CA1186106A - Process and apparatus for the oxygen delignification of pulp - Google Patents
Process and apparatus for the oxygen delignification of pulpInfo
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- CA1186106A CA1186106A CA000411807A CA411807A CA1186106A CA 1186106 A CA1186106 A CA 1186106A CA 000411807 A CA000411807 A CA 000411807A CA 411807 A CA411807 A CA 411807A CA 1186106 A CA1186106 A CA 1186106A
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Abstract
PROCESS AND APPARATUS FOR THE
OXYGEN DELIGNIFICATION OF PULP
Abstract of the Disclosure A medium consistency oxygen delignification process and apparatus are provided utilizing one or more substantially horizontal agitated tubular reac-tion zones. The process produces rapid delignifica-tion rates at low alkali charges, minimizes oxygen requirements, and yields pulps having high viscosi-ties. The use of rotary screws or paddles in the reaction zone or zones provides the agitation required to enable good mixing of oxygen with the medium consistency pulp and alkaline chemicals as well as controlling the pulp retention time in each reaction zone. The process tolerates the presence of up to 20% entrained black liquor solids without significant adverse effects on the pulp.
OXYGEN DELIGNIFICATION OF PULP
Abstract of the Disclosure A medium consistency oxygen delignification process and apparatus are provided utilizing one or more substantially horizontal agitated tubular reac-tion zones. The process produces rapid delignifica-tion rates at low alkali charges, minimizes oxygen requirements, and yields pulps having high viscosi-ties. The use of rotary screws or paddles in the reaction zone or zones provides the agitation required to enable good mixing of oxygen with the medium consistency pulp and alkaline chemicals as well as controlling the pulp retention time in each reaction zone. The process tolerates the presence of up to 20% entrained black liquor solids without significant adverse effects on the pulp.
Description
PRQCF.SS AND APPARATUS FOR THE
OXYGEN DE I GN I F I CAT I ON OF PULP
Cross-Reference_to Related Ap ~
This application is related -to co-pending Canadian Application Serial No. 385,272 filed September 4, 1981, entitled "Process and Apparatus for the Oxygen Delignification of Pulp". ThiS application is also related to Canadian Application Serial No. 365,411, filed November 25, 1980~ and entitled "Apparatus and Method for Medium Consistency Delignification of Pulp".
Back~round of the Invention This invention relates to a process and apparatus for the oxygen delignification of fibrous materials, and more particularly to the medium consistency oxygen delignification of bleachable grade pulp and other fibrous materials using a series of tubular reaction zones.
Conventional processes for chemical pulping of fibrous raw materials have in the past utilized sulfur-containing compounds while conventional bleaching processes have utilized chlorine containing compounds.
Today, environmental considerations have resulted in a search for nonpolluting processes which can offer the desired puLp yields and qualities. Much attention has been devoted to the use of oxygen in combination with alkaline chemicals to delignify pulp and other fibrous materials.
For example~ several workers have investigated oxygen delignification of high consistency pulp (i.e~, 20-30% consistency). See, Eachus, TAPPI Volume 58, p.
151-154 (Sept. 1975) and Hasvold, 1978 International Sulfite Conference, Montreal, Canada ~September 13, 1978)o Other workers have utilized ~æ
, , .
oxygen delignification in low consistency (i.e., 1-5~ consistency) pulping or bleaching processes.
See, Paper Trade Journal p. 37-39 (July 15, 1978).
Recently, workers have also investiqated processes for the oxygen delignification of pulp mill screen eejects and knots. Such screen rejects and knots have often been heretofore unusable and had to be dewatered and then burned or dumped.
However, Kirschner, Paper Trade Journal, p. 32 1~ (November 15, 1978), has reported the use of a low-consistency oxygen delignification process for kraft and sulfite screen rejects which produces a bleachable grade`of pulp. Hasvold, 1978 Interna-tional Sulfite Conference, Montreal, Canada 15 (September 13, 1978), has reported an oxygen process which deligniEies sulfite knots at a 25% pulp con-sistency.
While most workers have utilized either high or low consistency oxygen delignification processes in working either with pulp or with screen rejects and knots, both of these processes suffer from several disadvantages. Low consistency opera-tion requires a large reactor volume to maintain an acceptable retention time for the pulp. Operating at low consistency also produces large power demands for pumping large volumes of pulp and a high steam usage to heat the pulp in the reactor. Additional-ly, the low concentration of dissolved solids in the spent liquor increases evaporation costs for chemi-cal recovery processes. Operation at high consis-tency, on the other hand, usually requires special dewatering equipment to attain the higher consisten-cy. It is also known that high consistency opera-tion of an oxygen delignification system can result in overheating oP the pulp due to the exothermic ~elignification reaction, as well as pulp degrada~
tion and even comhustion of the pulp. Hiyh consis-tency operation is also sensitive to the presence of impurities such as black liquor solids and oxygen stage solids. These impurities result from incom-plete washing of the pulp and recycling of the oxygen stage filtrate, respectively. Even small concentrations of such impurities may adversely affect both pulp viscosity and yield selectivity.
Carrying out oxygen delignification of pulp at medium consistency (i.e., 8 20% consistency) would be advantageous in that much existing mill equipment, including pulp washing and thickening equipment, is designed to operate in that consisten-cy range and no special equipment would be required to attain that range. Some workers have reported satisfactory results operating at medium consistency on a laboratory scale using rotary autoclaves with no internal means of mixing (See, e.g., Annergren et 20 al, 1979 Pulp Bleaching Conference, Toronto, Canada, June 11-14, 1979; Saukkonen et al, TAPPI Volume 58, p. 117 (1975); and Chang et al TAPPI Volume 56, p.
97 (1973)). However, such equipment is not suitable ~or scale-up to handle large tonnages of pulp on a 25 commercial scale. Other workers have encountered serious problems even on a small laboratory scale.
For example, Eachus, TAPPI Volume 58, pO 151 (1975), reported that oxygen delignification at medium consistency was not practical because of a high 30 alkali requirement, o~ygen starvation, and a limited delignification.
Chang et al, TAPPI 57, p. 123 (1974), concluded that operation at medium consistency produced a considerably lower delignification rate 35 than high consistency operation and also resulted in nonuniform delignification. Although the authors BFN 7089-C ~4~
suggested that these problems could be overcome through the use of higher oxygen pressures in the reaction vessel, use of such higher pressures has several disadvantages. These include greater costs for a thicker-walled reaction vessel, greater diffi culty in feeding pulp against the higher pressure, and an increased danger of gas leakage.
Vertical tube oxygen reactors operating at medium consistency have been constructed for trial purposes. (See Annergren et al, 1979 Pulp Bleaching Conference, Toronto, Canada, June 11-14, 1979, Kleppe et al, TAPPI Vol. 59, p. 77 (1976) and Kleppe et al TAPPI, Volume 64, pp. 87-90(1981).) However, such vertical tube designs have serious deficiencies, including channeling of gas and pulp up through the tower and also the requirement for a high speed mechanical mixer to disperse oxygen into the pulp slurry. Such high speed mixing can lead to pulp degradation and additionally requires substantial power input.
Moreover, during an oxygen delignification process, the liquid feed to the oxygen stage will contain dissolved solids as impurities. These dissolved solids have two sources, black liquor solids entrainment (BLSE) which result from incom-plete washing of the pulp and oxygen stage solids entrainment ~OSSE) which result from recycling oxygen stage filtrate back to the brown stock washers upstream from the oxygen stage. A typical pulp stream from an initial kraft digestion stage will contain up to 170~ by weight dissolved solids based on pulp weight.
Prior high consistency oxygen delignifica-tion systems taught the necessity of thorough washing of the pulp stream from the digestion stage to reduce the black li~uor solids content and avoid BF~ 70B9-C ~5~
their adverse effect on the oxygen stage. For example, E~illstrom et al, Svensk Papperstidning, nr6 (1977), and Jamieson et al, Svensk Papperstidning, nr5 (1973), pp~ 187-181, pp. 167-170, indicate that S a BLSE o~ as little as 1~ has ad~erse effects on pulp viscosity and Kappa number. See also, 5SVL
Swedish Forest Industry Foundation for Water and Air Protection-Environmental Care Project, Technical Summary l1974) pp. 73-76. The medium consistency oxygen delignification system of Kleppe et al, supra, also u~ed highly washed pulp stock which was supplied to the oxygen delignification stage from the washing stage of a continuous digester or from an in-line diffuser washer positioned in front of the oxygen reactor. This need for thorough washing increases the capital costs for such systems because of the additional equipment which is required.
As can be seen, there is a need in the art for a simple and efficient process for oxygen delig-nification of Eibrous materials including pulp aswell as screen rejects and knots which avoids the problems which have plagued the prior art.
Summary of the Invention The present invention meets this need by providing a medium consistency process and apparatus utilizing one or more substantially horizontal agitated tubular reaction zones which produce rapid oxygen delignification rates at low alkali charges, minimize oxygen requirements, and yield pulps having high viscosities. The use of rotary screws or paddles in the one or more reaction zones provides the agitation required to enable good mixing of oxygen with the medium consistency pulp and alkaline chemicals as well as controlling the pulp retention time ;n each reaction zone.
By "medium consistency" it is meant that the consistency of the pulp supplied to and main-tained in the reaction zone is from 8-20~ and preferably 10~15~ This is to be distinguished from prior high (above 20% and preferably 25-30%) and low (less than 8~ and preferably 1-5%) consistency delignification systems. The oxygen delignification system of the present invention can be used to delignify any type of pulp includinq mechanical pulPs, thermomechanical pulps, semichemical or modi-fied mechanical pulps, chemical pulps, and secondary fiber. Additionally, straw, flax, and bagasse can also be delignified as well as pulp mill screen rejects and knots. Preferably, the starting materi-1~ als for the process are unbleached wood pulps suchas softwood Icraft pulps having Kappa numbers between 20 and 50 or hardwood kraft pulps having Kappa numbers between 10 and 30, high yield pulps (i.e., 55-60% yield) cooked to near the point of fiber liberation such as softwood kraft pulps having Kappa numbers between 50 and 80 or hardwood kraft pulps having Kappa numbers between 25 and 50, or fiberized pulp mill screen rejects and knots.
In accordance with the invention, the pulp or other fibrous material may be sent directly from the blow tank of a chip or raw material digester or cooker to brown stock washers which are typically operated in the medium consistency range. In instances where an initially high Kappa number pulp such as a high yield kraft pulp is utilized, the pulp may, optionally, be sent to a further refining stage after leaving the brown stock washers~ In instances where the pulp has been screened, the screen rejects and/or knots removed from the pulp stream may be fiberized in a further refining stage and then recombined with -~he main pulp stream either before or af~er the screeninq stage for the oxygen delignifica~ion process. Alternatively, the pulp from the brown stock washers may be sent directly into the oxygen delignification reactor and further S washed and/or screened aEter delignification. Those post-delignification screen rejects can be refined and recombined with the main pulp stream prior to the oxygen delignification reactor.
A significant advantage of the medium consistency prccess and apparatus of the present invention is its ability to tolerate significantly higher dissolved solids content pulps (both BLSE and OSSE) than prior systems. The system of the present invention can be operated using pulp having a BLSE
content as high as 6-20% by weight, preferably 8-10%, and OSSE content of up to 8% by weight with-out significant adverse effects on the pulp proper-ties or rate of delignification. This perrnits the system of the present invention to be operated with fewer washers, and in the case where the oxygen stage is retrofitted into an existing system, the oxygen stage can be positioned so that some washers which served as pre-bleach stage brown stock washers can be utilized downstream of the oxygen stage to remove oxygen stage impurities. Additionally, the unique system of the present invention is especially suited to supp:Ly any additional oxygen requirements in the deligni~Eication stage due to the presence of increased levels oE dissolved solids.
The pulp is then introduced, at a medium consistency of between 8 and 20~ and preferably 10-15%, into a substantially horizontal tubular reaction vessel where it is contacted with oxygen gas and alkaline chemicals. A thick stock pump is used to feed the pulp into the reaction vessel~ Use of the thick stock pump prevents the loss of gas BFN 7089~C -8-pressure fr~m the vessel and does not severely compact the pulp so that uniform oxygenation and delignification can occur.
Oxygen may be introduced into the delignif-ication s~ste~ either at one injection point or multiple injection points. Typically, oxygen gas will be injected on the lower side of the reaction vessel. Partially spent gas may, optionally, be removed from the delignification system by venting to the atmosphere or it may be collected for recycleO
Additionally, the partially spent gas may be drawn off and utilized for lime kiln enrichment, waste water treatmen~, or other suitable uses. Any organic compounds or carbon monoxide formed during the delignification reaction may be removed by passing the gas through a catalyst bed before reuse.
Alkaline pulping chemicals are also intro-duced into the reaction vessel to aid in the delig-nification. Examples of such alkaline chemicals which are suitable for use in the practice of the present invention include sodium hydroxide, sodium carbonate~ sodium borate compounds, ammonial oxi-dized kraft white liquor, and mixtures thereof.
Preferably, at least a portion of the total charge of alkaline chemicals is added to the pulp prior to its passage through the thick stock feed pump into the first reaction zone. This insures that the pulp has an alkaline pH when the pulp enters the Eirst reaction zone and also lubricates the pulp for easier pumping. An additional portion of the total charge is added to the first reaction zone from one or more injection points along the top of the vessel.
Magnesium sulfate or other known protector chemicals or catalysts for preserving the viscosity and strength of the pulp may be introduced into the pulp either before or after the thick stock feed pump.
Steam is al50 added to the pulp prior to its entry into the thick stock feed pump. The steam aids in expelling excess air from the pulp prior to delignification. Additional steam may be injected into the reaction vessel as needed in order to main-tain the desired reaction temperature, although the exothermic delignification reaction supplies a substantial fraction of the heat requirement.
As the pulp at 8-20% and preferably 10-15%
consistency is introduced into the reaction vessel through the thick stock pump, a rotary screw or series of paddles agitates the pulp, oxygen, and alkaline chemical mixture. It has been found that a solid flight helical screw extending the entire length of the reaction zone produces the gentle agitation necessary for uniform and rapid delignifi-cation. Satisfactory delignification is achieve~ by rotating the screw at a speed of less than about 15 rpm and preferably 1-6 rpm. In another embodiment of the invention, one or more additional substan-tially horizontal tubular reaction vessels are utilized to achieve an additional amount of delig-nification of the pulpo The reaction temperature, alkali charge, type of alkaline chemical, oxygen partial pressure, and retention time depend on the type of material being treated and the desired degree of delignifica-tion. Typically, temperatures may range from 80 to 160C, alkaline chemical char~es from 1 to 20%
calculated as Na2o on oven dry material, and oxygen partial pressures from 30 to 200 psi.
Appropriate retention times have been found to be 5 to 120 minutes.
Accordingly, it is an object of the present invention to provide a system for uniformly and rapidly delignifying pulp at medium consistencies ~FN 7089-C ~10-while minimizing alkali dosages and oxygen require-ments to provide a pulp having high strength proper-ties. It is a further object of the invention to provide a system which can be operated using pulp which contains higher dissolved solids levels that previous systems without adversely affecting pulp properties~ These and other objects and advantages of the invention will become apparent from the ollowing description, the accompanying drawings, and the appended claims.
Brief_Description of the Drawin~s Fig. 1 is a schematic flow diagram illus-trating the overall process of the present invention;
Figs. 2a, 2b, and 2c are schematic flow diagrams illustrating alternative embodiments of the invention;
Fig. 3 is a graph of pulp viscosity versus Kappa number Eor medium consistency o~ygen delignif-ication of pulp in accordance with the practice of the invention;
Fig. 4 is a graph of pulp viscosity versus Kappa number for different p~lp consistencies;
Fig. 5 is a graph of the change in Kappa number versus alkaline chemical charge for agitated and nonagitatecl delignification processes;
Fig. 6 is a graph of alkaline chemical charge versus Kappa number reduction for different pulp consistencies;
Fig. 7 is a graph of the effect of black liquor solids entrainment on Kappa number for medium and high consistency operation;
Fig. 8 is a graph of the effect of black liquor solids entrainment on yield selectivity for medium and high consistency operation;
Fig. 9 is a graph of the effect of black liquor solids entrainment on viscosity selectivity for medium and high consistency operation;
Fig. 10 is a graph of the effect of black liquor solids entrainment on oxygen consumption for medium consistency operation;
Fig. 11 is a graph of the effect of oxygen stage solids entrainment on yield selectivity; and Fig. 12 is a graph of the effect of oxygen 0 stage solids entrainment of viscosity selectivity.
Description_of_the_Pre~erred Embodiments In a preferred embodiment of the invention, pulp ha~ing a BLSE content of from 4.5-20% by weight, and preferably 8-10~, and an OSSE content of up to 8% by weight is introduced into the oxygen reaction ~essel. Because the system can tolerate larger amounts of dissolved solids than prior systems without significant adverse effects on either pulp properties or the rate of delignifica-tion, the system can be positioned more closelydownstream from an initial pulp digestion stage than would otherwise be possible, resulting in potential savings in capital costs of washing equipment.
As illustrated in Fig. 1, pulp at from 8-20% consistency and preferably 10-15~ consistency from the brown stock washers is introduced into a firs~ horizont:al reaction vessel or tube 10 by a thick stoclc pump 12. Inclined reaction tubes may also be employed, but the angle of incline should not exceed approximately 45 degrees to avoid compression and dewatering of the pulp in the lower end of the tube, which will interfere with uniform mixing of oxygen. Additionally, while the reaction vessel is illustrated as a cylindrical reactor tube, noncylindrical tubes such as a twin-screw system may be utilized.
I'g~
Pump 12 may be a Moyno (trademark) progressing cavity pump available from Robbins &
Myers, Inc., Springfield, Ohio. Alternatively, pump 12 may be a Cloverotor (trademark) pump available from the Impco Division of Ingersoll-Rand Co., Nashua, New Hampshire, or a thick stock pump manufactured by Warren Pumps, Inc., Warren, Massachusetts.
It has been found that these pumps are capable of feeding the pulp into the reaction tube against the pressure in that tube without severely compacting the pulp and without any gas losses from the tube. Other feeding devices such as rotary valves or screw feeders are not desirable for use in this invention. A rotary valve allows substantial gas loss from the reaction tube due to the rotation of valve sections which are alternately exposed to the high oxygen pressure in the reactor and then to atmospheric pressure external to the reactor. Use of a screw feeder results in the severe compression and dewatering of pulp so that efficient oxygenation at the proper consistency eange cannot occur.
Prior to introducing the pulp into thick stock pump 12, steam may be injected into the pulp via line 14. The steam aids in expelling excess air from the pulp and also raises the temperature of the pulp somewhat. Additionally, it is desirable to add at least a portion of the total amount of the charge of alkaline material prior to the introduction of the pulp into thick stock pump 12. This addition of alkaline material can be made through line 16. The alkaline material serves to lubricate the pulp for easier pumping as well as to insure that the pulp will have an alkaline p~l when it enters reaction tube 10~ Alternatively, all of the charge may be added at this point.
:~) v~
Generally, the tGtal alkaline material charge will amount to from 1 to 20~ by ~eight calcu-lated as Na2o of the oven dry weight of the raw fibrous material. Examples of alkaline materials suitable for use in this invention include sodium hydroxide, sodium carbonate~ sodium borate compounds, ammonia, oxidized kraft white liquor, and mixtures thereof although other known alkaline pulping liquors may also be used.
Once introduced into reaction tube 10, the pulp undergoes an oxygen delignification reaction.
Oxygen gas is introduced into reaction tube 10 through line 18. Alternatively, oxygen may be introduced at a number of points along the length of tube 10~ Typically, the oxygen partial pressure maintained in the system is from about 30 to 200 psi~ .
Spent gas may be removed from the system by venting it to the atmosphere. Alternatively, it may be recovered for recycle to the reaction tubes or may be used for other purposes such as lime kiln enrichment or waste water treatment. Any organic vapors or carbon monoxide produced during the delignification reaction can be removed by passing the gas through a catalyst bed.
The delignification reaction is carried out by mixing the pulp, oxygen, and alkaline liquor which is in~ected through line 20 and sprayed over the pulp along the length of the tube. By adding the alkaline liquor gradually along the length of the tube rather than all at once as is conventiona in high consistency (i.e., 20-30% consistency) oxygen delignification, better pulp viscosity and strength is achieved. Another advantage to gradual-ly adding the alkaline liquor is that the exothermic BFN 7089-C -14~
delignification reaction is more easily controlled and the risk of localized overheating is diminished.
Satisfactory gentle agitation can be achieved by rotating screw 22 with drive means 23 at a rate of less than about 15 rpm and preferably 1-6 rpm. Preferably, the system is operated so that a gas space remains at the top o reaction vessel 10 and the vessel i5 less than full of pulp. Total retention times of the pulp in th~ system may vary depending upon the nature and condition of the pulp and the desired amount of delignification to be accomplished. Retention times of between 5 and 12C
minutes have been found to be satisfactory. Steam may be injected into the reaction vessel through line 46 to maintain the temperature within the preferred 80-160C range.
Upon completion of the delignification reaction, the pulp exits vessel 10 through outlet 26 and is passed to blow tank 28. The pulp is then discharged using a conventional blow wiper discharg-er.
In another embodiment of the invention illustrated in Fig. 2a, where like components are indicated by like reference numerals, pulp from wasller 50 is sent through refiner 52 ~or further fiberization before bein~ fed to thick stock pump 12~ Since the consistency of the pulp lea~-incl washer 50 will be in the medium consistency range, the pulp can be refined and then fed to the reaction vessel at the same consistency without any need for any dewatering. Ref iner 52 may be utilized in instances where delignification is to be carried out on pulp having an initially high Kappa number such as high yield kraft pulp having an initial Kappa number greater than about 50.
Also illustrated in Fig. 2a is the use of one or more subsequent substantially horizontal reaction vessels such as vessel 30 to carry out ~urther delignification on the pulp. As shown, pulp exiting one end of vessel 10 drops into vessel 30 where it is transported along the length of the vessel with gentle agitation by rotary screw 32 having solid helical flights 34 and driven by a suitable drive means 33. Steam may be added through line 48 to maintain the temperature in vessel 30 within the preferred range of 80-160C. Addition-al oxygen may be injected through line 18a if requiredO
Yet another embodiment of the invention is illustra~ed in Fig. 2b in which like components are represented by like reference numerals. In this embodiment, pulp is transported from an initial cooking or digestion stage through line 54 to screens 56 where oversize slivers, shives, knots, and other impurities are removed. The accepted pulp passes through line 58 into pulp washer 50 while the rejected material is sent to refiner 52 for further fiberization before being recombined with the main pulp stream through line 60. This combined pulp stream is then washed and oxygen delignified as described above to yield a bleachable grade pulp.
Alternatively, the refined material from refiner 52 may be sent through line 62 and be recombined with the main pulp stream prior to screens 56.
In another embodiment of the invention which is illustrated in Fig. 2c, in which like components are represented by like reference numer-als, pulp is transported from an initial cooking or digestion stage through line 54 directly into pulp washer 50. From there, the pulp is fed into vessel 10 where it undergoes oxygen delignificationl Upon completion of the reaction, the pulp exits ~essel 10 through outlet 26 and is passed to blow tank 28.
The pulp may optionally be further washed in washer 61 before being screened by screens 56. Accepted pulp from screens 56 is sent via line 57 to further processing such as a Eurther bleaching stage.
Rejects from screens 56 are passed through line 59 and fiberized in refiner 52. The fiberized rejects are then returned through line 64 to be recombined with the main pulp stream entering vessel 10.
In order that the invention may be better understood, reference is made to the following nonlimiting examples.
Example 1 A northeastern softwood kraft pulp having an initial Kappa number of 29.3 and a viscosity of 26.9 centipoise (cps) was oxygen delignified in accordance with the process of the invention. The reaction conditions were 10~ p~lp consistency, 100 psig total gas pressure, and a 3~ sodium hydroxide dosage by weight based on dry pulpo Retention time ~ in the reaction zone was varied from 8 to 16 to 39 ; minutes by varying the speed of the rotary screw in the reactor. The pulp feed rate was set at either 1.7 ton/day ~T/D) or 5.0 T/D.
The results are illustrated in Fig. 3.
That graph shows a linear relationship between pulp viscosity and Kappa number at up to 60% delignifica-tion, where Initial Kappa No. -% Delignification = Final Kappa No. x 100 Initial Kappa No.
This result is surprising because high pulp viscosi-ties, which are indicative of high pulp strength, were obtained at a relatively high percentage of delignification. Commercial high consistency oxygen delignification systems are limited to a~out 50%
delignification due to severe losses in pulp strength ~measured as greatly lowered pulp viscosi-ties) beyond that point.
Thus, utilizing the medium consistency oxygen delignification process of the present inven-tion with substantially continuous gentle agitation of the pulp, more lignin can be removed from the pulp without loss of pulp strength. This can result in significant reductions in operating and capital costs over high consistency processes because of reduced bleaching costs and the elimina~ion of the need for a conventional chlorine bleaching stage.
Example 2 Medium ~15~) consistency oxygen delignifi-cation was carried out on a softwood kraft pulphaving an initial viscosity of 29.5 using the process of the present invention. The delignifica-tion reaction wa~ carried out for 20 minutes at 110C and at a total gas pressure of 150 psig.
For comparison purposes, the same pulp was deligni-- fied under the same conditions with the exception that in one instance the pulp was maintained at a low (2%) consistency throughout the reaction and in another instance was maintained at a high (28%) consistency throughout the reaction.
The rlesults are illustrated in Fig. 4. As shown by that graph, for the same Kappa number, the medium consistency delignified pulp exhibited higher viscosities than both the high and low consistency pulp.
Example 3 Softwood kraft pulp having an initial Kappa number of 29.5 ~as oxygen delignified in a 2 liter autoclave at 110C and an oxygen gas pressure of 150 psig for a time sufficient to achieve a final Kappa number of 18.5. Several tests were run with the consistency of the pulp varied from 2% to 15~ to 28~. The results are reported in Table I below.
Table I
Consistency % 2 Consumed % CO Evolved Hydrocarbon Based On Based On to CO
4Weiaht of PUl~ Weiqht of Pulp Ratio(Moles)
OXYGEN DE I GN I F I CAT I ON OF PULP
Cross-Reference_to Related Ap ~
This application is related -to co-pending Canadian Application Serial No. 385,272 filed September 4, 1981, entitled "Process and Apparatus for the Oxygen Delignification of Pulp". ThiS application is also related to Canadian Application Serial No. 365,411, filed November 25, 1980~ and entitled "Apparatus and Method for Medium Consistency Delignification of Pulp".
Back~round of the Invention This invention relates to a process and apparatus for the oxygen delignification of fibrous materials, and more particularly to the medium consistency oxygen delignification of bleachable grade pulp and other fibrous materials using a series of tubular reaction zones.
Conventional processes for chemical pulping of fibrous raw materials have in the past utilized sulfur-containing compounds while conventional bleaching processes have utilized chlorine containing compounds.
Today, environmental considerations have resulted in a search for nonpolluting processes which can offer the desired puLp yields and qualities. Much attention has been devoted to the use of oxygen in combination with alkaline chemicals to delignify pulp and other fibrous materials.
For example~ several workers have investigated oxygen delignification of high consistency pulp (i.e~, 20-30% consistency). See, Eachus, TAPPI Volume 58, p.
151-154 (Sept. 1975) and Hasvold, 1978 International Sulfite Conference, Montreal, Canada ~September 13, 1978)o Other workers have utilized ~æ
, , .
oxygen delignification in low consistency (i.e., 1-5~ consistency) pulping or bleaching processes.
See, Paper Trade Journal p. 37-39 (July 15, 1978).
Recently, workers have also investiqated processes for the oxygen delignification of pulp mill screen eejects and knots. Such screen rejects and knots have often been heretofore unusable and had to be dewatered and then burned or dumped.
However, Kirschner, Paper Trade Journal, p. 32 1~ (November 15, 1978), has reported the use of a low-consistency oxygen delignification process for kraft and sulfite screen rejects which produces a bleachable grade`of pulp. Hasvold, 1978 Interna-tional Sulfite Conference, Montreal, Canada 15 (September 13, 1978), has reported an oxygen process which deligniEies sulfite knots at a 25% pulp con-sistency.
While most workers have utilized either high or low consistency oxygen delignification processes in working either with pulp or with screen rejects and knots, both of these processes suffer from several disadvantages. Low consistency opera-tion requires a large reactor volume to maintain an acceptable retention time for the pulp. Operating at low consistency also produces large power demands for pumping large volumes of pulp and a high steam usage to heat the pulp in the reactor. Additional-ly, the low concentration of dissolved solids in the spent liquor increases evaporation costs for chemi-cal recovery processes. Operation at high consis-tency, on the other hand, usually requires special dewatering equipment to attain the higher consisten-cy. It is also known that high consistency opera-tion of an oxygen delignification system can result in overheating oP the pulp due to the exothermic ~elignification reaction, as well as pulp degrada~
tion and even comhustion of the pulp. Hiyh consis-tency operation is also sensitive to the presence of impurities such as black liquor solids and oxygen stage solids. These impurities result from incom-plete washing of the pulp and recycling of the oxygen stage filtrate, respectively. Even small concentrations of such impurities may adversely affect both pulp viscosity and yield selectivity.
Carrying out oxygen delignification of pulp at medium consistency (i.e., 8 20% consistency) would be advantageous in that much existing mill equipment, including pulp washing and thickening equipment, is designed to operate in that consisten-cy range and no special equipment would be required to attain that range. Some workers have reported satisfactory results operating at medium consistency on a laboratory scale using rotary autoclaves with no internal means of mixing (See, e.g., Annergren et 20 al, 1979 Pulp Bleaching Conference, Toronto, Canada, June 11-14, 1979; Saukkonen et al, TAPPI Volume 58, p. 117 (1975); and Chang et al TAPPI Volume 56, p.
97 (1973)). However, such equipment is not suitable ~or scale-up to handle large tonnages of pulp on a 25 commercial scale. Other workers have encountered serious problems even on a small laboratory scale.
For example, Eachus, TAPPI Volume 58, pO 151 (1975), reported that oxygen delignification at medium consistency was not practical because of a high 30 alkali requirement, o~ygen starvation, and a limited delignification.
Chang et al, TAPPI 57, p. 123 (1974), concluded that operation at medium consistency produced a considerably lower delignification rate 35 than high consistency operation and also resulted in nonuniform delignification. Although the authors BFN 7089-C ~4~
suggested that these problems could be overcome through the use of higher oxygen pressures in the reaction vessel, use of such higher pressures has several disadvantages. These include greater costs for a thicker-walled reaction vessel, greater diffi culty in feeding pulp against the higher pressure, and an increased danger of gas leakage.
Vertical tube oxygen reactors operating at medium consistency have been constructed for trial purposes. (See Annergren et al, 1979 Pulp Bleaching Conference, Toronto, Canada, June 11-14, 1979, Kleppe et al, TAPPI Vol. 59, p. 77 (1976) and Kleppe et al TAPPI, Volume 64, pp. 87-90(1981).) However, such vertical tube designs have serious deficiencies, including channeling of gas and pulp up through the tower and also the requirement for a high speed mechanical mixer to disperse oxygen into the pulp slurry. Such high speed mixing can lead to pulp degradation and additionally requires substantial power input.
Moreover, during an oxygen delignification process, the liquid feed to the oxygen stage will contain dissolved solids as impurities. These dissolved solids have two sources, black liquor solids entrainment (BLSE) which result from incom-plete washing of the pulp and oxygen stage solids entrainment ~OSSE) which result from recycling oxygen stage filtrate back to the brown stock washers upstream from the oxygen stage. A typical pulp stream from an initial kraft digestion stage will contain up to 170~ by weight dissolved solids based on pulp weight.
Prior high consistency oxygen delignifica-tion systems taught the necessity of thorough washing of the pulp stream from the digestion stage to reduce the black li~uor solids content and avoid BF~ 70B9-C ~5~
their adverse effect on the oxygen stage. For example, E~illstrom et al, Svensk Papperstidning, nr6 (1977), and Jamieson et al, Svensk Papperstidning, nr5 (1973), pp~ 187-181, pp. 167-170, indicate that S a BLSE o~ as little as 1~ has ad~erse effects on pulp viscosity and Kappa number. See also, 5SVL
Swedish Forest Industry Foundation for Water and Air Protection-Environmental Care Project, Technical Summary l1974) pp. 73-76. The medium consistency oxygen delignification system of Kleppe et al, supra, also u~ed highly washed pulp stock which was supplied to the oxygen delignification stage from the washing stage of a continuous digester or from an in-line diffuser washer positioned in front of the oxygen reactor. This need for thorough washing increases the capital costs for such systems because of the additional equipment which is required.
As can be seen, there is a need in the art for a simple and efficient process for oxygen delig-nification of Eibrous materials including pulp aswell as screen rejects and knots which avoids the problems which have plagued the prior art.
Summary of the Invention The present invention meets this need by providing a medium consistency process and apparatus utilizing one or more substantially horizontal agitated tubular reaction zones which produce rapid oxygen delignification rates at low alkali charges, minimize oxygen requirements, and yield pulps having high viscosities. The use of rotary screws or paddles in the one or more reaction zones provides the agitation required to enable good mixing of oxygen with the medium consistency pulp and alkaline chemicals as well as controlling the pulp retention time ;n each reaction zone.
By "medium consistency" it is meant that the consistency of the pulp supplied to and main-tained in the reaction zone is from 8-20~ and preferably 10~15~ This is to be distinguished from prior high (above 20% and preferably 25-30%) and low (less than 8~ and preferably 1-5%) consistency delignification systems. The oxygen delignification system of the present invention can be used to delignify any type of pulp includinq mechanical pulPs, thermomechanical pulps, semichemical or modi-fied mechanical pulps, chemical pulps, and secondary fiber. Additionally, straw, flax, and bagasse can also be delignified as well as pulp mill screen rejects and knots. Preferably, the starting materi-1~ als for the process are unbleached wood pulps suchas softwood Icraft pulps having Kappa numbers between 20 and 50 or hardwood kraft pulps having Kappa numbers between 10 and 30, high yield pulps (i.e., 55-60% yield) cooked to near the point of fiber liberation such as softwood kraft pulps having Kappa numbers between 50 and 80 or hardwood kraft pulps having Kappa numbers between 25 and 50, or fiberized pulp mill screen rejects and knots.
In accordance with the invention, the pulp or other fibrous material may be sent directly from the blow tank of a chip or raw material digester or cooker to brown stock washers which are typically operated in the medium consistency range. In instances where an initially high Kappa number pulp such as a high yield kraft pulp is utilized, the pulp may, optionally, be sent to a further refining stage after leaving the brown stock washers~ In instances where the pulp has been screened, the screen rejects and/or knots removed from the pulp stream may be fiberized in a further refining stage and then recombined with -~he main pulp stream either before or af~er the screeninq stage for the oxygen delignifica~ion process. Alternatively, the pulp from the brown stock washers may be sent directly into the oxygen delignification reactor and further S washed and/or screened aEter delignification. Those post-delignification screen rejects can be refined and recombined with the main pulp stream prior to the oxygen delignification reactor.
A significant advantage of the medium consistency prccess and apparatus of the present invention is its ability to tolerate significantly higher dissolved solids content pulps (both BLSE and OSSE) than prior systems. The system of the present invention can be operated using pulp having a BLSE
content as high as 6-20% by weight, preferably 8-10%, and OSSE content of up to 8% by weight with-out significant adverse effects on the pulp proper-ties or rate of delignification. This perrnits the system of the present invention to be operated with fewer washers, and in the case where the oxygen stage is retrofitted into an existing system, the oxygen stage can be positioned so that some washers which served as pre-bleach stage brown stock washers can be utilized downstream of the oxygen stage to remove oxygen stage impurities. Additionally, the unique system of the present invention is especially suited to supp:Ly any additional oxygen requirements in the deligni~Eication stage due to the presence of increased levels oE dissolved solids.
The pulp is then introduced, at a medium consistency of between 8 and 20~ and preferably 10-15%, into a substantially horizontal tubular reaction vessel where it is contacted with oxygen gas and alkaline chemicals. A thick stock pump is used to feed the pulp into the reaction vessel~ Use of the thick stock pump prevents the loss of gas BFN 7089~C -8-pressure fr~m the vessel and does not severely compact the pulp so that uniform oxygenation and delignification can occur.
Oxygen may be introduced into the delignif-ication s~ste~ either at one injection point or multiple injection points. Typically, oxygen gas will be injected on the lower side of the reaction vessel. Partially spent gas may, optionally, be removed from the delignification system by venting to the atmosphere or it may be collected for recycleO
Additionally, the partially spent gas may be drawn off and utilized for lime kiln enrichment, waste water treatmen~, or other suitable uses. Any organic compounds or carbon monoxide formed during the delignification reaction may be removed by passing the gas through a catalyst bed before reuse.
Alkaline pulping chemicals are also intro-duced into the reaction vessel to aid in the delig-nification. Examples of such alkaline chemicals which are suitable for use in the practice of the present invention include sodium hydroxide, sodium carbonate~ sodium borate compounds, ammonial oxi-dized kraft white liquor, and mixtures thereof.
Preferably, at least a portion of the total charge of alkaline chemicals is added to the pulp prior to its passage through the thick stock feed pump into the first reaction zone. This insures that the pulp has an alkaline pH when the pulp enters the Eirst reaction zone and also lubricates the pulp for easier pumping. An additional portion of the total charge is added to the first reaction zone from one or more injection points along the top of the vessel.
Magnesium sulfate or other known protector chemicals or catalysts for preserving the viscosity and strength of the pulp may be introduced into the pulp either before or after the thick stock feed pump.
Steam is al50 added to the pulp prior to its entry into the thick stock feed pump. The steam aids in expelling excess air from the pulp prior to delignification. Additional steam may be injected into the reaction vessel as needed in order to main-tain the desired reaction temperature, although the exothermic delignification reaction supplies a substantial fraction of the heat requirement.
As the pulp at 8-20% and preferably 10-15%
consistency is introduced into the reaction vessel through the thick stock pump, a rotary screw or series of paddles agitates the pulp, oxygen, and alkaline chemical mixture. It has been found that a solid flight helical screw extending the entire length of the reaction zone produces the gentle agitation necessary for uniform and rapid delignifi-cation. Satisfactory delignification is achieve~ by rotating the screw at a speed of less than about 15 rpm and preferably 1-6 rpm. In another embodiment of the invention, one or more additional substan-tially horizontal tubular reaction vessels are utilized to achieve an additional amount of delig-nification of the pulpo The reaction temperature, alkali charge, type of alkaline chemical, oxygen partial pressure, and retention time depend on the type of material being treated and the desired degree of delignifica-tion. Typically, temperatures may range from 80 to 160C, alkaline chemical char~es from 1 to 20%
calculated as Na2o on oven dry material, and oxygen partial pressures from 30 to 200 psi.
Appropriate retention times have been found to be 5 to 120 minutes.
Accordingly, it is an object of the present invention to provide a system for uniformly and rapidly delignifying pulp at medium consistencies ~FN 7089-C ~10-while minimizing alkali dosages and oxygen require-ments to provide a pulp having high strength proper-ties. It is a further object of the invention to provide a system which can be operated using pulp which contains higher dissolved solids levels that previous systems without adversely affecting pulp properties~ These and other objects and advantages of the invention will become apparent from the ollowing description, the accompanying drawings, and the appended claims.
Brief_Description of the Drawin~s Fig. 1 is a schematic flow diagram illus-trating the overall process of the present invention;
Figs. 2a, 2b, and 2c are schematic flow diagrams illustrating alternative embodiments of the invention;
Fig. 3 is a graph of pulp viscosity versus Kappa number Eor medium consistency o~ygen delignif-ication of pulp in accordance with the practice of the invention;
Fig. 4 is a graph of pulp viscosity versus Kappa number for different p~lp consistencies;
Fig. 5 is a graph of the change in Kappa number versus alkaline chemical charge for agitated and nonagitatecl delignification processes;
Fig. 6 is a graph of alkaline chemical charge versus Kappa number reduction for different pulp consistencies;
Fig. 7 is a graph of the effect of black liquor solids entrainment on Kappa number for medium and high consistency operation;
Fig. 8 is a graph of the effect of black liquor solids entrainment on yield selectivity for medium and high consistency operation;
Fig. 9 is a graph of the effect of black liquor solids entrainment on viscosity selectivity for medium and high consistency operation;
Fig. 10 is a graph of the effect of black liquor solids entrainment on oxygen consumption for medium consistency operation;
Fig. 11 is a graph of the effect of oxygen stage solids entrainment on yield selectivity; and Fig. 12 is a graph of the effect of oxygen 0 stage solids entrainment of viscosity selectivity.
Description_of_the_Pre~erred Embodiments In a preferred embodiment of the invention, pulp ha~ing a BLSE content of from 4.5-20% by weight, and preferably 8-10~, and an OSSE content of up to 8% by weight is introduced into the oxygen reaction ~essel. Because the system can tolerate larger amounts of dissolved solids than prior systems without significant adverse effects on either pulp properties or the rate of delignifica-tion, the system can be positioned more closelydownstream from an initial pulp digestion stage than would otherwise be possible, resulting in potential savings in capital costs of washing equipment.
As illustrated in Fig. 1, pulp at from 8-20% consistency and preferably 10-15~ consistency from the brown stock washers is introduced into a firs~ horizont:al reaction vessel or tube 10 by a thick stoclc pump 12. Inclined reaction tubes may also be employed, but the angle of incline should not exceed approximately 45 degrees to avoid compression and dewatering of the pulp in the lower end of the tube, which will interfere with uniform mixing of oxygen. Additionally, while the reaction vessel is illustrated as a cylindrical reactor tube, noncylindrical tubes such as a twin-screw system may be utilized.
I'g~
Pump 12 may be a Moyno (trademark) progressing cavity pump available from Robbins &
Myers, Inc., Springfield, Ohio. Alternatively, pump 12 may be a Cloverotor (trademark) pump available from the Impco Division of Ingersoll-Rand Co., Nashua, New Hampshire, or a thick stock pump manufactured by Warren Pumps, Inc., Warren, Massachusetts.
It has been found that these pumps are capable of feeding the pulp into the reaction tube against the pressure in that tube without severely compacting the pulp and without any gas losses from the tube. Other feeding devices such as rotary valves or screw feeders are not desirable for use in this invention. A rotary valve allows substantial gas loss from the reaction tube due to the rotation of valve sections which are alternately exposed to the high oxygen pressure in the reactor and then to atmospheric pressure external to the reactor. Use of a screw feeder results in the severe compression and dewatering of pulp so that efficient oxygenation at the proper consistency eange cannot occur.
Prior to introducing the pulp into thick stock pump 12, steam may be injected into the pulp via line 14. The steam aids in expelling excess air from the pulp and also raises the temperature of the pulp somewhat. Additionally, it is desirable to add at least a portion of the total amount of the charge of alkaline material prior to the introduction of the pulp into thick stock pump 12. This addition of alkaline material can be made through line 16. The alkaline material serves to lubricate the pulp for easier pumping as well as to insure that the pulp will have an alkaline p~l when it enters reaction tube 10~ Alternatively, all of the charge may be added at this point.
:~) v~
Generally, the tGtal alkaline material charge will amount to from 1 to 20~ by ~eight calcu-lated as Na2o of the oven dry weight of the raw fibrous material. Examples of alkaline materials suitable for use in this invention include sodium hydroxide, sodium carbonate~ sodium borate compounds, ammonia, oxidized kraft white liquor, and mixtures thereof although other known alkaline pulping liquors may also be used.
Once introduced into reaction tube 10, the pulp undergoes an oxygen delignification reaction.
Oxygen gas is introduced into reaction tube 10 through line 18. Alternatively, oxygen may be introduced at a number of points along the length of tube 10~ Typically, the oxygen partial pressure maintained in the system is from about 30 to 200 psi~ .
Spent gas may be removed from the system by venting it to the atmosphere. Alternatively, it may be recovered for recycle to the reaction tubes or may be used for other purposes such as lime kiln enrichment or waste water treatment. Any organic vapors or carbon monoxide produced during the delignification reaction can be removed by passing the gas through a catalyst bed.
The delignification reaction is carried out by mixing the pulp, oxygen, and alkaline liquor which is in~ected through line 20 and sprayed over the pulp along the length of the tube. By adding the alkaline liquor gradually along the length of the tube rather than all at once as is conventiona in high consistency (i.e., 20-30% consistency) oxygen delignification, better pulp viscosity and strength is achieved. Another advantage to gradual-ly adding the alkaline liquor is that the exothermic BFN 7089-C -14~
delignification reaction is more easily controlled and the risk of localized overheating is diminished.
Satisfactory gentle agitation can be achieved by rotating screw 22 with drive means 23 at a rate of less than about 15 rpm and preferably 1-6 rpm. Preferably, the system is operated so that a gas space remains at the top o reaction vessel 10 and the vessel i5 less than full of pulp. Total retention times of the pulp in th~ system may vary depending upon the nature and condition of the pulp and the desired amount of delignification to be accomplished. Retention times of between 5 and 12C
minutes have been found to be satisfactory. Steam may be injected into the reaction vessel through line 46 to maintain the temperature within the preferred 80-160C range.
Upon completion of the delignification reaction, the pulp exits vessel 10 through outlet 26 and is passed to blow tank 28. The pulp is then discharged using a conventional blow wiper discharg-er.
In another embodiment of the invention illustrated in Fig. 2a, where like components are indicated by like reference numerals, pulp from wasller 50 is sent through refiner 52 ~or further fiberization before bein~ fed to thick stock pump 12~ Since the consistency of the pulp lea~-incl washer 50 will be in the medium consistency range, the pulp can be refined and then fed to the reaction vessel at the same consistency without any need for any dewatering. Ref iner 52 may be utilized in instances where delignification is to be carried out on pulp having an initially high Kappa number such as high yield kraft pulp having an initial Kappa number greater than about 50.
Also illustrated in Fig. 2a is the use of one or more subsequent substantially horizontal reaction vessels such as vessel 30 to carry out ~urther delignification on the pulp. As shown, pulp exiting one end of vessel 10 drops into vessel 30 where it is transported along the length of the vessel with gentle agitation by rotary screw 32 having solid helical flights 34 and driven by a suitable drive means 33. Steam may be added through line 48 to maintain the temperature in vessel 30 within the preferred range of 80-160C. Addition-al oxygen may be injected through line 18a if requiredO
Yet another embodiment of the invention is illustra~ed in Fig. 2b in which like components are represented by like reference numerals. In this embodiment, pulp is transported from an initial cooking or digestion stage through line 54 to screens 56 where oversize slivers, shives, knots, and other impurities are removed. The accepted pulp passes through line 58 into pulp washer 50 while the rejected material is sent to refiner 52 for further fiberization before being recombined with the main pulp stream through line 60. This combined pulp stream is then washed and oxygen delignified as described above to yield a bleachable grade pulp.
Alternatively, the refined material from refiner 52 may be sent through line 62 and be recombined with the main pulp stream prior to screens 56.
In another embodiment of the invention which is illustrated in Fig. 2c, in which like components are represented by like reference numer-als, pulp is transported from an initial cooking or digestion stage through line 54 directly into pulp washer 50. From there, the pulp is fed into vessel 10 where it undergoes oxygen delignificationl Upon completion of the reaction, the pulp exits ~essel 10 through outlet 26 and is passed to blow tank 28.
The pulp may optionally be further washed in washer 61 before being screened by screens 56. Accepted pulp from screens 56 is sent via line 57 to further processing such as a Eurther bleaching stage.
Rejects from screens 56 are passed through line 59 and fiberized in refiner 52. The fiberized rejects are then returned through line 64 to be recombined with the main pulp stream entering vessel 10.
In order that the invention may be better understood, reference is made to the following nonlimiting examples.
Example 1 A northeastern softwood kraft pulp having an initial Kappa number of 29.3 and a viscosity of 26.9 centipoise (cps) was oxygen delignified in accordance with the process of the invention. The reaction conditions were 10~ p~lp consistency, 100 psig total gas pressure, and a 3~ sodium hydroxide dosage by weight based on dry pulpo Retention time ~ in the reaction zone was varied from 8 to 16 to 39 ; minutes by varying the speed of the rotary screw in the reactor. The pulp feed rate was set at either 1.7 ton/day ~T/D) or 5.0 T/D.
The results are illustrated in Fig. 3.
That graph shows a linear relationship between pulp viscosity and Kappa number at up to 60% delignifica-tion, where Initial Kappa No. -% Delignification = Final Kappa No. x 100 Initial Kappa No.
This result is surprising because high pulp viscosi-ties, which are indicative of high pulp strength, were obtained at a relatively high percentage of delignification. Commercial high consistency oxygen delignification systems are limited to a~out 50%
delignification due to severe losses in pulp strength ~measured as greatly lowered pulp viscosi-ties) beyond that point.
Thus, utilizing the medium consistency oxygen delignification process of the present inven-tion with substantially continuous gentle agitation of the pulp, more lignin can be removed from the pulp without loss of pulp strength. This can result in significant reductions in operating and capital costs over high consistency processes because of reduced bleaching costs and the elimina~ion of the need for a conventional chlorine bleaching stage.
Example 2 Medium ~15~) consistency oxygen delignifi-cation was carried out on a softwood kraft pulphaving an initial viscosity of 29.5 using the process of the present invention. The delignifica-tion reaction wa~ carried out for 20 minutes at 110C and at a total gas pressure of 150 psig.
For comparison purposes, the same pulp was deligni-- fied under the same conditions with the exception that in one instance the pulp was maintained at a low (2%) consistency throughout the reaction and in another instance was maintained at a high (28%) consistency throughout the reaction.
The rlesults are illustrated in Fig. 4. As shown by that graph, for the same Kappa number, the medium consistency delignified pulp exhibited higher viscosities than both the high and low consistency pulp.
Example 3 Softwood kraft pulp having an initial Kappa number of 29.5 ~as oxygen delignified in a 2 liter autoclave at 110C and an oxygen gas pressure of 150 psig for a time sufficient to achieve a final Kappa number of 18.5. Several tests were run with the consistency of the pulp varied from 2% to 15~ to 28~. The results are reported in Table I below.
Table I
Consistency % 2 Consumed % CO Evolved Hydrocarbon Based On Based On to CO
4Weiaht of PUl~ Weiqht of Pulp Ratio(Moles)
2 3.2 0.010 0.2 lS 1.5 0.015 0.2 1028 ~.5 0.024 0.2 For a working system, it is necessary to provide venting of the reactor gases in order to remove combustible reaction products such as carbon monoxide and hydrocarbons. The resulting dilution of the gas in the reactor with oxygen maintains a safe condition.
Using the data from Table I, material balance calculations were made to determine the amount of oxygen required to maintain the reactor in a safe condition of 30% of the lower explosive limit (LEL) of combustibles. The results are reported in Table II below~
Table II
Consistency 2 Consumed* 2 Required*
2 3.2 3.58 1.5 2.06 28 1.~ 2.42 *based on weight of pulp.
The results show that the medium consistency process has lower oxygen requirements.
Example 4 A high yield sof~wood kraft pulp near the point of fiber liberation and having an initial Kappa number of 59.4 was oxygen delignified in an autoclave at temperatures ranging from 100-130C
and at a total gas pressure of 120 psig. The pulp was maintained at a medium consistency for an approximately 15 minute reac~ion time as the charge of alkaline ~caustic) chemicals was varied from 2-6 ~ by weight based on oven dry pulp.
As shown in Fig. 5, the curve labeled A in which the pulp was continuously gently agitated in the autoclave shows a greater reduction in Kappa number (indicative of a greater delignification rate) than the curve labeled B in which no agitation was performed. The results show the importance of gentle agitation of pulp when delignifying at medium consistency to improve the rate of delignification of the pulp.
Example 5 ; Tests were made using a softwood kraft pulp having an initial Kappa number of 29.5 to determine the effect of pulp consistency on the extent of delignification for a given alkaline chemical (caustic) dosage and reaction time. The tests were carried out in a 2 liter autoclave at 110C and 150 psig gas pressure for 20 minutes. Low (2%) consistency tests were done under conditions of vigorous agitation (rotation of reactor at 1250 rpm) while the medium (15%) and high (28%) consistency tests were conducted without agitation. The results are shown in Fig. 6.
As can be seen, surprisingly the extent of delignification for the medium and high consistency tests were nearly identical at a given causti~
charge. The low consistency ~ests resulted in ~ 3 BFN 7089-C -~~
substantially less deligniEication. Therefore, longer reaction times would be required for a low consistency process to achieve the same reduction in Kappa number as for either a medium or high consis-tency process~
Example 6 Tests were made to evaluate the effects, ifany of black liquor solids entrainment (BLSE) on pulp viscosity and yield selectivities in an oxygen deliqnification reaction. Experimentally, a high yield northern softwood Kraft pulp having an initial Kappa number of 59.4 was oxygen delignified for a reaction time of 20 minutes at 110C at a total pressure of 120 psig. A caustic charge of 5% NaOH
based on dry pulp was us~d. No protector chemicals were added. The amount of black liquor solids added to each test cook is expressed as a weight percen-tage of pulp as follows:
20weight of total organic BLSE(%~ = and inorganic black solids x 100 weight of pulp The range of BLSE studied was 0-40%. The extent of delignification for both high (28%) and medium (15%) consistency operation was tested for comparison purposes. The results are shown in Fig.
As can be seen, surprisingly at medium consis-tency the final Kappa number drops and then rises as the BLSE loading is increased indicating an enhance-ment of the rate of delignification. The maximum extent of delignification appears to be in the range of 2-4% BLSE. When the BLSE reaches 16%, the final Kappa number reached is the same as that reached at BLSE=0%. Further addition of solids above 20% BLSE
results in a severe retarding effect on the extent of delignification. By comparison, BLSE has a severe negative effect on the rate of delignifica-tion over the entire range tested (0-20%) for high consistency operation. The data also show that high consistency processing is much more sensitive to the negative rate effect of BLSE than medium consistency operation, over the range of BLSE studied. Se~ Fig.
7.
The effect of BLSE on yield selectivity was tested. The results are illustrated in FigO 8. The solid curve represents a base case for 0% BLSE at 15% consistency with caustic charges ranging from
Using the data from Table I, material balance calculations were made to determine the amount of oxygen required to maintain the reactor in a safe condition of 30% of the lower explosive limit (LEL) of combustibles. The results are reported in Table II below~
Table II
Consistency 2 Consumed* 2 Required*
2 3.2 3.58 1.5 2.06 28 1.~ 2.42 *based on weight of pulp.
The results show that the medium consistency process has lower oxygen requirements.
Example 4 A high yield sof~wood kraft pulp near the point of fiber liberation and having an initial Kappa number of 59.4 was oxygen delignified in an autoclave at temperatures ranging from 100-130C
and at a total gas pressure of 120 psig. The pulp was maintained at a medium consistency for an approximately 15 minute reac~ion time as the charge of alkaline ~caustic) chemicals was varied from 2-6 ~ by weight based on oven dry pulp.
As shown in Fig. 5, the curve labeled A in which the pulp was continuously gently agitated in the autoclave shows a greater reduction in Kappa number (indicative of a greater delignification rate) than the curve labeled B in which no agitation was performed. The results show the importance of gentle agitation of pulp when delignifying at medium consistency to improve the rate of delignification of the pulp.
Example 5 ; Tests were made using a softwood kraft pulp having an initial Kappa number of 29.5 to determine the effect of pulp consistency on the extent of delignification for a given alkaline chemical (caustic) dosage and reaction time. The tests were carried out in a 2 liter autoclave at 110C and 150 psig gas pressure for 20 minutes. Low (2%) consistency tests were done under conditions of vigorous agitation (rotation of reactor at 1250 rpm) while the medium (15%) and high (28%) consistency tests were conducted without agitation. The results are shown in Fig. 6.
As can be seen, surprisingly the extent of delignification for the medium and high consistency tests were nearly identical at a given causti~
charge. The low consistency ~ests resulted in ~ 3 BFN 7089-C -~~
substantially less deligniEication. Therefore, longer reaction times would be required for a low consistency process to achieve the same reduction in Kappa number as for either a medium or high consis-tency process~
Example 6 Tests were made to evaluate the effects, ifany of black liquor solids entrainment (BLSE) on pulp viscosity and yield selectivities in an oxygen deliqnification reaction. Experimentally, a high yield northern softwood Kraft pulp having an initial Kappa number of 59.4 was oxygen delignified for a reaction time of 20 minutes at 110C at a total pressure of 120 psig. A caustic charge of 5% NaOH
based on dry pulp was us~d. No protector chemicals were added. The amount of black liquor solids added to each test cook is expressed as a weight percen-tage of pulp as follows:
20weight of total organic BLSE(%~ = and inorganic black solids x 100 weight of pulp The range of BLSE studied was 0-40%. The extent of delignification for both high (28%) and medium (15%) consistency operation was tested for comparison purposes. The results are shown in Fig.
As can be seen, surprisingly at medium consis-tency the final Kappa number drops and then rises as the BLSE loading is increased indicating an enhance-ment of the rate of delignification. The maximum extent of delignification appears to be in the range of 2-4% BLSE. When the BLSE reaches 16%, the final Kappa number reached is the same as that reached at BLSE=0%. Further addition of solids above 20% BLSE
results in a severe retarding effect on the extent of delignification. By comparison, BLSE has a severe negative effect on the rate of delignifica-tion over the entire range tested (0-20%) for high consistency operation. The data also show that high consistency processing is much more sensitive to the negative rate effect of BLSE than medium consistency operation, over the range of BLSE studied. Se~ Fig.
7.
The effect of BLSE on yield selectivity was tested. The results are illustrated in FigO 8. The solid curve represents a base case for 0% BLSE at 15% consistency with caustic charges ranging from
3-6% NaOH by weight based on dry pulp. The data indicate that BLSE has no effect on yield selectivi-ty at medium consistency operation for at least up to 20% BLSE. However, yield selectivity is adverse-ly affected at high consistency operation at about 10% BLSE and above. The data show that medium consistency operation is much less sensitive than high consistency operation to BLSE with respect to yield selectivity.
The effect of BLSE on viscosity selectivi~y was also tested. The results are illustrated in Fig. 9. The solid curve represents a base case for 0~ BLSE at 15~ consistency for caustic charges ranging from 3-6% NaOH by weight based on dry pulp.
The data indicate that at medium consistency opera-tion, a BLSE of up to about 8-10% could be tolerated without any significant loss in viscosity selectivi-ty. Moreover, the viscosity selectivity appears to decrease at a faster rate at high consistency opera-tion than at medium consistency as BLSE increases.
For example, at 20~ BLSE, essentially the same viscosities were achieved at both medium and high consistency while the medium consistency Kappa number was 2.5 points lower.
The effect of BLSE on oxygen consumption in a medium consistency system was tested using a softwood kraft pulp having an initial Kappa number of 60. The results are illustrated in Fig. 10. As can be seen, the consumption of oxygen increases substantially linearly with increasing black liquor solids content. As the results indicate, between about 0.1~% by weight oxygen (slope M,) and 0.18% by weight oxygen (slope m3) are consumed for each percentage unit by weight of black liquor solids present in the feed liquor~ The average amount of oxygen consumed was about 0.14% by weight (slope m2) per unit percentage of BLS.
These results are significant because the dissolved wood components and spent cooking chemi-cals which make up black liquor are present in the pulp leaving a batch digester in an amount which is about 175% of the weight of the pulp. Before a conventional bleaching operation, this amount must be dramatically reduced to a BLSE of about 1-3 based on pulp weight by countercurrent washing operations. The closer an oxygen stage reactor can be positioned to the digestion stage, the greater the savings in capital and operational costs of both pre-and post-oxygen stage washing equipment. The data indicate that an oxygen delignification reactor operating at medium consistency can tolerate a BLSE
of 10% without adversely affecting either the reac-tion rate or selectivity. High consistency opera-~ion is much more sensitive to the negative effectsof BLSE on both rate and selectivity.
Moreover, since oxygen consumption increases with increasing black liquor solids content, delignification systems in which only limited amounts of oxygen can be supplied will be especially sensitive to the presence of black liquor solidsO The system of the present invention, however, has provision for adding additional oxygen as needed at several locations. This also enables the system of the present invention to tolerate the presence of increased amounts of black liquor solids.
Example 7 Tests were also made to evaluate the effects, if any, oE oxygen stage solids entrainment (OSSE) on pulp yield and viscosity selectivities in an oxygen delignification reaction. Experimentally, repulped paper bag feedstock having an initial Kappa number of 57.5 was oxygen delignified for a reaction time of 20 minutes at 112C at a total pressure of 100 psig. The pulp was of 15~ consistency, and a lS caustic charge of 4~ ~aOH based on dry pulp was used. About 0.3% MgSO4 based on dry pulp was added as a protector. The amount of oxygen stage solids added to each test cook is expressed as a weight percentage o pulp as follows:
~0 weight of dissolved OSSE(~j = solids from oxy~en sta~e x 100 weight of pulp The effect of OSSE on yield selectivity was tested. The results are illustrated in Fig. 11.
The area within the lines labeled ~ = 0.15 and ~ = 0.17 represent the potential theoretical yield of the reaction. The dashed line indicates the results for the base case of 0% solids. As can be seen, surprisingly, the presence of up to 8~ OSSE
improved the yield selectivity slightly over the base case.
The effect of OSSE on viscosity selectivity was tested. The results are illustrated in Fig.
12. As can be seen, for medium consistency opera-BFN 708g~C -24-tion, the presence of up to 8~ OSSE had no adverse effect on the viscosity selectivity of the pulp.
While the methods and apparatus herein described constitute preferred embodiments of the invention, it is to be understood that the invention is not limited to these precise methods and appara-tus, and that changes may be made in either without departing from the scope of the invention, which is defined in the appended claims.
~0
The effect of BLSE on viscosity selectivi~y was also tested. The results are illustrated in Fig. 9. The solid curve represents a base case for 0~ BLSE at 15~ consistency for caustic charges ranging from 3-6% NaOH by weight based on dry pulp.
The data indicate that at medium consistency opera-tion, a BLSE of up to about 8-10% could be tolerated without any significant loss in viscosity selectivi-ty. Moreover, the viscosity selectivity appears to decrease at a faster rate at high consistency opera-tion than at medium consistency as BLSE increases.
For example, at 20~ BLSE, essentially the same viscosities were achieved at both medium and high consistency while the medium consistency Kappa number was 2.5 points lower.
The effect of BLSE on oxygen consumption in a medium consistency system was tested using a softwood kraft pulp having an initial Kappa number of 60. The results are illustrated in Fig. 10. As can be seen, the consumption of oxygen increases substantially linearly with increasing black liquor solids content. As the results indicate, between about 0.1~% by weight oxygen (slope M,) and 0.18% by weight oxygen (slope m3) are consumed for each percentage unit by weight of black liquor solids present in the feed liquor~ The average amount of oxygen consumed was about 0.14% by weight (slope m2) per unit percentage of BLS.
These results are significant because the dissolved wood components and spent cooking chemi-cals which make up black liquor are present in the pulp leaving a batch digester in an amount which is about 175% of the weight of the pulp. Before a conventional bleaching operation, this amount must be dramatically reduced to a BLSE of about 1-3 based on pulp weight by countercurrent washing operations. The closer an oxygen stage reactor can be positioned to the digestion stage, the greater the savings in capital and operational costs of both pre-and post-oxygen stage washing equipment. The data indicate that an oxygen delignification reactor operating at medium consistency can tolerate a BLSE
of 10% without adversely affecting either the reac-tion rate or selectivity. High consistency opera-~ion is much more sensitive to the negative effectsof BLSE on both rate and selectivity.
Moreover, since oxygen consumption increases with increasing black liquor solids content, delignification systems in which only limited amounts of oxygen can be supplied will be especially sensitive to the presence of black liquor solidsO The system of the present invention, however, has provision for adding additional oxygen as needed at several locations. This also enables the system of the present invention to tolerate the presence of increased amounts of black liquor solids.
Example 7 Tests were also made to evaluate the effects, if any, oE oxygen stage solids entrainment (OSSE) on pulp yield and viscosity selectivities in an oxygen delignification reaction. Experimentally, repulped paper bag feedstock having an initial Kappa number of 57.5 was oxygen delignified for a reaction time of 20 minutes at 112C at a total pressure of 100 psig. The pulp was of 15~ consistency, and a lS caustic charge of 4~ ~aOH based on dry pulp was used. About 0.3% MgSO4 based on dry pulp was added as a protector. The amount of oxygen stage solids added to each test cook is expressed as a weight percentage o pulp as follows:
~0 weight of dissolved OSSE(~j = solids from oxy~en sta~e x 100 weight of pulp The effect of OSSE on yield selectivity was tested. The results are illustrated in Fig. 11.
The area within the lines labeled ~ = 0.15 and ~ = 0.17 represent the potential theoretical yield of the reaction. The dashed line indicates the results for the base case of 0% solids. As can be seen, surprisingly, the presence of up to 8~ OSSE
improved the yield selectivity slightly over the base case.
The effect of OSSE on viscosity selectivity was tested. The results are illustrated in Fig.
12. As can be seen, for medium consistency opera-BFN 708g~C -24-tion, the presence of up to 8~ OSSE had no adverse effect on the viscosity selectivity of the pulp.
While the methods and apparatus herein described constitute preferred embodiments of the invention, it is to be understood that the invention is not limited to these precise methods and appara-tus, and that changes may be made in either without departing from the scope of the invention, which is defined in the appended claims.
~0
Claims (13)
1. A process for the continuous oxygen delig-nification of medium consistency pulp containing from 6% to 20% black liquor solids entrainment (BLSE) such that reduced preliminary washing is required comprising the steps of screening said pulp, fiberizing the screen rejects and recombining the fiberized rejects with said pulp prior to the screening step, introducing accepted pulp from a screening step at a consistency of from 8 to 20% and alkaline materials into a substantially horizontal reaction zone and maintaining said pulp at medium consistency throughout said reaction zone, adding oxygen to said reaction zone to delignify said pulp, and transporting the pulp through said reaction zone while gently agitating the mixture of pulp, oxygen, and alkaline materials for a time sufficient for delignification to occur.
2. A process for the continuous oxygen delig-nification of medium consistency pulp containing from 6% to 20% black liquor solids entrainment (BLSE) such that reduced preliminary washing is required comprising the steps of introducing said pulp at a consistency of from 8 to 20% and alkaline materials into a substantially horizontal reaction zone and maintaining said pulp at medium consistency throughout said reaction zone, adding oxygen to said reaction zone to delignify said pulp, transporting the pulp through said reaction zone while gently agitating the mixture of pulp, oxygen, and alkaline materials for a time sufficient for delignification to occur, screening the delignified pulp, fiberizing the screen rejects, and recombining the fiberized rejects with said pulp prior to the pulp being introduced into said reaction zone.
3. The process of claim 2 including the step of washing said delignified pulp before screening it.
4. A process for the continuous oxygen delig-nification of medium consistency pulp containing from 6% to 20% black liquor solids entrainment (BLSE) such that reduced preliminary washing is required comprising the steps of introducing said pulp at a consistency of from 8 to 20% and alkaline materials into a substantially horizontal reaction zone and maintaining said pulp at medium consistency throughout said reaction zone, adding oxygen to said reaction zone to delignify said pulp, and transport-ing the pulp through said reaction zone while gently agitating the mixture of pulp, oxygen, and alkaline materials for a time sufficient for delignification to occur.
5. The process of claim 4 in which said pulp is screened and the screened rejects are fiberized and recombined with said pulp immediately prior to being introduced into said reaction zone.
6. The process of claim 1, 2, or 4 wherein the medium consistency pulp to be oxygen delignified contains 8% to 10% by weight black liquor solids entrainment (BLSE).
7. The process of claim 1, 2 or 4 wherein said medium consistency pulp contains up to 8% by weight oxygen stage solids entrainment (OSSE).
8. The process of claim 1 in which the mixture of pulp, oxygen, and alkaline materials is agitated by rotating a helical screw extending along the length of said reaction zone at less than about 15 rpm.
9. The process of claim 8 in which said helical screw is rotated at between 1 to 6 rpm.
10. The process of claim 2 in which the mixture of pulp, oxygen, and alkaline materials is agitated by rotating a helical screw extending along the length of said reaction zone at less than about 15 rpm.
11. The process of claim 10 in which said helical screw is rotated at between 1 to 6 rpm.
12. The process of claim 4 in which the mixture of pulp, oxygen, and alkaline materials is agitated by rotating a helical screw extending along the length of said reaction zone at less than about 15 rpm.
13. The process of claim 12 in which said helical screw is rotated at between 1 to 6 rpm.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US31765081A | 1981-11-02 | 1981-11-02 | |
| US317,650 | 1981-11-02 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1186106A true CA1186106A (en) | 1985-04-30 |
Family
ID=23234653
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000411807A Expired CA1186106A (en) | 1981-11-02 | 1982-09-21 | Process and apparatus for the oxygen delignification of pulp |
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| CA (1) | CA1186106A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP1528149A1 (en) * | 2003-10-28 | 2005-05-04 | The Boc Group, Inc. | Low consistency oxygen delignification process |
-
1982
- 1982-09-21 CA CA000411807A patent/CA1186106A/en not_active Expired
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP1528149A1 (en) * | 2003-10-28 | 2005-05-04 | The Boc Group, Inc. | Low consistency oxygen delignification process |
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