CA1129697A - Process for controlling pulp washing systems - Google Patents

Abstract

PROCESS FOR CONTROLLING PULP WASHING SYSTEMS
ABSTRACT OF THE DISCLOSURE

A method of controlling the amount of a liquid shower flow introduced onto a slurry mat such as a pulp mat which is undergoing incomplete liquid separation on a vacuum filter drum.
The flow rate of the liquid being discharged with the slurry mat is determined by a capacitance measurement which is taken after the slurry mat has passed to a point on the filter means where liquid separation no longer occurs. The shower flow is controlled by a correlation of the flow rate of the liquid in the slurry mat with the rate of slurry mat transfer from the vacuum filter drum and the necessary liquid shower flow as expressed by a dilution factor.
The present control system may be combined with secondary apparatus to measure the flow rate and thickness of the total slurry mat in order to determine by correlation the slurry mat consistency, the rate of solid material produc-tion from the slurry mat and the amount of air contained in the slurry mat.

Description

~2~G~7 BACKGI~OUND OF ~`IIE: INVE:NTION

1. ~iel(l of the Inventioll The present invention relates to a method of controlling the amount oE water which is added into a `
pulp washing system by monitoring tlle amount of water in a washed pulp mat leaving the uulp washing system.

2. escription oE thc l'rior ~rt Pollution control depends upon the washing opera-tion control whereby if there is insufficient wash 'C water added to ~he system the washing stcp is inefficient and increases the pollution.

Poor control can also be in the other direction whereby an excessive a~:ount of wash water is applied and this excess water must be evaporated in the evauorator operation which then causes an excessive energy consumption.

Washing control systems presen-tly in use do not use the water efficiently in that for short periods of time there is an excess oE water used followed by a period whereby an insuEficient amount is used. rrllis leads to in-~o suEEiciencies of both kinds described above within the one continuous washing system.

Pulp washing systems are dcsicJned to minimize - the amount of frcsh water needed to wash tlle black licluor from the pulp produced from a pulp digester. Countercurrent ,., '~
;' pulp washing techniclues are almost exclusively used in order to increase efficiency in the pulp washin{l system. In con-ventional countercurrent washing operations, fresh water which is added to the system is generally referred to as shower water because it is sprayed as a shower upon a pulp mat which has been formed ln the last of a number of washillc~ operations.

I'wo important factors which must be under-stoo~-with relation to the present washing control system are the dilution factor and the displacement fact~r. These factors influence the determination of the Elow rate of shower water as calculated by the present control system and must necessarily be apprecia~ed to enable the control system to per~it a satisfactory pulp washing operation.

The dilution factor r,epresents a ratio of lS the amount oE fresh water sprayed onto a pulp mat undergoing a washing operation to the final volume of water contained in the pulp mat as it leaves the washing operation. The amoun~ of ~resh water entering the washing system may be e~pressed ln appropriate flow rate units such as liters per '~ minute. The volume of water leaving the washing system in the final washed pulp mat product is exuressed in the same units. In countercurrent washing operations recycled water is sprayed on the pulp mat in all but the final washing operation. The dilution Eactor for each washincJ operation step prior to the final fresh water treatment is expressed ~ 7 - -.. .. . _.

as the ra,tio ~ the ~ecycled w~te~ sp~ayed onto the pulp mat to the volume of water cont~ned in the pulp mat which has been treated in the individual washing step, The displacement ratio is equal to the fractional of the liquor entering the filter drum in the pulp mat which is displaced ~y water from the spray washer, The ideal dis-L~lacement ratio would be 1.0 where ideal plug flow existed;
; howev~er, the ideal situation is not obtained. The displacement factor is primarily a function of the dilution facto~ but 1~ is influenced ~y such factors as air entrainment in the pulp, web, pulp web sheet uniformity, and the temperature of the system. The displacement factor.can be determined by an analysis of the amount oE dissolved solids which remain in the pulp after washing compared to the dissolved solids..which !) would be in the pulp at the same collsistellcy without any shower Elow. In eEfect, this displacement ratio may be displaYed by a comparison between the dissolved solids con-centration in the water in the pulp mat exiting the washing system a~ter the washing treatmen-t, and the dissolved solids ~U concentration of the pulp before the ~inal shower water treatmen-t.

While the use of countercurrent washing systems reduces t}le amount of fresh water which is needed in a pulp washing system, previous attempts to minimize the to-tal amount ?~ of fresll sllower water i.ntro~uced into tlle wash system llave proved to be ine~ficient. Previous systems have not provided . 3 -, for ~ continuous moni.toriz~tion ~nd immedi~te shower flow response to produce a pulp washing system which is continuously efficient in minimizing the shower flow necessary to produce A satisfactorily washed pulp product, The present inv.ention OVercoMes the deficiencies of the control methods used in the past, Primarily~ two con-trol methods have been used to control pulp washin~ systems in the past~ In the first method a pulp flow rate on the entire set o~ washers is l.() estima-ted to be constant and the pulp flow rate is calculated for one washer by correlating a 10w measurement and a consistency, The consistency o~ the pulp~le~villg the washer clrums is not taken into account except in the design oE
~he s~stem. ~'he shower water ~low on the lask washer is then set by the operator based on hourly tests oE the solids contenk of the liquor in the early stages of the washing operation, The system~can be out of balance in both of the ways previously described several times during the hour without detection by the operator. l`he average liquor solids content can be on target ye-t the system can be inefficient in producing both hi~h losses to the sewer and excessive water to be evaporated. This can be explained by showing that an overwash for part of the time cannot make up for an insufficient wash the other part of the time, ~5 In the SeCOlld prior art control me~hod as described in U. S. Patent 4,046,621 to Sexton, the conductivity of the liquid displaced froM the pulp mat in the last washing step is measured and this measuremen-t is used to adjus-t the ,; _ 9 _ ., . ~f~

amount oE fresh washing liquid in the last washing stage.

This system is an improvement over the operator control alone but has several disadvantages. The first disadvantage is that conductivity is not precisely r) related to t~le liquor solids content as it is ~reatly in-fluenced by the composition of the solids. Secondly, the large volumes of liquor circulated in the wash system have a large buffering action on the rate of change of liquor conductivity with a change in washing efficiency. In a ]0 typical pul~ washil~g system operation at 500 metric tons of pulp per day the liquor volume maintained in eac~ stage filtrate tank will be in the order oE 200,000 liters which is recirculated in the wash system at a rate of about 30,000 liters per minut~. The shower flow.for 1.15 dilution 1~ factor would be 2928 liters per minu~e at 12 percent discharge consist~ncy. OE this 2928 liters per minute approximately 382 liters per minute would penetrate th~ mat with a ~erfect displacement system. In a normal balanced system this 382 liters per minute would be mixing continuously ,~ with the 200,000 liters in the filtrate tank.

If the shower flow was accidently cut complete;~y off the conductivity system of control would detect a rate of change of only ~100 x 382/200,000) = 0.19 I>ercent per minute. This small cllange in conduc~ivity would ;'~. not initiate a change in the shower se-t point until signifi-cant inefficiencies in the system had occurred.

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SUMMi'~RY OF TIIE: INVE:NTI~N

This invention oVercomes the problem o~ the prior art by providing a continuous monitoriza-tion sys~em which is used to immediately control the sllower flow of fresh water or liquid additives whicll is introduced into a pulp washing system. l`hroucJh the use oF a monitor~zation system featuring immediate response in the shower flow control greater efficiency is produced in regulatin~ the shower flow necessary to produce an ~cceptable washed pulp product. This improvement over the prior wash control processes alleviates the problems produced by wash systems using too much sllower water flow, tllereby causincJ excess water to be sent to the evaporators, and by wash systems using too little shower water flow, ~hereby producing pulp Ir~ which has not been sufficielltly washed cr~ating hicJh pollu-tion loads and econornic.loss of chemicals from the process, he foregoing ad~antages are obtained by the present invention by a process which determines the amount of water content present in a pulp web using a cap~citance ?O measurement technique after the web passes over a vacuum break on a rotary drum vacuum filter which is the final step before removal of the pulp in the form of a mat or a web from the waslling systeln. Once the water content of the ~` pulp mat is dctermined, i.t may be correlated to control the fresh water shower Elow rate, thus minimizing the amount of fresh water needed to satisfactorily clean paper pulp ~ r ~,. - . . .................... .
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in a countercurrent pulp washing operation. This correlation consists of combining the water content per unit area of the pulp mat, as determined by a capacitance measurement, with the drum filter area, rotational speed of the drum and a dilution factor.

~lternative or secondary measurement apparatus may be employed in the washing system either alone or in conj~nction with the capacitance measurement apparatus in order to determine other parameters of the washing operation such as consistency of the uulp mat and the production rate of the l)ulp mat from the washing systelll. 'rhese measurement n devices include apparatus to produce and monitor backscattered nuclear radiation and perturbation of microwave cavities. T}le nuclear radia-tion measurement apparatus measures the total mass of pulp and water per unit area oE the pulp mat.

Microwave cavity perturbation apparatus can be used to determine the liquid content of the pulp mat in place of the capacitance measurement apparatus when the conducti~ity of the liquid in the pulp mat is a si~JniEicant factor such as when the conductivity is affected by changes in chemical concentratiolls in the liquid in the slurry mat. Through the ?n use of microwave cavity perturbation measurement apparatus, changes ~in the dielectric losses of the liquid in the pulp mat may be separately detected from the remainder of the dielectric propcrties such as the diclec~ric cons~ant. The dielectric losses are responsive to the conductivity of the ~, . , ,__ ...... ..

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liquid in the pulp mat and are therefore responsive to the efficiency of the shower flow as a wash. The dilu-tion factor and thus the shower flow will be directly responsive to the measured dielectric losses.

The preferred embodiment for accuracy would combine capacitance measurement for the water and nuclear radiation used for the total mass but in some applications the microwave perturbation techniques may be suitable or even superior.

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By Usillc3 the seeondary measurement apparatus to determine the total mass per unit area of the pulp mat leavincJ the washing system, and by usinc3 the capacitance measurement apparatus to determine the water mass per unit area, the pulp mass per unit area leaving the waslling system may be determined by substraeting the water mass per unit area ~rom the total mass per Ullit area. Tllis determination perm~ts the calculation of the consistency of the pulp mat exiting the washing system as the consistency is the per-~) centage of solids of the total liquid-solid eontent of the pulp mat. A measure of the consistency o the pulp mat allows the user to evaluate clifferent conditions oE drum speed, ma-t dilution, press roll pressure, anti-foam acJents or drainage aids sueh that the maximum eonsisteney is obtained. The maximum consisteney at any given tol?nacJe rate is known to produce the best wash with the least water as shown by the dilution factor and displacernent faetor. Obviously the highest consistency a~ a given tonnage rate contaills the least amount of water in the sheet and at any given dilution faetor () the highest consistency also uses the least amount of water to be evaporated. The highest consistency at these conditions will also allow the best wash possible. It is important therefore to be able to quiekly and easily determine the consisteney of the pulp mat on the filter. Testing for :'5 this consistellcy by present hallcl samplin~ methods is very teclious ancl inaccurate since such a small samL)le must be taken. 'l`he eonsistellcy of the pulu ma~ is continuously ,. monitorecl by continuously evaluating the pulp mat characteristics as the pulp mat rotates on the filter drum. Capacitance ,;' ;~
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measurements of the pulp mat on the filter drum are preferrably recorded across the entire surface of the Filter drum.

B~IF,F D~SC3~IP'~'ION OF ~'IIE DR~WINGS

The foregoing objects, features, and advantages r~ of the invention will be more fully understood upon a considera-tion of the following detailed description of preferred forms of the invention, together with the accompanying draw.ngs, in which:

Fig. 1 is a flow schematic of a countercurrent 1'~ pulp washing operation;

Fiy. 2 is an ~nd view of a rotary drum vacuum filt~r us~d in conjunction Wit]l th~ pulp washing operation 03 ~ig. 1;

Fig. 3 is a side view of a rotary drum vacuum 1~ filter used in conjunction with the pulp washing operation ~ of Fig. l; and `~
~ Fig. 4 is a side view of a rotary drum - vacuum filter used in conjunction with the pulp washing , operation of Fig. l;

~U Fig. 5 is the end view of a rotary drum vacuum 3ilter used in conjunction witll the pulp washiny operation of Fig. 1.
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DESC~IPTION OF T11E PR~I'ER1~D EMsoDIl~lENTs T11e present invention is applicable to pulp washing operations in general, howeverl the main embodiment is applicable to the standard countercurrent pulp washing system as demonstrated in Fig. l. Referring to Fig. l, the r~ overall countercurrent pulp wash system as displayed in Fig. l consists of three rotary drum vacuum filters, 4, ~' and 4'', threg filtra-te tanks 8, 8', and 8'', two repulpers l0 and l0' and flow lines connecting these individual components in conventional manner.

~!) 1'he pulp and liquor elltering this system via a transfer line 20 comes from the digester operation in a wood pulpinc3 operation. The amount oE water added to the system up to this pOi11t is kept'to an absolute minimum consistent with good operating procedul-e in order to main-tain the least amount of water that must be evaporated in the subsequent evaporation and cllemical recovery system.

The fresh wash water enters the washing system in one location only, through a wash sprayer l. This fresh water s~ray through the wash sprayer l is hereafter 2~ referred to as shower water. The shower water is projected by the wash sprayer l onto a pulp mat 2 formed on a tertiary rotary drum vacuum filter 4 which rotates in the direction of the shown arrow. ~s will be seen, fresll shower water need only be intorudced in one location because a countercurrent -- 10 ~
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washing system maximizes the use of water in the system Eor pulp washing purposes by recycling the filtrate to the previous stages.

sefore the pulp mat is subjected to the action of the wash sprayer 1, the pulp mat consists mainly of water supplied from preceeding wash operations, pulp and black liqu~r. The majority of the water which is contained within the pulp mat formed on the tertiary rctary drum vacuum filter 4 and some of the shower water which is added to the ulp mat by the wash sprayer 1 is drawn from the pulp mat into the tertlary vacuum filter ~ where the water is transferred via a discharge line S to a filtra~:e tank 8. The water which is not removed from the pulp mat 2 by ~he operation oE ~he vacuum drum .Eilter 4 exits l:iie wash system as a ~5 washed pulp mat discharge 16.

The majority oE the wash water contained , in the filtrate tank 8 is recycled via a transfer line 9 ; into the intermediate repulper 10 which repulps a pulp mat 2' formed on a secondary vacuum drum filter 4' for feeding 2'` onto the tertiary vacuum filter ~. The remainder of the was~: water transferred from the filtrate tank 8 is recycled via a trHnsfer line 9' to be used to wash the pulp mat formed on the surface of the secondary vacuum drum filter 4' and is dispenscd on the mat 2 via a wash spray 1'. 'l`he secondary ~5 vacuum drum filter 4' removes most of the water from the pulp mat formed on its surface and transfers the water via a transfer line 12 into a filtrate tank 8'.

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Most of the wash water con tained in filtrate tank 8 ' is recycled via a transfer line 14 into the intermediate repulper 10 ' which repulps the pulp mat formed on a primary vacuum drum filter 4' ' for feeding onto the secondary vacuum drum filter 4 ' . The remainder oE the wash water t;ransferred from the Eiltrate tank 8 ' is recycled via a transfer line 14 ' to be used to wash a pulp ma-t 2"
~ormed on the surface of the primary vacuum drum filter 4 ' ', and is dispensed on the pulp mat 2" via the wash sprayer !.0 1' ' .

ï`he primary vacuum drum Eilter 9 ' ' removes some oE the water frorn the pulp mat on its surface and -transfers this water into the Eiltral:e tank 8 ' ' via a transfer line 16 .

I ~ Some of the water in tlle filtrate tank 8 ' ' is added via a transfer line 18 to pulp and liquor supplied via a transfer line 20 from pulp digesters (not shown~ for introduction of a pulp slurry into the primary vacuum drum filter 4 ' ' . The remainder of the water from the filtrate ~n tank 8' ' is transferred to evaporators via a transfer line 22 .

The improvement of the present invention in relation -to the countercurrent washing operal:ion as described in Fig. 1, resicles in a control apparatus shown in Figs.
2, 3 and 4 which demonstrate the location of the control apparatus in relation to the rotar~ drum vacuum ~ilter 4 as used in the washing operation described in Fig. 1.

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The countercurrent flow wash system as illustrated in part by the tertiary rotary drum vacuum filter 4 in Fig. 2 shows the entry of ~resh shower water into,th~
system via a displacement wash sprayer 1. The fresh water is dispensed by wash sprayer 1 onto the thill pulp mat 2 which is formed from a pulp slurry 39. 'rhe pulp mat 2 travels over the exposed surface 3 of a rotary-drum vacuum Eilter 4 in the direction of the shown arrow. The wash sprayer 1 is designed to apply a uniform application of fresh ]~ shower water in order to achieve a high decJree of dis-placement ratio. Wier type showers are sometimes used in place of or in conjunction with spray showers. .The water content per Ullit area of the pulp on the drum filter is measured using a capacitance measurement apparatus 6 as' the I5 pulp mat 2 travels over the discharge side of the drum ~ilter 4 and is between a vacuum break 5 and a discharge roller 38.

; The capacitance measurement apparatus 6 perEorms an accurate measurement of the li~uid content, that is,,the water,per square meter in the pulp mat 2 being 2'~ discharged from the drum filter 4. When the surface 3 of tlle drum filter 4 is made of metal, the capacitance measure-ment apparatus may consist of only one live probe 7 using the drum filter surface 3 as the other plate of the capacitance circuit whicll becomes the grounded electrode. Where the 2S filter drum is not metal, an a~ditional capacitance elcctrode plate 40 must be stationed between the filter drum surface 3 and the rotating pulp mat 2. The live probe 7 should be spaced from the drum filter surface 3 such that the pulp mat 2 fGrms a substantial portion of the dielectric medium betweell the live probe 7 and the grounded electrode.

Capacitance measurement of the water content S of the web is used -to permit greater i~nediate control of the washincJ system. After determinin~3 the water content oE the pulp mat exiting the washing system, a desired dilution facto~ is used to adjust the shower flow.

Capacitance measurement is used to de-termine l~) the water contellt of the pulp mat for the following reasons.
~ater has a dielectric constant of ~0 at 21C, paper pulp has a dielectric constant oE about 3, and air has a dielectric constant of 1. ~ pulp mat leaving a washer consists of 85 to 90 percent water and 10 to 15 percent pulp. ~ capacitance measurement alone at t}lese conditiolls cannot be used to determine the percentage of pulp or water in the mat even if the mat were freely suspended from the washer drum due to the very low dielectric constant of pulp compared to water. For the same reason a capacitance measurement of a pulp mat containing 85 to 90 percent of water will measure essentially the water alone.

~ Capacitance measurement of the water content is determined in the following manner. The capacitance is measured usin~ the formula in ~cluation I:

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(I) C = 0.0386 KA/t where:
C = capacitance in picofarads (pF) K = dielectric constant of the pulp m~t A = area of plates in square centimeters t = spacing between plates in centimeters It is well known that the dielectric constant of water which is the predominant factor effecting -the dielectric const~nt of -the pulp web, is variable with temperature. rrhe lC dielectriC constant of water at 100C is 55.33 and increases to 38.00 at 0C.

'l`his effect is compens~ted by temperature measurement of the [ulp mat in tlie system. In most uulp washincJ processes the temperature of the washillg water is Ic, held fairly constant and will not require a measuremen-t of the tempera-ture in the pulp mat itself. A normal tempera-ture for the shower water is about 65C. At a dilution factor exceeding 1.0,: the temperature o~ the water in the mat is very nearly 65C. A variation o~ 5C. in -the temperature of `:
the water in the mat would produce an error of 2.3 percent ,~'J in the measured amount of water in the mat. In instances where the water in the pulp mat has this degree of variability, the temperature should be constantly measured and the dielectric constant~must be determined for use in Equation I.

A preselected frequency will be used in ~5 measuring tlle capaci-tance, however, the use of multiple frequencies for more accurate determination of the water content is possible and could be done in systems requiring greater accuracy than single frequency determinations.
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Once the capacitance is determined, the water content in the discharging pulp mat as herein expressed in the terms liters of water per scluare metcr of pulp mat may be e~pressed by Equation II.

(II) L - CF
where:
L = liters of water per square meter C - capacitance of pulp mat in picofarads (pF) F = cell factor l~ The cell factor F of -the capacitor predeterlnilled by a calibra-tion test usin~ ~quation IIA:

F =-B/V
where:
V = meter reading in picofarads when a prepared sa~ple is in the capacitor B = water content oE the prepared sample above in liters per square meter.

~ This calibration of the capacitor and determina~
tion of the cell factor is performed by measuring the capacitance of air in the capacitance measuring apL>aratus and subsequently 3n measuring the capacitance oE a prepared sample of pulp mixed with water in a knowll proportion.

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After determining the liquid content L, the set point for the shower water flow on the washer may be calculated by Equation III with tile variables expressed in appropriate units:

(III) S = (L) (R) (A) (D) S where:
S = shower flow set point (li-ters/minute) L = liters of water per scluare meter R = revolutions of the filter drum per minute U A = area oE drum face surface in square meters D = dilution ~actor In tile use of ~quation III it should be noted that the area A of the Eilter drum surface 3 is completely ]r~ covered with pulp mat as the drum ma~kes one complete revolution and thus represents the area oE pulp mat on the surface oE the filter drum.

~`he capacitance measurement apparatus 6 is shown in alternative forms in Fig. 3 and Fig. 4.
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2.0 In Fig. 3 the live capacitance plate 50 mechanically tranverses across the surface of the pulp mat 2. The transverse movement of the live plate 50 is per-formed by the rotation of a screw bar 52 through a threaded opening 54 in a plate assembly 55. The screw bar is rotcted ~5 by a reversible electric motor 56. T}le plate assembly 55 is held in a vertical position through tlle use oE a pair of , guide rods 5a. The live plate 50 transverses back and forth .

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across the pulp mat on the drum filter ~ measuring the capacitance o~ the pump mat 2 using the metallic surface of the drum filter 4 as the grounded electrode while moving the trans-versing live capacitance plate 50 remains at the same relative distance between the vacuum break 5 and the pulp mat discharge on roller 38. The function oE the transversing live plate 50 is to obtain capacitance readings along the entire width oE the pulp mat.

In Fig. 4, a series of stationary live capacitance plates 60 are used to measure the capacitance along the width of a pulp mat 2 which rotates wi~h the drum filter ~ and the metallic surEace 3 of the filter drum acts as the grounded electro~e. The plates 60 are located on a support 62 and controlled by an electrical switchilig device 64 which can L~ activate any one of the individual plates 60 or any combination of the plates 60 to record either the capacitance at an individual plate position or take an average capacitance reading from two or more of the plates when a plurality of plates are activated.

The capacitance measurement apparatus has heretofore been illustrated as being located only on the tertiary~drum filter 4 in the series oE filter drums which are used in the countercurrent washing operation as shown in Fig. 1. Ilowcver, it is o~vious to one skilled in ?5 the art tha-t it is advantageous to control the shower flow on all oE the filter drums to increase the efficiency of the . - 18 -.

over-all wasll system. As represented in Fig. 1, it is noted tllat the flow of liquid dispensed from the fil-trate tank 8 via the transfer lines 9 and 9' is equal to the input into the filtrate tank 8 via the transfer line 5 from the tertiary ~, drumfilter 4. The level of liquid in the filtrate tank 8 must remain constant or the wash system will either overflow liquid from the filtrate tank 8 or stop due to a shortage of l~quid supply to the transfer lines 9 and 9' from the filtrate tank 8. Since the filtrate tank 8 contains a 1(1 large capacity o~ liquid compared to the flow through the wash sprayer 1' it is practical to rec~ulate the li~uid flow througll the wash sprayer 1' onto the pulp mat 2' formed on the secondary rotary drum vacuum filter q' throuyh the use of the same capacitance measurement tecllllique as is used on the tertiary rotary drum vacuuln fil~er ~. One problem which arises in Usill~ the same capacitance measurement technique is that a sligllt change in conditions such as pulp consistency in the washing o~erations will cause a net gain or loss in the liquid level in the filtrate tank 8. This problem .~t` is overcome through the use of an additional dilution control system 66 as shown in Fig. 1 which slowly alters -the dilution factor up or down to maintain a constant liquid level in the filtrate tank 8. The dilution control system 66 may consist of a flow regula-tor 67 which is responsive to a liquid level ~S sensor 6~3 whicll records l:hc level of tlle liquid i.n the filtrate tank 8 and correspondingly adjusts the flow rate of liquid to the wash sprayer 1'~ -~, .

The shower flow through the wash sprayer 1"
on the primary rotary drum vacuum filter 4'' is controlled in the same manner using a dilution control system 69 to regulate the liquid level in the filtrate tank 8'.

1`hese combined methods of controllinc3 the shower Elow Erom the wash sprayers 1, 1' and 1" onto their resp~ctive pulp mats 2,2' and 2" produces additional benefits in efficiency over excercising a control of the shower flow from the wash sprayer 1 alone in that a short term excessive wash will not compensate for an equal term of ,In underwash in a previous washinc~.stage.` Neither the control of the shower flow nor the control of the liqu`id level in the filtrate tank either alone or in combination can be usèd to control the dilution factor without the capacitance measure-' ments proposed herein. ~dditionally, through the use oE the dilution control systems 66 and 6g in the countercurrent pulp washing operation an added benefit is obtained in the early detection of faulty equipment as filtrate tank levels will be responsive to abnormal deviations in the washing operation.

While the use of capacitance measurement apparatus appears'to be the most accurate method of measuring the water content per unit area of the pulp mat on the Eilter drum, alternative systems may be used cither alone or in combination with the capacitance measuring appara~u,s.

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tr'p The first alternative system is shown in Fig. 5. ~ radiation source 76 transmits radiation which passes through tlle pulp mat 2 and strikes the metallic surface 3 of the filter drum 4 whereby radiation is reflected'and detected by a radiation detector 78. This system will measure the total mass per unit area of the ~ulp ma~ and is located in a position to monitor the pulp mat 2 aft~r the pulp mat rotates on the filter drum 4 over the vacuum break 5. In some cases this backscatter nuclear radiation device n can be used alone and will give a better control than previously used since only the consistency need be estimated rather than an estimated rate determined from the first washer ~eed rate along with the consistency.

The second alternati.ve system is essentially l~rj the same as the nuclear radiation system except that the nuclear source 76 is replaced with a microwave source 80 and the radiation detector 78 is replaced with a microwave ' sensor 82. With very sophisticated equipment and scanning microwave requellcies it is possible to determine both the dielectric losses and the water per unit area with this system.

The function of shower water control is so im-portant ~hat it may be desirable to use the capacitance measure~
ment aL~Laratus in conjunction with one of the alternative elnbodimellts. l`he caL~acitance Ineasuremellt is used to determine the water content per unit ar,ea in the pulp web and the radiation absorption measurement techniques can determine the total mass of the pulp mat per unit area.
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Through the combined use of the measurements as calculated from capacitance measurement apparatus and from either radiation or microwave absorption measurement apparatus, the following parameters of a pulp wash system '~ may be calculated from their respective determination equations.

Equation IV may be used to calculate the dry pulp ~nass per uni.t area in the pulp mat employiny both the total mass of pulp and water per unit area, as determined 1~ by radiation measurement -techniques and the mass of water per unit area oE the pulp mat as determined by capacitanc~
measurement:

(IV) P = M-.LG
where:
P = kilograms of p~;lp per square me~er of pul.p mat ~1 = kilo~rams of pulp and water per square meter of pulp mat L = liters of water per square meter of ~t! pulp mat G = speciEic ~3ravi.ty of water at existinc3 conditions such as the temperature of the pulp mat The pulp production rate of a pulp washing ~r~ system is determined by Equation V:

~V) Q = 1. 44 (P) ~R) (A) where:
Q = pulp production ratc in metric tons ~1!1 per day P = kilograms of pulp per square meter of pulp mat X = revo:lutions per minute of filter drum - A = area of surface of filter drum in square 3~ meters ,:

The consistency or the percentage of pulp in the mixture of pulp and water leaving the wash system in the form of a pulp mat is calculated by equation VI:

(VI) N = 100 P/M
where:
N - consistency of pulp mat expressed n percentage ~- P = kilograms of dry pulp per square meter oE pulp mat ` M = kiloyrams of pulp and water per square meter of pulp mat Conventional thickness measuring apparatus may be employed in a pulp washing system as demonstrated by thickness measuring device 88 in Fig. 5 which qetermines the l~ pulp mat thickness at a point between the vacuum break 5 and roller discharge 38. The air content of the mat may be determined by Equation VII through the use of the combined measurements of the apparatus demonstrated in Fig. 5. The percentage of air entrained in the pulp effects the displace-'~0 ment factor.

(VII) U = 100- 10 (L + (P/G)) T
where:
U = percentage of air in pulp mat L = liters of water per square meter ~r~ oE pulp mat P = ki]ograms of dry pulp per square meter of pulp mat G = specific gravity of cellulose T = thickness of pulp mat in centimeters ~ . .. . . . , . __ . . _. _. ..... . ... _ .

Some pulp washing systems en~ploy chemical additives to improve the overall washing operation. These additives perform a variety of functions such as prevention of foaming and air entrainment in the pulp slurry and include anti-foam agents, drainage aids and washin~ aids. The measurement and determination of the pulp consistency and the air content of the pulp mat can be used to minimize the amount of the above-identified additives which are added to the washing operation.

While the present invention has been described '!l in the context of a basic brown stock pulp washin~ operation for washing cellulose, it may be applied to a variety of operations such as a bleach p:Lant washing step. The liquid dispensed ~rom the wash sprayers may be water, recycled water or chemical treating agents. Tlle co,ntrol system is applicable !; ~0 systems which treat slurries o~ materials other than pulp such as lime mud feed to kilns or calciners.
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It is, oE course, understood that the foregoing description of the process of the present invention ~0 is intended to be illustrative and that modifications thereof as would be apparent to one skilled in the art are deemed to fall within the scope and spriit of the present invention as defined by the following claims.

.... . . .

Claims (14)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for controlling the flow rate of a liquid shower for a desired dilution factor in a counter-current pulp washing operation in which the shower is applied to a slurry mat consisting of a mixture of a liquid and a solid material which rotates on the surface of a rotary drum vacuum filter wherein the liquid content per unit area of the slurry mat is determined by directly measuring the di-electric properties of the slurry mat after it has passed over a vacuum break and before it is discharged from the vacuum filter and the flow rate of the liquid shower is controlled in relation to said measured liquid content per unit area of the slurry mat with the surface area and the rotational speed of the rotary drum vacuum filter and said dilution factor.

2. A process as defined in claim 1 wherein the slurry mat is a cellulose pulp mat consisting of a mixture of pulp and water.

3. A process as defined in claim 1 wherein the washing operation is a brown stock washing operation.

4. A process as defined in claim 1 wherein the washing operation is a bleach plant washing operation.

5. A process as defined in claim 1 wherein the flow rate of the liquid shower is controlled in each of a plurality of washing operations on the drum filters in a countercurrent washing system.

6. A process as defined in claim 1 wherein the liquid shower is fresh water.

7. A process as defined in claim 1 wherein the liquid shower is recycled wash water.

8. A process as defined in claim 1 wherein a measurement of the liquid continuity per unit area of the slurry mat is obtained by a capacitance measurement apparatus which measures the dielectric properties of the slurry mat after the slurry mat rotates over a vacuum break in the filter drum and before the slurry mat is discharged from the filter drum wherein the measurement of the liquid content per unit area of the slurry mat is determined in accordance with the following equation:
L = CF
where:
L = content of liquid per unit area of the slurry mat C = capacitance of the slurry mat F = cell factor of the capacitance measurement apparatus.

9. A process as defined in claim 8 wherein a production rate of the solid material in the slurry mat and a consistency of the slurry mat is determined from a correlation of the liquid content per unit area of the slurry mat and a combined liquid and solid material content per unit area of the slurry mat on the drum filter as measured by a total mass measurement apparatus.

10. A process as defined in claim 9 wherein the total mass measurement apparatus consists of at least a backscattered nuclear radiation apparatus.

11. A process as defined in claim 9 wherein the total mass measurement apparatus consists of at least a microwave cavity perturbation apparatus.

12. A process as defined in claim 9 wherein a means for measuring a thickness of the slurry mat is used in addition to the dielectric properties measurement apparatus and the total mass measurement apparatus in order to determine an air content of the slurry mat.

13. A process as defined in claim 1 wherein the liquid shower is supplied from a filtrate tank and the dilution factor is responsive to a liquid level in the filtrate tank.

14. A process as defined in claim 1 wherein the dilution factor is adjusted to be responsive to dielectric losses in the slurry mat.