1004
ITDUSTIIIAL 42.D EKCINEERIXG CHEMISTRY
AEROFORM DRYER
T
IIE mechanical separation of water costs less than evaporating moisture with steam heat, hut with a plastic chemical mass from which the water can be only partly removed hy filtering or mechanical separation, the problem has always been how to remove the remaining water. The material coming from a filter or from any mechanical dewatering device is wholly unsuited for quick drying. Even though separated into small forms advantageous for drying, many Jiltered materials, after being formed and then removed to any kind of a conveyor, will immediately settle hack into a solid mass formation and defeat the purpose of the special forming. The application of heat to the preforming meehanism, so as to evaporate srifhcieiit water in order that the
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formed material d l hold its shape, eliminates this difficulty. The oredried forms can then be removed fw drvinz on a con-
material by tho fins produces a multitude of crevices, froni which the steam can escape without blowing the cake from the surface of the drum. The sidesof thefinsheip tosecure the plastic material to thedrum. In the case of some relatively nonporous materials the individual strips are lifted slightly from the wooves, after they come underneath the first pasting roll. After this first "onrush" of evaporated moisture is released, a second re. pasting roll places the strip back securely into the fins where i t remains until it is suitably predried and finally removed from the grooves by means of the finger scrapers. When the material has been predried on the fin drum, it is removed from the grooves by a series of scraper fingers, which press like small ploughs between each of the fins. The s c r a p ers do not necessarily fit the sides of the fins but merely serve as a means for lifting the predried strips from the revolving
SEPTEMBER, 1938
INDUSTRIAL A S D ENGINEERING CHEMISTRY
1005
the screen. Special methods of manufacture were developed to obtain the accuracy required in production of this conveyor screen. The chain, which extends along the side of tlie conveyor, has links 8 inches long, and the hinging point of the screen and the chain coincide so that the body of the screen is never flexed. Supporting girts extend from the chain on one side to the chain on the other. The side links relieve the screen of any tension lengthwise, and the supporting girts keep the screen from sagging crosswise. On the two extreme sides of the screen conveyor t.here are secFronRE 2. PREFORMED MATERIAL ON THE CONVEYIXC SCRmW*tional side guards which allow material to be loaded over tho entire width of the conveyor, without tlie possibility of m a t e rial falling from the sides. drum. This action breaks the material up into a series of On the fin drum the material remains in contact with the short strips which, in turn, slide down a wide chute where fins for an average of from 20 to 60 seconds, whereas the time they are loaded uniformly to a depth of ahout l'/e to 2 inches to pass through the dryer runs from 20 to 30 minutes. This across the entire width of the screen conveyor. They fall gives a remarkably quick drying time. Former methods for loosely in such a way as to leave a multitude of spaces, tirougli filtered materials required from 2 to 24 hours. I n the case oE which the heated air is recirculatrri in intimate contact with materials which are sensitive to continued exposure a t high the small forms as they pass through the drying chamber tenrperatures this short drying time has improved the quality. (Figure 2). The steam pressures, usually applied to the fin drum, run It is desirahle, from the standpoint of efficiency and for the from 40 to 140 pounds per square inch gage. For the heating prevention of dust, to remove the predried strips from the coils of the conveyor air dryer, the pressure may pun from 5 grooves as soon as they are sufficiently dried to hold their shape. In practice it is found that this is the stage at which to 150 pounds. ahout one-third of the moisture in the filtered material has The aeroform dryer is particularly adapted to the drying been removed. From this point on, the fin drum drops off in of such materials as lithopone, white lead, lead sulfate, arefficiency, and it becomes more difficultto remove the prodsenates, bordeaux mixture, magnesium carbonate, calcium carbonate, clays, iron oxide, zinc sulfide, and many other uct from the gooves. For these reasons and because dust chemicals from which the iuiti$blwater has been mechanically losses would occur, the drying is riot completed on the fin removed. It is impracticable in the drying of certain matedrum hut on a screen-conveyor dryer designed for the purpose rials such as lake colors, where there is not sufficient produc(Figure 3). It is evident that the screen conveyor must have a fme tion to keep the dryer busy on one color, and where frequent mesh because a small percentage of the sticks unavoidably cleaning, between different kinds of material, would interfere b r e d down into fines. Since any continuous screen will soon with production. It is not well suited to the drying of materials which cannot be heated above 200" F. on the fin drum, fail due to flexing, it is necessary to employ a special sectional, hinged, fine-mesh screen conveyor. Fleretoforc all materials wliieh uill not qizickly dry into stick formation, of the hinged-joint screen conveyors have been made of materials which form a gummy inass under heat and stick to coarse-mesh screen, u s d to handle only fibrous materials. the fins, granular materials, such as salt, that will not p r e The micromesli hinge-joint conveyor developed for the form; or liquids or very wet materials. In some cases the purpose provides free passage of t.hc air, and has openings material to he dried cannot be preformed on the fin drum, around the joints wliidi are very clost: in size to those i n but other siiacessfirl means for preforming have been deI I
FIGURE3. C ~ o s SECTION s OF CONVEYIXG-SCREEN DRYER
INDUSTRIAL AND ENGINEEIIING CHEMISTRY
1006
veloped, so that the formed materials may be air-dried on the screen conveyor. The metals used in the combruetion of those parts of the fin drum coming in contact with the material have so far been bronze or stainless steel. Bronze has the advantage of less cost, casier maehiilability, and much better heat transfer. Plain steel has been avoided on account of corrosion. Aluminum is not sufficiently durable to last; tbe hardened grades lose their superior physical qualities under heat and working conditions. Platiiig, &sameans of protection on the fin drum, is impractical. For the screen conveyor, micromesh hinge, side guards, and supporting girts, the following metals have been used: galvanized iron, special electroplated iron, aluminum, bronze, and stainless steel. R ~ c m v e oMay 4, 1938.
DRUM DRYING D.S. VAN MARLE BuffaloFoundry and Machine Company, Buffalo, N. Y.
D
RUM drying is that process where the material is dried on the surface of an internally heated revolving drum. Various forms of drum dryers and their application were discussed by Harte (8) and IIarcourt ( I ) , and tlie effect of variation in operating conditions, by Reavell (4); IIirscli (S), in addition, gave a theoretical treatment of the subject. A twili-drum dryer is illust,rated in Figure 1. Major connideratiori and problenis in the use of drum drying have to do primarily with (u) nature of the product to he dried, (b) the drum dryer as an inst.rument of heat transfer, (c) design and construction of the dryer, (d) adaptation of the dryer to t.he product io hand, (e) conditioos under which the dryer operatas, and (f) economies to be effected witahin the drum dryer itself, as well as in co~ineetionwith the maririfactnriiig process in which it is being used.
Product to Be Dried
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the boiling point of the solvent as long as the material is completely wetted and then increases during the final stage of the drying cycle. The capacity of a drum dryer depends not only on the drying rate of a thin layer of the material but also on the degree of adherenee of the product to the drying surface. I n this respect there is such a wide variation in different products as to make impossible any definite prediction of drum dryer performance. The amount of liquid adhering to the drum surface varies, to a considwable extent, with the type of feed used, as well as with the steam pressure and drum speed. Feed temperature and concentration frequeiitly can be regulated; preheating and preconcent.ration are desirable wherever possible since they result in a reduction of the amount of heat to be transferred per unit weight of dry material. Concentration caniiot always be carried to the limit, because dhere is often an optimum concentration above which it becomes difficult to apply a uniform coating, and adherence becomes poor. Other properties, such as viscosity, surface tension, and wetting power, are undoubtedly important in governing layer formation but are generally fixed because any change in these properties may affect the quality of the dry product. Sometimes it is possible, however, to improve drying properties a great deal by pH control, removal of impurities, or addition of a small percentage of a foreign nubstance. In view of the many advantages of drum drying it is worth while to consider these possibilities whenever a difficult drying problem is to be solved.
Heat Transfer Viewed as a heat transfer problem, drum drying involves heat transfer from condensing steam through a metal wall of considerable thickness to a thin layer of material, from which the moisture is removed by direct evaporation a t or above the boiling point. Over-all rate of heat transfer varies greatly, largely on account of the difference in resistance to the removal of moisture with decreasing moisture content and the variation in film resistance of individual products. It is often particularly difficult to eliminate the last few per cent of moisture, and to do so would mean the installation of a greatly increased amount of drying surface. Under these circunrstances i t is more eeonomid to use the drum dryer for drying to an advantageous moisture content and to remove the balance of the moisture in a more suitable dryer. In one case it was actually found possible to triple the rapacity of the drum dryer by this method.
The produet to be dried is usually in fluid form, with solids either in solution or suspension, and the material is spread in a thin layer on the surface of the drum. Since the layer of material is coniparatively thin, the dryi n e rate is at, no time eovernrd bv " the ~diEusion of^ the vapor through the layer. The drying of solutions involves a change of state of the sohito in addtion to the evaporation of water. Three stages in this change may he observed: a first stage during which the material remains a liquid while its temperature is raised to the boiling point of the solution and becomes higher as the eoneentra.tion of the solute increases; a second stage, during which there is a gradual change from a liquid to a solid state, a t a eonstant temperature if the solute separates a t a definite concentration, or at an increasing temperature if the change is gradual; and a final stage, where thc material is in a solid condition and in which the temperature gradually approaches that of the heating surface. I n the case of slurriesand susprnsions the temperature remaiirs a t FEU~E I. TWIN-DHUM DRYER WITH SPLASX FEEDAND ~~
CHROMITIM-PIAT~DDRUMS
