Abstract: Phenol and acetone are prepared by a process comprising homogeneously cleaving cumene hydroperoxide in the presence of acid catalyst in a cleavage apparatus comprising at least one reactor having plug flow characteristics thereby producing a cleavage product; recycling some of the cleavage product stream by combining the recycled cleavage product stream with the cumene hydroperoxide-containing feed stream fed to said cleavage apparatus under the condition that the mass flow ratio of the recycled partial cleavage product stream to the cumene hydroperoxide-containing feed stream sent to the cleavage reactor is less than 10.

Abstract: The present invention provides a method of starting up a loop reactor system, which method includes heating a reaction mixture present in the loop reactor system with the heat of reaction of at least one exothermic reaction occurring in the loop reactor system. The method of the invention makes possible an inexpensive, reliable and simple start-up of loop reactor systems, since the heat liberated when carrying out an exothermic reaction is utilized for heating the reaction mixture present in the loop reactor. Use of the method of the invention makes it unnecessary to switch heat exchangers over from coolant to heat transfer media such as steam. Since the method of the invention is able to heat up the reaction mixture in the loop reactor system more rapidly than in conventional methods, the number and the amount of by-products formed by secondary reactions during the start-up phase is reduced.