Desiccants silica gel

Significant contamination of desiccants (silica gel, zeolite and CaS04) was observed in unsealed containers exposed to tritiated concrete powder at room temperature and in a refrigerator. This contamination was not seen for metals and plastics. 

W. A. Patrick, The Properties of Highly Desiccated Silica Gel, Symposium on Catalysts and Reaction Mechanism, sponsored by the Catalyst Club of Philadelphia, University of Pennsylvania, April 28, 1951. 

Figure 1 illustrates the overall block diagram. To raise S03/air with a volumetric content between 4 and 7% SO3, the process air should be dried to prevent the formation of sulphuric acid mist. Filtered ambient air is compressed (about 0.6 bar gauge), chilled (about +5 C) to remove the major part of water by condensation and subsequently dried with a desiccant (silica gel. Alumina) to arrive at a process air dewpoint of about -60 C. 

Fatty acid methyl esters will readily hydrolyse if stored in contact with water, the hydrolysis being acid catalysed. Therefore steps should be taken to exclude water from the storage vessel. It is therefore recommended that storage tanks be vented through desiccant (silica gel). 

Water vapor permeability (WVP) was determined gravimetrically according to the standard method E96-00 (ASTM 2000). The films were fixed on top of test cells containing a desiccant (silica gel). Test cells then were placed in a relative humidity chamber with controlled temperature and relative humidity (25°C and 75% RH). After steady-state conditions were reached, the weight of test cells was measnred every 12 hours over three days. The water vapor transmission rate (WVTR) was determined using equation (2) ... 

Nitrogen adsorption-desorption isotherms at 77 K for the desiccated silica gels examined by TEM in Fig. 3. (A) Two-step acid-catalyzed xerogel. (B) Two-step acid-base-catalyzed xerogel. (C) Particulate xerogel. (D) Two-step acid-base-catalyzed aerogel. absorption O desorption [7]. 

The beat method of drying, if time permits, is to place the crystals in a desiccator containing an appropriate substance (usually anhydrous calcium chloride, silica gel, or concentrated sulphuric acid) to absorb the solvent. More eflicient and more rapid drying is obtained with the aid of a vacuum desiccator (see Section 11,38 and Fig. 77, 38, 1). 

Into a 500 ml. round-bottomed flask, fitted with a reflux condenser, place 42 g. of potassium hydroxide pellets and 120 g. (152 ml.) of absolute ethyl alcohol. Heat under reflux for 1 hour. Allow to cool and decant the liquid from the residual solid into another dry 500 ml. flask add 57 g. (45 ml.) of A.R. carbon dtsulphide slowly and with constant shaking. Filter the resulting almost solid mass, after cooling in ice, on a sintered glass funnel at the pump, and wash it with two 25 ml. portions of ether (sp. gr. 0-720), followed by 25 ml. of anhydrous ether. Dry the potassium ethyl xanthate in a vacuum desiccator over silica gel. The yield is 74 g. If desired, it ma be recrystallised from absolute ethyl alcohol, but this is usually unneceasary. 

After drying or decomposing a sample, it should be cooled to room temperature in a desiccator to avoid the readsorption of moisture. A desiccator (Figure 2.9) is a closed container that isolates the sample from the atmosphere. A drying agent, called a desiccant, is placed in the bottom of the container. Typical desiccants include calcium chloride and silica gel. A perforated plate sits above the desiccant, providing a shelf for storing samples. Some desiccators are equipped with stopcocks that allow them to be evacuated. 

Methods for removing water from solids depends on the thermal stability of the solids or the time available. The safest way is to dry in a vacuum desiccator over concentrated sulfuric acid, phosphorus pentoxide, silica gel, calcium chloride, or some other desiccant. Where substances are stable in air and melt above 100°, drying in an air oven may be adequate. In other cases, use of an Abderhalden pistol may be satisfactory. 

Dihydroxybenzoic acid [490-79-9] M 154.1, m 204.5-205", pK 2.95. Crystd from hot water or benzene/acetone. Dried in a vacuum desiccator over silica gel. 

Depth loading The deposition of particles mainly within the filter interstices, rather than on the filter surface. Desiccator A sealed container containing a water-absorbing substance such as silica gel or calcium chloride used to dry test materials in the laboratory,... 

Silica gels will shatter in the presence of free water and are chemically attacked by many corrosion inhibitors. The chemical attack permanently destroys the silica gels. The other desiccants are not as sensitive to free water and are not chemically attacked by most corrosion inhibitors. However, unless the regeneration temperature is high enough to desorb the inhibitor, the inhibitor may adhere to the desiccants and possibly cause coking. 

Adsorption is the property of certain extremely porous materials to hold vapors in the pores until the desiccant is either heated or exposed to a drier gas. The material is a solid at all times and operates alternately through drying and reactivation cycles with no change in composition. Adsorbing materials in principal use are activated Alumina and silica gel. Molecular sieves are also used. Atmospheric dew points of minus 1000°F are readily obtained using adsorption. 

Desiccating agents used in corrosion prevention must be cheap, easy to handle and non-corrosive. These requirements rule out many of the familiar laboratory desiccants, and in practice the most common packaging desiccants are silica gel, activated alumina and quicklime (calcium oxide). Activated... 

Silica gel and activated alumina present few practical problems. They are easily reactivated after use by heating in a ventilated oven, to 130-300°C for silica gel, and 150-700°C for activated alumina. British standard specifications have been published for desiccants for packaging which regulate the contents of soluble chloride and sulphate, dust content and absorptive capacity. 

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