MXPA99000700A - Method and apparatus for forming a cellulose article that includes solve recovery means - Google Patents

Abstract

A method and apparatus for forming a cellulose film suitable for direct contact with food by precipitation from the cellulose solution, water and a solvent of cellulose tertiary amine oxide is disclosed. After precipitation of a cellulose gel from the solution, the gel is washed at even higher temperatures and then dried to provide a cellulose film having a solvent level of no more than 40 ppm. Portions of wash water are collected and treated to recover the solvent for reuse.

Description

METHOD AND APPARATUS FOR FORMING AN ARTICLE OF CELLULOSE THAT INCLUDES MEANS OF SOLVENT RECOVERY TECHNICAL FIELD The present invention relates to the production of cellulose tubes and films suitable for direct contact with foodstuffs using a cellulose dissolving process and with the recovery of the cellulose solvent to be reused in the process.

BACKGROUND OF THE INVENTION The manufacture of seamless cellulose tubes for sausage casings using cellulose derived from the so-called "viscous process" is well known in the art. Briefly, in the viscous process, a natural cellulose such as cotton wool or wood pulp reacts chemically to form a cellulose derivative (cellulose xanthate) which is soluble in a dilute caustic solution. The solution or "viscose" is extruded as a tube into an acid bath. The extruded viscose reacts chemically with the acid bath, which results in the regeneration and coagulation of a pure cellulose tube. The chemical reaction produces several undesirable byproducts including hydrogen sulfide and carbon disulfide.

More recently, a direct cellulose dissolving process has been adapted for the manufacture of cellulose food wrappers. In this process, no cellulose derivative is formed so that the chemical reactions that are required first to form a cellulose derivative and then to regenerate the cellulose of the derivative have been eliminated. Instead, a natural cellulose is directly placed in solution with the use of a tertiary amino oxide cellulose solvent such as N-methyl-morpholine N-oxide (N MMO). The solution is thermoplastic because it hardens when it cools and flows when it overheats. The solution, when melted, can be extruded as a film tube in a water bath. The N MMO solvent, when in contact with the water bath, is extra so that a regeneration of the cellulose solution occurs. Therefore, the use of NMMO as a solvent for cellulose eliminates the need for derivatization of cellulose, as in the viscous process. This eliminates certain disadvantages of the viscous process such as the generation of gaseous sulfur components during the regeneration process. Furthermore, the solvent is recoverable so that it can be reused for the dissolution of cellulose. U.S. Patent Nos. 2, 179, 181,; 4, 145,532; 4,426,228 and Canadian Patent No. 1, 171, 614 are concerned with the formation of a cellulose solution using the NMMO solvent and the subsequent formation of cellulose articles such as films and filaments using the resulting solution. An apparatus and method for preparing an extruded cellulose solution in a continuous process are described in Patent Nos. 5,094,690 and 5,330,567. In these patents, a suspension of cellulose in an aqueous solution of NMMO is fed from the top of a vessel having a wall that has been heated. Within the container, a rotating brush spreads the suspension through the heated wall and moves the suspension down into the container. As the suspension moves down, the water evaporates and the NMMO concentration increases. Finally, the temperature of the suspension and the NMMO concentration reach a level where the cellulose dissolves so that the cellulose solution flows from the bottom of the container. U.S. Patent Nos. 5,277,857; 5,451,364 and ,597,587 describe a tubular extrusion method and apparatus that uses the thermoplastic cellulose solution to manufacture tubular films. Such films, for example, could be used as sausage casings. As described in these patents, the cellulose solution is extruded through an annular die and in an insolvent liquid bath. When it comes into contact with the bath liquid, the cellulose solvent is extracted which causes the cellulose in the solution to precipitate or "regenerate" in the form of a cellulose hydrate or "ge!". The cellulose gel tube is washed to remove residual solvents, it is plasticized and dried to produce the tubular film of pure cellulose. Because of economic factors, it is essential to recover as much cellulose solvent as possible for reuse. For this reason, the aqueous solution of the precipitation bath and wash water is treated to evaporate or separate the water for the purpose of concentrating and recovering the solvent. In the production of tubular films, the process parameters that produced convenient film characteristics are often compromised by the process parameters to precipitate or regenerate the cellulose as fast as possible and to recover the solvent. For example, for purposes of rapid cellulose regeneration, a hot water precipitation bath is preferred. With water only in the precipitation bath, the solvent is extracted faster and additionally a hot bath accelerates the extraction.

However, due to economic factors of solvent recovery, it is preferable to keep the bath in a high concentration of solvent. For example, at a solvent concentration of 5 to 30%, the bath is still considered an insolvent liquid for the purposes of precipitating cellulose, however this concentration has the advantage of facilitating recovery of the solvent. A second consideration for the precipitation bath is temperature. Generally, a high temperature will accelerate the rate of cellulose precipitation by increasing the rate at which the solvent is extracted from the solution. However, it has been found that the desirable film properties are improved if the thermoplastic solution is extruded in a cold precipitation bath. It is possible that the cold precipitation bath promotes the formation of a liquid crystal structure that will improve the strength of the resulting film. The effect of the temperature of the precipitation bath on the properties of a tubular film is further set forth in U.S. Patent Nos. 5,658,525 and 5,658,524. Therefore, the conflicting purposes of the economic solvent recovery and a strong determination of the film so that the precipitation bath is maintained at a solvent concentration of 10 to 25% and that the precipitated bath is relatively cold being the preferred temperature from 0 to 8o C, with the precipitation bath maintained at a solvent concentration of 10 to 25%, not all the solvent contained in the extruded solution is removed in the precipitation bath so that the regenerated cellulose gel contains a amount of recoverable solvent. Consequently, the film, when it is still in the gel state, is washed to separate the greater amount of solvent that the economy allows from the film. This adds to the economics of the method in which the extracted solvent can be recycled into the system to dissolve the natural cellulose for extrusion. The washing conditions could further affect the characteristics of the resulting film. Particularly, it has been found that subjecting the regenerated cellulose gel to a drastic temperature change will adversely affect the properties of the film. Therefore, while the cellulose gel is washed with hot water to accelerate the extraction of the solvent, it is preferred that the cold cellulose gel leaving the precipitation bath is not immediately exposed to a hot water bath. Furthermore, if the cellulose film is intended for uses involving contact with food, it is essential that the residual solvent content of the film be reduced to levels below any level of economic recovery of the solvent and preferably not more than 40. ppm. Therefore, additional cellulose gel treatments are required to comply with government regulations for use in food contact applications. Therefore, it is an object of the present invention to provide a process for regenerating a cellulose film from a cellulose solution, a cellulose solvent and water including recovering the cellulose solvent for reuse. Another purpose is to provide a process for regenerating a cellulose film that is suitable for direct contact with food, since the level of solvent remaining in the film when ready to be used is no greater than 40 ppm on a dry cellulose base. . A further purpose is to provide such a process where the regenerated cellulose gel comes into contact with bath water at temperatures that increase to reduce the solvent levels in the regenerated cellulose at levels no greater than 40 ppm.

BRIEF DESCRIPTION OF THE NONDION According to the present invention, the original material is a cellulose solution comprising a natural cellulose which has been subjected to direct dissolution by a solvent comprising an aqueous solution of an amine oxide. Processes for such dissolution using an aqueous solution of N-methyl-morpholine N-oxide (N M NO) are known in the art and do not form part of the present invention. The product of said dissolution process is an extruded solution having a melting temperature of about 60-70% and comprising 10-20% cellulose, 70-80% of NMMO and 5-15% of water. The cellulose solution is hereinafter characterized as a non-derivatized cellulose solution. For the purposes of the specification, the term "non-derivatized" cellulose means a cellulose that has not been subjected to covalent bonding with a solvent or reagent but has been dissolved by association with a solvent or reagent through the Van der forces. Waals such as the hydrogen bond. The cellulose solution, which has a melting temperature of about 65 ° C, is extruded through an annular die and around a mandrel that hangs from the die. Suitable extension methods and apparatus for such extrusion are described in U.S. Patent Nos. 5,227,857; 5,451, 364 and in US Serial Nos. 08/827, 130 and 08/827, 152, the descriptions of which are incorporated herein by reference. Briefly, and as described in the aforementioned patents, the extrusion from the die is down through an air space and into a bath of non-solvent liquid, "non-solvent" as used herein represents a liquid which is not a cellulose solvent such as demineralized water or a non-solvent concentration of NMMO in water. In the bath, the NMMO is extracted from the extruded tube by coagulating in this way and regenerating the non-derived cellulose to form a tube composed of a cellulose gel. The gel tube is washed to remove the residual NMMO solvent. It is then plasticized with a polyol such as glycerin and dried to form a tubular film. For the purposes of economy in the recovery of the solvent, the precipitation bath is maintained at a solvent concentration of about 5 to 30%, with 10 to 20% being preferred. The solvent extracted from the extruded solution tends to raise this concentration so that it is a stable introduction of water into the bath and a stable removal of liquid from the bath to maintain this concentration. Also, the bath temperature is maintained between 0a and 20 ° C and preferably below 10 ° C. It has been found that these cold temperatures improve the desirable properties of the film such as the tensile strength and, in the case of tubular films, the pressure to the trap. The burst pressure is the internal pressure required to burst a wet tubular film. It is usually determined by filling the tube with water and then increasing the water pressure until the tube bursts. The diameter of the burst tube (burst diameter) and the amount of film extension to the burst diameter are other desirable properties. Likewise, US Pat. No. 5,216,144 mentions that for the production of fiber, a 0 ° C precipitation bath allows the operation at a higher concentration of solvent in the bath without detriment to the characteristics of the fiber. The extruded solution is initially composed of about 75% solvent. In the precipitation bath, this is reduced to approximately 45%. At this level of solvent, the cellulose in the extruded tube is precipitated to form a self-stable film of hydrated cellulose gel. After the precipitation bath, the cellulose gel is transported through a washing operation to remove as much of the 45% retained solvent as is economically possible. It is preferred to wash this solvent retained from the gel using hot demineralized water (35 to 70 ° C) in order to achieve extraction as quickly as possible. However, moving the cold cellulose gel from the cold precipitation bath (0 to 20 ° C) directly in contact with the hot wash water compromises the physical properties of the resulting film. For example, in a tubular film, the working limits of the casing, as represented by multiplying the bursting pressure of the tubular film by its diameter at the time of the burst, can be reduced if the temperature differential between the precipitation bath and the wash water is much higher than about 5 to 15 ° C. Accordingly, it has been found that washing the solvent from the cellulose gel film should be done by initially contacting the gel with cold wash water and then gradually increasing the temperature of the wash water to about 35 to 50 ° C. While it is desirable to wash as much of the N M MO cellulose solvent as possible, it should be appreciated that there are limits to the amount of solvent that can be recovered economically. For example, at least 95% must be recovered in order to economically justify the use of the direct dilution system to make a cellulose film with the limit of economic recovery that is not more than about 99.98%. so, the washing step should reduce the solvent content of the gel to at least about 250,000 ppm based on the weight of the dry cellulose (95% recovery of solvent) and preferably from about 1000 to 2000 ppm (99.98 to 99.96%) of solvent recovery). The attempt to reduce the solvent level below 1000 to 2000 ppm, is generally not more economical in terms of solvent recovery. In other words, below about 1000 to 2000 ppm there is so little solvent in the cellulose gel that it is no longer economical to process the wash water to recover the solvent. While a film containing 1000 to 2000 ppm of cellulose solvent is suitable for some applications, it does not have the approval of the United States government for direct contact with the food. The maximum solvent level for use as a direct contact film with the food, such as a sausage wrap, is 40 ppm based on the weight of the dry cellulose. Accordingly, after the first wash, the cellulose gel must be subjected to a second wash to provide a film having a solvent level that is no more than 40 ppm based on the weight of the dry cellulose. Since the solvent level is no more than 1000 to 2000 ppm after the first wash, the cellulose gel can be exposed to higher temperatures to facilitate the reduction of the solvent to acceptable levels without compromising the desirable film properties. Therefore, the second wash is at about 50 ° C and preferably at 60 ° C or more. Considering that the liquid from the precipitation bath and the first wash is processed to recover the solvent, the wash water from the second washing operation can be discarded without the need for additional treatment.

It is preferred that the second wash reduce the solvent content of the cellulose gel to no more than 40 ppm. However, after washing, the cellulose gel is dried and the drying process can possibly impel a portion of the solvent. Consequently, it may be sufficient if the second wash reduces the solvent content to 100 to 150 ppm. What matters is that after drying, the solvent content of the cellulose film for use in direct contact with the food should be no more than 40 ppm. Accordingly, the present invention can be characterized in one aspect thereof by a process that includes the formation of a cellulose film for direct contact with food from a cellulose solution, a tertiary amine cellulose solvent and water together with the recovery of the solvent, which comprises the steps of: a) extruding a solution of 10 to 20% cellulose, 70 to 80% solvent and 5 to 15% water in the form of a tube or film within a first aqueous bath and in the bath, extract the solvent from the precipitate solution of a cellulose gel in the form of a tube or film containing no more than about 45% solvent. b) maintaining the temperature of the first aqueous bath between 0 and 20 ° C and the concentration of the solvent of the bath between 5 and 30% during the course of the extraction of the solvent from the extruded solution; c) subjecting the cellulose gel tube or film from the first aqueous bath to a first wash with water to extract the solvent from the cellulose gel, the first washing which is conducted by washing water flowing countercurrent with respect to the passage of the tube or film and the temperature of the first wash which is not more than about 5 to 10 ° C greater than the first aqueous bath where the cellulose gel first makes contact with the countercurrent flow and between about 35 and 70 ° Where the cellulose gel finally makes contact with the countercurrent flow; d) subjecting the cellulose tube or film from the first wash to a second wash of hot water maintained at a temperature above the hottest temperature of the first wash and which keeps the cellulose gel in contact with the second wash for a sufficient time for further reducing the solvent content of the cellulose gel; e) drying the cellulose gel to provide a cellulose film or tube having a cellulose content of not more than 40 ppm based on the weight of dry cellulose; and f) treating the liquid from the first aqueous bath and the first wash to recover the solvent in those liquids. In another aspect, the present invention is characterized by an apparatus for forming a cellulose film suitable for direct contact of the feed from a cellulose solution including recovery of the solvent for reuse comprising: a) means for extruding a tube or a film composed of a thermoplastic cellulose solution of 10 to 20% cellulose, 5 to 15% in water and 70 to 80% of tertiary amine oxide cellulose solvent; b) a first aqueous bath to receive the extrusion of step a) and extract from the extrusion a sufficient amount of solvent to cause the precipitation of the cellulose from the solution and form a tube or film of hydrated cellulose gel containing no more of approximately 45% of the solvent; c) means for maintaining the first aqueous bath at a temperature of 0 to 20 ° C and at a solvent concentration of 5 to 30%; d) a second hot demineralized water bath to rinse the solvent from the cellulose gel to reduce the solvent content of the cellulose gel to 1000 to 2000 ppm based on the weight of the dry cellulose, the water of the second bath that moves countercurrent to the passage of the cellulose gel through the second bath where the second bath has an effluent rich in amine oxide; e) means for maintaining the temperature of the water entering the second bath at about 35 to 50 ° C and the temperature of the water leaving the second bath which is about 12 to 16 ° C; f) a tube to receive the cellulose gel from the second bath, the tube containing hot wash water to wash the cellulose gel and further reduce the total solvent content of the cellulose gel, the temperature of the water in the tube that it is greater than the temperature of the water that enters the second bath; g) means for drying the cellulose gel that passes from the tube to provide a cellulose film having a solvent content of not more than 40 ppm based on the weight of the dry cellulose; and h) means for treating the effluent from the second bath to coat the amine oxide contained in the effluent.

DESCRIPTION OF DI BUSHINGS The only figure is a view that shows in schematic form the stages of a process to extrude tubular films and recover the solvent.

DETAILED DESCRIPTION OF THE INVENTION Referring to the drawings, Figure 1 shows a schematic representation of a tubular extrusion system for the production of regenerated cellulose tubular films of the type that can be used for sausage casings or the like. In summary, the system includes the apparatus generally indicated at 12 to solubilize a natural cellulose. For example, the apparatus may be of the type described in U.S. Patent Nos. 5, 094,690 and 5, 330, 567 as referred to hereinbefore. In this apparatus, a mixture of natural cellulose, water and an amine oxide cellulose solvent such as an NMMO introduced into the upper part of a heated vessel 14 through an inlet 16 is converted to a cellulose solution, as successively referred to as "impurity", which is forced from a lower outlet 18. The cellulose solution or "impurity" is thermoplastic in that it is solid at room temperature and has a melting point of about 65 ° C. The impurity produced in the container 14 is pumped to an extrusion die 20 positioned to extrude the impurity downwardly into a first bath 22. For the purposes of the tubular extrusion, the die 20 is annular and the resulting tubular extrusion 24 passes over a mandrel 26 that connects the air space between the die and the regeneration bath. The mandrel is not part of the present invention and reference is made to the Patents of the United States Nos. 5,277,857; 5,451, 364 and US Serial Nos. 08/827, 130 and 08/827, 152 for a more detailed description of the operation and function of the mandrel structure.

In the first bath, the amine solvent is extracted from the extruded tube to coagulate and regenerate the cellulose from the solution to form a seamless tube 28 of hydrated cellulose gel. From the point of view of the process velocity, it would be preferred to extract as much solvent as possible from the extruded tube in the first bath 22 and to do this as quickly as possible. Favorable conditions for this purpose include the use of hot water in the bathroom. However, in order to recover solvency economically for reuse, the concentration in the bath 22 is preferably maintained at some higher level and in the range of 5 to 30% with 10 to 20% which is a preferred scale. At this concentration, the bath is considered non-solvent and allows the extraction of sufficient solvent from the extruded solution 24 to cause coagulation and regeneration of the cellulose from the solution and produce the tube 28 of hydrated cellulose gel. The desired concentration in the first bath can be maintained, for example, by the introduction of fresh demineralized water into the bath through a pipe 30 as necessary while continuing the removal of the aqueous solvent solution from the bath from an overflow and out through the flow line 32. It has also been found that the concentration of solvent in the bath which is in the range of 10 to 20%, the desirable properties of the film are improved by keeping the first bath 22 relatively cold, preferably below 10 ° C. For example, it is considered that colder temperatures reduce the speed of solvent extraction so that the cellulose gel that regenerates is denser than if it regenerates at a higher temperature.

In the first bath 22, the cellulose solvent content of the extruded solution 24 is decreased to about 45%.

While this concentration allows the precipitation and regeneration of cellulose, the resulting cellulose gel tube 28 still contains a considerable amount of solvent that must be removed. For this purpose the cellulose gel tube 28 is transported to a second bath 34 to remove as much amine solvent as is economically possible. A wash to reduce the solvent content of the cellulose gel tube 20 to about 1000 to 2000 ppm based on the dry weight of the cellulose has in effect the potential to recover 99% or more of the solvent contained in the initial extrusion 24. As it was written up, washing the cellulose gel tube 28 with hot water facilitates the extraction of the solvent. The hot water causes the regenerated cellulose structure to collapse which makes it easier to force the solvent from inside the cellulose structure. However, it has been found that the tube 28 of cellulose gel which is relatively cold when it leaves the regeneration bath can not be immediately subjected to a hot water wash. Doing this causes the resulting film to become dark and compromises tensile strength and burst pressure. One possible explanation is that contacting the regenerated cellulose gel with hot water causes such rapid extraction of the solvent that voids are created in the structure of the film which weakens the cellulose structure. The darkening of the film can also be caused by such voids. However, whatever the cause, it has been found that it is not desirable to contact the regenerated cellulose gel from the first bath 22 immediately with a hot water wash. Accordingly, it has been found that desirable film properties can be obtained if the tube 28 of the cellulose gel is exposed to a gradually rising water temperature. In this way, it is the cellulose gel that contains the lowest solvent that is exposed to the hottest water. Therefore, it has been found that washing the cellulose gel tube 28 with water that gradually increases in temperature obtains the benefits of extracting the solvent in a manner in which it produces a dense cellulose structure substantially free of solvent. To achieve exposure to increasingly hot water, the flow through the second bath 34 preferably is countercurrent to the passage of the tube 28. This countercurrent washing should reduce the amine content of the tube 28 to at least 250000 and preferably around from 1000 to 2000. The wash water leaving the second bath 34 through the drain 36 is rich in amine and has a concentration of about 6 to 8% amine cellulose solvent. This amine-rich wash water is subsequently combined with the aqueous solvent solution of the first bath 22 and the mixture is then treated to recover the solvent. Such treatment may involve contact with successive anion and cation exchangers 38 to remove ions that may have been generated during the elaboration of the impurity and cellulose regeneration processes. The purified mixture is then treated by any suitable means such as reverse osmosis or evaporation to concentrate the amine cellulose solvent (NMMO). The recovered concentrated cellulose solvent is then fed to the system for the purposes of solubilizing the cellulose and making the impurity to be extruded. For example, the recovered solvent can be fed to a mixer 42 where it is mixed with natural cellulose pulp and water to form a suspension that is fed through the inlet 16 to the container 14. The countercurrent wash in the second bath 34 It can be achieved in a cascade system. In such a system, the casing would move upwards through an inclined passage while the treated water introduced at the high end flows downward. In a preferred arrangement as shown in the figure, the cellulose tube 28 is afted through the tube 48. The tube is divided vertically by internal pairs 50 into a plurality of cells, such as the three cells 52, 54 and 56 as it shows. While three cells are illustrated, it should be appreciated that any number may be used within the context of the present invention. There is communication between the cells to accommodate the flow of wash water from the last cell 56 to the first cell 52. In addition, the temperature of the water in each cell can be controlled individually through any suitable heating / cooling means 58. With this provision, the hot demineralized wash water that enters the last cell 56 through the inlet 60, fills and then flows through the cells to the outlet 36 in the first cell 52. The temperature of the wash water entering the last cell 56 is on the scale of 45 to 70 ° C and preferably is not more than about 50 to 60 ° C. The temperature of the wash water in the second cell 54 is colder and the temperature in the first cell 52 is colder. The temperature in the first cell 52 is generally no more than about 5 to 10 ° C hotter than the first bath 22 and preferably is 12 to 16 ° C. Therefore, backwashing creates a plurality of temperature zones wherein each successive zone contacted by the cellulose gel tube 28 is larger than the preceding zone. In countercurrent washing, the solvent concentration of the cellulose tube is reduced to about 1000 to 2000 ppm and the concentration of the solvent in the wash water is increased to about 6 to 8%. At this level, the wash water from the drain 36 can be mixed with liquid from the overflow drain 32 of the first bath 22 and treated to recover the solvent. As an alternative, at least part of the wash water from the tub drain 36 can be fed directly into the first bath 22 to help maintain the desired solvent concentration. If this is done, it may not be necessary to add fresh demineralized water from outlet 30 to bath 22. As noted above, the cellulose tube 28 containing a solvent level as low as 1000 to 2000 ppm as it exits the second Bath 34 is not acceptable for direct contact with the food. Consequently, the tube must be washed a second time. The second washing is preferably achieved by aftenoning the tube through a washing tub 44 which allows sufficient residence time to separate the solvent level below 1000 ppm and preferably to no more than 40 ppm. The separation of this level is facilitated by using water at a higher temperature than the water in the bath 34. Therefore, the water entering the wash tub through the line 62 may be at 60-70 ° C or more . Since the wash water effluent from the wash tub 34 contains a level of solvent that is too low to recover economically, it is simply discharged through a drain 46 without further treatment. Upon exiting the second wash tub 44, the cellulose gel tube can be contacted with a glycerin solution (not shown) to add a plasticizer. This is conventional in the art. The cellulose gel tube 28 is then dried to a moisture content of about 6% based on the weight of the dry cellulose. This is achieved by inflating the gel tube 28 and passing it through a dryer indicated at 64. The dried cellulose film 66 is then collapsed to its flat width and wound on a reel 68. It has been found that the cellulose gel can be extracted part of the solvent. The amount extracted by drying may vary depending on the conditions although generally if the solvent content is higher, a greater portion of the solvent can be extracted can be extracted by drying than if the content is lower. For example, drying a cellulose gel containing 100 to 200 ppm solvent can result in a cellulose film containing no more than 40 ppm of the solvent. Accordingly, for use in a food contact application, the solvent content of the cellulose gel leaving the wash tub 44 can be greater than 40 ppm and still produce an acceptable film. Therefore, for the purposes of the present invention, it is sufficient if the second wash removes sufficient solvent so that after drying, the film contains no more than 40 ppm of solvent. In other words, ia The drying step and the second washing step should together remove enough solvent to bring the level of solvent from the cellulose gel that enters the washing tub 44 (approximately 1000 to 2000 ppm) to no more than 40 ppm in the film of dry cellulose. The total residence time of the cellulose tube in the second bath 34 and the washing tub 44 and the temperature of the washing water are factors in determining the amount of solvent removed. After more prolonged residence time and warmer temperature, more solvent is removed. Nevertheless, it has been found that the speed of the tube through the system is also important and that a faster speed is better than a slower speed. It is considered that a higher relative speed alters the limit layer to improve the mass transfer of solvent out of the extruded tube. One option to increase this relative velocity is to increase the water flow in the second countercurrent bath 34. However, at some point this becomes impractical as the quantity conforms to the amount of water used to achieve a high flow rate to a decrease in the concentration of the solvent contained in the wash water in the drain 36. A second option, and the preferred one, is to increase the extrusion rate of the cellulose solution so that the speed of the tube 28 through the system is increased. For example, with a tubular extrusion size, it was found that the speed is a control factor for the removal of the solvent in almost all instances where the concentration of the first bath is in the range of 12 to 20%, the washing temperature final in the tub 44 is in the range of 64 to 71 ° C and the total contact time of the extruded tube in the second bath 34 and the washing tub 44 is between 12 to 30 minutes. It should be noted that most of the time spent washing the cellulose tube 28 is used for the separation operation in the tub 44 to reduce the solvent content below 1000 to 2000 ppm of cellulose gel level entering the cellulose. tina 44. For example, at a typical extrusion rate of approximately 30 to 31 meters per minute, the total residence time of the extruded tube 24 in the first bath 22 would be approximately 20 seconds. In the second bath 34, a residence time of only 2 1/2 minutes is required to reach the limit of 1000 up to ppm of economic solvent recovery. The equilibrium of the residence time for the final wash in the vat 44 is required to bring the level of solvent in the tube 28 of the cellulose gel below 40 ppm or at least below a level where the subsequent drying step is capable of extracting enough solvent to provide a dry film having no more than 40 ppm of solvent. Therefore, it should be appreciated that the present invention attains the objects intended to provide the method and apparatus for producing a cellulose film suitable for direct contact with food using a solvent process wherein the solvent content of the films is no longer of 40 ppm based on the weight of dry cellulose. This is achieved in part by contacting the extruded and regenerated cellulose gel with an initial countercurrent wash that increases in temperature and then with a second wash at an even higher temperature and then the film is dried. The water in the first bath is treated to recover the solvent for reuse, thus adding efficiency to the process. The second hot water bath executes a separation operation to reduce the solvent levels to the point where such washing, together with the subsequent drying step, provides a cellulose film containing no more than 40 ppm of the solvent.

Claims (17)

1. CLAIMS 1 . A process for forming a non-derivatized cellulose film suitable for direct contact of food from a thermoplastic solution of cellulose, water and a tertiary amine cellulose solvent that includes recovering the solvent for reuse, the process comprising the steps of : a) extruding the solution comprising at least 70% solvent as a tube or film inside a first aqueous bath, the first bath causing the extraction of sufficient solvent to precipitate a cellulose gel from the solution in the form of a tube or film having a solvent content of less than 45% by weight of dry cellulose; b) maintaining the temperature of the first aqueous bath at a temperature of 0 to 20 ° C and the concentration of the solvent of 5 to 30% during the course of the extraction of the solvent from the extruded solution; c) subjecting the cellulose gel tube or film from the first aqueous bath to a first wash to extract the solvent from the cellulose gel, the first wash being conducted by washing water flowing countercurrent to the gel passage of cellulose, the temperature of the water which is not more than about 5 to 10 ° C higher than the temperature of the first aqueous bath where the cellulose gel first makes contact with the countercurrent flow of the first wash and between about 35 and 70 ° C where the cellulose gel finally makes contact with the countercurrent flow of demineralized water; d) subjecting the cellulose tube or film from the first wash to a second wash of hot water maintained at a temperature above the hottest temperature of the first wash and which keeps the cellulose gel in contact with the second wash for a sufficient time to further reducing the solvent content of the cellulose gel; e) drying the cellulose gel to provide a film or cell tube having a solvent content of not more than 40 ppm based on the weight of dry cellulose; and f) treating the liquid from the first aqueous bath and the first wash to recover the solvent in those liquids.
2. 2. A process as claimed in claim 1, comprising contacting the cellulose gel from the first aqueous bath with the first countercurrent wash in a plurality of successive controlled temperature washing zones and maintaining the temperature of each zone contacted by the cellulose gel greater than the temperature of the zone immediately preceding.
3. 3. A process as claimed in claim 2, wherein the temperature of the first wash zone contacted by the cell gel is 12 to 16 ° C and the temperature of the last wash zone is from 45 to 70 ° C.
4. 4. A process as claimed in claim 1, wherein the temperature of the first aqueous bath is about 7 ° C, the temperature of the water in said wash being about 15 ° C where the flow countercurrently. it is contacted first by the cellulose gel and approximately 50 ° C where it is contacted at the end by the cellulose gel.
5. A process as claimed in claim 1, comprising subjecting the cellulose gel to the first wash for a sufficient time to reduce the total solvent content of the cellulose gel to at least 250,000 ppm based on the weight of the cellulose dry
6. 6. A process as claimed in claim 5, wherein the total solvent content of the cellulose gel is reduced from 1000 to 2000 ppm based on the weight of the dry cellulose.
7. 7. A process as claimed in claim 1, wherein the total solvent content of the cellulose gel after the second wash is not greater than 40 ppm.
8. 8. A process as claimed in claim 1, wherein the temperature of said second wash is about 60 ° C.
9. 9. A process as claimed in claim 1, wherein drying the cellulose gel provides a cellulose film or tube having a moisture content of about 6% based on the weight of the dry cellulose.
10. 10. A process for forming a non-derivatized cellulose cellulose film suitable for direct contact with food from a cellulose solution, water and solvent of tertiary amine oxide cellulose and recover the solvent for reuse that includes the steps of: a) extrude a solution of 10 to 20% of cellulose, 5 to 15% of water and 70 to 80% of solvent in the shape of a tube or film within a first aqueous bath and maintaining the extruded solution in the first bath for a sufficient time to extract the solvent from the extruded solution and precipitate a hydrated cellulose gel in the form of a tube or film. Containing no more than about 45% solvent by weight of dry cellulose; b) maintaining the temperature of the first aqueous bath between 0 and 10 ° C and the concentration of the solvent between 10 and 20% during the course of the extraction of the solvent from the extruded solution; c) subjecting the cellulose gel tube or film to a first counter-current wash with demineralized water, the temperature of the water when it is first contacted with the cellulose gel which is 12 to 16 ° C and the temperature which is 35 to 50 ° C when it is contacted at the end by the cellulose gel and continue washing countercurrently for a sufficient time to reduce the solvent content of the cellulose gel from 1000 to 2000 ppm based on the weight of the dry cellulose; d) subjecting the cellulose tube or film from the first wash to a second wash at a temperature greater than the highest temperature of the countercurrent wash; and e) treating the demineralized water of the first wash to recover the washed solvent from the cellulose gel.
11. 11. A method as claimed in claim 10, wherein the second wash is at a temperature of at least 60 ° C.
12. A method as claimed in claim 10, which comprises drying the cellulose gel after the second washing, the second washing and the drying together providing a cellulose film having a solvent content of less than 40 ppm based on the dry cellulose weight.
13. 13. A method as claimed in claim 12, wherein the drying provides the cellulose film with a moisture content of about 6% based on the weight of the dried cellulose.
14. A method as claimed in claim 10 comprising: a) extruding the solution at a rate of at least 30 meters per minute; b) keeping the extruded solution in contact with the first aqueous bath for at least twenty seconds to precipitate the cellulose gel; c) subjecting the cellulose gel to backwashing for at least 2 Vz minutes; d) drying the cellulose gel from the second wash; and e) subjecting the cellulose gel to the second wash for a sufficient time to reduce the total solvent content of said cellulose gel to a level such that upon drying, a cellulose gel having a solvent content of less than 40 ppm based on the weight of the dry cellulose and a moisture content of at least 6% by weight of dry cellulose. 5.
15. Apparatus for forming a non-derivatized cellulose film from a solution of cellulose, water and tertiary amine cellulose solvent and recovering the cellulose solvent for reuse comprising: a) means for extruding a tube or a film Composed of a thermoplastic cell solution of 1 to 20% cellulose, 5 to 1.5% in water and 70 to 80% solvent of the tertiary amine oxide ulna cell; b) a first aqueous bath to receive the extrusion of step a) and extract from the extrusion a sufficient amount of solvent to cause the precipitation of the cell from the solution and form a tube or film of hydrated cellulose gel which contains no more than about 45% of the solvent; c) means for maintaining the first aqueous bath at a temperature of 0 to 20 ° C and at a solvent concentration of 5 to 30%; d) a second bath of hot demineralized water to rinse the solvent from the cellulose gel to reduce the solvent content of the cellulose gel to at least 250,000 ppm based on the weight of the dry cellulose, the second bath which includes means for moving the hot water countercurrent to the passage of the cellulose gel through the second bath where the second bath has an effluent rich in amine oxide; e) means for maintaining the temperature of the water entering the second bath at about 35 to 50 ° C and the temperature of the water leaving the second bath at about 12 to 16 ° C; f) a water wash tub for receiving the cellulose gel from the second bath to further wash said gel, the temperature in the wash tub that is higher than the temperature of the water entering the second bath; g) means for drying the cellulose gel that passes from the washing tube to provide a cellulose film having a solvent content of not more than 40 ppm based on the weight of the dry cellulose; and h) means for treating the effluent from the second bath to coat the amine oxide contained in the effluent.
16. 16. Apparatus as claimed in claim 15, wherein the second bath comprises a plurality of heated zones and means in each of the zones to maintain the temperature of each of said zones in contact with the cellulose gel at a higher temperature. temperature than the preceding zone contacted by the gel.
17. 17. Apparatus as claimed in claim 15, wherein the second bath reduces the solvent content of the cellulose gel between 1000 to 2000 ppm based on the weight of dry cellulose. SUMMARY A method and apparatus are described for forming a cellulose film suitable for direct contact with food by precipitation from the cellulose solution, water and a tertiary amine oxide cellulose solvent. After precipitation of a cellulose gel from the solution, the gel is washed at even higher temperatures and then dried to provide a cellulose film having a solvent level of no more than 40 ppm. Portions of the wash water are collected and treated to recover the solvent for reuse.