1002
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.
T
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).
SEPTEMBER, 1938
INDUSTRIAL AND ENGINEERING CHEMISTRY
1003
14 pounds per square inch back pressure and thus gives heat for the vacuum pans. ' Fuel cost is about $0.05 per ton of salt and includes: Steam for exhauster Steam for heating air
$0.015 $0.039 $0.064
Steam is figured at 200 pounds per square inch pressure and $0.017 per thousand. Power is figured as follows: Fan (booster) Filter drive
3 h. p. 2 h. p. (totaling 4 kw-hr.)
General costs are: Fuel Fixed charges (7% basis) Maintenance Labor (half of a man's time) Total
$0.054 $0.025 (approx.) $0.005 $0.025 $0,109 per ton
Steam cost is $0.10 per ton of salt, but 14 pounds per square inch back pressure allows $0.09 credit to be used in vacuum-pan operation. The operation of the crystal filter on grainer salt is as follows: FIGURE 1. TOP-FEED SALTFILTER
The top-feed salt filter is totally enclosed in a housing, into which a booster fan passes heated air. Air flows through steam coils for heating and then through the salt crystals on the filter surface. As the wet crystal magma is fed upon the top of the filter drum, nearly the whole perimeter of the filter may be used for drying. Dry salt may be taken off near the bottom or a t different points on the ascending side. The special construction of this design accomplishes two purposes : (a) It removes all possible brine from crystal surfaces by passing large volumes of air through the salt on filter surface; ( b ) the residual film of brine on the crystal surface is evaporated in place a t the temperature of 300' F. existing inside the filter housing, and thus produces bone-dry salt. Data showing the operation and approximate costs for a crystal filter operating on vacuum-pan salt are as follows:
Size of unit, 6 feet in diameter and 4 feet in width (72 square feet area) Dry salt per hour of operation, 4 tons Moisture in discharged salt, 0.1 per cent Vacuum used, 3 to 4 inches of mercury Air volume through filter, 10,000 to 12,000 cublc feet per minute Inlet temperature, 360° F.; exhaust temperature, 90' to 100' F. Thickness of cake on filter, 1 to 1?4inches Filter cycle of operations, 120 seconds Steam pressure, 120 pounds per square inch gage (minimum) Salt temperature in discharge, 225O F. and up
Size of unit, 6 feet in diameter and 4 feet in width (72 square feet area) Dry salt per hour of operation, 12 tons Moisture in discharged salt, 0.1 per cent Vacuum used, 3 inches of mercury Air volume through filter, 12,000 to 15,000 cubic feet per minute Inlet temperature, 360' F.; exhaust temperature, 90' F. Thickness of cake on filter, 2% inches Filter cycle of operation, 180 seconds Steam pressure, 120 pounds per square inch gage (minimum) Salt temperature in discharge, 225' F.
Any crystalline material whose particle dimension is suitable, such as crystal chemical products, may be dewatered and dried with this filter unit. Very fine materials encountered in precipitating and washing problems cannot be dried to advantage in this way. Coarser materials, such as washed coal, 3/8 to 3 / 4 inch, or 1 inch in diameter, form a different drying problem. This is solved by the horizontal bed dryer (Figure 2) which uses the general methods of the crystal filter, employing larger volumes of heated air which are drawn through the wet material by a suction fan. A succession of separate wire-covered horizontal screens or pallets are utilized which are supported on rollers as they traverse in a straight line the zone of heated gas application. Its field of use to date is found in drying washed coal from 10 to 15 per cent moisture in feed, to a discharged coal carrying 5 per cent or less moisture.
The exhau turbine, whic
RECEIVED May 4,1938.
FIGURE 2. HORIZONTAL BED DRYER
