Chemical wood pulping process with reduced pitch and VOC...

Details Chemical wood pulping process with reduced pitch and VOC... Chemical wood pulping process with reduced pitch and VOC...

2001-11-06

2003-11-04

Alvo, Steve (Department: 1731)

Paper making and fiber liberation

Processes of chemical liberation, recovery or purification...

With regeneration, reclamation, reuse, recycling or...

C162S037000, C162S072000, C162S077000, C162S082000, C162S083000, C162S090000, C162SDIG004

Reexamination Certificate

active

06641699

ABSTRACT:

FIELD OF THE INVENTION
A wood pulping process wherein wood particulates are subjected to a solvent extraction process that removes pitch as well as other wood extractives. The wood particulates are then subjected to a pulping process that is virtually free of volatile organic compound emissions.
BACKGROUND OF THE INVENTION
As a preliminary matter, wood can be viewed as consisting of two major components, carbohydrates and lignin. Other components constitute a minor part of the wood and manifest as intercellular material, and extraneous substances that are related to the growth of the cells of the tree. The cell walls of the wood are composed of polysaccharides, the chief of which is cellulose. Lignin, on the other hand, is an amorphous substance, partly aromatic in nature, that has been called a “cementing material” or an “encrusting substance.” It is insoluble in water and in most common organic solvents. It is also insoluble in acids, but undergoes condensation reactions in the presence of strong mineral acids. Lignin is partly soluble in alkaline solutions and is readily attacked and solubilized by oxidizing agents.
The extraneous substances of wood are deposited as cells grow, or after they reach maturity. Most of these substances are relatively simple compounds, having a low molecular weight. These low molecular weight substances include pectins, proteins, and like substances that are soluble in water or neutral organic solvents. The extraneous substances also include “wood extractives” that include pitch and volatile organic compounds. These naturally-occurring wood extractives are found in both resin canals within the structure of the wood, as well as within the parenchyma cells of the wood.
In general, wood extractives may be divided into a higher molecular weight, higher boiling point fraction, commonly known as “pitch”, and a lower molecular weight, lower boiling point fraction that falls within the definition of “volatile organic compounds.” The United States Environmental Protection Agency (EPA) has determined that volatile organic compounds (VOCs) pose an environmental hazard when they are released into the atmosphere. These VOCs are defined in 40 CFR Part 51(s) as “any compound of carbon, excluding carbon monoxide, carbon dioxide, carbonic acid, metallic carbides or carbonates, and ammonium carbonate, which participates in atmospheric photochemical reactions.” Typically these are volatile low molecular weight organic compounds. The EPA has promulgated regulations limiting the quantity of VOCs that a manufacturing facility may release into the atmosphere.
There is no equivalent regulation regarding pitch. However, pitch poses processing and product quality issues in the pulp and paper industry. In pulp mills, the pitch separates from the cellulosic fibers to form a colloid-like suspension that gradually deposits to build up a scale within the process equipment and ducting of the mill. Ultimately, the pulp mill must be shut down so that this pitch scale may be manually removed. In an effort to reduce the frequency of shut-downs to remove pitch scale, pitch scale control chemicals, such as sodium aluminate and alum, are added to the pulping process. While this strategy is partially successful in that it alleviates the equipment fouling problem, it does not eliminate all the problems caused by pitch. Indeed, the addition of scale control chemicals also poses a waste disposal problem since these chemicals are present in the process water. Although this water is recycled, a portion must be treated and disposed of. This, of course, entails additional operating costs for treatment chemicals, labor and facilities.
Colloidal pitch in the process water, as well as some pitch adhering to pulp fibers, cause significant equipment fouling problems in pulp dryers and papermaking machines. In these capital intensive high speed machines, the pulp is formed into continuous webs on high speed fabrics, dewatered, and dried. During these processes, pitch is gradually deposited onto the rolls and machine “clothing” of the papermaking machines to form a tacky, gummy surface deposit. This ultimately results in reduced product quality and machine efficiency. Removing the gum can require shutting down the papermaking machine, chemical cleaning or removing the clothing, and cleaning all affected surfaces. This results not only in cleaning costs and paper wastage losses, but also in significant machine downtime with consequent economic loss. Other methods of treatment include the use of continuous cleaning chemicals and equipment. Some of these chemicals may contribute to the release of VOCs and compositions with high biological oxygen demand (BOD) and/or high chemical oxygen demand (COD) into the environment.
Finally, pitch present in the pulp causes a loss of brightness in paper and absorbent products produced from the pulp. To overcome this, the pulp must be bleached, at an added chemical treatment cost.
The release of VOCs into the atmosphere is also a long-standing problem in the pulp and paper industry. To produce boards (oriented strand board, particle, veneer) composite wood products, and paper and pulp products, raw logs or wood fibrous material must be reduced to wood chips, flakes or sawdust. These wood particulates are then further processed, either by bonding together with a suitable glue to make board products, or undergoing pulping and forming processes to produce a variety of papers and absorbent products. However, the processing of logs into wood particulates, and thence into finished products, poses several challenges. Some of these arise from the nature of wood, namely, that it includes not only cellulosic fibers and lignin but also “wood extractives,” as discussed above. VOCs occur naturally in timber and the processing of timber into wood particulates facilitates the migration or diffusion of VOCs to chip surfaces from which the compounds vaporize into the surrounding atmosphere. As a practical matter, since the industry requires a large inventory of wood chips for processing into board products and as feedstock in the pulp and paper processes, significant amounts of VOCs are released into the atmosphere from wood chip storage piles. Further, VOCs are also released into the atmosphere during the processing of the wood chips into paper and pulp products.
The distribution of pitch and VOCs from raw wood into the environment and into products may be more easily understood with reference to FIG.
1
. As shown, logs
5
′ are processed into chips in chip mill
10
′ releasing VOCs
2
′ to the atmosphere. The chips are stored in mounds
7
′ that continue to release VOCs
4
′ to the atmosphere. Chips, naturally containing chemical compounds that may produce from about 1 to about 6 wt. % VOCs, for example, are processed in a pulp mill
12
′, which can be a mechanical, thermo-mechanical or a chemical pulp mill, to produce cellulosic and fibrous pulps. During this pulping process, cellulosic fibers of the wood are separated from each other thereby allowing entrapped VOCs to diffuse to fiber surfaces and vaporize into the surrounding atmosphere. However, the higher boiling point pitch material remains in the fibers. Treatment chemicals fix a portion of the pitch to the fibers to reduce the rate of mill equipment fouling. The cellulosic pulp produced may be bleached, such as by a chlorine bleaching process, and is then formed into a continuous web and dried on a pulp drier or paper machine
14
′. During these processes, a further significant amount of VOCs is released into the atmosphere. The combined chipping, crushing, pulping, and paper or absorbent product making processes release about one-third of the total natural extractives in the wood into the atmosphere (shown by arrows
2
′,
4
′,
6
′, and
8
′) as VOCs, and another one-third into effluent water (arrows
20
′,
22
′ and
24
′). The papermill product
15
′, such as newsprint, writing paper, or absorbent products, includes the residua

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