Fractionating powders

Industrial reactors generally operate at very high velocities (of order 1 m/s) much in excess of terminal falling velocity for at least the finest powder fractions. Powder is continually elutriated and returned to the bed via cyclones. Under these conditions there is disagreement as to whether or not bubbles retain their identity and such beds have been described as "turbulent" or "fast fluidised". What little evidence there is supports the continued existence of bubbles but now in a much disturbed or heterogeneous dense phase and with a less definite shape. Until more is known about this physical situation it is not easy to see how the bubbling bed reactor models should be modified correctly to describe this flow regime. 

The present study was initiated to eliminate the difficulties inherent in the previous experiments. It consists of measurements of the integral enthalpies (via immersional heat measurements) and entropies (via adsorption isotherms) for ground and fractionated powders of fused quartz with a wide spectrum of specific surface areas. 

Benyahia S, Syamlal M, O Brien T (2006) Extension of Hih-Koch-Ladd drag correlation over all ranges of Reynolds number and solids volume fraction. Powder Technology 162 166-174... 

Fig. 10. Temperature dependence of H spin-lattice relaxation times, T of B. mori silk fibroin fiber, film, powder, and the cry,stalline fraction powder, in a dry system.

Silk fiber Silk film o Silk powder A Crystalline fraction powder... 

Figure 3.9. A nest of sieves is used to conveniently fractionate powders into fractions of different size range. The equipment shown above is manufactured by the W. S. Tyler Company [13]. a) A nest of sieves mounted in a Ro-tap sieving machine, b) Cohesive powders can be fractionated using wet sieving equipment. (Photographs courtesy of W. S. Tyler Particle Analysis and Fine Screening Group.)...

Figure 5.1. In a vertical elutriator upward moving fluid fractionates powders into portions greater than and less than the cut size determined by the Stokes Equation from the properties of the parade, the fluid, and the fluid velocity.

It is common practice in the air elutriation of fine powders to add what is known as a dea lomerating j ent or a flow agent to improve the flow characteristics of the powder. A frequendy used flow agent is colloidal silica. The exact mechanisms by which the improvement of flow is achieved by the addition of these very fine powders is not fully under stood [10] In their discussion of the performance of the elutriator shown in Figure 5.4, Leschonski and Rumpf state that the equipment should not be used to fractionate powders smaller than 10 microns [10]. 

BeU RA (2000) Numerical modelling of multi-particle flows in bubbling gassohd fluidized beds. Licentiate thesis, Swinburne University of Technology, Melbourne Benyahia S, Syamlal M, O Brien T (2006) Extension of Hill-Koch-Ladd drag correlation over aU ranges of Reynolds number and solids volume fraction. Powder Technol 162 166-174 Benyahia S (2008) Verification and validation study of some polydisperse kinetic theories. Chem Eng Sci 63 5627-5680... 

The crude acetonitrile contains as impurity chiefly acetic acid, arising from the action of phosphoric acid on the acetamide. Therefore add to the nitrile about half its volume of water, and then add powdered dry potassium carbonate until the well-shaken mixture is saturated. The potassium carbonate neutralises any acetic acid present, and at the same time salts out the otherwise water-soluble nitrile as a separate upper layer. Allow to stand for 20 minutes with further occasional shaking. Now decant the mixed liquids into a separating-funnel, run off the lower carbonate layer as completely as possible, and then pour off the acetonitrile into a 25 ml, distilling-flask into which about 3-4 g. of phosphorus pentoxide have been placed immediately before. Fit a thermometer and water-condenser to the flask and distil the acetonitrile slowly, collecting the fraction of b.p. 79-82°. Yield 9 5 g. (12 ml.). 

Bromoform. Commercial bromoform should be shaken thoroughly with water, separated, dried over powdered anhydrous sodium sulphate and then fractionally distilled under reduced pressure using a water-condenser. It should be stored in a dark cupboard. It is an excellent solvent, has the advantage of a high Constant, and very seldom causes association of the solute. 

Preparation of benzyl cyanide. Place 100 g. of powdered, technical sodium cyanide (97-98 per cent. NaCN) (CAUTION) and 90 ml. of water in a 1 litre round-bottomed flask provided with a reflux condenser. Warm on a water bath until the sodium cyanide dissolves. Add, by means of a separatory funnel fitted into the top of the condenser with a grooved cork, a solution of 200 g. (181-5 ml.) of benzyl chloride (Section IV.22) in 200 g. of rectified spirit during 30-45 minutes. Heat the mixture in a water bath for 4 hours, cool, and filter off the precipitated sodium chloride with suction wash with a little alcohol. Distil off as much as possible of the alcohol on a water bath (wrap the flask in a cloth) (Fig. II, 13, 3). Cool the residual liquid, filter if necessary, and separate the layer of crude benzyl cyanide. (Sometimes it is advantageous to extract the nitrile with ether or benzene.) Dry over a little anhydrous magnesium sulphate, and distil under diminished pressure from a Claisen flask, preferably with a fractionating side arm (Figs. II, 24, 2-5). Collect the benzyl cyanide at 102-103°/10 mm. The yield is 160 g. 

Fine Powder Resins. Fine powder PTFE resins are extremely sensitive to shear. They must be handled gendy to avoid shear, which prevents processing. However, fine powder is suitable for the manufacture of tubing and wire insulation for which compression molding is not suitable. A paste-extmsion process may be appHed to the fabrication of tubes with diameters from fractions of a millimeter to about a meter, walls from thicknesses of 100—400 )J.m, thin rods with up to 50-mm diameters, and cable sheathing. Calendering unsintered extmded soHd rods produces thread-sealant tape and gaskets. 

The paste-extmsion process includes the incorporation of ca 16—25 wt % of the lubricant (usually a petroleum fraction) the mixture is roUed to obtain uniform lubricant distribution. This wetted powder is shaped into a preform at low pressure (2.0—7.8 MPa or 19—77 atm) which is pushed through a die mounted in the extmder at ambient temperature. The shear stress exerted on the powder during extmsion confers longitudinal strength to the polymer by fibrillation. The lubricant is evaporated and the extmdate is sintered at ca 380°C. 

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