rad upon the gas vapor mixture the gas vapor mixture is cooled there and the solidis desublimed the resulting gas solid mixture with the desublimed solid leaves the mixing zone through the orifice 6 in direction 9 the temperature of the gas solid mixture is measured by the thermocouple 16 the cooling gas feed rate fed is controlled to maintain a constant coolinggas temperature or the cooling gas temperature is controlled at constant cooling gas rate if desired the housing 17 can be temperature controlled the gas solid mixture is passed to a filter device which is not shown and in which the desublimed solid is filtered out of the gas another embodiment of the lower part 12 of the nozzle illustrated in fig2 is shown in fig3 by means of the additionally fitted plate 18 the two disc shaped air chambers 19 and 20 are formed the heat transport via the wall from the gas solid mixture to the cooling gas is made more difficult by these two chambers therefore the temperature of the material located underneath the air chamber 20 is virtually the same as the temperature of the gas solid mixture underneath the orifice 6 a cylindrical space is formed by the ring 21 the result is that the flow of the gas solid mixture emerging from the orifice 6 is extended before the gas solid mixture comes into contact with the temperature controlled housing 17 having generally described this invention a further understanding can be obtained by reference to certain specific examples which are provided herein for purposes of illustration only and are not intended to be limiting unless otherwise specified a gas vapor mixture containing pyromellitic dianhydride pmda was producedby oxidation of tetraalkylbenzenes.