Process for the production of transfer mixtures by direct dissolution and thin film evaporator

Abstract

The invention relates to a method for producing a transfer mixture by direct dissolution in a thin-film evaporator having a feed section, a housing and an outlet, wherein the feed section introduces a product consisting essentially of cellulose, water and a functional liquid into the housing, wherein an evaporator shaft arranged in the housing rotates over the heated interior of the housing, thereby subjecting the product to heat and evaporating a portion of the water, so that a transfer mixture is obtained which flows to the outlet in a general composition of (a) in which the product does not overheat upon contact with the surface of the housing, which is at least 20 K higher than the temperature of the product, or (b) in the case of NMMO as the functional liquid, the amount in the general composition is not greater than x H2O = -0.235 x Cell + 0.235, at least x H2O = -0.59 x Cell + 0.2047.

Description

Technical Field

[0001] The present invention relates to a method for producing a transfer mixture by direct dissolution, as described in the preamble of claim 1, 2, 11 or 12. Background Technology

[0002] For example, WO 1994 / 006530 A1 describes a method for producing molding solutions from cellulose on an industrial scale by direct dissolution via evaporation of a cellulose-water-functional liquid mixture (hereinafter also referred to as the product, where cellulose, the functional liquid, and water are referred to as product components), wherein N-methylmorpholine-N-oxide (hereinafter also referred to as NMMO or amine oxide) is used as the functional liquid (commonly referred to as the Lyocell molding solution method or the amine oxide method). A cellulose-water-functional liquid mixture or product refers to a mixture of all water contents or substances ranging from a high water content in the presence of a suspension to a low water content in a molding solution, and therefore includes transfer mixtures.

[0003] The aforementioned literature describes the use of thin-film evaporators for evaporating water from cellulose-NMMO-water mixtures. These thin-film evaporators are now widely used in lyocell-based solution processes and are known and commonly used in various forms and designs. They are all characterized by an evaporator shaft within the evaporator housing distributing the product onto an inner surface of the housing, which serves as the heating surface, thereby forming a thin film. This film can generate additional turbulence and reduce viscosity with increasing rotational speed, thus rapidly heating the product and causing some water to evaporate. In this context, a heating surface refers to any heated surface intended to introduce thermal energy into the product via the temperature difference between the heating surface and the product. However, this method discloses the production of molding solutions, which are referred to hereinafter as molding solutions as defined in DE 102012 103 296A1.

[0004] Although thin-film evaporators are typically designed with a vertical evaporator shaft, for example as described in WO 1994 / 006530A1, they can also be designed with a horizontal evaporator shaft, for example as described in WO 2020 / 249705A1.

[0005] Those skilled in the art are also familiar with methods for producing molding solutions from cellulose by direct dissolution, in which an ionic liquid (hereinafter referred to as IL) is used instead of NMMO as the functional liquid, but in other respects the molding solution is also produced by direct dissolution by evaporating water from a cellulose-IL-water mixture using a thin-film evaporator.

[0006] In this article, IL or ionic liquid refers to a group of organic compounds that, despite having ionic structures, possess low melting points (<100°C) and are therefore also referred to as molten salts. Thus, using IL as a functional liquid always implies an implementation scheme suitable for direct dissolution methods within the group of ionic liquids.

[0007] Those skilled in the art will know that some ILs tend to thermally decompose at elevated temperatures, therefore processes using heated ILs must keep the IL temperature below their decomposition temperature. As an example, IL[DBNH][OAc] can be mentioned as decomposing above approximately 100°C. Those skilled in the art will also know that NMMO begins to decompose as the water content of the NMMO-water mixture decreases at elevated temperatures. Typically, above 140°C, the risk of explosion increases with decreasing water content, for example, the risk of explosion due to explosive autocatalytic decomposition, thus requiring further temperature increases and therefore posing a serious explosion risk. Depending on the composition of the mixture, decomposition can also be observed at temperatures above 125°C, as the presence of reducing agents (such as cellulose) and heavy metal ions (such as iron ions) can lower the decomposition temperature. In the case of the direct dissolution method described above, although stabilizers are typically added, a lower decomposition temperature can be expected due to the presence of cellulose in the process and the use of iron materials in the machine manufacturing.

[0008] Those skilled in the art will also recognize the fact, referred to in the context of this invention, as a problem of substantial overheating: to avoid decomposition of the functional liquid, very careful control of the product temperature (hereinafter also referred to as product temperature) is necessary. Therefore, a process is chosen in which the desired temperature of the product and its equilibrium temperature are below the decomposition temperature.

[0009] In multi-component mixtures, such as cellulose-functional liquid-water mixtures, an equilibrium temperature is assigned to each component at the primary process conditions (pressure) under which the mixture begins to boil. If energy is added to the system, some volatile components (in this case, water) evaporate. Simultaneously, the composition of the mixture changes, and therefore the equilibrium temperature also changes. Thus, when energy is supplied, the mixture is heated, and its composition changes along its equilibrium curve due to the evaporation of volatile components. As the water content decreases, the viscosity increases due to the increased concentration of cellulose and the increased solubility of the functional liquid or functional liquid-water mixture. With increasing viscosity, the evaporation of water in the product increases the risk of transport limitations, making evaporative cooling insufficient, and thus the temperature of the water-functional liquid mixture and the product increases above the equilibrium temperature. The equilibrium temperature of the mixture varies with its composition and process pressure. Therefore, even without transport limitations, the temperature of the mixture increases throughout the process, and decomposition may occur even at the equilibrium temperature if process parameters are appropriately selected. Therefore, those skilled in the art choose process parameters in which the equilibrium temperature always appears below the decomposition temperature. For NMMOs as functional liquids, the equilibrium temperature is typically in the range of 50°C to 110°C throughout the process.

[0010] When NMMO is used as a functional liquid, the exothermic decomposition process can also lead to localized overheating (hot spots) of the material. If transport is restricted, the heat of reaction cannot be sufficiently dissipated. Subsequently, this can trigger a further exothermic decomposition process.

[0011] Significant overheating results in functional liquids, cellulose, or cellulose-functional liquid-water mixtures being heated to temperatures high enough to decompose. Overheating that causes the temperature of a product or product component to exceed its specific decomposition temperature will be referred to hereinafter as substantial overheating. Therefore, it is not important whether the overheating is due to insufficient evaporative cooling caused by transport limitations or to excessive drying of the product under equilibrium conditions.

[0012] The decomposition of functional fluids is as described above. On the one hand, it necessitates the replacement of lost (often very expensive) functional fluids; on the other hand, it also dramatically increases the risk of explosion when using NMMO as a functional fluid.

[0013] However, substantial overheating can also involve the decomposition of cellulose or cellulose-functional liquid-water mixtures (or, by definition, the product), due to a decrease in the degree of polymerization (DP, as the technicians call it) of cellulose at excessively high temperatures, possibly accompanied by an increase in shear—which typically occurs with increased viscosity. Therefore, substantial overheating of a product can have the effect of causing product damage or safety hazards by decomposing some parts of the product or the product itself.

[0014] Of course, due to non-thermally induced decomposition processes (such as free radical reactions), process control at temperatures below the decomposition temperature can also damage the product or compromise safety. However, in practice, this can be mitigated by common stabilizers.

[0015] Furthermore, aiming for a temperature control above the equilibrium temperature of decomposition can also lead to product damage and safety hazards. In practice, technicians do not aim for this operating mode.

[0016] However, when using a thin-film evaporator for direct dissolution, temperature control must be carefully considered due to the specific nature of the equipment. On one hand, the characteristic high heat input from the small amount of product present in the equipment leads to the effective evaporation of volatile components. On the other hand, due to the small amount of product in the equipment, process fluctuations (such as mixture flow rate, energy input, or product viscosity) can only be compensated for to a limited extent. Therefore, even a slight decrease in the supply mixture flow rate can lead to a significant increase in product temperature. This involves the risk of the temperature rising above the decomposition temperature.

[0017] WO 2008 / 154668 A1 also discloses a thin-film evaporator for producing lyocell molded solutions, wherein the conveying elements on the evaporator shaft of the thin-film evaporator are at a steep angle for rapid vertical product conveying. While this reduces the risk of decomposition due to substantial overheating, even at high heating temperatures and vacuums requiring high manufacturing capacity and process efficiency, this is compensated for by higher heating temperatures necessary to compensate for the higher heat flux density required for shorter residence times.

[0018] Finally, the thin-film evaporator is integrated into a screw located at the end, which feeds a pump that pumps the molding solution through pipes to molding devices such as spinnerets. One advantage of this is that the molding solution can accumulate upstream of the pump, which is necessary for the operation of many pressurized discharge devices to enable the thin-film evaporator to operate under vacuum.

[0019] On the other hand, those skilled in the art will understand that the stacking of the product is also crucial to ensure the required dissolution time, because the dissolution process does not occur instantaneously, but rather at a certain rate over a certain period of time. Those skilled in the art know that the dissolution rate is affected by various factors, such as temperature. In the case of the described direct dissolution method, those skilled in the art also know that, in addition to temperature, the concentration of the functional liquid and the mechanical load on the mixture also affect the dissolution rate. Therefore, for the same composition and the same temperature, different dissolution states may occur due to different mechanical treatments of the material within the same processing time.

[0020] Therefore, another reason for stacking the product in the thin-film evaporator before discharge, as described above, is that in order to produce the molding solution, in addition to water evaporation, a mechanical action must be applied to the product. This mechanical action is achieved through the specially selected geometry of the evaporator shaft of the thin-film evaporator, which is ensured by the stacking of the product, which must be carried out within a specific time.

[0021] In the event of an unexpected pump stoppage, whether due to a technical malfunction of one of the multiple process units following the process, resulting in an interruption of the entire production facility including the process itself, or due to a malfunction of components used in the process itself, the high viscosity of the molding solution accumulated before the pump will cause it to overheat rapidly due to the mechanical energy input from the evaporator shaft. This represents a serious risk of product damage, and in the case of NMMO, it particularly represents an explosion risk. If the rotation of the evaporator shaft stops, the product, distributed as a thin film in close contact with the inner surface of the heated housing, can also overheat rapidly, which also poses a serious risk of product damage, especially an explosion risk. In the context of this invention, the heated housing surface is referred to as the heated surface.

[0022] Technicians also recognized that large industrial thin-film evaporators have limited size, with their maximum heating surface typically around 50 m². 2 .

[0023] Those skilled in the art also recognize the following drawback: to avoid significant overheating of the product or functional liquid, the temperature of the heating surface in the thin-film evaporator (also known as the heating temperature or heating surface temperature) is significantly lower on the discharge side. Due to the higher product viscosity there, there is also a higher dissipated energy input compared to the upper region of the thin-film evaporator. Furthermore, the reduced product conveying speed due to build-up before the discharge section increases the available time for product heating, further increasing not only dissipated energy input but also thermal energy input.

[0024] Therefore, to avoid significant overheating of the product, the heating temperature must be reduced relative to the upper region. The temperature of the heated surface is typically reduced to a level roughly corresponding to the temperature at which the molding solution exits the thin-film evaporator (hereinafter also referred to as the molding solution temperature), typically 100°C to 105°C in the case of NMMO as the functional liquid, so as to temper only the product. Depending on the water content in the starting material, the discharge side region with the reduced heating temperature typically affects 20% to 50% of the heated surface of the thin-film evaporator. Conversely, avoiding this reduction in heating temperature would mean an increase of 25% to 100% in the area of ​​the thin-film evaporator available for heat input, thus effectively doubling it.

[0025] The purpose of reducing the temperature of the heated surface is specifically to compensate for this mechanical energy input in order to avoid the risk of substantial overheating of the product or product components and the associated product damage and decomposition.

[0026] Lowering the temperature of the heating surface to avoid substantial overheating has a drawback for all functional liquids (NMMO or IL): not all available heating surfaces can be used for heat input. Given the limited size of thin-film evaporators, this means that lowering the heating temperature reduces the maximum production capacity of the production line. This has significant competitive implications for cost-sensitive industries, which compete on the basis of economies of scale.

[0027] Another drawback is the increased energy cost due to the throttling of thermal energy input, which is more conducive to mechanical energy input, because instead of the normally inexpensive thermal energy input via the heating surface, the normally expensive electromechanical energy input is achieved through the energy dissipation of the rotating evaporation shaft of the thin-film evaporator.

[0028] Lowering the heating temperature can even cause heat to be carried away from the product in the discharge section of the thin-film evaporator, which means heat loss and further increases energy costs.

[0029] In the case of NMMO as a functional liquid, the explosion hazard is common to all the methods described above for preparing molding solutions by evaporating water from a cellulose-NMMO-water mixture.

[0030] WO 96 / 33302A now discloses an apparatus for producing cellulose membranes, fibers, and other molding products by an amine oxide process (i.e., using NMMO as the functional liquid). Preferably, two mixing units with two different pulpers are thus used. With the aid of these mixing units, the pulp is first defiberized or milled, wherein a pump pumps a first suspension of the pulp in an aqueous amine oxide solution (wherein the dry pulp density is not greater than 10% of the dry pulp mass) to a unit (hereinafter referred to as unit 1), wherein the unit reduces the amount of water present until the suspension is converted into a concentrated pulp suspension, wherein the concentrated pulp suspension is transferred from unit 1 to another unit (hereinafter referred to as unit 2), wherein the resulting concentrated pulp suspension is converted into a formable cellulose solution.

[0031] Both devices can be designed as thin-film evaporators in this paper. The reason for this two-stage evaporation is that the upstream process control of device 1 results in a high water content in the pulp-water-NMMO mixture. Therefore, the object of this invention is to pre-concentrate the concentrated pulp suspension to reduce the amount of water to be evaporated through device 2. As disclosed, the product after device 1 is in the state of a concentrated pulp suspension, meaning that the dissolution process has not yet begun and therefore does not contain dissolved cellulose components.

[0032] However, a discharge pump is provided between device 1 and device 2, before which the accumulation of concentrated pulp suspension in liquid form is exposed, which again increases the risk of explosion due to the risk of material accumulation.

[0033] WO 2013 / 156489 A1 also discloses two continuous apparatuses for evaporating water, the first being a thin-film evaporator and the second a thick-film evaporator, preferably a kneading reactor as described in DE 199 40 521A1, hereinafter referred to as a mixing kneader. Due to its excellent high-viscosity mixing properties and efficient mechanical energy input via its shaft (hereinafter referred to as the kneader shaft), its speed can be rapidly adjusted and reduced. The mixing kneader allows for very precise and reliable temperature control and generally avoids product cooling or heat dissipation. The thin-film evaporator and the mixing kneader are directly connected to each other via a connection, but WO 2013 / 156489A1 does not mention a specific embodiment of the connection.

[0034] During the dissolution process, the water content in the NMMO of the concentrated pulp suspension is divided into three parts. After the first part, the pulp suspension is discharged from the thin-film evaporator and fed into a mixing kneader. The first part shows no increase in viscosity and ends at the beginning of the dissolution window, which corresponds to 2.5 hydrates, which correspond to an NMMO-water concentration of about 72.2 wt% (NMMO relative to NMMO and water, in mass fractions, and where, as in WO 06 / 33302A, the tendency to explode is expected to be low due to the presence of high water content). The second part consists of the main dissolution process, where the viscosity begins to increase sharply, accompanied by the necessary evaporation of water, until 1.5 hydrates are formed, which correspond to an NMMO-water concentration of about 81.3 wt%. In the third part, homogenization occurs and water evaporates until 0.8 to 1.0 hydrates (monohydrates) are formed, which correspond to NMMO-water concentrations of about 89.1 wt% to 86.7 wt%, respectively.

[0035] The drawback of this method remains the high water content, which cannot be evaporated by the thin-film evaporator, corresponding to the untapped potential for improving process efficiency. Furthermore, in cases where the NMMO water portion still exists as a 2.5-hydrate in the cellulose-water-NMMO mixture, the subsequent mixing kneader, as described in WO 2013 / 156489A1, still requires the evaporation of a significant amount of water in the input zone.

[0036] To achieve an economically sized mixing kneader, a significant portion of the energy input, in addition to the mechanical energy input through the kneading action, must be provided by contact heat. Therefore, the heating surface must be maximized, at least in the input region. This means that the discs attached to the kneader shaft with the mixing rods (described in more detail below) must also be designed to be heated. This heating increases the design and manufacturing costs and time of the mixing kneader, and also results in a higher structural weight of the kneader shaft. The integrated heating system also reduces the mechanical stability of the design, leading to a reduction in the maximum size of the mixing kneader. The low viscosity of the concentrated pulp suspension entering the mixing kneader and its approximately 72.2 wt% NMMO water concentration further reinforce this design necessity, as it results in insufficient torque and speed in the input region of the subsequent mixing kneader to ensure adequate water evaporation through friction and heating of the resulting pulp suspension. Therefore, in summary, the high water content implies both a high evaporation load on the mixing kneader and low mechanical energy input due to the low viscosity.

[0037] Mixing kneaders are known to those skilled in the art. They preferably have exactly one or exactly two kneader shafts for high-viscosity and crust-forming processes, and they can operate under vacuum, atmospheric pressure, or overpressure, and can be heated or cooled via heat exchange surfaces. Furthermore, the kneader shafts and their structure can heat the product very effectively through rotation and friction at typical product viscosities.

[0038] If there is exactly one kneader shaft, a single-shaft mixing kneader is located in front of the housing, as described, for example, in CH 674 472A5. In this case, the shaft structure of the kneader shaft preferably engages with the static structure of the housing (e.g., so-called mating hooks) during operation. If there are exactly two kneader shafts, a dual-shaft mixing kneader exists, as described, for example, in DE41 18 884 A1. The shaft structures of the kneader shafts preferably engage with each other during operation.

[0039] At least one kneader shaft includes a disc-shaped shaft structure and a rod attached thereto, wherein the shaft structure of the at least one kneader shaft is configured to engage during operation with the shaft structure of a second kneader shaft or with a fixed mating element present in the kneading machine. Kneading machines having such meshing elements are known and are referred to as "self-cleaning" because the described meshing removes any buildup from the meshing element.

[0040] The kneader shaft with disc and rod described above is known from the prior art, for example, from DE4118884A1. For the present invention, the manner in which the rod is attached to the disc and in turn to the kneader shaft is not important. For example, the rod and disc (also referred to as "support") can also be manufactured as a single piece, therefore the term "fastening" should be interpreted broadly. Although DE 4118884A1 above shows a biaxial mixing kneader, CH 674472A5 also shows a single-axis mixing kneader with hook-like static kneading mating elements (so-called "kneading mating hooks") on the inner wall of the housing. The housing, kneader shaft, shaft structure, and static kneading mating elements can be designed as described in the aforementioned publications.

[0041] The inner side of the mixing kneader housing, the kneader shaft, and the disc cannot be specifically configured as heat exchange surfaces. In the case of the mixing kneader housing, the heat exchange surface is typically designed as a welded half-pipe, or preferably as a double-walled design. In the case of the kneader shaft, a heat exchange surface means that the kneader shaft is designed as a hollow shaft, and preferably has inflow and return of the heat transfer or cooling medium, for example, using an inner tube.

[0042] A disc with a heat exchange surface has a cavity or orifice for an electric heating element or a heat transfer medium or a cooling medium, wherein the latter has an inflow fed through an inflow in the kneader shaft and an outflow returning to the kneader shaft. The cavity can typically be double-walled or orifice-shaped, requiring a thicker wall and a larger shaft superstructure, as well as a thicker kneader shaft. This results in significantly higher manufacturing costs compared to cavityless (hereinafter also referred to as heated cavityless) discs, which can typically be made smaller due to the elimination of the heat exchange surface.

[0043] Purpose of the invention

[0044] The purpose of this invention is to overcome the shortcomings of the prior art.

[0045] Specifically, it aims to describe a method for producing a transfer mixture by evaporating water from a cellulose-water-functional liquid mixture, which firstly enables a significant improvement in the evaporation performance of a suitable evaporation device (preferably a thin-film evaporator) through thermal energy input, without the risk of overheating or explosion.

[0046] Solution of the objective

[0047] The solution for this purpose is formed according to the features of claim 1, 2, 11 or 12.

[0048] Advantageous embodiments are described in the dependent claims.

[0049] The object of the present invention is to produce a transfer mixture in a thin-film evaporator according to the direct dissolution method as the first process stage of a process of at least two stages, taking into account all the facts mentioned in the context of this object.

[0050] The method according to the invention for producing a transfer mixture by direct dissolution is carried out in an apparatus, preferably a thin-film evaporator having a feed section, a shell, and a discharge section, wherein the feed section includes a product entering the shell in the form of a starting material, which is essentially composed of cellulose, water, and a functional liquid; in the thin-film evaporator, the starting material is heated and some water evaporates to form a transfer mixture, wherein the transfer mixture flows together with the supply stream to the discharge section and is then transferred without delay to a subsequent processing unit.

[0051] In this method, a transfer mixture is produced instead of a molding solution. Therefore, the transfer mixture is characterized by a water content at which the temperature of the heating surface of the thin-film evaporator—particularly the temperature of the discharge section—is not significantly lower than that of the feed section, but there is no substantial overheating of the product. The transfer mixture differs from the molding solution in that it corresponds to the state of the product before it is transferred to a moldable, particularly moldable or spinnable solution. A moldable or spinnable solution exists when all the major cellulose components of the starting material have dissolved. In this context, "significant" means that the proportion of undissolved components is so low that the so-called filter life is economically acceptable. Filter life refers to the filter typically used after the process stage of producing the molding solution and before the process stage of molding or spinning. Filter life corresponds to the length of time, or service life, before the filter must be replaced because the pressure drop across the filter becomes too high due to the accumulation of components unwanted for further processing. These components can be, for example, undissolved cellulose fibers and non-cellulose impurities. Therefore, the specific choice of filter life depends on overall economic cost and quality optimization.

[0052] The transfer mixture differs from the starting material in that, due to the thermomechanical treatment in the thin-film evaporator, some of the cellulose present in the starting material is already present in the transfer mixture in solution form.

[0053] The method for producing transfer mixtures according to the invention has the following advantages: the product in the thin-film evaporator always has a sufficiently high water content, thus preventing substantial overheating. In the case of NMMO as a functional liquid, this prevents the risk of explosion. Therefore, the heating temperature on the discharge side of the thin-film evaporator is not reduced or substantially reduced, but there is always a temperature difference necessary for heat input, i.e., the difference between the heating temperature and the product temperature is at least 20K, preferably 50K, and ideally 70K, while avoiding substantial overheating. Another advantage is that this results in a lower mechanical load on the evaporator shaft due to the lower viscosity and avoids product accumulation on the discharge side of the thin-film evaporator before discharge, thus allowing the thin-film evaporator to be implemented at a lower cost. This increases the maximum water evaporation rate per heating surface of the thin-film evaporator while reducing energy costs and making the structure more advantageous. For example, the more advantageous design stems from the fact that conventional thin-film evaporators can be made larger, thus processing more material than before. This increases the water evaporation rate per heating surface and allows for a larger design, i.e., the production capacity of each production line.

[0054] Also advantageous in this paper is the improvement in energy efficiency and process reliability not only for the process steps of producing concentrated pulp suspension in thin-film evaporators, but also for combination with suitable post-processing facilities such as preferred mixing kneaders to completely transfer the starting material into the molding solution.

[0055] According to the present invention, a functional liquid is prepared by adding IL or NMMO as a starting material. The functional liquid is used to dissolve cellulose under suitable conditions.

[0056] In this method, for example, the transfer mixture can be conveyed to a subsequent processing unit. The subsequent processing unit can be one of the processing units already described, such as a mixing kneader or another thin-film evaporator. The subsequent processing unit is intended to further process the transfer mixture into a molded solution. A preferred embodiment of the transfer mixture is thus chosen where, in practice, the viscosity of the transfer mixture is not only low enough to avoid substantial overheating in the thin-film evaporator, but also high enough that the mechanical energy input through friction in the mixing kneader is high enough that the thermal energy input (supplementary mechanical energy input) in the mixing kneader is low enough that the upper structure of the kneader shaft does not need to be used for heat exchange and can be designed without a heating chamber. This makes the design of the kneader shaft significantly simpler, more advantageous, and faster.

[0057] Therefore, the water concentration of the transfer mixture is selected such that at least one of the following conditions is satisfied, or at least one of the following embodiments according to the invention is provided:

[0058] (1) In the case of a thin-film evaporator that reduces water concentration by evaporation to produce a transfer mixture, the temperature of the heating surface, especially the temperature of the discharge section, is not substantially reduced compared to the feed section, and the transfer mixture does not undergo substantial overheating.

[0059] (2) When NMMO is used as a functional liquid, the water content of the transfer mixture generally corresponds to the following conditions:

[0060] Maximum x H2O =-0.235x Cell +0.235,

[0061] minimum x H2O =-0.59x Cell +0.2047.

[0062] Even more preferably, the transfer mixture is applied with the following preferred composition:

[0063] Maximum x H2O =0.2864x 2 Cell -0.6786x Cell +0.2288,

[0064] Maximum x H2O =0.2864x 2 Cell -0.6786x Cell +0.2188.

[0065] Preferably, in addition to (1) and (2) above, the transfer mixture is selected such that its water concentration is also applicable.

[0066] (3) When the mixing kneader is used as a processing mechanism directly or indirectly after the thin film evaporator, the mixing kneader is equipped with at least one kneader shaft. The upper structure of the kneader shaft does not need to be heatable. In addition to the heat input via the heatable kneader shaft and the heatable outer shell of the mixing kneader, the viscosity of the transferred mixture is sufficient to achieve the evaporation performance required for producing the molding solution in the mixing kneader by means of mechanical energy input.

[0067] The starting materials exist as a mixture of cellulose, water, and functional liquids, and their composition can vary considerably.

[0068] The starting material is thus referred to as the transfer mixture from the feed section to the discharge section.

[0069] In the transfer mixture according to the invention, cellulose exists in a partially dissolved form, and in the case of NMMO as a functional liquid, the water content in NMMO can be derived from a mathematical formula.

[0070] The formula used to describe the transfer mixture refers to the production of the transfer mixture under thermomechanical conditions that allow for low-risk operation of the thin-film evaporator, and has been proven in practice. However, while the formula can potentially predict the spinnable solution due to the explained effect on the cellulose dissolution rate, in practice, due to the selection of specific thermomechanical treatments, such as a very short time between the feed and discharge sections, the transfer mixture according to the invention still contains undissolved cellulose components despite its low water content. However, at the same time, such thermomechanical treatment is accompanied by an increased process risk due to substantial overheating and explosive decomposition, due to the associated low water content in the transfer mixture.

[0071] Furthermore, according to the invention, this can also mean that the starting material is supplied to the thin-film evaporator with a composition even within a concentration range that describes a typical transfer mixture. Typically, a transfer mixture characterized by partial dissolution of pulp is only produced when the starting material undergoes a specific thermomechanical treatment (temperature increase and shear increase) in the thin-film evaporator.

[0072] The discharge leads to a subsequent processing unit, which is preferably more suitable than a thin-film evaporator for safely transferring the transfer mixture into the molding solution. In this context, such a subsequent processing unit can be a mixing kneader, as described above. The mixing kneader is particularly suitable as a subsequent processing unit due to its excellent high-viscosity mixing properties and efficient mechanical energy input via the kneader shaft, whose speed can be rapidly adjusted and again rapidly reduced—if necessary to zero degrees Celsius. Therefore, another advantage is that the mixing kneader allows for very precise and reliable temperature control, thus generally avoiding cooling or heat dissipation.

[0073] In another embodiment, the transfer mixture first passes through a subsequent transfer mechanism. The transfer mixture is then conveyed to a subsequent processing mechanism.

[0074] Therefore, the improved energy efficiency and process reliability apply not only to the process steps of producing transfer mixtures, but also when combined with suitable post-processing units to completely transfer the initial treatment to the molding solution.

[0075] The subsequent processing unit can also be another thin-film evaporator—accepting the aforementioned disadvantages—where the overall design of the thin-film evaporator can be adapted to realize the transfer mixture, and the additional thin-film evaporator can be adapted to process the transfer mixture by its other overall design (e.g., overall length, angle of its blade, etc.). Thus, the subsequent processing unit is limited to further processing the transfer mixture into a molding solution. For example, the molding solution can then be used for spinning.

[0076] The material properties described using the formulas listed above for the general transfer range are derived from the following studies. Maximum water content x H2O The composition corresponding to the dihydrate, taking into account water and NMMO, is described. NMMO molecules have the ability to form two hydrogen bonds. These hydrogen bonds result in the formation of a structure to which water molecules are bound, and also lead to the dissolution of cellulose molecules. Theoretically, NMMO has solubility for cellulose molecules, given that each NMMO molecule contains fewer than two water molecules. This upper limit on water content will separate the transfer mixture from a pure suspension. For example, it is also described in the literature that, through appropriate thermomechanical treatment, some cellulose can be dissolved in a solution with an NMMO water concentration of approximately 75%.

[0077] For those skilled in the art, based on US 4,196,282 A, the minimum water content x H2O The composition of cellulose completely dissolved in a cellulose-water-NMMO mixture, observed for the first time after visual inspection, is described. This distinguishes the transfer mixture from a complete molding solution. The statement in US 4,196,282A regarding NMMO as a functional liquid is confirmed by the inventors' results and observations from industrial applications and experiments conducted on a pilot-scale facility. Therefore, the two formulas given cover the general range of transfer mixtures.

[0078] For a description of the material properties for the preferred transfer range, refer to US 4,196,282 A regarding the minimum water content. Based on this, the curve describes the 95% confidence interval for the minimum water content in the general transfer range, thus further distinguishing it from the safe state of a fully molded solution. The maximum water content x used to describe the preferred transfer range... H2O The formula is based on results from industrial applications of the direct dissolution method and data collected from a series of experiments conducted by the inventors at a pilot-scale facility. It marks the upper limit of the transfer range within which optimal utilization of the process-specific thin-film evaporator characteristics is ensured.

[0079] In both the general and preferred compositions, the transfer mixture is in a pre-solution state. Therefore, a method for producing lyocell is also achieved under optimal operating parameters of the thin-film evaporator, avoiding the relatively undesirable material states of lyocell, such as when crusting, over-drying, or decomposition occurs in the thin-film evaporator. These problematic material states can be relatively easily prevented in the subsequent processing mechanism, which is typically designed for, for example, higher torque and to prevent crusting.

[0080] In a typical configuration, the discharge of the transfer mixture is already advantageous because sufficient water has been evaporated to form the transfer mixture, which can then be further processed into a solution in a subsequent processing unit within a relatively short processing time.

[0081] In the preferred composition of the transfer mixture, the maximum water content is closer to a complete solution than the general definition of a transfer mixture. On the other hand, the minimum water content is still quite far from this complete solution.

[0082] This allows for further processing within a shorter processing time, while further reducing the risk of excessive drying of the material in the thin-film evaporator. However, in both cases, removing the transfer mixture from the moldable or spinnable solution requires several minutes of processing time, during which the product is homogenized by mixing, and any remaining water can be evaporated if necessary, allowing the thin-film evaporator to operate with the aforementioned preferred operating characteristics.

[0083] The product, and therefore its starting materials, essentially comprise cellulose, water, and functional liquids. In addition, the product contains other chemical substances such as stabilizers, which, in the context of this invention, can be omitted from detailed enumeration as they are known to those skilled in the art and are applicable to each individual application.

[0084] It is also claimed that a thin-film evaporator is used to process starting materials into a transfer mixture by direct dissolution. The thin-film evaporator thus includes a feed section, a shell, and a discharge section, wherein the feed section introduces a product consisting essentially of cellulose, water, and a functional liquid into the shell, wherein an evaporator shaft located within the shell rotatably sweeps the product over the heated interior of the shell, in which the product is heated and some water evaporates to form a transfer mixture, wherein, according to the invention, all heated surfaces of the thin-film evaporator in contact with the product are subjected to a heating temperature at least 20 K higher than the temperature of the product.

[0085] In another thin-film evaporator according to the invention for processing starting materials into a transfer mixture by direct dissolution, the thin-film evaporator has a feed section, a shell, and a discharge section, wherein the feed section introduces a product consisting essentially of cellulose, water, and N-methylmorpholine-N-oxide (NMMO) into the shell, wherein an evaporator shaft located in the shell rotatably sweeps the product over the heated interior of the shell, in which the product is heated and some water evaporates to form a transfer mixture, wherein the following composition of the transfer mixture is achieved:

[0086] Maximum x H2O =-0.235x Cell +0.235,

[0087] Minimum x H2O =-0.59x Cell +0.2047,

[0088] The transfer mixture is continued to be conveyed through the discharge section. This means that the thin-film evaporator is configured such that when the parameters of the transfer mixture are reached, the discharge is automatically or guided through, and further processing or conveying is operatively connected to at least one subsequent transfer mechanism, or the transfer mixture is operatively connected to a subsequent processing mechanism.

[0089] When the transfer mixture contains the following components, even more preferred compositions of the transfer mixture are given:

[0090] Maximum x H2O =0.2864x 2 Cell -0.6786x Cell +0.2288,

[0091] Minimum x H2O =0.2864x 2 Cell -0.6786x Cell +0.2188.

[0092] Other embodiments of the method according to the invention are also applicable to thin-film evaporators, especially when the same features are specified in the same way. Attached Figure Description

[0093] Further advantages, features, and details of the invention will become apparent from the following description and drawings of preferred embodiments; these are as follows:

[0094] Figure 1 : A graph showing the preferred material properties of the transfer mixture;

[0095] Figure 2 : A graph showing the general material properties of the transferred mixture. Detailed Implementation

[0096] Example 1

[0097] The subject of this embodiment is the production of a molding solution with a cellulose content of 12 wt% using a direct dissolution method. The functional liquid used in this embodiment is NMMO. All subsequent proportions refer to the total mass of the cellulose-NMMO-water mixture. A starting material with a cellulose content of approximately 7.2 wt% was prepared from an aqueous solution of cellulose and NMMO, yielding an NMMO content of approximately 46.1 wt%. This starting material was introduced into a thin-film evaporator, where it was concentrated into a transfer mixture. In this embodiment, the thin-film evaporator was operated at a process pressure of 70 mbar, thus the equilibrium temperature of the starting material was approximately 43°C. The heating temperature of the thin-film evaporator was 130°C. In this embodiment, the transfer of the mixture occurred within a preferred transfer range with a cellulose content of approximately 11.5 wt% and an NMMO content of approximately 73.9 wt%. Under these process conditions, this corresponds to an equilibrium temperature of approximately 100°C for the transfer mixture. At this point, the water to NMMO ratio is approximately 1.3 hydrate ratio. The transfer mixture provided by the thin-film evaporator was then transferred to a mixing kneader. There, the mixture is finally concentrated by evaporation and homogenized to obtain a complete molding solution. The molding solution leaving the mixing kneader has a cellulose content of 12.0 wt%, an NMMO content of about 77.0 wt%, and a temperature of about 107°C.

[0098] Example 2

[0099] The subject of this embodiment is the production of a molding solution with a cellulose content of 12 wt% using a direct dissolution method. The functional liquid used in this embodiment is an ionic liquid. All subsequent proportions refer to the total mass of the mixture. A starting mixture with a cellulose content of approximately 8.2 wt% was prepared from cellulose and an aqueous IL solution, yielding an IL content of approximately 58.6 wt%. This starting mixture was introduced into a thin-film evaporator, where it was concentrated to form a transfer mixture. In this embodiment, the transfer of the mixture was carried out at a cellulose content of approximately 11.5 wt% and an IL content of approximately 79.6 wt%. The transfer mixture provided by the thin-film evaporator was then transferred to a mixing kneader. There, the mixture was finally concentrated by evaporation and homogenized to obtain a complete molding solution. The molding solution leaving the mixing kneader had a cellulose content of 12.0 wt% and an IL content of approximately 83.0 wt%.

[0100] Figure 1 A diagram showing the general and preferred material compositions of the transfer mixture is presented. Therefore, the general composition a has the following parameters:

[0101] Maximum x H2O =-0.235x Cell +0.235,

[0102] minimum x H2O =-0.59x Cell+0.2047.

[0103] Therefore, it is shown that a has a larger margin than the preferred composition b, which has the following parameters:

[0104] Maximum x H2O =0.2864x 2 Cell -0.6786x Cell +0.2288,

[0105] Maximum x H2O =0.2864x 2 Cell -0.6786x Cell +0.2188.

[0106] As the water content decreases, the solution first reaches the range of L, and as the water content continues to decrease, crystallization of NMMO occurs at K.

[0107] List of reference numerals

[0108] B Preferred composition of the transfer mixture K crystallization L solution

Claims

1. A method for producing a transfer mixture in a thin-film evaporator by direct dissolution, the thin-film evaporator having a feed section, a shell, and a discharge section, wherein the feed section introduces a product consisting substantially of cellulose, water, and N-methylmorpholine-N-oxide into the shell, wherein an evaporator shaft located in the shell rotatably sweeps the product over the heated interior of the shell, in the thin-film evaporator, the product is heated and some water evaporates to form the transfer mixture. in, (a) Discharging the transfer mixture before the product is substantially overheated; and (b) All heating surfaces of the thin-film evaporator that come into contact with the product have a heating temperature at least 20 K higher than the temperature of the product; The composition of the transfer mixture is as follows Maximum x H2O = -0.235 x Cell +0.235, minimum x H2O = -0.59 x Cell + 0.2047.

2. The method according to claim 1, wherein, The transfer mixture has the following composition: Maximum x H2O = 0.2864 x 2 Cell - 0.6786 x Cell + 0.2288, minimum x H2O = 0.2864 x 2 Cell - 0.6786 x Cell + 0.2188.

3. The method according to claim 1 or 2, wherein, The transfer mixture passes through at least one subsequent transfer mechanism.

4. The method according to claim 1 or 2, wherein, The transfer mixture is then conveyed to a subsequent processing facility.

5. The method according to claim 4, wherein, The subsequent processing unit further processes the transfer mixture to form a molding solution.

6. The method according to claim 1 or 2, wherein, The machine control system monitors the amounts of water, cellulose, and N-methylmorpholine-N-oxide in the composition of the transfer solution from the feed section to the discharge section.

7. The method according to claim 6, wherein, The machine control system monitors the composition of the product at all stages using sensors until the transfer mixture is obtained.

8. The method according to claim 4, wherein, The subsequent processing unit is a mixing and kneading machine.

9. The method according to claim 8, wherein, The mixing kneader further processes the transferred mixture into a molding solution by water evaporation and stirring using a direct dissolution method, wherein the structure of the kneader shaft is designed not to have a heating chamber.

10. A thin-film evaporator for processing starting materials into a transfer mixture by direct dissolution, the thin-film evaporator comprising a feed section, a shell, and a discharge section, wherein the feed section introduces a product consisting substantially of cellulose, water, and N-methylmorpholine-N-oxide into the shell, wherein an evaporator shaft located in the shell rotatably sweeps the product over the heated interior of the shell, in the thin-film evaporator, the product is heated and some water evaporates to form the transfer mixture, wherein... All heating surfaces of the thin-film evaporator that come into contact with the product have a heating temperature at least 20 K higher than the temperature of the product, and the following transfer mixture composition is achieved: Maximum x H2O = -0.235 x Cell+ 0.235, minimum x H2O = -0.59 x Cell + 0.2047, The transfer mixture is continued to be conveyed through the discharge section.

11. The thin-film evaporator according to claim 10, wherein, The transfer mixture has the following composition: Maximum x H2O = 0.2864 x 2 Cell - 0.6786 x Cell + 0.2288, minimum x H2O = 0.2864 x 2 Cell - 0.6786 x Cell + 0.2188.

12. The thin-film evaporator according to any one of claims 10 to 11, wherein, The transfer mixture can be operatively connected to at least one subsequent transfer mechanism.

13. The thin-film evaporator according to any one of claims 10 to 11, wherein, The transfer mixture can be operatively connected to a subsequent processing unit.

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