Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
104925~7
BACKG~OUND OF THE INVENTION
In many thermal processes, particularly those involv-
~ ing solids treatment or utilization, the amount of moisture
present in the feed has a significant effect on the processing
rate and process control, and, consequently, on the overall
practicality, attractiveness and economics of the process.
For example, most carbonizing furnaces can be operatedat a much higher through-put if the input material, such as wood
waste, is pre-dried before feeding into the unit.
Currently, many processes are under development involv-
ing the processing by pyrolysis and other means of various
waste materials into useful products and useful fuels. Solid
wastes, generally predominantly cellulosic in compositional
character, such as wood chips and the like, sawdust, paper and
other cellulosic goods, agricultural grain, fruit, veyetable and
meat materials, animal manures, municipal wastes, which
advantageously may be shredde~ and/or air classified, and so
forth including mixtures of such materials with one another (or
even other substances such as waste petroleum products and oil
stocks) are prominent amongst the varieties of things of
interest for utilization according to such recovery procedures
and techniques.
In practically all of these processes, the question
of whether or not the waste rnaterial is dry or wet (as regards
water content, of course) plays an important role in the overall
J performance of the system and on the desirability and economics
of the operation. For these reasons and others, the importance
- of drying of waste materials prior to conversion to valuable
recovery products is increasing. At the same time, the drying
~ 30 of waste solids for subsequent recovery and/or efficient and
beneficial disposal thereof relates to the environment and to
energy consumption. These are very important aspects of more
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1049Z57
beneficial and efficient recovery and profitable accountability
of resources.
Many of the problems involved in and with good drying
of waste prior to treatment are well brought forth and illus-
trated in the article entitled "Gasification Of Solid Wastes In
Fixed Beds" by A.C.W. Eggen and Ronald ~raatz appearing as
Publication 74-~A/Pwr-10 in the Transactions Of The American
Society Of Mechanical Engineers. Another interesting presen-
tation (which, in effect, serves to high-light the surprising
and unexpected results of the present invention) is in the
article entitled "Fuel Preparation Systems Using A Rotary Drier"
by Stanley P. Thompson which was presented in Denver, Colorado
during September 3-5, 1975 during a Conference there on Wood
Residues As An Energy Source which was sponsored by and is
available from the Forest Products Research Society. Other
references of interest include U.S. Patents Nos. 403,411; 920,-
050; 1,554,854; 1,642,469; 1,749,451; 2,057,681; 2,242,702;
2,365,890; 2,116,059; 2,981,528; 3,044,182; 3,289,318; 3,303,-
578; 3,475,832; 3,646,689; 3,827,158; and 3,837,271. Additional
are in the overall field appears, inter alia, in U.S. Search
Classes 34, 99, 110 and 432 and International Search Classes
F26b and A231.
Amongst the more salient problems which are most
frequently encountered in high capacity waste material (or
equivalent) driers are: undesirable degradation or decomposi-
.,
; tion of the material being treated; gas and solids pollution
from the drying operation; provision of dried material in an
optimum and most suitable condition for use as the pre-dried
feed stock to subsequent treatment and/or recovery applications
30 and operations; heat transfer achievement leaving something to
be desired as to results and effectiveness; deleteriously and/or
undesirably high temperature levels involved in the procedure;
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1049257
impracticable and cumbersome materials handling requirements and
involvements; and overall efficiency and economy of both
equipment and the operation involved.
FIELD AND OBJECTIVES OF THE INVENTION
The present invention pertains to and resides in the
overall field(s) of an improved, high-capacity and large volume
drier and drying method for reducing and eliminating moisture in
various generally particulate or disintegral materials, parti-
cularly waste materials of this sort and most especially waste
intended for processing and treatment (including chemical under-
goings and transformations) in subsequent thermal steps and
operations such as, but not limited to, pyrolysis.
Amongst the essential and more predominant purposes
and objectives of the invention are to provide a technique,
including both apparatus and procedure therefor, for better and
more attractively and desirably drying waste or other equivalent
-
materials to most beneficially and most sensitively condition
them for subsequent handling and/or treatment and processing,
especially as prepared optimally-dry feed stock for thermal
processes, wherein and whereby there is materially and r.lost
advantageously reduced if not entirely avoided or eliminated
the foregoing alluded-to problems of possible pollution,
moisture-removal ineffectiveness, undesirable heat transfer
efficacies, excessive heat and temperature requirements, diffi-
cult materials handling and inefficiency and/or impractical and
~ exhorbitant apparatus and processing costs and other disadvan-
- tageous and/or detrimental incidences.
Practice of the present invention generally provides
and achieves the indicated novel and heretofore realistically
~ 30 unobtainable results and possibilities, with great attendant
practicality and expanded utility thereabout; all this being
achieved with great ease, simplicity, versatility, economy and
1049Z57
effectiveness; and all predictably according to well-engineered
and basically sound concepts and applications of fundamental
technological postulations.
- The above-indicated and many more and other of the
benefits and advantages in the obtainable by and upon practice
of the present invention are more particularly set forth and
easily evident in the ensuing Specification, taken in conjunc-
tion with the accompanying Drawing (inwhich, insofar as possible,
like reference nurnerals and letters refer to like or analogous
parts and constituents and all the representations are depicted
in more or less generally schematic and/or symbolic and/or
fanciful portrayal with many obviously purposefully simplified
exaggerations or unnecessary replications of parts or elimina-
tion of details non-essential to or especially meaningful for
expostulation of the ru~iments of the invention), wherein:
Figure 1 is an exploded, perspective view of the
essential components of a drier assembly according to the
invention;
Figure 2 is a schematic lay-out view in side elevation
of an associated drier assembly;
Figure 3 is a.schematic cross-sectional view of the
screw conveyor means and trough taken along Line 3-3 in Figure 2;
Figure 4 is a broken-out cross-sectional view of a
plate and screen combination providing both bottom support for
the material being dried in apparatus according to the invention
as well as admission for passage through the material being
- dried of the drying fluid utilized for the purpose in practice
of the invention;
Figure 5, which appears on the same sheet of drawings
~ 30 as Fig. 1, is a fanciful flow-diagram type of view depicting, in
:; side elevation showing, the general processing scheme and
approach for drying material pursuant to the invention; and
104925~
Figure 6 in graphical representation illustrates
typical advantageous results obtained in use of the drying
assembly and method of the invention.
GENERALIZED REVELATION OF THE INVENTION
In basic essence, the present invention and the
findings and discoveries upon which it is based involve a
relatively low temperature, modestly sub-atmospheric pressure
technique in which the wet, disintegral material feed is
conveyed by agitating and forwarding screw conveyor and/or
auger-like handling and transfer means through at least two
distinct and separate, yet interconnected, multiple dryin~ zones~
The material being dried is sequentially subjected to the drying
action and influence of hot gas streams of various temperatures
(the total heat content of which will generally decrease in
sequential zones). The hot gas streams are substantially uni-
formly passed through the material (resulting from appropriaté
adaptation of the bed-forming means containing the material and
the gas flow control means) at a predetermined and optimum
transverse velocity with the outlet temperature of the gas in
the last zone (from which the finally dried material is removed)
being fixed to be most efficient according to the critical
moisture level of the material being treated.
With greater particularity, the salient features and
advantageous characteristics of associated and interacting
components and elements utilized in the distinctive cooperative
apparatus assembly and means combination with and upon which the
associated method steps and procedure are followed in embodi-
ments and practice of the invention are as in the following
amplified delineations:
I. In general, the invention contemplates assembly and use of
a conveyor dryer in which the auger-like or screw conveying
mechanism serves to very thoroughly mix the material while it is
1049Z57
being conveyed. This avoids stratified drying as so frequently
occurs with a batch operation or in such conveyor-type dryers
as belt or vibrating bed conveyors where the material is not
thoroughly mixed while it is being conveyed and dried. This is
of particular importance with combustible type materials, such
as wood and paper wastes. It is of significance that in the
screw-type conveyer dryers employed pursuant to the present
, invention, the mixing can be further improved or increased by
; providing and positioning various types of supplemental agita-
~ 10 tion devices along the length of the screw. A noteworthy and
; meritorlous aspect of operation according to the invention is
that moisture variation from the top to the bottom of the
material in the bed moving along in its drying passage along
the screw length, as a result of the thorough mixing thereby
imparted, results in only small and literally negligible
variations in moisture content measured vertically with bed
depth.
; II. The present conveyor dryer operates at sub-atmospheric
pressure. Drying gases are sucked through the dryer by a fan
; 20 or equivalent fluid suction means located on the off gas side of
the dryer. This particular arrangement offers several advan-
tages over pressurized systems, namely: (a) The physical size
of the fan or fluid moving and withdrawing installation, since
the same is used on the "cold" side of the dryer, is significan-
tly reduced. The volumetric quantity of drying gases is signif-
icantly less than that which is supplied on the "hot" side thus
; allowing a smaller fan to be utilized and one suitable for low
temperature operations. (b) It greatly simplifies and enhances
dryer design from a sealing standpoint. The dryer does not have
to contain a pressurized gas, and small leaks into the dryer are
not detrimental to the dryer's operation. (c) Operations at sub-
atmospheric conditions allows floating (or unlocked or unsealed)
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access doors to be utilized so that quick and efficient pressure
release is available to prevent or avoid damage in cases of
~ explosion and the like.
III. The design of the screw trough in the dryer, as mentioned,
I is altered to reduce the amount of entrained material that
leaves the top of the bed in thedrying gases. One side of the
screw trough is flared out from the base so that in a most
beneficial embodiment the gas flow area is approximately about
three times that of a standard screw trough to reduce the velo-
10 city of the leaving gases. The flaring of one side of the
trough is coupled with the inherent conveying characteristics of
a screw conveyor ! The screw tends to cause the conveyedmaterial
to ride up on one side of the trough considerably higher than on
hl the other side. Consequently, the side that is flared out is
the low side; and the flaring has no effect on the conveying
characteristics of the screw trough arrangement. In addition,
this inherent feeding characteristic of a screw conveyor (i.e.,
the drying material riding high on one side of the trough and
I low on the other side) must be accorded proper account in
designing and positioning the porous portion of the base of the
screw trough so that ~he gases will flow uniformly through the
bed of materials.
IV. The use of a screw conveyor for a dryer most advantageously
enables the dryer to be operated with various bed depths in
different compartments or sections of the dryer. For example, a
very shallow bed can be operated in the first portion of the
dryer by a suitable selection of screw pitch. In another section
of the dryer, the screw pitch can be changed so that material
will run deeper so as to thereby considerably increase the
residence time. These features can be included in the design of
the dryer to give most advantageous operating conditions for any
given instance and to meet particular requirements in practice
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1049257
of the invention; considerin~ in and f~r all of this the type
of material to be dried and the inherent drying chaxacteristics
of the material itself which dictate the conditions under which
optimum drying is to be accomplished. Contemplation must be
¦ made of the fact, however, that a bed which is too deep will not
¦ mix as thoroughly as desired without involving necessity for
¦ additional agitation.
V. The dryer has a porous base in the material handling trough.
The design of this generally foraminous substrate and its
position relative to the bed depth is an important feature in
, practice of the invention. Firstly in this connection, the part
of the base through which the drying gases flow is located to
one side of the dryer trough so that it is centered with respect
; to the bed depth of the material being conveyed down the dryer.
The position is usually better off-center with respect to the
trough, since the material tends to ride up on one side of the
j~ trough and low on the other side. The foraminous base is made
j from a perforated plate with benefically uniformly distributed
~ and patterned holes, slots or other open or protected (as in
¦ 20 forms of expanded metal lathe and the like) apertures of any
suitable opening geometry which cover anywhere from say 25 or so
to 75 or so, most advantageously approximately 40, percent of
the overall total area of the porous base. The plate serves as
both a structural part of the trough and as a means of passing
j the drying gases into the bed. The holes or other openings,
which may be of any desired effective diameter or other opening
size, are ordinarily larger than the particle sizes of much of
i the material being conveyed above the perforated plate.
¦ Ordinarily, actual individual hole sizes (or equivalent open
areas) of between about 1/6th of an inch to about l/lOth of an
inch - advantageously 1/8 or so inch - in diameter are effective
for most materials being handled for drying. The open area for
!
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1049257
flow of the drying gases through these holes or the like is
small, so that the gas velocity is high and prevents the holes
from plugging. This is a self-cleaning feature of dryers
according to the invention. The perforated plate alone ordinar-
..
ily does not provide enough pressure drop to properly distribute
the drying gases along the base of the dryer. To increase the
pressure drop across the foraminous base, a fine mesh screen is
stretched across the outside of the trough area over the perfor-
ated plate. This mesh is sized so that the pressure drop across
10 the base region is generally and for a preponderance of the
materials to be treated in the approximate neighborhood of 1-1/2
to 2 inches of water, whereas the pressure drop across the bed
itself is typically of 1/2 to 1 inch of water. This can of
,i
course be varied to best meet the exigencies of any given
~ situation, largely depending on the characteristics of the mater-
j ial being dried as will be apparent to and readily determinable
3 by those skilled in the art. The gases can be uniformly dis-
tributed along the bottom of the bed of the material being dried.
This means prevents blow-through or channeling in the material.
20 The size of the mesh can be varied so that a desired and most
effective and efficient flow distribution can be produced along
the length of the trough. A great benefit of drying practice in
accordance with the present invention results from the fact that
¦ wire mesh can be cleaned periodically by simple brushing. Over
a long period of time, dust and other foreign material may get
sucked into the hot gas system and lodge on the dryer screen.
The simple servicing procedure is to then brush the screen free
of these particles. Alternatively, resort may be had to a type
¦ of vacuum system to clean the material from the screen. In this
a 30 connection, the material to be cleaned generally collects on the
hot gas side of the trough which is on its outside, this greatly
facilitating the cleaning operation. It is feasible in the
_ g _
. .
104925'7
i practice of the invention t~ use any appropriate type of fine
¦ mesh or other style l~w pressure drop fllter at the base of the
unit and in the inlet gas stream; although replaceable and
easily cleaned filter rigs are most beneficial.
VI. A very unique and most advantageous feature of dryers
according to the invention is the overall drying efficiency
i attained with relatively low temperature and consequently much
easier to handle drying gases. This is accomplished by having
a low bulk temperature of the off-gases from the dryer, while at
the same time drying to a very low moisture content in the outlet
feed. Contrary to initial impression (which offhand seems in-
consistent with equilibrium conSiderations of moisture content,
relative humidity, and temperatures of exiting drying gases),
such a low final moisture content is attained by passing only a
small fraction of the total drying gas flow through the last
. section or zone of the screw conveyor dryer. The temperature at
which the gas is taken off from the last drying zone, so as to
achieve the desired level of dryness in the product, is dictated
by the nature of the material (particularly its water-diffusion
characteristic, since its diffusion rate is limiting when the
material is relatively dry), its water content, the inlet humid-
ity of the drying gas, and the temperature of the material being
dried and of the drying gas (which are assumed to be at equil-
ibrium). For every material of a given moisture content, there
is only one temperature at which it can exist in equilibrium
with a ~as of a given humidity; therefore, if the desired
moisture content is to be attained in that material, the drying
gas must be taken off at a temperature above the equilibrium
value. Assuming, however, adequate flow rates and residence
times, it has been found that highly effective drying to rela-
tive low moisture levels can be achieved even though the tem-
¦ perature of the dryer off-gas is only somewhat higher than the
., --10--
1049Z57
equilibrium temperature discussed above. For a typical wood
waste, in which the final moisture content attained is about
five percent, the gas is usually taken off the top of the bed
in the final zone at approximately 200F. to 250F. This gas is
mixed back with the bulk of the flow coming from the other
~ sections of the dryer. The overall bulk temperature is ordinar-
¦~ ily in the range of only 135F. to 150F. or so. This is
extremely low for a dryer operating with a typically achievable
output moisture of 5 or so percent (and frequently less) mois
ture in the waste, such as wood waste. Also, the moisture
carrying capability of air is a very strong function of temper-
ature. Consequently, the bulk off-gas temperature should
usually be maintained in the region of, say, 120F. to 150F. or
so for highest efficiency. At off-gas temperatures in this
relatively low range, the effect of moisture concentration in
the inlet drying gas will be very significant on the amount of
moisture picked up from the solids per pound of drying gas flow-
ing. The economically keneficial effect of this is to greatly
¦ reduce the losses which would normally be associated with
¦ 20 diluting hot gases with atmospheric air for low temperature
. drying. It is well known that dry~ng gases are often combustion
gases having relatively high moisture content. When mixed with
atmospheric air, the moisture content is, of course, greatly
reduced on a per pound basis; and more moisture can thereby be
picked up as a result of greater air flow. Efficiency is not
j affected significantly by adding air, due to the extremely low
temperatures of the bulk off-gas and the increase of total gas
flow. Were the gases to exit at a temperature in the range of
¦ 300F. to 400F. the effect of mixing and increasing mass flow
as a result would be detrimental to efficiency, because of the
wasted energy content of the off-gases.
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PARTICULARIZED EXEMPLIFICATION OF THE INVENTION
The further features and characteristics of the im-
proved and much more efficient and practicable technique for the
dryer assembly and the drying procedure of the present invention
as well as the way in which the same so nicely achieve(s) and
1 fulfill(s) the aims and objectives of the invention and con-
j tribute to the relevant art are brought out in the Drawing, of
which initial reference is had to Figures l, 2, 3, and 4 thereof
in which a two-stage unit is shown. In this connection and
10 since the same reference numerals are generally used throughout,
the identified Figures are elucidated simultaneously by gener-
alized explanation, as follows, of the reference characters
I associated with each of the thereby-designated parts or elements
;¦ (whensoever set forth in one or more of Figures 1, 2, 3, and 5)
of the depicted assembly as it is to be operated.
Reference DESCRIPTION AND FUNCTION
Numeral Or
! Letter
9. Generally identifies the apparatus assembly.
10. Identifies housing.
20 ll. Designates the supply duct for the hot gases
~, used in the drying process. These gases may be
supplied from:
(a). A suitable combustion stack with ambient
air mixing to generate the desired tem-
. perature;
j (b). A separate combustion burner fired with oil,
gas, or other fuel;
(c). Exhaust from a boiler; or
¦ (d). Any hot process gas stream, including such
things as incinerator gases and the like.
12. Locates the high temperature plenum. This is
positioned on the inlet end of the screw 17 so
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_ .
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Reference DESCRIPTION AND FUNCTION (cont.)
Numeral Or
Letter (cont.)
. that the hottest gases are exposed to the solids
which are highest in moisture.
13. Locates the low temperature plenum which is
positioned on the exit end of the screw 17 so that
lower temperature gases are used in drying the
solids which are already partially dried. This
. serves to reduce smoke and/or haze and greatly
diminishes the possibility of unwanted decomposi- -
tion and fires.
14. This indicates the damper used to distribute the
flow between plenums 12 and 13 to optimize the
drying process.
15. Indicates the damper used to supply additional
ambient air to the hot gases entering plenum 13
to lower the temperature to the desired value.
. In connection with this, good mixing in plenum 13
of the flows through dampers 14 and 15 can be
achieved by baffles (not shown).
16. Generally designates the overall porous ba'se. The
base of the trough is formed of a metal plate 16a
with, as mentioned, the generally regular holes or
equivalent openings 16p drilled or punched so that
it has the desired 40 or other percent opcn area.
~s indicated, l/a inch diameter holes are usually
quite satisfactory for such things as pine bark or
sawdust. The gas flow upward through these holes
¦ prevents clogging by the material. The size and
¦ 30 distribution of the openings 16p can be varied in
j accordance with the type of material, the particle
size, and density of the solids being dried so as
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.
1049Z57
Reference DESCRIPTION AND ~UNCTION (cont~)
l~umeral Or
Letter (cont.)
16. to accomplish this desirable effect. In many
instances, it is desirable to better accommodate
the changing characteristics of the drying mater-
~ ial - during its complete traverse through the
¦ dryer - to have the downstream apertures more or
less slanted or slotted even if those upstream are
circular- This materially helps to avoid clogging
¦ l0 of the material in the porous base in the final
drying zones(s). At least one metal screen 16s is
attached to the bottom of the perforated plate 16a.
This serves to produce a pressure drop approxim-
ately as high, and preferably higher, across the
base as across the bed of solids being conveyed
through the trough 24 by means of the advancing
screw 17 in the screw. This prevents channelling
and assures a good distribution of gas flow
~ through the material in the screw. The pressure
¦ 20 drop through the bed of solids in the screw de-
creases with decreasing moisture content, which
can cause a flow variation with axial position i~
the "base" pressure drop is too low. Usually,
I this flow variation is small. However, it varies
¦ with particle size, moisture, density, bed depth,
etc. It is preferable to make the "base" pressure
drop high so that the effects of changes in feed
are minimized and channelling is avoided.
17. This identifies the screw that is used to convey
i 30 the solids through the dryer 9. Residence time
of the material in the dryer is determined by the
rate of rotation of the screw. Usually, the
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1049Z57
Reference
i Numeral Or
Letter(cont.) DESCRIPTION AND FUNCTION (cont.)
17. clearance between screw tips and trough should be
on the order of 1/2 inch or so; this, or course,
` depending on particle size being handled. Bed
¦ depth is controlled by the input feed rate and
rotation rate of the screw 17. AS illustrated in
Figure 2, one or more portions 17f of the screw
may have varied flight pitch to alter the movement
and passage of the material in portions of its
travel through the dryer 9. The screw also serves
to continually stir and mix the material so that
the stratified drying problems which occur when
¦ using a deep bed are avoided. These problems, as
has been noted, include complete drying low in the
bed with attendant decrease in drying rate,
increase in fire hazard and smoke, and a conse-
quent limitation on inlet temperature as the lower
portion of the bed dries. Along this line, it is
understood in the art that a so-called "deep" bed
is any bed wherein the physical and/or chemical
composition of the transitting drying gas is sub-
stantially and/or appreciably changed upon passage,
regardless of actual bed depth dimensions.
i All this has bearing on heat transfer rates; and
property variations in the drying gases can occur
. in a "deep" bed of any thickness appropriate to
cause the phenomenon even when relatively shallow
material layers are being treated. In dryers
according to the invention, the solids in any
short section taken along the axis of the screw
have a relatively constant moisture content, i.e.,
,.
10492S7 ----
~eference
Numeral Or
Letter(cont.) DESCRIPTION AND FUNCTION (cont.)
. 17. the moisture content does not vary appreciably in
the vertical direction at a fixed axial position.
It must be noted that proper agitation is the best
means to avoid stratification, and different
j drive ratios imparted to discontinuously connected
segments of a given overall flight pitch screw can
also be utilized to make desired regional varia-
tions in residence time and/or agitation intensity
along the length of the screw. It should be
observed that when the bed depth in the trough is
about half full or less a screw is sufficient and
usually desirable for accomplishing the desired
conveying and mixing.
18. These are plenums to aid in keeping the flow from
and the bed relatively uniform and at a
19. low velocity to reduce particulate entrainment.
¦ 20. Represents the duct for connecting plenums 18
and 19 to the exhaust fan 21.
21. Designates an exhaust fan or equivalent fluid
suction means which is connected here so that the
entire dryer 9 operates at below atmospheric
pressure. This allows cooling air to be admitted
by damper control. Other advantages of having the
fan 21 on the downstream, or "cool" side of the
` dryer are the reduced fan size and operating temp-
erature requirements. The cubic feet per minute
("cfm") of gas on the "hot" side in Zone "A" will
be much higher due to the high temperature there
encountered as compared in Zone "B" to the "cool"
side. This is the case even when the additional
-16-
.
10492S7
Reference DESCRIpTION AND FUNCTION (cont.)
Numeral Or
I Letter (cont.)
21. moisture is added to the gas stream on and from
drying. Thus, a smaller fan or equivalent can be
used; and it can be one that does not have to
~ withstand the higher temperatures on the inlet
j side of the dryer.
22. Identifies air lock or seal on the inlet feed to
the dryer, which feed is shown by the arrow
¦ 10 designated "FI".
23. Identifies air lock or seal on the exit feed from
the dryer, which feed is shown by the arrow desig-
y nated "FX".
24. With particular reference to Figure 3, represents
the cross-section of the screw trough. The trough
24 is inclined on one side 24s as shown. This
3 reduces the velocity of the gases coming off the
3 bed of solids, designated by reference letter "B",
in the screw 17 which consequently reduces the
entrainment of particulate matter. The inclined
disposition of trough side 24s may be between
about 30 and 60 from the vertical, although a
slope of about 45 or so is generally desirable
and utilized. Usually the drying gas velocities
are thus reduced to a few feet/second at the top
Ç of the trough. The requirements for this are
determined by the material being dried, the part-
¦ icle size and density, and the flow rate of drying
¦ gases. Since a screw inherently lifts the material
to one side as it pushes the material forward, it
should be operated so that the material being
dried "rides up" on the vertical wall and not on
! -17-
1049Z57
Reference DESCRIPTION AND FUNCTION (cont,)
Numeral Or
Letter(cont.)
24. the inclined wall. The lower the bed depth, the
more pronounced will this become.
j 25. Identifies a moisture-laden gas outlet damper.
! This used to increase or decrease the total flow
of drying gases through the bed of material in the
screw 17.
26. Designates a drive pulley which provides means for
¦ 10 turning the dryer screw 17.
27. r~ith particular reference to Figure 1, a hanger
bearing is identified for supporting and coupling
the screw 17 which can be made up in two sections
respectively extending through Zones "A" and "B".
Directional arrows "M" (shown in Figure 1) illustrate
the direction of movement in the screw of the material passing
through trough 21. Arrows "HA" and "HZ" in Figures 2 and 5
illustrate hot gas inlet flow in Zones A and Z, respectively, of
dryer 9; while the analogy applys to the flow of exit gases
illustrated by arrows "CA" and "CZ". Arrows "OG", also in
- Figures 4 and 5, point out the off-gas flow in duct 20 to the
fan or other suction device to the outlet; it being frequently
desirable before venting or other diversion for use to pass the
cool off-gas stream through a cyclone or other suitable particle
collector and/or gas cleaner or scxubber 29.
Figure 4 particularly illustrates an advantageous
construction for the porous base 16. As shown, the porous base
is comprised of the foraminous metal plate 16a which is covered
on the bottom by two conforming screens 16s separated by a glass
fiber cloth or the like 16f. Advantageously, the screen mater-
ial is stainless steel of the 40 mesh size; although coarser or
finer mesh sizes may also be employed for the screen over a
!
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1049257
general 10-20 mesh range encompassing the 40 mesh construction.
Asbestcs and other woven and nonwoven cloth constructions may be
utilized for the interscreen separater layer 16s. It should go
without mention that, so long as a sufficiently high gas inlet
pressure drop is achieved, metal screens alone or those separ-
ated by mechanical means other than cloth separators can also be
~ utilized.
q There is no absolute necessity for the screen underlayto the foraminous plate 16a in the porous base unit 16, despite
the great advantage in utilization of same. The screen and its
particular mesh size hàs an obviously significant influence on
the inletting pressure drop imparted to the hot drying gas feed
during its entry into the trough. Likewise, the separating
layer and/or employment of a double screen may also be eliminat-
ed, even though the savings in so doing may not compensate for
j the benefits of a complete construction.
While the porous base unit 16 may extend the complete
distance of the underside portion of the trough 24, it is
generally more desirable for the base to be in more than a
i 20 single separate porous base section constituting the trough or
screw-confining vessel through which the material being dried
is advanced~ Thus, two and even more porous base sections can
be provided to constitute the bottom of trough 24 in each drying
zone or gas flow region of dryer 9. This allows for easier
handling of same.
Most advantageously, each porous base unit is readily-
detachably affixed, or at least hingedly and swingably mounted,
¦ in the bottom portion of trough 24 over longitudinaly extending
¦ rectangular openings 24a. Quick opening couplers and/or non-
J 30 permanent-type fastening means such as screws 16h bayonet-
fittings, dog-like clips or latches and other snap fasteners may
¦ be well utilized to mount detachable porous base units on the
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s
.: .
1049257
perforated plate bottom of the trough, especially when screens
I are involved. This greatly ameliorates interior accessibility
J to the dryer and substantially facilitates unit maintenance
3 including porous base cleaning and/or replacement. It also and
with at least as great benefit allows ready change of the screen
mesh size utilized in a given porous base so as to accommodate
desired inlet gas pressure drop(s) under different hot gas
feedin~ conditions and when changes are made in the material
being" ried in and with a given apparatus assembly and corres-
ponding method procedure according to the invention. In this
i connection, detatchable porous base units further allow for the
~' possibility, frequently of great advantage,,to utilize different
'¦ gas-pressure-drop-controlling screen mesh sizes for processing
..,
, the same material through different sequential drying zones or
regions (or parts of same) in the dryer.
Although placement in a centered position on the under-
~' side of the trough is tolerable, it is usually more desirable
, and beneficial for the porous base 16 to be offset from the ver-
~ tical centerline of trough 24 by about 30 to 60, most advan-
,~ ~ 20 tageously 45 or so, from the vertical centerline in a direction
~ upwards and away from the general exact underside opposite the
,j:,
sloped or inclined side wall 24s of the trough or material bed
container 24. Such off-centered porous base situation gives
between gaspassage through the material bed to most nicely
balance an even dehumidifying fluid flow through the drying mass
given the bed configuration imparted thereto as a consequence
of influence by and from the sloped through side wall.
The off-centered or cocked disposition of the porous
base, a most significant feature in preferred embodiments of
the invention, also minimizes the necessity to have excessively
deep bed levels and fillings of the material being processed
through the dryer in order to approach utmost heat transfer and
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~ .. , ~
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1049257
moisture removal efficiencies. Greater trough filling does
tend to avoid the possibility of encountering areas in the bed
of relatively sparse material volume; but overfilling ordinarily
sacrifices optimum mixing and causes unevenness and stratifica-
tion, giving rise to problems in good treatment of the drying
j material in the bed. This is usually much more readily achieved
j when shallower bed depths are utilized; although in the overall
uniformity of bed depth is also very important in order to best
achieve all desired influences and results.
In this connection and as is schematically portrayed
in Figure 3, the depth of material bed "B" should advantageously
¦ be not more than about 2/3, and more advantageously less than
~¦ 1/2 or so, of the effective total action height (such as diame-
ter of the flights of screw 17) of the mechanical means utilized
to agitate and forward the bed "B" through dryer 9. It is
very advantageous, along this line, to so combine the pitch or
slope angulation of the inclined wall 24s in the trough 24 with
_ the particular offset positioning of the porous base 16 in
trough 24 so that the upper surface contour of bed "B", at any
given combination(s) of drying gas velocity and vigor of mixing
~ agitation plus speed of drying material advance per particular
¢ involved volume for involved unit residence time relationships,is such that the surface at least roughly lies and terminates in
l a generalapproximate plane that is more or less normally (i.e.,
j right angularly) disposed to the slope angle of inclined wall
24s of trough 24.
Although in most instances it will be highly advanta-
geous to have only one of the side walls of trough 24 set in an
inclined or flared disposition, it is possible to have both
trough walls so sloped. Opening-up relief of both sides of the
trough will have an influence on entrainment by the hot drying
fluid of the drying material in the bed "B"; but such construc-
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~049257
tion also unfortunately and somewhat disadvantageously tends to
cause occurrence of "dead" spaces in the bed due to the conse-
quence excessive spacing around too much of the screw periphery
between the circumference of a large portion of the screw
flights and the adjacent inner trough surfaces; this being
particularly pronounced because of gradual upward spacing
increase between the lower semi-circle cross-section of the
screw as its contiguity with trough wall progressively decreases
j upwardly with respect to the outwardly flared trough wall(s).
Needless to mention, for most efficient material forwarding
action the spacing between screw flight periphery and inner
trough surface should be as close as possible and practicable
(even as little as 1/4 inch or so) in order to effectively move
the drying material in the screw and without unnecessary scrap-
ing through the trough; taking into account in the mean or
J average disintegral particle or individual chunk or piece size
of the material being dried. Naturally, all of this does depend
on involved particle size; it being obvious that larger particles
may need bigger conveyor means for best results. In actuality,
a near scraping relative spacing is oftentimes most advantageous
to utilize for the purpose.
The present invention lends many unprecedented and
heretofore unattainable benefits, advantages and desiderata to
material drying means and operations. In this, the fundamental
requisites to be achieved are to heat the material to be dried
and supply a properly conditioned gas which will carry away the
moisture evaporated, so as to most efficiently accomplish the
wanted drying. In this, most efficient and effective heat
¦ transfer becomes a controlling factor, especially when desirably
~ 30 relatively low drying temperatures are involved. For the
3 purpose, one needs to supply a properly conditioned drying gas
which will effectively carry the moisture from the material
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... .
1049Z57
being treated, Greater specific heat content of the gas with
minimized saturation levels thereof obviously facilitates
dryïng. This, as is readily appreciable, provides the most
facile means of eliminating moisture from and most properly
conditioning the material being treated. Practice of the pres-
! ' ent invention, as is evident in and from the foregoing, gives
pragmatic realization of this.
By way of some amplification of the above, it is
rudimentary that at any given off-gas temperature, the off-gas
¦ 10 can only have a certain maximum moisture content. Also, the
higher the moisture content of the material being dried the
higher the temperature which can be utilized in the drying gas
while, at the same time, minimizing problems of stratification
and/or fire and explosion hazards. Following of the present
contribution to the art permits utilization of varying temper-
~ atured inlet drying gases in the sequential drying zones
j employed enabling the temperature of the gas supplied to each
!~ zone to be tailored to the characteristics of the material
therein. While equal quantities of moisture could be removed
by use of a smaller volume, single stream of gas at a higher
temperature, the latter has an inordinate propensity for and
likelihood of heat loss, not to mention possibility of undesir-
able deterioration or decomposition of the drying material.
The instant invention materially avoids or at least vastly
diminishes these disadvantageous factors. A relatively low
3 temperature operation is always involved in practice of the
invention so that heat loss consequences are relatively trivial.
The efficient and effective high volume drying fluid throughout
achieved in embodiments of and by practice in accordance with
the present invention and all the enumerated factors and asso-
ciations constituting and giving rise thereto permits the
! very desirable, albeit most efficacious and easily obtainable,
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1049Z57
low temperature conduction of material drying pursuant to and
characterizing of this inVention.
In further substantiation of the foregoing and with
particular reference again back to Figure 5 of the Drawing, a
very typical drying operation with a mixed pine bark and sawdust
material practiced in accordance with the present invention will
¦ utilize:
.
(i) In Zone "A", hot inlet air (or equivalent gas) "HA" of
relatively low humidity is fed in at a temperature for drying of
¦ 10 only between about 550F. and about l,000F.; and amost satur-
ated outlet gas "CA" at an approximate 120F.;
(ii) In Zone "B", hot "dry" inlet air "HZ" of relatively low
humidity fed in at a temperature less than that of Zone A and
between about 250F. and 550F.; and relatively dry outlet gas
at about 200F.; and
(iii) A total off-gas in stream "OG" output through duct 20
drawn out of the assembly at a mean, and obviously quite cool,
temperature of only about 140F.
As appears in the foregoing, the present invention
provides many associated advantages and benefits in the tech-
- nology of drying waste materials for subsequent utilization.
For most desirable benefits in adaptations of the invention, as
is herein evident, relatively hot drying gases are utilized in
preliminary zones or stages to remove significant portions of
the moisture in the waste product being pre-dried. In later
stages (although, as explained, variations depending on involved
conditions may be effectuated), the temperature is usually lower
to avoid excessive smoking and/or actual decomposition of the
drier material as it is obtained in the final zone(s) of the
operation. Gas flow through the materials being dried is so
controlled as to substantially, if not entirely, eliminate
channeling.
- -24-
1049257
Embodiments of apparatus according to the present
invention provide siqnif~cant capital investment advantages;
these frequently being to an extent of reduction by as much as
half or more of the cost of that required for the heretofore
known and conventional waste driers. A good part of the reason
for this is that, in practice of the present invention, a
J relatively low temperature drying assembly is taken advantage ofwhich can, in actuality and for practical purposes, utilize
available waste heat at relatively low temperature levels (for
example, in the 300-500F. input range) while, at the same
time, being capable of using only slightly modified standard
conveyor systems (such as rotary drums, rotary screens, drag
chains, mesh belt conveyers) and small fan or equivalent
suction devices for drawing the drying gases through and out of
the assembly.
Equipment according to and the method of the invention
can readily handle as much as 8,000 or more lbs./hr. of waste
containing as much as 50~ water, yielding, as dried product,
¦ 4,000 lbs. or more per hour of 5~ moisture content waste.
Typical (and overall expectable without limitations
thereon) screw diameter, length, trough sizes, screw speeds
(including seg~ental variations) etc. include apparatus as long
as 25 or more feet with a screw diameter of about 2 feet.
Anticipatable material volume/unit time figures for
different operable types of materials include slower drying
garbage taken from about 25~ to about 10% (wet basis) moisture
content and wood chips dried to about 3-10% (wet basis) moisture,
both with residence times of 15 to 20 minutes or so. Some
materials such as pine bark can be dried to about 5% (wet basis)
in as little as 5 or so minutes.
3 In a dryer having 2 drying zones typically approximat-
ely 75%-80% of the drying gas goes through the first zone and
-25-
104~ZS7
the remainder through the second zone.
The usual ratio of porous base area to trough semi-
half bottom is about 1/2 to 3/4. This, of course, depends to
some extent on the particular material being handled and its
j precise particle size, density and initial moisture content.
I Overall and zone-by-zone operation can utilize as many
¦ as 3 or 4 (or even more) zones, particularly if it is desired
. , .
to operate at higher temperatures while avoiding smoking
problems.
In general, moisture removal expectabilities are such
- that about 1/3 to 1/2 lb. of water per minute per pound of dry
;~ solid can be realized, except in the final zone where critical
moisture content is encountered causing significant reduction in
j the drying rate. Expectable dryness (as to precise moisture
content) of dried product is usually in the 5% or better range
for wood and the like waste and 8-10% for garbage.
Except for the screens, mild carbon steel materials of
construction can be employed for the apparatus of the invention.
Parallel trough arrangements can be made to increase
f 20 capacity.
PRACTICAL ILLUSTRATION OF THE INVENTION
¦ Using a two zone apparatus of the type illustrated
in the Drawing, an equal weight mixture of pine bark and saw-
¦ dust containing about 47% water (wet basis) was processed with a
1/2 flight pitch screw of 2 ft. diameter and 25ft. length
operated at 5 RPM through both zones. The output volume of
dried material (having about 5% moisture) was 4,000 lbs./hr.
The rate of drying gas flowing into the dryer was about 24,000
cfm at about 550F. proportioned in a ratio of about 3:2 as
between the first and second zones. The excellent results
obtained are set forth and discernible in the graphical repres-
', entation made in Figure 6 of the Drawing. The Figure also shows
-26-
~04925~
the excellent degree of homogeniety which is attainable in the
I apparatus due primarily to the highly effective mixing action
! of the screw.
Analogous good results are obtainable with other
~' materials and under varied operating conditions within the
explained parameters of practice in accordance with the present
invention.
~ Many changes ànd modifications can readily be made and
1 adapted in embodiments in accordance with the present invention
¦ 10 without substantially departing from its apparent and intended
spirit and scope, all in pursuance and accordance with same as
it is set forth and defined in the hereto appended Claims.
In any event and as is apparent in and from the fore-
going Specification and by virtue of any objective appraisal
and realization of the intrinsic great value of and most merit-
~ orious moment associated with the instant contribution to the
i art, the present invention obviously fulfills a pressing and
, heretofore unsatisfied need and provides many attractive and
. very desirable opportunities and possibilities in and for very
many major uses and requirements for relatively low cost andextremely efficient and very adaptable material drying opera-
tions; this bearing particular significance and importance in
` view of currently impending fuel and energy problems that must
¦ necessarily be faced and hopefully satisfactorily resolved.
I
1 .
,,
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