CA1186492A - Carbon dioxide removal method employing packed solid calcium hydroxide - Google Patents

Abstract

A B S T R A C T

In order to remove carbon dioxide from an off-gas stream and immobilize it in solid stable form, the gas is passed through a packed bed of calcium hydroxide maintained at a temperature in the range 10°C - 50°C, the moisture con-tent of the gas being controlled so as to correspond to a relative humidity from 40% to 100% at the bed temperature.

Description

~8~92 This invention relates to a me-thod of removing car-bon dioxide from a gas stream. The invention is especially applicable to the immobilization of radioacti.ve carbon dioxide present i.n industrial off-gas streams, for example waste gas generated in nuclear power plants and the like. ~owever the invention is not limited to such applicakion but is suited to the removal of carbon dioxide from gas streams generally.

The ]cnown methods of removing carbon dioxide from gases include (a) contacting the gas with a lime slurry, (b) contacting the gas with solid calcium hydroxiae at elevated temperatures, typically 350C - 450C, and ~ c) contacting the gas with solid barium hydroxide hydrate at ambient temperatures.

The reaction product calcium carbonate, or barium carbonate, r.-sulting from these methods is highly stable and ~, 4~3i2 ~ -2-well suited for long term storage. This offers a very at-tractive chemical form for the fixation and disposal of ra-dioactive carbon isotopes. However, each of the three methods has disadvantages. The lime slurry method has a contaminated liquid efEluent; the solid calcium hydroxide system requires operation at elevated temperatures; while the barium hydroxide hydrate system has the serious disadvantage that barium hy-droxide is both toxic and expensive. Applicants have tested a solid calcium hydroxide system at ambient temperatures (20C - 250C), but this was found to be quite unsatisfactory as the conversion of Ca(OH)2 to CaCO3 was found to be only about 3%.

However, the applicants have found, ~uite unexpec-tedly, that the solid calclum hydroxide system is most eEfec-tive even at ambient temperatures if the humidity of the gasis raised to a value corresponding to a relative humidi-ty of about ~0% or higher measured at the bed temperature. Below 80%
relative humidity there is absorption of the carbon dioxide, with consequent conversion of Ca(OH)2 to CaCO3 as one would expect, and the conversion increases with relative humidity, but it is only when the relative humidity reaches about 80%
that the conversion becomes hiyh enough to be commercially useful. In fact, the conversion continues to increase rapidly throughout -the 80% - 100% relative humidity range. If the

2~ moisture content exceeds the upper limit of this range, how-ever, the utilization of the Ca(OH)2 is diminished.

Further investigation showed that the bed tempera-ture may be as low as 10C provided that the moisture content of the gas is suitably increased. In this case the rela-tive humidity at the bed temperature should be between about ~0%
100% for there to be useful conversion, i.e. utilization, of the calcium hydroxide. On the other hand, at higher bed tem~
peratures the ~elative humidity of the gas may be considerably lower, and may be as low as about 40% if the temperature of the bed is 50C. The moisture content of the gas may be fur-ther reduced for higher bed temperatures, a useful conversion of the calcium hydroxide being obtained, buc at bed tempera-tures above about 50C one sacrifices the main advantage of the invention, namely the effective utilization of a calcium hydroxide bed operated at a conveniently low temperature. For the benefit of the invention to be fully realized the bed should be operated in the -temperature range 10C -- 50C, and preferably in the temperature range 20C -- 30C, the moisture 10 content of the gas stream being controlled accordingly.

U.S. Patent No. 4162298 issued to David W. Holladay and Gary L. Haag, dated July 24, 1979, discloses a method for removing carbon dio~ide from industrial off-gas using a par-ticulate bed of barium hydroxide monohydrate wherein the gas 15 is treated so that its relative humidity is in the range 106 --100%. However/ no attempt was made to apply this method to a solid calcium hydroxide system, which was believed to be un-suitable. Moreover, subsequent work by Holladay and Haag, as reported in a presentation at the 16th DOE Nuclear Air Clean-20 ing Conference, San Diego, Callfornia, October 19-24, 1980, inaicated that even the barium hydroxide system would be of little practical use if the relative humidi.t~ of the yas were too hiyh, owing to degradation of the Ba(O~I)2 particles with resultant capillary condcnsation of water vapour and a conse~
25 quent high pressure drop across the packed bed. The present appl.icants have discovered that the calcium hydroxide system does not have this drawback and will be most effective at very high relative humidit:Les at which the barium hydroxide would be of little use.

Accordin~J to the present invention, therefore, there is provided a method of removing carbon dioxide from a gas stream by passing the gas stream through a packed bed of cal-cium hydro~ide, wherein the bed is maintained at a tempera-ture in the range 10C -- 50C, and wherein the moisture content of 6~9~

the gas is controlled tG a value corresponding to a relative humidity in the range 40% - 100% at the bed temperature. To be con~ercially useful the conversion of the calcium hydroxide should be a-t leas-t 0.15 (i.e. 15~) calculated according to the formula . RCT / W
Conversln = 22.4/ 74 where R is the gas flow rate, C is the carbon dioxide concentration upstream ].0 of the bed, T is the time required for the downstream con-centration of carbon dioxide to reach 5~ of the upstream concentration, and W is the weight of calcium hydroxide in the bed.
1.5 The numbers 22.4 and 74 are respectively the molar volume of the carbon di.oxide and the molecular weight of -the calcium hydroxide.

The calcium hydroxide should be in a form which per-mits a substantial flow of the gas through it without exces-:20 s.ive pressure drop, and for this it is preferred that -the cal-cium hydroxide be prepared by hydra-ting CaO, followed by dry-ing and crushing to the required particle size, 0.25mrn - 3~n.

~ n order to improve the gas flow it has been found advantageous to provide a plurality of packed beds in the form o~ relatively ~lat, spaced apart layers of the crushed calcium hydroxide arranged in series with respect to the gas flow.

One exemplary embodiment of the invention is illus-trated schematically in the accompanying drawing, which repre-sents a system for removing radioactive carbon dioxide from off--gas generated by a nuclear pol~er plant.

~ eferring to the drawing, the nuclear power plant is denoted by the reference numeral 10, and off-gas is led away by pipes 11 to a humidifying unit 12. The humidifying unit may be a gas-water contactor, a sprayer or an evaporator, by which the moisture content of the qas is raised. The humi-dified gas is led from the humidifying unit 12 via piping 13 -to an absorption bed system 14, where it is passed through successive beds of calcium hydroxide, and exhausted via piping 15, after removal of the carbon dioxide. The bed temperature ]o is preferably maintained at about 25C, i.e. between 20C and 30C, in which case the moisture content of the humidified gas must correspond to a relative humidity of between 80'~ - 100%
at that temperature~
The unit 14 in the present example comprises a ver-tical column 16 in which a number of packed beds 17 of thecalcium hydroxide are arranged so that the gas will flow through each. ~ach bed 17 is in the form oE a relatively flat layer and is spaced from the adjacent bed so as to permit mix-ing and redistribution of the gas.
In an experimenta] pilot plant gas streams containing from 20 -to 50000 ~1 of carbon dioxide per litre of gas were treated in the carbon dioxide removal s~stem. The gas streams were humidified to greater than 80% relative humidity at ambi-ent temperature and then passed through a stationary bed of the packed solid calcium hydroxide. The chemical analysis of -the absorbent bed is given in Table l; ho~ever it was found that the composition is not critical.
Table 1 _ Chemical Analysis of the Calcium Hydroxide Used 30CompoundsWt Fraction (g/kg) Ca~OH)2 870 900 CaO 24-33 CaCO3 57~86 The condltions and results of the tests are set out in Table 2. $~

.~ Y

c~ ~ V V V~ V .c $
8~ ~o ~
o _ ~ ' ~ ~P O o o O I I ~ ~

_~ _ ____ ___ ~ ~
-~rO ~ O
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4~ ~:) O ~ ~r h ~ h r`h r` u ) u~ O
O 0 ~) ~1 ~1 ID a~ h h ~j C ) O ^ t ~a~ r~ ~ ~

~-1!~;U O (d O U Ei E~ ~ U ~ U 13 U ~ U lt h 5~ ~ n In u n U n u E~ C n ~n ~n n n r~~n~ O
0 N r-- N r~ N ~IN r~l N ~r) N ~) C rt O _ N ~ t LO~
r~ ~ a)~ ~Ln n ~ .C
.) O rt ~ u~ o In o ~n ~n O
U h Lnn m N n ~n O
.~ _ o o o o o o _ o CJU oU oU U O rtCD

~t rl N ~ N N r r ~ ~O

r~, ~ _ rtrt ~t rt . O a) O r~
o ~ ~ a~a O ~ 1 O O O O ! rt U U~ n ~n O ~ N (~ ~
U ~ 0 0

3 ~

~_ N O O O O

~6~

The tests were also carried out at other bed temper-atures and different values of moisture content of the humidi-fied gas. The conditions and results of these further tests are set ou-t in Table 3.

,C ~ ~ _ ~ o *~ x ~1 0 ~ ~C`.
P~ -r P; O ~ t~l ~1 a) ~ rc~ ~ ~) (~I ~D r-l d~ (~ U) ~ ~1 ~ ~) 1`` (~ ~
~ )O ......... ... .. ~ ~
E~ ~ ~ ~) a l ID r Ir-l ~l r-l t~) ~ r~ r~l (~ ~ ~ CO Ll') ~ r I IJ') tJl 2i ~ r-~ ~1 r-l ~J ~ ~ ~ r-i ~ 15~ <~1 ~ ) C) ~1 .C (~ O
H ~) t~ O H ~1 ~ _~ o )~ a) ~0 oO ~ ..0~ ~ ~ ~ ~ .
~ U~ C,) C ) Il~ O -r~ m li ~ __ __ ~ dP
m (~10 _~ ~ ~ 11 ) 11~ O O a) r~
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~- H td ~. r~ r~ ~ r~ r~ r~l r~l r~l r~ Ln u~ ~ . . . ~ E~ ~::
~ ~ o ~ . tS~ ~ X ~

tr) H ~3 _ C~ ~_) fd ~1 O
r H O O o o ~ td r) rd ~q~ c~ ~ r~l r~ l l rd ~ 4~
E~ ~ O ~ >1 O O ~ I Orl~D ~ tl) O
-r~ -r~ ~) ~ ) r~ ~,) r~ ) r~ r~l ~ _) r~o t~ a~ 11~ ~ .1_) ~ rd rd r~ 0~ O O O O O O O O C~ ~ ~ .~ O 3 E I ~1 ~ ~_ ~1 r~ rd r~l rd r~ r~l rd rd u) n ~r td td O r h O ~ l ~ ~1 O~o ~ o\o ~ ~ Op O~O ~ ~ ~ fd o\ o~P ,1 O
O rd td ,~ rd ~r rd rd r~J ~o rd r~l rd r.~ tn 0 C) n O~o 0~O 1 0~,O 1 O`p 0~ 1 1 O~p 0~O 0~ o I I v X
E~ `1 ~ ~ o ~ ~ In rn ~D ~D ~D rn o r~ r~
~ ) t~ r~l r~l ~ u') ~D tXI r tX~ r~o r~ D r-t r i~ O
~ ___._ __ ___ _ ~ ~
a:~ ~o x ~
rd :1 o o o n ,Jl ~~ h ~:1 r~l~ ~r~ r~l r~ rr) r~7 r~
n ~ O ~ ~l l l l l c~ ul Z ~ C,~ 1 o ~ ul o o o o o o o o o o o o ~c rd (D ~-- r ~D O r~ rJ~ O --~1 tY7 0 O O O r-l r-l r-l H ~ rY') r.~ r~l r~') r~l r~ r ~) t~l r~) r~) r~ rr) ~) r ~) H
a c~ ~
o r) _ _ __ . _ O~P

3 _~ O
,0~ ~ ~ ., rd ~ o o o o o o o o o o o o o o o ~V
u~ ~ ~ r~l rJ r.~l r~l r~ r~l r~J r.~l r~J r~l r~l r~l r~l r; r~
r~ C) The results of these -tests demons-trate that carbon dioxide can be removed from gas streams at ambient tempera-ture, and indeed at temperatures in the range 10~C - 50C by calcium hydroxide and a high conversion of the Ca(OH)2 to CaCO3 can be achieved if the gas is first conditioned to high humidity.

It is to be understood that the gas stream may be at a temperature substantially hi.gher than ambient, in which case its relative humidity at that higher temperature may be substantially less than 80~. The essential thing is that the moisture conten-t of the gas, prior to passage through the bed, should correspond to a relatlve humidity of between 40%
and 100~ at the bed temperature and be controlled according to the bed temperature so as to achieve a conversion of the calcium hydroxide of at least 15~.

For the treatment of industrial off-gas it is gen-erally necessary to humidify the gas stream to bring its moisture content up to the required value prior to passing the gas stream through the bed, as described above. However, in cases where the initial humidity of the gas is higher than the required value, as for e~ample in rebreathing apparatus, it is generally necessary to control the humidity by extrac-ting moisture.

Claims (10)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of removing carbon dioxide from a gas stream by passing the gas stream through a packed bed of calcium hydroxide, wherein the bed is maintained at a temper-ature in the range 10°C - 50°C, and wherein the moisture con-tent of the gas is controlled to a value corresponding to a relative humidity in the range 40% - 100% at the bed temper-ature and such that the resultant conversion of calcium hy-droxide is not less than 0.15, the conversion being defined by where R is the gas flow rate, C is the carbon dioxide concentration upstream of the bed, T is the time required for the downstream con-centration of carbon dioxide to reach 5% of the upstream concentration, and W is the weight of calcium hydroxide in the bed.

2. A method according to claim 1, wherein the bed is maintained at a temperature in the range 20°C - 30°C, the moisture content of the gas stream corresponding to a rela-tive humidity in the range 80% - 100% at the bed tempera-ture.

3. A method according to claim 1 or claim 2, wherein the initial moisture content of the gas stream corresponds to a relative humidity greater than 100% at the bed temper-ature, said control comprising dehumidifying the gas prior to passage through the bed.

4. A method according to claim 1 or claim 2, wherein the initial moisture content of the gas stream corresponds to a relative humidity less than the minimum of said relative humidity range at the bed temperature, said control comprising humidifying the gas stream prior to passage through the bed.

5. A method of removing carbon dioxide from a stream of industrial off-gas by passing the gas stream through a packed bed of calcium hydroxide maintained at a temperature in the range 10°C - 50°C, including the step of humidifying the gas stream to raise its moisture content to a value corresponding to a relative humidity in the range 40% - 100% at the bed temperature and such that the resultant conversion of calcium hydroxide is not less than 0.15, the conversion being defined by where R is the gas flow rate, C is the carbon dioxide concentration upstream of the bed, T is the time required for the downstream concentration of carbon dioxide to reach 5% of the upstream concentration, and W is the weight of calcium hydroxide in the bed.

6. A method according to claim 5, wherein the bed is maintained at a temperature in the range 20°C - 30°C, the moisture content of the gas stream corresponding to a relative humidity in the range 80% - 100% at the bed temperature.

7. A method of removing and immobilizing carbon dioxide from a gas stream including carbon dioxide containing radioactive carbon as a constituent, which comprises providing a gas filter comprising a packed bed of solid Ca(OH)2 prepared by hydrating CaO
and drying, crushing and packing the resultant hydroxide, maintaining the temperature of the bed in the range 10°C - 50°C, contacting the gas stream with water to raise its moisture content to a value corresponding to a relative humidity in the range 80% - 100% at the temperature of the bed, and flowing the humidified gas stream through the gas filter to effect conversion of the calcium hydroxide to calcium carbonate the conversion of the calcium hydroxide being not less than 0.15, the conversion being defined by where R is the gas flow rate, C is the carbon dioxide concentration upstream of the bed, T is the time required for the downstream concentration of carbon dioxide to reach 5% of the upstream concentration, and W is the weight of calcium hydroxide in the bed.

8. A method according to claim 7, wherein the temperature of the bed is in the range 20°C - 30°C.

9. A method according to claim 7, wherein the gas filter is formed by a column comprising a plurality of relatively flat, spaced apart beds of crushed solid calcium hydroxide, the column defining a gas flow path and the beds being arranged in series with respect to said flow path, the spacings between the beds modifying the gas flow to effect redistribution thereof between the filter stages.

10. A method according to claim 9, wherein the temperature of the bed is 20°C - 30°C.