Thesis for the Degree of Master of Science


Year 1996
Jouni Mäkelä

A study on the possibility of controlling the crystal size distribution of the continuous crystallizer using fuzzy logic control

In this work the continuous operation of the pilot-scale evaporative DTB crystallizer was studied when crystallizing ammonium sulfate.

In the literature section of the thesis the possibilities to apply fuzzy logic control for tuning the PID controller and also the benefits obtained by applying fuzzy logic control in industrial applications are discussed.

In the research part of the study a data acquisition and control system for the DTB-crystallizer of the chemical engineering laboratory was built. The system controls the pressure, level, condensate mass flow, steam mass flow, by-pass flow, recycle flow and the feed temperature of the crystallization process.

The system was used when five crystallization runs with an average run time of 35 hours were made. The goal of the experiments was to study the stability of the crystallization and the possibilities for on- or off-line measurement and control of the crystal size distribution. With the proper selection of the magnitude of the by-pass flow, it was possible to temporarily stabilize the crystal size distribution.

The laser backscattering device, Partec-200, was used for making a total of sixty experiments with three materials: sodium chloride, polystyrene and ammonium sulfate. In the dissolution experiments of sodium chloride the measurement device was able to follow the changes in crystal sizes but with ammonium sulfate the results were not satisfactory. The experiments indicated that the material to be measured significantly affects the ability of the device to follow the changes.

The Partec-200 was also tested in on-line monitoring of the crystallization runs. However, because of the disturbance caused by air bubbles, the measurements could not be used for control.

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