Hydrocyclones reverse flow

The reverse-flow hydrocyclone shown in Figure 1.9 is a relatively cheap, compact and versatile device. The basic unit has no moving parts and comprises an inverted conical bottom section attached to a cylinder containing a tangential inlet port. Feed is injected through the port at a mean velocity between 10 and 30 m s whence geometry-induced motion causes the (usually denser) suspended particles to experience centrifugal forces of between... 

Figure 1.9 Cross-section through a reverse-flow hydrocyclone showing the typical flow patterns. The inset photograph (Axsia-Mozley) shows a bank of six cyclones connected to a common feed manifold system.

The idea of utilizing the centrifugal force for separation of liquid from gas also finds application in gas-oil hydrocyclone separators (Fig. 2.9). In order to create a centrifugal stream inside these separators, an attached unit - a hydrocyclone - is installed on their lateral surface (Fig. 2.9, a, b, c). A hydrocyclone is usually a vertical device with a fiat tangential input and directing branch pipe for gas flow reversal in the top part, and a liquid cross-flow section in the bottom part. The technological reservoir is made in the form of a horizontal separator with various devices for additional separation of liquid from gas that are typical for oil-gas separators. 

The pattern of fluid flow within the hydrocyclone body is best described as a spiral within a spiral with circular symmetry. A schematic view of the spiral flow inside a hydrocyclone is shown in Fig. 23b. The entering fluid flows down the outer regions of the hydrocyclone body. This combined with the rotational motion creates the outer spiral. At the same time, because of the wall effect, some of the downward moving fluid begins to feed across toward the center. The amount of inward motion of fluid increases as the fluid approaches the cone apex, and fluid that flows in this inward stream ultimately reverses its direction and flows upward to the cyclone overflow outlet via the vortex finder. This reversal applies only to the vertical component of velocity, and the spirals still rotate in the same circular direction. In the meantime, the downward flow near the wall carries solid particles to the apex opening (bottom outlet). 

Though the term "slurry refers to a suspension of fine solid particles in a liquid, the term slurry reactor is often used for a three-phase system, where both gas bubbles and solid particles are suspended in a liquid phase. For a solid/liquid/gas process, slurry reactors have two obvious advantages the possibilities for very large solid/liquid surface areas and for good heat transfer to the reactor wall. Therefore the volumetric capacity of slurry reactors can be relatively large. However, effective separation of the fine catalyst from the liquid phase may offer considerable technical problems. One possibility is an external separation, e.g. with centrifuges or hydrocyclones, and a transport of a concentrated catalyst slurry back into the reactor. More often internal filters are used, usually consisting of porous tubes (sintered stainless steel, or ceramics), that are cleaned every few minutes by a periodic reversal of the flow. 

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