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| United States Patent |
5,096,687
|
|
Agrawal
|
March 17, 1992
|
Catalyst pack for ammonia conversion to HCN
Abstract
The manufacture of hydrogen cyanide by the catalytic reaction of air,
ammonia and a hydrocarbon gas employing a catalyst pack which consists of
a top layer of platinum-rhodium alloy catalyst gauze or granular catalyst
and a bottom layer of platinum-rhodium alloy metal catalyst gauze or
granular catalyst, wherein the top layer is a finer mesh than the bottom
layer.
| Inventors:
|
Agrawal; Jitendra P. (Memphis, TN)
|
| Assignee:
|
E. I. Du Pont de Nemours and Company (Wilmington, DE)
|
| Appl. No.:
|
592177 |
| Filed:
|
October 9, 1990 |
| Intern'l Class: |
C01C 003/02 |
| Field of Search: |
423/376
|
References Cited
U.S. Patent Documents
| 2607663 | Aug., 1952 | Perry et al. | 423/376.
|
| 2666689 | Jan., 1954 | Heider | 423/376.
|
| 3033658 | May., 1962 | Gross et al. | 423/376.
|
| 3056655 | Oct., 1962 | Inman | 423/376.
|
| 3215495 | Nov., 1965 | Jenks et al. | 423/376.
|
| 4469666 | Sep., 1984 | Stephenson et al. | 423/376.
|
| Foreign Patent Documents |
| 566835 | Dec., 1958 | CA | 423/376.
|
Primary Examiner: Langel; Wayne
Attorney, Agent or Firm: Krukiel; Charles E.
Claims
I claim:
1. A process for producing hydrogen cyanide comprising feeding a gaseous
mixture comprising nitrogen compounds, oxygen and carbon compounds to a
reactor containing two superposed layers of catalyst comprising one or
more platinum group metals, the first layer of catalyst containing from 33
to 67 weight percent of the platinum group metal present in the total
catalyst bed in the form of 40 to 100 mesh gauze or 0.2 to 1.0 mm size
particles and the second layer of catalyst containing from 33 to 67 weight
percent of the platinum group metal present in the total catalyst bed in
the form of a 20 to 40 mesh gauze or 2 to 10 mm size particles, provided,
however, that when the form of catalyst is gauze, the mesh size of the
first layer is finer than the mesh size of the second layer.
2. The process of claim 1 wherein both of the layers of the catalyst bed
are in the form of particles and the second layer contains particles from
5 to 10 times the diameter of the particles in the first layer.
3. The process of claim 2 wherein the reaction is carried out at from about
1000.degree. to 1200.degree. C.
4. The process of claim 3 wherein the catalyst contains between 50 and 90
weight percent platinum.
5. The process of claim 4 wherein the feed to the reactor consists
essentially of a mixture of ammonia, methane and air.
6. The process of claim 5 wherein the catalyst consists essentially of
about 90 weight percent platinum and 10 weight percent rhodium.
7. The process of claim 1 wherein both layers of the catalyst bed are in
the form of a mesh gauze and the mesh size of the first layer is from 1.5
to 5 times the mesh size of the second layer.
8. The process of claim 7 wherein the reaction is carried out at from about
1000.degree. to 1200.degree. C.
9. The process of claim 8 wherein the catalyst contains between about 50
and 90 weight percent platinum.
10. The process of claim 9 wherein the feed to the reactor consists
essentially of a mixture of ammonia, methane and air.
11. The process of claim 10 wherein the wires forming the mesh in both
layers are from about 1 to 5 mils in diameter.
12. The process of claim 11 wherein the catalyst consists essentially of
about 90 weight percent platinum and about 10 weight percent rhodium in
the catalyst pack.
13. In a process for preparing hydrogen cyanide by reacting a mixture of
air, ammonia and a hydrocarbon gas in the presence of a catalyst
comprising one or more platinum group metals, a method for improving the %
single pass conversion of ammonia to hydrogen cyanide in said process
which comprises:
(a) forming the catalyst into a catalyst bed comprising two superposed
layers, said first layer containing from 33 to 67 weight percent of the
platinum group metal present in the total catalyst bed and having the form
of 40 to 100 mesh gauze or from 0.02 to 1.0 mm size particles, and said
second layer of catalyst containing from 33 to 67 weight percent of the
platinum group metal present in the total catalyst bed and having the form
of a 20 to 40 mesh gauze or from 2 to 10 mm size particles; and
(b) passing said mixture through said first catalyst layer and then through
said second catalyst layer, with the proviso that when the catalyst is
gauze, the mesh size of the first layer is finer than the mesh size of the
second layer.
14. The process of claim 13 in which the catalyst is an alloy comprising
90% platinum and 10% rhodium, the first catalyst layer is an 80 mesh
gauze, and the second catalyst layer is a 40 mesh gauze.
15. In a process for producing hydrogen cyanide wherein a reactant mixture
comprising ammonia, oxygen and a hydrocarbon gas is reacted in a bed
containing a platinum metal gauze catalyst, the improvement for increasing
the % single pass conversion of ammonia to hydrogen cyanide by the steps
comprising:
forming the catalyst into a bed comprising two superposed layers, said
first layer containing from 33 to 67 weight percent of the platinum metal
catalyst present in the total catalyst bed and the gauze is from 40 to 100
mesh, said second layer containing from 33 to 67 weight percent of the
platinum metal present in the total catalyst bed and the gauze is from 20
to 40 mesh, with the proviso that the mesh size of the first layer is
finer than the mesh size of the second layer; and
(b) passing said reactant mixture through said first catalyst layer and
then through said second catalyst layer.
Description
FIELD OF THE INVENTION
The present invention relates to the manufacture of hydrogen cyanide by the
reaction of air, ammonia and a hydrocarbon gas in the presence of a
catalyst consisting of one or more of the platinum metals. It relates
particularly to an improved catalyst pack for ammonia conversion to
hydrogen cyanide.
The catalyst employed in commercial processes for the formation of hydrogen
cyanide from air, ammonia and a hydrocarbon gas is generally some form of
platinum or one of its alloys. The catalyst may take the physical shape of
wire gauze, metallic particles, plates, spirals or may be a metallic
coating on various inert substrates. Wire gauzes may take the form of flat
gauzes, cylindrical type gauzes or conical structures of gauzes.
The primary object of this invention is to provide a method for making a
catalyst pack which yields a higher conversion of ammonia to hydrogen
cyanide than has been realized by employing a single mesh size of
platinum-rhodium alloy catalyst gauze. The improved catalyst pack consists
of a plurality of top layers of platinum-rhodium alloy catalyst gauzes or
granular catalyst and a bottom layer of platinum-rhodium alloy catalyst
gauzes or granular catalyst, wherein the top layer is a finer mesh than
the bottom layer.
BACKGROUND OF THE INVENTION
U.S. Pat. No. 3,244,479 discloses a method for regenerating gauze catalysts
in situ. The discussed catalyst, of 90% platinum and 10% rhodium, consists
of three layers of 80 mesh, 3 mil gauze.
U.S. Pat. No. 2,831,752 discloses a combination catalyst body employing
gauze and granular material and a method for activating new catalysts and
reactivating spent catalysts. The catalyst comprises at least one metallic
gauze superimposed on a granular refractory material covered with a
metallic coating. In normal practices, the granular material is about 6 to
10 mesh and is formed into a bed of about 3/4 to 11/2 inch (1.9 to 3.8 cm)
thick and the overlayer comprises a single gauze. When more than one layer
of gauze is employed, the layers are separated by 1/4 inch (0.635 cm)
layers of 6 to 10 mesh porcelain chips.
U.S. Pat. No. 2,975,144 discloses a method of catalyst support which
eliminates contact of the catalyst with heat conductive holders, clamps or
other metallic or heat conductive surfaces. The apparatus is thus
constructed in such a manner that the catalyst temperature is as uniform
as possible and that heat dissipation and loss of heat due to radiation is
minimized. In a preferred embodiment, the wires that make up the gauze are
of a thickness of about 0.16 to 0.20 mm. In addition, several layers of
such catalyst gauzes, of unspecified mesh size, are placed one over the
other to permit the use of a higher velocity of reaction gases.
U.S. Pat. No. 3,056,655 discloses a reactor design in which either catalyst
gauze or finely divided catalyst particles are supported on layers of
inert pellets which range in size from fine particles (1/8 inch [0.32 cm]
cylinders) immediately underneath the catalyst layer to relatively coarse
pellets (5/8 inch [1.59 cm] cylinders) at the downstream size of the
reactor which are, in turn, supported on an insulating refractory plate.
When catalyst gauze is used, the layer of graded pellets is optional. The
purpose of the refractory plate is to insulate the catalyst from the metal
surfaces of the heat exchanger. The inert pellets serve as catalyst
support.
U.S. Pat. No. 3,545,939 discloses a support system for the catalyst used in
the production of hydrogen cyanide in a conventional reactor. The support
system consists of sillimanite grid tiles which support a bed of graded
ceramic pellets--a layer of 5/8 inch (1.59 cm) pellets cover the tiles, on
top of which is a layer of 3/8 inch (0.95 cm) pellets, which is covered by
a layer of 1/4 inch (0.635 cm) pellets. One additional layer of ceramic
pellets cover the 1/4 inch (0.635 cm) pellets. Covering the ceramic
pellets is a corrugated ceramic material which supports several layers of
platinum-rhodium gauze pads.
U.S. Pat. No. 4,469,666 discloses the design of a catalyst pack the purpose
of which is to improve the durability of platinum group metal catalyst
gauzes used in the production of hydrocyanic acid. Improved durability
implies that the same conversion efficiency of ammonia with less precious
metal or that with the same quantity of precious metal, greater conversion
is obtained. The catalyst pack is described in U.S. 4,469,666 as "a pack
of woven metallic gauzes wherein at least some of the gauzes disposed at
or towards the front of the pack are made from wire having a
cross-sectional area greater than at least some of the gauzes disposed at
or towards the rear of the pack. The ,front, of the pack is considered to
be that portion of the pack which gas entering the pack contacts first.
... [For more than two stages,] the cross-sectional area of the wire of
the gauze or gauzes in any one stage is greater than the cross-sectional
area of the wire of the gauzes or gauzes in the next succeeding stage
considered in the direction of the flow of reactants through the unit".
SUMMARY OF THE INVENTION
The present invention relates to the manufacture of hydrogen cyanide by the
reaction of air, ammonia and a hydrocarbon gas in the presence of an
improved catalyst pack, by use of which it has been found that the single
pass conversion of ammonia to hydrogen cyanide can be increased. The
catalyst pack consists of a top layer of platinum-rhodium alloy catalyst
gauze or granular catalyst and a bottom layer of platinum-rhodium alloy
metal catalyst gauze or granular catalyst, wherein the top layer is a
finer mesh or particle size than the bottom layer.
DETAILED DESCRIPTION OF THE INVENTION
The platinum metal catalyst employed in the process of this invention may
be one of the platinum group metals, defined to be platinum, rhodium,
iridium, palladium, osmium or ruthenium, or a mixture or alloy of two or
more of these metals. The preferred alloy contains between 50% and 90%
platinum. The most preferred alloy consists of 90% platinum and 10%
rhodium. The platinum metal catalyst packs may be employed in the form of
a top layer segment of catalyst gauzes and a bottom layer segment of
catalyst gauzes, wherein the top layers are a finer mesh than the bottom
layers. The top of the catalyst pack is considered to be that portion of
the pack which gas entering the pack contacts first. Each layer may
consist of one or more catalyst gauzes. It is not necessary that each
layer be composed of an equal number of catalyst gauzes or of an equal
weight of catalyst, but generally the catalytic metal in the upper layer
of gauzes will comprise from 33 to 67 wt. % of the catalytic metal present
and the bottom layer of gauzes also contains 33 to 67 wt. %. In addition,
it may be desirable to include a sheet of catalyst gauze made of a thicker
wire at the bottom of the pack to add structural support to the catalyst
pack, and constructed with a coarse mesh so that the flow distribution of
the gases is not disturbed.
A granular catalyst may be used to form an equivalent catalyst pack, i.e.,
two different "mesh" sizes of granular catalyst may be layers such that
the finer mesh is on the top of the coarser mesh. A granular catalyst may
also be used in combination with a catalyst gauze, an example of which is
given in U.S. Pat. No. 2,831,752 but with two different "mesh" sizes.
Platinum metal catalyst employed as a granular catalyst may be one of the
platinum metals as defined above. The granular catalyst may be in the form
of metal pellets, spheres, chips, turnings, etc., or in the form of
platinum metal alloy catalyst coating on an inert substrate such as beryl
(beryllium aluminum silicate), alumina, sillimanite, etc.
It is believed that the increased ammonia conversion results from improved
flow distribution. The preferred 80 mesh upper layer acts as a flow
distributor which improves the flow distribution to the 40 mesh lower
layer. The net result is that the mixed bed outperforms an equivalent 80
mesh bed. In a preferred aspect of the invention, the wire forming the
mesh in both layers is about the same diameter of about 1 to 5 mils
(2.54.times.10.sup.-5 to 12.7.times.10.sup.-5 m). The preferred mesh size
is from 40 to 100 mesh for the upper or front section and from 20 to 40
mesh for the bottom or back section of the catalyst bed. Generally the top
or front section should have a mesh size from 1.5 to 5 times the mesh size
of the bottom or back section of the catalyst bed. When using a
particulate catalyst bed, the particle size of the top layer should be
from 0.2 to 1.0 mm and the particle size of the bottom layer should be
from 2.0 to 10.0 mm. The average particle size of the bottom layer
particles should be from 5 to 10 times the particle size of the top layer
particles. Generally, the total catalyst bed will be from 0.5 to 6.00
inches in thickness.
This invention is particularly useful when employed in combination with the
hydrogen cyanide reactor and process disclosed in U.S. Pat. No. 3,104,945.
The reaction mixture, in the temperature range 400 to 525.degree. C., of a
controlled composition containing air, ammonia and methane or a gas
consisting predominantly of methane, preferably preheated, is passed over
a platinum metal catalyst. The composition of the said reaction mixture is
controlled to contain about 1 volume of methane or natural gas to 1 volume
of ammonia and air equivalent of 25% to 40% of the amount that would be
required for complete oxidation of the ammonia and methane to water,
nitrogen and carbon dioxide. In carrying out the hydrogen cyanide
synthesis, maximum yields are obtained when reaction conditions are
maintained in such a manner that the product gas always contains unreacted
hydrocarbon equivalent to 0.1 to 0.3% by volume of methane. Variations in
preheat temperature or methane feed cause variations in the catalyst bed
temperature which is maintained in the range of from 1000.degree. to
1200.degree. C.
The reactant mixture which is preferred for the synthesis of hydrogen
cyanide is a mixture of ammonia, methane or natural gas, and air. It is to
be understood, however, that the process of this invention may be employed
with mixtures of nitric oxide and hydrocarbons; ammonia, methane and
oxygen; and other mixtures of gases comprising nitrogen compounds, oxygen
and carbon compounds, including hydrocarbons and carbon oxides. Another
reaction which may be carried out in the presence of the combination
catalyst pack of this invention is the oxidation of ammonia to nitric
acid. Other reactions in which this combination catalyst pack may be
employed will be apparent to those skilled in the art.
The process may be carried out at any pressure, i.e., at atmospheric,
superatmospheric or subatmospheric. In practicing this process of making
hydrogen cyanide, conventional gas mixtures, reaction conditions,
materials of reactor, preheater, etc. and methods of working up the
combustion products are employed and need not be described in detail.
U.S. Pat. No. 3,215,495 discloses a method for preventing heat loss from
the catalyst bed by the use of inert refractory fiber and/or refractory
particles on the surface of the catalyst which faces the reactant gas
flow. The use of refractory materials on the surface of the catalyst pack
of the current invention in no way detracts from the scope of the
invention and may in fact prove to be beneficial. Many variations in
conditions from those given in the examples can be made without departing
from the scope of the invention.
EXAMPLES
The data from a side-by-side comparison of a 40 mesh catalyst pack (B
converter), a 80 mesh catalyst pack (A converter) and equal weight of a 40
and 80 mesh catalyst pack (C converter) are reported in Examples 1, 2 and
3. The process was based on U.S. Pat. No. 3,104,945 in which the
reactants, air, ammonia and natural gas were passed, on a single pass
once-through basis, through the catalyst pack.
The same weight loading of 90% platinum-10% rhodium alloy catalyst was used
in each converter. In each catalyst pack, a single 20 mesh sheet of 9 mil
(2.3.times.10.sup.-4 m) wire was placed at the bottom to provide
structural support of the catalyst pack. For the mixed bed catalyst pack,
the 80 mesh sheets were layered on top of the 40 mesh sheets with the 20
mesh sheet on the bottom, downstream of the gas flow. The catalyst gauze
for the 40 and 80 mesh gauze was made with 3 mil (7.62.times.10.sup.-5 m)
wire.
All these tests were done at moderate preheat temperatures. The preheated
gas temperature was between 308.degree. to 350.degree. C.
EXAMPLE 1
______________________________________
Catalyst Bed:
80 mesh - 2 sheets
40 mesh - 32 sheets
20 mesh - 1 sheet
Total number of sheets = 35
Total weight of catalyst = 1037 Troy ounces
B Converter Data
Forty Mesh Bed
Conv Yield Unreacted
Day % % NH.sub.3 %
______________________________________
1 1 62.4 78.1 20.1
2 3 65.4 81.7 20.0
3 8 66.1 79.1 16.4
4 10 65.4 77.8 16.0
5 15 63.4 77.3 18.0
6 22 67.1 82.1 18.3
7 32 64.6 78.8 18.1
8 36 64.5 80.7 20.1
9 37 66.9 81.7 18.2
10 43 65.3
11 52 68.6 82.3 16.6
12 57 65.3 83.7 22.0
13 59 64.5 80.0 19.4
14 64 63.8 79.7 20.0
15
16 MEAN 65.2 80.2 18.7
17 STDEV 1.6 2.0 1.7
18 66 60.1 76.9 21.8
19 68 59.8 76.5 21.9
20 77 61.0 75.9 19.6
21 84 63.3 78.6 19.5
22 91 67.8 86.5 21.6
23 96 62.6 84.2
24 98 66.4 85.7 22.6
25 103 61.9 80.4 23.0
26 105 62.4 77.5 19.6
27 110 60.4 78.1 22.7
28 112 61.7 78.3 21.3
29 117 60.5 77.6 22.0
30
31 MEAN 63.9 80.0 20.0
32 STDEV 2.5 2.9 2.1
______________________________________
EXAMPLE 2
______________________________________
Catalyst Bed:
80 mesh - 9 sheets
40 mesh - 18 sheets
20 mesh - 1 sheet
Total number of sheets = 28
Total weight of catalyst - 1037 Troy ounces
C Converter Data
Mixed Bed of 80 and 40 Mesh (50:50 by wt.)
Conv Yield Unreacted
Day % % NH.sub.3 %
______________________________________
1 1 72.8 87.3 16.6
2 6 72.6
3 13 75.4 88.3 14.5
4 15 74.3 87.8 15.3
5 20 72.0 89.1 19.1
6 27 72.7 85.7 15.1
7 36 72.6 87.1 16.7
8 41 72.6
9 43 72.6 86.5 16.1
10 48 69.6 86.3 19.3
11 50 70.8 85.6 17.3
12 55 69.1 84.0 17.7
13 57 68.5 82.5 17.0
14 62 64.6 78.3 17.5
15 69 63.5 79.0 19.6
16 74 65.3 83.4 21.8
17
18 MEAN 70.6 85.1 17.4
19 STDEV 3.5 3.3 2.0
20 76 65.1 77.5 16.0
21 83 68.6 81.8 16.1
22 90 67.3 79.5 15.3
23 97 66.9 75.8
24 102 63.5
25 109 66.8 82.9 19.3
26 111 69.2
27 116 63.3 75.3 16.0
28 118 66.1 76.3 13.4
29 123 64.9 75.8 14.4
30
31 MEAN 68.9 82.6 16.9
32 STDEV 3.7 4.6 2.0
______________________________________
EXAMPLE 3
______________________________________
Catalyst Bed:
80 mesh - 18 sheets
40 mesh - 0 sheet
20 mesh - 1 sheet
Total number of sheets = 19
Total weight of catalyst = 1037 Troy ounces
A Converter Data
80 Mesh Bed
Conv Yield Unreacted
Day % % NH.sub.3 %
______________________________________
1 1 79.2 24.3
2 3 71.9 21.6
3 8 66.3 82.8 20.0
4 15 68.4 83.4 18.0
5 17 68.3 87.2 21.7
6 22 65.7 83.7 21.5
7 24 22.9
8 29 66.8 86.9 21.0
9 31 68.5 86.9 21.2
10 38 71.0 22.6
11 43 69.5 23.6
12 45 69.5 23.4
13 50 67.6 86.9 22.2
14 52 20.5
15 57 70.8 23.2
16 59 22.0
17 64 69.7 89.7 22.3
18 68 66.3 85.2 22.3
19 73 69.1 24.1
20
21 MEAN 68.2 85.2 22.0
22 STDEV 1.8 3.0 1.5
23 75 64.2 86.9 26.1
24 79 64.3 81.3 23.4
25 82 65.8 83.1 20.8
26 87 21.1
27 94 63.1 80.1 21.3
28 99 65.4 89.6
29 101 63.8 86.1
30
31 MEAN 67.3 84.9 22.1
32 STDEV 2.7 3.1 1.6
33 106 59.6 81.4
34 108 58.9 74.2
35 113 57.6 75.1
36 115 59.7 77.8 23.3
37 120 59.7 77.0 22.5
38
39 MEAN 65.6 83.1 22.2
40 STDEV 4.1 4.5 1.6
______________________________________
COMPARISON OF EXAMPLES
After 21/2 months of operation:
B converter (40 mesh): 65.2/80.2 (% ammonia conversion/yield)
A converter (80 mesh): 68.2/85.2
C converter (mixed): 70.6:85.1
Although a natural gas quality upset, after 21/2 months of operation,
caused the performance of the process to drop off, the general trend of
the increased ammonia conversion to hydrogen for the improved catalyst
pack when compared to the 40 or 80 mesh catalyst pack alone remained the
same.
After 4 months of operation:
B converter (40 mesh): 63.9/80.0
A converter (80 mesh): 65.6/83.1
C converter (mixed): 68.9/82.6
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