Sieving Surfaces

To allow for these practical difificulties it is usually necessary to rigidly specify and adhere to sieve fractionation study protocols. One also may fix limitations on the permitted variations in sieve apertures and sieving times according to empirical limits. In the manufacturing of sieve surfaces using wire woven cloth most major industrial nations have established specifications for sieves used in test studies [3, 7—10]. See Table 3.1. 

Permitted Variations in Aperture Dimensions for Wire-Woven Sieves Conforming to U.S. Sieve Series (Fine Series)  

Width of Opening urn Average Screen No. Maximum Opening ( ) Wire Opening (+) Diameter mm 

F uie 3.2. The range of apertures permitted in a wire woven sieving surface are strictly limited by ASTM and similar standaids [10]. Shown above are the ap erture size distributions derived from two sets of 100 measurements made at the midpoint of the apertures of a 63 mesh ASTM sieve. Note that the aperture size has been normalized by dividing the measurements by the nominal aperture size of the 65 mesh sieve. 

Equivalent Mesh Openings Mesh Openings DIN Mesh/cm Openings 

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Sieving Time Because sieving time depends upon the type of sieving motion and the provision for continuous movement of the sample over all parts of the sieving surface, the only requirement for sieving time appears to be that shaking be continued until equilibrium conditions are reached... 

Hillock, A.M.W., Miller, S.J., and Koros, W.J. (2008) Crosslinked mixed matrix membranes for the purification of natural gas effects of sieve surface modification./. Memhr. Sci., 314, 193-199. 

The hydrophobic character of molecular sieves surface and high resistance to acidic and basic media increased their competitiveness for use as adsorbents for separation of gases. They proved themselves even in such processes as separation of gaseous mixtures where the critical size of molecules differed only by 0.02 nm. The possibility to control molecular sieve properties during their preparation is the next reason for the interest in these materials as industrial adsorbents [4]. 

If we define in that way the phenomena taking place on the molecular sieves surface, the first step to characterize the type of separation mechanism of a specific gas mixture should be determination of the pore sizes. However, selection of the measurement method for characterization of porous structures also has to take into account these mechanisms. 

The geometry of the sieving surface, e.g. fractional open area. 

Whether or not the particle will pass the sieve when it is presented at the sieving surface will then depend upon its dimensions and the angle at which it is presented. The size distribution given by the sieving operation depends also on the following variables. 

Kaye, B.H. Yousufzai, M.A.K. Calibrating and monitoring woven wire sieving surfaces. Powder Bulk Eng. 1992, (Jan.), 29-34. 

The results from the IGC experiment on Upper Freeport Coal (APCS No. 1) are presented in Figure 3. There are three retention mechanisms likely to affect the IGC results. These mechanisms include molecular sieving, surface adsorption, and the dissolution of the probe in the stationary phase. The observed retention behavior results from the combination of the effects operating under the given conditions. The specific retention volume, Vg, is given by the sum of the retention terms in Equation 1 ... 

Four-A molecular sieve surface area 640 to 80 nf/g pore volume 0.27 cmVg, temperature <300°C superficial gas velocity 150 to 250 dmVm -s usually adsorb 480 to 720 kg/m 0.05 kg nitrogen/kg dry solid 0.22 to 0.36 kg water/kg dry solid. Lifetime 400 cycles. 

Sorption n. The process of one substance taking up and holding another by physical or chemical action. Usually the sorbed substance is mobile, a gas or vapor, and the sorbing phase is dense, a liquid or solid. However, components of liquids can also be sorbed by solids, as by a molecular sieve. Surface phenomenon which may be either absorption or adsorption or a combination of the two. Often used when the specific mechanism is not fully known. Wypych G (ed) (2001) Handbook of solvents. Chem-tec Publishing, New York. 

There are many transport mechanisms Poiseuille, Knudsen, molecular sieving, surface diffusion, multilayer diffusion, and capillary condensation. 

In a size analysis by screening, material is successively passed over a series of sieves having progressively smaller openings. Particles are passed or retained on a particular aperture size of sieve surface. The object of the experiment is to determine the size distribution of coarse-sized dolomite and compare the results with those of the microscopy counting method. 

An icosahedral sieving chamber can be rotated on two great circular supports. Ten triangular surface elements between these can carry several sieving surfaces. The system operates in continuous or in batch mode, the latter being preferable for powder characterization. The residue on the sieve after a prolonged period can be calculated. 

In this section we will concern ourselves with the manufacture and characterization of sieving surfaces. The first type of sieving surface to be widely used in powder technology was woven wire. In the manufacture of sieves for analytical studies, it is usual to have the wire used to weave the cloth forming the surface of the sieve the same diameter as the aperture created by the weaving process. The sieve apertures created by such a process have the appearance shown in Figure 3.1(a). The aperture defined... 

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