USE and REGENERATION of DRIERITE W. A. HAMMOND Antioch College, Yellow Springs, Ohio
Although Drierite (anhydrous calcium sulfate) is aL ready i n use in hundreds of institutions and industries throughout the United States and C a d and i n a few foreign countries, it is still essentially a new product, and some directions and suggestions for its use and regeneration seem just$ied. These are presented very briejly herein. STORING AND HANDLING
D
RIERITE absorbs atmospheriwmoisturerapidly. The new or freshly regenerated material should therefore be protected from exposure to the air during handling, charging of absorption units, etc. Moisture absorbed in handling obviously reduces the capacity in service. TEST FOR ACTMTY
The instantaneous action of Drierite with water is accompanied by a sharp rise in temperature. To test the activity of a given lot, place about 10 grams in a small container such as a crucible, insert the bulb of a tbermometer, and moisten the granules with water from a wash bottle. If a sudden rise in temperature (10' to 20°C.) is not noted, the material needs regeneration. ESTIMATION OF CAPACITY
Drierite absorbs water both by hydration and capillary action. The conversion of the anhydrous salt, CaSOd,to the'hemihydrate, CaSO4.'/,HeO, involves the
absorption of 6.6 per cent. by weight of water. In the drying of organic liquids, either in the liquid or vapor phase, this is the limit of capacity. In drying gases the capillary absorption varies-both with the moisture content of the gas and with'the temperature. From a gas of medium to high moisture content Drierite absorbs from 9 to 12 per cent. by weight a t 2 5 T . to 20 per cent. or above a t temperatures close to O°C. As a margin of safety in important operations the use of an excess of 25 to 50 per cent. b a y he advisable. USE IN DESICCATORS AND DRYING ROOMS
The moisture which remains in air in equilibrium with Drierite at ordinaly temperatures (25OC.) amounts to 0.005 mg. per liter, ( I ) , ( 3 ) ,and one pound of Drierite will dry approximately 140 cubic feet of air of 50 per cent. relative humidity. It is easy therefore to maiutain a dry atmosphere in any closed compartment, large or small. Since Drierite does not become wet, disintegrate, nor change in volume, and is not corrosive nor harmful, even by contact, to other materials, it is well adapted to use in balances, display cases, drying cabinets, and in large drying rooms. In large units the Drierite should be spread 1 to 2 inches deep in large pans or trays and air circulation should be maintained by fans. Or the Drierite may be held in towers or columns and forced circulation may be effected by blowers. THE DRYING OF GASES
Any of the ordinary and well-known absorption units
may be used for the drying of gases with Drierite. The granule size and the size of the unit should be determined by the volume of gas and the velocity of flow required by the particular operation. All units should be protected from flooding with liquid water, since flooding causes hydration to the &hydrate with resultant hardening of the mass in the absorption unit. INDICATING DRIERITE
For the drying of gases in transparent units, such as the common laboratory drying columns, U-tubes, etc., Indicating Drierite gives a clear visual indication of the progress of absorption and exhaustion by the change in color from blue to rose-red. A suitable unit containing Indicating Drierite may be used a t the end of a large unit of Regular Drierite to indicate the exhaustion of the charge. A gas which bas passed through active Regular Drierite will not change the color of the Indicating variety. The latter thus becomes a test reagent for moisture in a gas. THE DRYING OF NON-AQUEOUS LIQUIDS
The last detectable traces of water are removed by Drierite from any liquid .which is sufficiently fluid to make intimate contact with the porous solid. Several procedures are available for different conditions. To Remove Traces of Water.-When water is present in traces to less than 1 per cent., the liquid is readily dried a t normal temperatures by use of powdered Drierite followed by filtration or decantation. Add an excess of the coarse powder and agitate for a minute or so. Filter or decant into a dry container. Since calcium sulfate is insoluble in organic liquids and inert except toward water, many solutions as well as pure liquids may be dried in this way. To R e m e Larger Amounts of Water.-Liquids, such as 95 per cent. alcohol, containing, say, 3 to 10 per cent. of water, are best dried by Granular Drierite. For drying in the liquid phase, fi1l.a distilling flask fourfifths full of the granules and pour in the liquid until the granules are nearly covered. Rod the flask gently to secure contact, and note the immediate rise in temperature. No refluxing is necessary. Submerge the whole bulb of the flask in a water bath and distil by heating the bath to 95' to 100°C. If the liquid boils above 100' to 105"C., it is best to distil under reduced pressure to avoid any possibility of partial dehydration of the Drierite. Drying Liquids in the Vapor Phase.-An efficientand rapid method of obtaining anhydrous liquids is afforded by drying in the vapor phase. The operation involves merely passing the vapor of the boiling liquid through any suitable coil or column containing granular Drierite maintained a t or above the temperature of the boiling liquid. This method is applicable to either small- or large-scale operation. For illustration and details see original papers ( I ) , (2). REGENERATION
After any normal use any of the forms of Drierite
may be regenerated for re-use. The operation is simple and involves only standard equipment. The used and exhausted material should be ventilated to remove vapors, if any, and stored in a convenient container until a sufficient amount is accumulated to justify the work of regeneration. A very convenient heating device for regeneration on the laboratory scale is the ordinary, inexpensive metal oven which is available in any hardware store. These are usually provided with two or three shelves and are heated over a twoburner gas grid or electric hot-plate. Insert a longstemmed thermometer from one side through a hole made for the purpose so the bulb extends between the shelves to about the center of the oven. The exhausted Drierite should be spread about 1 inch deep in shallow metal pans or trays and heated in the oven to effect complete dehydration. Heating in this way a t 225' to 250°C. for 1 to 2 hours is sufficient even for the large granules. The regenerated material should be placed in the original container and sealed while hot. The date of the regeneration should be recorded on a label attached to the container. For the regeneration of Indicating Drierite, as well as for small lots of Regular Drierite, the granules may he spread in layers '/%-inch or less in thickness and heated for the shorter time and lower temperature in the ranges suggested above. The colors of the Indicating variety may become less distinct on successive regenerations due to the movement of the pigment into the body of the granule. In large-scale operations where the granular material is used in columns or beds regeneration in situ may be effected by passing heated air through the unit, continuous operation being obtained by use of a multipleunit system. SELECTION OF GRANULE SIZE
The smallest granule size of Drierite available in quantity, the &mesh, offers adequate contact and a t the same time a negligible resishce to flow for convenient use in small laboratory units for the drying of air and other gases. The most widely applicable for general drying operations is the &mesh size. This size is recommended for the drying of liquids either in the liquid or vapor phase, for large gas-drying units, and for many industrial operations. The 6-mesh size suffers no loss of capacity due to the "filtering effect" and offers very slight resistance to the flow of gases. The 4-mesh size is also suitable for the drying of liquids and is recommended for use in desiccators and drying cabinets. The 2-mesh size is suitable for desiccators and is available for other large drying operations. LITERATURE CITED
HAMMOND, W. A. AND WITHROW, J. R.. "Soluble anhydrite as a desiccating agent, I. Preparation and general characteristics,"Ind. Eng. Chem., 25, 653 (1933). HAMMOND, W. A. AND WITHROW, J. R.. "Soluble anhydrite as a desiccating agent. 11. Drying of organic liquids," Ind. Eng. Chem., 25, 1112 (1933). BOWER.J. H., "Comparativeefficiencies of various dehydrating agents for drying gases," Bur. Standards I. Research, 12,241 (1934); Rerearch Paper 649.
