wear and tear on walls or pipes inside of equipment due to friction of moving solid particles; specially in areas where flow direction changes or velocity increases. Reduction is possible through smooth transitions or the application of a protective coating. The protective layer on the blades of a conveyor screw is called armoring.
Means that a specified minimal particle size has to be retained with absolute certainty.
The transport of gases or vapours by diffusion into a condensed phase (i.e. a liquid or a solid) and forming homogeneous solutions. The gas-specific equilibrium concentration as determined by temperature and pressure limits the amount adsorbed.
Common test material for the evaluation of a filter medium. Arizona Control Test Dust is available in two relatively narrow fractions, i.e. Fine (0–80 µm) and Coarse (0–200 µm).
Activated Carbon (Charcoal)
Activated carbon is made from wood, peat, hardened coal or fruit shells by carbonization in an oven. They are highly porous with a large specific surface area and have excellent adsorption properties. Activated carbon is used for liquids as a filter medium in deep bed filtration for clarification, decolorization, and for flavor adjustment.
Foreign matter added to a partially demoistured bulk, in order to maintain a specified mean product moisture. One employs this method e.g. in the processing of municipal sewage that must be deposited in dry form, however, this method may lead to a technical success but is not desirable because additional material is needed for deposition.
Adherence of solids caused by molecular attraction forces. Originates from Adsorption.
The liquid portion in the bulk of a suspension that is still bound by adhesion after mechanical demoistering. The adsorbed quantity is a function of the solids specific surface, the type of liquid and its structure, respectively, reaching up to 10 molecule diameters, i.e. about 30 x 10-8 cm. This liquid cannot be removed by mechanical means.
One can distinguish between Physisorption and Chemisorption. Physisorption means an enrichment of gases and dissolved substances (adsorbates) due to molecular forces (Van-der-Waals forces, electrostatic forces) at phase boundaries, e.g. a solid surface (adsorbens) or a liquid surface. The larger the interface the more can be adsorbed. The adsorption is normally limited to a mono-molecular surface layer. An adsorption process is accompanied by a release of energy (adsorption energy). The so-called adsorption isotherm correlates the concentration of adsorbate in the fluid around the boundary with the amount already adsorbed. During Chemisorption molecules are bound by chemical bonds at a solid surface.
Changing of product properties by time depending effects under the influence of oxygen, microbial activity, ultraviolet radiation and others. Very important for taking and analyzing samples of materials. I.e. a sample of fresh sewage sludge can alter its filtrability completely after some hours or days if it is not preserved carefully.
Binding of solid particles by adhesive forces which in turn can be generated by Van-der-Waals forces, hydrogen bridges, or cross-linking polymerics, such as flocculants. Agglomerates formed like flakes enhance the separation of solids by increasing their sedimentation velocity, and often improve the permeability of a filter cake by increasing its porosity.
Air Blow Back
Measured air flow through a filter medium disregarding the thickness of the medium. It is quantified by the flow rate of air per unit area under a defined differential pressure which may vary from 12.5, 20 to 25 mm WC (WC = water column). Measured values require care since they may refer to different definitions such as l/m²/s, m³/m²/h, l/dm²/min and cfm/sq.ft.
Factor characterizing the filter cake permeability as derived from the Darcy equation. Typical alpha values of cakes range from 1011 m–2 to 1016 m–2. They represent an integral mean over the entire cake thickness. Individual alpha values can be estimated for instance by the Karman & Kozeny equation. However, it can be done quantitatively only experimentally (Filtratest).
Chemical compound, which can react both as an acid or a base, for instance alumi-num hydroxide, or amino acids.
Chemical compounds behave in an amphoteric manner when they respond against stronger acids as bases, or against stronger bases as acids; e.g. oxides and hydroxi-des of aluminum, zinc or lead, or amino acids.
Angle of Wetting
A molecule with either a single or multiple negative charge. Salts can dissociate in a solution into anions and positively charged cations. In an electrolyte, anions travel to the positive terminal under the application of a direct current. Anionic flocculation agents or anionic tensides carry functional groups with a negative charge.
used for estimating the ‚ settling velocity of particles in ‚ sedimentation when the ‚ Reynolds Number is unknown and therefore ‚ Stokes’ law can not readily be applied. The dimensionless Ar number containing the particle size x on the ordi-nate is plotted against the Ω number involving the settling velocity w, as follows:
where g = earth‘s gravitational acceleration, x = particle diameter, ρs = solids density, ρL= liquid density, ν = kinematic viscosity. Starting with a particle diameter x, the Ar number is calculated and the Ω number is read from the diagram, from which the settling velocity can be calculated.
Type of membrane with an asymmetric pore structure across its thickness. Generally, the smallest pore size structure is facing the suspension while the larger pores face the permeate. Asymmetrical membranes are preferably employed in ultrafiltration. The fine pore size membrane should be minimized in thickness to reduce flow resistance; a coarse pore size membrane layer serves as mechanical support underneath.
Solid-liquid separation apparatus employing candle-shaped sieves that are cleaned in-place by periodic back flushing, or with a mechanical device.