This invention relates to a process for purifying unsymmetrical dimethyl hydrazine (UDMH). More specifically, hydrazone is extracted from a UDMH solution by distillation of the solution in the presence of a codistillation agent.
UDMH is employed primarily as a fuel for liquid propellant rockets and only relatively small amounts are used industrially for the preparation of agricultural chemicals. As a rocket fuel, UDMH is usually not used neat; it is mixed with other hydrazines or amines to obtain liquids of the proper freezing points and viscosities. These mixtures are used in several rockets and other propulsion devices throughout the Department of Defense and NASA. Unsymmetrical dimethyl hydrazine was originally prepared on a commercial scale by the Raschig method. The disadvantage of this process was that the UDMH has to be recovered from a dilute aqueous solution, a complicated and energy consuming operation. The increased demand for UDMH during the 1950's from an expanding aerospace industry generated the impetus for the development of a more economical manufacturing procedure. It is based on the catalytic hydrogenation of n,n-dimethylnitrosamine (DM-Nitroso). The method involved the catalytic reduction of n-nitrosodimethylamine which was prepared from dimethylamine and nitrous acid. EQU (CH.sub.3).sub.2 NH+HONO.fwdarw.(CH.sub.3).sub.2 NNO+H.sub.2 O EQU (ch.sub.3).sub.2 nno+(h) .sup.catalyst (CH.sub.3).sub.2 NNH.sub.2 +H.sub.2 O
udmh was produced in large quantities by this method. In 1973, when DM-Nitroso was identified by the Occupational Safety and Health Administration (OSHA) as a known carcinogen, the production facility was shut down and modified to meet more stringent Environment Protection Agency (EPA) requirements. Other interesting innovations for the preparation of UDMH reported in the literature which do not involve a carcinogenic intermediate are shown below:
a. Sisler Gas Phase Chlorination of Ammonia: EQU 2NH.sub.3 +Cl.sub.2 .fwdarw.NH.sub.2 CL+NH.sub.4 Cl EQU NH.sub.2 Cl+(CH.sub.3).sub.2 NH+NaOH.fwdarw.(CH.sub.3).sub.2 NNH.sub.2 +NaCl+H.sub.2 O PA0 b. Alkylation of Hydrazine with an Alkyl Halide or Alkyl Sulfate: EQU 2RCl+NH.sub.2 NH.sub.2 .fwdarw.R.sub.2 NNH.sub.2 +2HCl PA0 c. 1,1-Dimethylurea Process: ##STR1## Both the Sisler and dimethylurea methods lead to the production of relatively large amounts of the formaldehyde hydrazone of UDMH because an oxidizing agent is used in the process. The oxidative interaction between NH.sub.2 Cl, NaOCl or other oxidizing ingredients and UDMH form the hydrazone as shown below: ##STR2## The second method does not appear to have any advantage over the abandoned Raschig method. A sizable research effort has been devoted to the Sisler route. Good yields of UDMH have been obtained with this method. However, for UDMH to be produced by this method on a commercial scale, the hydrazone must be removed from the UDMH or a method must be found to prevent its formation. The hydrazone cannot be removed from UDMH by simple distillation.
It is important to note that the three amine fuels, UDMH, monoethylhydrazine, and anhydrous hydrazine are each very dissimilar materials. The hydrazines are so named because of the nitrogen bonding within the molecule. The differences between the three hydrazines are attributable primarily to either the absence of or presence of one or two carbon atoms with bonds to adjacent hydrogen atoms. Unsymmetrical dimethyl hydrazine does not form an azeotrope with water while both of the other materials do form these azeotropes. There are numerous techniques available for dehydration of the azeotrope called hydrazine hydrate. U.S. Pat. Nos. 2,963,407 and 2,698,286 teach these techniques, however none of the prior art references teach the process for removing the hydrazone contaminant. In one of the above patents various alcohols are added to break the azeotrope form by allowing water to migrate up to the top of the column from where it can be removed as overhead product. In the other case, materials such as aniline are used in order to break the azeotrope. The difference between the dehydration techniques employed for anhydrous hydrazine and monomethylhydrazine are slight even though the major difference between these materials is that in one case for hydrazine there is a purely inorganic compound, while in the case of monomethylhydrazine there are carbon and hydrogen atoms involved within the molecule. In the case of separation by addition of a alcohol, the water is removed from the azeotrope by taking it up the column and removing water/alcohol as a product. This situation does not occur in the case of UDMH. The isopropanol treated within the present invention is utilized as a means for isolation between the water and UDMH thereby keeping both the water and the isopropanol in the distillation pot. Since unsymmetrical dimethyl hydrazine does not form an azeotrope but only has an affinity for water (as evidenced by a pinch point at the low concentrations of UDMH in water) it is necessary to treat UDMH differently as it really behaves differently than the other hydrazines.
The main problem encountered in trying to produce UDMH is that relatively high amounts (5-20%) of the formaldehyde hydrazone of UDMH is formed. Because of the similarity of the boiling points of the side product and UDMH, it is extremely difficult to effectively separate the two materials by distillation. Since the military specification for the rocket fuel calls for a minimum content of 98% UDMH, any material prepared in the above mentioned fashion is useless.
The prior art, included herein by reference, and defined in "Reaction of Chloramine with Anhydrous Primary and Secondary Amines," by G. M. Onnietonski, A. D. Kelmers, R. W. Shellmann, and H. H. Sisler, J.A.C.S. 78,3847 (1956) and in U.S. Pat. Nos. 2,806,851, 2,963,407 and 2,698,286, do not differentiate between UDMH and its formaldehyde hydrazone contaminant. The reason for this is that these test results were analyzed by oxidation with Potassium Iodate, and this system is unable to make that distinction.