VOL.3, No. 5
DRYERS AND DRYING
547
SEPARATION WITH PHASE CHANGE: DRYERS AND DRYING* 1.K. TOWERS. RUGGLES-COLES ENGINEERING CO.,YORK, PENNSYLVANIA In the short time allotted for a subject of this nature, i t would be impossible to give anything but a brief reference to various phases of drying problems. That part with which I will have to do, will be more particularly that applying to industrial practice not only in the chemical but other industries as well, since practically every industry bears some relation to the chemical industry. For more specific theoretical data on this subject, I refer you to pages 619-625, 1923 edition of "Kent's Mechanical Handbook." In practical application, the only safe course to pursue is to consult companies that specialize in drying equipment. Trade papers and equipment catalogs list these companies. Dryers are used primarily to facilitate further handling of materials, to reduce grinding and shipping costs, to prevent possible ruin and increase in repairs to supplementary equipment, to aid in mixing and to allow for more rapid and less expensive calcining or burning. While a dryer is an apparatus for driving off free moisture from materials, it does so by the application of heat or by air absorption, and is thus distinct from a dehydrator which extracts moisture by mechanical means, and from a calciner which drives off the combined moisture and other volatiles usually by roasting a t high temperatures. The more common problems of drying have to do with the drying of nonmetallic minerals, as for instance, the drying of coal before pulverizing in cement plants; sand and stone for use as a mix with hot asphalt in paving plants; phosphate rock before grinding in fertilizer plants; clays in plants producing for the china, paint, and rubber industries; Fuller's earth in the oil-refining industries; gypsum in the plaster manufacturing plants; and salts, coke, and chemicals in chemical works. Let us take a concrete example: The use of dryers in cement plants. While a cement plant cannot be termed a chemical industry, chemistry is finding a greater application in this field, and the chemical engineer has become a part of every up-to-date cement organization. For this reason it would be well to consider phase change of the raw materials entering into the process of cement manufacture and thus study dryers and drying in this one industry. Drying problems in this industry comprise raw material dryers-these only in dry process plants; and coal dryers-these only in plants using coal as fuel in kiln. The raw material consists of stone, shale, and clay. Quantities vary according to the size plant. For instance-a 3600-barrel plant requires from 50 to 55 tons per hour of the raw mix in which case a rotary dryer * Student lecture delivered at the Tenth Exposition of Chemical Industries, New York City, October 1, 1925.
7' 6" diam. X 75' 0" long would give a hot and dry stone when drying from 6% moisture. The cost of this machine installed would run from $15,000.00 to $20,000.00 depending on the type of construction used and local conditions. The coal, which is pulverized for use in the rotary kilns in burning the raw mix slurry into clinker, is dried prior to grinding. Because of the unusual conditions involved in the drying of a combustible material of this character, the most careful consideration of details of construction of a dryer for coal is necessary. The most common type of dryer used for this purpose is the double shell dryer, about which information will be given further on. A 3600-bbl. cement plant, dry process, requires approximately 8 tons per hour of coal for kiln and coal dryer requirements. A double shell dryer 5' diam. X 30' long would fill the requirements when drying from 10yo moisture. Such a dryer installed would cost approximately $7000.00. As previously stated, drying is accomplished by driving off free moisture by the application of heat or by air absorption. There are two forms or methods of drying by mechanical means: direct heat and indirect heat. These may be classified according to the type dryer used, as follows: Direct heat
Indirect heat
Direct fired rotary dryer. Semi-direct fired rotary dryer.
Indirect rotary dryer (steam) Indirect fired rotary dryer (with combustion
Direct fired stationary dryer, (Vertical or tower type)
Indirect rotary dryer (steam-drum or vacuum type) Indirect dryer (steam-pan or tray type) Indirect dryer (steam-tunnel or conveyor type) Indirect dryer (steam or combustion garespaddle or screw type)
gases)
By direct heat drying is meant the application of heat from gases of combustion directly on the material being dried, an instance of this being in the drying of some clays in a rotary single shell dryer, the heat for drying being generated in a Dutch oven furnace and drawn through the cascading material passing through the dryer shell. A fan or stack is placed a t the opposite end of the furnace for removing the moisture and drawing gases through the material. A modification of this form of drying with a resultant increase in thermal efficiency of about 20 to 25% was brought about by Mr. Wm. B. Rnggles some thirty years ago, and has since been elaborated on in the production of the double-shell rotary dryer. In this type of dryer, known as the semidirect heat or two-pass dryer, the heat is first drawn through the inner shell and returned through the showering material in the annular space between the inner and outer shell. By such a method the heat is used a t
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DRYERS AND DRY IN^
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the higher temperatures by radiation and a t the lower temperatures by direct contact with the returning gases of combustion. The increase in thermal efficiency, literally meaning decrease in fuel required per unit of product dried, is brought about by this utilization of the gases "going and coming," so to speak. The material is fed a t the high or furnace end of the shell and discharged a t the opposite or low end. By indirect heat drying is meant the drying by heat produced indirectly from the gases of combustion, as for instance, steam generated a t a boiler and transported through pipes or coils producing the heated air necessary for drying by passage over or through the material being dried. An instance of this is found in the drying of tobacco stems in a rotary steam dryer having return bend pipe coils on the interior of the shell, steam being supplied through a rotary steam valve a t one end. As the moisture is liberatgd, it is drawn off by a fan or stack placed at one end of the dryer shell. The material, being fed at one end, is showered through the heated air passing through the shell and is discharged at the opposite end. Another form of indirect heat drying consists in the passing of hot combustion gases through an inner shell of a double shell dryer similar to the semi-direct type, and returning them through ducts. The air in the drying chamber between the two shells is heated by radiation from the inner shell and ducts and thus the products being dried are not contaminated by products of combustion. Drying is accomplished by radiation and contact with heated air, not gases. In the indirect rotary drum dryer, the steam is supplied to the interior of the drum, thus heating the whole surface of the drum. The drum, revolving a t slow speed in a pan containing the wet material, takes up the material in a thin layer, drying in one rotation. The material is discharged with the aid of a scraper on the downward side of rotation. The action is similar to that of the rotary continuous filter press. This dryer must not he mistaken for the commonly known rotary dryer, and is used only where the material is in a liquid state and primwily for pasty materials. In the other forms of indirect heat dryers, such as the pan or tray and tunnel dryers, the material is placed in pans either on trucks or conveyors and remains in a heated air chamber for the required length of time to dry the material. These are used extensively in the chemical industry, and their greatest field is doubtless in the drying of finely divided and sticky materials that usually involve mechanical difficulties when usmg the rotary dryer. I t has been my purpose to convey to your minds an impression of the outstanding features of the various types of dryers and how they are applied to different problems. It is to he hoped, however, you will consider these descriptions only as a basis for your calculations when confronted with a problem in drying, as nearly all the types described can be arranged
differently and thus produce an entirely different effect. For instance, in the ordinary single shell dryer some materials require drying with the furnace at the feed end, thus giving a parallel flow dryer as distinguished from the counter-flow more commonly used. Some materials are more successfully handled in a dryer using a stack in place of the fan for carrying off the moisture. Then too, while a tray or drum dryer may find a greater field in handling finely divided materials, the rotary dryer may he successfully used by correct arrangement under similar conditions. As previously stated, I suggest a liberal use of trade papers and catalogs listing firms handling drying equipment. Use these as you do your chemical dictionary and your handbook on structural shapes, and you will doubtless find a way out of your difficulties. I n submitting your problem, always give all available data and particularly the following: Trade name of material. Chemical constituents. Wet or dry weight in pounds or tons per hour. Moisture in material as delivered to dryer. Moisture that may remain if possible. Temperatures (F. or C.) to which material may be heated without injury. Whether or not combustion or furnace gases are injurious to the material. Physical characteristics. Submit, if possible, a small sample of about 2 lbs. in sealed container, of nature in which it would be delivered to dryer. If steam is available for drying, and at what pressure.
