Feed zone acceleration

The power required to bring the feed material up to bowl speed at the pond 

Power available in the feed stream at the pond surface is Pa. where  

The power lost on entry is thus the difference between equations (4.124) and (4.12 5), and this is dissipated in heat and turbulence on entry. Thus to minimise turbulence and power loss, it is necessary to design the decanter with the pond surface as close as practicably possible to the centre line. Nevertheless, other process considerations may require the taking of a different view. 

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This requires pre-accelerating the catalyst to the feed zone. Steam or fuel gas is often used to lift the catalyst to the feed injection. Figure 7-1 shows the design criteria of using steam as a lift media to deliver a dense supply of catalyst to the feed nozzles. 

The feed slurry enters through a stationary central pipe into a feed accelerator/distributor, then is introduced onto the (in this case) oscillating inner basket just in front of the pusher plate. In the feed zone, most of the liquid is drained, forming a cake sufficiently stiff to transfer the push force through the bed of solids and transport the cake without shearing. This is why it requires fast-draining materials and is liquid-limited, since it must form a cake within the period of one stroke. 

In extreme cases of wear, parts inside the feed zone are hard surfaced or specific erosion resistant components are used. Hard surfacing is often used on the accelerator plate, particularly on leading edges and the tips of the vanes, where most wear takes place. Some shaped accelerators have been made completely of urethane rubber. 

When the feed leaves the feed tube, in most cases it is at a high axial velocity. When it hits the rotating target, some splashing inevitably occurs. In fact a dense aerosol mist is often produced. At the back of the feed zone a tube is sometimes built in, to surround the end of the feed tube. On the outside of this tube, small accelerator vanes are welded to accelerate and condense the mist and also to accelerate liquor up to speed, should the feed zone become flooded. Ideally, air is allowed to enter the feed zone from around the feed tube. It will be drawn in by the fan effect of the feed zone and thrown out of the feed zone exit ports. The air would then pass along the bowl to exit over the centrate. This draught helps to prevent splash back of feed from the feed zone. 

New feed zones have been introduced recently to reduce feed particle attrition, by slowing and extending the acceleration time to bring the feed up to speed, and to reduce the inlet turbulence in the separation zone. 

In its simplest form the feed tube is a plain cylindrical tube. A clamp or flange holds it on a support extension from the main frame. It extends to the feed zone and within a few centimetres of the accelerator in the feed zone. 

Simple variations in the design of the feed zone include the number of exit ports, the number and shape of the accelerator vanes and the size of the chamber. 

The smaller the feed zone is relative to the feed rate, then the more sophisticated has to be its design, to ensure that the feed is accelerated to speed without flooding and consequent spillage back through the feed entrance. To this end, sometimes there will be seen accelerator blades to the rear of the feed zone. Also the entrance to the feed zone will sometimes have a tube fitted to surround the feed tube. This ensures a high velocity for the air sucked in around the feed tube to prevent escape of liquor from the feed chamber. 

If large tramp material enters the feed zone, from the feed tube, it may lodge in the feed zone if this zone contains substantial dead areas. Thus, the walls and shape of the feed zone must be designed, not only to accelerate the feed adequately, but also so as to avoid plugging. 

There can almost be an infinite number of feed zone designs with different shaped inlets and outlets, different number of outlets, erosion protection, accelerators and linings. 

The Matsuzaka Elbow-Jet classifier (Fig. 11) is based on a transverse flow principle (26). The stream of feed particles are accelerated to minimize the effect of gravity, and introduced into an air jet at right angles. The particles are fanned out in the classification zone with the trajectories for particles of the same hydrodynamic behavior, ie, size and shape, being the same. Classification is achieved by mounting one or more cutters in the classification zone, thus dividing the feed into two or more fractions. A stream of fine particles of less than 5 Jm can be produced in this manner. 

In the RCC process (Figure 8-13), the clean regenerated catalyst enters the bottom of the reactor riser where it contacts low-boiling hydrocarbon lift gas which accelerates the catalyst up the riser prior to feed injection (Hydrocarbon Processing, 1996). At the top of the lift gas zone the feed is injected through a series of nozzles located around the circumference of the reactor riser. 

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