Hydrogen cyanide synthesis process

Details Hydrogen cyanide synthesis process Hydrogen cyanide synthesis process

2001-07-13

2003-07-22

Silverman, Stanley S. (Department: 1754)

Chemistry of inorganic compounds

Nitrogen or compound thereof

Carbon containing

Reexamination Certificate

active

06596251

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an improvement of the Andrussow method for synthesis of hydrogen cyanide (HCN).
2. Description of the Related Art
The synthesis of hydrogen cyanide by the Andrussow method is described in Ullmann's Encyclopedia of Industrial Chemistry, Volume 8, VCH Verlagsgesellschaft, Weinheim, 1987, pp. 161-162. The educt gas mixture, which comprises methane or a methane-containing natural-gas stream, ammonia and oxygen is passed into a reactor over catalyst gauze and reacted at temperatures of about 1000° C. The necessary oxygen is usually introduced in the form of air. The catalyst gauzes comprise platinum or platinum alloys. The composition of the educt gas mixture corresponds approximately to the stoichiometry of the overall equation of the reaction
CH
4
+NH
3
+3/2 O
2
→HCN+3 H
2
O  dHr=−473.9 kJ,
which takes place exothermically.
The discharged reaction gas contains the product HCN, unreacted NH
3
and CH
4
, as well as important by-products CO, H
2
, H
2
O and CO
2
, and a large proportion of N
2
.
The reaction gas is cooled rapidly to about 150 to 200° C. in a waste-heat recovery boiler and then passed through a scrubbing column, in which the unreacted NH3 is removed with dilute sulfuric acid and some of the steam is condensed. Also known is the absorption of NH
3
with sodium hydrogen phosphate solution followed by recycling of the ammonia. HCN is absorbed in cold water in a downstream absorption column and then purified to better than 99.5 wt % by mass in a downstream rectification unit. The HCN-containing water present in the column bottoms is cooled and recycled to the HCN absorption column.
A broad spectrum of possible embodiments of the Andrussow method is described in German Patent 549055.
As an example, a catalyst comprising a plurality of fine-mesh gauze pieces of Pt with 10% rhodium disposed in series is used at temperatures of about 980 to 1050° C. The HCN yield is 66.1% based on NH
3
used.
A method for maximizing the HCN yield by optimal adjustment of the air
atural gas and air/ammonia ratios is described in U.S. Pat. No. 4,128,622.
In addition to the standard operating procedure with air as the oxygen supply, the use of oxygen-enriched air is described in various documents. Table 1 lists some patents with the operating conditions described therein.
U.S. Pat. No. 5,882,618 describes the synthesis of hydrocyanic acid by the Andrussow method using oxygen-enriched air.
To circumvent the problems that occur under these conditions, such as proximity to the explosion limits of the mixture of NH
3
, CH
4
and oxygen-enriched air, as well as the elevated temperature of the catalyst gauze, which can lead to yield losses and shortened catalyst life, the following measures are proposed:
In a first process step, the system is started up with air as the oxygen source. During this first process step, the catalyst mesh reaches a defined temperature.
In a second process step, oxygen is then metered in and, at the same time, the contents of ammonia and methane are adjusted such that the mixture is situated above the upper explosion limit and the catalyst temperature corresponds to within 50 K of the reference temperature determined in step
1
. The temperature of the catalyst gauze is about 1100° C. to 1200° C.
By means of this procedure, safe use of the system is achieved during operation with oxygen-enriched air.
International Patent WO 97/09273 overcomes the disadvantages of high N
2
dilution of the reaction gases by the use of preheated, mixtures of methane, ammonia and oxygen-enriched air or pure oxygen which are capable of detonation.
In order to be able to safely handle the mixtures that are capable of detonation, a special reactor is used that prevents detonation of the reaction mixture. The use of this solution in industrial practice necessitates intensive investment for converting existing HCN plants.
TABLE 1
List of various references with operating conditions
German Examined Application
German Patent 1283209, 1968,
1288575, 1968,
Società Edison
Società
International Patent WO
Netherlands Patent 6604519,
Edison
97/09273, 1997,
U.S. Pat. No. 5882618,
Belgian Patent 679440
Netherlands Patent 6604697,
ICI
1999,
corresponds to
U.S. Pat. No. 3,379,500 (italics)
Belgian Patent 679529
special reactor
Air Liquide
Educt gas
—
200 to 400° C.
200 to 400° C.
preheating
300 to 380° C.
further temperature data for
Gauze temperature
1100 to 1200° C.
1100 to 1200° C.
individual educt gas streams
1100 ± 50° C.
(O
2
+ N
2
)/CH
4
6.5 to 1.55
6.0 to 1.6
Ratios reported as relative
molar ratio
4.55 to 2.80
4.5 to 2.6
to the mode of operation
(O
2
+ N
2
)/NH
3
6.8 to 2.0
6.0 to 2.0
with air
molar ratio
4.8 to 3.65
4.5 to 3.0
CH
4
/NH
3
1.4 to 1.05
1.3 to 1.0
1.0 to 1.5
molar ratio
1.3 to 1.1
1.25 to 1.05
O
2
/(O
2
+ N
2
)
0.245 to 0.4
0.245 to 0.35
0.3 to 1.0
molar ratio
0.27 to 0.317
0.25 to 0.30
Disadvantages of the Related Art Regarding Operation With Air as the Oxygen Supply
If air is used as the oxygen supply in the starting-gas mixture, the HCN concentration in the reaction gas reaches only about 6 to 8 vol %. Because of establishment of equilibrium, the low HCN concentration in the reaction gas leads to a relatively low HCN concentration of 2 to 3 wt % by mass in the aqueous discharge stream from the sump of the HCN absorber column. Thus, high expenditure of energy is necessary for cooling and separating the large mass flow of absorption water. Furthermore, the high inert-gas content necessitates relatively large apparatus volume and substance streams in the working-up part of the process. Because of the dilution with nitrogen, the hydrogen content in the residual-gas stream is lower than 18 vol %. Thus the hydrogen cannot be economically isolated as a valuable product.
Disadvantages of the Related Art With Oxygen Enrichment in the Starting Gas
The known processes with oxygen enrichment of the educt gas (see Table 1) represent an improvement over the cited disadvantages of operating with air, but they also lead to other limitations. Examples are:
1. If the O
2
/NH
3
or O
2
/CH
4
educt ratios (vol/vol) of the starting gas are not adapted to the degree of enrichment with oxygen, the NH
3
/CH
4
/N
2
/O
2
mixture is not sufficiently far from the upper explosion limit, and safe operation of the reactor is no longer assured. Possible consequences are:
(a) danger of explosion
(b) danger of deflagration (damage to the catalyst gauze)
(c) danger of local temperature spikes, which damage the catalyst gauze.
2. The increased oxygen supply to the catalyst leads to increased oxidation of NH
3
to N
2
and thus to decrease of the HCN yield relative to the feed NH
3
.
3. In the known processes the degree of enrichment with oxygen is limited to an enrichment of up to about 40% O
2
in the oxygen-nitrogen mixture (German Patent 1283209, German Patent 1288575, U.S. Pat. No. 5,882,618).
4. Because of enrichment of the educt gas with oxygen, the catalyst gauze can reach a higher temperature, which leads to faster damage to and deactivation of the catalyst.
5. Possible solutions that counter the existing disadvantages with a specially constructed reactor (International Patent WO 97/09273) require large investments and are not suitable for increasing the performance of existing plants at favorable costs.
BRIEF SUMMARY OF THE INVENTION
One object of the invention was therefore to develop a procedure for performing the Andrussow process for synthesis of hydrogen cyanide with which, by extensive enrichment of the combustion air in existing plants to as much as 100 vol % of oxygen, there are ensured
increased HCN productivity (metric tons of HCN per hour), accompanied by
higher HCN yield relative to feed NH
3
and
lower energy consumption per metric ton of HCN as well as
long operating life of the catalyst gauze and
safe plant operation.
These and other objects are achieved by a process for synthesis of hydrogen cyanide comprising reacting methane or me

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