Implementation of a feedback control for a continuous tubular crystallizer

Objective: The aim of the study is the prove of concept of a feedback control loop for a
continuous tubular crystallizer. Therefore an experimental set-up for the implementation
of a hardware as well a software technical part had to be designed and implemented.
Methods: Due to the implementation of a CSD (crystal size distribution) analyzer, it
was possible to develop a feedback controller capable of tuning crystal sizes. For this
purpose a measurement environment which contains a valve system, a dilution concept
and the development of a control software, was designed and implemented. In addition,
a cleaning concept was implemented to provide long term runs.
Results: The results show that for a consistent process set-up the CSD of the product
crystals remained constant, that product crystals with the mean size of 140 ± 2 μm
were achievable for two different seed suspensions and that the controller was capable
of tuning the mean crystal size accurately within a range of 90 μm–140 μm.
Conclusion: This work demonstrates crystal size tuning in a continuously operated
tubular crystallizer via a simple model free control strategy. It was shown that the
reproduction of product crystals of a certain mean crystal size over a longer period of
time and crystal size tuning were feasible.