Elutriated particles

A lot of attempts have been made to describe the time dependence of the attrition rate in batch fluidized bed processes. Gwyn (1969) studied the degradation of catalysts in a small-scale test apparatus and defined the elutriated particles as the only attrition product. He described the increase of the elutriated mass, Wel, with time, t, based on the initial solid bed mass, Wbed 0, by the now widely known Gwyn equation ... 

These difficulties are avoided in Gwyn s (1969) design (Fig. 3). Here, the attrition products are not kept inside the system but it is rather assumed that they are elutriated. In the enlarged diameter top section, gravity separation defines the limiting diameter of the elutriable particles. The attrition rate is assumed to be given by the elutriation rate. The steady-state elutriation rate can, therefore, be used as a friability index. 

Elutriation is important in most industrial fluidized beds and is generally thought of as a disadvantage. In addition to the small particles which may be present in the initial particle size distribution, fines may be created in the course of operation by the attrition of bed particles. Elutriated particles usually need to be collected and recovered either because they represent the loss of product particles of a given size, because they must be separated from the exhaust gas for environmental reasons, or because of safety concerns there is a considerable risk of a dust explosion with very fine particles and perhaps especially so with many food particulates. Therefore the fluidized bed plant will require ancillary gas cleaning equipment such as a cyclone, filter or electrostatic precipitator to separate the fines from the gas. The loss of a particular size fraction from the bed may change fluidized bed behaviour and it then becomes important to return the fines to the bed continuously. 

The bed mass loss is caused by elutriated particles (dust) generated by attrition, breakage and non-deposited spray drops (overspray). The overspray depends on the position of the nozzle. 

Sizing by elutriation is the reverse of the technique used for sedimentation. While any fluid may be used for elutriating particles, air has been found most satisfactory thus far. The basis of elutriation methods is... 

Crystal Size Distribution. The measured crystal size distribution followed a log-normal form, suggested as characteristic for most small particles by Herdan (6). Figure 1 shows results obtained with the 4A crystal powder and with the 2 types of pellets formed from it. Here and below, the Linde crystal powder as received, the pelletized powder, and the pellets formed from the elutriated powder will be referred to as CPR, CPS, and ECPS, respectively. Clearly, the pelletizing process did not alfect the size distribution of the original material. Furthermore, the elutriated particles do have a somewhat larger average size. 

Typical fluid bed processors are illustrated in Fig. 1. Fig. lA is a simple fluid bed, usually with a conical shape at the bottom of the chamber, leading to inducing spouted particle flow more or less depending on the angle of the conical chamber. The spray is supplied from the top toward the fluidized bed in agglomeration and into the bed in a tangential direction in coating. Elutriated particles from the fluid bed are... 

Bubbling bed combustors conduct almost all of their combustion and, simultaneously, considerable heat transfer within the relatively dense bubbling bed. The freeboard is primarily used to complete CO and volatiles combustion and to disengage solids from the raw combustion gas before the gas reaches the convective heat transfer surface. Elutriated particles that are captured by cyclones may be recycled, at relatively low temperature, to the fluidized bed combustor. 

Figure 6.3.1. Separation systems where the bulk flow is parallel to the direction offorce (a) elutriation (particle separation in liquids) (b) capillary electrophoresis (c) centrifugal elutriation (d) inertial impaction in particle filtration (e) distillation (flash/batch) (f) liquid extraction (separating funnel) (g) zone melting (h) normal freezing (i) cake filtration (j) ultri tration separation of proteins (dead-end) (k) batch cell reverse osmosis separation of brine (1) gas permeation.

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