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Etherification technology and innovation of natural cellulose ether

Views: 6     Author: Site Editor     Publish Time: 2022-03-07      Origin: Site

Etherification technology and innovation of natural cellulose ether

Cellulose ether and its further derivatives are extremely important chemical raw materials and auxiliaries after refining, alkalization, epoxy compounds, halogenated alkane etherification. With the development of social economy, people have a deeper understanding of its performance, the demand in various fields is increasing, and the variety and quality are also put forward higher and higher requirements.

Talk about improving our country from the following several Angle of natural cellulose ether and its derivatives manufacturing level further, some opinions and Suggestions to improve market competitiveness, further understanding of cellulose, strengthen basic research, reform and improvement of production technology and equipment, increase product variety and gear, improve product testing and analysis of the application fields of the intermediate links.

1. Further understanding of cellulose

The use of a substance depends on the properties of the material of which it is composed, which in turn is determined by its structure. Therefore, structure research has always been the hot spot, difficult point and focus of cellulose scientific research. Understanding the molecular and supramolecular structure of cellulose is the basis of modification, synthesis and application of cellulose. Now there is no debate that the base ring of cellulose macromolecule is dehydrated glucose, whose molecular formula is :(C6H10O5)n. Due to different sources, the number of glucose residues in cellulose molecule, that is, the degree of polymerization varies from 100 to 14000. It contains 44.44% carbon, 6.17% hydrogen and 49.39% oxygen.

The natural cellulose molecule is a long chain giant molecule composed of n D-pyranoid glucose residues connected by β -glycosidic bond at position 1 and 4, and each glucose residue has three alcohol hydroxyl groups. From the perspective of molecular structure, natural cellulose should be easily dissolved and chemically modified and derived by -oh group. But in fact, cellulose is not only insoluble in water, and even insoluble in NaOH solution, only soluble in special solutions such as copper ammonia in the form of complexation. Chemical derivatization and modification reactions are mostly carried out in multiphase systems, and the fundamental reason lies in the structure of cellulose. Cellulose macromolecules can form high density intermolecular and intramolecular hydrogen bonds, coupled with the β-1, 4-linked glucose residues in cellulose molecules in the chain is up and down interactive arrangement, so that the long chain becomes "hard and straight", the formation of "double tile" structure; In addition, in oligosaccharide glycosides, although β -glycoside is easier to hydrolyze than α -glycoside, for polymer materials, the hydrolysis rate of the polymer with β -glycoside bond in acid is only 1/3 of the α -glycoside bond. Moreover, cellulose macromolecular chains are easily arranged in parallel with each other, and the existence of intra-chain and inter-chain hydrogen bonds makes them doubly stable. Because of this toughness, cellulose serves as the basic skeleton in plant tissues, both withstanding high osmotic pressure and supporting the organism. But also because of this, the natural cellulose ether industry has faced many difficulties for many years, there is a widespread problem of low degree of product replacement, uneven replacement, seriously affecting product quality and performance.

Cellulose ether production of the main raw material is cellulose, the good raw material is the key to produce high quality products. Foreign cellulose ether large factories have their own plant cellulose planting, picking and refining processing base and enterprises, strictly regulate the source of raw materials, which is the first step to control the quality of cellulose ether. Usually, for cellulose raw materials, the concern is its viscosity (degree of polymerization), ash, moisture, etc. In fact, from a chemical point of view, the content of sulfuric acid insoluble substance, iron content and maturity of cellulose raw materials will also have a direct impact on the product. In the same process, the type and source of cellulose have direct influence on the product. Figure 2 is the sem photo of Huian cotton cellulose, Russian broadleaf wood (hardwood) and Sun brand coniferous wood cellulose (cork). See not hard, surface of natural cork fibrin is irregular and porous, fibrin bundle is irregular arrangement; The natural hardwood fibrils have very small pores and pits on the surface. The fibrils are arranged in regular order, almost parallel to the fiber axis. Cotton wool fiber has net structure, smooth surface, no pit, high fibrin density. Fibrin voidage of cork paddle, hardwood paddle, cotton down in turn. The reason is that cotton fiber is almost pure cellulose, and trees in addition to cellulose, there are 40% lignin, pentocant and resin impurities, the cork paddle after the extraction of impurities has a higher void.

The difference of voidage will directly affect the degree of basification and etherification rate of raw material. But from the perspective of molecular structure, no matter which kind of cellulose as cellulose ether raw material, cellulose macromolecular chains are easy to build side by side, intra-molecular and inter-chain hydrogen bonds will exist in high density, and are alkalized and etherized in heterogeneous conditions.

Second, raw material pretreatment technology and new solvents in people's attention

In order to get higher quality products and reduce costs, people have taken many measures and methods. Including cellulose pretreatment technology is a hot research area, involves pretreatment methods include physical, chemical and comprehensive method, the purpose is to change the state of aggregation structure of cellulose, such as the degree of crystallinity decreased, the crystallite size decreases, and pores increases, polymerization degree decline, intramolecular hydrogen bond rupture between molecules, improve the chemical reagent and enzyme reagent accessibility and reactivity of cellulose. It includes biological technology, pre-swelling treatment of alkali metal hydroxide aqueous solution, liquid ammonia treatment, copper ammonia solution and other chemical reagents treatment.

At present, all kinds of pretreatment reagents are used cross, to the direction of less pollution, less consumption. There are mechanical methods such as grinding and fine breaking, high-energy electron radiation treatment, CO2 supercritical flash Explosion treatment, Steam flash Explosion technology.

The existing cellulosic ether industrial cellulose crushing process, in fact, also belongs to a pretreatment method, that is, through mechanical energy, so that the intramolecular and intermolecular hydrogen bonds of cellulose weaken, cellulose bundle dispersion, so that the morphology and microstructure of cellulose changes, resulting in decreased crystallinity and increased accessibility, Tossinari, Koshijimat, Hon and others have made quantitative studies in this respect. Taking fine cutting as an example, when the cotton fiber is finely broken from 6mm to 0.5mm, the accessibility (expressed by iodine adsorption value) increases from 10.06% to 17.3%. With the extension of fine breaking time, the accessibility also increases, and the accessibility increases from 10.06% to 18.16% after 6 times of cutting.

But as a mechanical grinding process to improve the efficiency of etherification, most domestic enterprises suffer from the poor performance of the grinder system, compared with foreign grinding machinery, rapid heating, low life, frequent knife change, greatly affects the grinding efficiency and production efficiency, but also raise the cost of the product. Pallmann Maschinenfabrik GmbH & Co.KG produces cellulose material crusher with high overall performance. For the humidity of 6%~10% cellulose grinding to 0.35mm, its production capacity of 400~500kg, double drive infinitely variable gear transmission, speed of 0~118rpm, speed in the range of 0~29m/min adjustable. In addition, as users put forward the performance requirements of the grinding knife, more and better high-quality wear-resistant, high-strength domestic mill steel will soon be widely used in the new mill system.

Japanese scholar Kamide et al. activated pulp under a specific condition by Steam Explosion to destroy the internal hydrogen bonds of cellulose molecules and form alkali-soluble cellulose, which could be completely dissolved in a certain concentration (8%-10%) of NaOH aqueous solution at 4℃. Professor Tan Huimin and Shao Ziqiang et al. used SE technology (see Figure 3) to treat cellulose, and found that under the same reaction conditions, carboxymethyl cellulose with high substitution degree and excellent solubility was obtained by using the flash-burst cellulose as raw material. At the same time, the influence of high-pressure steam flash on the polymerization degree of cellulose was quantitatively studied.

The second basic research focus and idea is to develop a new solvent that can dissolve cellulose to achieve homogeneous etherification of cellulose. Current concerns include NaOH aqueous solution, NMMO (N-methyl-morpholine n-oxide), DMAO (N,N '-dimethylethanolamine-N-oxide), DMCAO (N,N' -dimethylcyclohexamine-N-oxide), AND DMF/N2O4 (dimethylformamide/nitrogen tetroxide). DMSO/ (CH2O) X (dimethyl sulfoxide/paraformaldehyde), NH3/NH4SCN/H2O (ammonium thiocyanate/liquid ammonia), DMAc/LiCl (dimethyl acetamide/lithium chloride), etc.

In the patent, Zhang Lina et al. introduced the use of 6wt%NaOH/4% urea aqueous solution system to dissolve cellulose with a certain molecular weight through a specific process, and successfully prepare water-soluble hydroxyethyl cellulose by hydroxyethylation of chloromethane and cellulose in homogeneous system (MS=0.5~1.6). The results show that the molecular weight of cellulose is 5.4´104~8.7´104, and there is no obvious degradation in the reaction process. This process has low cost, no pollution, high purity and good uniformity.

LiCl/DMAc system is an excellent non-aqueous solvent for dissolving cellulose, which was first reported by McCormick and Turban et al. Cellulose in the LiCl/DMAc solvent system can be prepared into a solution with a concentration of 15%-17%, in which LiCl content is 3%-18%. In the process of dissolving cellulose, In order to dissolve the cellulose completely, the solvent exchange treatment with water, methanol and DMAc is often needed to activate the cellulose. The solvent system has little pollution and good dissolving ability to activated cellulose with certain degree of polymerization. The results show that there is no derivatization of cellulose molecules in the solution, which indicates that cellulose is dissolved in molecular form. Cellulose ethers have been successfully synthesized in DMAc/LiCl using triethylamine and pyridine as catalysts. Cellulose 4,4 '-bis (dimethylamino) diphenyl methyl ether (DMF) films synthesized in homogeneous system exhibit photoconductivity behavior. Although these examples have demonstrated that cellulose etherification can take place in DMAc/LiCl. However, in this solvent system, the solubility of strong base becomes poor, so that many typical etherification reactions cannot be carried out. And compared with the traditional heterogeneous cellulose ether production process, it does not show special advantages. HPLC analysis showed that the distribution of substituents of CMC prepared by traditional method was related to the reaction conditions of CMC synthesized in NAOH-DMAC /LiCl suspension system. Compared with the traditional slurry method of cellulose suspended in isopropyl alcohol/water, the distribution of substituents of CMC prepared by DMAc/LiCl was related to the reaction conditions. Both unsubstituted and fully tricarboxylmethylated units are abundant, and preliminary results indicate that the DMAc/LiCl solvent system appears to increase the nonstatistical distribution of monomeric units.

In addition, DMSO/SO2/ diethylamine is a solvent that uses a wider range of homogeneous cellulose etherification systems. Isogai et al. studied the reaction of cellulose DMSO/SO2/ diethylamine solution with NaOH and benzyl chloride. Compared with N2O4/DMF and DMAc/LiCl solvent systems, this system shows unmatched advantages in reaction rate and yield. Among the cellulose derivatives reported, trioxy-aryl methyl ether and trioxy-naphthalyl methyl ether with double bonds have liquid crystal properties. The homogeneous system derivatization of cellulose aqueous solution is widely concerned, because the water system has low pollution cost and is easy to realize industrialization. Th.Heinze et al. used nickel hydroxide, trethylamine aqueous solution and lithium perchlorate melt to dissolve cellulose, and successfully carried out completely homogeneous carboxymethylation derivatization of cellulose in this system. This has great inspiration for cellulose ether industry.

In actual production, industrial cellulose ether production is mostly organic (mixed)/aqueous solvent system for cellulose dispersion, alkalization, etherification in heterogeneous system. For example, alcohol/water system, alcohol/toluene/water system and so on, it is very important to master the best proportion of cellulose ether to produce high quality. The effects of methanol, ethanol, tert-butanol, propanol, acetone, dioxane, DMSO and DMF on the degree of substitution, the length of substituted chain segment and the length of substituted and unsubstituted glucose ring groups of cellulose carboxymethylation have been studied. The results showed that the degree of hydrolysis of tert-butanol, acetone, dioxane and DMSO as solvents was small. In isopropanol/water system, when the content of isopropanol was between 75% and 95%, the degree of substitution of the product increased with the increase of the content of isopropanol. In the production of non-ionic cellulose ether, the amount of solvent and ratio of the product substitution degree and distribution have a direct impact, such as HMPC production, the amount and proportion of alcohol/benzene solvent have a direct impact on the product performance, in-depth study of some problems in the field of cellulose ether production in China has important guiding significance.

3. Production process and equipment need to be improved and reformed

Look from whole, cellulose ether industry in our country, the technology and equipment has a great progress in recent years, a lot of links to get updates and transformation, from cellulose raw material crushing, basification, etherification, abstersion and solvent recovery, drying and confusion, new equipment, new technology, and constantly research and application of the technology.

In terms of process, etherification mainly includes slurry method or liquid slurry method, and gas-solid reaction process is rarely used. The former mostly uses alcohol/water system or alcohol/benzene/water system for solvent recovery, mainly the recovery of alcohol, alcohol and benzene mixed solvent recovery, and then distillation separation of alcohol and water mixed system. At present, the treatment and recovery of side reaction products and low boiling matter should be solved. The purpose is to reduce costs, reduce pollution, improve the safety of the use of recycled solvents, so as to avoid repeated accumulation of continuous production, resulting in the unpredictable improvement of product indicators. The latter is mainly through the liquid phase recovery, gas phase recovery etherifying agent most recovery, a large number of heat exchange makes the product performance can control poor, in addition, the gas-solid reaction contact is not sufficient, low degree of substitution, uniformity is poor, the safety of the system is also poor. The alkalization process can be carried out in stages, either alkalization ® etherization or pre-alkalization ® etherization ® supplementary alkalization, in a variety of ways.

There is a certain gap between the domestic main reactor and foreign ones, and the system control technology level is not high. There are two kinds of vertical and horizontal reaction kettle from the structure. The horizontal reaction kettle is mostly used for gas-solid reaction system. The vertical reaction kettle is more suitable for liquid slurry or slurry cellulose ether production process. In the design of agitator, there is still room for improvement.

Rodger machinery Manufacturing company in Paderborn, Germany, has rich experience in cellulose ether and other chemical industry equipment manufacturing, which is worth our reference. Horizontal plough shovel type mixer can ensure that in a relatively short period of time get a mixing material, shape, special plough shovel type (see figure 4) mixer in a special way is arranged on the horizontal axis, the size of the hybrid components between the size, the quantity, geometry and linear velocity matching the mixture in the reaction tank for three dimensional turbulent flow movement (see figure 5), To prevent dead Angle or low speed movement area inside the mixer, speed up high speed and accurate mixing of materials. When necessary, auxiliary components can be used to support the crushing and shearing of aggregate materials and long fibers, such as individually driven high-speed crushing knives. The horizontal Rodger DVT10000 reactor is shown in Figure 6.

The company has three production of vertical mixing machine installation type impeller stirrer, with its special shape and linear velocity, eddy current ways of material circulation flow mixing, accelerate in the horizontal and vertical direction at the same time, fully mixing of material, also in the necessary cases, driven by a separate high-speed broken knife, materials for reunion