On the use of periodic operation for improvning chemical processes

Doktorsavhandling, 1987

In this thesis, some methods of improving the product distribution from chemical processes are compared. The first two parts contain an experimental investigation of how the product distribution of a process is influenced by the choice of catalyst. The ability of different forms of zeolites, such as 13X, 4A, Y and AWSOO, to catalyze the formation of mono-, di- and triethanolamines from ethylene oxide and ammonia in the liquid phase has been examined and compared with that of an organic ion-exchange resin. The ion-exchange resin, Amberlite 200, gives the highest yield of monoethanolamine (MEA), while the zeolite 13X has the highest capacity of producing MEA. Furthermore, it is demonstrated that any contributions from homogeneous reactions to the total reaction rate are negligible. A fast and highly reproducible method for the direct GC analysis of ethanolamines using fused-silica capillary columns is also presented. The third part of the thesis examines a more unusual method of changing the product distribution, the so-called periodic operation. Periodic operation means that one or more of the control variables, such as inlet concentrations or temperature, are intentionally varied with time in a periodic manner. By expanding the treatment of two well-known theoretical examples it is in one case shown that there exist price relations which can make periodic operation worthwhile. In another case it is illustrated how the comparison of periodic operation with steady-state operation is influenced by the restrictions under which the comparison is made. A number of experimental investigations given in the literature claim positive influence of periodic operation on selectivity or yield. It is shown that only a handful of these papers survive a critical examination of how the comparison with steady-state operation has been made. Even these, however, are not complete. What often remains is to cover a larger field of operating conditions. For the reaction system studied in the first two parts, no influence of periodic operation on yield could be detected.