Patent Description:
As it is known from the literature and practice, microcrystalline cellulose (MCC) is partially depolymerized cellulose with low degree of polymerization (DP). When it is free from organic and inorganic pollutants, microcrystalline cellulose has the structure of a powder with white color, without taste and smell, insoluble in water, diluted acids and in most of the organic solvents.

It's obtaining from plant fibers, the lack of toxicity, hygroscopic capacity and physiological inertness ensure not only the harmlessness, but rather the beneficial influence on the human body as excipient for healthy foods and source of dietary fibers. As nonabsorbing and unassimilated bulking fiber it supports the foods caloricity reduction, without influencing the organoleptic properties, and it is a proven fact that it brings to normal the chemical reactions in the organism, and even decreases the risk of appearance of new neoplasms.

Due to the above described as well as other properties resulting from its chemical nature, the microcrystalline cellulose is a multifunctional product with a huge widely spread and diverse appliance.

In the food industry, for example, MCC is used for improving the fluid products consistency, increasing the terms for conservation of the bulky foods and as anti-caking agent, for caloricity decreasing, increasing the expiry dates and softness of the pastry products and confectionery. It is used also as a filler for preservation and increasing the useful mass of the end products, as a texturizer, emulsifier and improver during heat treatment in meat processing, as a component for cream, cheese, ketchup, mayonnaise, mashed soups, pates, sauces, sweet fruit matters, etc..

Due to the possibility for achieving a high solidity at low pressure levels during pressing, the low friability and physical inertness, microcrystalline cellulose is used in the pharmaceutical industry for example as binding agent and filler for production of tablets and self-adhesive dental products, as well as disintegrator for powdered medical products. It is also used in biotechnologies, for example as component for the food means for in vitro cultivation.

In cosmetics, the microcrystalline cellulose is used for example for the production of creams, hair dyes and masques, tooth pastes, and powders.

In agriculture, the microcrystalline cellulose is mainly used as an additive in foods for birds and domestic animals.

Microcrystalline cellulose finds a wide appliance in the chemical industry, for example in the process of ceramics, composite materials, resins, polyurethane sorbents, thermo-stable and bituminous coverage, plastics, fire-resistant materials, emulsions, faux leather, biological protection means, welding electrodes, thickeners, papers, non-woven materials, and etc..

There are different methods for production of microcrystalline cellulose described in the literature through combination of acid hydrolysis and different other processes.

Patent <CIT> describes the obtaining of pulp with constant DP (degree of polymerization) through acid hydrolysis and mechanical treatment. The pulp is hydrolyzed during boiling in <NUM>,<NUM> normal hydrochloric acid (HC1), at a temperature of about <NUM>, for approximately <NUM> minutes up to <NUM> hour, followed by a mechanical fragmentation of the pulp hydrolysate in water medium. Disadvantage of this method is the high consumption of acid and the necessity of additional energy-intensive mechanical treatment.

From BG19462, an analogue with Patent <CIT> is known a method for production of microcrystalline cellulose with raw materials and assistance of the mill of the present applicant and with the participation of one of the present inventors, in which the hydrolysis of the pulp is implemented with dissolved sulphuric acid at temperature from <NUM> to <NUM>. Simultaneously with the hydrolysis is implemented also a chemical disaggregation of the product, which after that is a subject of additional bleaching, cleaning, washing and drying.

Patent <CIT> reveals a process for microcrystalline cellulose production through adding of acid solution to the pulp and feeding of the thus obtained reaction mixture through an extruder, at which the pulp undergoes an acid hydrolysis and generates microcrystalline cellulose. The temperature of the extruder's drum during the hydrolysis is from <NUM> to <NUM>. The ratio of condensation of the extruder' screw is between <NUM>:<NUM> and <NUM>:<NUM>, preferably about <NUM>:<NUM>. The disadvantage of the method is that the extruders are expensive, and the costs for the maintenance are quite high and require a high mechanical energy, approximately <NUM> kWh or more per ton dried pulp.

In international patent application <CIT> is described the method which allows bleached microcrystalline cellulose to be obtained, either from pulp for chemical processing, or from paper grade pulp which includes neutralization or alkalization of suspension of acid microcrystalline reaction mixture, usually obtained through acid hydrolysis or through cathode-ray irradiation, followed by adding of oxidizer to this reaction mixture.

In Patent <CIT> is described the method for microcrystalline cellulose production, which includes hydrolysis of fibrous pulp material with an acid at increased temperature or acidification of fibrous pulp material, followed by washing and hydrolysis of the washed pulp material at increased temperature, in order a mixture from microcrystalline cellulose and hydrolysate to be obtained, followed by separation of microcrystalline cellulose from the hydrolysis, during which the mixture or the separated hydrolysate or the microcrystalline cellulose could be a subject to neutralization. A scheme for flows organization is also presented, so that part of the used chemicals during the acidification, the acid hydrolysis and/or the neutralization to be recovered.

On the grounds of the above mentioned it could be summarized that most of the known methods for microcrystalline cellulose production through acid hydrolysis of the pulp are not suitable for treatment of pulp with high hemicelluloses content, requires usage of energy-intensive additional mechanical treatments and/or chemicals for neutralization and additional bleaching, due to which exists a necessity of developing more efficient and simple production processes.

A problem of the current invention is a simple method for production of microcrystalline cellulose to be developed, through acid hydrolysis of paper grade pulp, so that as an end-product microcrystalline cellulose to be produced, which is not required to be a subject to additional neutralization and bleaching.

The invention problem is solved through method for production of bleached microcrystalline cellulose from paper grade pulp, which includes two separate stages of acid hydrolysis with intermediary washing of the pulp with water and separation of the dissolved sugars.

The method in accordance to the invention includes the following operations:.

The nature of the method, in accordance to the invention consists in that as the first stage of treatment of the input material, i.e. pulp (as bone dry) with pentosan content of <NUM> to <NUM> weight % against its total weight and with consistency from <NUM>% to <NUM>%, compared to the total weight of the reaction mixture, an acid hydrolysis is carried out, in which the acid is added in amount from <NUM> to <NUM> weight percent, on the basis of the bone dried pulp weight. Hydrolysis is carried out at temperatures from <NUM> to <NUM>, preferably at temperatures from <NUM> to <NUM>, and even more advisable from <NUM> to <NUM>. The reaction time in the first stage of acid hydrolysis is from <NUM> to <NUM> minutes, preferably from <NUM> to <NUM> minutes, and even more advisable from <NUM> to <NUM> minutes.

Between the two stages is performed washing of the pulp with water, but neutralization is not required.

In the second stage of the method in accordance to the invention an acid hydrolysis is carried out with the washed after the first stage pulp with concentration of <NUM>% to <NUM>% (as bone dry), in accordance to the total weight of the reaction mixture. The acid is added in amount of <NUM> to <NUM> weight percent, on the basis of the weight of the bone dry pulp. Hydrolysis is carried out at temperatures from <NUM> to <NUM>, preferably at temperatures from <NUM> to <NUM>, and even more advisable from <NUM> to <NUM>. Reaction time in the second stage of hydrolysis is from <NUM> minutes to <NUM> minutes, preferably from <NUM> minutes to <NUM> minutes, and even more advisable from <NUM> to <NUM> minutes.

After the second stage of acid hydrolysis a final washing of the obtained microcrystalline cellulose with desalinated water is implemented. Part of the washing waters from second stage could be used during the intermediary washing.

The two stages of acid hydrolysis could be carried out in one and the same reactor or separately as batch or continuous process.

Unexpectedly for the person skilled in the Art it has been found that in the first step of acid hydrolysis the pulp brightness is further increased with <NUM> - <NUM>%. The second stage of acid hydrolysis has also a purifying effect, during which from the microcrystalline cellulose are removed the adsorbed in the first stage chromophoric structures, which leads to production of microcrystalline cellulose with brightness about and above <NUM>%, due to which is not required implementation of additional bleaching of the end product.

In accordance to the invention as an input fibrous material is used pulp, which is bleached, semi-bleached or unbleached sulphate pulp, bleached or unbleached sulphite pulp, natron pulp, pulp obtained after cooking with organic solvents or after processing of waste paper. The used pulp is from softwood, hardwood and non-wood plant raw materials, for example as straw, hemp, kenaf, ramie, bamboo, and other agricultural plant wastes containing cellulose.

The hydrolysis in the first and second stage of acid hydrolysis in the method in accordance to the invention is carried out with diluted organic acids (oxalic, formic, acetic, etc.), solvents of sodium bisulphite and sodium bisulphate and with diluted inorganic acids, preferably mineral acids as hydrochloric acid, sulfuric acid and nitric acid.

In another option of method implementation according to the invention, the first stage of acid hydrolysis is more continuous than the second one but the intermediary brightness of the pulp obtained as a reaction product should be controlled so as not to fall under <NUM>%. After washing and removing of the dissolved sugars, shorter second stage of acid hydrolysis is carried out where the pulp degree of polymerization is decreased and after final washing by water the pulp is purified by the adsorbed chromophore structures.

The advantages of method according to the innovation are:.

The invention is presented by the following examples that clarify it without limiting its scope of protection.

The tests are carried out by bleached hardwood pulp identifying by high brightness and pentosan content in the pulp over <NUM>%.

The acid hydrolysis at first and second stage is carried out in rotating <NUM> liters laboratory autoclaves.

The autoclaves are charged with preliminary disintegrated pulp containing <NUM> bone dry pulp with definite consistence and to which required amount of diluted acid is added.

The autoclaves are placed and rotated in a vat with polyethylene-glycol that was previously heated to the required temperature. The time for autoclaves tempering is <NUM> minutes.

When the hydrolysis is completed the autoclaves are removed and cooled down by cold water.

The obtained microcrystalline cellulose is washed at buchner funnel by vacuum pump through filter with pores size <NUM>-<NUM>. The first filtrate is separated for sugars and furfural analyze at chromatographic system HPLC Dionex according to NREL Technical Report (NREL/TP-<NUM>-<NUM>). Each sample is washed twice by deeply deionized water.

The particles size and their distribution by size is identified by laser analyzer Fritsch Analysette <NUM> Micro Tec Plus. The average degree of polymerization DP is defined according to European Pharmacopoeia standard and the brightness according to ISO <NUM>.

Table <NUM> and Table <NUM> show the results from two-stage acid hydrolysis with diluted sulfur acid with pulp consistency <NUM>% at first stage and <NUM>% at second stage. The temperature is <NUM>, reaction time at maximum temperature <NUM> and <NUM> and acid charge (in regard to bone dry pulp) is <NUM>% for both stages. The pulp is washed after the first acid stage with desalted water and the second stage is carried out with duration of <NUM> minutes.

In the initial period of first acid stage it is observed pulp cleaning effect and the brightness increasing by nearly <NUM> %. This is due to the removal by hydrolysis of xylanrelated residual chromophores. During the one-stage hydrolysis the high pentosan content in hardwood paper grade pulp generates a lot of xylose in the next step of the process. The formation of caramelized sugars take place and microcrystalline cellulose brightness decreases significantly to the end of the hydrolysis process (samples <NUM> and <NUM> from Table <NUM>).

Two-stage acid hydrolysis is a decision of that problem, due to removing by washing of the dissolved sugars and by-products. Thus the possibility for caramelization and formation of new chromophore structures, at relatively mild conditions in second stage of hydrolysis, is minimized and the microcrystalline cellulose brightness remains very high until the end of the second stage (Table <NUM>).

Table <NUM> shows the results from two stage acid hydrolysis at short first stage with duration of <NUM> minutes and prolonged second stage of <NUM> and <NUM> minutes. In both stages of acid hydrolysis the acid charge in regard to bone dry pulp is <NUM>%, the pulp consistence is <NUM>% and the reaction temperature is <NUM>. The pulp after first stage of acid hydrolysis is washed by desalted water.

The results presented in Table <NUM> confirm that by extending the duration of the second stage of acid hydrolysis leads to obtaining of microcrystalline cellulose with very high brightness and decreased degree of polymerization. It is observed better particle uniformity of microcrystalline cellulose obtaining by two-stage hydrolysis compared to the one-stage process (samples <NUM> and <NUM> from Table <NUM>).

Table <NUM> shows results from two-stage acid hydrolysis at 120minutes duration of first stage acid hydrolysis and shorter second stage of acid hydrolysis from 60minutes. In both stages of acid hydrolysis sulfur acid charge is <NUM>% and the temperature <NUM> however the pulp consistency in the second stage is <NUM>% and <NUM>%. After the first stage the pulp is washed by deionized water.

Claim 1:
Method for microcrystalline cellulose production including first stage of acid hydrolysis of pulp with pentosan content of <NUM> to <NUM> weight % against its total weight and pulp consistency from <NUM>% to <NUM>% against the total weight of the reaction mixture with diluted acid in amount of <NUM> to <NUM> weight percents on the base of bone dry pulp weight where the hydrolysis is carried out at temperatures within the range from <NUM> to <NUM> for reaction time from <NUM> to <NUM> minutes, characterized with the fact that the obtained pulp after first acid hydrolysis is washed by water, after that the washed pulp, with <NUM>% to <NUM>% pulp consistency against the total weight of the reaction mixture, is subject to second stage of acid hydrolysis with diluted acid in amount of <NUM> to <NUM> weight percents on the base of bone dry pulp weight where the hydrolysis is carried out at temperatures from <NUM> to <NUM> for reaction time from <NUM> to 180minutes, followed by water washing of the final reaction product.