Processes control Crystallizers, design

In this paper, three methods to transform the population balance into a set of ordinary differential equations will be discussed. Two of these methods were reported earlier in the crystallizer literature. However, these methods have limitations in their applicabilty to crystallizers with fines removal, product classification and size-dependent crystal growth, limitations in the choice of the elements of the process output vector y, t) that is used by the controller or result in high orders of the state space model which causes severe problems in the control system design. Therefore another approach is suggested. This approach is demonstrated and compared with the other methods in an example. 

Batch crystallization process control using the first-principles and direct design approaches were discussed. The first-principles approach utilizes crystallization process models, which require the associated determination of crystallization kinetics. The optimal seed characteristics and/or supersaturation profile to obtain the desired product characteristics are then computed. The direct design approach involves feedback control of a state measurement, in this case the solution... 

Doki, N. Seki, H. Takano, K. Asatani, J. Yokota, M. Kubota, N. Process control of seeded batch cooling crystallization of the metastable a-form glycine using an in-situ ATR-FTIR spectrometer and an in-situ FBRM particle counter. Crystal Growth Design 2004, 4 (5), 949-953. 

One of the key challenges of the development of a task based design approach for crystallization processes is flie ability to control crystallization tasks independently from each other, which makes optimization of that particular task possible. To demonstrate the practical feasibility of the approach an experimental program has been designed and conducted. The objectives of the experimental work are as follows ... 

This chapter includes a discussion of several special topics on crystallization, including ultrasound for crystallization, crystallization using supercritical fluids, and experimental design and process control. It also contains examples of CrystaUization operations that were developed to meet special requirements. 

The principal variables that must be controlled in crystallization are the solution supersaturation, the crystal surface area available for growth, and the nucleation rate. These are affected by multiple interacting secondary variables, which may be divided into two categories—equipment design variables and dynamic variables affecting the crystallization. It is the secondary dynamic variables, such as those listed in Table 9.1, to which automatic process control is applied in a typical crystallizer for these vari-... 

A crystallizer containing >1000 tubes, all 12 m in length, was designed and built to give an economically viable production capacity. Carefiil attention was ven to the design of the crystallizer, the equipment for providing heat energy at the correct rate, and precise process control. 

Except in special cases, such as those described in sub-paragraphs 4, 5, and 6 above, crystallization is of primary importance in ammonium sulfate production. Fortunately, there are several pdblications reviewing the jfuudamentaLprocess.. ind design features that control crystal formation and influence plant performance [50,51,52,531. Some processes used to produce ammonium sulfate are described below. 

For bonding there are two principle phosphates available. These are calcium modified and tri-cation modified zinc processes. Both are designed to yield low coating weights and fine crystal structures. Each system has its merits and is critical for the control of the baths. 

The particle size measurement of the raw materials is usually done by the supplier or the manufacturer. For the interpretation of those results it is necessary to know the principles of some methods. In the design phase, during in-process controls and at final testing, it is important to choose the particle size measurement method that is relevant to the property for which the particle size is investigated. Next to the determination of the particle size, a description of the nature of particles (crystals, agglomerates or aggregates) may be recommended. 

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