U.S. patent number 3,741,273 [Application Number 05/170,535] was granted by the patent office on 1973-06-26 for spray drying apparatus.
This patent grant is currently assigned to The Pillsbury Company. Invention is credited to Reginald E. Meade.
United States Patent |
3,741,273 |
Meade |
June 26, 1973 |
SPRAY DRYING APPARATUS
Abstract
Apparatus for drying fluids containing dissolved or suspended
material composed of a spray chamber, a spray nozzle, a forminous
element such as a screen positioned to receive the spray while the
droplets are in a tacky condition to form a mat on the screen and a
blower for drawing air from the enclosure through the mat and
thence through the screen to dry the mat.
Inventors: |
Meade; Reginald E. (Stillwater,
MN) |
Assignee: |
The Pillsbury Company
(Minneapolis, MN)
|
Family
ID: |
26866196 |
Appl.
No.: |
05/170,535 |
Filed: |
August 10, 1971 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
13501 |
Feb 24, 1970 |
|
|
|
|
553101 |
May 26, 1966 |
3520066 |
|
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Current U.S.
Class: |
159/4.01;
159/4.08; 159/4.03 |
Current CPC
Class: |
B01D
1/18 (20130101); F26B 3/12 (20130101); A23P
30/40 (20160801); Y10S 159/28 (20130101) |
Current International
Class: |
A23L
1/00 (20060101); B01D 1/16 (20060101); B01D
1/18 (20060101); F26B 3/12 (20060101); F26B
3/02 (20060101); B01d 001/16 () |
Field of
Search: |
;159/4R,4B,4C,4CC,4E,4F,D28,47R,48R,49,45 ;34/9 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Yudkoff; Norman
Assistant Examiner: Sofer; J.
Parent Case Text
This application is a continuation-in-part of a prior application
having the same title, Ser. No. 13,501 filed Feb. 24, 1970 and now
abandoned, which is in turn a continuation-in-part of application
Ser. No. 553,101, filed May 26, 1966, now U.S. Pat. 3,520,066, for
"Spray Drying Method".
Claims
What is claimed is:
1. A spray drying apparatus comprising in combination: a liquid
supply, dispersing means connected to the liquid supply for
reducing the liquid to a multiplicity of minute gas-suspended
droplets, a spray enclosure surrounding the dispersing means to
contain the drying gas and maintain the gas in contact with the
droplets, a source of drying gas communicating with the enclosure,
a draft producing means connected to communicate with the enclosure
for forcing the drying gas from the gas source through the
enclosure, an outlet communicating with the enclosure and with the
draft producing means for exhausting the drying gas from the
enclosure, an endless foraminous collecting screen located within
the enclosure, rolls supporting the collecting screen so that the
latter defines a collecting surface extending from one side of the
enclosure to the other across the enclosure whereby substantially
all of the solids expelled from the dispersing means are deposited
as a mat upon the screen, drive means for advancing the collecting
screen from one side of the enclosure to the other at a speed with
the respect to the size of the collecting surface and the spray
rate such that the mat accumulates in a quiescent condition to a
thickness of at least about a quarter of an inch whereby the
particles present in the mat itself serve as a collecting medium
for additional particles, said collecting surface extending
entirely across the outlet and being sealed at all times along its
edges whereby a pressure differential of at least one-half inch of
water is developed thereacross by means of the draft producing
means and the gas that passes through the outlet flows steadily
through the collecting screen and so as to deposit the droplets on
the collecting surface as a homogeneous mat having no holes therein
other than pores between the individual dried droplets and composed
of the partially dried droplets of the liquid bonded together at
their points of contact, the nozzle being spaced from the
collecting surface for a particular temperature and drying capacity
of the gas so that the particles are tacky but contain insufficient
liquid to flow together when they impinge upon the collecting
surface and means for removing the mat from the screen operative
outside of the enclosure after the screen has been withdrawn from
communication with the upstream portion of the enclosure.
2. The apparatus according to claim 1 wherein the foraminous screen
comprises a plurality of perforated plates positioned adjacent and
coplanar to one another.
3. The apparatus according to claim 1 wherein a chamber
communicates with the downstream side of the screen and a duct
means communicates between said chamber and the upstream side of
the screen for returning particulate material from the chamber to
the upstream side of the screen.
4. The apparatus of claim 1 wherein a plurality of linearly
distributed suction plenums are positioned on the opposite side of
the foraminous collecting member from the dispersing means.
5. The apparatus according to claim 1 wherein a means is provided
for precoating the screen on its upstream surface with a porous
material adapted to serve as a collecting bed for the tacky
droplets.
6. The apparatus according to claim 5 wherein said precoating means
comprises an imperforate element positioned on the side of the
screen closest to the said outlet, said imperforate element being
positioned adjacent to a portion of the foraminous member to block
the flow therethrough until a deposit of the sprayed material forms
thereupon.
7. An apparatus according to claim 1 wherein there is provided a
second enclosure, a means for forcing a second gas into said second
enclosure and through the screen and the screen is driven so as to
move the accumulated mat thereon from said enclosure into said
second enclosure whereby the mat upon the screen is exposed first
to the gas within the enclosure and thereafter to the gas within
the second enclosure.
8. The apparatus of claim 7 wherein a duct means is provided for
exposing the mat upon the screen in the second enclosure to a gas
having a composition that is different from the gas in the
enclosure.
9. The apparatus of claim 7 wherein a source of cooling gas is
provided and a duct means is provided for circulating the cooling
gas through the mat on the screen in the second enclosure after the
gas within the first mentioned enclosure has passed through said
mat.
10. The apparatus of claim 1 wherein a brush is provided to remove
material from the screen.
11. The apparatus of claim 10 wherein a conveyor is provided for
receiving pieces of the mat leaving the end of the screen after the
pieces have been removed.
12. A spray drying apparatus comprising in combination: a liquid
supply, dispersing means connected to the liquid supply for
reducing the liquid to a multiplicity of minute gas-suspended
droplets, a spray enclosure surrounding the dispersing means to
contain the drying gas and maintain the gas in contact with the
droplets, a source of drying gas communicating with the enclosure,
a blower connected to communicate with the enclosure for forcing
the drying gas from the gas source through the enclosure, an outlet
communicating with the enclosure and with the blower for exhausting
the drying gas from the enclosure, a relatively fixed foraminous
collecting screen located within the enclosure whereby
substantially all of the solids expelled from the dispersing means
are deposited as a mat upon the screen, the lower portion of the
enclosure comprising a hopper having side walls that converge
toward the bottom thereof, said foraminous collecting screen being
disposed vertically and sealed along all of its edges across the
enclosure between the source of drying gas and the blower, a gate
valve within the hopper at the bottom of the screen for allowing
material that is collected upon the screen to flow periodically
when the gate valve is in an open position to the outlet at the
bottom of the hopper whereby gas in the enclosure can be forced to
pass exclusively through the foraminous collecting screen when the
gate valve is in a closed position, whereby the mat accumulates in
a quiescent condition to a thickness of at least about a quarter of
an inch whereby particles present in the mat itself serve as a
collecting medium for additional particles, said collecting surface
extending entirely across the outlet and being sealed at all times
along its edges whereby producing a pressure differential of at
least one-half inch of water is developed thereacross by means of
the blower and the gas that passes through the outlet flows
steadily through the collecting screen and so as to deposit the
droplets on the collecting surface as a homogeneous mat having no
holes therein other than pores between the individual dried
droplets and composed of the partially dried droplets of the liquid
bonded together at their points of contact, the nozzle being spaced
from the collecting surface for a particular temperature and drying
capacity of the gas so that the particles are tacky but contain
insufficient liquid to flow together when they impinge upon the
collecting surface and means for removing the mat from the screen.
Description
The present invention relates to an apparatus for drying liquids
containing dissolved and suspended material and more particularly
to an apparatus for drying liquids in the form of a spray entrained
in a gas stream.
In the past several decades the drying of materials has been
carried out on an ever-increasing scale by spraying liquids that
are to be dried into a hot dry stream of gas passing through a
drying chamber and collecting the dried solids. This process,
frequently referred to as "spray drying," has become particularly
widely used in the food industry for drying such products as milk,
cream, instant coffee, cocoa, fruit and vegetable juices, extracts
and flavorings. A variety of other products such as drugs,
detergents, soaps, cosmetics, etc., have also been subjected to the
process in other industries. The equipment now used in this process
has several important disadvantages.
Product accumulation on the walls of the drying chamber is a major
disadvantage. In spite of efforts to prevent this accumulation, the
unintentional deposition of materials on the walls of the chambers
has remained a problem, particularly with products having low
melting points and sensitivity to heat.
Resulting from the accumulation of material is the necessity for
interrupting the drying operation so that a workman can enter the
drying tank and manually scrape the walls. This operation is not
only a time-consuming and unpleasant task but is an expensive
procedure and one which increases the opportunity for contaminating
a product. Moreover, even after the tank has been scraped, the
residual material remaining on the walls of the tank can produce a
sanitation problem and produce off flavors in food products. The
tendency of materials to accumulate has also limited the
application of spray drying in general to those materials which
either have high melting points or in which heat damage can be
tolerated.
Prior spray drying equipment also releases a substantial amount of
dust into the atmosphere or in the alternative requires an
elaborate dust collecting apparatus such as a cyclone or textile
bag filter frequently larger than the drying apparatus itself.
Moreover, the texture of the product produced in conventional spray
drying varies to some extent; that is to say, the product lacks
uniform consistency, some parts being hard, dense and brittle while
others may be fine loose dust. In some areas of the drying chamber
where the product accumulates in corners, very hard or even
crystalline masses can be formed whereas in other areas relatively
loosely packed powdered materials will accumulate.
Still another shortcoming of a conventional spray dryer is the loss
of heat which is experienced. While a portion of the lost heat is
carried away in the particles of solid material escaping from the
dryer, an additional amount is also lost because there is
insufficient contact time to bring about saturation of the drying
atmosphere.
It has been previously proposed in spray drying equipment of the
kind in which more than half of the material collects on the floor
of a drying chamber to employ hanging screens as a lighting place
for spray dried particles. A slowly rotating cam subjects the
screens to periodic shocks which cause the powder to fall from the
screens. These screens are not sealed at either their upper and
lower ends. Consequently air is not forced to pass through the
screens. Equipment of this kind is not suitable for the purposes of
the present invention for several reasons. First, relatively little
drying takes place once the material falls to the floor and becomes
covered by a layer of falling particles. This is particularly
important where the removal of the last bit of removable moisture
may make the difference between commercial success and failure due
to product deterioration. Furthermore, continuous removal of
product from the screens prevents the development of a mat
structure which applicant has found important in achieving high
product dryness and improved drying efficiency. Moreover, there is
no way to force the air through the screen when one or both of its
ends swing up and down along an arc that is at times spaced from
the top or floor of the drying chamber. Consequently, if a
relatively thick bed or mat of particles were to accumulate on one
of hanging screens, most of the air would go around the top or
bottom of the screen due to the air space present, thereby
defeating the objective of maintaining a steady current of air
through the collected mat.
A variety of other drying systems previously proposed have been
either costly or largely ineffective in eliminating the aforesaid
problems and accordingly have not been widely used.
In view of the deficiencies of the prior art, it is the general
object of the invention to provide an improved spray dryer having
the following advantages and characteristics: (a) a reduction in
the loss of material in the form of dust escaping through exhaust
vents; (b) the reduction of hard, difficult-to-remove and
heterogeneous cakes of dried material upon the walls of the drying
chamber; (c) suitability for the production of a porous dried mat
that can be subdivided into pieces of a predetermined size which if
water dispersible will become moistened quickly when added to a
liquid; (d) has increased drying capacity and better thermal
efficiency; (e) is able to produce a dried product of a uniform
consistency; and (f) makes provision for removing the collected mat
from contact with the heated drying gas and exposing it to a
cooling gas until the dried product reaches a chemically or
thermally stable condition.
These and other more detailed and specific objects will be apparent
in view of the following specification and attached drawings
wherein:
FIG. 1 is a flow chart showing the steps performed in a preferred
embodiment of the invention.
FIG. 2 is a diagram illustrating the method employed in accordance
with one embodiment of the invention.
FIG. 3 is a drawing illustrating the appearance of a piece of spray
dried material after being removed from the screen of FIG. 2.
FIG. 4 is a perspective view showing the appearance of another type
of spray dried material after being removed from the screen of FIG.
2.
FIG. 5 is a side elevational view partially broken away of an
apparatus employed for carrying out the present invention.
FIG. 6 is a transverse sectional view taken on line 6--6 of FIG.
5.
FIG. 7 is a greatly enlarged side elevational view of the
collecting screen housing partly broken away.
FIG. 8 is a side elevational view of another embodiment of the
invention partly broken away.
FIG. 9 is a side elevational view partly in section of still
another embodiment of the invention.
FIG. 10 is a side elevational view of yet another embodiment of the
invention.
FIG. 11 is a semi-diagrammatic side elevational view of a modified
form of a dryer in accordance with the invention including a
provision for precoating the collection screen.
FIG. 12 is a modified form of the apparatus in accordance with the
invention including another provision for precoating the collection
screen.
FIG. 13 is a partial side elevational view of the upper portion of
the spray dryer of FIG. 8 showing the modified form of air flow
control means.
FIG. 14 is a semidiagrammatic vertical longitudinal sectional view
of another form of apparatus embodying the invention.
FIG. 15 is a vertical sectional view of another embodiment of the
invention.
FIG. 16 is a sectional view on a reduced scale taken on line 16--16
of FIG. 15.
FIG. 17 is a side elevational view partly broken away of another
embodiment of the invention.
FIG. 18 is a side elevational view partly broken away of still
another embodiment of the invention.
FIG. 19 is a sectional view taken on line 19--19 of FIG. 18.
FIG. 20 is a vertical sectional view of another embodiment of the
invention .
Refer now to FIG. 1. The liquid to be dried contains a solid
material either dissolved or suspended therein, e.g., milk,
vegetable or fruit juices. This liquid is sprayed into a drying
atmosphere composed, for example, of air heated to
200.degree.-400.degree.F. or of air having a relatively low
humidity level or of heated dry air. The moisture present in the
drying atmosphere is substantially below its saturation point. The
term "drying atmosphere" used herein is intended to mean a gas at
such conditions of temperature, pressure and vapor content as to be
capable of absorbing a substantial quantity of liquid from the
fluid to be dried.
In accordance with the invention, the liquid to be dried is sprayed
into an enclosure containing a drying atmosphere and is collected
upon a foraminous element such as a screen. A blower or other
draft-producing device draws the drying atmosphere through the
accumulated mat that forms on the screen and thence through the
screen itself. The pressure differential produced across the screen
in this way results in a steady flow of gas through the screen and
the accumulating mat. This was found highly effective in assuring
product uniformity, improving thermal efficiency and in achieving a
low moisture level in the finished product.
It is important to have the screen placed with respect to the spray
nozzle such that the surfaces of the particles are still in a tacky
condition at the time they strike the mat on the screen but do not
contain sufficient liquid to enable them to coalesce or run
together to form a relatively impermeable continuous material. It
was surprising to find that the tacky condition of the particles at
the time they reached the screen was effective in preventing an
excessive pressure drop across the screen (screen blinding). Thus,
by properly positioning the screen with respect to the sprayer
(under a given set of drying conditions and spray droplet size),
the porosity of the mat is preserved.
By the term "coalesce" as used herein I mean a flowing or fusing
together of the particles to form a relatively solid material in
which the individual particles cannot be clearly recognized without
magnification and wherein the spaces or interstices between them
are relatively small (if present at all) compared with the size of
the particles.
The particles are thus brought into contact with one another while
the surfaces thereof are in a tacky condition to form bonds at the
points of contact between the particles. This produces a relatively
porous lacy reticulum in which a substantial portion of the
agglomerate thus formed consists of communicating spaces or pores
between the contacting and bonded particles.
Once the particles have been deposited on the collection screen, a
sudden change will take place in the relation between the particles
and the surrounding air; the air will begin to stream over and
around the particles and through the minute microporous interstices
between the particles in the agglomerate. As this streaming takes
place, moisture is removed from the particles at a much higher
rate. Removal of the last traces of removable moisture from a
material that is being dried is much slower than during the earlier
drying period. It is commonly referred to as the falling rate
drying period. In accordance with the present invention, after the
movement of the particles has been arrested by deposition on the
bed or screen, a substantial increase in the velocity of the drying
gas relative to the particles is established. This provides a more
effective means for removing moisture during the falling rate
drying period.
In summary, the invention has the following advantages: First,
filtration of air-entrained particles from the drying medium is
accomplished by those particles already deposited in the mat, thus
reducing and in some cases eliminating the need for auxiliary
filters in the stream of exit drying gas. Since this phenomenon in
the course of a drying operation has heretofore been unknown, it
will be referred to as "autofiltration." Second, greater drying
efficiency can be achieved than was heretofore possible. While the
precise reason for this result is not known with certainty, it is
believed to be, in part, due to the accelerated evaporation that
results from the streaming of the drying gas around and through the
particles that make up the mat and, in part, to the increased
driving force across the solid and gaseous interface. Third,
greater product dryness is accomplished with the invention because
of the increased exposure of the material to the drying atmosphere.
Fourth, the invention is capable of reducing the undesired
accumulation of the product on the walls of the dryer. This allows
the invention to be used with products which were heretofore
difficult or impossible to dry by conventional spray drying
techniques or, for that matter, by any previous drying process of
which I am aware. The invention also increases the product quality
because of the reduction of heat damage. Fifth, the product that is
being dried is placed, while in a practically dried state, in a
form which can be transferred from the drying zone to one or more
additional zones in which further treatment may be performed.
Sixth, the invention places the product in a novel physical form (a
porous, self-supporting mat which can be further processed if
desired to provide products having advantageous properties).
Refer now to FIGS. 2, 3 and 4. In FIG. 2 is shown a stationary
enclosure or vessel 20 which in this instance forms a portion of a
horizontally disposed duct. A gaseous suspending medium such as
heated air 22 is forced through the duct from left to right and
through a screen 24 which is supported to extend over the entire
cross section of vessel 20 so that all of the heated air 22 must
pass through it. Upstream of the screen 24 is a nozzle 26 for
producing a spray 28 of the fluid material that is to be dried. The
material being dried collects as a highly porous mat or agglomerate
30.
The thickness of this deposit upon the screen will vary. I have
found that when a vertical stationary screen is employed, the
thickness of the material deposited on the lower end of the screen
is sometimes much greater than at the top. For example, the
thickness of the material deposited in one instance on a vertical
screen was one-half inch thick at the top and 3 inches thick at the
bottom.
The appearance of the agglomerate 30 formed on the screen 24 is
shown in FIGS. 3 and 4. The surface 30a which is positioned
adjacent to the screen 24 bears the configuration of the screen
surface. If the material is relatively tacky at the time it was
deposited, marked striations and channels 30b normal to the surface
30a as shown in FIG. 4 will be present. If the material contains
less moisture or is only slightly tacky, it will have the irregular
surface with no visible striations or markings as shown in FIG.
3.
Process variables are described in detail in the above-noted
patent.
Refer now to FIGS. 5, 6 and 7 which illustrate one form of drying
apparatus in accordance with the invention. As shown in the
figures, there is provided a drying chamber or enclosure 40
including front and rear walls 42 and 44 and side walls 46 which
are generally cylindrical but include a downwardly and centrally
inclined wall portions 46a and 46b connected by a hemispherical
trough 48 within which is mounted an auger 50 driven by means of a
suitable motor 52. The auger 50 when rotated in the proper
direction will convey dried material that falls to the bottom of
the apparatus toward the right as seen in FIG. 5 to a collecting
hopper 54 which is connected by means of a duct 56 to a storage
container 58. The auger 50 is provided for collecting any of a
material which is deposited upon the walls of the vessel 40 and
which thereafter becomes loosened from the walls and falls down the
inclined surfaces 46a and 46b. A portion of the material will be
collected on the foraminous collecting means described below. The
enclosure 40 can be supported in any suitable manner as by means of
tubular supporting frame members 60 connected in any suitable
manner such as by welding to the enclosure 40.
In the wall 44 is provided a drying gas inlet duct 62 which
communicates with a duct 64 that extends vertically and
communicates with the outlet of an air heater 66 of any suitable
known construction such as a gas burner. Air is supplied to the
burner 66 from the atmosphere by means of a blower 68. Positioned
concentrically within the air supply duct 62 is a spray nozzle 70
that communicates through a line 72 with a pump 74 which supplies
the fluid to be dried from a supply tank 76. The fluid material
being dried emerges from the nozzle 70 in the form of a fine spray
78. The sprayed particles are carried in the heated drying gas from
left to right as seen in FIG. 5 toward a collecting device
indicated generally at 80 which will not be described.
The collecting device 80, as shown in FIG. 7, comprises a tapered
duct or funnel 82 and a pair of wire collecting screens 84 and 86
extending across funnel 82 in parallel spaced relationship. The
screens 84 and 86 can be supported in any convenient manner but I
have found that they will be suitably held in position by mounting
screen 86 between the outward end of the funnel 88 and a square
spacer 90 and the screen 84 between the spacer 90 and a short duct
section 92. Bolts or other suitable fasteners can be used to secure
the duct section 92 to the spacer 90 and the spacer 90 in turn to
the outward end of the funnel 82. The entire collecting apparatus
80 is mounted upon a baffle 94 which is supported from the upper
end of the vessel 40 upon brackets 96 and 98.
As shown in FIG. 7, a mat 100 of the type described hereinabove
will form on the outward or central surface of the primary
collecting screen 84. Screen 86 is not essential for proper
operation but is included as a back-up screen for starting. The mat
100 will be highly porous and will contain channels that extend
from the interior of the vessel 40 to the interior of the funnel 82
thereby allowing a substantial portion of the heated drying gas
passing through the inlet 92 to pass through the mat 100 thereby
further drying it after it has been deposited. The portion of the
drying gas which does not pass through the screen will be exhausted
through an outlet 102 (FIG. 5). Generally the exhausted gas passing
through the outlet 102 will have a small amount of particulate
material entrained in it and will consequently be passed through a
suitable filtering mechanism such as a combination of cyclone
collector and textile bag filter. Connected to the outlet end of
the funnel 82 is a duct 104 to which is secured a flow control
valve 106 and an exhaust blower 108 to reduce the pressure within
the duct 104 and funnel 82. The exhaust blower 108 can be driven in
any suitable manner as by means of a motor 110.
The operation of the apparatus of FIGS. 5, 6 and 7 will now be
described. To begin the operation, the heater 66 and blower 68 are
started causing heated air to be introduced into the enclosure 40
through the inlet duct 62. The liquid to be dried is then admitted
through nozzle 70. Simultaneously, the motor 110 is started with
the valve 106 in the "open" position thereby causing a reduced
pressure condition within the line 104 and funnel 82. This, in
turn, causes a current of heated gas to pass through the screens 84
and 86 as clearly shown in FIG. 7. As the operation of the device
continues, a deposit 100 of a porous product will begin to be
produced upon the outward or central surface of the screen 84.
After the deposit 100 is formed, little if any material will be
added to the deposit 100 deposited on the screen 86 since the mat
already formed upon the screen 84 acts as its own filter and
thereby effectively removes all entrained particulate material from
the stream of air passing through screen 84. However, because the
openings in the screen 84 are much larger than the openings between
the finely divided air entrained particles striking screen 84, a
small amount of material will strike the screen 86 when the
operation is started. Thus, as soon as the openings in the screen
84 become covered by the particles of the mat 100, the accretion of
material on the screen 86 will be interrupted. In an apparatus of
the type described in FIGS. 5, 6 and 7 used for drying a cake mix
composition consisting primarily of shortening, flour and sugar, a
screen having 16 openings to the square inch and having more than
70 percent open area was employed as a primary screen and another
as a secondary or backup screen having 10 openings per square inch
was employed. While the primary screen had an accumulation of about
11/2 inches of material, the backup screen had less than one-eighth
inch of material deposited upon it and this material was deposited
during the first 5 seconds of running. When sugar is being dried
with an air flow of 100-400 feet per minute through the dryer and
an air temperature of 320.degree.F. entering the dryer, good
results were obtained by placing the nozzle from about 9 to 10 feet
from the screen. At shorter distances, 5-6 feet, the particles were
wet and tended to coalesce. At greater distances, the particles
were insufficiently tacky and tended to pack or pass through the
screen.
Refer now to FIG. 8 which illustrates another embodiment of the
invention. The apparatus of FIG. 8 comprises an enclosure such as a
vertically disposed chamber 120 communicating through an inlet port
123 with a supply duct 122 and a heater 124 which in turn
communicates through a duct 126 with a damper 128 to which air is
supplied by blower 130. Mounted concentrically within the upper and
of chamber 120 is a nozzle 132 that communicates through a supply
duct 134 with a fluid product which is to be dried. If desired, the
fluid traveling through duct 134 may be aerated or any suitable air
entrained therein to facilitate the drying operation and reducing
the density of the material. The fluid is supplied from a storage
tank 136 under pressure by a pump 138. The liquid sprayed from the
nozzle 132 is initially present in the form of wet droplets 140.
These droplets are entrained in the heated air passing downwardly
from the duct 122. As the particles pass downwardly, they strike a
moving foraminous element or belt 144 entrained over a pair of
horizontally disposed spaced rollers 146 and 148, the former being
driven by means of a belt 150 connected to a speed reducer 152
which is, in turn, operated by means of a belt driven by a motor
154. The foraminous belt 144 can comprise a variety of materials
such as a woven wire screen or perforated metal sheet. The
foraminous member or screen 144 is depicted diagrammatically. It
can comprise a plurality of perforate plates positioned adjacent
and coplanar to one another. A variety of other collecting elements
will be apparent to those skilled in the art.
A mat 160 forms on the upward surface of the foraminous belt 144.
The air flowing into the chamber 120 from the duct 122 passes
through this mat 160 as indicated by arrows into a compartment 162
below the screen and is exhausted through a duct 164. The
foraminous screen 144 is moved so that the upper reach thereof
travels toward the right as seen in FIG. 8 carrying the accumulated
mat on its upward surface into a second chamber 166 through an
opening 168 for purposes hereinafter described. The duct 164 is
connected to an exhaust blower 170 driven by a speed reducer 172
and motor 174. From the blower 170 the air passes through a
suitable damper 176 which like damper 128 can be opened or closed
as conditions of operation require. The chamber 166 communicates
with an inlet duct 180 which is connected to an air cooler 182 of
any suitable known construction. Prior to entering the air cooler
182 the air is passed through a suitable filter 184. On the
opposite side of the screen 144 from the chamber 166 is a chamber
186 which communicates with an outlet duct 190 to a blower 192
driven by means of motor 194 and speed reducer 196. Air passing
through the blower 192 is exhausted through a damper 198. The
foraminous member 144 and the rollers are mounted within a chamber
210. At the lower end of the chamber 210 is provided a rotating
brush 212 for removing any remaining material from the foraminous
belt 144 before the belt returns to the inlet end of the chamber
120. The mat may be removed by any other suitable technique such as
a blast of air. Air can be forced through the apparatus by any
suitable draft producing device connected to the upstream or
downstream end of the drier.
During operation, as the foraminous member 144 travels over the
roller 148, the pieces of the mat fall and pass between a pair of
parallel driven sizing rolls 200 and 201 spaced appropriately to
break the mat into smaller pieces of the desired size. The
resulting pieces pass downwardly into a collecting trough 204. If
desired, a conveyor can be provided below rolls 200, 201 to convey
the broken material laterally. A doctor blade similar to 242 (FIG.
9) can be used ahead of the brush and the screen 144 can then be
passed through a washer of suitable known construction. A suitable
sifter can be provided for separating and grading various desired
particle sizes and exhausting them through a pair of ducts 206 and
208 according to size.
As mentioned above, it is necessary to construct the apparatus with
a predetermined spacing between the nozzle and the mat assuming a
particular set of conditions of temperature and drying capacity of
the gas introduced to the drying chamber. Under a given set of
conditions, if the nozzle 132 is too close to the mat 160, the
droplets will be of such high moisture content that they will
coalesce or flow together to form large drops or pools of moisture.
On the other hand, if the distance between the nozzle and the mat
is too great for a given set of operating circumstances, the
particles, due to their condition of dryness will be insufficiently
tacky to become bonded to one another when they reach the mat 160.
This will cause them to pass through the screen or to pack tightly
together and blind the screen. Accordingly, it should be understood
that the distance of the nozzle from the screen 144 (or mat 160) is
established with respect to the temperature and drying capacity of
the drying gas, so that the particles are sticky upon reaching the
screen.
The upper section of the screen 144 between rolls 146 and 148
defines a collecting surface upon which the mat 160 accumulates and
it should be understood that except for relatively minor amounts of
material which collect upon the inside walls of the apparatus or
manage to pass through the screen and are collected in bag filters,
substantially all of the sprayed solids accumulate as a mat upon
the screen 144. Even when conditions of operation are relatively
unfavorable, with present apparatus at least 92-95 percent of the
spaced solids are deposited in the mat 160. The expression
"substantially all of the solids" are used herein shall mean that
90 percent or above of the total sprayed solids expelled through
the spray nozzle are deposited in the mat.
It is important to note that the foraminous collecting screen 144,
as in the embodiment of FIGS. 2, 5, 6 and 7, extends entirely
across the outlet of the spray drier which communicates with the
exhaust duct 164 and is sealed at all times along its edges to the
outlet so that a pressure differential of at least a half an inch
of water can be developed across the screen by the blower 170. To
this end, the upward edges of the front and rear vertical walls of
the chamber below the top run of the screen and the corresponding
vertical side walls (only one of which is shown in FIG. 8) either
abut against the lower surface of the screen 144 or are positioned
so closely adjacent to it that only insignificant amounts of air
can pass through the gap. By thus sealing the collecting screen
across the outlet 164, air exhausted by the blower 170 is forced to
pass through the mat as the mat begins to build in thickness and
regardless of the build-up, the air will be drawn steadily through
the mat in spite of the increasing pressure drop across it.
Depending upon the conditions of operation, the pressure
differential may be as high as 10 to 15 inches of water but more
typically is from about 1/2 to 5 inches of water but should not be
so high as to crush or collapse the collected mat. In any event, it
has been found that by sealing the edges of the screen at all times
to the housing, it is possible to provide for a steady flow of
drying gas through the collected mat in spite of increases in the
thickness of the mat. The importance of sealing the screen along
all of its edges can be understood better when it is realized that
in the case of a typical mat, if an opening were made between the
upstream and downstream sides of the mat which had an area of about
10 percent of the area of the mat, the air flow through the mat
would drop to about one-fiftieth or so of its previous level.
It should be noted that the mat 160 just as described earlier in
connection with FIGS. 3 and 4 is homogeneous and has no holes in it
other than the pores between the individual dried droplets. In this
way the air is forced to pass through the minute interstices
between the particles themselves which is important in obtaining
the improvements in drying efficiency and product moisture
uniformity that are achieved in the present invention. While the
mat can be as little as about one-quarter of an inch in thickness,
in most commercial applications, it is usually from about 1 inch to
about 6 inches in thickness.
The speed of the drive motor 154 is determined with respect to the
amount of material being sprayed and the cross-sectional area of
the collecting surface so that the particles are able to accumulate
without being subjected to jarring or vibration to a minimum
thickness for practical purposes of at least about a quarter of an
inch. By allowing the particles to accumulate as a mat in an
essentially quiescent condition to a substantial thickness, e.g.,
at least about a quarter of an inch, it is possible for the
particles present in the mat to serve as a collecting medium for
additional particles and at the same time for the mat to take on a
friable self-supporting integral structure. In this way if the
spray rate is increased, other things being equal, the speed of
drive motor 154 should be increased. Similarly, if the area of the
screen within the dryer enclosure 120 is increased, the speed of
the motor 154 should be reduced to assure development of the mat
structure as described herein above to at least about a quarter of
an inch thickness.
It has also been found that a preferred relationship exists between
the exposed area of the screen 144 within the enclosure 120 and the
air flow therethrough in a given period of time. Thus in a typical
commercial sized unit, when the air flow is usually on the order of
20,000 to 30,000 cu. ft./min. the area of the screen exposed to the
air flow should not be less than about 120 sq. ft.
The present invention in addition to achieving a higher drying
efficiency makes possible lower operating temperatures which helps
to preserve the quality of temperature sensitive materials as well
as improving the production capacity of the apparatus for a given
quantity of air. Both improvements appear to result from the
improved contact of the particles with the drying medium which
flows continuously through the minute interstices between them.
It can also be seen by reference again to FIG. 8 that the screen
144 and mat is completely removed from the drying chamber 120
before the mat 160 is itself removed from the screen. Thus the
advance of the mat 160 around the roll 148 can be thought of as a
means operative outside of the enclosure 120 for removing the mat
from the screen after the screen has been withdrawn from
communication with the upstream portion of the enclosure 120. If
this were not the case, because of the substantial pressure drop
across the mat, much of the air would pass through the part of the
screen where the mat had been removed. This would defeat the
objective of achieving a steady flow through the mat.
A slightly modified form of air flow control is shown in FIG. 13.
This flow control consists of a grating 125 composed of a plurality
of closely spaced parallel vertically disposed fins for
establishing a laminar air flow which reduces the turbulence within
the chamber 120 and the consequent nonuniform accumulation of dried
product on the walls of the apparatus. The flow through the
foraminous collecting member 144 is also laminar. It may be seen
that my invention makes possible the provision of a spray drying
apparatus without constrictions in the air flow between the spray
nozzle and the point at which the drying air is withdrawn from the
drying chamber as in some prior devices.
In accordance with another modified form of the invention, an
additional drying agency is employed consisting of a microwave
heater of any well-known commercially available kind. The microwave
heater is suitably mounted to heat that portion of the mat within
either chamber 120 or the chamber 166 of FIG. 8 to assist in
removing residual moisture from the mat. As electromagnetic wave
energy from the microwave heater is transferred to the water
molecules, their increased energy will accelerate evaporation and
as soon as the water is converted to the vapor state it is removed
by the current gas such as air passing through the mat from chamber
166 to chamber 186.
Refer now to FIG. 9 which illustrates another embodiment of the
invention composed of an enclosure or chamber 220 having a top wall
222, side walls 224, 226, 227 and 229, inclined lower wall 228 and
outlet duct 230 which communicates with a star valve 232. A drying
gas inlet duct 234 is provided at the upward end of the chamber.
Within the duct 234 is a spray nozzle 236 that communicates with a
source of fluid to be dried (not shown). Mounted for rotation
within the chamber 220 is a cylindrical collecting drum 236
supported on a shaft 239 which is itself journalled in the walls
227 and 229 and driven by means of a motor (not shown) at a uniform
speed in a clockwise direction as seen in FIG. 9. The cylindrical
wall of the drum 236 comprises a foraminous member 240 which can
consist, for example, of perforated sheet metal. The openings may
suitably be one-fourth inch in diameter. A doctor blade 242 is
mounted within the housing 220 in position to scrape deposited
material 244 which has accumulated on the surface of the drum as
shown. Material thus scraped from the drum falls downwardly in the
form of porous chunks and pieces 246 along the inclined surface 228
into the star valve 232 and are exhausted through the duct 233. On
the upper wall 222 is also provided an exhaust duct 248. Extending
downwardly between the exhaust duct 248 and the inlet duct 234 is a
baffle 250, the lower end of which contacts the upward surface of
the drum 240. In this manner, the air passing through the duct 234
must pass through the accretion 244 to the interior of the drum
before being exhausted thereby drying the material deposited upon
the surface of the drum. The air passing through the drum section
between the doctor blade 242 and the baffle 250 is exhausted
through the duct 248.
In FIG. 10 is shown a drying chamber of vessel 260 including
imperforate side, top and bottom walls. On each of the walls is
mounted a foraminous collecting element each comprising an endless
screen entrained over a pair of horizontally disposed vertically
spaced rolls 262 and 264. On the bottom of the chamber is provided
a similar collection surface comprising an endless screen 266
entrained over horizontally spaced rolls 268. A suitable driving
means is provided for turning each pair of rolls in given feed
directions. At the upper end of the apparatus is an inlet duct 270
for conveying heated air 272 to the upward end of the apparatus. A
spray nozzle 274 is provided at the upward end of the chamber 260.
The fluid to be dried is supplied to nozzle 274 from a storage
container 276 through a duct 278. A metering pump 280 controls the
flow of fluid from the storage container 276 to the nozzle 274.
During operation, the air 272 passing through into the chamber 260
flows downwardly and outwardly through each of the screens 262.
Material expelled from the nozzle 272 accumulates on the inward
surfaces of each of these foraminous collecting members 262. The
solid material collecting on the surface of each of the screens
either falls from the screens where the screens pass over the rolls
or can be removed therefrom by means of a doctor blade (not shown).
After being removed, the material falls downwardly and is
discharged through a duct 282. The drying air is discharged through
a duct 284. The apparatus of FIG. 10 has the advantage that all
exposed walls are covered by one of the foraminous collecting
surfaces. This effectively prevents the walls of the chamber 260
from being covered with cakes of dried material sprayed from the
nozzle 274.
It was found that under some operating circumstances and in the
case of certain materials, there is a tendency for the dried or
semidried particulate material to pass through the portions of the
screen at the point where they enter the apparatus prior to the
formation of a coating on the screen. In accordance with the
modified forms of my invention illustrated in FIGS. 11 and 12, a
means is provided for reliably preventing the passage of dried
particulate material through the screen and out of the apparatus
through the outlet duct which, if allowed to take place, would be
wasteful and would create a dust problem. To prevent this
occurrence, I provide a preliminary coating layer on the upstream
surface of the collection screen prior to passing a drying gas
through the screen and expelling the gas into the atmosphere. This
can be accomplished in various ways. For example, an external
supply of particulate material can be applied to the outside
surface of the screen prior to passage into the drying apparatus.
Alternatively, a portion of the material collected on the screen
can be allowed to remain at all times on the surface of the screen
thereby providing a permanent preliminary coating layer. Two other
alternative means for accomplishing this objective are described in
connection with FIGS. 11 and 12 to which reference will now be
made.
FIG. 11 shows a modified form of the invention including a
provision for coating the collection screen with a predetermined
thickness of the material that is to be dried prior to the passage
of the drying gas through the screen and into the atmosphere. There
is provided a drying enclosure or housing 300 including side, top
and bottom walls, an inlet duct 302 through which a drying gas such
as heated air is introduced and a duct 304 for supplying a moist
material which is to be dried to a nozzle 306 positioned at the
upward end of the housing 300 in the center of the duct 302. The
housing 300 includes horizontally disposed and laterally spaced
openings 308 and 310 through which passes a suitable foraminous
member such as an endless woven foraminous screen 312 entrained
over a pair of horizontally spaced parallel and laterally extending
drive rolls 314 and 316, each suitably journalled for rotation and
driven in given feed directions for advancing the screen through
the housing 300.
Below a portion of the screen 312 within the housing 300 is a
chamber designated 300a that defines an outlet plenum which
receives the drying gas after it is passed through the screen 312.
The drying gas within the chamber 300a is exhausted through a duct
318 connected to the inlet of a suitable blower (not shown). Below
the first portion of the screen 312 entering the slot 308 is a
blocking plate 320 which will ordinarily rest in engagement with
the lower surface of the screen 312 or closely adjacent thereto. A
portion of the plate 320 can extend out of the apparatus through
the slot 308. The plate 320 is preferably slidably mounted within
the slot 308 for movement toward either the left or right as seen
in FIG. 11 so that the area of the plate 320 underlying a portion
of the screen 312 within the housing 300 can be controlled as
desired. A handle 322 is provided for permitting manual adjustments
to be easily made.
During operation of the apparatus illustrated in FIG. 11, the
droplets sprayed from the nozzle 306 will be carried downwardly by
the air passing through duct 302 toward the bottom of the housing.
When the operation is started, the rolls 314 and 316 are turned in
given feed directions thereby advancing the screen 312 toward the
right through the housing. The particles will become tacky as
moisture is removed from them as they travel toward the lower end
of the housing and will form a deposit on the upper surface of the
screen 312. The particles which are either extremely small or have
been dried to the point where they are no longer tacky may
initially pass through the screen 312 and will be exhausted through
duct 318. A deposit will, however, immediately begin to accumulate
upon the upper surface of the screen 312 adjacent the plate 320
since substantially no air will be able to pass through that
portion of the screen. This coating layer is designated 11a in FIG.
11. Because the layer 11a will be formed upon the screen prior to
the time that the drying gas is allowed to flow through the screen,
the loss of particulate material which would otherwise pass through
the screen prior to the buildup of a coating layer is
prevented.
Refer now to FIG. 12 which is similar to FIG. 11 and in which the
same numbers have been used for corresponding parts except for the
differences which will now be explained. As clearly seen in FIG.
12, no blocking plate has been employed. Instead, there is provided
a collection chamber 324 defined by the side and bottom walls of
the housing 300 and by a plate 326 which separates the chamber 324
from the chamber 300a. A duct 327 connects the chamber 324 to the
inlet of a blower 328. The outlet of the blower 328 is, in turn,
connected by means of duct 330 with the chamber 300 thereby
recirculating gas from chamber 324 to the upper chamber of housing
300.
The operation of the apparatus illustrated in FIG. 12 will now be
described. To begin the operation, the rolls 314 and 316 are driven
in given feed directions to convey the upper reach of the screen
312 from left to right as seen in the figures so that it travels
first over the chamber 324 and next over the chamber 300a. The
sprayer 306 is then operated as described hereinabove such that the
droplets are carried downward and partially dried in the stream of
drying gas entering through the inlet 302. The screen 312 is first
contacted by partially dried particles upon entering the chamber
from the left. Any particles which happen to pass through the
screen at this point are recirculated through the duct 326, blower
328 and duct 330. The particles which remain on the screen form an
initial coating layer or deposit designated 12a, the thickness of
which can be controlled by the speed of the screen 312 and the
characteristics of the blower 328 and by other factors which will
be apparent to those skilled in the art.
In this manner, a coating layer of semidried product designated 12a
will be formed upon the upward surface of the collection screen 312
prior to the final deposit of the dried product thereon in the area
designated 12b. In this manner, the product which initially passes
through the screen will be recirculated to the drying chamber
upstream of the collecting screen instead of being lost through the
outlet duct 318.
The formation of the preliminary coating layer as described herein
is highly effective in preventing the loss of dried products from
the dryer. Moreover, the preliminary coating layers were found
surprisingly porous and uniform in consistency.
Refer now to FIG. 14 which illustrates a modified form of the
invention embodying an endless moving collecting element. The
apparatus includes a nozzle 26 connected to a source of liquid to
be dried by a duct 29. The liquid to be dried is expelled as a
spray 31 into a drying chamber 33. At the lower end of the chamber
33 is a pair of laterally spaced horizontally disposed rolls 35
over which is entrained a porous collecting element such as a woven
screen 37. During operation, the heated drying gas enters the
apparatus in the area of the nozzle 26, passes through the screen
37 and is exhausted through a duct 39. The screen is thus
interposed between the air inlet and air outlet of the enclosure.
The screen 37 is preferably driven in a direction of travel
opposite the flow of material through the nozzle, i.e., from right
to left in FIG. 2. It will be seen that the particles containing
the greatest amount of moisture travel the greatest distance in the
air and are therefore deposited furthest from the nozzle 26. They
consequently are subjected to drying for a greater period of time
as they travel toward the outlet of the machine on the screen. A
greater number of relatively small particles will be deposited near
the outlet of the machine and consequently dried for a shorter
period of time.
A variety of measures can be taken to prevent the undesired
depositions of material upon the walls of the dryer. For example,
the walls of the dryer can be coated with a poorly adherent
material sometimes referred to as a release composition. This
composition should, of course, be compatible with the product being
processed and in the event that the product is food, it should have
no toxic characteristics. One suitable material which may be
mentioned by way of example is an acetylated monoglyceride such as
the product sold under the tradename "Myvacet" by the Distillation
Products Industries of New York. In the alternative, a foraminous
or louvered liner can be provided within the drying chamber and a
gas passed from the space between the liner and the outer portion
of the chamber through the pores or louvers as the case may be into
the interior of the dryer thereby preventing suspended particles
from impinging against the walls. Other methods for preventing the
deposition of particles on the wall of the dryer will be apparent
to those skilled in the art.
I have found that in some instances the mat formed in accordance
with the invention contains relatively fine particles which are
collected by autofiltration from the gaseous drying medium as the
medium passes through the agglomerate. These fine particles may be
adhesively bonded to the mat but in some cases owing to their
relatively low moisture content are merely mechanically trapped in
the agglomerate. Moreover, a dry pulverulent material can be
introduced into the drying chamber and intermixed with the
dispersed air entrained liquid particles and collected by
autofiltration on the mat of agglomerated material. The pulverulent
material can have either the same or different compositions from
the liquid particles.
Refer now to FIGS. 15 and 16 which illustrate how the invention can
be applied to modify existing spray drying equipment such as spray
dryers manufactured by the Rogers Company. The dryer of FIGS. 15
and 16 includes a housing 500 formed from sheet metal composed of a
rectangular upper portion 500a and a hopper 500b defining the lower
portion of the housing. The bottom wall of the Rogers dryer which
normally is located at 502 is removed and replaced with the hopper
section 500b. A vertically disposed plate 504 is also placed within
the enclosure 500 between the spray nozzle 506 which is supplied
with liquid to be dried by pump 508 through line 510, and an outlet
plenum 512 which contains vertically disposed bag filters 514 of
suitable known construction. The bag filters 514 are periodically
subjected to agitation by vibrators 516 to shake loose dust that
has collected within them. The separator plate 504 has provided in
its center a collection screen 518 which is subjected to agitation
periodically during operation by a vibrator 521 connected to the
screen by means of a rod 522. The rod is secured to the screen with
retaining discs 523 and 524. A blower 520 forces air into the dryer
through heater 522, duct work 524 and inlet tube 526. Air is
exhausted from the plenum 512 through an exhaust fan 528. At the
lower end of the separator 504 is a gate valve 530 that can be
opened or closed during operation to permit material to fall
downwardly through an outlet duct 532 from either side of the
screen 518.
During operation, a mat 534 will accumulate on the screen 518. The
vibrator 521 will periodically shake portions of this mat 534 to
the bottom of the hopper section 500b. The vibrators 516 will also
shake material from the bag filters 514. In this way, the
relatively large droplets and those that contain a substantial
percentage of moisture will accumulate within the mat 534 and will
be dried by a continuous stream of hot air that passes through the
mat and screen 518. Any relatively dry or small particles that are
not collected upon screen 518 will be collected within the bags
514; and both the material from the bags and from the screen 518
will together pass through the outlet duct 532 and fall into a
receptacle 533.
Refer now to FIG. 17 which shows a modified form of the apparatus
shown in FIGS. 5, 6 and 7. In the embodiment of FIG. 17, the
collection device 80 has been replaced by a vertically disposed
screen 550 which extends across the entire height and width of
dryer 40 to divide it into two chambers which communicate with each
other only through the screen 550. During operation, the mat is
allowed to accumulate on the screen 550 as described hereinabove in
connection with the screen 84. Material which is collected upon the
screen 550 is periodically removed by agitation imparted to the
screen 550 by a vibrator 552 through a connecting rod 554 and
anchoring plates 556 and 558. In a modified form of construction,
vibration is imparted to the screen 550 by means of a vibrator 558a
at the top of the screen. If this modification is made, the
vibrator 552 can be eliminated.
Refer now to FIGS. 18 and 19 which illustrate a modified form of
dryer in which a horizontally disposed circular collection screen
600 is mounted within a cylindrical drying chamber 602 for
collecting material sprayed from a nozzle 604 into a stream of hot
drying gas introduced through duct 606. The screen 600 is provided
with a radially disposed opening 608 (FIG. 19) below which is
mounted an auger 610 and trough 612. A radially disposed scraper
blade 614 supported at its inward end upon a drive shaft 616 is
rotated during operation by a motor 618 connected to the lower end
of the shaft 616. As the blade 14 rotates, the accumulated material
on the upper surface of the screen 600 is scraped off the screen
and allowed to fall into the trough 612 (FIG. 19) where it is
removed by the auger 610. The liquid to be dried is fed from the
supply tank 620 through line 622 to the nozzle by means of a pump
624. The warm air that passes downwardly through the product which
has accumulated upon the screen 600 and the screen itself is
exhausted through an outlet duct 626.
Refer now to FIG. 20 which illustrates one way in which the
invention can be used in a multiple bank dryer of the type
manufactured, for example, by the Rogers Company. To modify an
existing dryer, the bottom wall of the rectangular dryer enclosure
700 (normally located along the line designated 702) is removed and
replaced by a housing 704 containing a parallel horizontally
disposed transversely extending screen supporting rolls 706 and 708
over which is entrained a screen 710. Beneath the upper reach of
screen 710 is a suction plenum 712 directed into compartments
(which serve as a plurality of linearly distributed suction
plenums) by vertical plates 712a that communicate through a
manifold 712 with a suction blower 716. Dampers 717 make it
possible to vary the suction applied to the mat above each
compartment of the suction plenum 712. Hot air is fed into the
dryer through a manifold 718 by a blower 720 which is connected to
an air filter 722. The air passes from the manifold 718 to the
enclosure 700 through inlet ports 724, each of which has located at
its center a spray nozzle 726. The sprayed material collects as a
mat (not shown) on the upper surface of the screen 710 and is
conveyed from left to right in the figures as the rolls 706 and 708
are driven in the feed direction shown. The mat is removed by
suitable doctor blade 730. Bag filters 732 contained within an
exhaust plenum 733 are periodically vibrated by means of vibrator
734. Hot air is drawn from the enclosure 700 through the bag
filters 732 by a blower 736. The general operation is the same as
the embodiment of FIG. 8 except that no cooling gas is forced
through the collected mat.
The invention will be better understood by reference to the
following example which illustrates the preparation of a specific
product in accordance with my invention.
EXAMPLE I
A yellow cake premix was prepared and dried in accordance with the
invention using the apparatus generally similar to that illustrated
in FIGS. 5, 6 and 7. The mix consisted of 54 parts flour, 29 parts
sucrose, 14 parts fat including both shortening and emulsifiers and
3 percent moisture by weight. The dry mix was added to water to
produce a suspension consisting of 44.2 percent solids. Mixing was
carried out in a high-speed mixer of the type known as an Oakes
mixer and air was entrained in the feed stream at a pressure of
3,800 psig. The resulting aerated mixture had a density of 0.85
gms/cc and when entrained air was removed from the mix, the density
was 1.15 gms/cc. The viscosity of the aerated suspension at
125.degree.F. was 1,200 Cp. The resulting suspension was heated and
fed at a pressure of 3,500 psig. to the spray nozzle at which point
the temperature was 124.degree.F. The dry bulb temperature of the
drying gas fed to the dryer was maintained between 315.degree.F.
and 325.degree.F., and the wet bulb temperature was less than
110.degree.F. The air exhausted through the duct 102 (FIG. 5) had a
temperature between about 190.degree.F. and 200.degree.F. The air
exhausted through the duct 102 had a dew point of about
94.degree.F. The amount of entering air estimated at the inlet was
about 2,500 C.F.M. The foraminous collecting screen 84 had an area
of 2 square feet and consisted of a woven stainless steel screen
formed from 0.150 inch round wire of 10 mesh/inch. The distance of
the screen from the nozzle was 9 feet 3 inches.
In the first run, a dry friable highly porous agglomerated mass was
deposited on the collecting screen having a moisture content when
removed of 2.3 percent moisture by weight. By comparison, the
product collected from the interior wall of the dryer had a
moisture content of 2.6 percent by weight. All percentages given
below will be represented in parts by weight.
A second run was conducted at a somewhat reduced exit temperature.
In this run, a product was obtained on the collecting screen having
a moisture content of 3.6 percent. The material collected on the
wall of the dryer had a moisture content of 4.2 percent.
In the third run, a product was obtained on the screen having a
moisture content of 1.4 percent. The corresponding product obtained
on the interior wall of the dryer had a moisture content of 3.6
percent. In each of these three runs the product obtained on the
screen was highly porous, foraminous, and friable agglomerate or
mat consisting of a multiplicity of microscopic spheroidal
particles bonded together at their points of contact and having a
multiplicity of interconnecting passages extending therethrough
from the free surface thereof up to the screen and communicating
through the screen with the portion of the dryer in the opposite
side of the screen. The product was dry to the touch and could
readily be either broken or otherwise subdivided into pieces of the
required size or reduced to a fine powder as required.
* * * * *