Delignification is the removal of lignin from plant tissue for papermaking. The Kraft process dissolves lignin with sodium hydroxide and sodium sulfide, producing brown stock. Bleaching removes residual lignin, but oxygen delignification is a newer, more efficient process. Lignin is being researched for its potential in biofuel production, and scientists are studying microorganisms and enzymes to improve its digestibility and industrial applications.
Delignification is the removal of structural polymer lignin from plant tissue, so that it can be used for applications such as papermaking. The process mainly refers to the chemical process for removing the pulp from wood. This can also be done mechanically.
Lignin is a mixture of phenolic compounds that intertwine in the secondary walls of plants, cross-linking cellulose carbohydrates that can be used to form paper fibers. This complex forms a hydrophobic matrix, meaning it repels water, allowing the plant to transport water through its system. Lignin adds a lot of mechanical strength to cell walls and reduces their digestibility, both by animals and by the chemical process of delignification. This polymer also reduces the susceptibility of plants to attack by insects and phytopathogens. It is one of the last compounds left when plants decompose and accumulates in the soil in the form of humus.
The removal of lignin from wood is traditionally done by a method called the Kraft process. This name comes from the German word for strong. The mass of fibers that remains after the removal of the lignin is known as pulp. The Kraft process produces a stronger pulp than previously used methods and removes 95% of the lignin from the wood.
This process usually involves digesting wood chips under high temperature and pressure and in a solution of sodium hydroxide and sodium sulfide in water, which is a combination known as white liquor. Chemically dissolves the bonds that connect cellulose fibers. The delignification of wood takes place in a vessel called a digester, which can withstand high pressures. There are two types of digesters: batch and continuous, while the more recently developed ones are continuous.
At this stage, the solid pulp is brown and is called brown stock. The combined liquids are known as black liquor and contain the lignin fragments along with chemicals and by-products. The pulp is separated from the cooking liquids used through a series of washings. The shards are collected and burned to fuel the factory. The remainder of the process is designed to recycle heat and cooking chemicals.
Some byproducts of the Kraft process are turpentine and tall oil, which is a resin with a variety of industrial uses. Today’s Kraft factories are self-contained and reclaim most of their chemicals, producing very little water pollution. They can, however, produce air pollution.
The brown stock produced by the Kraft process contains approximately 5% residual lignin, and is further delignified by a series of bleaching steps. Bleaching removes the additional lignin, making the paper brighter. Sometimes, whitening needs are minimal. For example, if the paper is meant to be a brown paper bag, it doesn’t have to be bright white. There is an incentive to avoid bleaching, as it decreases the mass of pulp produced, increases cost, and decreases fiber strength.
Oxygen delignification is a newer process that removes more lignin and uses fewer chemicals. It consists of treating the pulp with oxygen in a high temperature pressurized vessel in an alkaline solution. This process is followed by a washing step. The amount of residual lignin can be reduced to about 1.5% with this method, limiting the degree of bleaching needed to make paper from the pulp.
While this type of engineering has traditionally focused on wood pulp for paper and wood fiber, more recent efforts have involved the use of biomass, large amounts of plant material, as a source of ethanol and an alternative to fossil fuels. This plant material must undergo delignification before it can be used for this purpose. Microbial systems have been designed that combine the removal of lignin with the conversion of cellulose to ethanol. Research into biofuel production is a very active area.
Numerous biotechnological researches are underway both on the synthesis of lignin in plants and on its degradation by microorganisms. Scientists are trying to alter the structure and content of lignin in hopes of improving its digestibility for animals and increasing the usefulness of cell walls for paper and biofuel production.
There is also great interest in developing industrial applications for microorganism enzymes to degrade lignin. Some fungi are quite adept at living on wood because they produce enzymes, such as peroxidases, which catalyze the degradation of lignin in the presence of oxygen. Other microbial enzymes being studied for the production of biofuels act in the absence of oxygen. Genetic engineering experiments are underway to improve the properties of these enzymes for the industrial use of lignin degradation.
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