US2000663A - Process of drying materials - Google Patents

Description

May 7, 1935. w. A. DARRAH PROCESS OF DRYING MATERIALS Sheets-Sheet 1 Filed Oct. 15, 1930 May 7, 1935. w. A. DARRAH PROCESS OF DRYING MATERIALS Filed Oct. 15, 1930 2 Sheets-Sheet 2 ||l| Ll I III \h w- I W5 Illlll ,fllllirvlllili 124 612 for.

Patented May 7, 1935 UNITED STATES PROCESS OF DRYING HAW William A. Dan-ah, Chicago, 111., asslgncr to United States Gypsum Company. a corporation of Illinois cum Application October-.15, 1930, Serial No. mass:

14 Claims. (01. 34-24) This application is in part a continuation of my co-pending application, Serial No. 361.920,

filed May 10, 1929 for Process of drying materials and apparatus therefor. Certain new features have been added to the previous applicationas will be set forth below.

This invention relates to equipment, methods and processes of drying materials and is particularly applicable to such objects as boards. sheets, plates and similar flattened shapes in which the ratio of surface to volume is very great. Some of the objects of this invention are to provide economical, simple and eflicient means for drying such articles as wallbourd, insulation board, veneer, paper, cloth and similar materials without damage. Another object of this invention is to carry on drying, economically, rapidly and without discoloration of the material being dried.

This invention provides simple, continuous and automatic means for accomplishing the above results. Other objects of this invention will be apparent from a perusal of the drawings, specification and claims attached hereto.

Referring to the drawings:

Figure 1 shows a side elevation partly in section of one form of my device;

Figure 2 shows a vertical section of one form of my device;

Figure 3 shows a plan view partly in section of one'form of mydevice. I

Referring to the drawings, i indicates a housing or container through which the material to be dried is conveyed. 2, I and 4 indicate several of a series of conveying rolls for moving material through my device. In place of the rolls shown. I may use any desired form of conveyor mechanirm such as a series of chains, or other commonly known mechanisms. The rolls 2, 8 and 4 indicated may each be driven by a sprocket as indicated at 5, 'over which a continuous length of chain 8 is passed serving to keep the above rolls in continuous movement at uniform rate and in the same direction. These constructional details form no fundamental part of my invention and are merehr illustrated to make clear one form of mechanism which I have found convenient to use. 7 I

Housing I is preferably insulated with heat retarding material I, formed in a shell or cover around housing i. Reference numeral 0 indicates a board about to enter my device and t represents a board leaving my device, the 'direc-' tion of travel being indicated by the arrow on the board. A heating means is shown diagrammatically at in connected to a fan or gas moving device I I. The heater may consist of an enclosed shell of steel or other material properly insulated and arranged to burn a desired fuel such as gas,

oil, coke, coal,.etc. I prefer to use gas or oil although in many cases powdered coal or other fuels may be employed to advantage by reason of the lower cost. The exact details of construction of my heater are not of particular importance in this case, although I prefer an arrangement in which the fuel is supplied by means of a burner l2. Burner l2 mixes the fuel with air and delivers it to a combustion chamber It. The products of combustion leave chamber 13 by means of an outlet H and enter mixing chamber It between fan and combustion chamber. The fan which may be of any commercial type is driven by a motor or equivalent indicated by l8 and serves to maintain a substantially continuous circulation of drying gases and products of combustion through the system. The gases leaving fan Ii pass through discharge duct I l and enter dryer housing I, where they are distributed by a series of nozzle ducts II, is, 20 and 2| arranged substantially symmetrically. These distributing 'ducts serve to direct the flow of hot gases toward and provided with dampers for exhausting a portion of the moist gases. I

In certain cases where the permissible temperatures are low, I may exhaust the gases entirely at the ends of the equipment instead .of returning them to be reheated. In other cases. I may pass the circulating and drying gases in one direction only,'these being details subject to practical operatingv conditions.

A tank or container 24 for liquid is arranged above the entrance portion of the dryer and connected with a discharge duct SI controlled by a valve :0 in sucha manner as to continuously deliver liquid to the rotating distributor 21 which rubs or-brushes the surface of the article I being dried. A similar brush or roll I! supplied with liquid from a reservoir it serves to apply liquid to the underside of the board.

This equipment is particularly applicable to those installations in which the drying process is carried to substantial completion within the dryer and in which the operating temperatures exceed 212 F. In these cases the discoloring effect of sulphur and other acids is particularly marked. This equipment is also of great importance in those cases in which the products of combustion are recirculated either all or in part, being reheated and delivered back to the dryer.

Instead of applying the neutralizing liquid from tank 34 onto the sheets or boards being treated, I may neutralize the effect of the acid by adding directly to the circulating gases a proper material for absorbing or combining with the acid gases. This result may be accomplished by spraying into the discharge of circulating fan II or introducing into the intake it of said fan a finely powdered material such as soda ash, lime, etc. I prefer to use slaked lime although'quicklime will accomplish a similar result, but apparently slightly less effectively. I'have found that ammonia is particularly eifectlve. andI may also spray a solution of caustic soda. soda ash. ammonia, etc.

The material should be so added as to become distributed fairly uniformly through the circulating gasesp'rior to the time that they come into contact with the material being. treated.

According to tests' which I have made, the addition of the neutralizing ma erial to the gases does not result in wholly neutralizing the sulphur in the gases. It appears. however, that the materials added collect in part upon the sheets which are passing through the dryer and in this manner serve to neutralize the acid fumes in the circulating gases at the time that the fumes appreach the sheets.. I have made tests of the gases taken from the system when the neutralizing effeet on the sheets was quite completeand have found that the gases still give ample evidence of a large acid content. In the case of adding a solid material in quantity. such as hydrated lime, soda ash, etc., a certain amount will collect and leave the dryer with each sheet. This is somewhat troublesome in some cases and in order to overcome this difllculty I have provided a series of scrapers or brushes 38 to remove the excess of material. I may also add to one of the rolls on the discharge end of the dryer, as for example 40, a cushion or scrubber made, for example.

from a strong fabric. This will serve to remove the excess of neutralizing material from the under side of the sheet.

4| indicates a tank or container for holding the alkaline material, as for example liquefied ammonia. 42 represents asimilar tank. Tanks 4i and 42 are connected by ducts 43 and 44 respectively to the air ducts entering the dryer I. As an alternative-arrangement, duct 26 is shown leading from tank 42 to the intake of fan ll.

Ducts 43 and 44 are provided with control valves 45 and 46 respectively. These valves make it possible to control the relative amounts of neutralizing material delivered into the ducts. Oil burner I 2 is provided with a control valve 48 which may be automatically or manually operated as required. Valve 48 is mechanically interlocked with valves 45 and 48, so that the opening or closing of valve 48, thus delivering more or less fuel to heater l0, will automatically -deliver more or less neutralizing agent to the dryer I. This mechanical interlocking is indicated diagrammatically by the cables 50 and II although obviously any similar mechanical expedient may be employed. The lines indicating the dotted cable 52 and 53 show a similar method of control in the alternative case in which the neutralizing material is added into the intake l6 of-fan H. Obviously in most cases, either one or the other alternative would be employed, as it is unnecessary to use the two.

In Figure 2 I have shown an alternative arrangement in that hopper 54 delivers a supply of powdered material to the circulating gases Hopper 54 may be-controlled automatically as to volume if desired. Hopper 54 indicates a simple method of adding finely divided solids to the circulating gases, accomplishing substantialLv the same result as in the case of adding atomized liquids or gaseous ammonia.

In most cases I prefer to use gaseous ammonia as I have found that very much smaller amounts are required to accomplish proper neutralization. Furthermore the products which result from the neutralization when ammonia is used consist essentially of ammonium sulphate and sulphite. These substances appear under ordinary conditions as very fine powders whose quantity is so small as to be substantiall; unimportant. In many instances therefore whengaseous ammonia is used to neutralize the acid conditions, .it is unnecessary to remove the resultant solid and in most cases a small air blast directed onto the sheet will readily remove all traces of this substance.

By way of example, I have found that when drying gypsum wallboard with this system, and burning in the neighborhood of from forty to sixty gallons L Y fuel oil per hour, it is advisable to use between fifteen and twenty pounds of slaked lime per hour in case the 011 contains around 56-01 1% of sulphur. The samev result may be obtained using between one-half pound and one pound of liquid ammonia (gasifled). While the liquid ammonia costs in herently much more than lime, the ease of hendling it and applying it, the smaller amount of residue and the elimination. of the necessity for cleaning, usually make the ammonia a more economical material.

Owing to the inherent expense per pound for ammonia, it is advisable to apply only the amount required to neutralize the acid conditions resulting from the fuel. In some cases. the use of a large excess of ammonia causes a dimculty, in interfering with the sizing of the sheet or board, influencing color, etc. It is therefore highly desirable .both for reasons of economy and also in order to obtain the desired quality of product to roughly proportion the amount of neutralizing material added to the amount of sulphur delivered. Under any given set of conditions, this would mean that the amount of neutralizing material should be varied roughly with the amount of fuel delivered to the heater, since with a constant grade of oil the amount of sulphur will vary with the amount of fuel burned. v

I therefore have found it to great advantage to provide automatic means as shown for varying the amount of neutralizing material with the amount of oil.

It is to be understood that the ratio should be held substantially constant under any set of conditions, but that a different ratio is required for different sets of conditions. I

For example, if the ratio proves to be correct when one pound of ammonia is added to the circulating gases for each sixty gallons of oil burned, 4

assuming the oil to contain one-half of 1% of sulphur the amount of ammonia required would using it to described my invention because it is beatleasttwiceasgreatortwopoundspereach sixty gallons burned. In either case, the ratio,

should be held constant when burni' varying amounts of oil having the same sulphur content.

I have further found that the amount of neutralizing material required is greater when operating at higher temperatures than when operating at lower temperatures, independent of the amount of sulphur contained in the oil or the amount of oil burned. As a theory, I account-for this difference as being due to the much more rapid action of the sulphur acids at "the higher temperatures.

Referring now to the operation of the equipment and process which I have invented, it should be understood that to satisfactorily dry boards and similar materials they should be subjected to a stream of warm gases. The maximum temperature to which the boards may be subjected varies of course with the nature of -the material from which they are formed and various other factors. tures ranging from 300 to 400 F. are not unusual. while in the case of fiber or insulating board temperatures as high as 500 to 600 F. are frequently permissible. It will be noted that at these temperatures the gases which convey the heat and remove the moisture are well above the dew point regardless of the amount of moisture carried by them.

I have found that drying may be accomplished satisfactorily when operating at the temperatures mentioned above with little regard to the amount of water carried by the circulating gases. In

other words, an atmosphere consisting almost entirely of water vapor would form a very effective drying medium under these conditions. By returning the flow of circulating gases and reheating them, I am able to materially reduce the fuel requirements for a given installation. Further, by adding the products of combustion directly to the circulating gases I avoid the so-called stack loss due to exhausting these products of combustion, which has hitherto been common practice.

In general, therefore, 'I prefer to obtain my drying results by passing a continuous stream composed principally of air and water vapor but containing also afew percent of carbondiox'ide.

It will be obvious that since the products of combustion are mixed with the circulating gases and caused to continually pass in contact with the surface of the material being dried, any substance within the products of combustion-which,

may have a tendency to attack or combine with thematerial being dried is likely to cause damage.

It is, of course,'well known that ordinary fuel oil contains appreciable amounts of sulphur and in some cases sulphuric acid as well as other materials such as chlorides, etc.

Under the conditions existing in the type of equipment here disclosed, any sulphur present will be oxidized to sulphur dioxide which in contact with the moisture in the air and in the board will tend to form both sulphurous and sulphuric acid. Any sulphates present in the oil due to the neutralization of the acids used in refining, or due to the natural, impurities will tend to form sulphur trioxide when the oil is burned which, of course, in contactwith the moisture present will normally form sulphuric acid.

I am emphasizing the effect of sulphur and In the case'of gypsum board, tempe'ra-' a common and marked condition and serves to clearly illustrate my invention. I do not, however, wish to be confined to means for neutral izingthe effect of sulphur only as under some conditions other materials may be troublesome.

I have found that. when traces of sulphuricacid are present in the circulating gases a portion of the acid is absorbed by the moisture on the surface of the material being dried with the result that the surface of the materiallie-' comes decidedly acid.

As thedrying progresses further, I have found that the water at the surface of the board is evaporated while the sulphuric acid produced is evaporated to a very much lesser extent. Thia. of course, results in the formation of a fairly concentrated acid on the surface of the material being dried. If the material being dried contains large quantities of organic material such as cellulose, starch, dextrine or other carbohydrates, the addition of heat will rapidly, cause discoloration which is usualb objection able particularly in the case of dryingwallboard, insulating board, gypsum board, etc.-

I have found.that a concentration of acid as little as three parts in one hundred will cause a dry sheet of paper similar to that used in the case of gypsum wallboard to becomejet black when the temperature is raised to around 400 F. The same paper' witho'ut the acid will withstand a temperature of 400 1''. for a limited time without serious change of color.

When it is considered that most wallboards ecually are used as building materials for finishing the interior of buildings and, therefore, are subjected to rigid purchasing specifications as to color,"

imiformity, etc., it will be appreciated that the effect of acid discoloration may be so serious as to render the product unsalable. 7

In order to overcome'this diillculty,'I have developed the equipment and process here disclomd. I have found that by adding to the surface of the board or sheet before it enters'tha dryer, a solution designed to neutralize the effect of the acid without discolor-ing the sm'face of the board or sheet it is possible tov rapidly dry at high temperatures with direct products of combustion under conditions which would otherwise be impossible.

A'wide range of materials may be employed depending upon the conditions which met. a

In the case of neutralizing the effect of sulphuric acid on gypsum wallboard, I have foimd thatbywashing'the surfacein the manner dis-' mixed together in solution will give desirable re-- suits as for example by the prevention of the formation of crystals in the surface of the board, etc.

It will be evident from the'above, that the essential feature is to add a material which will combine with and neutralize the acid condition. thus maintaining a concentration of acid so low that it will not discolor the'orsanlc materials must be" found that the addition of several materials present at the maximmn that the board or'sheetis subiected to duringthe drying Process.

It is, of course. desirable to apply the mate- 6 rialtobothtopandbottomsurfacesoftheboard. and while I have shown the material applied just prior to the time that the board enters the dryer. it will be apparent, of course, that the material may be applied at any time prior to 10 dryinm. Thus the material may be'added in the manufacture of the board or sheet or in the case of-the gypsum wallboard in the manufacture of the paper covering which is on the board. I have found in'commercisl practice that it is 16 entirely impossible to produce a commercially clean or salable board with certain grades of fuel, unless the surface of the board is previously treated in the manner here outlined.

Certain materials are more satisfactory than others as neutralisers. In the case of a gypsum wallboard a solution of lime in water is quite satisfactory as the net result of the reaction is to produce calcium sulphate which is chemically similar to gypsum and which as produced in this.

process is a fine white powder not directhr notice able on the surface of the board.

Solutions of caustic soda when passed through a dryer under the conditions here specified frequently form sodium carbonates which are in turn converted to sulphates by the action of the acid. Under some conditions large' amounts of sodium sulphate will form visible dition of a water solution of thevarious mateneedles-or crystals which are objectionable.

The addition of small amounts of sodium sil- 38 icate or other gelatinous or colloidal materials will frequently prevent the formation of noticeable crystals on the surface of the board or sheet and I consider as one of the decided advantages of my'invention the use of a mixture of colloidal 40 materials with the neutralizing compound so that the resultant product does not form large or unsightly or otherwise objectionable crystals. Traces of soap solution will serve the same purpose under certain conditions as will also the ad- 5 dition of small amounts of commercial borax.

It should be understood that there are many possible modifications of my invention without departing from the spirit of this disclosure.

While I have referred primarily to "fuel oil as a source of heat and also the source of discoloration, it should be understood that other fuels such as gas, coal or coke will frequently give similar results. My invention, therefore, should not be confined to devices burning oil only,

While I prefer to practice my invention by returning the gases for further recirculation as this method gives greater economy, it should be understood that in general the same conditions exist and the same results are obtained in case a stream of hot gases containing products of combustion are circulated through the dryer and then discharged without employing the recirculating principle.

Obviously many forms of dryer may be employedsuch as chain conveyors, oscillating mechanism, cars, etc.. or the process may be employed in the so-called batch type of dryer in which a load of material is placed in a'chamber. dried and then removed. I prefer the type of roller conveyor disclosed in the drawings. but do not wishtobeconilnedtothistypconly.

-: wish it to be particularly understood that 1 have not described all of the possibleneutralizing v, materials as such a list would be extremely 76'. lengthy. I have indicated the general class of thesematerialsandthoseskilledintheartwill readily select additional materials which will be eifective. In general any compound which soare controlled by doors "A and "A respectively.'

Airdrawninatthispointservesthepurposeof cooling the combustion-chamber of the heater and also dilutes the active materials which tend to attack the-board. I have not foimd. however, that in most cases where a considerable amount of sulphur is present in the fuel, the addition of air in practical quantities will overcome the difilculties here described.

In order to give a specific statement of condl- I tiom encountered, I wish to state that I have found thatfuel oil containing four percent of sulphur will occasion serious diiliculties in certain classes of paper covered gypsum board and this dlmcultycan be overcome by the proper adrials mentioned above.

It is frequently possible to operate without the addition of neutralizing materials in case the amount of sulphur in fuel oil is one half percent or less. I do not, however. wish to be confined to any specific limitations as to quantities as obviously these will vary widely with the temperatures used, nature of the material being dried, acid formed and other formations which must be determined in each individual case.

Havlngnow fully described my invention, what I claim as new and wish to secure by Letters Patent, in the United States, is as follows:

1. The process 'of drying materials by subjecting said materials to the action of a moving stream of hot products of combustion to which free ammonia has been added. to neutralize some of the various acids contained in the products of combustion.

2. The process of drying materials which consists in'subjecting said materials to the action of a stream'of hot products-of. combustion containing sulphur compoimds and ammonia, said ammonia serving to neutralize some of the various acids contained in said products of combustion.

8. 'Ihe'process of drying materials which consists in subjecting said materials to the action of a hot stream ofgases containing products of combustion and simultaneously adding a finely divided alkaline material to the hot stream of gases thereby forming a layer of acid neutralizing substance on the surface-of said material being dried, said alkali material 7 8 to neutralize some of'the various acids contained in said products of combustion.

4. The process of drying materials which consists in subjecting said material to the action of a stream of hot gases containing products of combustion and sulphur containing compounds and simultaneously adding ammonia to said gases thereby forming on the surface of said materials a layer containing to neutralize some of the acids resulting from said products combustion.

5. Intheprocessofdrymgthestepofheating the material being driedbyastreamofhot gases and sulphur containing compounds and simultaneously diswibuting in said combustion gases compounds having an alkali reaction therebyapplylnstothe surfaceofsaidmaterialalayerof acid neutralizing substance, capable of neutraliz ing some of the acids produced from said prod ucts of combustion.

6. Intheprocessofdryinmthestepofheating thematerlalbeingdrledbyastreamofhotgases containing sulphur compounds and simultaneously adding to said combustion gases ammonia. thereby forming on the surface of said material a layer of a substance containing ammonia, capable of neutralizing some of the acids produced from said products of 7. In the process of drying materiala the step of circulating warm combustion gases-in contact with said materials, adding to said gases a substance having an alkali reaction thereby formingalayeronthesurfaceofsaidmateriaLsaid layer containing ammonia, and adapted to neutralize some of the acids present in said gases.

8. The process of drying materials by subjecting said materials to the action of a moving stream of gas containing hot products of combustion, to which a substance having an alkaline reaction has been added, to neutralize some of the various acids contained in said products of combustion.

9.Intheproceasofprotectinga.rticleshaving a cellulosic surface, while being driedin an atmosphere containing sulfuric acid, the step which comprises forming a minute layer of an alkaline dust on sald'surfacesso as to insulate the same fromthesuifm'icacidandtoneutralisethelatter;

lmrntheprocessofproteetingartieieshsving aoellulosicsurfaoe.duringdryinginahotoxidizing atmosphere containingsulturic acid, the stepwhichcomprisesformingaminutelayerof limedustonsaidsurfaoessoastoinsulatethe sametromthesulfuricacidandtoneutralisethe latter.

11. The process of protecting celluloeicarticlee under ing dryingln a hot atmosphere containingfreeoxygenandsulfuricaeidwhichoomprises thestepofchargingsaidawithsui'iicientamountsofanaikalinedusttoneutralize saidmlfuricacidandtosettleonsaidarticlesto protect the same againstthecorrodingefi'ect of anyacid settlingthereonwhichmayhave escaped neutralimtion.

12. The process of protecting articles havinga cellulosicsmfaceduringdryinginamoistatmosphere containing sulfuric acid, which comprlsesthestepofformingathinlayerofadust onsaid surface, said dust being capable of reactingwithsaidsulfuricacidtoformaneutralsalt.

13.1nthedryingofarticleshavingasuflace composed substantially of cellulose in anatmosphere containing hotgasesandentralnedsmfur trioxide and sulfur dioxide, the stepwhich comprises interpcsing between said a and saidsurfaoeapulvemlentthlnlayerofasubstancecapableofcombiningwithsaidsulfur trioxideandsulfurdioxide.

14..t'heprocessofdryingboardscompoeedat leastpartlallyofcellulosawhichcomprisessubjectingthesametotheactionofgasescontaining water vapor and water-solublesulfuracids, while protecting the cellulcsic component of said boards fromtheeifectofsaidsulfuracidsbycoveringthe outersurfaceofsaidcellulosewithathinfilmof analkallne-reactingcompotmd.

WILLIAHLDARRAB.

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