US2132656A

US2132656A - Flash drying control

Info

Publication number: US2132656A
Application number: US60456A
Authority: US
Inventors: Alfred R Smith
Original assignee: Combustion Engineering Inc
Current assignee: Combustion Engineering Inc
Priority date: 1936-01-23
Filing date: 1936-01-23
Publication date: 1938-10-11
Anticipated expiration: 1955-10-11

Description

A. n'. SMITH 2,132,656

FLASH DRYING CONTROL oct. 11', 193s.

Filed Jan; 25, 195s 2 sneets-sheet 1 u 40h WiL-CIE FURNACE "8 5 j ,Tf 5

BY @fw ATTORNEY Oct. 1l, 1938. A, R 5M|TH 2,132,656

FLASH DRYING CONTROL Filed Jan. 23, 1936 2 Sheets-Sheet 2 ma d 'mz C mi l t. t; t5 t, 1,-:

INVENTOR ALF/n50 R. SM//v,

ATTORNEY Vimitated oci. 11, ,193s

V UNITED ysrafrlas d t c I,2,132.656.

PATENT oEFics-f c FLASH DRYINGcoN'rRoL c c l Alfred R. Smith,fForest HillsQrLong* Island, N.

assignor to Combustion Y' v Inc., New York, N.Y.

Engineering Company, I

ApplicationJanuary `2.3, 1936i Serial No. 60,456 f This invention relates to improvements in apparatus fondrying materials by `the so-called ,flash-drying method. Inl'this `method,-the' Wet material in` comminuted i form lis intimately mixed withV anhotjgaseous drying'` medium and 4is .iaintainedin suspension :until it is'dified" to the desired point. The yprocess may be used' not only when materials are merely dried without changing any ofy their other characteristicsfbut also where one or another of such otherfcharacteristics is to be changed,fas for example,"in` the reduction or removal' of water of crystallization, in. distillation or in sublimation.` f

It is important in the flash/drying process, as

distinguished from the usual drying process such as vby means of a rotary drierfthatv all thema-k terial particles be maintained in suspension and agitation during the process,` thereby subjecting themfto continuous" contact with the drying medium,` which assuresto all'particles `a substantially equal land uniform rate of drying. The accumulation of materialk Whichmay occur within the usual forms of 'driersjis thereby avoided and c therewith the danger of Aoverheating particles' that linger within thedrier longer than the time required for their drying.

Itis also important for rapid or viiash drying to have the particle size small for 'various reasons, one being that a relatively large surface of material is thus exposed to the drying medium. The ner the particle size the greater will be the ratio of particle surface area ,to weight of particle and hence weight of moisture.` Another reason for having small particle sizeis to assure a substantially uniform temperature throughout the particle during drying, for should the rate of surface drying be too `fast this willjtend to overheat and scorchthe surface, and this danger is greater if the particles are large.

40 In` accordance with the .presentinvention the temperature Aof the material being dried'lby 'suchrangement shown in Fig. 1;l

control is present `of the Atemperature* of all the particles.V I A f "An object ofthe invention then istoy provide in adrying process employing a high temperature` drying medium, means for'maintaining aV subcstantiallyuniforrn'temperature of all particles ofthev materi'albeing dried and to maintain said particles temperature at a predeterminedvalue throughout the drying periodi.` How the foregoing together with such other objectsand advantagesas may hereinafterappear,

or are incident to my invention, are realized,- is

illustrated in preferredform in the accoiripanx'rirff*A drawings, wherein: I

Fig. 111s a diagrammatic illustration of a flash dryingsystem embodying the invention; I

Figures2 and 3V show modifications of the ar- Fig.'4 is a partial reproduction` 0f tionalI-ps'ychrometricchart. l

Fig. 5, shows one control means;

Fig. 6 showswanother control means.

vThe wet material' to be dried may be admitted throughy conduitY IIJ into a feeder Ilwhere-it is preferably mixed ywith iii-predetermined amount of dried material from the surge bin I2'and thence delivered tothe drierJI3. If a pressure difference exists between the mixer and drier a seal or pressure lock I4 may be provided. On arriving `in drier I3 the material is caught by the current of drying medium -entering through conduit I5 and maintained in flotation' through the drier and `thence delivered by fan lIE to separator-Silvia conduit I8, in-which separatorthe material and drying medium are separated. The Vmaterialis delivered to the surge bin` I2 for use'in` mixing it with the raw material in mixer II,f\orffor -delivery ofthe finished product'through spout 20. A seal2| may be provided in the feeder line from the separator to thebin yin casea difference of pressure exists between thetwo.- g I On` `leaving the separator, the separated drying m-ediumis'in controlled part conducted -tothe drier I3 through conduit I9,- I5 and in'controlled part conducted to condenser 22. That'part of the drying medium led to' c0ndenser22'has substanaconventially all of its moisture removed by condensation and the remaininggas is rdeodorized as indicated at 23, by lsuitable means such as by dilution; by adsorption by activated materials such'as carbon, bone black or by other means; The condensate may be disposed to waste or to the recovery of certain ingredients l'if desired. VBefore entering the drier I3, the recirculated dryingmedium merges with'vcontrolled amounts ofcold gases through inlet 24 and/or hot gases from furnace 25, through conduit 26. The drier I3 is of such construction as to maintain the material in sus- Ipension and agitation throughout the drying process and may be of the spray type, an impact type of pulverizer mill or an agtitator as shown in a copending patent application Ser. No. 720,417, now Patent No. 2,033,757, dated March v10, 1936. The theory underlying the invention is that a moisture carrying particle which is to be dried resembles a wet-bulb thermometer and therefore that when dried' in a chamber by hot gas or heated vapor introduced thereinto the temperature of the particle corresponds with the wet bulb temperature of the drying medium. These 'observations have led to the further discovery that by maintaining a constant wet-bulb temperature in a drying apparatus the heat of the dryingmedium may be utilized to absorb moisture from the particle while the latter is protected by maintaining it at a constant Wet-,bulb temperature.

It is known that when moisture in the form of a spray or associated with solid particles is introduced into a chamber containing airand superheated moisture the `wet-bulb temperature of the air-vapor mixture remains constant. During the time that moisture is being introducedY (up to thesaturation point which is ne`ver reached in practical operation) the dry-bulb temperature is dropping. This drop in temperature represents sensible heat of the `air-vapor mixture passing into latent lheat of vaporization for the water being introduced; Since there is substantially no addition ,or withdrawal of heat inA this process, the total heat remains-substantially the same and the wet-bulb temperature remains substantially constant. Y

` The wet-bulb temperature is a measure of the temperature lof the liquid within a wick surrounding a thermometer some of which liquid is evaporatng to the surrounding air-vapor mixture. Similarly a wet particle being dried within the-drier is substantially at or near the wet-bulb temperature. Ifthen the wet-bulb temperature of the drying medium is kept at a predetermined point, the materials temperature will be kept at substantially the same temperature and the material will thereby be protected from over-heating. It is wellto note that wet bulb temperatures may be measured at dry bulb temperatures well above theboiling point of the moisture being evaporated whereas. relative humidity is non-existent at temperatures above the boiling point and consequentlyvwould'not be indicated by a hygrostat or similar device.

One may use various drying media -such as dry gas, a mixture of gas and Water vapor or superheated steam or other vapor alone. When using a mixture of gas and water-vapor, the psychrometric laws mentioned apply and the tempera ture of the material vto be dried Ymay be controlled bythe wet-bulb temperature which is a measure of the temperature of the material. When using a superheated vapor alone, the temperature of the material to be dried may be controlledby the vapor temperature corresponding to the vapor pressure maintained during the drying period within the drier, this being substantially the nal temperature of thematerial after being dried to the desired degree. Should for example atmospheric pressure prevail within .the drier, the wet-,bulb temperature of the vapor will he 212 deg. F. and as long as a residual of moisturf: if; present in the material to be dried, its temperature will remain substantially at 2l2deg.

F. In the use of either a gas and vapor mixture or superheated vapor for a drying medium, the wet-bulb temperature, which is one equivalent of the nal dried materials temperature, serves according to the invention as a control means for protection against overdrying and possible injury to the material.

According to the invention, means responsive to the wet-bulb temperature of the' drying circuit are provided for controlling the heat input to the drier. Such means may be placed in Fig. l at locations such as A, B or C for if substantially no further heat is added or abstracted beyond A in the direction of flow, the wet-bulb temperature will be substantially the same for the purpose of the control, en route to and at these various locations. The means may be such as shown in Fig. 5 in whichl gask is withdrawn from the circuit through a connection 30 at one of the mentioned locations, preferably passed through a filter 3| and then over an element enveloped in a wetted wick, the expansion and contraction of the element being responsive to variations in the wetbulby temperature and actuating damper control means. For example, a bellows 32, enveloped by wick 33 which dips into a liquid in a reservoir34 and maintained wet thereby, is provided with stem 35, which with rise and fall of the bellows, may contact one or thegother of the

terminals

36, 31 of a switchlcontrollng a.` reversable motor 33. Motor 38 maybe connected by any suitable means to act to open or closea'damper or dampers in the conduits of the drying circuit. d

Referring toA Figurel, the numerals 40, 4|, 42 represent control valves or dampers; damper 40 being in the hot gas supply line 26 from the source of heat 25; 4| in the

air supply line

24, and 42 in the return line I9. Damper 40 may be alternatively replaced by dampers 40a, 4Gb in the air and fuel lines to furnace 25. The dampers 40, 4|, 42 are arranged to be operated by the `motor, or motors, 38 inresponse to wet-bulb temperature at B, or other convenient point, as follows:

Y Movement of dampers Change in wetbulb tem rature pe 40 4l 42 Rising Closing Opening Closing. Falliug Opening Closing Opening.

A means for controlling damper `movements as tabulated above is shown in Fig. 1 where 32 representsv the wet-bulb device of Fig. 5 and

dampers

40, 4l and 42 are actuated by a common connecting rod 39 operated by motor 38.

Fig. 4 is a partial reproduction of a conventional psychrometric chart on which the line ae Arepresents, a chosen limitingt constant wet bulb through conduit I9 at a temperature of about t: and a moisture content m5. While commingling there is an interchange of moisture and heat between the recirculated mixture and hot gas which Y stabilizes in a resulting mixture at a temperature ta and moisture content ma as indicated at a in Fis. 4e f Wh n mixed'with `thexnaterial which is to .be dried part of `the sensibl'efheat of the drying medilllrl passes into latent heat of vvaporization for the water .introduced `with the material and the dry bulb temperature of the drying medium drops. 'I'he portion of the drop in dry bulb temperature used for sensible heat to bring the introduced moisture and material to the wet bulb temperature is but a small proportion of the total drop in dry bulb temperature and takes'place in a small part of the time for drying the materials so` that during the drying process the total moisture present 'is substantially constant at the optimum wet-bulb temperature. .i i During the drying period the drying `medium entering at dry-bulb temperature ta with a `moisture content ma, as represented at pointra.v on Fig. 4, absorbs moisture from the material as its dry bulb temperature falls from t3 to t2 while its wet bulb` temperature is maintained constant, or substantially so ,y as mentioned above, along the line af-e. l

The return of ,part of the drying medium through conduitk I9 increases the amount of moisture introduced into the drier and if the amount of recirculated medium commingled with the fresh hot gas is excessive as compared with the amount offreshy hot gas with which it is commingled the wet bulb temperature of the drying medium on entering the drier would'be above the optimum value a, as indicated for example at bin Fig. 4.

'If during the drying process the temperature of the drying medium von entering the drier exceeded the optimum temperature t: `for the moisture content ma by entering the drier at a temperature t4 the wet bulb temperature of the drying medium wouldbehigher as indicated at pointid andy this departure from desired. conditions also would be detected by `the, wet-bulb apparatus 32. i f

However, as thewet-bulbtemperature rises to b or d the control mechanism acts to close

dampers

40 and 42 and to open damper 4I. Closing of damper 42 reducesthe amount of recirculated medium andy lowers the amount of moisture entering the drier. Opening of damper 4I increases the quantity of cool air and thusy rcducesthe temperature of the drying medium, the amount of whichlis also reduced by closing damper 40.` These regulating movements of the

dampers

40, 4I, 42 act to restore the wet-bulb temperature to the selected value or along line cz--e and tend to maintain a substantially constant wet-bulb temperature in the drier so that thematcrial being driedis likewise maintained at fa substantially constant'ltemperature.

It is known that heretofore attempts have been made toy maintain determined input and outlet temperatures such as t3 and and t2 by measuring the dry-bulb temperatures at inlet and outlet of the drier and then `regulating the temperature and amount of drying medium admitted to the drier.` However, a dry-bulb temperature t3 might prevail and be so indicated whilethe moisture content of `they drying medium might be anywhere along the line t3 `(as1 too'high at b ortoo low at c)` without any indication being given of thesey conditions since the dry-bulb device is incapable of detecting the variations inr moisture content of the drying medium. Furthermore, although an outlet dry bulb temperature tz for the of the material to determinewhether it hasbeen wet-bulb apparatus A32 tocontrol the amountof recirculated ldrying :medium by regulating damper- 42' andin Figure `3 ythetemp'erature of the flash drying medium is` controlled by dampers 49 and 4I or. dempers '4Ila,4,0b. LIn these arrangements, one or two respectively of the variables affecting the wet-bulb temperature of the drying medium arev automatically controlled by the wet-,bulb device rather than lall as in Fig. l.

[The total heat required from the' drying medium for eliminatinga fixed amount of moisture from the material is a definite quantity. If the quantity of the drying medium. is more than required', the resulting dry-bulb temperature leaving the drier may be relatively `high and heat will be wasted.` Ifthe amount'of heated drying medium were excessive as compared with the amount of materials to be dried, the particles might have all theirmoisture absorbed by the drying medium when the dry bulb temperature of the latter had fallen to point g for example. Thereafter the. contact of the drying medium with the material would raise the temperature of the. latter as heat passed yfrom the drying mediumto the material with consequent danger of `dryingl the latter beyond the point desired or even scorching the material. When the quantity isy less than required, i the resulting dry bulb temperature may be relatively low and the rate of drying, which falls off with drop in the temperature of the drying medium, may become too small' anddrying may not. beA carried far enough to'dry the material to the extent desired. In accordance with the invention, I may control the quantity of drying medium by the dry-bulb temperatures in the circuit before and after the drier, in addition.A to the above mentioned controls, thereby effecting an economical use of theheat supplied as well as a substantial controlof the extent of drying.

. In. the arrangement 4of Fig. 2, a suitable thermostaticdevice 44 at the inlet of drier I3 isconnected to controla motor 45 operating air damper 4I and damper 40 which regulates the amount of hot gas, or in place ofI these dampers the air and fuel dampers 40a, 4|Jb, thereby maintaining a substantially. constant inlet temperature t3. A thermostat 46 at the outlet of the drier, or in conduit I9, controlsa motor 41 operating a :la`1nper`48regulating` the amount of drying medium entering the drier thereby cooperating to maintainra substantially constant outlet temperature t2. Thermostat 46 might also controll the speed of ian I6,`through a rheostat 46 as limits for the drying medium between which the drying process takes place along line a-e for each unit quantity of drying medium. Assuming a uniform flow and moisture content of the material to be dried through the drier, the control of temperatures t2 and ta then proportions'the quantity ofdrying medium so that each unit thereof will absorb a predetermined amount of moisture from a to e, which range is preferably such that most economical operation of the process will result.

The novel features of this invention may be applied to driers which receive previously ground material and are constructed to maintain the material in substantial fiotation and agitation during the drying process, or to mills which serve as driers and which may receive coarse material and grind it while it is simultaneously being dried within the mill. v

The return of vapor from the separator I1 to the drier for diluting hot gas results in an increase in efficiency and it is therefore desirable to recirculate a maximum of said gas. By the controls in accordance with this invention, such maximum recirculation may be safely employed without the danger of overheating the material being dried. K

What I claim is:

l. In flash drying apparatus of the type described in which finely comminuted moisturecarrying material is fed into a drying chamber and intimately commingled with and carried by a heated gaseous drying medium at above 212 F. that absorbs moisture from said material and wherein the material and drying medium are withdrawn from the drying chamber and separated and at least a portion of the separated moisture retaining drying medium is mixed with a fresh supply of heated drying medium and returned to the drying chamber for drying wet incoming material; means for supplying a predetermined constant amount of freshly heated drying medium to said chamber; means for regulating the amount of returned moisture 'carrying drying medium; and means for maintaining said material at a substantially constant temperature during the drying period consisting of means responsive to the Wet bulb temperature of said drying medium for controlling said regulating means to Vary the amount of heat retaining moisture carrying drying medium lreturned to the drying chamber.

2. In flash drying apparatus of the type described in which finely comminuted moisture carrying material is fed into a drying chamber and intimately commingled with and carried by a heated gaseous drying medium at above 212 F. that absorbs -moisture from said material and wherein the material and drying medium are withdrawn from the drying chamber and separatedand at least a portion of the separated moisture retaining drying medium is mixed with a fresh supply of he'ated drying medium and returned to the drying chamber for drying wet incoming material; means for returning a pre- .determined constant amount of moisture-carrying medium to said chamber; means for regulating the amount of freshly heated drying medium supplied to said chamber; and means for maintaining said material at a substantially constant temperature during the drying period consisting of means responsive to the Wet bulb temperature of saiddrying medium for controlling said regulating means to vary the amount of freshly heated drying medium supplied to the drying chamber.

3. In flash drying apparatus of the type described in which finely comminuted moisturecarryingr material is fed into a drying chamber and intimately commingled with and carried by a heated gaseous drying medium'at above 212 F. that absorbs moisture from said material and wherein the material and drying medium are withdrawn from the drying chamber and separated and at least a portion of the separated moisture retaining drying medium is mixed with a fresh supply of heated drying medium and cool gas and returned to the drying chamber for drying wet incoming material; means for returning 'a predetermined constant amount of moisturecarrying drying medium to said chamber; means for regulating the amount Yof freshly heated drying medium supplied to said chamber; means forregulating the amount of cool gas supplied to said chamber; and means for maintaining said material at a substantially constant temperature during the drying period consisting of means responsive to the wet bulb temperature of said drying medium for controlling said regulating means to vary the amounts of cool gas and freshly heated drying medium entering the drying chamber.

4. In ash drying apparatus of the type described in which finely comminuted moisture carrying material is fed into a drying chamber and intimately commingled with and carried by a heated gaseous drying medium at above 212 F. that absorbs moisture from said material and wherein the material and drying medium are Withdrawn from the drying chamber and separated and at least a portion of the separated moisture retaining drying medium is mixed with a fresh supply of heated drying medium and returned to the drying chamber for drying wet incoming material; means for regulating the amount of separated drying medium returned to the drying chamber; means for maintaining said material at a substantially constant temperature during the drying period consisting of means responsive to the Wet bulb temperature of said drying medium for controlling said regulating means to vary the amount of heat retaining moisture carrying drying medium returned to the drying chamber; and means responsive to the dry bulb temperature of the drying medium at the inlet of said chamber for regulating the amount of freshly heated drying medium supplied to said chamber.

5. In flash dryingl apparatus of the type described in which nely comminuted moisture carrying material is fed into a drying chamber and intimately commingled with and carried by a heated gaseous drying medium at above 212'F. that absorbs moisture from said material and wherein the material and drying medium are withdrawn from the drying chamber and separated andrat least a portion of the separated moisture retaining drying medium is mixed with a fresh supply of heated drying medium and returned to the drying chamber for drying wet incoming material; means for regulating the amount of separated drying medium returned to the drying chamber; means for maintaining said material at a substantially constant temperature during the drying period consisting of means responsive to the Wet bulb temperature of said drying medium for controlling said regulating means to vary the amount of heat retaining moisture carrying drying medium returned to the drying chamber; and means responsive to the dry bulb temperature of the drying medium after a substantial amount of drying has occurred for regulating the amount of drying medium entering said chamber to vary the relative amounts of drying medium and materials entering said chamber.

6. In flash drying apparatus of the type described in which finely comminuted moisture carrying material is fed into a drying chamber and intimately commingled with and carried by a heated gaseous drying medium at above 212 F. that absorbs moisture from said material and wherein the material and drying medium are withdrawn from the drying chamber and separated and at least a portion of the separated moisture retaining drying medium is mixed with a fresh supply of heated drying medium and returned to the drying chamber for drying wet incoming material; means for regulating the amount of separated drying'medium returned to the drying chamber; means for maintaining said material at a substantially constant temperature during the drying period consisting of means responsive to the wet bulb temperature of said drying medium for controlling said regulating means to vary the amount of moisture-carrying drying medium returned to the drying chamber; and means responsive to the dry bulb temperatures of the drying medium at the inlet and outlet of the drying chamber for controlling the amount of drying medium entering said chamber to cause the drying of the material in said chamber vto take place between determined limits of dry bulb temperature while at the constant wet bulb tem perature attained by regulation of the amount of moisture carrying medium returned to the drying chamber.

7. In flash drying apparatus of the type described in which finely comminuted moisture carrying material is fed into a drying chamber and intimately commingled with and carried by a heated gaseous dryingmedium at above 212 F. that absorbs moisture from said material and wherein the material and drying medium are withdrawn from the drying chamber and separated arid at least a portion of the separated, moisture-retaining dryingmedium is mixed with a fresh supply of heated drying medium and cool gas and returned to the drying chamber for dry- AIJ'RED R. SMITH.