CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation-in-part application of pending U.S. patent application Ser. No. 10/618,119, filed Jul. 11, 2003, which is a continuation application of U.S. patent application Ser. No. 09/747,469, filed Dec. 20, 2000 and which has issued as U.S. Pat. No. 6,627,784, which claimed priority to the filing date of U.S. Provisional Patent Application Ser. No. 60/204,838, filed May 17, 2000. U.S. patent application Ser. Nos. 10/618,119; 09/747,469; and No. 60/204,838 are incorporated herein by reference as if set forth herein in their entireties.
The present invention generally relates to methods for processing lignocellulosic pulp, and more particularly to delignifying lignocellulosic pulp.
Wood is comprised of two main components, a fibrous cellulose and a non-fibrous component. The polymeric chains forming the fibrous cellulose portion of the wood are aligned with one another and form strong associated bonds with adjacent chains. The non-fibrous portion of the wood comprises a three-dimensional polymeric material formed primarily of phenylpropane units, known as lignin. The lignin is interspersed both between and in the cellulosic fibers, bonding them into a solid mass.
Processes for the production of paper and paper products generally includes a pulping stage in which wood, usually in the form of wood chips, is reduced to a fibrous mass by removing a substantial portion of the lignin. Some of these processes include digestion of the wood by a Kraft or modified Kraft process resulting in the formation of a dark colored slurry of cellulose fibers known as “brownstock.” The dark color of the brownstock is attributable to the presence in the pulp after digestion of lignin that has been chemically modified during pulping to form chromophoric groups. In order to lighten the color of the brownstock pulp sufficiently to make it suitable for use in various paper applications, it is necessary to remove much of the remaining lignin.
Further reduction of the concentration of lignin in the lignocelluosic pulp is carried out in specific delignification processes, bleaching processes, or combinations of the two. Delignification processes include, for example, chemical treatment with chlorine-containing compounds, such as sodium hypochlorite in a caustic medium, or oxygen delignification with oxygen-containing compounds. Both types of delignification processes typically are followed by bleaching operations in which the delignified pulp is bleached or brightened with chlorine or chlorine dioxide.
The use of chlorine or chlorinated compounds in paper making processes is common. The use of such compounds usually result in the production of effluent containing substantial quantities of color, BOD (biological oxygen demand), COD (chemical oxygen demand) and chlorides, which require additional processing before being discharged. Therefore, reductions in the amount of chlorinated compounds used the paper making processes can reduce the amount of pollutants produced by the processes.
Conventional oxygen delignification processes, in some cases, have produced smaller amounts of chlorinated organic compounds and reduced levels of pollutant discharge, as compared with other types of delignification processes. However, conventional oxygen delignification processes typically require significant amounts of oxygen-containing materials and significant processing time to produce desired levels of delignification. Furthermore, these oxygen delignification processes usually require that the concentration of lignocellulosic pulp in the pulp slurry that is produced in the initial stages of the paper making process be increased so that the oxygen-containing materials can diffuse sufficiently through the pulp to effect delignification. The concentration of pulp usually must be lowered again for further processing. The concentration manipulations are time-consuming and energy intensive.
Accordingly, alternative delignification processes are needed that have the potential to reduce or eliminate some or all of the above disadvantages.
Methods for treating lignocellulosic pulp with cavitation are disclosed. The methods generally include delignifying lignocellulosic pulp in a slurry in the presence of cavitation.
In one aspect of the present invention, a method of processing lignocellulosic pulp is provided that comprises contacting an oxidizing agent with a slurry comprising pulp in the presence of cavitation to produce a delignified pulp.
In another aspect of the present invention, a method for processing lignocellulosic pulp is provided that comprises delignifying a lignocellulosic pulp in a cavitation zone to produce a delignified pulp.
In a further aspect of the present invention, a method of treating lignocellulosic pulp is provided that comprises mixing a slurry comprising pulp with a non-alkaline oxidizing agent in the presence of cavitation to produce a delignified pulp.
In still a further aspect of the present invention, a method of treating lignocellulosic pulp is provided that comprises mixing an oxidizing agent and a slurry containing lignocellulosic pulp in the presence of cavitation to produce a delignified pulp, wherein the mixture of the oxidizing agent and the slurry exhibits a pH above 7.
In another aspect of the present invention, a method of making paper is provided that comprises pulping a lignocellulosic feedstock to produce a lignocellulosic pulp and preparing a slurry of the lignocellulosic pulp. The method also comprises delignifying the lignocellulosic pulp in the presence of cavitation to produce a delignified pulp, and forming paper from the delignified pulp.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other aspects of the present invention are set forth in greater detail in the description below and in the accompanying drawings which are briefly described as follows.
FIG. 1 illustrates a system in which lignocellulosic pulp can be delignified in the presence of cavitation.
FIG. 2 is a cross-sectional view of the reactor shown in FIG. 1.
FIG. 3 is a diagram showing results of delignification trials charted as percent delignification versus residence time.
FIG. 4 is a diagram showing results of delignification trials charted as percent delignification versus residence time.
The present invention includes methods for delignifying lignocellulosic pulp. The delignification generally is carried out by oxidizing the lignocellulosic pulp contained within a slurry in the presence of cavitation to produce a delignified pulp.
As used herein, the term “delignification” refers to a process of reducing the amount of lignin contained within a material, particularly by a chemical reaction, such as oxidation, which can be carried out in conjunction with one or more other mechanical process or chemical reactions. The term “delignified pulp” refers to a pulp derived from lignocellulosic material that, as a result of delignifying, exhibits a smaller Kappa Number than the pulp exhibited prior to delignification. The amount of delignification that occurs typically is determined by comparing the kappa value of the pulp slurry before delignification and the Kappa Number of the delignified pulp. As used herein, the term “Kappa Number” refers to the volume (in millimeters) of 0.1N potassium permanganate solution consumed by one gram of moisture free pulp. The amount of delignification can be expressed as a percent reduction in the two Kappa Numbers. The methods of the present invention can effectuate delignifications of greater than 5% and, in some instances, about 5% to about 55% and above.
While a variety of oxidants, such as sodium hydroxide (NaOH), sodium hydrosulfite, chlorine, chlorine dioxide, and hydrogen peroxide (H2O2), can be used in delignifying the pulp, the methods particularly are directed to the use of gaseous oxidizing agents, such as air, molecular oxygen (O2), ozone (O3) and combinations thereof to carry out the delignification.
The methods of the present invention generally are directed to delignifying lignocellulosic pulp in a slurry at any consistency, but particularly at medium or low consistencies. As used herein, the term “consistency” refers to the concentration by weight of pulp in a pulp slurry on a dry weight basis. The term “high consistency” refers to pulp slurry containing greater than about 15% by weight pulp. The term “medium consistency” refers to pulp slurry containing about 6% to about 15% by weight pulp. The term “low consistency” refers to pulp slurry containing about 0.1% to about 6% by weight pulp. Consequently, pulp being processed to make paper can be maintained at or near the consistency level in previous steps, delignified and further processed without raising the consistency prior to delignification and then lowering the consistency for further processing.
The methods generally include contacting an oxidizing agent with a slurry comprising pulp in the presence of cavitation to produce a delignified pulp. The oxidizing agent can be a non-alkaline agent, such as gaseous oxidants air, molecular oxygen, and ozone, an alkaline agent such as sodium hydroxide, or a combinations thereof. The oxidizing agent can be added to the pulp slurry during mixing, immediately before mixing, or can be remaining in the slurry from a previous process. Contacting the slurry and oxidizing agent in the presence of cavitation can be done under pressure. In one example, contacting is done under pressure in the range of about 480 kPa to about 1035 kPa.
Furthermore, the contacted oxidizing agent and slurry also can be heated, either in the presence of cavitation or before being exposed to cavitation, to further effectuate delignification. In one aspect, either one or both of the oxidizing agent and slurry, or the mixture of the two, can be heated to a range of about 50° C. to about 120° C. In another aspect, either one or both of the oxidizing agent and slurry, or the mixture of the two, can be heated to a range of about 80° C. to about 100° C. This contacting also can take place in a pH range of about 9 to about 12.
The methods also can include directing the delignified pulp to a retention tank and holding the oxidant/slurry mixture containing delignified pulp in the retention tank for a predetermined period of time to effectuate additional delignification. The oxidant/slurry mixture can be held under pressure, for example in the range of about 480 kPa to about 1035 kPa, for a period of time. The time period in which the mixture can be held, either under pressure or otherwise to effectuate further delignification, can be in the range of about 1 minute to about 2 hours. In another aspect of the present invention, the time period is about 1 minute to about 30 minutes. In yet another aspect of the present invention, the time period is about 1 minute to about 20 minutes.
The amount of oxidant used to delignify the pulp depends on the type of lignocellulosic material that is being treated. For example, when molecular oxygen is being used to delignify pulp before washing, molecular oxygen is combined with the slurry in a range of about 3% to about 15% by weight based on the weight of the dry pulp fiber in the slurry. For delignification of pulp after washing, molecular oxygen is added in a range of about 1% to about 4% by weight. The specific amount of oxidant that is used to delignify the lignocellulosic pulp can be affected by the type of material it is, whether softwood or hardwood, and the amount of sodium sulfide (Na2S) that remains in the slurry from previous process steps. In one aspect, the methods of the present invention generally include adding oxidant to the slurry in the range of about 1% to about 20% by weight on a dry pulp fiber basis. In another aspect of the present invention, the methods include adding oxidant to the slurry in the range of about 5% to about 15% by weight on a dry pulp fiber basis. In yet another aspect of the present invention, the methods include adding oxidant to the slurry in the range of about 1% to about 4% by weight of the dry pulp fiber.
The slurry to which the oxidant is added can have a consistency in the range of about 0.1% to about 15%. In another aspect of the present invention, the slurry can have a consistency in the range of about 0.1% to about 12%. In yet another aspect of the present invention, the slurry can have a consistency of less than about 6%. In still another aspect of the present invention, the slurry can have a consistency of about 0.1 to about 6%.
Referring now in more detail to the drawings, in which like numerals refer to like parts throughout the several views, FIG. 1 illustrates a system 100 comprising an apparatus in which a lignocellulosic pulp can be delignified. The system 100 includes a reactor 11 in which the slurry is exposed to cavitation. The system 100 also includes a feed tank 50 which contains the pulp slurry that is to be delignified. The pulp slurry can be low or medium consistency. The feed tank 50 is in flow communication with the reactor 11 by delivery line 55, which has a flow meter 60 disposed therein for monitoring the flow rate and/or amount of slurry flowing through the delivery line 55. A feed pump 65 also is provided in flow communication with the delivery line 55 to pump the slurry from the feed tank 50 to the reactor 11. A gas inlet 28 is provided in flow communication with the delivery line 55 to allow the introduction of gaseous oxidizing agents into the slurry stream as it flows to the reactor 11.
An electric motor 70 is operably connected to the shaft 18 of the cavitator 20 so as to provide the driving force for rotating the rotor 17 of the cavitator 20. As used herein, the term “cavitator” refers to a device that can induce cavitation in a fluid. Also, as used herein, the term “mechanical cavitator” refers to a device that induce cavitation in a fluid by moving a body through the fluid. An exit line 73 is in flow communication with the reactor 11 and routes the mixed slurry/oxidizing agent stream to a retention tank 80. The mixture of slurry and oxidizing agent can be retained in the retention tank for a predetermined period of time or simply until an appropriate amount of delignification has occurred, as can be calculated from determining the Kappa Numbers of the slurry over time. Sample lines 77 can be provided in-line with the exit line 73 and the product line 75 to allow samples to be taken to determine Kappa Numbers of the slurry and monitor quality.
As shown in FIGS. 1 and 2, the reactor 11 comprises a cylindrical housing 12 defining an internal cylindrical chamber 15. In the figures, the housing 12 is formed of a wall 13 capped by end plates 14 secured to each other by bolts 16. The wall 13 is sandwiched between the plates 14.
The cylindrical rotor 17 is disposed within the cylindrical chamber 15 of the housing and is mounted on the axially extending shaft 18. The shaft 18 is journaled on either side of the rotor within bearing assemblies 19 that, in turn, are mounted within bearing assembly housings 21. The bearing assembly housings 21 are secured to the housing 12 by means of appropriate fasteners such as bolts 22. The shaft 18 projects from one of the bearing housings 21 and is coupled to the electric motor 70 or other motive means. It will thus be seen that the rotor 17 may be spun or rotated within the cylindrical chamber 15 in the direction of arrows 23 by activating the motor 70 coupled to the shaft 18.
The rotor 17 has a peripheral surface that is formed with one or more circumferentially extending arrays of irregularities in the form of relatively shallow holes or bores 24. As shown in FIG. 2, the rotor 17 is provided with five arrays of bores 24 separated by voids 26, the purpose of which is described in more detail below. It should be understood, however, that fewer or more than five arrays of bores may be provided in the peripheral surface of the rotor as desired depending upon the intended fluids and flowrates. Further, irregularities other than holes or bores also may be provided. The rotor 17 is sized relative to the cylindrical chamber 15 in which it is housed to define a space, referred to herein as a cavitation zone 32, between the peripheral surface of the rotor and the cylindrical chamber wall 13 of the chamber 15.
An inlet port 25 is provided in the housing 12 for supplying from the delivery line 55 the slurry to be delignified in the interior chamber 15. Gaseous oxidizing agents, such as air, molecular oxygen, ozone, chlorine, chlorine dioxide and combinations thereof, can be introduced into the delivery line 55 through the gas supply conduit 28 and entrained in the form of bubbles within the stream of slurry flowing through the delivery line 55, if desired. Alternatively, the oxidant can be introduced into the slurry in liquid form. Oxidants such as sodium hydrosulfite, chlorine, chlorine dioxide, hydrogen peroxide, and combinations thereof can be introduced into the delivery line 55.
At the junction of the delivery line 55 and the gas supply conduit 28, the slurry and oxidant form a gas/slurry mixture in the form of relatively large gas bubbles 31 entrained within the flow of slurry 29. This mixture of slurry and gas bubbles is directed into the cylindrical chamber 15 of the housing 12 through the inlet port 25 as shown.
An outlet port 35 is provided in the housing 12 and is located in the cap 14 of the housing opposite to the location of the inlet port 25. Location of the outlet port 35 in this way ensures that the entire volume of the gas/slurry mixture traverses at least one of the arrays of bores 24 and thus moves through a cavitation zone prior to exiting the hydrosonic mixer 11. The outlet port 35 is in fluid communication with the exit line 73, which directs the gas/slurry mixture to the retention tank 80.
In operation, the reactor 11 functions to mix the pulp slurry with the oxidizing agent and induce cavitation in the slurry to effectuate thorough mixing. A slurry containing lignocellulosic pulp exhibiting a first Kappa number is pumped from the feed tank 50 through the delivery line 55. A gaseous oxidant is supplied through the gas supply conduit 28 to the slurry stream, which then form a mixture comprised of relatively large gas bubbles 31 entrained within the slurry 29. The slurry/gas bubble mixture moves through the delivery line 55 and enters the chamber 15 through the supply port 25.
From the supply port 25, the mixture moves toward the periphery of the rapidly rotating rotor 17 and enters the cavitation zones 32 in the region of the bores 24. As described in substantial detail in U.S. Pat. No. 5,188,090, the disclosure of which is hereby incorporated by reference, within the cavitation zones 32, millions of microscopic cavitation bubbles are formed in the mixture within and around the rapidly moving bores 24 on the rotor. Since these cavitation bubbles are unstable, they collapse rapidly after their formation. As a result, the millions of microscopic cavitation bubbles continuously form and collapse within and around the bores 24 of the rotor, creating cavitation induced shock waves that propagate through the mixture in a violent albeit localized process.
As the mixture of slurry and relatively large gas bubbles moves into and through the cavitation zones 32, the gas bubbles in the mixture are bombarded by the microscopic cavitation bubbles as they form and further are impacted by the cavitation shock waves created as the cavitation bubbles collapse. This results in a “chopping up” of the relatively large gas bubbles into smaller gas bubbles, which themselves are chopped up into even smaller gas bubbles and so on in a process that occurs very quickly. Thus, the original gas bubbles are continuously chopped up and reduced to millions of tiny microscopic gas bubbles within the cavitation zone.
The dispersement and random flow patterns within the cavitation zone 32 provide a high degree of mixing of the oxidant and slurry. Some conventional systems do not achieve a thorough mixing of the oxidant and slurry, thus requiring the addition of substantially more oxidant into the slurry, resulting in increased costs and still not guaranteeing even mixing of the combination. The turbulence of the fluids within the cavitation zone 32 leads to more complete mixing of the oxidant with the slurry.
The term “cavitation zone” is used herein to refer to any region in which cavitation is induced in the lignocellulosic pulp slurry, and, more particularly, a region specifically established for the generation of cavitation within the slurry. In regards to the reactor 11, shown in FIGS. 1 and 2, the term “cavitation zone” refers to the region between the outer periphery of the rotor wherein the bores are formed and the cylindrical wall of the housing chamber. This area is where the most intense cavitation activity occurs. It should be understood, however, that cavitation may occur, albeit with less intensity, in regions other than this space such as, for example, in the reservoir or region between the sides or faces of the rotor and the housing.
The process of cavitating the oxidant/slurry mixture can be on a substantially continuous basis in that a continuous flow of slurry is pumped into the hydrosonic mixer 11, treated by cavitation and then discharged from the reactor 11. Alternatively, the process can be conducted on a batchwise basis, wherein a specified amount of slurry and oxidant is charged to the reactor 11, cavitated, and then discharged before any additional material is charged to the mixer.
Delignification occurs within the cavitation zone of the reactor 11 and can continue in the retention tank 80 if so desired. The oxidant and slurry are mixed within the reactor 11 under pressure, typically in the range of about 480 kPa to about 1035 kPa. The residency time of the slurry within the reactor generally is within the range of about 20 seconds to about 60 seconds, although this range can vary depending upon the flowrate and size of the mixer.
The pulp contained within the slurry is delignified within the mixer. In one example, a delignified pulp exhibiting about 20% to about 25% delignification can be produced within the reactor. If additional delignification is desired, the delignified oxidant/slurry mixture can be discharged from the reactor 11 through outlet port 35, exit line 73 and into retention tank 80. The mixture can be retained in the retention tank for a period of time under pressure to further delignify the pulp. In one example, further delignification in the range of up to about 52% can be effectuated by retaining the mixture in the retention tank 80 for a time period in the range of about 5 to about 30.
When the desired amount of delignification is achieved, the slurry can be directed through the product line 75 for further processing, such as washing, bleaching, etc.
- Example 2
Softwood slurry with consistency of 5% was heated to 195° F. Either black liquor or sodium hydroxide was added to the slurry to adjust the pH of the slurry to about 12, so as to mimic process conditions for before and after washing, respectively. Hot slurry was pumped through the cavitating reactor and the line pressure was increased to about 95 psig. Oxygen was added to the slurry before it entered the cavitating reactor. The rotor of the reactor included thirty holes or bores per row and was driven by a variable electric motor. The cavitating reactor was made of 316 stainless steel and was sixteen inches in length and three inches in width. The rotor was set at about 1200 rpm during the trials. The slurry went to a retention tank for residence time after leaving the reactor. Samples were taken at the feed tank and after the retention tank.
|TABLE 1 |
| ||Slurry ||Slurry ||Slurry ||Slurry || || ||Kappa ||Kappa # ||% Kappa |
|Source of ||Consistency ||pH ||Pressure ||Temperature ||Oxygen ||Oxygen ||# ||Final ||Drop |
|Alkalinity ||% wt ||# ||Psig ||° C./° F. ||SCFM ||Psig ||Initial ||5 Min ||5 min |
|Sodium ||5% ||11.6 ||93 ||95.2/203 ||3 ||105 ||34.5 ||24.3 ||29.6 |
|Sodium ||5% ||11.6 ||94 ||69.5/157 ||3 ||105 ||34.5 ||23.5 ||31.9 |
|Black ||5% ||11.62 ||92 ||69.1/156 ||5 ||105 ||34.5 ||24.5 || 29% |
|Black ||5% ||11.6 ||98 ||88.5/191 ||5 ||105 ||34.5 ||21.22 ||38.50% |
Oxygen Delignification of Softwood Pulp Before Washers
Trials were run in the cavitating reactor of Example 1 to determine the amount of oxygen necessary to achieve desired delignification. Before washing of lignocellulosic pulp slurry using cavitation, pulp slurries typically carry black liquor solids that are oxidizable by the oxidant intended for delignification. Consequently, the amount of oxygen provided to delignify the pulp should be sufficient to allow for the competing reaction of the oxidation of the black liquor.
Table 2 shows the process conditioning for the trials to determine oxygen requirement for other oxidizeable compounds in black liquor. FIG. 3
shows the results of the trial.
| ||TABLE 2 |
| || |
| || |
| ||Property ||Range |
| || |
| ||Slurry Flow gpm ||12-52 |
| ||Consistency ||2.5% Softwood |
| ||Slurry Inlet Temperature ° C. ||81-88 |
| ||Pressure Psig ||85-110 |
| ||Slurry pH || 9-12 |
| ||Reaction Temperature ° C. ||89-93 |
| ||Reaction Pressure ||90 psig |
| ||End pH ||>11.5 |
| || |
- Example 3
Results show that black liquor solids (excluding fiber) oxidize in the presence of oxidizing agent as does the lignin. The results shown in FIG. 3 indicate that the rate of delignification is independent of the competing reaction as long as there is enough oxidant for all the reactions to occur.
In this example about 2.5% hardwood slurry after washers was sent through the reactor and then to the retention tank. The slurry temperature was about 90° C. and the pH was about 11.8. The starting Kappa Number was about 13. Oxygen was added to the slurry prior to the slurry entering the cavitating reactor, which was the same as described in Example 1. Samples were taken at the feed tank, after the reactor and before the retention tank, and after the retention tank. The results shown in FIG. 4 indicate that during the brief time period of about one minute in the reactor, the slurry exhibited up to about 20% of delignification.
Although the methods of the present invention have been illustrated being carried out using a reactor as shown in FIGS. 1 and 2 and additionally described in the incorporated references, the methods for delignifying lignocellulosic pulp in the presence of cavitation are not limited to being carried out only with such devices. Rather, any apparatus or system that can generate cavitation in the pulp slurry can be used in conjunction with the methods of the present invention. For example, systems employing venturi nozzles, sonic wave generators, or other mechanical mixers that produce cavitation in the slurry can be used.
Although certain aspects of the invention have been described and illustrated, it should be understood by those skilled in the art that the foregoing and various other changes, omissions and additions may be made therein and thereto, without parting from the spirit and scope of the present invention.