SEPTEMBER, 1938
INDUSTRIAL AND ENGINEERING CHEMISTRY
1001
SPRAY DRYING
side walls in a wet or damp condition. Several types of atomizers are shown in Figures 1, 2, and 3.
W. S. BOWEN Bowen Research Corporation, Garwood, N . J.
Evaporation In order to promote rapid vaporization, mechanical removal of the outer envelope of locally saturated air surrounding each particle is imperative. This is effected by highly controlled turbulence. Uncontrolled turbulence, such as exists in slow-moving air, will result in the deposit of moist solids, and lead ultimately to shutdowns and damaged products. The Bowen spray dryer meets this problem of continuous high-speed evaporation by means of controlled turbulence established in the following manner : Hot air enters the top center of a cylindrical chamber (Figure 4) with a clockwise whirl through an opening surrounding the spray machine and above the sheet of atomized spray. Its motion is spiral downward with both rotational and axial components, deflecting the sheet of spray down and away from the ceiling of the chamber. It flows a t FIGURE^. STEAM sufficiently high velocity to form, in ATOMIZER effect, a dynamic cap over the top of the No moving parts+ spray which the spray cannot penetrate. liquid preheat (apecia1 sewage atomizer) Similar hot air is admitted simultaneously through a manifold to thirty-two vertical slots forming a band of nozzles opposite and below the level of the spray. The axis of each of these nozzles is set 15 O to the radius so as to produce a violent vortexwith dynamically rigid walls of the same hand of rotation. Thus atomization takes place within a true dynamic cylinder closed a t the top with a dynamic cap, and the whole rotates at high speed within, but not in contact with, a rigid structure, the steel chamber. It is Iike the western tornado with dynamically rigid walls within which heavy solids are maintained in suspension. This vortex is also under a considerable draft as the velocities are high, and this also aids evaporation by increasing the potential difference between the liquid and vapor phases. This mechanism makes possible almost instantaneous evaporation and consequently reduces the size of the drying chamber itself. Another problem closely allied to evaporation arises in cases where some solids have low melting points such as banana, glucose, fruit juices, etc. To meet this situation a manifold of nozzles forming a cooling band exactly similar to that for the hot air is provided to cool the soft dried solid particles while still in suspension in the central vortex. This mechanism is of great importance in special cases.
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PRAY drying consists primarily in the fine atomization of a liquid or slurry containing solids, in either solution or suspension, in the presence of a desiccating medium so as to evaporate the liquid phase and leave the solid phase in free suspension in the gaseous phase consisting of the desiccating medium and the evaporated liquid phase Secondarily it consists of removal of the solid phase in powdered form from the gaseous and vapor phase. The process consists of producing the desiccating medium, atomizing the liquid containing solids, bringing the two into contact in such a manner as to remove the moisture while all individual particles are in free suspension, and removing the dry solids from the system.
Heating the Desiccating Medium The necessary air heating device may be direct-fired or heated indirectly by steam coils. I n England and on the Continent hot oil has been substituted for steam in the coils, and there are also several heat transfer fluids that can be employed. A t present the best procedure appears to be the use of a specially designed “Carbofrax”-lined oil furnace which turns all the products of combustion into the main incoming air stream. By this means and suitable dampers it is possible to control the condition of the desiccating medium with great accuracy and a steadiness of temperature that is very satisfactory. Furthermore, such a device is flexible in control and will produce an airflue-gas mixture that is quite clean. By careful design of the furnace it is possible to obtain a high heat liberation (30,000 B. t. u. per cubic foot per hour) and a t the same time, by means of high turbulence and restricted refractory passages, all free carbon is consumed. F I G U R E1. CENTRI F u GA L ATOMIZER
Atomization
I n the atomization of such fluids as oil and milk, the utilization of high-presVariable speed, sure atomizers having small orifices is perremote oontrol (9,000 to 8,000 missible. The use of these devices with r. p. m.) materials such as blood, clay slips, cooked potato, or peas (including the skins), etc., led to obvious difficulties-and resulted in the development of centrifugal atomization in its many forms. An atomizing spinner, disk, or spray wheel is mounted on a vertical high-speed axis. Although most designers have tended toward heavy construction, the author believes in lightness as a means of overcoming bearing troubles a t high speed. When the atomizer travels at a relative speed of between 6 and 7 miles a minute, the air surrounding it becomes a dense wall of great relative inertia that materially assists in pulverizing the fluid. The properly designed atomizer is nonologging. Various devices at the periphery of the rotating disk are used to “bat” the liquid into a spray, although disks with smooth edges are also employed, These latter must travel at higher peripheral speeds than the type using peripheral “gadgets” and tend to produce a uniform particle size in the spray. This is important in preventing particles from reaching the
Dust Removal Tracing the development of the art, we find the older conception of dust removal to consist in dust settlement within
A.
Closed type
B.
Open type
FIGURE 3. HOMOGENIZING ATOMIZERS
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INDUSTRIAL AND ENGINEERING CHEMISTRY
VOL. 30, NO. 9
such as milk, blood, black liquor, and banana; and sIurries and sludges, such as sewage and clay slips. For years milk has been spray-dried, and numerous plants exist all over the country for the purpose. Soap has also been a spray-dried product for household use for a long time. Soap powder is an analogous product but is different in that it is spraychilled so as to retain as much water of crystallization as possible. RECEIVED May 4,1938
FILTER DRYING D. F. IRVIN Oliver United Filters, Inc.,New York, N. Y.
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FIGURE 4. SPRAY CHAMBER A , B. Hot air inlets Radial inlet for directing flow of air C. Vertical inlet D. Sloping passages for air E. Manifold; hot air entering a t B passes through F t o vertical F. inlet nozzles D ,so arranged as t o direct their flow just to the left of the axial center of the chamber, thus producing a violent central vortex, closed a t the t o p b y the hot air entering a t C H. Inlet for solid t o be dried Homogenizing atomizer which discharges a horizontal eheet I. of spray into the vortex of hot gases Manifold carrying cold air, used only when the dried product J. is t o be cooled Air sweeper to remove dust M. Inlet for air t o operate sweeper N. Outlet for dust P. Q. Exhaust manifold R,S . Typical aluminum foil insulation t o prevent charring of dust particlea on internal surfaces
the drying chamber itself and in auxiliary chambers. Then came dust collectors of both the centrifugal and bag type in place of the auxiliary chambers, and finally, complete removal of all dust from the drying chamber and separation of dust from the entire air stream in standard dust collectors. Numerous methods of dust removal from the drying chamber and incorporation of this dust in the air stream have been devised. In the Bowen spray dryer the dust is swept from the floor of the chamber continuously by means of a rotating-reaction air sweeper operating on the difference in pressure between the chamber and the outside atmosphere. In leaving the chamber all products leave through a scroll manifold maintaining the uniform direction of rotation so carefully adhered to within the chamber.
Applications of Spray Drying The materials which can be successfully spray-dried include true solutions, such as monocalcium phosphate, trisodium phosphate, and sodium sulfate; colloidal solutions,
HE development of the modern rotary filter dryer originated with attempts to reduce the moisture content of the cakes discharged from continuous rotary filters. The use of vacuum in the early rotary filters covered with woven fabrics (usually of cotton) gave compact dense cakes with residual moisture contents varying with the physical nature of the solids. With rarely as low as 10 per cent water content, these cakes in extreme cases might contain 25 to 30 per cent. Drying on the filter, per se, was impossible in Some cases; in others, it was a t least impracticable. The first semidrying, which employed the so-called rotary horizontal sand table, was used successfully in various plants to dewater sand products, sulfides, etc., by applying vacuum underneath the flat annular surface covered with cotton cloth. A further application for handling even coarser material was the hopper dewaterer. This machine was essentially a conventional Oliver filter, provided with a rim on each end of the drum and along each section of the drum. Since these rims were about 18 inches high, each section became a hopper (whose bottom was the fabric covering of the drum), and the wet feed was kept about 15 inches deep. Within its limitations this machine was successful, and some are in use today. Use of the sand tables and hopper dewaterers led to the first real attack on crystal dewatering. This took place in the salt industry, on vacuum pan salt. Since slimy solids are absent in evaporated salt, cotton fabrics, were discarded in favor of woven-wire screen of about 50-mesh size, as this allows free passage of air and liquid. Large air volumes are needed in this work to sweep through the layer of wet crystals on wire cloth surface of filter. Since the porosity of the crystal layer prevents more than a lorn differential vacuum from existing beneath the filter surface, the design of the filter differed widely from the preceding types. Vacuum drum piping was replaced by a cellular drum, and vacuum pumps were replaced by rotary exhausters which provided up to 5 inches of mercury vacuum. Early filters of this design discharged salt with about 3 per cent moisture, but the demand for still lower moisture contents soon led to the addition of heat, after vacuum alone had reached its maximum effect. Steam and heated air were employed separately or jointly and thus reduced moisture contents materially. The additional improvement of using a “booster” fan for positive pressure on the hot gases passing through the cake of salt crystals compelled the use of a substantially air-sealed housing on the filter. Further mechanical changes were made until the latest model of the crystal filter dryer, known as the top-feed salt filter, was evolved (Figure 1).
