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Literature Cited Harcourt, G . N., Chem. & Met. Eng., 45, 179 (1938). Harte, C. H., Jr., IND.ENQ.CHEM.,28, 7 (1936). Hirsch, M . , “Die Trockentechnik,” Berlin, Julius Springer, 1927. Reavell, J. A., Trans. I n s t . Chem. Engr8. (London), 6 , 115 (1928). Roeser, W. F., and Mueller, E. F.. Bur. Standards, Research Paper 231 (1930).
FIGURE4. DOCBLE-DRUM DRYER WITH TROUGH FEED sufficiently even to produce a uniformly dry product, and spreader bars or spreading rolls must be installed. While most of these designs can be applied equally well to dryers with one and with two drums, there is an advantage to the two-drum type. By setting the drums close together and regulating the clearance between them, a layer of uniform thickness can be obtained without removing any of the partially dry material from the drum surface. This serves to increase capacity and eliminates a source of trouble. A double-drum dryer with trough feed is illustrated in Figure 4. All the designs described so far suffer from the disability that a residue always remains in the feed pan which has to be disposed of when the dryer is stopped a t the end of the day. With two drums set close together and feeding the liquid to the reservoir formed between the drums, all available material can be dried without residual loss. In addition, the liquid is preheated to the boiling point and a considerable amount of concentration takes place in this reservoir, which in this case is a distinct advantage. Products over a wide range of concentration and viscosity can be handled without the necessity of providing circulating systems, and spreading devices are rarely required.
Operation and Economics Operating conditions of the drum dryer are of great importance and can be readily adapted to the product. h-aturally steam pressure in the drums is set as high as possible. Frequently temperatures can be higher than that to which the product may be subjected without injury to quality or color, because the time element is exceedingly short, and rapid cooling takes place as soon as the material is removed from the drum surface. The influence of steam pressure on capacity is very complicated and is by no means in proportion to over-all temperature differepce. In general, increase in capacity on account of increase in steam pressure is more than might be expected, because with most products the layer adhering to a revolving heated surface increases in thickness, the higher the temperature of the surface. Increase in drum speed reduces the time of contact between the drum and the liquid and tends to reduce the thickness of the layer of ad-
RECEIVED .May 4, 1938.
ROTARY VACUUM
DRYING L. H.BAILEY F‘. J. Stokes Machine Company, Philadelphia, Pa.
T
HE rotary vacuum dryer consists of a stationary cylindricalsteam-jacketed vessel in which an agitator revolves.
A modification is the rotating vacuum dryer in which the agitation is produced by revolving the cylindrical jacketed shelI. The first mentioned type is used when possible, as it is much more convenient to operate (Figure 1). Several types of agitators are in use in rotary vacuum dryers. Fixed flat paddles, usually set a t a slight angle to the radius, are the least expensive. Flat paddles may also be mounted on hinges and be equipped with springs to keep them in contact with the shell, although these paddles tend to lift and drop the charge in the dryer instead of producing thorough mixing and agitation. Various modifications have been made to produce better agitation and to decrease power requirements, with better mixing and rapid automatic discharge. One of the most satisfactory types consists of narrow spiral or helical shaped paddles mounted on arms of two different lengths (Figure 2). The long arms carry sectional spirals which tend to move the material towards the center of the dryer; the shorter arms carry continuous spirals of the *opposite hand carrying the material away from the center and maintaining a fairly even depth of charge in the dryer. The outside spirals provide rapid discharge as soon as the outlet of the dryer is opened. If the dryer is over 15 feet long, two outlets are used and the arrangement of the spirals is modified to suit the need. I n conjunction with a single inlet this double outlet arrangement of the spirals facilitates charging,
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mitted by the cooling water a v a i l a b l e . F o r example, bleached shellac which has a softening point of 85" F. is now being dried on a commercial scale by means of hot water heat. Sufficientvacuum is maintained to boil the water a t 75" F. The steam economy of a vacuum dryer is not much greater than that of other types. While less steam is used in heating the charge to the low boiling point under vacuum, the increased latent heat at this low boiling point offsets the advantage to some .._ extent. In general. the steam requirement; are '1.1 to 1.2 pounds per pound of water evaporated. T h e chief economy, as indicated previously, is in the possibility of using exhaist steam-as the heating medium. The advantages of vacuum drying are that materiala scrisitive to heat and oxidation can be rapidly diicd without injury, and that drying rates are entirely independent of variations in atmospheric temperature and humidity. Fuel economy is a secondary consideration. Evaporation rates are usually expressed as pounds evaporated per square foot of heating surface per hour. This figure varies over a wide range, depending on the nature of the material, the percentage of moisture originally present, the h a 1 moisture desired, and many other factors. It also depends on the size of the dryer, since a dryer of large diameter holds more material in proportion to its heating surface than a small one; this means that the dryer of large diameter gives the product better contact with the heating surface. Average rates on finely divided material run about one pound of evaporation per square foot per hour. Poor heat conductors, such as wood flour, dry much more slowly. A material such as cornstarch, where the original moisture content is fairly high and the h a 1 moisture needs to be reduced to only about 10 per cent, shows an evaporation rate of about twice the average. I n some cases initial rates are high but final rates are very low; for example, white lead dries rapidly from its I
FIGURE1. ROTARY VACUUMDRYER
FIGURE 2. SPIRALAGITATOR
since the material is carried away quickly from the center of the dryer. It has been found best not to allow the spirals to cross a t any point. In order to increase evaporation rates and to prevent the wet material from sticking to the agitator and not drying properly, the paddles are usually mounted on a hollow steam-heated shaft. As an example of the effectiveness of the spiral-type agitator, over 9000 pounds of dry white lead can be discharged in 7 minutes from a 4 x 20 foot dryer with a single 10-inch outlet. Savings of 20 to 30 per cent in power requirements over the flat-type agitator have also been obtained. Automatic discharge is also possible with the flat paddle typeof agitator. An extra set of blades, set a t an angle to the axis of the dryer, is provided, and the agitator is reversed for discharging. To meet corrosion conditions, dryers have been made with the inner shell of copper, aluminum, nickel, stainless steel, and clad metals. The outer shell can usually be made of steel, but in dryers with very long shells it is best to duplicate the metal in both inner and outer shells to minimize unequal expansion and contraction. Heat is usually applied by introducing steam into the jacket and hollow shaft. In most cases low-pressure or exhaust steam can be utilized since ample temperature difference is made available by the low boiling points of liquids under high vacuum. When drying heat-sensitive materials, circulating hot water can be employed to heat the dryer, and this water can be heated by blowing steam into i t or by using electric immersion heaters. Drying rates are stilI satisfactory with circulating water as low as 120" F., for with modern high-vacuum pumps the boiling ROTARY VACUUMDRYERFOR STARCH, SHOWING EXPLOSION-PROOF MOTOR point can be readily kept as low as perDRIVE,SPECIAL DUSTSEPARATOR, AND ENCLOSED DISCHARGE
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VACUUM ROTARY DRYERFOR DRYING A CHEMICAL, SHOWI N G D U S T F I L T E RC , ONDENSER, VACUUM PUMP, AND LOADING D.EVICE
original pasty condition and is reduced to 1 per cent in about 5 hours. Another 5 hours is required to reduce the moisture to 0.10 per cent, the requirement for d x i n g with oil. This is quite different from the behavior of reclaimed rubber scrap, which maintains a nearly uniform rate and has a sharp end point. In nearly all cases drying rates are roughly four times as great as in the vacuum-shelf type of dryer where no agitation i s used. The ratio is still more unfavorable to the shelf type of dryer when the material being dried is a poor heat conductor. The ratio between the two types is not sufficiently definite to permit the calculation of drying rates in one type from data obtained in the other type. Drying rates are reduced to an uneconomical point if the material scales onto the inner shell and insulates the charge from the heating medium, although a scraper agitator may in some cases keep the scale down to a negligible thickness. Since drying characteristics are so variable, the manufacturers of equipment find it impossible to guarantee drying capacities without making actual tests. Small-scale apparatus is maintained and drying tests are run on nearly every material to be handled. Evaporation rates in small apparatus run about 40 to 60 per cent of those to be expected in large equipment. Actual drying time, however, is longer in larger equipment, because of the increased ratio of charge to heating surface, as previously mentioned. The speed of the agitator is usually not critical. A speed of 6 to 10 revolutions per minute provides satisfactory agitation and gives rapid discharging rates. Lower speeds are used for dusty materials. Dusting is undesirable as the condensing and vacuum equipment may become fouled. A dry dust filter containing a bag of canvas or other suitable material is inserted in the vapor line and is very effective when maintained a t a temperature which will prevent condensation. The dried dust can be added to the main charge. In some cases it is impossible to keep a bag from plugging with dust, and wet dust scrubbers are used. The dusty vapor is passed upward through a column carrying Raschig rings or other packing material, while water is introduced in a spray a t the top and trickles down to a receiver below, carrying the dust
with it. The wet dust is added to a subsequent batch and then dried. An important application of vacuum drying with agitation is in the recovery of volatile solvents from materials which have been extracted. Usually the extracted material will contain nearly an equal weight of the solvent. The chief advantage of the use of vacuum is that the troublesome problem of separating solvent from an air mixture is avoided. Air is first pumped out of the system and then heat is applied and the solvent is boiled off and condensed in a pure state. This method has long been widely used in the pharmaceutical industry to recover alcohol from dregs. The percentage recovery runs from 95 to 98 per cent, and the recovered solvent is not diluted as in the older steaming-off procedure and so does not require rectification. The residue is dry enough to be burned, whereas the steaming-off method leaves a wet mass not so readily disposed of. A wide variety of solvents has been handled in vacuum apparatus. The degree of vacuum used depends on the volatility of the solvent and the temperature of available cooling water. Another excellent point in favor of vacuum solvent recovery is the reduction of explosion risks. Explosive mixtures of air and solvent are eliminated by the original removal of the air. Also any leaks present are inward and not outward. The percentage of recovery depends to a large extent on the tightness of the apparatus. Each pound of air passing through the system comes out saturated with solvent vapor a t the temperature of the exhaust from the vacuum pump, and this exhaust is heated by the compression of the pump. It should be passed through a coil or multipass condenser having a long path of travel and effective cooling. The condensate can be added to the solvent from the main condenser; or if contaminated with oil from the pump, it is added to the next charge and so redistilled. In some cases the extraction operation can be carried out in the same equipment as the recovery operation. The practicability of the combination depends on a satisfactory rate of drainage of the solvent through the filter screens installed in the end heads of the dryer. RECEIVED M a y 4, 1938.
Previous paperg i n this symposium appeared on pages 384 and 388 of the April issue and on page 506 of the May issue. Other papers will be printed i n subsequent issues.
