Vacuum, drying

Rgure 26.4 Microwave assembly for microwave-vacuum-convection dryer. (From Anon., 1990c.) 

One limitation of this technique is that it uses electricity, which is generally expensive. The cost of magnetrons has declined significantly over the past decade. Change of operation pressure may permit changes in the quality of 

Rgure 26.5 Cross-view of the microwave-vacuum dryer with revolving rack. (From Anon., 1995c.) 

In order to characterize the structure of RF and carbon xerogels, a combination of nitrogen adsorption (for micro- and meso-pores) and mercury porosimetry (for pore diameters from 7.5 to 150 nm) was used to obtain the BET surface area and pore volume (microporous and total) helium and mercury pycnometry were applied to determine the skeletal and bulk density. 

All aerogels stayed monolithic, but some showed volumetric shrinkage, which was attributed to residual water and ethanol after thewashing steps andcanreachupto 40% for high R/C and high dilution ratio. By variation of the synthesis conditions, a wide range of apparent densities (as low as 0.19 gcm ) and pore sizes can be covered. 

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The main features in which the Radford process differs from the batch operation are in thermal dehydration and compounding. Water-wet nitrocellulose on a continuous vacuum belt filter is vacuum-dried followed by hot air transfusion (80°C) to reduce the moisture to less than 2%. After cooling, alcohol is sprayed on the nitrocellulose to a concentration of 15—20%. The alcohol-wet nitrocellulose is then transferred from a surge feeder to a compounder by a continuous weigh-belt along with the other ingredients of the composition, which are also weighed and added automatically. 

The physical and chemical properties are less well known for transition metals than for the alkaU metal fluoroborates (Table 4). Most transition-metal fluoroborates are strongly hydrated coordination compounds and are difficult to dry without decomposition. Decomposition frequently occurs during the concentration of solutions for crysta11i2ation. The stabiUty of the metal fluorides accentuates this problem. Loss of HF because of hydrolysis makes the reaction proceed even more rapidly. Even with low temperature vacuum drying to partially solve the decomposition, the dry salt readily absorbs water. The crystalline soflds are generally soluble in water, alcohols, and ketones but only poorly soluble in hydrocarbons and halocarbons. 

The esterification reaction in making ester oils is commonly carried out with a catalyst at about 210°C while removing excess water as it forms (32). Excess acid or alcohol is then stripped off, and unreacted acid is neutrali2ed with calcium carbonate or calcium hydroxide before final vacuum drying (qv) and filtration (qv). 

In vacuum drying or other processes containing atmospheres of 100 percent vapor, the temperature of liquid vaporization will equal or exceed the saturation temperature of the liquid at the system pressure. (When a free liquid or wetted surface is present, drying will occur at the saturation temperature, just as free water at I0I.325 kPa vaporizes in a 100 percent steam atmosphere at I00°C.)... 

The vacuum plate drwer is provided as pari of a closed system. The vacuum dryer has a cylindrical housing and is rated for fiill-vacuum operation (typical pressure range 3-27 kPa absolute). The exhaust vapor is evacuated try a vacuum pump and is passed through a condenser for solvent recovery. There is no purge-gas system required for operation under vacuum. Of special note in the vacuum-drying system... 

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. 

Use alternate drying method (ex. vacuum drying instead of atmospheric drying vacuum tray dryer, freeze drying, cryogenic CO2 drying, instead of vacuum rotary dryer) where material is subdivided in multiple locations... 

The elements thus formed are then inserted into a sheet steel container, vacuum dried and impregnated, with suitable non-PCB dielectric, which may be an oil dielectric or epoxy resin. The capacitor shell is then hermetically sealed, in oil dielectrics, to avoid any leakage of dielectric during operation. 

Crown-6 [17455-13-9] M 264.3, m 37-39 . Recrystd from acetonitrile and vacuum dried. Purified by pptn of 18-crown-6/nitromethane 1 2 complex with Et20/nitromethane (10 1 mixture). The complex is decomposed in vacuum and distilled under reduced pressure. Also recrystd from acetonitrile and vacuum dried. 

Diacetyl benzene [1009-61-6] M 162.2, m 113-5-114.2 . Crystd from benzene and vacuum dried over CaCl2. Also dissolved in acetone, treated with Norit, evapd and recrystd from MeOH [Wagner et al. J Am Chem Soc 108 7727 1986]. 

Dibenzo-18-crown-8 [14174-09-5] M 448.5, m 103-106 . Recrystd from EtOH, and vacuum dried at 60° over P2O5 for 16hours. [Delville et al. J Am Chem Soc 109 7293 1987.]... 

Dibenzyl sulfide [538-74-9] M 214.3, m 48.5 , 50 . Crystd from EtOH/water (10 1), or repeatedly from Et20. Also chromatographed on AI2O3 (pentane as eluent), then recrystd from EtOH [Kice and Bowers J Am Chem Soc 84 2390 7962]. Vacuum dried at 30° over P2O5, fused under nitrogen and re-dried. 

Diphenylguanidine [102-06-7] M 211.3, m 148°, pK 10.12. Crystd from toluene, aqueous acetone or EtOH, and vacuum dried. 

Recrysts slowly from aqueous 80% EtOH, then vacuum dried over P2O5. Alternatively, crystd from water at 55°, then dried for 6h in a vacuum oven between 60-70° at 2mm. 

The propionamide can be dried over CaO. H2O and unreacted propionic acid were removed as their xylene azeotropes. It was vacuum dried. Material used as an electrolyte solvent (specific conductance less than 10 ohm cm" ) was obtained by fractional distn under reduced pressure, and stored over BaO or molecular sieves because it readily absorbs moisture from the atmosphere on prolonged storage. [Hoover Pure Appl Chem 37 581 I974 Recommended Methods for Purification of Solvents and Tests for Impurities, Coetzee Ed., Pergamon Press, 1982.]... 

Alternatively, it is dissolved in twice its weight of water at 55-60°, filtered, coned to half its volume and poured slowly, with stirring, into about twice the volume of EtOH. The crystals which separate on cooling to 3-4° are filtered off, washed with a little MeOH, air dried by suction, then finally ground and dried in a vacuum desiccator over P2O5. It has also been crystd from water, MeOH or aq MeOH, and vacuum dried at 80° for 2 days. 

Also purified by dissolving in hot 95% EtOH (14mL/g), filtering and cooling, then drying in a vacuum desiccator. Alternatively, it was crystd from H2O, vacuum dried, washed with anhydrous Et20, vacuum dried. These operations were repeated five times [Maritato J Phys Chem 89 1341 1985 Lennox and McClelland J Am Chem Soc 108 3771 1986, Dressik J Am Chem Soc 108 7567 1986]. 

Sodium ethylsulfate [546-74-7] M 166.1. Recrystd three times from MeOH-Et20 and vacuum dried. 

The solid tenaciously holds a small amount of chloroform which can be detected by proton magnetic resonance (S 7.25). Vacuum drying overnight at 60 removes this impurity. 

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