How is dimethylurea made?

BACKGROUND OF THE INVENTION

1. Field of the Invention

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This invention relates to an improvement in a process for indirectly alkylating urea.

2. Description of the Prior Art

In order to appreciate the manner in which the present invention represents a marked improvement over the closest prior art, it warrants considering briefly the usefullness N, N-dimethylurea potentially offers as a chemical intermediate. The indicated utility is that of producing unsymmetrical dimethylhydrazine (UDMH), in turn a versatile intermediate for the preparation of surfactants, insecticides, dyes, monomers, etc.; but the most important current use thereof being in the field of liquid propellants for rockets.

Recently, an alternate method to that of the present commercial practice for producing UDMH involving the hydrogenation of nitrosodimethylamine, has been actively sought. This is so because nitrosodimethylamine has been identified as such a powerful carcinogen that in order to provide absolute protection for plant workers a prohibitively expensive installation would be required. An environmentally acceptable alternate method appearing to have commercial merit resides in the modification of the Scheslakoff process (J. Russ. Phys. Chem. Soc., 37, pgs. 1-7, ) wherein N, N-dimethylurea is rearranged in accordance with the Hoffman mechanism. As is characteristic of such type rearrangement reactions, optimum yields of product are substantially less than quantitative.

It has hitherto been proposed to prepare N, N-dimethylurea by reacting dimethylamine sulphate with urea in an aqueous system capable of effecting solubilization of the urea. Besides recovery problems, the method suffers because the optimum yields attainable are reportedly in the order of only about half of theoretical. Notwithstanding that the indicated reactants are readily available and relatively inexpensive, the commercial attractiveness of the modified Scheslakoff process for preparing UDMH depends largely on realizing a highly efficient method for preparing N, N-dimethylurea. Accordingly, the foremost objective of the instant invention is to provide such a method.

SUMMARY OF THE INVENTION

In accordance with the broadest aspect of the present invention urea is reacted with dimethylamine in a substantially anhydrous system to provide N, N-dimethylurea. Beyond the essential anhydrous feature of the process, a critical requirement is that of maintaining the reactants in an essentially liquified form during the course of reaction without resorting to the use of added solvent media. The stated requirement is realized through the use of the combination of an appropriately selected elevated reaction temperature and at least a onefold molar excess of dimethylamine based on the extant amount of urea available at any time for reaction. The preferred pressure conditions contemplated correspond to the particular vapor pressure of the dimethylamine at the selected operating temperature.

The salient advantage of the process of this invention over the prior art is that an essentially quantitative yield of N, N-dimethylurea is provided. Another advantage is that the product conversion rates experienced are excellent. Additionally, the product recovery feature of the process together with product purity realized renders the overall process highly efficient.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The reaction between urea and dimethylamine in accordance with the present invention proceeds, from all indications, by the urea first equilibrating into ammonia and cyanic acid, whereupon the latter reacts with dimethylamine to yield the unsymmetrical dimethylurea. This proposed reaction scheme serves to account for the reported poor results obtained pursuant to the prior art practices. Thus where there is water present, a competing reaction is prone to occur whereby water reacts with cyanic acid to yield carbon dioxide and ammonia. The absence of water in the instant process, therefore, primarily accounts for the essentially quantitative yields being obtained. As implied in the foregoing discussion, water does not poison the reaction in the usual sense. In order to anticipate the extent of water build-up that can be tolerated, which build-up would indubitably occur in constant recycling of unreacted dimethylamine in a commercial operation, it was found that amounts up to 5% based on the weight of the amine reactant had no preceptible effect upon yield or conversion rates. A quantum of water, however, in excess of about 10% based on the amine reactant can be expected to have a progressively noticeable adverse effect on yield. Therefore, in the context of the present invention the phrase "substantially anhydrous system" connotes those reaction systems wherein the amount of water present does not exceed about 10% of the weight of the dimethylamine.

In light of the nature of the reaction between urea and the dimethylamine, the presence of a polar solvent which at the same time serves as a mutual solvent for the reactants is paramount. In the practice of the present invention, the excess of dimethylamine contemplated serve as the required polar solvent. Of course, in this instance the use of an elevated temperature is necessary in order to effect the requisite solubilization of the urea which is much less soluble in dimethylamine than in water. Further discussion with respect to the influence of temperature will be more appropriately set forth hereinbelow. The minimum amount of dimethylamine applicable should be at least a 100% molar excess based on the maximum amount of urea available at any time during the reaction. Substantially larger excesses of dimethylurea can obviously be used but no practical advantages are provided by employing more than three moles of the amine to urea. The optimum ratio noted has been that of 2.5 moles of the amine to urea. Accordingly, the preferred molar ratio of dimethylamine to urea is from about 2.5:1::3.0:1, respectively.

The applicable temperature range for conducting the reaction as aforesaid is between about 110° and 150° C. The lower temperature limit specified permits a substantially homogenous liquid system when employing the least excess of dimethylamine contemplated. The applicable maximum temperature is governed by the critical temperature of the dimethylamine. But from a practical standpoint it is desirable to limit the upper temperature to somewhat less than said critical temperature; namely, in the order of about 150° C. The preferred temperature range is from 125° to 130° C.

In view of the low boiling point of dimethylamine, a closed reaction system for carrying out the present invention is indicated. The applicable pressure conditions are autogenic and for the most part depend upon the operating reaction temperature. During the course of the reaction, however, ammonia is produced in amount which can cause the pressure to rise considerably. Beyond resulting in the need for expensive high pressure equipment, the presence of ammonia adversely affects the conversion rate insofar as the underlying reaction is an equilibrium one. Consequently, the preferred procedure is to vent the reaction system periodically in order to allow by-product ammonia to escape. In following this procedure the system should be vented for only the time needed for the pressure to be lowered to that of dimethylamine at the observed reaction temperature.

As indicated hereinabove, the practice of the present invention permits the realization of essentially theoretical yield of the unsymmetrical dimethylurea. Apart from yield, the conversion values attainable are excellent. Specifically in this regard, employing the preferred conditions noted above in a commercial type reactor, a conversion of an excess of 90 percent in less than an hour can be expected. In light of this feature, the process is ideally suited for batch operations. This is not to say, however, that the process can not be conducted continuously; but as inferred this may not be economically justified.

Another important advantageous aspect of the present process resides in the fact that the dimethylurea precipitates from the reaction mixture in a crystalline mass upon formation. Thus upon completion of the reaction, the unreacted dimethylamine can be readily recovered for recycling purposes and thereupon the crystalline product can be slurried with water and conveniently pumped or otherwise discharged from the reactor. The product can be further purified by a conventional crystallization procedure if desired although reactor purity of in excess of about 95% can be readily achieved.

In order to illustrate to those skilled in the art the manner in which the present invention can be implemented, the following working examples are set forth. The primary purpose of the first working example is to illustrate the effect of certain variables on the process, all as discussed hereinabove. The succeeding example represents the best mode contemplated for carrying out the invention. It is to be understood that these examples are provided solely by way of illustration and accordingly, any enumeration of details set forth therein is not to be interpreted as limiting the invention except as such limitations appear in the appended claims. All parts are parts by weight unless otherwise indicated.

EXAMPLE I

In carrying out the runs of this example a 316 SS Parr bomb reactor was used having a capacity of about 2 liters. The amount of reactants charged in each instance totalled from 150 to 575 g. A uniform charging procedure was observed consisting of first chilling the bomb to approximately -5° to 0° C. and thereupon adding the urea followed by the addition of liquid dimethylamine at -5° to 0° C. The reaction mixture was briefly stirred and the reactor sealed. Heating was then applied to achieve the selected operating temperature, and the reaction was stirred continously. The combining ratio of reactants observed in each run together with processing parameters applicable therein are noted in Table I set forth hereinbelow. The conversion values given are based on the crude product, a determination of which was afforded by means of a melting point phase diagram of mixtures of authentic N, N-dimethylurea and pure crystalline urea. The yield in each instance was quantitative. The indicated pressure of Runs 2 and 4 was maintained by venting the reactor periodically whereas autogenic pressure prevailed in the other runs.

              TABLE I                                                     
______________________________________                                    
Run  DMA/Urea  Time(Hrs.)                                                 
                         Temp. °C                                  
                                 Press.                                   
                                       Conversion                         
______________________________________                                    
1    2.7       1.5       130     350   65                                 
2    2.5       2.67      124     460   78                                 
3    2.5       1.5       120     500   75                                 
4    2.5       1.25      124     460   70                                 
5    2.5       3.0        90     195    0                                 
6    2.5       1.5        70     150    0                                 
7    2.1       1.5       110     420   70                                 
8    2.1       1.75      105     210   55                                 
______________________________________                                    

EXAMPLE II

In the run of this example a 30 gallon stainless steel pressure reactor was used. The reactor was equipped with a stirrer, cooling coils, electric heating elements, vacuum means and condenser. Crystalline urea in the amount of 45.3 lbs. was first charged to the reactor. The reactor was then sealed and thereupon 84.8 lbs. of dimethylamine were added. Stirring was commenced when the fluidity of the reactor contents permitted. With continued stirring the temperature was raised to 127° C. During the course of the reaction the ammonia produced was vented in order to maintain the pressure at 450 psig. After 45 minutes reaction time, ammonia formation had practically subsided indicating the completion of the reaction. The reactor was then cooled and the excess dimethylamine vented and condensed. Residual amine was finally drawn off under vacuum following which 70 lbs. of water were added, and an aqueous slurry of 135 lbs. was discharged. Analysis of the crystalline product by chromatography indicated it to be free of urea or biuret. The product exhibited a melting point range of 180°-185° C. indicating that essentially complete conversion had been achieved. After recrystallization from water, the product melted at 181.5°-183.5° C. The recrystallized product was shown by infrared and magnetic resonance analysis to be 100 percent pure.

Patented June 29, PREPARATION OF DIMETHYL UREA August H. Homeyer, Webster Groves, Mm, as-.

signor to Mallinckrodt Chemical Works, St. Louis, Mo., a corporation of Missouri No Drawing. Application November 18, , Serial No. 710,413

7 Claims. (Cl. 260-55 3) This invention. relates to the preparation of ureas and more particularly to the preparation of dimethyl urea.

Among the objects of this invention are the provision of eihcient methods for preparing dimethyl urea; the provision of methods of the type indicated which may be easily carried out; the provision of methods of the type referred to which utilize relatively inexpensive media; and the provision of methods of the type indicated which produce a highyield of dimethyl urea. Other objects will be in part apparent and in part pointed out hereinafter.

The invention accordingly comprises the steps and sequence of steps, and features of synthesis, analysis, or metathesis, which will be exemplified in the processes hereinafterdescribed, and the scope of the application of which will be indicated in the following claims.

Dimethyl urea has been previously prepared he employment of an organic solvent has heretofore been considered. absolutely essential. Because ffthe well-known reactivity of phosgene with watenit has been heretofore believed impossible to cause phosgene to react in any great proportion with an amine in an aqueous solution. In an aqueous solution it would have beenpredictedthat the phosgene would react preferentially with the water.- In accordance with the present invention I have made the surprising discovery that the reaction of methyl amine and phosgene to form dimethyl urea, in the presence of a caustic alkali, such as sodium or potassium hydroxide,

reaction may be easily carried out in a single aqueous medium. The methyl amine is merely dissolved in the aqueous medium, which maybe water or water containing one or more inert components. The phosgene is slowly added as such by merely adding it to the methyl amine solution; Contrary to the teachings of the art I have found that it is not necessary to dissolve the phosgene in a vehicle or solvent, but that the reaction easily takes place with high yields where the phosgene is merely added to an aqueous solution or methyl amine.

. In this way, the obvious diiflculties of handling a solution of phosgene in an organic solvent are avoided, and, although such a solution affords a means for controlling the reaction, it has been found that the reaction may be effectively controlled in other ways to utilize the advantages of the present invention.

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The following examples illustrate the invention:

Example 1 A twenty-gallon, jacketed, glass-lined kettle, fitted with an emcient stirrer, thermometer, an inlet tube arranged for delivery of phosgene below the surface of the liquor, and provided with valves to control the rate of flow. Sodium hy droxide solution (7.55 gallons), 9.7 normal, was placed in a calibrated tank and connected through i a valve to the inlet tube. Phosgene was admitted to the well-stirred reaction mixture at the rate of about five pounds' per hour. The cooling water was circulated through the jacket to keep the reaction mixture at about 18 C. After several pounds of phosgene had been admitted, the gradual addition of sodium hydroxide solution was begun and maintained at such a rate that at no time was there an excess of sodium hydroxide present over the amount necessary to react with the chloride produced by the reaction of the phosgene with methyl amine. The rate of addition of phosgene was balanced against the cooling provided by the circulation ofwater so that the temperature was maintained constant at about 18 C.

After about 80% of the theoretical amount of phosgene had been introduced at a maximum rate of about flvepounds per hour, the rate of addition was decreased somewhat and the last part of the reaction was conducted at a lower solution was adjusted so that it exactly balanced the proportion of phosgene at the endof. the reaction.

The reaction product consisted of a solution of dimethyl urea containing some sodium chloride in suspension. The reaction mixture weighed 155 pounds and analysis showed that it contained 24 pounds of dimethyl urea having a freezing point of 101 C. This corresponds to a yield of 90% of theoretical based on the methyl amine used as starting material. It was found further that the reaction mixture contained about of unreacted methyl amine which could be recovered and re-used by appropriate procedures. Taking into account the recovered methyl amine,

, than a theoretical amount of phosgene.

the yield of dimethyl urea was practically Example 2 Materials were combined in the same manner and proportions as described in Example 1 except that the temperature of the reaction was maintained at -35 C. and an excess of phosgene amounting to 5.5% was employed. The product was shown by analysis to contain 81% of the theoretical amount of dimethyl urea and 14% of unreacted methyl amine. The-dimethyl urea was contaminated by by-products so that the crude material had a freezing point of 89.9 C. The dimethyl urea can be recovered as-described in Example 1, but the crude product obtained under these conditions usually was less pure.

Example 3 Example 1 was repeated but,-in lieu of the aqueous methyl amine solution, the kettle was" charged with water, and methyl amine in gaseous form was delivered below the surface of the .water at the rate of about five pounds per hour through another inlet tube. Comparable results were obtained.

fective stirring prevents overheating and aids heat transfer to control the temperature so that it is not substantially above 50 C.

It is preferred that the reaction mixture be naintained at a temperature of approximately 18 ,C. Lower temperatures are not detrimental C. deleterlously affects the yield and quality 0! the product.

In general it is preferred that not more than a theoretical proportion of phosgene be used, since an excess tends to promote side reactions. Satisfactory results can be obtained by using less For example, excellent results are obtained with 90% of theoretical. 1

The caustic should be added at such a rate that it is consumed as it is added; that is, the rate of addition is controlled so that at no time durin the reaction is there an excess of caustic present. Preferably, the reaction is permitted to occurto the extent of about 10-25% before the addition of the caustic is begun. It may be added continuously as the reaction takes place, or peri ably of a concentration of about 35% by weight.

However, more concentrated or more dilute solutions may be employed, although in general it is preferred that the concentration not be substantially below 10% or above 50%.

In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

As many changes could be made in the above processes without departing from the scope of the invention, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

I claim:

1. The method of preparing dimethyl urea which comprises reacting methyl amine and not substantially in excess of the theoretical proportion of phosgene in an aqueous medium and adding a caustic alkali to the aqueous solution.

2. The method of preparing dimethyl urea which comprises mixing not substantially in excess of the theoretical proportion of gaseous phosgene and an aqueous solution of methyl amine and adding a caustic alkali to the aqueous solution.

3. The method of making dimethyl urea which comprises bubbling'not substantially in excess of the theoretical proportion of gaseous phosgene into an aqueous solution of methyl amine and adding a caustic alkali to the aqueous solution.

4. The method of making dimethyl urea which comprises mixing not substantially in excess of the theoretical proportion of gaseous phosgene with an aqueous solution of methyl amine and neutralizing the acid formed by the reaction by adding a caustic alkali to the reaction mixture.

5. The method of making dimethyl urea which comprises bubbling not substantially in excess of the theoretical proportion of phosgene into an aqueous solution of methyl amine and simultaneously neutralizing the acid formed by the reaction by adding a caustic alkali.

6. The method of making dimethyl urea. which comprises mixing together phosgene and an aqueand even higher temperatures up to as high as 50 C. may be employed, although in general increasing the temperature substantially above 18 ous solution of methyl amine, said phosgene being added in not substantially more than the theoretical proportion and said mixing being carried out at a temperature not substantially above 50 C., while agitating the mixture, and neutralizing the acid formed by the reaction by adding to the reaction mixture while the reaction is taking place a caustic alkali at such a rate that at no time during the progress of the reaction is there a substantial excess of caustic present.

7. The method of making dimethyl urea which comprises simultaneously introducing into an aqueous medium, gaseous phosgene and methyl amine, said phosgene being introduced in not substantially more than the theoretical proportion, maintaining the aqueous medium at a temperature not substantially above 59 C. and adding a caustic alkali to the aqueous-medium at such a rate that at no-time during the progress of the reaction is there a substantial excess of caustic present.

AUGUST H. HOMEYER.

REFERENCES CITED The iollowing references are of record in the file of this patent:

O'ZQHER. REFERENCUES' 10 Marckwald, Ber. Deut. Chem, vol. 23, page

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