Cyclone centrifuge

Fig. 9 Elutriators can use gravity or centrifugal force to separate fine particles into undersize and oversize fractions. (A) Gravity elutriator (B) cyclone (centrifugal elutriator) and (C) impactor. (From Ref. l)...

The effect of contactor design on the selectivity of amine solutions for absorbing H2S in tire presence of CO2 has been smdied by Darton et al. (1987). They conclude that selectirity, as represented by the ratio of overall mass transfer coefficients for H2S and CO2, is about the same (100) for trays and 2-in. Pall rings in a MDEA contactor operating at atmospheric pressure. Their work indicates that even higher selectivities should be possible with alternative contactor designs, such as cyclones, centrifuges, and cocunent gas/hquid flow tubes. 

The simplest type of centrifugal device is the cyclone separator (Fig. 3.4), which consists of a vertical cylinder with a conical bottom. The centrifugal force is generated by the fluid motion. The mixture enters in a tangential inlet near the top, and the rotating motion so created develops centrifugal force which throws the particles radially toward the wall. 

Figure 3.4 A cyclone generates centrifugal force by the fluid motion.

Cyclones. Cyclones are also primarily used as prefilters. These also were discussed in Chap. 3 and illustrated in Fig. 3.4. The particle-laden gas enters tangentially and spins downward and inward, ultimately leaving the top of the unit. Particles are thrown radially outward to the wall by the centrifugal force and leave at the bottom. 

Centrifugal demister or cyclone) devices rely on high velocities to remove liquid particles and substantial pressure drops are required in cyclone design to generate these velocities. Cyclones have a limited range over which they operate efficiently this is a disadvantage if the input stream flowrate is very variable. 

Hydrocylones have become common on offshore facilities and rely on centrifugal force to separate light oil particles from the heavier water phase. As the inlet stream is centrifuged oil particles move to the centre of the cyclone, coalesce and are drawn off upwards, while the heavier water is taken out at the bottom. 

Other Centrifugal Collectors. Cyclones and modified centrifugal collectors are often used to remove entrained Hquids from a gas stream. Cyclones for this purpose have been described (167—169). The rotary stream dust separator (170,171), a newer dry centrifugal collector with improved collection efficiency on particles down to 1—2 pm, is considered more expensive and hence has been found less attractive than cyclones unless improved collection in the 2—10-pm particle range is a necessity. A number of inertial centrifugal force devices as well as some others termed dynamic collectors have been described in the Hterature (170). 

The reaction mixture can either be crystallised, centrifuged, and dried, or spray-dried and cyclone-separated to produce a fine crystalline powder having a particle size of 50 p.m. Metal analysis of the AP produced by this method is reported to be less than 0.02 p.g/g. 

Hydrolysis of Dimethyl Terephthalate. Hoechst Celanese and Eormosa Chemical Eibers Corp. produce a polymer-grade terephthahc acid by hydrolysis of high purity dimethyl terephthalate. Hbls-Troisdorf AG hcenses a process with this step (70). Hydrolysis occurs at 260—280°C and 4500—5500 kPa (45—55 atm) in a hydrolysis reactor without catalysis. The overhead methanol and water vapor is separated and the methanol is returned to the dimethyl terephthalate section for reuse. The reactor hquid is crystallized, cycloned, washed, and further cooled. Einahy, the slurry is centrifuged and dried. The product has less than 25 ppm of 4-formylbenzoic acid and very low levels of other impurities. There may be several hundred parts per million of monomethyl terephthalate, which is incompletely hydrolyzed dimethyl terephthalate. 

Etibank is the sole producer of boron minerals and derivatives in Turkey, which is second in production only to the United States. The open-pit Kirka mine in the Eskisehir Province is the only source of Turkish sodium borate ore. A tincal concentrate is produced from ore that has been blasted and carried in tmcks to the concentration plant. The ore is screened and cmshed to reduce it to —100 mm and then hammer milled to —25 mm. The stockpiled material is further milled and screened to 6 mm. A fraction of +1 mm (+18 mesh, U.S. Standard) is washed and classified to remove fine clay. Clay is removed from the —1 mm faction bypassing it through cyclones and then through a classifier. This material is centrifuged and combined with the washed + 1 mm faction to produce a final product which is a —6 mm concentrate having 6—8 wt % moisture and 32% 2 3 1987 production was... 

Glassification. Classification (2,12,26,28) or elutriation processes separate particles by the differences in how they settle in a Hquid or moving gas stream. Classification can be used to eliminate fine or coarse particles, or to produce a narrow particle size distribution powder. Classification by sedimentation iavolves particle settling in a Hquid for a predetermined time to achieve the desired particle size and size distribution or cut. Below - 10 fim, where interparticle forces can be significant, gravitational-induced separation becomes inefficient, and cyclone and centrifugation techniques must be used. Classification also separates particles by density and shape. Raw material separation by differential sedimentation is commonly used in mineral processiag. 

Centrifugal Separation Centrifugal force can be utilized to enhance particle collection to several hundredfold that of gravity. The design of cyclone separators for dust removal is treated in detail in Sec. 17 under Gas-Solids Separations, and typical cyclone designs are shown in Fig. 17-43. Dimension ratios for one family of cyclones are given in Fig. 17-36. Cyclones, if carefully designed, can be more efficient on hquids than on solids since liquids coalesce on capture and are easy to drain from the unit. However, some precautions not needed for solid cyclones are necessary to prevent reentrainment. 

FIG. 14-108 ( 7)Liqi lid entrainment from the bottom of a vessel by centrifugal flow. (Rietema and Veroer, Cyclones in Industry, Elsevier, Amsterdam, 1961. ) (h) Gas-oiitlet sldrt for liquid cyclones. (Stern et al. Cyclone Dust Collectors, Ametican Petroleum Institute, New York, 1955. )... 

For determination of the aerodynamic diameters of particles, the most commonly apphcable methods for particle-size analysis are those based on inertia aerosol centrifuges, cyclones, and inertial impactors (Lundgren et al.. Aerosol Measurement, University of Florida, Gainesville, 1979 and Liu, Fine Paiiicles—Aerosol Generation, Measurement, Sampling, and Analysis, Academic, New York, 1976). Impactors are the most commonly used. Nevertheless, impactor measurements are subject to numerous errors [Rao and Whitby, Am. Ind. Hyg. A.s.soc.]., 38, 174 (1977) Marple and WiUeke, "Inertial Impactors, in Lundgren et al.. Aerosol Measurement and Fuchs, "Aerosol Impactors, in Shaw, Fundamentals of Aerosol Sci-... 

Cyclones work by using centrifugal force to increase the gravity field experienced by the solids. They then settle to the wall under the influence of their increased weight. Settling is improved as the path the solids traverse under centrifugal flow is increased. This pam is... 

Cyclone Separators Finer feed sohds, from 0.04 to 0.0005 m (1.5 in to 28 mesh), may be treated in dynamic separators of the Dutch State Mines cyclone type (Fig. 19-36). In cyclone separators, the medium and the feed enter the separator together tangentially at the feed inlet (1) the short cyhndiical section (2) carries the central vortex finder (3), which prevents short circuiting within the cyclone. Separation is made in the cone-shaped part of the cyclone (4) by the action of centrifugal and centripetal forces. The heavier portion of the feed leaves the cyclone at the apex opening (5), and the hghter portion leaves at the overflow top orifice (6). 

For any cyclone, regardless of type, the radius of motion (curvature), the particle mass, and the particle velocity are the three factors which determine the centrifugal force exerted on the particle. This centrifugal force may be expressed as... 

For. 1 given cyclone collector, plot centrifugal force as a function of particle specific grav ity (0.5U-3.00), gas velocity (175-1750 m min ), and radius of curvature (30-250 cm)... 

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