US2636555A - Spray drying method and apparatus therefor - Google Patents

Description

April 28, 1953 E.YKLEPETKO E'l-AL 2,

SPRAY DRYING METHOD AND APPARATUS THEREFOR Filed April 5, 1947 4 She ets-Sheet 1 Emai 7517i; 4' .A aye BY P... 4 LAMA M-.IBM

ATTORNEYS April 28, 1953 E. KLEPETKO ETAL SPRAY DRYING METHOD AND APPARATUS THEREFOR Filed April 5, 1947 4 sheets-sheet 2 INV NTO 5;

WM K a u M M 5% 5 Y E N m n A April 28, I953 SPRAY DRYING METHOD AND APPARATUS THEREFOR Filed April 5, 1947 4 Sheets-Sheet 5 1.4.. W um? fit-MM ATTORNEYS A ril 28, 1953 E. xL'gPfiTko EEAL 2,636,555

SPRAY DRYIRG METHOD AND APPARATUS THEREFOR Filed April 5, 1947 .4 Sheets-Sheet 4 lNVENTO 5 Ernest lire v Phi/:7 de Kqe BY ATTORNEYS Patented Apr. 28, 1953 UN E s AT PATENT OFFICE SERAYDRYING METHOD AND APPARATUS I THEREFOR Ernest Kleiietlio and Philipde' BIK'aye; Bauer,

' Utah, assignors toCombin'ed Metals Reduction cmpany,St0ckton, Utah, acorporation oi Utah.

s ent-anon April 5,1947, Serial no. 739,550

Spray drying is a method quite commonlyused for recovering in solid and substantially dry forma product that has been dissolved in a solvent or otherwise incorporated in a thin liquor; The

basic feature of the spray-drying process resides in spraying the liquor tobe dried into contact .s Claims. (01. 1.5

with a hot gas, heated to a temperature-above the boiling point of-the solvent or other-liquid: The sprayed solution breaks up into finely divid-' the difficultie inherent in heretofore known'apparatus and methods. One feature of the invention, for example, is the-provision of apparatus more or less similar to a spray-dryingvessel; but in which one or more heated and rabbledhearths are provided in the lower portion-olthe vessel. Spray dried solid material fallingirom-j the upper portion or thevessel to the heated hearths is further dried as his rabbled thereacross. Another feature of the invention involves uti'-' lizing vapor volatilized from the liquor as the gaseous medium in which the spray-drying op-f oration is carriedout. When this feature of the invention is employed (it is particularly useful when recovery of the vapor from the liquid'component of the liquor by condensation is important), provision is made for collecting at least part of the volatilized vapor,-- heating it toasufficiently high temperature, and recycling itfor reusein drying a further portion of the liquor.

The advantages of; the new; apparatusv and method will be apparent from a consideration of; some of the limitations inherent'in the, spraydrying techniques heretofore known and einp'loyed, and which are overcome .by the present invention s In the usual spray-dryin process, the liquor to be dried is sprayed into the upper part Of a; tall chamber, wherein it comes in contact with a current of hot gas (frequently heated air or combustion gases) The spraying head by which theliquor is introduced into thechamber atomizes it (breaks it up into fine droplets) and the resulting particles settle toward the bottom of thechamber. The..liquid componentis volatil I material successively across each hearth "andandthe dried solid material collects at the bottcm o-i the chamber. It is characteristic of a spray-drying operation of this sort that complete drying must take place within the time it takes iorthe finely divided droplets tdfall to the bottom of the chamber. some liquorsdo not dry very completely in this'rather short'ti'me of fall, and consequently it has not been considered tea sible to spray-dry such liquors economically.

fllhe present invention meets this-limitation byproviding drying apparatus comprising a chem her, means for injecting into the upper portion of the chamloer and atomizinga'liquor'to bedried,

av heated hearth in the lower portion'of the chamher in position to receive spray-dried material falling from the upper portion thereof, and means tor rabbling the dried material on the heated hearth. A liquor to be dried isi'njected by spraying or otherwise into the upper portion of the: chamber and therein is spray-dried sufficiently. to free it'from' the bulk'oi the liquid component The resulting partiallysolidparticles, still moist. with unvolatilized liquid, fall to the heated hearth in the lower portion of th'e'vessel'. As themoist material is rabbled or raked on? the heated hearth, it is' eiiectivelyfreed from residual liquid. If the material is particularly difiicul t to freecomp-letely from residual liquid, the provision of I several heated and rabbled hearths, with the rabbles arranged to rake the from hearth. to hearth, allows for heating thespray-dried material for a sufiicient lengthlof time to insuresubstantially complete drying;

While hearths provided in spray-drying apparatv-s according to the invention will most often be heated, usesare contemplated where it will be advantageous .to 0001, rather than heat, one or more of the. hearths. Hence, while hearth-heating isirequently mentioned herein, the invention in its more general a'spectc'ontemplates; maintaining the. hearth or .hearthsl at a predetermined temperature that differs from the temperature at which it or they would "normallybein the-absence of such means. whether uch temperature. --maintenance or control involve heating v or cooling.

, Mention may be made. attthispointuthat pro-.

ized as the. particlesialithroughthe heated gas, ooled hearth asherein described. It istem- Spray-drying methods as heretofore known have been most successful for drying aqueous liquors. They have been less successful generally for drying solutions or other liquors in an organic solvent or vehicle, particularly where recovery of the volatilized organic vapor by condensation is important. is that the hot gaseous medium in which the liquor is atomized so dilutes the vapor that subsequent recovery thereof by' condensation is made very difiicult. heating medium is itself condensible, as when steam is employed for this purpose, condensation and recovery of the organic vapor is handicapped by the need for largecondensing capacity so as to condense not only the volatilized solvent but also the steam with which it is admixed.

-A feature of the present invention that pro-- vides for meeting this limitation is based on using the vaporvolatilized from the liquor as the gaseous heating medium with which the solution is brought in contact for spray drying. Apparatus embodying this feature of the invention comprises a heating unit in association with the spray-drying chamber, and means for passing vapor from the spray-drying chamber through the heating unit and back into contact with additional liquor injected into the spraydrying chamber. The heated vapor serves to volatilize additional liquid from the injected liquor. It is advantageous, in apparatus embodying this feature of the invention, to provide means for separating finely divided solid or,

liquid particles from the vapor before the latter is heated and recycled to the spray-drying chamber. By such provision, the danger that the heating unit will rapidly become fouled by an accretion of particles of low heat conductivity is minimized.

By using liquor vapor as the gaseous heating medium, the vapor volatilized from the liquor introduced into the drying apparatus is not diluted with any foreign material. Consequently whatever part of the vapor that is not recycled may be recovered readily by condensation, with condensing apparatus of minimum capacity and cost. Contrary towhat one might expect, we have found that the vapor usually need not be heated much above theboiling point of the liquid component of the liquor in order to secure effective and economical volatilization of additional vapor from the liquor'in the spraydrying portion of the apparatus.

Throughout this specification and in the claims, the term spray drying is used. This term connotes that the liquid is atomized by being sprayed through an atomizing nozzle or spray head. We use the term, however, in a broader sense, to include any appropriate means for injecting the liquor into the apparatus. For example, instead of spraying it into the drying vessel, it may be introduced as a more or less solid stream which is broken up into small droplets by a blast of the hot gaseous drying agent or by mechanical means.

Inaddition to What has been described above, tl 1 e inven tion provides anumber of. special fea- One reason for this limitation tures that are advantageous in particular circumstances. One such feature involves heating the solid material on at least the lowermost of the hearths to a temperature above its melting temperature, so that the solid material is melted for convenience in handling and packing, and tcinsure maximum elimination of moisture, before it is withdrawn from the drying apparatus. This provision is especially useful when resins of low melting point, and which are very bulky in the spray-dried solid condition, are dried in accordance with the invention.

Another feature, of especial value when economizing on the use of heat warrants some additional investment in equipment, involves preevaporating the solution to be spray-dried, to

concentrate it, by passing hot gas and vapors from the spray-drying operation into heat- Even when the gaseous exchange'relation with the solution prior to introducing the latter to be spray-dried.

Still another feature is the provision for atomizing the solution or liquor in the spraydrying operation by directing a small stream of the solution vertically into the path of a jet of the gaseous heating medium introduced horizontally and at high velocity. This feature is particularly useful when steam is employed as the gaseous heating agent in the spray-drying operation.

In some instances it is desirable to employ in a single drying operation all of the apparatus and method features provided by the invention. For instance, when the liquor to be dried comprises an organic vehicle that must be recovered by condensation, and the spraydried solid material is not adequately freed from residual liquid, it is generally best to make use of all features of the invention. In other cases, however, conditions may favor the use of only one of these features. For example, with liquors incorporated in a vehicle that need not be recovered, it is frequently advantageous to make use of the heated and rabbled hearths in the lower portion of the spray-drying vessel, but to effect the preliminary spray-drying operation by means of a hot gas other than the vapor of the liquid being dried. Again, there are instances when it is desirable to circulate liquor vapor through a heating unit and back into the spray-drying apparatus to volatilize additional vapor from the liquor, in accordance with the invention, but to dispense with the heated hearths.

Advantageous embodiments of the invention, in which the foregoing features are exemplified, are described below with reference to the accompanying drawings, in which Fig. 1 is a vertical section through spraydrying apparatus constructed in accordance with the invention, taken substantially along line l-l of Fig. 2;

' Fig. 2 is a horizontal section taken substantially along the line 2-2 of Fig. 1;

Fig. 3 is a detail showing an arrangement for sealed transferring material from an out-hearth to an in-hearth in the lower portion of the apparatus; a

Fig. 4 is a diagrammatic view showing an arrangement for utilizin hot gases from the spraydrying operation to pre-evaporate and concentrate solution to be spray-dried;

Fig. 5 is a longitudinal section through appathrough the. spray-drying zonethereof;

re circ ati giiquor vapo and was; 1111 a. i w. were were. reassess sellminl'theupperportioniof which pr 7 ed a. spray-drying zone U r stageouslyis made or a sheet steel or 'other metal shell lzafind is provid d w th heatin u a ion 1 S serieisioi ports ,1 4 ex end :a un i r r i hs r' helspr r-dying zone 11; r ese ports provide fore :t 'ii'n ef ra -iir e zo e r i qlr io conduits fi jahdrsteam (or other'heating mediuml conduits it. I

the. i ,orm or. par t s sh vi g, 1.; small 's'tefadyi streams 'of solutionto be dried are fed t" the spra'y dr'ying zone l 'l.througl r the solunduits l5, and :are delivered vertically downwardly directly in front of nozzle apertures l'lfkatthe ends ofthe steam conduits 'l'o gThe eteamjror other heating medium is introduced under "sufli'cient pressure so that the impact of the fjtbf steam issuing from the nozzles on the. stream or incoming solution atom'izes the ,lla'tt'e'r 'i'a'ndcreat'es afhot turbulent 'at'mos were in whichfsettlement of "the atomized'solu t'ion droplets is somewhat hindered. The heat of the steam or other heating medium causes th'e solventto' jvolatilize from the solution droplets, andvolatilization proceeds quite rapidly in consequenceo'f the smalljsi'ze and hence large surface to-volume ratioof the droplets.

' An exhaust flue. I8 is provided "at the top of the vessello. "The volatilized' solvent and the heatin; medium escape from thevessel through, this flue; A damper l9 may be provided in the flueto' permit some control overthe flow of 'gastherethrough. The gas and solventvaporwithdrawn through the flue l8 'rlr ay'be exhausted tothe atmo'sphere if they are inert, or, if it is desired'to recover-"either the heating medium or the solvent, or both; they may he conducted to suita erefcoveryfapparatus.

In the m of apparatus shown in Fig. '1, the

steam jet nozzlesj arej arranged so that they are directedfhorizontally substantially at right angles to aradiusof the vessel "Ill (and, as described, against 'thedownwardly delivered'stre'am of solution); Thisarrangement causes the atomized solution to be given'a cyclonic motion within the spray-drying zone H, which motion may be correlated with the flow of steam and vaporfrom the apparatus through the flue l8 to insure more uniform juse of the entire volume of the spray drying zone ii, and to minimize the amount of dried particles swept out through the flue l8 with "the exiting stream of steam and solvent vapor.

"A "series of vertically spaced, heat-jacketed hearths 2e are mounted inthe lowenportion of the vessel It beneath the spray drying lzone 'fl thereof. Three such hearths are shown in Fig. 1. Each hearth is provided with its set of rabble's (or rakes) 2| secured to rabble arms 22-, which in turn are mounted on acentral columnflil. "The column 2 3 is supported on a thrust bearing '10- dated below the bottom of the vessel H1, and carries a ring geared through which it maybe slowly rotated by'sultable'driving mechanism 2 5-.

that is; the rattles .2-] are set at 'anangleeash that as the rabble arm's -rotate with' the column 23; any solid material on' these 'hearths' isslowly raked toward-their outer periph'eries: The center hearth 2-0, on the otherha'nd, is-an in-heartnithat is, material on this hearth isadvanced by the rabble's' fronithe outer periphery toward the center "of the hearth; The uppermost hearth is carried on brackets 26 which are quite short,-so thatspaces 21 j-Fig} 2) exist- "between the :outer periphery orgthe hearth and the steel shell 12 or the vessel luthr'ouehout thegreatest part or the hearth "periphery: Thus material that is raked to the outside "of the" upper hearth fallsithrough the openings 21 to the "central hearth. Therouter periphery bf the central hearth fits close to the shell l2 ofthe vessel 'ln, but itsinneredge 2% is spaced from the central column 23'. '1 Consequently. material ialling'to'this hearth and raked inwardly by" the rabble-s falls over the inner :edge as to the lowermost hearth. Here the "solid ma! terial is again rakedto'ward theouterxperiphery, and v ultimately reaches and falls "through "one" or more suitably locatedfd'ischarge openings '29 at the outer periphery of this'hearth. An altern'at'ive arrangement for conveying material from the upper 'o'ut he'arth to the central in-he'a'rth is shown in'Fi'g. "3. "In thls'modifica'tion, the upper hearth; likethe central hearthjhasii'ts outer periphery fitted closely. against, the. steel:

shell 12 of the vessel. An enclosedheat-insulat'ed chute so securedto the outerisurf'ace oi the yes sel shell i2 provides a passageway for material raked to the outside ofthe. upper 'hearthtoufall to the outside or the central hearth. The cross section area of the-Chute. is suficientlyrsmall in relation to the volume of material transferred therethrough so that such material. backing :up from the lower of the two hearths,.substantially fills the chute. In conseduencethe 'chuteis substantially blocked to the .flow of gas therethrough. Chute transfer meansfof this sort are particularly advantageous when it is desired to prevent the gas or vapor atmosphere maintained in thespraydrying Zone from flowing or diffusing into, the spaceover the hearths below thechute, or vice VglSa. u .r

1 indicated above, the several hearths 20 are jacketed for heating .by steam or other heating medium. In the particular construction shown in the drawings, the hearths aremade up of fiat, hollow metal segments, each segment beinglapproximately one quarterof a circle. Four-such segments secured together and mounted in a common plane form each'fhearth. Steam or other heating fluid is admitted to eachsegment through inlet conduits 3i, and is exhausted throughoutlet conduits 32 (Fig. 2). Bafiles (not shown) may be positioned within the'interior of each hearth segment so as to direct the flow of steam to secure optimum efilciency and uniformheating ofthe hearths.

vvhue jacketing the hearths as shown is a particularly satisfactory means for providing for hearth temperature control,.-it is not-the only means available (j-Joils through whichza heating or cooling fluid 'flows may be fastened to flat metal plate hearths for .thispur-pose, or, insome cases, it may even-be desired to heat electrical-1y by resistance elements secured in heat-exchange relation to the hearths. v In apparatusof'this sort it is generally desirable for the vessel I3-to :bB- sealed as completely as possible from the atmosphere. -Accordingly the outer periphery 'ofthe lowermost hearth is accepts connected in-gas-tight relation to the vessel i2,-

anda cylinder 33 is connected in gas-tight relation to its inner periphery, about the opening through which the column 23 extends. The lower-end of the cylinder 33 projects into an annular cup 34 fastened in gas-tight relation to the column 23. The cup 34 is filled with a suitable liquid, such as a high-boiling oil, to a level above the lowermost edge of the cylinder 33. Thus the vessel interior is effectively sealed from the outside atmosphere, without hindering rotation of the column 23, except through the openings properly provided for introducing and withdrawing materials treated or used in theapparatus, and these openings may readily be controlled by valves. It is of course evident that the cylinder 33 may be mounted .on the column 23, and the cup 34 may be mounted on the lowermost hearth, instead of in the positions shown.

The spray-dried solute that settles from the upper spray-drying zone of the vessel I collects on the uppermost of theheated hearths 20. As it collects, it is slowly advanced by the "rabbles 2| across the hearth to its outer periphery, where it falls to the central hearth. It is then raked across the central hearth. falls to the lowermost hearth, and 'finally is raked .to the discharge opening. Because of the slow rotation of the column 23 and rabble arms 22, the spray-dried material is retained on the heated hearths for an appreciable length of time. In consequence, with the hearths heated to a temperature above the boiling point of the solvent, any residual solvent which still moistens the spray-dried solute collecting on the uppermost hearth is quite completely volatilized by the time the solute is finally discharged from the lowermost hearth. The rabbling action of the rabbles 2| serves to turn the material on the hearths over and over again, thus promoting efiective fina1 drying. The solid material finally discharged through the outlet 29 therefore is substantially completely dried.

The provision of the rabbled hearths at the bottom of the spray-drying zone ll does not reduce the drying capacity of the apparatus. Although the movement of the spray-dried material across the hearths is slow, the amount of material on each hearth may be quite large, and hence the final drying capacity of the hearths may readily be made as large asthe spray-drying capacity of the spray-drying zone i l.

Spray-dried material often is bulky and diflficult to handle and package. For this reason (and also to insure elimination of tthe last traces of moisture) it is frequently desirable to melt it and r cast it into convenient shapes. This is true, for example, of fossil resins that are spray-dried from hexane solutions. Accordingly it is sometimes advantageous to arrange the heating of the hearths so that the lowermost one or more hearths are maintained at a temperature above themelting point of the resin or other spraydried material. If this is desired, it is only necessary to make the lowermost hearth in the form of a shallow pan capable of holding a body of the inolten'resin or other material, and to circulate a 'sufficiently hot fluid through its heating jacket. In the apparatus shown in Fig. l, the cylinder 33, where it projects above the hearth surface, and the shell 12 of the vessel serve to make the lowermost hearth a pan for holding molten resin, if so desired. The discharge opening 29 may, in such case be equipped with a heat-jacketed fluidtight valve for controlling out-flow of the melted resin. If the resin is melted on one or more of .the hearths, the rabbles may, if desired. be

omittcdfrom those hearths,- but often it is adabove with the molten resin.

.Once a pool of molten resin or other material has beenestablished on one or more of the hearths, the melting of additional resin, as it falls from the hearth above, is greatly expedited. Hence if the spray-driedsolid product from the upper hearth or hearths is light and fluffy and diiiicult to melt, and if it is desired to melt it for convenience in handling and shipping or for any other reason, the procedure of melting the product on the lowermost hearth or hearths is often advantageous.

To achieve maximum heat economy in operating apparatus of the character described above, provision may be made for utilizing the hot steam (or other heating medium) and solvent vapors passing out through'the fiue ill to pro-evaporate and thus concentrate the liquor that is tobe spray-dried. Fig. 4 illustrates apparatus with which the spray-drying zone .ll of the vessel [0 may be combined in order to effect this economy. As shown in Fig. 4, the flue i8 is connectedto a heat-exchange unit 35, which, in association with an expansion chamber 36, constitutes a preevaporator for fresh solution. The heat-exchanger may be of the type having tubes 31 extending. between headers. Fresh solution to be concentrated is introduced through an inlet pipe 37' which delivers it to the interior'of the pipes 31. The hot steam and vapor from the spraydrying zone flows through the unit 35 outside the pipes 31, thus heating the solution inside them.

The steam and solvent vapor are withdrawn through a condiut 38 to a cooler or condenser 39, wherein they are condensed to liquid form. The condensed solvent and steam are delivered by .a pipe 40 to apparatus for separating the solvent from the water with which it has condensed. -A damper-controlled by-pass 4| may be provided to control the amount of steam and hot solvent vapor that is permitted to pass through the heat-exchange unit 35.

Heated solution from the unit 35 is delivered by a conduit 4'2 to the expansion chamber 36, wherein space is provided for solvent vapors to evaporate from the solution. A considerable portion of the solution may be recirculated from the expansion chamber through a conduit'43 back into the heat-exchange unit 35, to be reheated and returned to the expansion tank for further concentration. Intermittently or continually a portion of the concentrated solution may be withdrawn from the chamber 36 through a pipe 44 to a cooler 15, wherein it may be cooled to a low enough temperature for safe storage in a tank 46. The amount of concentrated solution withdrawn through the pipe 44 is balanced by the amount of fresh solution introduced through the inlet pipe 31'. v

Solvent vapor liberated in the expansion chamber 36 is conveyed by a conduit 46 to a cooler or condenser 37, from which condensed solvent may be withdrawn through a pipe 48.

The concentrated solution is pumped or otherwise delivered from the solution storage tank 46 to the solution inlet conduits 15 in the spraydrying zone I I of the vessel l0.

By means of the arrangement shown in Fig. 4, a substantial saving of steam or other gaseous heating agent may be effected. A further small saving could be efiected by pumping the warm concentrated liquor withdrawn from the expan 350 near the top of the spray-drying zone. vent vapor from the spray-drying zone is drawn siontankv through the pipe llli directly-to the solution inlet conduit 15, omitting the cooler 45 and the solution storage tank 35. This arrangement, while feasible in some cases, has the disadvantage of making the spray-drying operation Ernest Klepetko. Philip de B. Kaye and Lester J.

Hartzell, Jr. In such 2: case, the additional investment in plant facilities necessary to make use of the phase of the invention described in connection with Fig. i is mostly confined to making the pre-evaporator equipmentjof adequate size, and to providing the pipes and pumps for circulating the gases and solutions in the manner described.

The apparatus described above and shown in Figs. 1 to 3 is designed primarily for use with a gaseous heating medium other than solvent vapor to effect spray drying of the solution introduced through the conduits l5. Forexample, such apparatus may be used for drying solutions of coal resin in hexane, using steam as the heating medium in the spray-drying stage (as described in above-mentioned Patent No. 2,506,300). It may also be used with advantage where no pro- 'vision'is deemed necessary to recover the heating medium or thesolvent vapor, as when an aqueous solution isspray-dried with steam,

-' heated air, or combustion gases.

Fig. 5 shows a modified form of apparatus which may be used with advantage when the [solvent vapor is to be recovered. This modified form of v apparatus is designed to achieve circulation of solvent vapor through heating units, so that the circulated vapor serves asthe sole heating medium in the spray-drying stage, or is used to reduce the amount of heating medium shown in Fig. 5. These elements may'jbe constructed and arranged similarly to the corresponding parts of the apparatus shown in jFigs'l and 2.

A major feature of the apparatus shown in :Fig. 5 is the provision'of a vapor exhaust-conduit 53 connected to the side wall of th'eve'ss'el Solthrough the exhaust conduit 53, and is blown over a'heating unit 55 with which the conduit .53 connects, by means of fans 55 driven through sel 50, near the bottom of its spray-drying'zone .511, through a vapor returnconduit 51.

-10 The, solvent vapor withdrawn throughthe conduit 53 will generally be .at a temperature not much abovethe boiling point of the solvent. As

the vapor passes in contact with heating coils 58 in the heating unit 54, it is heated to a substantially higher temperature, and is reintroduced into the spray-drying zone through the conduit 5? at this higher temperature. It thereby is enabled to supply the heat of volatilization necessary for vaporizing a further quantity of solvent from freshly introduced solution to be spray-dried.

Since, in some cases, an appreciable amount of solvent may be volatilized from the spray-dried solute as the latter is rabbled across the hearths 52, a second return conduit 59 may be provided for circulating heated solvent vapor over the lowermost hearth, and upwardly from hearth to hearth. A. heating unit Ellis arranged in the conduit 59 and communicates with the heating unit 54, so that the fans 55 may serve to deliver solvent vapor through both of the return conduits 5i and 50. By passing some of the vapor withdrawn through the conduit 53 through the heating coils 5! of the heating unit and circulating it upwardly over the several hearths, solvent vapor liberated on the hearths is swept up into the spray-drying zone 5|, and complete drying of the solid material on the hearths is facilitated. A damper 52 in the return conduit 5'! at the bottom of the spray-drying zone, and a second damper 63 in the conduit over the lowermost hearth, are provided to regulate the total amount of vapor recirculated through the heating units 54 and by the fans 55,'and to regulate the proportion of vapor delivered through the respective exhaust conduits 5i and 59.

The solvent vapor circulated through the heating units 5 and 60 and back into the spray-drying zone 5! replaces part of the gaseous heating medium that must be supplied from extraneous sources to the spray-drying zone, and in some cases may render it unnecessary to supply any gaseous heating medium-from extraneous sources. In either case, the result is to reduce substantially the extent to which the solvent volatilized from the solution being spray-dried is diluted by such extraneous heating medium. The solvent vapor that is finally withdrawn from the vessel .50 (for example, through an exhaust flue-not shown-similar to the exhaust flue [8 of Fig. l) is therefore vmore concentrated than it would otherwise be, and may be recovered in liquid form by condensation more readily and, with-less extensive condensing equipment than would be necessary were the vapor largely diluted with other gases or vapors.

A; modified form of apparatus embodying similarprinciples to that shown in Fig. 5, but more satisfactory in'many cases, is shown in Figs. 6 .and 7. This apparatus, like that shown in Figs.

able central column (59, serve to-advance material the hearths and from hearth to hearth in the same manner as described above 'ence to Figs. land 2.

dome it, to which a vapor exhaust flue H is. conwith refer- The vessel 55 is provided at the top with a vapor nested. Also connected to the dome l0 isan exhaust conduit F2 leading to the intake side of a blower it. The blower forces vapor withdrawn through they conduit 12 through a separator M into the spray-drying zone 66.

The separator 14 is provided to separate solid and liquid particles from the vapor withdrawn through the conduit 72. Any suitable apparatus capable of serving this purpose may be employed. For example, such apparatus as is known by the trade name Multiclone may be used advantageously. This type of separator comprises a chamber arranged so that the vapor driven through it is rapidly whirled about, and any relatively heavy solid and liquid particles suspended in the vapor are caused to impinge on and be collected by suitably arranged collecting plates.

The collected solid and liquid particles may be periodically withdrawn from the bottom of the separator.

The vapor emerging from the separating apparatus, substantially freed from solid and liquid particles in suspension, is driven by the pressure developed by the blower E3 to and through the heating unit 15, in which it comes into intimate heat-exchange contact with heating coils 16. The

temperature of the vapor is substantially increased in the course of its passage through the heating unit, so that upon reintroduction into the spray-drying zone, it may provide the heat necessary for vaporizing solvent from freshly introduced solution.

' of the heating coils should occur, and particularly 'if the solid and liquid particles entrained in the vapor possess low heat conductivity, the eniciency of the heating unit would soon be reduced to an uneconomically low value.

The heated vapor emerging from the heating unit 15 is reintroduced into the spray-drying zone 66 through a conduit 1! having a discharge aperture 18 at its lower extremity. Advantageously this discharge aperture is arranged to direct the reintroduced vapor in a substantially horizontal direction, and at a right angle to a radius of the spray-drying zone. Solution to be spray-dried is delivered under pressure through a conduit 19 provided with a spray nozzle 80. finely divided solution droplets issuing from the nozzle 80 is intercepted by the current of heated vapor blown out through the conduit 11, and is thereby directed into a cyclonic path of circulation within the spray-drying zone 65.

As in the case of the apparatus described above with reference to Fig. 5, the apparatus shown in Fig. 6 since it provides for utilizing solvent vapor as the gaseous heating medium in which the solution is spray-dried, minimizes the amount of gaseous heating medium that need be introduced from extraneous sources, and so reduces to a minimum the extent to which the solvent vapor is diluted. While in some cases some extraneous gaseous heating medium may have to be introduced in order to efiect spray-drying at the rate desired, the amount of such extraneous gaseous heating medium is reduced proportionally to the amount of solvent vapor that is recirculated by the blower 13 through the heating unit 15. Unrecirculated solvent vapor withdrawn through the exhaust flue 1| may be condensed readily, with condensing equipment of minimum capacity, as a result of the small extent to which the vapor is diluted by extraneous gaseous heating agent.

A spray of 12 In both nigsys and s, only one'complete unit for heating and recirculating the solvent vapor is shown. Actually several duplicate sets of heating and recirculating apparatus may be employed in both embodiments of the invention, in order to attain increased spray-drying capacity. Fig. 7, by way of example, shows in cross section three vapor-return conduits Tl, associated with three vapor-recirculating circuits and three solution inlet conduits 19. Similarly, the apparatus shown in Fig. 5 may be provided with several sets of heating units 54 and 60, and associated conduits and fans, arranged about the periphery of the vessel 50.

As indicated above in connection with Fig. 1, it may be desirable in some cases to employ spraydrying apparatus having heated hearths on which the spray-dried material collects, without making use of any vapor recirculating apparatus such as that described above in connection with Figs. 5 to 7. Use of this embodiment is generally advantageous when the solution to be dried contains a solvent such as water which it is not desired to recover, but where it is difiicult to remove the final moistening amounts of solvent from the spray-dried solute. Such apparatus also may be used when the solvent vapor is to be recovered, provided it is not considered objectionable to efiect such recovery from diluted solvent vapor. As indicated above by way of example, such apparatus, having the heated hearths but no provision for solvent vapor recirculation, has been used successfully in drying solutions of coal resin inhexane, where steam was employed as the gaseous heating agent in the spray-drying stage.

It is also true that in some cases it maybe advantageous to eifect spray drying in apparatus equipped for circulating solvent vapor through a heating unit, as described in connection with Figs.

5 to 7, but without making use of the heated hearths. This is generally so when the solvent component of the solution is to be recovered by condensation, but when it is not difiicult to effect desirably complete vaporization of the solvent b spray drying alone. i In still a third class of cases, where it-is desired to recover the solvent and where final complete drying of the solute is diflicult, it is advantageous to employ apparatus embodying both the arrangement for recirculating solvent vapors and theheated hearths on which final complete drying of the spray-dried solute is effected. This embodiment of all of the major features ofthe invention may be applied successfully to drying solutions of coal resin in hexane (and other solutions behaving similarly). It has the advantage that recovery of the hexane or other solvent vapor by condensation is facilitated, and at the same time a, well-dried product, that is difiicult to dry adequately by spray drying alone, is obtained.

We claim:

1. Drying apparatus of the character described comprising a chamber, means for injecting into the upper portion of the chamber and atomizing therein a liquor to be dried and comprising a hot gas jet nozzle extending into the chamber and directed horizontally at right angles to a radius of said chamber and a liquor delivery pipe so positioned with respect to the jet nozzle as todeliver a small stream of the liquor perpendicularly into the path of a horizontal jet of hot gas issuing from the nozzle, a hearth in the lower portion of the chamber in position to receive spray-dried material originating in said liquor and falling from the upper portion thereof, heat exchange means associated with the hearth I3 and adapted to efiect heat transfer between said at exchange .meansand. said fallen material on the hearth, andmeans for. rabbling the spraydriedmaterialcnthe heated hearth.

-2. Dryingapparatus of the character described comprising-av chamber, means for injecting into the upperportion of the chamber and atomizing therein a liquor to be dried ahdcomprising a: hot gas jetnozzle, extending into the chamber and directed horizontally at right angles to a radius ofsaid chamberand a liquor delivery pipe so positioned with respect to the jet nozzle as to deliver a small stream of the liquor perpendicularly into the pathof a horizontal jet or hot gas issuing from the, nozzle, a pluralityof vertically spaced hearths in thelower portion of the chamber, the uppermost of the hearths being in position to receive. spray-dried material originating in said liquor; and falling from they upper portion of the chambenmeans for heating the material on each H the chamber and atomizing therein a liquor to be dried and comprising a gaseous heating agent jet nozzle extending into the chamber and directed horizontally at right angles to a radius of said chamber and a liquor deivery pipe so positioned with respect to the jet nozzle as to deliver a small stream of the liquor perpendicularly into the path of a horizontal jet of the gaseous heating agent issuing from the nozzle, means for withdrawing the gaseous heating agent and vapor volatilized from the liquor in the upper portion of the chamber, a heated hearth in the lower portion of the chamber in position to re ceive spray-dried material originating in liquor and falling from the upper portion thereof, and means for rabbling the dried material on the heated hearth.

4. Apparatus for drying solid material dissolved in-a solvent therefor comprising a preevaporator wherein the solution is concentrated by evaporation of the solvent, a chamber having a spray-drying zone in its upper portion, means for introducing said solution into the pre-evapora tor, means for injecting and atomizing concentrated solution from the pro-evaporator into the spray-drying zone of said chamber in contact with a gaseous heating medium therein and comprising a gaseous heating medium jet nozzle extending into the chamber and directed horizontally at right angles to a radius of said chamber and a liquor delivery pipe so positioned with respect to the jet nozzle as to deliver a small stream of the liquor perpendicularly into the path of a horizontal jet of the gaseous heating medium issuing from the nozzle, means for passing the gaseous heating medium and solvent vapors from the spray-drying zone into heat-exchange relation with the solution in the pre-evaporator, and a heated and rabbled hearth immediately below the spray-drying zone of said chambe in position to receive spray-dried solid material falling from the spray-drying zone.

5. Drying apparatus of the character described comprising a chamber, means for introducing a liquor to be spray-dried into the upper portion of the chamber and for atomizing the liquor therein and comprising a hot gas jet nozzle extending into the chamber and directed horizontally at right angles to a radius of said chamber and a liquor delivery pipe so positioned with respect to the jet nozzle as to deliver a small stream 14 o'fth'e liquor perpendicularly intothe path of a horizontal jet of hot gas issuing from thenozzle, a heating unit associated with the upper portion of said chamber, means for passing vapor volatilized from; the liquor in the upper portion of the chamber through the heating v unit and back into contact with additional liquor introduced into the chamber, a heated hearth in the lower portionof the chamber in position toreceive spray dried material originating in said liquor and falling from the upper portion thereof, and means for rabbling the material on the heated hearth. V

6, Drying apparatus of the character described comprising a chamber, a spray-drying zone in the upperportion of said chamber, a plurality of vertically spaced hearths in the lower portion of said chamber, means for injecting into the spray-drying zone and atomizing therein a liquor-to be dried, a heating unit positioned outside the spraydryin'g zone, means for withdrawing vapor volatilized fromsaid liquor in the spray-drying zone from the upper portion of said zone and for passing said vapors in contact with said heating unit, and means for returning said vapor from the heating unit in part to the lower portion of the spray-drying zone above said hearths and in part to the space between two of said hearths.

'7. In spray-drying apparatus of the character described, a chamber having a spray-drying zone in its upper portion and a plurality of hearths in its lower portion, means for injecting into the upper portion of the chamber and atomizing therein a liquor to be dried and comprising a hot gas jet nozzle extending into the chamber and directed horizontally at right angles to a radius of said chamber and a liquor delivery pipe so positioned with respect to the jet nozzle as to deliver a small stream of the liquor perpendicularly into the path of a horizontal jet of hot gas issuing from the nozzle means for moving solids across as least one hearth from the central protion to the periphery thereof, means for transferring material from said one hearth to the hearth next below comprising an enclosed chute connecting the space over said One hearth adjacent its periphery with space over the next lower hearth adjacent its periphery, said chute being 01 sufficiently small cross section area relative to the volume of solid material being transferred therethrough to said next lower hearth so that it may be maintained sufiiciently full at all times of said material to prevent any substantial flow of gas therethrough.

8. Drying apparatus fo the character described comprising a chamber, a spray-drying zone in the upper portion of said chamber, a plurality of vertically spaced hearth-s in the lower portion of said chamber, means for injecting into the spraydrying zone atomizing therein a liquor to be dried and comprising a hot gas jet nozzle extending into the chamber and directed horizontally at right angles to a radius of said chamber and a liquor delivery pipe so positioned with respect to the jet nozzle as to deliver a small stream of the liquor perpendicularly into the path of a horizontal jet of hot gas issuing from the nozzle, a heating unit positioned outside the spray-drying zone, means for withdrawing vapor volatilzed from said liquor in the spray-drying zone from the upper portion of said zone and for passing said vapors in contact with said heating unit, and means for returning said vapor from the heating unit in part to the lower portion of the spray-drying zone above said hearths andln part to the space between two of said hearths.

ERNEST KLEPETKO. PHILIP DE B. KAYE.

References Cited. in the file of this patent UNITED STATES PATENTS Number Name Date Duryea Jan. 3, 1882 Schroeder Mar. 13, 1883 Schumacher July 17, 1883 Repath et a1 Mar. 21, 1905 Ellis Aug. 1, 1911 Gray Nov. 18, 1913 Gray May 14, 1918 Thunholm Dec. 9, 1919 Stutzke Aug. 17, 1920 Stutzke Aug. 17, 1920 Pike Aug. 28, 1923 Faber July 21, 1925 Sprague Sept. 22, 1925 Nishina Aug. 17, 1926 Number Number 16 'Name 'Date' Jones May 13, 1930 Novotny July 22, 1930 Dickson June 30, 1931 Doolittle Jan. 29, 1935 Baird Feb. 11, 1936 Bowen et a1. Jan. 26, 1937 Schilling Feb. 22, 1938 Hartley Aug. 2, 1938 Raiseh Aug. 30, 1938 Thurman Mar. 12, 1940 Lewers Aug. 12, 1941 Hall Jan. 6, 1942 Hall July 7, 1942 Pattee Jan. 20, 1948 FOREIGN PATENTS Country Date Switzerland Dec. 16, 1921 Great Britain Nov. 4, 1926 Germany Sept. 2, 1935 France Mar. 29, 1934

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