WO2025216985A1 - Digester aid for the kraft process - Google Patents

Abstract

A useful digester aid for a Kraft pulping process is an aromatic polycarboxylic acid that: (a) contains an aromatic moiety having on average at least 2 carboxylic acid groups per molecule pendant from the aromatic moiety; and (b) has an electron affinity below -2.80eV and at least -3.6eV. or is a functional equivalent of the aromatic polycarboxylic acid.

Description

DIGESTER AID FOR THE KRAFT PROCESS

FIELD

This invention relates to the field of pulp and paper-making.

INTRODUCTION

Wood and other plant matter that is used to make cellulose pulp (called lignocellulosic materials) contain cellulose intermixed with lignin. Lignin is a natural, rigid organic polymer that is insoluble in water. To make the cellulose pulp, the lignin must be dissolved and removed, while minimizing damage to the cellulose.

The Kraft process is a common process to make cellulose pulp by removing lignin from lignocellulosic materials. Conventional Kraft processes are known and described in many publications such as US Patents 4,507,172; 5,595,628 and 11,390,990 and in Section 10.2.2 “ Kraft Pulping” in Publication AP-42, “Compilation of Air Pollutant Emissions Factors from Stationary Sources” published by the US Environmental Protection Agency (2024).

The first step in the Kraft process is a digestion step, in which the lignocellulosic material is treated under high temperature and pressure with a “cooking liquor’ that contains aqueous sodium hydroxide and sodium sulfide. Sodium hydroxide decomposes the lignin and renders it water soluble. Sodium sulfide increases the speed of the cooking reactions and reduces the degradation of cellulose caused by sodium hydroxide. Temperature of the digestion step is normally from 150°C to 170°C. The pressure of the digestion step is normally from 110 to 150 psi. The digestion time is often from 2 to 6 hours, such as 1 to 3 hours to get to the desired temperature and pressure and then 1 to 3 hours at the desired temperature and pressure.

The digestion step produces cellulose pulp with reduced lignin and a “black liquor” which is the cooking liquor with dissolved lignin, cellulose and their degradation products plus other materials that were in the lignocellulosic material. The black liquor is separated from the cellulose pulp and is sent to recovery steps described later. The cellulose pulp is washed and optionally bleached. Bleaching removes further lignin. However, chemicals used in bleaching are expensive compared to the cooking liquor and potentially more damaging to the cellulose. Therefore, it is desirable to maximize the removal of lignin and the yield of cellulose in the digestion step.

The effectiveness of the digestion step is judged by three measurements: yield, kappa number and pulp viscosity.

• “Yield” is the percentage of dry cellulose recovered after the digestion process, based on the quantity of dry lignocellulosic materials that enters the process. The production of cellulose is a primary goal of the digestion step, and so the yield is desirably as high as practical. Pulp yield in commercial facilities is typically from 49 percent to 53 percent for hardwood and from 46 percent to 49 percent for softwood. • “Kappa number” measures the quantity of lignin that remains in the recovered cellulose after the digestion process. Kappa is measured by the consumption of dilute potassium permanganate during oxidation of pulp samples in an acidic environment. One kappa number corresponds to about 0.12 weight percent lignin in the cellulose pulp. The removal of lignin is a primary goal of the digestion step, and so the kappa number is desirably as low as practical. The kappa number in commercial facilities is typically from 14 to 20 for hardwood pulps and from 25 to 30 for softwood pulps.

• “Viscosity” of pulp is measured in solution, such as 0.5% cellulose solutions using 0.5M cupriethylenediamine as a solvent. Viscosity indicates the degree of polymerization for cellulose in the pulp, and loss of viscosity indicates damage to the pulp during digestion.

The black liquor from the digestion step normally contains about 10 to 20 weight percent dissolved solids, which includes degraded and dissolved wood constituents and inorganic salts. The black liquor is concentrated in one or more evaporators and then combusted in a furnace. The combustion step burns off organic components and drives off water to produce a molten “smelt” that contains sodium hydroxide, sodium carbonate and sodium sulfide.

The smelt is dissolved in water to make “green liquor.” The green liquor is treated with calcium hydroxide, which reacts with the sodium carbonate to make more sodium hydroxide. After treatment, the resulting liquor is called “white liquor.” Sodium sulfide in the white liquor reacts with water to form sodium hydroxide and sodium hydrosulfide (NaSH). Sodium hydroxide and sodium hydrosulfide dissociate to produce hydroxide (-OH) and hydrosulfide (-SH) ions.

The white liquor can be used as cooking liquor in the digestion step, or it can be mixed with black liquor to make the cooking liquor, such as about 10 to 50 weight percent black liquor.

The potency of the white liquor and cooking liquor is measured using the following measurements:

• “Active alkali content” is the combined concentration of sodium hydroxide and sodium sulfide in the liquor. Active alkali concentration in the white liquor is typically from 140 to 170 g/1.

• “Sulfidity” is the weight percent ratio of sodium sulfide to active alkali in the liquor. Sulfidity is typically 35 percent to 45 percent.

• “Alkali charge” is the weight percent of active alkali to dry wood in the digestion step. The alkali charge is usually from 20 percent to 23 percent.

As previously discussed, it is desirable to increase yield of cellulosic pulp, reduce kappa number of lignin in the Kraft process and maintain good viscosity. It has been common to add small quantities of anthraquinone to the cooking liquor as a digester aid. For example, the addition of 0.02 to 0.05 weight percent anthraquinone can increase pulp yield by about 2 percent. However, US and European regulatory authorities have warned of health concerns associated with anthraquinone. See, for example, Hart et al., “Anthraquinone a Review to the Rise and Fall of a Pulping Catalyst”, 13( 10) TAPPI Journal 23-31 (2014). New digester aids are needed that can replace anthraquinone. SUMMARY

One aspect of this invention is a process to produce cellulose pulp from a lignocellulosic material that contains cellulose fibers and lignin, which process comprises a digestion step in which the lignocellulosic material is contacted with an aqueous cooking liquor containing alkali metal hydroxide and alkali metal sulfide in the presence of a digester aid, under conditions such that lignin or its reaction products are dissolved into the cooking liquor to leave a cellulose-containing pulp, characterized in that the digester aid is an aromatic polycarboxylic acid that:

(a) contains an aromatic moiety having on average at least 2 carboxylic acid groups pendant per molecule from the aromatic moiety; and

(b) has an electron affinity below -2.80 eV and at least -3.6 eV, or is a functional equivalent of the aromatic polycarboxylic acid.

A second aspect of this invention is a process to produce cellulose pulp from a lignocellulosic material that contains cellulose fibers and lignin, which process comprises a digestion step in which the lignocellulosic material is contacted with an aqueous liquor containing alkali metal hydroxide and alkali metal sulfide in the presence of a digester aid which is an aromatic polycarboxylic acid that:

(a) contains an aromatic moiety having on average at least 2 carboxylic acid groups per molecule pendant from the aromatic moiety; and

(b) has an electron affinity below -2.80 eV and at least -3.6 eV; or is a functional equivalent of the aromatic polycarboxylic acid, under conditions such that lignin or its reaction products are dissolved into the cooking liquor to leave a cellulose-containing pulp; and

The digester aids of this invention are an alternative to anthraquinone.

DETAILED DESCRIPTION

This invention may be used in the digestion step of a Kraft process for making cellulose pulp. The base Kraft process is well-known and summarized in the Introduction. The invention does not require changes from the known Kraft process or its equipment, aside from adding the inventive digester aids. However, slight changes in conditions may be desirable to optimize the process with the inventive digester aids.

The process begins with a lignocellulosic material, such as wood chips or other plant matter, which contains cellulose with lignin. The size of wood chips and plant matter is not critical, as long as the raw material can readily be impregnated with the aqueous liquor and can provide cellulose strands of desired length. For information on chip size, see Rajesh et al., “Chip Size Distribution - A Lot Can Happen Over Its Variation” , 22(3) J. IPPTA 93-96 (2010). In some embodiments, the average long dimension of the chips is at most 50 mm or at most 45 mm. In some embodiments, the average long dimension the chips is at least 20 mm or at least 25 mm or at least 30 mm. In some embodiments, the average thickness of chips is at least 4 mm or at least 5 mm. In some embodiments, the average thickness of chips is at most 10 mm or at most 8 mm or at most 7 mm. In some embodiments, the wood chips contain hardwood chips such as oak, maple, cherry, poplar and eucalyptus, and in some embodiments, the wood chips contain softwood chips such as pine.

The lignocellulosic material is contacted with a cooking liquor that contains alkali metal hydroxide and alkali metal sulfide or their reaction products. In some embodiments, the alkali metal is sodium, and in some embodiments, the alkali metal is potassium. In some embodiments, the alkali metal hydroxide is sodium hydroxide. In some embodiments, the alkali metal sulfide is sodium sulfide.

It is known that the sodium sulfide reacts with water in the cooking liquor to form sodium hydroxide and sodium hydrosulfide. It is also known that sodium hydroxide and sodium hydrosulfide dissociate to form sodium ions and hydroxide ions or hydrosulfide ions, respectively. The industry terminology commonly ignores these reactions and refers to the “sodium hydroxide” and “sodium sulfide” content of the cooking liquor, rather than their reaction products. This application will follow the same terminology.

In some embodiments, the active alkali content of the cooking liquor is at least 120 g/L or at least 130 g/L or at least 140 g/L. In some embodiments, the active alkali content of the cooking liquor is at most 200 g/L or at most 180 g/L or at most 170 g/L.

In some embodiments, the sulfidity of the cooking liquor is at least 25 percent or at least 30 percent or at least 35 percent. In some embodiments, the sulfidity of the cooking liquor is at most 50 percent or at most 45 percent.

In some embodiments, the alkali charge in the digestion step is at least 16 percent or at least 18 percent or at least 20 percent. In some embodiments, the alkali charge in the digestion step is at most 25 percent or at most 24 percent or at most 23 percent.

In some embodiments, the temperature of the digestion step is at least 135°C or at least 145°C or at least 150°C. In some embodiments, the temperature of the digestion step is at most 200°C or at most 180°C or at most 170°C.

In some embodiments, the pressure of the digestion step is at least 90 psi (620 kPa) or at least 100 psi (690 kPa) or at least 110 psi (750 kPa). In some embodiments, the pressure of the digestion step is at most 200 psi (1400 kPa) or at most 175 psi (1200 kPa) or at most 150 psi (1000 kPa).

The time of digestion may vary based on the conditions used and the desired cellulose yield and kappa number. Digesting for too short a time may fail to solubilize lignin and result in a high kappa number. Digesting for too long a time may cause cellulose to break down and reduce yields. In some embodiments, the digestion is performed for at least 30 minutes at the selected temperature and pressure, or at least 60 minutes or at least 90 minutes. In some embodiments, the digestion is performed for at most 4 hours at the selected temperature and pressure, or at most 3 hours.

In some embodiments, the temperature in the digestion step is ramped up slowly to minimize formation of hot spots and avoid damage to the cellulose. In some embodiments, the temperature of the digestion step is increased from ambient temperature to digestion temperature over a time period of at least 30 minutes or at least 1 hour. In some embodiments, the temperature of the digestion step is increased from ambient temperature to digestion temperature over a time period of at most 3 hours or 2 hours.

The optimum time and temperature for removal of lignin in the digestion step are inter-related. Higher temperatures require less time, and lower temperatures require more time. Time and temperature are often expressed as a single measurement called the “H factor,” which relates time and temperature to the rate of lignin removal. See Segura et al., “Effectiveness of the H-factor for Controlling Eucalyptus Kraft Pulping”, 47 (124) Sci. For., Piracicaba 791 -798 (2019) and Singh et al., “The H Factor for Bamboo Sulfate Pulping: A device to control pulp mill operation,” 8(1) IPPTA 57 (1976). The relationship of H factor to specific times and temperatures varies depending on the content of the cooking liquor and the lignocellulosic material. In some embodiments, the H factor for the digestion step is at least 800 or at least 900 or at least 950 or at least 1000. In some embodiments, the H factor for the digestion step is at most 1600 or at most 1400 or at most 1200 or at most 1100 or at most 1050.

Suitable equipment to perform the digestion step is known and commercially available. See “Types of Equipment in Pulp Cooking Process” published by Ochre Digi Media Pvt Ltd. at https://www.pulpandpapcr-tcchnology.com/articlcs/typcs-of-cquipmcnt-in-pulp-cooking-proccss. In some embodiments, the digester is made of 316L steel, stainless steel, carbon steel, duplex stainless steel, and/or composite plates. Both continuous and batch process vessels are known and can be used in this process.

The digestion is performed in the presence of a digester aid which is an aromatic polycarboxylic acid or its functional equivalent. The aromatic polycarboxylic acid: (i) contains an aromatic moiety having on average at least 2 carboxylic acid groups pendant from the aromatic moiety; and (ii) has an electron affinity that is below -2.80 eV and at least -3.6 eV.

Digestion takes place in an aqueous, highly-alkaline cooking liquor, and it is expected that the aromatic polycarboxylic acid will convert to its equivalent carboxylate anion in the cooking liquor. A functional equivalent to the aromatic polycarboxylic acid is a compound that will convert to the same carboxylate anion as the aromatic polycarboxylic acid, when the functional equivalent is added to the cooking liquor. Examples of functional equivalents to the aromatic polycarboxylic acid may include salts of the aromatic polycarboxylic acid such as alkali metal salts, esters of the aromatic polycarboxylic acid such as lower (Ci to C&) alkyl esters, and anhydrides of the aromatic polycarboxylic acid such as cyclic anhydrides. For example, functional equivalents of pyromellitic acid may include pyromellitic anhydride, tetrasodium pyromellitate and pyromellitic acid tetramethyl ester. However, the requirement that the electron affinity must be below -2.80 eV and at least -3.6 eV refers only to the aromatic polycarboxylic acid form of the digester aid and not to its functional equivalents.

Suitable aromatic polycarboxylic acids may meet Formula 1 :

(1) Ar-(CO₂H)_X wherein Ar is an aromatic moiety and x is a number of pendant carboxylic acid groups per molecule equal on average to at least 2. Examples of suitable aromatic moieties (Ar) include phenyl, naphthyl and biphenyl moieties, and moieties having alkyl substitutions such as tolyl and cumenyl moieties. In some embodiments, the aromatic moiety comprises a single aromatic ring. In some embodiments, the aromatic moiety comprises two or more unfused aromatic rings. In some embodiments, the aromatic moiety comprises two or more fused rings. In some embodiments, the aromatic moiety is unsubstituted. In some embodiments, the aromatic moiety contains one or more pendant alkyl substituents, such as methyl, ethyl, n-propyl, isopropyl or t-butyl groups. In some embodiments, the aromatic moiety contains a pendant nitro group.

In some embodiments, the aromatic polycarboxylic acid contains on average at least 2.5 pendant carboxylic acid groups per molecule or at least 3 carboxylic acid groups or at least 3.5 carboxylic acid groups or at least 4 carboxylic acid groups, (“x” is on average at least 2.5 or 3 or 3.5 or 4.) In some embodiments, the aromatic polycarboxylic acid contains on average at most 6 pendant carboxylic acid groups per molecule or at most 5 carboxylic acid groups or at most 4 carboxylic acid groups, (“x” is on average at most 6 or 5 or 4.)

In some embodiments, the aromatic polycarboxylic acid has an electron affinity of at most -2.82 eV or at most -2.85 eV or at most -2.9 eV or at most -3.0 eV or at most -3.1 eV or at most -3.2 eV. In some embodiments, the aromatic polycarboxylic acid has an electron affinity of at least -3.5 eV or at least -3.4 eV or at least -3.35 eV or at least -3.3 eV. For example, the electron affinity of the aromatic polycarboxylic acid may be from -2.85 eV to -3.5 eV or from -2.9 eV to -3.4 eV or from -3.0 eV to -3.3 eV.

In some embodiments, the digester aid has a negative test for causing genetic mutation under the Ames test.

Examples of suitable digester aids include 1,2,4,5-benzenetricarboxylic acid (also called pyromellitic acid) and its functional equivalents.

The quantity of digester aid should be suitable to improve cellulose pulp yield, kappa number and/or pulp viscosity as compared to digestion with no digester aid. In some embodiments, the cooking liquid contains at least 0.01 weight percent digester additive, or at least 0.02 weight percent or at least 0.03 weight percent or at least 0.04 weight percent or at least 0.05 weight percent. Too much digester aid can add cost that exceeds the benefit provided by the digester aid. In some embodiments, the cooking liquid contains no more than 0.5 weight percent digester aid, or no more than 0.3 weight or no more than 0.2 weight or no more than 0.1 weight percent or no more than 0.08 weight percent or no more than 0.06 weight percent.

In some embodiments, the cooking liquor contains less than 0.05 weight percent anthraquinone or less than 0.02 weight percent or less than 0.01 weight percent. In some embodiments, the cooking liquor contains essentially no (0 weight percent) anthraquinone.

After the digestion step is completed, the cooking liquor can be separated from the cellulose pulp by known methods, such as filtering or centrifuging. The cellulose pulp may be further processed, such as by washing, bleaching and/or drying according to known methods. The cooking liquor, which is now black liquor, can be sent for further processing and recovery according to known methods. Depending on cooking conditions, the yield of pulp obtained using the digester aid may be similar to a similar digestion without the digester aid, or 1 percent higher or 2 percent higher or 3 percent higher. In some embodiments, the yield of pulp from softwood is at least 46 percent or at least 47 percent or at least 48 percent. In some embodiments, the yield of pulp from hardwood is at least 49 percent or at least 50 percent or at least 51 percent or at least 52 percent. There is no maximum desired pulp yield, but in some cases yields above 70 percent may be uneconomical to obtain.

Depending on cooking conditions, the kappa number obtained using the digester aid may he similar to a similar digestion without the digester aid, or 0.5 lower or 1 lower or 2 lower. In some embodiments, the kappa number for hardwood is at most 19 or at most 18 or at most 17 or at most 16. In some embodiments, the kappa number for softwood is at most 28 or at most 26 or at most 25 or at most 23 or at most 21 or at most 20. There is no minimum desired kappa number, but in some cases kappa numbers below 10 may be uneconomical to obtain prior to bleaching. In some embodiments, higher kappa numbers may be acceptable to obtain higher yield, or lower yield may be acceptable to obtain lower kappa number.

Depending on conditions of use, the digester aids of this invention can improve the yield or kappa number or both for a digestion step. Properly selected digester aids may have little or no environmental or health concerns, at the residual concentrations for which they remain in the cellulose pulp.

Test Methods

Unless stated otherwise, measurements listed in this application are made using the following test methods:

Calculation of Electron Affinity: The electron affinity (EA) for a molecule is computed by calculating the difference in energy of the neutral molecule and the corresponding anion, using Gaussian 09 software, from Gaussian Inc. • Ground geometries and vibrational frequencies of the neutral molecule and its corresponding anion (where an electron is added to the neutral molecule) are calculated using density functional theory (DFT) at the B3LYP level of theory. See: A. D. Becke, “Density-Functional Thermochemistry. I. The Effect of the Exchange-Only Gradient Correction,” 96 J. Chem. Phys. 2155-60 (1992) and A. D. Becke, “Density-Functional Thermochemistry. II. The Effect of the Perdew- Want; Generalized-Gradient Correlation Correction,” 97 J. Chem. Phys. 9173-77 (1992)).

• The calculation uses 6-31G* basis sets. See Petersson, et al, “A Complete Basis Set Model Chemistry. I. The Total Energies of Closed-Shell Atoms and Hydrides of the First-Row Atoms,” 89 J. Chem. Phys. 2193-218 (1988) and Petersson et al, “A Complete Basis Set Model Chemistry. II. Open-Shell Systems and the Total Energies of the First-Row Atoms,” 94 J. Chem. Phys. 6081-90 (1991).

• The effect of a dielectric medium (E = 78.3553) is calculated using a conductor like polarizable continuum model (cpcm). See: V. Barone et al., “Quantum Calculation of Molecular Energies and Energy Gradients in Solution by a Conductor Solvent Model,” 102 J. Phys. Chem. A 1995- 2001 (1998) and M. Cossi, et aL, “Energies, Structures, and Electronic Properties of Molecules in Solution with the C-PCM Solvation Model,” 24 J. Comp. Chem. 669-81 (2003).

• During the geometry optimization of neutral and anions, various conformations of the neutral molecule and its corresponding anion are evaluated. Reported energies or structures correspond to the minimum in the potential energy surface of a chosen conformation. For molecules with different possible conformations, the computed EA of at least one conformation will be within the chosen range (-2.8 eV to -3.4 eV).

Examples

The following examples illustrate specific embodiments of the invention, but do not limit the broadest scope of the invention.

Commercial mixed hardwood chips for pulping are obtained. The chips have an average length and width of 1.5 in to 2 in (38 mm to 50 mm) and an average thickness of 5 to 7 mm. The wood chips are oven dried.

A cooking liquor is made whose active alkali content is about 20% NaOH and sulfidity is about 24.6% (NazS). In Comparative Example A (CE A), no digester aid is added. In Comparative Example B (CE B), 0.2 weight percent anthraquinone is added. In Comparative Example C (CE C), 0.2 weight percent 2-(2-hydroxy-4-methoxybenzoyl)benzoic acid is added. In Inventive Example 1 (IE1), 0.2 weight percent pyromellitic acid is added. In Inventive Example 2 (IE2), 0.2 weight percent of 5-nitroisophthalic acid monoethyl ester is added.

Each cooking liquor is loaded into a laboratory digester with 750 g of wood chips in a quantity to provide an alkali charge of 17.5 percent and is sealed in. Each sample is heated to 170°C over a period of about 70 minutes and then cooked at 170°C for 65 minutes to a constant h-factor of 1133. The resulting pulps are cooled, washed and dried. Yield, viscosity and kappa numbers are measured. Results are shown in Table 1.

Table 1

Claims

CLAIMS: We claim:

1. A process to produce cellulose pulp from a lignocellulosic material that contains cellulose fibers and lignin, which process comprises a digestion step in which the lignocellulosic material is contacted with an aqueous cooking liquor containing alkali metal hydroxide and alkali metal sulfide in the presence of a digester aid, under conditions such that lignin or its reaction products are dissolved into the cooking liquor to leave a cellulose-containing pulp, characterized in that the digester aid is an aromatic polycarboxylic acid that:

(a) contains an aromatic moiety having on average at least 2 carboxylic acid groups per molecule pendant from the aromatic moiety; and

(b) has an electron affinity below -2.80 eV and at least -3.6 eV, or is a functional equivalent of the aromatic polycarboxylic acid.

2. The process of Claim 1, wherein the aromatic polycarboxylic acid meets formula 1:

(1) Ar-(CO₂H)_X wherein Ar is an aromatic moiety, and x is a number of pendant carboxylic acid groups per molecule equal on average to at least 2, and wherein the cooking liquor contains at least 0.01 weight percent digester aid.

3. The process of Claim 2 wherein the aromatic moiety (Ar) comprises a single phenyl ring.

4. The process of Claim 3 wherein the aromatic moiety (Ar) has no pendant substitutions other than the pendant carboxylic acid groups.

5. The process of Claim 2 where in the digester aid contains on average from 3 to 5 pendant carboxylic acid groups per molecule.

6. The process of Claim 2 wherein the electron affinity of the digester aid (in acid form) is no more than -2.9 eV.

7. The process of Claim 2 wherein the electron affinity of the digester aid (in acid form) is from -3.0 eV to -3.5 eV.

8. The process of Claim 2 wherein the digester aid comprises the acidic form of the aromatic polycarboxylic acid.

9. The process of Claim 2 wherein the digester aid comprises an anhydride, salt or ester of the aromatic polycarboxylic acid.

10. The process of Claim 2 wherein the digester aid comprises pyromellitic acid or its functional equivalent.

11. The process of Claim 10 wherein the cooking liquor contains from 0.03 to 0.1 weight percent of pyromellitic acid or an equivalent amount of its salt derivative.

12. The process of Claim 2 wherein the cooking liquor contains from 0.03 to 0.1 weight percent of the digester aid.

13. The process of any one of Claims 1 through 12 wherein:

(a) The alkali metal hydroxide is sodium hydroxide, and the alkali metal sulfide is sodium sulfide;

(b) The cooking liquor has an alkali charge from 16 percent to 24 percent;

(c) The cooking liquor has an active alkali content from 130 g/L to 180 g/L;

(d) The cooking liquor has a sulfidity from 25 percent to 50 percent; and

(e) The temperature of digestion is from 145°C to 180°C.

14. The process of Claim 13 wherein the time and temperature provide an H factor from 900 to 1400.

15. The process of Claim 13 wherein the cooking liquor contains less than 0.02 weight percent anthraquinone.