US3190343A

US3190343A - Method and apparatus for the spray drying of heat sensitive liquiform materials

Info

Publication number: US3190343A
Application number: US227673A
Authority: US
Inventors: Hussmann Peter
Original assignee: Birs Beteiligungs- und Verwaltungsgesellschaft AG
Current assignee: Birs Beteiligungs- und Verwaltungsgesellschaft AG
Priority date: 1961-10-05
Filing date: 1962-09-28
Publication date: 1965-06-22
Anticipated expiration: 1982-06-22
Also published as: DE1492733A1

Description

`lune 22, 1965 P. HussMANN 3,190,343

METHOD AND APPARATUS FOR THE SPRAY DRYING OF HEAT SENSITIVE LIQUIFORM MATERIALS Filed Sept. 28, 1962 2o and 2b Dehumdifying Zones 5 1l .f4 s e n o Z su 7 .uw mo 0T S g 1m E u ew v HD 6 .D 3 d n n a 3| United States Patent 3,190,343 METHOD AND APPARATUS FOR THE SPRAY DRYENG @Il HEAT SENSITVE MQUWGRM MA- TERIALS Peter Hussmann, Florence, Italy, assignor to Birs Beteiliguugsund Verwaltungsgesellschaft AG., Basel, Switzerland, a corporation of Switzerland Filed Sept. 2S, 1962, Ser. No. 227,673 Claims priority, application Germany, Get. 5, 1961, is 64,255 30 Claims. (Si. 159--4) The present invention relates to improvements in a method and apparatus for drying particulate solid materlals, such as dehydrating aqueous dispersione, suspensions or solutions of solids. More particularly, it is concerned with improvements in the dehydrating procedures disclosed in my copending applications Serial No. 735,299, filed May 14, 1958, for Dehydration Process and Apparatus Therefor, now abandoned, and Serial No. 87,808, filed February 8, 1961, for Dehydration Process, Product Obtained Thereby, and Apparatus for Carrying Out Said Process.

With the procedures disclosed and claimed in these applications, it iirst became possible to obtain instant powders merely by drying. The instant powders obtained by these procedures have all the properties of the solids contained in the starting material. They are not denatured and, in the case of food products, they show no change of taste. They may be considered as instant products because the powders will instantly dissolve without leaving a residue, even in cold Water, if they are colloidally soluble or, if they are only dispersible, they will be instantly dispersed or wetted, while swelling.

In this previously disclosed method, the material to be dried is dispersed in the upper region of a high tower in the form of a dense umbrella of droplets descending in the tower through a counter-currently owing stream `of relatively cool or cold drying air, the air being introduced in the lower region of the tower in a highly dehumidifed state, for instance, with a water content of 0.35 g./cu. m., at a temperature not exceeding about 60 C. In view of the low temperature of the drying gas, the residence time of the material in the tower must be relatively long and the volume of drying gas must be accordingly large.

It is one of the primary objects of the present invention to increase the residence time of the materialrto be dried in the countercurrently owing drying gas without increasing the height of the drying tower.

It is another object of this invention to operate the drying apparatus with a minimum of heat requirements.

It is still another object of the invention to handle the dried material so that it sustains substantially no damage due to denaturing or abrasion until it is packaged in irnpermeable containers.

It is a further object to improve the quality of the dried material further and to control the apparent density of the instant powder.

The above and other objects and advantages may be obtained according to the present invention due to a singular property of the instant powders obtained by my cool air drying method. I have found that these instant powders consist of fully spherical granules with holes reaching deeply into the interior of the spheres even when they still contain about 5% to 20% water, depending on the type of material. Once the material has reached this spherical structure, the powder may be conveyed by air streams or mechanically without destroying the spherical granules or appreciably abrading the same. Furthermore, an air stream may upwardly pass through a layer of spherical granules of this type and having a thickness up to about 20 cm. without causing turbulence in the layer.

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.In one aspect of this invention, special means is prov1ded to prevent undue turbulence in such dried goods which should remain as little disturbed as possible until they are packed and to convey them from the drying tower to the packing zone in the shortest possible path. If the dried powder is of the type which tolerates stirring, the invention provides special means for removing the powder from the drying tower and for conveying it in the shortest possible path to the packing station which may be, if desired, any conventional vacuum-packing system.

If the starting material is particularly hygroscopic, such as tornato concentrate which may still contain tomato peels and which tends to stick to the bottom Vof the drying tower, brushing means must be placed there to avoid the formation of a solid material layer on the tower bottom. In the various embodiments of this aspect of the invention, the brushes serve partially not only for conveying the dried material but also for imparting turbulence thereto.

Different types of apparatus will be used for different materials, depending on their characteristics, the method and apparatus of the invention being adapted for use with any starting material which may be dried to form a powder. Types of useful apparatus include (l) A drying tower with a flat or substantially ilat perforated bottom through which the drying gas flows upwardly into the tower, the sieve-like bottom taking a variety of forms.

(2) A drying tower with a conically converging impermeable or perforated bottom having the material outlet at its apex, with or without brushes.

(3) A drying tower with a hat or substantially flat impermeable bottom with brushes for imparting turbulence to the powder.

(4) A drying tower with a partitioned jacket to obtain a temperature equilibrium between the day and night air used as drying gas, on the one hand, and the air dried chemically, for instance, by silica gel or molecular sieves, so as to store heat and simplify the servicing of the tower.

(5) A drying tower with a special material distributing arrangement for controlling the bulk density of the powder.

If desired, the drying tower may be combined with other driers, for instance, freeze driers for very sensitive materials. Also, aromatic substances may be admixed with the drying gas because the porous granular structure of the dried powder obtained according to my method makes it possible for the powder effectively to adsorb aromas.

The above and other objects, advantages and features of the present invention will become more apparent in the following detailed description of certain preferred embodiments thereof, taken in conjunction with the accompanying schematic drawing wherein In all embodiments of the present invention, use is made of a drying tower according to my above mentioned copending applications, wherein the throughput of the material to be dried, its droplet size and distribution, the drying gas velocity and temperature are so correlated that the material falling through the dehumidied and upwardly streaming drying gas reaches the tower bottom in the form of porous and fully spherical granules which remain on the bottom without collapsing or adhering to each other. This novel granular structure of the dried material makes it possible to introduce the gaseous drying agent into the tower through the bottom, even if it carries a layer of dried powder up to 20 cm. thick and without imparting turbulence to the powder layer. The powder remains lying on the tower bottom until it has reached the desired degree of dehydration and is then conveyed on the shortest possible path to the vacuum from the bottom of the tower as soon as it may be mechanically'or pneumatically conveyed to a` sieve-like support on which it is additionally dried to the desired degree of dehydration without turbulence.

The drying tower used in the invention may be provided with a false bottom which is a horizontal porous, sievelike support. l The false bottom deines a space with'the tower bottom and an inlet pipe supplies a gaseous dryingl medium to the-tower. The Vliquid material to Ybe dried in tower. The used drying medium, which carries most or all of the water from the liquid material dried in the tower, may escape into the atmosphere through a ilue or it may be recovered in a pipe leading to a gas dehumidifying plant so that the drying medium maybe returned to the drying tower in a closed cycle after it hasbeen dehuniiditied. Adjustable valves may be mounted in gas outlets to regulate the draft of the drying medium according to requirements. Y

The granular powder is collected on the false bottom until it has reached a desired layer thickness and after it has reached a predetermined degree of dehydration. A scraper, which is rotatable about the axis of the tower, is turned to push the dried material through a radial slo't arranged in the false bottom.Y TheA screw conveyor is mounted ina housing which has an upper opening facing the radial slot so that the dried material will'fall through the slot Ainto the housing and will be transported out of the housing and the tower by the screw conveyor directly into a vacuum packing device withoutv contacting the atmosphere. p .v Y

It the scraper has brushes for pushing the dried ymaterial, it may be useful intermittently to blow a dried gasV through linlet pipe to remove material adhering Vto the brush bristles,V Adjustable vvalve means are mountedfin inlet pipe to control the gas stream therethrough.

In another embodiment of the invention, the dried ma-` terial is not removed fromvffalse ybottom by a mechanical scraper but by a pneumatic'conveyor. The pneumatic conveyor comprises a radially extending pipe arranged for rotation about the axis-of the tower to sweep over fase bottom'with a suction slot facing the falseA bottom'. The radially extending pipe is in communication with an axially extending tube whosefoutput'i's connected to the in; terior of a container which'is under sub-atmospheric'presdiuxn isintroduced into the,y tower through inlet opening.V

sure. The tube is rotated through a gear train by a power SOUICe.

If desired, the mechanical andV pneumaticY conveyors Y Y may be so combined that lthe mechanical scraperrpushes the dried material towards the circumference of the tower while the pneumatic conveyor has suction slots only at'the by awplurality of sector-shaped sieve 'boxes'. The boxes may be turned about their axe-s.r Y Y l The Vbrushes* are also rotatable about theirax'es by suitable motors and sweepthe undersidesy of thesieve boxes. In this manner, thevdried spherical granules fall first on the upper talsebottom, vpassthroiigh its sieve` boxes, are swept oit 'their undersides, fall lon the under-Y lying talse bottoni, pass through the iatters sieve boxes; areswept 'oit their -undersides and drop to the bottom of Vthe tower wheretliey are swept vby Vawe'ptby rotating scraper or brush into a central outlet opening through of the tower.

d lwhich the dried powder is'immediately gravity-fed into packing station without contacting the atmosphere,

Ihedehumidtiedv dryinggas may be introduced into the b-ottom of the drying Itower a pipe and a baie plate may Ibey mounted over the outlet of the pipe to distribute the drying gas over the entire cross section of the tower so that it will rise trom the bottom in fa column.

The sieve yboxes may be bounded by two generally horizontally extendingperforated or sieve-like walls and one of these walls may haveplaced tliereover a tine-mesh web. The sieve boxes may be turned about their taxes.

The introduction of drying gas is preferably initiated onlyshortly :after the-'sieve boxes have been turned so that the tine-mesh sheet is Afully dried before( another layer of dried'fmaterial is formed thereon.

-In anotherV embodiment of the tower used in the invent-ion,there are mounted two false sieve bottoms in horizontally rectangular cross section. f

The two talse bottoms are spaced apant and each bottom consists of a plurality of adjacently mounted sieve boxesV bounded by horizonatlly extending sieves covered by tine-mesh sheets. A support extends horizontally below each talse bottom and carries las brushing device tor movement thereon so as to sweep the underside of each .false bottom.u ,Another scraping or brushing device is movably mounted on a support labove the-tower bottom for `sweeping the bottom -or door of the tower and con- Vey the ydried materialV to anY outlet opening.

Whiletwo superposed false bottoms may be used, a single false sieve bottom will :suffice for materials which may be dried easily. Also, the lower false bottom may cons-ist of a :single sieve instead of the illustrated sieve boxes. In many instances, the scrapingV or brushing de- .|vice maybe omitted at the lower sieve 'bottom because all the dried powder will drop olf the sieve boxes when Ithe same are turned. f

yIn Ianother embodiment of the tower used in the invention, the tower may have Ia false sieve bottom constitiuted Vby a Vperfo-rated conveyor band.4 The bottom of this tower extends laterally to form an after-drying chamber Whose length is about twice the diameter of the tower. The conveyor is about twice the -diameter of 'the tower. The conveyor band extendsA into the after-drying chamber and transports the dried material from the tower, through the |after-dryingchamber to the packing station Without contact Withthe atmosphere. The gaseous drying mevmove in opposite directions so that the dried material is moved yfrom oneV band to the other beforethe last band conveys the material to the packing station.

`In, another embodiment of ther-tower of the invention, a false sieve bottom is rotatably mounted about the axis The drying gas enters the tower through inlet opening in the bottom of the tower and a 4radially extending b-aillevplate extends from the center of the .tower to the material outlet. vIt desired, ascrew conveyor may be. mounted under, the' bal'lle plate to transport the dried material ,out of the tower to the packing station. Alternatively, adried gas may be used toblowthe dried v"material into material outlet, to which suction may be applied i-f needed or desired. A scraping or brushing device is larranged abovethe bafile plate to sweep dried material olf the plate'and onto, the sie-ve bottom. In anotherdry- AingY tower in accordance'with the invention a false sieve bottom maybe yieldingly mounted on the tower wall by means of spring supports. The spring supports may be excitedby an oscillating device so that the spring-suspended bottom is oscillfate-dl andk dried material is passed therethrough to theY bottom of the tower. I The underside of the Asieve bottom is swept by ascraping or brushing device rotatable about the axis of thetower. Asimilar rotating `scraping or brushing'device sweeps'over'the botythere into the tower itself. ymaterial losses are encountered because any nes are tom or floor of the tower `and conveys the dried material to outlet opening.

Another embodiment of the tower of the invention may have a conical bottom converging towards a central outlet opening leading to an after-drying chamber wherein three perforated conveyor bands are mounted one above the other.

In another embodiment, the tower may be itted with an imperforate and almost fiat false bottom. This ernbodiment is particularly useful for very hygroscopic materials or materials of different components, such as tomato concentrate containing tomato peels. The peels dry more slowly than the remainder of the concentrate and tend to clog any sieve after extended operation. Therefore, it is necessary to dry such a material to a large extent on an imperforate support before drying is completed on a sieve.

An imperforate false bottom may have the configuration of an annular trough with only slightly inclined side walls.

In accordance with the invention, the dehumidified drying gas is blown into the tower through an inlet pipe whose vertically extending portion surrounds a drive shaft and leads to a baie plate which deilects the drying gas in the direction of the rotating brushes. A brushing device is mounted above the baflie plate to sweep the same and remove therefrom any dried material and drop it to a false bottom.

A stirrer may be rotated by a motor to move the dried material through a sieve in the after-drying chamber. If the dried material is not too sensitive to abrasion, the sieve in the after-drying chamber may be oscillated for more rapid removal of dried material therefrom. Useful operating conditions of this embodiment of thc invention include a speed of two to ten rotations per minute of the brush elements about the tower axis while they are rotated about their own axes at Such high speeds as 300 to 1400 r.p.m. In this manner, the brushes serve not only for the mechanical conveyance of the dried material from the false bottom of the tower but also keeps the material from adhering to the false bottom and maintains it in a turbulent state. In view of the high rotational speed of the brushes, it is preferred to use nylon bristles or strips. Like synthetic resins may be used instead of nylon.

In another embodiment the drying tower may have a slightly conical imperforate false bottom converging toward central material outlet opening. The false bottom is swept by a brushing device which conveys the dried material into an opening. An inlet pipe for a dry gas containing a suitable aromatic substance leads to a radially extending pipe adjacent each brush and having a downwardly open outlet slot. In this manner, the porous spherical granules on the bottom will be impinged directly by the aroma-containing gas and will adsorb the aroma. The drying gas proper may be introduced into the tower by an inlet conduit whose vertical portion coaxially surrounds a pipe and leads to a battle plate which detlects the drying gas and distributes it uniformly about the false bottom of the tower. A funnel is rotatably `mounted on the pipe inlet and leads from the outlet opening to a selected one of three after-drying charnbers. The three after-drying chambers may be so operated that one is continuously filled with dried material coming from the tower while the second one is used for after-drying and the third one is emptied. In such an operation, the drying gas is first introduced into the chamber used for after-drying whence it is led into the charnber filled with the material from the tower and from ln such an operation, no

returned to the tower by the drying gas and are removed from the upwardly streaming drying gas by the liquid material sprayed into the top of the tower.

Enrichment of the dried material with an aromatic CIK substance has been particularly useful in the produc'- tion of instant coffee. With the use of different aromatic substances, coffee of different quality may be obtained in this manner. For instance, enrichment of the coffee aroma may be obtained with the gas obtained from the end phase of the coffee roasting just at the beginning of point at which the coffee beans turn brown. Alternatively, the gases following the cooling of the coffee beans after roasting may be used for this purpose or a gas Conducted through the coffee while it is ground. Finally, use may be made of the gases evolving during any stage of extraction. In each instance, the instant coffee will have a different aroma.

Since the drying method of the invention with relatively cool air of the invention requires drying towers of large volume and correspondingly large volumes of drying gas, the present invention also provides for operation of the drying tower with as little heat requirement as possible and with a maximum recovery of the required heat. For this purpose, the drying tower is connected with a special air conditioning plant.

Drying towers useful for the practice of this invention must have a height in the range of 50 m. to 200 m., tower heights of about 70 m. having been found most useful for most purposes.

In accordance with the present invention, a minimum of structural units are used for the heat recovery and the dehumidification of the air and these units for conditioning the drying air are part and parcel of the drying tower. By placing the dehumidification housing into the drying air and heating cycle, the invention maintains a constant temperature of the drying air entering into the tower over each cycle of dehumidification and regeneration of the drying medium, with no, or a minimum of heating or cooling of the drying air. This temperature equilibrium is obtained inside the drying housing in multiple steps. If desired, the drying air may be additionally heated in the drying tower from the walls of the tower so that the air keeps its entry temperature while streaming upwardly in the tower and thus maintains its drying power almost until it reaches the region where the liquid material is released.

The dehumidification of the drying air is effected with the aid of adsorption media, such as silica gel or molecular sieves and experience has shown that a packing of such adsorption media may remove water from the air passed therethrough for about 5 to 6 hours. This must then be followed by a regeneration cycle of about equal duration during which hot air of constant temperature is passed through the adsorption medium packing, the temperature of the regenerating air ranging between about C. and 300 C., depending on the adsorption medium.

The cold or cool air drying of the present invention requires a constant supply of dehumidified air of a temperaturc of about 25 C. to 30 C., for instance, for introduction into the drying tower and of about C. for regenerating the dehumidifying packing if silica gel is used for this purpose, for instance. Thus, the invention provides improved means for conditioning the air in this manner with a minimum of servicing.

This is accomplished in accordance with the invention by passing the fresh air through a chamber defined by the outer wall of the drying tower and a heat-storing intermediate wall between the outer and inner walls of the tower, while the dehumidified fresh air is passed through the chamber defined by the inner tower wall and the intermediate wall, which has an inwardly facing heatinsulating lining, after it has been conducted through the dehumidifying adsorption medium and before it is introduced into the drying tower. If additional cooling is required, fresh air for regenerating the adsorption medium may also be passed through the first-named chamber on its way to the adsorption medium and/or through a heatstorage device in the regenerating cycle.

perature difference of the atmospheric air'between night" and day time, on the one hand, and the rise of the temperature of the adsorption medium during the to 6 hours dehumidifying cycle from about to 20 C. to about' 40 to 50 C. When therfresh Vair is passed through the outer chamber 'of the double wall or jacket ofthe drying Y tower during vthe night, the heat-storing intermediate wall in the jacket will be cooled, together with the heat-` storing outer wall of tower, to such a degree that the fresh air conducted to the adsorption medium will Anever exceed a maximum temperature of about C. during the entire day. The lower the temperature of the air passing through the adsorption medium packing, the higher the dehumidication eciency of the packing. In the initial time period of dehumidification, the dried air leaves the packing with an average temperature of about C. In the preceding cycle corresponding to the final time period of dehumidication, the dried air left the adsorption medium at a temperature of about 50 C. and caused the inner wall of the drying tower jacket to be heated to an average temperature of 45 C., as the said air is passed through the inner jacket chamber before introduc-A tion into the drying tower. Heatexchange accordingly causes the initially dehumidied air to be heated to C. during its passage through the inner jacket chamber on its way to the drying tower, atewhich temperature it then enters the interior of lthe tower. In the second period ofdehurnidiiication, the dried air leaves the packing with Van averagetemperature of about 35 C. and this rises to C. or so in the tinaldehumidication period. The

warmer air raises the temperature of the previously cooled Y tower wall so thatheat exchange continues to keep the temperature of the fdehumidified air entering the drying tower at about 35 CQ This cycle is repeated asvadditional fresh air is passed through a secondY adsorption Ymedium packing which has, meanwhile,- been regenerated.

Temperature control may be obtained by imparting a zigzag configuration to thel intermediate wall inthe drying towerY jacket, which `increases its Vheat-storage capacity andY also sub-divides the space between the Vouter and inner tower walls into individual cells. The heat exchange may then be controlled byrsuitable control of the passage and amount of air streams into the individual cells. Thus, a variety Vof drying air temperaturesmay be obtained independently of the temperature of the" atmospheric air, in addition to the temperature of 35 C; given hereinaboveby way of example. In case of rextreme climatic conditions or speciallyrlowdrying 'air'ternperatures required for certain materials, for instance, 18V C.

for drying butter, specialV heatexchangers for cooling or lheating the fresh air may be positioned inthe fresh vair conduits ahead of the adsorption medium or the yinlet into the drying tower. In rn'ostV instances, however, it is posv Y `aros. 1 amamantar@ a sV f Y structural embodiment of .the plant for conditioning the drying air -in the indicated man- Y ner, fFIG. 1 being a vertical section of the tower and FIG.

2 show-ing a horizontal section thereof.' To facilitate a schematic showing of the conduits,

theV elements

2a, 2b, and 3a, 3b are .shown in FIG. 1 one ybehind theV other, while they are illustrated adjacent one another in FIG. 2. In practicethey are-arranged only at one ofthe two locations. Elernents '6 land A7 also are positioned atthe same location.l

The drying 'air enters the drying tower at inlet opening 9 and passes upwardly to outlet opening 11;" The liquid material distributing means '1(n`ot' shown)'is mounted in the 'tower below the drying gas outlet opening. The tower,

VVwhich may have circulary or rectangular walls, is surrounded by a jacket formed by 'an inner and` outer' tower wall defining a space therebetween. This space` is divided into outer .cells 4 and inner cells 5 by an intermediate zig- Y the adsorption mediumis passedv into the plant fromthe atmosphere through theA outer cells 4 ofthe tower jacket.

YIf desired, the vair may be led upr and rdown the cells several times `lbefore lleaving the jackets. For instance, it may be blown into the cells 4 of one side wall of the tower by eight fans mounted in frontfof suitable Vinlet openings in the wall.rv It may then be directed, for instance from the lower -ce-lls of the one wall into the outer cells 4 ofthe two adjacent walls, passing .upwardly through these cells and being conducted by suitable conduits from the upper cells into; the outer'cclls of the adjacent wall opposite to the `first-named wall, whence the air-is led through openings v in the lower pontion of the latter wall into la header.

sible to dispense with such special heat exchangers or to operate the same only'for short periods of time at certain Y pointsjof the cycle.

.Since `the temperature of the dehumidicati'on packing rises at 4the endv of the dehumidication and regeneration cycles, suitable thermocouples may `be used for automati-` cally switching the'packings from-the dehumidication to the regeneration cycle,and vice versa. VIf desired-,- the entry temperature ofthe drying gas into the' drying tower may lbe used to initiate this switching a little sooner, particu--V lar-ly if the 'latter temperature V'is also used to direct the drying ai-r suitably through the individual jacket cells and `to control its throughput .to obtain the desired drying gas temperature. If a thermostat indicates this desired tern` peratur-e to be exceeded, it may lthen automatically switch the gas `stream through 'a freshly regeneratedads'orption medium while causing regeneration of the exhausted rnc-V dium. The entire vair, circulation -throughthedrying plant Vmay be eifected by fans mountedin the fresh air inlet open- Y ings of the outer wall of the drying tower.

As'shown in iEIG.V 1, the header is sub-divided into two branch conduits. In oneV of the operating cycles,'the fresh air, which has Vbeen pre-heated by passage through jacket cells 4,1 is fled through. the upper branch conduit into a lregenerated :adsorption medium packing 2a consisting of two vertical packingsarranged in parallel. The other por- 'tgion of thev pre-heated `fresh airis conducted into a heat storage device y3a. `The lentryvvalveY of the lower branch .conduit then remains closed. In the followingoperating cycle, Va portion .of the fresh air coming from cells 4 is conducted by the lower branch conduit to adsorption medium packing 2b which 'is similar to .structure to packing 2a; The other portion of the fresh air `is then led from valve of the 'upper Ibranch circuit VDepending on the'cycle, the dehumidied air coming from packing 2a orb is conducted to the inner cells 5 of the tower jacketand is led up and down thesejcells Vin a manner Vsimilar to that described in connection with the passage ofthe fresh lair through cellsY 4. Finally, the de- 'humiditied and conditioned drying air is'introducedv into the tower through opening 9. f

The fresh vair' further: heated in heat storage devices 3a or. Sbis led through the flue of the heating element of another heat exchanger 6, Iwhere it is additionally heated, Iand isy finally directed into 1 a heat exchanger -7 where it is Abrought to the required: regeneration temperature. This hot air is ythen directed to the adsorption medium packing requiring'regeneration, whence ilt is passed into one of heat storage ydevicesrz -or 3b to heattheysarne. The 4regenerating air then-passes from the heat storage :devices into the atmosphere through iiue 10. The inlet openings for cells 4, which admitithe fresh :air thereto, and for cells 5,

provided with :suitable filters.

If the temperature of the adsorption packing used for dehumidifying the air exceeds a predetermined value, for instance 50 C., a thermosensitive relay may automatically operate suitable valves in the air conduits to direct the fresh air to a previously regenerated adsorption packing and to direct the regenerating air to the exhausted packing. The control -of the next cycle may then be taken over by a thermostat in the -regenerated packing. However, the cycle control Kmay also be effected in an obvious manner by a thermostat in the conduit directing the condtioned air to cells 5.

By -using a .tower wall jacket as heat storage and balancing device, it is possible -to equalize the air temperatures so as to obtain a drying air of nearly constant temperature throughout the operation of the tower. Since the atmospheric air is first led through outer jacket cells 4, its temperature may `be raised sufliciently on its way to the adsorption packing so that the conditioned and dehumidified air will never drop below a given temperature, for instance 2-5 t-o 30 C., at its entrance into the tower. Temperature control may be achieved by creating zones 4of different temperatures in the tower jacket. -For instance,

the fresh air supply fans, which may be eight or nine in number, for instance, may each supply a separate air conduit leading to the header so that, depending on the fan speed and the corresponding air throughput of each fan, the air entering the adsorption packing from each conduit has a different temperature. In a similar manner, the drying air may `be `prevented from reaching a temperature higher than desired by conducting the air on 4its way to the tower through more or fewer cooled cells 5. Excess air temperature toward the end of the dehumidifying cycle may also be avoided by switching prematurely to the subsequent cycle as soon as the temperature reaches a certain point if it would `be impossible to change the air path through the jacket cells. This switch may be automatically etiected by a thermostat at inlet opening 9.

Since the time period during which drying air is introduced into the tower at a temperature substantially exceeding the desired temperature level is relatively short in each cycle, the inner wall of the tower jacket will be maintained `at substantially constant temperature. This may be accomplished by suitable dimensioning the wall thicknesses. Furthermore, additional control may be obtained, if desired, by lining the outwardly facing surface of the intermediate jacket wall with an insulating layer in `some of the `jacket cells. Heat storage is effected primarily in the zigzag intermediate wall in the tower jacket. It is also possible to make the Volume of cells 4 and 5 different, for instance to make the cells 5 larger than cells 4. The amount `of air passing through cells 4 may be one and a half or two times that passing through cells 5, depending on the ratio of drying ai-r to regenerating air, so that the cool night air may be used to an increased extent for cooling the heat exchangers.

As indicated hereinabove, the adsorption medium regenerating air is not led through cells 5 but is passed into the atmosphere at it). The rising drying air tends to decrease in Itemperature from the bottom to the top of the tower and if desired, the jacket `space may be subdivided by horizontal partition walls so that the inner wall of the tower jacket will be kept at different temperatures at different levels.

Some water will be removed from the fresh .air by condensation as it passes through outer jacket `cells and a suitable wa-ter outlet is provided at the bottom of cel-1s 4 in the jacket.

While the invention has been described in connection with certain now preferred embodiments, it will `be clearly understood that many variations and modifications may occur to be `skilled in the art, particularly after beneiitt-ing from `the present teaching, without departing from the spirit and scope of the present invention as defined in the appended claims.

I claim:

1. The method of supplying air into a drying tower having a jacket formed by an inner and an outer tower wall defining a space therebetween, said space being divided into a first and a second region by an intermediate wall and the walls of the iirst region having been preconditioned by heat-exchange with atmospheric air at one temperature, which comprises subsequently passing atmospheric air having another temperature through the rst region to exchange heat with the walls of the first region; then dehumidifying the air after it has left the first region by feeding it through a dehumidifying zone; then passing the dehumidiiied air through the second region to exchange heat with the walls of the second region; and then feeding the dehumidiiied air having a temperature not exceeding about 60 C. into the drawing tower continuously for at least one diurnal period.

2. The process of claim 1 in which the temperature of the dehumidified air which is fed into the tower does not exceed 30 C.

3. The process of claim 1 in which the dehumidifying zone has a temperature in the range of 15 to 50 C.

d. The process of claim 1 in which the dehumidilied zone has a temperature in the range of 20 to 40 C.

5. The method of supplying air into a drying tower having a jacket formed by an inner and an outer tower wall defining a space therebetween, said space being divided into a first and a second region divided by an intermediate wall and the walls of the first region having been preconditioned by heat-exchange with atmospheric air at one temperature, which comprises cooling atmospheric air having a higher temperature in the first region by releasing heat to the walls of the first region; then dehumidifying the cooled air by feeding it through a dehumidifying zone; then passing the dehumidifying air through the second region to exchange heat with the walls thereof; and

feeding the dehumidified air having a temperature not exceeding 60 C. into the drying tower.

6. The method of supplying air into a drying tower having a jacket formed by an inner and an outer tower wall defining a space therebetween, said space being divided into a first and a second region by an intermediate wall and the walls of the first region being preconditioned by heat-exchange with atmospheric air at one temperature, which comprises heating atmospheric air having a lower temperature in the first region by absorbing heat from the walls of the first region; then dehumidifying the heated air by conducting it through a dehumidifying zone; then passing the dehumidified air through the second region to exchange heat with the walls thereof; and feeding the dehumidied air having a temperature not exceeding 60 C. into the drying tower.

7. The method of supplying air into a drying tower having a jacket formed by an inner and an outer tower wall defining a space therebetween, said space being divided into a first and a second region by an intermediate wall and the walls of the first region having been precondtioned by heat-exchange with atmospheric air at one temperature, which comprises heating atmospheric air having a lower temperature in the first region by absorbing heat from the walls of the first region; then dehumidifying the heated air by conducting it through a dehumidifying zone;

heating the dehumidified air by absorbing heat from the walls of the second region;

feeding the heated dehumidied air having a temperature not exceeding about 60 C. into the drying 'i l l tower, then cooling the dehumiditied air by .releasing its heat to the walls of the second region and feeding the dehumidiied air having a temperature not exceeding about 60 C. into the drying tower. 3. The method of supplying air into a drying tower havalemania ing a jacket formed by an inner and an outer tower .wall

defining a space therebetweenfsaid space being divided into a rst and a second region by an intermediate wallV and the walls of the first region having been preconditioned by heat-exchange with atmospheric air at one temperature, which comprises .f f cooling atmospheric air havinga higher temperaturein the first region by releasing heat to the walls of the first region; then Y dehumidifying the-.cooled air by feeding it through a dehumidifying Zone; then Y v cooling the dehumiditied air by releasing heat to the walls of thesecond region; feeding the cooled dehumidied air having a temperature not exceeding 60C. into the drying tower, then heating the dehumidied air by absorbing heat from the walls of the second region and feeding the dehumidi-V tied air having a temperaturernot exceeding about 60 v C. into the drying tower. 9. The method of supplying air continuously ceeding about 60 C. into a drying tower having a jacket formed by an'inner and; an outer tower wall deiining a` space therebetween, said space being divided into a rst Y i second region by an' intermediate wall and the Walls of*l the first region being preconditioned by heat-exchange with atmospheric air at one temperature, which comprises Y cooling atmospheric day air-having ahigher. temperature in theV rst region by releasing its heat to therwalls Y of the lirst region; then dehumidifying the cooled air by conducting it through a dehumidifying zone; then passing the dehumidiied air through the second` region to exchange heatwith the walls of the Vsecond region;` then feeding the dehumiditied air having a temperature not exceeding about 60 fC. into the drying tower;

heating atmospheric nocturnal air in the tiret' region by absorbing heat from the walls of the'rst region; then l dehuinidifying the warmed air by conducting it through ing a jacket formed by an inner and an outer'towervwall defining aV space therebetween, said space being dividedY into a first and a second region by an intermediate wall and the walls of the irst region being preconditioned by for atleast l one diurnal period at a temperature in the range notex- :iaY

defining a space therebetween, said Vspace being divided into a iirsty and asecond region by an intermediate wall and the walls of the rst region being ,preconditioned by heat-exchange rwith atmospheric air aty one temperature,

which comprises (a) during the day y. cooling atmospheric air having a higher temperature in the fiirstV region by releasing its heat to the walls of the tirst region; then dehumidifying the air by feeding it through a dehumidifying Zone; then Y n passing the dehumidiiied air through the second region to exchangeV heat with the walls thereof, andthen feeding the dehumidiied air having a temperature not exceeding about 60 C. into the drying tower continuously, and

(b) duringthe night Y heating'atmospheric airrhaving a lower temperaturc inthe first region by absorbing heat from the wallsfof ythe first region; then f dehumidifyingthe air by feeding itthrough a de- Y Vhurnidifying zone; then Vpassing the dehurnidiie'dl air through thesecond region to exchange heat with the walls thereof;

and Y feeding the dehumiditied air having a temperature not exceeding about 60 C. into the drying tower r v continuously. i2'. The method of supplying-air into a drying tower having a jacket formed by an'innerfand an outer tower wall defining a space therebetween, saidvspace being divided into a-first and a second region by an intermediate wall and the walls of the iirst region being preconditioned by heat-exchange with atmospheric'air at one temperature, which comprises y passing atmospheric air having another temperature throughthe. first region to exchange heat with the Vwalls of the lirstregion; f y dehumidifying the airafter it has left the yiirst region y f by feeding itV through-a dehumidifying zone; then subjecting the dehumidied air to an alternating heating and cooling cycle during which' heating cycle, the dehumiditied air is heated Vby absorbing heat from the walls of the second region and during .which cooling cycle, `the dehumidiiied air is cooled by releasing heat to the walls'of the` second region; and then feeding dehumidilied airhaving'a temperature not ex- Y eeeding about V60 C. into the drying tower.

i3. The processof claim y12 -in which the air which is fed into the drying Ytower is essentiallyrkept ata constant f temperatureV not exceeding about 60 C.

1d. The method of supplying airv continuously for at least one diurnal period at a temperature not exceeding about 60 C.' into a drying, tower in which material to be dried is descending from the upper region of'v the tower pas a dense umbrella of dropletsy and is accumulating as a heat-exchange with atmospheric air at one temperature,

which comprises Y y i passing atmospheric air havingV another Vtemperature through the iirst region to exchange .walls of the first region; then dehumidifyingthe air by feeding itlthrough a dehumidi-V hreat with the tying zone which has a dehuniidifying and a regenj eration cycle;

y passing 'the'dehumidiied airiithrough the second regionV to'exchange heat with the wallsthereof; andi. l feeding thedehumidiiied air atatemperature substantially constant and not exceeding about 60 C; during Y each cycle of the dehumidifying apparatus `continuously into the drying tower for at leastl one diurnal layerrof dried material on the bottom of the tower, said drying tower having a jacket formed by an innerv and an outerV tower Vwall deiining'a space therebetween; said space being divided into a first andra second region by an intermediate ,wall and the walls of the ir'st regionbeing preconditioned by heat-exchange with atmospheric air at one temperature, which comprises Y v v v passing atmosphericair havinganother temperature vthrough the iirst region to yexchange heat with the walls ofthe first region; Y Y A dehumidifying'the air after it has lett the iirst region byfeeding it through. a dehumidifying zone; then vpassing the dehumidiiied air through `thersecond region to exchange heat with-the wallsyof the second region; Vandthen i Y feeding thedehumidiied air'having a temperature not exceedingabout 601`C. into the drying tower; and

feeding the air upwardly within the drying tower for at least one diurnal period. 15. The process of claim 14 which comprises feeding the dehumidiiied air into the bottom of the tower and then upwardly through the layer of pre-dried material accumulated at the bottom of the tower.

16. The process of claim 14 which comprises feeding the dehumidiiied air upwardly through a layer of pre-dried material accumulated at the bottom of the drying tower, then upwardly as an airstrearn conntercurrently and through the umbrella of droplets of material to be dried descending in the tower.

17. The tower of claim 15 which has air-conduct means interconnecting a plurality of the cells.

18. The method of supplying air into a drying tower having a jacket formed by an inner and an outer tower wall defining a space therebetween, said space being divided into a first and a second region by an intermediate wall which comprises preconditioning by heat-exchange the wails of the first region with atmospheric air at one temperature,

passing atmospheric air having another temperature through the first region to exchange heat with the walls of the first region; dehumidifying the air after it has left the first region by feeding it through a dehumidifying zone; then passing the dehumidified air through the second region to exchange heat with the walls of the second region; and then feeding the dehnmidied air having a temperature not exceeding about 60 C. into the drying tower continuously for at least one diurnal period.

19. A drying tower using dehnmidified atmospheric air as a drying medium, said tower comprising a heat-exchange jacket surrounding the tower, said jacket being deiined by an outer and an inner Wall of heat-exchanging material, an intermediate wall of heat-exchanging material in said jacket, said intermediate wall dividing the jacket into two chambers adjacent the outer and inner wall, respectively.

20. The drying tower of claim 19 in which the thickness of the intermediate wall exceeds that et the inner and of the outer tower walls, respectively.

21. The tower of claim 19 which has a heat-insulating layer on the intermediate wall on the side facing the outer tower wall.

22. The tower of claim 19 which has a heat-insulating layer on the intermediate wail on the side facing the inner tower wall.

23. The tower of claim 19 in which the intermediate wall divides each chamber into a plurality of cells extending along said walls.

24. The tower of claim 23 in which the intermediate wall has a zigzag coniguration.

25. The tower of claim 19 in which the two chambers have a dierent volume.

26. The tower of claim 25 in which the volume of the inner chamber exceeds that of the outer chamber.

27. A drying tower using dehumidified atmospheric air as a drying medium, said tower comprising a heat-exchange jacket surrounding the tower, said jacket being defined by an outer and an inner wall of heat-exchanging material, an intermediate heat-insulating wall of heat-exchanging material in said jacket, said intermediate wall dividing the jacket into two chambers adjacent the outer and inner wall, respectively.

28. A system which comprises a drying tower using dehumidied atmospheric air as a drying medium, said tower comprising a heat-exchanging jacket surrounding the tower, said jacket being defined by an outer and an inner wall of heat-exchanging material, an intermediate wall of heat-exchanging material in said jacket, said intermediate wall dividing the jacket into a first chamber adjacent the outer wall and a second chamber adjacent the inner wall and dehumidifying means, the two chambers, the dehumidifying means and the tower being so operatively connected that during operatic-n of the tower atmospheric air passes through the first chamber, then through the dehumidifying means, then the second chamber and then into the bottom part of the tower at a temperature not substantially exceeding 60 C.

29. The system of claim 28 in which the dehumidifying means comprise two zones, one for dehumidifying the air as it passes from the first to the second chamber, and a second zone that undergoes regeneration as the rst zone dehumidies the air.

30. A system which comprises a drying tower using dehumidified atmospheric air as a drying medium, said tower comprising a heat-exchanging jacket surrounding the tower, said jacket being defined by an outer and an inner wall of heat-exchanging material, an intermediate heat-insulating wall of heat-exchanging material in said jacket, said intermediate wall dividing the jacket into a first chamber adjacent the outer wall and a second charnber adjacent the inner wall and dehumidifying means, the two chambers, the dehumidifying means and the tower being so operatively connected that during operation of the tower atmospheric air passes through the first chamber, then through the dehumidifying means, then the second chamber and then into the bottom part of the tower at a temperature not substantially exceeding 60 C.

References Cited bythe Examiner UNITED STATES PATENTS 1,289,779 12/ 18 Howard.

1,730,048 10/29 Zizinia 159-4 1,870,423 8/32 Richards.

1,946,566 2/ 34 Bowen.

1,959,301 5/ 34 Northcutt.

2,253,319 8/ 41 Batterman.

2,494,644 1/50 Clement 34-80 X 2,584,573 2/52 Gay 165-18 X 2,589,262 3/52 Keith.

2,621,966 12/52 Jehlicka.

2,635,684 4/53 Joscelyne.

2,776,562 1/57 Davie et al 16S-4 X 2,842,193 7/ 58 Ballestra.

2,851,097 9/5 8 Ledgett.

2,990,011 6/ 61 Stratford.

3,007,254 11/61 Schuster 34-35 X NORMAN YUDKOFF, Primary Examiner.

Claims

11. THE METHOD OF SUPPLYING, FOR AT LEAST ONE DIURNAL PERIOD, A CONTINUOUS SUPPLY OF AIR INTO A DRYING TOWER HAVING A JACKET FORMED BY AN INNER AND AN OUTER TOWER WALL DEFINING A SPACE THEREBETWEEN, SAID SPACE BEING DIVIDED INT A FIRST AND A SECOND REGION BY AN INTERMEDIATE WALL AND THE WALLS OF THE FIRST REGION BEING PRECONDITIONED BY HEAT-EXCHANGE WITH ATMOSPHERIC AIR AT ONE TEMPERATURE WHICH COMPRISES (A) DURING THE DAY COOLING ATMOSPHERIC AIR HAVING A HIGHER TEMPERATURE IN THE FIRST REGION BY RELEASING ITS HEAT TO THE WALLS OF THE FIRST REGION; THEN DEHUMIDIFYING THE AIR BY FEEDING IT THROUGH A DEHUMIDIFYING ZONE; THEN PASSING THE DEHUMIDIFIED AIR THROUGH THE SECOND REGION TO EXCHANGE HEAT WITH THE WALLS THEREOF, AND THEN FEEDING THE DEHUMIDIFIED AIR HAVING A TEMPERATURE NOT EXCEEDING ABOUT 60* C. INTO THE DRYING TOWER CONTINUOUSLY, AND (B) DURING THE NIGHT HEATING ATMOSPHERIC AIR HAVING A LOWER TEMPERATURE IN THE FIRST REGION BY ABSORBING HEAT FROM THE WALLS OF THE FIRST REGION; THEN DEHUMIDIFING THE AIR BY FEEDING IT THROUGH A DEHUMIDIFYING ZONE; THEN PASSING THE DEHUMIDIFIED AIR THROUGH THE SECOND REGION TO EXCHANGE HEAT WITH THE WALLS THEREOF; AND FEEDING THE DEHUMIDIFIED AIR HAVING A TEMPERATURE NOT EXCEEDING ABOUT 60* C. INTO THE DRYING TOWER CONTINUOUSLY.