Electrolysis: processes – compositions used therein – and methods – Electrolytic material treatment – Organic
Reexamination Certificate
1998-06-05
2001-02-13
Phasge, Arun S. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic material treatment
Organic
C205S690000, C205S691000, C205S742000, C204S242000
Reexamination Certificate
active
06187170
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a system for the electrochemical delignification of lignin-containing materials and a process for its application.
2. The Prior Art
The term ‘lignin-containing materials’ summarizes a multiplicity of renewable raw materials, for example wood, grass, and other non-wood-forming plants such as hemp or cotton. This term also includes the intermediate and final products produced therefrom, for example pulp, chemical pulps, paper and textiles. The lignin-containing materials are in general water-insoluble. In these materials, lignin is incorporated into complex structures, for example fibers. Frequently, lignin-containing materials must be delignified, for example when producing high-quality papers. Thus, the lignin present must be wholly or partly depolymerized so that it can be wholly or partly extracted from the lignin-containing materials. This process must depolymerize lignin as selectively as possible, since the substances combined with lignin, such as celluloses and hemicelluloses, are not generally to be destroyed.
In the industrial production of paper, delignification is an essential and necessary process step. The majority of the lignin present in the wood is removed by a primary process step in the current processes for production of chemical pulp. A number of such digestion processes have been developed; the process most frequently used industrially is based on an alkaline boiling of wood with sulfide (Kraft process). After the boiling, the residual lignin content remaining in the resulting pulp must be further reduced. This also applies to other digestion processes, such as the ‘ASAM’ process or sulfite boiling.
The usual multistage process for removing the residual lignin is termed bleaching. In this process, lignin is removed and/or decolorized. Essentially three different bleaching processes can be differentiated. In what is termed chlorine bleaching, lignin can be removed highly selectively and inexpensively by elemental chlorine. In what is termed ECF bleaching (‘elemental chlorine free’), chlorine-free bleaching is achieved using chlorine dioxide. To reduce the chlorine dioxide demand, and thus the environmental pollution, in this process, the ECF bleaching is in part combined with an oxygen delignification. In the third process, what is termed the TCF bleaching (‘total chlorine free’), the bleaching is carried out completely in the absence of chlorine-containing compounds. Lignin oxidation is achieved, for example, by a treatment with oxygen and/or ozone and/or peroxide and/or peracids. Chlorine bleaching is now still only employed in old plants. Although technically and economically advantageous, this process must be replaced, since the associated environmental pollution is no longer accepted. In particular, the release of chlorinated aromatic hydrocarbon is an environmental problem. In the ECF process, although the environmental pollution with chlorinated compounds is markedly lower than with chlorine bleaching, chlorinated hydrocarbons are also formed with this process. Furthermore, the Cl
−
content makes ‘closing the cycle’ more difficult. That is operating ECF-bleaching plants with no waste water or a reduced amount of waste water is more difficult. When Cl
−
concentrates, plant corrosion can occur. From environmentally-relevant aspects, TCF bleaching is to be preferred to the two processes described. However, it is a problem that the totally chlorine-free bleaching agents, in comparison to chlorine-containing compounds, have a lower selectivity, That is, in addition to lignin depqlymerization, damage to the cellulose and the hemicelluloses also occurs. As a result, there are losses of yield and fiber damage, which can only be minimized by not carrying out the delignification completely. Paper from TCF-bleached chemical pulp has either lower fiber quality or a lower brightness than paper from ECF-bleached chemical pulp. In addition, TCF processes are economically unfavorable, since they require large amounts of relatively expensive process chemicals (e.g. H
2
O
2
, peracetic acid etc.).
In addition to such purely chemical delignification processes, biological catalysts, namely enzymes, are being used for industrial delignification. Such enzymes can attack the lignin either directly or indirectly and thus facilitate the delignification.
Hemicellulases, such as xylanases or mannanases, reinforce the delignification of chemical pulp by an indirect mechanism of action. Wood essentially consists of cellulose, lignin and hemicelluloses. The enzymatic hydrolysis of hemicellulose can facilitate the chemical bleachability of chemical pulp (Chang & Farrell (1995) Proceedings of the 6th International Conference on Biotechnology in the pulp and paper Industry: Advances in Applied and fundamental research, p. 75 ff; Suurnäkki et al. (1995) Proceedings of the 6th International Conference on Biotechnology in the pulp and paper Industry: Advances in Applied and fundamental research, p. 69 ff). As a result of such an enzymatic pretreatment, the requirement of bleaching chemicals can be decreased by a maximum of up to 35% (Chang & Farrel (1995) Proceedings of the 6th International Conference on Biotechnology in the pulp and paper Industry: Advances in Applied and fundamental research, p. 75 ff). However, a disadvantage in this case is particularly that the hydrolysis of the hemicellulose leads to a loss in yield. Furthermore, all of the disadvantages listed below of enzymatic systems also apply to hemicellulases.
In addition, some enzymes exist which are produced by naturally wood-degrading fungi (the so-called white rot fungi) and which can depolymerize lignin with the interaction of what are termed mediators. Enzymes of this type are, for example, lignin peroxidase and manganese peroxidases. These enzymes require H
2
O
2
for their activity. Since H
2
O
2
at an excessive dosage also leads to inactivation of the peroxidases, such systems are badly suited to industrial application (Paice et al. (1995) Journal of pulp and paper science. Vol. 21(8) p. 280 ff).
Bourbonnais and Paice (Bourbonnais & Paice (1990) FEBS Letters 267: p. 99 ff) and Call (WO 94/29510) described a system in which a usually lignin-polymerizing enzyme, a laccase, can be used for lignin depolymerization. The process is based on an indirect action of the laccase (Paice et al. (1995) Journal of pulp and paper science. Vol. 21(8) p. 280 ff). In this process, the laccase oxidizes a chemical molecule, what is termed a mediator, producing a free-radical form of the mediator. This mediator free radical is then able to oxidize lignin. In this oxidation the mediator molecule is regenerated. Active mediators are, for example ABTS (Bourbonnais & Paice (1990) FEBS Letters 267: p. 99 ff), HOBT (WO 94/29510) and phenothiazines (WO 95/01426).
The laccase is able to oxidize four mediator molecules, accepting in this process four electrons which ultimately originate from the lignin. Subsequently, in one reaction step, the four electrons are transferred to oxygen and two molecules of water are formed. The system of laccase and mediator thus catalyzes an oxygen-dependent lignin oxidation. The oxidized lignin can subsequently be extracted, for example, by an alkaline treatment (WO 94/29510). In contrast to peroxidases, laccases do not require an addition of H
2
O
2
and can thus be used industrially.
General problems with the use of enzymes in the chemical pulp industry are the temperature and pH ranges at which the chemical wood digestion processes are carried out. Most chemical bleaching processes are carried out at temperatures above 80° C. and under strongly alkaline conditions at pHs >10.0 or under strongly acidic conditions below pHs of 4.0. However, most enzymes have optima which differ greatly from these values. For economical use of enzyme systems, it is necessary to adapt these systems to appropriate conditions. Thus the thermal stability of at least 80° C., needs to be ensured. Thermostable xylanases, for example, which
Collard & Roe P.C.
Consortium fur Elektrochemische Industrie GmbH
Phasge Arun S,.
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