CONTINUOUS PROCESS FOR THE PRODUCTION OF METHOMYL

Abstract: Disclosed is a continuous process for the production of methomyl comprising the steps of: continuously forming, in an inert organic solvent and at an elevated reaction-initiating temperature of from 30-70°C, a liquid mixture of methyl­ thioacetaldoxime and with methyl isocyanate in an amount in excess of stoichiometric proportions; passing the mixture through a plug-flow, adiabatic, tubular reactor, maintained at a pressure of from 30 to 70 psig, to produce reaction product at a temperature up to a reactor discharge temperature of from 80 to 120°C; and withdrawing the reaction product from the reactor and stripping unreacted methyl isocyanate and solvent therefrom to provide product methomyl, while recycl­ ing at least a portion of the unreacted isocyanate and solvent to the mixture.

Description: CONTINUOUS PROCESS FOR THE PRODUCTION OF METHOMYL The present invention relates to a process for the production of methomyl (l-methylthioacetaldoxime N-methylcarbamate) which is known to have broad pesticidal activity. Heretofore, a number of batch processes have been proposed for the production of methomyl. Certain of such processes, together with the nematocidal activity of this composition, are disclosed in U.S. - A - 3,459,866 to Buchanan, issued August 5 1969 and entitled "Hydroxamate Carbamates As Nematocides". In that disclosure, the processes may involve the conversion of thiolhydroxamate esters to any one of a broad class ot O-dialkylcarbamylydroxamate esters and treating the resulting intermediate with sodium hydride in tetrahydrofuran and subsequently with a dialkylcarbamyl chloride. The resulting esters are members of a class having the following formula: EMI1.1 wherein: R1 is alkyl of 1 through 4 carbon atoms, alkoxylalkyl of 2 through 5 carbon atoms, alkylthioalkyl of 2 through 5 carbon atoms 4-halophenyl or benzoyl, R2 is alkyl of 1 through 4 atoms; R is hyarogen, methyl, ethyl, propyl, allyl, butenyl or propargyl; and R4 is hydrogen or methyl. When R1 R2 and R are methyl groups 3 and R4 is hyarogen, the compound of the formula is methomyl. A further reference, U.S. - A 3,506,698 issued April 14, 1970 to Jelinek and entitled 1preparation of Thiolhydroxamate Carbamates", discloses the reaction of thioihydroxamate esters with methyl isocyanate to provide a class of compounds which includes methomyl. It is, therefore, known in the prior art to product methomyl by the reaction of methyl thioacetalooxime ana methyl isocyanate. Other different ana more exotic carbamates may be prepared in accordance with the teachings of U.S. - A - - 3,574,736 to Fuchs; 3,576,834 to Buchanan; 3,639,633 to Buchanan; and 3,890,362 to Alverez. All of the processes referred to above contemplate the production of methomyl employing a batch process requiring relatively high reaction temperature operations, substantial cooling requirements and severe limitations on the total production time required for the process. These batch process llmitatwons, in turn, require certain secondary limitations which further reduce the overall efficiency of tne process with respect to heat input requirements and efficiency based on methyl isocyanate use. Accordingly, it is an object of the present invention to provide a process for the production of methomyl in which the heat input and efficiency (based on methyl isocyanate usage) are improved over those obtainable in the processes of the prior art. Other aims and advantages of the invention will be apparent from the following description and appended claims. In accordance with the present invention, a continuous process is provided for the production of metnontyl comprising the steps of: continuously forming, in an inert organic solvent and at an elevated reaction-initiating temperature of from 30-70 C, a liquid mixture of methylthioacetaldoxlme and methyl isocyanate in an amount in excess of stoichiometric proportions; passing the mixture through a plug-flow, adiabatic, tubular reactor, maintained at a pressure of from 30 to 70 psig, to produce reaction product at a temperature up to a reactor discharge temperature of from 80 to 120or; and withdrawing the reaction product from the reactor anu stripping unreacted methyl isocyanate and solvent therefrom to provide product methomyl, while recycling at least a portion of tfle unreacted isocyanate and solvent to the mixture. The continuous process of the present invention is illustrated by the following equation: EMI3.1 <tb> <SEP> O <SEP> H <tb> CH3 <SEP> - <SEP> C-NOH <SEP> + <SEP> CH3- <SEP> NCOC3 <SEP> - <SEP> C <SEP> = <SEP> N <SEP> - <SEP> 0-- <SEP> C <SEP> - <SEP> N <tb> <SEP> S-CH3 <SEP> S-CH3 <SEP> CH3 <tb> <SEP> 3 <SEP> 3 <SEP> 3 <tb> (methylthio <SEP> (methyl <SEP> (methomyl) <tb> <SEP> acetaldoxime) <SEP> isocyanate) <tb> To ensure full methylthioacetaldoxlme conversion, an amount of methyl isocyanate in excess of stoichiometric proportions is fed to the reaction. In the continuous process of the invention, the reaction is carried out at an elevated reaction initiation temperature from about 30-700C, preferably of the order of 5O0C. Since the reaction is adiabatic, the temperature at the discharge end of the plug-flow, tubular reactor is about lO()0C. Tne pressure is between 30 ana 70 psig ana preferably of the order of between 40 and 60 psig. A tubular reactor is selected having an internal diameter and length capable of providing a residence time for passage of the reaction mixture througn the reactor in a time of the order of about 30 minutes. The reaction of the continuous process of the present invention is carried out in an inert organic solvent which is preferably a lower aliphatic ketone, chlorinated or aromatic hydrocarbons, ether, or paraffin, or mixtures thereof. methyl ethyl ketones, methyl isobutyl ketone and vinyl methyl pyridine or mixtures thereof are preferred solvents. Whereas the reaction of the process of the present invention may be carried Out without tne need for a catalyst, any one or a mixture of a number of reaction catalysts known to be active for catalyzing this reaction may be employed in low concentrations, e.g. of the order of one-tenth of one weight percent. Tertiary amines and aromatic amines are preferred. In tne drawing, the single figure schematically represents a flowsheet of apparatus suitable for the practice of the continuous process of the invention. As there shown, me thy lthioacetaldoxime (MSAO) is introduced into the system through line 10. The start-up and later make-up methyl isocyanate (MIC) is concurrently introducea into the system through line 12. The inlet reactants mix in line 14 and are passea through heater 16 to a tubular reactor 18, having the usual plurality of serpentine bends 20, before discharging the reaction product into stripper 22 at point 23. A separation is effected in stripper 22 between the top fraction containing essentially of MIC and a portion of the solvent which stream is recycled through line 24 to the incoming MIC stream and the bottom fraction discharge or stripper 22 through line 26 and containing product methomyl and the remainder of the solvent. As to materials of construction of the tubular reactor, it is preferred that a Type 304 stainless steel reactor be employed. The reactor dimensions are selectea based on the material throughput rate. For example, or a 1000 lbs. per hour rate, a 6-inch internal diameter and a length of 250 ft is preferred. The following Table 1 sets forth, in weight percent, various fluid compositions for the various streams in the lines of the flow diagram of the drawing. In an example or the process of the invention, apparatus in accordance with that of the flow sheet of the drawing is employed at the temperatures, pressures, compositions ana flow rates set forth in Table I to produce product methomyl. The reactor residence time for such example is 31 minutes for a 6-inch, 250 foot length type 304 stainless steel reactor. TABLE 1 Stream (Line No.) 12 10 14 24 23 26 Fresh HSAO Reactor Recycle MIC Stripper Product MIC Charge Feed MIC/MIBK Feed Outlet (wt. %) (wt. %) (wt. %) (wt. %) (wt. %) Component Light Ends - 0.11 0.13 0.38 0.88 Methyl Icocyanate 100.00 - 9.98 11.14 1.19 Solvent/MIBK 80.50 74.50 88.48 74.81 Water 0.04 0.04 - 0.01 1,3 Dimethyl UREA 0.16 Syn - Methlthio Acetaldoxime 18.03 15.33 0.05 Anti - Methylthio Acetaldoxime 0.12 0.10 Syn - Methomyl 22.07 Anti - Hethomyl 0.23 Catalyst/Dimethyldodecylamine 0.10 0.08 0.08 Heavy Ends 0.03 0.24 0.52 Total 100.00 100.00 100.00 100.00 100.00 Flow (1bs. per hour) 403 3,690 4,531 438 4,531 4,093 Temperature ( C) 2 50 50 50 98 - Pressure (psig) 105 105 60 105 40 - Residence time in Reactor = 31 minutes In the comparison of the continuous process of the present invention with the batch process of the prior art, certain basic parameter differences, which result in highly significant efficiency differences, should be noted. In the batch process, MSAO ana MIC mixing and reaction initiation results in the development of high temperatures in short periods of time. Heat is constantly removed to maintain the reactor at the desired reaction temperature level for the entire reaction period. Upon completion of the reaction, the products are cooled and transferred to holding tanks prior to passage to a stripper. In the continuous process of the present invention, MSAO and MIC are continuously mixed and heated to a lower reaction initiating temperature. Heat removal during the reaction is not necessary and, as a result, the temperature of the material increases to a substantially higher temperature than that obtainable in a batch process operation. This use of higher reaction temperatures and pressure in the continuous process operation permits the use of a simplified continuous tubular reactor resulting in lower capital investment and lower operating costs due to a subslantlal reduction in vent losses (and consequent MIC losses) together with lower heating and cooling requirements. The following Table 2 sets forth an operating comparison between a typical batch process system and the continuous process system of the present invention. TABLE 2 Batch Continuous Reaction Temperature (QC) Initial 43 50 Reaction 77(1) 50-98(2) Stripper Feed 46 98 Reactor Pressure (psig) 1-5 40-60 Time Required (minutes) Charge MSAO 45 0(3) Charge Recycle MIC 15 0(3) Charge Fresh MIC 10-30 Reaction 110 30 Empty Reactor 35-40 o() TOTAL 215-240 30 (1) Essentially constant temperature (2) Adiabatic. Temperature increases through reactor (3) Continuous raw material feeds and product withdrawal In the batch process operation, reactor materials are charged into or out of: weighing tanks, the reactor, recycle tanks and holdlng tanks. This result in vapors being vented from these vessels which contain MIC, a relatively expensive raw material. Estimates based upon material balance calculations indicate 0.261 of the MIC feed to the reaction system is lost in these vents. Continuous process operation encounters no corresponding MIC loss since there are no vapor vent streams employed. The continuous process operation employs no heat removal, thereby allowing the reaction temperature to rise. This produces a significantly lower utility usage for continuous operation. Other batch process operations, such as addition of heat to the reactor and subsequent removal, imposes a higher heat input to the MIC stripper, rather than the constant feed stripper temperature employed in the continuous process of the invention. The difference in temperature between batch and continuous operation requires the introduction of more heat to the batch process operation with the calculated resulting effect that the continuous process operation employs only 188 of the heat input required by the batch process operation. In summary, it has been estimated by material balance calculations that the continuous process operation, as against a batch process operation, results in: a 33% lower capital investment; an MIC efficiency gain of 0.26%; and a lower utility usage of 18%. It is, however, believed that, in spite of the improved economic efficiencies in the use of the continuous processes of the present invention over the batch processes of the prior art, the most significant improvements of the present invention are the efficiency of oxime-to-methomyl and of hIC-to-methomyl due to the reduced reaction time and less thermal decomposition of the methomyl. As pointed out hereinabove, the MIC efficiency is further enhanced due to the absence of venting losses.

Claims: CLAIMS. 1. A continuous process for the production of methomyl comprising the steps of: continuously forming, in an inert organic solvent and at an elevated reaction-initiating temperature of from 30-70 C, a liquid mixture of methylthioacetaldoxime and with methyl isocyanate in an amount in excess of stoichiometric proportions passing said mixture through a plug-flow, adiabatic, tubular reactor, maintained at a pressure of from 30 to 70 psig, to produce reaction product at a temperature up to a reactor discharge temperature of from 80 to 120 C; and withdrawing said reaction product from said reactor and stripping unreacted methyl isocyanate ana solvent therefrom to provide procuct methbmyl, while recycling at least a portion of saia unreacted methyl isocyanate and solvent to the mixture. 2. A process as claimed in claim 1, wnerein said reaction-initiating temperature is about 500C. 3. A process as claimed in claim 1 or 2 , wherein said pressure is between about 40 and 60 psig. 4. A process as claimed in any one of the preceding claims wherein said discharge temperature is about 1000C. 5. A profess as claimed in any one of the Preceding wherein said tubular reactor is selected to have -an internal diameter and length so as to provide a residence time for said mixture within said reactor or the oraer o about 30 minutes. 6. A process as claimed in any one of the preceding claims wherein said inert organic solvent is a lower aliphatic ketone. 7. A process as claimed in any one of the preceding claims wherein methomyl is formed in the presence of a catalyst. 8. A process as claimed in claim 7, wherein said catalyst is an aliphatic amine.

Inventors: EWBANK, RODGER MAURICE, NELSON, JAMES RUSSELL
Applicants: UNION CARBIDE CORPORATION
Application year: 1984
Main classification: C07C153
IPC codes: C07C313, B01J31, C07C67, B01J19, C07C327