The drying process

Once the final crosslinked gel is formed, it becomes necessary to remove the gelation solvent (e.g., water). The different methods used to remove the gelation solvent have dramatic effects on the properties of the RF organic aerogels, as outlined in Table 36.4. 

Solvent-exchange Necessary for supercritical drying with CO2 or freeze-drying Facilitates replacement with drying media Reduction of surface tensions upon subcritical evaporation [29] [60-62] 

Supercritical drying with acetone Such as supercritical drying with CO2, but with lower pressures Eliminates necessity for exchanging solvent with CO2 shortens processing time significantly Requires high temperatures to shift acetone to supercritical state May cause partial thermal decomposition of dried gels [69-71] 

Freeze-drying Production of dried light polymers called cryogels based on sublimation of frozen solvents Cryogels mostly mesoporous Density of solvents must be invariant with freezing [69, 72, 73] 

Drying can be performed at 50°C or room temperature with an open container. Research has shown that the RF aerogels with RIC ratio more than 500 can be successfully dried at ambient pressure in a large range of densities. All linear shrinkage is smaller than 10%, and in some cases it is even smaller than 2%, which can be considered negligible. 

Inexpert drying can lead to cracks in the green product, the causes 

Much of what needs to be done about the techniques of removing water from glass apparatus and from solvents and reagents has been set out above in a more general context. It remains to point out a few pecularities which characterise the drying process rather than any other purification procedure and to discuss some drying agents. 

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Fig. 6. The dry-process hardboard (and medium-deasity fiberboard) process.

The mats are moved along the line to the press loader. When the loader is filled and the press opens to remove the load of freshly pressed boards, the loader pushes the new boards into the unloader and deposits the load of mats on the press platens. The press closes as quickly as possible to the desired panel thickness. More pressure, as much as 4.8—6.9 MPa (700—1000 psi) is required to press high density dry-process hardboard, because the dry fiber exhibits much more resistance to compression and densification than wet fiber. Press temperatures are also higher, in the range of 220—246°C. No screens are used in the dry-process, but the moisture in the mats requires a breathe cycle during pressing to avoid blowing the boards apart at the end of the cycle. Because no screens are used, the products are called smooth-two-sides (S-2-S), in contrast to the wet-process boards, which have a screen pattern embossed into the back side and are known as smooth-one-side (S-l-S). 

Solventless Extrusion Process. The solvendess process for making double-base propellants has been used ia the United States primarily for the manufacture of rocket propellant grains having web thickness from ca 1.35 to 15 cm and for thin-sheet mortar (M8) propellant. The process offers such advantages as minimal dimensional changes after extmsion, the elimination of the drying process, and better long-term baUistic uniformity because there is no loss of volatile solvent. The composition and properties of typical double-base solvent extmded rocket and mortar propellant are Hsted ia Table... 

The formation of the metallic salts is a pyrometaHurgical process, and is commonly referred to as the dry process. The separation of the salts from each other is accompHshed by selective dissolution in water, and is named the wet process. 

For fine pulverization, both dry and wet processes are utilized, but increasingly the dry process is more popular because wet grinding ultimately requires drying and is much more energy intensive. A sensitive fan swirls the dust sizes into the air separator and permits coarse particles to recycle to the grinding mill or be rejected as tailings the fines are drawn into cyclones where the dust is collected. 

The dry-processed, peel-apart system (Fig. 8b) used for negative surprint apphcations (39,44) is analogous to the peel-apart system described for the oveday proofing apphcation (see Fig. 7) except that the photopolymer layer does not contain added colorant. The same steps ate requited to produce the image. The peel-apart system rehes on the adhesion balance that results after each exposure and coversheet removal of the sequentially laminated layer. Each peel step is followed by the apphcation of the appropriate process-colored toners on a tacky adhesive to produce the image from the negative separations. The mechanism of the peel-apart process has been described in a viscoelastic model (45—51) and is shown in Figure 8c. 

After curing, the plates are allowed to finish the drying process in ambient or elevated temperature air. The moisture and metallic lead content of the cured plates should be substantially reduced to less than 2%. 

Portland cement is manufactured by two basic processes, the wet process and the dry process. The dry process uses approximately 25% less energy per ton of Portland cement and is used to produce about 68% of the U.S. Portland cement. Both processes start by mixing selected raw materials, cmshed and/or milled to approximately s in. (1.9 cm) diameter, in the correct ratios to give the final desired chemical composition. 

Temperature, pH, precipitation conditions, and the drying process determine the amount of water in the sohd. In addition the species Ce(OH)... 

Convection Drying Modeling. Models of the drying process have been developed to estimate whether a particular coating can dry under the conditions of an available dryer. These models can be mn on desktop personal computers (see Computer technology). 

Mass Transfer and Useful Capacity. The term useful capacity, also referred to earlier as breakthrough capacity, differs from the equihbrium capacity shown on Figures 9a and 9b. The useful capacity is a measure of the total moisture taken up by a packed bed of adsorbent at the point where moisture begins to appear in the effluent. Thus the drying process cycle must be stopped before the adsorbent is fully saturated. The portion of the bed that is not saturated to an equihbrium level is called the mass-transfer 2one. 

After dewatering the cmmb it is fed to the drying process which is usually carried out in a continuous tunnel dryer. The cmmb is spread on a perforated stainless steel bed through which hot air is passed to evaporate the remaining water. Typically, in the first portion of the dryer, air at 110—140°C is used, with lower temperatures being used as the product approaches dryness. A typical target for final moisture content is 0.5% or less. At the exit of the dryer the product is cooled and conveyed to a baler which shapes it into bales for packaging and shipment. 

As drying proceeds, the point is reached where the evaporating surface is unsaturated. The point of evaporation moves into tne sohd, and the dry process enters the second faJhng-rate period. The drying rate is now governed by the rate of internal moisture movement the influence of external variables diminishes. This period usually predominates in determining the overall diying time to lower moisture content. 

Comparison Data—Plate Dryers Comparative studies have been done on products under both atmospheric and vacuum drying conditions. See Fig. 12-79. These curves demonstrate (1) the improvement in drying achieved with elevated temperature and (2) the impact to the drying process obtained with vacuum operation. Note that cui ve 4 at 90°C, pressure at 6.7 kPa absolute, is comparable to the atmospheric cui ve at 150°C. Also, the comparative atmospheric cui ve at 90°C requires 90 percent more diying time than the vacuum condition. The dramatic improvement with the use of vacuum is important to note for heat-sensitive materials. 

Wet-Process Cement Ball, tube, and compartment mills of essentially the same construction as for the dry process are used for grinding. Water or clay shp is added at the reed end of the initial grinder, together with the roughly proportioned amounts of limestone and other components. 

Figure 9.5(a) Typical values of 1- and 10-minute insulation resistances during the drying process of a class B insulated winding of a large machine... 

The purification of diethyl ether (see Chapter 4) is typical of liquid ethers. The most common contaminants are the alcohols or hydroxy compounds from which the ethers are prepared, their oxidation products (e.g. aldehydes), peroxides and water. Peroxides, aldehydes and alcohols can be removed by shaking with alkaline potassium permanganate solution for several hours, followed by washing with water, concentrated sulfuric acid [CARE], then water. After drying with calcium chloride, the ether is distilled. It is then dried with sodium or with lithium aluminium hydride, redistilled and given a final fractional distillation. The drying process should be repeated if necessary. 

The richer a plant part is in nutrients and the slower the drying process, the higher the bacterial count of the resulting drug root drugs, which of course are to begin with more heavily contaminated, always have a higher bacterial count than do flowers, for example, which are a less suitable as a nutrient medium. 

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