Hazardous or toxic waste destruction or containment – Containment – Solidification – vitrification – or cementation
Reexamination Certificate
2000-08-04
2002-01-29
Griffin, Steven P. (Department: 1754)
Hazardous or toxic waste destruction or containment
Containment
Solidification, vitrification, or cementation
C588S249000, C423S220000, C423S244090, C423S523000, C423S563000, C423S564000, C423S443000
Reexamination Certificate
active
06342651
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to the oxidation of ammonium salts of sulfuric acid contained in aqueous media; and, more particularly, to a reductive combustion process which produces a combustion gas containing a divalent sulfur compound such as hydrogen sulfide and/or carbonyl sulfide, and further to the conversion of such divalent sulfur compound to other useful sulfur products.
Various industrial processes produce aqueous by-product streams comprising ammonium salts of sulfuric acid. For example, Ruest U.S. Pat. No. 4,524,077 and Hernandez U.S. Pat. No. 4,912,257 both describe processes for the preparation of 2-hydroxy-4-methylthiobutanoic acid (HMBA) by sulfuric acid hydrolysis of 2-hydroxy-4-methylthiobutanenitrile (HMBN). In each process, an aqueous hydrolyzate is produced comprising HMBA and ammonium bisulfate. In Ruest, the aqueous hydrolyzate is extracted with a substantially water-immiscible solvent for recovery of the product HMBA. Raffinate from the extraction is stripped for recovery of solvent, producing a bottoms fraction which comprises ammonium bisulfate. Depending on hydrolysis conditions, the raffinate stripper bottoms may also contain some ammonium sulfate or free sulfuric acid.
In Hernandez, the hydrolyzate is neutralized with ammonia, causing separation of an organic phase containing HMBA from an aqueous phase containing ammonium sulfate.
The by-product ammonium salt solutions produced in the Ruest and Hernandez process generally lack economic value, and must be disposed of in some manner. U.S. Pat. Nos. 5,498,790 and 5,670,128 describe processes for the regeneration of sulfuric acid from the by-product ammonium salt solutions, and recycle of the regenerated acid for further hydrolysis of HMBN to HMBA. Processes are known for the recovery of ammonium sulfate for use in fertilizers or other applications. However, such processes are complicated, and the market value of ammonium sulfate is generally not sufficient for recovery of the processing costs.
Other processes which produce by-product ammonium salts of sulfuric acid include the preparation of caprolactam and the preparation of methyl methacrylate. Sulfuric acid regeneration processes have been proposed for treating the by-product salt solutions in these instances as well. See, for example, U.S. Pat. Nos. 3,549,320 and 4,490,347 directed to the treatment of waste ammonium sulfate solution produced in the preparation of methyl methacrylate.
In all the sulfuric acid regeneration processes, a solution or slurry of by-product ammonium salt, or solid particulate salt, is introduced together with fuel into a combustion furnace wherein the salt is pyrolyzed to produce a combustion gas comprising sulfur dioxide, carbon dioxide, water vapor, nitrogen, excess oxygen, and typically oxides of nitrogen. Ammonia released from the salt is burned in the process to yield water vapor and nitrogen. After the gas stream has been cooled and cleaned, it is typically passed to a converter in which sulfur dioxide is catalytically converted to sulfur trioxide. Absorption of sulfur trioxide in sulfuric acid yields concentrated sulfuric acid which may be recycled or otherwise used or sold. Because the object of the sulfuric acid regeneration processes is to produce a gas stream containing both SO
2
and O
2
for further oxidation in the catalytic converter, all these processes introduce at least a slight excess of oxygen, typically in the form of air.
German Offlengungsschrift 197 54 562 A1 describes an alternative process for the recovery of sulfuric acid from sulfur-containing secondary products of a process for the preparation of HMBA. In that process, an aqueous mixture comprising ammonium sulfate or ammonium bisulfate is introduced into a combustion zone and burned to produce a combustion gas containing sulfur dioxide. The cooled combustion gas is contacted with a hydrogen peroxide solution to produce sulfuric acid. The acid produced can be used for the hydrolysis of HMBN to HMBA.
Certain of the known sulfuric acid regeneration processes use two stage combustion as a means to reduce the concentrations of oxides of nitrogen in the combustion gas leaving the combustion operation. Such operation is described, for example, in U.S. Pat. Nos. 5,498,790 and 5,670,128 wherein the first stage is operated with a slight deficiency of air so that nitrites, ammonia, and any amines contained in the sulfate feed solution are oxidized to nitrogen gas and carbon oxides but not to oxides of nitrogen. The partial combustion gas leaving the first stage contains unburned combustibles and carbon monoxide. To fully convert the carbon, hydrogen and sulfur content of combustibles and carbon monoxide to carbon dioxide, water, and SO
2
, oxidizing conditions are established in the second stage of the combustion by further injection of air. The sum of the air provided to the two stages is preferably sufficient to provide an oxygen content between about 0.5% and about 5% in the gas leaving the combustion chamber.
BASF DE Offenlegungsschrift 41 01 497 describes a process for thermal workup of waste water containing ammonium sulfate by metering the waste water through a nozzle/burner system, consisting of a burner and a centrally located atomization nozzle, into an adiabatic combustion chamber using a supporting fuel and a suitable oxidizing agent, typically air. Two stage combustion is carried out first at reducing, then at oxidizing conditions. In the first stage, aqueous ammonium sulfate is burned reductively to produce hydrogen sulfide in a combustible mixture of reducing gases. Nitrogen bound in the ammonium sulfate is mainly reacted to nitrogen (N
2
) and not to nitrogen oxides. In the second stage additional air is added to facilitate complete combustion of the reduced species in the offgas, H
2
S being converted to SO
2
. The process is said to be characterized by the fact that with reducing and/or oxidizing reaction conditions at a temperature between 600° C. and 2000° C., particularly at temperatures between 900° C. and 1100° C., the nitrogen bound in the ammonium sulfate is mainly reacted to N
2
with the simultaneous recovery of gaseous sulfur compounds (SO
2
, SO
3
, COS, H
2
S, etc.). In Example 1 of the BASF patent, a 10% by weight ammonium sulfate solution is introduced through a burner with a centrally located atomization lance into an adiabatic, vertically placed combustion chamber, in which reducing conditions are established by appropriately controlling combustion. Natural gas is used as the fuel with 80% theoretical air, producing a combustion gas containing CO, H
2
, H
2
S, and zero NO. The flue gas is said to be conducted to a gas workup in which the gaseous sulfur compounds (COS and H
2
S) are separated. The reference further reports that the purified sulfur-containing gases can be carried through a working unit in a subsequent process, e.g., reaction to elemental sulfur in a Claus unit. The remaining gas is used for the production of steam by the under-firing of a boiler or can be used for heating purposes by subsequent combustion. Other examples describe oxidative combustion of the ammonium sulfate solution.
The ammonium sulfate solutions used in the process of DE 41 01 497 are relatively dilute, thereby requiring a substantial energy input for vaporization of water, and producing a combustion gas in which sulfur-bearing gases are diluted with water vapor and other products of combustion. Although Example 1 of the '497 publication states that the hydrogen sulfide produced in the reductive combustion can be converted to sulfur in a Claus unit, the hydrogen sulfide content of the combustion gas is relatively low. The '497 publication does not describe measures to maximize either the hydrogen sulfide, carbon monoxide, or hydrogen content of the combustion gas.
In U.S. Pat. No. 4,208,390, Hirabayashi et al. describe a process for the recovery of ammonia and sulfur dioxide from an aqueous mixture containing an ammonium salt of sulfuric acid obtained as a by-product of the preparation of &egr;-caprolact
Flanigan Virgil J.
Irvine Lewis B.
Kapila Shubhender
Liske Yvonne M.
Lorbert Stephen J.
Novus International, Inc.
Senniger Powers Leavitt & Roedel
Vandy Timothy C.
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