Dry surface area

Here, a, is the total dry surface area of packing per unit volume of column. It is interesting to note that the dependence of Peclet number on the Reynolds number and Galileo number predicted by Eqs. (6-48) and (6-50) are very similar. 

Table 3.4 Comparison of solvated and dry surface areas Sorbent Type Surface area, m /g...

Vacuum band dryers are suitable for all types of pastelike materials. Wet material is dried as it is transported on a moving band. Fleat is supplied by infrared low-temperature radiators or by contact with heated bands. A multiband vacuum dryer is shown in Figure 33.15. To allow continuous production, such dryers are equipped with an automatic discharge system that prevents dehermetization of the dryer chamber. Drying chambers that provide drying surface areas up to 120 m are available commercially. 

This type of dryer is usually designed for specialty tea processing. It is an oven with about four layers of trays. The drying surface area is 1-2 m. Electrical heating or heated air can be the source of drying energy. The wet tea leaves are placed on the top layer tray. The product is obtained in the bottom layer tray. The moving of the trays is accomplished manually... 

In the Prefaces of both the 4th and the 5th editions the senior author commented on the tendency of wet and dry surface chemistry for differentiation into separate schools. This remains the case today also, academic research in wet surface chemistry continues to move from chemistry departments to engineering ones. On the other hand, new connections between the two areas have been forming apace with the current prominence of scanning microscopies. 

It is less well known, but certainly no less important, that even with carbon dioxide as a drying agent, the supercritical drying conditions can also affect the properties of a product. Eor example, in the preparation of titania aerogels, temperature, pressure, the use of either Hquid or supercritical CO2, and the drying duration have all been shown to affect the surface area, pore volume, and pore size distributions of both the as-dried and calcined materials (34,35). The specific effect of using either Hquid or supercritical CO2 is shown in Eigure 3 as an iHustration (36). 

Fibrillated Fibers. Instead of extmding cellulose acetate into a continuous fiber, discrete, pulp-like agglomerates of fine, individual fibrils, called fibrets or fibrids, can be produced by rapid precipitation with an attenuating coagulation fluid. The individual fibers have diameters of 0.5 to 5.0 ]lni and lengths of 20 to 200 )Jm (Fig. 10). The surface area of the fibrillated fibers are about 20 m /g, about 60—80 times that of standard textile fibers. These materials are very hydrophilic an 85% moisture content has the appearance of a dry soHd (72). One appHcation is in a paper stmcture where their fine fiber size and branched stmcture allows mechanical entrapment of small particles. The fibers can also be loaded with particles to enhance some desired performance such as enhanced opacity for papers. When filled with metal particles it was suggested they be used as a radar screen in aerial warfare (73). 

M ass Transfer. Mass transfer in a fluidized bed can occur in several ways. Bed-to-surface mass transfer is important in plating appHcations. Transfer from the soHd surface to the gas phase is important in drying, sublimation, and desorption processes. Mass transfer can be the limiting step in a chemical reaction system. In most instances, gas from bubbles, gas voids, or the conveying gas reacts with a soHd reactant or catalyst. In catalytic systems, the surface area of a catalyst can be enormous. Eor Group A particles, surface areas of 5 to over 1000 m /g are possible. 

DRI can be produced in pellet, lump, or briquette form. When produced in pellets or lumps, DRI retains the shape and form of the iron oxide material fed to the DR process. The removal of oxygen from the iron oxide during direct reduction leaves voids, giving the DRI a spongy appearance when viewed through a microscope. Thus, DRI in these forms tends to have lower apparent density, greater porosity, and more specific surface area than iron ore. In the hot briquetted form it is known as hot briquetted iron (HBI). Typical physical properties of DRI forms are shown in Table 1. 

Allowing DRI to become wet does not necessatily cause it to overheat. When large pdes of DRI are wetted with rain, the corrosion reactions are limited to the outer surface area of the pde and the resultant heat from the corrosion reactions is dissipated into the atmosphere. However, if water penetrates into the pde from the bottom, or if wet DRI is covered with dry DRI, the heat from corrosion reactions can budd up inside the pde to the point where rapid reoxidation begins. Corrosion occurs significantly faster with salt water than with fresh water. DRI saturated with water can cause steam explosions if it is batch charged into an electric arc furnace. 

Surface Area. Surface area is measured by determining the quantity of nitrogen gas that adsorbs on the particle/crystal surfaces of a dry sample. Determination of surface area by measuring adsorption at gas—soHd interfaces is covered extensively in the Hterature (84). Instmments such as the FlowSorb 2300 are used to control the adsorption/desorption within specific conditions of temperature and pressure. 

The newly formed y-Mn02 actually coats the surfaces of the particles of the soHd phase the MnSO dissolves in the Hquid phase, along with the majority of the ore impurities. The effective surface area is expanded by the etching action of the sulfuric acid. Following the acid treatment step, the slurry is filtered and the cake is carefiiUy washed and dried at a controlled temperature. 

The second step is to disperse the core material being encapsulated in the solution of shell material. The core material usually is a hydrophobic or water-knmiscible oil, although soHd powders have been encapsulated. A suitable emulsifier is used to aid formation of the dispersion or emulsion. In the case of oil core materials, the oil phase is typically reduced to a drop size of 1—3 p.m. Once a suitable dispersion or emulsion has been prepared, it is sprayed into a heated chamber. The small droplets produced have a high surface area and are rapidly converted by desolvation in the chamber to a fine powder. Residence time in the spray-drying chamber is 30 s or less. Inlet and outlet air temperatures are important process parameters as is relative humidity of the inlet air stream. 

Iron oxide yellows can also be produced by the direct hydrolysis of various ferric solutions with alkahes such as NaOH, Ca(OH)2, and NH. To make this process economical, ferric solutions are prepared by the oxidation of ferrous salts, eg, ferrous chloride and sulfate, that are available as waste from metallurgical operations. The produced precipitate is washed, separated by sedimentation, and dried at about 120°C. Pigments prepared by this method have lower coverage, and because of their high surface area have a high oil absorption. 

Preparation of Pillared Clay Catalysts. PAG products are used for the preparation of zeolite-like catalysts by intercalation, the insertion of Al polycations molecules between the alurninosiHcate sheets of clay (3,33). Aqueous clay suspensions are slowly added to vigorously stirred PAG solutions, and the reaction mixture is aged for several hours. The clay is separated from the PAG solution and washed free of chloride ion. The treated clay is first dried at low temperature and then calcined in air at 450—500°G, producing a high surface area material having a regular-sized pore opening of about 0.6 to... 

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