Spray Drying - American Chemical Society

FIGURE1. GRAY-JEXSEN SPRAYDRYINGUNITWITH PRECONCENTRATION CHAMBER (THE DOUTHITT CORPORATION)

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S P R A Y DRYING’ BEN B. FOGLER AND ROBERT V. KLEINSCHMIDT Arthur D. Little, Inc., Cambridge, Mass.

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PRAY drying is the process by which the solids are recovered from a liquid solution or slurry by spraying the liquid into a stream of drying gas under conditions which permit recovery of a dry granular product. The term “spray drying” is usually applied to those spray-processing operations in which the stream of drying gas is heated above atmospheric temperature; however, there are a number of commercial processes using essentially similar equipment in which the atomized liquid is a hot, very highly concentrated solution of some material that is solid a t atmospheric temperatures. When such a solution, usually heated in some previous operation, is sprayed into a stream of air at normal temperatures, the combined effect of congelation by cooling and evaporation produces a dry granular product. I n this “spray cooling,” the capacity of the equipment may be increased by precooling the air supply if that is economically practicable. A common and important feature of all spray processing is the direct conversion of the spray liquor to a granular product suitable for packaging without grinding or other intermediate handling. The individual particles of spray-dried products are generally rounded in shape and either hollow or spongy in the interior. Another important aspect is the unusually high rate of drying attained by atomizing the spray liquor directly into the drying medium; thus direct contact, unusually high 1 This paper was presented as part of the Symposium on Drying and Air Conditioning, the Fourth Chemical Engineering Symposium held under the auspices of the Division of Industrial and Engineering Chemistry of the American Chemical Society, at the University of Pennsylvania, Philadelphia, Pa., December 27 and 28, 1937. Other papers in the symposium appeared in April (pages 384 to 397), May (page 506), September (pages 993 to 1010). Ootober (pages 1119 to 1138). and December (page 1385).

specific surface, and a n effective relative velocity are provided between the product and the drying gas. In a welldesigned system 15 to 30 seconds is a fair time for the passage of the sprayed particle through the drying zone. This is particularly important in handling heat-sensitive organic products which need not rise materially above the wet-bulb temperature for the spray liquor in the chamber. The equipment for spray processing consists primarily of a chamber and spray mechanism for the contact between spray liquor and the gas stream, supplemented by accessories such as the pump and piping for the liquor stream, the fan, duct work, and heater in the case of spray drying for supplying the drying gas, and means for recovering the dry solid product.

The Spray Chamber Many of the earlier commercial spray-drying installations, notably those of the Merrell-Soule and Fleischer type, were rectangular horizontal chambers into which the spray liquor was introduced a t one end. I n recent construction, however, the typical spray-drying unit has a vertical cylindrical spray chamber, frequently with a hopper bottom. The spray liquor is generally introduced near the top of the chamber, and the major portion of the dry product removed from the bottom, either mechanically or entrained in the exhaust gas. Representative designs of this type used extensively in this country are The Douthitt Corporation’s Gray-Jensen unit with an almost equidimensional spray chamber (Figures 1 and 2), and the tower type as developed by Dickerson and built by the Spray Process Dryer Corporation shown in Figure 3. 1372

DECEMBER, 1938

INDUSTRIAL AND ENGINEERING CHEMISTRY

The path of the drying gas through the spray chamber is still the greatest point of variation between the more common designs, although in the majority of cases the mass flow is generally downward and concurrent with the product. The entering gas stream is frequently split; one portion enters the top centrally in a n annular stream around the spray, while the balance is introduced around the top circumference

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through a series of louvers. Although countercurrent flow of drying gas and spray liquor offers theoretical advantages in heat utilization in comparison with concurrent flow, the latter has a number of practical advantages which have led to its more general use. I n many installations supplementary cooling air is introduced partway down the tower to chill the product, particularly in the case of materials which are sticky above room temperature. Most drying chambers are operated under at least a slight suction to avoid having the product blow out when inspection ports are opened, since it is almost impossible to keep glass ports free from the fine air-float material, produced to some extent by practically all atomizing devices. From a technical viewpoint the size of spray dryers, particularly in diameter, lies between a minimum a t which particles from the atomizing device hit the wall before they are dry, and a maximum beyond which atomization and the flow of drying gas cannot be so controlled as to give a fairly uniform reaction between the spray and the drying media. The minimum diameter will depend on the uniformity of performance of the atomizing device and the average size of the particles; the larger particles travel much farther before their energy and velocity are lost by friction with the surrounding gas. On extremely fine products (150 to 300 mesh) drying chambers of 5 feet in diameter or even less may be operated successfully with atomizers which can be depended upon for uniform performance. Chambers ranging in diameter from 10 to 20 feet are much more common and are ordinarily preferred, as these larger chambers with their much greater production require little if any more operating attention than a smaller one. The height of the tower is determined by the trajectory of the atomizing device, the path of the air stream, and the drying characteristics of the product handled. I n general, spray chambers need not be as high in systems using centrifugal atomizers and tangential entry gas streams as in those designs in which spray nozzles point downward, and in which the inlet gas enters the chamber centrally with a substantial downward velocity component. Where the nature of the product dried requires the admission of cooling air below the drying zone to prevent the particles from sticking together, additional height is usually required if the diameter of the chamber is to be used advantageously.

In spray drying, a solution or slurry containing the desired solid material is continuously sprayed into a chamber and subjected to the action of a stream of drying gas, usually preheated air or diluted products of combustion. One of the most important aspects of the process is the physical form of the product, which is usually granular ; the individual particles are generally rounded in shape and hollow to a degree which is controllable over a rather wide range in the operation of the process. For many materials a product in this form has important advantages, chief of which are rapid solubility, lessened hygroscopicity, and unusual mobility in handling. The hollow form of the spray-dried particle usually gives a more bulky product than is obtained by other drying processes. This is an asset only when the spray-dried product will command a price which will more than offset the higher cost of packaging. The other important advantage of spray drying is the rapid rate and relatively low material ternperature a t which drying is accomplished. From 15 t o 30 seconds is a fair estimate of the time a particle stays in the spray-drying chamber when passing from liquid to solid form; the particle temperature need not rise materially above the wet-bulb temperature of the liquid of the solution. This makes the process particularly adapted to the drying of heat-sensitive materials, some of its most important applications being the drying of milk, eggs, potato flour, soap, and blood.

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FLOW SHEET GRAY-JENSEN PROCESS DRYINQ MILK (THE DOTJTHITTCORPORATION)

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PATENT (24) FIGURE 3. TOWERTYPEUNIT WITH AUXILIARIESBASEDON HOLLIDAY

Spray Liquor System

now available, gear drives appear to be satisfactory for speeds beyond those possible with direct drive from 60-cycle a. c. motors. It is good practice to provide spray-liquor lines with water and steam connections for cleaning the lines prior to shutdowns. Many operators, particularly when drying expensive materials, prefer to bring the drying chamber up to operating temperature on water and then switch to spray liquor; thus a large part of the off-specification material likely to be pro-

The performance of any spraydrying system, both in thermal efficiency and continuity of operation, depends in large part upon the proper design of the spray liquor system. This consists essentially of the actual spraying mechanism and the liquid handling system which supplies it with the liquor containing the solids to be recovered. Three types of equipment are commonly used to disperse the spray liquor in the drying chamber-the pressure nozzle, the atomizing or two-fluid nozzle, and the centrifugal or spinning-disk atomizer. Pressure nozzles are usually of the tangential-entry spin-chamber type (Figure 4); the character of the spray depends on the rotative velocity created in the spin chamber immediately behind the nozzle orifice by the pressure in the spray liquor line. This pressure may range from 100 to 1000 pounds per square inch, depending on the viscosity of the liquid and the particle size wanted in the product. In the atomizing or two-fluid type nozzle (Figure 5) the compressed air or steam, which furnishes the energy for atomizing, is supplied a t pressures ranging up to 100 pounds per square inch, depending on the requirements of the spray liquor and final product. Centrifugal atomkers (Figure 6) may be driven either by electric motors or directconnected steam turbines. In some of the earlier direct-connected electric drives, frequency changers were used to obtain the desired operating speed above 3600 r. p. m. OF MERRELL-SOULE PRESSURE NOZZLE (FROM FIGURE 4. DEITAILS DRAWING, 11) With the better materials and designs I.

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DECEMBER, 1938


duced in starting t h e dryer directly on spray liquor is eliminated. All atomizing devices should be so mounted in the dryer as to be readily inspected and quickly removed. When nozzles are used, ((spared’ should be available; even under the most carefully controlled conditions the satisfactory action of most types of nozzles is likely to be impaired either by partial clogging inside or the building up of dry material outside. From a design and operating viewpoint the pressure nozzle has a relatively lower capacity per nozzle in general spray-drying practice than either the two-fluid nozzle or the centrifugal atomizer. When they are used in large chambers, it is usual to find several nozzles arranged symmetrically to cover the cross section of the chamber. Both the two-fluid nozzle and FIGURE 5. TWOthe centrifugal atomizer can be IZING TYPE) designed t o handle a large throughput, so that only a single spraying station is needed for even a relatively large chamber. These Same two types are less susceptible to clogging than the pressure nozzle and are better adapted to handling either viscous solutions or heavy suspensions. Either the two-fluid nozzle or the centrifugal atomizer can be designed to disperse practically any material that can be pumped to it. I n uniformity of particle formation the pressure nozzle and the centrifugal atomizer will generally give a narrower range of particle sizes from a given spray liquor than the twofluid nozzle. The centrifugal atomizer now seems to be the choice of most manufacturers offering - prefabricated _ spray-drying equipment.

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within its temperature range, particularly if the dryer is so located that a furnace supplying products of combustion makes a fire risk. In connection with the use of steam in air heaters, it is relatively difficult and expensive to take advantage of superheat to raise the temperature of the air above that attainable with saturated steam a t the same pressure. While superheated steam, as the drying gas itself, has many attractive theoretical aspects, its use on a commercial scale in some of the early installations developed such serious condensation difficulties as to render the system impracticable. When drying air is required above the temperature obtainable from the available steam supply, it may be advisable to carry the temperatures as far as possible with steam heat exchangers and then use a small oil- or gas-fired indirect heater for a final booster. The choice of a source of heat for the drying gas must be made on the basis of the conditions of the individual installation, as no general rating of preference can be given the several methods.

Collection of Product The bulk of the powder may be removed from the system in either of two ways, both being commonly used. In the first the bottom of the chamber terminates either in a flat belt or in hoppers feeding one or more conveyors which continuously remove the powder that falls to the bottom of the chamber. However, with practically all types of atomizing mechanisms a wide range of particle sizes is produced from some rather controllable upper limit down to droplets which give dry particles of 200 to 300 mesh or smaller. As the specific volume of a material is increased by the characteristically hollow particle form of a spray-dried product, practically all spraydrying operations give a substantial amount of air float material which is carried out with the exhaust gases; unless recovered, this is likely to prove a considerable dust nuisance to the surrounding neighborhood.

Heating the Drying G a s The temperature of the drying gas entering the spray chamber ranges from 300” to 1200” F., depending on the product handled. It is important to operate the dryer with the highest temperature of gas entering the chamber permissible for the product handled. For self-contained drying plants the use of products of combustion, diluted to the required temperature, as the drying gas is becoming increasingly common. These products of combustion may be obtained either from gas or clean grades of oil and coal, although with the latter by-passes are frequently used in starting to ensure that only clean gases enter the chamber. Many food products are successfully dried in combustion gases, although these have not the same chemical composition as the atmosphere. Where products of combustion cannot be used satisfactorily, the drying gas is air-heated by some form of heat exchanger; on the primary side, products of combustion, steam, or a hot liquid, such as oil or diphenyl, are utilized. Under special conditions electric heat may be used. All indirect heaters or heat exchangers involve a stack loss of the order of 10 per cent from the heat of the original fuel. When steam can be obtained from a n existing boiler plant, it is likely to be one of the more attractive sources of heat

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Closed type

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OpenItype

FIQURE6. HOMOGENIZING ATOMIZERS In the second method of removal, the bottom of the drying chamber tapers into a n exhaust duct sufficiently small in size to give conveying velocities to the outgoing spent drying gas. This discharge duct usually leads initially to some form of cyclone separator which may subsequently be supplemented by a shaking fabric filter, scrubbing tower, and electrostatic separator. I n many commercial installations, particularly where the product is not very high in unit value and where the nuisance situation is not acute, reasonably satisfactory recovery is secured with the more effcient type of cyclone separators such as the Sirocco type D and the Western Precipitation Company’s “Multicone.” Where the main stream of product is recovered by cyclonetype separators, it is important that the fan handling the exhaust gases from the chamber should not be located in the

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duct between the chamber and the separator on account of the shattering effect of the fan blades on the product, as well as the fire hazard from sparks caused by the impact of entrained foreign solids on them. No general figures can be given for the total recovery of product with the various types of collecting equipment, although on many installations this varies between 90 and 95 per cent of the solids in the original spray liquor. Higher collection efficiency is almost invariably obtained at the expense of increased pressure drop across the collecting system with correspondingly increased power costs. In efficient installations the fan horsepower for the collecting unit may be double that of the rest of the drying equipment. The use in spray drying of fabric filters for cleaning the exhaust gas, of which there are several types of continuous units of excellent mechanical design, is frequently limited by the effect of the relatively high temperature of the exhaust gas on the life of the fabric; condensation wetting also impairs the permeability of the fabric. The cost of product collection is important in spray drying, and careful consideration must be given to both fixed charges and operating cost of collection equipment, as well as to the value of the uncollectable product.

Formation of Particles

rounding medium, they break u p into particles. The characteristics of particles formed from the breaking u p of films or sheets of liquid are essentially different from those of particles formed by the breaking up of filaments or threads. This fact furnishes the basis of a classification of nozzles for spray-drying purposes. Film-forming nozzles are represented by four common types of construction: (a) the common pressure nozzle (Figure 4) in which the fluid issuing from an orifice is given a whirling motion and thereby forms a widening conical film of spray; (b) the impinging nozzle in which a jet of fluid a t high velocity strikes a surface or edge and spreads into a flat fan-shaped spray; (c) the double-jet nozzle in which two jets of fluid impinge, forming a flat fan-shaped spray similar to type b; and (d) the spinning cup or disk spray (Figure 6) in which a sheet of fluid is thrown from a rapidly revolving edge by centrifugal action. I n all of these types the energy for breaking up the fluid is supplied directly to the fluid itself in the form of pressure or velocity. I n the atomizing type of nozzle (Figure 5 ) the fluid to be sprayed is delivered to the nozzle a t a relatively low pressure and is torn into threads or filaments by a high-velocity jet of compressed air or steam. These may be internal or external mixing nozzles, according as the atomizing air or steam issues from the nozzle through the same orifice as the material to be sprayed or through a separate jet, The action of both nozzles is practically the same, although under some circumstances the internal mixing type has a tendency to discharge a film of material from the edge of the orifice. A comparison of the action of filming and atomizing nozzles reveals extremely important differences. The energy which can be imparted to a pound of liquid in the form of pressure is very small in comparison with the energy available in a pound of air or steam. This is shown in Table I. Therefore, the pressure or filming-type nozzle is limited in its action to relatively fluid materials that are easily broken up. On the other hand, this type of nozzle has perfect distribution of energy to each particle of the fluid, resulting in very uniform spraying action. This is especially true of the double-jet nozzle where the action is not modified by drag on metal surfaces.

One of the chief justifications for spray drying is the peculiar form of the finished product. At the same time, the form in which the material is dried has a major effect on the rate of drying. The important characteristics of particles treated by spray drying include size and uniformity of size, shape, density, and structure. Spray-dried products may be obtained in a wide range of sizes from a coarse granular material down to the finest dust. The characteristic particle form of commercially dried products is round and either hollow or porous in the interior to a varying degree. The density of such products may differ widely also. These characteristics are affected by the nature of the material to be dried, the manner of spraying, and the subsequent treatment during the drying operation. As far as particle form is concerned, the most important characteristic of the material to be dried is its fluidity. A high viscosity is the result of strong TP I. BEHAVIOR OF AIR, STEAM, AND WATERIN NOZZLES (DISCHARGE TO ATMOSPHERIC PRESSURE) internal forces which resist disruption of the ,-Air . Steamfluid. Initial Expansion Initial Expansion Water An important factor, less obvious in its action, , temp. temp. Energy temp. temp. Energy Energy is surface tension. The formation of small parL b . / s q . in. F. F. Ft-lb./lb. F. F. Ft-lb./lb. Ft-lb./lb. QaQe ticles involves the production of a large area of 250 15 17 7,200 3,900 35 80 150 surface. The energy of the surface (the product 30 113 10,800 6,300 69 275 - 65 80 14,800 138 308 -119 60 74 9,300 80 of area and surface tension) must be supplied by 11,500 207 331 -150 37 17,100 80 90 13,400 312 358 -180 the spray mechanism. Although apparently very 135 19,400 80 4- 14 392 15,600 485 -210 21,600 210 - 8 80 small, it becomes important owing to the low ef285 417 23,100 16,800 -230 -14 658 80 ... .... 4600 ... .. 2000 ficiencv of mdication of the enerm in the ordinary spray nozzle. Surface tension also has a marked effect on forming the particle. In general, An even more important characteristic of the pressure high fluidity and high surface tension tend to produce spherinozzle lies in the manner in which the film of fluid breaks u p cal particles and to permit easy control of particle size. into drops. The motion of air past a surface a t high velocity The action of the nozzle is the most important single facgives rise to a n unstable condition commonly referred to as tor in determining both the character of the final product and turbulence. If the surface is nonrigid, tjhe reaction of this the action of the spray chamber in the drying operation. turbulence on the surface creates a series of waves, as when The spray nozzle performs three distinct functions-breakthe wind blows over the ocean. When the surface is a thin ing up the fluid into particles of proper size, forming those parfilm acted upon on both sides, as a flag flying in the breeze, ticles into suitable external shape and internal structure, and the waves build up very rapidly and cause an actual whipping distributing the particles in the drying gases. of the surface so that it curls back on itself. If the surface is The mechanism of spray formation consists in first draga thin film of liquid shot out a t high velocity into the air, this ging out the fluid into thin flat sheets or filaments. Both phenomenon causes the liquid film to curl up on the free forms are unstable, and under the combined action of surface edge until it may actually join in a hollow tube. Such a tension, internal viscous forces, and turbulence in the sur-

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just behind the discharge orifice sets up a rotative motion in tube immediately breaks off and, since it is statically unstable, themassof spray liquor. necks down in places and breaks into a series of hollow When the pressure in the spray-liquor line is too low to spheres. We have ample experimental demonstration of this create any appreciable whirl in the spin chamber, the spray tendency of filming nozzles to form hollow particles. Among liquor is forced out in an initially smooth solid stream, the the more important demonstrations is the following: molten surface of which soon begins tallow may be sprayed to ripple under the combined into cold air to proeffect of internal and external duce, under suitable forces as shown in Figure SA. conditions, a large B illustrates the effect of increasmajority of the paring the pressure at the nozzle ticles in the form of to the point where the hollow hollow spheres (Figcone of spray film is just beure 7). This same ginning to form and hreak, alaction has hindered though the pressure is still inthe spray drying of sificient for good atomization. gelatin solutions, as This photograph shows in a t h e final material striking manner the ”wavingencloses so much air flag” surface of the cone of that it has a low bulk film and its final rupture under density and produces the combined effect of attenuaa f r o t h y solution tion, as the cone diameter inwhen redissolved. creases, and of increasing wave Edgerton and Geraction of the film. C shows the meshausen h a v e effect of a high pressure on the succeeded in photonozzle, giving s a t i s f a c t o r y graphing the formaatomization of a viscous liquor. tion of a hollow parFigure 10 is a spark phototicle by this means. graph of a pressure nozzle in Figure 8 is a section action on a less viscous surav of their motion picFIGURE 7. PHOTOMICROGRAP~ OF H ~ L L P ~A W~ T I CFORMED LE~ liquor. The film in the iiitill ture of a nozzle spray BY A PRESSITRE Nozzra snrav taken at six thousand . “ cone is thinner and more responsive to the forces tending frames per second. to hreak it down. The transition from the conical film a t the u e d . it is necesTo nhotoeraoh action af . .this hieh rate of s~, nozzle to round particles is complete within a few inches of sary to secure illumination from behind, which results in the nozzle. In the action shown in Figure 10 the atmosphere silhouette pictures. The dark areas represent either solid about the spray nozzle was approximately 70’ 1”. and cooler liquid as contrasted to a thin film, or a thin film wheu its surthan the spray liquor itself. This eliuiinates the possibility face is perpendicular to the camera. that the hollow particles of product recovered and pbotoAt the left is a section of the film, showing successive frames, graphed in Figure 7 were formed by expansion of vapor beginning at the bottom. A wave of liquid film enters the within the particle. field of vision on the left at No. 1 and in forty frames (‘/m The atomizing nozzle has its own distinct field (Figure 11). second) progressively changes to the spheres leaving the First, the vast aniount of energy available, which can be righbhand side at No. 10. varied by changing the ratio of atomizing air or steam to the At the right are enlargements of every fourth frame. These liquid sprayed, permits the atomization of almost any mafollow the liquid film which can be seen entering edgewise at terial that can he fed to the nozzle. It is also possible to ohA in KO. 2 and curling up on itself to make the tubular form tain extremely fine atomization-in fact, much too fine for shown in No. 4. In KO. 5 this tube closes to the hollow most spraydrying processes. Fine atomization is usually egg shape, B, which either breaks away from or absorbs its avoided because of the difficulty of recovering the product appendages, as at C in No. 9. The final form assumed is from the drying air. that of the small sphere in No. 10. Particles formed in this way are usually perfect hollow The mechanism of atomization hy air or steam is more spheres and often very thin-walled. Variations on this form complex tlian that. in the film-forming nozzle. The priinclude hollow spheres with one or two tails, resulting from mary atomizing action is again due to the turbulent action of a high-velocity jet of gas moving past a liquid surface. rapid drying of the thin filament formed during the breaking of the rolled-up tube of solution, and collapsed spheres formed This causes particles of the fluid to be blown off, frequently by subsequent cooling of the entrapped air and vapors. A dragging out filaments of fluid behind them, which then curious freak which is not uncommon in this type of particle break down into sinall particles. In this case a primary parformation is the formation of one complete hollow sphere ticle may he much larger than the particles formed by hreakwithiu another larger one. The smaller particle already ing down of the filament and, since the action is somewhat formed and dried is caught in the film as it wraps up into a fortuitous, the result is a considerable range of particle sizes. tube and remains in one of the larger spheres when the tube This simple action is complicated by secondary effects. breaks up into particles. Several of these may be seen in The rush of air or steam from a nozzle produces a hissing Figure 7. sound composed of very high-frequency sound vibrationsFigures 9 and 10 are spark photographs, showing several that is, alternate compressions and rarefactions in the stream aspects of the action of two of the more common types of of gas, often of marked intensity. The effect of these violent pressure nozzles. and rapid pressure changes on the character of the particles Figure 9 illustratos the effect of pressure in the spray-liquor formed is difficult to estimate. It is known that such viline and of viscosity on the format.ion of the spray in one of tbe brations tend to liberate dissolved gases in fine bubbles most common types of pressure nozzles. A spin chamber throughout the material. Many atomizing nozzles also have Y

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FORMATIOK OF ROUND, HOLLOW PARTICLES AS SEIOWN BY “STILLS” FROM MOTION PICTURE FILMAT SIX THOUSAND FRAMES PER SECOND

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a slow pulsation determined by the relative sizes of the passages in the nozzle and of the supply piping. These pulsations, which are noticeable on an ordinary pressure gage connected to the nozzle, have the effect of causing atomization to occur under continually varying conditions. When this happens, uniform spraying action is not to be expected. The temperature effects due to the expansion of steam or air in an atomizing nozzle are considerable, although frequently overlooked. Table I indicates the adiabatic temperature drop due to expansion to atmospheric pressure for both air and steam. This temperature is approached by a particle moving with the high-velocity jet of gas resulting from the expansion. Steam temperatures below freezing are indieated and probably actually exist in the jet. Owing to the relative velocity and intimacy of contact, enormous rates of heat transfer are to be expected; therefore, even when materials are supplied hot to such a nozzle, they must he chilled by these low temperatures many degrees a t the actual point of particle formation. This tends to set the primary particles rapidly if they are thermoplastic and prevents surface tension from acting to round the particles. An irregular shredded particle results. It also prevents the rapid drying action that would otherwise be expected a t the nozzle, since the expanding gases tend to become mturated. Finally, the particles are cool on entering the spray chamber and d tend to remain so during the drying process. This cooling action in the nozzle explains why there is little practical difference between spraying with steam and with air.

Drying of e Particle The one fundamoutal principle to remember in removing water from a single particle suspended in hot air is that a

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definite amount of heat i s required to evaporate water, regardless of the surroundings, dry air or moist air, still air or moving air. The temperature of the particle a t which evaporation occurs will vary with the air conditions. There are three possible sources of this heat-the internal beat of the material itself, heat transferred by conduction and convection from the surrounding medium, and radietion from hot surfaces a t a distance. The fust source is necessarily limited in capacity, any draft on the sensible heat of the particle being accompanied by a marked lowering of temperature. Evaporation of 5 per cent of water from a water particle will lower ita temperature by 50" F. However, the ready availability of this heat makes it immediately effective. The second source is of major importance in spray drying, but the third source is not to be ignored, since it permits instantaneous heat transfer from a considerable distance. When a body having a moist surface is exposed to air,an equilibrium condition is set up in which the body and the surrounding air are cooled by evaporation until the air is saturated a t the familiar wet-bulb temperature. This can never be so high as the boiling point of the liquid and is usually much lower. Since conduction of heat through a solid or liquid is rapid compared with that through a gas (in most ordinary oases), the temperature of the interior of the particle will be very little higher or lower than the surface. As the surface continues to lose water, a concentration gradient is set up which starts diffusion of water from the interior to the surface. This diffusion is modified materially in some cases by changes of diffusion resistance due to variation in temperature or irreversible dehydration (casehardening). If the rate of evaporation (heat transfer to the surface) is

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low, diffusion takes place normally, but if irreversible dehydration takes place at the surface, high resistance to diffusion may be set up. This phenomenon is well recognized in the spray drying of certain colloids. It permits the particle to heat up to the temperature of the surrounding gas, all evaporation having ceased. If at this stage the gas is above the boiling point of the water in the particles, steam may be generated within the particle, and expansion or disruption may occur, depending on the plasticity of the dehydrated surface. Evaporation of as little as 1 per cent of moisture in the particle in this way will expand it to seventeen t.imes its original volume. A small amount of air or other permanent gas in the particle modifies this effect and causes a small internal expansion to take place much more readily. As drying proceeds, the particle, if suspended in still air, becomes surrounded by a thicker and thicker laycr of cool air-vapor mixture, and the rate of heat transfer falls off rapidly with time. For 1 per cent of water removed, the steam formed is seventeen times the volume i f the particle; the air cooled is of the order of 40,000 divided by the temperature drop, or with air at 500" F. it will he approximately one hundred times the particle volume. Thus the first 1 per cent of moisture evaporated will surround the particle with a layer of cool gases q u a l to about five times the radius of the particle. If the pariicle moves t.hrough the gas, the cool air and vapor will continually flow off the particle. Since, however, even a thin layer of cool gas will decrease the rate of drying (heat transfer) greatly, vigorous motion of the particle through the drying gases is important. As the particles arc usually so small that they are readily conveyed by t,he gases, some form of centrifugal act.iou, such as turbulence, is necessary to produce the relative motion which is essential to spray drying at a reasonablc rate.

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through the air, and the relatively larger effect of the resulting layer of cool gas, so that evaporation from the surface of small particles is slower than from that of large particles. When diffusion of moisture through the particles is the controlling factor, the small part.icles will dry much more thoroughly than larger ones.

Conditions in the Spray Chamber We are interested in the efficiency of heat utilization and in tlie capacity of the chamber per unit of volume. The efficiency of a spraydrying process is determined by the theoretical thermal efficiency of the cycle employed, the extent to which the equipment approaches the theoretical efficiency, and the economy of the cycle as affected by the relative value of high- and low-temperature heat. Figure 12 shows, for various temperatures of drying air, the maximum thermal efficiency that can be obtained when evaporating water from a nonsoluhle substance. The factors which prevent realization of the efficiency in Figure 12 are failure to attain equilibrium due to poor contact of spray and air, the low rate of diffusion of moisture t,hrough the material, heat loss from the chamber, moisture in the drying air, soluble substances in the liquid, and failure to utilize the advantages of countercurrent operation. The attainment of equilibrium between the spray particles and the drying air as a whole presupposes uniform contact of every portion of air with the spray. This is never possible. One advantage of spray drying is that i t permits the drying of sticky or pasty materials, and the spray in these cases must not touch the walls until the drying is beyond a certain stage. All drying air Bhich passes near the walls of the spray chamber is therefore prevented from contacting its proportiouato part of the spray. Also, when several nozzles are spraying into a single tower, it is difficult to prevent open spaces between the spray zones. This by-passed air is partially utilized as a result of turbulence, but since a11 the sprayed material must he dry even where the ratio of spray to air is greatest, where less spray is present the air will contain wasted heat. In materials which tend to caseharden or in which diusion is slow, the attainment of equilibrium with the surrounding gases is much slower than with more easily dried materials. For this reason the efficiency of a given spray chamber may vary considerahly with the material treated, even with similar atomization and distribution of t.he particles in the drying gas.

FIGURE 10. PRE~SURE NOZZLEIN ACTIONON MATERIAL OF Low VISCOSITY

A

A knowledge of the relative rate of drying of particles of different sizes is very important in designing for a uniformly dried product from a heterogeneous spray, such as is produced by most commercial nozzles. Since the finer particles have high ratio of surface to volume, they frequently dry much more rapidly than the coarser particles. But this effect is modified by the relatively lower velocity of small particles

Heat losses through the walls of the tower and the ducts leading to it are also a major source of loss. Since velocities of the drying air in the spray chamber are usually kept low, the heat handled per square foot of chamber surface is also small, with the result that radiation losses are large. The losses due to moisture in the drying air are very small, and, except when using recirculated air, can be neglected. When the character of the materials being dried permits, the use of countercurrent drying allows us to approach the theoretical equilibrium. This is true even when soluble salts are present which on concentration raise the equilibrium temperature, since in this case final equilibrium will be with the fresh material before concentration occurs. Capacity of the chamber per unit of volume is determined both by the efficiency of heat utilization and by tlie amount of heat handled per unit volume per unit time. This is proportional to the velocity and temperature of the drying gases. The velocity is limited, especially in any attempt at countercurrent operation, by t.he conveying velocities of the material, and in concurrent drying the time oi contact in the tower is reduced as the velocity increases. On the other hand, high velocities properly handled will cause high relative mo-

DECEMBER, 1938


tion of particles through tlie drying gas. A recognition of the value of such action in the spray chamber characterizes recent developments in equipment.

Factors Controlling the Efficiency of Spray Drying With the exception of the relatively sniall amount of heat

that can he put directly into t,he spray liquor as it is pumped to the atomizer, the primary heat cycle of the spraydrying opemiion consists in heating the drying gas from the atmosphcric datum, To, to the temperature, T I , at which it enters the drying zone of the spray chamhcr. Here heat is given up in heating the spray liquor, in evaporating solvent from it, and in radiation losses from the chamber walls, following which the gas leaves the drying zone at a temperature, T,. If the radiation loss is stat.ed as a percentage, E , of tlie total temperature drop in the drying zone, then the useful work done in the drying zone is proportional to (100 E ) per cent of (Ti- "%),andthe thermal efficiency of the spray chamhcr operation may he stated as:

-

Practical experience has shown that over a rather wide range of inlet temperature (say from 300" to 1ooO" F. for 2;) the temperature at the end of the spraying zone, T,; may be held approximately in the range of 170" to 300' F. Taking these data into account, it will be seen that the following factors make for and control the efficiency of the spraydrying o p eration: high inlet temperature, low exhaust temperature, recovery of heat from exhaust gas beyond the drying zone, and reduced radiation loss.

Inlet Temperature The question of inlet temperature is usually determined by two factors-(a) the temperature which can safely he used without damaging the product or creating operating hazards and (b) the comparative cost of high- and low-temperature sources of heat. In the case of most organic products extreme care should

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be taken in estahlishing the temperature of the incoming drying gas to avoid damage to the product as well &s danger from either fire or explosion. Theoretically, relatively high temperatures may be used, particularly when such products are dried with concurrent flow of drying gas and product through the drying chamber, as a result of the continued evaporation from the surface of the atomized particles. In actual practice it is difficult to avoid the accumulation of small qumtities of material either by their heing plastered on the walls at times of faulty atomizing or by their being lodged on rough spots or slight projections from the wall. Such material usually overdrics in place and gradually falls off. With many materials these overdried portions are likely to lower the quality of the product in taste or color, and all too frequently such accumulations are the cause of serious fires and explosions. Great care should be exercised in the design of spray towers to reduce the chance for such accumulations by carefully controlling atomization and air flow and by avoiding recesses or projections on the inside walls. When flammable materials are to he dried, good operating practice requires frequent inspections to see that the walls of the spray chamber are clean, as well as shutdowns whenever it is necessary to remove accumulations from the walls. In some cases it may bc desirable to operate a t an inlet tempcrature somewhat lower than is technically possible, owing to the difference in cost of alternative sources of heat; for example, it may be desired to dry a product, which might stand an inlet temperature of 400' F., with air heated to 350" F., tlie maximum temperature possihlc with a n available steam siipply when in this particular instance the cost of heat units is niuch lower than with oil. Each case of this sort must be figured on its merits, and no general statement is possible.

Exhaust Temperature In practical operation, the operating control on the dryer is usually the exhaust temperature, Ta, or the temperature at the end of the drying zone in those chambers in which eooling air is subsequently introduced. With the inlet gas temperature fixed by the heat sensitivity of the product handled, and the volume of drying gas established by a fan of fixed speed giving the proper gas circulation in the dryer, the c&paeity of the dryer is determined by a rate of feed which will give a properly dried product and keep the chamber walls clean under normal operation. The exhaust temperature obtained under these conditions is established as the control index of the dryer, and the operator holding other conditioas constant, including the concentration of spray liquor, adjusts the rate of feed to hold the exhaust temperature coustant at this point. It is almost impossible to predict this exhaust temperature in advance for a given product, since i t depends on the drying characteristics of the material. As this temperature represents the point at which the useful work of the drying medium stops in the average installation, most equipment manufacturers insist on the opportunity of making a test run io their pilot plants before stating the capacity of their dryer on the product in question.

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VOL. 30, NO. J2

Many materials, of which soap is a notable example, have such a low initial water content as to preclude this use of the exhaust gases for preliminary concentration. Many such materials are so viscous a t atmospheric temperature that it is necessary to heat them to attain sufficient fluidity for spraying. This heat is used to unusual advantage although there is a definite limit to the amount that can be so applied. Heating much beyond the boiling point of the spray liquor prevents satisfactory atomization and particle formation. In any contemplated heat-recovery installation a careful check should be made to see that the value of heat and product recovered will not be offset by the capital and operating charges of the recovery equipment.

Radiation Loss Heat insulation should be used to prevent losses from the chamber and all ducts leading both to and from the dryer as long as there FIQURE12. MAXIMUM THERMAL EFFICIENCY DURINQ EVAPORATION OF WATERFROM NONSOLUBLE SUBSTANCE is a substantial heat head above atmosphere which can be advantaneouslv used. The exposed surface of these varilous parts of the Heat Recovery from Exhaust Gas spray-drying installation is usually relatively large with a corresponding chance for radiation loss. However, the usual There are three approaches to improving the over-all effieconomic balance must be considered between the cost of ciency of the spray-drying operation on the low end of its insulation and the value of the heat saved. working cycle-i. e., the temperature of the exhaust gas: (a) recirculating exhaust gas to the drying chamber, ( b ) Improved Chamber Design preheating fresh drying air, and (c) preheating and concentrating spray liquor. The factors just discussed bear on the efficiency of any of Particularly where high-temperature primary heat is availthe established types of dryers. Within the past ten years able, a marked gain in efficiency would result from recircuthe inherent attractiveness of the spray-drying process has lating a substantial part of the exhaust gas and boosting its led to a n intensive study of the possibility of improving the temperature by adding a relatively small amount of very hot chamber design in order to secure a larger output from a unit fresh gas. With heat-sensitive organic materials this is selof given size and cost. These efforts have been directed to dom possible because of the difficulty of completely freeing obtaining more uniform atomization and a controlled path the exhaust gas from suspended dried material, which would be overheated a t the instant of contact with the hot booster gas and carry through into the drying chamber to contamiDATAOF COMMERCIAL SPRAY-DRYINQ TABLE11. PERFOEMANCE nate the product and frequently cause a fire hazard. EQUIPMENT ON REPRESENTATIVE MATERIALS" I n general, a sufficiently thorough cleaning of the exhaust Temp. Heat Required gas to permit its recirculation is likely to result in a heavy Solids Water in Inlet per Lb. Water Type of Msterial in Spray Product Gas Evaporatedb capital investment and operating cost for the scrubbing % % F. 8. t. u. equipment and frequently a reduction in the gas temperature 2 300 Organic 10 2250 which nullifies a large part of the potential gain. The fore3 Organic 35 3800 325 2 33 350 Organic 3300 going does not apply to the Peebles design in which a portion 2 450 10 2700 Organic 15 375 3000 Organic 65 of the gas stream is withdrawn from the top of the chamber 4 750 55 2250 Organic. and returned to the bottom without reheating. Inornanic 4 750 2500 60 -----a Four different commercial types and five sizes of dryers are represented The difficulty just noted of completely cleaning the exhaust by the data given. gas of entrained product without seriously reducing its temb B. t. u.'s in drying gas above 65' F. datum temperature at entering spray chamber. perature makes it almost impossible to use it in a heat exchanger for heating either incoming gas or spray liquor, as the air-float solids which the exhaust gas contains will quickly for the drying gas in the spray zone, which will bring about foul its side of the exchange surface. However, where the intimate and uniform reaction of spray and drying medium spray liquor has a relatively high water content, as in milk, and ensure the drying of all of the spray to solid nonsticking blood, and many water extracts, the weak spray liquor may material before it comes in contact with any portion of the be concentrated and heated by being sprayed into a precondryer wall. centration chamber through which the exhaust gas from the Among the more advanced equipment embodying these spray tower is circulated prior to venting it to the atmosphere. ideas are the English Kestner dryer and the unit developed When such a chamber is properly designed, a high proportion in this country by the Bowen Research Corporation (Figure of the air-float solids carried from the tower by the exhaust is 13). Both of these units, as well as the comparatively new recovered, and although put back in liquid form, the cost of Peebles dryer of the Western Precipitation Company, use redrying is usually negligible in comparison with the value of centrifugal atomizers. the material. Such a preconcentration chamber is standard When these smaller dryers give the same output as might practice in the well-known Gray-Jensen milk-drying unit formerly have been expected from a substantially larger (Figure 1). O

DECEMBER, 1938


unit, the fan horsepower per ton of product usually will be somewhat greater. On the other hand, the radiation loss, a n item of real importance in the conventional dryer, is certain to be reduced. It also seems probable that with these smaller units, which lend themselves more readily to standardization and fabrication, plant construction should also show a substantial reduction in capital investment and fixed charges as compared to the tower type unit. Economics of the Spray-Drying Process It is particularly difficult to draw any general conclusions as to the economics of spray drying because of the extreme variation in the thermal efficiency of the drying operation over the range of materials which may technically be handled by the process. Data recently published on a well-designed commercial unit 10 feet in diameter show theoretical evaporations, when spraying water a t 212" F., ranging from 24 to 64 pounds per minute a t inlet temperatures of 300" to 1000" F., respectively, with allowances for radiation. Assuming a datum of 65" F. for the temperature of air taken into the system through the year, these data show heat requirements a t the point of entering the spray chamber ranging from approximately 2100 to 1600 B. t. u. per pound of water evaporated. Spray liquors of different materials and concentrations vary over a wide range in their rate of giving off water under the conditions of the spray process; and in commercial practice actual evaporations obtained in spray-drying materials commonly handled will range from 50 to 90 per cent of the theoretical water evaporations just noted. Table 11, based on performance records of commercial units of different sizes and types, affords a fair idea of the heat input to the spray chamber per pound of water evaporated from spray liquors of materials commonly spray-dried. The data given in Table I1 show only the thermal efficiency of the spraydrying chamber. Losses incident to heating the drying gas, such as stack loss, condensation, and furnace .and duct radiation, must be deducted in determining the aver-all thermal efficiency. Similarly, credit should be given

-a

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for the heat recovered from the exhaust gas through such measures as a preconcentration chamber. If a product is made up of very fine mesh particles, it is possible to build and operate a spray dryer of relatively small size-5-foot diameter or less with correspondingly low capacity; but the majority of commercial units are much larger. Coupled with the possibility of operating a comparatively large dryer with practically the same labor cost as a small one, this makes the spray drying of most material produced on a competitive basis a n operation which is almost necessarily on a relatively large scale. Capital costs for a spray dryer approximately 10 feet in diameter of the evaporative capacity previously stated will range from $20,000 to $30,000 installed, depending on materials of construction, the type of auxiliaries such as air heaters, collection equipment, etc., and the drying cycle. Energy requirements are primarily in the fan horsepower which may range from 25 to 75 h. p. for a dryer of the size just noted, depending on the amount of drying gas used, whether or not cooling air is required, and the type of collection equipment. I n addition, from 5 to 15 h. p. must be allowed for pumping and atomizing spray liquor. For the direct operation of a dryer of ordinary size, one operator and a helper are usually required; when there is a battery of several dryers, the amount of labor is proportionately less. A careful study of a large number of commercial spraydrying operations leads to the conclusion that the direct economies of the drying operation itself have seldom been the determining factor in the decision to use the process. In many cases an important consideration is found in the form of the product which is not duplicated by any other drying operation, and which frequently gives physical properties of substantial value in the keeping qualities and use. Note has already been made of the ability of the process to handle heat-sensitive materials which is paralleled only by the vacuum dryer. The spray process is particularly attractive for drying solutions of sticky amorphous materials which are required in a dry pulverulent form; grinding operations are eliminated, and under proper operation there need be no stage between the original liquor and the final product where sticking of material to equipment presents an operating problem. Such factors are likely to make the question of whether or not to spray dry a rather complex problem; considerations of the merits of alternative physical forms for the dry product and of rather different operations elsewhere in the manufacturing process are involved, as well as the comparative cost of spray processing and some other method of drying. 0

FIGURE13. CROSSSECTION OF BOWEN SPRAYCHAMBER A , B . Hot air inlet ducts Central annular inlet for hot air entering at A C.

Vertical inlet nozzles Sloping radial passagea in C Manifold; hot air entering at B passe8 through F to vertical F. inlet nozzlea D , EO arranged as t o direct their flow j w t to the left of the axial center of the chamber, thus producing a violent central vortex, closed at the top by the hot air entering at C H. Inlet for Eolid to be dried Homogen;zing atomizer which discharges a horizontal sheet I. of spray into the vortex of hot gases Manifold carrying cold air, wed only when the dried product J. is to be cooled Air aweeper to remove duat M. Inlet for air t o operate aweeper N. Outlet for dust P. Exhauat manifold Q. R. S. Typical aluminum foil insulation to prevent charring of duet particlea on internal aurfaces

D.

E.


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Acknowledgment The following individuals and organizations have contributed data or illustrations used in this discussion: Bowen Research Corporation, Martin Dennis Company, The Douthitt Corporation, Harold E. Edgerton and Kenneth J. Germeshausen, Percy Landolt, Lever Brothers Company, and Spray Engineering Company.

(17) 1,312,019, 0. Carr, 1919;

(18) (19)

(21) (22)

U. S. Patents (in Chronological Order) (1) 51,263, C. A. LaMont, 1865; earliest reference to spray drying,

(23)

relates to egg powder.

(2) 125,406, S. R. Percy, 1872; spray-processing conditions de-

scribed in detail. (3) 666,711, R. Stauf, 1901;

atomized material introduced into hottest zone of drying air (patent acquired by Merrell-Soule Go.). (4) 860,929, L. C. Merrell, I. S. Merrell, and W. B. Gere, 1907; preconcentration before spraying shown. (5) 890,078 and 940,398, W. Luring, 1908-9; use of steam or other heated gaseous medium to assist atomization of soap, which is first superheated. (6) 962,781, W. S. Osborne, 1910; concurrent spraying with hot air; dried product recovered in cyclone collector, exhaust from which is scrubbed with weak spray liquor, thus recovering dust and entrained solids. (7) 997,950, F. P. Bergh, H. J. Loebinger, and H. C. Neuberger, 1911; countercurrent drying. ( 8 ) 1,074,264, G. Jebsen and C. Finkenhagen, 1913; high drying-air temperatures, 250’ to 600° C . (9) 1,078,848, C. E. Gray and A. Jensen, 1913; 1,107,784, C. E. Gray, 1914; fundamental apparatus and process patents of Gray-Jensen system; atomization at center of chamber, essentially countercurrent. (10) 1,090,740, W. B. McLaughlin, 1914; current of relatively cold air introduced a t predetermined point in the drying stream to solidify product. (11) 1,183,393, I. S. Merrell and 0. E. Merrell, 1916; high-pressure spray nozzle, adapted for pressures of 2000 lb./sq. in. and over. (12) 1,188,755, W. B. Gere, 1916; first of four patents covering spray-dried milk and milk-mixture products. (13) 1,198,203, L. B. Edgerton, 1916; spray-dried sodium silicate and process. (14) 1,214,160, F. J. Hemig, 1917; spray-dried maltose (malt sugar) and process. (15) 1,215,889 (1917), 1,350,248 (1920), R. W. G. Stutzke; use of superheated steam directly as drying medium, recirculation of drying medium by closed system. (16) 1,301,288, 1,394,035, and 1,398,735 (1919-21), J. C. MacLachlsn; relate, respectively, to apparatus, product, and process aspects for drying heavy viscous substances.

liquid preconcentrated by being for scrubbing out dry fines from spray-drying This and two subsequent patents-l,321,362 1,405,756 (1922)-were basis of Cardem process for liquor. 1,361,238-9, R. S. Fleming, 1920; relate to spray-dried fruit juices; glucose added before spraying to obviate sticking together of powder particles. 1,396,028 and 1,397,039, W. H. Dickerson, 1921; spray drying waste sulfite liquors. 1,428,526, C. E. Bradley and J. G. Coffin, 1922; process and apparatus for spray drying latex. 1,472,472-3, J. C. Ingram, 1923, and 1,484,271, S. B. Murdock, 1924; relate to spray processing soap. 1,497,168 and 1,512,461, W. L. Fleisher, 1924; represent, respectively, apparatus and process aspects of Fleisher & Company’s later developments, introduced about 1921. 1,600,503, W. H. Dickerson, 1926; continuation, in part, of application which became U. S. Patent 1,397,039; covers broad process aspects of producing glazed particles by spraying into hottest part of drying air, which may be 1000° F. Reissue 16,749 of 1,621,506, R. L. Holliday, 1927; 1,652,900, D. R. Lamont, 1927; production of round, hollow particles of soap. 1,698,505, C. D. Marlatt and W. H. Dickerson, 1929; process for spray drying basic chromium sulfate. 1,734,260, D. R. Lamont, 1929; methods for control of sprayprocessed product characteristics. 1,797,055, F. H. Douthitt, 1931; improvements on and modifications of Gray-Jensen process (see 1,078,848). 1,914,895, D. D. Peebles, 1933; use of two tangential currents of drying air of different characteristics, one current outside the other, each moving in opposite directions (process used by Western Precipitation Co.). 2,081,909, W. S. Bowen, 1937; air introduced through long vertical slots into drying chamber in such a manner and angle that dried product will not accumulate on walls. used f i s t operation. (1919) and drying tan

(20)

Bibliography of Some of the Important Patents in t-he Spray-Drying Art

VOL. 30, NO. 12

(24) (25) (26) (27) (28)

(29)

Foreign Patents (30) British 28,767 (1906) and 18,549 (1907), P. Bevenot; counter(31) (32) (33)

(34)

current and concurrent spray drying, respectively; comesponding patents include French 372,581 and U. s. 1,020,632. British 109,471, H. C . Booth, J. L. Baker, and Solac Ltd., 1917; Solac process for making milk powder. British 169,471, F. E. Beeton and Trufood Ltd., 1921; method and nozzle for spray drying very viscous materials, overcoming stringiness of product as made by prior processes. German 203,193 (1906) and 232,698 (1907), Gebr. Koerting A.-G.; see corresponding U. S. patents to Luring ( 5 ) . British counterparts are 25,276 (1907) and 26,133 (1908) to Lindemann. German 338,924, 343,955, 348,333, and Austrian 76,782, Metallbank und Metallurgische Ges. A,-G. (1918-19) ; representative of large number of patents issued in Germany and elsewhere on Krause process; concerned fundamentally with use of centrifugal disk for atomizing; extensively used commercially.

RECEIVE~D April 16,1938.

Soviet Trucks to Use Distilled Wood Fuel Large-scale production of an ingenious type of autotruck t h a t generates its own gas from wood picked up as it goes along has been ordered b y Soviet authorities as t h e result of a successful test run, according t o t h e New York Times. Various Soviet automobile a n d tractor plants are expected t o turn out 56,000 such trucks in t h e next two years. It is claimed t h a t these trucks are specially suitable for t h e vast expanses of Soviet Russia, since they can operate in remote areas where gasoline is unobtainable, can reduce demands for precious gasoline, and can be run more cheaply t h a n gasoline-consuming vehicles. At one point in t h e 7000-mile run it is stated t h a t t h e trucks ran out of fuel in a sparsely inhabited region b u t simply thrust old railway ties into t h e gas-generating apparatus a n d continued. T h e route traversed crossed t h e Ural Mountains into Siberia and

then turned back through northern European Russia and t h e Ukraine, following t h e course Moscow-Magnitogorsk-Omsk-

Kiroff-Gorky-Leningrad-Minsk-Kiev-Moscow.

Twelve trucks began t h e circuit, manned b y cheery and sturdy young women. It was said t h a t there were no accidents or serious breakdowns, though only seven trucks were counted crossing t h e finish line. A brass band and trucks, busses, and automobiles filled with automobile executives a n d relatives and friends met t h e touring party near t h e city and escorted them in. This report is interesting in connection with t h e article on “Motor Fuel Economy of Europe” by Gustav Egloff, published AND ENGINEERING in t h e October, 1938, issue of INDUSTRIAL CHEMISTRY, pages 1091 t o 1104.

Spray Drying - American Chemical Society

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