CA1210813A - Process control method - Google Patents
Process control methodInfo
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- CA1210813A CA1210813A CA000471697A CA471697A CA1210813A CA 1210813 A CA1210813 A CA 1210813A CA 000471697 A CA000471697 A CA 000471697A CA 471697 A CA471697 A CA 471697A CA 1210813 A CA1210813 A CA 1210813A
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- washer
- filtrate
- alkaline
- sodium ion
- pulp
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Abstract
PROCESS CONTROL METHOD
ABSTRACT OF THE DISCLOSURE
The invention is centered on the discovery of a straight line relationship between sodium ion content and total dissolved solids in the liquor or washer filtrate from a sodium based alkaline pulp treatment stage.
Sodium concentration can most conveniently be determined by the use of sodium ion selective electrodes. The relationship holds true in any stream within a given generalized system; e.g., a brownstock washer system or the washer following a caustic extraction stage in a bleach sequence. The dissolved solids information can be used as input for process control. As one example, the drop leg on the final stage of a brownstock washer system can be instrumented and used to control shower water. This enables the system to turn out a washed product in which the residual dissolved solids are held within close limits. An Important co-benefit lies in the uniform composition of the washer filtrate and the corresponding weak liquor stream to the recovery system evaporators. An Instrumented brownstock washer can also provide feed forward information and control to a bleach plant or brown paper sheeting operation.
ABSTRACT OF THE DISCLOSURE
The invention is centered on the discovery of a straight line relationship between sodium ion content and total dissolved solids in the liquor or washer filtrate from a sodium based alkaline pulp treatment stage.
Sodium concentration can most conveniently be determined by the use of sodium ion selective electrodes. The relationship holds true in any stream within a given generalized system; e.g., a brownstock washer system or the washer following a caustic extraction stage in a bleach sequence. The dissolved solids information can be used as input for process control. As one example, the drop leg on the final stage of a brownstock washer system can be instrumented and used to control shower water. This enables the system to turn out a washed product in which the residual dissolved solids are held within close limits. An Important co-benefit lies in the uniform composition of the washer filtrate and the corresponding weak liquor stream to the recovery system evaporators. An Instrumented brownstock washer can also provide feed forward information and control to a bleach plant or brown paper sheeting operation.
Description
Plog ~ Q~3 PROCESS CONTROL METHOD
BACKGROUND OF THE INVENTION
The pre3ent invention comprises a method OI determining the tot~l dissolved solids in the black liquor or washer filtr~te from a sodium-based extractive-type allcaline pulp treatment stage~ The method further u~ilizes total dissolved solids information as feedbaclc or feed forward d~ta for washer and chemical recovery plant control and QS feed forward data Ior bleach plant or paper mill operations.
Good control of the washers following the various chemical treatments in pulp mills is critically important to the economics of the mill and to the quality of the ultimate product. This is especially true for the brownstock washers which remove the spent digestlon liquors from the pulp.
Most typicRlly, the brownstock washer system will consist of several sequentlal or c~sca~ed countercurrent vacuum-type drum washers. Fresh water, evaporator condensate, or another suitable pulp mill water SQUrce is used on the showers of the ~inal washing stage. This is recycled counter~
curren~ly through the washer system where it is combined vvlth the bluck liquor discharged from the digester with the pulp chips. The filtrate ~rom the first washing stage contains rom 12-20% total solids content, o~ which a portion consists of inorganic pulping chernicals and another portion of orgflnic materi&ls removed from the wood chips during the p~ping process.
This stream Is ultimately directed to a bank of multi-effect evaporators where it is concentrated to about 50-65% solids content. Concentrated llquor Is then sent to a recovery furnace where it is burned to create process steam and to recover the inorganic chemicals for reuse in pulping liquor makeup.
The amount of fresh water used on the washers Is critical to pulp quality and to the economics of the operation. On the one hand, it is desirable that the pulp leaving the brownstock washers should be as clean as possible. The term "clean" in~this context re~rs to the relative freedom from water soluble di~solved solids. A well washed pulp has a significantly lower demand ~or bleaching chemicalis when ~ white product is being produced. Where an unbleached product is being ma~e, an excessive carryover of pulping liquor into the sheeting operation deleteriously affects P109 ~ 3LQ~3 paper machille running rate because of poorer sheet drainage on the wire, and water removal in the press sections, and lower dryillg rate. The physical properties of the product are also reduced, particularly burst strength. On the other hand, producing a very clean pulp from the washers results in more 5 water being sent to the evaporators with accompanying higher steam consumptionO Thus, there is an economic balance point between pulp cleanliness and evaporation cost.
The above problem was discussed by Fisher, _up-and-P~p-er,6 ~9):101-103 (1979) who proposed control of brownstock washers using a 10 relatively complex instrumentation array involving conductivity and flow measurements in most of the liquid streams in the system. Fisher historically points out that the typical control procedure on a brownstock washer system involves specific graYity measurements taken periodically on the first-stage washer filtrate and conductivity measurements macle on the 15 fin~l-stage filtrate or washer mat squeezings. These re~dings might be taken at intervnls ranging from 1-8 hours so thnt major upsets in the system could easlly pass by wlthout being noticed for long periods of time. The normal response for an upset would be for the operator to manipulate incoming stock flow rate and/or shower water flow on the final stage. This 20 is admittedly a very haphazard method of controlling the system~ one reason being that there is a 30-45 minute time lag in filtrate movement between the first and the last washer stages. Others besides Fisher have described washer control using conductivity and ~low measurements. Freyaldenhoven and McSweeney, TapFi, 62 ~h59-61(1979~, write about what is apparently 25 the identical pulp mill system described by Fisher. The latter allthors also supply A useful Flow ~iagrarn not present in the earlier article.
The instrumentation and control system using conductivity measurements finds its origin somewhat earlier in time. A paper by Gossage and McSweeney, ap~i 60 (4~:110-112 ~1977), notes that conductivity can be 30 used to indicate the soda or salt cake left in the la~t stage washer pulp mat.
These writers note a correlation between conductivity measurement and nonvolatiles in the washer streams and also note a correlation between conductivity and sodium concentration in the same streams. In one of the three mills studied, the relationship between nonvolatiles and conductivity 35 appeared to be linear although the relationships were curvilinear in the other two mills. The same situation prevailed for the relationship of P109 ~ L3 conductivity to sodium concentration. These writers did not propose a control strategy based upon their observations. However, in a patent filed during the same time period, lRosenberger, in U.S. 4,096,028, does detail a brownstock washer system control str~tegy bflsed on conductivity and ~ow 5 measurements. This appears to be the same system described later by Fisher and Freyaldenhoven and McSweelley. This system was offered commercially and installed in several mills although, to the best of the inventors' present knowledge, it has since been withdrawn from the market.
Another proposal for monitoring a pulp washer system was made 10 almost a decade prior to the work of the authors cited abosre. This was based on the direct measurement of sodium ion in various liquor and filtrate streams using the then newly-ava31able glass electrodes specific toward sodium ions. Camacho, So; Pulp--an~-l?ap~r, 30 (6):96-101(1967), notes that the methods available prior to that time measured some other physical 15 property which was then relatable to sodium concentrations in the p~p mat ~nd in the filtrate streams. He speculated that it might be possible some day to contlnuously mollitor wnshed stock cleanliness and soda losses by direct measurerrlent of sodium ions using the ion specific electrodes.
However, his own work was limited to the establishment of a system for 20 spot check measurements. Camacho did note that the voltage output between the sodium specific electrode and the reference electrode corre-lated linearly with the log of the soda content of the stream being measured. A few years later, Len~ and Mold, Tappi, 54:2051-2555 (1971) showed that scdium ion selective electrodes could determine sodium in both 25 kraft and neutral sulfite semichemical green and white liquors, dregs, and black liquors. They further noted that their observed data were close to a Mernstian rela$ionship. These observations apparently lay dormant until the matter was ~gain picked up by DeBerry, Tappi, 63 (7):123-L24 (1980), who observed that a sodium ion selective electrode can be used to rne~sure the 3~ total sodium content of a low consistency aqueous pldp suspension.
Other workers have been well aware of the need for good control of the brownstock and other washers in the p~p mill and various computer based analysis and control systems have been proposed.
Virtually all of these systems are concerned only with the 35 estin ation of sodium ion concentration. They tend to ignore the fact that a major part of the water soluble dissolved solids are organic in nature. This P109 ~ 3 information is equally critical if proper washer control is to be achieved. It is th~s organic material that gives the black liquor stream its heat value and the carried over organic portion is also a significant consumer of bleaching chemicals.
None of the systems discussed aboYe have found general acceptance within the industry and, in most cases, their implementation appears to have been of a transient or temporary nature. Despite all of the ad-,rances in instrumentation and control technology that have been made in the last two decades9 the control of pulp mill washer systems depsnds 10 heavily on the intuitive skill of operators who must work with infrequent batch sarnples and with data which are usually well a~ter the fnct.
The present inventors have found a surprising ancl totally unexpected direct relationship between sodium ion concentration in various washer liquid streams and the total dissolved`solids present in the liquid.
15 This ability to predict total dissolved solids makes it possible to develop sirnple and inexpensive control systems for pulp washers regardless of whether they are multistage or single-stage.
SUMMARY OF THE INVENTION
The ~llowing deflnitions apply to tePms used with~n the specifî-20 cation and clnims of this application.
"Tot~l dissolved solids" refers to all water soluMe extractable material, whether organic or inorganic, that is dissolved in the water physically and/or mechanically associated with the pulp fiber.
"Liguor" refers either to spent pulping liquor or washer ~iltrate 25 which contains measurable amounts of dissolYed solids.
'IExtractive-type alkaline pulp treatment" refers to any sodium bAsed alkaline pulping or bleachirlg stage which extracts and solubilizes measurable amounts of organic materials ~rom the pulp fibers. In the context of the present invention~ the pulping process can be any of the well-30 known alkaline processes such as kraft, soda, soda-oxygen, alkaline sulfite, or any of the alkaline ~nthraquinone types. During the bleaching sequence, an extractiv~type alkaline pulp treatment would normally refer îo a caustie extraction stage in which significant amounts of free chlorine or hypo-chloride ion were absent.
"Brownstock washer" is the washing system, whether single-stage or multistage, which immediately follows the pulp digesters.
P109 :~2~
11~77 5 A ~Igeneralized system" refers to any individual unit process used to process a given type of furnish within the pulp mill. The brov~nstock washers in a kraft mill pulping softwoods would be considered one generalized type of system. Brownstock washers in a kraft srill pulping 5 hardwoods would be considered as a second generalized type of system. A
decker following high density storage, or a washer following an alkaline extraction in the bleach plant would be considered third and fourth types of generalized systems. The term implies a common type of chemistry for the process being considered.
A 1'washer system" relates to all of the washer equipment which is used in integrated fashion ~ollowing any process operation. A washer system ean consisl of either a single washer or several washers operated in a countercurrent sequential ~ashion.
"Liquid ~ssociated with the pulp" broadly means the free liquid 15 pbysically associated with the pulp fibers at any point in time.
Broadly stated, the present invention comprises a method OI
determining total dissolved solids in the liquor following ~ sodium based extrnctive-type alkaline pulp treatment stage. The invention further comprises a method of estimatlng the carryover of water soluble dissolved 20 sollds remaining in a cellulosic p~p product leaving the washers which follow such a treatment stage. The invention also comprises a method of controlling the carryover of water soluble dissolved solids remaining in a cellulosic pulp product leaving the washers. The imrention may 3~rther be consideted to comprise a method of controlling washing efficiency of a 25 multistage countercurrent pulp washer system which follows a sodium based extractive-type alkaline pulp treatrnent stage.
A key element in all of the aspects of the present invention is the unpredictable and unexpected discove~y that within any generalized system there is a ~tratght line relationship between sodium ion concentra-30 tion and total dissolved solids measured in any individual liquor streamwithin the generalized system. With this knowledge, it is possible to determine total dissolYed solids in any liquor stream within the generalized system by first experimentally establishing a calibration algorithm relating sodium ion concentration to total dissolved solids in one of the liquor 35 streams within the system~ The algorithm will be a llnear relationship of the ~orm `: ~
.
rrotal Dissolved Solids - a + b (Sodium Concentration) where a is a constant which approximate3 0 and b is a constant between about 0.1 ~nd 0O35. It is assumed that dissolved solids and sodium concentrations are expressed in equivalent units. The eql-ation thus defines a straight line which will pa.ss through or near the origin. When the algorithm has been established for any given generaliged system, total solids can then be determined by simply measuring the sodium ion concentration in the liquor in any stream within the system. This measurement is entered into the algorithm which is then readily solved to give a value for total dissolved solids.
Knowledge of the above relationship, which enables total dissolved solids to be determined continuously, is the key which opens the door to a number of different control means for any given washer system.
As one example, it is now possible to calculate the water soluble dissolved solids carried over with the pulp to the next st.sge in the process.
A consistent relationship has been discovered between sodium lon content of washer filtrate and sodium ion content in the liquid associated with the pulp on the fllter mat leaving the washer. This ls giYen by the general equation Namat = (lop)(m + n log Nafilt)q where Nam!lt represents sodium ion in the liquid associated with the filter mat, Nafilt is the sodium ion content of the washer filtrate, m and n are constants of the generalized system, p is a constant between about 0.15 and 0.45 and q is a constant between about 1.5 and 2~t)D Values for all of the constants can be readily determined experimentally for each ~eneralized system.
In a system using a sodium ion specific electrode for measurement, the sodium concentration may be represented by a voltage analog. The voltage readout signal from filter mat squeezings is linearly related to the log of the voltage readout signal from washer filtrate. This enables the above relationship to be more simply expressed as Emat ~ c ~ d log Efilt .
. . .
~2~4~8~3 11~77 7 where Emat is the voltage readout from a measurement on mat s~ueezings, Efilt is a similar reading on washer filtrate, and c and d are constants of the generalized system. Either voltage reading can be readily converted to sodium concerltration by using the appropriate electrode calibration curve.
It is an important feature of the present invention that by measuring the sodium ion concentrfltion of washer filtrate9 the sodium content of the liquid associated with the ~ilter mat can also be determined.
This can then be directly related to the total dissolved solids in the mat liquid.
If consistency o~ the filter mat is known then the total dissolved solids carried over with the mat are shown. This makes it a simple matter to instrument and control downstream processes in a ~eed ~orward manner.
It is also possible to use the knowledge of carried over dissolved solids as a feedback or feed forward signal for controlling f1OW rate of t5 washQr shower water in order to maintain a predetermined range of water soluble clissolveà solids In the washer mat. Thls type sf control is effective on eIther a single-stage or a multistage countercurrent pulp washer system By controlling the total dissolved solids in the washer filtrate, control of mat carryover is automatically achieved ns is control of dissolved solids in the weak liquor sent to the recovery system evaporators. While in its broadest form, the invention contemplates control oE shower water at any stage of a multistage washer, most typically orly the final stage shower water would be controlled.
The methods of the invention are readily adapted to any of the common types of pulp washer systems now In use. They are based on the instantaneous or continuous measurement of sodium ion concen$ration by any means. Most convenienMy this will be a sodium ion specific electrode.
The output signal between the electrode and its corresponding re~erence electrode is a voltage which relates linearly to the common logarithm of the sodium concentrAtion. This ~an be readily converted by use of the particular algorithm or conversion factor of the generalized systern being measured to total dissolved solids concentration. The voltage analog may be directly used through appropriate inter~aciDg hardware to control shower valves or other control devices within the washer system The output from the electrode may with equal convenience be directed to A mill process computer or a programmable controller so as to give an instantan~ous or continuous reading of total dissolved solids to the appropriate operating ..
lt~77 8 personnel while, at the same ffme~ serving flS a sensor for a process control function.
Washers may be instrumented with elqual convenience at any of several points. Total dissolved solids readings can be made on mat squeezings, on the washer filtrate in the vacuum leg o~ a drum-type washer, or in the filtrate seal tank. It has been found preferable to sample the filtrate in the lracuum leg of the washer where this is possible. The use of mat squeezings poses sampling problems since it is well known that a pulp mat is not necessarily OI uniform composition along a traverse direction 10 taken across a washer drum. Sampling the filtrate tank introduces a time delay so that the re~ding may be indicative of pulp which left the washer as much as 10-20 minutes earlier. By sampling at this point, short term upsets in the system will not be readily noticed, and accommodated by the washer controIs. The time delay is minimized by sampling iltrate in the vacuum lS leg so that for all practicRl purposes it is lnsigni~icant. By utilizing the methods of the present Invention, a new measure of control can be brought to any washer system in a plilp mill which follows a sodium based extractive-type nlkaline treatment.
The method has the great advantage that once the calibration 20 algorithm relating sodium to total dissolved solids has been determined for ageneralized system, this ~lgorithm is valid for any mill in which the ger~eralized process system is practiced. Thus, for all practical purposes, the algorithm for a brownstock w~sher on a kraft mill pulping Northwestern softwoods will be the same for the brownstock washer on a s~uthern pine 25 kraft mill.
Additional advantages of the method are its simplicity, depend-ability and the relatively low cost involved in instrumenting and controlling a given washer system.
It is an object of the invenffon to pr~ide a simple and 30 inexpensive method for determining total dissolved solids in the liquor generated in any sodium based e~tractive-type alkaline pulp treatment stage.
It is a further object to provide a method ~or estimating the carryover o~ water soluble dissolved solids in the pulp mat leaving a washer 35 system.
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P1~9 It is another object to provide a method for controlling a washer system so that the carryover o water soluhle dissolved solids is rnaintained within a predetermined range.
It is yet another object of the invention to provide a method of 5 controlling cost-effectiveness of washing in a multistage countercurrent pulp washer system.
It is still another object to provide a measurement and control system for pulp washers whieh is simple, dependable, and relatively inexpen-sive.
It is yet a further object to provide a rnethod for eontrolling pldp washing where, within the confines of a given generalized system, the ratio of sodium to dis.solved solids is constant in any liquid stre~m within the system.
These and many other objects will become readily app~rent to 15 those skilled in the art upon reading the following detailed description taken in conjunction with the attached drawings.
BRIEF DESCRIPTION OF THE DF~AWINGS
Pi~ure 1 is a diagram showing a generalized countercurrent multistage washer system following a p~p digester using the methods of the 20 present invention in a feedback control system.
Figure 2 is a typical calibration curve eor an indi~idual sodium lon specific electrode showing the linear relationship between the readout signal in millivolts and the common 1~ of the sodium concentration as determined by atomic absorption spectroscopy.
Figure 3 is a ~urve ~owing the correlation between sodium and total dissolved solids measured in samples from mat squeezings, washer rat liquor, and washer filtrate t~ken from brownstock washers in six p~p mills from widely separated geographic locations.
Figure 4 is a graph showing the straight line relationship between 30 the electrode signal nneasured on mat s~ueezings and the common logarithm of the electrode sign~l of washer filtrate collected at the vacuum leg, in a mill processing softwood grades of p~p.
Figure 5 is a graph which shows the straight line relationship between sodium and dissolved solids in brownstock liquor samples taken 35 from the washer vat, from mat squeezings, and From the vacuum leg on the washers in a mill pulping softwood grades.
I
8~3 11~77 10 Figure 6 is a graph similar to Figure 5 but taken from similar locations in a mill pulping hsrdwoods.
Figure 7 is a graph showing the straight line relationship between the solids concentration in the filtrate of the firslt washer in a three-washer 5 sequence and the common log of the sodium ion concentrate in the third washer filtrate, in a mill pulping softwoodsO
Figure 8 is a similar curve to that shown in Figure 7 but determined in a mill pulping hardwoods.
Figure 9 is a curve showing the straight line relationship between 10 sodium concentration and dissolved solids in mat squeezirtgs from a single stage washer following a caustic extraction stage in a bleach sequence.
Figure 10 is a diagram of a washer system similar to that shown in Figure 1, but usi~ a feed forward control system.
I)ESCRIPTION OF THE PREFERRED EMBODIMENTS
We have discovered a principal reason why conductivity measurements have not proved to be particularly satis~actory for brown-stock washer control. The logic was based on the faulty premise OI constant relative concentrations of each sodium compound present in the liquor UnPortunntely, this ratlo varies widely. As one example, the conductivity of 20 sodiurn hydroxicle in a 1% by weight solution is about 48 mmhos/cm while a similar solution oi~ sodium carbonate or sulfate will have a conductivity of about 11 mmhos/cm. At 4% concentration, the sodium hydroxide solution will huve risen to a conductlvity of about 170 mmhos/cm while the equivalent sodium carbonate or sul~a~e will be at about 37 mmhos/cm.
25 Rxperiments using conductivity measurements at different points in time on a given black liquor stream have shown errors falling in the range of abou 5.7-10.2% ~om actual sodium values. Similar measurements using a sodium ion speciPic elec~rode varied between 0-0.3% from true values.
The disco~rery that there is a direct linear relationship between 30 sodium ion concentration and the total dissolved solids in the pulp liquor stream of a sodium based extractive-type alkaline pulp treatment stage opens a new vista for pulp mill process contro~. Sodium ion concentration is most conveniently measured with a sodium ion specific electrode used in con~unction with a suitable reference electrode. These may be used with 35 any conventional pEI meter or similar device which will give an output reading in millivolts. The sodium electrode is manufactured with a glass tip . . , ,j .
(?8~3 11477 ~1 that is sensitive to sodium ions. Suitable electrodes are the Model No. 97-12, manufaetured by Orion Research, Inc., Cambridge, Massachusetts, or the Corning Model X-EL available from the Corning Glass Works, Medfield, Massachusetts. These both contain the sodium ~sensing 5 electrode and the re~erence electrode in one body. Other ions, such as silver, potassium, and hydrogen ions, can potentially cause interference. In a typical pulp mill process stream, neithe~ silver nor potassium are usually present in interfering quantities. Hydrogen ion interIerence normally is not a problem unless the concentration of sodium ions being measured is 10 extremely low. Electrodes are also somewhat temperature sensitive and can introduce ~n error of about û.6% in the sodium ion reading for each degree C of temperature change. Again, this is nor~ally not a problem during measurements of a pulp washer stream since temperatures tend to be qulte stable over very long periods of time. Where temperature variation is 15 encountered, it may be desirable to run the stream being measured through smnll heat exchanger immersed In a constant temperature bath beEore taking readlngs.
Figure 1 is a simplified diagrammatic presentation of a typical brownstock washsr systemO This would be essentially the same whether the 20 pulp was to be bleached or sheeted as an unbleached product. The system shown uses a series of three sequential vacuum~type drum washers followed by a high-density storage chest. This in turn is followed by a plilp decker which may be identical to one of the preceding washers. The system works essentially as follows. Pulp Irom the screens or knotters is pumped in line a 25 to the first washer 4. It is diluted with recycled washer filtrate arri~ing in line 54 to about 1% conslstency before it ~lows into the washer vat. The pulp rnat will leave the washer drum at about 12~16% consistency. It is then reslurried and diluted again to about 1% consistency where it is pumped to the second stage washer 6. The sequence is again repeated and the pulp 30 slurry is brought to the final stage 8 o~ the brownstock washer system.
From there~ it will usually be directed to a high-density storage chest 10, although this is not essential. Once again the pulp is diluted as it is taken ~om high-density storage and ultimately receives a final washing sequence 12 at the pulp decker. The washed pulp is again diluted and led 35 through line 14 to the next appropriate process station.
P1~9 11~L77 12 In the example being shown, fresh wash water arrives through line 20 to the showers of des~ker 12 where it displaces liquid entrained in the pulp fibers. The interior of the washer drum is maintained under negative pressure by a water or vacuum leg 24. The proximal end of this leg communicates with the interior of the washer drum, while the distal end is maintained under the surface of washer effluent held in a seal tank 26. A
similar arrangement is present on all of the other washers. Higher levels of vacuum may be provided by the use oE external vacuum pumps in communication with the seal tanks. ~ince the vacuum on the washer drum is lD dependent on the level of water in the seal tank, in the absence of an auxiliary vacuum pump, each tank is equipped with a level control. In the case of the seal tank, level control 60 operates a valve 62 to cor.trol the flow of makeup water through line 22. Filtrate from the seal tank 26 is pumped through line 28 as dilution water for stock being withdrawn from 15 high-density storage chest 10. It is also directed through line 30 to the showers of the third washer 8 in the brownstock washer system. A portion ot the w~ter delivered to the showers through line 30 i9 drawn off through line 32 as dilution water ~or the ilter mat ~rom third-stage washer 8 in order to dilute it sufficiently so that it can be pumped to the highdensity 20 storage chest. Third-stage washer 8 is equipped with vacuum leg 34 and seal tank 36. Filtrate is withdrawn Erom the seal tank through lin~ 38 where it acts as shower water on secon~stage washer 6. A portion of this water is drawn off to dilute the filter mat from the second-stage washer 6 to a suitable consistency for handling on the thir~stage w~sher. Filtrate from 25 the second~stage washer is delivered through vacuum leg 42 to se~l tank 44.
From here it is pumped through line 46 to the first-stage washer showers and through line 48 as dilution water for the filter mat from first-stage washer 4. E~iltrate from Iirst-stage washer 4 flows through vacuum leg 50 into seal tank 52. The efnuent from seal tank 52 is pumped to three points.
30 A portion of it flows through line 54 as dilution water for unwashed stock arriving through line 2 to the washer system. Another portion is directed through line 58 to the blow tan3c for stock dilution. A third portion of this seal tank water ~lows through line 56 to the evaporatnrs in the chemical recovery system.
Third washer seal tank 36 has a level control 64 which serves to control the flow of shower water flowing through valve 66 to the showers on .
.
P109 ~ 3 secon~stage washer 6. Sect)nd-stage seal tank 44 has a similar tevel control 68 which controls shower water on the first stage by means of control valve 70. The first stage seal tank 52 is equipped with a level control 72. This regulates the weak liquor ultirnately leaving the system via control valve 74 and line 56 leading to the evaporators.
The system exemplified here contains two sodium ion detectors.
A first sodium ion measurement means 80 is attached to drop leg 34 of the third-stage washer. This serves to control third-stage brownstock washer shower water arriving through line 30 by acting on control valYe 82. A
lD second sodium ion detector means 76 is similarly located on the drop leg of tbe pulp decker 12. This controls fresh water corning to the showers of the decker by acting on control valve 78.
The three brownstock washers, when washing pulp produced from a given type of furnish, may be considered as one generalized system. The 15 pulp decker may Ibe considered as a second generalized system when used with the same type of furnish. If no high density storage chest was used between the brownstock wnshers and the p~p decker, the decker could be considered as a part of the same generalized system as the pulp washers and would simply be considered as ~ fourth stage. However9 the interposition of 20 high density storage gives a tlme peeiod in which soluble organic and inorganic solids ~re able to diffuse from the fibers into the liquid phase associated with the pulp. This may somev~hat change the slope of the curve relating sodium to total solids and the curve for the brownstock washer ~eneralized system will not be exactly coincident with the curve for the 25 decker generalized system.
When sodium ion selective electrodes are used for measurement, each electrode will normally be individually culibrated to establish a curve relating readout voltage to sodium ion content oF the solution being measured. Such a curve is shown in ~igure ~. While different electrodes 30 will have very similar response curves, accuracy of a system can be improved somewhat by making individual calibration deterrninations. The curve shown in Figure 2 was plotted using mat squeezings ~rom the first washer in a brownstock washer system. Sodium content was determined by atomic absorption spectroscopy.
One of the most remarkable discoveries is that the relationship of sodium ion concentration to total dissolved solids in any given gener~ ed P109 ~ 3 system appears to be universal. Figure 3 shows a eurve plotting sodium concentration in samples obtained from brownstock washer mat squeezings, washer vat liquor, and washer filtrate against total dissolved solids in the liquid. This curve is ôased on ~pproximatelLy 11~ measurements taken in six 5 difPerent kraft pulp mills. Three of these mills were in the Pacific Northwest coastal region of the United States, one was in interior British 1: olumbia, and two were in North Carolina. The correlation coefficient for this plot is 0.95. This correlation is remarkable in view of the different tree species used in the furnish and in view of both inter-and intra-mill variations 10 in cooking liquor sulfidity and active alkali as well as other digestion conditions that affect pulp yield. Other mill variables which affect liquor makeup are reduction efficiency in the recovery furnace and causticizing efficiency. These variables all affect sodium concentration and black liquor solids concentration in varying degrees. Rosenberger in U.S. 4,096,028 15 notes the great variability in composition ancl physical characteristics of input streams to brownstock washers.
It is well known that the composition oE a filter mat has signiîicant v~riability across the drum and at any given location nt different tlmes. Despite this knowledge, squeezings from the filter m~t from the 20 washer drums are still the principal s~urce of liquor taken for washer systemanalysis and control. Because of the variability, and bec~use it is difficult to take mat squeezings on a continuous basis, other sources of sampling are more desirable. The filtrate in the vacuunn leg of a washer represents an integrRted sample of liquor that is passed through the filter mat. It also 25 represents a vely short time interval between the passage o~ a given unit of pulp and the time of measurement. The use of tracer compounds added ~t the washers hAs shown that the interval between passage of a unit of pulp and time of sarnpling in a vacuum leg is less than two minutes. However9 what is essential if the vacuum leg is sampled is that the ratio of sodium to 30 total dissolved solids measured in the filtr~te is the same as, or directly relatable to, the ratio of sodium to dissolved solids present in the liquid associated with the filter mat. Figure 4 shows a c~ve taken from the third stage of a three-stage brownsto~k washer during a tirne when softwood gradss were being run in a kraft mill. There is a linear semilogarithmic 35 relationship betuveen the readout signals obtained on mat squeezings and the filtrate salTIpled from the racuum leg of the washer. Since the relationship ., /
of these readout signals to sodium ion concentration and thus the total solids is known, it can be said with confidence that sodium ion readings taken on filtrate sampled from the vacuum leg are indicative OI total solids in the liquor at that point. The same readings can also be used to calculate the 5 total solids in the liguor associated with the pulp mat as it leaves the washer. In the past, it has been a largely unfulIilled desire to have pulp with a known controlled amount of dissolved solids entering the bleach plant or paper mill. This problem can now be brought under control using the methods of the present invention. Total dissolved solids information can 10 therefore be used for feed ~orward control of chemical usage in the bleach plant.
While the curve in Figure 4 relates voltage readings taken on washer filtrate and mat squeezings, the relationship can also be readily expressed as an equation that relates sodium ion concentrations, or total 15 dissolved solids, in these two liquid streams. In the generalized system from which Figure 4 was determined this equation was lo0.29 (_114.96 ~ 46.49 log Nafiltrate) 20 The above equation can be readily determined for each generalized system being measured and can take into account the individual calibration curve for the sodium ion specific electrode. Once determined, the above relationship will hold constant over time for the generalized system.
Small differences are noted in the slope of the line relating any 25 brownstock washer lîquor sodium concentration and dissolved solids~
depending on whether softwoods or hardwoods are being pulped. These differences are basic to the chemistry of the p~ping process and to the nature of the materials being solubiliæed in the wood chips. Figures 5 and 6 show curves relating sodium content to dissolved solids for any stream in the 30 brownstock washers in kraft mills processing softwood and hardwood grades, respectively. While the difference of the slope oi these curves is not great, it is sufficient so that it should be accounted for if highest accuracy is desired. Differences of this type were not taken into account when the data were gathered from which Figure 3 was plotted, accounting for the slightly 35 different slope of that curve as compared to the pr~sent ones.
11~77 16 Other factors besides furnish that affect the basic chemistry of the pulping process will bring about changes that require definition of a new generalized system. Among these might be mentioned the use of anthra-quinone as a pulping catalyst.
Now that a relationship between mat squeezings and filtrate dissolved solids has been established for the last washer of a multistage system, it could be hypothesized that there wvuld also be a relationship between third-stage filtrate and first-stage filtrate total dissolved solids~ aslong as the time lag between them is also considered. Further experimental work showed that this was indeed the case. The relationship between third-stage filtrate and first-stage Eiltr~te for softwood and hardwood grudes is shown in Figuree 7 and 8, respectively. The log of the sodium ion concentration measured in ppm in No. 3 washer filtrate relates linearly to the solids concentration in the filtrate of the No. 1 washer. Controllin~ the total solids concentration in the thir~stage îiltrate also implies controlling the solids content in the weak liquor stream sent to the evapor~tors.
The relationship between sodium ion concentration and dissolved solids also appears to hold for extractive-type alkflline pulp treatments other than digestion. Figure 9 shows the relationship between sodium ion concentration and dissolved solids in the washer filtrate from the alkaline extraction stage in a bleach plant. This particular bleach sequence employs a first-stage chlorination, secon~stage alkaline extraetion, and thir~stage hypochlorite treatment. The curve shown was based on 75 individual samples, all of which fell closely along the regression line shown in the Figure.
In any washer system, whether it is single-stage or multistage, the water entering the system must be equal to the water leaving the system. The normal method o controlling water, and also of controlling pulp cleunliness, is by control of the flow rate of shower ~Nater. In a multi-stage washerl this will be the shower water flowing onto the final stage.
Control of shower water can be easily achieved by total solids measurement in either the filtrate or the liquid associated with the pulp mat. These measurements can act as acc!Jrate feedback or feed forward information.
One alternati~re to shower water control by itself is seen in Figure 10. This is a revision of the system seen in Figure 1 in that the sodium ion flnd thus total dissolved content are measured by sensor 80 in the . . .
P109 ~Z~ 3 liquid portion of the slurry flowing into the first washer vat. This signal is sent to control valve 7~ on the first washer shower system. Level control 68 now controls shower valve 6B on the secon~stage washer and level control 64 on the third washer filtrate tank controls valve 82 on the third washer showers. The decker is instrumented as shown in ]Figure 1 but, in addition, it feeds forward the total dissolved solids content of the washed pulp to the bleach plant or paper mill. A feed forward system of this type is advantageous in that it offsets the effect of the time lag across the system when process up~ets occur. These upse ts ean now be anticipated at 10 downstream stations.
For the first time, on a practical basis, the present invention gives the information needed for the control of defoamer addition to the pulp system. The evaluation of defoamer effectiveness and the optimum point of application has been highly subjective to date. Now the efficiency 15 of a defoamer can be accurately determîned by measurement of total solids in the final stage filtrate. Use of the present metnods also enables a stream having uniform total solids concentration to be sent to the recovery area, a portion of the mill notoriously subject to upsets from concentration fluctuations.
Having thus described the best kaown modes of the invention known, it will be immediately apparent to those skilled in the art that many variations are possible without departing from the spirit of the invention.
As one example, the methods of tbe invention are equally applicable to the liquor streams from any lignocellulosic material and should not be 25 ~onsidered as limited to wood. The invention should be considered ~s being limited only by the following claims.
BACKGROUND OF THE INVENTION
The pre3ent invention comprises a method OI determining the tot~l dissolved solids in the black liquor or washer filtr~te from a sodium-based extractive-type allcaline pulp treatment stage~ The method further u~ilizes total dissolved solids information as feedbaclc or feed forward d~ta for washer and chemical recovery plant control and QS feed forward data Ior bleach plant or paper mill operations.
Good control of the washers following the various chemical treatments in pulp mills is critically important to the economics of the mill and to the quality of the ultimate product. This is especially true for the brownstock washers which remove the spent digestlon liquors from the pulp.
Most typicRlly, the brownstock washer system will consist of several sequentlal or c~sca~ed countercurrent vacuum-type drum washers. Fresh water, evaporator condensate, or another suitable pulp mill water SQUrce is used on the showers of the ~inal washing stage. This is recycled counter~
curren~ly through the washer system where it is combined vvlth the bluck liquor discharged from the digester with the pulp chips. The filtrate ~rom the first washing stage contains rom 12-20% total solids content, o~ which a portion consists of inorganic pulping chernicals and another portion of orgflnic materi&ls removed from the wood chips during the p~ping process.
This stream Is ultimately directed to a bank of multi-effect evaporators where it is concentrated to about 50-65% solids content. Concentrated llquor Is then sent to a recovery furnace where it is burned to create process steam and to recover the inorganic chemicals for reuse in pulping liquor makeup.
The amount of fresh water used on the washers Is critical to pulp quality and to the economics of the operation. On the one hand, it is desirable that the pulp leaving the brownstock washers should be as clean as possible. The term "clean" in~this context re~rs to the relative freedom from water soluble di~solved solids. A well washed pulp has a significantly lower demand ~or bleaching chemicalis when ~ white product is being produced. Where an unbleached product is being ma~e, an excessive carryover of pulping liquor into the sheeting operation deleteriously affects P109 ~ 3LQ~3 paper machille running rate because of poorer sheet drainage on the wire, and water removal in the press sections, and lower dryillg rate. The physical properties of the product are also reduced, particularly burst strength. On the other hand, producing a very clean pulp from the washers results in more 5 water being sent to the evaporators with accompanying higher steam consumptionO Thus, there is an economic balance point between pulp cleanliness and evaporation cost.
The above problem was discussed by Fisher, _up-and-P~p-er,6 ~9):101-103 (1979) who proposed control of brownstock washers using a 10 relatively complex instrumentation array involving conductivity and flow measurements in most of the liquid streams in the system. Fisher historically points out that the typical control procedure on a brownstock washer system involves specific graYity measurements taken periodically on the first-stage washer filtrate and conductivity measurements macle on the 15 fin~l-stage filtrate or washer mat squeezings. These re~dings might be taken at intervnls ranging from 1-8 hours so thnt major upsets in the system could easlly pass by wlthout being noticed for long periods of time. The normal response for an upset would be for the operator to manipulate incoming stock flow rate and/or shower water flow on the final stage. This 20 is admittedly a very haphazard method of controlling the system~ one reason being that there is a 30-45 minute time lag in filtrate movement between the first and the last washer stages. Others besides Fisher have described washer control using conductivity and ~low measurements. Freyaldenhoven and McSweeney, TapFi, 62 ~h59-61(1979~, write about what is apparently 25 the identical pulp mill system described by Fisher. The latter allthors also supply A useful Flow ~iagrarn not present in the earlier article.
The instrumentation and control system using conductivity measurements finds its origin somewhat earlier in time. A paper by Gossage and McSweeney, ap~i 60 (4~:110-112 ~1977), notes that conductivity can be 30 used to indicate the soda or salt cake left in the la~t stage washer pulp mat.
These writers note a correlation between conductivity measurement and nonvolatiles in the washer streams and also note a correlation between conductivity and sodium concentration in the same streams. In one of the three mills studied, the relationship between nonvolatiles and conductivity 35 appeared to be linear although the relationships were curvilinear in the other two mills. The same situation prevailed for the relationship of P109 ~ L3 conductivity to sodium concentration. These writers did not propose a control strategy based upon their observations. However, in a patent filed during the same time period, lRosenberger, in U.S. 4,096,028, does detail a brownstock washer system control str~tegy bflsed on conductivity and ~ow 5 measurements. This appears to be the same system described later by Fisher and Freyaldenhoven and McSweelley. This system was offered commercially and installed in several mills although, to the best of the inventors' present knowledge, it has since been withdrawn from the market.
Another proposal for monitoring a pulp washer system was made 10 almost a decade prior to the work of the authors cited abosre. This was based on the direct measurement of sodium ion in various liquor and filtrate streams using the then newly-ava31able glass electrodes specific toward sodium ions. Camacho, So; Pulp--an~-l?ap~r, 30 (6):96-101(1967), notes that the methods available prior to that time measured some other physical 15 property which was then relatable to sodium concentrations in the p~p mat ~nd in the filtrate streams. He speculated that it might be possible some day to contlnuously mollitor wnshed stock cleanliness and soda losses by direct measurerrlent of sodium ions using the ion specific electrodes.
However, his own work was limited to the establishment of a system for 20 spot check measurements. Camacho did note that the voltage output between the sodium specific electrode and the reference electrode corre-lated linearly with the log of the soda content of the stream being measured. A few years later, Len~ and Mold, Tappi, 54:2051-2555 (1971) showed that scdium ion selective electrodes could determine sodium in both 25 kraft and neutral sulfite semichemical green and white liquors, dregs, and black liquors. They further noted that their observed data were close to a Mernstian rela$ionship. These observations apparently lay dormant until the matter was ~gain picked up by DeBerry, Tappi, 63 (7):123-L24 (1980), who observed that a sodium ion selective electrode can be used to rne~sure the 3~ total sodium content of a low consistency aqueous pldp suspension.
Other workers have been well aware of the need for good control of the brownstock and other washers in the p~p mill and various computer based analysis and control systems have been proposed.
Virtually all of these systems are concerned only with the 35 estin ation of sodium ion concentration. They tend to ignore the fact that a major part of the water soluble dissolved solids are organic in nature. This P109 ~ 3 information is equally critical if proper washer control is to be achieved. It is th~s organic material that gives the black liquor stream its heat value and the carried over organic portion is also a significant consumer of bleaching chemicals.
None of the systems discussed aboYe have found general acceptance within the industry and, in most cases, their implementation appears to have been of a transient or temporary nature. Despite all of the ad-,rances in instrumentation and control technology that have been made in the last two decades9 the control of pulp mill washer systems depsnds 10 heavily on the intuitive skill of operators who must work with infrequent batch sarnples and with data which are usually well a~ter the fnct.
The present inventors have found a surprising ancl totally unexpected direct relationship between sodium ion concentration in various washer liquid streams and the total dissolved`solids present in the liquid.
15 This ability to predict total dissolved solids makes it possible to develop sirnple and inexpensive control systems for pulp washers regardless of whether they are multistage or single-stage.
SUMMARY OF THE INVENTION
The ~llowing deflnitions apply to tePms used with~n the specifî-20 cation and clnims of this application.
"Tot~l dissolved solids" refers to all water soluMe extractable material, whether organic or inorganic, that is dissolved in the water physically and/or mechanically associated with the pulp fiber.
"Liguor" refers either to spent pulping liquor or washer ~iltrate 25 which contains measurable amounts of dissolYed solids.
'IExtractive-type alkaline pulp treatment" refers to any sodium bAsed alkaline pulping or bleachirlg stage which extracts and solubilizes measurable amounts of organic materials ~rom the pulp fibers. In the context of the present invention~ the pulping process can be any of the well-30 known alkaline processes such as kraft, soda, soda-oxygen, alkaline sulfite, or any of the alkaline ~nthraquinone types. During the bleaching sequence, an extractiv~type alkaline pulp treatment would normally refer îo a caustie extraction stage in which significant amounts of free chlorine or hypo-chloride ion were absent.
"Brownstock washer" is the washing system, whether single-stage or multistage, which immediately follows the pulp digesters.
P109 :~2~
11~77 5 A ~Igeneralized system" refers to any individual unit process used to process a given type of furnish within the pulp mill. The brov~nstock washers in a kraft mill pulping softwoods would be considered one generalized type of system. Brownstock washers in a kraft srill pulping 5 hardwoods would be considered as a second generalized type of system. A
decker following high density storage, or a washer following an alkaline extraction in the bleach plant would be considered third and fourth types of generalized systems. The term implies a common type of chemistry for the process being considered.
A 1'washer system" relates to all of the washer equipment which is used in integrated fashion ~ollowing any process operation. A washer system ean consisl of either a single washer or several washers operated in a countercurrent sequential ~ashion.
"Liquid ~ssociated with the pulp" broadly means the free liquid 15 pbysically associated with the pulp fibers at any point in time.
Broadly stated, the present invention comprises a method OI
determining total dissolved solids in the liquor following ~ sodium based extrnctive-type alkaline pulp treatment stage. The invention further comprises a method of estimatlng the carryover of water soluble dissolved 20 sollds remaining in a cellulosic p~p product leaving the washers which follow such a treatment stage. The invention also comprises a method of controlling the carryover of water soluble dissolved solids remaining in a cellulosic pulp product leaving the washers. The imrention may 3~rther be consideted to comprise a method of controlling washing efficiency of a 25 multistage countercurrent pulp washer system which follows a sodium based extractive-type alkaline pulp treatrnent stage.
A key element in all of the aspects of the present invention is the unpredictable and unexpected discove~y that within any generalized system there is a ~tratght line relationship between sodium ion concentra-30 tion and total dissolved solids measured in any individual liquor streamwithin the generalized system. With this knowledge, it is possible to determine total dissolYed solids in any liquor stream within the generalized system by first experimentally establishing a calibration algorithm relating sodium ion concentration to total dissolved solids in one of the liquor 35 streams within the system~ The algorithm will be a llnear relationship of the ~orm `: ~
.
rrotal Dissolved Solids - a + b (Sodium Concentration) where a is a constant which approximate3 0 and b is a constant between about 0.1 ~nd 0O35. It is assumed that dissolved solids and sodium concentrations are expressed in equivalent units. The eql-ation thus defines a straight line which will pa.ss through or near the origin. When the algorithm has been established for any given generaliged system, total solids can then be determined by simply measuring the sodium ion concentration in the liquor in any stream within the system. This measurement is entered into the algorithm which is then readily solved to give a value for total dissolved solids.
Knowledge of the above relationship, which enables total dissolved solids to be determined continuously, is the key which opens the door to a number of different control means for any given washer system.
As one example, it is now possible to calculate the water soluble dissolved solids carried over with the pulp to the next st.sge in the process.
A consistent relationship has been discovered between sodium lon content of washer filtrate and sodium ion content in the liquid associated with the pulp on the fllter mat leaving the washer. This ls giYen by the general equation Namat = (lop)(m + n log Nafilt)q where Nam!lt represents sodium ion in the liquid associated with the filter mat, Nafilt is the sodium ion content of the washer filtrate, m and n are constants of the generalized system, p is a constant between about 0.15 and 0.45 and q is a constant between about 1.5 and 2~t)D Values for all of the constants can be readily determined experimentally for each ~eneralized system.
In a system using a sodium ion specific electrode for measurement, the sodium concentration may be represented by a voltage analog. The voltage readout signal from filter mat squeezings is linearly related to the log of the voltage readout signal from washer filtrate. This enables the above relationship to be more simply expressed as Emat ~ c ~ d log Efilt .
. . .
~2~4~8~3 11~77 7 where Emat is the voltage readout from a measurement on mat s~ueezings, Efilt is a similar reading on washer filtrate, and c and d are constants of the generalized system. Either voltage reading can be readily converted to sodium concerltration by using the appropriate electrode calibration curve.
It is an important feature of the present invention that by measuring the sodium ion concentrfltion of washer filtrate9 the sodium content of the liquid associated with the ~ilter mat can also be determined.
This can then be directly related to the total dissolved solids in the mat liquid.
If consistency o~ the filter mat is known then the total dissolved solids carried over with the mat are shown. This makes it a simple matter to instrument and control downstream processes in a ~eed ~orward manner.
It is also possible to use the knowledge of carried over dissolved solids as a feedback or feed forward signal for controlling f1OW rate of t5 washQr shower water in order to maintain a predetermined range of water soluble clissolveà solids In the washer mat. Thls type sf control is effective on eIther a single-stage or a multistage countercurrent pulp washer system By controlling the total dissolved solids in the washer filtrate, control of mat carryover is automatically achieved ns is control of dissolved solids in the weak liquor sent to the recovery system evaporators. While in its broadest form, the invention contemplates control oE shower water at any stage of a multistage washer, most typically orly the final stage shower water would be controlled.
The methods of the invention are readily adapted to any of the common types of pulp washer systems now In use. They are based on the instantaneous or continuous measurement of sodium ion concen$ration by any means. Most convenienMy this will be a sodium ion specific electrode.
The output signal between the electrode and its corresponding re~erence electrode is a voltage which relates linearly to the common logarithm of the sodium concentrAtion. This ~an be readily converted by use of the particular algorithm or conversion factor of the generalized systern being measured to total dissolved solids concentration. The voltage analog may be directly used through appropriate inter~aciDg hardware to control shower valves or other control devices within the washer system The output from the electrode may with equal convenience be directed to A mill process computer or a programmable controller so as to give an instantan~ous or continuous reading of total dissolved solids to the appropriate operating ..
lt~77 8 personnel while, at the same ffme~ serving flS a sensor for a process control function.
Washers may be instrumented with elqual convenience at any of several points. Total dissolved solids readings can be made on mat squeezings, on the washer filtrate in the vacuum leg o~ a drum-type washer, or in the filtrate seal tank. It has been found preferable to sample the filtrate in the lracuum leg of the washer where this is possible. The use of mat squeezings poses sampling problems since it is well known that a pulp mat is not necessarily OI uniform composition along a traverse direction 10 taken across a washer drum. Sampling the filtrate tank introduces a time delay so that the re~ding may be indicative of pulp which left the washer as much as 10-20 minutes earlier. By sampling at this point, short term upsets in the system will not be readily noticed, and accommodated by the washer controIs. The time delay is minimized by sampling iltrate in the vacuum lS leg so that for all practicRl purposes it is lnsigni~icant. By utilizing the methods of the present Invention, a new measure of control can be brought to any washer system in a plilp mill which follows a sodium based extractive-type nlkaline treatment.
The method has the great advantage that once the calibration 20 algorithm relating sodium to total dissolved solids has been determined for ageneralized system, this ~lgorithm is valid for any mill in which the ger~eralized process system is practiced. Thus, for all practical purposes, the algorithm for a brownstock w~sher on a kraft mill pulping Northwestern softwoods will be the same for the brownstock washer on a s~uthern pine 25 kraft mill.
Additional advantages of the method are its simplicity, depend-ability and the relatively low cost involved in instrumenting and controlling a given washer system.
It is an object of the invenffon to pr~ide a simple and 30 inexpensive method for determining total dissolved solids in the liquor generated in any sodium based e~tractive-type alkaline pulp treatment stage.
It is a further object to provide a method ~or estimating the carryover o~ water soluble dissolved solids in the pulp mat leaving a washer 35 system.
~"
.
., . ~
~23~
P1~9 It is another object to provide a method for controlling a washer system so that the carryover o water soluhle dissolved solids is rnaintained within a predetermined range.
It is yet another object of the invention to provide a method of 5 controlling cost-effectiveness of washing in a multistage countercurrent pulp washer system.
It is still another object to provide a measurement and control system for pulp washers whieh is simple, dependable, and relatively inexpen-sive.
It is yet a further object to provide a rnethod for eontrolling pldp washing where, within the confines of a given generalized system, the ratio of sodium to dis.solved solids is constant in any liquid stre~m within the system.
These and many other objects will become readily app~rent to 15 those skilled in the art upon reading the following detailed description taken in conjunction with the attached drawings.
BRIEF DESCRIPTION OF THE DF~AWINGS
Pi~ure 1 is a diagram showing a generalized countercurrent multistage washer system following a p~p digester using the methods of the 20 present invention in a feedback control system.
Figure 2 is a typical calibration curve eor an indi~idual sodium lon specific electrode showing the linear relationship between the readout signal in millivolts and the common 1~ of the sodium concentration as determined by atomic absorption spectroscopy.
Figure 3 is a ~urve ~owing the correlation between sodium and total dissolved solids measured in samples from mat squeezings, washer rat liquor, and washer filtrate t~ken from brownstock washers in six p~p mills from widely separated geographic locations.
Figure 4 is a graph showing the straight line relationship between 30 the electrode signal nneasured on mat s~ueezings and the common logarithm of the electrode sign~l of washer filtrate collected at the vacuum leg, in a mill processing softwood grades of p~p.
Figure 5 is a graph which shows the straight line relationship between sodium and dissolved solids in brownstock liquor samples taken 35 from the washer vat, from mat squeezings, and From the vacuum leg on the washers in a mill pulping softwood grades.
I
8~3 11~77 10 Figure 6 is a graph similar to Figure 5 but taken from similar locations in a mill pulping hsrdwoods.
Figure 7 is a graph showing the straight line relationship between the solids concentration in the filtrate of the firslt washer in a three-washer 5 sequence and the common log of the sodium ion concentrate in the third washer filtrate, in a mill pulping softwoodsO
Figure 8 is a similar curve to that shown in Figure 7 but determined in a mill pulping hardwoods.
Figure 9 is a curve showing the straight line relationship between 10 sodium concentration and dissolved solids in mat squeezirtgs from a single stage washer following a caustic extraction stage in a bleach sequence.
Figure 10 is a diagram of a washer system similar to that shown in Figure 1, but usi~ a feed forward control system.
I)ESCRIPTION OF THE PREFERRED EMBODIMENTS
We have discovered a principal reason why conductivity measurements have not proved to be particularly satis~actory for brown-stock washer control. The logic was based on the faulty premise OI constant relative concentrations of each sodium compound present in the liquor UnPortunntely, this ratlo varies widely. As one example, the conductivity of 20 sodiurn hydroxicle in a 1% by weight solution is about 48 mmhos/cm while a similar solution oi~ sodium carbonate or sulfate will have a conductivity of about 11 mmhos/cm. At 4% concentration, the sodium hydroxide solution will huve risen to a conductlvity of about 170 mmhos/cm while the equivalent sodium carbonate or sul~a~e will be at about 37 mmhos/cm.
25 Rxperiments using conductivity measurements at different points in time on a given black liquor stream have shown errors falling in the range of abou 5.7-10.2% ~om actual sodium values. Similar measurements using a sodium ion speciPic elec~rode varied between 0-0.3% from true values.
The disco~rery that there is a direct linear relationship between 30 sodium ion concentration and the total dissolved solids in the pulp liquor stream of a sodium based extractive-type alkaline pulp treatment stage opens a new vista for pulp mill process contro~. Sodium ion concentration is most conveniently measured with a sodium ion specific electrode used in con~unction with a suitable reference electrode. These may be used with 35 any conventional pEI meter or similar device which will give an output reading in millivolts. The sodium electrode is manufactured with a glass tip . . , ,j .
(?8~3 11477 ~1 that is sensitive to sodium ions. Suitable electrodes are the Model No. 97-12, manufaetured by Orion Research, Inc., Cambridge, Massachusetts, or the Corning Model X-EL available from the Corning Glass Works, Medfield, Massachusetts. These both contain the sodium ~sensing 5 electrode and the re~erence electrode in one body. Other ions, such as silver, potassium, and hydrogen ions, can potentially cause interference. In a typical pulp mill process stream, neithe~ silver nor potassium are usually present in interfering quantities. Hydrogen ion interIerence normally is not a problem unless the concentration of sodium ions being measured is 10 extremely low. Electrodes are also somewhat temperature sensitive and can introduce ~n error of about û.6% in the sodium ion reading for each degree C of temperature change. Again, this is nor~ally not a problem during measurements of a pulp washer stream since temperatures tend to be qulte stable over very long periods of time. Where temperature variation is 15 encountered, it may be desirable to run the stream being measured through smnll heat exchanger immersed In a constant temperature bath beEore taking readlngs.
Figure 1 is a simplified diagrammatic presentation of a typical brownstock washsr systemO This would be essentially the same whether the 20 pulp was to be bleached or sheeted as an unbleached product. The system shown uses a series of three sequential vacuum~type drum washers followed by a high-density storage chest. This in turn is followed by a plilp decker which may be identical to one of the preceding washers. The system works essentially as follows. Pulp Irom the screens or knotters is pumped in line a 25 to the first washer 4. It is diluted with recycled washer filtrate arri~ing in line 54 to about 1% conslstency before it ~lows into the washer vat. The pulp rnat will leave the washer drum at about 12~16% consistency. It is then reslurried and diluted again to about 1% consistency where it is pumped to the second stage washer 6. The sequence is again repeated and the pulp 30 slurry is brought to the final stage 8 o~ the brownstock washer system.
From there~ it will usually be directed to a high-density storage chest 10, although this is not essential. Once again the pulp is diluted as it is taken ~om high-density storage and ultimately receives a final washing sequence 12 at the pulp decker. The washed pulp is again diluted and led 35 through line 14 to the next appropriate process station.
P1~9 11~L77 12 In the example being shown, fresh wash water arrives through line 20 to the showers of des~ker 12 where it displaces liquid entrained in the pulp fibers. The interior of the washer drum is maintained under negative pressure by a water or vacuum leg 24. The proximal end of this leg communicates with the interior of the washer drum, while the distal end is maintained under the surface of washer effluent held in a seal tank 26. A
similar arrangement is present on all of the other washers. Higher levels of vacuum may be provided by the use oE external vacuum pumps in communication with the seal tanks. ~ince the vacuum on the washer drum is lD dependent on the level of water in the seal tank, in the absence of an auxiliary vacuum pump, each tank is equipped with a level control. In the case of the seal tank, level control 60 operates a valve 62 to cor.trol the flow of makeup water through line 22. Filtrate from the seal tank 26 is pumped through line 28 as dilution water for stock being withdrawn from 15 high-density storage chest 10. It is also directed through line 30 to the showers of the third washer 8 in the brownstock washer system. A portion ot the w~ter delivered to the showers through line 30 i9 drawn off through line 32 as dilution water ~or the ilter mat ~rom third-stage washer 8 in order to dilute it sufficiently so that it can be pumped to the highdensity 20 storage chest. Third-stage washer 8 is equipped with vacuum leg 34 and seal tank 36. Filtrate is withdrawn Erom the seal tank through lin~ 38 where it acts as shower water on secon~stage washer 6. A portion of this water is drawn off to dilute the filter mat from the second-stage washer 6 to a suitable consistency for handling on the thir~stage w~sher. Filtrate from 25 the second~stage washer is delivered through vacuum leg 42 to se~l tank 44.
From here it is pumped through line 46 to the first-stage washer showers and through line 48 as dilution water for the filter mat from first-stage washer 4. E~iltrate from Iirst-stage washer 4 flows through vacuum leg 50 into seal tank 52. The efnuent from seal tank 52 is pumped to three points.
30 A portion of it flows through line 54 as dilution water for unwashed stock arriving through line 2 to the washer system. Another portion is directed through line 58 to the blow tan3c for stock dilution. A third portion of this seal tank water ~lows through line 56 to the evaporatnrs in the chemical recovery system.
Third washer seal tank 36 has a level control 64 which serves to control the flow of shower water flowing through valve 66 to the showers on .
.
P109 ~ 3 secon~stage washer 6. Sect)nd-stage seal tank 44 has a similar tevel control 68 which controls shower water on the first stage by means of control valve 70. The first stage seal tank 52 is equipped with a level control 72. This regulates the weak liquor ultirnately leaving the system via control valve 74 and line 56 leading to the evaporators.
The system exemplified here contains two sodium ion detectors.
A first sodium ion measurement means 80 is attached to drop leg 34 of the third-stage washer. This serves to control third-stage brownstock washer shower water arriving through line 30 by acting on control valYe 82. A
lD second sodium ion detector means 76 is similarly located on the drop leg of tbe pulp decker 12. This controls fresh water corning to the showers of the decker by acting on control valve 78.
The three brownstock washers, when washing pulp produced from a given type of furnish, may be considered as one generalized system. The 15 pulp decker may Ibe considered as a second generalized system when used with the same type of furnish. If no high density storage chest was used between the brownstock wnshers and the p~p decker, the decker could be considered as a part of the same generalized system as the pulp washers and would simply be considered as ~ fourth stage. However9 the interposition of 20 high density storage gives a tlme peeiod in which soluble organic and inorganic solids ~re able to diffuse from the fibers into the liquid phase associated with the pulp. This may somev~hat change the slope of the curve relating sodium to total solids and the curve for the brownstock washer ~eneralized system will not be exactly coincident with the curve for the 25 decker generalized system.
When sodium ion selective electrodes are used for measurement, each electrode will normally be individually culibrated to establish a curve relating readout voltage to sodium ion content oF the solution being measured. Such a curve is shown in ~igure ~. While different electrodes 30 will have very similar response curves, accuracy of a system can be improved somewhat by making individual calibration deterrninations. The curve shown in Figure 2 was plotted using mat squeezings ~rom the first washer in a brownstock washer system. Sodium content was determined by atomic absorption spectroscopy.
One of the most remarkable discoveries is that the relationship of sodium ion concentration to total dissolved solids in any given gener~ ed P109 ~ 3 system appears to be universal. Figure 3 shows a eurve plotting sodium concentration in samples obtained from brownstock washer mat squeezings, washer vat liquor, and washer filtrate against total dissolved solids in the liquid. This curve is ôased on ~pproximatelLy 11~ measurements taken in six 5 difPerent kraft pulp mills. Three of these mills were in the Pacific Northwest coastal region of the United States, one was in interior British 1: olumbia, and two were in North Carolina. The correlation coefficient for this plot is 0.95. This correlation is remarkable in view of the different tree species used in the furnish and in view of both inter-and intra-mill variations 10 in cooking liquor sulfidity and active alkali as well as other digestion conditions that affect pulp yield. Other mill variables which affect liquor makeup are reduction efficiency in the recovery furnace and causticizing efficiency. These variables all affect sodium concentration and black liquor solids concentration in varying degrees. Rosenberger in U.S. 4,096,028 15 notes the great variability in composition ancl physical characteristics of input streams to brownstock washers.
It is well known that the composition oE a filter mat has signiîicant v~riability across the drum and at any given location nt different tlmes. Despite this knowledge, squeezings from the filter m~t from the 20 washer drums are still the principal s~urce of liquor taken for washer systemanalysis and control. Because of the variability, and bec~use it is difficult to take mat squeezings on a continuous basis, other sources of sampling are more desirable. The filtrate in the vacuunn leg of a washer represents an integrRted sample of liquor that is passed through the filter mat. It also 25 represents a vely short time interval between the passage o~ a given unit of pulp and the time of measurement. The use of tracer compounds added ~t the washers hAs shown that the interval between passage of a unit of pulp and time of sarnpling in a vacuum leg is less than two minutes. However9 what is essential if the vacuum leg is sampled is that the ratio of sodium to 30 total dissolved solids measured in the filtr~te is the same as, or directly relatable to, the ratio of sodium to dissolved solids present in the liquid associated with the filter mat. Figure 4 shows a c~ve taken from the third stage of a three-stage brownsto~k washer during a tirne when softwood gradss were being run in a kraft mill. There is a linear semilogarithmic 35 relationship betuveen the readout signals obtained on mat squeezings and the filtrate salTIpled from the racuum leg of the washer. Since the relationship ., /
of these readout signals to sodium ion concentration and thus the total solids is known, it can be said with confidence that sodium ion readings taken on filtrate sampled from the vacuum leg are indicative OI total solids in the liquor at that point. The same readings can also be used to calculate the 5 total solids in the liguor associated with the pulp mat as it leaves the washer. In the past, it has been a largely unfulIilled desire to have pulp with a known controlled amount of dissolved solids entering the bleach plant or paper mill. This problem can now be brought under control using the methods of the present invention. Total dissolved solids information can 10 therefore be used for feed ~orward control of chemical usage in the bleach plant.
While the curve in Figure 4 relates voltage readings taken on washer filtrate and mat squeezings, the relationship can also be readily expressed as an equation that relates sodium ion concentrations, or total 15 dissolved solids, in these two liquid streams. In the generalized system from which Figure 4 was determined this equation was lo0.29 (_114.96 ~ 46.49 log Nafiltrate) 20 The above equation can be readily determined for each generalized system being measured and can take into account the individual calibration curve for the sodium ion specific electrode. Once determined, the above relationship will hold constant over time for the generalized system.
Small differences are noted in the slope of the line relating any 25 brownstock washer lîquor sodium concentration and dissolved solids~
depending on whether softwoods or hardwoods are being pulped. These differences are basic to the chemistry of the p~ping process and to the nature of the materials being solubiliæed in the wood chips. Figures 5 and 6 show curves relating sodium content to dissolved solids for any stream in the 30 brownstock washers in kraft mills processing softwood and hardwood grades, respectively. While the difference of the slope oi these curves is not great, it is sufficient so that it should be accounted for if highest accuracy is desired. Differences of this type were not taken into account when the data were gathered from which Figure 3 was plotted, accounting for the slightly 35 different slope of that curve as compared to the pr~sent ones.
11~77 16 Other factors besides furnish that affect the basic chemistry of the pulping process will bring about changes that require definition of a new generalized system. Among these might be mentioned the use of anthra-quinone as a pulping catalyst.
Now that a relationship between mat squeezings and filtrate dissolved solids has been established for the last washer of a multistage system, it could be hypothesized that there wvuld also be a relationship between third-stage filtrate and first-stage filtrate total dissolved solids~ aslong as the time lag between them is also considered. Further experimental work showed that this was indeed the case. The relationship between third-stage filtrate and first-stage Eiltr~te for softwood and hardwood grudes is shown in Figuree 7 and 8, respectively. The log of the sodium ion concentration measured in ppm in No. 3 washer filtrate relates linearly to the solids concentration in the filtrate of the No. 1 washer. Controllin~ the total solids concentration in the thir~stage îiltrate also implies controlling the solids content in the weak liquor stream sent to the evapor~tors.
The relationship between sodium ion concentration and dissolved solids also appears to hold for extractive-type alkflline pulp treatments other than digestion. Figure 9 shows the relationship between sodium ion concentration and dissolved solids in the washer filtrate from the alkaline extraction stage in a bleach plant. This particular bleach sequence employs a first-stage chlorination, secon~stage alkaline extraetion, and thir~stage hypochlorite treatment. The curve shown was based on 75 individual samples, all of which fell closely along the regression line shown in the Figure.
In any washer system, whether it is single-stage or multistage, the water entering the system must be equal to the water leaving the system. The normal method o controlling water, and also of controlling pulp cleunliness, is by control of the flow rate of shower ~Nater. In a multi-stage washerl this will be the shower water flowing onto the final stage.
Control of shower water can be easily achieved by total solids measurement in either the filtrate or the liquid associated with the pulp mat. These measurements can act as acc!Jrate feedback or feed forward information.
One alternati~re to shower water control by itself is seen in Figure 10. This is a revision of the system seen in Figure 1 in that the sodium ion flnd thus total dissolved content are measured by sensor 80 in the . . .
P109 ~Z~ 3 liquid portion of the slurry flowing into the first washer vat. This signal is sent to control valve 7~ on the first washer shower system. Level control 68 now controls shower valve 6B on the secon~stage washer and level control 64 on the third washer filtrate tank controls valve 82 on the third washer showers. The decker is instrumented as shown in ]Figure 1 but, in addition, it feeds forward the total dissolved solids content of the washed pulp to the bleach plant or paper mill. A feed forward system of this type is advantageous in that it offsets the effect of the time lag across the system when process up~ets occur. These upse ts ean now be anticipated at 10 downstream stations.
For the first time, on a practical basis, the present invention gives the information needed for the control of defoamer addition to the pulp system. The evaluation of defoamer effectiveness and the optimum point of application has been highly subjective to date. Now the efficiency 15 of a defoamer can be accurately determîned by measurement of total solids in the final stage filtrate. Use of the present metnods also enables a stream having uniform total solids concentration to be sent to the recovery area, a portion of the mill notoriously subject to upsets from concentration fluctuations.
Having thus described the best kaown modes of the invention known, it will be immediately apparent to those skilled in the art that many variations are possible without departing from the spirit of the invention.
As one example, the methods of tbe invention are equally applicable to the liquor streams from any lignocellulosic material and should not be 25 ~onsidered as limited to wood. The invention should be considered ~s being limited only by the following claims.
Claims (24)
1. A method of determining total dissolved solids in the liquor following a sodium based extractive-type alkaline pulp treatment stage which comprises: establishing a calibration equation relating sodium ion concentration to total dissolved solids in the liquor of a generalized system of the type being measured, measuring sodium ion concentration in the liquor being tested, and determining the total dissolved solids in the test liquor by entering the measured sodium ion concentration into the calibra-tion equation and solving the equation, wherein the equation expresses a linear relationship of the form Total Dissolved Solids = a + b (Sodium Concentration) where a is a constant which approximates 0 and b is a constant between about 0.1 and 0.35.
2. The method of claim 1 in which the sodium ion concentra-tion is measured using a sodium ion specific electrode.
3. The method of claim 1 in which the liquor is the black liquor of an alkaline pulping process.
4. The method of claim 1 in which the liquor is washer filtrate from the brownstock washer of an alkaline pulping process.
5. The method of claim 1 in which the liquor is washer filtrate from an alkaline extraction stage in a bleach sequence.
6. The method of claim 3 in which the alkaline process is selected from kraft, soda, soda-oxygen, alkaline sulfite, and alkaline anthra-quinone processes.
7. The method of claim 4 in which the alkaline process is selected from kraft, soda, soda-oxggen, alkaline sulfite, and alkaline anthra-quinone processes.
8. A method of determining the carryover of water soluble dissolved solids remaining in a wood pulp product leaving the washers following a sodium based extractive-type alkaline pulp treatment stage which comprises:
a. measuring the sodium ion concentration in the washer filtrate;
b. relating the sodium ion concentration in the filtrate to the sodium ion concentration In the liquid associated with the filler mat using the following equation Namat = (10P)(m + n log Nafilt)q where Namat represents sodium ion concentration in the liquid associated with the filter mat, Nafilt is the sodium ion concentration of the washer filtrate, m and n are constants of the generalized system p is a constant between about 0.15 and 0.45 and q is a constant between about 1.5 and 2.0;
c. determining the total solids in the liquid associated with the mat using the equation Total Dissolved Solids = a + b (Sodium Concentration) where a is a constant which approximates 0 and b is a constant between about 0.1 and 0.35.
d. measuring the consistency of the pulp as it leaves the washer; and e. further determining the water soluble dissolved solids carried over with each unit of pulp.
a. measuring the sodium ion concentration in the washer filtrate;
b. relating the sodium ion concentration in the filtrate to the sodium ion concentration In the liquid associated with the filler mat using the following equation Namat = (10P)(m + n log Nafilt)q where Namat represents sodium ion concentration in the liquid associated with the filter mat, Nafilt is the sodium ion concentration of the washer filtrate, m and n are constants of the generalized system p is a constant between about 0.15 and 0.45 and q is a constant between about 1.5 and 2.0;
c. determining the total solids in the liquid associated with the mat using the equation Total Dissolved Solids = a + b (Sodium Concentration) where a is a constant which approximates 0 and b is a constant between about 0.1 and 0.35.
d. measuring the consistency of the pulp as it leaves the washer; and e. further determining the water soluble dissolved solids carried over with each unit of pulp.
9. The method of claim 8 in which the sodium ion concentra-tion is measured using a sodium ion specific electrode.
10. The method of claim 8 in which the alkaline process is a pulping process selected from kraft, soda, soda-oxygen, alkaline sulfite and alkaline anthraquinone processes.
11. The method of claim 8 in which the alkaline process is an alkaline extraction in a bleach sequence.
12. The method of claim 8 which further comprises feeding information on dissolved solids carryover forward for downstream process control.
13. A method of controlling a pulp washer system following a sodium based extractive-type alkaline pulp treatment stage which comprises:
a. continuously sampling the filtrate from the washer, b. measuring sodium ion concentration in the filtrate, and c. feeding back the sodium ion concentration as a signal to means controlling the shower water on the washer system in order to maintain a predetermined level of dissolved solids in the filtrate and in the liquid associated with the pulp mat leaving the washers.
a. continuously sampling the filtrate from the washer, b. measuring sodium ion concentration in the filtrate, and c. feeding back the sodium ion concentration as a signal to means controlling the shower water on the washer system in order to maintain a predetermined level of dissolved solids in the filtrate and in the liquid associated with the pulp mat leaving the washers.
14. The method of claim 13 wherein the washer system comprises sequential vacuum drum washers in which the vacuum is provided by a water leg whose proximal end is in cooperation with the interior of the drum and whose distal end is sealed below the surface of washer filtrate held in a filtrate seal tank.
15. The method of claim 14 which further comprises measuring the sodium ion concentration of the washer filtrate standing in the vacuum leg as the source of the feedback signal.
16. The method of claim 14 which further comprises measuring the source ion concentration of the filtrate in the filtrate seal tank as the source of the feedback signal.
17. The method of claim 14 which further comprises sending the feedback signal to means controlling shower water on the final washer stage to maintain a predetermined range of total dissolved solids in the pulp mat leaving the washer and in the washer filtrate.
18. The method of claim 13 in which the sodium ion concentra-tion is measured using a sodium ion specific electrode.
19. The method of claim 13 in which the alkaline treatment stage is a pulping process selected from kraft, soda, soda-oxygen, alkaline sulfite, and alkaline anthraquinone processes.
20. The method of claim 13 in which the alkaline treatment stage is an alkaline extraction in a bleach sequence.
21. A method of controlling a pulp washer system following a sodium based extractive-type alkaline pulp treatment stage which comprises:
a. continuously sampling the liquid component of the pulp slurry entering the washer system, b. measuring the sodium ion concentration in the liquid component, and c. feeding forward the sodium ion concentration as a signal to washer control means in order to maintain a predetermined level of dissolved solids in the pulp mat leaving the washer.
a. continuously sampling the liquid component of the pulp slurry entering the washer system, b. measuring the sodium ion concentration in the liquid component, and c. feeding forward the sodium ion concentration as a signal to washer control means in order to maintain a predetermined level of dissolved solids in the pulp mat leaving the washer.
22. The method of claim 21 wherein the washer system comprises sequential vacuum drum washers in which the vacuum is provided by a water leg whose proximal end is in cooperation with the interior of the drum and whose distal end is sealed below the surface washer filtrate held in a filtrate seal tank.
23. The method of claim 22 wherein the washer control means comprises liquid level controls acting on the level in the filtrate seal tanks.
24. The method of claim 22 wherein the washer control mean is a valve controlling shower flow rate on the final stage of the washer.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US57377084A | 1984-01-24 | 1984-01-24 | |
| US06/573,770 | 1984-01-24 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1210813A true CA1210813A (en) | 1986-09-02 |
Family
ID=24293329
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000471697A Expired CA1210813A (en) | 1984-01-24 | 1985-01-08 | Process control method |
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| Country | Link |
|---|---|
| CA (1) | CA1210813A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN103868960A (en) * | 2014-03-10 | 2014-06-18 | 佛山市南海Tcl家用电器有限公司 | Water purification machine, measuring method and measuring device of TDS (Total Dissolved Solids) value of water purification machine |
-
1985
- 1985-01-08 CA CA000471697A patent/CA1210813A/en not_active Expired
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN103868960A (en) * | 2014-03-10 | 2014-06-18 | 佛山市南海Tcl家用电器有限公司 | Water purification machine, measuring method and measuring device of TDS (Total Dissolved Solids) value of water purification machine |
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