WO2009048525A2

WO2009048525A2 - Selectivity improvement in oxygen delignification and bleaching of lignocellulose pulp using singlet oxygen

Info

Publication number: WO2009048525A2
Application number: PCT/US2008/011403
Authority: WO
Inventors: Martha G. Hollomon; Huai N. Cheng
Original assignee: Hercules Incorporated
Priority date: 2007-10-05
Filing date: 2008-10-02
Publication date: 2009-04-16
Also published as: TW200934924A; US20090090478A1; WO2009048525A3

Abstract

A process to enhance delignification or bleaching in chemical or mechanical pulps comprising generation of singlet oxygen as a separate step and subsequent transport of singlet oxygen to pulp to effect bleaching or brightening of the pulp.

Description

Selectivity Improvement in Oxygen Delignification and Bleaching of Lignocellulose Pulp Using Singlet Oxygen

FIELD OF INVENTION

[0001] The present invention relates to a process for delignification and bleaching of lignocellulose pulp and improving selectivity by means of which a stream of molecular oxygen is treated with visible light and a chemical agent generating singlet oxygen and then applying the stream of singlet oxygen to pulp during the pulping procedure.

BACKGROUND OF INVENTION

[0002] Oxygen delignification is the use of molecular oxygen (i.e., ordinary oxygen) and alkali to remove a substantial fraction of lignin in unbleached chemical pulp before conventional bleaching. Oxygen bleaching is considered synonymous with oxygen delignification. The process is usually conducted with molecular oxygen under high pressure and at elevated temperatures. Molecular oxygen is usually applied to kraft unbleached pulps but can also be used for other chemical pulps. Oxygen delignification is now a commercial process, and a number of patents have been published. These patents describe the use of molecular oxygen, arrangements, number of stages, process conditions — time and temperature, and addition of alkali/oxygen charge.

[0003] Oxygen bleaching typically is not applied to mechanical pulp. In general, mechanical pulps are bleached with a reducing agent, such as sodium hydrosulfite (sodium dithionite) or an oxidizing agent, such as hydrogen peroxide, or a combination thereof. [0004] Oxygen delignification involves a rather complex chemistry. Under alkaline conditions at high pressure, oxygen reacts to form both free radicals and anions, including possibly superoxide radicals, peroxide anions, and hydroxyl radicals. These species interact with lignin in a complex fashion; however they can also degrade cellulose causing yield loss and decreased pulp strength.

[0005] Degradation of the cellulose resulting in yield loss can be classified into two categories: random chain cleavage and the "peeling" reaction. Random chain cleavage which may occur at any point along the polymeric chain is more significant in oxygen delignification due to the amount of free radicals present. "Peeling" causes monosaccharide units at the end of the chain to be attacked and successively removed. Peeling is generally not an issue in kraft pulp since the acidic chain ends have been converted to the more stable oxidized form. However, when random chain cleavage is excessive, peeling can become a problem since two new chain ends are formed which have a reducing end group [0006] The chemistry of oxygen is unique. Molecular oxygen has a normal (lowest energy) configuration that is a triplet state containing two unpaired electrons, i.e. a free radical. However, the first excited state of oxygen molecules is a singlet state. The lifetime of singlet oxygen in solution is in the microsecond range (3 μsec in water) but is extremely long lived (72 minutes) in the gaseous phase. Singlet oxygen can react selectively with carbon-carbon double bond either by the Diels-Alder reaction or by the ene reaction. Native lignin or modified lignin after various pulping processes contain functional groups such as olefϊnic, phenolic, and non-phenolic groups, which can potentially react with singlet oxygen (Frimer, Singlet O₂, Volume W - Polymers and Biomolecules, pp. 21-24, CRC Press, 1985). [0007] Prior art on singlet oxygen chemistry in the pulp and paper industry generally falls into one of the following categories:

1. Processes that generate singlet oxygen in situ by using chlorine-containing compounds in combination with oxygen-containing compounds (U.S. Patent 4,008,120; Szabo et al, Cellulose Chem. Technol, 28, 183, 1994). In these processes, bleaching can occur from both the chlorine-containing compounds (e.g., hypochlorite) and from the singlet oxygen.

2. Processes of direct irradiation of the lignocellulosic pulp mixture in a pH between 8 and 13 with ultraviolet light in the presence of oxygen (Turner, U.S. Pat. No. 4,294,654). Turner's Example 1 suggests that a photosehsitizer is not required for his process, and in fact the reported results are the same with or without photosensitizer in his process. Direct irradiation of pulp using specific ultraviolet light at 300-420 nm to facilitate bleaching has been previously reported in U.S. Pat. No. 2,161,045.

3. Processes using corona (or other) discharge methods to produce "active" gas, which is used for delignification and bleaching of chemical and mechanical pulp at a consistency of 15-95% (U.S. Pat. No. 3,806,404, Liebergott et al). The use of activated oxygen with softwood was disclosed by Liebergott in Example 1, but the decrease in Kappa number was modest. As pointed out by Turner (U.S. Pat. No. 4,294,654), the activated nitrogen as found by Liebergott was extremely effective, while activated oxygen under the same reaction conditions was far less effective.

4. Processes using pure model lignin compounds using singlet oxygen generated via photo-oxygenation, primarily related to color reversion of cellulosics. (G. Gellerstedt, K. Krinstad, and E. L. Lindfors, "Singlet Oxygen Oxidation of Lignin Structures" in Singlet Oxygen reaction with organic compounds and polymers, eds. B. Ranby and J. F. Rabek, pp. 302-310, John Wiley & Sons, 1978)

[0008] Liebergott dealt with the bleaching of chemical pulps by the use of a high consistency gaseous phase bleaching process. The high consistency is defined as 15-95% (i.e., 5 -85% water). In their own words, the "'active' or 'electronically excited' singlet oxygen or high energy oxygen triplet is ... produced by passing the respective gas...through an electrode-less discharge, or through a microwave discharge, or through a condensed pulsed discharge or by a 'plasma jet.'" The use of electronically excited states of oxygen with softwood is disclosed by Liebergott in Example 1-(1), where 'oxygen, at a rate of 0.009 liter/min, at 55 kPa, was passed through a corona discharge, where it was acted upon by a primary potential of 120 volts.' However, they indicated that gaseous mixture formed in their process is not singlet oxygen, but ozone. Furthermore, as pointed out by Turner, the data in Table I in Liebergott indicate that activated oxygen [generated per Liebergott's process] was only marginally effective in delignifying and bleaching lignocellulosic pulps. Reference to Table I of the Liebergott patent indicates that subsequent to bleaching the Kappa number of the pulp was 22.6, which represents a reduction of only 1.4 units, which translates into a percentage reduction of only 5.8%.

[0009] Finally, Szabo suspended pulp in an aqueous solution OfH₂O₂ and then add

NaClO to the solution. H₂O₂ and NaClO react chemically to generate about 10% singlet oxygen. This is a single-step process. Furthermore, there is no assurance that residual H₂O₂ or NaClO may not carry out some bleaching. Thus, the observed bleaching results may not necessarily be due to singlet oxygen. In addition, their data (Table 2) did not show any significant increase in activity of H₂0₂/NaC10 versus NaClO alone. In fact, NaClO alone shows higher delignification and higher selectivity than H₂O₂/NaClO. BRIEF DISCRIPTION OF THE INVENTION

[0010] Disclosed is a novel process to enhance delignification or bleaching in chemical or mechanical pulps comprising photochemical generation of singlet oxygen in a separate step and subsequent transport of the singlet oxygen to pulp to achieve bleaching or brightening. Preferably, we enhance bleaching of lignocellulose pulp in the oxygen delignification process by treating a stream of molecular oxygen with visible light and at least one chemical agent converting molecular oxygen into singlet oxygen and applying the resulting stream of singlet oxygen to pulp during the pulping procedure.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 is a block diagram of a laboratory system using ambient pressure for producing singlet oxygen that may be used to enhance delignification or bleaching in accordance with one embodiment of the invention.

[0012] Figure 2 is a block diagram of a laboratory system using high pressure for producing singlet oxygen that may be used to enhance delignification or bleaching in accordance with one embodiment of the invention.

[0013] Figure 3 is a block diagram of a possible industrial scale system using high pressure with a five chamber device for producing singlet oxygen that may be used to enhance delignification or bleaching in accordance with one embodiment of the invention. More or less chambers can also be realized.

[0014] Figure 4 is a block diagram of a portion of a possible two-stage industrial system using high pressure for producing singlet oxygen that may be used to enhance delignification or bleaching in accordance with one embodiment of the invention. The same setup with one or multiple stages can also be realized. DETAILED DESCRIPTION OF THE INVENTION

[0015] The present invention provides for a process for the delignification or bleaching of lignocellulosic pulp comprising the steps of a) providing an aqueous or non-aqueous mixture of pulp, b) providing a stream of molecular oxygen, reacting the molecular oxygen in the presence of visible light and at least one chemical agent photochemically converting a portion of the molecular oxygen to singlet oxygen and c) subsequently contacting pulp or pulp mixture with the singlet oxygen stream.

[0016] The present invention also provides for pulp or paper product produced using singlet oxygen.

[0017] The conversion of raw lignocellulosic material to unbleached and then bleached pulp requires an extremely complex and intricate series of chemical reactions and physical processing, usually requiring two or more stages in which different reactions are involved. The first is referred to as pulping, and the second as bleaching. Both, however, include delignification.

[0018] Chemical or mechanical treatments of wood or other plant materials are used to generate pulp, which is then made into paper products. In chemical pulping, a "digestion" process is used (e.g., kraft or sulfite) where solutions of various chemicals dissolve or modify lignin, non-fibrous materials and other impurities to liberate pulp. In mechanical pulping, a mechanical grinding motion separates cellulose-containing fibers from a wood matrix. One method of mechanical pulping uses a disc refiner, which typically has two metal plates where at least one of the plates is rotating at high speed; the wood chips is fed into the refining area and are broken down at high pressure and temperature between the center (eye) of the refiner and the other refiner zones. Another method (called "groundwood") is to press the wood against the face of a rapidly revolving grindstone, where the abrasive action tears the fibers from wood matrix. Many variations of these methods are known, e.g., semi-chemical, thermo-mechanical, and chemi-thermo-mechanical pulping.

[0019] "Oxygen delignification" is the use of molecular oxygen and alkali to remove a substantial fraction of lignin in unbleached chemical pulp before conventional bleaching. Oxygen bleaching is considered synonymous with oxygen delignification. The process is usually conducted with molecular oxygen under high pressure and at elevated temperatures. Molecular oxygen is usually applied to unbleached kraft pulp but can also be used for other chemical pulps. [0020] Oxygen delignification as single-stage or multiple-stage process is well known in the art. Oxygen bleaching typically is not applied to mechanical pulp. The present invention can be used at one of more stages in an oxygen delignification process. [0021] In general, mechanical pulps are bleached or brightened with a reducing agent, sodium hydrosulfϊte (sodium dithionite) or an oxidizing agent, such as hydrogen peroxide, or a combination of sodium hydrosulfite and hydrogen peroxide.

[0022] The lignocellulosic pulp employed in the present process can be prepared from any lignocellulose-containing material derived from natural sources such as, but not limited to, hardwood, softwood, gum, straw, bagasse and/or bamboo by various chemical, semichemical, mechanical or combination pulping processes. Chemical and semichemical pulping processes include, but not limited to kraft, modified kraft, kraft with addition of sulfur and/or anthraquinone, and sulfite. Mechanical pulping processes include, but not limited to stone groundwood, pressurized groundwood, refiner mechanical, thermo-refiner mechanical, pressure refined mechanical, theπno-mechanical, pressure/pressure thermo- mechanical, chemi-refiner-mechanical, chemi-thermo-mechanical, thermo-chemi-mechanical, thermo-mechanical-chemi, and long fiber chemi-mechanical pulp. Handbook for Pulp and Paper Technologist, ed. G. A. Smook (Atlanta, GA, TAPPI Press, 1989) describes both chemical and mechanical pulping.

[0023] The term "oxygen" or "diatomic oxygen" is considered synonymous with molecular oxygen (i.e., ordinary oxygen). Oxygen has an electron configuration of an open- shell triplet ground state ( O₂) having two unpaired electrons occupying two degenerate molecular orbitals and is one of two major components of air.

[0024] The term "singlet oxygen" refers to the first electronically excited state of oxygen (¹O₂) (also known as singlet delta - a'Δ_g) in which all the electron spins are paired. Singlet oxygen has a higher energy than triplet ground state oxygen. Singlet oxygen has a limited lifetime in solution (microsecond range) and gaseous phase (less than 2 hours). [0025] The present invention provides for a method to generate singlet oxygen that consists of the steps of providing a molecular oxygen source, a visible light source, a chemical agent wherein the molecular oxygen source provides a stream of molecular oxygen into a chamber or a series of chambers containing the chemical agent, contacting the molecular oxygen with the chemical agent in the presence of visible light from the light source.

[0026] The singlet oxygen employed in the process of the invention is generated with visible light and a chemical agent. Molecular oxygen in the presence of a photosensitive chemical agent is exposed to visible light in a chamber resulting in the generation of singlet oxygen. The singlet oxygen stream is then brought into contact with the pulp mixture. The singlet oxygen reacts with the pulp resulting in delignification.

[0027] Visible light employed in the process of the invention has a wavelength between

400 nm to 700 nm. Sources of visible light are selected from the group consisting of, but are not limited to, halogen lamps, tungsten or fluorescent lamps, light emitting diodes (LED), lasers, or combination thereof. Any light source with filters to limit the wavelength to visible light may be used.

[0028] The method of the invention provides increased selectivity of the oxygen delignification process by converting a portion of the molecular oxygen to singlet oxygen. Selectivity is defined as the ratio of the intrinsic viscosity of the pulp versus the Kappa number of the pulp. Intrinsic viscosity is a measure of cellulose molecular weight and cellulose degradation. Kappa number is a well known indicator of the lignin content or bleachability of pulp. It indicates the amount of bleach needed during digestion of wood pulp to obtain a pulp with a given level of whiteness. A higher selectivity indicates a better process, and more desirable pulp product. The present invention provide for an improved selectivity based on molecular weight data of at least about 9%.

[0029] Chemical agents useful in the present invention include, but are not limited to, photo sensitizer dye, pigment, aromatic hydrocarbon, coenzyme or biochemical, metallic salt, and transition metal complex. Photosensitizer dyes include, but are not limited to, methylene blue, rose bengal, eosin, crystal violet, and acridine orange. Pigments useful in the present invention include, but are not limited to, chlorophyll, hematoporphyrines, and flavin. Aromatic hydrocarbons useful in the present invention include but are not limited to rubrene and anthracene. Coenzymes or biochemical agent useful in the present invention include, but not are limited to, pyridoxals and psoralenes. Metallic salts useful in the present invention include, but are not limited to, cadmium sulfide and zinc sulfide. Transition metal complexes useful in the present invention include, but are not limited to, ruthenium and bipyridine. The chemical agents useful in the present invention can absorb light in the 380 - 900 nm, preferably 400 - 700 nm range (visible light). The chemical agent can be affixed to gas filters, glass beads, wire mesh, or a catalytic bed.

[0030] The photosensitizer dye exhibits fluorescence and phosphorescence reflecting two separate electronically excited states, namely, the singlet state and the triplet state. The singlet state is produced first by the absorption of light, but it has a short lifetime, decaying by fluorescence to the ground state and by electronic intersystem crossing to the triplet state. The triplet state of these photosensitizers decays to the ground state at a slower rate. The most effective photosensitizers have a high quantum yield of a long-lived triplet state. [0031] The conversion of molecular oxygen into singlet oxygen is dependent on the solvent and chemical agent used and measured as quantum yield. For example, quantum yield for singlet oxygen formation Φ('θ₂) = 0.76 for rose bengal in methanol. The quantum yield for the formation of the excited triplet state of rose bengal is 0.76. Therefore, all triplet oxygen (molecular oxygen) is converted into singlet oxygen. It is preferred that a least 10% of the molecular oxygen is converted into singlet oxygen. The conversion of molecular oxygen into singlet oxygen can be from 5% to 100%. Preferably the conversion of molecular oxygen into singlet oxygen is 10% to 100% and most preferably the conversion of molecular oxygen into singlet oxygen is 20% to 100%. The supply of molecular oxygen and/or air into the chamber containing the chemical agent(s) and visible light contains preferably less than 6 molar ppm of water (H₂O), preferably less than 3.5 ppm.

[0032] In a kraft oxygen delignifϊcation system, singlet oxygen may be generated and added either to the "mixer" before to the first oxygen reactor and/or added in the main charge position before the first oxygen reactor in the reactor system and separate from the steam line. [0033] In mechanical pulps, the singlet oxygen can be applied to the mechanical pulp during the pulping process at the eye of a mechanical pulp refiner or through the dilution water for stone ground wood. It can also be applied at other points in the pulping process such as at the storage tanks, at a washing step, at a beaching step. Typical mechanical pulping conditions are well known in the art.

[0034] The consistency of solids of the. pulp mixture can be from 0.5% to 28%, based upon the weight of oven-dried pulp. The consistency can be as high as 28% or as high as 20% or preferably as high as 14%. The consistency is at least 0.5 %, or at least 3% or at least 5 %, or at least 8%. Preferably the consistency of the pulp mixture is 5% to 28% and, most preferably, the consistency of the pulp is 9% to 14%. The pulp mixture useful in the present invention comprises the pulp, water and/or an organic solvent. The organic solvent is selected from the group consisting of acetone, acetonitrile, ethanol, methanol, isopropanol, acetic acid or combination thereof. The ratio of water to organic solvent is about 100:0 to about 1:99, preferably 100:0 to 80:20, and more preferably 100:0 to 90:10. [0035] The chemical pulping consists of several unit operations, one of which is oxygen delignification. In the oxygen delignifϊcation process for chemical pulp, the caustic consumption ranges from 0 to 24 kg/ton corresponding to a pH range of 7 to 12, preferable at least 8.5; the reactor temperature ranges from 20⁰C to 16O⁰C or 25⁰C to 160⁰C, preferably 80⁰C to 150⁰C, and more preferably 85°C to 125°C; and the pressure ranges from atmospheric pressure to 1 MPa, preferably 0.1-1.0 MPa, and more preferably 0.5-0.8 MPa.- Preferably the pressure is at least 0.1 MPa. It is under these conditions in the oxygen delignification process of chemical pulp that the singlet oxygen would be contacted with the pulp. Generally in an oxygen delignification process the consumption of molecular oxygen can be from 1 to 100 kg per ton pulp, preferably from 5 to 50 kg per ton pulp, and most preferably between 5 to 25 kg per ton pulp.

[0036] In the present invention, oxygen is treated in a separate stream in a chamber via a combination of visible (non-UV) light and a chemical agent, which is then applied to the pulp. This is in contrast to the prior patent by Turner, which uses UV or corona discharge. UV light generates more reactive and less selective species such as free radicals which can cause degradation to the pulp as compared to visible light; UV light is more expensive to produce then visible light; and UV light is more hazardous to the skin and to human eyes and is more difficult to install in a pulp or paper mill than is visible light. The present invention which uses visible light is therefore more beneficial than the process of U.S. Pat. 4,294,654. Turner suggests that a photosensitizer is not required for his process; see Example 1 in Turner where the reported results are the same with or without photosensitizer in his process. In the present invention, chemical agent/photosensitizer is a required element of the process. In contrast to Turner's process, the present invention never applies the light energy directly to the pulp, thereby obviating the absorption of light by cellulose as described in U.S. Pat. 2,161,045.

[0037] In contrast to the results obtained by Liebergott using activated oxygen produced by discharge methods having consistencies from 15-95%, it has now surprisingly been found, as shown in the present invention, that a highly effective process for the delignification and bleaching of low consistency pulps is achieved by using visible light and a chemical agent to specifically generate singlet oxygen in a mild reaction and exposing the resulting singlet oxygen to the pulp. The data set forth in this invention for unbleached kraft softwood pulp in Table 1 for methylene blue and Table 3 for rose bengal illustrate the selectivity improvement of oxygen delignification in the presence of singlet oxygen of the present invention compared to oxygen delignification with molecular oxygen. For methylene blue, the Kappa number decreases from 22.7 with molecular oxygen to 20.2 with singlet oxygen showing greater degree of bleaching and bleachability with singlet oxygen. A decrease in Kappa number is desirable. As shown in Table 1 for methylene blue, the intrinsic viscosity drastically decreases from 778 g/mL for the starting pulp to 668 g/mL during oxygen delignification with molecular oxygen whereas the intrinsic viscosity decreases much less to 773 g/mL during oxygen delignification with singlet oxygen. Molecular oxygen causes more cellulose degradation than singlet oxygen. The process in this invention produces significant selectivity improvement of 23.5% with methylene blue as the chemical agent as shown in Example 1 and of 36.9% with rose bengal as the chemical agent as shown in Example 3. The selectivity improvement obtained with this invention provides pulp and/or paper prepared from such pulp with higher degree of polymerization which is evidenced by a higher viscosity of the cellulose, at the same Kappa number.

[0038] Unbleached mechanical pulp as shown in Example 11 and Table 11 of this invention, surprisingly showed selectivity improvement was found at 9.49% with singlet oxygen as compared to molecular oxygen. The Kappa number for oxygen delignification with molecular oxygen shows an increase from 57.9 to 67.6 whereas oxygen delignification with singlet oxygen shows a decrease in Kappa number from 57.9 to 52.0. Whereas the weight average molecular weight as measured by size exclusion chromatography for the starting pulp was 689,000 dalton, for oxygen delignification with molecular oxygen was 621,000 dalton and for oxygen delignification with singlet oxygen was 523,000 dalton. [0039] The present invention 1) uses visible light and a chemical agent (a photochemical reaction) to produce singlet oxygen, and 2) carries out the process in two steps, separately generating the singlet oxygen and then transporting the singlet oxygen to pulp. [0040] Examples shown below are used for illustration but are not limiting.

EXAMPLES

[0041] Examples 1-12 and the corresponding Tables 1-12 show the following application of singlet oxygen:

Effect of pressure - ambient pressure (Example 1) and 40 psig (Examples 2-12).

Effect of chemical agent - methylene blue (Examples 1 & 2), rose bengal

(Examples 3, 6-12), eosin Y (Example 4), and mixture of chemical agents

(Example 5)

Effect of manifold length - 6.25 cm (Example 6) and 12.5 cm (Examples 2-5, 7-

12)

Effect of gas - oxygen (Examples 1-12) and compressed air (Example 3)

Effect of additives in combination with singlet oxygen - guar (Example 7), peroxide (Example 8) Effect of light source - halogen 500W (Examples 1-8, 10-12) and fluorescent light

(Example 9)

Effect of furnish - softwood (Examples 1-9, 12), hardwood (Example 10), and mechanical pulp (Example 11).

Effect of pulp consistency - low, medium, and high (Example 12).

[0042] General Procedure Employed in Examples

[0043] Size Exclusion Chromatography in DMSO:

[0044] The procedure for preparing cellulose samples for SEC in dimethylsulfoxide

(DMSO). Cellulose was made soluble by converting it to the methylol cellulose derivative. Approximately 75 mg of sample and 1.6 g of paraformaldehyde was added to 56 g of DMSO containing 1.0% LiCl and 500 ppm BHT. This mixture was heated with stirring at 110⁰C for 40 minutes. This solution is referred to as the cook solution. Paraformaldehyde decomposes into formaldehyde during the sample preparation step, which subsequently reacts with cellulose to form the derivative which is soluble in DMSO.

[0045] The chromatographic conditions used were:

Chromatograph: Waters Alliance 2000 GPCV

Primary Detector: Waters Differential Refractometer, 45°C

Secondary Detector: Waters Single Capillary Viscometer, 45°C

Columns: 1 PL-Gel Mixed A

Column Temperature: 45⁰C

Mobile phase: DMSO with 0.5% LiCl and 3.0% Formalin

Flow rate: 0.2 mllmin

Run Time: 100 min

Sample Concentration: 0.06 wt%

Injection volume: 325 μl

Internal standard: THF

Mw Calibration Standards: Pulullan

[0046] The analyte solution was prepared by adding 9.6 g of mobile phase to 0.4 g of cook solution. The analyte solution was filtered through 0.45 μm nylon filter. The refractive index chromatograms were processed using a pullulan calibration and Waters Empower software. The weight-average molecular weight (Mw) is sued in the calculation of selectivity. [0047] TAPPI brightness index and CIE L*a*b* color space values of compressed cellulose sheet samples were determined by standard method. (TAPPI Method of Test T452 -

"Brightness of pulp, paper & paperboard (directional reflectance at 457 nm)". For each sample, five readings were taken on the sheet surface using X-Rite 532 spectrodensitometer.

The observer angle for L*a*b* measurement is 2°.

[0048] Kappa number: Kappa number of the pulp was determined according to TAPPI

Standard Method T236 cm-85.

[0049] Intrinsic viscosity: The intrinsic viscosity of the pulp was determined according to ISO 5351 — "Pulps — Determining of limiting viscosity number in cupriethylenediamine (CED) solution".

[0050] Selectivity: Selectivity is defined as the ratio of intrinsic viscosity versus Kappa number. Selectivity is used to assess the benefits of pulp bleaching or delignifϊcation since the viscosity is related to the cellulose molecular weight and Kappa number provides a measure of the degree of bleaching and bleachability. In Tables 2-12, the selectivity listed is calculated from the ratio of intrinsic viscosity versus Kappa number and also from the ratio of SEC weight-average molecular weight versus Kappa number.

Selectivity = Intrinsic Viscosity/Kappa number or = Weight-average molecular weight/Kappa number

Selectivity Improvement =

(Singlet Oxygen Selectivity Datum - Control Molecular Oxygen Selectivity

Datum) / Control Molecular Oxygen Selectivity Datum * 100%

Selectivity Improvement can be calculated based on the data from the intrinsic viscosity or from the data from the molecular weight.

Example 1

[0051] The equipment included a three-necked round bottom flask equipped with a mechanical stirrer, a condenser with gas bubbler, gas inlet with a U-tube (or reduced ends tube) filled with methylene blue prior to entering the round bottom flask, and a 500 W portable halogen worklight source (Stanley Tools, Manufacturer No. XG-1009) as shown in Figure 1. The flask was charged with 110.82 g deionized water, 3.0 g sodium hydroxide (NaOH), 0.18 g magnesium sulfate (MgSO₄) and 6.0 g softwood pulp with a starting Kappa number of 54.4. Softwood pulp was added after the sodium hydroxide and magnesium sulfate completely dissolved. The reaction flask was heated to reflux (100⁰C) for four hours. Molecular oxygen flows from the oxygen tank and is treated in a separate stream in a chamber via a combination of visible light and a chemical agent, which is then applied to the pulp mixture. The flow rate of the molecular oxygen was between 110 to 140 mL/min and more typically 125 mL/min as measured by Agilent flowmeter ADM 2000 (Agilent Technologies, Inc., Wilmington, DE). For the molecular oxygen experiments, the methylene blue was covered with aluminum foil. The light from a halogen lamp (500 W) was applied during the singlet oxygen experiments. Initial experiments were performed at atmospheric pressure.

[0052] Unbleached, prehydrolyzed kraft softwood (Canadian spruce from J. D. Irving,

St. John, NB, Canada) pulp was used as received after drying. Its starting Kappa number was 54.4, its starting intrinsic viscosity 778 mL/g, and its weight-average molecular weight 984,000 daltons. Sample LD. # IA and IB were obtained using methylene blue as the photo sensitizer.

TABLE 1

EFFECT OF AMBIENT PRESSURE OF MOLECULAR OXYGEN VERSUS SINGLET OXYGEN ON SOFTWOOD USING METHYLENE BLUE OR ROSE BENGAL AS THE PHOTOSENSITIZER.

[0053] It is apparent from Example 1 - Table 1 of the present invention that a non- optimized, ambient pressure system can achieve significant selectivity improvement in the presence of singlet oxygen generated by passing molecular oxygen through a chamber with a chemical agent and visible light to produce singlet oxygen which is then applied to the pulp. The Kappa number decreases from 22.7 with molecular oxygen to 20.2 with singlet oxygen showing greater degree of bleaching and bleachability with singlet oxygen. A decrease in Kappa number is desirable. Table 1 demonstrates that with molecular oxygen the intrinsic viscosity drops drastically from 778 g/mL to 668 g/mL whereas, with singlet oxygen, the intrinsic viscosity drops only slightly to 773 g/mL. Molecular oxygen causes more cellulose degradation than singlet oxygen. The process produces significant selectivity improvement of 23.5% (as measured by the ratio of the intrinsic viscosity versus Kappa number).

[0054] In Examples 2- 12, the following high-pressure experimental apparatus consists of compressed oxygen tank which feeds pressured oxygen into a one-piece pressure manifold (Ace Glass, Vineland, NJ, Cat. No. 6448) packed with glass wool, Drierite® anhydrous calcium sulfate (W. A. Hammond Drierite Company LYD, Xenia, OH), and borosilicate glass beads (size 3 mm) coated with the photosensitizer dye up to 12.5 cm length. The manifold feeds pressurized singlet oxygen into a 300-mL reactor with a programmable controller (Parr Instrument Co., Moline, IL, Model No. 4561 and 4843) to a back pressure regulator (Swagelok, Huntingdon Valley, PA). A safety shield (RAD-GARD®, Instruments For Research and Industry, Inc., Cheltenham, PA) was placed between the 500 W Stanley® portable halogen worklight source and the pressure manifold as shown in Figure 2. Molecular oxygen flows from the oxygen tank through the chemical agent chamber and then into the pulp mixture. The flow rate of the molecular oxygen was between 110 to 140 mL/min and more typically 125 mL/min as measured by Agilent flowmeter ADM 2000 (Agilent Technologies, Inc., Wilmington, DE). For the molecular oxygen experiments, the chemical agent was covered with aluminum foil. The light from a halogen lamp (500 W) was applied during the singlet oxygen experiments.

[0055] In Example 9 (one of the singlet oxygen experiments), two Craftsman® 23 W fluorescent worklight sources were used.

Example 2

[0056] The Parr reactor was charged with 180 g deionized water, 19.31 g softwood pulp (18.0 g oven dried pulp), and 0.0072 g NaOH. Softwood pulp was added after the sodium hydroxide and magnesium sulfate completely dissolved. The reaction flask was heated to 100⁰C at 275 kPa for 90 minutes. The one-piece pressure reactor manifold (Ace Glass, Vineland, NJ) was packed with glass wool, Drierite® anhydrous calcium sulfate, and glass beads coated with the methylene blue photosensitizer. For the control molecular oxygen experiments, the methylene blue photosensitizer was covered with aluminum foil. The light from a halogen lamp (500 W) was applied during the singlet oxygen experiments. The pulp consistency was 10%.

[0057] After a specified time, the reacted pulp was isolated on a Buchner funnel with a

Whatman #41 filter paper under aspirator vacuum. The pH of the filtrate was measured between 7.9 and 9.4. The pulp was rinsed by the addition of about 500 mL D.I. water to the funnel with aspirator vacuum. The pulp was rinsed again by the addition of about 500 mL D.I. water with aspirator vacuum. The pulp was then air dried overnight followed by vacuum oven drying at 40⁰C for 2 hours. The amount OfMgSO₄ added for sample LD. # 2A and 2B was 0.0g. The amount OfMgSO₄ added for sample I.D. # 2C and 2D was 0.009 g. TABLE 2

EFFECT OF HIGH PRESSURE OF MOLECULAR OXYGEN VERSUS SINGLET OXYGEN ON SOFTWOOD USING METHYLENE BLUE AS THE PHOTOSENSITIZER.

[0058] Table 2 demonstrates that using a high pressure experimental apparatus, the selectivity improvement is 10.3% without MgSO₄ and 13.5% with MgSO₄ using methylene blue as the chemical agent. In the remaining tables, MgSO₄ was added in the molecular and singlet oxygen examples. MgSO₄ is a selectivity agent as described in Example 7.

Example 3

[0059] In this example, the Parr reactor was charged as described in Example 2. The pressure manifold was prepared as described in Example 2 except that the photosensitizer was rose bengal and compressed oxygen (or compressed air) was used as the gas. The amount OfMgSO₄ added for sample I.D. # 3 A, 3B, 3C, and 3D was 0.009 g.

TABLE 3

EFFECT OF HIGH PRESSURE OF MOLECULAR OXYGEN VERSUS SINGLET OXYGEN ON SOFTWOOD USING ROSE BENGAL AS THE PHOTOSENSITIZER.

M O = molecular oxygen an = sing e oxygen

[0060] The data set forth for unbleached kraft softwood pulp in Table 3 for rose bengal illustrates the selectivity improvement of 36.9% (Sample LD. 3B) for oxygen delignification in the presence of singlet oxygen of the present invention compared to oxygen delignification with molecular oxygen. The high selectivity improvement of 39.9% (based on the ratio of intrinsic viscosity versus Kappa number) was obtained by a decrease in Kappa number from 24.9 (Sample LD. 3A) with molecular oxygen to 21.3 (Sample LD. 3B) with singlet oxygen showing greater degree of bleaching and bleachability with singlet oxygen. A decrease in Kappa number is desirable. Table 3 also shows that the intrinsic viscosity drastically decrease from 778 g/mL for the starting pulp to 607 g/mL (Sample LD. 3A) during oxygen delignifϊcation with molecular oxygen whereas with singlet oxygen the intrinsic viscosity had a much smaller decrease to 711 g/mL (Sample LD. 3B). Molecular oxygen causes more cellulose degradation than singlet oxygen.

Example 4

[0061] In this example, the Parr reactor was charged as described in Example 2. The pressure manifold was prepared as described in Example 2 except the photosensitizer was eosin Y. The amount OfMgSO₄ added for sample LD. # 4A and 8B was 0.009 g.

TABLE 4

EFFECT OF HIGH PRESSURE OF MOLECULAR OXYGEN VERSUS SINGLET OXYGEN ON SOFTWOOD USING EOSIN Y AS THE PHOTOSENSITIZER.

Example 5

[0062] In this example, the Parr reactor was charged as described in Example 2. The pressure manifold was prepared as described in Example 2 except the photosensitizer was a mixture of equal amounts of methylene blue and rose bengal. The amount OfMgSO₄ added for sample LD. # 5A and 5B was 0.009 g.

TABLE 5

EFFECT OF HIGH PRESSURE OF MOLECULAR OXYGEN VERSUS SINGLET OXYGEN ON SOFTWOOD USING MIXTURE OF METHYLENE BLUE AND ROSE BENGAL AS THE

PHOTOSENSITIZER.

[0063] Table 5 shows that when a mixture of methylene blue and rose bengal is used as the chemical agent, the selectivity improvements more closely matches that of methylene blue.

Example 6

[0064] In this example, the Parr reactor was charged as described in Example 2. The pressure manifold was prepared as described in Example 2 except the photosensitizer was rose bengal at a length of 6.25 cm. The amount OfMgSO₄ added for sample LD. # 6A and 6B was 0.009g.

TABLE 6

EFFECT OF HIGH PRESSURE OF MOLECULAR OXYGEN VERSUS SINGLET OXYGEN ON SOFTWOOD USING ROSE BENGAL (length = 6.25 cm) AS THE PHOTOSENSITIZER.

[0065] Table 6 demonstrates decreasing the surface area of the rose bengal (chemical agent) changes the selectivity improvement proportionally.

Example 7

[0066] In this example, the Parr reactor was charged as described in Example 2. The pressure manifold was prepared as described in Example 2 except the photosensitizer was rose bengal. Guar was also added. The amount OfMgSO₄ added for sample I.D. # 7 A and 7B was 0.009 g and the amount of guar added was 0.19 g.

TABLE 7

EFFECT OF HIGH PRESSURE OF MOLECULAR OXYGEN VERSUS SINGLET OXYGEN ON SOFTWOOD USING ROSE BENGAL AS THE PHOTOSENSITIZER - GUAR ADDITION.

[0067] Examples 7 and 8 show the effect of singlet oxygen in the presences of two different selectivity agents - guar and hydrogen peroxide. Selectivity agents are generally chemicals that are added during oxygen delignification. The selectivity agents reported in prior art are grouped into five categories: (1) oxidizing agents, such as, but not limited to hydrogen peroxide, chlorine, chlorine dioxide; (2) complexing agents that either remove or inactivate transition metal ions, such as, but not limited to magnesium sulfate (MgSO₄), sodium or magnesium gluconate, muconic acid, ethylenediaminetetraacetic acid (EDTA) and its salts, diethylenetriaminepentaacetic acid (DTPA) and its salts; (3) radical scanvengers which reduce the amount of free radicals present such as, but not limited to, muconic acid, sodium or magnesium gluconate, sodium l-hydroxyethylidene-l,2-diphosphonate (HEDP); (4) cellulose protective materials which absorb onto the cellulose fibers such as, but not limited to, magnesium sulfate (MgSO₄), phenol, phenol and magnesium sulfate, guar and hemicellulose; and (5) others which include, but not limited to enzymes, surfactants, organic solvents (organosolv process), and spent liquor.

[0068] In the presence of guar as shown in Table 7, the selectivity improvement is

23.3% (Sample LD. 7B) whereas hydrogen peroxide which generates hydroxide radical was detrimental to the selectivity improvement (Sample LD. 8B).

Example 8

[0069] In this example, the Parr reactor was charged as described in Example 2. The pressure manifold was prepared as described in Example 2 except the photosensitizer was rose Bengal in combination with hydrogen peroxide. The amount OfMgSO₄ added for sample LD. # 8A and 8B was 0.009g. The amount OfH₂O₂ added for sample LD. # 8A and 8B was 0.6437 g.

TABLE 8

EFFECT OF HIGH PRESSURE OF MOLECULAR OXYGEN VERSUS SINGLET OXYGEN ON SOFTWOOD USING ROSE BENGAL AS THE PHOTOSENSITIZER - H₂O₂ Addition.

Example 9

[0070] In this example, the Parr reactor was charged as described in Example 2. The pressure manifold was prepared as described in Example 2 except the photosensitizer was rose bengal. As light source, two Craftsman® 23W fluorescent work lights were used. The amount OfMgSO₄ added for sample LD. # 9A and 9B was 0.009g.

TABLE 9

EFFECT OF HIGH PRESSURE OF MOLECULAR OXYGEN VERSUS SINGLET OXYGEN ON SOFTWOOD USING ROSE BENGAL AS THE PHOTOSENSITIZER - Fluorescent Light.

[0071] The data in Table 9 illustrates that fluorescent lamp can also be used as a light source to generate singlet oxygen.

Example 10

[0072] In this example, the Parr reactor was charged as described in Example 2. The pressure manifold was prepared as described in Example 2 except the photosensitizer was rose bengal. Unbleached, prehydrolyzed kraft hardwood (from Appleton Paper Inc., Roaring Springs, PA - 40-45% RED OAK, 10-15% WHITE OAK along with popular and ash) pulp was used as received after drying. Its starting Kappa number was 15.2, its starting intrinsic viscosity was 668 mL/g, and its weight-average molecular weight was 1,080,000 daltons. The amount OfMgSO₄ added for sample LD. # 1OA and 1OB was 0.009g.

TABLE 10

EFFECT OF HIGH PRESSURE OF MOLECULAR OXYGEN VERSUS SINGLET OXYGEN ON HARDWOOD USING ROSE BENGAL AS THE PHOTOSENSITIZER.

[0073] The data in Table 10 demonstrates enhanced selectivity improvement in oxygen delignification and bleaching of hardwood. The selectivity improvement is not as large as with softwood because hardwood contains less lignin than softwood. A selectivity improvement of 9.03% (Sample LD. 10B) based on the ratio of SEC Mw versus Kappa number is obtained.

Example 11

[0074] In this example, the Parr reactor was charged as described in Example 2. The pressure manifold was prepared as described in Example 2 except the photosensitizer was rose bengal. Unbleached, prehydrolyzed mechanical pulp was used as received after drying. Its starting Kappa number was 57.9, its weight-average molecular weight was 689,000 daltons, and TAPPI brightness 47.6. The amount OfMgSO₄ added for sample LD. # 11 A and HB was 0.009g.

TABLE 11

EFFECT OF HIGH PRESSURE OF MOLECULAR OXYGEN VERSUS SINGLET OXYGEN ON MECHANICAL PULP USING ROSE BENGAL AS THE PHOTOSENSITIZER.

[0075] The data set forth in Table 11 uses mechanical pulp as opposed to chemical pulp used in Examples 1-10 and Example 12. Table 11 illustrates the selectivity improvement of oxygen delignification with singlet oxygen on mechanical pulp compared to the results obtained with molecular oxygen. Sample LD. # 1 IB shows 9.49% selectivity improvement (based on the ratio of SEC Mw versus Kappa number) with singlet oxygen was obtained. The intrinsic viscosity of Sample LD. # 1 IA and 1 IB was not obtained due to the fact that the samples would not completely dissolve in solution.

Example 12

[0076] In this example, the Parr reactor was charged as described in Example 2. The pressure manifold was prepared as described in Example 2 except the photosensitizer was rose bengal and the consistency was varied - 25% high consistency, sample LD. #12A and 12B ; 10% medium consistency, sample LD. #12C and 12D; and 5% low consistency, sample LD. #12E and 12F. The amount OfMgSO₄ added for sample LD. # 12A-12F was 0.009g. TABLE 12

EFFECT OF HIGH PRESSURE OF MOLECULAR OXYGEN VERSUS SINGLET OXYGEN ON SOFTWOOD USING ROSE BENGAL AS THE PHOTOSENSITIZER AT VARIOUS

CONSISTENCIES.

[0077] The data in Table 12 demonstrate the enhanced selectivity improvement with singlet oxygen at various consistencies. The best selectivity improvement with singlet oxygen was obtained at medium consistency, 10%.

Claims

1. A process for the delignification or bleaching of lignocellulosic pulp comprising the steps of a) providing an aqueous or non-aqueous mixture of pulp, b) providing a stream of molecular oxygen, reacting the molecular oxygen in the presence of visible light and at least one chemical agent photochemically converting a portion of the molecular oxygen to singlet oxygen and c) subsequently contacting pulp or pulp mixture with the converted oxygen stream.

2. The process of claim 1 wherein the chemical agent comprises at least one photosensitizer dye.

3. The process of claim 2 wherein at least one photosensitizer dye is selected from the group consisting of methylene blue and rose bengal or combination thereof.

4. The process of claim 1 wherein at least 10% of the molecular oxygen is converted to singlet oxygen.

5. The process of claim 1 wherein the mixture of pulp is at a consistency of 14% or less.

6. The process of claim 1 wherein the mixture of pulp is at a temperature of from about 20°C to 160⁰C.

7. The process of claim 1 wherein the selectivity improvement is at least 9% based on intrinsic viscosity.

8. A process for the delignification or bleaching of lignocellulosic pulp comprising the steps of: a) providing an aqueous or non-aqueous mixture of pulp under typical oxygen delignification conditions, b) separately providing a stream of molecular oxygen, reacting the molecular oxygen light in the presence of visible light and a chemical agent, photochemically converting a portion of the molecular oxygen to singlet oxygen, and c) subsequently contacting pulp or pulp mixture with the converted oxygen stream.

9. The process of claim 8 wherein the chemical agent comprises at least one photosensitizer dye.

10. The process of claim 8 wherein at least 10% of the molecular oxygen is converted to singlet oxygen.

11. The process of claim 8 wherein the mixture of pulp is at a consistency of 14% or less.

12. The process of claim 8 wherein the mixture of pulp is at a temperature of from about 80°C to 150⁰C.

13. The process of claim 8 wherein the mixture of pulp is at a pressure of at least 0.1 MPa.

14. The process of claim 8 wherein the mixture of pulp is at a pH of at least about 8.5 or higher.

15. The process of 8 wherein the singlet oxygen is added to one of more stages of an oxygen delignification process.

16: A process for the delignification or bleaching of lignocellulosic pulp comprising the steps of: a) providing an aqueous or non-aqueous mixture of mechanical pulp under typical mechanical pulping conditions, b) separately providing a stream of molecular oxygen, reacting the molecular oxygen in the presence of visible light and a chemical agent, photochemically converting a portion of the molecular oxygen to singlet oxygen and c) subsequently contacting pulp or pulp mixture with the converted oxygen stream.

17. The process of claim 16 wherein the chemical agent comprises at least one photosensitizer dye.

18. The process of claim 16 wherein at least 10% of the molecular oxygen is converted to singlet oxygen.

19. The process of claim 16 wherein the mixture of pulp is at a consistency of 14% or less.

20. The process of claim 16 wherein the singlet oxygen is applies to the mechanical pulp during the pulping process at the eye of a mechanical pulp refiner or through the dilution water for stone groundwood.

21. A method to generate singlet oxygen that comprises the steps of providing a molecular oxygen source, a visible light source, a chemical agent wherein the molecular oxygen source provides a stream of molecular oxygen into a chamber or a series of chambers containing the chemical agent, contacting the molecular oxygen with the chemical agent in the presence of visible light from the light source.

22. The process of claim 21 wherein the chemical agent comprises at least one photosensitizer dye.

23. The process of claim 21 wherein the chamber contains the chemical agent wherein the chemical agent is affixed to gas filters, glass beads, wire mesh, or a catalytic bed.

24. A pulp or paper product produced by the process of Claim 1.