EP4041777A1

EP4041777A1 - Method of bleached microcrystalline cellulose production from unbleached pulp

Info

Publication number: EP4041777A1
Application number: EP20761133.6A
Authority: EP
Inventors: Ivo Vladimirov VALCHEV; Krassimir Banchev DACHEV; Nikolay Asenov YAVOROV
Original assignee: Svilosa Ad
Current assignee: Svilosa Ad; UNIVERSITY OF CHEMICAL TECHNOLOGY AND METALLURGY(UCTM)
Priority date: 2019-07-25
Filing date: 2020-07-14
Publication date: 2022-08-17
Also published as: BG67533B1; BG112971A; WO2021012023A1

Abstract

The invention is related to a method of bleached microcrystalline cellulose production from unbleached pulp by two-stage acid hydrolysis and washing and removal of dissolved products after each stage, both, during the hydrolysis, and during the subsequent bleaching of the microcrystalline cellulose. This method allows using unbleached paper grade pulp with high content of hemicelluloses allow the production of bleached microcrystalline cellulose with low pentosan content and properties suitable for use in for use in the pharmaceutical, cosmetic and food industries.

Description

METHOD OF BLEACHED MICROCRYSTALLINE CELLULOSE PRODUCTION FROM UNBLEACHED PULP

FIELD OF THE INVENTION

This invention refers to a method for production of microcrystalline cellulose from unbleached pulp by two-stage acid hydrolysis with intermediate washing and removal of the dissolved pentose sugars, and the subsequent bleaching of the microcrystalline cellulose obtained. This method allows from paper grade pulp, which contains considerable amount of hemicelluloses, to obtain bleached microcrystalline cellulose with low pentosan content and suitable parameters for use in the pharmaceutical, cosmetic and food industries.

BACKGROUND OF THE INVENTION

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.

Thanks to the above-specified and other properties related to its chemical nature, the microcrystalline cellulose is a multifunctional product with extremely broad and varied application in the food industry, pharmaceutical industry and biotechnologies, in the cosmetic industry, the chemical industry, and agriculture.

The references available reveal different methods of obtaining MCC through combination of acid hydrolysis and various other processes, wherein usually a bleached pulp is used for chemical processing, which is characterized by low pentosan content and which is subjected to further bleaching using specific oxidants for achieving of high brightness. US patent US9469695B2 reveals a method for production of microcrystalline cellulose, including acidification and cleaning of any heavy metals from the pulp, followed by washing and hydrolysis of the washed cellulose 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. The suggested method does not take into account the brightness of the microcrystalline cellulose, which is significantly reduced at the high temperature of hydrolysis.

BG19462 and the analogous US patent 3,954,727 describe 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 120 to 160°C. 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.

The international application of patent WO2016182867 describes a method, which allows to obtain bleached microcrystalline cellulose both, from cellulose for chemical processing, and from paper grade pulp, which includes neutralisation or alkalization of an acidic microcrystalline slurry, usually obtained by acid hydrolysis or by electronic beam irradiation, followed by adding of oxidant to such reaction mixture.

Patent RU2298562 Cl describes a method for production of microcrystalline cellulose out of unbleached pulp with output brightness up to 65%, which is subjected to a single-stage hydrolysis with sulphuric acid and to subsequent bleaching of the microcrystalline cellulose obtained with chlorine dioxide within a single stage. As a result, a maximum brightness of up to 80% is achieved. The problem with the brightness of the microcrystalline cellulose produced by this method results mainly from the absorption of chromophoric structures on the cellulose in the process of single-stage acid hydrolysis.

The international application of WO2019095024 reveals a method for producing microcrystalline cellulose by two-stage acid hydrolysis and washing with water after each of the hydrolysis stages, which allows obtaining microcrystalline cellulose with sufficiently high brightness, (i.e. without the need of additional bleaching) and suitable parameters for use in in the pharmaceutical, cosmetic and food industries, in the agriculture and the chemical industry, from bleached paper grade pulp.

On the basis of the above facts it can be summarized that most of the known methods for microcrystalline cellulose production by acid hydrolysis of the pulp are not suitable for treatment of pulp with high hemicelluloses content. Carrying out of acid hydrolysis within a single stage results in considerable absorption of chromophoric structures on the cellulose and is accompanied by significant increase of the Kappa number and reduction of the cellulose brightness. All of this determines the need of development of effective production processes, through which high brightness and low pentosan content microcrystalline cellulose can be produced from unbleached pulp, which is suitable for use in the pharmaceutical, cosmetic and food industries.

SUMMARY OF THE INVENTION

The problem of the current invention is an efficient method for production of microcrystalline cellulose by acid hydrolysis of unbleached paper grade pulp, so that the end product will be bleached microcrystalline cellulose with low pentosan content, as that obtained from cellulose for chemical processing.

The problem of the invention is resolved through method for production of bleached microcrystalline cellulose out of unbleached paper grade pulp, which includes two separate stages of acid hydrolysis, each one of which is followed by washing with water and removal of the dissolved pentose sugars, and the subsequent bleaching of the microcrystalline cellulose to a high degree of brightness.

The method, according to the invention, includes the following operations:

a) first stage of acid hydrolysis during which are dissolved mainly the hemicelluloses and in lower extent the cellulose;

b) washing of the pulp after the first stage of acid hydrolysis with water, during which the dissolved sugars are removed;

c) second stage of acid hydrolysis during which the pulp degree of polymerization and particles size are decreased to typical for the microcrystalline cellulose, while the pentosan content is reduced to levels, characteristic of cellulose for chemical processing; d) washing of the microcrystalline cellulose obtained after the second stage of acid hydrolysis;

e) bleaching of the microcrystalline cellulose, obtained after two-stage acid hydrolysis, with oxidizing bleaching reagents.

f) washing of the pulp after each bleaching stage and obtaining the high brightness microcrystalline cellulose.

DETAILED DESCRIPTION OF THE INVENTION

The nature of the method, in accordance to the invention consists in that as the first stage of processing, when the input material - unbleached pulp (as bone dry) with consistency from 5% to 30% relative to the total weight of the reaction mixture, is subjected to acid hydrolysis, wherein the acid is added in amount from 1.0 to 5.0 weight percent, based on the basis of the bone dried pulp weight. Hydrolysis is carried out at temperatures from 125°C to 180°C, preferably, at temperature from 130°C to 160°C and even more advisable, from 135°C to 150°C. The reaction time in the first stage of acid hydrolysis is from 10 to 180 minutes, preferably, from 20 to 150 minutes and even more advisable, from 40 to 120 minutes.

Intermediate washing of the pulp with distilled water is conducted between both stages of acid hydrolysis, but neutralization is not required.

In the second stage of the method according to the invention an acid hydrolysis is carried out of with the washed after the first stage pulp with consistency from 5% to 30% (as bone dry), in accordance to the total weight of the reaction mixture. The acid is added in amount of 0.5 to 5.0 weight percent, on the basis of the weight of the bone dry pulp. Hydrolysis is carried out at temperatures from 100°C to 180°C, preferably, at temperature of 120°C to 160°C and even more advisable, from 130°C to 150°C. Reaction time in the second stage of hydrolysis is from 10 minutes to 180 minutes, preferably, from 20 minutes to 150 minutes and even more advisable, from 40 to 120 minutes.

Washing of the unbleached microcrystalline cellulose obtained after the second stage of acid hydrolysis is carried out with distilled water.

Part of the filtrate waters from the second acid stage can be used for the intermediate washing. The two stages of acid hydrolysis could be carried out in one and the same reactor or separately as batch or continuous process.

Unobvious for the person skilled in the art, it was found that during the first stage of acid hydrolysis, as function of the reaction time and temperature, there was a moment when the Kappa number was substantially reduced and significant part of the hemicelluloses and chromophoric structures are dissolved. Completion of the first stage of hydrolysis at that moment provided their extraction from the system by the intermediate washing. Thus, in the second stage of the acid hydrolysis the destructive processes related to the dissolved sugars resulting in formation of new chromophoric structures were significantly reduced, and the option for their absorption on the cellulose was minimized. The unbleached microcrystalline cellulose obtained had considerably lower Kappa number, compared to the input unbleached pulp and to the one obtained after a single-stage acid hydrolysis. This result is a pre-condition for a more efficient bleaching with lower consumption of reagents. Furthermore, considerably lower was also the pentosan content in microcrystalline cellulose which was comparable to the levels in the cellulose for chemical processing.

Bleaching of the obtained unbleached microcrystalline cellulose is carried out by the traditional oxidizing bleaching reagents such as: hydrogen peroxide in alkaline medium; pressurized oxygen in alkaline medium (without or together with hydrogen peroxide); chlorine dioxide and peracetic acid. The conditions of bleaching are standard for the respective reagents. The temperature of the bleaching is from 50°C to 110°C, preferably, from 60°C to 100°C and even more advisable, from 70°C to 100°C. Bleaching duration at each stage is from 10 minutes to 240 minutes, preferably, from 20 minutes to 180 minutes and even more advisable, from 30 to 120 minutes. The charge of bleaching reagents and NaOH is from 0.2 to 5.0 weight percent, on the basis of the weight of the bone dry pulp. The pressure, when oxygen is used, is from 0.2 to 7.0 atm. The number of the bleaching stages and the charge of bleaching reagents is less than the one required for bleaching of the initial pulp which is determined by the considerable reduction of the Kappa number, as a result from the acid hydrolysis. Washing with water is carried out after each stage of bleaching.

Final washing of the bleached microcrystalline cellulose is performed with deionized water. Part of the filtrate waters can be used for washing at the preceding stages. According to the invention, the input fibrous material used is unbleached pulp, which can be: sulphate pulp; sulphite pulp; natron pulp; pulp obtained after cooking with organic solvents or after processing of waste paper.

The used pulp could be 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 used pulp can have pentosan content above 15%.

The hydrolysis in the first and second stage of acid hydrolysis in the method in accordance to the invention could be 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.

THE ADVANTAGES OF METHOD ACCORDING TO THE INNOVATION ARE:

- obtaining high brightness microcrystalline cellulose by acid hydrolysis of unbleached pulp and subsequent bleaching with considerably lower consumption of reagents and reduced number of stages in comparison to the ones, which would have been required for the bleaching of the input unbleached pulp;

- the carried out acid hydrolysis in two stages and the pulp washing after the first stage leads to limitation of the destructive processes related to formation of chromophoric structures from the dissolved sugars, and as a result the option for absorption of these structures on the cellulose is minimized;

- the obtained unbleached microcrystalline cellulose has significantly lower Kappa number than the input unbleached pulp, which also determines the more efficient bleaching process;

- using suitable temperature-time conditions of the two-stage acid hydrolysis and after additional bleaching it is obtained bleached microcrystalline cellulose with low pentosan contents, which is characteristic of the cellulose for chemical processing and with parameters, suitable for use in pharmaceutical, cosmetic and food industries;

- washing of the reaction products between the acidic hydrolysis stages is sufficient and no correction of pH associated with additional chemical consumption is required; - an additional significant advantage of two-stage acid hydrolysis process is that the process flows more uniformity.

ENBODYMENT OF THE INVENTION

The invention is illustrated by the following examples, which clarify it without limiting its scope of coverage.

All tests are done using unbleached and oxygen delignified hardwood pulp, which is characterised by pentosan contents of more than 18%.

The acid hydrolysis in first and second stage is carried out in rotating 2 litters laboratory autoclaves made of stainless steel.

The autoclaves are charged with preliminary disintegrated pulp containing 30g bone dry pulp with definite consistency and to which required amount of diluted acid is added.

The autoclaves are placed and rotated in a vat with polyethylene-glycol that are previously heated to the required temperature. The time for autoclaves tempering is 5 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 2-3 mm. The first filtrate is separated for sugars and furfural analyze at chromatographic system HPLC Dionex according to NREL Technical Report (NREL/TP-510-42623). Each sample is washed with distilled water. Partial intermediate washing is also performed between the acid hydrolysis stages, wherein the mass is diluted with distilled water down to a 5% concentration, and then filtered.

Bleaching with CIO₂, hydrogen peroxide and peracetic acid is carried out under the respective conditions in polyethylene bags placed inside a water bath. Bleaching with oxygen is conducted under pressure in rotating autoclaves.

Washing after each stage of bleaching is carried out with distilled water, and the final washing with deionized water.

The particles size and their distribution by size is identified by laser analyzer Fritsch Analysette 22 Micro Tec Plus. The average degree of polymerization DP is defined according to European Pharmacopoeia standard and the brightness according to ISO 2470. Pentosan content in the pulp is determined according to TAPPI T223 cm-10, and the Kappa number— according to ISO 302:2004. MCC yield is determined by weight.

Example 1

Table 1 presents the results of single-stage and two-stage acid hydrolysis of oxygen delignified pulp, characterised by Kappa number of 8.83% and pentosan content of 19.70%. The acid hydrolysis is carried out with diluted sulphuric acid with charge of acid (in regard to bone dry pulp) of 3.0% for the first stage and 2.0% for the second stage. The reaction temperature is 150°C, and the pulp consistence 12.5% for both stages. Process duration is 60 minutes for the first stage and 60, and 90 minutes for the second one. After the first acid stage the pulp is washed with distilled water.

Under these conditions of the process, after the first stage of the acid hydrolysis, no absorption of caramels and sugar decomposition products on the MCC is observed. After 60 min. at this temperature, pentosan content is reduced to 11%, and the Kappa number - to 4.6. The average degree of polymerisation is reduced to 460.

After the second stage of hydrolysis, pentosan content is additionally reduced to 6%. The average degree of polymerisation is slightly above 300. The Kappa number after the second stage of acid hydrolysis reached the levels of 4 units, which is a precondition for more efficient bleaching.

The bleaching with peroxide is carried out at pulp consistence 10%, temperature 70°C, duration 120 min., charge of peroxide 1.0% and carge of NaOH - 2.5%.

The bleaching with CIO₂ is carried out at pulp consistence 10%, temperature 70°C, duration 60 minutes, charge of aCl 2.0% and 2.5%.

The microcrystalline cellulose, obtained after two-stage acid hydrolysis of oxygen delignified unbleached pulp is bleached to a high degree of brightness in two stages of bleaching (table 2), where an additional slight reduction of the pentosan content and of the average degree of polymerisation of the cellulose is observed.

The degrees of bleaching and the consumption of bleaching reagents is less than the one required for bleaching of the input pulp, which was determined by the considerable reduction of the Kappa number, as a result of the acid hydrolysis.

It is found that performing the CIO₂ bleaching stages in an acid medium, as the last stage ensures the required low ash content of the microcrystalline cellulose.

Higher speed of bleaching of the microcrystalline cellulose is observed in comparison with the bleaching of pulp for paper.

The example presented shows, that by two-stage acid hydrolysis of oxygen delignified pulp and subsequent bleaching, can be obtained microcrystalline cellulose, that meet the requirements of the European pharmacopoeia and characterized with pentosan content comparable to the one of the commercial microcrystalline cellulose, which is produced from bleached pulp for chemical processing.

Example 2

Table 3 presents the results of a single-stage and two-stage acid hydrolysis of unbleached hardwood pulp, characterized by Kappa number of 18.01% and pentosan content of 19.90%. The acid hydrolysis is carried out with diluted sulphuric acid with charge of acid (in regard to bone dry pulp) of 3.5% for the first stage and 2.0% for the second stage. The reaction temperature is 150°C, and the pulp consistence - 12.5% for both stages. Duration of the process is 60 minutes for the first stage and 60, and 90 minutes for the second. The pulp is washed after the first acid stage with distilled water.

Under these conditions of the process, after the first stage of the acid hydrolysis, no absorption of caramels and sugar decomposition products on the MCC is observed. After 60 min. at this temperature, pentosan content is reduced to 10%, and the Kappa number - to 13. The average degree of polymerisation is reduced to 460.

It could be seen that after the second stage of acid hydrolysis pentosan content is additionally reduced to levels below 6%. The average degree of polymerisation is around 300. The Kappa number is reduced after the second stage of acid hydrolysis to levels of around 12 units, which is a precondition for more efficient bleaching.

The bleaching with peroxide is carried out at pulp consistence 10%, temperature 70°C, duration 120 min., charge of peroxide 2.0% and ca of NaOH - 2.5%.

The bleaching with CIO₂ is carried out with pulp consistence 10%, temperature 70°C, duration 60 minutes, charge of aCl 0.5%, 1.0% and 2.0%.

The example presented shows, that by two-stage acid hydrolysis of unbleached pulp and subsequent bleaching, can be obtained microcrystalline cellulose, that meet the requirements of the European pharmacopoeia and characterized with pentosan content comparable to the one of the commercial microcrystalline cellulose, which is produced from bleached pulp for chemical processing. The charge of bleaching reagents is less than the one required for bleaching of the initial unbleached pulp and higher speed of the bleaching of microcrystalline cellulose is observed.

Example 3

The experiments are carried out with unbleached hardwood pulp, characterized with Kappa number of 16.62% and pentosan content of 19.6%.

The acid hydrolysis is carried out with diluted sulphuric acid at charge of acid (in regard to bone dry pulp) of 3.5% for the first stage and 2.0% for the second stage. The reaction temperature is 150°C, and the pulp consistence 12.5% for both stages. Duration of the process is 30, 60, 90, 120 and 150 minutes for the first stage and 60 and 90 minutes for the second one (table 5). The pulp is washed after the first stage of acid hydrolysis with distilled water, and a partial intermediate washing is also carried out.

In the course of the first stage of acid hydrolysis, significant reduction of the average degree of polymerisation of the cellulose and of pentosan content are observed, and the process is also characterized by the significant reduction of the yield.

Unexpectedly for the person skilled in the art it is found that the pulp Kappa number is sharply reduced by around 4 units in the initial step of the acid hydrolysis, but after the 60^th minute of processing the Kappa increases significantly and at the 150^th minute its value exceeds by 2 units that in the initial unbleached pulp. This effect can be explained with the process of absorption of caramels and products from the decomposition of sugars on the MCC.

After the second stage of acid hydrolysis with duration 60 and 90 minutes pentosan content is additionally reduced to 2.8% - 3.8%, and the average degree of polymerisation is to 247 - 281. The Kappa number of the microcrystalline cellulose is virtually unchanged after the second stage of acid hydrolysis.

Bleaching with peroxide is carried out at pulp consistence 10%, temperature 70°C, duration 120 min., charge of peroxide 2% and charge of NaOH 2.5%.

Bleaching with CIO₂ is carried out at pulp consistence 10%, temperature 70°C, duration 60 minutes, and charge of aCl 2.0%.

The example presented shows, that by two-stage acid hydrolysis of unbleached pulp and subsequent bleaching, can be obtained microcrystalline cellulose with high brightness, that meet the requirements of the European pharmacopoeia and characterized with pentosan content comparable to the one of the commercial microcrystalline cellulose, which is produced from bleached pulp for chemical processing. Due to the higher values of the Kappa number of the unbleached microcrystalline cellulose, the single-stage acid hydrolysis process leads to a 7% lower final brightness.

Claims

PATENT CLAIMS

1. Method for microcrystalline cellulose production, characterizing with the fact that unbleached pulp is used for that with consistency from 5% to 30% against the total weight of the reaction mixture by two-stage acid hydrolysis with diluted acid, including intermediate washing with water of the cellulose obtained after the first stage of acid hydrolysis, where the first stage of acid hydrolysis is carried out with diluted acid in charge of 1.0 to 5.0 weight percent on the base of bone dry pulp at temperatures from 125°C to 180°C for reaction time from 10 to 180 minutes, after that the washed pulp with 5% to 30% pulp consistency against the total weight of the reaction mixture, is subject to second stage of acid hydrolysis with diluted acid of 0.5 to 5.0 weight percent on the base of bone dry pulp weight where the hydrolysis is carried out at temperatures from 100°C to 180°C for reaction time from 10 minutes to 180 minutes, followed by washing with water and multistage bleaching of the obtained unbleached microcrystalline cellulose, where after each stage of bleaching the microcrystalline cellulose is washed with water, and final washing of the bleached microcrystalline cellulose is done by deionized water.

2. Method according to claim 1 characterizing with the fact that the preferably temperature range of first stage acid hydrolysis is from 130°C to 160°C and even more preferably from 135°C to 150°C.

3. Method according to claim 1 and 2 characterizing with the fact that the preferably temperature range of second stage acid hydrolysis is from 120°C to 160°C and even more preferably from 130°C to 150°C.

4. Method according to claim 1 characterizing with the fact that the preferably reaction time for first stage acid hydrolysis is from 20 to 150 minutes and even more preferably from 40 to 120 minutes.

5. Method according to claim 1 and 4 characterizing with the fact that the preferably reaction time for second stage acid hydrolysis is from 20 to 150 minutes and even more preferably, from 40 to 120 minutes.

6. Method according to claim 1 characterizing with the fact that the acid in the first and second stage acid hydrolysis could be diluted organic acid, water solution of sodium bisulfite, water solution of sodium bisulfate, diluted inorganic acid, and preferably mineral acid.

7. Method according to claims 1 and 6 characterizing with the fact that the diluted organic acid could be oxalic, acetic, formic acid.

8. Method according to claim 1 and 6 characterizing with the fact that the diluted mineral acid could be hydrochloric acid, sulfuric acid, nitric acid.

9. Method, according to claim 1 characterizing with the fact that the pulp could be unbleached sulphate cellulose; unbleached sulphite cellulose, natron cellulose, cellulose obtained after boiling with organic solvents or after processing of waste paper.

10. Method, according to claim 1 characterizing with the fact that the pulp could be from softwood, hardwood, non-wood plant materials like straw, hemp, kenaf, ramie, bamboo and other cellulose-containing agricultural plant waste.

11. Method, according to claim 1 characterizing with the fact that the pulp could be with pentosan content above 15 weight % against its total weight.

12. Method, according to claims lfom 1 characterizing with the fact that both stages of acid hydrolysis can be carried out in one and the same reactor or separately as a periodic or continuous process.