Method for producing cellulose ether

Abstract

To provide a method for producing cellulose ether in which consumption amounts of water and energy are reduced compared with a conventional producing method, that is, a method of obtaining cellulose ether having low yellowness and ash content, and having high bulk density.SOLUTION: A method for producing cellulose ether, comprises at least: a reaction step of reacting alkaline cellulose with an etherifying agent to obtain a reaction product; a step of washing and dewatering the reaction product to obtain a first hydrated cellulose ether; a step of cooling the first hydrated cellulose ether; a step of heating the cooled first hydrated cellulose ether to 60 to 110°C; a step of dewatering the heated first hydrated cellulose ether using a screw press to obtain immediately after discharge from the screw press a solid second hydrated cellulose ether; and a step of drying and pulverizing the second hydrated cellulose ether.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing a cellulose ether. [Background technology]

[0002] Cellulose ethers are widely used in pharmaceuticals, food products and various industrial applications as coating agents for tablets, base materials for capsules, thickeners for various solvents, suspension stabilizers, dispersants and the like.

[0003] The cellulose ether can be obtained by reacting alkali cellulose with an etherifying agent, washing and deliquoring the reaction product, drying, and pulverizing the reaction product.

[0004] When the cellulose ether is used as an additive for pharmaceuticals or foods, it is preferable that the cellulose ether has a low yellowness in terms of appearance. In addition, it is preferable that the cellulose ether has a low ash content. As a method for reducing the ash content in cellulose ether, there is a method of increasing the amount of washing water or the number of washings in the washing and deliquor steps (Patent Document 1).

[0005] In addition, cellulose ether is generally provided as a powder. In this case, a high bulk density is preferable from the viewpoint of space for storage and transportation, and the flowability of the powder during use. A method for obtaining a cellulose ether with a high bulk density includes mixing the cellulose ether with water at 70°C or higher to adjust the water content of the cellulose ether to 55 to 90% by mass, and then cooling the hydrous cellulose ether (Patent Document 2).

[0006] The cooled hydrous cellulose ether has a dissolved particle surface and is therefore very adhesive and tacky. Therefore, when drying hydrous cellulose ether, methods for reducing the adhesive and tackiness have been used, such as a method of pulverizing the hydrous cellulose ether while heating it using a high-speed impact mill (Patent Document 3) or a method of drying the cooled hydrous cellulose ether in two stages using a flash dryer and a conduction heat transfer dryer (Patent Document 4). [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Special Publication No. 2002-541945 [Patent Document 2] Japanese Patent Application Publication No. 2017-186557 [Patent Document 3] Japanese Patent Application Laid-Open No. 2001-240601 [Patent Document 4] Japanese Patent Application Laid-Open No. 2015-214683 Summary of the Invention [Problem to be solved by the invention]

[0008] The method of Patent Document 1 requires a large amount of hot water to wash the cellulose ether, which increases water and energy consumption and requires a large space for washing and dewatering equipment. Furthermore, even if the amount of washing water or the number of washings is increased, there is a limit to the reduction in yellowness and ash content. In the methods of Patent Documents 3 and 4, a large amount of hot air must be generated in order to dry a highly water-containing cellulose ether with hot air, and there is room for improvement in the energy consumption in the drying step. Therefore, an object of the present invention is to provide a method for producing a cellulose ether that has low yellowness and ash content and high bulk density, and that consumes less water and energy than conventional production methods. [Means for solving the problem]

[0009] As a result of intensive research to solve the above-mentioned problems, the present inventors have found that by heating cooled cellulose ether and then dehydrating it using a screw press, it is possible to reduce the yellowness and ash content of the cellulose ether while also reducing the amount of steam consumed in the production process of the cellulose ether, and have thus completed the present invention. According to the present invention, there is provided the following method for producing a cellulose ether. [1] a reaction step of reacting alkali cellulose with an etherifying agent to obtain a reaction product; washing and deliquoring the reaction product to obtain a first hydrous cellulose ether; cooling the first hydrous cellulose ether; a step of heating the cooled first hydrous cellulose ether to 60 to 110°C; deliquoring the heated first hydrous cellulose ether using a screw press to obtain a second hydrous cellulose ether that is solid immediately after being discharged from the screw press; drying and pulverizing the second hydrous cellulose ether; A method for producing cellulose ether comprising at least [2] The method for producing a cellulose ether according to [1], wherein the first hydrous cellulose ether to be subjected to the cooling step has a water content of 50 to 90% by mass. [3] The method for producing a cellulose ether according to [1] or [2] further comprises, after the step of washing and deliquifying the reaction product to obtain a first hydrated cellulose and before the step of cooling the first hydrated cellulose ether, a step of adjusting the moisture content of the first hydrated cellulose obtained by the washing and deliquifying to 50 to 90 mass%. [4] The method for producing a cellulose ether according to any one of [1] to [3], wherein the second hydrous cellulose ether has a water content of 10 to 35% by mass. [5] The method for producing a cellulose ether according to any one of [1] to [4], wherein the first hydrous cellulose ether is cooled to 0 to 40°C in the step of cooling the first hydrous cellulose ether. [6] The method for producing a cellulose ether according to any one of [1] to [5], wherein the viscosity of a 2.0% by mass aqueous solution of the obtained cellulose ether at 20° C. is 100 to 100,000 mPa·s. [Effects of the Invention]

[0010] According to the present invention, it is possible to reduce the yellowness and ash content of cellulose ether and also reduce the amount of energy consumed in the process for producing the cellulose ether. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a schematic diagram of an example of a screw press used in a deliquoring step. DETAILED DESCRIPTION OF THE INVENTION

[0012] The present invention will be described in detail below. The method for producing a cellulose ether includes at least the following steps: a reaction step of reacting alkali cellulose with an etherifying agent to obtain a reaction product; a step of washing and deliquifying the reaction product to obtain a first hydrous cellulose ether; a step of cooling the first hydrous cellulose ether; a step of heating the cooled first hydrous cellulose ether to 60 to 110°C; a step of deliquifying the heated first hydrous cellulose ether using a screw press to obtain a second hydrous cellulose ether in a solid state immediately after being discharged from the screw press; and a step of drying and pulverizing the second hydrous cellulose ether. A screw press is a device that performs solid-liquid separation by using a compressive force generated by utilizing the volume change between the deliquified raw material inlet and the deliquified product outlet.

[0013] [Reaction process] In the reaction step, alkali cellulose obtained by contacting pulp with an alkali metal hydroxide solution is reacted with an etherifying agent to obtain a reaction product.

[0014] Examples of pulp include wood pulp, linter pulp, etc. Pulp may be in the form of powder, sheet, chips, etc. The intrinsic viscosity of the pulp is preferably 300 to 2500 ml / g, more preferably 350 to 2300 ml / g, even more preferably 400 to 2000 ml / g, and most preferably 400 to 1500 ml / g. If the intrinsic viscosity is less than 300 ml / g, the 2% by mass viscosity at 20°C of the obtained cellulose ether may be low. On the other hand, it is difficult to obtain pulp with an intrinsic viscosity exceeding 2500 ml / g.

[0015] From the viewpoints of economy and operability, the alkali metal hydroxide solution may be an aqueous solution of alkali metal hydroxide such as an aqueous sodium hydroxide solution or an aqueous potassium hydroxide solution. From the viewpoints of economy and operability, the concentration of the alkali metal hydroxide in the alkali metal hydroxide solution is preferably 23 to 60 mass%.

[0016] The mass ratio of the alkali metal hydroxide in the alkali cellulose to the solid components in the pulp (alkali metal hydroxide / solid components in the pulp) is not particularly limited as long as the desired degree of substitution is achieved, but is preferably 0.30 to 1.50, and more preferably 0.35 to 1.45. The amount of the alkali metal hydroxide solution used may be appropriately selected depending on the above mass ratio. The mass of the alkali metal hydroxide component can be calculated by neutralization titration. The solid components in pulp refer to the components in pulp other than water. In addition to the main component cellulose, the solid components in pulp include organic matter such as hemicellulose, lignin, and resin, and inorganic matter such as silicon and iron. The solid components in pulp can be calculated from the dry matter content determined by the JIS P8203:2010 Pulp - Dry Matter Content Test Method. The dry matter content is the ratio of the mass of a sample when it has reached a constant weight after drying at 105±2°C to the mass before drying, and is expressed as a percentage by mass.

[0017] Examples of the etherifying agent used in the reaction step include alkyl halides such as methyl chloride and ethyl chloride, and alkylene oxides such as ethylene oxide and propylene oxide. The etherifying agent may be added to the alkali cellulose all at once, in several divided portions, or continuously. In order to suppress a local temperature rise in the reactor during the etherification reaction, an organic solvent that is not used in the etherification reaction can be added to the reaction system. Examples of the organic solvent that is not used in the etherification reaction include dimethyl ether. The temperature for the etherification reaction is preferably 40 to 120° C. The reaction time for the etherification reaction is preferably 1 to 5 hours.

[0018] [Cleaning and draining process] In this step, the reaction product is washed and deliquified to obtain a first hydrous cellulose ether. The washing and deliquification can be performed using known techniques. For example, hot water at a temperature of preferably 85 to 100°C is added to the reaction product to form a slurry having a cellulose ether concentration of preferably 1 to 15% by mass, which is then washed and deliquified. Washing and dewatering may be carried out separately or simultaneously. For example, filtration or squeezing may be carried out after washing, or filtration or squeezing may be carried out while pouring washing water. As the apparatus for washing and deliquoring, a vacuum filter, a pressure filter, a centrifugal dehydrator, a filter press, a V-type disk press, etc. can be used. From the viewpoint of productivity, as the apparatus for washing and deliquoring, a continuous pressure filter is preferred, a pressure rotary filter is more preferred, and a pressure rotary filter whose interior is divided into multiple compartments and has a structure that allows for the supply of slurry, the production of cake by filtering the slurry, the supply of steam, the supply and filtration of wash water, the discharge of the washed product, and the washing of the filter after discharge to be performed in each compartment is even more preferred. An example of a pressure rotary filter is a rotary pressure filter manufactured by BHS of Germany.

[0019] The water content of the first hydrous cellulose ether obtained by washing and deliquoring is preferably 30 to 80% by mass, more preferably 35 to 70% by mass, from the viewpoints of the ash content and yellowness of the cellulose ether. On the other hand, from the viewpoint of the bulk density of the obtained cellulose ether, the water content of the first hydrous cellulose ether when subjected to the cooling step described below is preferably 50 to 90 mass%, more preferably 52 to 85 mass%, even more preferably 55 to 80 mass%, and most preferably 55 to 75 mass%. Therefore, in order to obtain a cellulose ether having a low ash content and low yellowness and a high bulk density, if necessary, a step of adjusting the moisture content of the first hydrous cellulose obtained by the washing and deliquescence to preferably 50 to 90 mass%, more preferably 52 to 85 mass%, even more preferably 55 to 80 mass%, and most preferably 55 to 75 mass% may be provided between the step of obtaining a first hydrous cellulose ether by washing and deliquescence and the step of cooling the first hydrous cellulose ether.

[0020] The water content of the first hydrous cellulose ether can be adjusted by mixing the first hydrous cellulose ether obtained by washing and deliquoring with water or hot water. The first hydrous cellulose ether and water or hot water can be mixed either batchwise or continuously, but the continuous method is preferred from the viewpoints of uniform water distribution in the first hydrous cellulose ether and productivity. The temperature of the water or hot water is preferably 5 to 95°C. The moisture content of the first hydrous cellulose ether was measured using a heat-drying moisture meter (MX-50, manufactured by A&D Co., Ltd.) under the conditions of 5 g of the target sample, a heating temperature of 105°C, and a heating time of 120 minutes. The moisture contents of the second hydrous cellulose ether and the final cellulose ether described below were also measured using the same method as the moisture content of the first hydrous cellulose ether. Note that the moisture content does not refer to the ratio of water to the cellulose ether, but rather to the ratio of water to the total of water and cellulose ether.

[0021] [Cooling process] In this step, the first hydrous cellulose ether, preferably a first hydrous cellulose ether having a water content of 50 to 90% by mass, is cooled. The first hydrous cellulose ether can be cooled by any known technique. Examples include a method of placing the first hydrous cellulose ether in a container and contacting it with a cooled jacket, a method of contacting it with cold air, and a method utilizing the heat of vaporization generated when water evaporates. The cooling method can be either a batch method or a continuous method. From the viewpoints of uniform temperature distribution of the first hydrous cellulose ether and productivity, a continuous method is preferred. In the cooling step, the first hydrous cellulose ether is cooled so that its product temperature falls within a range of preferably 0 to 40°C, more preferably 5 to 37°C, and even more preferably 7 to 35°C. It is preferable to maintain the temperature within the above range even after cooling and subject the resulting mixture to the subsequent heating step. In other words, the temperature of the first hydrous cellulose ether after cooling is preferably 0 to 40°C, more preferably 5 to 37°C, and even more preferably 7 to 35°C, from the viewpoint of the bulk density of the cellulose ether.

[0022] [Heating process] In this step, the cooled first hydrous cellulose ether is heated. Examples of heating methods for the first hydrous cellulose ether include contacting the first hydrous cellulose ether with hot water or steam, placing the first hydrous cellulose ether in a container and contacting it with a heated jacket, and irradiating the first hydrous cellulose ether with electromagnetic waves such as microwaves or infrared rays. In any of these heating methods, either a batch or continuous heating system can be used. To ensure uniform heating, it is preferable to stir the first hydrous cellulose ether during heating. The temperature of the first hydrous cellulose ether after heating is 60 to 110°C, preferably 70 to 110°C, more preferably 80 to 110°C, and even more preferably 90 to 105°C, from the viewpoint of preventing adhesion of the cellulose ether to the screw press used in the deliquification step described below, and thus preventing clogging of the screw press due to adhesion of the cellulose ether, and from the viewpoint of efficiently performing deliquification.

[0023] Furthermore, the viscosity of a 2.0% by mass aqueous solution of the heated first hydrous cellulose ether at 20°C before the deliquoring step (screw press) is preferably 100 to 100,000 mPa·s, more preferably 150 to 80,000 mPa·s, even more preferably 200 to 50,000 mPa·s, and most preferably 300 to 30,000 mPa·s. A 2.0% by mass aqueous solution of the first hydrous cellulose ether at 20°C within the range of 100 to 100,000 mPa·s is preferred because the second hydrous cellulose ether obtained by the deliquoring step is solid. The viscosity of a 2.0% by mass aqueous solution of the first hydrous cellulose ether at 20°C within this range can be achieved by using pulp with an intrinsic viscosity within this range or by preventing depolymerization of the alkali cellulose or cellulose ether from occurring between the reaction step and the deliquoring step. In order to prevent depolymerization of the alkali cellulose or cellulose ether from occurring during the period from the reaction step to the deliquoring step, for example, the amount of oxygen in contact with the alkali cellulose can be adjusted, or an oxidizing agent such as hydrogen peroxide or an acid such as hydrogen chloride can be prevented from being present in the system during each step. If the viscosity of a 2.0% by mass aqueous solution of the first hydrous cellulose ether at 20°C is less than 100 mPa·s, the 2.0% by mass viscosity of the finally obtained cellulose ether at 20°C may be very low (see Comparative Examples 5 and 6 described later). Furthermore, if the deliquor step is carried out in the presence of the oxidizing agent or acid, depolymerization of the first hydrous cellulose ether may proceed, and the 2.0% by mass viscosity of the finally obtained cellulose ether at 20°C may be very low.

[0024] The viscosity of a 2.0% by mass aqueous solution of the first hydrous cellulose ether at 20°C can be measured by drying the first hydrous cellulose ether after the heating step using a tray dryer to a moisture content of 5.0% by mass or less, coarsely pulverizing it, and then preparing a 2.0% by mass aqueous solution. The viscosity of a 2.0% by mass aqueous solution of the first hydrous cellulose ether at 20°C can be measured using a single-cylinder rotational viscometer in accordance with the rotational viscometer method in the viscosity measurement method of the general test methods described in the Japanese Pharmacopoeia, Eighteenth Edition, when the 2.0% by mass viscosity is 600 mPa s or more. On the other hand, when the 2.0% by mass viscosity is less than 600 mPa s, it can be measured using an Ubbelohde viscometer in accordance with the capillary viscometer method in the viscosity measurement method of the general test methods described in the Japanese Pharmacopoeia, Eighteenth Edition.

[0025] [Deliquoring process] In this step, the first hydrous cellulose ether heated in the heating step is deliquored by a screw press to obtain a second hydrous cellulose ether. Note that this step may be referred to as a "second deliquoring step" to distinguish it from the "washing and deliquoring step" that is performed after the reaction step and between the cooling step and the reaction step.

[0026] A screw press is a device that separates solids and liquids using a compressive force generated by utilizing the volumetric change between the deliquified raw material inlet and the deliquified product outlet. Figure 1 shows a schematic diagram of an example of a screw press used in the deliquifying process. The screw press 1 comprises a substantially cylindrical filter cylinder 2 having slits or perforations formed by punching or the like for discharging the liquid, and a screw disposed concentrically within the filter cylinder 2. The screw comprises a screw shaft 4 and screw blades 3 welded to its periphery. The volume of the screw shaft 4 gradually increases from the deliquified raw material inlet 5 to the deliquified product outlet 8. The screw shaft 4 can control temperature by passing a liquid or a gas from the temperature-adjusting liquid inlet 6 to the temperature-adjusting liquid outlet 10. The screw shaft 4 also comprises a straight section 12 without screw blades 3 in a section preceding the deliquified product outlet 8, in order to extend the residence time of the deliquified raw material A within the apparatus and thereby enhance dehydration performance. Specifically, deliquified raw material A, transported from the deliquified raw material inlet to the deliquified product outlet due to volumetric changes, remains in the straight section 12, which lacks screw blades. This applies pressure to the deliquified raw material A, which is subsequently transported from the deliquified raw material inlet 5, from the deliquified product outlet 8, further reducing the moisture content of the deliquified product B. The length of the straight section 12 is called the straight length (or plug length), and this length can be adjusted by moving the screw, which is horizontally movable along the main shaft. The deliquified product outlet 8 is equipped with a truncated cone-shaped back pressure plate 11 concentrically arranged on the screw shaft 4. The opening of the back pressure plate 11 is adjustable. Furthermore, pressure can be applied from the deliquified product outlet 8 to the deliquified raw material inlet 5 using an air cylinder 9 (hereinafter also referred to as "back pressure"). The compressed liquid C, the liquid component compressed by the screw press 1, passes through the perforations of the filter cylinder 2 and is discharged from the compressed liquid outlet 7. Examples of the screw press include the FKC screw press (manufactured by Fukoku Kogyo Co., Ltd.), the ISGKV hybrid press-fit screw press (manufactured by Ishigaki Co., Ltd.), and the YSP screw press dehydrator (manufactured by Yamato Sangyo Co., Ltd.).

[0027] A method for deliquoring the heated first hydrous cellulose ether using a screw press will be described based on the screw press 1 illustrated in FIG. First, if necessary, select a screw that will achieve the desired compression ratio. The compression ratio of a screw is the ratio of the area between the thickest part of the screw shaft 4 and the inner diameter of the filter cylinder 2 to the area between the thinnest part of the screw shaft 4 and the inner diameter of the filter cylinder 2. Next, the desired screw rotation speed, straight length, back pressure plate opening, back pressure, and screw shaft inlet temperature are set. After setting, the screw press 1 is operated to rotate the screw shaft 4. After the screw rotation speed reaches the desired speed, heated first hydrous cellulose ether is introduced as deliquified raw material A through the deliquified raw material inlet 5. Deliquified raw material A supplied through the deliquified raw material inlet 5 undergoes gravity filtration through the holes in the filter tube 2 directly below the deliquified raw material inlet 5, resulting in solid-liquid separation of a portion of the liquid component. The partially solid-liquid separated deliquified raw material A is transported by the rotating screw to the deliquified product discharge section 8. During this process, deliquification occurs due to volume changes between the deliquified raw material inlet 5 and the deliquified product discharge section 8. Deliquified product B either falls from the deliquified product discharge section 8 due to its own weight, or falls from the deliquified product discharge section 8 after being roughly crushed by collision with the back pressure plate 11. The compressed liquid C, which is the liquid component obtained by solid-liquid separation, is discharged through the holes in the filter cylinder 2 and recovered from the compressed liquid discharge section 7.

[0028] The temperature at the inlet of the screw shaft of the screw press is preferably 100 to 160°C, more preferably 110 to 150°C, and even more preferably 115 to 145°C. If the temperature at the inlet of the screw shaft is less than 100°C, the moisture content of the second hydrous cellulose ether may become high. If the temperature at the inlet of the screw shaft exceeds 160°C, the cellulose ether may burn on the surface of the screw shaft, causing foreign matter.

[0029] The compression ratio is preferably 1.1 to 2.0, more preferably 1.2 to 1.8, from the viewpoint of the water content of the second hydrous cellulose ether.

[0030] The straight length is preferably 100 to 150 mm, more preferably 120 to 150 mm, from the viewpoint of the water content of the second hydrous cellulose ether.

[0031] The back pressure is preferably 0 to 0.5 MPaG, more preferably 0.1 to 0.45 MPaG, from the viewpoint of the water content of the second hydrous cellulose ether. The opening of the back pressure plate is not particularly limited, but is preferably 20 mm or more from the viewpoint of the dischargeability of the second hydrous cellulose ether.

[0032] The screw rotation speed is preferably 0.5 to 6 rpm, more preferably 1 to 4 rpm, from the viewpoint of obtaining a second hydrous cellulose ether having a low moisture content while maintaining high production efficiency. The moisture content of the second hydrous cellulose ether obtained after the deliquoring step using a screw press is preferably 10 to 35% by mass, more preferably 12 to 30% by mass, and even more preferably 13 to 27% by mass. If the moisture content of the second hydrous cellulose ether is less than 10% by mass, the dehydrated product may be too hard, which may result in poor grindability in the grinding step. If the moisture content exceeds 35% by mass, the cellulose ether may have a high ash content and yellowness, and may require a large amount of steam in the drying step.

[0033] The second hydrous cellulose ether is in a solid state immediately after being discharged from the screw press. In this specification, the term "solid" refers to a solid state without fluidity, such as a cake, a lump, or a granular form, and does not include a form that is fluid in a molten state. If the second hydrous cellulose ether is in a fluid state in a molten state or a liquid state immediately after being discharged from the screw press, the compressing force of the screw press may not be effectively transmitted to the cellulose ether, which may increase the moisture content of the second hydrous cellulose ether, and the cellulose ether may leak from the slits or punchings of the filter cylinder of the screw press, making it impossible to continue continuous operation. Furthermore, as a result of various investigations, it was found that when the second hydrous cellulose ether is in a molten, fluid state or liquid immediately after being discharged from the screw press, the viscosity of a 2.0% by mass aqueous solution of the final cellulose ether at 20°C is low. Therefore, in order to make the second hydrous cellulose ether solid immediately after being discharged from the screw press, as described above, the viscosity of a 2.0 mass% aqueous solution of the heated first hydrous cellulose ether at 20°C before being subjected to the screw press is preferably 100 to 100,000 mPa·s, more preferably 150 to 80,000 mPa·s, even more preferably 200 to 50,000 mPa·s, and most preferably 300 to 30,000 mPa·s.

[0034] [Drying and grinding process] In this step, the second hydrous cellulose ether is dried and pulverized to obtain a cellulose ether. In the drying and pulverization steps, drying and pulverization may be performed separately or simultaneously. For example, pulverization may be performed after drying, or pulverization may be performed after drying, or pulverization may be performed simultaneously with drying. The dryer is preferably a conduction heat transfer type dryer such as a groove type agitator dryer, a cylindrical agitator dryer, or a rotary dryer with a steam heating tube, or a fluidized bed dryer, an airflow type dryer, a band dryer, or a tray dryer, etc. Drying may be carried out using a combination of two or more types of dryers. The pulverizer is preferably a feather mill, a cutter mill, an impact pulverizer, a ball mill, a vibration mill, a roller mill, a jet mill, etc. The pulverization may be carried out using a combination of two or more types of pulverizers. As a method for simultaneously drying and pulverizing, a method in which heated gas is introduced into an impact pulverizer or roller mill together with the second hydrous cellulose ether can be mentioned. After drying and pulverization, the product is sieved and mixed as necessary to obtain a cellulose ether.

[0035] [Cellulose ether] Examples of the cellulose ether include alkyl cellulose, hydroxyalkyl cellulose, and hydroxyalkyl alkyl cellulose. From the viewpoint of the form of the second hydrous cellulose ether obtained by deliquoring using a screw press, alkyl cellulose or hydroxyalkyl alkyl cellulose is preferred. Examples of alkyl cellulose include methyl cellulose having a DS of the methoxy group of 1.0 to 2.2, and ethyl cellulose having a DS of the ethoxy group of 2.0 to 2.6. Examples of hydroxyalkyl cellulose include hydroxyethyl cellulose having a hydroxyethoxy group MS of 1.0 to 2.2, and hydroxypropyl cellulose having a hydroxypropoxy group MS of 2.0 to 4.0. Examples of hydroxyalkyl alkyl cellulose include hydroxyethyl methyl cellulose having a methoxy group DS of 1.3 to 2.2 and a hydroxyethoxy group MS of 0.1 to 0.6, hydroxypropyl methyl cellulose having a methoxy group DS of 1.3 to 2.2 and a hydroxypropoxy group MS of 0.1 to 0.6, and hydroxyethyl ethyl cellulose having an ethoxy group DS of 1.3 to 2.2 and a hydroxyethoxy group MS of 0.1 to 0.6. The degree of substitution (DS) refers to the number of alkoxy groups per anhydroglucose unit of cellulose, and the molar substitution (MS) refers to the average number of moles of hydroxyalkoxy groups substituted per mole of anhydroglucose unit. The DS of the alkoxy group and the MS of the hydroxyalkoxy group of the cellulose ether can be calculated from the contents of the alkoxy group and the hydroxyalkoxy group obtained by the quantitative method listed in the 18th edition of the Japanese Pharmacopoeia.

[0036] The viscosity of a 2.0% by mass aqueous solution of the cellulose ether at 20° C. is 100 to 100,000 mPa·s, more preferably 150 to 80,000 mPa·s, even more preferably 200 to 50,000 mPa·s, and most preferably 300 to 30,000 mPa·s. If the viscosity of a 2.0% by mass aqueous solution of the cellulose ether at 20° C. is less than 100 mPa·s, the compressive force of the screw press is not effectively transmitted to the cellulose ether, which may increase the water content of the second hydrous cellulose ether and may cause leakage of the cellulose ether from the slits or punchings in the filter cylinder of the screw press, making continuous operation impossible. The viscosity of a 2.0% by mass aqueous solution at 20°C can be measured using a single cylinder rotational viscometer in accordance with the rotational viscometer method in the viscosity measurement method of the general test methods described in the Japanese Pharmacopoeia, 18th Edition, if the 2.0% by mass viscosity is 600 mPa s or more. On the other hand, if the 2.0% by mass viscosity is less than 600 mPa s, it can be measured using an Ubbelohde viscometer in accordance with the capillary viscometer method in the viscosity measurement method of the general test methods described in the Japanese Pharmacopoeia, 18th Edition.

[0037] The ash content of the cellulose ether is preferably 0.0 to 0.20% by mass, more preferably 0.0 to 0.15% by mass, from the viewpoint of increasing transparency when the cellulose ether is used as a film and reducing residue when the cellulose ether is ultimately burned. The ash content can be measured using the measurement method prescribed in the 18th Edition of the Japanese Pharmacopoeia.

[0038] From the viewpoint of appearance, the yellowness index of the cellulose ether solution is preferably 1.0 to 2.8, more preferably 1.0 to 2.7. The method for measuring the yellowness index of the solution is as described in the Examples below. From the viewpoint of appearance, the yellowness index of the cellulose ether tablet is preferably 1.0 to 8.0, more preferably 1.0 to 7.0. The method for measuring the yellowness index of the tablet is as described in the Examples below.

[0039] The loose bulk density of the cellulose ether is preferably 0.2 to 0.6 mL / g, more preferably 0.3 to 0.5 mL / g, from the viewpoints of fluidity and storage space. The loose bulk density refers to the bulk density in a loosely packed state and can be measured using a powder property evaluation device by passing a sample through a 1 mm mesh sieve into a cylindrical container (made of stainless steel) with a diameter of 5.05 cm and a height of 5.05 cm (volume of 100 ml) and uniformly feeding it from above (23 cm), and then leveling the top surface to weigh it. An example of a powder property evaluation device is the Powder Tester PT-S manufactured by Hosokawa Micron Corporation. [Example]

[0040] The present invention will be specifically explained below with reference to examples and comparative examples, but the present invention is not limited to the following examples. In each example below, unless otherwise specified, each evaluation indicates a value measured by the respective method described above.

[0041] Example 1 Wood-derived pulp chips with an intrinsic viscosity of 450 ml / g were immersed in a 49% by mass aqueous solution of sodium hydroxide. The mass ratio of the 49% by mass aqueous solution of sodium hydroxide to the solid components in the pulp at the time of immersion was 200. Subsequently, excess 49% by mass aqueous solution of sodium hydroxide was removed to obtain alkali cellulose. The mass ratio of the alkali metal hydroxide in the obtained alkali cellulose to the solid components in the pulp (sodium hydroxide / solid components in the pulp) was 1.25. The alkali cellulose (100 parts by mass of solid components) was charged into a pressure vessel (reactor) equipped with a Plossier-type internal stirring blade, and the pressure was reduced to -97 kPaG. After that, nitrogen gas was introduced and the pressure was returned to atmospheric pressure. The pressure was then reduced again to -97 kPaG. Next, 40 parts by mass of dimethyl ether, 220 parts by mass of methyl chloride, and 28 parts by mass of propylene oxide were added to the reactor using a pressure pump. The reaction was allowed to proceed for a total of 2 hours while the temperature was increased from 60°C to 100°C, yielding a reaction product. Hot water at 95°C was added to the obtained reaction product to form a slurry. The concentration of cellulose ether in the slurry was 10% by mass. The slurry was supplied to a rotary pressure filter, washed, and dewatered to obtain a first hydrous cellulose ether. The water content of the first hydrous cellulose ether was 35.0% by mass. Subsequently, the first hydrous cellulose ether was charged into a Plosser mixer equipped with a spray nozzle and having a jacket temperature of 90° C. While stirring the cellulose ether, hot water at 90° C. was sprayed from the spray nozzle. After the hot water spraying was completed, stirring was continued for 5 minutes, and the water content of the first hydrous cellulose ether was adjusted to 61.8% by mass. Subsequently, the first hydrous cellulose ether was charged into a Plosser type mixer having a jacket temperature of 10°C, and stirred for 10 minutes to cool the product temperature of the first hydrous cellulose ether to 15°C. Subsequently, the cooled first hydrous cellulose ether was charged into a Plosser mixer with a jacket temperature of 130°C, and the first hydrous cellulose ether was heated by contacting it with steam at 130°C. The temperature of the first hydrous cellulose ether after heating was 95°C. Next, the heated first hydrous cellulose ether was deliquored using a screw press (SHX-200, manufactured by Fukoku Kogyo Co., Ltd.) under conditions of a screw shaft inlet temperature of 130°C (steam supplied at a vapor pressure of 0.3 MPaG), a compression ratio of 1.47, a straight length of 130 mm, a back pressure of 0.3 MPaG, a back pressure plate opening of 20 mm, and a screw rotation speed of 1 rpm, to obtain a second hydrous cellulose ether having a moisture content of 27.0% by mass. The second hydrous cellulose ether was crushed using a feather mill (FM-1, manufactured by Hosokawa Micron Corporation) equipped with a 5 mm mesh screen, and then dried by feeding it to a paddle dryer (NPD-1.6G, manufactured by Nara Machinery Works) at a paddle temperature of 130°C (steam was supplied at a steam pressure of 0.3 MPaG). The dried cellulose ether was crushed using a victory mill (VP-1, manufactured by Hosokawa Micron Corporation, 0.5 mm mesh). The crushed cellulose ether was sieved through a 100 μm mesh sieve to obtain a cellulose ether. The DS of the methoxy (MeO) group, MS of the hydroxypropoxy (HPO) group of the obtained cellulose ether, and the viscosity of a 2.0% by mass aqueous solution at 20°C were measured using the methods described above. Furthermore, the total steam consumption, yellowness of the solution, yellowness of the tablet, ash content, and loose bulk density were measured using the methods described below. These results are shown in Table 1. In Table 1, "cellulose ether" is abbreviated as "CE."

[0042] <Measurement of total steam consumption> The total steam consumption was determined by measuring the amount of steam consumed in each step (i.e., the heating step, the second deliquifying step or the preliminary drying step, and the drying step) with a flow meter and adding them up, and the relative value to the total steam consumption in Example 1 was calculated and shown in Table 1. The second deliquifying step refers to a step of deliquifying the first hydrous cellulose ether using a screw press or other dehydrator, and the preliminary drying step refers to a step of pre-drying the first hydrous cellulose ether using a dryer such as a flash dryer before the drying step.

[0043] <Ash content measurement> The ash content was measured according to the method described in the 18th Edition of the Japanese Pharmacopoeia, section "General Test Methods 2.44 Ignition Residue Test Method."

[0044] <Measurement of yellowness of solution> A 2% by mass aqueous solution of cellulose ether was prepared, and the sample solution was analyzed using an SM Color Computer SM-4 (manufactured by Suga Test Instruments Co., Ltd.) to measure the yellowness index. <Measurement of yellowness of tablets> Using an automatic tableting tester AUTOTAB-200W (manufactured by Ichihashi Seiki Co., Ltd.), 450 mg of cellulose ether powder was compressed into tablets with a diameter of 12 mm at a tableting pressure of 15 kN. The same procedure was repeated to produce a total of three tablets. Next, using a spectrophotometer / colorimeter SE-7700 (manufactured by Nippon Denshoku Industries Co., Ltd.) and a 6 mmφ 0 box, the yellowness of the above three tablets was measured a total of 18 times on the front and back of the tablets, three points on each side, and this was recorded as the yellowness of the tablets.

[0045] <Loose bulk density measurement> The loose bulk density was measured using a powder property evaluation device, Powder Tester PT-S type (manufactured by Hosokawa Micron Corporation), by passing the sample through a 1 mm mesh sieve into a cylindrical container (material: stainless steel) 5.05 cm in diameter and 5.05 cm in height (volume 100 ml) and feeding it evenly from above (23 cm), and then leveling the top surface and weighing it.

[0046] Example 2 Cellulose ether was obtained in the same manner as in Example 1, except that the straight length of the screw press was changed to 150 mm. The results are shown in Table 1.

[0047] Example 3 Wood-derived pulp chips with an intrinsic viscosity of 550 ml / g were immersed in a 49% by mass aqueous solution of sodium hydroxide. The mass ratio of the 49% by mass aqueous solution of sodium hydroxide to the solid components in the pulp at the time of immersion was 200. Subsequently, excess 49% by mass aqueous solution of sodium hydroxide was removed to obtain alkali cellulose. The mass ratio of the alkali metal hydroxide in the obtained alkali cellulose to the solid components in the pulp (sodium hydroxide / solid components in the pulp) was 1.25. The alkali cellulose (100 parts by mass of solid components) was charged into a pressure vessel (reactor) equipped with a Plosser-type internal stirring blade, and the pressure was reduced to -97 kPaG. After that, nitrogen gas was introduced and the pressure was returned to atmospheric pressure. The pressure was then reduced again to -97 kPaG. Next, 40 parts by mass of dimethyl ether, 222 parts by mass of methyl chloride, and 43 parts by mass of propylene oxide were added to the reactor using a pressure pump. The reaction was allowed to proceed for a total of 2 hours while the temperature was increased from 60°C to 100°C, yielding a reaction product. The resulting reaction product was slurried by adding hot water at 95°C. The concentration of cellulose ether in the slurry was 10% by mass. The slurry was fed to a rotary pressure filter for washing and dewatering, yielding a first hydrous cellulose ether. The water content of the first hydrous cellulose ether was 36.0% by mass. Subsequently, the first hydrous cellulose ether was charged into a Plosser mixer equipped with a spray nozzle, the jacket temperature of which was 90° C. While stirring the cellulose ether, hot water at 90° C. was sprayed from the spray nozzle, and stirring was continued for 5 minutes to adjust the water content of the first hydrous cellulose ether to 61.8% by mass. Subsequently, the first hydrous cellulose ether was charged into a Plosser type mixer having a jacket temperature of 10°C, and stirred for 10 minutes to cool the product temperature of the first hydrous cellulose ether to 15°C. Subsequently, the cooled first hydrous cellulose ether was charged into a Plosser mixer with a jacket temperature of 130°C, and the first hydrous cellulose ether was heated by blowing steam at 130°C into it. The temperature of the first hydrous cellulose ether after heating was 95°C. Next, the heated first hydrous cellulose ether was deliquored using a screw press (SHX-200, manufactured by Fukoku Kogyo Co., Ltd.) under conditions of a screw shaft inlet temperature of 130°C (steam supplied at a vapor pressure of 0.3 MPaG), a compression ratio of 1.47, a straight length of 150 mm, a back pressure of 0.3 MPaG, a back pressure plate opening of 20 mm, and a screw rotation speed of 1 rpm, to obtain a second hydrous cellulose ether having a moisture content of 24.0% by mass. The second hydrous cellulose ether was dried, pulverized, and sieved in the same manner as in Example 1 to obtain a cellulose ether. The results are shown in Table 1.

[0048] Example 4 A cellulose ether was obtained in the same manner as in Example 3, except that chip-like pulp having an intrinsic viscosity of 1200 ml / g was used. The results are shown in Table 1.

[0049] Example 5 100 parts by mass of powdered pulp with an intrinsic viscosity of 800 ml / g was placed in a pressure vessel (reactor) equipped with a Plossia type internal stirring blade. The pressure was reduced to -95 kPa and then returned to atmospheric pressure with nitrogen gas twice, after which the pressure was reduced to -95 kPa. Next, 143 parts by mass of a 49% by mass aqueous sodium hydroxide solution was added to obtain alkali cellulose. The mass ratio of the alkali metal hydroxide to the solid components in the pulp (sodium hydroxide / solid components in the pulp) in the obtained alkali cellulose was 0.70. Next, 40 parts by mass of dimethyl ether, 124 parts by mass of methyl chloride, and 30 parts by mass of propylene oxide were added to the reactor using a pressure pump, and the reaction was carried out for a total of 2 hours while the temperature was raised from 60°C to 100°C, yielding a reaction product. Hot water at 95°C was added to the obtained reaction product to form a slurry. The concentration of cellulose ether in the slurry was 10% by mass. The slurry was supplied to a rotary pressure filter, washed, and dewatered to obtain a first hydrous cellulose ether. The water content of the first hydrous cellulose ether was 44.0% by mass. Subsequently, the first hydrous cellulose ether was charged into a Plosser mixer equipped with a spray nozzle, the jacket temperature of which was 90° C. While stirring the cellulose ether, hot water at 90° C. was sprayed from the spray nozzle, and stirring was continued for 5 minutes to adjust the water content of the first hydrous cellulose ether to 55.0% by mass. Subsequently, the first hydrous cellulose ether was charged into a Plosser type mixer having a jacket temperature of 30°C, and stirred for 10 minutes to cool the product temperature of the first hydrous cellulose ether to 34°C. Subsequently, the cooled first hydrous cellulose ether was charged into a Plosser mixer with a jacket temperature of 130°C, and the first hydrous cellulose ether was heated by blowing steam at 130°C into it. The temperature of the first hydrous cellulose ether after heating was 95°C. Next, the heated first hydrous cellulose ether was deliquored using a screw press (SHX-200, manufactured by Fukoku Kogyo Co., Ltd.) under conditions of a screw shaft inlet temperature of 130°C (steam supplied at a vapor pressure of 0.3 MPaG), a compression ratio of 1.47, a straight length of 150 mm, a back pressure of 0.3 MPaG, a back pressure plate opening of 20 mm, and a screw rotation speed of 1 rpm, to obtain a second hydrous cellulose ether with a moisture content of 24.0% by mass. The second hydrous cellulose ether was dried, pulverized, and sieved in the same manner as in Example 1 to obtain a cellulose ether. The results are shown in Table 1.

[0050] Example 6 100 parts by mass of powdered pulp with an intrinsic viscosity of 800 ml / g was placed in a pressure vessel (reactor) equipped with a Plossia type internal stirring blade. The pressure was reduced to -95 kPa and then returned to atmospheric pressure with nitrogen gas twice, after which the pressure was reduced to -95 kPa. Next, 187 parts by mass of a 49% by mass aqueous sodium hydroxide solution was added to obtain alkali cellulose. The mass ratio of the alkali metal hydroxide to the solid components in the pulp (sodium hydroxide / solid components in the pulp) in the obtained alkali cellulose was 1.01. Next, 35 parts by mass of dimethyl ether and 172 parts by mass of methyl chloride were added to the reactor using a pressure pump, and the reaction was carried out for a total of 2 hours while the temperature was raised from 60°C to 100°C, yielding a reaction product. Hot water at 95°C was added to the obtained reaction product to form a slurry. The concentration of cellulose ether in the slurry was 10% by mass. The slurry was supplied to a rotary pressure filter, washed, and dewatered to obtain a first hydrous cellulose ether. The water content of the first hydrous cellulose ether was 35.0% by mass. Subsequently, the first hydrous cellulose ether was charged into a Plossar mixer equipped with a spray nozzle, the jacket temperature of which was 90° C. While stirring the cellulose ether, hot water at 90° C. was sprayed from the spray nozzle, and stirring was continued for 5 minutes to adjust the water content of the first hydrous cellulose ether to 59.5% by mass. Subsequently, the first hydrous cellulose ether was charged into a Plosser type mixer having a jacket temperature of 10°C, and stirred for 10 minutes to cool the product temperature of the hydrous cellulose ether to 15°C. Subsequently, the cooled first hydrous cellulose ether was charged into a Plosser mixer with a jacket temperature of 130°C, and the first hydrous cellulose ether was heated by blowing steam at 130°C into it. The temperature of the first hydrous cellulose ether after heating was 95°C. Next, the heated first hydrous cellulose ether was deliquored using a screw press (SHX-200, manufactured by Fukoku Kogyo Co., Ltd.) under conditions of a screw shaft inlet temperature of 130°C (steam was supplied at a vapor pressure of 0.3 MPaG), a compression ratio of 1.47, a straight length of 150 mm, a back pressure of 0.3 MPaG, a back pressure plate opening of 20 mm, and a screw rotation speed of 1 rpm, to obtain a second hydrous cellulose ether having a moisture content of 20.0% by mass. The second hydrous cellulose ether was dried, pulverized, and sieved in the same manner as in Example 1 to obtain a cellulose ether. The results are shown in Table 1.

[0051] Comparative Example 1 The procedure up to the cooling step was the same as in Example 1, to obtain a first hydrous cellulose ether at 15°C and with a water content of 61.8% by mass. The first hydrous cellulose ether was then pre-dried in a flash dryer together with hot air at 135°C to obtain a second hydrous cellulose ether. The water content of the second hydrous cellulose ether was 40% by mass. The second hydrous cellulose ether was dried, pulverized, and sieved in the same manner as in Example 1 to obtain a cellulose ether.

[0052] Comparative Example 2 The procedure up to the cooling step was the same as in Example 1, to obtain a first hydrous cellulose ether at 15°C and with a water content of 61.8% by mass. Subsequently, the first hydrous cellulose ether was charged into a Plosser type mixer with a jacket temperature of 50°C and stirred. The temperature of the first hydrous cellulose ether after stirring was 30°C. Next, the first hydrous cellulose ether was deliquored using a screw press (SHX-200, manufactured by Fukoku Kogyo Co., Ltd.) under the conditions of a screw shaft inlet temperature of 130°C (steam was supplied at a vapor pressure of 0.3 MPaG), a compression ratio of 1.47, a straight length of 150 mm, a back pressure of 0.3 MPaG, a back pressure plate opening of 20 mm, and a screw rotation speed of 1 rpm. However, significant adhesion of the hydrous cellulose ether to the screw was observed. Furthermore, the adhesion grew and clogged the screw, making continuous deliquoring difficult.

[0053] Comparative Example 3 A first hydrous cellulose ether was obtained in the same manner as in Example 1 up to the step of adjusting the water content to 61.8% by mass. The temperature of the first hydrous cellulose ether was 80°C. Without performing a cooling step, the first hydrous cellulose ether was charged into a Plosser mixer with a jacket temperature of 130°C, and heated by blowing steam at 130°C into the first hydrous cellulose ether. The temperature of the first hydrous cellulose ether after heating was 95°C. Next, the first hydrous cellulose ether was deliquored using a screw press, dried, pulverized and sieved in the same manner as in Example 1 to obtain a cellulose ether.

[0054] Comparative Example 4 The first hydrous cellulose ether was obtained in the same manner as in Example 1 up to the heating step. The temperature of the first hydrous cellulose ether at the end of the heating step was 95°C. The first hydrous cellulose ether was deliquored using a rotary pressure filter to obtain a second hydrous cellulose ether. The water content of the second hydrous cellulose ether was 50.0% by mass. The second hydrous cellulose ether was dried, pulverized, and sieved in the same manner as in Example 1 to obtain a cellulose ether. The results are shown in Table 1.

[0055] Comparative Example 5 Wood-derived pulp chips with an intrinsic viscosity of 550 ml / g were immersed in a 54% by mass aqueous solution of sodium hydroxide. The mass ratio of the 54% by mass aqueous solution of sodium hydroxide to the solid components in the pulp at the time of immersion was 200. Next, excess 54% by mass aqueous solution of sodium hydroxide was removed to obtain alkali cellulose. The mass ratio of the alkali metal hydroxide in the obtained alkali cellulose to the solid components in the pulp (sodium hydroxide / solid components in the pulp) was 1.21. The alkali cellulose (100 parts by mass of solid components) was placed in a pressure vessel (reactor) equipped with a Plosser-type internal stirring blade. Hot water at 95°C was passed through the reactor jacket. Subsequently, air was passed through the reactor for 30 minutes while stirring. The product temperature of the alkali cellulose at the start of the aeration was 50°C, and at the end of the aeration, it was 85°C. After reducing the pressure to -97 kPaG, nitrogen gas was introduced and the pressure was returned to atmospheric pressure. The pressure was then reduced again to -97 kPaG. Next, 40 parts by mass of dimethyl ether, 197 parts by mass of methyl chloride, and 41 parts by mass of propylene oxide were added to the reactor using a pressure pump, and the reaction was carried out for a total of 2 hours while increasing the temperature from 60°C to 100°C, thereby obtaining a reaction product. In the same manner as in Example 1, the reaction product was slurried, subjected to first washing and deliquation, adjusted for water content, cooled, and heated to obtain a first hydrous cellulose ether at 95°C. Next, the heated first hydrous cellulose ether was deliquored using a screw press (SHX-200, manufactured by Fukoku Kogyo Co., Ltd.) under the following conditions: screw shaft inlet temperature 130°C (steam supplied at a vapor pressure of 0.3 MPaG), compression ratio 1.47, straight length 150 mm, back pressure 0.3 MPaG, back pressure plate opening 20 mm, and screw rotation speed 1 rpm. The second hydrous cellulose ether discharged from the screw press was in a form having fluidity in a molten state. Furthermore, the second hydrous cellulose ether having fluidity in a molten state leaked from the slit in the filter cylinder of the screw press, making it impossible to operate the screw press continuously. The leaked second hydrous cellulose ether was dried in a tray dryer set at 80°C, pulverized in a Willey mill (manufactured by Yoshida Seisakusho, 1.0 mm screen), and then the physical properties were measured.

[0056] Comparative Example 6 Wood-derived pulp chips with an intrinsic viscosity of 550 ml / g were immersed in a 49% by mass aqueous solution of sodium hydroxide. The mass ratio of the 49% by mass aqueous solution of sodium hydroxide to the solid components in the pulp at the time of immersion was 200. The mass ratio of the alkali metal hydroxide in the resulting alkali cellulose to the solid components in the pulp (sodium hydroxide / solid components in the pulp) was 0.70. The alkali cellulose (100 parts by mass of solid components) was placed in a pressure vessel (reactor) equipped with a Plosser-type internal stirring blade. Hot water at 95°C was passed through the reactor jacket. Subsequently, air was passed through the reactor for 40 minutes while stirring. The product temperature of the alkali cellulose at the start of the aeration was 50°C, and at the end of the aeration, it was 85°C. After reducing the pressure to -97 kPaG, nitrogen gas was introduced and the pressure was returned to atmospheric pressure. The pressure was then reduced again to -97 kPaG. Next, 40 parts by mass of dimethyl ether, 122 parts by mass of methyl chloride, and 30 parts by mass of propylene oxide were added to the reactor using a pressure pump, and the mixture was reacted for a total of 2 hours while being heated from 60°C to 100°C to obtain a reaction product. In the same manner as in Example 1, the reaction product was slurried, subjected to first washing and deliquation, adjusted for water content, cooled, and heated to obtain a first hydrous cellulose ether at 95°C. Next, the heated first hydrous cellulose ether was deliquored using a screw press (SHX-200, manufactured by Fukoku Kogyo Co., Ltd.) under the following conditions: screw shaft inlet temperature 130°C (steam supplied at a vapor pressure of 0.3 MPaG), compression ratio 1.47, straight length 150 mm, back pressure 0.3 MPaG, back pressure plate opening 20 mm, and screw rotation speed 1 rpm. The second hydrous cellulose ether discharged from the screw press was in a molten state and had fluidity. The second hydrous cellulose ether was dried, pulverized, and sieved in the same manner as in Example 1 to obtain a cellulose ether. The results are shown in Table 1.

[0057] [Table 1]

[0058] As shown in Table 1, Examples 1 to 6, which included predetermined steps, had a small total steam consumption. In addition, the obtained cellulose ether had a low yellowness index and ash content, and a high bulk density. On the other hand, in Comparative Examples 1 and 4, in which a screw press was not used, the yellowness and ash content of the tablets increased, in Comparative Example 2, in which the heating step to 60 to 110°C was not performed, continuous operation became impossible, and in Comparative Example 3, in which the cooling step was not performed, the loose bulk density was low. In addition, in Comparative Examples 5 and 6, depolymerization was carried out during the reaction process, which significantly reduced the 2.0% by mass viscosity at 20°C of the first hydrous cellulose ether to be subjected to the screw press. As a result, the second hydrous cellulose ether immediately after being discharged from the screw press became in a molten state with fluidity, making it impossible to continuously perform the drainage process using the screw press or increasing the yellowness of the tablets. [Explanation of symbols]

[0059] 1. Screw press 2 Filter tube 3 screw blades 4 screw shaft 5 Deliquified raw material input section 6 Temperature control liquid inlet 7. Squeezed liquid discharge section 8 Deliquid product discharge section 9 Air Cylinder 10 Temperature control liquid outlet 11 Back pressure plate 12 Straight section A. First hydrous cellulose ether B. Dehydrated product (second hydrous cellulose ether) C. Pressed liquid D Temperature control liquid to be added E Discharged temperature control liquid

Claims

1. a reaction step of reacting alkali cellulose with an etherifying agent to obtain a reaction product; washing and deliquoring the reaction product to obtain a first hydrous cellulose ether; cooling the first hydrous cellulose ether; heating the cooled first hydrous cellulose ether to 60 to 110°C; deliquoring the heated first hydrous cellulose ether using a screw press to obtain a second hydrous cellulose ether that is solid immediately after being discharged from the screw press; drying and pulverizing the second hydrous cellulose ether; A method for producing cellulose ether comprising at least

2. 2. The method for producing a cellulose ether according to claim 1, wherein the first hydrous cellulose ether to be subjected to the cooling step has a water content of 50 to 90% by mass.

3. 3. The method for producing a cellulose ether according to claim 1 or 2, further comprising a step of adjusting the water content of the first hydrous cellulose obtained by the washing and deliquification to 50 to 90 mass % after the step of washing and deliquifying the reaction product to obtain a first hydrous cellulose and before the step of cooling the first hydrous cellulose ether.

4. 3. The method for producing a cellulose ether according to claim 1, wherein the second hydrous cellulose ether has a water content of 10 to 35% by mass.

5. 3. The method for producing a cellulose ether according to claim 1, wherein the first hydrous cellulose ether is cooled to 0 to 40° C. in the step of cooling the first hydrous cellulose ether.

6. 3. The method for producing a cellulose ether according to claim 1, wherein the viscosity of a 2.0% by mass aqueous solution of the obtained cellulose ether at 20° C. is 100 to 100,000 mPa·s.

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