Patent Publication Number: US-6216706-B1

Title: Method and apparatus for producing reconstituted tobacco sheets

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method and apparatus for producing reconstituted tobacco sheets. More particularly, the present invention relates to a method and apparatus for producing reconstituted tobacco sheets in a relatively rapid and uniform manner. 
     BACKGROUND AND SUMMARY 
     In the manufacture of tobacco products, tobacco by-products such as tobacco stems, leaf scraps, and tobacco dust produced during the manufacturing process can be recycled to reclaim their useful tobacco content. In the past, such tobacco by-products have been formed into what is known in the industry as reconstituted tobacco sheets. 
     Commonly owned U.S. Pat. No. 5,203,354 discloses a process for manufacturing reconstituted tobacco sheet by casting a tobacco containing slurry containing 20% solids and 80% liquid onto a rotating cylindrical dryer which is heated by gas, electricity or steam and the dryer reduces the moisture content of the slurry to 12-40%. In order to avoid deterioration of flavor and other properties of the tobacco the dryer surface temperature is from 180 to 200° F. A process for making reconstituted tobacco sheet by casting or extruding a tobacco slurry at 80-200° F. onto a continuous stainless steel belt and drying the slurry at 200-700° F. is described in commonly owned U.S. Pat. No. 5,724,998. Commonly owned U.S. Pat. No. 5,584,306 discloses a process for making reconstituted tobacco sheet by reverse-roll coating a slurry at 120-160° F. onto a stainless steel belt traveling at a line speed of 20-400 feet/minute and drying the slurry in a primary dryer and secondary dryer after which the sheet is doctored from the belt. In the examples of the &#39;306 patent the belt speed was 30 fpm, the dryer was a gas-fired, air impingement dryer, and the sheet emerging from the secondary dryer had a moisture content of approximately 15% and sheet weight of 11.6-14.6 g/ft 2 . 
     U.S. Pat. No. 5,099,864 describes a process for making reconstituted tobacco sheet wherein a tobacco slurry at 140-180° F. and solids content of 14-20% is cast as a 15-40 mil thick sheet on a stainless steel belt moving at 1.5 m/min and dried to a moisture content of about 14% and weight of 75-150 g/m 2 . U.S. Pat. Nos. 4,306,358 and 5,743,022 describe apparatus for the drying of a reconstituted tobacco slurry. 
     During the drying process, prior art processes have utilized air impingement dryers and other drying equipment to remove the excess moisture. In order to effect rapid removal of water from the slurry, gas-fired air impingement dryers have been placed both above and below the conveyor belt downstream of the casting device. In such gas-fired, air impingement dryers, the thin layer of slurry arrives in a relatively cool state due to heat transferred to the steel belt and during heating by the dryers, a skin or “film” can form on the surface of the slurry which lowers the rate at which moisture can be evaporated from the slurry. 
     In view of the state of the art of making reconstituted tobacco sheet, there is a need for a processing apparatus and method which overcomes the slow evaporation problem caused by prior art techniques for drying reconstituted tobacco slurries. It would be desirable if such apparatus could be incorporated in existing processing equipment. 
     SUMMARY OF THE INVENTION 
     The invention provides an apparatus for producing a reconstituted tobacco sheet from an aqueous slurry wherein the apparatus includes an endless conveyor belt, a source supplying slurry to the conveyor at a deposition point, a steam heating arrangement which preheats the slurry as it enters a drying apparatus which reduces the moisture content of the thus-produced sheet to a desired level. The sheet can be further dried in a secondary drying apparatus which reduces the moisture content to a lower level. The steam heating arrangement can comprise a steam box arranged such that a portion of the conveyor above an upstream portion of the steam box is exposed to the open air and a portion of the conveyor above a downstream portion of the steam box is located in an air lock of the heating arrangement. 
     The invention also provides a method for producing a reconstituted tobacco sheet from an aqueous slurry, the method including steps of depositing the slurry on a conveyor, heating a portion of the conveyor with a steam heating arrangement so that the slurry is preheated to a preheat temperature, and drying the sheet in a drying apparatus which reduces the moisture content of the sheet to a desired level. The conveyor can be a stainless steel belt and the steam heating arrangement preferably heats the belt such that moisture evaporates from the slurry from the inside to the outside thereof. Due to the steam heating arrangement the speed of the belt can be increased considerably compared to heating arrangements which omit the steam heating arrangement. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects and advantages of the invention will be apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a schematic of an apparatus in accordance with the invention wherein a process line for drying a reconstituted tobacco slurry includes a steam box which preheats the slurry as it enters a gas-fired air dryer; 
     FIG. 2 is an enlarged schematic of the steam box shown in FIG. 1 wherein only a portion of the gas-fired dryer is illustrated; 
     FIG. 3 is a perspective view of a steam box in accordance with the invention wherein details of pipes for supplying steam to the interior of the box and a peripheral edge seal for contacting the underside of the steel conveyor belt are shown; 
     FIG. 4 is a perspective view of a steam box in accordance with the invention wherein a perforated plate is recessed below the upper edge thereof; 
     FIG. 5 is a diagram of a hole pattern in the perforated plate shown in FIG. 4; and 
     FIG. 6 is a schematic flow diagram of a steam distributing system which can be used to supply steam to the steam box shown in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention provides improvements in productivity in manufacture of reconstituted tobacco sheet. In general, reconstituted tobacco sheet has been manufactured by casting a tobacco slurry onto a stainless steel belt which carries the slurry through air impingement driers to reduce the water content of the slurry from about 80% when it is deposited on the belt to less than about 40% on exiting the driers. In accordance with the invention, the air impingement driers are preceded by a steam heating apparatus which effects preheating of the slurry in a manner which allows the desired moisture content of the reconstituted tobacco sheet to be achieved while increasing the speed of the belt. Thus, the process and apparatus according to the invention can provide substantial improvement in the rate of production of reconstituted tobacco sheet. 
     According to one embodiment of the invention, the steam heating apparatus comprises at least one steam box positioned beneath the belt at a location between a slurry caster and the entrance to an air drying arrangement. The slurry on at least an upstream portion of the belt above the steam box can be exposed to the open air. Heat generated by the steam box is effective to minimize temperature gradients in the slurry by either rapidly increasing the slurry temperature to a substantially uniform preheat temperature prior to entry into the air drying arrangement or by maintaining the slurry at substantially the slurry casting temperature until the slurry enters the air dryer. The air drying arrangement preferably includes a series of air drying zones at different temperatures which supply heat to the slurry and reduce the water content thereof. For example, in the case where the steam box is located between the slurry caster and the air dryer, the slurry deposited on the belt can be at a temperature of about 150 to 180° F. (e.g., about 170° F.) and have a water content of about 70 to 85% (e.g., about 78%), the slurry deposited on the belt can lose heat to the belt and rapidly cool to about 70 to 120° F. (e.g., 80 to 90° F.), the steam box can heat the slurry to a preheat temperature of about 170 to 190° F. (e.g., 180 to 185° F.), and the air drying arrangement can supply heat to the slurry to reduce the moisture content of the sheet to below about 40% (e.g., 30 to 40%) upon exit from the air drying zones. The air drying zones are preferably preceded by an air lock which prevents the hot gas in the air drying zones from escaping into the open atmosphere. 
     FIG. 1 shows an embodiment of the invention wherein an apparatus  10  for drying a reconstituted tobacco slurry includes a slurry casting device  12 , a conveyor belt  14  supported by first and second rollers  16 ,  18 , a steam box  30  and a drying apparatus  40 . The slurry casting device  12  is located above the belt  14  at an upstream portion thereof and the belt  14  carries the slurry downstream in the direction of arrow  20 . The casting device  12  may be any suitable device for depositing the slurry on the conveyor such as a casting box, reverse roll coater or other coating apparatus. The casting device preferably deposits a thin layer of a tobacco slurry comprising a mixture of tobacco scraps produced during a cigarette making process, one or more binding agents and water, e.g., a layer ranging in thickness from 0.01 to 0.2 inch thick, preferably 0.01 to 0.04 inch thick. Commonly owned U.S. Pat. Nos. 4,341,228; 4,421,126; 5,203,354; and 5,724,998, the disclosures of which are hereby incorporated by reference, describe various techniques for preparing reconstituted tobacco slurries which can be cast into the slurry. It will be apparent, however, that the drying apparatus according to the invention can be adapted for processing other types of slurries such as food products or powder compositions. 
     The conveyor belt  14  preferably defines a flat upper horizontal support and is driven in the direction of arrow  20  at a predetermined rate of speed, e.g., by rotation of one or both of the rollers  16 ,  18 . The conveyor belt  14  can be made of any suitable material, e.g., stainless steel or other material suitable for withstanding the conditions in the drying apparatus. In the preferred embodiment, the belt is made of an impervious sheet of stainless steel. 
     The steam box  30  is arranged to heat the underside of the belt  14  such that the slurry on the portion of the belt passing over the steam box is rapidly heated. The steam box can be located between the casting device  12  and the drying apparatus  40  or combined with the drying apparatus  40 . For instance, in the preferred embodiment, a downstream portion of the steam box is located in an upstream portion of the drying apparatus  40 . 
     In the case where the slurry temperature drops due to heat loss to the conveyor belt, the steam box  30  can function to raise the temperature of the cast tobacco slurry to a preheat temperature before its entry into one or more heating zones of the drying apparatus  40 . The steam can provide intense heating to the underside of the belt with the result that moisture can be driven from the inside to the outside of the slurry. In contrast, air drying arrangements remove moisture from the outside to the inside of the slurry with the result that a dried surface skin (“film”) is formed which inhibits evaporation of moisture from inside the slurry. Accordingly, by passing the belt over the steam box prior to drying the slurry in the drying apparatus  40 , it is possible to effect more rapid removal of moisture from the slurry in the drying apparatus than in the case where the drying apparatus is used without the steam box. For example, if the slurry can be dried to a desired moisture content by passing the belt through the drying apparatus at a speed such as 35 feet/minute, by incorporating the steam box in accordance with the invention prior to the drying apparatus, it is possible to increase the speed of the belt by at least 10% (e.g., 40 feet/min), preferably at least 25% (e.g. 45 feet/min), and more preferably at least 40% (e.g., 50 feet/min). 
     The drying apparatus  40  can be any suitable drying device. A preferred drying apparatus is a gas fired drier wherein gas such as natural gas is burned lean and a fan blows the combusted fuel/air mixture into a heating zone. According to a preferred drying technique, the belt passes through a plurality of heating zones which dry the slurry to a moisture content suitable for removing a partially dried sheet of the reconstituted tobacco from the end of the belt by a doctor blade. In the embodiment shown in FIG. 1, the drier includes an air lock portion  42 , and four heating zones,  44 ,  46 ,  48 , and  50 . 
     The air lock portion  42  forms an entrance to the drier and prevents hot air from escaping from the heating zones into the open atmosphere around the belt. For instance, the interior of the air lock can be placed under vacuum by a suitable pumping arrangement, e.g., an exhaust fan can be used to withdraw air from the air lock. In an arrangement wherein the downstream portion of the steam box is located in the air lock, as the slurry is carried by the belt through the air lock the slurry is only heated by the steam heat transmitted through the belt. Thus, like the slurry on the upstream portion of the steam box located outside the air lock, the slurry heated by the steam beneath the belt is not subjected to any additional heating above the belt until the belt passes into the heating zones. 
     In the embodiment shown in FIG. 1, the four heating zones  44 ,  46 ,  48 , 50  can be maintained at progressively lower temperatures, e.g., zone  44  can be at 500-625° F., zone  46  can be at 400-500° F., zone  48  can be at 325-400° F. and zone  50  can be at 300-375° F. In an experimental arrangement wherein the drier is 40 feet long and the belt is 60 foot long and 1.5 feet wide, a slurry deposited on the belt at about 150-180° F. (e.g., about 170° F.) and a moisture content of about 60-90% (e.g., about 80%) can be heated by the steam box to about 150-212° F. (e.g., about 180° F.) and then hot air dried in the gas-fired air drier to a moisture content of about 30-50% (e.g., about 35-40%) while moving the belt at speeds of 40 to 60 feet per minute. In a scaled up version suitable for commercial production of reconstituted tobacco sheet wherein the belt is wider (e.g., 4-6 feet wide) and several hundred feet long, it is contemplated that the drier may have over 5 heating zones (e.g., 9 heating zones). However, other types of drying apparatus could be substituted for the air dryer such as electric, steam or gas heated cylindrical driers, electric heating furnaces, or the like. 
     The reconstituted tobacco sheet can be removed from the end of endless conveyor belt  14  by a doctor blade  60 . As indicated above, the slurry can be reduced to a moisture content of approximately 35% to 40% in order to facilitate the doctoring of the dried slurry sheet from off the conveyor belt  14 . The partially dried sheet is then passed through a secondary drying arrangement  62  such as a multi-zone gas-fired heating furnace or drum heater to further dry the reconstituted tobacco sheet to a lower moisture content of about 10-20% (e.g., about 15%). However, it should be understood that the slurry may be dried to any desired moisture content and that the secondary drier can be omitted or another treatment substituted therefor. 
     The steam box can comprise a single enclosure or plurality of adjacent enclosures located along the underside of the belt. While each enclosure can have any desired geometry, according to the embodiment shown in FIGS. 1-3, the steam box  30  is substantially rectangular in shape. In order to withstand the high temperature steam environment, the steam box is preferably made of stainless steel. However, any material or materials suitable for such an environment can also be used for the steam box. 
     As shown in FIG. 3, the steam box  30  includes a transverse upstream wall  64 , two longitudinally extending side walls  66  and  68 , and a transverse downstream wall  70 . A bottom wall  72  extends between the lower edges of walls  64 , 66 , 68 , 70 . The lower edges of the side walls  66 , 68  are sloped downwardly towards the end walls  64 , 70  and the bottom wall  72  conforms to the lower edges of the side walls. As a result, the steam box  30  includes recesses adjacent the end walls  64 , 70  which collect steam condensate. The condensate can be removed from the steam box by suitable drainage holes such as holes  74 . Preferably, at least one condensate drain  74  is located at each recessed portion of the steam box. 
     The steam box  30  is open at the upper end thereof and a seal  76  is provided around the upper edge to prevent steam from escaping from between the upper edge of the box and the underside of the belt. The seal  76  can comprise a single piece of material or individual sections of material such as a ceramic or resilient material which can withstand a high temperature steam environment and exhibit suitable wear properties due to contact with the moving steel belt. Further, the steam box preferably includes one or more wiping arrangements which remove steam condensate from the underside of the belt. Such wipers can be of the same material as the seal  76 . In the embodiment shown in FIG. 2, a wiper  75  extends completely across the steam box at a location adjacent the downstream portion of the box. Depending on the length of the box it may be desirable to provide additional wipers at longitudinally spaced apart locations along the box. 
     The seal  76  and wipers  75  can have any desired geometry such as the shape of a bar having an oval cross-section as shown in FIG. 2. A suitable material for the seal and/or wipers is a resilient elastomeric material such as natural or synthetic rubber and a preferred material is a cured ethylene propylene diene monomer (EPDM). Other materials which can be used for the seal and wipers include any high temperature materials such as blends of polymer, ceramic, rubber and/or metal materials. However, it is preferred that the seal and wiper materials permit the conveyor belt  14  to slide over the steam box  30  without undue wear of the belt and/or the seal and wipers, yet provide an adequate sealing arrangement. 
     A highly advantageous feature of the invention is that the steam box can be attached to an existing processing line in a convenient and inexpensive manner. For instance, in a processing line having an air lock at the entrance to a gas-fired air dryer, the steam box can be mounted in a manner such that the steam box extends into the air lock, as shown in FIG.  1 . The steam box  30  can be attached by any suitable technique preferably by securing the steam box directly to the drying apparatus. 
     As shown in FIG. 2, the steam box  30  can include one or more support brackets, such as brackets  77 , welded or otherwise attached to portions of the steam box such as side wall  68 . As an example, a pair of brackets can be attached to side wall  68  and another pair of brackets can be attached to side wall  66 . Each bracket  77  can include a clip  78  which is welded onto the wall of the drying apparatus  40  and a clip  79  which is welded onto the side wall  68 . Holes in the clips  78  and  79  can be aligned and bolted together. However, it should be understood that the steam box  30  may be affixed to the apparatus  10  in other ways which hold the steam box  30  stationary relative to the moving conveyor belt. For instance, the steam box  30  may be affixed to the stationary supports of the conveyor belt  14  or can be supported by a free standing support arrangement. Because the steam box apparatus can be incorporated in existing production lines, it is possible to realize substantial savings in production costs of making reconstituted tobacco sheet with minimal costs associated with manufacture and installation of the steam box. 
     Steam can be supplied to the steam box  30  by any suitable arrangement. For example, steam can be supplied by a pair of steam tubes  80  and  82  which are conveniently located close together and extend through openings  86  and  88  in side wall  68  near one end of the steam box. In the embodiment shown, steam tube  80  is an L-shaped tube which supplies steam to a first end of the steam box and the other steam tube  82  is a straight tube which supplies steam to the opposite end of the steam box. However, steam can be supplied by other arrangements which achieve a desired distribution of steam in the box. According to a preferred embodiment of the invention, the steam is distributed uniformly in the box to achieve uniform heating of the belt. Nonuniform heating of the belt can result in uneven heating of the slurry with the result that portions of the slurry are dried considerably more than other areas. In extreme cases of nonuniform heating, the belt can become warped which may allow steam to escape into the open atmosphere. 
     According to one embodiment of the invention, uniformity of heating of the belt is enhanced by providing a perforated plate  90  between the steam supplied to the box and the underside of the belt. The plate  90  preferably encloses an upper portion of the steam box  30 , just below the seal  76 , as shown in FIGS. 2 and 4. The perforated plate  90  includes a plurality of openings such as circular holes  92  which are evenly distributed across the plate  90 . As illustrated with regard to FIG. 5, the holes can be provided in a pattern having staggered centers such that a horizontal distance  94  between two adjacent holes  92  is smaller than the vertical distance  96  between adjacent holes  92 . The holes preferably provide a total open area of about between 5 to 15% or more, preferably about 10 to 12% in the case where steam is supplied to the box at 5 psig. The plate  90  can be 18 gage stainless steel having one-sixteenth inch diameter holes with centers thereof spaced 0.216 inch apart in a transverse direction and 0.250 inch apart in a longitudinal direction. 
     The temperature rise of the slurry as it travels over the steam box can be predicted according to the formula ΔT=0.86(w)(t) where ΔT is the temperature rise of the slurry from the entrance to the exit of the steam box (assuming a slurry thickness of 0.015 to 0.030 inch), w is the steam flow in pounds per hour (pph) per inch width of the steam box (assuming 5≦w≦30), and t is the residence time (in seconds) of the slurry film over the steam box (assuming t is greater than 2). Thus, the steam box can be designed taking into account the desired temperature rise of the slurry, the length of the steam box and the amount of steam supplied to the steam box. For example, with a given size steam box, the amount of steam supplied to the box can be adjusted by changing the size of the pipes or number of pipes supplying steam to the box. However, the amount and/or pressure of steam supplied to the steam box is preferably below that which lifts the belt away from the seal around the top of the steam box. A useful range for the pressure of steam supplied to the box is 1 to 15 psig, preferably about 5 psig. 
     FIG. 6 shows one embodiment of a steam supply line for supplying steam to the steam box. Steam is suppled to steam box  30  by way of plant steam which is represented by reference numeral  100 . For example, the steam can enter the steam line at 100 psig. In the embodiment shown, the steam travels through the steam line and preferably through a double-block and bleed station  102 . The station  102  preferably provides a safe shut off and isolation of the steam box system from the high pressure plant steam during system maintenance. 
     The steam line can include a strainer  104  for filtering out foreign suspended impurities in the plant steam and one or more two pressure gages  106  such as prior to and after a pressure regulator  108 , to indicate local steam pressure at those locations. The pressure regulator  108  functions to decrease the pressure before it enters steam box  30 . For example, the pressure regulator  108  can provide pressure reduction of the 100 psig plant steam to the desired range of between 1-10 psig, preferably around 5 psig. 
     A condensate drain station  110  can be located between the second pressure gage  106  and the pressure regulator  108  to remove condensed water in the steam, and thus provide dry steam to the steam box  30 . A temperature gage  112  is preferably disposed at the steam box  30  to indicate the local steam temperature within the steam box  30 . Finally, there is preferably another condensate drain station  114  which provides quick removal of steam condensate from the steam box  30 . 
     With reference to FIG. 1, the operation of the apparatus  10  begins with the casting of an aqueous slurry onto the conveyor belt  14 . Typically, the slurry has a moisture content between 70-90%, e.g., 78.5%. The cast slurry is conveyed in the direction of arrow  20  and is moved across the steam box  30 . The steam box  30  is provided with steam from the steam plant (not shown) and functions to heat the underside of the conveyor belt  14 , which transfers heat to the slurry. Preferably, steam is introduced into the steam box  30  at a rate of approximately 5 psig. The temperature of the slurry is raised to about between 170° F. to about 190° F., and preferably around 185° F., where drying initiates in tobacco slurries without causing degradation of the tobacco product. 
     The pre-treated slurry is then conveyed through the drying apparatus  40 , thereby further reducing the moisture content, so that the dried slurry may be doctored off by the doctor blade  60 . The slurry is then conveyed to a secondary drying arrangement  62  which further reduces the amount of water in the slurry. Once the slurry is dried into a sheet, it may be cut to a desired size and used in place of natural tobacco leaves. 
     While the invention has been particularly shown and described with reference to preferred embodiments, it would be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention. For instance, the steam box could be located beneath the slurry caster and extend up to the air dryer. Such an arrangement would reduce or eliminate any drop in temperature of the slurry as it is deposited on the conveyor belt and is transported into the hot air dryer. As in the earlier embodiments, the steam box would minimize temperature gradients across the thickness and/or width of the slurry due to the intense steam heating on the underside of the belt. In the conventional hot air drier without the steam box, temperature gradients in the slurry could cause layer formation and/or film splitting since the slurry would drop in temperature when deposited on the belt and then the center of the slurry would be cooler than the upper and lower surfaces of the slurry during heating in the air drier.