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Title: Mathematical modelling of the automatic control characteristics of a fluidized bed dryer
Author: Nehal, K.
Awarding Body: University College of Swansea
Current Institution: Swansea University
Date of Award: 1991
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Maxiheat Ltd uses a fluidized bed dryer heated via a direct combustion system to dry coal as part of its briquetting process. The company wishes to improve its efficiency and solve some problems encountered in the drying section. The objective of this investigation was to develop input/output mathematical models which can be used to predict the performance of the drying system and to design a system to control the outlet coal moisture content. These models were based on the theories of combustion, fluidization and drying and the resulting nonlinear differential equations were solved by computer to predict both the steady state and dynamic behaviour of the dryer. A linearized form of the model was developed to obtain plant transfer functions which may be used for control system design. Models of the controlled dryer based on both the nonlinear and linearized models were developed to simulate a feedback controller and near optimal values of control parameters were obtained. However such a system cannot yet be implemented in practice because there is no satisfactory method of measuring coal moisture content on line. Because of this, inferential control using feedback of other plant variables was simulated. This was found to give unsatisfactory control of the moisture particularly following external plant disturbances. The model equations were also used in a preliminary study of plant corrosion due to sulphur dioxide in the combustion gases and recommendations to alleviate the problem have been made. A small scale dryer has been developed to study the fluidization quality and the heat and mass transfer in the bed, and to validate correlations on minimum fluidizing velocity etc. which were used in the models. Transient performance of this dryer, but used in batch rather than continuous mode, was measured experimentally. The forms of the transfer functions obtained agreed closely with those predicted by the linearized model of the main dryer although it was not possible to validate the heat and mass transfer coefficients used in the models. These results suggest that model validation must be done on the full size plant on an input/output basis and not by validating the design correlations.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available