Fine particle carry-over

Particle carry-over. To determine whether particles will be carried over, the superficial liquid up-flowrate must be detenuined and compared with the settling velocity of the slowest settling particles i.e. the fines. 

To protect the condensote coalescer from plugging by any fine solids carried over from the primary water/condensote separator a Condensate Filter was installed upstream of the coalescer to remove all particles larger thon 5 micron. 

The particle carry-over into the downstream equipment is the main gas production limit It strongly depends on the oxygen load, fines fraction in the fuel or fragmentation of the fuel, and operating stability (e.g., partly bed fluidization). 

Earlier on when we described the cutting action of the drill bit we learned about the drilling fluid or mud. The mud cools the bit and also removes the cuttings by carrying them up the hole outside the drill pipe. At the surface the mud runs over a number of moving screens, the shale shakers (Fig. 3.11) which remove the cutting for disposal. The fine particles which pass through the screens are then removed by desanders and desilters, usually hydrocyclones. 

The length of time the particles stay in the pool determines the distance the particles settle in the pool. Thus, the feed entrance must be located so that the velocity of the pulp toward the weir, together with the distance, allows sufficient time for the fine particles to be carried out over the weir as the... 

Mechanical Scrubbers Mechanical scrubbers comprise those devices in which a power-driven rotor produces the fine spray and the contacting of gas and liquid. As in other types of scrubbers, it is the droplets that are the principal cohectiug bodies for the dust particles. The rotor acts as a turbulence producer. An entrainment separator must be used to prevent carry-over of spray. Among potential mainte-... 

The finest particles composed of fine silt and clay are carried over much larger distances (Fig. 1.6). Using oxygen isotopes of aeolian quartz, it has been shown that significant quantities of dust with an average diameter of 1-10 microns are transported by jet streams from desert zones over the rest of the world. Eolian admixtures have been referred to explain the presence of Si02 in soils developed in non-quartz-containing volcanic rocks in Hawaii and Israel. 

The need to remove suspended dust and mist from a gas arises not only in the treatment of effluent gas from a plant before it is discharged into the atmosphere, but also in processes where solids or liquids are carried over in the vapour or gas stream. For example, in an evaporator it is frequently necessary to eliminate droplets which become entrained in the vapour, and in a plant involving a fluidised solid the removal of fine particles is necessary, first to prevent loss of material, and secondly to prevent contamination of the gaseous product. Further, in all pneumatic conveying plants, some form of separator must be provided at the downstream end. 

Where there is a wide range of particle sizes in the powder, fluidisation will be more even in a bed that is tapered so as to provide the minimum cross-section at the bottom. If the pressure gradient is low and the gas does not therefore expand significantly, the velocity will decrease in the direction of flow. Coarse particles which will then tend to become fluidised near the bottom of the bed assist in the dispersion of the fluidising gas. At the same time, the carry-over of fines from the top will be reduced because of the lower velocity at the exit. 

In these devices classification occurs in the space over a fluidized bed from which fine particles are carried away with the air stream. The air velocity must essentially exceed the maximal fluidization velocity of fine fraction. These classifiers operate on the gravitational counterflow separation principle with fluidized bed used to increase particles residence time in the separator. Coarse particles can move horizontally across the bed to the exit, which is another advantage of this device. 

We have ass ned that v is determined by conditions of turbulence. However, this is not always the case and it is an observed phenomenon that fine particles are not necessarily set in motion with the same fluid intensities needed to move the larger particles. Greater intensities are often necessary for fine than for large particles. This is due to the fact that when the surface is composed of small particles the drag is distributed over the whole surface. A large exposed particle, on the other hand, may carry not only the drag on the area it occupies but also for an area about it as much as twenty times its projected area. 

For polydisperse solids, the higher velocity at the lower section of a cone guarantees adequate fluidization of the coarser particles, while the lower velocity at the top section prevents excessive carry-over of the fines ... 

The fluidized-bed reactor allows catalyst to be regenerated while the unit is in operation by continuously removing a portion of the catalyst from the reactor for regeneration treatment and subsequent flow-back into the reactor. Because there is a tendency for the catalyst particles to be carried over in the product stream, auxiliary units, such as cyclone separators or dust collectors, must be provided for separating out the solid particles or catalyst fines. 

In the fluidised-bed gasifier, finely ground coal particles (<5mm) enter the top and are fluidised in upward flowing steam and oxygen, fed from the bottom of the gasifier. Fluidisation results in some carry-over of particles which are separated from the synthesis gas in a cyclone and returned to the gasifier. 

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