PDF | 3rd Int. Conf. on Biotechnology in the Pulp and Paper Industry. Stockholm, 16 - 19 June , 67 - PDF | Pretreatment of hemp pulp with xylanase was investigated. Unbleached hemp pulp was treated with commercial xylanase, and then bleached with. biotechnology to the bleaching of kraft pulp. Biobleaching research focuses on using white rot fungi, and ligninase and hemicellulase enzymes. Bleaching with.
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Abstract: The utilization of hemicellulases in bleaching of kraft (sulphate) pulp is considered as one of the most important new large-scale industrial applications. The pulp bleaching technologies entered a new era in late 80s due to well as bleaching results obtained using various hemicellulases, pulps and bleaching. Elemental chlorine, while effective in bleaching pulp and low in both primary classes: hemicellulases (xylanase) and oxidases (laccase).
Also, a method and apparatus for treating wood pulp containing incompletely washed brownstock in which the brownstock is treated at a pH range of approximately 6. Description This application is a continuation of application Ser. Field of the Invention The present invention relates to processes for treating paper pulp and particularly relates to a method for enzyme treatment of paper pulp. One of the biggest challenges facing the pulp and paper industry is to reduce the use of chlorine in the bleaching process. The effluent from the pulp bleaching plant, that portion of a mill that converts brown pulp to white, contains numerous chlorinated organic substances including toxic chlorinated phenols and dioxin. Pulp and paper processors worldwide are under intense regulatory pressure to reduce these emissions. The present invention relates to an improved method for using enzymes in the processing of paper pulp to boost the efficiency of the bleaching process.
This is a preview of subscription content, log in to check access. Preview Unable to display preview. Download preview PDF. References 1. Tolan JS The use of enzymes to enhance pulp bleaching.
Rydholm SA Pulp processes. Interscience, New York, p Google Scholar Kallmes O Tappi Google Scholar Koeppen A von Tappi Google Scholar Poutanen K Characterization of xylanolytic enzymes for potential applications. Elsevier, Amsterdam, p Google Scholar The present inventors have discovered that incompletely or partially washed brownstock can be efficiently delignified with hemicellulase enzymes having a pH optima for activity below 6.
Applicants have further discovered that enzyme preparations with a pH optima for hydrolysis of below 6. The present inventors have discovered that, contrary to all expectations, in the Kraft brownstock that has not been fully washed, and contains residual dilute black liquor i.
This is the complete opposite of what it does at the normally preferred conditions of operation for enzyme treatment. In fact, it has such a strongly positive impact that it almost completely, and unexpectedly, cancels out the well known negative effects of increasing pH beyond the optimum level of performance for the enzymes. As a result, the preferred pH for enzyme treatment is significantly higher than the enzyme's optimum pH for hydrolysis.
In fact, the preferred optimum is in a range normally believed to lead to rapid enzyme inactivation. The inventors have found, for example that bleaching results using Trichoderma xylanase are three times better at pH 7.
It is very surprising that the enzyme works better at a pH significantly higher than its pH optimum in a system containing black liquor. Even more surprising is that, while the black liquor appears to inhibit enzyme action at optimum pH, it enhances enzyme performance at elevated pH. To our knowledge, this result is completely unexpected and no other enzyme system demonstrates these properties.
We can only speculate that several complex factors are working together to cause this effect. For example, at high pH levels, a component in the black liquor may stabilize the enzyme and modify the properties of the substrate, thereby making it more susceptible to attack. It might further be speculated that a change in pH modulates this process by affecting the charge on some acid substituent groups in the black liquor or on the xylan substrate that have a pKa in the range of 5 to 7.
The present inventors have further discovered that dilute or weak black liquor, which might previously have been expected to be harmful to enzyme action, can be used as a buffer solution and mixed with acid and enzyme for simultaneous addition to the brownstock. This eliminates the need for expensive buffer solutions at this stage of processing while allowing optimal hemicellulase activity. A further objective of this invention, therefore is to obtain an improved means of using acid hemicellulases, i.
It has previously been found that these enzymes do not work well on the partially washed brownstock that is typical of commercial Kraft pulp mills. Another objective is to overcome the inhibition of enzyme activity that is observed in the presence of dilute Kraft black liquor.
The present invention makes possible a three to four fold improvement in the "brightness boosting" power of the enzymes, to produce strong pulp that is light in color. Therefore yet another object of the present invention is to provide an improved process for making paper that uses the bleached pulp of the novel enzyme process, including apparatus for performing the improved process. Preferably, the incompletely washed pulps should have a residual soda in pulp of between 1 and 50 kg per ton of pulp.
For effective enzyme treatment, the pH of the pulp should be reduced to below at least 9. The enzyme treatment should preferably last at least 30 minutes.
Referring to FIG. Wood chips are debarked and then fed into a digester where they are cooked in a concentrated solution of sodium hydroxide and sodium sulfide. The purpose of this process, known as kraft pulping, is to separate the wood chips into individual fibers and to substantially dissolve the lignin portion of the wood.
After the cooking is completed, the resulting slurry of fibers, dissolved lignin, and pulping chemicals is blown from the digesters into a blow tank. Knots and incompletely cooked chips are removed from the pulp slurry in specialized machines called knotters. At this point, the fibers are in a solution of dissolved lignin and pulping chemicals, called dilute or weak black liquor. In the next unit operation, a series of rotary drum filters are used to wash the bulk of the weak black liquor away from the fibers.
The partially washed fiber, or brownstock, is then stored in a high density brownstock tank, screened, washed again, and then pumped into a storage tank to await bleaching.
The bleaching process may involve anywhere from one to thirteen stages. The specific process described in FIG. The chlorinated pulp is then washed before entering the alkaline extraction stage E. Sodium hydroxide is added to the pulp to remove the residual reaction products that were not solubilized in the acidic chlorination stage but readily dissolve in an alkaline medium.
The extracted pulp is then washed with water to remove residual caustic. The C and E bleaching stages reduce the lignin content of the pulp to less than 0. The delignified pulp, however, still has an unacceptable dull tan color that requires further processing to reach an acceptable "brightness".
The process outlined in FIG. In the process of the present invention, an acid or buffer solution is added to the brownstock at a point after the first stage of brownstock washing but before the last brownstock storage tank. This is intended to reduce the pH of the brownstock slurry to below 9.
The brownstock slurry should be mixed, for example with a mixing pump, to ensure uniform distribution of enzyme and then held in a storage tank or line for a period of at least 15 minutes, and preferably at least 1. The preferred range of pulp kappa numbers is between 20 and 40 for softwood and 10 to 20 for hardwood, however, the process of this invention can be applied to oxygen delignified pulps with even lower kappa numbers. The enzymes added should be from the class of hemicellulose degrading enzymes that have a pH optimum for hydrolysis between 3.
They may include, but are not limited to: xylanase, endo-xylanase, beta-xylosidase, mannanase, and arabinase. This invention preferably concerns the use of xylanase or other hemicellulase enzymes that have pH optima for hydrolysis of below 6. In this preferred embodiment, the invention relates to enzyme preparations wherein the total cellulase activity added to the pulp is not more than about 10, filter paper units FPU of cellulase per ton of pulp using the IEA standard filter paper assay see Example 2.
Measurement of the cellulase and xylanase activities is described in Examples 1 and 2. The acid used for pH adjustment may include sulphuric, sulfurous, hydrochloric, phosphoric or any other appropriate acid. These acids may be buffered so as to reduce extremes of pH. In some instances, the pulp slurry may be so thick that it will take as long as 60 minutes for the pH of the free liquid in the pulp to stabilize.
The pH should be at least 1 point higher than the apparent pH optimum for the enzyme when it is hydrolyzing its target substrate. In one embodiment of this invention, the process is carried out using a xylanase enzyme preparation produced by the fungus Trichoderma reesei.
T reesei also produces a group of cellulase and hemicellulase enzymes. It is preferred in the practice of this invention that the specific cellulase content of the enzyme preparation contemplated be very low so that not more than about 10, FPU of cellulase activity is added per ton of pulp see Examples 1 and 2 , and more preferably about 2, FPU or even about FPU or less per ton of pulp.
In a further embodiment of this invention, as it may be applied to a mill with a flow sheet as outlined in FIG. The amount of acid should be chosen so that the pH of the brownstock slurry will stabilize at roughly 7.
After the acid has been sprayed onto the pulp a xylanase enzyme made by T.
The pulp should have a residence time of preferably over one hour in this brownstock storage tank. A further embodiment of this invention, as it may be applied to a mill with a flow sheet as outlined in FIG.
After the weak black liquor has been sprayed onto the pulp, a xylanase enzyme made by T. Alternatively, the enzyme may be included in the spray going onto the pulp with the black liquor and the sulfuric acid. The amount of sulfuric acid to be added would be chosen using feedback control techniques to adjust the pH at which the brownstock slurry will stabilize to between 6. A xylan substrate was made using oat spelt xylan from the Sigma Chemical Co.
Catalog X in the following manner. The suspension was vacuum-filtered and the filter cake was washed with ml deionized water to remove all the soluble xylan. The insoluble portion was then resuspended in 70 ml of deionized water and uniformly distributed by gentle mixing. The reaction was then stopped by adding 0.
The resulting samples were then centrifuged to remove insoluble substrate and assayed for the total amount of reducing sugar as xylose released in the reaction using the DNS method. The activity of the enzyme was calculated based upon the amount of enzyme that is needed to produce 0. These results are shown in Table 1. The results are shown in Table 2. From the results shown in Examples 1 and 2, the relative cellulase and xylanase activity for Applicants' Iogen xylanase preparation is Cellulase activity added per ton of pulp was calculated based on the relative cellulase activity of the enzyme preparation, as shown in Table 2.
Xylanase focuses in a band corresponding to an isoelectric point pI of 9. Cellulase enzymes are found on the gels at positions corresponding to lower pI levels. These slurries are so thick as to make pH measurement by the usual method i. To avoid these problems, the liquor was squeezed out of a sample of the pulp and the pH of this liquor measured.
The pulp sample was squeezed manually, so that at least one-third of liquor in the pulp sample was separated for the pH measurement. Prior to any addition of sulphuric acid, the pH was Adjustment of the pH of pulp has the added difficulty of the slow mass transfer within the pulp fibers, which delays the attainment of an equilibrium pH after acids are added to the pulp. It is also important that acids be well dispersed within the pulp. The acidified pulp is then allowed to sit undisturbed.
Typical measurements of pulp pH over time after acidification are shown in Table 3. Because of the finite time for diffusion of acid into the fibers, the pH rises over time. In the subsequent testing, pulp was used that had been allowed to sit and have its pH equilibrate, as well as pulp that had just had acid added to it.
It was found that the relevant pH for the enzyme reaction appears to be the pH at which the pulp equilibrates. This is the relevant pH for the invention, and is the pH referred to in the following examples. The pulp Kappa number was A sample of pulp of g dry basis at 8.
The filtrate was discarded and the pulp cake was resuspended in 10 L of water and filtered a total of four times. This procedure produced "well-washed" pulp with a soda level of 0.
The pH was adjusted to equilibrate at various levels between 5 and 8. Iogen Xylanase enzyme, with activities described in Examples 1 and 2, was then added to the pulp. In this case, 12 micro-liters of the enzyme were added to each 17 g sample of pulp. Pulp that did not receive enzyme treatment was brought through the procedure, except enzyme was not added.
After enzyme treatment, each sample of pulp was washed with 3.
Chlorination was carried out at 2. The caustic charge was 3. The chlorine dioxide usage was 0. The pulp was washed thoroughly between stages.
The bleached pulp was formed into handsheets and the brightness measured by an Elrepho instrument calibrated to an ISO scale. In the absence of enzyme treatment, the bleached pulp was 71 ISO brightness.
The degree of enhanced brightness due to enzyme treatment relative to an untreated control sample is shown in FIG. As expected, the largest benefit of enzyme treatment occurred at pH 5 8 ISO points , and the bleaching benefit decreased as the pH increased.
The procedures were as described in Example 5, except the initial multistage water washing was omitted. The pulp was adjusted to equilibrate at pH 5 with 6 ml of 1N sulfuric acid.
The results are shown in Table 5. The enzyme boosted the brightness of the bleached pulp by 3 ISO points, as compared to 8 ISO points with pH 5 enzyme treatment on well-washed pulp.
This result is not surprising, because black liquor contains many aromatic and sulfide compounds that would be expected to be detrimental to enzyme activity. The subsequent enzyme treatments and bleaching were carried out as described in Example 6.
The results are shown in FIG. Surprisingly, the benefit of enzyme treatment increases as the pH is increased. Above roughly pH 6. That is, as the equilibrated pH values increased for pulp containing black liquor, the bleach boosting increased, whereas, for well-washed pulp, the bleach boosting decreased correspondingly when the pH was increased to basic.
The amounts of sulfuric acid added were sufficient to bring the steady state equilibrium pH to between 5. The subsequent bleaching was carried out as described in Example 5. The results are shown in Table 7. The pH of the pulp increased about 1 unit in two hours after addition of the acid, then maintains a steady value.