A method for manufacturing transfer mixtures based on direct dissolution and a thin-film evaporator

Abstract

This invention relates to a method for producing a transfer mixture in a thin-film evaporator having an inlet, a shell, and an outlet according to a direct dissolution method, wherein the inlet feeds a product consisting essentially of cellulose, water, and a functional liquid into the shell, wherein an evaporator shaft arranged in the shell rotates and causes the product to pass over the heated interior of the shell, wherein the product is heated and a portion of the water evaporates, thereby producing a transfer mixture, the transfer mixture flowing to the outlet with a general composition, (a) under the aforementioned general composition, the product does not overheat when the shell surface in contact with the product is at least 20 K higher than the product temperature, or (b) when NMMO is used as the functional liquid, the aforementioned general composition is: maximum x H2O = -0.235 x Cell + 0.235 minimum 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 means of a direct dissolution method as described in the preamble of claims 1, 2, 11 or 12, and a thin-film evaporator. Prior Technology

[0002] A method for manufacturing molding solutions from pulp on an industrial scale using a direct dissolution method by evaporating water from a cellulose-water-functional liquid mixture (hereinafter also referred to as the product, and cellulose, functional liquid, and water are referred to as product components) is described, for example, in WO 1994 / 006530 A1, wherein N-methylmorpholine-N-oxide (hereinafter referred to as NMMO or also as amine oxide) is used as the functional liquid (generally referred to as the Lyocell molding solution process or amine oxide process). The cellulose-water-functional liquid mixture or product can be understood as encompassing all water contents or mixture states, i.e., from high water content present in a suspension to low water content present in a molding solution, and therefore includes transfer mixtures.

[0003] Currently, thin-film evaporators are widely used in Lyocell molding solution processes to evaporate water from cellulose-NMMO-water mixtures. Thin-film evaporators are now known and used in various forms and designs, characterized by an evaporator shaft within the evaporator housing distributing the product onto the inner surface of the housing, which serves as the heating surface, thus forming a thin layer. With increasing rotational speed and decreasing viscosity, this thin layer may also generate turbulence, rapidly heating the product and causing some water to evaporate. The heating surface, as referred to above and below, means any heated surface designed to transfer energy thermally to the product through the temperature difference between the heating surface and the product. However, this process discloses the manufacture of moldable solutions, hereinafter referred to as molding solutions as defined in DE 10 2012 103 296 A1.

[0004] Although, as described in WO 1994 / 006530 A1, most thin-film evaporators have their evaporator shafts oriented vertically, they can also be equipped with horizontally oriented evaporator shafts, as described in WO2020 / 249705 A1.

[0005] Those knowledgeable in the relevant field are also familiar with the method of manufacturing molding solutions from pulp using a direct dissolution method, in which an ionic liquid (hereinafter referred to as IL) is used instead of NMMO as the functional liquid. However, in addition, the direct dissolution method is also used to manufacture molding solutions by evaporating water from a cellulose-IL-water mixture using a thin-film evaporator.

[0006] IL, or ionic liquid, here refers to a group of organic compounds that, although possessing ionic structures, have low melting points (<100°C) and are therefore also known as molten salts. In other words, using IL as a functional liquid implies that there is always a suitable implementation method for direct dissolution within the group of ionic liquids.

[0007] Those skilled in the art generally know that some solvents (ILs) are prone to thermal decomposition at elevated temperatures; therefore, processes using heated ILs must maintain the IL temperature below their decomposition temperature. For example, IL[DBNH][OAc] decomposes at temperatures above approximately 100°C. Those skilled in the art also know that NMMO begins to decompose when the water content of an NMMO-water mixture decreases with increasing temperature. When temperatures are typically above 140°C, the risk of explosion, such as from explosive autocatalytic decomposition, increases with decreasing water content, leading to further temperature increases and an imminent explosion hazard. Depending on the mixture composition, decomposition may 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) lowers the decomposition temperature. In the case of the aforementioned direct dissolution process, although stabilizers are usually added, the presence of cellulose in the process and the iron materials used in the machine structure can both contribute to a decrease in the decomposition temperature.

[0008] Those skilled in the art will also understand the following facts regarding the serious overheating problem within the framework of this invention: to avoid decomposition of the functional fluid, the product temperature (hereinafter also referred to as product temperature) must be carefully controlled. Therefore, those skilled in the art will choose a process that ensures both the ideal temperature and equilibrium temperature of the product are below the decomposition temperature.

[0009] In multi-component mixtures such as cellulose-functional fluid-water mixtures, each component corresponds to an equilibrium temperature under the dominant process conditions (pressure), at which the mixture begins to boil. If energy is supplied to the system, some volatile components (in this case, water) will evaporate. Simultaneously, the composition of the mixture changes, thus altering the equilibrium temperature. Therefore, when energy is supplied, the mixture is heated, and due to the evaporation of volatile components, the composition changes along the equilibrium curve. As the water content decreases, the viscosity increases due to the increased cellulose concentration and the enhanced solubility of the functional fluid or functional fluid-water mixture. With increasing viscosity, the water evaporating from the product is increasingly likely to be transport-restricted, preventing sufficient evaporative cooling and causing the temperature of the water-functional fluid mixture and the product to rise above the equilibrium temperature. The equilibrium temperature of the mixture varies with its composition and process pressure. Therefore, even without transport-restriction effects, the temperature of the mixture will rise as the process continues, and thus, with appropriate selection of process parameters, decomposition can occur even at the equilibrium temperature. Therefore, those knowledgeable in the field typically select process parameters that ensure the equilibrium temperature always remains below the decomposition temperature. For example, when using NMMO as a functional fluid, the process usually produces an equilibrium temperature in the range of 50°C to 110°C.

[0010] When NMMO is used as a functional liquid and transport is restricted, the exothermic decomposition process can lead to localized overheating (hot spots), preventing sufficient heat dissipation from these areas. This can subsequently trigger further exothermic decomposition processes.

[0011] Severe overheating can cause functional liquids, cellulose, or cellulose-functional liquid-water mixtures to be heated to temperatures that cause their decomposition. Overheating that causes the temperature of a product or product component to exceed its characteristic decomposition temperature is referred to below as severe overheating. It is not important whether the overheating is due to insufficient evaporative cooling caused by transport limitations or due to excessive drying of the product in equilibrium conditions.

[0012] The decomposition of functional fluids is as described above. This decomposition leads to the need to replace the lost (often very expensive) functional fluid, and, when NMMO is used as a functional fluid, it significantly increases the risk of explosion.

[0013] However, severe overheating can also affect the decomposition of cellulose or cellulose-functional liquid-water mixtures (or products, as defined above), because at excessively high temperatures, and under conditions of increased shear forces (typically present with increased viscosity), severe overheating can lead to a decrease in the degree of polymerization of cellulose (often referred to as DP by those knowledgeable in the field). In other words, severe overheating of a product can have detrimental or safety-critical effects due to the decomposition of parts of the product or the product itself.

[0014] Of course, process control at temperatures below the decomposition temperature can also have adverse effects on the product or endanger safety due to non-thermally induced decomposition processes (such as free radical reactions). However, in practice, this effect is offset by commonly used stabilizers.

[0015] Furthermore, targeting a temperature above the equilibrium temperature for decomposition can damage the product and create safety hazards. Those knowledgeable in related fields typically do not pursue such operating procedures in practice.

[0016] Nevertheless, when using a thin-film evaporator for a direct dissolution process, special attention must be paid to temperature control due to its unique equipment characteristics. With a small amount of product in the equipment, the inherently high heat input effectively evaporates volatile components. However, because the product volume is small, any process fluctuations (such as changes in the feed rate, energy input, or product viscosity) can only be compensated for within a limited range. Therefore, even a slight decrease in the feed rate can lead to a significant rise in product temperature. This creates the risk of the temperature rising above the decomposition temperature range.

[0017] WO 2008 / 154668 A1 also discloses a thin-film evaporator for manufacturing Lyocell molding solutions, wherein the conveying elements on the evaporator shaft are angled to enable rapid vertical transport of the product. While this reduces the risk of decomposition due to severe overheating under high heating temperatures and vacuum conditions (required for high throughput and high process efficiency), this is offset by the higher heating temperatures required to achieve higher heat flux densities, which are necessary for shorter residence times.

[0018] The thin-film evaporator is connected at one end to a screw that feeds a pump that delivers the molding solution through a pipe to the molding device (e.g., a spinneret). One advantage is that the molding solution can accumulate before the pump, which is necessary for the operation of many pressurized discharge components, allowing the thin-film evaporator to operate under vacuum.

[0019] As those skilled in the art generally understand, product accumulation is also necessary to ensure the required dissolution time, because the dissolution process does not occur instantaneously but rather at a certain rate over a period of time. As is known to those skilled in the art, the dissolution rate is affected by various factors, including temperature. Regarding the aforementioned 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, under the same composition and temperature, different mechanical treatments applied to the material for the same processing time may result in different dissolution states.

[0020] Therefore, another reason for accumulating the product in the thin-film evaporator as described above before discharge is that, in addition to evaporating water, the production of the molding solution also requires applying mechanical action to the product (this is achieved by specifically selecting the geometry of the evaporation shaft of the thin-film evaporator), and this mechanical action must last for a certain period of time, which is ensured by the product accumulation.

[0021] When the pump stops operating, whether due to a technical failure in one of the many process components following the process or due to a failure in the component itself used in the process, the molding solution accumulated before the pump, due to its high viscosity, rapidly overheats because of the mechanical energy input from the evaporator shaft. This means a serious risk of product damage, and especially an explosion risk when using NMMO. If the evaporator shaft stops rotating, the product, as a thin layer in close contact with the heated inner surface of the shell, will also rapidly overheat, which similarly means a serious risk of product damage, particularly an explosion risk. Within the framework of this invention, the heated shell surface is referred to as the heating surface.

[0022] Those familiar with the field also know that large industrial thin-film evaporators have limited dimensions, with their maximum heating area typically around 50 m².

[0023] As is further known to those skilled in the art, to avoid severe overheating of the product or functional liquid, the temperature of the heating surface (also known as the heating temperature or heating surface temperature) on the discharge side of a thin-film evaporator is significantly reduced. Due to the higher viscosity of the product at this location, dissipation also results in higher energy input compared to the upper region of the thin-film evaporator. Furthermore, the product transport velocity is reduced by the accumulation effect before discharge, thus prolonging the product heating time. This not only increases the energy input due to dissipation but also the thermal input.

[0024] Therefore, to avoid severe overheating of the product, it is necessary to reduce the heating temperature relative to the upper region. The temperature of the heated surface is typically reduced to roughly match the temperature at which the forming solution leaves the thin-film evaporator (hereinafter also referred to as the forming solution temperature), which is usually 100°C to 105°C in the case of NMMO as the functional liquid, simply for temperature regulation of the product. Depending on the water content in the starting material, the reduced heating temperature on the discharge side typically affects 20% to 50% of the heating area 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, or, at its maximum, a doubling.

[0025] The purpose of reducing the temperature of the heating surface is, in particular, to compensate for this mechanical energy input, so as to avoid severe overheating of the product or product components, and to avoid the associated risks of product damage and decomposition.

[0026] Lowering the heating surface temperature to avoid severe overheating has the following drawbacks for all functional liquids (NMMO or IL): not the entire available heating surface can be used for heat input. Given the limited size of thin-film evaporators, this means that such a reduction in heating temperature will reduce the maximum capacity of the production line, which is crucial in cost-sensitive industries where economies of scale are the primary drivers of success.

[0027] Another drawback is that while the aforementioned restrictions on heat input are beneficial for mechanical energy input, they lead to increased energy costs because heat is no longer input in a low-cost manner through the heating surface, but rather in a more expensive manner through the energy dissipation of the rotating evaporator shaft of the thin-film evaporator.

[0028] Lowering the heating temperature can even lead to heat loss in the discharge side of the thin-film evaporator, which means heat loss and will further increase energy costs.

[0029] When NMMO is used as a functional liquid, all of the above methods for producing molding solutions by evaporating water from a cellulose-NMMO-water mixture pose an explosion risk.

[0030] WO 96 / 33302 A discloses an apparatus for manufacturing cellulose membranes, cellulose fibers, and other molded articles using an amine oxide process (i.e., using NMMO as the functional liquid). Preferably, two mixing units with two different pulpers are used. These mixing units are first used to separate or grind the pulp fibers. A pump pumps a first pulp suspension in an aqueous amine oxide solution containing dry pulp with a dry solids density not exceeding 10 mass% into a unit (hereinafter referred to as Unit 1), which reduces the existing water volume until the suspension becomes a concentrated pulp suspension. The concentrated pulp suspension is then transferred from Unit 1 to another unit (hereinafter referred to as Unit 2), and the resulting concentrated pulp suspension is converted into a moldable cellulose solution.

[0031] Both devices can be implemented as thin-film evaporators. This two-stage evaporation is employed because of the high water content in the pulp-water-NMMO mixture, caused by process control upstream of device 1. Therefore, the present invention aims to pre-concentrate the pulp suspension to reduce the amount of water to be evaporated in device 2. The product state after device 1 is revealed as a concentrated pulp suspension, meaning the dissolution process has not yet begun and therefore does not contain dissolved cellulose components.

[0032] However, a discharge pump is installed between device 1 and device 2. As disclosed in the case, concentrated pulp suspension accumulates in front of the discharge pump in the form of liquid accumulation. Due to the danger of material accumulation, this increases the risk of explosion.

[0033] WO 2013 / 156489 A1 also discloses two consecutive 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 40 521 A1, hereinafter referred to as a mixing kneader. The mixing kneader has excellent high-viscosity mixing performance and provides efficient mechanical energy input through a shaft (hereinafter referred to as the kneader shaft), the rotational speed of which can be quickly adjusted and rapidly reduced again, thus allowing for highly precise and safe temperature regulation and generally avoiding product cooling or heat dissipation. The thin-film evaporator and the mixing kneader are directly connected to each other by a connector, but WO 2013 / 156489 A1 does not specify a particular embodiment of this connector.

[0034] The dissolution process of NMMO in concentrated pulp suspension is divided into three stages. After the first stage, the pulp suspension is discharged from the thin-film evaporator and fed into a mixing kneader. The first stage shows no increase in viscosity and ends with the opening of the dissolution window, which corresponds to 2.5 hydrates, or approximately 72.2 wt% NMMO-water concentration (NMMO is related to the mass percentage of NMMO and water), and as described in WO 06 / 33302 A, there may be a slight tendency for explosion at this stage due to the high water content. The second stage consists of the main dissolution process, where the viscosity begins to rise sharply, triggering the necessary water evaporation until approximately 1.5 hydrates are formed, which corresponds to approximately 81.3 wt% NMMO-water concentration. In the third stage, homogenization and water evaporation occur until 0.8 to 1.0 hydrates (monohydrates) are formed, which corresponds to approximately 89.1 wt% to 86.7 wt% NMMO-water concentrations, respectively.

[0035] The drawback of this method is that the water content not evaporated by the thin-film evaporator remains high, meaning that there is untapped potential for increasing process efficiency in the thin-film evaporator. Furthermore, for cellulose-water-NMMO mixtures where NMMO-water content is still in the form of 2.5 hydrates, (as described in WO 2013 / 156489 A1) the downstream mixing kneader still needs to evaporate a significant amount of water in the inlet zone.

[0036] To achieve economies of scale in mixing kneaders, in addition to the mechanical energy input through the kneading effect, a significant portion of the energy input must be provided by contact heat. Therefore, it is necessary to maximize the heating surface area, at least within the inlet zone. This means that the discs mounted on the kneader shaft (with mixing rods mounted on them, see below) must also be designed to be heated. This heating increases the design and manufacturing costs and extends the manufacturing time of the kneader, while also increasing the structural weight of the kneader shaft. An integrated heating system also reduces the mechanical stability of the structure, resulting in a reduction in the maximum size of the mixing kneader. The necessity of this design is further reinforced by the low viscosity of the concentrated pulp suspension entering the mixing kneader at approximately 72.2 wt% NMMO-water concentration. This results in insufficient torque and speed in the downstream mixing kneader's inlet zone to ensure adequate water evaporation through the resulting friction and the heating of the pulp suspension. In summary, high water content means both a high evaporation load for the mixing kneader and a low proportion of mechanical energy input due to low viscosity.

[0037] Mixing kneaders are known to those skilled in the art. Preferably, a mixing kneader has exactly one or exactly two kneading shafts for performing high-viscosity processes that produce crusts, can operate under vacuum, atmospheric pressure, or overpressure, and can be heated or cooled through heat exchange surfaces. Furthermore, at typical product viscosities, the kneading shafts and their structure can heat the product very effectively through rotation and friction.

[0038] If there is exactly one kneading shaft, it is a single-shaft mixing kneader, for example, as described in CH 674 472 A5. Preferably, the shaft structure of the kneading shaft engages with the static structure of the housing (e.g., a so-called anti-hook) during operation. If there are exactly two kneading shafts, it is a dual-shaft mixing kneader, for example, as described in DE 41 18 884 A1. Preferably, the shaft structures of the kneading shafts engage with each other during operation.

[0039] At least one kneader shaft includes a disc and a shaft structure in the form of a rod fixed to the disc, wherein the shaft structure of the at least one kneader shaft is adapted to engage with the shaft structure of a second kneader shaft or with a static mating element present in the mixing kneader during operation. Mixing kneaders having elements that engage in this manner are known and are referred to as "self-cleaning" because the engagement removes any adhering substances that may be present on the mating elements.

[0040] The aforementioned kneader shaft with disc and bar is prior art, disclosed for example in DE 41 18 884 A1. For the purposes of this invention, the manner in which the bar is fixed to the disc, and the manner in which the disc is fixed to the kneader shaft, is irrelevant. For example, the bar and disc (also called "support") can be manufactured as a single unit, therefore the term "fixed" should be interpreted broadly. DE 41 18 884 A1 discloses a biaxial mixing kneader, while CH 674 472 A5 provides a single-axis mixing kneader with hook-shaped static mating kneading elements (so-called "kneading hooks") on the inner wall of the housing. The housing, (plural) kneader shafts, shaft structure, and static mating kneading elements can be designed according to the descriptions in the aforementioned disclosures.

[0041] Heat exchange surfaces can be designed including, but are not limited to, the inner side of the kneader housing, (multiple) kneader shafts, and discs. Regarding the kneader housing, the heat exchange surface is typically designed as a welded half-tube, or preferably as a double-walled structure. Regarding the kneader shaft, a heat exchange surface means that the kneader shaft is designed as a hollow shaft, and preferably has inlets and outlets for the heat transfer or cooling medium, for example, by incorporating an inner tube.

[0042] A disc with a heat exchange surface has a cavity or aperture for an electric heating element, a heat transfer medium, or a cooling medium. The latter has an inlet for feeding material through an inlet in the kneader shaft and an outlet for returning material to the kneader shaft. The cavity is typically designed as double-walled or apertured, requiring greater wall thickness, larger shaft dimensions, and a thicker-walled kneader shaft. Therefore, it is significantly more difficult to manufacture than a disc without a cavity (hereinafter also referred to as a disc without a heating cavity), which can typically be manufactured smaller due to the omission of the heat exchange surface. Summary of the Invention

[0043] The purpose of this invention is to overcome the shortcomings of prior art.

[0044] In particular, a method for producing a transfer mixture by evaporating water from a cellulose-water-functional liquid mixture is described. This method significantly improves the evaporation efficiency of a suitable evaporation device (preferably a thin-film evaporator) via heat input, without the risk of overheating or explosion.

[0045] The features described in claim 1, 2, 11 or 12 are solutions for achieving this purpose.

[0046] Advantageous technical solutions are described in the appendix.

[0047] The subject of this invention is the production of a transfer mixture in a thin-film evaporator by a direct dissolution method as the first process stage of a at least two-stage process, taking into account all the circumstances described above when the intended purpose was proposed.

[0048] The method for producing a transfer mixture in a direct dissolution method according to the present invention is carried out in an apparatus preferably a thin-film evaporator having an inlet, a shell, and an outlet, wherein the inlet feeds a product in the form of a starting material consisting essentially of cellulose, water, and a functional liquid into the shell, wherein the starting material is heated and a portion of the water evaporates, thereby producing a transfer mixture, wherein the transfer mixture flows to the outlet at the feed flow rate and is then transferred without delay to a downstream process component.

[0049] In this method, a transfer mixture, rather than a forming solution, is to be produced. The transfer mixture is characterized by a certain water content at which the temperature of the heating surface of the thin-film evaporator (especially towards the outlet) does not decrease significantly compared to the inlet, and the product does not overheat severely. The transfer mixture differs from the forming solution in that it represents the state of the product before it is transformed into a formable, particularly a formable or spinnable solution. A formable or spinnable solution is produced when all the basic cellulose components of the starting material have dissolved. In this case, "basic" refers to the proportion of undissolved components so low that the so-called filter life is economically acceptable. Filter life is related to the filter typically used after the process stage for producing the forming solution and before the process stage for forming or spinning. Filter life is equivalent to the length of time or usable period before the filter needs replacement, as components unwanted for further processing accumulate on the filter, causing the pressure drop to rise until it becomes excessive. These components can be, for example, undissolved cellulose fibers or non-cellulose impurities. Therefore, the specific choice of filter life is subject to macroeconomic cost and quality optimization.

[0050] The transfer mixture also differs from the starting material; that is, due to the thermomechanical treatment performed in the thin-film evaporator, some of the cellulose present in the starting material is already in solution in the transfer mixture.

[0051] The method for manufacturing transfer mixtures according to the present invention has the following advantages: the product in the thin-film evaporator always has a sufficiently high water content, thereby preventing severe overheating and, in the case of NMMO as the functional liquid, preventing the potential explosion risk therefrom. Thus, it is not necessary to reduce or significantly reduce the heating temperature on the discharge side of the thin-film evaporator, but there is always a temperature difference required for heat input, i.e., the difference between the heating temperature and the product temperature, at least 20 K, preferably 50 K, and ideally 70 K, while avoiding severe overheating. Another advantage is that, due to the lower viscosity and the avoidance of product accumulation in front of the outlet on the outlet side of the thin-film evaporator, the mechanical load on the evaporator shaft of the thin-film evaporator is lower, thereby enabling more economical implementation of the thin-film evaporator. This increases the maximum water evaporation per unit heating area of ​​the thin-film evaporator while reducing energy costs, making the structure more advantageous. The more advantageous structure stems, for example, from the fact that conventional thin-film evaporators can be implemented on a larger scale, thus processing more material than before. This increases both the water evaporation per unit heating area and the structural size, i.e., increases the production capacity of each production line.

[0052] Furthermore, the improved energy efficiency and process safety not only facilitate the process steps of producing concentrated pulp suspensions in thin-film evaporators, but also, when combined with suitable downstream process components (preferably such as mixing kneaders), facilitate the entire process of transforming starting materials into spinning solutions.

[0053] According to the present invention, IL or NMMO is added as a functional liquid for use as a starting material. The functional liquid system is used to dissolve cellulose under appropriate conditions.

[0054] In this method, the transfer mixture can be conveyed, for example, to a downstream process component. The downstream process component can be a mixing kneader as described above, or other thin-film evaporators, etc. The downstream process component is used to further process the transfer mixture into a molded solution. This selection of the transfer mixture is preferred because, in practice, it has been shown that the viscosity of the transfer mixture is not only low enough to avoid severe overheating in the thin-film evaporator, but also high enough that the mechanical energy input through friction in the mixing kneader is so high that the demand for thermal energy input (supplementing the mechanical energy input) in the mixing kneader is so low that the structure of the kneader shaft does not need to serve heat exchange, and can be designed without a heating chamber. This makes the kneader shaft structure simpler, cheaper, and faster.

[0055] Therefore, the transfer mixture is selected to have a water concentration and to satisfy at least one of the following conditions, or to form at least one of the following embodiments according to the invention: (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 towards the outlet, does not decrease significantly compared to the feed inlet, and the transfer mixture does not experience severe overheating. (2) When NMMO is used as the functional liquid, the water content of the transfer mixture generally meets the following conditions: Maximum x H2O = -0.235 x Cell + 0.235 Minimum x H2O = -0.59 x Cell + 0.2047. More preferably, the transfer mixture is produced with the following preferred 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.

[0056] It is preferable to select the transfer mixture in such a way that it (as a supplement to (1) and (2) above) has a water concentration, in addition: (3) When the mixing kneader is used as a downstream process component that is immediately following or not immediately following the thin film evaporator, the mixing kneader is equipped with at least one kneader shaft whose structure does not need to be designed to be heated, and the transfer mixture has sufficient viscosity so that the evaporation efficiency required to produce the molding solution in the mixing kneader can be achieved by means of mechanical energy input (as a supplement to the heat input achieved through the heated kneader shaft of the mixing kneader and the heated shell of the mixing kneader).

[0057] The starting material exists as a mixture of cellulose, water and functional fluid, and its composition can vary greatly.

[0058] In this process, the starting material is transformed into a transfer mixture from the inlet to the outlet.

[0059] Cellulose is partially dissolved in the transfer mixture according to the invention, and the water content in NMMO can be derived from a mathematical formula when NMMO is used as the functional liquid.

[0060] The formula used to describe the transfer mixture relates to the manufacture of the transfer mixture under proven thermomechanical conditions that allow for low-risk operation of the thin-film evaporator. However, since the rate of cellulose dissolution is affected by the aforementioned factors, it is possible that the formula predicts a spinnable solution, while in practice, despite low water content, undissolved cellulose components still exist in the transfer mixture according to the invention because of the selected special thermomechanical treatment, such as a very short time between the product feed and outlet. However, since this results in a low water content in the transfer mixture, this thermomechanical treatment also introduces greater process risks due to severe overheating and explosive decomposition.

[0061] According to the present invention, this also means that the starting material is fed into the thin-film evaporator with a composition within the concentration range that describes a typical transfer mixture. Normally, a transfer mixture characterized by partial dissolution of pulp is only produced by subjecting the starting material to process-specified thermomechanical treatment (increased temperature and shear effect) in a thin-film evaporator.

[0062] The outlet leads to a downstream process component, which is better suited than a thin-film evaporator for safely converting the transfer mixture into a molded solution. As previously mentioned, this downstream process component can be a mixing kneader. Specifically, the mixing kneader has excellent high-viscosity mixing performance and provides efficient mechanical energy input through the kneader shaft, whose rotational speed can be quickly adjusted and rapidly reduced (to zero if necessary), making it particularly suitable as a downstream process component. Another advantage is that the mixing kneader allows for highly precise and safe temperature regulation, thereby avoiding cooling or heat dissipation under normal circumstances.

[0063] In another embodiment, the transfer mixture first passes through a downstream transfer member. Then, the transfer mixture is conveyed to a downstream process member.

[0064] Therefore, the improved energy efficiency and process safety are not only applicable to the process steps in manufacturing transfer mixtures, but also, when combined with suitable downstream process components, applicable to the entire process of transforming starting materials into molding solutions.

[0065] While accepting the aforementioned drawbacks, other thin-film evaporators can also be considered as downstream process components. These evaporators can be designed to transfer the mixture, and further customized in terms of overall design, such as overall length and the angle of their wiping blades, to suit the processing of the transfer mixture. The downstream process component is defined as a processing component that further processes the transfer mixture into a molding solution. This molding solution can, for example, be used for spinning in the future.

[0066] The material properties described using the formulas listed above for the general transfer range are based on the following research. Maximum water content x H₂O describes the composition corresponding to the dihydrate when considering water and NMMO. NMMO molecules are capable of forming two hydrogen bonds. These hydrogen bonds lead to the formation of structures with bound water molecules and, conversely, to the dissolution of cellulose molecules. Thus, theoretically, NMMO has a certain solubility for cellulose molecules starting from a ratio of less than two water molecules per NMMO molecule. This defines the upper limit of water content in the transfer mixture, distinguishing it from a pure suspension. For example, the literature also records that, under appropriate thermomechanical treatment, a portion of the cellulose is already in solution at an NMMO-water concentration of approximately 75%.

[0067] For those skilled in the art, the minimum water content x H2O described in US 4,196,282 A represents the first visually observable composition of cellulose completely dissolved in a cellulose-water-NMMO mixture. This composition distinguishes the transfer mixture from a fully formed solution. The statement in US 4,196,282 A regarding NMMO as a functional liquid has been confirmed by the inventors' results and observations from industrial applications and experiments conducted on a pilot plant scale. Therefore, the aforementioned two formulas encompass the general range of transfer mixtures.

[0068] For a description of the material properties for the preferred transfer range, please refer to the description of minimum water content in US 4,196,282 A. Accordingly, the curve describes the 95% confidence interval for the minimum water content within the general transfer range, further distinguishing it from the safety state of a fully formed solution. The formula for describing the maximum water content x H2O within the preferred transfer range 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 plant scale. It marks the upper limit of the transfer range within which optimal utilization of the specific process-related thin-film evaporator characteristics can be ensured.

[0069] The transfer mixture is in a pre-dissolved state under both normal and preferred compositions. This enables the manufacturing of Lyocells under optimal operating parameters of the thin-film evaporator and avoids unmanageable material states of Lyocells, such as crusting, over-drying, or decomposition in the thin-film evaporator. Such troublesome material states can be relatively easily prevented in downstream process components, as these components are typically designed for higher torque and to prevent crusting.

[0070] Producing a transfer mixture with a typical composition is already advantageous because enough water is evaporated to produce the transfer mixture, which downstream process components can then further process into a solution within a relatively short process time.

[0071] In a preferred composition of a transfer mixture, the maximum water content is close to complete dissolution compared to the general definition of a transfer mixture. On the other hand, the minimum water content is further away from complete dissolution.

[0072] This allows for further processing within a shorter process time, while also reducing the risk of the material over-drying in the thin-film evaporator. However, in both cases, there is a processing time of several minutes between the transfer mixture and the formable or spinnable solution, during which the product is homogenized by stirring and residual water is evaporated if necessary, allowing the thin-film evaporator to operate with the aforementioned optimal operating characteristics.

[0073] The product, i.e., the starting material, is essentially composed of cellulose, water, and functional fluid. In addition, the product also contains other chemicals such as stabilizers, which need not be listed in detail within the framework of this invention, as they are known to those skilled in the art and can be adapted for each individual application.

[0074] Furthermore, a thin-film evaporator for processing starting materials into a transfer mixture according to a direct dissolution method is also claimed. This thin-film evaporator has an inlet, a shell, and an outlet. The inlet feeds the starting material or product, which is essentially composed of cellulose, water, and a functional liquid, into the shell. An evaporator shaft arranged in the shell rotates to allow the product to pass over the heated interior of the shell, whereby the product is heated, a portion of the water evaporates, thereby producing a transfer mixture. 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 product temperature.

[0075] The present invention also provides a thin-film evaporator for processing starting materials into a transfer mixture according to a direct dissolution method, having an inlet, a shell, and an outlet, wherein the inlet feeds a product consisting essentially of cellulose, water, and N-methylmorpholine-N-oxide (NMMO) into the shell, wherein an evaporator shaft arranged in the shell rotates to allow the product to pass through the heated interior of the shell, wherein the product is heated and a portion of the water evaporates, thereby producing a transfer mixture, wherein the transfer mixture can be conveyed through the outlet when the following composition is achieved: Maximum x H2O = -0.235 x Cell + 0.235 and Minimum x H2O = -0.59 x Cell + 0.2047 This means that the thin-film evaporator is configured to automatically or via guided passage through the outlet when the parameters for transferring the mixture are reached, and to be operatively connected to at least one downstream transfer component for further processing or transport, or to operatively connect the transfer mixture to a downstream process component.

[0076] A preferred transfer mixture composition is one that has the following characteristics: 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.

[0077] The remaining description of the method according to the invention also applies to thin-film evaporators, especially when the same features are indicated by the same symbols. Simple Explanation of the Diagram

[0078] Further advantages, features, and details of the invention will become apparent from the following description of preferred embodiments, with reference to the accompanying drawings; wherein: Figure 1 is a view showing the preferred material properties of the transfer mixture. Figure 2 is a view showing the general material properties of the transfer mixture. Implementation

[0079] [Example 1]

[0080] This example focuses on the production of a molding solution with a cellulose content of 12 wt% using a direct dissolution method. NMMO is used as the functional liquid in this example. All percentages given below relate to the total mass of the cellulose-NMMO-water mixture. A starting material with a cellulose content of approximately 7.2 wt% was prepared using an aqueous solution of cellulose and NMMO, resulting in an NMMO content of approximately 46.1 wt%. This starting material was fed into a thin-film evaporator, where it was concentrated into a transfer mixture. In this example, the thin-film evaporator was operated at a process pressure of 70 mbara, resulting in an equilibrium temperature of approximately 43°C for the starting material. The heating temperature of the thin-film evaporator was 130°C. In this example, the transfer of the mixture was carried out within a preferred transfer range with a cellulose content of approximately 11.5 wt% and an NMMO content of approximately 73.9 wt%. Under the current process conditions, this corresponds to an equilibrium temperature of approximately 100°C for the transfer mixture. At this point, the ratio of water to NMMO is approximately equal to the ratio of 1.3 hydrates. The transfer mixture provided by the thin-film evaporator was then fed into a mixing kneader. Here, the mixture is finally concentrated by evaporation and homogenized to produce a fully formed solution. The formed solution leaves the mixing kneader with a cellulose content of 12.0 wt%, an NMMO content of approximately 77.0 wt%, and a temperature of approximately 107°C.

[0081] [Example 2]

[0082] This example focuses on the preparation of a molding solution with a cellulose content of 12 wt% using a direct dissolution method. An ionic liquid is used as the functional liquid in this example. All percentages given below relate to the total mass of the mixture. A starting mixture with a cellulose content of approximately 8.2 wt% is prepared using an aqueous cellulose IL solution, resulting in an IL content of approximately 58.6 wt%. This starting mixture is fed into a thin-film evaporator, where it is concentrated into a transfer mixture. In this example, the transfer of the mixture is carried out with 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 is then fed into a mixing kneader. Here, the mixture is finally concentrated by evaporation and homogenized to produce a fully molded solution. The molding solution leaves the mixing kneader with a cellulose content of 12.0 wt% and an IL content of approximately 83.0 wt%.

[0083] Figure 1 is a view showing the general material composition and preferred material composition of the transfer mixture. The general composition a has the following parameters: Maximum x H2O = -0.235 x Cell + 0.235 Minimum x H2O = -0.59 x Cell + 0.2047. This demonstrates that the preferred composition b has a greater margin, and this preferred composition has the following parameters: 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.

[0084] As the water content decreases, the solubility of L is first reached. As the water content decreases further, NMMO crystallizes into K.

[0085] A: General composition of the transfer mixture B: Preferred composition of the transfer mixture K: Crystallization L: Dissolve

Claims

1. A method for producing a transfer mixture in a thin-film evaporator (D) having an inlet, a shell (4) and an outlet according to a direct dissolution method, wherein the inlet feeds a product consisting essentially of cellulose, water and a functional liquid into the shell (4), wherein an evaporator shaft (5) arranged in the shell (4) rotates to allow the product to pass over the heated interior of the shell (4), wherein the product is heated and a portion of the water evaporates, thereby producing the transfer mixture, wherein the temperature of the heated surface toward the outlet is not significantly lower than that of the inlet of the thin-film evaporator (D), characterized in that (a) the transfer mixture is discharged before the product is severely overheated, and (b) a heating temperature is applied to all heated surfaces of the thin-film evaporator (D) in contact with the product, the heating temperature being at least 20 K higher than the temperature of the product.

2. A method for producing a transfer mixture in a thin-film evaporator (D) having an inlet, a shell (4), and an outlet, wherein the inlet feeds a product consisting essentially of cellulose, water, and N-methylmorpholine-N-oxide (NMMO) into the shell (4), wherein an evaporator shaft (5) arranged in the shell (4) rotates to allow the product to pass over the heated interior of the shell (4), wherein the product is heated and a portion of the water evaporates, thereby producing the transfer mixture, wherein the temperature of the heated surface toward the outlet is not significantly lower than that of the inlet of the thin-film evaporator (D), characterized in that the composition of the transfer mixture is: maximum xH2O = -0.235 xCell + 0.235; and minimum xH2O = -0.59 xCell + 0.2047, wherein xH2O represents the feed flow rate of water, and xCell represents the feed flow rate of cellulose.

3. The method as described in claim 2, wherein the aforementioned transfer mixture has the following preferred composition: maximum xH2O = 0.2864 x2Cell – 0.6786 xCell + 0.2288; minimum xH2O = 0.2864 x2Cell – 0.6786 xCell + 0.2188.

4. The method as described in claim 1 or 2, wherein the aforementioned transfer mixture passes through at least one downstream transfer member.

5. The method as described in claim 1 or 2, wherein the aforementioned transfer mixture is conveyed to a downstream process component.

6. The method as described in claim 5, wherein the aforementioned downstream process component further processes the aforementioned transfer mixture into a molding solution.

7. The method as described in claim 1 or 2, wherein the machine controller monitors the proportions of water, cellulose, and functional liquid in the composition of the transfer solution from the aforementioned inlet to the aforementioned outlet.

8. The method as described in claim 7, wherein the aforementioned machine controller utilizes sensors to monitor the composition of the aforementioned product in all states prior to becoming the aforementioned transfer mixture.

9. The method as described in claim 5, wherein the aforementioned downstream process component is a mixing kneader.

10. The method as described in claim 9, wherein the aforementioned mixing kneader further processes the aforementioned transfer mixture into a molding solution by means of evaporating water and stirring according to a direct dissolution method, wherein the shaft structure of the kneader shaft is implemented without a heating chamber.

11. A thin-film evaporator (D) for processing starting materials into a transfer mixture according to a direct dissolution method, having an inlet, a shell (4) and an outlet, wherein the inlet feeds a product consisting essentially of cellulose, water and a functional liquid into the shell (4), wherein an evaporator shaft (5) arranged in the shell (4) rotates to allow the product to pass over the heated interior of the shell (4), wherein the product is heated and a portion of the water evaporates, thereby producing the transfer mixture, wherein the temperature of the heated surface toward the outlet is not significantly lower than that of the inlet of the thin-film evaporator (D), characterized in that all heated surfaces of the thin-film evaporator (D) in contact with the product are subjected to a heating temperature at least 20 K higher than the temperature of the product.

12. A thin-film evaporator (D) for processing starting materials into a transfer mixture according to a direct dissolution method, having an inlet, a shell (4), and an outlet, wherein the inlet feeds a product consisting essentially of cellulose, water, and N-methylmorpholine-N-oxide (NMMO) into the shell (4), wherein an evaporator shaft (5) arranged in the shell (4) rotates to allow the product to pass through the heated interior of the shell (4), wherein the product is heated and a portion of the water evaporates, thereby producing the transfer mixture, wherein the temperature of the heated surface toward the outlet is not significantly reduced compared to the inlet of the thin-film evaporator (D), characterized in that the transfer mixture can be conveyed through the outlet when the following composition of the transfer mixture is achieved: maximum xH2O = -0.235 xCell + 0.235; and minimum xH2O = -0.59 xCell + 0.2047, wherein xH2O represents the feed flow rate of water, and xCell represents the feed flow rate of cellulose.

13. The thin-film evaporator (D) as claimed in claim 12, wherein the aforementioned transfer mixture has the following preferred composition: maximum xH2O = 0.2864 x2Cell – 0.6786 xCell + 0.2288; minimum xH2O = 0.2864 x2Cell – 0.6786 xCell + 0.2188.

14. The thin-film evaporator (D) as claimed in claim 11 or 12, wherein the aforementioned transfer mixture is operatively connected to at least one downstream transfer member.

15. The thin-film evaporator (D) as claimed in claim 11 or 12, wherein the aforementioned transfer mixture is operatively connected to a downstream process component.

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