Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – Gas – vapor or mist contact
Reexamination Certificate
2000-11-28
2002-10-15
Alvo, Steve (Department: 1731)
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
Gas, vapor or mist contact
C162S082000
Reexamination Certificate
active
06464827
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to production of pulp useful in the manufacture of paper.
BACKGROUND OF THE INVENTION
Wood represents about 30-45% of total pulp production costs. Thus, increasing the yield of wood conversion into pulp, i.e. the percentage of the wood fed to a pulping operation that usefully becomes part of the pulp solids, is an effective way of achieving the desirable goal of decreasing overall pulp manufacturing costs, by decreasing wood consumption.
Increasing pulp yield provides other benefits such as increased pulp mill through-put and decreased load of black liquor solids to recover. As a consequence there occurs a debottlenecking of the kraft recovery system.
Another important factor in pulp manufacturing costs is the bleaching chemical cost, that can represent 15-20% of the total costs. Total bleach chemical consumption is influenced by the so-called pulp bleachability which is defined as the bleach chemical requirement to achieve a given level of final pulp brightness (e.g. 90% ISO brightness). Thus, increasing pulp bleachabllity during the manufacturing process results in decreased bleach chemical costs.
Those investigating in this field have made many attempts to increase pulp yield and bleachability. Most of the processes proposed are either effective to increase yield or to increase bleachability, but not both at the same time. As a matter of fact, many of the alternate methods proposed to increase pulp yield result in decreased pulp bleachability.
Prior attempts by others with regard to improving yield in the kraft pulp industry can be divided basically into two categories.
One category is processes that use additives together with the wood pulping chemicals to enhance pulping efficiency and to protect pulp carbohydrates. These include: (1) the so-called kraft-anthraquinone pulping whereby anthraquinone (AQ) or any of its derivatives are added together with the pulping chemicals during wood pulping. Besides enhancing the rate of removal from the pulp of the undesirable fractions (lignins) present in the wood, AQ has the property of protecting the desirable fraction (carbohydrates) present in the wood. Anthraquinone acts by oxidation of the carbohydrate reducing end groups, thus increasing overall pulp yield. (2) The so-called kraft-polysulfide pulping whereby polysulfides (PS) are added together with the pulping chemicals to protect the wood carbohydrate fraction. Purportedly, PS have the ability of oxidizing carbohydrate reducing end groups, thus avoiding the so-called “peeling reaction” promoted by the alkali and increasing process yield. (3) The so-called kraft-anthraquinone-polysufide pulping whereby AQ and PS are added together with the wood pulping chemicals. The benefits of these two additives in improving pulp yield have been considered to be synergistic.
It should be noted that the kraft-AO, kraft-PS and kraft-PS-AO processes are effective in improving pulping yield but they have no reported positive effect on pulp bleachability.
In a second category, yield improvements have been based in a more accurate control of kraft pulping kinetics, so that the losses of carbohydrates through peeling reactions and hemicellulose dissolution are minimized. The processes developed for this purpose, the so-called extended delignification processes or modified kraft cooking processes, include among others the isothermal cooking (ITC®), the low solids cooking (Lo-solids®), the extended modified continuous cooking (EMCC®), the rapid displacement heating cooking (RDH®″), and the Super-Batch® cooking processes. The four basic principles that are used more or less extensively in these processes are: (1) a constant temperature profile throughout the cook, (2) a constant alkali profile throughout the cook, (3) a high sulfidity throughout the cook, particularly in the beginning and the end of the cook and (4) a low content of solids in the cooking medium throughout the cook.
It should be noted that all these new processes result in yield improvements in the order of 1-2% at most.
Considering that wood and bleach chemicals have the largest impact on pulp manufacturing costs, and that through-put limitations as well as bottlenecked recovery systems are major problems in modern pulp mills, there remains a need for a method to increase pulp yield and bleachability in the manufacturing process so that overall pulp production cost is decreased.
BRIEF SUMMARY OF THE INVENTION
One aspect of the invention is a method for producing pulp, comprising
digesting lignocellulosic wood, containing one or more xylan derivatives selected from the group consisting of xylan bound with lignin, xylan bound with hexenuronic acid, and mixtures thereof, with an aqueous alkaline pulping solution containing sulfide and having an initial free hydroxyl ion concentration of at least 1 mole per liter, under conditions whereunder xylan is dissociated from said one or more xylan derivatives, and the pH of said solution remains above 12.5; and then while the pH of said solution is above 12.5, adding a sufficient amount of an acidic agent to said pulping solution to precipitate dissociated xylan from said pulping solution while minimizing precipitation of lignin from said pulping solution.
DETAILED DESCRIPTION OF THE INVENTION
As used herein, “xylan” and “xylans” means polysaccharides composed of repeating units of the formula:
The term “xylan derivatives” means one or more polysaccharides wherein xylan having the aforementioned structure is substituted with one or more substituents and particularly with one or more sugars or with a hexenuronic acid, which is a uronic acid. Examples of xylan derivatives include L-arabino-D-xylans, L-arabino-D-glucurono-D-xylans, L-arabino-(4-O-methyl-D-glucurono)-D-xylans, O-acetyl derivatives of any of the foregoing, xylan bound with lignin, and xylan bound with hexenuronic acid (such as wish hexenuronoxylan or 4-deoxy-&bgr;-L-threo-hex-4-enopyranosyluronic acid-xylan).
The term “bound” is used herein to mean that two entities, such as xylan and lignin, are “bound” to each other if they are held together covalently, ionically, by another attractive force, or by being physically engaged with each other such as by intermolecular entanglement. Two entities are considered “bound” if the “bound” form precipitates under conditions under which one of the entities, unbound, would precipitate and one of them, unbound, would not.
Two entities are considered “disassociated” if they are no longer “bound”; thus, disassociation can occur by cleaving of covalent bonds, by neutralizing ionic or other attractive forces, or by disentangling or otherwise disengaging the two entities.
Wood is a complex raw material comprising four major components: cellulose (45-50%), hemicelluloses (15-30%), lignin (20-30%) and extractives (1-5%). The hemicellulose fraction includes two major groups: xylan and xylan derivatives; and glucomannan and glucomannan derivatives. The xylan derivatives are predominant in the hardwoods (about 20%) and are present in appreciable amounts in the softwoods.
Conventionally, the aim of wood pulping is to remove into the pulping liquid the lignin and the extractive fractions while retaining in the solid fraction as much as possible of the carbohydrates (cellulose and hemicelluloses). Nevertheless, during pulping a significant fraction of the carbohydrates are also dissolved into the pulping liquid and their value to the pulp is lost. The lignin, he extractives and a significant fraction of the carbohydrates go into solution and become part of the so-called black liquor. The non-dissolved carbohydrate fraction remains as pulp fibers. Conventionally, about 50% of the wood is dissolved in the black liquor, of which about 25-30% is lignin and extractives and the balance 20% are carbohydrates. In the case of hardwoods, about 10% of the dissolved carbohydrates are xylan and xylan derivatives. The yield of the pulping process is directly related to the amount of cellulose and hemicelluloses that are retained in the pulping operation.
Wood pu
Alvo Steve
Black Donald T.
Praxair Technology Inc.
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