May, 1921
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t h e branch duct in each room. T h e unit system of apparatus, however, is usually preferable. For low temperature work, i t is necessary t o use refrigeration for t h e removal of moisture, and this is accomplished b y dehumidification, t h e air being brought into contact with a spray of refrigerated water a t low temperatures, removing t h e excess moisture by condensation, as indicated in Fig. 17. T h e air is then reheated b y means of heater coils, which supply t h e necessary heat for drying. Fig. 18 shows a humidifier with interchanger, by means of which most of t h e heat of t h e warm return air is transferred t o t h e cool dehumidified air, before i t is supplied t o t h e room. This permits a reduction of about one-third in t h e refrigerating capacity required for a given d u t y in low temperature drying.
saturation, and then reheated t o maintain a definite room temperature, or t h e control may be regulated b y admixture of fresh and return air, controlled by a hygrostat located in t h e room. By either system, a very accurate humidity control may be secured. The temperature may be controlled independently.
U
EXFICIENCY O F DRYERS
The general theory of t h e efficiency of dryers has been discussed in t h e preceding paper on t h e “Theory of Drying.” It is generally found t h a t about 2 lbs. of steam are reqgired t o evaporate 1 lb. of water, under t h e most favorable, conditions, while t h e more usual figure for steam consumption is 2 . 5 lbs. of steam t o 1 lb. of water evaporated. The principal losses i n air drying are radiation and escape of unsaturated air, either through t h e usual vent ducts or b y leakage through t h e kiln walls.
-EL EVA TrON FIG.18- DEHIIXIDIFXER wirn INTERCHANGER
Fig. 16 shows a central station, where one apparatus supplies a group of dry rooms. For accurate humidity control with this system, a dew-point control is necessary, and various relative humidities may be obtained in each room, by regulation of t h e separate heater in
The Spray Process of Drying By R. S. Fleming &IERRELL-SOULE Co., SYRACUSE, NEW YORK
’
The spray process of drying; has been developed in connection with t h e manufactureof dried milk. I n 1901 a patent Was granted to Robert staufof posen, Germany, relating to the drying of blood, milk, and Other Organic liquids’ One Of the chief claims was described as follows: The process of obtaining the solid constituents of mifk, in the form of powder, said process consisting in converting the liquid into a fine spray, bringing such spray or atomized liquid into a regulated current of heated air so that the liquid constituents are completely vaporized, conveying the dry powder into a suitable collecting space away from the air current and discharging the air, a vapor, separately from the dry powder. This patent was purchased b y a n American company which had independently developed a spray process, b u t was antedated b y Stauf. It soon became apparent t h a t a better product could be obtained and t h e process materially cheapened if t h e material t o be dried could be condensed t o a considerable degree b y t h e vacuum method before it was dried b y t h e spraying process. One of t h e chief claims of t h e patent covering this point‘ is as follows: The process of obtaining the solid constituents of liquids and semiliquids in the form of powder, which process consists in concentrating the substance by removing a large percentage of water therefrom, converting the concentrated mass into a spray, bring1
L. C. Merrell, I. S. Merrell, and W. B. Gere, U.
(1907).
s. Patent
860,929
ing such spray into a current of dry heated air or gas, having an avidity for the moisture of the substances treated, retaining‘ the atoms momentarily in said current so that substantially all the remaining moisture is converted into vapor and the product is prevented by the cooling effect of such evaporation from un. dergoing chemical change, conveying the dry powder into a suit. able collecting space away from the vaporizing current, and discharging the air or gas separately from the dry powder: *
APPARATUS REQUIRED
The apparatus for successful spray drying necessarily requires t h e following equipment: A N AIR FILTER-This is necessary in order t o have clean air which will not contaminate t h e product undergoing desiccation. Various forms may be used, such as washing t h e air with water. But t h e cheapest, and i n general a very satisfactory, method is t o filter through cotton. BLOwER-This is necessary t o propel t h e air through t h e desiccating apparatus. HEATER-The air is usually heated by passing over steam heated radiators. A direct heat b y gas has been used, and even a’ coal burner, t h e air passing over t h e burning coal. The latter, however, is unsatisfactory. D R Y I N G CHAMBER-The heated air is passed into a drying chamber where t h e hot air mingles with t h e sprayed liquid. There are various styles of drying chambers, t h e usual form being a rectangular room,
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t h e hot air entering one end and passing out the other. The liquid is generally sprayed in with t h e ingoing air, sometimes one spray being used and sometimes several. Another form of drying chamber is vertical. I n this form t h e air frequently enters a t t h e bottom and passes out a t the top. T h e liquid enters a t t h e top. ATOMIZERS-The liquid may be turned into a spray by several different forms of apparatus. One is t h e so-called air spray by which t h e liquid is atomized by t h e action of compressed air which, passing through a n orifice, draws t h e liquid through, much on the principle of an ejector. On emerging from t h e ejector t h e liquid breaks into fine droplets a n d is t h u s atomized. A better method is t h a t disclosed in t h e Bevenot a n d DeNeveu patent1 b y which t h e liquid t o be dried is atomized under high pressure, a hydrailic pump being used for this purpose. Sometimes as much as several thousand pounds per inch is required. D U S T COLLECTOR-Adust collector is necessary, for t h e reason t h a t t h e product is frequently very fine a n d light, and there would be serious loss if the outgoing air were not filtered or some other means taken for collecting t h e fine particles which are carried in suspension by t h e outgoing moisture-laden air. mxm" DEVICES-It will be seen t h a t in order t o use efficiently t h e heat of t h e heated air which enters the desiccating chamber, it is necessary t o mix t h e heated air and sprayed particles so t h a t all of t h e heated air shall come in contact with some of t h e sprayed liquid particles. Different devices have been used for this purpose.2 NATURE O F SPRAY EVAPORATION
When t h e atomized liquid is mixed with t h e air drying takes place practically instantaneously. We can think of each atom of liquid as a spherical droplet, on t h e surface of which a n intensive evaporation is going on. It is t o be noted t h a t t h e moisture passes away from t h e surface t o which heat is applied. This is t h e reverse of what occurs in most systems of drying. I n t h e latter t h e heat is applied t o one surface and t h e moisture passes away a t another. I n such a system the whole body of t h e liquid must become heated. I n t h e former case, however, with the evaporation taking place on t h e surface t o which the heat is applied, t h e whole body of t h e liquid in t h e droplet does not become heated while the evaporation is going on. We believe t h a t t h e evaporation is SO rapid t h a t the droplet is actually kept cool until t h e dry state is reached; this is due of course t o the absorption of heat in vaporizing t h e liquid. After t h e evaporation has ceased, t h e temperature of t h e particle rises t o the general temperature of t h e drying chamber. I n the dry state there is much less likelihood of injury from heat. I n fact, i t is well known t h a t chemical changes produced by heat are usually much niore effective in t h e presence of moisture. If we are correct in our argument t h a t the rapid evaporation keeps t h e droplet cool during t h e drying process, i t appears t h a t the spray process is especially useful U. S. Patent 1,020,632. One of these is covered by t h e U . S. Patent 1,183,098 granted to I. S. Merrell and 0. E. Merrell. This does the mixing very effectively. 1
2
1701.
13, No. 5
in t h e desiccation of materials which are easily injured b y heat. I n t h e ordinary concentration of liquids by boiling, t h e greatest injury usually occurs just before t h e d r y state is reached. T h e greatest injury of all probably occurs between t h e sirupy stage and absolute dryness. I n spray drying this stage is passed almost instantaneously and, if our theory is correct, in a fairly cool state. The results of many careful tests seem t o prove t h a t t h e above conclusions are correct. For instance, albumin which is coagulated a t 65' C. can be dried by the spray process without injury a t 75' C. or higher. Bacterial cultures can be dried a t temperatures far above their thermal death points. COST O F O P E R A T I O X
It is very difficult t o give t h e cost of operating a spray dryer either in dollars or in heat units, for t h e reason t h a t this will depend t o a very great extent on the character of t h e material t o be handled, and t h e properties desired in t h e dried product. It can be said, however, t h a t spray drying is comparatively expensive, mainly because i t is impossible t o utilize all t h e heat going through t h e drying chamber. There is a limit t o the temperature t o which t h e ingoing air may be heated, while t h e outgoing air is necessarily fairly warm. I n other words, there is a serious loss of heat units in t h e outgoing air. For instance, if we commence with air a t 15' C., heat i t t o , say, 135'C., dry a material with i t , a n d then let it pass away a t , say, 75" C., we shall have put in 135 - 15 = 120 ', used 135-75 = G O ', a n d lost 75 - 15 = 60". I n other words, we have lost half our heat. With some materials which are very easily injured this would represent actual operating conditions; with others not so sensitive, t h e loss of heat would be less. P R E C O N C E N T R AT1 OK
As indicated above, t h e cost of drying is considerably reduced if t h e article t o be dried is first concentrated in vacuum. Usually t h e limit of concentration is reached just before t h e substanc.e becomes so viscous t h a t i t will not readily pass through a pump. Preconcentration not only reduces the cost of drying b u t improves t h e quality of t h e product. On spraying a very dilute liquid the solids are obtained in a very finely powdered condition. T h e product is bulky, requires large packing space, is hard t o dissolve, a n d usually does not have as good keeping qualities a s t h e product obtained by drying t h e more concentrated material. The reason appears t o be t h a t the more concentrated liquid gives larger and heavier d r y particles, with less surface per unit of weight. When such a powder is mixed with water or other solvent, it tends to sink and pass into solution, the spaces between t h e particles being sufficient t o allow the liquid t o penetrate. With t h e finer powder t h e particles tend t o hang together a n d form an impervious layer which the solvent does not so readily penetrate. T h e finer particles offering greater surface are more readily oxidized, or subject t o change from outside influences. Preconcentration leads t o a higher recovery. There is always a slight loss of solids in t h e form of fine particles which get by t h e dust collector. As precondensing
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makes larger, heavier particles, this tendency is minimized. SIZE O F U N I T
Drying units may vary a great deal in size, although there seems t o be a certain minimum below which it is not satisfactory t o operate. The unit with which t h e writer is most familiar requires a floor space of about 54 f t . X 15 it. X 14 f t . This allows for working space. With two units there would be some saving on this. Such a unit would evaporate 200 lbs. of water per hour from a material which is fairly sensitive t o heat, a n d give a product containing less t h a n 3 per cent moisture. The dryness, however, will depend on t h e character of t h e material, especially on whether it has a tendency t o retain water of crystallization. RECOVERY O F PRODUCT
The recovery of product will depend on various factors, b u t mainly t h e concentration before drying and t h e efficiency of t h e dust collector. When operating under favorable conditions in these respects, i t is quite possible t o recover from 95 t o 98 per cent of t h e total solids. TEMPERATCRE O F DRYING
The temperature of drying is affected by three things: steam pressure in t h e heater, amount of air passed through t h e drying chamber, and t h e amount of liquid sprayed. I n practice, t h e amount' of air is constant and t h e steam pressure is usually also constant. When such is t h e case, a sufficient amount of t h e liquid is sprayed in t o reduce t h e temperature t o t h e desired degree. There seems t o be a limit t o t h e temperature which i t is permissible t o use, both in t h e entering flue and in t h e drying chamber.
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APPLICATIONS O F T H E SPRAY PROCESS
The spray process may be used for t h e desiccation of a wide variety of substances. Whether it is t h e best method t o use in any particular case will depend on t h e value of t h e material and on t h e properties required in t h e dried product. It is sometimes difficult t o predict with certainty whether a material will dry satisfactorily by t h e spray process. Some materials have a tendency t o form a gummy mass on t h e floors and walls of t h e drying chamber. It is not always a question of hygroscopicity. Consider two substances like refined cane sugar and commercial glucose. The glucose will dry quite readily t o a fine white powder, b u t t h e cane sugar has a marked tendency t o give a gummy product. Normally, dried glucose is hygroscopic; cane sugar is not. It appears t o be a question of t h e rate a t which t h e material solidifies. Substances which solidify slowly are likely t o give trouble. I n general, i t may be said t h a t substances dried by t h e spray process are likely t o retain their natural properties. I n t h e case of milk, for instance, when t h e dried product is restored b y t h e addition of water, it again becomes normal milk. It has the milk flavor. There is no sediment. The albumin is not coagulated. The. casein has its colloidal character. The butter f a t is in natural emulsion. The enzymes and vitamines are active. Commercially, this process gives t h e greatest advantages in drying substances which are injured by temperatures and methods ordinarily used in desiccation. On these substances t h e slightly greater cost of drying will be much more t h a n made u p for by t h e quality of t h e resulting product.
Direct Heat Rotary Drying Apparatus By Robert G. Merz AMERICANPROCESS COMPANY, NEW YORK, N. Y.
The removal of moisture by vaporization from t h e various materials employed in t h e different industrial processes is a subject of great commercial and technical importance, especially for chemical engineers, and hence a more or less intimate knowledge of t h e types and characteristics of mechanical drying apparatus, available for such work, is very desirable. I n many cases t h e utilization of waste products is made possible only by t h e cheap and rapid elimination of t h e large quantities of water which such materials usually contain, and hence t h e mechanical dryer is a major factor in most by-product recovery processes. For those not thoroughly acquainted with t h e necessity for drying many of t h e materials employed in manufacture, i t may be well t o review briefly t h e reasons for such treatment. T h u s drying may be required: 1-To permit pulverizing or fine grinding. 2-To permit screening or grading. %-To permit uniform mixing. &-To permit magnetic and electrostatic' separation or dry table concentration.
increase capacity in later operations. reduce weight in shipment. ?'-To prevent decomposition of organic material due to high moisture content. 8-To permit improved conditions of combustion when burned in a furnace, etc. 5-To 6-To
T H E DIRECT HEAT ROTARY DRYER
Although there are many kinds of mechanical dryers in use, t h e present paper will be confined largely t o t h e discussion of a type which has a wider field of application t h a n a n y other single type, and which a t t h e same time possesses certain advantages which make its use practically universal in many industries. The direct heat rotary dryer is undoubtedly one of t h e oldest forms of mechanical drying apparatus and probably originated in satisfying a demand for a rapid and economical method of eliminating t h e natural moisture which occurs in minerals when taken from t h e earth or when exposed t o t h e elements. For many years t h e use of t h e direct heat rotary dryer was limited t o t h e drying of such natural inorganic materials. With t h e advent, however, of t h e more complex in-
