Radiant energy – Invisible radiant energy responsive electric signalling – Infrared responsive
Reexamination Certificate
1999-05-19
2002-01-15
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Infrared responsive
C162S049000
Reexamination Certificate
active
06339222
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to a method for determining ionic species, particularly anionic species in aqueous solution, particularly pulp process liquors of cellulosic pulp manufacturing processes, by near infrared spectrophotometry and more particularly to the use of an on-line method for determining concentration parameters of said process liquors, and subsequent control of said cellulosic pulp manufacturing process by use of said determined parameters.
BACKGROUND OF THE INVENTION
Kraft pulping is performed by cooking wood chips in a highly alkaline liquor which selectively dissolves lignin and releases the cellulosic fibers from their wooden matrix. The two major active chemicals in the liquor are sodium hydroxide and sodium sulfide. Sodium sulfide, which is a strong alkali, readily hydrolyses in water to produce one mole of sodium hydroxide for each mole of sodium sulfide. The term “sulfidity” is the amount of sodium sulfide in solution, divided by the total amount of sodium sulfide and sodium hydroxide and is usually expressed as a percentage (% S) which varies between 20 and 30 percent in typical pulping liquors. The total amount of sodium hydroxide in solution, which includes the sodium hydroxide produced as the hydrolysis product of sodium sulfide, is called either “effective alkali” (EA), expressed as sodium oxide, Na
2
O before pulping, or residual effective alkali (REA) after pulping. Timely knowledge of these parameters would enable good control of the pulping process.
At the beginning of the kraft process, “white liquor” is fed to a digester. This white liquor contains a high amount of effective alkali up to 90 g/L, as Na
2
O. At intermediate points in the digester, spent liquor, or “black liquor,” is extracted from the digester. This spent liquor contains low levels of effective alkali—less than 30 g/L, as Na
2
O and also contains large amounts of organic compounds which, generally, are burned in a recovery furnace. Resultant inorganic residue, called smelt, is then dissolved to form “green liquor” which has a low concentration of effective alkali and a high concentration of sodium carbonate—up to 80 g/L, as Na
2
O. White liquor is regenerated from the green liquor by causticizing the carbonate through the addition of lime. After the recausticizing operation, a small residual amount of sodium carbonate is left in the white liquor. The combined amount of sodium hydroxide, sodium sulfide and sodium carbonate is called total titratable alkali (TTA). The causticizing efficiency (CE) is usually defined as the difference in the amounts, as Na
2
O of sodium hydroxide between the white and green liquors, divided by the amount, as Na
2
O of sodium carbonate in the green liquor. Sodium sulfate, sodium carbonate and sodium chloride represent a dead load in the liquor recycling system. The reduction efficiency (RE) is defined as the amount, as Na
2
O of green-liquor sodium sulfide, divided by the combined amounts, as Na
2
O, of sodium sulfide, sodium sulfate, sodium thiosulfate and sodium sulfite in either green liquor or the smelt.
The timely knowledge of the white-liquor charge of EA and of black-liquor EA would close the control loop in the digester and optimise for example, production and product quality and chemical utilization, of alkali and lime consumption. The control of sodium sulfide, TTA and of non-process electrolytes, such as sodium chloride and potassium chloride would also have a beneficial impact on closed-cycle kraft-mill operations. For example, environmentally-driven reduction of sulfur losses generally increases liquor sulfidity, thereby creating a sodium:sulfur imbalance that needs to be made up through the addition of caustic soda. Another important need is the control of TTA in green liquor, which is most easily done by adding weak wash to a smelt dissolving tank. The value of the green-liquor TTA is important because it is desirable to maintain the TTA at an optimal and stable level so as to avoid excess scaling while obtaining a high and stable white liquor strength. The ongoing development of modern chemical pulping processes has thus underscored the need for better control over all aspects of kraft-mill operations and more efficient use of all the chemicals involved in the process by knowledge of the concentration of aforesaid species in the liquors.
Sodium carbonate is difficult to characterise and quantify in situ because of a current lack of on-line sensors which can tolerate long-term immersion in highly alkaline liquors. Important economic benefits could result from causticizing control with a reliable sensor for sodium carbonate. Accurate causticization is critical for the uniform production of high-strength white liquor in that adding too much lime to the green liquor produces a liquor with poorly settling lime mud, whereas adding too little produces a liquor of weak strength. Determining the relative quantities of EA and carbonate in green and white liquor is thus important for controlling the causticizing process.
The recovery furnace of a recovery process produces a molten salt (smelt) that contains, in part, oxidized and reduced sulfur compounds. This smelt is dissolved in water to produce raw green liquor. The oxidized sulfur compounds are mainly in the form of sodium thiosulphate (Na
2
S
2
O
3
) and sodium sulfate (Na
2
SO
4
), while the reduced sulfur is in the form of sodium sulphide (Na
2
S). Since only the sodium sulphide is useful in the pulping process, it is desirable to keep the proportion of sulfur that is reduced, known as the reduction efficiency, as high as possible. Timely measurement of sulphate and thiosulphate in the raw green liquor would allow improved control of the recovery boiler's reduction efficiency.
Some mills produce fully oxidized white liquor for use in the bleach plant. In this process, the sodium sulphide ions in the white liquor are first partially oxidized to sodium thiosulphate (Na
2
S
2
O
3
), and then filly oxidized to sodium sulfate (Na
2
SO
4
). Timely measurement of the sodium thiosulphate concentration that is remaining in the liquor would allow improved control of the oxidation process.
It is known that an increase in carbohydrate yield in a kraft cook can be achieved by the addition of sodium polysulphide to conventional white liquor. Reference is made to this process in an article published in
Svensk Papperstidn
, 49(9):191, 1946 by E. Haegglund. Sodium polysulphide acts as a stabilizing agent of carbohydrates towards alkaline peeling reactions. Thus, polysulphide-cooking results in a significant pulp yield gain, which provides increased pulp production, and reduces the cost of wood chips.
A common method for producing polysulphide is to convert the sodium sulphide already present in the white liquor to polysulphide by an oxidation process. Several variants of this method are reported by Green, R. P. in
Chemical Recovery in the Alkaline Pulping Process
, Tappi Press, pp. 257 to 268, 1985 and by Smith, G. C. and Sanders, F. W. in the U.S. Pat. No. 4,024,229. These procedures generally involve redox and catalytic or electrochemical processes.
A typical polysulphide process is carried out in the recausticizing tank, which has a residence time of approximately 60 minutes. An example of such a process is described in G. Dorris U.S. Pat. No. 5,082,526. The main product, polysulphide, is produced through an oxidation reaction which also creates sodium thiosulphate through over-oxidation. Process conditions must therefore be controlled so that a maximal amount of polysulphide is produced. With a closed-loop control system, this is best achieved with a minimum sampling rate of 4 samples per unit of residence time. The traditional methods presently available for polysulphide are based on wet chemical methods and all take several hours. Therefore, they are not suitable for control methods. A spectrophotometric method had been reported by Danielsson et. al,
Journal of Pulp and Paper Science
, 22(6), 1996. Unfortunately, this method must either use a short pathlength,
Dylke Edward A.
Kester Michael
Leclerc Denys F.
Trung Thanh P.
Gabor Otilia
Kvaerner Canada Inc.
Pillsbury & Winthrop LLP
LandOfFree
Determination of ionic species concentration by near... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Determination of ionic species concentration by near..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Determination of ionic species concentration by near... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2838477