Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – Gas – vapor or mist contact
Reexamination Certificate
2001-03-02
2003-05-27
Nguyen, Dean T. (Department: 3629)
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
Gas, vapor or mist contact
C162S063000, C162S066000
Reexamination Certificate
active
06569285
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for the removal of lignin from lignocellulose containing material comprising treating the lignocellulose containing material prior to a chemical pulping process with a delignifying gas, wherein the gas comprises chlorine dioxide.
BACKGROUND OF THE INVENTION
In chemical pulping processes the objective is to remove the lignin present in lignocellulose containing material while minimising the damage and loss of the cellulose and hemi-cellulose fibres. In these processes, often also referred to as cooking or digestion processes, the lignocellulose containing material reacts with pulping chemicals at an elevated temperature over a time period sufficient to effect a specified degree of delignification. As such, the digestion process is a complex kinetic balance relating delignification to inter alia the cooking chemical(s), time and temperature. These variables are balanced to produce a pulp with the highest strength, greatest yield and the lowest lignin content.
In chemical pulping processes, pulping chemicals are introduced into the digester along with the lignocellulose containing material and subsequently heated to a specific digestion temperature for a predetermined period of time. As the digestion proceeds the chemical concentration, temperature and digestion time all effect the removal of the lignin from the lignocellulose containing material.
However, as the chemical concentration increases there is more of a tendency for chemical attack on the cellulose and hemicellulose fibres instead of the lignin. The chemicals react with the carbohydrates and break or cleave the fibre chains resulting in shorter polymer lengths and overall lower fibre strength, i.e. impaired viscosity. This effect is further increased as the temperature and reaction time increase. Consequently, more cellulosic material is present in the pulping liquor. When the spent pulping liquor is removed from the pulp in the post-washing step, these materials are lost resulting in lower pulp yields.)
Side reactions in the cooking process are very temperature dependent. Lignin removal proceeds slowly at first but accelerates markedly as the temperature rises above 160° C. Cellulose removal starts at 120-130° C. and levels off when the maximum temperature is reached. Hemicellulose is composed of two main components, glucomannan and xylan. Glucomannan removal is rapid at first and becomes even greater as the temperature increases above 100° C. Xylan on the other hand follows the same pattern as lignin removal starting slowly at first and increasing rapidly as the temperature increases.
The rate of penetration and diffusion of the pulping chemicals into the fibre source determines the efficiency of the digestion. A too short digestion time results in non-uniform cooking and a poor pulp product, while excessive digestion generates overcooked pulps with poor viscosity.
From the above it can be seen that changes within the digestion process to increase lignin removal usually have a negative impact on the product pulp properties. Higher chemical concentrations and/or temperature result in lower strength and yield. Longer digestion times reduce the throughput through the digester and lower the pulp production.
To achieve a given degree of delignification, various combinations of heat-up time, maximum temperature and time at the maximum temperature can be used. To simplify comparisons of different cooking conditions, the pulp and paper industry has developed the H-factor. This value is the sum of the relative rates of reaction occurring in the digestion. For a closer definition of the H-factor we refer to Kirk-Othmer, Encyclopedia of Chemical Technology, Vol. 20, 4
th
Ed., p. 535 which is hereby incorporated by reference. Using the H-factor and the kappa number after the cook, different treatment and cooking processes can be compared on an equivalent basis. For example, reducing the H-factor for the process would allow for shorter cooking/digesting times and increase pulp production in existing equipment or allow the digestion to be accomplished at a lower temperature resulting in improved yield and strength properties.
Thus, from the reasons set out above it is preferable to pretreat the lignocellulose containing material in such a way as to facilitate the removal of lignin in a subsequent pulping process and/or break and softer the lignin bonded to the cellulose and hemicellulose fibres.
If the pulping process is a chemical pulping process the H-factor can be significantly reduced when preterating the lignocellulose containing material according to the invention.
U.S. Pat. No. 4,172,006 refers to the pretreatment of wood chips with oxygen prior to adding a cooking liquor.
U.S. Pat. No. 4,750,973 relates to a process for reducing carbohydrate losses in sulphate pulping of wood using sodium hydroxides and sodium sulfide, wherein the wood is pretreated in presence of water with oxygen gas and nitrogen oxides.
GB 567774 discloses a process for the treatment of cellulosic raw material where wood chips are contacted with a aqueous solution of a wetting agent prior to subjecting the chips to a solution containing sodium chlorite thereby using sufficient acid to insure the liberation of chlorine dioxide.
WO 8908165 refers to a method for the pretreatment of wood chips with sulphur dioxide gas prior to alkaline deligification operations.
DE 1049220 discloses a method comprising subjecting wood chips to carbon acid before sulfite cooking.
JP 49020241 refers to a pulping process comprising the steps of reacting chlorine dioxide or a mixture consisting of chlorine dioxide and chlorine with wood chips in the presence of water soluble cellulose derivatives and thereafter removing inter alia the oxidised lignin by extraction.
U.S. Pat. No. 5,474,654 refers to a delignification process where chlorine dioxide gas is used on pulp obtained from pulping processes such as chemical kraft, sulfide or mechanical processes.
U.S. Pat. Nos. 3,591,451 and 3,919,041 disclose the use of gaseous chlorine dioxide subsequent a pretreatment step which may be either mechanical, chemical or a combination thereof.
SUMMARY OF THE INVENTION
In accordance with the present invention it has surprisingly been found that the lignin content of lignocellulose containing material can be reduced prior to a chemical pulping processes by providing a process according to the claims. More specifically, the invention relates to a process for the removal of lignin from lignocellulose containing material, where the material prior to a chemical pulping process is treated with a delignifying gas comprising chlorine dioxide.
DETAILED DESCRIPTION OF THE INVENTION
An advantage by treating lignocellulose containing material prior to a pulping process in accordance with the present invention is that the pulp yield and pulp properties such as pulp viscosity subsequent a pulping process are significantly improved at given (corresponding) H-factors.
Thus, by implementing the present invention in an existing pulping process, improved pulp yield and pulp viscosity is obtained at corresponding Kappa numbers.
Another advantage is that effluent streams originating from the pulping process are reduced since in-specific properties such as pulp yield and viscosity can be obtained at decreased pulping chemical dosages (lower H-factors of the cooking operation can be used).
A still further advantage of the present invention is that by pretreating cellulose containing material prior to a chemical pulping process with chlorine dioxide or any gas comprising chlorine dioxide the lignin content of the pulp (expressed by the kappa number) dramatically decreases after chemical pulping processes having similar H factors while not significantly impairing the viscosity over pretreatments where oxidising gases other than chlorine dioxide have been used.
Further advantages of the present invention are apparent from the specification.
Suitable lignocellulose containing material used in the present invention can be any lignocellulose containin
Bryant Patrick S.
Sokol John C.
Akzo Nobel N.V.
Burke Michelle J.
Nguyen Dean T.
Parker Lainie E.
LandOfFree
Process for gas phase pretreating of lignocellulosic... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Process for gas phase pretreating of lignocellulosic..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Process for gas phase pretreating of lignocellulosic... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3053158