Wet materials

Tunnel dryers are shown in Fig. 3.15a. Wet material on trays or a conveyor belt is passed through a tunnel, and drying takes place by hot air. The airflow can be countercurrent, cocurrent, or a mixture of both. This method is usually used when the product is not free flowing. 

Drying is an operation in which volatile Hquids are separated by vaporization from soHds, slurries, and solutions to yield soHd products. In dehydration, vegetable and animal materials are dried to less than their natural moisture contents, or water of crystallization is removed from hydrates. In freeze drying (lyophilization), wet material is cooled to freeze the Hquid vaporization occurs by sublimation. Gas drying is the separation of condensable vapors from noncondensable gases by cooling, adsorption (qv), or absorption (qv) (see also Adsorption, gas separation). Evaporation (qv) differs from drying in that feed and product are both pumpable fluids. 

Wet basis is a material s moisture coateat expressed as a perceatage of the weight of wet material. Although commonly employed, this basis is less satisfactory for dryiag calculatioas than the dry basis for which the percentage change of moisture per unit weight of dry material is constant at aH moisture contents. 

Before drying can begin, a wet material must be heated to such a temperature that the vapor pressure of the contained Hquid exceeds the partial pressure of vapor already present in the surrounding atmosphere. The effect of a dryer s atmospheric vapor content and temperature on performance can be studied by constmction of a psychrometric chart for the particular gas and vapor. Figure 2 is a standard chart for water vapor in air (6). 

Convection heat transfer is dependent largely on the relative velocity between the warm gas and the drying surface. Interest in pulse combustion heat sources anticipates that high frequency reversals of gas flow direction relative to wet material in dispersed-particle dryers can maintain higher gas velocities around the particles for longer periods than possible ia simple cocurrent dryers. This technique is thus expected to enhance heat- and mass-transfer performance. This is apart from the concept that mechanical stresses iaduced ia material by rapid directional reversals of gas flow promote particle deagglomeration, dispersion, and Hquid stream breakup iato fine droplets. Commercial appHcations are needed to confirm the economic value of pulse combustion for drying. 

Contact Drying. Contact drying occurs when wet material contacts a warm surface in an indirect-heat dryer (15—18). A sphere resting on a flat heated surface is a simple model. The heat-transfer mechanisms across the gap between the surface and the sphere are conduction and radiation. Conduction heat transfer is calculated, approximately, by recognizing that the effective conductivity of a gas approaches 0, as the gap width approaches 0. The gas is no longer a continuum and the rarified gas effect is accounted for in a formula that also defines the conduction heat-transfer coefficient ... 

For drying calculations, it is convenient to express Eq. (12-25) in terms of the decrease in moisture content rather than in the quantity of water evaporated. For evaporation from a tray of wet material, if no change in volume during diying is assumed, Eq. (12-25) becomes... 

The value of equilibrium moisture content, for many materials, depends on the direction in which equilibrium is approached. A different value is reached when a wet material loses moisture by desorption, as in drying, from that obtained when a diy material gains it by adsorption. For diying calculations the desorption values are preferred. In the general case, the equilibrum moisture content reached by losing moisture is higher than tnat reached by adsorbing it. 

Diying equipment may be classified in several ways. The two most useful classifications are based on (1) the method of transferring heat to the wet solids or (2) the handling characteristics and physical properties of the wet material. The first method of classification revels differences in diyer design and operation, while the second method is... 

Heat is transferred to the wet material by conduction through a solid retaining wall, usually metalhc. 

Initial. selection of diyers. Select those diyers which appear best suited to handhngthe wet material and the diy product, which fit into the continuity of the process as a whole, and which will produce a product of the desired physical properties. This preliminaiy selection can be made with the aid of Table 12-9, which classifies the various types of dryers on the basis of the materials handled. 

Frequently, particularly in tray diying, heat arrives at the evaporating surface from the tray walls by condiiction through the wet material. For this case, in which both radiation and conduction are significant, the total heat-transfer coefficient is given by Shepherd,... 

Design Methods for Vactium-Shelf Dryers Heat is transferred to the wet material by conduction through the shelf and bottom of the tray and by radiation from the shelf above. The critical moisture content will not be necessarily the same as for atmospheric tray drying [Ernst, Ridgway, and Tiller, Jnd. Eng. Chem., 30, 1122 (1938)]. 

Turbo-Tray Dryers The turbo-tray diyer is a continuous diyer consisting of a stack of rotating annular shelves in the center of which turbo-type fans revolve to circulate the air over the shelves. Wet material enters through the roof, falling onto the top shelf as it rotates beneath the feed opening. After completing one revolution, the mate-... 

The second type is a stable dispersion, or foam. Separation can be extremely difficult in some cases. A pure two-component system of gas and liquid cannot produce dispersions of the second type. Stable foams can oe produced only when an additional substance is adsorbed at the liquid-surface interface. The substance adsorbed may be in true solution but with a chemical tendency to concentrate in the interface such as that of a surface-active agent, or it may be a finely divided sohd which concentrates in the interface because it is only poorly wetted by the liquid. Surfactants and proteins are examples of soluble materials, while dust particles and extraneous dirt including traces of nonmisci-ble liquids can be examples of poorly wetted materials. 

Other applications Desliming—Removal of extremely fine particles from a wet material hy passing it over a screening surface. Conveying—In some instances transport of the material may he as important as the operation. Media r ecovery—A comhination washing and dewatering operation. 

Talc milling is largely a grinding operation accompanied by air separation. Most of the industrial talcs are diy-ground. Diyers are commonly employed to prediy ahead of the milling operation because the wet material reduces mill capacity by as much as 30 percent. 

Pneumatic, conveying and storage Wet process materials handling, grinding, storage Particulates (dust) Wet materials, no dust and baghouse... 

Equipment should be supported in such a way that it will not rest in pools of hquid or on damp insulating material. Porous insulation should be weatherproofed or otherwise protected from moisture and spills to avoid contact of the wet material with the equipment. Specifications should be sufficiently comprehensive to ensure that the desired composition or type of material will be used and the right condition of heat treatment and surface finish will be provided. Inspection during fabrication and prior to acceptance is desirable. 

Ion-exchange resins swell in water to an extent which depends on the amount of crosslinking in the polymer, so that columns should be prepared from the wet material by adding it as a suspension in water to a tube already partially filled with water. (This also avoids trapping air bubbles.) The exchange capacity of a resin is commonly expressed as mg equiv./mL of wet resin. This quantity is pH-dependent for weak-acid or weak-base resins but is constant at about 0.6-2 for most strong-acid or strong-base types. 

Moishire is defined as the ratio of the fluid s weight retained by solids to the weight of the wet material ... 

If the wet material is exposed to air at a give temperature, it will either lose or... 

The XK column system is a medium-pressure jacketed glass column system designed for operating pressures up to 5 bar (0.5 MPa). Column dead volumes are less than 0.1% of the total column volume. Wetted materials include EPDM, TEFZEL, superpolyoxymethylene, and flurorubber. Columns use nylon nets of 10-)Lim mesh size and may be used with most SEC media with particle diameters >20 /xm. Columns are intended for use with aqueous solutions and... 

The HR column series was designed for high-resolution applications at higher pressures than the C, XK, or SR column series. Wetted materials include... 

BioProcess stainless-steel columns are fixed bed height columns designed for the most stringent requirements in the routine production of biopharmaceuticals. Wetted materials include stainless steel, polypropylene, and EPDM. The BPSS series may be operated at pressures up to 3 bar (0.3 MPa) and are supplied with sanitary fittings of 10 or 22 mm i.d. The available column sizes and specifications for the BPSS column series are given in Table 2.18. 

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