CA2081907A1 - Liquid composition analyzer and method - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
An apparatus (30) which determines concentrations of each of three components that are intermixed in a homogeneous solution (34). Each component is detectable by at least one characteristic and more than one characteristic is associated with more than one component. First, characteristics that are quantitatively detectable in relation to the concentrations of the components are identified. A
mathematical relationship is then developed between the components and the detectable characteristics using the detectable characteristics as independent variables. A
sample of the solution is then analyzed to obtain quantitative data of each of the characteristics. The obtained quantitative data is then employed in the mathematical relationship to obtain the concentration of each of the components.

Description

W091/173Q5 P~T/~Sg1/02933 t~

LIQUI~ COMPOSI~TON ANALY~ZE~ ~13 ~-~HOD
~CKGRO~D OF T~E INVEN~ION
Accurate knowledge of white and green liquor composition is necessary for close con~rol Oc kraLt pulping and recausticizing operations. Firs., if changes in the green liquor composition ca~ be monitored, feed forwaxd control of the lime feed rate in the causticizing plant can be achieved. second, compositional information of the white liquor can b~
used as feedback to compensake for varia~ions in the lime quality or reactivity, and as feed forward compens~tion for pulping.
In the past, white and green liquox compositions have been determined by laboratory titrations. However, both the analysis rate and the accuracy of routine titrations are not sufficient to take full advantage of modern control systems and strategies.
The Wallin U.S. Patent 3,941,649 describes an attempt to control the pulping time and pulping temperature by taking a sample of the pulping liquor after initial digestion has occurred. The pulping sample is titrated to provide an alkaline content of the liquor. From this alkaline content, the pulping inte~sity expressed as -~-s ~actor is determined and used to obtain the desired K~PPA number.
The ~ultman et al U.S. Patent 4,236,960 describes a proces~ for controlling the degree of causticization of white liquor~ The process of the Hultma~ et al pat~nt includes determining the sodium carbona~e co~centration o~ green liquor fed to ~he caus~iciza~ion, then de~ermining tha sodium carbonate concentratio~ of white liquor resulting from the WO9l/17305 P~T/US91/~2933 ~r$~

-2-causticization and thereby controlling the degree of caus~icization within a predetermined range while taking both sodium carbonate concentrations into account.
The Bertelsen U.S. Patent 4,536,2S3 describes a process for controlling the properties of white liquor by measuring the electric conductivity of the green liquor before causticization in addition to me~suriny of the conductivity of the white liquor. The conduc~ivity of the green liquor is measured both before the slaker lo and gradually as it passes through th~ slaker to determine the reaction of the ~arbonate.
SUMMARY OF THE INVF~TION
The present inv~ntion includes a method for determining the concentration of each of at least thre~
components intermixed in a homogeneous solution. For example, in th~ case of th~ kraft pulping or reca~-sticiæing operations, th¢ white or gre~n liquor co~position includes three major components, sodiu~
hydroxide, sodium sulfidç, and sodium carbonate~
~he method includ~s identi~ying characteristiGs of the co~ponent~ that are quantitatively detectable in relation ~o the concentration o~ th~ compon~nts. A mathematical relationship is then developed between the concentration o~ each o~ the com~onents and the d~tectable characteris~ics using ~he characteristics as independent variablas. The solutlon is then s~nsed usiny detectors to cbtain quan~itative data for each of the characteris~ics. The quanti~a~ive data is ~hen employed in the math~mati~al relationship ~o obtain the concentration o~ each Or the components.
The present lnven~ion also includes an analyzer having de~ectors which ~ense the three W091/17305 P~r/US91/02933

3--components and provide quantifiable data to a co~puter ~or emplo~ment in the mathematical relationship that was developed~ In one preferr~d mode, a sample is extracted - from the process and analyzed by the detectors. In another preferred mode, the process solution is continuously passed by the detectors ~or analysis.
~ F:D~SCRIP~ON OF T~E DRA~tINÇS
Figure 1 is a sch2matic diagram of a liquid analyzer of the present invention.
Fiyure 2 is a graphical ~iew of a ~ypi~al response of the analyzer of Figure 1 to a kraft liquor.
Figure 3 is a graphical view illustrating de~iations between titrated and predicted industrial white liquor compositions using the analyzer o~ ~iqure l.
Figure 4 is a graphical view o~ the deviations between titrated and predicted industrial green llquor compositions using the analyz~r o~ Figure l.
Figure 5 is a schematic diagram of an lternative ~bodi~ent o~ the liquor analyzer of the present invention.
Figure 6 is a graphical view of a comparison between the titrated and predicted sodium sulfide concent~ations for the analyzer of Figure 5 ~or white liquor-~ype solutions..
Figur~ 7 is a graphical view of the comparison b~tw~en tikrat~d and predic~ed sodium hydroxide concentration~ for ~he analyzer ~g Figure 5 for white liquor typ~ solution~.
Figur~ 8 is a ~raphical ~iew o~ the comparison b~tween o the tltrated and predicted sodiu~ carbonate conc~ntration~ ~or th~ analyzcr o~ Figure 5 rOr white liquor-type solution3~

W~91/17305 PCr/USgltO~933 Figure 9 is a graphical view of the co~parison between the titrated and predicted sodium sulfide concen~rations for the analyz~r of Figure S for green liquor-type solu~ions.
Figure 10 is a graphical view of ~he comparison between the titrated and predicted sodiu~
hydroxide concentrations ~or the analyzer of Figure 5 for green liquor-type solutionsO
Figure 11 is a graphical view o~ the comparison b~tween the titrated and predicted sodium carbonate concentrations for the analyzer of Figure 5 for green liquor-type solutions.
Figure 12 is a diagra~matical view o~ one exa~ple of a control system using the analyzer of the pres~nt invention.

In a pre~erred emhodiment, the present invention includ~s ~n on-line au~omatic liquor analyzer for a kraft pulp-paper mill application. Timely knowledge of liquor co~position is n~cessary ~or close control o~ the digesting and recovery operations in a pulping prccess. ~lthou~h the procsss described herein is a kra~t (alkali based) proc~ss, the analyzer of ~he present invention ~ay very well be used in other proceC~ such as a sulfite process.
` In a kra~t pulping op~ration, the three p~imary co~ponents of th~ liquor include sodiu~
hydroxide, sodium sulfide, and so~iu~ carbonate. The present invention provides a non-invasive t~pe o~
meAsureme~t o~ the green liquor (the liquor exiting the re~overy furnace) or the white liquor (th~ liquor exiting th~ causticizer) or green, whita, or weak liquor solutions in other par-ts of the proc~ss can also be ~r'~

measured. Detectors ar2 chosen for sensing a characteristic of each of the components. For example, in one of the preferred embodiment~, W absorption at 25~ nm was used to detac~ sodium sulfide which hydrolyz~s into sodium hydrosulfide in kraft liquors.
Conductivity and refractive index were used to detect sodium hydroxide, sodium sul~ide, and sodium car~onate in differing proportions.
The present invention also includes a process for obtaining the concentrations of components of a process solution by initially identifying the characteristics of the components that are quantitatively detectable in relation to the concentration o~ the component. A mathematical lS technique such as re~ression analysis is used to develop a mathemakical relationship between the relative concentration of the components and the detectable chara~teristics. For example, eq~ations are developed using the detectable oharacteristics as independznt variables. A sa~pl~ o~ the solution is then analyzed to obtain quantitative data for each o~ ~he detecta~le characteristics. That quantitative data is then employed in the mathematlcal relationship developed pr~vio~sly to obtain the concentration of sach ~f the co~ponents in the sampl Using the analyzer of the present invention, whit~ or gr~en l~quor is drawn without interrupting the proc~ss or contaminating the process soiution. Since a ampl~ an be taken at any time and an analysis done quickly, ~or example in les~ than three minutes, th~
present invention provide~ ~or close monitor.ing of the proc~ss that was pr~viously not po~sible.

WO91/17305 P~ /US~1/02933 -6- ~'~J~

The analyzer of the present inventio~ can be used in at least one og two preferred modes. In a ~irst mode, a sa~ple is extracted from the process and analyzed. In a second mode, the proces~ solution is continuously pa~sed by the detectors.
EX~RACTIv~ SAMPLE~ANALYZER
ANI~I~YZE~ - D~SIGN
The extractive sample analyzer 30 is illustrated in Figure 1. The analyzer 30 includes a Valco EC6W 6-port sa~ple injection valve wit~ electric actuator 32 for extracting a sample from a sample stream at 34 fro~ the process of-the present invention. A
Wat~rs S10 HPLC pump 36 is usEd o pump wa~er 38 into the valve 32. A Waters zero dead volume ~ee 40 is disposed upstream from a Waters high pressure gradient mix~r 42. A Waters 510 HP~C pump 44 pumps ~ater 46 through the tee 40. A Waters column heater 4R is.
dispos~d downstr~am ~rom the mixer for maintaining a selected temperature of the extracted sample. Located downstream ~rom the h~ater are a WatQrs 481 varia~le wavelength W spectrophotom~ter 50, a W~ters 430 enhanced conductivity detec~or 52, and a Wa ers 410 di~erential refracto~e~er 54. Data fro~ the three d~t~ctors 50, 52, and 54 is collected by a Keithley 570 d~ta acqui~ition system 56 and a Zenith 248 roco~uter 58.
Th~ operation of the analyz~r proceeds as follows. A liquor stream is run t~rough the extrac~ive valve 32 and a very small sample (5 microliters) is captured i~ a constant volume loop in ~he valve 32. The sampl~ i~ flushed. from the valve 32 by a stream o~
dis~llled, d~gassed water 38 provided by pump 36~ The flowing sample is dilut~d by additional distilled, -7~ 3~" ~j!~

degassed water ~6 en~ering through the ~ee 40. The resulting sample is mixed thoroughly in the gradient mixer 42. The mixed sample is heated to a unifor~
temperature by passing through the column heater 48.
The sample then flows through the W spectrophotometer 50, conductivity detector 52, and differential refractometer S4. The responses from the detectors are sent to the computer 5~ by the data acquisition system 56 where the responses are integrated over time. The areas are calculated by the computer in units of volt-sec X 10. The conc2ntrations of sodiu~ hydroxide, sodium sulfide, and sodium carbonate in the liquor are then calculated by correlations with the detector response areas.
ANALYZER OP_R~T~QN
Experi~ents were carried out with aqueous solutions containing sodium hydroxide, sodium sulfide, and sodium carbonste, th~ three major components in ~ost kraft pulp mill liquors. The exp~riments were divided into two groups, white liquor typ~ solutions and green liquor solutions. The white liquor solutions containing between 60 and 120 g/1 o~ NaOH, 10 to 40 g/l Na2S, and O to 40 g/l of Na2C03, all expressed as Na20 ~qui~alentsO
The gre~n liguor solutions contained between 60 and 120 g/l o~ Na2C03, 0 to 40 g/l of NaO~, and O to 40 g/1 Na2S, all expre ~ed as Na20 @quivalents.
The test solutions were prepared from concentr ted stock solutions of the individual compounds. The ~tock solutions wer~ prepar~d in distill~d, dQga~ed water, u~ing rea~ent qrade chsmicals. The~e solutions wer~ kept ~ightly capped and the runs w~r~ ~ad~ within 10 days o~ stock solution preparation. The concentrations o~ th~ stock solutions W091~17305 PCr/US91/OZ933 ~8~ 7 were determined by titration with HCe. The solution den~ities were deter~ined by weighing known volumes.
The concentration and density of each solution were periodically checked and no changes were detected during S the course o~ the experiments.
The test solutions were prepared by mixing speci~ic masses of the stock solutions and water, if necessary, to produce the desired ~oncentration of each component. The solutions were injected into the analyzer 30 imm~diately ~fter preparation to minimize any compositional changes du~ to sulfidè oxidation, carbo~ate formation, or evaporation.
The operating param~texs ~or the analyzer components are listed in Table 1. The inputs ~o the data acquisition -~yst~m w~re carri~d by twisted, shielded pairs, and filtered by first-order RC ~ilters with a 207 K ohm r~sistor and a 10 yF non-polar capacitor.
T~LE 1 Operating Para~ters for ~he Ex~racti~e Sample ~hite and Green Liquor Analyzer Components Pu~p~: Sample flow = 0.1 ~l/min Dilution flow = 5.0 ~l/min H2ater: Temperature = 30~0 C
UV detector: ~avelength - 254 nm ~ime consta~t = 1 sec.
Ou~pu~ ~ lV / ~g Condu~tivity: Output ~ 2V / ~S
detQctor Temperatt~e control on Re~racto~e~er: Ti~e con~tant ~ 1 sac.
S~nsi~ivity ~ 64 Scale ~actor - ~0 Temperature ~ 32.0 C

WO91~17305 P~l/U~91/02g~3 _9_ ~ ,~'~J~

~ fter the sample injection, the detector responses were recorded for five minutes by the data acquisition system. A sample response is illustrated in Figure 2. The response shows a dead time of just over one minute, where the sample zone was traveling from the injection valve to the tee. The analysis was run for over a minute after the responses returned to the baseline although this additional time is not necessary.
Thus, the minimum analysis time per sample is approximat~ly three minutes.
AN~YZER_~LI_RATION
For white liquor ~ype solutions, 21 experi~ental runs were made to establish correlations ~or t~e individual components. The concentration of each co~pon~nt and the deteotor re~ponse areas w2re recorded ~or each run. The data was analyzed in two ways. First, the concentrations were taken as the independent variables to give an indication of how the liquor component concentrations a~fect the deteotor responses.
Stepwise multiple linear regression was used to determine the best co~bination of factors to describe the det~ctor output. The results for white liquor indicat~:
R~ ~ ~.5~59 O~ ~ 5.g751 S + ~.9~313 C ~ 4.4202 (l) W 2 19. 0756 S - 0.~3ll s2 1 0.1586 OH + ll.lB99 (2) CO = l5.9066 OH - 0.0120 oH2 ~ 10.5559 S (3) ~ 7.2305 C ~ 50.0796 where RI = di~ferential re~ractome~er re~ponse W = W sp~ctrophotome~er response CO = conductivity detector respon~
OH - ~odium hydroxide concsntra~ion (g/l Na~O) S = sodium sulZide concentra~ion (g/l Na20) C ~ s~dium carbonat~ concentration (g/l Na20) WO 91~17305 P~/U.591tO2~33 ~10- z~ ~7 The standard deviations for the predic~ions are:
RI = 1.28, W = 3.97, and Co = 4.74 area units. The coefficients of varia~ion ~or each detector response are: ~I = 0.23%, W = 1.05~, and C0 = 0.32%. These values are very close to the rep atability deviations of each detector respon~e as determined by multiple tests of the same solution. The repeatability limits are approximately: RI - +1, W = ~3, and C0 = +4 units.
This analysis in~icates that the error in the predicted responses is pri~arily due to random errors introduc2d by the analyzeE, including injection volume differemces, carri~r flow fluctuations, detector respo~se variations, and data acquisition noise. This conclusion is verified by regression residual analysis which show~ no residual pattern with respect to the solution concentrations or test order.
For th~ purpose of con~entration prediction, a ~ore useful regression involves the use of detector respons~s as independent variables. In this way, the solution composition could be calculated directly from the detector responses. Stepwis~ multiple linear regression produces the following equations:
0~ = O.il63 C0 ~ 4.721X10-6 Co2 ~ 0.1882 RI (4) 25- 0.~2067 W - 1.210 S ~ 0.~2955 UV + 6.487XlOs w2 - 2.882Xl07 C0~ (5) + ~.113 1~ æ 0~36484 P~I ~ 0~107~? C0 ~ 2.923X10 6 Co2 (6 - 0~02529 W ~ 6.354X10 5 w2 ~ 1.614 The standard devlation~ for ~he predic:a~ions are: OH -0.35, ~ s 0.37, and c - 0.60. Equatic~ns (4, s, and 6) re~ul~ in an approximate 90% con~i~enc~ lnterv~l o~ ~0.5 g/l for bo~h sodium hydroxidQ and ~odiu~ sul~idQ, and +0.8 g/l tor sodium carbonate.

WO91/17305 PC~/US91/02933 These correl tions were entered into the computPr data acquisition system in order to calculate component concentrations for subsequent trials. A set of four additional samples were run as a test set to check the prediction capability. The predictions were also compared wi~h values obtained by manual titration of the samples. The results are shown in Table 2. ~he results verify the error limits as predicted by thP
regression analysis.

Comparison of Extractive Sample Analyzer Results with Titration Results for Synthetic White Liquor Solutions Test Component Actual Analyzer Titration No. Conc. Conc. Conc.
.......... ~ _ . .
Sodium Hydroxide 79.9 79.9 79.2 1 Sodiu~ Sul~ide27.5~7.8 27.4 Sodium Carbonate 32.8 32.7 32.9 ~ , . . .
Sodium Hydroxide 56.7 s6.7 56.7 2 Sodi~m Sulfide13.813.9 13.4 Sodium Carbonate 30.9 30.8 30.9 Sodium Hydroxide 40.4 40.1 40.4 - 3 Sodium Sulfide20.B20.8 20.3 Sodium Carbonate 23.7 23.6 23.9 Sodiu~ Hydroxide 98.5 99.l 99.0

4 Sodium Sulfide4l.l~l.5 40.O
Sodium Carbonate 0.0 0.0 0.5 _ _ .
NOTE: all concentrations in g/l as Na2O

A similar set of ~xperiments was run ~or green liquor type solutions. In this case, 23 experiments ~091~17305 PCT/US~1/02933 -12- ~

were run to establish correlations. The r~gression using concentrations as independent variables yielded tAe following equations:
RI = 4.869 OH 6.873X103 oH2 + 50857 S (7) ~ 4.910 C + 3.821 W = 20.483 S - 0.1519 s2 + 0.1576 C - 9 533 (8) CO = 13.680 OH + 10.266 S ~ 6.813 C (9) + 3.080X103 ~ 134.665 .

The standard deviations for the predictions are: RI =
1.?1, UV = 4.30, and CO = 3.32 units. The coefficients of variation are: RI = 0.27%, W = 1.28~, and CO =
0.28%. These errors are again close to the analy2er repeatability error. It should be noted tAat equations (7, 8, and 9) are not exactly the same as eqUatiQnS (1, 2, and 3) for white liquor but are very similar. Much of the observed di~ferences can be attributed to interactions between the liquox components which can cause nonlinearities in detector responses both white and green liquor samples.
The regression with detector responses as the independent variables produces:
OH - O. 1483 CO - 0.2219 RI - 0.01069 W (~0) - 8.050X10-6 w2 - 15.5~3 S = 0.04006 UV = 4.9~3X10-5 w2 (11) - 1~893X10~ RI2 + 0.6781 C = 0.4180 RI o 0.1415 CO - 0.03506 W (12)

5.538XlOs w2 + ~2.10~

The standard deviations for these predictions are: OH
~ 0.35, S - 0.37, and C = 0.54. ~his gives approximate 90% con~idence intervals o~ ~0.5 g/l for sodium hydroxide and sodium sulfide and ~0.7 g/l ~or so~ium carbonate. These prediction errors are similar ~o those WO~1/17305 PCT/US91/02933 .

for white liquor, as would be expected since the basic equations are very similar.
Analyses were conducted on four additional samples as a test set to check these correlations. The samples were also manually titrated to compare with the predictions. The results are shown in Table 3. As with the white liquor, the results are excellent, with the analyzer error less than the predicted error ~ounds.

Comparison of Extractive Sample Analyzer Results with Titration Results for Synthetic Green L~quor Solutions Test Co~ponent Actual Analyzer Titration No. Conc. Conc. Conc.

Sodium Hydroxide21.4 21.4 21.S
1 Sodium Sulfide 10.4 10.1 10.2 Sodium Carbonate115.9 116.1 115.4 __ _ Sodium Hydroxide30.0 29.9 30.2 2 Sodium Sulfide 0.O O.3 0.O
Sodium Carbonate78.9 78.4 78.9 .
Sod~um Hydroxide 9.9 9.9 9.8 3 Sodium Sulfide 24.9 25.1 24.7 Sodiu~ Carbonate86.3 86.2 ~6.6 Sadiu~ Hydroxide21.7 22.0 21~5 4 Sodium Sulfide 15.3 15.3 ~5.2 Scdium Carbonate94.2 94.6 94.3 NOTE: all concentrations in gJl as Na20 The results show that the analyzer works well for white and green liquor type samples prepared from pure chemicals. It is expected that the responses would W091tl7305 PCT/USg1/02933 -14- 2~

cha~ge if other compounds were present in the liquor.
In ind~1strial white and green liquor, there could be trace amounts o~ sodium sulfate, sodium sul~ite, sodium thiosulfate~ and polysulfide sulfur. The detector response to these impurities was tested by lnjecting solutions containing ~hese rontaminants into the analyzer. The results are shown in Table 4. The data indicates that in each case the presence of impurities will add area to each of the detector respo.nses of a pure liquor. Thus, ~or industrial use, the analyzer must be calibrated to accommodate the concentration of impurities in the liquor. In all cases with kra~t liquors, the impurities will be present in only minor a~ounts and will be present at nearly constant levels.

Effect of ~hite and Green Liquor Impurities on Extractive Sampl~ Analyzer Detector Responses ~0 Compound Concentration W RI Cond (g/l as Na20) Area Area ~rea ~ Na2S203 10.5 42.7 69.7 90.7 Na2S3 9.6 5.3 53.0 73.5 Na2S~4 lO~0 0.2 4603 82.1 5~ CIl~_o~lC~L~L
Kr~t Liquors Analyses were per~ormed on various white and green liquors obtained fro~ kraft mills. The object of the tests was to deter~ine the ef~ect of actual mill liquor impurities on the analyzer results. Ten white and green liquor samples wexe analyzed.
The results ~rom the industrial white liquor analysis are ~hown in Figure 3. The results indi~ate that each componen~ is generally overpredicted when . .

using ~he correlations developed for pure white liquor.
The sodium hydroxide estimate is the least affected by the impurities. The observed deviations ~re reasonable consid~ring the types of impurities which may be present. Some thiosulfates and polysulfides may be present which would contribute to the absorbance at 254 nm. This would result i~ overprediction o~ sodium sulfide. Sodium sulfate and sulfite would basically appear to the detectors as sodium carbonate. Sodium sulfate and sulfide are species with low conductivity contribution, but significant refractive index contribution. The average deviations for the industxial white liquors are shown in Table 5.
TABLE S
Analysis ~rrors for Industrial ~hite Liquors Using the Extractive Sample Liquor Analyzer ~ .
Component Avg. rror Std. Deviation Sodium Hydroxide 0.14 g~l 0.50 g/l Sodi~m Sulfide l.07 g/l l.lO g/l Sodium Carbona~.e 6.21 g/l 2.77 g/l . ~ . ... . _ . . _ . _ The results from the industrial green liquor analyses are shown in Figure 4. The results are ~imilar to the industrial white liquor analysis. The sodiu~
carbonate i5 always overpredicted. This is again caused by the influence of impurities having small contribu~ions to solution conduc~ivity. The sodium sulfide error is small~r, and the sodium hydro~id~ error is larg~r than that o~ the white liquor. Th~ average deviations for industrial green liquors are shown in Table 6.

WO91/17305 PCT/USg1/02933 -16- 2r ~ 7 Analysis Errors for Industrial Green Liquor Using the Extractive Sample Liquor Analyzer Component Avg. Error Std. Deviation lOSodium Hydroxide 1.23 g/l O.S7 g/l Sodium Sulfide 0.61 g/l 0.~6 g/l Sodium Carbonate 5.81 g/l 2.1~ g/l Analyzer Desian A schematic diagram of the in-situ analyzer 60 is illustrated in Figure 5. The analyæer 60 includes a Rosemou~t Model 222 Toroidal Conductivity Sensor with a y 20 Model ~054T Toroidal Conductivity Analyzer/Transmitter 62, a Micro Motion Model D25 Mass Flow M~ter with a Micro Motion DMS Liquid Densitometer 64, and a Rosemount Model 340A Selective Ion Sensor with a Model 1033 Selective Ion AnalyzPr/Transmitter with a Phoenix Silver/Sulfide Ion Electrode 66. Temperature data was transmitted throuyh a Ros mount Series 78S platinum RTD
with a ~odel 444 Temperature Transmitter 82. Data acquisition was a~complished with a Keithley 570 data acquisition ~ystem 68 and a Ze~ith 248 microcomputer 70.
Th~ conductivlty sensor ~2, the densitometer 64 and the sul~lde ~lectrode 66 are disposed serially along a ~ypass conduit 72 that prQvides a ~tream of liquor from a vessel 80. The bypass stream 72 is maintain~d a~ a uniform temperature by a heater 74 with temperature control 76. A pump 78 provides the mode o~ ~orce for circula~ing the bypass stream 72.
The analyzer operation involves pumping the liquor through the various sensors and processing the WO91/17305 PC~/U$91/02933 -17~ ~J.'.1,,~

sensor data to calculate liquor composition.
ANALYZER OPERATION
Experiments were carried out with aqueous solutions containing sodium hydroxide, sodium sul~ide, and sodium carbonate. The experiments were divided into two groups, white liquor type solu~ions and green liquor type solutLon The white liquor solukions contained between 50 and l00 g/l of NaOH, 50 to 40 g/l of Na2S, and 0 to 25 g/l of NazCo3, all expressed as Na2O
e~uiYalents. The green liquor solutions contained between 65 and 105 g/l of Na2CO3, 0 to 30 g/l o~ NaOH, and 5 to 35 g/l o~ Na2S, all expressed as NazO
equivalents.
The solutions were prepared from reagent grade chemicals in distilled water and were used immediatPly after preparation. A liquor sample was taken from the vessel 74 and titrated ir. duplicate using HCe before each experiment was begun. The vessel 80 was capped and the contents heated sequentially to 70, 80, and 90C.
2 0 This temperature range was chosen because it is the typical temperature range in which white and green liquors are transported throughout a pulp mill. The liguor was held at each temperature until all of th~
sensor responses had stabi1ized. In each case, the 25 sul~ide electrode had the slowest, and thus limiting, recpo~se time. Sensor data wa~ recorded at ten second intervals throughout each experiment as averages of ten consec~tive readings. The da~a acquisition rate was 3.33 ~Z.
The proc~ss trans~itters were configured to provide good signal resolution over the expected range of liguor concentrations. Each transmitter ou~put was connected to the data acquisition system as a 4-20 mA
current loop. R load resistor o~ ~50 ohms was used ~o WO91/17305 ~CrJUS91/0~33 convert the signal into a voltage of 1-5 V. The transmitter ranges and maximum signal resolutions are shown in Table 7.

In~Situ Liquor Analyzer Sensor Configurations and Maximum Signal Resolutions S~nsor . Range Max. Resolution .
Temperature 0-210 C 0.13 C
Conductivity 0-1000 mS/cm 0.61 mS/cm Density 950-1150 g/l 0.12 g/l Sulfide Ion 730-~80 mV 0.09 mV
. ~
~NALYZER_CALIBRATION
~O For white liquor type sslutions, 15 experimental runs were made to establish correlations for the individual components~ The data consisted o~
the concentration of each component and the sensor readings at each temperature level.
The procedure for reducing the data into corr21ations involved two steps. First, the tempera~ure effect on the detec or responces was de~ermined. This allowed the final rsgression to be made on temperature co~pensated data. This approach was chosen based on the eventual ~i~ld application of the analyzer. In ~he ~i~ld, ~he te~perature compensation could possibly be performed prior to data trans~ission.
Examination of the white liquor data indica~es that the temperature e~fec~ on both density and conductivity i5 approximately linear over the range 70-90C. The values at 80C were used as the re~erence values. The da~ at o~her tempera~ures were adjusted to the reference. The linear slope relating denslty divided by re~erence density to ~emperature tooX on , W~91/17305 PCT/US9lJ02933 ,7 values between -4.6X10-4 and -5.4X104Cl. The slope was found to be a linear function of the reference density.
The regression equation is:
Dr a DT t13) [m Dr + n]~T - 80) + 1 where t = temperature (C) DT = density at temperature T (g/1) D, = density at reference temperature (g/1) m = 3.9752Xl07 15 n = -9.3618X10-4 This relationship can be expressed as a quadratic ~quation in Dr. Thus, Dr can be solved for explicitly using the quadratic formula:
Dr = -b ~ ~(b2-4ac) (14) where 2a 25 a = m (T ~ 80) b = n (T - 80) ~ l C = ~ DT
The conductivity divided by reference 30 conductivity data exhibited slopes between 8.6X103 to 11.5X103C1 which were also a linear function of the reference density. Since the reference density can be calculated by equa~ion (14), the regression equation is:
C, ~ CT, _ (15) [m Dr + n~ (T ~ 80j ~ 1 where0 T = te~perature ( C ) CT = conductivity at temperature T (mS/cm) C, = conductivity at reference temperature (mS/cm) m = 2 . 9106X10~S
n = -2 . 2055X10~2 The liquor tempera~ure i not a signi~icant factor in 'che sulfid~ electrode response for the white liquor solutions in the range of 70-90C. T}~e da~a -2 O~ 3~ f~7 indicat~s that there is a small deviation ~f the electrode response within this region, but no trend was observed with temperature.
The co~plete set of data adjusted o the reference temperature was analyzed using stepwi e multiple linear regression to obtain best regression equations for the component concen rations. The relationship between sulfide concen~ration and sul~ide electrode voltage is logarithmic:
V = V0 + B ln(X) (16) where V = ~ulfide electrode voltage (m~) V0 = reference potential (mV) B - electrode slop (mV/decade) X = sulfide activity (M) The activity coefficient relating the activity and the concentration is dependent upon the total ionic strength o~ the liquor b~ing measured. This indicat~s that additional terms involving the other components in the liquor ~ay be required to adequately fit the electrode response to measured sulfide concentration.
The ~est reqression equations for sulfide concentration in the white liquor composition range are:
ln(S) = -~0.2719 t ~.7718X102 V + 9.5084Xl03 Cr

- 6.8623Xl06 Cr2 (17) whe~e S = sodium sulfide conc~n~ra~ion (g/l Na20) V a sul~ide electrode voltage (-m~) Cr ~ re~erence conductivity (mS/cm) and ln(S) ~ --39.18~3 + 4.6sl8xl0 2 V ~ 8.3142Xlo 3 Cr -- 6.0181X10 6 Cr2 (1~) WO~1/1730~ PCT/US91/02933 -21- ~r'~

Equation (17) is the best fit for liquor only at 800C, equation (18) is best for the te~perature range 70-90oc.
These correlations indica~e that the basic relationship between electrode voltage and sul~ide concentration is exponential with some correction for ionic strength effects. The choice of conductivity for ionic strength correction was made by xamination of residuals obtained by fitting only ~he ~lec~rode voltage. The co~ductivity showed a clear trend with the residuals. The sodium hydroxide ion concentration also showed a trend, but it is not a measured variable. No other correction term, including sodium hydroxide, provided a significant regression improvement after conductivity was included.
The prediction ability of the sulfide electrode is shown in ~igure 6. The error at R0C
expressed as a 90% confidence interval is approximately ~2.3 g/l, while that over 70-90C is nearly +3 g/l.
Both errors are substantial.
The regr~ssions involving sodium hydroxide and sodium carbonate were carried out for three di~ferent cases. In the first case, the sodium sulfide concentration was assumed to be known with accuracy corresponding to that of the liquor titration. The second case involved regression of the data at 80C
u~ing equation (17) ~or sul~ide prediction. The third case was a regression of all data using equation ~183 for sul~ide prediction. The equation form de~ermined by stepwise regression to be optimum was:
o~ or CO3 - a ~ b Df ~ C ~r + d Dr (19) Th~ regression coe~icients for both ~aOH and Na2CO3 in all ~hree cases are shown in Table ~. The . ..

WO91/173G5 P-,~/US91~Z933 coefficients are of the same order of magnitude for each case, reflecting the reasonable fit of the sulfide data as compared to the known values. The accuracy of the predictions as 90% con~idence interYals for each compon~nt are shown for each case in Table 9.

Regr2ssion Coefficients for NaOH and Na2CO3 ~rediction Using the In-sitll Liquor Analyzer Sodium Hydroxide Case a b c d e f Xlo~2 Xl03 Xl0-4 . ~
1 2851.68 -5.2877 ~3.184S 2.4756 1~3644 -5.7230 2 2310.31 -4.287~ -9.0360 2.0341 1.728g ~6.2667 3 2459.77 -4.5606 -7.0792 2.1521 1.6000 -~.9363 Sodium Carbonate 25 Case a b c d e f x~o-2 Xl03 Xl04 XlO1 1 -2546.~3 4.1842 -~.4066 -1.6334 -7.314~ -2.6597 2 -2595.0~ 4.2732 -3.1885 --1.6707 -6.9820 -~.7991 3 -2733.89 4.5340 -4.3062 -1.7902 -6.1359 -2.70~7 TA~LE 9 Prediction Errors (g/e) for ~ite Liquor Using th~ ~n-situ Liquor Analyzer Case NaOH Na2S Na2CO3 . _ _ _ _ _ _ _ _ _ _ n _ _ _ _ _ 1 0.58 - 0.51 2 0.97 2.30 0.71 3 1.61 2.9~ 1.17 WO91/1730S PCr/US91~2933 .J, ~ ,b ~J ~7 It is clear that the prediction ability is excell~nt if the sulfide concen~ation i5 known. The errors in this case are significantly less than l g/l over the entire temperature range. The large error in sulfide pre~iction clearly degrades the other predictions. Simulation was used to examin~ the ef~ect of a smaller sulfide error on the prediction of sodium hydroxide and sodium carbonate. ~he sulfide error was represented by a normal distribution N(0,0.25). This distribution produoes a sulfide error of approximately l g/l at 95% confidence. Trials using the correlations obtain2d Por case l indicate that the errors in NaOH and Na2CO3 prediction would not be inflatsd sisni~ica~tly at this level of sulfide error. Thus, if a more accurate detector for sulfide detection was available, the in-situ analyzer should perform as well as the extractive sample analyzer with white liquor. ~he results for all three cases are shown in Fi~ures 7 and 8.
A similar set of experiments was run for green liquor type ~olutions. In this case, 12 experimen~s w~re run to establish correlations.
- The procedure for data regression parallels that of th2 white liquor. Examination of the data indicated that the temperakure effect on both density 25 a~d conductivity was linear ~nd the slope was dependent on tAa re~rence den~ity. The equation for density te~perature compensation is:
D - -b + ~b2-4ac) t20) r _ 2a where a = In (T - 89) b - n (T - 80) ~ 1 m - 1, 027~Xl0'6 n = 1. 6784X10 3 W091/17305 PC~/US91/02933 -~4~ 7 The equation for conductivity temperature compensation is:
Cr = T (21) where T = Temperature (C) C~ = conductivity at ~emperature T (mS/cm) C, = conductivity at reference temperature (mS/cm) m = 1.479~X10-s n = -5.862lX10-3 The sulfide electrode response was regressed as an exponential correlation at 80C, and the best fit equation is:
ln(S) = -21.1551 ~ 3.6220Xl02 V - 5.8771X103 Dr (22) where s = sodium sulfide concentration (g/l Na20) V = sulfid~ electrode ~oltage (-mV) D, = refer~nce density (mS/cm) Equation (22) is similar ~o equation fl7) for white liquor. However, the ionic strength correction in this case is density rather ~han conductivity. This dependence reflects the ef~ect of sodium carbonate, the primary green liquor component, on density elevation rather than conductivity elevation. The standard deviation o~ the prediction with equation (22) is 2.54 g/l, re~ulting in a 90~ confidence interval of ~4.60 g/l for sodiu~ ~ulfide.
The regressions ~or sodium hydroxide and sodium carbonate were carried out ~or ~wo cases. Firs~, the sulfide conc~ntration was assumed to be known with titration accuracy. Second, the dat~ at 80C was used with equation (2Z) ~or sul~lde prediction. Equation (19) was ~ound to best represent the sensor response to the gr~en liquor solutions. The reqression coefficients WO91/17305 PCr/US91/02933 -25- 2~

for both NaOH and Na2CO3 in both cases are shown in Ta~le l0. The accuracy of the predictions as 90%
confidence intervals for each compcnent are shown for each case in Table ll.
TABLE l0 Regression Coe~ficients for NaOH and NazCO3 Prediction Using the In-situ Liquor Analyzer on Green Liquor Sodium Hydroxide Case a b c d e f Xl01 Xl0-3 Xl0-5 Xl01 l2608.g0 -4.6430 l.3885 2.0328 7.~224 -6.8430 22056.60 -3.7289 2.2~89 l.6464 -6.0929 -6.14~5 ~
Sodium Carbonate Case a b c d e f xlo~2 Xl0'4 XlOs xlo~l l-1738.50 2q533l 9.8034 -7.7130 -7.6584 ~2.0~2 2-1958.50 2.9015 -7.0751 -9.2~09 -l 7 09~3 -2.7331 TABLE ll Pxediction Errors for Green Liquor Using the In-situ Liquor Analyzer Case NaO~ Na2S Na2C~3 ~
l 0.~6 -- 0.61 2 2.70 4.60 0.94 As with white liquor, the prediction ability is excellent if the sul~ide concentra~ion is known.

WO~1/17305 PCT/US~1/02933 -26- 2~ 7 However, due to the presently available sulfide electrode, the sulfide predictions adversely ~ffect the NaOH and Na2CO3 predictions when the correlation is used. If the sodium sul~ide measurement could be made S with aocuracy of +l.0 g/l or better, then sodium hydroxide and sodium carbonate m~asurements would be within 90% confidenc~ intervals of +0.7 g/l.
Mathematical manipulation of sodium sulfide data shows this to be trueO The Na2S prediction results are shown in Figure 9. The results ~or NaOH and Na2CO3 are shown in Figures l0 and ll.
S~A~,Y
The novel extractive sample liquor analyzer o~
the present invention has the ability to analyze kraft white and green liquor samples for sodium hydroxide, sodium sulfide, and sodium carbonate concentrations with accuracy comparable to titration. The in-situ liquor analyzer also has comparable accuracy if a reliable sulfide electrode that can withstand the continuous hostile en~ironment is developed. The design o~ both types of analyzers permits handling of other pulp mill liquors based on the same components such as soda, soda-AQ, ~eutral and alkaline sulfit~, and controlled alkali ssmi-chemical liquors. It will be further understood th~t the analyzer of this invention may be used in processes other than paper pulping processes.
The liquor analyzer o~ the pres~nt invention is suitabl~ for use in both eed forward and feed back control ~yste~s. Many types of control systems can be configured to help control ~he concentra~ion o~ the green liquor in a kraft paper proc~ss. One simple control sys~em is illustrated in Figure l~. C~ntrol ~or a causticizer 80 includes analysis of the green li~uor stream 82 entering the caustic~zer and the liquor stream WO91/17305 PCr/USgl/0~933 -27~ 't~

~4 exiting the causticizer~ The addition of lime 86 is re~ulated by a computer control system 88 which uses the data received from detectors 90.
The minimum analysis time for the extractive sample liquor analyzer is approximately three minutes.
The 90% confidence intervals for white and green liquors are approximately +0.5 g/l f or sodium hydroxide and sodium sulfide, and ~0.8 g/l for sodium carbona~e expressed as equivalents of Na20.
The analyzer o~ this invention has featur~s which make it advantageous as compared to current types of analysis. One advantage is speed. The time for analysis is approximately three minutes. This is signi~icantly faster than automatic titrators or ion lS chromatography~ Another advantage is that a minimal amount o~ maintenance is required. There is no sensitive chromatography column that must be periodically regenerated or replaced. There are no chemical reagents required that must be prepared and standardized. ~he accuracy of analysis is also good over a wide range of operating conditions.
- The in-situ liquor analyzer, speci~ically the conductivity and density portion, has the advantages of co~tinuous liquor monitoring and simplicity of design.
The question of temperature compensation has been an~wered, and the accuracy could be comparable to that o~ the extractive sample analyzer once a s~lfide electrode is developed to withstand the harsh en~ironment for an extended period of time.
Although the present inv~ntion has been described with re~er~nce to preferred embodiments, workers sk~lled in the art will ~cognize that changes may be made in ~orm and detail without departing from the spirit and scope of the invention.

Claims (21)

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

1. A method for determining information characteristic of concentration of each of at least three components that are intermixed in a homogeneous solution, the method comprising:
identifying detectable characteristics that are quantitatively detectable in relation to the concentration of the components;
developing a mathematical relationship between the components and the characteristics with the characteristics employed as independent variables;
analyzing the solution to obtain the quantitative data of each of the detectable characteristics; and employing the quantitative data in the mathematical relationship to obtain the information characteristic of the concentration of each of the components.

2. The method of claim l wherein a regression analysis is used to obtain the mathematical relationship.

3. The method of claim l wherein the solution is a kraft pulping process liquor and the components for which characteristics are identified include sodium hydroxide, sodium carbonate, and sodium sulfide.

4. The method of claim 3 wherein the characteristics that are identified include UV
absorption, conductivity, and refractive index.

5. The method of claim 1 wherein the characteristics that are identified include conductivity, density, and sulfide ion concentration.

6. The method of claim 3 wherein the solution is analyzed by extracting a sample.

7. The method of claim 3 wherein the solution is analyzed by passing the solution on a continuous basis passed detectors that detect the characteristics.

8. The method of claim 1 and further including:
controlling the concentrations of the three components by utilizing the obtained information characteristic of the concentration of each of the components.

9. A method for determining information characteristic of concentration of each of at least three components intermixed in a homogeneous solution, each component being detectable by at least one detectable characteristic and more than one of the characteristics being associated with more than one of the components, the method comprising:
determining the relationship of the three components by employing the characteristics as independent variables in a regression analysis;
passing the solution past detectors, the detectors adapted to detect each characteristic;
analyzing each of the characteristics to obtain a quantifiable value for each characteristic; and incorporating the obtained quantifiable values for each characteristic in equations obtained through the regression analysis to obtain the information characteristic of the concentration of each of the components.

10. The method of claim 9 wherein the relationship that is determined is between sodium carbonate, sodium hydroxide, and sodium sulfide.

11. The method of claim 10 wherein the detectors detect UV absorption, conductivity, and refractive index.

12. The method of claim 10 wherein the detectors detect conductivity, density, and sulfide ion concentration.

13. The method of claim 10 wherein a sample of the solution is extracted.

14. The method of claim 10 wherein the solution is continuously passed past the detectors.

15. The method of claim 9 and further including:
controlling the concentration of each on the three components by utilizing the obtained information characteristic of the concentration of each of the components.

16. An apparatus for detecting the concentration of each of at least three components in a homogeneous solution, the apparatus comprising:
detecting means for detecting each of the three components and for providing a signal that is representative of the concentration of those components in solution;
computing means for receiving and storing said signal; and a mathematical expression that relates the characteristics as independent variables and is constructed such that when quantifiable data is employed in the mathematical expression, the solution includes information characteristic of concentration of each of the components.

17. The apparatus of claim 16 wherein the detecting means includes a conductivity detector, a density detector, and sulfide ion concentration detector.

18. The apparatus of claim 16 wherein the detecting means includes a UV absorption detector, a conductivity detector, and a refractive index detector.

19. The apparatus of claim 16 and further including valving means and conduit means for extracting a single sample of solution.

20. The apparatus of claim 16 and further including valving and conduit means for obtaining a continuous sample of the solution.

21. The apparatus of claim 16 and further including control means for controlling the relative concentration of each of the three components by utilizing the information characteristic of the concentration of each of the components.