A CONTINUOUS PROCESS FOR PRODUCTION OF CELLULOSE PULP FROM GRASS-LIKE PLANT FEEDSTOCK

Abstract

A continuous process for production of cellulose pulp from grass-like plant feedstock for paper making. The process includes: preparing the grass-like plant feedstock by comminuting, dedusting, continuous digestion, disperging, diluting the cellulose pulp, screening and fractionation, concentration to remove black liquor, dilution with fresh water, and, optionally, pulp bleaching processes. The continuous digestion is performed in the vertical column at 70-100 C. for 40 minutes to 2 hours; average composition of thus formed digestion suspension is maintained within the following ranges: 0.9-1.5% w/w of NaOH; 0.15-0.4% w/w of NaCl or Na.sub.2SO.sub.3; and 15-18% w/w of grass-like plant feedstock; concentrations of ingredients being calculated on the weight of the liquid phase. Screening and fractionation are resulting with two fractions; one being further processed by milling, and another being further processed to final pulp with or without bleaching processes. The preferred grass-like feedstock for the process is miscanthus/Miscanthusgiganteus, Andersson/.

Claims

1. A continuous process for production of cellulose pulp from grass-like plant feedstock, comprising the steps of: (i) preparing the grass-like plant feedstock by comminuting to produce a feedstock with longitudinal size distributed from 1.5-30 cm and diameter of 0.5-15 mm, and with removed fine dusty particles by dedusting of said feedstock with fan; (ii) continuous digestion of a grass-like dust-free plant feedstock prepared in step (i) in a digester formed as a vertical column internally having only smooth side walls; where dust-free grass-like plant feedstock is continuously fed directly on the top of the said digester via conveyor; wherein, in parallel with said dust-free feedstock top-feeding, the chemicals for digestion selected from the group consisting of NaOH and NaCl or Na.sub.2SO.sub.3, fresh water, regenerated water, and steam are introduced continuously on the top of said digester; maintaining the digestion temperature from 70-100 C. and average composition of thus formed suspension during said continuous digestion within the following ranges: (a) 0.9-1.5% vv/w of NaOH; (b) 0.15-0.4% w/w of NaCl or Na.sub.2SO.sub.3; and (c) 15-18% w/w of grass-like plant feedstock; where concentrations of ingredients being calculated on the weight of the liquid phase; where dissolution of noncellulosic substances from the grass-like plant feedstock is performed during the mass transfer from the top to the bottom of the said digester performed only by the gravity that lasts 40 minutes to 2 hours, and where cooked pulp is concentrated at the bottom of the digester, and continuously, by equal rate as being feedstock fed into the digester, discharged from the bottom of the said digester by the conveyor having the hydraulic pressure compensation.

2. A continuous process for production of cellulose pulp according to the claim 1, further comprising the steps of: (iii) disperging, where the suspension of cooked pulp discharged from the step (ii) is processed through a disperger; (iv) diluting, where pulp being disperged in step (iii) is diluted with water in the dilution vessel from a starting concentration 15-18% w/w to a concentration of 3-6% w/w of pulp; and (v) screening and fractionation, where diluted suspension from the step (iv) is processed through a screening and fractionation device equipped with 0.1-0.5 mm sieve, yielding two fractions; the first fraction that does not passed through the 0.1-0.5 mm screen, in amount of maximally 50% w/w; and the second fraction that does passed through the 0.1-0.5 mm screen, in amount of minimally 50% w/w, which is considered as a good material for further processing that is transferred into the auxiliary vessel.

3. A continuous process for production of cellulose pulp according to the claim 2, wherein the first fraction obtained in step (v) is further processed by milling in 1-3 mills, and then: (a) returned back to the auxiliary vessel in step (iv) for the reprocessing via steps (iv) and (v); or (b) transferred directly into the auxiliary vessel.

4. A continuous process for production of cellulose pulp according to the claim 2, wherein the material collected in the auxiliary vessel is further processed in the process comprising the steps of: (vi) concentration in a dewaterer to remove black liquor, dilution in auxiliary vessel with fresh water, and preparation of cellulose pulp in connected vessel; and (vii) optionally, a pulp bleaching step in the bleaching vessel regulated via valve yielding cellulose pulp suitable for paper manufacturing or production of cellulose sheets; where the used water regenerated in the dewaterer is returned through manifold back to the digester in step (ii) and into the dilution vessel in step (iv).

5. A continuous process for production of cellulose pulp according to claim 1, wherein the chemicals for digestion in step (ii) are introduced as a mixture of chemicals or each chemical separately in the form of crystalline solids or pellets.

6. A continuous process for production of cellulose pulp according to claim 1, wherein the chemicals for digestion in step (ii) are introduced as a mixture of chemicals or each chemical separately in the form of concentrated aqueous solutions of 30-50% w/w NaOH; and 20-30% w/w of NaCl or Na.sub.2SO.sub.3.

7. A continuous process for production of cellulose pulp according to claim 1, wherein the digestion temperature is 94-98 C.

8. A continuous process for production of cellulose pulp according to claim 1, wherein the grass-like feedstock includes stems of plant species selected from at least one of: wheat/Triticum vulgare, Linne/; rice/Oryza sativa, Linne/; barley/floredum vulgare, Linne/; oat/Avena sativa, Linne/; flax/Linum usitatissimum, Linne/; maize/Zea mays, Linnen millets: proso millet/Panicum miliaceum, Linne/, pearl millet/Pennisetum glaucum, Linne/, browntop millet/Panicum ramosum, Linne/, and barnyard/Echinochloa frumentaceae, Linne/; triticale/Triticosecale, Wittm. ex A. Camus/; buckwheat/Fagopyrum esculentum, Moench/; miscanthus/Miscanthusgiganteus, Andersson/; switchgrass/Panicum virgatum, Linne/; sorghum/Sorghum species, Linne/; common reed/Phragmites australis, Cav./, giant reed/Arundo donax, Linne/, burma reed/Neyraudia reynaudiana, Kunth./, reed-mace/Typha spp., Linne/, paper reed/Cyperus papyrus, Linne/, bur-reed/Sparganium spp., Linne/, thatching reed/Thamnochortus insignis, Linne/; esparto grass/Stipa tenacissima, Linne and Lygeum spartum, Linnen jute/Corchorus olitorius, Linne/, bamboo/Bambusoideae spp., Linne/, bagasse, or mixtures thereof.

9. A continuous process for production of cellulose pulp according to the claim 8, wherein the grass-like feedstock is miscanthus/Miscanthusgiganteus, Andersson/.

10. A continuous process for production of cellulose pulp according to the claim 3, wherein the material collected in the auxiliary vessel is further processed in the process comprising the steps of: (vi) concentration in a dewaterer to remove black liquor, dilution in auxiliary vessel with fresh water, and preparation of cellulose pulp in connected vessel; and vii) optionally, a pulp bleaching step in the bleaching vessel regulated via valve, yielding cellulose pulp suitable for paper manufacturing or production of cellulose sheets; where the used water regenerated in the dewaterer is returned through manifold back to the digester in step (ii) and into the dilution vessel in step (iv).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0068] FIG. 1 shows a process for production of cellulose pulp from comminuted dust-free grass-like feedstock according to the invention; the following manufacturing phases: (ii) digestion, (iii) disperging, (iv) dilution, and (v) screening and fractionation.

[0069] FIG. 2 shows further processing of cooked pulp according to the invention; the phases: (v) milling from the phase of screening and fractionation, (vi) concentration, dilution, and preparation of pulp for further production, and (vii) pulp bleaching (optional).

DETAILED DESCRIPTION OF INVENTION

[0070] The invention is related to an improved continuous process for production of cellulose pulp from dust-free comminuted grass-like feedstock such as miscanthus/Miscanthusgiganteus, Andersson/.

[0071] The process is performed in a continuous manner through several manufacturing phases, i-vii, as shown in FIG. 1 (phases i-v), followed by FIG. 2 (phases v-vii):

(i) Preparation of the Grass-Like Plant Feedstock

[0072] The feedstock is prepared by comminuting, to produce a material with longitudinal size distributed from 1.5-30 cm and diameter of 0.5-15 mm. Comminuting of starting grass-like material is carried out by conventional comminuting machines or supplied directly from the fields, if the crops were collected by suitable combine harvester, equipped with adequate chopping device yielding the plant material of above-stated particles dimensions.

[0073] Primarily, the comminution should be performed in a mild manner yielding fibrous material predominantly comminuted along the fibers, in order to preserve them.

[0074] This is the reason why comminution is not shown in FIG. 1, because it represents either conventional pre-treatment or may be even carried out during harvesting in the field.

[0075] Then, the comminuted material is subjected to dedusting by removal of fine, dusty, non-fibrous plant material, which would otherwise reduce the quality of resulting cellulose pulp. This is done by suitable fan which produces a strong air circulation that enable blowing away of fine light particles.

[0076] The latter fine dust does not enter into the process at all, thus saving significant amounts of chemicals that would be otherwise spent through reaction of this material with NaOH. Additionally, the effluents are not contaminated with such level of organic matter, what would significantly negatively affect the environment.

[0077] This non-fibrous fine material mainly comes from central part of plant stalks. In the case of miscanthus, the percentage of this fraction is roughly 8-9% w/w.

[0078] Thus collected fine non-fibrous dust can be used in the process as a fuel in energy production or, alternatively, as raw material in manufacturing of xylan.

(ii) Continuous Digestion of Grass-Like Plant Feedstock

[0079] The feedstock prepared in step (i) is further processed in a digester (10), which is a vertical column internally having only smooth side walls, without any screen, loops, or stirring device, where grass-like plant feedstock is continuously fed directly on the top of the said digester via conveyor (1).

[0080] In parallel with said dust-free feedstock top-feeding, the following materials are also introduced continuously on the top of said digester: [0081] (a) chemicals for digestion are fed through conveyer (2); [0082] (b) fresh water is fed through manifold (30); [0083] (c) steam is introduced through manifold (31); as well as [0084] (d) regenerated water which is fed through manifold (32).

[0085] From all these materials a suspension of dust-free comminuted plant material in a liquid phase consisting of aqueous solution of digesting chemicals is generated.

[0086] The digestion is preformed at temperature of 70-100 C., wherein average composition of thus formed suspension during said continuous digestion has to be maintained within the following ranges: [0087] (a) 0.9-1.5% w/w of NaOH; [0088] (b) 0.15-0.4% w/w of NaCl or Na.sub.2SO.sub.3; and [0089] (c) 15-18% w/w of grass-like plant feedstock;
where concentrations of ingredients are being calculated on the weight of the liquid phase.

[0090] From this specification of the digestion suspension one can state that the digestion according to this invention is performed: [0091] under very mild conditions; the concentration (w/w) of the key chemicals, are very low, 0.9-1.5 w/w NaOH and 0.15-0.4% w/w of NaCl or Na.sub.2SO.sub.3, what is drastically lower than is usually employed in the prior art, see for instance references EP 2003241 and GB 770,687; this makes the process from the present invention environmentally friendly; and, in the same time, [0092] the working concentration of grass-like plant feedstock is relatively high; of 15-18% w/w of comminuted grass-like feedstock to solution of digestion chemicals, or 5.5-6.6:1 w/w, what is far more higher than is in the known prior art, see GB 892,277; this results in high output, less water usage, and energy consumption.

[0093] The digestion chemicals are selected from the group consisting of: sodium hydroxide (NaOH) and sodium chloride (NaCl) or sodium sulfite (Na.sub.2SO.sub.3). In this case, digestion chemicals are continuously fed via conveyor (2) separately, or as previously prepared solid mixture of commercially available crystals or pellets.

[0094] Alternatively, the digestion chemicals can be added as previously prepared concentrated aqueous solutions: as 30-50%) w/w NaOH, and 20-30% w/w of NaCl or Na.sub.2SO.sub.3. In later option, instead of conveyor (2), suitable manifold for addition of digesting chemicals is employed on the top of digester (10).

[0095] Within the operationally acceptable digestion temperature range of 70-100 C., the optimal range is 94-98 C.

[0096] The digestion is the most important part of overall process of the invention. Dissolution of noncellulosic substances from the dust-free grass-like plant feedstock takes place during mass transfer from the top of digester (10) to its bottom; this transfer eventually occurs only by the gravity that lasts from 40 minutes to 2 hours which, in the same time, represents the average duration of the digestion in this invention. During this time, the dissolution of noncellulosic substances is facilitated by the digestion chemicals.

[0097] At the top zone of the digester (10) the suspension is consisting of fresh comminuted grass-like plant material in a liquid phase of solution of digesting chemicals, which is commonly called the white liquor. As the plant material is transferred by the gravity through the suspension from the top to the bottom of the digester, what is accompanied with progression of the digestion process, the final product that is concentrated (precipitated) at the bottom of the said digester is consisting of cooked cellulose pulp which is suspended in the used chemicals, usually termed black liquor.

[0098] The cooked pulp which is concentrated at the bottom of the digester (10) is continuously, by approximately equal rate as being feedstock fed into digester, discharged from the bottom of said digester (10) by the conveyor (3), which is positioned next to the digester (10) vertically, and forms, together with the vertical digester (10), a U-type vessel, thus forming the hydraulic pressure compensation. This type hydraulic pressure compensation is know in the art and enables continuous extraction of the cooked pulp.

[0099] Thanks to this technical design, the level of the digesting suspension in the digester (10) is approximately the same as is the level of cooked pulp in vertical part of conveyor (3) that transfers the cooked pulp to the further processing in the disperger (11).

(iii) Disperging

[0100] The cooked pulp is further processed in the disperger (11), in order to separate mutually connected plant fibers from the pulp into fine suspension of separated fibers. Thus processed pulp is collected into auxiliary vessel (12). The suspension in this phase is of concentration range of 15-18% w/w of dry matter.

(iv) Dilution

[0101] The disperged pulp is discharged by a conveyor (4) into the dilution vessel (13), in which, the suspension is diluted with addition of fresh water via manifold (44) and regenerated water that comes from a manifold (32), to the concentration of 3-6% of dry matter (pulp). Such diluted pulp suspension is transferred by a line (33) into the auxiliary vessel (14).

(v) Screening and Fractionation

[0102] The suspension of pulp of 3-6% w/w concentration is pumped from the auxiliary vessel (14) by the line (34) to the screening and fractionating device (15) equipped with 0.1-0.5 mm sieve.

[0103] In this device, the pulp is screened and fractioned into two distinct fractions: [0104] (a) the first fraction, that does not passed through the 0.1-0.5 mm screen, in amount of maximally 50% w/w, that is further transferred by line (36) and processed by milling in 1-3 mills (16), and then: [0105] returned back to the auxiliary vessel (14) by the line (36) in step (iv), for the reprocessing via steps (iv) and (v); or, optionally, [0106] transported by line (37), which is operated by the valve (51), directly into the auxiliary vessel (17) for further processing; and [0107] (b) the second fraction; that does passed through the 0.1-0.5 mm screen, in amount of minimally 50% w/w, which is considered as a good material for further processing; this material is transported by the line (35) directly into the auxiliary vessel (17), which serves for collection of processed, yet not concentrated pulp.

(vi) Concentration, Dilution, and Preparation of Pulp for Further Production

[0108] The suspension of pulp, of concentration of 3-6% w/w, is transferred from the auxiliary vessel (17) by line (38) to dewaterer (18). In this device, the excess of liquid phase is removed from the pulp yielding: [0109] (a) concentrated pulp suspension of concentration of up to 30% w/w; and [0110] (b) regenerated water phase (black liquor) that contains also a traces of digesting chemicals, and, which is regenerated back to the process by manifold (32) to either digester (10) or dilution vessel (13).

[0111] After concentration in dewaterer (18), the cellulose pulp is transferred by line (39) into the auxiliary vessel (20).

[0112] Auxiliary vessel (20) serves for collection of concentrated, or optionally bleached, pulp of concentration of up to 30% w/w, and its dilution to the working concentration of 3-6% w/w, which is required for further paper manufacturing. In this manner, the auxiliary vessel (20) is equipped with addition manifold (43) for fresh water addition.

[0113] Finally, the pulp suspension is transferred from auxiliary vessel (20) by the line (41) to the connected vessel (21, 22), wherein the pulp suspension is diluted up to the concentration of 3-6% w/w, and prepared to reach acceptable hydrostatic pressure for further processing in the paper manufacturing machine. The role of connected vessel (21, 22) is to integrate the fluctuation of the pulp via maintaining its hydrostatic pressure constant in the manner know in the art.

[0114] Final cellulose pulp suitable for paper manufacturing is pumped/transported from the connected vessel (21, 22) by the line (42) to the further processing into unbleached (brown) paper.

[0115] In the case that final cellulose pulp is not used for paper manufacturing, then, the suspension that comes out from the auxiliary vessel (20) is processed by conventional pressing and drying to yield unbleached pure cellulose in the form of sheets (not shown in the FIG. 2).

(vii) Pulp BleachingOptional

[0116] In the case of manufacturing of bleached pulp for manufacturing of white papers or bleached cellulose sheets, the concentrated pulp suspension is transferred by line (39) to the bleaching vessel (19), wherein the pulp is bleached by any conventional process, e.g. by either hydrogen peroxide (H.sub.2O.sub.2) or sodium hypochlorite (NaOCl)-based technologies. This is controlled by the valve (52); closed valve direct the concentrated cellulose pulp into the bleaching vessel (19). The bleaching process yields in bleached cellulose pulp that is further transferred from the bleaching vessel (19) by the line (40) and the part of line (39) into the auxiliary vessel (20); FIG. 2.

[0117] As described above for unbleached cellulose pulp, in this case, the bleached cellulose pulp is also diluted with fresh water in the auxiliary vessel (20) down to the concentration of 3-6% w/w, and prepared in connected vessel (22, 23) for further production of white paper.

[0118] Alternatively, the bleached cellulose pulp is processed by conventional pressing and drying into the bleached sheets of pure cellulose, that is not shown in the FIG. 2.

Grass-Like Feedstock

[0119] The grass-like feedstock that can be used in the production of cellulose pulp according to this invention includes stems of plant species selected from the group consisting of: wheat/Triticum vulgare, Linne/; rice/Oryza sativa, Linne/; barley/Horedum vulgare, Linne/; oat/Avena sativa, Linne/; flax/Linum usitatissimum, Linne/; maize/Zea mays, Linne/; millets: proso millet/Panicum miliaceum, Linne/, pearl millet/Pennisetum glaucum, Linne/, browntop millet/Panicum ramosum, Linne/, and barnyard/Echinochloa frumentaceae, Linne/; triticaleTriticosecale, Wittm. ex A. Camus/; buckwheat/Fagopyrum esculentum, Moench/; miscanthus/Miscanthusgiganteus, Andersson/; switchgrass/Panicum virgatum, Linne/; sorghum/Sorghum species, Linne/; common reed/Phragmites australis, Cav./, giant reed/Arundo donax, Linne/, burma reed/Neyraudia reynaudiana, Kunth./, reed-mace/Typha spp., Linne/, paper reed/Cyperus papyrus, Linne/, bur-reed/Sparganium spp., Linne/, thatching reed/Thamnochortus insignis, Linne/; esparto grass/Stipa tenacissima, Linne and Lygeum spartum, Linne/; jute/Corchorus olitorius, Linne/; bamboo/Bambusoideae spp., Linne/; bagasse; or mixtures thereof.

[0120] The preferred grass-like feedstock is miscanthus/Miscanthusgiganteus, Andersson/.

[0121] The cellulose pulp from this invention is further manufactured into the paper. The latter is of significantly improved properties than is the paper manufactured from conventional wood-based feedstock obtained, for instance, from poplar/Populus alba, Linne/by the conventional technology, other than is the process from this invention.

[0122] Comparative results of key parameters of the paper manufactured from the cellulose pulp from this process obtained from 100% miscanthus/Miscanthusgiganteus, Andersson/feedstock, in comparison with conventional poplar-based paper, are given in Table 1.

TABLE-US-00001 TABLE 1 Comparative results of papers obtained from the cellulose pulp from the process of this invention (column 4) in comparison with conventional poplar-based paper manufactured by the conventional cellulose pulp (column 3). Conventional 100% miscanthus- poplar-based paper based paper - (this No. Paper parameter (state-of-the art) invention) 1 Freeness (mlCSF) 250 126 2 WRV (%) 217 3 Breaking length (m) 2.800 4.095 4 Scott Bond (J/m.sup.2) 31 5 Tear (mN m.sup.2/g) 3.6 6 Bulk (cm.sup.3/g) 2.4 2.2 7 Shives 0.1 0.5 (S-ville 0.15 mm; %)

[0123] The results from the development phase show that, in comparison with conventional wood-based processes, the technology from the present invention does result in 44% less (thermal) energy consumption at the same material output of cellulose pulp.

INDUSTRIAL APPLICATION

[0124] This invention is used as the manufacturing technology for production of cellulose pulp for paper making. Thus, industrial applicability of this invention is obvious.

LIST OF REFERENCES

[0125] 1conveyor [0126] 2conveyor [0127] 3conveyor [0128] 4conveyor [0129] 10digester [0130] 11disperger [0131] 12auxiliary vessel [0132] 13dilution vessel [0133] 14auxiliary vessel [0134] 15screening and fractionation device [0135] 16pulp mill [0136] 17auxiliary vessel [0137] 18dewaterer [0138] 19bleaching vessel [0139] 20auxiliary vessel [0140] 21,22connected vessel [0141] 30fresh water manifold [0142] 31steam manifold [0143] 32regenerated water manifold [0144] 33line [0145] 34line [0146] 35line [0147] 36line [0148] 37line [0149] 38line [0150] 39line [0151] 40line [0152] 41line [0153] 42line [0154] 43fresh water manifold [0155] 44fresh water manifold [0156] 51valve [0157] 52valve [0158] Mdriving electromotor