Process for producing hydrogen cyanide

Hydrogen cyanide is commercially produced by reacting in the vapor phase ammonia, air, and a hydrocarbon gas, e.g., natural gas, in the presence of a platinum group metal catalyst. The operation of this process to obtain optimum hydrogen cyanide production is accomplished by determining HCN production at various natural gas feed rates, i.e., varying the air/natural gas ratio, while maintaining a constant air/ammonia ratio. Once the production peak is located, the natural gas feed is maintained at the corresponding rate.

BACKGROUND OF THE INVENTION 
This invention relates to an improvement in a process for making hydrogen 
cyanide (HCN). 
HCN can be produced by a process comprising contacting in vapor phase, 
ammonia, a hydrocarbon, e.g., methane or natural gas, and an 
oxygen-containing gas in the presence of a platinum-group metal catalyst 
at high temperature. In commercial operations, the reactants are generally 
ammonia, natural gas and air. This one-stage synthesis of HCN from ammonia 
and natural gas in which heat is supplied by simultaneous combustion 
reactions with air is disclosed in U.S. Pat. No. 1,934,838 issued to 
Andrussow on Nov. 14, 1933. Numerous modifications and improvements 
pertaining to this process have been described and patented. 
It is known that in the reaction of ammonia, natural gas and air to produce 
HCN, one of the most important variables with respect to HCN yield and 
conversion is the composition of the feed gas. While any two ratios of 
reactants can fix the feed gas composition, various combinations of these 
ratios can and have been employed. Optimum ratios vary depending upon 
operating conditions, such as throughput, catalyst type and age, preheat, 
and reactor geometry. Hence, the optimum ratio needs to be determined and 
regulated periodically so as to assure maximum productivity and yield. 
Various process techniques have been developed for obtaining improved HCN 
yields. For example, several patents, e.g., U.S. Pat. No. 3,104,945 and 
British Pat. No. 956,200, describe preheating the feed gases and 
maintaining specified ratios of reactants. With the use of preheat it is 
possible to add more ammonia to the reaction without reducing the reaction 
or catalyst bed temperature, thus increasing production of HCN. 
U.S. Pat. No. 3,370,919 issued to Pan on Feb. 27, 1968 discloses that 
control of the reaction of methane, ammonia and air to produce HCN is 
effected by measuring the reaction or flame temperature and/or off-gas 
temperature produced at a given air/(CH.sub.4 + NH.sub.3) ratio and 
adjusting the CH.sub.4 /NH.sub.3 ratio until a minimum reaction or flame 
temperature and/or a minimum off-gas temperature is obtained. Yield of HCN 
is said to be maximized at the minimum reaction or flame temperature while 
conversion of HCN is said to maximize at the minimum off-gas temperature. 
Additional details on this process can be found in an article by B. Y. K. 
Pan and R. G. Roth entitled "Optimization of Yield Through Feed 
Composition," I & EC Process Design and Development, Vol. 7, No. 1, pages 
53-61 (January 1968). A further article on maximizing yield in the HCN 
process authored by B. Y. K. Pan and entitled "Elaboration and Extension 
of Experimental Results Through Mathematical correlations and Fundamental 
Knowledge" can be found in I & EC Process Design and Development, Vol. 8, 
NO. 2, pages 262-266 (April 1969). 
In practice the process disclosed by Pan has not provided optimum HCN 
production in all Andrussow-type processes. In some such processes, 
controlling the reaction at minimum reaction or flame temperature has not 
resulted in optimum HCN production; increased production of HCN can be 
obtained at temperatures other than minimum bed temperature. Furthermore, 
in many HCN converters the reaction or flame temperature will vary 
considerably across the gauze catalyst, the uneven heating thus creating 
problems in the use of the Pan technique. 
SUMMARY OF THE INVENTION 
An improved process for producing HCN by contacting ammonia, a hydrocarbon 
and an oxygen-containing gas has been developed. 
This process is based on determining HCN production by varying the air-gas 
feed ratio within limits while maintaining the air-ammonia feed ratio 
constant. Once the maximum production rate is obtained, the process feed 
ratios are maintained at the corresponding settings. Specifically, the 
process involves: 
(1) feeding to the converter a ratio of oxygen-containing gas, e.g., air, 
to hydrocarbon gas, e.g., natural gas, within the range of 4.5:1 to 5.5:1, 
the flow of air being at its maximum rate, as determined by the physical 
constraints, 
(2) feeding to the converter a ratio of air to ammonia within the range of 
3.0:1 to 9.0:1, 
(3) measuring the converter off-gas to determine the weight percent of HCN 
present at the selected ratios, 
(4) varying the natural gas feed while maintaining the air to ammonia ratio 
constant, i.e., either increasing or decreasing the amount of natural gas, 
(5) continuing to vary the natural gas feed until the highest weight 
percent of HCN in the off-gas is obtained, and 
(6) maintaining the natural gas feed at the rate corresponding to the 
highest weight percent of HCN in the off-gas.

DETAILED DESCRIPTION OF THE INVENTION 
In the process of the invention HCN is produced by contacting in a vapor 
phase, ammonia, a hydrocarbon and an oxygen-containing gas in the presence 
of a platinum-group metal catalyst at a high temperature and controlling 
the conversion of NH.sub.3 and production of HCN as set forth herein. 
Suitable hydrocarbons, as disclosed in U.S. Pat. No. 1,934,838, include 
aliphatic, cycloaliphatic and aromatic hydrocarbons. Methane or a 
methane-containing gas, such as natural gas, is a preferred hydrocarbon 
for the present process. Ammonia used in the process of the invention is 
substantially pure ammonia. Suitable oxygen-containing gases include air 
and oxygen-containing inert gases having an oxygen content roughly 
equivalent to that of air. In the following paragraph the invention will 
be discussed in terms of the usual commercial process which employed 
natural gas, air and ammonia; however, it should be understood that the 
invention is not so limited. 
The platinum-group metal catalyst employed in the process of the invention 
is selected from the group consisting of platinum, rhodium, iridium, 
palladium, osmium, ruthenium, and a mixture or alloy of two or more of the 
foregoing metals. The platinum-group catalyst can be employed in the form 
of sheets, wires, turnings, etc., the preferred form being one or more 
layers of a fine wire gauze. The metal can also be used in the form of a 
coating on an inert substrate, such as beryl (beryllium aluminum 
silicate), alumina, sillimanite, etc. 
In the process of the invention the gaseous ammonia, natural gas and air 
are fed into a converter containing a bed of the previously described 
catalyst, which is maintained at a temperature of from about 1000.degree. 
C.-1200.degree. C. 
The feed of air into the converter should be set at the maximum flow 
obtainable for its design, i.e., the blowers, converter design, i.e., 
converter diameter, and process interlocks will determine this rate. It is 
known that increased air flow increases ammonia conversion to HCN; thus 
when operating to obtain production rates greater than 65% of the maximum, 
maximum air flow should be maintained. During period of low production 
rates, it may be desirable to use less than maximum air flow. 
The initial air to natural gas ratio should be in the range of 4.5:1 to 
5.5:1; this ratio will subsequently be adjusted by adding or decreasing 
natural gas to achieve maximum production. 
It is preferred that air and natural gas be preheated prior to being mixed 
with the ammonia and fed into the converter. Such preheating is described 
in detail in U.S. Pat. No. 3,104,945 to Jenks et al. When preheating is 
employed, the gases are usually heated to temperatures between 300.degree. 
and 500.degree. C., preferably 450.degree.-490.degree. C. In some 
embodiments ceramic materials of construction can be employed and the gas 
may be preheated up to temperatures of 1100.degree. C. 
The air to ammonia ratio is then set between 3.0:1 to 9.0:1. The ratio used 
is based on prior experience and will usually be in the range of 3.0:1 to 
6.0:1. If preheat is employed, the range will usually be between 3.3:1 to 
5.0:1. With preheating additional ammonia can be added which in turn 
increases HCN production and lowers the reaction temperature, i.e., bed 
temperature. In most embodiments of the process of the invention it will 
be desirable to set the ammonia feed as high as possible. The constraints 
on ammonia feed are, first, ammonia cools the reaction or bed temperature 
and this cannot be cooled below the minimum reaction temperature and 
second, unreacted ammonia can cause an environmental problem if present to 
a large extent in the converter effluent. 
The air to ammonia ratio is fixed at selected setting and thereafter 
constantly maintained at this ratio. The maximum HCN production is then 
determined for this fixed ratio. If this ratio is ever changed, the entire 
sequence of steps must be conducted to determine the peak production feed 
ratios for the new air/ammonia ratio. 
In keeping the air/ammonia ratio constant, it is important that a constant 
mass flow of air and ammonia be fed into the converter. This can be 
accomplished by measuring the pressure and temperature of these feeds and 
using these measurements to maintain constant mass flow. One method of 
doing this is to continuously monitor the pressure and temperature and 
employ a computer to adjust the controller to maintain constant mass flow. 
The output of the converter is then sampled by an on-line process analyzer 
to determine the volume percent of HCN in the reactor off-gas. On-line 
process analyzers that can be used are gas chromatographs or infra-red 
analyzers. 
The volume percent is then converted into weight percent by any appropriate 
means, e.g., by measuring the density directly using a density meter or by 
using an experimentally determined average molecular weight at the 
off-gas. The resulting weight percent of HCN times the off-gas flow is the 
production for the existing feed conditions. 
The air to natural gas ratio is then varied to determine the HCN production 
at a different air/gas ratio for comparison. The ratio is changed by 
changing the rate of natural gas feed, by either adding or reducing 
natural gas. 
The weight percent of HCN in the off-gas is then determined for the new gas 
feed rate. This result is then compared with the prior one. This process 
is repeated until the results indicate the feed rate wherein the maximum 
amount of HCN is being produced. The natural gas is then maintained at the 
feed rate corresponding to the maximum HCN production. As set forth 
previously the optimum point will change depending on various factors such 
as throughput and catalyst age, accordingly the process will have to be 
repeated at some interval to maintain maximum production. The interval to 
be employed will be based on experience with the particular converter 
involved, and may vary from a few hours to 24 hours or more. 
FIG. 1 illustrates a typical operation of the process of the invention. The 
HCN production is first determined for a specific air/gas ratio (1) on the 
Figure. Natural gas feed is decreased, and a second reading (2) of HCN 
production is obtained. Result (2) is compared with (1) and further 
decreases of natural gas are made (3), and (4), until the comparison 
indicates that the production peak has been passed. In this case an 
additional change was made by increasing the natural gas feed (5) and the 
peak identified. In some instances, it will be desirable to pass over the 
peak at least two times to reduce the possibility of locating a false 
maximum. This embodiment is illustrated in FIG. 2 and the Example. 
The identification of the peak can be accomplished manually by plotting or 
the like; however, one of the advantages of the process is that it lends 
itself to computer operation. Thus, a computer with a suitable program can 
be employed to determine HCN production and adjust the air/gas ratios 
until the production peak is obtained. 
The off-gas from the converter is treated in conventional ways in order to 
recover the hydrogen cyanide produced; or if desired, it can be further 
treated to make alkali or other metal cyanides. One further advantage of 
the process of the invention is that by improving HCN production, less 
nitriles and other byproducts will be produced, thus reducing separation 
problems in these subsequent steps. 
The process of the invention is further illustrated by the following 
example. 
EXAMPLE 
Air and natural gas are preheated to 455.degree. C. and fed into a 
converter containing a platinum-rhodium aloy catalyst (10% rhodium and 90% 
platinum). The catalyst is maintained at a temperature between 
1100.degree. and 1150.degree. C. The converter is circular in cross 
section and has a diameter of 6 feet. 
The air/natural gas ratio is initially set at 5.05. The flow rate of air is 
9170 scfm. 
Ammonia is fed into the converter to obtain an air to ammonia ratio of 
4.80:1. 
The reactor off-gas is analyzed and the volume percent of HCN present is 
determined. The weight percent of HCN is then found using a predetermined 
average molecular weight and converted into lbs. of HCN per hour and 
plotted on FIG. 2 at point (1). 
While maintaining air to ammonia ratio constant, natural gas feed is 
decreased to provide three additional HCN production readings, (2), (3) 
and (4). The natural gas feed was then increased to provide readings (5) 
and (6). A final reading (7) was made at an air to gas ratio of 5.15 and 
the peak was identified, i.e., approximately at an air/gas ratio of 5.14. 
The natural gas feed is maintained at this rate, and maximum HCN 
production is obtained. 
The bed temperature was also measured for the various air/gas ratios 
compared. The readings are indicated by the dotted line on FIG. 2. This 
plotting indicates that for the particular converter involved and the 
air/ammonia ratio employed; minimum bed temperature does not correspond to 
maximum HCN production.