PROCESS FOR PRODUCING MICROCRYSTALLINE CELLULOSE

Abstract

A process for producing microcrystalline cellulose from fibrous cellulosic material by acid hydrolysis at an MCC plant. In this process a suspension of cellulosic pulp is thickened, so that filtrate is formed. A consistency of the thickened suspension is adjusted to 5-40% by adding dilution liquid, and the pH of the suspension is adjusted to a value of 4 or less with acid. The pulp suspension is subjected to acid hydrolysis of at least 120 C. in a reactor to produce a mixture of microcrystalline cellulose and hydrolysate. The mixture is discharged from the reactor to a lower pressure and lower temperature to flash the mixture and generate flash steam. Microcrystalline cellulose is separated from hydrolysate. The separated cellulose is washed in one or several washing apparatuses. The consistency of the microcrystalline cellulose is increased by thickening and/or drying for further processing.

Claims

1. A method for producing microcrystalline cellulose from fibrous cellulosic material by acid hydrolysis at a microcrystalline cellulose (MCC) plant, the method comprising: a) thickening a suspension of cellulosic material to form a filtrate, b) adding a dilution liquid to the thickened suspension of cellulosic material to adjust a consistency of the thickened suspension to a range of five to forty percent, and adding an acid to the thickened suspension to adjust a pH to no more than four of the thicken suspension, c) increasing a temperature of the thickened suspension to a range of 120 to 200 C., d) subjecting the thickened suspension to acid hydrolysis a reactor while the thickened suspension is in the range of 120 to 200 C. to produce a mixture of microcrystalline cellulose and hydrolysate, e) flashing the mixture by discharging the mixture from the reactor to reduce the pressure and temperature of the mixture and generate flash steam, f) separating the mixture to separate the microcrystalline cellulose from the hydrolysate, and washing the separated microcrystalline cellulose in at least one washing apparatus, and g) increasing a consistency of the separated and washed microcrystalline cellulose by thickening.

2. The method according to claim 1, wherein the dilution liquid in step b) comprises a weak hydrolysate from the washing of microcrystalline cellulose, and/or condensate from a heat recovery system of the MCC plant.

3. The method of claim 1, wherein the flash steam from step e) is led to a condenser so that condensate is formed which is used in step b).

4. The method of claim 1, wherein the MCC plant is integrated with pulp production of a pulp mill.

5. The method of claim 4, wherein the filtrate formed in the thickening of the suspension of cellulosic material in step a) is recovered and led to the pulp mill for use in pulp treatment.

6. The method of claim 4, wherein acid hydrolysate and/or acid filtrates from step f) are used in a prehydrolysis kraft pulp production, in an acid stage of pulp bleaching or at a tall oil plant of the pulp mill.

7. The method of claim 1, wherein the temperature in the hydrolysis in step d) is in a range of 120- to 185 C.

8. The method of claim 1, wherein step of thickening the suspension of microcrystalline cellulose includes increasing a consistency of the suspension to within a range of forty to fifty percent.

9. The method of claim 1, wherein the step of increasing the consistency of the separated microcrystalline cellulose includes thickening and drying the separated microcrystalline cellulose to have a dry content of at least 93 percent.

10. The method of claim 1, wherein the step of separating includes diluting the mixture with a weak hydrolysate recovered from the washing of the separated microcrystalline cellulose before the separation of the microcrystalline cellulose from the hydrolysate.

11. A method to produce microcrystalline cellulose from fibrous cellulosic material comprising: thickening a suspension of cellulosic material; adding a dilution liquid to the thickened suspension of cellulosic material to reduce a consistency of the thickened suspension of cellulosic material to a consistency in a range of five to forty percent; adding an acid to thickened suspension such that a pH is no greater than four of the thickened suspension; heating the thickened suspension in a reactor to a temperature in a range of 120 to 200 C. to subject the thickened suspension to acid hydrolysis to produce a mixture of microcrystalline cellulose and hydrolysate, flashing the mixture by discharging the mixture from the reactor to reduce a pressure and temperature of the mixture; separating microcrystalline cellulose and hydrolysate in the flashed mixture; washing the separated microcrystalline cellulose, and thickening the separated microcrystalline cellulose.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0037] To enable a better understanding of the present invention, and to show how the same may be carried out into effect, reference will now be made, by way of example only, to the accompanying drawing, in which:

[0038] FIG. 1 is a schematic view illustrating an exemplary system for practicing a process according to the invention.

DETAILED DESCRIPTION

[0039] FIG. 1 shows an exemplary system of an MCC plant that may be used according to the present invention. In this embodiment the MCC plant is integrated to a chemical pulp mill. The new process can be practiced also at a standalone MCC plant.

[0040] In an initial stage of the new process fibrous cellulosic material is fed to a pre-thickening stage 2 through line 1. The material may be derived from wood plant material, such as softwoods or hardwoods. A preferred fibrous cellulosic material is a bleached or unbleached chemical pulp, such as kraft pulp, or dissolving pulp.

[0041] The thickening can be done with a screw press, or with other known dewatering apparatus suitable for pulp suspension, e.g. a drum filter, a disc filter, a filter press, a belt press, a twin wire press, a dewatering press, a wash press. Part or all of the filtrate from the thickening stage can be recovered and led through line 3 to the chemical pulp mill, if the MCC plant is integrated to the pulp mill. The filtrate can be used in pulp bleaching or brown stock area.

[0042] After the thickening stage 2, the pulp consistency for acid hydrolysis is controlled by dilution. The pulp is led through line 4 where dilution liquid is added into the pulp which is further directed to a dilution screw 5. The dilution liquid may comprise condensates from the heat recovery system of the MCC plant. The dilution may be carried out e.g. at a screw conveyor when the pulp is transported from the pre-thickening stage towards a hydrolysis reactor. The dilution screw drops the pulp to a stand pipe 6 from which the pulp is pumped forward at a consistency of 5-40%, preferably 10-30%, with a pump 8, such as a medium consistency (MC) pump or a screw pump.

[0043] The pulp suspension is then pumped towards the hydrolysis reactor 12. The pulp is acidified by injecting acid 7 into the pulp after or before a feed pump 8. The pulp temperature is raised with one or several direct steam injectors 9, e.g. a steam injector heater described in U.S. Pat. No. 6,361,025. The steam is typically medium pressure steam 10. Acid and steam are mixed evenly into the pulp suspension by using a mixer 11. The pulp pressure is increased with the feed pump 8 and, if needed, also e.g. with a booster pump, to the reactor pressure, which typically is 3-12 bar(g) at the reactor top.

[0044] The acid may be organic acid or inorganic acid. Typically it is sulphuric acid. Sulphuric acid can be produced at the pulp mill. Acidic liquid flows, such as bleaching filtrates, from the pulp mill can also be used for the acidification of the pulp suspension.

[0045] Hydrolysis takes place in the reactor 12 under acidic conditions (pH is 4.0 or below, preferably 2-3), and at an elevated temperature (120-200 C., typically 120-185 C., preferably 150-170 C.). During the residence time of 5-240 minutes, typically 10-60 minutes, in the reactor 12, a mixture of microcrystalline cellulose and hydrolysate is formed.

[0046] The reactor 12 is typically a vertical vessel, which may be an up-flow or down-flow reactor.

[0047] The mixture of microcrystalline cellulose and hydrolysate is blown from the reactor 12 to a lower pressure and temperature. The mixture is typically discharged through line 13 into a blow tube 14 in which it will flash to lower temperature and pressure. Generated flash steam 33 is condensed in a condenser 35 to produce hot water 34. Condensate can be led through lines 36 and 38 to pulp dilutions in line 4 or led to the chemical pulp mill through line 37 and utilized there.

[0048] After the blowing the mixture is treated for separating hydrolysate from microcrystalline cellulose. The mixture is first diluted, e.g. at the bottom of the blow tube 14, before it is pumped forward.

[0049] Dilution liquid may be weak hydrolysate 44 from the washing department, condensate from a dryer condenser, or process water.

[0050] The dilution liquid can be cooled in a cooler 43 in order to bring the mixture temperature down to a level suitable for the subsequent washing department. The mixture is pumped through line 15 forward to a first washer 16 at a suitable feed consistency, such as 3-10%. Using a process pump is possible because the hydrolyzed product is easily fluidized and suspended, even at medium consistency.

[0051] Washing of the hydrolysis product takes place in one or several washing apparatuses. The washing principle can be dilutionthickening or displacement washing. In this embodiment a dilution-thickening method is used. The washers 16 and 18 may be drum washers.

[0052] The washing is carried out counter-currently. The cleanest washing liquid, such as hot water, is fed from line 20 to a last washing or thickening stage 21, and filtrates 47 and 42 from washing/thickening stages 21 and 18 are led counter-currently to the previous washing stage in a direction of the MCC flow.

[0053] The washer apparatus thickens the product to a consistency of 25 . . . 30% and discharges it into a subsequent repulper screw conveyor washer or to a drying department.

[0054] When the washing requires predilution for washer(s), this is done with filtrate of the subsequent washer, condensate from a dryer condenser, and/or process water. For the first washer predilution may take place at the blow tube. For subsequent washers predilution takes place at the repulper screw conveyor of the preceding washer. The repulper after the first washer drops the diluted end product into a stand pipe, from which the product is pumped to a subsequent washer typically at a consistency of 3-10%.

[0055] In displacement washing a washing liquid may also be filtrate of the subsequent washer, condensate from a dryer condenser, and/or process water.

[0056] Hydrolysate is separated from MCC in the first washer 16. The hydrolysate, which is rich in carbohydrates, can be used in the production of energy at the pulp mill where it is led through line 39, 41. A portion of the hydrolysate can be recycled through line 40 into the pulp suspension for the pH adjustment before hydrolysis in the reactor 12. It may be recycled also directly to the hydrolysis reactor.

[0057] Filtrate (weak hydrolysate) from the second washer 18 may be used as dilution liquid in the screw conveyor 5 where it is led through line 46. The filtrate may be heated in a direct steam heater 48. Filtrate from the second washer may be added also to the hydrolysis reactor 12 through line 45.

[0058] The final apparatus 21 in the washing department operates as a washer or only as a thickener adjusting a dry consistency of the MCC product to a suitable level depending on a subsequent dryer. It is beneficial for the drying energy consumption that the consistency of the product is high before drying, preferably 40% or over, typically 40-50%.

[0059] The next step in the process is drying, if the MCC has to be dried to a high dry content before further processing or use. The dryer type can be: a paddle dryer, a flash dryer, a fluid bed dryer, a belt dryer, a drum dryer, a helix dryer, a plate dryer, a spray dryer. The dryer 23 utilizes power systems, e.g. steam, electricity, of the chemical pulp mill, typically, low or medium pressure steam 24. This steam is condensed in the dryer 23, and condensate 26 can be returned to the boiler plant and used as boiler water. As the product progresses through the dryer, its moisture is evaporated and vented through line 27 to a condenser 29, where hot water 28 is generated. Condensate in line 30, 31 is utilized in pulp dilution in line 4 or led through line 32 to the chemical pulp mill. Dry product typically at a consistency over 93% is led from the dryer 23 through 25 to a subsequent product treatment, e.g. a packaging step.

[0060] Optionally, after the thickening the MCC at a consistency of 40-50% can be led directly to a further processing or use, if no drying is needed.