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
2002-10-22
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
06469163
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-based 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 frequently referred to as the “conventional” or “Allied Signal” process. Such process is disclosed, for example, in Weissermel and Arp, Industrial Organic Chemistry (VCH Verlagsgesellschaft mbH 1993), pp. 249-258. In the conventional process, hydroxylamine sulfate ((NH
2
OH)
2
.H
2
SO
4
) and aqueous ammonia are reacted to synthesize the oxime. The hydroxylamine sulfate is produced by the Raschig process:
Catalytic air oxidation of ammonia to form nitric oxide:
4NH
3
+5O
2
→4NO+6H
2
O (I)
Continued oxidation of nitric oxide to form nitrogen dioxide:
NO+½O
2
→NO
2
(II)
Synthesis of ammonium nitrite:
NO+NO
2
+(NH
4
)
2
CO
3
→2NH
4
NO
2
+CO
2
(III)
Reduction of ammonium nitrite to hydroxylamine diammonium sulfate:
2NH
4
NO
2
+4SO
2
+2NH
3
+2H
2
O→2HON(SO
3
NH
4
)
2
(IV)
Hydrolysis of hydroxylamine diammonium sulfate to hydroxylamine sulfate:
2HON(SO
3
NH
4
)
2
+4H
2
O→(NH
2
OH)
2
.H
2
SO
4
+2(NH
4
)
2
SO
4
+H
2
SO
4
(V)
Oximating the cyclohexanone with the hydroxylamine sulfate to produce cyclohexanone oxime:
C
6
H
10
O+(NH
2
OH)
2
.H
2
SO
4
+NH
4
OH→C
6
H
11
NO+(NH
4
)
2
SO
4
+H
2
O (VI)
The process for forming hydroxylamine sulfate in the conventional 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
2
. The water formed in the ammonia oxidation is thereafter removed from the process stream in a condenser
30
. Some of the NO
2
is absorbed in the water as it is condensed, producing a weak nitric acid condensate
5
.
The NO
x
-rich process gas stream
7
exiting the condenser
30
is then contacted countercurrently with an aqueous ammonium carbonate stream
9
in a trayed absorption tower
40
, referred to as a “nitrite tower”. In the conventional process additional, “secondary” air is added either directly into the nitrite tower through line
11
or into the NO
x
process stream through line
13
. The amount of secondary air fed to the nitrite tower affects the relative concentrations of NO and NO
2
in the tower. An ammonia stream
15
a
may also be added to the tower to recover CO
2
.
Ammonium nitrite is desirably formed in the nitrite tower, according to the reaction:
NO+NO
2
+(NH
4
)
2
CO
3
→2NH
4
NO
2
+CO
2
(VII)
The CO
2
liberated in this reaction can be recovered in-situ as ammonium carbonate by reaction with the ammonia stream
15
, according to the reaction:
CO
2
+2NH
3
+H
2
O→(NH
4
)
2
CO
3
(VIII)
An undesired product, ammonium nitrate, is also formed in the nitrite tower by the following reactions:
2NO
2
+H
2
O→HNO
3
+HNO
2
(IX)
HNO
3
+HNO
2
+2NH
3
→NH
4
NO
2
+NH
4
NO
3
(X)
Ammonia participating in reaction (X) may be derived from the dissociation of the ammonium compounds formed in these reactions.
The nitrite tower
40
must be operated to minimize the formation of nitrate. To accomplish this, an approximate 1:1 molar ratio of NO to NO
2
should be maintained in the tower. In order to maintain such ratio, secondary air is added to the nitrite tower in the conventional process in amounts of about 5 to 10 volume % of the total air flow into the system.
The vent gas
17
exiting the nitriting tower must be properly regulated to minimize the emission of NO
x
. An increase in production of hydroxylamine sulfate typically results in a corresponding increase in NO
x
emission in the vent gas
17
.
The nitrite-rich aqueous solution
19
is then reacted with a sulfur dioxide stream
21
and an ammonia stream
15
b
fed into a disulfonate column
50
to form hydroxylamine diammonium sulfate. In some systems the ammonia may rather be admixed with the nitrite-rich aqueous solution
19
from the nitrite tower and the mixture then introduced into the disulfonate column.
The hydroxylamine diammonium sulfate stream
23
removed from the disulfonate column is conventionally hydrolyzed in a hydrolysis column
60
to form hydroxylamine sulfate. A portion of the hydroxylamine diammonium sulfate is recycled through line
27
to the disulfonate column
50
. The hydroxylamine sulfate solution exiting the hydrolysis column is then recovered from line
25
for use in the oximation process.
In view of the strict environmental regulation of NO
x
emissions, the quantity of NO
x
gases vented through line
17
cannot be increased. Accordingly, any increased hydroxylammonium sulfate 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 nitrite 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 conventional process for producing caprolactam, by which increased amounts of hydroxylamine sulfate and, consequently, caprolactam can be produced without large capital investment, and without increasing NO
x
emissions.
SUMMARY OF THE INVENTION
The present invention provides just such an improvement in the conventional process for the production of caprolactam involving:
(a) reacting air with ammonia gas in an ammonia conversion zone to produce nitric oxide;
(b) oxidizing a portion of the nitric oxide to nitrogen dioxide to produce an NO
x
-rich process gas stream;
(c) reacting the NO
x
-rich stream with ammonium carbonate in a nitriting zone to produce ammonium nitrite;
(d) reducing the ammonium nitrite to hydroxylamine diammonium sulfate;
(e) hydrolyzing the hydroxylamine diammonium sulfate to hydroxylamine sulfate;
(f) oximating the hydroxylamine sulfate with cyclohexanone to produce cyclohexanone oxime; and
(g) converting the cyclohexanone oxime to caprolactam.
In accordance with the invention, the foregoing process is improved by adding supplemental oxygen downstream of the ammonia conversion zone to increase the quantity and rate of formation of nitrogen dioxide in the NO
x
-rich process gas stream. Desirably, secondary air, normally introduced into the nitriting zone (or into the NO
x
-rich gaseous stream feeding into the nitriting zone) is rerouted to the ammonia conversion zone to increase the production of nitric oxide formed in the ammonia conversion zone without increasing the level of NO
x
contained in the
Echegaray Diego Fernandez
Velloso Antonio Augusto M.
Wagner Matthew Lincoln
Langel Wayne
Lau Bernard
Praxair Technology Inc.
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