Patent GB-691352-A (GB)
Title: Improvements in and relating to the production of ketenes
IPC: C07C45/88
Filed: 1949-08-12
Abstract: Ketene or its homologues are obtained from acetone or the corresponding ketone by pyrolysing the appropriate ketone in a ceramic reactor in the presence of moving ceramic material comprising pebbles or particulate solid.  The ceramic reactor is defined as one in which the surfaces of the reaction zone are of refractory material substantially free from iron containing compounds which would yield metallic iron under the pyrolysis conditions and in which there is no contact of the reactants with metallic iron.  Specified materials suitable for the reactor and moving ceramic material are mullite, sillimanite, sillimanite cement (fired) and vermiculite.  The pyrolysis temperature may be 570 DEG  to 750 DEG  C., preferably 675 DEG  to 725 DEG  C.  The ceramic material in pebble or particulate form is passed continuously through the reactor as a compact bed moving under the effect of gravity or is employed in finely divided form and maintained in the fluidized condition.  The pyrolysis is preferably effected by employing a pebble heater using balls of ceramic material which may be of 1/8 to 3/4 inch diameter, thus, when acetone is used, it is preheated and fed as vapour into the bottom of a ceramic reactor through which a stream of heated ceramic pebbles at a controlled temperature is descending countercurrently.  The pebbles give up their sensible heat to the acetone vapour in the reactor and pyrolyse it to ketene and other compounds, the products being drawn off from the top of the reactor and treated for the recovery of ketene, e.g. by rapidly cooling the products, separating the bulk of the acetone in a catchpot and absorbing the ketene in the vapour from the catchpot by means of a liquid such as water.  The pebbles may be heated by flue gases in a vessel situated above the reactor and fall through the latter by gravity.  The pebbles are withdrawn from the bottom of the reactor through a tube fitted with a device capable of maintaining their rate of flow substantially constant, e.g. a slide aperture valve, a star feeder, or preferably a rotating table.  The withdrawn pebbles are then returned to the heating vessel in which they are brought to the requisite temperature and are then recycled to the pyrolysis vessel.  It is preferred to operate with a pressure in the pyrolysis zone not exceeding atmospheric, preferred conditions being a pressure of 10 lb. per sq. inch gauge and less at the inlet to the reactor and a pyrolysis temperature of 570 DEG  to 750 DEG  C. In the fluidized bed technique the particulate material is maintained in the reactor in a fluidized bed by means of the acetone in vapour form, the particulate material heated to the requisite temperature in a heating zone being continuously fed to the bottom of the reactor as an entrained stream, e.g. in acetone vapour, and being continuously withdrawn from the bottom of the reactor and introduced as an entrained stream in a mixture of air and fuel gas to the bottom of the heating zone in which it is heated to the requisite temperature by combustion of the gas and is then returned to the reactor.  The ketene is taken off overhead from the reactor and the gaseous products of combustion of the fuel gas from the top of the heating zone.  In an example a pebble heater is employed, the heating chamber consisting of an upper cylindrical portion and a lower centrally located conically-shaped portion in the form of a frustum below which is situated a cylindrical pyrolysis chamber terminating in a bottom conical portion, the two chambers being connected by a vertical throat. The whole unit is of fired sillimanite brickwork and the pebbles used are of fired mullite.  Heat is supplied to the upper chamber by burning a mixture of coke oven gas and in both chambers the flow of pebbles is countercurrent to the gas stream.  The pyrolysis temperature is maintained at 650 DEG  C. and the pressure at atmospheric.  The unchanged acetone is condensed out and the yields of ketene based on various rates of flow of liquid acetone and various pass conversions are stated.