EP0554391B1

EP0554391B1 - Pulping process

Info

Publication number: EP0554391B1
Application number: EP92900022A
Authority: EP
Inventors: Ethan K. Andrews; Fernando E. Mera; Lawrence K. Swift
Original assignee: Beloit Technologies Inc
Current assignee: Beloit Technologies Inc
Priority date: 1990-10-24
Filing date: 1991-10-07
Publication date: 1997-04-16
Anticipated expiration: 2011-10-07
Also published as: EP0554391A1; ZA918441B; NO302187B1; DE69125722D1; FI931854A0; CN1028250C; AU9022291A; RU2068904C1; CN1061452A; NO931486L; CA2094819C; CA2094819A1; PL168216B1; ES2101830T3; DE69125722T2; FI931854A; WO1992007994A1; MX9101706A; NO931486D0; ATE151824T1

Abstract

A pulp preparation process in which pulp treatment is expanded to include in-digester treatments (Figures 7, 8, 9) previously performed outside of the digester (12) in discrete process equipment. Specifically, a manner for in-digester bleaching is disclosed, preferably using non-chlorine containing bleaching reagents.

Description

i. Technical Field

The present invention relates generally to the field of chemical cooking processes, and more specifically to a pulp cooking process in which fibrous material is cooked at elevated temperatures and pressures in the presence of chemicals and subsequently further treated in the digester to separate fibers from the bonding agents in the material and to whiten the pulp.

ii. Technical Background

In typical cellulose chemical pulping operations, wood fibers are separated from lignin by cooking wood chips at elevated temperatures and pressures in the presence of chemicals. A certain level of delignification can be achieved in these cooking vessels, known as digesters, with the pulp resulting therefrom being passed from the digesters to subsequent operations. The subsequent operations often include various stages of washing to remove residual chemicals and other pulp constituents; as well as various stages of bleaching wherein the pulp is subjected to still further chemical treatments to further remove lignin and increase the brightness of the resulting pulp. The bleaching operations take place in separate bleaching vessels and are followed again by various stages of washing, to remove bleach chemicals from the pulp. The washing stages are frequently performed in drum type washers wherein the slurry is washed on a drum in a continuous operation. Bleach plants and the associated washing stages require vast expanses of real estate for equipment, and significant capital investment.
In recent years, enhanced environmental awareness has subjected the pulp and paper industry to close scrutinization in that the various pulping, washing, and bleaching stages can result in the discharge of spent aqueous solutions potentially harmful to the environment. Typical bleaching processes included the use of chlorine, and bleaching effluents can contain residual chlorine, which is perceived as undesirable in the environment. Chlorine recovery processes are difficult in that the corrosive nature of the effluents is harmful to common metals, and requires expensive equipment for chemical recovery. Additionally, such recovery processes are extremely complex and expensive, while yielding no financial return on the capital outlay required for their construction.
In typical batch cooking processes, upon completion of the cook, a digester blow valve is opened and the high temperature, high pressure liquid in the digester flashes, thereby emptying the digester. In recent years, displacement heating and cooking techniques have become popular wherein, before or after completion of the cook, liquor displacements have been utilized to conserve energy and chemicals.
In a known process, as taught in U.S. patent 4,578,149, at the completion of the cook, hot spent cooking liquors in the digester are displaced with effluent from subsequent washing stages, with the hot spent cooking liquors being made available for preheating and pretreating subsequent chip charges in a digester. This cooking process has lower steam requirements, is more acceptable environmentally, produces higher quality pulp and recovers more of the energy and chemicals put into the process than conventional batch processes. After a displacement at the end of the cook, cooked pulp chips are emptied from the digester by the application of pressure at the top of the digester, with the pulp then being passed to subsequent conventional treatments such as further pulp washing, bleaching, additional washing, and the like.
Pending U.S. application 07/068,721 teaches a process in which the equivalent of one or several independent stages of washing can be achieved by the utilization of displacement washing techniques in the digester.

Summary of the Invention

It is therefore one of the principle objects of the present invention to further expand the process steps which take place in a cellulose pulp digester, to minimize equipment and space required for subsequent treatment of the fibrous material after it leaves the digester.
Another object of the present invention is to provide a cellulose pulping process operable in a batch process which can complete various washing and bleaching stages without having to transport the pulp from a batch digester to subsequent process apparatus.
A further object of the present invention is to provide a pulping process wherein real estate requirements for equipment are minimized, making pulping plants of smaller expanse while producing acceptable quality pulp.
A still further object of the present invention is to provide a pulp bleaching process which does not require the use of chlorine and is environmentally preferred to previously known processes utilizing chlorine.
Yet another object of the present invention is to provide a pulping process which reduces total process time from beginning to end as compared with conventional equivalent processes.
These and other objects are achieved in the present invention by utilizing a batch cooking process wherein at the completion of a batch cook at elevated pressures and temperatures, liquor displacement techniques are used to wash the chips and remove residual liquors therefrom, and to further delignify the chips through the use of in-digester displacements with chemicals for pulp bleaching. In a preferred process, bleaching is performed with chemicals other than those containing chlorine, and acceptable brightness pulp is achieved.
Further objects and advantages of the present invention will become apparent from the following detailed description and the accompanying drawings.

Brief Description of the Drawings

Figures 1 through 10 illustrate a preferred process sequence for carrying out the expanded treatment of pulp in a digester according to the present invention.

Figure 1 illustrates the chip filling step;
Figure 2 illustrates a cool liquor fill;
Figure 3 illustrates a warm liquor fill;
Figure 4 illustrates a hot liquor fill;
Figure 5 illustrates the time-to-temperature and time-at-temperature steps in the digesting process;
Figure 6 illustrates the end of cook displacement step;
Figure 7 illustrates a post cook oxidized green liquor treatment;
Figure 8 illustrates a peroxide treatment step;
Figure 9 illustrates a peroxyacetic acid treatment; and
Figure 10 illustrates the pulp discharge step.

Detailed Description of the Preferred Embodiment

A process to meet the objectives outlined previously herein should have familiar unit operations and be compatible with subsequent and prior process steps. In the aforementioned U.S. patent 4,578,149, a batch cooking process is known utilizing familiar equipment and resulting in energy conservation, extended cooking capabilities, effective reuse and recycle of chemical alkali charges resulting in acceptable delignification levels requiring lesser fresh chemical liquor charges, and an in-digester brown stock washing stage.
The present invention entails the continuation of process steps in the digester, wherein additional displacements are made utilizing various additional chemicals and/or spent liquors to further dissolve lignin and brighten the pulp. The chemicals selected for the further delignification and brightening preferably include reagents of non-chlorine types, including but not limited to hydrogen peroxide, oxygen, sodium polysulfide, sodium sulfite, sodium borohydride, sodium hydrosulfite, ozone, and peroxyacetic acid. Other reagents may be useful for the same purpose, so long as the general selection criteria are met. The group of appropriate bleaching agents includes bleaching agents that will dissolve lignin and brighten pulp, that are compatible with process metallurgy, that are compatible with practical chemical recovery and material balance, and preferably the by-products of which are relatively innocuous to the environment if accidentally lost or spilled.
The process can be carried out in a variety of manners wherein additional in-digester displacements are utilized to affect both pulping and bleaching operations, with necessary washing, so as to reduce downstream processing required for similar delignification results, as compared with common and conventional processes.
The following examples of the present invention represent trials done in a pilot plant operation.

Example 1

Southern yellow pine was cooked in a conventional kraft displacement heating process according to Rapid Displacement Heating (RDH) techniques as marketed by Beloit Corporation. This process is essentially that as disclosed in U.S. patent 4,578,149. The RDH cook continued to a target Kappa level of 16, after which a conventional RDH displacement occurred. Following displacement, rather than discharging the pulp as is conventionally known in the RDH process, a solution of hydrogen peroxide was displaced through the chip column at 80° C for a period of 30 minutes. A noticeably whiter pulp was obtained, measuring 31.9% I.S.O. versus 30.3% I.S.O. in the conventional RDH pulp at 16 Kappa.

Example 2

Again southern yellow pine was cooked in a conventional RDH kraft process in conventional manner to a target Kappa of 16. Following the conventional RDH displacement, oxidized green liquor (sodium polysulfide at low alkalinity) was displaced through the chip column followed by two more subsequent displacements and treatments with moderate amounts of hydrogen peroxide. The resulting pulp was found to have a Kappa number of 7.5, with a digester discharge brightness of 55% I.S.O.

Example 3

A cook similar to that of Example 2 was performed, after which 2% hydrogen peroxide was used in 2 separate post cooking treatments. A Kappa of approximately 10 was achieved with a brightness of 40.7% I.S.O. after the treatments. As a final treatment, an aqueous solution of peroxyacetic acid was displaced through the chip column in a pH4 buffer solution of acetic acid sodium acetate. The pulp discharged from this treatment had a Kappa of approximately 6 and a brightness of 50.8% I.S.O.
Additionally, several other bleaching sequences using peroxide (P) and peroxyacetic acid (A) were compared with similar sequences using chlorine dioxide (D), which is analogous to peroxyacetic acid, but having the undesirable chlorine atoms. The following results were obtained:

Sequence Conventional RDH Sample 4 Sample 5

(EOP)-A-P-A 84.8 86.3 ----

A-P-A ---- 82.2 86.9

(EOP)-D-P-D 90.6 89.7 ----
It can be seen that market pulp brightness can be obtained from the more environmentally friendly sequences containing only hydrogen peroxide and peroxyacetic acid comparable to similar sequences using chlorine dioxide. However, the advantages of in-digester treatment are obtained, including lower installed bleach plant costs and fewer process steps; and the undesirable use of chlorine based reagents is eliminated. While higher chemical utilization may result due to less washing between stages, these disadvantages are offset since all chemicals used are recycled to the chemical recovery cycle. If desired, additional washings by displacement may be used between the various process steps, including the use of chemicals to neutralize or otherwise prepare the digester contents between successive chemical treatments, to make more effective use of reactants displaced through the chip column. An effective process for performing such washing or preparation is to precede a treatment displacement with spent chemical of that treatment from a previous treatment, perhaps in a different digester.
The in-digester bleaching following RDH cooking processes results in shorter process times for a single chip charge. Whereas a conventional process from chip charge at the digester prior to cooking to the time bleached pulp goes to high density storage requires approximately 15 hours, using the present in-digester bleaching process reduces the total elapsed time to approximately 9 hours. While more process time is required in the digester for a single charge, additional digester capacity can be provided in a pulp mill at lower installed cost than is necessary for typical bleaching and washing sequences. Time is saved due to the inherent efficiencies in the displacement processes, as compared to the conventional washing and bleaching process which require thickening, mixing, dilution, and reacting in multiple repetitive steps.
Referring now more specifically to the drawings, various steps in a preferred process according to the present invention will be described.
In Figure 1, a suitable process equipment layout 10 is illustrated for carrying-out the present invention. A plurality of digesters 12 are provided for cooking wood chips according to displacement heating techniques and for carrying out the subsequent reaction steps with the pulped chips according to the present invention. The process layouts in Figures 1 through 10 illustrate numerous essential vessels, tanks, and the like, some of which operate at super atmospheric pressure and others of which operate at atmospheric pressure. Those vessels operating under pressure are illustrated by circular tanks, and square vessels are representative of atmospheric vessels. Various alternative processes are illustrated by optional tanks and vessels shown in dotted lines and to be described more specifically with respect to later drawings.
In Figure 1, the chip filling step of the pulping process is illustrated wherein chips from a chip storage location 14 are transferred to any one of the digesters 12 for beginning the pulping process. The chips may have been pretreated by known processes such as steaming or the like, depending on the chips being used and the desired pulping results. Normally, steam packers or the like, also not part of the present invention, will be used to fill the digester.
Figure 2 illustrates the second step of the conventional Beloit Corporation RDH process which is referred to as the cool liquor fill. Cool black liquor from the cool black liquor accumulator 16 is pumped into the digester at the bottom thereof, in such a manner as to overfill the digester with the cool black liquor flowing from the digester at the top thereof, and back to the cool black liquor accumulator 16. This cool liquor fill expels air from the digester and immerses the chips in liquor. The direct contact of the chips with the liquor preheats the chips, and the residual chemicals remaining in the cool black liquor begin initial reactions for the delignification of the chips. During this fill with cool black liquor, to enhance the precooking reactions, the cool black liquor or a portion of it can be fortified with the addition of cool spent green polysulfide liquor from a green polysulfide liquor storage vessel 18. Additionally, oxidized green liquor from an oxidized green liquor storage tank 20 can be added.
Figure 3 illustrates the conventional warm liquor fill associated with the RDH process, wherein warm black liquor from an accumulator 22 is pumped to the digester containing chips and cool liquor, with the cool black liquor being thus displaced by subsequent quantities of warm black liquor and being transferred back to the cool black liquor accumulator 16. The warm black liquor provided to the digester can be fortified with the addition of anthraquinone from an optional storage tank 24.
For the purpose of generating hot water and/or reducing the volume of warm black liquor in the accumulator 22, the warm black liquor can be passed through a heat exchanger 26 in heat exchange relationship with cold water supplied at 28 for generating hot water removed at 30 and additional volumes of cool black liquor being passed to the cool black liquor accumulator 16.
Figure 4 illustrates the hot liquor fill of the RDH process, in which hot white liquor from a hot white liquor storage vessel 32 is pumped into the digester, perhaps with the addition of hot black liquor from a hot black liquor accumulator 32. The hot white liquor is heated by passing cool white liquor from a cool white liquor tank 36 through a heat exchanger 38 in heat exchange relationship with hot black liquor from the hot black liquor accumulator 34. The hot black liquor leaving the heat exchanger 36 is passed to the warm black liquor accumulator 16. As the hot white liquor is pumped into the digester, warm black liquor in the digester is displaced and is passed to the warm black liquor accumulator 22. Upon complete displacement of the warm black liquor from the digester, some hot liquor may pass from the digester, which is then transferred to the hot black liquor accumulator 34.
Figure 5 illustrates the cooking sequence wherein liquor in the digester is circulated through the digester by means of a pump 40. In the process illustrated, liquor is withdrawn from the digester at a point intermediate the top and bottom ends by the pump 40, and is supplied to inlets at the top and bottom of the digester simultaneously. An external source of heat, such as steam (not shown) may be used to raise the liquor to the desired temperature level. After the desired temperature is reached, the temperature is held for a specified period of time until the desired level of delignification has been reached. These time-to-temperature and time-at-temperature operations are well-known for various wood species, processes and desired results.
Figure 6 illustrates the end of the cooking stage wherein cool spent green polysulfide liquor from the storage vessel 18 is pumped into the bottom of the digester to displace therefrom the cooking liquor present at the end of the cook. The liquors displaced from the digester are passed to the hot black liquor accumulator, the warm black accumulator, or back to the cool spent polysulfide storage vessel, as appropriate.
Figure 7 illustrates a post-cook green polysulfide liquor treatment in the digester. Green polysulfide liquor from the storage unit 18 is pumped into the bottom of the digester after having had added thereto oxidized green liquor from the oxidized green liquor storage tank 20. Displaced liquors are passed to the cool black liquor storage accumulator.
Figure 8 illustrates a post cook peroxide treatment wherein cool spent hydrogen peroxide liquor from a cool spent hydrogen peroxide storage vessel 42 is mixed with fresh hydrogen peroxide from a storage tank 44, the mixture then being pumped into the bottom of the digester to displace therefrom spent polysulfide liquor which is passed to the polysulfide liquor storage vessel 18.
Figure 9 illustrates an alternative peroxyacetic acid treatment wherein peroxyacetic acid from a storage tank 46 is combined with displacement liquor from a storage tank 48 and is pumped into the bottom of the digester, to displace hydrogen peroxide to the hydrogen peroxide storage vessel 42.
Figure 10 illustrates a suitable pulp discharge cycle wherein, upon completion of treatment, pulp from the digester is removed to a dump tank 50 and then to washing stages 52 which may include the addition of hot water at 54. Evacuation of the digester to the dump tank can be by any of several means, including the application of fluid pressure at the top of the digester, either liquid or gaseous, with the pulp and remaining liquid in the digester being passed from the bottom of the digester to the dump tank 50. The digester also can be evacuated by pumping the contents from the digester, either in a continuous pumping manner, as known previously, or through intermittent cyclic pumping as taught in U.S. serial no. 07/412,079. Washing can be performed in any of the known conventional belt or drum vacuum and pressure washers, or a final wash may be performed consistent with the teaching of the aforementioned U.S. patent application 07/068,721. Liquor from the washing stages is passed to a brown stock washer filtrate tank 56 and to the displacement liquor storage tank 48 for subsequent use.
It has been found that various reagents may be utilized to continue pulp processing in the digester, while the pulp is maintained in a substantially stationary column. Following the final displacement in the conventional RDH process, bleaching chemicals can be displaced through the column of pulp to continue the removal of lignin and the whitening and brightening of the pulp. Effective bleaching reactions occur without the previously required separate steps of mixing, blending, thickening, and reacting. Since time is not needed to transfer the pulp between various stages, continuing the bleaching and washing processes in the digester saves overall process time from beginning to end, for a given volume of pulp. While additional digester time is required for any given chip charge, the reduced overall process time minimizes, to some degree, the increased time required in the digester.
It will be recognized that in a typical pulping operation, the various digesters in the plurality of digesters 12 will be operating at different process stages at different times. However, the various accumulators and storage vessels and tanks will receive from and/or supply liquor and chemicals to each of the various digesters, as required. This provides the capability to enhance and make more efficient the various treatments with selected reagents by preparing the chip column for treatment prior to the addition of the reagents. For example, if a peroxide treatment in one digester is required, the actual peroxide treatment displacement can be preceded by a displacement using spent peroxide liquor obtained from the peroxide treatment in a different digester. By preceding the actual treatment displacement with a displacement of the spent liquor, process efficiencies are obtained in that initial reactions are begun with the spent liquors, thereby making the fresh liquor treatment in the following displacement more effective. More complete use of residual reagents in spent liquors is realized. This enables certain treatments to be performed using displacement volumes less than the digester volume, as the necessary treatment will occur as the layer of treatment liquor progresses upwardly through the digester as it is displaced therethrough by the addition of other liquids subsequently. Thus, while some processes may require liquor volumes as great as or greater than the digester volume, others may be performed by displacement through the digester of liquor volumes substantially less than the digester volume, and at a given time in the digester, a plurality of layers of different liquors may be present.
It has been found that various bleaching sequences not using chlorine produce similar advantageous bleached pulp results, without the use of chlorine. These environmentally more acceptable reagents are preferred for the bleaching displacements.

Claims

A cellulose pulp preparation process (10) for removing lignin from cellulose fibers, to free the fibers for subsequent use, such as the formation of paper products, said process comprising:
charging a batch digester (12) with a mass of cellulosic material and digesting liquor;

cooking the mass of cellulosic material and digesting liquors;

quantitatively displacing hot spent liquor under pressure by pumping into the digester (12) liquid under pressure to displace the cooking liquor, while maintaining a column of delignified pulp substantially intact in the digester; and

pumping through the digester additional treatment liquors,
characterized in that said pumping through the digester (12) additional treatment liquors includes pumping nonchlorine containing bleaching reagents into the digester (12) to further delignify, whiten and brighten the pulp in the digester (12) by displacing liquid previously supplied to the digester, while continuing to maintain the delignified pulp as a column in the digester, and
in that said pumping bleaching reagents into the digester is performed in a plurality of steps, successive steps of which being with different bleaching reagents.
The cellulose pulp preparation process (10) as defined in claim 1, characterized in that said step of bleaching the pulp by pumping bleaching reagents into the digester (12) is preceded by pumping into the digester a volume of spent bleaching reagent.
The cellulose pulp preparation process (10) as defined in claim 2, characterized in that said bleaching reagent is selected from the group of non-chlorine containing reagents consisting of hydrogen peroxide (44), oxygen, sodium polysulfide (20) sodium sulfite, sodium, borohydride, sodium hydrosulfite, ozone, and peroxyacetic acid (46).
The cellulose pulp preparation process as defined in claim 1, characterized in that each said pumping bleaching reagent into the digester (12) is preceded by pumping spent similar bleaching reagent into the digester.
The cellulose pulp preparation process as defined in claim 1, characterized in that each of said successive steps of said pumping bleaching reagent into the digester (12) is performed utilizing bleaching reagent selected from the group of non-chlorine containing reagents consisting of hydrogen peroxide (44), oxygen, sodium polysulfide (20), sodium sulfite, sodium borohydride, sodium hydrosulfite, ozone, and peroxyacetic acid (46).