RU2771348C1 - Cellulose production method - Google Patents

Abstract

FIELD: cellulose and paper industry.

SUBSTANCE: invention relates to the cellulose and paper industry; it can be used in the production of cellulose for the manufacture of paper and cardboard. Heating of water and technological chips to the reaction temperature is carried out in a stationary mode by heat of exothermic reactions of the oxidized part of wood chips, while hydro-classification of technological chips by size is carried out in two stages, and water is used as cooking liquid at a temperature from 374 to 550°C and a pressure from 20 to 30 MPa, while the destruction of a lignin-carbohydrate complex is carried out in a closed volume with the organization of a fluidized layer and simultaneous hydro-classification of obtained cellulose by size with constant control of the density of settling cellulose fibers, according to the change in which a conclusion is made about the readiness of the product, after which obtained cellulose is purified from chemical impurities in the presence of an oxidizer – air – with gasification, while preserving its main frame, α-cellulose.

EFFECT: increase in the productivity of the cellulose production process due to reducing the reaction time of wood chips, while maintaining the quality of cellulose, while reducing harmful emissions and the negative impact of cellulose and paper production on the environment.

2 cl, 1 tbl

Description

The invention relates to the pulp and paper industry and can be used in the production of pulp from softwood and hardwood for the manufacture of paper and cardboard.

The closest in technical essence and purpose to the proposed invention is a method for producing sulfate pulp, including heating the cooking liquid with sized technological chips to the reaction temperature, holding in the digester until the destruction of the lignin-carbohydrate complex, purifying the resulting product from impurities to obtain α-cellulose, and black liquor is used as an extractant. This extract is mixed with black liquor after the chip processing is completed and fed into the cooking chemicals recovery system. The aqueous extract is sent for disposal (Patent RU2 477346 C1 publ. 2013.03.10 IPC D21C 1/06 D21C 3/02).

The disadvantages of this method of obtaining cellulose are:

1 - the use of black liquor for the preparation of cooking liquid, which requires regeneration with the corresponding costs for loss compensation, high capital investments, which determine the cost of equipment as a whole for organizing production and the environmental hazard of the technology.

2 - reduction of the mechanical properties of pulp and yield of pulp due to the large variation in the size of the supplied technological chips.

3 - decrease in the whiteness of cellulose as a result of lignin condensation on the fibers, and low-quality used technological chips.

4 - long exposure time of technological chips (up to 2-3 hours) at low speed in the downward flow. This leads to large capital investments in the manufacture of the digester and difficulties in its operation. The efficiency of leaching decreases, since the technological chips under these conditions behave as a quasi-solid body, which makes it difficult for the solvent to reach the capillaries.

The objective of the invention is to eliminate the disadvantages of known methods for producing cellulose.

The technical result of applying the proposed method is to increase the productivity of the pulp production process by reducing the time of reaction exposure of wood chips, while maintaining the quality of pulp, while reducing harmful emissions and the negative impact of pulp and paper production on the environment.

The task is achieved by the fact that the ongoing method of obtaining cellulose includes heating the cooking liquid with sized technological chips to the reaction temperature, holding in the digester until the destruction of the lignin-carbohydrate complex, cleaning the resulting product from impurities to obtain α-cellulose, and heating water and technological chips to the reaction temperature is carried out in a stationary mode by the heat of exothermic reactions of the oxidized part of the wood chip, while the hydroclassification of the chip by size is carried out in two stages, and water is used as the cooking liquid at a temperature of 374 to 550 ° C and a pressure of 20 to 30 MPa , while the destruction of the lignin-carbohydrate complex is carried out in a closed volume of the digester with the organization of a fluidized bed and simultaneous cleaning hydroclassification of the obtained cellulose by size with constant control of the density of the settling cellulose fibers, according to the change of which, a conclusion is made about the readiness of the product, after which the obtained cellulose is purified from chemical impurities in the presence of an oxidizing agent - air with gasification while maintaining its main frame α-cellulose. At the same time, the classification of technological chips by size is carried out in two stages in a water flow with the production of two products of a plus and minus fraction at each stage, while the length of the plus fraction of the first stage is more than 25 mm, the length of the minus fraction of the second stage is less than 25 mm.

The essential difference of the proposed method is an inseparable set of technological methods with regime indicators of temperature and pressure, which ensure the achievement of the technical result indicated above.

The technological scheme of the proposed method is shown in Fig. 1, it includes the following blocks:

1 - Heat recovery unit.

2 - Hydroclassification block.

3 - Technological block.

4 - Block of mechanical and chemical cleaning.

5 - Energy block.

The method is carried out as follows.

Water purified from mechanical and chemical impurities from unit 4 at a temperature of 20°C is sent to unit 1 for regenerating the heat of water from the digester. The water heated in the heat recovery unit is sent to the pulp production unit 3.

Technological block 3 is a closed volume of a coaxial cylindrical-conical boiler, which consists of two stages, each of which is represented by a cone and a cylinder. The cone of the first stage is used to collect and unload classified pulp; in the cylinder of the first stage, mixing of water, oxidizing agent and steaming of wood chips is carried out; in the truncated cone of the second stage organize the movement of the flow in the fluidized bed due to the difference in the sections of the lower and upper parts of the truncated cone; the cylindrical part of the second stage is a reaction zone where an exothermic oxidation reaction of wood chips occurs while maintaining its frame, α-cellulose. The exothermic oxidation reaction makes it possible to heat the water leaving block 1 to a reaction temperature of 374 to 550°C.

Block 3 includes 3 products: water from block 1, tangentially supplied to the cone of the first stage; oxidizer (air) tangentially brought into the conical part of the first stage; technological wood chips from block 2, summed up in a truncated cone of the second step.

3 products come out of block 3: water from the upper part of the cylinder, the second stage, which is sent to block 1 for cooling in screw recuperative heat exchangers; pulp obtained in the lower cone of the first stage; gas consisting of hydrogen, methane and carbon dioxide, which is sent to unit 5.

Heated water from block 1 is sent to block 3, where it is heated to a reaction temperature of 374 to 550°C by the heat of exothermic oxidation reactions. Pressure from 20 to 30 MPa in the digester is regulated by a valve at the water outlet from unit 1.

Technological chips entering the production are subjected to hydroclassification in block 2. The block consists of two cylindrical-conical devices connected in series. In the first apparatus, technological chips are classified in a water flow to obtain a plus fraction with a length of more than 25 mm and a minus fraction with a length of 25 mm or less. The technological chips of the plus fraction are unloaded from the lower cone of the first apparatus and removed from the circuit for additional grinding. In the cone of the second apparatus, the main fraction of standard technological chips with a length of 25 mm is concentrated, which is sent to block 3 by a system of screws into the truncated cone of the digester. The minus fraction of chips of the second stage with a length of less than 25 mm is withdrawn from the technological process. Next, the technological chips treated with water, saturated with an oxidizing agent at a speed of 1.0-1.5 m/s are fed upwards into the truncated cone of the 2nd stage to form a flow movement in the fluidized bed. From the truncated cone, the wood chips, picked up by the water flow with the oxidizing agent, enter the cylindrical part, where within 2-5 minutes the complete destruction of the lignin-carbohydrate complex, hemicellulose and other forms of organic compounds occurs without destroying the α-cellulose wood frame. Wood chips in the reactor move in a fluidized bed together with water at a speed determined by the removal of particles with a length of less than 10 mm. In the volume of the boiler, a cellulose fraction is deposited along a length of 25 ± 10 mm with constant control of the density of the settling cellulose fibers, by changing which it is concluded that the product is ready, after which the resulting cellulose is cleaned of chemical impurities in the presence of an oxidizing agent - air with gasification while maintaining its main frame α-celluloses. Pulp is unloaded from the lower cone of the digester through an unloader.

Three streams are formed in the digester after the reaction for 2-5 minutes: a water stream after classifying wood chips with a length of less than 10 mm, cellulose, which is concentrated in the lower cone of the apparatus, and a gas stream consisting of hydrogen, methane and carbon dioxide. Water and gases are removed from the upper part of the boiler, cellulose - from the lower cone of the first stage of the apparatus.

The water flow after the second stage of hydroclassification is sent to the heat recovery unit. At the outlet of the flow from the heat recovery unit, water is sent to the mechanical and chemical treatment unit 4. Purified water after adjustment with additional water is sent to the tubular part of the screw heat exchangers of the first stage of block 1. The resulting pulp after the digester is sent for further processing. Air is used as an oxidizing agent. The oxidizer is fed tangentially into the conical part of the first stage of the cylindrical-conical apparatus at the level of water inlet by a high-pressure compressor. The oxidizing agent is supplied in an amount 20-30% higher than calculated for the complete oxidation of organic compounds, excluding the wood frame α-cellulose.

The resulting gases after the gasification of process chips from the upper part of the digester are sent for further processing to an energy unit to produce heat and electricity or subsequent processing into other products that have consumer demand and price, for example, liquid fuel.

The following example illustrates the effectiveness of the proposed method.

Example.

Purified water at a temperature of 40°C with a pressure of 25 MPa is fed into the pipe part of series-connected screw heat exchangers where it is heated to a temperature of 354°C. Further, water is fed through a tangential inlet into a closed volume of a two-stage cylindrical-conical digester, where it is heated to 374°C in a stationary mode by the heat of exothermic reactions during the oxidation of the lignin-carbohydrate complex of technological chips, that is, without heat supply from an external source. The specified temperature and pressure in the digester are maintained by changing the water flow, that is, by changing the ratio of oxidizable organics in wood chips and the amount of water with correction for pressure in the digester, oxidizer flow and gas flow. The resulting α-cellulose from the digester is sent to the annular part of the screw heat exchangers, where after passing through the screw it is cooled to 55°C and removed for cleaning from mechanical and chemical impurities. After cleaning, the water flow, corrected for the amount of losses, enters again into the pipe part of the screw heat exchangers.

Taking into account the fact that the time of chemical reactions is 10-15 minutes, the hydroclassification of technological chips is carried out in two stages in two series-connected two-stage cylindrical-conical apparatuses at atmospheric pressure to obtain the main fraction of technological chips with a length of 25 mm, a width of up to 25 mm and a thickness of 2-5 mm .

The obtained technological chips are sent to a two-stage cylindrical-conical digester operating at supercritical parameters, where it moves together with the flow of water and oxidizer in a fluidized bed.

To carry out gasification processes, water and an oxidizing agent are fed tangentially from the bottom of the digester. Air is used as an oxidizing agent. The main criterion for carrying out the technological process is the preservation of the carcass of α-cellulose wood chips. This requirement is met by the technological parameters of the process: pressure and temperature. It should be noted that under conditions of supercritical parameters, complete oxidation of wood chips is achieved, incl. and its α-cellulose backbone. At temperatures and pressures below the parameters of the critical point of the process, the decomposition of sugars prevail over the processes of cellulose hydrolysis. Therefore, by varying the temperature and pressure, chemical cleaning of wood chips is carried out without destroying its frame. During gasification of a mixture of lignin and cellulose in water at T=374°C and P=25 MPa, hydrogen, methane and carbon dioxide are formed.

The table shows the characteristics of the obtained pulp from softwood for different process modes.

As a result of the processing of technological chips in a fluidized bed at a temperature of 374 to 550°C and a pressure of 20 to 30 MPa without the use of chemicals with additional hydroclassification of technological chips and purification by gasification from the organic component of α-cellulose, product quality is maintained, and specific energy and capital costs decrease, as the technology lacks technological processes and apparatuses such as a soda recovery boiler, black liquor evaporation, limestone kilns and green liquor causticization, the process of bleaching and pulp sorting is reduced or absent. It should be noted that the duration of the process of obtaining pulp from coniferous and hardwood species for the manufacture of paper and cardboard is also significantly reduced.

Claims (2)

1. A method for producing cellulose, including heating the cooking liquid with sized technological chips to the reaction temperature, holding in the digester until the destruction of the lignin-carbohydrate complex, cleaning the resulting product from impurities to obtain α-cellulose, characterized in that heating water and technological chips to the reaction temperature is carried out in a stationary mode by the heat of exothermic reactions of the oxidized part of the wood chip, while the hydroclassification of the chip by size is carried out in two stages, and water is used as the cooking liquid at a temperature of 374 to 550 ° C and a pressure of 20 to 30 MPa, at the same time, the destruction of the lignin-carbohydrate complex is carried out in a closed volume of the digester with the organization of a fluidized bed and simultaneous cleaning hydroclassification of the obtained cellulose by size with constant control of the density of the settling cellulose fibers, by changing which a conclusion is made about the product is ready, after which the obtained cellulose is purified from chemical impurities in the presence of an oxidizing agent - air - with gasification while maintaining its main frame, α-cellulose. 2. The method for producing pulp according to claim 1, characterized in that the classification of technological chips by size is carried out in two stages in a stream of water with the production of two products of plus and minus fractions at each stage, while the length of the plus fraction of the first stage is more than 25 mm, the length the minus fraction of the second stage is less than 25 mm.

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