Fluidized bed reactors for polyethylene and polypropylene production are gaining increasing industrial interest because of the relatively low production costs and the ability to produce a wide range of polymer grades. However, agglomeration of particles is a major problem in fluidized bed polymerisation of olefins. The initial formation of particle clusters deteriorates the otherwise uniform heat distribution in the bed. In the resulting hot spots further agglomeration is promoted due to the increased reaction rate and particle stickiness. Without proper control, this process leads to excessive growth of the agglomerate. This ultimately results in complete defluidization, forcing unplanned plant shutdown and subsequent expensive cleaning of the reactor. To control agglomeration, operators commonly rely on average pressure drop or temperature measurements. Unfortunately, these techniques are insensitive to the actual agglomeration process: the detection is often too late to take corrective measures.

We developed a method for monitoring and controlling fluidized bed hydrodynamics: DyMonT (Dynamics Monitoring Toolkit). This method detects agglomeration at an early stage and allows operator actions to control the agglomeration process and prevent defluidization. DyMonT is based on recognising significant differences between a reference pressure fluctuation signal and successive measurements of pressure fluctuations during plant operation using an analysis method called ‘attractor comparison'. Several tests have demonstrated the selectivity and effectiveness of DyMonT to detect agglomeration in various types of systems. Small-scale tests also demonstrated the potential to control agglomeration by means of the bed material make-up. The use of DyMonT is not limited to early agglomeration detection in fluidized beds for polymerisation. It can also be used for detecting undesired misbehaviour in other multiphase systems, such as bubble columns and slurry reactors.

A prototype of the detection and control system is currently being tested in a full-scale fluidized-bed reactor for the production of polyethylene of a major European chemical company. We will present some of the first operational results with the DyMonT system.