Method and Apparatus for Removing Hydrogen Cyanide, Cyanogen and Cyanogen Chloride From Air

The following description and examples provide details of the manner in which the embodiments of the present invention can be made and used to effectively remove cyanogen chloride, hydrogen cyanide and cyanogen from air without the presence of chromium. The examples shown are based on actual experimental work. While exemplary of the present invention, the examples should not be construed as specifically limiting the invention and such variations which would be within the purview of one skilled in the art are to be considered to fall within the scope of the invention.

Activated carbon was impregnated with copper in combination with molybdenum and/or vanadium according to the formulae reported by Grabenstetter and Blacet, op.cit., as shown in TABLE 1.

2.25 liters of the impregnant solution shown in TABLE 1 were used to treat 750 gm of carbon. Two groups were prepared (Sample Sets I, II). For each sample set, the carbon was soaked in an appropriate amount of solution, drained, and dried in an oven at 220.degree. F. Samples 83-135 to 83-138 in Set I were heat treated in a fluidized bed at 375.degree. F. and 575.degree. F. in an atmosphere of simulated flue gas. Samples 83-151 to 83-154 in Set II were prepared as type ASVT (including vandadium with tartaric acid). Samples were also prepared with vanadium and molybdenum but without the tartaric acid additive. After drying, the products were heat treated in a fluid bed at 400.degree. F., in simulated flue gas, for 10 minutes. Half of these products received an additional fluid bed treatment in air at 260.degree.-270.degree. F. Samples made with and without air treatment were also impregnated with 6% triethylenediamine (TEDA). Air treatment was done as a supplemental operation at relatively low temperatures because the molybdenum and vanadium impregnated carbons were found to be susceptible to ignition during principal heat treatment.

Tube life results measured for hydrogen cyanide are shown in TABLE 2 for the members of sample Sets I and II. The addition of molybdenum and vanadium appeared to supplement the action of copper, and the performance of the products was significantly improved over that of activated carbon impregnated with copper and silver alone (type AS) and compared favorably with chromium impregnated carbon (type ASC). Type AS carbon has an average life of 13-14 minutes compared with 30 minutes, for type ASC impregnates.

For type AS carbon it has also been found that some hydrogen cyanide is converted into the toxic gas cyanogen which escapes capture. Cyanogen is not readily detected by the standard hydrogen cyanide test using a sodium bicarbonate absorbent and iodine indicator. However addition of chromium prevents escape of cyanogen (Grabenstetter and Blacet, op.cit.,). It is reported that molybdenum and vanadium also prevent escape of cyanogen when in combination with copper. This was tested for the current experimental products using samples inpregnated with vanadium. A special test using sodium hydroxide absorbent and chloramine-T indicator, which is sensitive to both hydrogen cyanide and cyanogen, showed no evidence of cyanogen escape. The new formulations therefore protect against premature breakthrough of cyanogen under challenge with hydrogen cyanide. It may also be inferred that these filter materials will protect against cyanogen when challenged with that gas.

For unaged products, those made with vanadium (ASV) were consistently better than the ones using molybdenum (ASM), some of which would appear to meet the hydrogen cyanide specification life of 25 minutes. The average life of all products made without vanadium was 21 minutes while that for products containing only vanadium was 27 minutes.

The aged products which contain molybdenum gave hydrogen cyanide life results of over 30 minutes, a substantial improvement over the unaged analogs. These samples had been treated at 85% relative humidity at 113.degree. F. for 168 hours.

Similar tests for cyanogen chloride life effectiveness are shown in TABLE 3. Products containing TEDA demonstrated useful protective life. The products made with vanadium and 6% TEDA gave tube life results in the 45-60 minute range. This compares with the specification minimum of 40 minutes (in canister tests).

Ammonia desorption test results for the vanadium and molybdenum products from Sample Set II are shown in TABLE 4. The ASV and ASM carbons without TEDA released about 0.3-0.4 .mu.g NH.sub.3 /1/100 cc C. This is about 1/3 of the desorption for experimental ASC carbons made under similar conditions. Addition of 6% TEDA raised the apparent ammonia desorption of the new products, but it is still below the level typical of commercial type ASC products (about 2 .mu.g NH.sub.3 /1/100 cc C).

Based on test results for these samples, chromium free whetlerites can be made which meet hydrogen cyanide and cyanogen chloride specifications, show little or no detrimental effect of aging on cyanogen chloride life, and desorb less than half the amount of ammonia typically produced by conventional type ASC whetlerite.

Additional type ASVT carbons (Sample Set III) were prepared from activated carbon using the sorption contacting method ("dip-and-drain") as employed previously. Additional products were made using the addition ("spray-on") contacting method. For the latter, the impregnant solution formula was the same as that used for soaking, except that the amount of silver was increased to 0.000402 gm Ag/ml solution from the concentration of 0.000127 gm Ag/ml used in sorption. In the addition method, the solution was added as a spray to just wet the carbon without yielding any excess solution which could be drained. In the sorption method, three times this amount of solution was used in contacting. The solution loading on carbon was 1.0 ml/gm carbon for the addition method, a litter less than for sorption.

The impregnated products were heat treated in a fluidized bed at 450.degree. F. and subsequently impregnated with TEDA by spraying with 20 ml of an aqueous TEDA solution per 100 gm activated carbon. The solutions contained sufficient TEDA to yield ASVT products containing 2%, 4%, and 6% TEDA. These products were dried at 220.degree. F. in an oven and the results on hydrogen cyanide tube life are shown in TABLE 5.

The results indicate that contacting by sorption gave much better hydrogen cyanide life performance. It is likely that product which was soaked contains a little more (probably less than 10% more) metal. However, additional prior testing of standard ASC whetlerites showed no difference between these contacting methods. Nor were there any significant effects observed on the hydrogen cyanide life as a function of TEDA loadings.

It is believed that the improved cyanogen chloride activity is due to the presence of TEDA, and that cyanogen chloride life increases with TEDA loading. A loading of 6% TEDA gave a tube test life greater than 40 minutes for these samples. As a practical matter, however, a loading on the order of 4% may be sufficient to give satisfactory performance in canister life tests, especially since this product does not undergo aging as with type ASC whetlerite. The optimum TEDA loading will be that which gives good cyanogen chloride activity without interfering with sorption of other gases.

In order to test vanadium (ASVT) and molybdenum (ASTM) impregnated material, the activated carbon precursor was impregnated with silver, copper and vanadium or molybdenum by soaking. These products were dried at 220.degree. F., and then heat treated at 450.degree. F. in an atmosphere of steam and flue gas using a fluidized bed reactor. TEDA was applied by spraying on an aqueous solution to provide a 6% loading. For each whetlerite type, product from several individual runs was composited to make 5 pound demonstration samples.

The ammonia desorption results in TABLE 6 correspond to a test interval of 2 hours, which was run without any evidence of a colorimetric end-point. Previous tests have shown that valid end-points can be obtained for long test intervals using this method.

It has now been found that molybdenum and vanadium are alternatives to chromium, which offer similar catalytic properties with respect to HCN removal when used in combination with copper. These metals are reported to prevent breakthrough of cyanogen gas, and are not considered carcinogenic. Thus, a chromium-free product, about equivalent to conventional type ASC whetlerite in HCN protection, but improved in cyanogen chloride after aging can be made by impregnation of carbon with copper, silver, molydenum and/or vanadium and TEDA. Prepared under the conditions illustrated, these products also release only 10-50% of the ammonia odor typical of type ASC whetlerite according to the standard test.

Although only preferred embodiments are specifically illustrated and described herein, it will be appreciated that many modifications and variations of the present invention are possible in light of the above teachings and within the purview of the appended claims without departing from the spirit and intended scope of the invention.