Processing, Application Methods and Techniques for Tea Seed Shells

Oil tea is a woody oil crop unique to China. According to statistics, China currently has more than 3.47 million hectares of oil tea area, with an annual output of 56.6 billion kg of oil tea seeds. Tea seed shells contain 52.15% lignin, 30.27% pentosan, 5.43% saponin, 2.47% tannin, 0.43% water, 0.13% oil, and 9.12% other components. Because tea seed shells are high in lignin and pentosan, research on their utilization is currently focused on the preparation of furfural, activated carbon and xylitol.

1. Preparation of Furfural from Tea Seed Shells

(1) Properties and uses of furfural. Furfural is a widely used chemical, a light yellow to amber transparent liquid whose color gradually darkens during storage until it turns brown, with a bitter almond odor. Furfural is an organic chemical raw material, mainly used for refining lubricating oil and producing furfuryl alcohol. It can also be used to produce medicines and veterinary drugs, as well as preservatives, disinfectants, insecticides and rust removers, and is also used in food flavorings, dyes and other industries.

(2) Preparation of furfural. Furfural is a widely used chemical. When extracted from tea seed shells, its theoretical yield is 47%, while the actual yield is 19%, the highest yield among all crops. Its production process is: acid hydrolysis -> neutralization -> evaporation -> crude distillation -> neutralization -> refining -> finished product.

(3) Application of downstream derivative products of furfural. ① Furfuryl alcohol. Furfuryl alcohol is the most important product among the hydrogenation products of furfural, mainly used to prepare furan resins. It can be used as a binder in the casting industry for internal combustion engines such as automobiles and tractors, to improve casting quality and promote mechanization and automation of the casting process. In addition, it can be used as a solvent for varnishes and pigments and as rocket fuel, as well as in the synthesis of the intermediate levulinic acid for the nutritional drug calcium fructose acid. ② Fumaric acid and its dimethyl ester. Furfural can be oxidized with potassium chlorate to produce fumaric acid, which is then esterified with methanol to obtain dimethyl fumarate. Fumaric acid is mainly used to produce unsaturated polyester resins, which have good chemical corrosion resistance and heat resistance. Dimethyl fumarate is a new type of high-efficiency, low-toxicity food and feed preservative and anti-mold fresh-keeping agent developed abroad in the 1980s with a wide applicable pH range. Its effect is superior to preservatives such as benzoic acid, sorbic acid, dehydroacetic acid, propionic acid and their salts, and it can be widely used for the preservation and fresh-keeping of food, feed, cosmetics, fruits, vegetables and other products. ③ Furoic acid. Furoic acid can be prepared by air oxidation of furfural, mainly used to synthesize tetrahydrofuran, furamide and furoates and their salts. It is used as a plasticizer in the plastics industry and as a preservative in the food industry, and can also be used as a coating additive and an intermediate for medicines and spices. ④ Furan. Furan can be prepared by decarbonylation of furfural, mainly used in organic synthesis. It is the main raw material for producing drugs such as furapromide, nitrofurazone, nitrofurantoin and furazolidone, and can also be used to prepare chemical products with promising market prospects such as thiophene, pyrrole, tetrahydrothiophene, tetrahydrofuran and tetrahydropyrrole. ⑤ Tetrahydrofurfuryl alcohol. Tetrahydrofurfuryl alcohol, also known as tetrahydrofuran methanol, is prepared by gas-phase hydrogenation of furfural in the presence of nickel, chromium and copper catalysts. It is an important chemical raw material that can be used in the synthesis of succinic acid, glutaric acid, dihydrofuran, lysine, pyran and pyridine. Tetrahydrofurfuryl alcohol is colorless, has good stability to light and heat, and is an excellent plasticizer for polyvinyl chloride. It has a high boiling point and is difficult to volatilize, so it can be used as a solvent for resins, coatings and oils, and also as a decolorizing and deodorizing agent for fatty acids and some medicines. ⑥ 2-Methylfuran. 2-Methylfuran is prepared from furfural under the action of a Cu-Cr-Al catalyst at high temperature and pressure. It is an important organic synthesis intermediate and can be used to prepare pentanediol, acetylpropanol, pentadiene, ketones and methyltetrahydrofuran, and is also an indispensable raw material for producing vitamin B1, chloroquine phosphate and primaquine phosphate. In addition, 2-methylfuran is also an important parent for synthesizing food flavorings such as 2-methyl-5-acylfurans, 2-methyl-3-furanthiol and 2-methyl-5-alkylthiofurans.

2. Preparation of Activated Carbon from Tea Seed Shells

(1) Properties of activated carbon. The surface of activated carbon has many micropores. Good-quality activated carbon can reach a surface area of 25 million sq m per 1 kg, while ordinary activated carbon can reach 10 million sq m. Activated carbon produced from tea seed shells and the residue from their furfural production as raw materials has good overall performance, and all quality indicators can fully reach the level of activated carbon from other fruit shells. It also has high activity yield, and raw material consumption and production cost are close to or better than those of other lignin raw materials. Its appearance is black and shiny, hard in texture, and in the form of irregular particles. Under a microscope, many micropores can be seen. This activated carbon can adsorb tiny particles and bacteria in water, and can sterilize, remove toxins, decolorize and soften water quality, making it a good chemical filtration material.

(2) Preparation of activated carbon. Tea seed shells -> carbonization -> water cooling -> boiling -> filtration -> pH adjustment -> activation -> cooling -> pulverization -> finished product.

(3) Applications of activated carbon. Activated carbon is mainly used in air purification, exhaust gas adsorption at sewage treatment plants, beverage water treatment, power plant water pretreatment, pretreatment before wastewater recovery, biological sewage treatment, toxic wastewater treatment, non-alkaline mercaptan removal in petrochemicals, solvent recovery, chemical catalyst carriers, filter poison canisters, gold extraction, exhaust gas purification in chemical storage, food decolorization, automobile exhaust purification, and gas purification in PTA oxidation units.

(4) Regeneration of activated carbon products. At present, activated carbon has been widely used in environmental protection and in industrial and civil fields, and considerable results have been achieved. However, after activated carbon becomes saturated with adsorption and is replaced, users discard, bury or burn it, causing waste of resources and secondary pollution to the environment. Activated carbon adsorption is a physical process, so high-temperature steam can also be used to desorb impurities from used activated carbon and restore its original activity, thereby achieving reuse with obvious economic benefits. Regenerated activated carbon can still be continuously reused and regenerated.

3. Preparation of Xylitol from Tea Seed Shells

(1) Properties of xylitol. Xylitol, also known as pentitol, tastes sweet and appears as a white crystalline powder. Its sweetness is equivalent to that of sucrose, and its physicochemical indicators such as solubility, solution density and refractive index are basically the same as those of sucrose, making it an ideal substitute for sucrose. Xylitol is a naturally occurring five-carbon sugar alcohol and a common intermediate in carbohydrate metabolism in humans and animals. It is widely present in various fruits and vegetables, but in very small amounts.

(2) Production of xylitol by fermentation. Tea seed shells -> pulverization -> pretreatment -> hydrolysis (acid catalyst) -> hydrolysate -> purification -> xylose solution -> detoxification -> fermentation -> refining -> finished product.

(3) Functions of xylitol. As a sweetener, the year-by-year increase in market consumption of xylitol is closely related to its own characteristics. ① Preventing dental caries. According to a national oral health epidemiological sampling survey, half of the people in China suffer from dental caries. Dental caries has become one of the common and frequently occurring oral diseases. Dental caries is caused by people frequently eating sucrose or foods high in sugar; the sucrose remaining in the oral cavity produces acidic substances under the action of bacteria, thereby causing mineral loss, corroding teeth and forming caries. Regular consumption of xylitol can effectively prevent dental caries, mainly because xylitol cannot be fermented and utilized by bacteria in the oral cavity. ② Lowering blood sugar. After entering the human body, xylitol can pass through cell membranes and be absorbed and utilized without the help of insulin, and it does not increase blood sugar or urine sugar. Within 12 h after consumption of xylitol, 50%-60% of the xylitol is oxidized to carbon dioxide and water, releasing heat that can be used by the body; 2%-10% of the xylitol is excreted in urine and feces; and 20%-30% of the xylitol exists in the body as liver glycogen or other intermediate products and will not increase blood sugar values. ③ Other health functions of xylitol. Xylitol can regulate human fat metabolism and reduce the content of free fatty acids in the blood, thereby reducing the formation of fat tissue. Its low-calorie nature will not increase the activity of human lipase and can reduce fat accumulation in the body. Xylitol can also promote the synthesis of liver glycogen, lower transaminases, improve the utilization rate of amino acids from glucose, and protect and repair the liver. Xylitol can also eliminate the formation of harmful ketone bodies in the blood and can be used as an anti-ketone drug. Xylitol can promote the proliferation of beneficial intestinal flora and regulate gastrointestinal function. As a new factor for improving gastrointestinal function, xylitol has high functional activity and is a promising functional additive factor. Because of its excellent properties, xylitol can be widely used in foods, alcoholic beverages, drinks and other products. In 1973, xylitol was officially listed as a Grade A food additive by the International Food Additives Conference.

In summary, the whole tea seed shell is a treasure. Both its economic value and its utilization prospects are very attractive. However, oil tea seed processing in China generally does not include shelling treatment, but instead directly presses oil with the shells. Because the shells contain more pigments and a small amount of wax, these components dissolve in tea seed oil during oil production, which not only affects the quality of tea seed oil and tea seed meal, but also causes great waste of resources. The fundamental reason for this phenomenon is that people have not paid attention to the usable value of tea seed shells. In fact, as long as a shelling step is added before pressing oil tea seeds, tea seed shells can be obtained. Therefore, the raw material is easy to obtain, the processing technology is simple, the investment is low, and the results are quick. It is a typical short, flat and fast project, especially suitable for production by small and medium-sized enterprises in oil tea producing areas, and is a shortcut for tea farmers in mountainous areas to escape poverty and become rich as quickly as possible. Therefore, as long as we fully recognize the utilization value of tea seed shells and attach importance to scientific research on their development and utilization, we can bring into play the positive role of oil tea seed by-product resources in increasing enterprise benefits and farmers' incomes.