Drying of porous solids

In the materials processing industry, size reduction or comminution is usually carried out in order to increase the surface area because, in most reactions involving solid particles, the rate of reactions is directly proportional to the area of contact with a second phase. Thus the rate of combustion of solid particles is proportional to the area presented to the gas, though a number of secondary factors may also be involved. For example, the free flow of gas may be impeded because of the higher resistance to flow of a bed of small particles. In leaching, not only is the rate of extraction increased by virtue of the increased area of contact between the solvent and the solid, but the distance the solvent has to penetrate into the particles in order to gain access to the more remote pockets of solute is also reduced. This factor is also important in the drying of porous solids, where reduction in size causes both an increase in area and a reduction in the distance... 

Newman, A. B. Trans. Am. Inst. Chem. Eng. 27 (1931) 203. The drying of porous solids diffusion and surface emission equations. 

Section V deals with the drying of porous solids in continuous operations. The study of drying in rotary and tunnel dryers is presented based on the relationships derived from basic theory. The effect of the operating variables on drier performance is discussed. A suitable procedure is developed for sizing rotary and tunnel driers. 

A number of problems such as the drying of porous solids of various shapes fall into this category. Solutions to such similar problems may be fonnd in Crank. Moihemaiics of Diffusion and also in Carslaw and Jneger, Heat Conduction in Solids. since the unstesdy-slate diffusion equation often takes on the same form as that fur heat conduction. 

The diameter or the radius of the pores is one of the most important geometric characteristic of porous solids. In terms of lUPAC nomenclature, we can have macropores (mean pore size greater than 5 x 10 m), mesopores (between 5 x 10 and 2 x 10 m) and micropores (less than 2 x 10 m). The analysis of species transport inside the porous structure is very important for the detailed description of many unit operations or applications among them we can mention suspension filtration, solid drying and humidification, membrane processes (dialysis, osmosis, gaseous permeation. ), flow in catalytic beds, ion exchange, adsorp-... 

In this section we shall briefly review some phenomena related to the penetration [imbibition] of porous solid materials by a liquid. These phenomena have great practical relevance. Numerous processes come to mind in which they play a role uptake of water in pharmaceutical powders, imbibition of dry soils by rain water, absorption of ink by paper, and tertleiry oil recovery. In the last example one fluid (mostly gas or aqueous) displaces the other (crude) and the process is therefore forced, in contradistinction to the other examples where penetration is spon-... 

Waananen et al. (1993) have provided an extensive bibliography of over 200 references dealing with models of drying for porous solids. Basically, such models are useful to describe... 

Amato G, Bullara V, Brunetto N, Boarino L (1996) Drying of porous silicon a Raman, electron microscopy and photoluminescence study. Thin Solid Films 276(l-2) 204-207... 

Isolation of Product. Cool the mixture to room temperature, whereupon the product aystaUizes from solution. Cool the vial and contents in an ice bath for 10 min and collect the solid by vacuum filtration using a Hirsch funnel ( -). Rinse the filter cake carefully by dropwise addition of 0.5 mL of cold hexane (Pasteur pipet) and continue the suction for several minutes. Complete the drying of the solid product by placing the crystals on a porous clay plate or on filter paper. 

Figure 6.3.20. (a) Conduction drying of porous wet solids on a plate/tray heated from below and subjected to vacuum at the top surface, (b) Typical equilibrium moisture content behavior vs. percent relative humidity. 

Where U is the overall heat-transfer coefficient, T. is the cylinder temperature (usually very close to that of the steam), Ts is the surface temperature of textile and A is the area of the drying surface on the cylinder. The value of U can be estimated from the conductivity of the cylinder material and of the layer of porous solid. 

Method 2. Mix 1 0 g. of 3 5-dinitrobenzoic acid with 1 5 g. of phosphorus pentachloride in a small, dry test-tube. Warm the mixture gently over a small smoky fiame to start the reaction when the reaction has subsided (but not before), boil for 1-2 minutes or until the solid matter has dissolved. Pour the mixture while still liquid on a dry watch glass (CAUTION the fumes are irritating to the eyes). When the product has solidified, remove the liquid by-product (phosphorus oxychloride) by transferring the pasty mixture to a pad of several thicknesses of filter paper or to a small piece of porous tile. Spread the material until the liquid has been absorbed and the residual solid is dry. Transfer the 3 5 dinitrobenzoyl chloride to a test-tube, add 0-5-1 ml. of the alcohol, and continue as in Method 1. 

I) When working with larger quantities of material, it is more convenient (and a better yield is obtained) to purify the air-dried product by distillation under diminished pressure. Use the apparatus pictured in Fig. II, 19, 4, and add a few fragments of porous porcelain to the solid. No air inlet can be employed to prevent bumping since this may lead to explosive decomposition. Collect the pure m-nitrophenol at I60-I65°/I2 mm. always allow the flask to cool before admitting air otherwise the residue may decompose with explosive violence. The recovery is over 90 per cent, of the pure m-nitroplienol. 

Diffusion in porous solids is usually the most important factor con-troUing mass transfer in adsorption, ion exchange, drying, heterogeneous catalysis, leaching, and many other applications. Some of the... 

Mass is transferred in drying as a hquid and vapor within the solid and as vapor from the exposed surfaces. Movement within the solid results from a concentration gradient which is dependent on the characteristics of the solid. A solid to be dried may be porous or non-porous. It can also be hygroscopic or nonhygroscopic. Many solids fail intermediately between these two extremes, but it is generally convenient to consider the solid to be one or the other. 

Absorption - Processes water can be removed from a material by the capillary action of porous bodies. An example is the cream of clay and water used for casting pottery, which is deprived of the greater part of its water by placing it in molds of plaster of Paris. The capillary character of this mold withdraws the water from the liquid clay mixture and deposits upon itself a layer of solid clay, the thickness of which is controlled by the time of standing. Certain types of candies, such as gumdrops, are dried mainly by contact with the starch molds in which they are cast. The drying effect of sponges, towels and materials of this kind is due to this same action. 

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