Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – Including recovery of organic by-product
Reexamination Certificate
2000-08-31
2001-07-17
Nguyen, Dean T. (Department: 1731)
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
Including recovery of organic by-product
C162S029000, C162S030100, C162S030110
Reexamination Certificate
active
06261411
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a process for recovering kraft chemicals from spent cooking liquors in the paper pulping process. In particular, the invention relates to a novel way of processing the liquors as a slurry rather than as a homogeneous solution.
2. The Prior Art
In a typical papermaking process, pulp is prepared from wood by either chemical or mechanical pulping processes. In the more common chemical process, wood chips are loaded into a pressure vessel known as a digester which is charged with a chemical reagent. Heat is supplied to the digester to “cook” the wood chips to remove undesirable substances such as lignin from the wood and to liberate the desirable cellulosic fiber.
A widely used chemical pulping process is the “kraft” pulping process, which uses an alkaline chemical reagent referred to as “white liquor” to act upon the lignin in the material. Typically, the white liquor is an alkaline solution of sodium hydroxide (NaOH) and sodium sulfide (Na
2
S), provided by an aqueous solution typically containing between about 80-90 grams/liter of NaOH and about 20-25 grams/liter Na
2
S with minor amounts of sodium carbonate, sulfate and thiosulfate. Depending upon the wood species used and the desired end product to be manufactured, white liquor is added to the wood chips in sufficient quantity to provide a total charge of alkali of 15-20% Na
2
O based on the dried weight of the wood.
Typically, the temperature of the wood/liquor mixture is maintained at about 165°-170° C. for a total reaction time of about 2-3 hours. When digestion is complete, the pressure in the digester is released and the resulting wood pulp is separated from the spent liquor, called “black liquor” in a series of washing operations.
The pulping chemicals are recovered from the black liquor by a process that is commonly referred to as the recovery process. The black liquor, which contains organic constituents and sodium, is concentrated and then burned in a process referred to as smelting to reduce the organic constituents to CO
2
and H
2
O. The ash from the smelting operation containing sodium carbonate, or soda ash, is mixed with water to form a weak solution known as “green liquor”. The solid particles in the green liquor, the “dregs” are then removed from the green liquor. The green liquor is subsequently causticized through the addition of lime to regenerate white liquor. This entire process can be time consuming and expensive.
It has been known that low grade fuels such as industrial waste may be more efficiently converted to energy if the fuel is provided in a slurry and heated at high pressure until the chemically bound oxygen is converted to CO
2
gas. Such a process is described in U.S. Pat. No. 5,485,728 to Dickenson.
Thermal treatment thus appears to have beneficial properties in the processing of waste materials. Its application to the chemical recovery in paper pulping processes has not been done prior to the present invention but would appear to offer beneficial results.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an improved method for the recovery of the kraft chemicals using fewer steps than the conventional method.
It is anther object of the present invention to provide an improved method for kraft chemical recovery that reduces black liquor solids, water and liquor volume.
It is a further object of the present invention to provide an improved method for kraft chemical recovery that increases the capacity of the existing recovery process.
It is yet a further object of the invention to provide an improved method for kraft chemical recovery that is less expensive than traditional methods.
These and other objects of the invention are accomplished by a method for recovering kraft chemicals comprising heating black liquor used in the kraft pulping process and comprising organic materials having oxygen containing functional groups for a sufficient time and at a sufficient elevated temperature and corresponding pressure (such that there is no or substantially all boiling) to convert all or a portion of the mass of oxygen into carbon dioxide. The elevated temperature may vary widely and is a temperature which is sufficiently high to carry out the desired conversion of oxygen to carbon dioxide. The upper temperature limited may be of any value and is primarily limited by the capability of the reaction or heating vessel or apparatus to withstand the resulting pressure. The temperature is preferably at least about 475° F., more preferably from about 475° F. to about 625° F. and most preferably from about 500° F. to about 575° F. In the embodiments of choice, the temperature is about 550° F. Heating times may very widely. The black liquor is heated at the elevated temperature for a time sufficient to convert the desired amount of oxygen into carbon dioxide. In general, the higher the temperature, the shorter the heating time required to convert a given mass of oxygen into carbon dioxide; and conversely, the lower the temperature, the longer the heating time required for such conversions. Usually, when employing preferred temperatures, heating times may be from 0 to about 60 minutes or longer converted from a homogeneous alkali lignin/soda-sulfur/water solution into a microparticulate char slurry in a green liquor suspension.
The reaction chemistry of this process is as follows:
Na
2
CO
3
→Na
2
CO
3
Na
2
SO
4
→Na
2
S+2O
2
Alkali Lignin→Char+NaOH+Na
2
S+Unknowns
In the reducing environment (absence of O
2
) of the thermal treatment, salt cake is reduced to sodium sulfide in a similar manner to the reduction in the smelt bed of a recovery boiler. Sodium carbonate, as in a recovery boiler, is retained as the same. Alkali lignins are reduced to carbonaceous char, sodium hydroxide, sodium sulfide and other unknown soda compounds.
During this process, the following two characteristics are observed:
1) All or a portion of the oxygen containing functional groups such as C—O, C═O and O—C═O terminal and conjugate groups contained in the organic material component of the spent black liquor are split of as CO
2
, thus decreasing the degree of polymerization and thus the average molecular weights of components such as such as lignins and carboxylic acids.
2) All or a portion of the oxygen present in the organic material component of the spent black liquor is converted to carbon dioxide in the reaction. The actual amount of oxygen converted will vary depending upon a number of factors as for example, the types of organic components in the black liquor as for example resins and acids. In general, higher percent conversions of oxygen are preferred. Usually, at least about 50% of the total mass of oxygen is converted into carbon dioxide. The percent conversion is preferably at least about 60%, more preferably at least about 75% and most preferably from about 75% to about 85%.
The process according to the invention first involves passing black liquor into an evaporator train where it passes through one or more evaporators. The liquid is then pumped into a high pressure pump where the pressure is increased so that the liquid does not boil when heated. The liquid is then fed into a heat exchanger and then into a heated reactor for a specified amount of time. The reacted product is then cooled by a reactor feed and the flash gas is returned to the evaporators where it is condensed. The pressure is then reduced to atmospheric pressure and the product is fed into an agitated product tank. At this point several methods of additional processing are available.
For example, the reacted black liquor, after filtration, may be fired in a coal or bark boiler since the green liquor has been removed. This increases the number of options for a recovery limited mill and likewise increases the pulping capacity of a mill, since more black liquor solids do not present a bottleneck at the recovery boiler.
The filtrate from the reacted liquor may be passed directly into the green liquor
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