Process for removal of HCN from synthesis gas

Hydrogen cyanide is removed from an HCN containing gas, e.g., a gas containing CO+H.sub.2, by contact with a metal oxide catalyst comprised of the oxides of molybdenum, titanium, and aluminum in the presence of water vapor, and subsequently water washing the resulting gas.

FIELD OF THE INVENTION 
This invention relates to the removal of hydrogen cyanide, HCN, from 
synthesis gas streams. More particularly, this invention relates to the 
use of a composite catalyst for HCN removal from essentially sulfur free 
streams containing hydrogen and carbon monoxide. 
BACKGROUND OF THE INVENTION 
Synthesis gas, hydrogen and carbon monoxide, is used in a variety of 
hydrocarbon synthesis processes, e.g., Fischer-Tropsch. However, trace 
components that find their way into the synthesis gas are often poisons 
for hydrocarbon synthesis catalysts. An example of a trace component that 
poisons Fischer-Tropsch catalysts is hydrogen cyanide. This component is 
difficult to remove from synthesis gas because of its low solubility in 
common solvents, e.g., water, and because of its low concentration, 
usually less than about 100 ppm, removal by adsorption is difficult. Also, 
chemical removal by, for example, alkaline scrubbing, is hampered by the 
presence of other acidic materials, e.g., CO.sub.2. Consequently, a need 
exists for the economic removal of HCN from synthesis gas at levels of at 
least about 95%, and particularly at temperatures similar to those 
employed in the hydrocarbon synthesis step. 
SUMMARY OF THE INVENTION 
In accordance with this invention hydrogen cyanide is substantially removed 
from an HCN containing gas, preferably a synthesis gas, by contacting the 
gas at HCN removal conditions with a composite metal oxide catalyst 
containing the oxides of molybdenum and titanium, and alumina in the 
presence of water vapor. A gas of reduced HCN content, such as synthesis 
gas, is then produced for use in subsequent hydrocarbon synthesis 
reactions, such as the Fischer-Tropsch reaction to prepare C.sub.5 +, 
preferably predominantly C.sub.10 + hydrocarbons. Reaction conditions for 
HCN removal include elevated temperatures and elevated pressures, and at 
these conditions at least about 95% of the HCN contained in the synthesis 
gas feed stream is removed, preferably at least about 98%, more preferably 
at least about 99%, removal of HCN is obtained by this process. Subsequent 
to the contacting step, the gas is scrubbed with water to remove NH.sub.3 
originally present or as converted, for example, by hydrolysis, from HCN. 
Preferably, the catalyst is characterized by the absence or substantial 
absence of Group VIII metals or the compounds, e.g., oxides, thereof.

Curve A is always 100 (HCN out/HCN in), or % HCN conversion, Curve B is 
always temperature, Curve C is always ppm HCN, and Curve D is always ppm 
CH.sub.4. In all figures the left ordinate is 100 (HCN out/HCN in), the 
abscissa is days on stream, and the right ordinate is ppm methane or ppm 
HCN in effluent. 
The HCN removal process proceeds via the reaction of hydrogen cyanide with 
water: 
HCN+H.sub.2 O.fwdarw.NH.sub.3 +CO 
and whereas NH.sub.3, ammonia, is also a Fischer-Tropsch catalyst poison, 
NH.sub.3 can be readily removed, e.g., by a washing of the treated 
synthesis gas. 
Feed gases for the process generally and primarily contain hydrogen, carbon 
monoxide and water vapor, while small amounts of CO.sub.2 may also be 
present. The feed gas is essentially free of sulfur. That is, sulfur 
levels in the feed are generally about 10 ppm (wt) or less, preferably 
about 5 ppm or less, more preferably about 1 ppm or less, and most 
preferably less than about 50 wppb. 
In a preferred embodiment, the catalyst is treated with hydrogen at 
elevated temperatures and pressures, thereby causing an increase in 
catalytic activity. While the metal oxides utilized in this invention are 
generally known as difficulty reducible oxides, there is evidence that at 
least a portion of the oxides of both molybdenum and titanium are in 
reduced oxidation states. It is unlikely, however, that the hydrogen 
treatment leads to the reduction of either of these metals to the 
elemental state, and the catalyst is essentially, and preferably devoid of 
any elemental metal. The alumina, acting in the manner of a support, is 
likely not reduced at all, alumina being a particularly difficult oxide to 
reduce. 
The composite oxide catalyst generally contains &gt;0 to 30 wt % molybdenum as 
the oxide, at least a portion of which is in the reduced valence state, 
i.e., less than the valence of +6, preferably 10-20 wt % oxide of 
molybdenum; &gt;0 to about 30 wt % titanium as the oxide, at least a portion 
of which is in the reduced valence state, and preferably 4-20 wt %, more 
preferably 8-16 wt %, e.g., about 8 wt % of the oxide of titanium; the 
remainder being alumina. 
The catalyst may be treated with hydrogen, or a hydrogen containing stream, 
although the effects of hydrogen treatment are not always manifest. 
Because the materials of the composite catalyst are essentially difficulty 
reducible oxides, we believe that there is virtually no metal present in 
the zero valence state, e.g., less than 0.1 wt % zero valence metal. 
However, some of the molybdenum, in particular, and perhaps some of the 
titanium, may be in a reduced valence state after hydrogen treatment. When 
hydrogen treatment is effected, temperatures may range from about 
200-600.degree. C., preferably 230-550.degree. C. for periods of about 
1-24 hours. 
The catalyst of the present invention may be readily prepared by depositing 
suitable sources of molybdenum and titanium on an alumina support. After 
deposition, for example, by impregnation or incipient wetness techniques, 
the material is dried, e.g., overnight at about 100-150.degree. C., 
followed by calcination at temperatures of about 250-500.degree. C., 
preferably 350-450.degree. C. to produce the oxide form. 
Suitable sources of molybdenum include ammonium heptamolybdate, ammonium 
molybdate, molybdenum trioxide; while suitable titanium sources include 
titanium isopropoxide, titanium oxychloride, titanium sulfate, titanium 
chloride, potassium titanium oxalate and other similar sources well known 
to those skilled in the art. However, chloride containing materials are 
preferably avoided since chlorides are poisons for Fischer-Tropsch 
processes; the process of this invention is then preferably conducted in 
the absence of chlorides. Water washing can often reduce chlorides to 
acceptably low levels. 
The alumina may be any alumina useful as a catalyst support, including eta 
and gamma forms of alumina, and may have surface areas ranging from about 
100-400 m.sup.2 /gm. 
HCN removal can be carried out over a relatively wide temperature range, 
e.g., 150-400.degree. C. However, the preferred temperature range is that 
compatible with the subsequent process step, e.g., Fischer-Tropsch 
processing, preferably 170-250.degree. C., more preferably about 
170-250.degree. C., and still more preferably about 180-235.degree. C. 
Pressures are similarly wide ranging, e.g., 1-100 bar, although preferred 
pressures are in the range of 10-50 bar, more preferably 15-40 bar. 
The converted HCN and product NH.sub.3 can then be removed from the feed 
synthesis gas by any applicable method well known to those skilled in the 
art, for example, water scrubbing or absorption onto a solid absorbent. 
The synthesis gas of relatively low HCN concentration may then be employed 
in a Fischer-Tropsch hydrocarbon synthesis process using shifting or 
non-shifting catalysts. Preferred suitable Fischer-Tropsch catalysts 
include non-shifting Group VIII metals, preferably cobalt or ruthenium in 
bulk or supported form. In supported form, supports may be silica, 
alumina, silica-alumina, or titania. Promoters may also be employed, e.g., 
zirconium, rhenium, hafnium, etc. 
The process of this invention will be more fully appreciated by the 
following examples which serve to illustrate, but not limit, the 
invention. 
CATALYST TESTING 
The following general procedure was used for testing Catalysts A and C: A 
weighed amount of catalyst was mixed with 2 cc (about 2.7 gms) of 
14.times.35 mesh crushed inert material (Denstone). This was placed in a 
0.4 inch I.D. tubular stainless steel reactor. The catalyst/diluent was 
supported by a plug of Pyrex wool at the bottom of the reactor. On top of 
this bed, 4 cc (about 5.4 gms) of 14.times.35 mesh crushed Denstone was 
placed. A thermocouple was inserted into the bed for temperature control. 
The charged reactor was then placed into a vertically mounted infrared 
furnace and connected to supply and withdrawal tubing. The flow path was 
downflow, first through the diluent layer and then to the catalyst/diluent 
bed. Product analysis was by gas chromatography. HCN conversion was 
determined by comparison vs. an internal standard. Gases were supplied by 
electronic mass flow controllers. Water, HCN, and internal standard were 
supplied via a liquid feed pump. The liquid feed was mixed with the gas 
feed and vaporized in the upper section of the reactor. 
All of the catalysts described below were prepared using an extruded 
alumina support. The properties of this support were: 
______________________________________ 
Surface Area 169 m.sup.2 /gm 
Pore Volume (H2P) 0.725 ml/gm 
Compacted Bulk Density 0.50 gm/ml 
Median Pore Diameter (Hg) 13 mn 
Shape Asymmetric Quadralobe 
Major Diameter 1.40 mm 
Minor Diameter 1.14 mm 
Average Extrudate Length 4.6 mm 
Non-volatile matter (%) 90.7 
______________________________________ 
The catalysts were prepared as follows: 
Catalyst A 
Ammonium heptamolybdate (22.229 gms) and citric acid (30.5 gms) were 
dry-mixed in a 300 ml flask. To this was added 60 ml of deionized water. 
The solution was mixed with mild heating until it was clear. Deionized 
water was then added to a final volume of 83.8 ml. This solution was added 
to 110.25 gms of the alumina extrudates. The flask was stoppered and 
allowed to sit overnight. It was then dried at 120.degree. C. in flowing 
air for 4 hours and then calcined in air for 2 hours at 1000.degree. F. 
Catalyst B 
Titanium Isopropoxide (42.028 gms) was dissolved with 30 ml of isopropyl 
alcohol in a 300 ml flask. Isopropyl alcohol was then added to bring the 
solution volume to 88 ml. Then 110.25 gms of the alumina extrudates were 
added to the solution in the flask. The flask was stoppered and shaken 
until all of the extrudates were wet. The wet extrudates were then put 
into an evaporation dish and dried in air at room temperature for 2 hours. 
The air dried extrudates were then further dried in an oven overnight. The 
extrudates were then calcined in air for 2 hours at 1000.degree. F. 
Titanium isopropoxide (26.849 gms) was dissolved in isopropyl alcohol to 
give a total solution volume of 78 ml. To this, 97.2 gms of the above 
calcined extrudates were added and the flask stoppered. The flask was 
shaken until all of the extrudates were wet. The wet extrudates were then 
put into an evaporation dish and dried in air at room temperature for 2 
hours. The air dried extrudates were then further dried in an oven at 
120.degree. C. in flowing air overnight. The extrudates were then calcined 
in air for 2 hours at 1000.degree. F. to give Catalyst B. 
Catalyst C 
Ammonium molybdate (9.704 gms) and citric acid (12.8 gms) were dry-mixed in 
a 250 ml flask. To this was added 25 ml of deionized water. The solution 
was stirred with mild heating until it was clear. Deionized water was then 
added to give a final solution volume of 42 ml. While the solution was 
still warm, 50.0 gms of dry Catalyst B was added to the flask. The flask 
was stoppered and shaken until all of the catalyst particles were wet. The 
stoppered flask was allowed to sit overnight. The catalyst then was put in 
an evaporation dish and dried in flowing air at 120.degree. C. for 4 
hours. The dried catalyst was then calcined at 1000.degree. F. in air for 
2 hours to give Catalyst C. 
Catalyst D 
COMATIVE EXAMPLE 
To activated alumina (LaRoche Chemicals A-2, surface area 299 m.sup.2 /gm, 
0.65 gm/ml bulk density, 12.times.32 mesh) sufficient titanium oxychloride 
was added to give 10% by weight TiO.sub.2. This material was calcined in 
air for 3 hours at 751.degree. F. This material was then impregnated to 
incipient wetness with a water solution of ammonium heptamolybdate to give 
a loading of 7.5 wt % Mo03. This material was dried in air at 220.degree. 
F. and then calcined in air at 751.degree. F. for 3 hours. This material 
was then crushed to &lt;150 microns to give Catalyst D. 
Example 1 
Catalyst A Without Prior Reduction 
Catalyst A (1.08 gms, 2 cc) was charged as described above to the reactor. 
Gas flows were established and the reactor temperature was set at 
450.degree. F. Liquid flow was then established. The operating conditions 
were: 
______________________________________ 
Feed Gas Composition 
______________________________________ 
Hydrogen 44.1 mole % 
Carbon Dioxide 8.3 mole % 
Water 19.1 mole % 
Argon 9.1 mole % 
Carbon Monoxide 18.8 mole % 
HCN 637 ppm 
Pyrrole (Internal Std) 50 ppm 
Pressure 320 psig 
GHSV 10360 1/hr 
______________________________________ 
Results of this operation are shown in the FIG. 1. HCN conversion improved 
with time on stream. Increasing the temperature to 650.degree. F. resulted 
in HCN conversion of &gt;99%. A portion of this activity was retained when 
the temperature was reduced back to 450.degree. F., suggesting the 
catalyst activation was not complete at the end of the first 450.degree. 
F. period. At 450.degree. F. the methane content of the product gas was 
about 5 ppm, comparable to the methane observed at 450.degree. F. with 
only inert Denstone in the reactor (blank run). Methane at 650.degree. F. 
was about 8 ppm, showing a net methane make of 3 ppm. 
Example 2 
Catalyst A With Prior Reduction by Hydrogen at 850.degree. F. 
Catalyst A (1.08 gms, 2 cc) was charged as described above to the reactor. 
Hydrogen flow was established (9435 GHSV) and the temperature increased to 
850.degree. F. The reactor was held for two hours at this condition. The 
pressure averaged 158 psig. At the end of this period, the reactor was 
cooled under flowing hydrogen to 450.degree. F. and the pressure increased 
to 320 psig. The color of the catalyst changed from pale yellow to black 
indicating a lowered oxidation state of at least a portion of the 
molybdenum. The other gas flows and liquid flow were then established. The 
operating conditions were: 
______________________________________ 
Feed Gas Composition 
______________________________________ 
Hydrogen 43.0 mole % 
Carbon Dioxide 8.5 mole % 
Water 19.0 mole % 
Argon 9.5 mole % 
Carbon Monoxide 19.3 mole % 
HCN 640 ppm 
Pyrrole (Internal Std) 50 ppm 
Pressure 320 psig 
GHSV 10590 1/hr 
______________________________________ 
The catalyst showed very high HCN conversion immediately, in contrast with 
the previous example where a "break-in" period was apparent. Furthermore, 
the activity at 450.degree. F. was significantly improved. In Example 1 
the best HCN conversion at 450.degree. F. was 91.8%. In this example HCN 
conversion at 450.degree. F. was 99.1%. Activity maintenance was also 
good. The final test temperature was 450.degree. F. and HCN conversion was 
as high as during the initial part of the test at 450.degree. F. 
Example 3 
Catalyst A With Prior Reduction by Hydrogen at 1000.degree. F. 
Catalyst A (1.08 gms, 2 cc) was charged as described above to the reactor. 
Hydrogen flow was established (9330 GHSV) and the temperature increased to 
1000.degree. F. The reactor was held for 2 hours at this condition. The 
pressure averaged 152 psig. At the end of this period, the reactor was 
cooled under flowing hydrogen to 400.degree. F. and the pressure increased 
to 320 psig. The color of the catalyst changed from pale yellow to black, 
indicating a change in the oxidation state of at least a portion of the 
molybdenum. The other gas flows and liquid flow were then established. The 
operating conditions were: 
______________________________________ 
Feed Gas Composition 
______________________________________ 
Hydrogen 43.8 mole % 
Carbon Dioxide 8.5 mole % 
Water 19.1 mole % 
Argon 9.4 mole % 
Carbon Monoxide 19.3 mole % 
HCN 670 ppm 
Pyrrole (Internal Std) 50 ppm 
Pressure 320 psig 
GHSV 10540 1/hr 
______________________________________ 
The results are shown in the figure. Similar to Example 2, the catalyst 
immediately showed good HCN conversion. The activities at all temperatures 
were marginally higher than in Example 2. Consistent with Examples 1 and 
2, net methane make was very low. 
A comparison of Example 1 with Examples 2 and 3 (in Table I) clearly shows 
prior reduction with hydrogen improves catalyst activity and that hydrogen 
reduction is preferred. A comparison of Examples 2 and 3 shows a small 
benefit for increasing reduction temperature to 1000.degree. F. from 
850.degree. F. 
Example 4 
Catalyst C With Prior Reduction by Hydrogen at 850.degree. F. 
Catalyst C (1.14 gms, 2 cc) was charged as described above to the reactor. 
Hydrogen flow was established (9460 GHSV) and the temperature increased to 
850.degree. F. The reactor was held for 2 hours at this condition. The 
pressure averaged 155 psig. At the end of this period, the reactor was 
cooled under flowing hydrogen to 400.degree. F. and the pressure increased 
to 320 psig. The color of the catalyst changed from pale yellow to black, 
indicating a change in the oxidation state of at least a portion of the 
molybdenum. The other gas flows and liquid flow were then established. The 
operating conditions were: 
______________________________________ 
Feed Gas Composition 
______________________________________ 
Hydrogen 43.8 mole % 
Carbon Dioxide 8.5 mole % 
Water 19.1 mole % 
Argon 9.4 mole % 
Carbon Monoxide 19.3 mole % 
HCN 670 ppm 
Pyrrole (Internal Std) 50 ppm 
Pressure 320 psig 
GHSV 10550 l/hr 
______________________________________ 
Similar to Example 2, the catalyst immediately showed good HCN conversion. 
The activities at all temperatures were significantly higher than in 
Example 2. Consistent with Examples 1 and 2, net methane make was very 
low. This example shows the combination of titania and molybdenum oxide on 
an alumina support provides a particularly active catalyst for HCN removal 
from syngas. Comparison of the first and second periods at 400.degree. F. 
shows good activity maintenance, with only a slight loss occurring over 
the test period. 
Example 5 
Catalyst D Without Prior Reduction 
Catalyst D (0.4625 gms, 0.5 cc) was mixed with crushed high purity tubular 
alpha alumina (10.72 gms, 5.5 cc). This was placed in a 0.4 inch I.D. 
tubular stainless steel reactor. The catalyst/diluent was supported by a 
plug of Pyrex wool at the bottom of the reactor. A thermocouple was 
inserted into the bed for temperature control. The charged reactor was 
then placed into a vertically mounted infrared furnace and connected to 
supply and withdraw tubing. The flow path was downflow through the 
catalyst/diluent bed. Product analysis was by gas chromatography. HCN 
conversion was determined by comparison vs. an internal standard. Gases 
were supplied by electronic massflow controllers. Water, HCN, and internal 
standard were supplied via a liquid feed pump. The liquid feed was mixed 
with the gas feed and vaporized in the upper section of the reactor. The 
operating conditions were: 
______________________________________ 
Feed Gas Composition 
______________________________________ 
Hydrogen 53.7 mole % 
Carbon Dioxide 6.4 mole % 
Water 18.2 mole % 
Argon 8.4 mole % 
Carbon Monoxide 13.3 mole % 
HCN 200 ppm 
Pyrrole (Internal Std) 500 ppm 
Pressure 320 psig 
GHSV 41000 l/hr 
______________________________________ 
The results are shown in the table. Very good HCN removal activity was 
achieved. HCN conversions were lower than those in Examples 1-4, but this 
is due to the much higher space velocity (41000 vs. 10500) in this 
Example. This example shows the benefit of higher alumina support surface 
area (299 m.sup.2 /gm vs. 169 m.sup.2 /gm for Catalysts A-C) and smaller 
particle size (&lt;150 micro vs. 1/20" extrudates for Catalysts A-C). 
Example 6 
Catalyst B With prior Reduction by Hydrogen at 850.degree. F. 
Catalyst B (0.98 gm, 2 cc) was mixed with 2 cc (about 27 gms) of 
14.times.35 mesh crushed inert material (Denstone). This was placed in a 
0.4 inch I.D. tubular stainless steel reactor. The catalyst/diluent was 
supported by a plug of Pyrex wool at the bottom of the reactor. On top of 
this bed, 4 cc (about 5.4 gms) of 14.times.35 mesh crushed Denstone was 
placed. A thermocouple was inserted into the bed for temperature control. 
The charged reactor was then placed into a vertically mounted infrared 
furnace and connected to supply and withdraw tubing. The flow path was 
downflow, first through the diluent layer and then to the catalyst/diluent 
bed. Product analysis was by gas chromatography. HCN conversion was 
determined by comparison vs. an internal standard. Gases were supplied by 
electronic mass flow controllers. Water, HCN, and internal standard were 
supplied via a liquid feed pump. The liquid feed was mixed with the gas 
feed and vaporized in the upper section of the reactor. 
Hydrogen flow was established (9830 GHSV) and the temperature increased to 
850.degree. F. The reactor was held for 2 hours at this condition. The 
pressure averaged 90 psig. At the end of this period, the reactor was 
cooled under flowing hydrogen to 450.degree. F. and the pressure increased 
to 320 psig. The other gas flows and liquid flow were then established. 
The operating conditions were: 
______________________________________ 
Feed Gas Composition 
______________________________________ 
Hydrogen 43.8 mole % 
Carbon Dioxide 8.5 mole % 
Water 18.6 mole % 
Argon 9.7 mole % 
Carbon Monoxide 19.4 mole % 
HCN 670 ppm 
Pyrrole (Internal Std) 50 ppm 
Pressure 320 psig 
GHSV 10830 l/hr 
______________________________________ 
The results are shown in the figure. As can be seen, HCN conversion 
initially decreased with time, finally stabilizing at about 70%. 
Table I below shows a compilation of the results of Examples 1-6. Example 
4, using a molybdenum-titania on alumina catalyst showed the best HCN 
reduction at the lowest temperature, i.e., the highest activity catalyst. 
TABLE I 
______________________________________ 
Example 1 2 3 4 5 6 
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Catalyst A A A C D B 
Reduction -- 850 1,000 850 -- 850 
Temperature, 
.degree. F. 
(.degree. C.) (454.4) (537.8) (454.4) -- (454.4) 
Reduction, -- 9,440 9,330 9,460 -- 9,830 
GHSV 
Reaction, 10,380 10,590 10,540 10,520 41,000 10,830 
GHSV 
HCN 
Conversion 
(%) at 
Temperature, 
.degree. F. (.degree. C.) 
650 (343.5) 99.3 -- -- -- -- -- 
550 (287) -- -- -- -- 93.7 -- 
450 (232) 91.8 99.1 99.2 -- -- 69.1 
400 (205) -- 92.4 95.6 99.4 -- -- 
375 (190.5) -- -- -- 95 -- -- 
350 (176.5) -- 59.7 63.4 83 67.8 -- 
325 (163) -- -- -- -- 55.7 -- 
300 (150) -- -- -- -- 42.7 -- 
First Order 
Rate Constant 
(1/hr) at 
Temperature, 
.degree. F. (.degree. C.) 
650 (343.5) 5,381 -- -- -- -- -- 
550 (287) -- -- -- -- 10,811 -- 
450 (232) 2,253 4,374 4,436 -- -- 1,103 
400 (205) -- 2,239 2,703 4,359 -- -- 
375 (190.5) -- -- -- 2,509 -- -- 
350 (176.5) -- 743 819 1,441 3,540 -- 
325 (163) -- -- -- -- 2,469 -- 
300 (150) -- -- -- -- 1,633 -- 
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