Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – With testing – sampling or analyzing
Reexamination Certificate
2001-03-05
2002-09-10
Alvo, Steve (Department: 1731)
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
With testing, sampling or analyzing
C162S052000, C162S068000, C162S061000, C162S062000, C162S238000, C162SDIG006, C250S282000, C250S287000, C422S003000, C422S062000, C422S108000, C700S117000, C700S127000, C700S128000
Reexamination Certificate
active
06447639
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a novel intelligent on-line pulp digester control system. More particularly, this invention pertains to a novel intelligent control system which employs heat and ion mobility spectrometry to automatically, rapidly and continuously determine on-line in real time the physical and chemical characteristics of feedstock being fed into a pulp digester and thereby maximize process efficiencies and minimize chemical and energy consumption.
BACKGROUND OF THE INVENTION
Many pulp mills use a chemical pulping process for producing pulp from wood. In such a process, it is important that the physical and chemical characteristics of the wood chip feedstock that is fed into a pulp digester be taken into account in order to ensure consistent and optimal quality pulp production. The chemical pulping process includes a digester which is a vessel that utilizes steam and chemicals in the form of “liquor” to “cook” the wood chips under pressure and break them down to cellulose and hemi-cellulose fibres. Currently, there is no accurate or feasible way to measure the physical and chemical characteristics of the wood chip feedstock, including moisture and wood species type, continuously and on-line. The physical and chemical characteristics of the wood chip feedstock must be estimated according to empirical formulae.
The moisture content of the wood chips feeding a digester depend on such factors as their source (residual chips or whole log chipping), the wood species involved, the retention time of the wood chips on the wood chip piles and the time of year. These variables often result in wood chip moisture deviations of between 5 and 10 percent, which is significant for pulp production purposes. The practice is to err on the conservative side in estimating moisture levels.
The nature of wood species in the wood chips also is important because it too affects the “cooking” process. Certain species of wood chips require higher levels of alkali chemicals to digest the chips and break them down into cellulose and hemi-cellulose fibres. Because of these significant variables and the inability with current instrumentation to continuously monitor chip moisture content and wood species type on-line, operators of pulp mills tend to run higher than required liquor to wood ratios in the digester in order to ensure consistent quality and that the produced pulp will meet required specifications. The downside of running elevated liquor to wood ratios and active alkali charges, however, is that more than necessary levels of chemicals and high levels of steam are required to heat the liquor, which in turn increases energy costs. Running less than conservative liquor to wood chip ratios and active alkali charges runs the risk of reducing wood pulp quality due to loss of circulation and the occurrence of dirty or uncooked blows. On balance, to ensure high quality pulp, it is preferred to follow a conservative approach and run at higher ratios, which are more costly.
Apart from the handicap of having to operate the digester empirically under conservative parameters which raise energy and processing chemical costs, there is an inherent shortcoming with current testing procedures. Currently, there is no instrumentation or methodology available which can rapidly monitor wood chip feedstocks on-line and determine wood species and moisture content. The common procedure to determine wood species is to run tests on the pulp that is produced by the digester to determine if the pulp meets specifications for a specific grade type. This is known as a “feedback” testing approach. The problem with this approach is that in many cases approximately three hours of pulp production are jeopardized if the resultant pulp quality is not up to specifications. If the pulp does not meet specifications, it must be downgraded to a lower grade which attracts a lower price.
It would be highly advantageous if lower amounts of chemicals and less liquor were used in a digester cook, so that less steam would be required to heat the liquor to cooking temperature. Typically, if this could be done, it is conservatively estimated that liquor to wood ratios would be reduced by a minimum of 6 percent on hemlock or Douglas fir wood chips, and by 11 percent on western red cedar wood chips. This would result in a reduction in energy consumption per cook of approximately 4 to 5 percent on hemlock or Douglas fir wood chips and of about 8 percent on western red cedar. Benefits will be mill specific and dependent upon the wood species they cook, the moisture content of their chips and their current cooking strategy.
For the foregoing reasons, it would be extremely advantageous if a continuous rapid “feed-forward” on-line testing procedure were available so that wood chip species and moisture content could be continuously monitored on-line. This information would facilitate improved chip species control, active alkali charge and liquor to wood ratio control and would ensure consistent production of pulp to specified standards. In addition, to the economic benefits of improved control, the environmental impact of the process would be reduced.
In recent years, pulp mill owners have become increasingly aware of the importance of accurately determining the wood species of the feedstock. Previously, it was common practice, particularly in northwestern North America, to mix different wood species together. However, different wood species have different wood fibre properties. For example, Douglas fir has coarse, stiff fibres which, when pulped, forms an open, free-draining pulp mat with very high tear strength. Western red cedar, on the other hand, has flexible flat fibres which, when pulped, yields a closely grouped fibre mass which drains very slowly but has excellent tensile strength properties. Certain pulps are made from a mixture of western hemlock and balsam feedstock. The properties of these blends fall between those of Douglas fir and western red cedar. In recent years, purchasers have become increasingly demanding in asking for specific quality pulps made from specific wood species. It is therefore important for pulp producers to be able to control the wood fibre type. This is done by segregating wood species and controlling the species of wood in the feedstock. The benefits of segregating wood chips according to their species applies equally to both hardwoods and softwoods.
For the foregoing reasons, there is a strong and long felt need for a quick and accurate intelligent feed forward control system for determining moisture content and wood species of wood chips on-line as the wood chips are fed via a conveyor from a wood chip source to a digester to ensure consistent, optimum quality pulp production with minimum energy and chemical consumption. Real time measurement of the physical and chemical characteristics of the wood chip feedstock (moisture content and species) would enable the minimization of chemical and energy requirements.
Ion mobility spectrometry (IMS) is relatively recently developed technology that is capable of separating ionized compounds based on differences in their drift velocity through a gas under an applied electric field. This IMS technique has the ability to produce, in a matter of milliseconds, a characteristic spectrum of a specific series of high molecular weight compounds. It is known, for instance, that IMS can produce identifiable signatures for such items as narcotics and explosives and hence it is being developed for use by customs, airlines and police forces to detect such substances.
Initial tests have been carried out to determine if IMS can be used to identify different bulk wood species, such as lumber, on a static basis. A report on these tests, entitled “Rapid Characterization of Wood Species by Ion Mobility Spectrometry”, was published by A. H. Lawrence on Feb. 2, 1989 at the 75th Annual Meeting of the Technical Section of the Canadian Pulp and Paper Association. Some tests were carried out in the “positive mode” and some in the “negative mode”. Positive mode is when po
McKenzie Douglas John
Warren Sita Ruby
Alvo Steve
Oyen Wiggs Green & Mutala
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