US2763478A - Apparatus for drying solids in a fluidized bed - Google Patents

Description

Jii? E8, 1956 v. F. PARRY A APPARATUS FOR DRYING SOLIDS IN A FLUIDIZED BED Original Filed Aug. 22, 1949 Fig. 1

2 INVENTOR Vernon F. Pav'ry ATTORNEY United States Patent APPARATUS FOR DRYING SOLIDS IN A FLUIDIZED BED Vernon F. Parry, Golden, States of America as the Interior Original application August 22, 1949, Serial No. 111,752, new Patent No. 2,666,269, dated January 19, 1954. Divided and this application December 28, 1953, Serial No. 400,825

5 Claims. (Cl. 263-21) (Granted under Title 35, U. S. Code (1952), sec. 266) Colo., assignor to the United represented by the Secretary of the rapid continuous heating of crushed solid materials by direct heat exchange with hot gases of high velocity. It relates more particularly to a continuous process characterized by high through-put and high etficiency for heating carbonaceous solids, including lignites, low-rank noncoking coals, and oil shales to drive off the more readily volatile products but without appreciable thermal decomposition of the volatile products. This invention also relates to the apparatus for carrying out the process.

Many coals in the United States as mined have a high percentage of mineral inorganic substances, or extraneous ash. It is recognized that a substantial portion of the ash from such coals can be removed by various cleaning methods, most of which involve washing the coal with aqueous solutions or suspensions under controlled conditions. Such coals after cleaning with an aqueous medium, are discharged from the cleaner with a film of water on each particle, and hence the reduction in ash content is accompanied by a substantial increase in the percentage of moisture in the treated coals. Other coals, including lignites and some of the low-rank non-coking bituminous coals, are characterized by a high moisture content, some times amounting to thirty-five percent or more.

For many industrial uses, moisture in excess of about five percent is undesirable, particularly for hydrogenation processes and in some gas manufacturing operations at high temperatures.

It has been proposed to dry crushed solids, including coal, with temperature control by passing a heated gas upwardly through the mass of crushed coal contained in a chamber, at such a velocity that the pieces of coal are in constant motion and the mass assumes in the fluidized condition the properties of a boiling liquid. The particles assume limited motion, without being entrained in the gas. United States Reissue Patent No. 21,526 broadly described such a process, mentioning that a temperature above 212 F. is preferred. This same patent also mentions that oil shale can be treated in like manner with steam whereby its oil is distilled.

The well known fluidized bed technique for reacting solids with gas usually employs space velocities in the range of l to 5 feet per second to maintain a dense, stable bed. If the reaction within the fluid bed is highly endothermic, such as that involved in moisture evaporation, the capacity of the system is low because the sensible heat transferred from the carrier gas is limited by the relatively low space velocity of the normal fluid bed. In order to increase the rate of heat transfer between the gas and solids, it is necessary to increase the space velocity of the carrier gases.

Flash drying or the evaporation of moisture from granular materials by entrainment in hot gases is a well known industrial process which operates successfully on many materials, current flash drying processes extract hot gases from a combustion system with a suction fan which directs the gas upwardly through the drying column. One such circulation drier is described in United States Patent No. 2,156,924 and another in United States Patent No. 1,585,511. The fine material is entrained and dispersed in the drying column where heat is transferred from the gas to the solids. Such systems are limited as to the temperature of the hot gas in order to protect the fan from deterioration. Furthermore the efliciency of fans operating on hot gases is low.

In practice, flash drying of entrained materials is usually accomplished with inlet hot gases at temperatures less than 1300 F. In the generation of hot gases in a combustion chamber excess air is admitted to reduce the flame temperature the desired amount whereby the generated gases are within the proper temperature range. By so doing much useful heat is wasted. The efiiciency of fuel utilization and the rate of drying entrained materials would be considerably higher if hot gas, at say 2600 F. could be employed. This, however, is not feasible in prior flash drying systems.

An object of the present invention is to provide an improved method of heating granular solid material, to volatilize water therefrom by entraining: said solids in gases of a high-temperature range impossible of utilization in conventional flash drying. It is an additional object of the present invention to remove water from wet coals and other solid substances by heating at optimum speed to temperatures just short of their thermal decomposition, which is attained by initially heating these solids in a fluidized bed by employing a gas of that high temperature range not conventionally employed then en training them by increasing the velocity. It is moreover an object of the present invention to treat lignite and reactive low rank coals at maximum capacity by heating under controlled conditions to temperatures close to, yet below, their point of thermal decomposition by dispersing such pulverized solids by employing a very high temperature gas, then entraining them in the same gas by increasing the velocity. A still further object is to separate the highly heated carbonaceous solids from which the water has been removed to secure one portion which is finer than the rest and return this to the furnace as a source of fuel in generating the highly heated gases for the process. Likewise it is an object to provide an apparatus for carrying out this process of removing volatiles from carbonaceous solids below point of thermal decomposition at optimum speed, using less gas for entrainment than any now known and returning to the furnace the more finely divided recovered solids as a source of fuel.

Further objects and advantages of the invention will be apparent from the following description of a preferred form of embodiment thereof taken in connection with the attached drawing which is diagrammatic view.

In accordance with my invention granular carbona ceous material, such as oil shade, lignite or wet low rank coal, is continuously fed into the heat transfer chamber where it is jetted with upwardly directed gases which have a temperature of the order of 2000-6000 F. This impingement with such hot, high velocity gases causes virtually instantaneous heat transfer to the solids. The space velocity of the gases which is between 5 and 15 feet per second puts the solids in a state of ebullient motion, or fluidized state resembling a boiling liquid. The particles at this stage are not entrained in the gas but are in vibrant motion and possess the turbulent motion of a Boiling fluid. The rate of feed of the solids and gases is adjusted to result in a temperature of the fluidized mass at the top of this chamber of about 300 F.

The boiling mass of fluidized solids is then entrained ass result of the higher space velocities produced by a decrease in the diameter of the superimposed chamber and moved upwardly. The velocities will usually be in the order of 30 feet per second so as to move the solids upwardly vertically. A greater space velocity than 30 feet per second is necessary, if material larger in size than A; inch is originally processed. The entrained treated solids flow to suitable separators where they are separated from the spent, hot gases and volatized products.

Where it is desired to recover the coarser product with out admixture with dust and finer product, this may be effected readily by means of two separators in series, the coarser material being collected in the first separator and the finer material being collected in the second separator.

When drying coal, oilshale or like carbonaceous substances, the finer solids may be returned to serve fuel to the combustion zone located below the chamber where the incoming granular solids are brought into a hot fluidized state. The amount of finer material returned will decrease to the same extent the quantity of fuel which would be required from an outside source.

The proportion of air to total fuel is gauged to generate the highest practical flame temperature. The fuel and air are admitted under pressure to the combustion zone. The primary products of combustion are then mixed with recirculated spent gases from the tail end of the process i. e., coming from the cyclone. Only a portion of the spent gases are however re-cycled; the greater volume is discarded.

The proportion of re-circulated spent gases is adjusted to develop the desired mixture temperature of the hot gases which pass through the series of openings in the diaphragm separating the combustion chamber from the heat transfer chamber (fluidi'zing chamber). The diameter of the openings and the pressure of the gas mixture should be so related that the velocity of gases issuing through them is greater than 30 feet per second. This will prevent the granular material undergoing treatment from falling into the combustion chamber.

The accompanying drawings show, more or less diagrammatically, a system embodying my invention. ure 1 shows aheater and associated solids handling apparatus, 'while Figure 2 is an enlarged view of a hot gas inlet port at the bottom of the heater and Fig. 3 is an alternate form.

Referring to Figure 1, a substantially cylindrical vertical feed hopper is shown at 2, preferably with a sloping bottom portion in open communication with a feeding Fig means 6 for transferring solids to the heater 3. Although r merrier feeding means 6 is shown as a worm conveyor, othermechanical feeding devices may be employed. For some types of operation it is preferable to utilize gravity orpneumati'c feeding devices. Feeder 6 discharges material into the base of heater 3.

The heater 3 is a substantially vertical cylinder, having an inwardly sloping conical top in open communication with an ofltake 4 of relatively smaller cross section D2 as compared with cross section D1 of heater 3. Heater 3 may be constructed of metal or refractory material, capable of resisting the erosion of material movingtherethrough. The floor 15 of heater 3 is provided with a'plurality of ports 14, preferably substantially over the entire surface, to provide uniform distribution therethrough, of hot gases 'from furnace 5. Furnace 5 is provided with fuel inlet 7, and air inlet 6, having means (not shown) for controlling the ratios and amounts of fuel and -air supplied thereto.

Vertical ofitake 4-leads to a-separator-S. Whichmay be aconventionalcyclone separator as shown, or other suitable means for removing solids from the gas stream. Solids accumulating in 8 may be discharged at a controlled rate by means of star wheel 10.

Gases and vapors from separator 8 are discharged at 16. A portion of these fluids may be recycled to furnace 5, if desired by means of conduit 11 and blower 12.

As shown in Figure 2, the ports 14 at the bottom of heater 3 are preferably of a diameter (d) less than the thickness of partition 15 and are venturi-shaped so as to provide a jetting action as the hot treating gases flow' therethrough into the bed of fluidized solids in the heater.

Ports 14 need not be formed in plate 15 as indicated in Figure 2. An alternate form is shown in Figure 3, wherein the venturi-shaped port 14 is formed in a plug 17 which is attached to plate 14 by threads 18. If desired, of course, plugs 17 may be force-fitted or welded to plate 15.

Although my invention is applicable to a wide variety of materials, it is primarily designed for drying and preheating coal (or lignite) and oil shale. A practical embodiment of the invention is the drying of a Texas lignite in the above-described apparatus. This fuel contains 37.5 percent moisture as received.

The lignite to be dried is crushed to pass a one-half inch screen. For very high drying rates, the particle size of raw fuel should not exceed one-eight inch. The crushed and sized fuel is charged to hopper 2, through valve 13, and is forced into heater 3 by feeder 6. In heater 3, the fuel is maintained in a state of violent agitation by streams of hot combustion products passing through the parts 14 from furnace 5. Air and fuel under pressure are injected into furnace 5, preferably in ratios and amounts to produce temperatures ranging up to 3000" F. The velocity of hot gases entering heater 3 at the ports 14 should exceed thirty feet per second to prevent solids dropping through the ports 14 into furnace 5.

Under these conditions, there is substantially instantaneous heat transfer to the fresh incoming solids. The rate of feed of solids and volume of hot gases are adjusted to produce a temperature of about 300 F. in the upper, restricted, zone of heater 3. Space velocity of gases required for agitation and mixing in the bed of solids being treated in heater 3 is from five to fifteen feet.

By restricting the diameter at the upper portion of the heater, as shown, space velocities in excess of thirty feet per second are secured in the upper part of the bed and through otftake 4. Treated solids are thus entrained in th higher velocity effluent gases and vapors and are carried out of heater '3 into separator 8.

The height of the heating zone, shown at H in Figure 1, 'determinesthe residence time in heater 3, since the time of contact or residence of solids in the fluidized heating zone is directly proportional to this height. For drying lignite under the described conditions a satisfactory residence time is effected with a zone of from live to six feet. The drying capacity of the heat transfer zone is proportional to the space velocity and temperature of the gases injected into the fuel bed, in accordance with the following empirical formula:

where C pounds/hour/square feet (excluding radiation losses).

S V:space velocity of gases in the column at 300 F. and

T1=temperature of hot gas injected into fluid bed, F.

M1=moisture content of coal feed, percent.

In drying a Texas lignite as described, the following data were obtained:

Coalcharging rate, lb./hr./sq. ft 1,3 58 Moisture in rawcoal, 'pereenttMi') 37:6 Moisture in dried coal, percent (M2) -'4.6

Furnace fuel (natural gas), cu, ft./lb. coal .597 Air for combustion, cu. ft./lb. coal Recirculated products of combustion, cu. ft./lb.

The drying of lignite in accordance with my process is accompanyed by a shrinkage in particle size, amounting in some cases to about 90 percent of the equivalent liquid volume of the moisture removed. This reduction in particle size is not due to attrition, as the amount of fines produced is too small to account for the change in particle size, and has not been observed in other drying processes. The dried fuel, particularly in the smaller sizes, has little tendency to pack, and readily flows through conduits like a liquid, under static pressure head concurrently with moisture removal, it is also possible in my process to modify the chemical properties of fuels being treated, if desired. Thus, by control of the fuel-air ratio in furnace 5, the hot gases injected into heater 3 may have sufficient excess oxygen to effect a mild oxidation of the fuel. In this manner, for example, the tendency of certain fuels toward spontaneous combustion when dry may be matcrially reduced. Furthermore, a mild oxidation of certain high-volatile coals may be obtained by my process while preheating and/or drying them for use in cokeovens, thereby improving the coking properties of such coals.

With or without modification of the chemical properties, preheating and/or drying of coal for coke-oven charges is highly desirable. By drying and preheating to 500 F., in accordance with my invention, a typical coke-oven charge, the heat for carbonization may be reduced from 1,265 B. t. u. to 980 B. t. u. per pound of coke produced. This represents a saving of about 25 percent of the oven heat, or approximately equivalent increase in the oven capacity.

While I have thus described my invention for the drying of carbonaceous material such as coal and oil shales, it should be understood that it is not limited thereto. Other solid substances may likewise be treated with hot gases to effect physical and/or chemical changes therein at low temperatures. Temperatures at the top of the bed may be as low as 250 F. for some coals. Having thus described the nature of my invention and the means of practicing the same, but without intending to limit the invention to such details save as expressly set forth in the appended claims.

I claim:

1. Apparatus for drying finely divided solids in a gas entrained bed comprising a substantially horizontal, elongated combustion chamber having a burner at one end thereof directed toward the opposite end thereof, an outlet for gaseous products of combustion in the upper wall of said chamber adjacent to said opposite end, a plate having a plurality of gas ports therethrough mounted in said outlet, a heat transfer chamber mounted on said chamber and arranged to receive gas passing through said plate in the bottom portion thereof, said heat transfer chamber including a lower portion and an upper portion interconnected by a conical portion, the diameter of said upper portion being substantially less than the diameter of said lower portion so as to produce a substantial increase in velocity of gas in passing from the lower to the upper section of said heat transfer chamber, feed means for introducing wet solids to the lower portion of said heat transfer chamber whereby said solids are entrained in said gas and are passed through said heat transfer zone, separator means communicating with said heat transfer chamber and arranged to separate large particles from said gassolids mixture and to exhaust a mixture of dust and fine particles in spent gas, and conduit means for passing a portion of said dust and fine particles suspended in said spent gas into said combustion chamber whereby to provide dust for burning therein and to provide spent gas for increasing the volume of drying gas Without increasing volume of combustible gas.

2. Apparatus for drying finely divided solids in a gas entrained bed comprising a substantially horizontal combustion chamber having burner means for generating gaseous products of combustion, an outlet for said gaseous products of combustion in an upper wall of said chamber, a plate having a plurality of gas ports therethrough mounted in said outlet of said chamber, a heat transfer chamber mounted on said chamber and arranged to receive gases passing through said plate in the bottom portion thereof, said heat transfer chamber consisting of a lower portion and an upper portion interconnected by a conical connector, the diameter of said upper portion being substantially less than the diameter of said lower portion so as to produce a substantial increase in velocity of gas in passing from the lower to the upper section of said heat transfer chamber, means for introducing wet solids adjacent the bottom of the lower portion of said heat transfer chamber whereby said solids are entrained in said gas and are passed upwardly through said heat exchange zone, separator means communicating with said heat transfer chamber and arranged to separate large particles from said gas-solids mixture and to exhaust a mixture of dust and fine particles in spent gas passing therefrom, and conduit means for passing a portion of said dust and fine particles suspended in said spent gas into said combustion chamber whereby to provide dust for burning therein and to provide spent gases for increasing the volume of drying gas without increasing volume of combustible gas.

3. Apparatus according to claim 2 wherein the gas ports are venturi-shaped whereby gas passing therethrough is injected into said heat transfer chamber in a plurality of high velocity streams.

4. Apparatus according to claim 2 wherein the upper portion of said heat transfer chamber is less than about one-half the diameter of said lower portion.

5. Apparatus according to claim 2 wherein the separator means is a cyclone separator.

References Cited in the file of this patent UNITED STATES PATENTS 2,357,901 Lewis et al Sept. 12, 1944 2,503,788 White Apr. 11, 1950 2,520,637 Henwood Aug. 29, 1950 2,529,366 Bauer Nov. 7, 1950 FOREIGN PATENTS 723,886 France Apr. 16, 1932

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