Organic compounds -- part of the class 532-570 series – Organic compounds – Unsubstituted hydrocarbyl chain between the ring and the -c-...
Reexamination Certificate
1998-12-24
2001-12-25
Langel, Wayne (Department: 1754)
Organic compounds -- part of the class 532-570 series
Organic compounds
Unsubstituted hydrocarbyl chain between the ring and the -c-...
C423S400000, C423S405000, C540S536000
Reexamination Certificate
active
06333411
ABSTRACT:
BACKGROUND OF THE INVENTION
Caprolactam can be produced from three hydrocarbon feedstocks: cyclohexane, phenol, and toluene. Approximately 68% of the world's caprolactam capacity is produced from cyclohexane, 31% from phenol, and 1% from toluene. All of the cyclohexane and phenol-based production proceeds via the formation of cyclohexanone oxime. In 94% of the cyclohexane and phenol-based caprolactam capacity, the formation of this oxime requires an ammonia oxidation step.
In the processes involving ammonia oxidation, caprolactam production from cyclohexane or phenol can be broken down into the following steps:
Oxidation of cyclohexane or hydrogenation of phenol, to synthesize cyclohexanone;
Oxidation of ammonia to form nitric oxide, followed by various reaction steps to form a hydroxylamine salt;
Synthesis of cyclohexanone oxime by reaction of cyclohexanone and the hydroxylamine salt; and
Treatment of the cyclohexanone oxime with sulfuric acid followed by neutralization with aqueous ammonia to form caprolactam.
One such method for producing caprolactam is the DSM-HPO (Dutch State Mines-Hydroxylammonium Phosphate-Oxime) process, also known as the Stamicarbon process. Such process is disclosed, for example, in Weissermel and Arp, Industrial Organic Chemistry (VCH Verlagsgesellschaft mbH 1993), pp. 249-258. In the DSM-HPO process, hydroxylammonium phosphate (NH
3
OH.H
2
PO
4
) is reacted with cyclohexanone in toluene solvent to synthesize the oxime.
The hydroxylammonium phosphate is synthesized in the DSM-HPO process in the following manner:
Catalytic air oxidation of ammonia to form nitric oxide:
4 NH
3+5
O
2
→4 NO+6
H
2
O (I)
Continued oxidation of nitric oxide to form nitrogen dioxide, among other nitrogen oxides:
NO+½ O
2
→NO
2
(II)
Reactive absorption of nitrogen dioxide in a buffered aqueous phosphoric acid solution to form nitrate ions:
3 NO
2
+H
2
O→2 HNO
3
+NO (III)
HNO
3
+H
2
PO
4
→NO
3
+H
3
PO
4
(IV)
Catalytic hydrogenation of nitrate ions to form hydroxylammonium phosphate:
NO
3
−
+2 H
3
PO
4
+3 H
2
→NH
3
OH.H
2
PO
4
+H
2
PO
4
−
+2 H
2
O (V)
Oximating the cyclohexanone with hydroxylammonium phosphate to produce cyclohexanone oxime:
C
6
H
10
O+NH
3
OH.H
2
PO
4
→C
6
H
11
NO+H
3
PO
4
+H
2
O (VI)
The process for forming hydroxylammonium phosphate in the DSM-HPO process is shown in the flow sheet depicted in
FIG. 1
of the attached drawing. As shown therein, an air stream
3
is initially compressed in a compressor
10
, introduced as a “primary” air stream through feed line
12
into admixture with a gaseous ammonia stream
1
, and thereafter fed to a catalytic ammonia converter
20
. Typically, 100% ammonia conversion and 95% selectivity to NO are achieved in that reaction. Upon exiting the converter, some of the NO is further oxidized to NO
2
to form an NO
x
-rich process gas stream
15
. Some of the NO
2
in the NO
x
-rich process stream
15
dimerizes to form N
2
O
4
.
The NO
x
-rich process gas stream
15
is contacted countercurrently with an aqueous inorganic acid stream
37
in a trayed absorption tower
40
. In the conventional DSM-HPO process, a “secondary” air stream
11
is added into a degasser
50
in amounts of from 5 to 20 volume % of the total air flow to the system. The secondary air stream
11
becomes laden with nitric oxide and the resulting nitric oxide laden air stream
17
is added to the base of the absorption tower
40
. A nitrate-rich liquid stream
13
exiting the absorption tower
40
is routed to the degasser
50
, and an NO, containing vent gas
5
exits the absorption tower.
The vent gas
5
exiting the absorption tower
40
must normally be properly regulated to minimize the emission of NO,. An increase in production of hydroxylammonium phosphate typically results in a corresponding increase in NO
x
emission in the vent gas
5
.
The aqueous inorganic acid stream
37
added to the top of the absorption tower
40
contains a mixture of water, phosphoric acid (H
3
PO
4
), ammonium nitrate (NH
4
NO
3
), and monoammonium phosphoric acid (NH
4
H
2
PO
4
). The acid stream
37
is continuously cycled from the oximator train (consisting of an oximator
70
, oxime extractor
80
, and a hydrocarbon stripper
90
) to the hydroxylamine train (consisting of the absorption tower
40
, degasser
50
, and a nitrate hydrogenator
60
). Nitric oxides in the NO
x
-rich process gas stream
15
reactively absorb in the phosphoric acid solution in the absorption tower
40
to form nitrate ions.
The nitrate-rich liquid stream
13
exiting the absorption tower
40
is passed through the degasser
50
, where it is contacted countercurrently with secondary air
11
entering the degasser
50
. The secondary air
11
removes unreacted nitric oxides from the nitrate-rich liquid stream
13
. The nitric oxide-containing air stream
17
exiting the degasser
50
is routed to the absorption tower
40
.
A nitrate-rich liquid stream
19
exiting the degasser
50
is combined with an aqueous inorganic acid stream
21
from the oximator train, and the combination
31
fed to the nitrate hydrogenator
60
. A hydrogen stream
7
is also added to the nitrate hydrogenator
60
. Nitrate ions are reduced with hydrogen in the nitrate hydrogenator
60
over a palladium catalyst to form hydroxylammonium phosphate. An aqueous stream of hydroxylammonium phosphate, phosphoric acid, ammonium nitrate, and monoammonium phosphoric acid
23
exits the nitrate hydrogenator
60
.
The hydroxylammonium phosphate containing aqueous stream
23
then reacts with a stream of cyclohexanone in toluene solvent
25
in the oximator
70
to produce cyclohexanone oxime. An oxime-toluene stream
9
exits the oximator
70
and is processed into caprolactam. An aqueous stream
27
also exits the oximator
70
, and is routed to a oxime extractor
80
which removes entrained oxime
39
, and adds it to the stream of cyclohexanone in toluene solvent
25
. An aqueous stream
29
exiting the oxime extractor
80
is routed to a hydrocarbon stripper
90
where entrained cyclohexanone and toluene
33
are removed and added to the stream of cyclohexanone in toluene solvent
25
, which is routed to the oximator
70
. Thus, the entrained oxime
39
obtained in the oxime extractor
80
and the cyclohexanone-toluene
33
obtained in the hydrocarbon stripper
90
are returned to the oximator
70
. The aqueous stream
35
leaving the hydrocarbon stripper
90
is routed back to the hydroxylamine train, where a portion
21
is distributed to the nitrate hydrogenator
60
and a portion
37
is distributed to the absorption tower
40
. Typically, about 90% of aqueous stream
35
is routed to stream
21
, and about 10% routed to stream
37
.
In view of the strict environmental regulation of NO
x
emissions, the quantity of NO
x
in the vent gas
5
cannot be increased. Accordingly, any increased hydroxylammonium phosphate production (and subsequent caprolactam production) must be obtained without any increase in NO
x
emissions. This can be accomplished by increasing the amount of air and ammonia fed to the process while increasing the plant size, e.g., the size of the absorption tower
40
and air compressor
10
. However, such an increase in equipment capacity requires substantial capital investment.
There is therefore a need for the development of improved techniques in the DSM-HPO process for producing caprolactam, by which increased amounts of hydroxylammonium phosphate and, consequently, caprolactam can be produced without large capital investment, and without increasing NO
x
emissions.
SUMMARY OF THE INVENTION
The present invention provides an improvement in the DSM-HPO process for production of caprolactam involving:
(a) reacting air with ammonia gas in an ammonia conversion zone to produce nitric oxide;
(b) oxidizing at least a portion of the nitric oxide to nitrogen dioxide to produce an NO
x
-rich process gas stream;
(c) re
Echegaray Diego Fernandez
Velloso Antonio Augusto M.
Wagner Matthew Lincoln
Langel Wayne
Lau Bernard
Praxair Technology Inc.
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