Hydroclones

Sodium carbonate monohydrate crystals from the crystallizers are concentrated in hydroclones and dewatered on centrifuges to between 2 and 6% free moisture. This centrifuge cake is sent to dryers where the product is calcined 150°C to anhydrous soda ash, screened, and readied for shipment. Soda ash from this process typically has a bulk density between 0.99—1.04 g/mL with an average particle size of about 250 p.m. 

Eastman Chemical Co. uses only cobalt and bromine, and lower temperature oxidations are held at 175—230°C (83). Solution of 4-formylbenzoic acid is obtained by using hydroclones to replace the mother hquor from the first oxidation with fresh acetic acid. A residence time of up to 2 h is used in order to allow for sufficient digestion to take place and to reduce the 4-formylbenzoic acid content to 40—270 ppm (83). Recovery of dry terephthahc acid is as described above. 

A BrasiHan company has reportedly produced ammonium chloride from hydrogen chloride gas (16). Hydrogen chloride is mixed with air and iatroduced iato a saturated ammonium chloride suspension at 80 °C. Excess ammonia is added to a conical section of the saturator to maintain a pH of 8. The ammonium chloride is recovered from the suspension by thickening ia a hydroclone, foUowed by centrifugation and dryiag. Mother Hquor and the water used to scmb waste gases, are returned to the saturator. 

In another cocrystalHzation process, lime is mixed with 50% caustic and recycled filtrate and chlorinated to yield a slurry of calcium hypochlorite dihydrate and NaCl crystals that are separated in a hydrauHc classifier. The underflow is mixed with centrate mother Hquor and sent to a wet screen classifier the overflow is recycled to the hydroclone and the salt-rich bottoms are centrifuged. The centrate is recycled to the chlorinator and the salt used as feed to chloralkaH ceUs. The Ca(OCl)2-rich overheads from the hydroclone are centrifuged, the cake going to a dryer and the filtrate sent to the wet screen classifier (207). 

The termination of the cone section is the apex orifice. The critical dimension is the inside diameter at the discharge point. The size of this orifice is determined by the application involved and must be large enough to permit the solids that have been classified to underflow to exit the cyclone without plugging. The normal minimum orifice size would be 10% of the cyclone diameter and can be as large as 35%. Below the apex is normally a splash skirt to help contain the underflow slurry in the case of a hydroclone. 

In determining the proper size and number of cyclones required for a given application, two main objectives must be considered. The first is the classification or separation that is required, and the second is the volume of feed slurry to be handled. In the case of hydroclones, before determining whether these objectives can be achieved, it is necessary to establish a base condition as follows Feed liquid - water at 20 C. Feed solids - spherical particles of 2.65 specific gravity Feed concentration - less than 1 % solids by volume Pressure drop - 69 kPa (10 psi) Cyclone geometry - "standard cyclone" as described above. 

Figure 52. Plot reported by Arterburn on particle diameter vs. particle recovery for hydroclone applications.

The separation that a cyclone/hydroclone can achieve can be approximated from the following relation. The 0 50 (base) for a given diameter cyclone is multiplied times a series of correetion faetors designated by C, Cj, and C ... 

Arterbum, R. A., The Sizing and Selection of Hydroclones, Krebbs Engineering, Menlo Park, CA (1999)... 

The liquid bulk flow limits the upward flow of small particles from the internal side and has a significant influence on the separating effect. Hydroclones are applied successfully for classification, clarification and thickening of suspensions containing particles from 5 to 150 tm in size. 

Using SI units, and from some literature reported values for hydroclones with D between 125 and 600 ram, and a cone angle of 38°, a value for coefficient K is 2.8 X 10. The maximum size of particles in the cleared liquid can be estimated from ... 

The power required to operate a hydroclone is the horsepower needed for a pump that supplies the capacity with an acceptable head of pressure. 

Six-tenths factor, 47 Yearly cost indices, 47 Critical flow, safety-relief, 438 Back pressure, 440 Sonic flow, 438 Critical flow, see Sonic Cyclone separators, 259-269 Design, 260-265 Efficiency chart, 263 Hydroclones, 265-267 Pressure drop, 263, 264 Scrubber, 269 Webre design, 265 Deflagration venting nomographs,... 

Recoverable catalysts also offer a promising way to economize the cost of liquefaction catalysts (74, 75). Dow designed a process that utilized fine powders of MoS2 that were reported to be recoverable by hydroclone however, specific details have not been published (76). 

Iron and chloride catalysts are basically disposable because they are considered to be rather cheap and difficult to recover from residual products, while Ni-Mo and Co-Mo catalysts are too expensive to be considered disposable (82). Recovery of very fine particles of MoS2 by hydroclone separation has been shown to be promising (83). Disposable catalysts added at levels similar to that of ash mineral contents significantly reduce the potential recovery of oil in both distillation and extraction. This is problematic because equal volumes of oil adhere to solid particles after separation. Slurry transportation of residues suffers from the same problem. Even if the cost of the disposable catalysts is affordable, adding 1 to 5% of the catalyst to the... 

Figure 9.13 Battery of hydroclones used for separation of germ from milled, steeped corn. (Courtesy of Dorr-Oliver, Inc., Stamford, Connecticut)...

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