US2099160A - Method and apparatus for drying - Google Patents

Description

Nov. 16, 1937. w. H. CHARCH METHOD AND APPARATUS FOR DRYING Filed 001-. 25, 1955 2 Sheets-Sheet l INVENTOR. Mlliam Hale Charch W ATTORNEY.

W. H. CHARCH METHOD AND APPARATUS FOR DRYING Nov. 16, 1937.

2 Sheets-Shet 2 Filed Oct. 23, 1935 myENToR. William Hale Chart/1 ATTO NEY.

Patented Nov. 16,1937

METHOD AND APPARATUS FOR DRYING William Hale Charch, Buffalo, N. Y., assignor, by

mesne assignments, to E. I. du Pont de Nemours & Company, Wilmington, DeL, a corporation of Delaware Application October 23, 1935, serial No. 46,325

1 Claim.

This invention relates to the drying of nonfibrous sheets and films which are subject to shrinkage during the drying thereof. This invention will be particularly described in relation 5 to the drying of regenerated cellulose sheeting cast from viscose and adapted to be used as a wrapping tissue.

Heretofore, in the manufacture of such sheets or films of regenerated cellulose the film was cast from viscose into a coagulating and/or regenerating bath and subsequently passed through additional baths such as regenerating, desulfuring, washing, bleaching and glycerinating baths. Thence, it was passed through a drying apparatus and wound up into rolls. In. passing through the said drying apparatus, the film loses moisture, and in losing this moisture tends to shrink to a great extent, thus diminishing the area of the regenerated cellulose film. and tending to produce puckers or wrinkles therein. In order to produce a relatively thin sheet or film of comparatively large area without puckers or wrinkles, the drying apparatus customarily consists of a series of rotating heated rolls around which the film passes and which, due to the friction between the film and roll surfaces, keep it at all times under a marked tension and thus' prevent or reduce shrinkage of the film. I

It has now been found particularly advantageous to employ various solutions or suspensions to wet sheets or films, hereinafter to be termed generally as webs, and particularly webs in the gel state, for the purpose of imparting certain, special, desirable properties thereto. When certain of the above named solutions or suspensions are applied to the web, difllculty is encountered in the subsequent drying stage by reason of the fact that the applied material tends to concentrate in the web as the liquid is removed until it reaches the point where it tends to stick to the roll in whole or in part. This sticking of the applied material to the roll surface results in an increasing fouling of the roll surface so that the machine must soon be shut down for cleaning. At the same time, there is a constant interchange of the material on the rolls with that in the web, some of the surface material in the web sticking to the rolls and some of the previously deposited material on the rolls again sticking to the web. When this happens, the web has a speckled or patched appearance due to the deposit thereon of small particles of the applied materials, or due to the removal of small sections of the applied material therefrom. When it is realized that the ultimate product (Cl. 3H8) finds its greatest utility because of its glass-clear transparency, it will be readily seen that difiiculties of this kind materially interfere and in some cases forbid the successful manufacture of films to which otherwise desirable solutions and 5 suspensions have been applied.

Although the present invention is applicable to non-fibrous, shrinkable sheets and films to which have been applied all types of materials which may during drying induce speckling of the finished web, I am primarily concerned with the drying of the films to which certain recently developed, non-heat-plastic materials have been applied, that is to say, materials which are not flowable under the application of a reasonable degree of heat, and consequently wherein there is no opportunity for removing the speckled or patched appearance therefrom.

It is therefore an object of this invention to provide a method and apparatus for drying nonfibrous, transparent sheets or films which are subject to shrinkage during the drying-thereof.

It is anotherobject of this invention to provide a method for drying non-fibrous, transparent sheets or films, which contain substances for imparting special characteristics to said film, without the appearance of speckles or patches thereon.

It is a more specific object of this invention to provide a method and apparatus for drying a regenerated cellulose sheet or film containing a substance which may adhere to an adjacent surface during the drying thereof by passing the same over drying rolls which are provided with means for maintaining the roll surface in a dry state.

Other objects of the invention will appear hereinafter.

The objects of this invention can be achieved generally by passing the web into contact with drying rolls which are provided with means for maintaining the samei-in a substantially dry state; preferably, however, the web is first preliminarily dried out of contact with any surface by an application of intense heat until a large part of the liquid has been removed therefrom.

The invention will be more easily understood by reference to the following detailed description taken in connection with the accompanying illustrations, and in which: 50

Figure 1 is a fragmentary, diagrammatic, elevational view of one form of drying apparatus suitable for use in accordance with the present invention.

Figure 2 is a fragmentary, diagrammatic, elevational View showing a modified form of drying apparatus.

Figure 3 is a plan view on an enlarged scale taken along the line 3-3 of Figure 2.

Referring to Figure 1 of the drawings, reference numeral l0 designates a comparatively wide web of non-fibrous, cellulosic material, such as regenerated cellulose which has been cast, regenerated, desulfured, bleached, washed and glycerinated, but undried. This film is led through the liquid bath in vat H by passing the same over rolls I2. From rolls I2 the web is passed through squeeze rolls l3, after which it is led through a narrow opening l4 of the drying enclosure l5, which is of the type described and claimed in the copending application of Walter C. Eberlin, Serial No. 46,326 filed of even date. The web has no contact with any solid surface throughout its passage through the enclosure I5, it being supported at opposite ends thereof by the squeeze rolls I3 and supporting roll It.

-A plurality of headers l8 are positioned to project from the top and bottom of the enclosure l5. These headers, instead of being directed perpendicularly against the web are preferably arranged to project forwardly at an angle in the direction of the movement of the web. A large plurality of hot air or gas jets or open flame gas burners H are arranged on each header and are preferably positioned to project forwardly in the same direction as the headers. The headers 18 are connected to a common chamber which is supplied with a heated gas or a combustible gas by an inlet conduit (not shown). The burners or jets are so arranged in multiple, units that the whole width of the web is exposed to the hot gases or products of combustion. When burners are used, they are so spaced that sufficient passage exists between flames for an ample supply of secondary air to reach the flames and for the products of combustion to escape. The burners or jets may, if desired, be made individually adjustable so that wet hands may be eliminated from the films and the latter be dried to produce a fiat sheet.

A plurality of narrow outlet openings 22 for the hot gases or products of combustion are ar-' ranged intermediate the headers. The openings 22 in the upper portion of the enclosure l5 are connected to a common chamber 24 from which the gases or combustion products are withdrawn through the conduit 28 and suction fan 32. The openings 22 in the lower portion of the enclosure l5 are connected to the chamber 26 from which they are withdrawn through conduit 30 and a suction fan (not shown).

The film passes from the enclosure l5 through the narrow opening 34, thence over the supporting roll it to a series of heated drying rolls 40 which may, if desired, be of the type disclosed in United States Patent No. 2,046,553 to D. H. Edwards, or the type disclosed in the United States patent application of F. P. Alles and D. H. Edwards, Serial No. 728,378, or both.

The first section of drying rolls which the web will contact is preferably confined within enclosure 46 which is provided with additional hot gas jets 42 and suction means 4| and 44 for withdrawing said gases therefrom. The hot gas jets 42 are connected to common headers 43 and supply conduit 45 (see Figure 3). The suction means 4| and 44 are connected to suction chambers 41 and 48 respectively by means of which the gases may be drawn off through conduits 49 and 50 by any appropriate means. The jets 42 are shown directing the hot gas against the section of the drying rolls 40 which are out of contact with the web l0, whereas the suction means 44 are arranged adjacent the webs which are moving over sections of the rolls 40. The

hot gas jets 42 serve to keep the first section of drying rolls dry and thereby to prevent any adherence between the web and the drying rolls. After being led over the drying rolls within the enclosure 46, the film will be sufficiently dry to overcome any tendency thereof to adhere to the additional unenclosed drying rolls 40 which are provided in suflicient number to remove such remaining moisture content of the web as may be most desirable. The web is then collected on the winding shaft 50 in the conventional manner.

The web may be drawn through the drying apparatus by imparting rotation to any of the rolls I2, l3, I6 and 40 or to all of them, as is well known in the art. If desired, succeeding driven rolls may be regulated with any desired or expedient speed differential means to compensate for film shrinkage or to prevent excessive shrinkage. For example, the circumferential speed of the drying rolls may be slower than the circumferential speed of squeeze rolls l3 to allow for web shrinkage in the drying of the film as described and claimed in the co-pending application of William Hale Charch and Francis P. Alles, Serial No. 46,324 filed of even date.

The requisite heat may be supplied to enclosure l5 in a variety of ways, such as, direct heating by open gas flames, direct heating by electrically heated units arranged to heat by convection and radiation to the web, heating by hot gases supplied by the passage of air orother gas over steam coils, electrical or gas heated" units, or otherwise heating by hot gases supplied by the combustion of gas, oil, coal or other fuel. The ultimate transfer of heat to the Web is in all cases by convection and/or radiation since it would defeat the objects of the invention to permit the web to touch any surface which might transfer heat to' it by conduction. Since it is necessary to transfer a large amount of heat to the web in a short time, it is desirable, with any method used, to cause the heated gas, air or flames to impinge against the web with good velocity, thus reducing the surface film of air or gas on the web to-a minimum and in accord with well known heat transfer principles obtain a maximum rate of heat transfer. When hot air or other gas is used for drying the web, the gas impinging on the bottom surface thereof may be given sufficient velocity to support the film, and the velocity of the gas directed against the top surface of the film regulated so as to compensate for any excessive upward thrust of the first mentioned jets.

The film of regenerated cellulose leaving the wet end of the casting machine and entering the dry end is largely swollen with water and contains in excessof 300% water, all figures based on dry cellulose) whereas the film as marketed contains less than 10% water. In its passage through the enclosure 15, the film must be dried to the point where it will no longer adhere to the drying rolls 40 and at the same time it must not be dried to the point where it will tend to pucker or wrinkle or to the point Where material shrinkage will take place. a When the film is led directly from the drying enclosure l5 to conventional drier rolls,. the moisture content range of the Web as it passes from the enclosure will be much smaller than where the film is led through the enclosure [5 to the drying rolls against the surface of which hot gases are being directed. Slightly moist webs which will adhere to the conventional drier roll will not adhere to rolls against the surface of which hot gases are directed by reason of the fact that the roll surface in the latter case is lrept dry. Therefore, the inter-position of a plurality of drier I'Olls against which hot air jets are directed also serves to eliminate the necessity for controlling and maintaining the moisture content of the web within a comparatively small range.

With the described arrangement it is possible to remove the surface layer of water from the web while the latter is entirely out of contact with any surface, and after the surface layer of water has been removed, but before the web has dried to the point where wrinkles or packers are imparted thereto, or before material shrinkage has taken place the web is further dried while in contact with a smooth, solid surface without danger of sticking of the film to the roll surface. When fouling of the rolls takes place, the web surface becomes covered with specks or patches due to the deposi= tion of material from the dirty rolls or the removal of portions of the applied material from the web. In webs to which heat plastic material has been applied, these specks or patches may be removed by further application of the heated roll surfaces, but when the applied material is nonheat-plastic, the specks or patches will be permanent and non-removable. Consequently, in the former case the web must be subjected to further comparatively expensive steps to remove the specks from the heat plastic coating, and in the latter case the Web is permanently damaged and of little or no value.

Referring to the modified form of apparatus lllustrated in Figure 2, the web this passed from the tank i through the squeem rolls l3 and thence directly over a series of drying rolls to. The first drying rolls over which the web is led are positioned within the enclosure t5. These drying rolls are provided with means for directing hot gas against the sections thereof which are not in contact with the web in the same manner as above described with reference to Figure 1. This modified form of apparatus finds particular utility .in the manufacture of webs which are coated with substances which need not be preliminarily dried in a freely suspended condition, but which may have a. slight tendency to stick to the roll surfaces unless they are kept substantially.

' trated in Figure 2.

The temperature of the hot gases directed against the drier rolls, the speed of the passage of the film through the drier, and the velocity of the gases may be varied within wide limits to accomplish the objects of the present invention.

drier at the rate of about 125 ft. per minute. Va-' rlous other factors may sheet the above preferred conditions and the temperature, velocity and volurne of the gases may be varied from the preferred ranges set forth above to produce material improvement in the resulting products, but it is to be understood of course in all cases that relatively high velocities, with the gas directed against the rolls, as distinguished from mere circulation of the circumambient atmosphere, are necessary to secure the desired result.

Although the present invention has been discussed with particular reference to regenerated cellulose, it is nevertheless applicable to webs made from other non-fibrous, transparent, shrinkable, cellulosic materials which are selfsupporting during the drying thereof, or to nonfibrous, transparent, shrinkable, non-cellulosic;

sheets and films of similar nature.

Although the present invention is particularly dried with great advantage in accordance with this invention include moistureproonng'compositions, materials which will prevent the transmission of ultra-vlolet light. compositions which may be applied for anchoring subsequent coatlugs to the dim, and the like.

The use of very intense heat to carry out the early stages of the drying leaves a very much smaller amount of drying for the later steps which follow the hot treatment. This makes possible the design of a much shorter drier with its attendant lower investment costs and maintenance and, more especially, with the reduction of the factors which make for non-uniformity of drying and which are of course magnified as the size and number of components of amachine are increased. The present invention permits of a very much higher drying speed for a given drier range and thus becomes important due to current competitive conditions.

This invention has the further advantage of the production of sheets having coatings thereon adapted to lend very desirable characteristics to the webs, but which would otherwise be extremely dimcult, it not impossible of application with previously mown apparatus and process steps.

It is to be understood that the specific embodiments of the invention illustrated and specifically described may be varied in many details of its construction within wide limits without departing from the nature and spirit of the invention, and that the invention is to be limited only as set forth in the appended claim.

I claim:-

lln the method of drying a highly shrinkable, non-fibrous, transparent, cellulosic web adapted to be used as wrapping tlssue, which web has been saturated with a treating liquid, the steps comprising passing said web substantially saturated with treating bath liquid through a drying zone comprising a plurality of open flamesin which said web is maintained out of contact with any solid surface whereby to preliminarily dry the same, continuing the drying thereof by passing said web into contact with a drying roll, and simultaneously passing a hot gas against a section of said roll in such a quantity and velocity as to prevent adherence of the said web to said roll.

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