US2666269A - Method of drying solids in a fluidized bed - Google Patents

Description

Jan. 19, 1954 v. F. PARRY METHOD OF DRYING SOLIDS IN A FLUIDIZED BED Filed Aug. 22, 1949 ATTORN ess, mentioning that r'ar tmperatiite Iabbv 2r2" v t i ium spedtqtemp me res" Patented Jan. 19;1954 I '7 I PATENT oFFlcE- v I ,l'ileezse w METHGD" tit'IiRmNG-s'buns m FLIJIQIZEDNB'ED t s fAmeric-a asr if V "Duvetfe? 1 ziiinlicattieii Align-SE22, 1 949,"'Se1"ia1 N6. 111,752

.1 i'ftiita" Title 35, 15: SJ-Cod' (1952), 4 see. 266) std-biebet'i; the r'tetibii ne we. r r as i i t df re t f tralhfr" btweerfthe-as ahd solids, it is nece'sj pace velocity of the carrier erettes successfnil produets.

er 7 particles 1 assume" 'li'rifitetl rri t entrained in the gas. I I it L Patent N 0".i21 ,52G"brQEd1/' .des'tribed Such F. p -efe 7red.;LtThia samezspatenfialsb menti nsf their" thermal d d nii iatqi ha1etcamn treated in=1ik mariner with t utk wn n timebeai'tethni u t a ting 011 W gs usugnyge'mploys space ..ing-"them" by in'ci'ea'smg'the 've10city.'-- It -'is"more-' veiiieitie in the 'range of 1 to 5 feet per second 55 over an object of the present invention to treat 3 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.

As used herein, the term fluidized bed has the conventional 'meaning of a mass ofsolid particles which exhibits the liquid-like characteristics of mobility, hydrostatic pressure, and an observed upper free surface or boundary zone across which a marked change in concentration of particles occurs. This fluidized condition is also characterized by a turbulent motion whereby the bed of solids has the appearance of a boil ing liquid.

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 a diagrammatic View.

In accordance with my invention granular carbonaceous material, such as oil shale, 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 20003000 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 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. V

The boiling mass of fluidized solids is then entrained as a 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 inch is originally processed. The entrained treated solids flow to suitable separators where they are separated from the spent, hot gases and volatilized products.

, Where it is desired to recover the coarser products without 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, oil shale or like carbonaceous substance, the finer solids may be returned to serve as fuel to the combustion zone located below the chamber where the incoming granular 4 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 (fluidizing 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. Figure 1 shows a heater and associated solids handling apparatus, while Figure 2 is an enlarged View of a hot gas inlet port at the lfaottom of the heater and Figure 3 an alternate orm.

Referring to Figure l, a substantially cylindrical vertical feed hopper isshown at 2, preferably with a sloping bottom portion'in open communication with a feeding means 6 for transferring solids to the heater 3. Although the fuel feeding means 6 is shown as a worm conveyor, other mechanical feeding devices may be employed. For some types of operation it is preferable to utilize gravity or pneumatic feeding devices. Feeder B 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 offtake l of relatively smaller cross section D2 as compared with cross section D! of heater 3. Heater 3 may be constructed of metal or refractory material, ca pable of resisting the erosion of material. moving therethrough. The floor I5 of heater 3 is provided with a plurality of ports I4, preferably substantially over the entire surface, to provide uniform distribution therethrough, of hot gases from furnace 5. Furnace 5 is provided with fuel inlet 1, and air inlet 3, having means (not shown) for controlling the ratios and amounts of fuel and air supplied thereto.

Vertical offtake 4 leads to a separator 8. Which may be a conventional cyclone 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 Ii].

Gases and vapors from separator 23 are discharged at IS. A portion of these fluids may be recycled to furnace 5, if desired by means of conduit H and blower [2.

As shown in Figure 2, the ports 94 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 not treating gases flow therethrough into the bed of fluidized solids in the heater.

Ports ill need not be formed in plate it as indipercent moisture as received.

'lihezligrrite. lJQbQdXiQdi is crushed toapasaaonehalf inch screen. For: very high: drying. rates, the particle size of raw fuel should not exceed oneeighth inch. The crushed and sized fuel is charged to hopper- 2-, through valve It, and is forced into heater 3 byfeeder e.- In heater 3, the-fuel is maintained in a stateof violent" agitation by= streamsof hot combustion products passing through thepar-ts W from furnace- 5. and fuel underpressure" injected into furnace 5'; prefer-ablydnratios and amount-s to producetemperaturosrangingup'to 3000"" The telocity of; hot gases enteringheater 3 at" the per-ts [4 should exceedthirty feet per second to prevent solids dropping through the ports" 14 into furnace 5;. V

Under these conditions there is substantially instantaneous heat transfer-to the fresh. incoming-solids.- Therate 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 offtake 4. Treated solids are thus entrained in the higher velocity efiluent 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, determines the 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 five 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:

3.2(SV)T1.96 &

Where C=pounds/hour/square feet (excluding radiation losses) SV=space velocity of gases in the column at 300 F. and 30 He 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:

Coal charging rate, lb./hr./sq. ft 1,358 Moisture in raw coal, percent (M1) 37.6 Moisture in dried coal, per cent (M2)"--- 4.6 Furnace fuel (natural gas), cu. ft./1b. coal- .597 Air for combustion, cu. ft./lb. coal. r 5.9

tat-2a Recirculated products oi combustion, e11.

ft./1b-.. al v w 4.5 Net heat supplied, B. t. m/lb. coal 525- Temperature of hot gas, "Ft {Tr}- 1,950 Te r u f flu ed. F; (In- -c--- 300 Dry coal-- recovered, lbt/hn/sq ft 887 Drying, efliciency (excluding. radiation) percent W V V s 92.2 Overall efliciency,. percent sm s"- 8-7.2

The drying of lignite in accordance with my process is accompanied by a shrinkage in partfcfe size, amounting some cases to about percent of the equivalent liquid volume of the moisture removed. This reduction in particle size is not duetoa'ttritiom. as the amomit of fines produced is toosmall to account for the change in particle size, and has not been observed in other drying processes. The dried fuel, particularly in the maller sizes,- has little tendency to pack, and readily flows through conduits like ali'quid; under staticpressurehead' 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 suificient excess oxygen to client a mild oxidation of the fuel. In this manner; for example, the tendency of certain fuels" toward spontaneous combustion when dry may be -materiaily reduced. Furthermore, a mildoxidation of certain high-volatile coals may be obtained by* my processwhile pieheating 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 22 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 describedthe 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. A method of drying solids in a fluidized bed comprising continuously feeding a moist solid in granular form to the lower portion of a fluidized bed, injecting substantially inert gases at a temperature of the order of 2000-3000 F. into the said moist solid, controlling the rate of feed of said moist solid and the volume and temperature of said gases whereby the temperature at the top of said bed is of the order of 300 F., entraining dried solid at the top of said bed by increasing the velocity of the efliuent gas stream and then separating the entrained solid from the eflluent gases.

2. The method according to claim 1 in which the moist solid is lignite.

3. The method according to claim 1 in which the moist solid is a low-rank bituminous coal.

4. The process according to claim 1 in which the moist solid is oil shale.

5. In a method for drying carbonaceous minerals in a fluidized bed, the improvement which comprises continuously feeding a granular carbonaceous mineral containing a substantial moisture content to the lower portion of said bed, and rapidly heating said mineral by impingement with combustion products at a temperature of the order of 2000-3000 F., the rate of feed of said mineral and the volume and temperature of said combustion products being so proportioned that the temperature in the upper portion of said bed is of the order of 300 F.

6. The method set forth in claim 5 in which the carbonaceous mineral is a low-rank fuel.

' 7. The method set forth in claim 5 in which the carbonaceous mineral is lignite.

8. The method set forth in claim 7 in which the dried lignite is removed from the fluidized bed by entrainment in the gases flowing from the upper portion of the fluidized bed.

9. The process of drying moisture containing low-rank solid fuels which comprises generating combustion gases under pressure and'at a temperature substantially Within the range of 2000-3000 F., passing said hot gases, without further compression, upwardly through a column of the granular fuels at a velocity of from about 5 to about 15 feet per second to maintain said fuel as a fluidized bed, continuously dispersing fresh granular fuel to be dried in the lower portion of said fluidized bed with the hot combustion products entering said bed, controlling the rate of feed of said hot gases and of said fresh fuel so that the temperature of the upper portion of the bed is in the order of 300 F., substantially increasing the velocity of the gas stream adjacent the upper portion of said bed, thereby entraining solids of substantially reduced moisture content, and separating the entrained solids from the efiluent gases.

VERNON F. PARRY.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 748,893 Trump Jan. 5, 1904 748,894 Trump Jan. 5, 1904 1,585,511 Rigby May 18, 1926 1,599,952 Brown et a1. Sept. 14, 1926 1,759,702 Koon May 20, 1930 2,119,790 McGehee et a1. June '7, 1938 2,330,545 Benoit Sept. 28, 1943 2,343,780 Lewis Mar. 7, 1944 2,357,901 Lewis et al Sept. 12, 1944 2,381,119 Dill Aug. 7, 1945 2,513,369 Shaw July 4, 1950 FOREIGN PATENTS Number Country Date 723,886 France Jan. 23, 1932

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