Multi-stage process with adiabatic reactors for the preparation of isocyanates

#U.S. Pat. No. 4,207,251 discloses the gas phase production of C.sub.1 -C.sub.24 organo-isocyanates by oxidative dehydrogenation of the corresponding N-monosubstituted formamides. This reaction is extremely exothermic, and consequently some method of temperature control or heat removal during the reaction has been employed to maintain the reaction temperature below that which would accelerate side reactions or actual decomposition of the isocyanate product. U.S. Pat. No. 4,207,251, for example, discloses the use of small diameter reactors or the introduction of relatively large volumes of an inert gas to the reaction mixture to absorb the heat liberated during the reaction. Disadvantages attend both of these methods, however. Small-diameter reactors that are designed for cooling are expensive and have relatively high surface-area/volume ratios, which often necessitates the use of several reactors in parallel, an arrangement which is difficult to operate. The use of an inert heat sink at the high volumes disclosed in the patent (over 50 volumes of inert gas per volume of oxygen), presents difficulty and expense in separation of the desired product. The improvement of the present invention eliminates the need for active cooling of the reactors and permits the reduction in volume, or entire elimination, of the inert gas.

#The present invention provides an improved process for the preparation of isocyanates of the formula R--(NCO).sub.n where R is a C.sub.1 -C.sub.10 alkyl, cycloalkyl, aryl, alkaryl, or aralkyl group and n is 1 or 2, by reacting a corresponding formamide of the Formula R(NH--CO--H).sub.n with oxygen or an oxygen-containing gas at 400.degree.-700.degree. C. in the presence of a silver catalyst, wherein the improvement comprises performing the reaction in at least two essentially adiabatic reaction stages in series and wherein the improvement is characterized by:

#(a) introducing the formamide and up to 70% of the stoichiometric amount of oxygen to the first reaction stage at a temperature of 200.degree.-350.degree. C.;

#The general procedure employed in Examples 1-7 was as follows: Monomethyl formamide (MMF) was continuously vaporized and mixed in a heated vessel with air and, in some examples as noted, nitrogen. This gas mixture was then heated to 230.degree. C. and fed continuously to a reactor at gradually increasing rates until a rate of 35 grams/hour of MMF, with the corresponding amounts of air and nitrogen, as shown in the table below, was established. The reactor was a quartz tube reactor (10 mm inside diameter; 18 cm in length) containing 8.0 grams of crystalline silver catalyst, at a catalyst bed depth of 2.5 cm. The reaction tube was equipped with a thermocouple to measure the temperature of the catalyst bed, and the bed was equipped with an electric heater to initially elevate the temperature to a level sufficient to start the reaction. When reaction was proceeding steadily, the electric heater was adjusted so as to make up the heat lost to the surroundings from the uninsulated reaction tube, to simulate adiabatic conditions of commercial-size reactors. The reaction mixture effluent of the first reactor was mixed with additional air, charged at ambient temperature, and the interstage temperature of the resultant mixture was recorded. In each case, the addition of air, and the loss of heat through the walls of the interstage piping, caused the temperature of the resultant mixture to drop to between 240.degree. C. and 265.degree. C. The reaction mixture was then passed through a second reaction tube which was equipped and operated identically to the first reaction tube. The entire reaction sequence was operated at about atmospheric pressure. The reaction mixture effluent of the second reactor was cooled to about 240.degree. C. and analyzed for methyl isocyanate (MIC) content and MMF conversion. The process conditions and results are summarized in the Table.

#______________________________________ Reactor Reactor 1 2 Overall Ex- Equiv. Equiv. MMF MIC am- Oxy- Temp. Oxy- Temp. N.sub.2 / Con- Ob- ple gen.sup.a (.degree.C.) gen.sup.a (.degree.C.) O.sub.2.sup.b version.sup.c tained.sup.d ______________________________________ 1 0.5 550 0.6 565 7.0 90 86 2 0.5 525 0.6 530 12.5 85 87 3 0.5 510 0.7 525 12.0 93 88 4 0.5 565 0.75 575 4.0 95 84 5 0.6 575 0.7 555 8.0 90 85 6 0.6 550 0.8 540 11.0 93 89 7 0.6 560 1.0 570 8.0 97 87 ______________________________________ .sup.a Equivalent amount of oxygen, in air, charged to the reactor indicated, based on amount of MMF originally fed to Reactor 1. .sup.b Total volume of nitrogen fed to the reactors, separately or in air per total volume of oxygen fed to the reactors. .sup.c Moles of MMF consumed per mole of MMF fed, expressed as percentage .sup.d Moles of MIC in effluent of Reactor 2 per mole of MMF consumed overall, expressed as percentage.