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Penetration of water and black liquor into overthick and treated wood chips
Abstract
In this paper, the penetration of water and black liquor into overthick and pre-treated overthick chips (cracked and re-chipped according to thickness) was studied with a laboratory impregnator to investigate ways to improve penetration. Overthick chips were divided in three categories: knot-containing chips, chips without knots and original overthick chips. The results of the penetration experiments were evaluated in terms of the penetration degree and liquor uptake. The results showed that cracking and re-chipping with a sizer are very effective ways to improve penetration in overthick chips. These pre-treatments gave higher and faster penetration than that achieved with normal pulpwood chips. With knot and original overthick chips, penetration degree values were at a lower level compared to normal pulpwood, but still over 92-97.5% depending on penetration conditions. Liquor uptake and penetrated chemical charge were almost 50% lower with overthick, knot and sizer chips compared to normal, knot free and cracked chips, the main reasons being high basic density and in the case of sizer chips high initial moisture content. Cracking decreased initial chip moisture content and therefore liquor uptake was about 60% higher with these cracked chips than with sizer chips. Although a high penetration degree (> 95%) was achieved quite quickly (within 20 min) with all chip grades, penetrated EA-charge was very low. In normal cooking conditions penetration is much less effective way to transport chemicals into chips than diffusion, but a high penetration degree is a prerequisite for fast diffusion. Therefore chips should be impregnated with conditions preferable to a fast diffusion of cooking chemicals. The penetration method used proved to be suitable only for water penetration studies. If liquors other than water are used then diffusion of chemicals and dissolution of wood material are involved in the mass transfer process at the same time as penetration. This means that some changes to the calculations and additional information about impregnation are needed if penetration of liquors alone is to be studied.
... Much can be learned about the porosity of wood by considering the dimensions of a typical wood chip used in pulping operations (Gustafson et al. 1997;Maattanen and Tikka 2008) or in a dilute acid pretreatment step in the production of cellulosic biofuels (Kim and Lee 2002). A wood chip for use in pulping often has a thickness in the range of 2-10 mm, whereas the length and breadth will be much larger (Schultz et al. 1999). ...
... The thickness of chips has to be small enough so that diffusion in not the rate-limiting factor. Over-thick chips can be rechipped in order to overcome such problems (Maattanen and Tikka 2008). ...
The rate and extent of transport of macromolecules and other solutes into cellulosic materials and fibers have important applications in such fields as papermaking, textiles, medicine, and chromatography. This review considers how diffusion and flow affect permeation into wood, paper, and other lignocellulosic materials. Because pore sizes within such materials can range from nanometers to millimeters, a broad perspective will be used, also considering some publications related to other porous materials. Factors that limit the rate or extent of polymer or other solute transport into pores can involve thermodynamics (affecting the driving motivation for permeation), kinetics (if there is insufficient time for the system to come to equilibrium), and physical barriers. Molecular flow is also affected by the attributes of the solute, such as molecular mass and charge, as well as those of the substrate, such as the pore size, interconnectedness, restricted areas, and surface characteristics. Published articles have helped to clarify which of these factors may have a controlling influence on molecular transport in different situations.
... Size fractionation of wood chips gives oversized chips, overthick chips, accept chips (large and small), pin chips, and fines (Brännvall 2009). Oversized and overthick wood chips are further processed before being used for production of pulp (Määttänen and Tikka 2008), whereas pin chips and fines to a large extent represent a loss. Even when removed, fines can cause dust problems and increased fire risk. ...
Wood chips from a novel type of drum chipper were compared to wood chips from a conventional disc chipper in an evaluation based on demonstration-scale and industrial-scale machinery. The evaluation was performed as the wood chippers were used as production machines in a kraft pulp mill using softwood. The average bulk density of the wood chips from the disc chipper and the drum chipper was similar and within the range of 138–140 kg/m ³ . The size distribution of the wood chips was investigated using a conventional screening method, and by using an automatized image-analysis system based on laser scanning. The average length was set to be the same, but the wood chips from the drum chipper had a more uniform length. The average thickness was similar, but the drum chipper generated slightly more thinner wood chips. Compared to the disc chipper and using the screening method, the drum chipper generated a similar fraction of oversized and overthick wood chips, 51% more large accept chips, 11% more total accept chips, and 74% less pin chips and fines. Image analysis resulted in similar data. The results indicate that drum chippers warrant further attention as an alternative to conventional industrial-scale disc chippers.
... However, this approach has limitations not only because of the higher energy demand for chip production but also for higher amounts of pin chips and fines, which result in yield losses. Another way to improve impregnation is by introducing cracks into the chips, which is a measurement equivalent to chip thickness reduction (Määttänen and Tikka 2008). ...
A pulp yield increase up to 2% can be achieved by impregnation with a liquor containing 2 M effective alkali (EA) concentration instead of 1 M. The yield increase is due to higher cellulose and glucomannan contents in
the pulp, which can be rationalized by less yield loss by peeling, as impregnation is more effective at an elevated EA level. A rapid loading of chips with alkali can be realized due to a high diffusion rate. When the temperature becomes higher in the cooking stage, enough alkali is available for delignification reactions without the risk of alkali depletion in the chip core, so that the delignification is more homogeneous.
... However, this approach has limitations not only because of the higher energy demand for chip production but also for higher amounts of pin chips and fines, which result in yield losses. Another way to improve impregnation is by introducing cracks into the chips, which is a measurement equivalent to chip thickness reduction (Määttänen and Tikka 2008). ...
A pulp yield increase up to 2% can be achieved by impregnation with a liquor containing 2 M effective alkali (EA) concentration instead of 1 M. The yield increase is due to higher cellulose and glucomannan contents in
the pulp, which can be rationalized by less yield loss by peeling, as impregnation is more effective at an elevated EA level. A rapid loading of chips with alkali can be realized due to a high diffusion rate. When the temperature becomes higher in the cooking stage, enough alkali is available for delignification reactions without the risk of alkali depletion in the chip core, so that the delignification is more homogeneous.
In earlier studies, the fundamentals of the liquor penetration process in the case of hand made, homogenous, identically shaped wooden blocks have been studied. However, the efficiency of liquor penetration into real industrial chips may differ from the efficiency of penetration into ideal handmade chips. In this paper, impregnation experiments with three industrial softwood chip grades including sawmill chips, juvenile chips from 1(st) thinning and normal pulpwood chips from 3(rd) thinning (final felling) were carried out using a new impregnation vessel concept. The effects of chips' physical properties and differing penetration conditions on the penetration process were investigated. The highest degree of penetration was achieved with sawmill chips. The lowest degree of penetration was achieved with 3(rd) thinning chips, because of the high heartwood and initial air content. A lower initial degree of penetration resulted in higher liquor uptake at the same process conditions. The influences of both penetration conditions and wood permeability on the penetration process were pronounced with 3(rd) thinning chips. The experiments showed, also, that the final degree of penetration and liquor uptake can be predicted for different process conditions based on the initial penetration degree calculated from the moisture and basic density properties of the wood chips used.
The elimination of oversize chips, which would lead to a much more uniform pulp, is now possible. The disc screening principle has been adapted to wood chip handling and a relatively new device, the slicer, has been developed to convert oversize to thin chips by a positive cutting action. This article is a first step in understanding more fully the effects of chip thickness screening. It includes data from the major scientific papers on the subject, from the viewpoint of process requirements. It relates entirely to the kraft process, because that is what has been intensively investigated. There are a few reports which indicate that the concept of critical thickness applies also to sulphite and TMP.
This study investigates the possible benefits of treating Scandinavian birch chips for kraft pulp cooking with one commercially available destructuring device, namely BMH Wood Technology's Chip OptimizerTM. Effects on cooking and penetration characteristics as well as on bleaching response and physical properties were studied under laboratory and pilot conditions. Treating chips with the Chip OptimizerTM resulted in better cooking liquor penetration, easier and faster cooking, improved screened yield, much less reject and slightly improved bleaching response. Reject dropped by 60-75% and screened pulp yield increased by 1-1.5% points. Savings of 5-10% of active chlorine are possible in bleaching. The effects become greater when a greater proportion of the total chip flow is treated. Not only the overthick chips but also `accept' sized chips clearly benefit from the treatment. Up to 55% of the total sample was treated with no effect on physical properties, but treating the total sample simultaneously seemed to impair the physical properties of the fibers. Treating the whole chip flow together with single OptimizerTM settings is not, however, the optimum way to carry out the treatment. No attempt was made in this study to optimize treatments of different fractions.
Crushing machines, which are used to destructure overthick chips, can reduce the rejects otherwise generated in kraft pulping by 90% or more. They can also cause losses in fibre length and pulp strength. Here is a procedure to measure such effects.
Importance of the chip impregnation stage is widely recognised in chemical pulping processes. However, there is limited amount of direct data on mass transfer during the impregnation stage. Most methods used for studying the liquid penetration process were based on indirect approaches. A new laboratory impregnator with a special weight sensor was developed, which allowed the direct measurements of the chip weight during impregnation process. First part of the research included testing of the system under various process conditions and method development. Experiments with water showed that the impregnator could be successfully used in studying the effect of different factors on the liquid penetration into wood chips. The new system can also be used as a tool to investigate different "penetration aid" techniques, including presteaming, evacuation, addition of surfactant, and various mechanical and biotreatments of wood chips.
The uniformity of delignification of laboratory-cut and mill wood chips can be markedly improved by reducing chip thickness or by impregnating standard chips with concentrated white liquor and cooking them in a liquor with a low active alkali concentration. More uniform cooking improves bleached pulp yield by 2-3 percentage points while maintaining pulp strength properties, even when softwood chips are cooked to kappa numbers below 10. The results can be explained on the basis of current theoretical delignification models.