bottom portion of a Moredana 
scrubbing column having a diameter of 500 mm and provided with three 
perforated plates without weir and downcomer. The free-space ratio (Fc) of 
each plate was 0.34, and the hole diameter of the plate was about 8 mm. 
Into the top portion of the Moredana column, 123,000 kg/m.sup.2 hr of 
water were simultaneously introduced, whereby the gas containing dust was 
countercurrently contacted with water. The superficial gas velocity in the 
column was 3.73 m/sec, and the liquid-gas ratio (L/G) was 7.7 (kg/kg). 
From the measurement of the dust content of the treated off-gas, the rate 
of the dust removal was determined to be 92%. The pressure drop of the 
plate was 125 mm H.sub.2 O. In addition, it was observed that only a 
little entrainment of water in the off-gas occurred. 
EXAMPLE 5 
408,000 m.sup.3 /hr of exhaust gas from a boiler containing 1275 ppm of 
SO.sub.2 were continuously fed into the bottom portion of a Moredana 
scrubbing column having a diameter of 5600 mm and provided with four 
perforated plates. The free-space ratios of the plates were 0.40, 0.32, 
0.32 and 0.38 from the bottom. On the other hand 130,000 kg/m.sup.2 
.multidot.hr of an aqueous absorbing liquid containing 0.08 mol/liter of 
calcium carbonate (CaCO.sub.3) were simultaneously fed into the top 
portion of the column, whereby the gas containing SO.sub.2 was brought 
into countercurrent contact with the aqueous absorbing liquid containing 
CaCO.sub.3. The superficial gas velocity in the column was 4.9 m/sec, and 
the liquid-gas flow ratio (L/G) was 7.0 (kg/kg). From the measurement of 
the SO.sub.2 content of the treated exhaust gas, i.e., off-gas, the 
desulfurization efficiency (i.e., SO.sub.2 removal percent) was determined 
to be 93.3% on the average. The total pressure drop of the four plates was 
195 mm H.sub.2 O. It was further observed that only a little entrainment 
of the absorbing liquid occurred and a stable operation could be continued 
for a long period of time. 
EXAMPLE 6 
350,000 m.sup.3 /hr of exhaust gas from a boiler containing 1510 ppm of 
SO.sub.2 were continuously fed into the bottom portion of a Moredana 
scrubbing column having a diameter of 6600 mm and provided with four 
perforated plate. The free-space ratios of the plates were 0.32, 0.32, 
0.32, 0.32 and 0.35 from the bottom. On the other hand 126,000 kg/m.sup.2 
.multidot.hr of an aqueous absorbing liquid containing 0.08 mol/liter of 
calcium carbonate (CaCO.sub.3) were simultaneously fed into the top 
portion of the column, whereby the gas containing SO.sub.2 was brought 
into countercurrent contact with the aqueous absorbing liquid containing 
CaCO.sub.3. The superficial gas velocity in the column was 2.84 m/sec, and 
the liquid-gas flow ratio (L/G) was 11.7 (kg/kg). From the measurement of 
the SO.sub.2 content of the treated exhaust gas, i.e., off-gas, the 
desulfurization efficiency (i.e., SO.sub.2 removal percent) was determined 
to be 98.0% on the average. The total pressure drop of the four plates was 
199 mm H.sub.2 O. It was further observed that only a little entrainment 
of the absorbing liquid occurred and a stable operation could be continued 
for a long period of time.