Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Organic component
Reexamination Certificate
2001-02-12
2003-12-30
Bos, Steven (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Organic component
C423S437100, C095S143000, C095S146000, C095S147000
Reexamination Certificate
active
06669916
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention generally relates to a method and apparatus for recovering carbon dioxide from a feed stream and reducing the costs of carbon dioxide purification systems. More particularly, the invention relates to a carbon dioxide purification method and apparatus utilizing an adsorbent bed, such as activated carbon, in combination with a catalytic oxidation system.
2. Description of the Related Art
Table 1 lists the concentrations of various components of a carbon dioxide feed stream from, for example, a well or an exhaust stream from a chemical process.
TABLE 1
FEED GAS COMPOSITION
O
2
Required
Btu's/hr.
For
Conc.
(from
Oxidation
Constituent
Ppm (v)
Lbs./hr.
1
combustion)
Lbs./hr.
Nitrogen
130
NA
NA
NA
Methane
2
7,730
27.13
569,730
108.5
Ethane
1,230
8.10
165,261
17.2
Propane
940
9.07
178,361
23.1
Iso-butane
260
3.31
64,922
11.9
N-butane
360
9.57
89,636
16.4
Iso-pentane
150
2.37
46,213
8.4
N-pentane
180
2.84
44,378
10.1
Hexane
380
7.17
139,034
25.4
Cyclohexane
290
5.34
100,638
18.3
Benzene
4,800
82.12
1,432,621
252.7
Toluene
1,000
20.18
355,188
63.2
Carbon
Remainder
NA
NA
Dioxide
TOTAL
177.2
3,185,982
555.2
Combustibles
1
Based on a 100 metric ton per day facility
2
Only partially removed by catalytic oxidation due to choice of catalyst and operating conditions.
As shown, the feed stream contains various hydrocarbons that must be removed to provide relatively pure carbon dioxide. Currently, technologies such as scrubbers, adsorption systems and catalytic oxidation systems are employed to remove the hydrocarbons from the gas stream. These technologies are briefly discussed below.
Scrubbers generally utilize a water wash and are sufficient for removing water soluble hydrocarbons such as, for example, ethanol and methanol from feed streams. However, scrubbers are not effective for removing hydrocarbons that are not water soluble. Instead, adsorption beds and catalytic oxidation systems are generally used to remove non-water soluble hydrocarbons.
A typical adsorption bed includes activated carbon as an adsorbing medium. Such an adsorption bed is generally effective for inexpensively removing large quantities of hydrocarbons having boiling points greater than 20° C. However, activated carbon beds are relatively inefficient in terms of capital and operating costs when the subject feed stream contains both weakly and strongly adsorbing species.(Basmadjian, p.75) For example, the weakly adsorbing species are typically displaced by the strongly adsorbing species and, consequently, not all hydrocarbon species are effectively adsorbed. Additionally, compounds such as ethane, propane, butane, methyl ether, etc., are not removed to the levels required (low ppm and/or ppb) for food grade carbon dioxide. For at least the foregoing reasons, an activated carbon system alone removes the contaminants only partially, for example only about 70% by weight of the hydrocarbon impurities in a feed stream described in Table 1 will be effectively removed by adsorption and therefore will not meet the specification of food grade carbon dioxide.(Table 5)
In addition to scrubbers and adsorption beds, catalytic oxidation systems may also be used to remove hydrocarbons from the feed stream. Catalytic oxidation systems are used for destroying volatile organic hydrocarbons and odorous compounds in exhaust air streams. Typically, for a feed stream containing ethane, propane and butane, catalytic oxidation over a platinum or palladium catalyst alone is effective even if water soluble and/or high boiling point hydrocarbon components are present. Since the feed gas may not contain adequate oxygen, oxygen must often be added to the feed stream to assure complete combustion of the hydrocarbons, and the amount and cost of the oxygen increases as the hydrocarbon level in the feed stream increases.
A significant amount of heat is generated when combusting high levels of hydrocarbons, and the heat must be removed to protect the catalyst and vessels. To limit heat generation, combustion is performed in multiple combustion stages. Heat generation in each stage may be controlled by limiting the amount of oxygen fed to each combustion stage, and by recycling carbon dioxide to reduce the concentration of hydrocarbons entering each combustion stage. Features such as multiple combustion stages, and oxygen limiting and heat removing systems, increase the complexity and costs associated with prior art catalytic oxidation systems.
By way of example,
FIG. 1
illustrates a block flow diagram of a conventional three stage catalytic oxidation system for purifying the previously described feed stream. Table 2 contains an example of typical characteristics as the feed stream is being processed by the catalytic oxidation system depicted in FIG.
1
.
TABLE 2
TYPICAL STREAM CHARACTERISTICS OF A FEED GAS
Caloric
Value (Btus/
Stream
Press.
Temp.
Flow
Lbs. of
Standard
No.
Psig
° F.
SCFH
Hydrocarbon
Cubic Foot
1
300
100
80,208
177.20
39.72
2
298
500
82,123
177.20
38.80
3
297
875
82,273
121.93
27.00
4
297
875
46,722
69.24
27.00
5
294
200
46,722
69.24
27.00
6
297
875
35,551
52.69
27.00
7
294
489
82,273
121.93
27.00
8
291
500
84,193
121.93
26.39
9
290
875
84,595
70.66
12.41
10
290
875
3,166
2.64
12.41
11
289
589
3,166
2.64
12.41
12
290
875
81,429
68.01
12.41
13
289
865
84,595
70.66
12.41
14
286
530
86,293
70.66
12.17
15
285
875
86,968
27.13
6.55
16
282
115
86,968
27.13
6.55
17
325
70
1,915
0
0
18
325
70
1,920
0
0
19
325
70
1,698
0
0
Referring to
FIG. 1
, oxygen from a first oxygen source
30
(stream
17
) is injected into a feed gas
10
stream
1
) entering catalytic oxidation system
5
prior to the feed gas
10
entering a first heat exchanger
20
. This oxygen provides an oxidant source for subsequent combustion of the feed gas
10
in a first reactor
40
. The feed gas
10
is warmed in heat exchanger
20
as will be discussed below, enters the first reactor
40
(stream
2
) and undergoes a catalytic oxidation process. The temperature of feed gas
10
in the first reactor
40
is measured by a first thermometer
50
and the amount of oxygen injected into the feed gas
10
by the first oxygen source
30
is controlled in accordance with the measured temperature. The temperature of the first reactor
40
is controlled to be about 875° F. to ensure favorable reaction kinetics for combusting the hydrocarbons in the feed gas
10
.
Feed gas
10
(stream
2
) entering the first reactor
40
is brought up to the necessary activation temperature, about 500° F., by passing through first heat exchanger
20
. The first heat exchanger
20
uses a portion (stream
4
) of the feed gas
10
exiting the first reactor
40
(stream
3
) as a warming medium to warm the feed gas
10
entering the first reactor
40
. The portion of the feed gas
10
used as the warming medium is then returned (stream
5
) to join the remainder of the feed gas
10
(stream
6
) exiting the first reactor
40
.
As shown in Table 2, the feed gas
10
enters the first reactor
40
(stream
2
), at 500° F., with approximately 177.2 lbs. of hydrocarbons and a caloric value of 39.72 Btus/cubic foot of feed gas, and exits the first reactor
40
(stream
3
) with approximately 121.93 lbs. of hydrocarbons and a caloric value of about 27.00 Btus/cubic foot of feed gas. Thus, approximately 29% by weight of the original hydrocarbons and about 32% of the caloric value are removed by the first catalytic combustion process. In this example, methane is not removed from the feed gas
10
, but can be removed in a later processing operation in the carbon dioxide plant, such as in a stripper column where it is removed by distillation of the liquid carbon dioxide.
Following combustion in the first reactor
40
, the feed gas
10
is successively fed to second and third reactors
80
and
120
(streams
7
and
13
), respectively. More specifically, as shown in
FIG. 1
, the feed gas
10
receives oxy
Gupta Amitabh
Heim Carl Joseph
Black Donald T.
Bos Steven
Kuhar Anthony
Praxair Technology Inc.
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