CA1271123A - Dryer and drying method - Google Patents

Abstract

Abstract Dryer and drying method A finely atomized liquid is projected in a stream and dried by contact with drying gas. The drying gas is projected in turbulent flow from opposite sides of the stream such flow being distributed along the length of the stream. The material may be atomized and projected by a jet of gas issuing from a nozzle, and the drying gas may be projected around the jet so that the drying gas meets the entrainment demand of the jet, thereby preventing recirculation of gases outside the jet.

Description

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DRYER AND DRYING METHOD

The present invention relates to drying methods and apparatus.
P~rticulate materi~ls ~r~ commonly formed by spray drying. A liquid i9 atomized to form droplets, and the droplets are e~posed to a dry gas such as hot air. The liquid constituants of the droplets evaporate, l~aving particles of dried material. In the food and beverage indu9try~ this procass i9 used to prepare products such as soluble coffee powder, dried milk and dried milk sub3titutes.
Spray drying typically has been performed by contacting the atomized liquid with hot air in a large, vertically extensive chamber or "tower". A tower-type dryer having sufficient capacity for an industrial process may be 20 meters high and 6 meters in diameter. Such large apparatus is expensive to construct.
Heating of the material in spray drying typically has adversely affected the quality of the dried product.
Recirculation of air within the dryer may c~use prolonged retention of dried particles within the dryer and hence may exacerbate the damage caused by heating. These difficulties are particularly significant in the food and beverage industry, as many comestible matsrials incorporate flavor constituents susceptible to loss or degradation upon heating.
U.S. Patent 3,038,533 discloses a variant of the spray drying process wherein hot atomizing or "primary" air is discharged through a nozzle at a relatively high velocity.
A liquid to be dried is atomized to minute dropl2ts by the primary air as it pas3es through the nozzle. The minùte droplets pass do~nstream in the jet of primary air issuing from the nozzle, and dry rapidly~
The primary air jet tends to create a ragion of partial vacuum adjacent the jet and hence tends to cause recirculation of the surrounding air. To avoid such recircu-lation, the patent proposes to direct the jet along the axis of a tubular chamber, and to blow additional or "secondary"
air into the chamber co-directionally with the jet so that the jet is surrounded by the stream of secondary ~ir.
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Je~ spray drying ~echnlques as disclosed in che patent would appear to overcome some of the difficulties in conventional spray drying. However, such jet spray drying cechniques entail drawbacks of their own. Dried material tends to accumulate on the wall of the tubular chamber. Moreover, jet spray drying techniques have not been suitable for processing materials which resist atomization to v~ry fine droplets, such as highly concentrated beverage extracts. Cons~quently, there have been significant needs ; for improvements in spray drying methods and apparatus.
SUMMA~Y OF THE INVENTI~N
The presenc invention provides such improvements In a drying method according tO the present invention, the liquid material co be dried is atomized, and a stream of atomized ~ material is projected in a downstream direction. A drying gas is -~ 15 projected inwardly, coward che stream of atomized material, in turbulent flow transv~rsely tO che downstream direction from opposite sides of the scream. The inwardly flowing drying gas is distributed at least partly throughout a porous diffuser, along the length or upstream-to-downstream extent of the stream so Ihac as the atomized material moves downstream, it passes becween opposing inward flows of drying gas~ The atomized material and the gas are thus continually and vigorously agitaced as che material passes downscream, chereby promoting rapid drying. The size of the pores of the diffuser are between 0.1 and 10.0 times the average diameter of che droplecs being discributed.
The macerial may be projected downstream by passing an impelling gas through a nozzle so that che impelling gas issues from the nozzle as a jet, and entraining the material in the jet.
~: The material may be entrained in the impelling gas upstream of the nozzle and a~omized to fine droplets by the impelling gas as the gas and liquid pass chrough che nozzle.
When a jet of impelling gas is employed, the drying gas preferably is projected so chac over at leasc the upstream porcion of ics lengch, che jec is encirely surrounded by inwardly flowing drying gas. Accordingly, entrainmenc of gas by the jet canno~
crea~e a region of }

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partial v~cuum adjacent the jet. Recirculation of gases and dried mat~ri~l ~ssociated with such regions of partial v~cuum are therefora substantially eliminated. Moreover, the inwardly flowing drying gas t0nds to dissipate the jet, S thereby further suppressing recirculation.
The drying gas thus induces turbulence but suppresses r2circulation. Turbulence differs from recirculation. As u~ed in thi~s disclosure, the term "recircul~tion" rQfers to the action of an eddy which is 10 stable and remains in ~ substantially fixed location for an appreciable time. The term "turbulence", as used in this di~sclo~sure, refers to the ~ction of an eddy encompassed within a larger, surrounding, gas flow, so that the eddy move~ with the larger flow. On a macroscopic basis, 15 turbulence does not affect the movement of material entrained in the gas and does not promote repeated passage of mat~rial through the dryer.
According to a further aspect of the present invention, the temperature of the drying environment at each ~o location along the length or upstream-to-downstream ext~nt of the stream may be controlled as desired. Because the drying gas is distributed along the length of the stream and directed transversely of the stream, the atomized material is exposed at each loc~tion along the length of the stream, to 25 gas at a temperature which varies with the temperature of the drying g~s directed toward that location. By supplying drying gas at different temperatures to di~ferent regions of - the stream, the atomized material may be exposed to different temperatures in pre-determined sequence as the material 30 passes downstream. For example, by supplying relatively hot drying gas to an upstream region and relatively cool drying gas to a downstream region, the temperature in the downstream region may be limited to control the product temperature at the dryer outlet, while maintaining very high temperatures in the upstream region to promote rapid drying.
~ rying is also promoted by fine atomization of the liquid. Processes according to the pre.sent invention, however, will still provide effective and rapid drying with droplets largsr than those typically employed in prior ' ~'7~

jet-spr~y drying procasses. Consequently, materials which are viscous or otherwise resistant to very fine atomization may be dried eEfectively. It is believed that the improved mixing and desirable gas temperatures attained in the 5 preferred forms of the present invention contribute to this advantageous result.
The present invention also provides improved drying apparatus. The apparatus incorporates means for atomizing the material to be dried and projecting ~ stream of atomized 10 material in a downstream direction. The apparatus also includes mean~ for projecting a drying gas in turbulent flow towards the stream from opposite sides thereof, transversely to the downstream direction, so that the inwardly flowing drying gas i9 distributed along the length of the stream.
15 Preferably, the drying gas projecting means is arranged to provide drying gas at different temperatures along the length of the stream. The atomizing and projecting means may include a no~zle, means for supplying an impelling gas to the nozzle and means for entraining the material to be dried in 20 the impelling gas upstream of the nozzle throat.
Apparatus according to the present invention may be extraordinarily compact for a given drying capacity. In its preerred forms, the apparatus may have only one one-hundredth the volume of a standard~ spray dryer having 25 equiv~lent capacity.
Other objects, features and ~dvantages of the present invention will be apparent from the detailed de~cription of certain embodiments set forth below, taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
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Figure 1 is a schematic, partially sectional, view o appar~tus according to one embodiment of the present invention.
Figure 2 is a fragmentary, schemat c, sectional view ~ on an enlarged scale, showing a portion of the apparatus -~ illustrated in Fig. 1.

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~7~3 Figure 3 i9 ~ schematic sectional ~iew taken along line 3-3 in Fig~ 1.
Figure 4 is a schematic sectional view depicting a portion of apparatus according to a further embodiment of the 5 pra~ent invention.
Figures 5 and 6 ars fragmentary, schematic perspective views depicting apparatus according to still further embodiments of the present invention.

DETAILED DESCRIPTION OF T~E EM~ODIMEMTS

Apparatus according to one embodiment of the present invention includes a nozzle assembly 10 mounted at the upstream end of a porous diffuser in the form of an elongated tubular shell 12. The nozzle assembly includes ~ gas tube 14 having a conical transition piece 16 (Fig. 2) at its downstream end. A nozzle 18 is mounted at the downstream end of the transition piece. The interior surface of the nozzle is a surface of revolution about the central axis 22 of the 20 nozzle, converging to a thro~t or narrowest portion 24 at the - downstream extremity of the nozzle.
A housing 26 is supported within the gas tube by centPring screws 27, the housing terminating upstream of the ~;~ nozzle. A feed tube 28 is mounted to the housing, the downstream end of the feed tube extending into the nozzle and termin ting slightly up~tream from the throat 24 of the nozzls. Thermal insulation surrounds the feed tube within the housing. Centering screws 27 maintain the housing, and hence the down~tream end of the feed tube, coaxial with the nozzle.
The nozzle assembly is mounted qo that nozzle 18 is coaxial with 3hell 12 and the downstream extremity of the nozzle is ~ligned with the upstream end of the shell. An end wall 29 extends between the nozzle and the wall of the shell, the end wall having a planar downstream face 30 flush with the downstrQam extremity of the nozzle.
The wall of ~hell 12 includes a frustoconical upstream collar 31 adjacent the nozzle assembly, and a cylindrical downstream collar 32 (Fig. 1) coaxial with the , . - ~ . .

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upstream collar. Each collar is formed from a porous sintered ~et~l and defines many tiny pores or apertures uniformly distributed over its surface. Upstream collar 31 is disposed within a first housing 34 defining an annular 5 channel or space 36 surrounding the up~tream coll~r and confronting its exterior surface. Downqtream coll~r 32 is disposed within a similar housing 38 defining a further ~nnular channel 40, channels 36 and 40 being separated from one another by a wall 41. The downstraam end of collar 38 is 10 connected via an exit tube 42 to a conventional cyclone separator 44.
The feed tube 28 of the nozzle assembly is connectad to a source 46 for supplying a liquid to be dried, which source may incorporate a conventional storage tank, pump and 15 metering device. Gas tube 14 is connected to a source 48 for supplying a gas at a controllable temperature under a preselected pressure, and channels 36 and 40 are connected to similar independently controllable gas sources 50 and 52, respectively. The gas sources may incorporate conventional 20 comprsssors, regulators, heat exchangers, and flow measurement devices.
In one proces3 according to the present invention, an impelling g~s supplied by source 48 through tube 14 flow~
through nozzle 18 at a high velocity. A liquid to be dried 25 is forced by source 46 through the ~eed tube 28. As the liquid exits from the downstream end of the feed tube, it is entrained by impelling gas passing through nozzle 1~ and atomized as the gas passes through the throat 24 o the nozzle so that a stream of dropIets i9 projected from the ~ 30 nozzle along with the impelling gas. The impelling gas, with - entrained droplets passes downstream from the nozzle as a generally conical jet 54 havinq its upstream-to-downstream axis coincident with the axis 22 of the nozzle and hence coincident with the lengthwise axis of the diffuser or shell 12.
Source 50 supplies a first portion of drying ga~ to annular channel 36. As channel 36 offers little resistance to flow, the pressure within the channel is substantially uniform. Accordingly, the e~tgrior surfsce of col1gr 31 is ' .

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~'7~3 expo3ed to a substantially uniform gas pressure around its entire circumference. The wall of collar 31 has substan-tially uniform porosity over lts entire circumference, 90 that drying ga~ passes through the wall of the collar at an 5 essentially uniform rate per unit of wall are~ about the circumferonce of the collar. As the pores of collar wall are microscopic and closely spaced, the gas streams emerging from adjacent pores merge with one another at microscopic distances from the interior surface of the collar, before the 10 drying gas encounters the jet. Thus, the upstream region of the jet i~ surrounded by a continuous flux of drying gas moving radially inwardly toward the axis of the jet from outside the periphery of the jet, as depicted ~y the arrows in Fig. 3. The drying gas also has a low vslocity 15 down~tream, parallel to the axis.
A second portion of the drying gas, supplied by source 52, passes through channel 40 and through the wall of downstream collar 32, 90 that the downstream region of the jet i3 surrounded by a similar continuous flux oE drying gas.
20 Adjacent the juncture of the two collars at boundary wall 41, the flux includes drying gas ~upplied through both collars.
The drying gas pas~es downstream with the impelling gas and the droplets. As the droplets pass downstrsam, the moisture in the droplets evaporates, 90 that the droplets are converted to dried particles before reaching exit tube 42.
The particles and gases pass through the exit tube to separator 44, where the particles are separated from the gas and removed from the system.
The drying gas supplied through the wall of the shell penetrates into the jet and mixes with the impelling ga~ in the ~at. Moreover, the flow of drying gas toward the axis of the jet promotes turbulence in the jet and hence promotes exchange of gases between the central or core region of the jet adjacent the axis and the peripheral region of the ~ 35 jet, remote from the axis. Such thorough mixing and - continual addition of drying gas maintains the gases in all regions o the jet at the desir-ed low humidity de~pits the continual transfer of mQisture from the material being dried to the gases.

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The tsmperature o~ each portion of the dryinq gas, and the temperature of the impelling gas, may be controlled independently~ The first portion of the drying gas, supplied through upstream collar 31, mixes with the impelling gas and 5 the matsrial being dried in the upstream region, adjacent the nozzle, whereas the 3econd portion of drying gas supplied through downstream coll~r 32 mixes with the other gases and material in the downstream region. Thus, the heat input to each region of the dryer, and hence the pattern of gas 10 temperatures to which the material is exposed as it passes downstream, may be controlled as desired.
In many cases, it is preferred to expose the m~tsrial to a relatively high gas temperature in the upstream region and a relatively low gas temperature in the downstream 15 region. In the upstream region, the material has a relatively high moisture content and a large amount of heat is consumed in conversion of moisture to the vapor phase. In ; the downstream region, the material iq relatively dry, ~o that less moisture i5 available for evaporation.
20 Consequently, less heat is required for avaporation in the downstream region. By supplying the impelling gas and the drying gas to the upstream region at relatively high temperatures, and supplying the drying gas to the downstream rsgion at a relatively low temperature, the heat input is 25 clo~ely matched to the heat requirement in each region of the drier. Thu~, the heat supplied in the gases is efficiently u~ed to effect the desired evaporation rather than wasted.
Morsover, by supplying the drying gas at different temperatures along the length of the jet, effective drying 30 may be attained without raising the temperature of the ~ material being dried beyond desirable limits. In the -~ upstream region, where the material has substantial moisture content and substantial amounts of heat are consumed in evaporation, the temperature of the droplets is substantially lower than the temperature of the surrounding gases. The gases in the upstream region therefore may be maintained at a relatively high temperature to promote evaporation without unduly raising ths temperature of the material being dried.
As the material passes downstream and becomes progressively -" ~'' ' . ' ' - : :

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_g_ drier, the rate of evaporation decreases. With continued heat transfer from t~e gases to the material being dried, the temperature of the material approaches the temperature of the surrounding gases. The gas tamperature in the downstream 5 region should be limited to avoid overheating the material.
By supplying the drying gas at different temperatures along the length of the stream, a high gas temperature may be maintained at the upstream region while maintaining a low gas temperature in the downstream ragion.
If the tsmperature of the drying gas supplied to the downstream region is low enough, the temperature of the mixed gases in the downstream region may be lower than the temperature of the material being dried. In that case, heat is tr3nsferred from the material to the gases. ~he dried 15 material is thus cooled in the stream, before it is collected. The intimats exposure of the material to the gases achieved by projecting the drying gas transversely to the direction of the stream provides effective and rapid cooling. Although the term "drying gas" is used for the sake 20 of convenience to designate the gas projected transversely of the stream, it should be understood that some or all of the drying gas utilized in cooling the matarial may contact the material after it is already dry. Some or all of the drying gas u-tilized in cooling may be projected downstream of the ~5 point along the length of the stream where the material reaches its final moisture content. I
When the material is cooled in the stream, prior to collection, it is exposed to elevated temperatures only during the time required for drying. By contrast, in 30 conventional spray drying processes, it is typically imprac-ticable to cool the dried material prior to collection. In conventional processes, the dried material typically is collected at an elevated temperature and hence is susceptible to thermal degradation after collection.
In the embodiments described above, two portions of drying gas at the two different temperatures are employed.
~- The desirable effects attained by supplying the drying gas at different temperatures to different regions along the length oi the stream may b~ enbanced by supplying the drying gas in ;: .

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~LZ~ 3 more than two portions to more than two regions. For example, three portions of the drying ga~ may be supplied at high, intermediate and low temperatures.
Other patterns of gas temperature may also be 5 employed. Thus, if the impelling gas i9 supplied at a lower temperature than the drying gas projected towards the upstream region of the stream, the temperature of the mixed gases will be low immediately adjacent the nozzle ~nd will increase progressively in the downstream di~ection within the 10 upstream region. Thu3, the temperature o~ the atomized material will be very low during the initial portion of the drying process. This ~ffect may be enhanced by ~upplying the drying gas at a low temperature to the most upstream region and at a higher temperature to the next region. Low 15 temperature during the initial phases of drying is useful with materials which are particularly sensitive to heat when moist but which became less sensitive as they become drier.
For example, it i9 believed that coffee and tea axtracts become less susceptible to loss of volatile aromas upon 20 heating as they become drier.
In the arrangements described above, the flux or flow of drying gas is symmetrical about the axis of the jet.
The inward flow of drying gas from each portion of the diffuser or shell wall is balanced by a like inward flow in the opposite diraction from the diametrically opposed portion of the wall. The opposing flows of drying gas do not deflect the tomized material from the axis. However, as best - appreciated with reference to Fig. 3., the drying gas tends to keep the atomized material away from the shell wall. Any droplet or particle moving outwardly toward the shell wall encounters the inwardly flowing gas and is deflected back towards the axis.
A jet of gas tends to entrain the surrounding gases, and hence tends to create a partial vacuum adjacent the - 35 upstream end of the jet~ The partial vacuum in turn tends to cause the surrounding gases to flow upstream outside of the jet, thus creating recirculation. The inwardly flowing Iry1ng ga~ prevents such recirculation.

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- A jet is characterized by a distinctive profile o~
gas velocities, with higher velocities in the central region adjacent the a~is and lower velocities in the peripheral reyion, remote from the axi~ As the distinctive velocity S profile as30ciated with the jet dissipates, its entrainment demand or tendency to entrain gases from the surroundings diminishas, and hence its tendency to cause recirculation also diminishes. It is believed that the thorough mixing induced by the drying gas promotes trans~er of momentum 10 bstween the central region of the jet and the periphery of the jet. The drying gas thus promotes dissipation of the velocity profile of the jet and hence reduces its entrainment demand.
Moreover, the inwardly flowing drying gas supplied 15 through the shell wall meets the remaining entrainment demand of the jet. The rate of drying gas flow through the shell wall per unit axial length adjacent the upstream end of the shell desirably exceeds the entrainment demand rate of the jet per unit length. Thus, there is some downstream flow of 20 dryinq gas outside the jet. If the rate of drying g~s flow per unit axial length through a downstream region of the shell wall i3 less than the entrainment demand rats of the jet, the excess drying gas flowing downstream can make up the deficiency. The reverse situation, with a deficiency of 25 drying gas in the upstream region and an excess in the ~ downstream region, is lass desirable. There would ~e ~n ; upstream flow of gas outside of the jet, which could cause recirculation. Stated another way, the total rate at which the drying gas is projected towards any portion of the jet 30 upstraam of an arbitrary location along the length of the jet, and hence the total drying gas flow rate through the shell wall upstream of that arbitrary location, preferably equals or exceeds the to'tal entrainment demand for that portion of the jet.
The actual entrainment demand of a jet decaying under the influence of the inwardly projected drying gas is not readily calculable. However, for a jet creatad by a -~ given flow of impelling gas through a given nozzle, the ~ actual entrainment demand rate will be less than the '' .

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~.~'711i~3 entr~inment demand of the corre~ponding free jst, i.e., a jet of impelling gas having the same flow rate and i3suing from the same nozzle into ~n infinite space without any inwardly project3d drying gas. The entr~inment demand per unit length S E of such a free jet is approximated by the formula:

E = 0~26 (Mo) D

Where: Mo is the mass flow rate of impelling gas through the nozzlQ, and D is the diameter of the nozzle throat.

As used in this disclosure, the term "theoretic~l 15 entrainment demand" refer~ to the entrainment demand for the corresponding free jet calculated according to the above formula. If the inward drying gas flow equals or exceeds the theoretical entrainment demand of the jet, then the drying gas flow will exceed the actual entrainment demand of the 20 jet. The desired relationship may be expressad by the following formula:

~-q x=q Rdx Edx , x=o x=o : ~ .
Where: R is the rate of drying gas flow through the shell w~ll per unit axial length;
- x is axial distance downstream from the throat of the nozzle, and q is an arbitrary value.
: ~, The aforementioned rel3tionship between entrainment demand and drying gas flow preferably is maintained in the ~- 35 region immediately adjacent the nozzle, over an axial distahce equal to or greater than 10 times the nozzle diameter, i.e., for any value of q between 0 and lOD. As the ~- ` jet decays appreciably in that region, it is less important to maintain that relationship further downstream. However, - ~ .
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~t7~ 3 maintainence of the ~pecified rel~tionship for distances greater than 10 nozzle diameters down~tream, typically up to about 30 to 60 nozzle diameters, provides even greatar assurance against recirculation.
It is believed that in drying processes ~ccording to preferred embodiments of the prssent invention, several factors coact to suppress adhesion of the material being dried to the walls of the drying chamber. Adhesion typically results from impingement of moi3t material on the chamber 10 wall, dry material typic~lly will not adhere to the wall.
The improved mixing and desirable gas temperature distribu-tions attainable according to the present invention promote rapid drying and hence promote drying of the material before it can impinge on the wall. Rocirculation tends to carry the 15 material being dried outwardly, toward the chamber wall, and hence promotes impingement and adhesion. Suppression of recirculation by the drying gas thus serves to suppress adhesion. Also, the inwardly flowing drying gas tends to blow the matarial being dried inwardly, away from the chamber 20 wall, further suppressing adhesion of ths material to the wall.
To minimize impingement of moist material on the shell wall, the shell wall in the upstream region preferably is disposed outside the lateral boundary of the jet. A jet 25 does not have discrete physical boundaries, at progressively greater distances from the axis of the jet, the velocity of the gas in the downstream direction declines, with no appreciable discontinuity between the jet and the surroundings. Ths lateral boundary of a free jet, undis-30 turbed by inwardly flowing gas, is ordinarily taken asdefined by a theoretical frustum of a cone spreading outwardly from the throat of the nozzle and having an included angle of about 23.S degrees. Although the actual jet will spread to a lesser degree due to the effect of the 35 drying gas, the free-jet approximation may be used in design of drying apparatus according to the present invention.
Thus, the shell wall may be arranged so that the upstream rsgion of the shell wall lies outside of the aforementioned frustum. In the downstream region, where the atomized ' -.
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material is substanti~lly dry and impingement of the material on the shell wall is unimportant, the shall wall may be disposed withi~ the theoretical frustum.
The sheLl wall desir~bly should either be parallel to the lengthwise axis of the shell or diverge from such axis at an angle less than or equal to about 3~ degrees. Thus, a frustoconical shell wall preferably has an included angle no greater than about 7 degrees. If the shell w~ll is in the form of a surface of revolution other than a frustum of a cone, the generator of such surface should not diverge from the lengthwise axis at an angle greatèr than about 3 degrees.
To provide the best interaction between the drying gas and the droplets, and to minimize the volume of the apparatus, the wall of the shell or diffuser should be di posed as close to the lengthwise axis as possible in keeping with the considerations set forth above~ Although the present invention i5 not limited by any theory of operation, it is believed that turbulent eddies are created as the drying gas passes through the pores in the diffuser wall, and that these eddies contribute to the interaction between the drying gas and the droplets. Further, it is believed that these eddies decay as they move inwardly with the drying gas. Close juxtaposition of the diffuser wall and

2~ the stream of atomized material is believed to provide better exposure of the material to the eddies. Preferably, at least part of the diffuser wall is disposed within about 25 cm of the center of the stream to provide effective propagation of the eddies into the center of the stream. It is believed that the most effective interaction between the eddies and the dispersed mat~rial occurs adjacent the wall of the shell or diffuser. Accordingly, it is believed that those droplets which approach the wall are dried most rapidly, thus further suppressing adhesion of the atomized material to the wall.
- 35 The size of the pores is also believed to be ; significant. The size of the eddies created as the drying gas issues from the pores is directly related to the size of the pores. It is believed that the optimum interaction between the eddies and the atomized mat~rial occurs when the - : , ' ~7~3 ~ize of the eddies approximates the diameter of the droplets.
To achieve thi~ relationship, the sizes of the individual pores should be predominantly from about 0.1 to ~bout 10 times, and preferably about 1.0 to about 5.0 times, the 5 average diameter of the droplets produced in the atomization step. In this context, the l'size'l of a pore means the diameter of the largest rigid spherical particle ~hich will pas3 through the pore.
Apparatus according to a further embodiment of the 10 present invention, schematically illustrated in Fig. 4, employs a nozzle 60 having an elongated, r2ctangular opening, a feed tube 62 having a rectangular outlet being disposed within the nozzle. The no%zle is directed into a tubular porous shell 64 of generally rectangular cross-section, the 15 walls of the shell flaring outwardly toward the downstream end of the shell. Impelling gas supplied to nozzle 60 entrains and atomizes liquid supplied to feed tube 62. The gas exits from the nozzle as a jet of generally rectangular cross-section, c~rrying a 3tream of droplets with it. ~rying 20 gas is supplied through both the narrow sides and the wide sides of shell 64. ~ust as in the embodiments described above with reference to Figs. 1-3, the jet is entirely surrounded by the drying gas projected inwardly from the porous shell, and the atomized matarial passes between 2S opposing inward flows of drying gas.
The aforementioned relationships betwesn the entrainment demand of the jet and the rate of drying ga~ flow through the shell apply to arrangements such as that of FigO
4, employing a nozæle of noncircular cross-section. With a rectangular nozzle, the narrow dimension of the nozzle should be taken as the diameter of the nozzle. A jet issuing from a rectangular nozzle spreads outwardly in much the same manner as a jet is~uing from a circular nozzle. The theoretical lateral boundary of a free je~ issuing from a rect~ngular nozzle, unaffected by any inwardly-flowing gas, is in the form of an obelisk having sides extending from the edges of the nozzle, opposing sides of the obelisk defining included angles of about 23.5. Again, although the jet is .
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constrained to some extent by the drying gas, the theoretical free jet boundary may be used in initial design of the shell.
The apparatus ill~lstrated in Fig. S incorpoeates a plurality of frustoconic~l porous shells 66 and associated 5 gas tubes 68. Each ga~ tube is arranged to discharge an impelling ga~ into the a3sociated shell through a nozzle, and liquid material to be dried i3 supplied through a feed tube (not shown) within each ga~ tube. Drying gas supplied through chambers (not shown) encircling the shells is 10 projected through the shells. Thus, each shell operates in the same fashion as the upstream portion of the drying chamber described above with reference to Fig~. 1-3: the jet issuing from each nozzle is entirely surro~nded, over the upstream portion of its length, by inwardly projected drying lS gas. The down9tream ends of the shells are disposed between a pair of opposed porous plat~s 70, and additional drying g~s `` i9 supplied via chambers 72 through the porous plates.
Gases and atomized material issuing f-rom the shells pass between opposing inward flows of drying gas projected 20 from the porous pl~tss, as indicated by the arrows in Fig. 5.
The drying gas issuing from the plates mixes under turbulent condition~ with the atomi~ed material and with the gases issuing from the shells. The streams of gas and atomized material issuing from the shells are not entirely surrounded 25 by inwardly-projected drying gas as they pass between the plates. To minimize the possibility of recirculation in the regions between adjacent streams, the configuration of shells 66 and the drying gas flow through the shells are arranged so that the jet issuing from each nozzle is substantially dis3ipated within the associated shell. Thus, e~ch shell preferably extends downstream from the associated nozzle for ~ distance at least 10 times the diameter of the nozzle, and drying gas prefer~bly is supplied through each shell at a rate at least equal to the theoretical entrainment demand of the jet.
In the embodiments described above, the material is projected in a jet of impelling gas. Howevsr, the matsrial may be atomized and projected without using an impelling gas.
The apparatus illustrated in Fig. 6 includes a drying chamber 731 ~

-~7 7~ which is enclosed at itg upstrsam end and at its sides~
Two opposed ~ides of the drying chamber are defined by porous difuser plates 76. Each porous plate communicates with chambers 78, the chambers being connected to gas sources (not 5 shown). Atomizing nozzles ~0 are mounted to the upstream w~ll of the chamber. Each atomizing nozzle has a plurality of fine orifices opening to the interior o-f the chamber. The atomizing nozzles are connected to a high-pressure pump 82.
A liquid to be dried is forced through nozzles 80 by lO pump 82~ so that a strea~ of fine droplets passes downstream from each nozzle. Drying gas ~upplied via chambers 78 i~
projected through porous plates 76 towards the streams of droplets. The streams pas~ between opposing inward flows of drying gas di 5 tributed along the length of the streams and 15 the dispersed liquid is effectively exposed to the drying gas under turbulent conditions. Preferably, porous plateq 76, and hence ths opposing inward flows of drying gas, extend downstream beyond the point where the material is substan-tially dry.
; 20 As the upstream end of the chamber is closed, continued flow of drying gas into the chamber forces the drying gas in the chamber downstream. The dried material is - collected in a separator (not shown) connected to the downstream end of the chamber.
As there are no jets of impelling gas, there is no need to arrange the drying gas flow so as to prevent recirculation inducsd by such jets. Thus, the strsams of droplets are not entirely surrounded by the inwardly-projected drying gas. Also, the heat requirad for drying is supplied entirely by the drying gas. Any liquid atomizing nozzle capable of providing the desired degree of atomization may be employed. In other respects, the drying operation is similar to those described above.
Regardless of the method of atomization, the required drying time varies markedly with the diameter of the droplets; larger droplets take longer to dry. Accordingly, atomization to an average droplet size of about 70 micron~ or less is preferred. Very large droplets which dry relatively slowly may impinge on the walls of the dryer while still .
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~71~3 moist, and hence may adhere to the walls. Ordinarily, the proportion o~ very large droplets varies directly with the degree of nonuni~ormity in the droplet diameter distribution and also varies diractly with the average droplet diameter.
5 Thus, the more uniform the droplet size distribution, the larger the average droplet diameter may be without encountering adhesion. Atomization to an average droplet diameter of about 63 microns, with 3~ of the droplets above 212 microns diameter and 12.2% between 150 and 212 microns, 10 may be employed successflllly. Insofar the drying process is concerned, there is no lower limit on droplet diameter. The droplet diameters referred to herein are determined by mea~uring the diameter of the dried particles, on the assumption that the diameters of the dried particles ars the lS same as the diameters of the droplets produced in the atomiz~tion 3tep.
Liquid materials which are viscous or otherwise difficult to atomize, such as aqueous comestible materials of about~40~ solids content or more, can best be atomized by 20 entraining the liquid in impelling gas and passing the impelling gas through an orifice as described above with reference to Figs. 1-5. Typically, the most effective atomization is attained when th2 impelling gas approaches or reaches sonic velocity at the nozzle throat.
The impelling and drying gases ordinarily are air, but other gases may be employed. Although the drying process ordinarily involves evaporation of water, materials bearing liquids other than water may ~lso be dried. Sugar-based materials, such as solutions of sugar in water, honey and 30 molasses enter ~ tacky, non-crystalline state upon drying and remain in that state for an appr2ciable period after drying is complete. To avoid adhesion problems in processing such materials, other m~terials may be admixed with the sugar-based material to provide nucleation sites and accelerate crystallization.
The present invention is particularly suitable for drying comestible liquids such as milk, aqueous extracts of coffee, chicory and tea, mixtures of such extracts, and combinations including such extracts together with sugar, :

~ ~'7~1~3 molasses or honey. It is believed that the rapid drying, substanti~l absence of recirculation within the dryer and controlled product temperatures attainable with the present invention preserve the flavor of the product.
In typical embodiments of the invention for drying aqueous comestible liquids, the impelling gas temperature upstream of the nozzle typically is about 500C or less, drying gas is supplied to the upstream region at about 120C
to about 210C, and drying gas is supplied to the downstream 10 region at less than about 80C. The total mass flow r~te of drying gas ordinarily is about 10 to 20 times the mass flow rate of the impelling gas. Under these conditions, typical comestible liquids can be dried effectively in industrial quantities with ~ rssidance time of the material in the dryer 15 on the order o~ 50 milliseconds or less. The shell or drying chamber for such a process may be 1 to 2 meters in length, and less than 1 meter in diameter.
The following examples illustrate certain aspects of the present invention. Solids contents and moisture contents 20 3t~ted in the examples as percentages are percentages by weight.

Apparatus similar to that illustrated in Figs. 1-3 i3 employed. The nozzle has a throat diameter of 18 mm. The shell is frustoconical, about 1 metsr in length, about 7 cm inside diameter at its up~tream end and about 33 cm inside diameter at its downstream end. The average pore size of the shell is about 30 microns. Aqueous coffee extract containing 30 about 45~0 solids is pumped through the feed tube at about 70 kg/hr. 267 kg/hr of impelling air at about 410C are ~upplied to the nozzle; as the impelling air passes through the nozzle, it is cooled by expansion to about 310C. 1662 kg/hr of drying air at about 160C are supplied through the upstream half of the shell, and 1948 kg/hr of drying air at about 42C are supplied through the downstream half o-f the ~hell. Air and dried material exit from the downstream end of the shell at about 80C. The coffee extract is dried to :, .

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- particles of about 20-30 microns diametsr and about S%
moisture.
Thers is no detectable accumulation of material on the shell wall. The beverage prepared from the dried product 5 is preferred by t~sters in ~ blind study o~er a beverage prepared rom a powder obtained by conventional tower spray drying of the same extr~ct.

The apparatus is as employed in Exampls 1, except that the shell has a frustoconical upstream section of about 22 cm inside diameter at it~ upstream end and about 32 cm inside diameter at its downstream end, and a cylindrical downstre~m section of about 32 cm inside diameter. Both 15 sections have pores of about 30 microns a~srage size. Air flow rates are measured as the air enters the system at room temperature and atmospheric pressure, prior to heating and compression. The impelling gas pressure and temperature are measured immediately upstream of the nozzle. The exit 20 temperature is the temperature of the mixed gases and dried product in the exit tube, measured approximately l meter from the downstream end of the shell. In each case, about, 28,300 liters of drying gas are passed through each section of the - shell per minute. The other parameters for each example are ~ 25 set forth in the table, below.

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___ A tea extract containing 44% solids i9 dried at a liquid feed rate of 115 kg/hr using the ~ame equipment a9 in Examples 2-6, and using air as the impelling and drying 5 gases. The impelling air flo~ rate is 317 kg/hr, and the total drying gas flow rate iq 2860 kg/hr, evenly divided between the upstream and downstream sections of the porous shell. Immediately upstream of the nozzle, the impelling gas is at 93C; as it pas3es through the nozzle, it is cooled to 10 about 28C. Drying air is supplied through the upstream section of the shell at 246C, and through the downstream section at 93C. The atomized extract is thus exposed to mixed gases ~t low, high and intermediate temperatures in that order. The exit temperature is about 85C. The product has a moisture content of about 3.5%.

Milk preconcentrated to about 48~o solids is dried using the same procedure and equipment as in Example 7, save that the liquid feed rate is 170 kg/hr and the impelling air flow rate is 476 kg/hr. The exit temperature is about 65C
and the product moisture content is about 5%.

Claims (19)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR
PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of drying a liquid material comprising the steps of:
(a) atomizing the material and projecting a stream of droplets of the material in a downstream direction; and (b) projecting a drying gas in turbulent flow transversely to said downstream direction and towards the stream from opposite sides thereof, the drying gas being distributed along the length of the stream and being at least partly supplied through a porous diffuser disposed adjacent the scream so that the atomized material passes between opposed inward flows of drying gas so that the drying gas mixes with the atomized material which is dried as it moves downstream essentially without recirculation, the size of the pores of the diffuser being between 0.1 and 10 times the average diameter of the droplets of the atomized material.

2. A method as claimed in claim 1 wherein the material is projected downstream and atomized by passing an impelling gas through a nozzle and entraining the material in the impelling gas upstream of the throat of the nozzle, so that the material is atomized by the impelling gas as it passes through the nozzle and the impelling gas passes downstream from the nozzle in a jet, the atomized material being entrained in said jet.

3. A method as claimed in claim 2 wherein said drying gas is projected so that in an upstream region adjacent the nozzle, the jet is entirely surrounded by inwardly flowing drying gas.

4. A method as claimed in claim 3 wherein the drying gas is projected so that for any arbitrary location between the nozzle and 10 nozzle diameters downstream, the total rate at which drying gas is projected toward that portion of the jet between the nozzle and said arbitrary location equals or exceeds the total theoretical entrainment demand of that portion of the jet between the nozzle and said arbitrary location.

5. A method as claimed in claim 2 , 3 or 4 wherein the impelling gas reaches sonic velocity at the throat of the nozzle.

6. A method as claimed in claim 1 wherein drying gas at different temperatures is projected toward different regions along the length of the stream.

7. A method as claimed in claim 6 wherein the drying gas projected towards an upstream region is at a higher temperature than the drying gas projected towards a downstream region.

8. A method as claimed in claim 7 wherein the dried material is cooled by the drying gas projected towards said downstream region.

9. A method as claimed in any one of claims 1, 2 or 3 wherein the material is atomized to droplets of an average diameter less than about 70 microns.

10. A method as claimed in any one of claims 1, 2 or 3 wherein the ratio of pore to droplet size is from 1.0 t 5.0:1.

11. A method as claimed in claim 1 wherein the liquid material is an aqueous comestible material selected from the group consisting of milk, coffee extract, chicory extract, tea extract and mixtures thereof or a mixture of one or more of the aforesaid aqueous comestible materials with sugar, molasses or honey.

12. A method as claimed in claim 11 wherein the liquid material contains at least about 40% solids by weight.

13. Apparatus for drying a liquid material comprising:
(a) means for atomizing the material and projecting a stream of the atomized material in a downstream direction; and (b) means including a porous diffuser disposed adjacent the scream for projecting a drying gas in turbulent flow, transversely to said downstream direction, towards the stream from opposite sides thereof so that the drying gas is distributed along the length of the stream and the drying gas mixes with the atomized material, the size of the pores of the diffuser being between 0.1 and 10 times the average diameter of the droplets of the atomized material.

14. Apparatus as claimed in claim 13 wherein the diffuser has opposed porous surfaces extending in said downstream direction and means for forcing the drying gas through said opposed surfaces, said atomizing and projecting means being operative to project the stream of atomized material between said opposed surfaces.

15. Apparatus as claimed in claim 14 wherein said diffuser includes a porous elongated tubular shell, said means for forcing the drying gas through the opposed surfaces includes means for applying the drying gas under pressure to the exterior of the shell, said atomizing and projecting means being operative to project the stream of atomized material into the shell in the lengthwise direction thereof.

16. Apparatus as claimed in claim 15 wherein said atomizing and projecting means includes a nozzle aligned with said shell, means for supplying an impelling gas to the nozzle under pressure so that the impelling gas exits from the nozzle as a jet directed downstream within said shell, and means for entraining the liquid material in the impelling gas upstream of the throat of the nozzle.

17. Apparatus as claimed in claim 16 in which the interior surfaces of said nozzle and said shell are surfaces of revolution and are coaxial with one another.

18. Apparatus as claimed in any one of claims 14, 15 or 16 wherein said drying gas projecting means includes means for projecting the drying gas at different temperatures through different portions of said opposed surfaces along the upstream-to-downstream extent thereof.

19. Apparatus as claimed in claim 17 wherein said drying gas projecting means includes means for projecting the drying gas at different temperatures through different portions of said opposed surfaces along the upstream-to downstream extent thereof.