Some safety aspects on the cleavage of cumene hydroperoxide in the phenol process

Abstract

The acid‐catalyzed cleavage of cumene hydroperoxide (CHP) to phenol and acetone is an exothermic reaction in homogeneous liquid phase. The reaction occurs within a minute. Therefore, a reliable heat removal and temperature control is essential in commercial units. In addition, an uncomplete conversion of CHP must be detectable to avoid any accumulation of the peroxide in the reaction system which can cause dangerous conditions. This article describes some process safety aspects of the CHP cleavage unit.The first part of the cleavage is a circulation loop with one or several heat exchangers in series to remove the heat from the exothermic reaction at around 50°C. CHP is added to the circulation loop and so diluted with cleavage product. The cleavage product consists of mainly phenol, acetone and cumene. Sulfuric acid in low concentrations is used as catalyst. CHP decomposes to phenol and acetone while passing through the cooler(s). In a specific side reaction, dicumylperoxide (DCP) is formed from CHP. Potential hazards in this unit rise from feeding too much CHP into the loop, which could lead to high temperatures and an impermissible high pressure due to the fast exothermic reaction. A failure of the equipment with product release could be the consequence. A countermeasure is for example a ratio control between the flows of the circulating cleavage product and the CHP feed stream. Another hazard for the circulation loop is the accumulation of CHP due to a deactivation of the sulfuric acid. A delayed start of the reaction could then lead to a severe runaway reaction when the accumulated CHP starts to decompose. This can cause a complete destruction of the unit. A “low temperature” interlock can be used to early stop the CHP feed in case of an acid deactivation.After this first cleavage unit, the product from the circulation loop is then treated in a second unit with a plug‐flow reactor operating at temperatures above 100°C. The temperature can be increased by adding CHP to use the exothermic reaction from decomposed CHP for heating‐up. In the downstream section of this second reactor, DCP reacts back to CHP at the higher temperature and this CHP immediately further decomposes to phenol and acetone. As well as for the circulation loop, too high concentrations of CHP can be avoided by a proper ratio‐controlled dosing of the CHP. A non‐complete conversion of CHP due to a loss of acid activity will result in a decrease of the temperature. This can be detected by either “low temperature” interlocks or, preferably, by permanently monitoring and verifying the “swan‐neck” temperature profile across the plug‐flow reactor. Finally, a sufficiently high conversion of DCP can be observed by the slope of the temperature profile at the end of the plug‐flow reactor. © 2019 American Institute of Chemical Engineers Process Saf Prog: e12034 2019