Moisture Measurement in Paper Pulp Using Fringing Field….doc

This work was supported in part by the Center for Process Analytical Chemistry, NSF CAREER Grant # 0093716, and Metso Automation. K. Sundararajan is with the University of Washington, Seattle, WA 98195 USA. Phone: 206-221-6673; fax: 206-543-3842; email: kishoreee.washington.eduL. Byrd II is with the University of Washington, Seattle, WA 98195 USA. email: leibu.washington.eduA.V. Mamishev is with the University of Washington, Seattle, WA 98195 USA. email: mamishevee.washington.eduEstimation of Moisture Content in Paper Pulp Containing Titanium Dioxide Using Interdigital Fringing Field Impedance SpectroscopyKishore Sundara-Rajan, Leslie Byrd II, and Alexander. V. MamishevAbstract Currently used methods for estimation of moisture content in paper pulp is restricted to levels of moisture concentration under 90%, and also assume that there are no additives in the pulp. This paper presents a technique that uses fringing field interdigital sensors to measure moisture concentration in paper pulp at levels as high as 94% in the presence of titanium dioxide. The method proposed in this paper uses single-sided measurements, offers high sensitivity, and does not require special operating conditions. The accuracy of the proposed method is also demonstrated. Index Terms Additives, impedance spectroscopy, fringing electric field, moisture measurement, paper pulp.I. INTRODUCTIONPaper manufacturers are looking for non-invasive, non-contact sensing technologies that can accurately measure the fiber content of paper pulp at the wet end of the paper machine. The fiber content of the paper pulp at the wet end ranges from 1% to 30%. This low concentration of fiber in the pulp makes it hard to detect concentration fluctuations with adequate resolution. In addition to fiber and water, the paper pulp at the wet end contains high quantities of chemical additives 1. One of the most commonly used additives is titanium dioxide. It is used as whitening agent in common paper, and sometimes as filler in very high quality paper. References 1-4 discuss the uses of titanium dioxide in paper manufacturing in detail.Microwave techniques 5-9, electromagnetic field perturbation 10,11, and a few other electrical methods 12-14 are currently used to measure moisture in paper pulp. These techniques have been analyzed in detail in 15.Fringing field impedance spectroscopy is a sensing technology that could be used to estimate the moisture content of the paper pulp at the wet end of a paper machine 16. Interdigital fringing field sensors were used for the experiments reported in this paper.The interdigital fringing field sensor operates in a way that is very similar to a conventional parallel plate capacitor. Fig. 1 shows the transition from a parallel plate capacitor to a fringing field sensor. It can be seen from Fig. 1 that the electric field lines always penetrate the bulk of the material under test, irrespective of the position of the electrodes. Hence, in addition to the electrode geometry, the capacitance between the electrodes also depends on the materials dielectric properties and geometry.Fig. 1. A fringing field dielectrometry sensor can be visualized as a parallel plate capacitor whose electrodes open up to provide a one-sided access to material under test.As seen from Fig. 1(c), the electrodes of a fringing field sensor are coplanar. Hence, the signal-to-noise ratio of measured capacitance is considerably low that in the case of Fig. 1(a). To strengthen the measured signal, the electrode pattern can be repeated several times. The resulting structure of the sensor is known as an interdigital structure. The term “interdigital refers to a digit-like or finger-like periodic pattern of parallel in-plane electrodes used to build up the capacitance associated with the electric fields that penetrate into a material sample 17.Fig. 2 shows a generic interdigital sensor. The wavelength of the sensor is defined as the distance between the centers of two adjacent electrodes of the same type. For a semi-infinite homogeneous medium placed on the surface of the sensor, the periodic variation of the electric potential along the X-axis, creates an exponentially decaying electric field along the Z-axis, which penetrates the medium. The possible variation in the properties of the material under test along the Z-axis, and hence is complex dielectric permittivity, *(), is schematically represented in Fig. 2 by the variation in shading. The model for analyzing such multi-layered systems is discussed in detail in 17.Fig. 2. A generic interdigital sensor with a periodicity 18,19.II. Experimental SetupThe experiments reported in this paper emulate the operational conditions in a paper machine. The pulp in the wet end of the paper machine is primarily a suspension. This pulp suspension is spread onto a semi-permeable membrane made of nylon or similar polymer, and is unavailable for contact measurements. To emulate this setup in the laboratory, the pulp is blended to a consistency of a suspension and is placed on a tray. The tray wall prevents contact with the pulp, and hence is equivalent to the wire on the paper machine.The sensor used for these measurements is an interdigital sensor tray with a spatial periodicity of 40 mm, finger length of 160 mm, and penetration depth of 7 mm. The sensor electrodes are not in direct contact with paper pulp. Instead, the sensor is attached to the outer side of the base of an acrylic tray with a wall thickness of 5 mm. A guard plane is placed underneath the sensor electrodes to provide shielding from external electric fields. The geometry of the sensor is shown in Fig. 3.Measurements reported here were taken using the Fluke manufactured RCL meter (model PM 6304). It generates a one-volt sinusoidal AC voltage in the frequency range from 50 Hz to 100 kHz.Known quantities of paper, titanium dioxide and water are mixed in a commercial blender to obtain the paper pulp. The pulp is then cooled to ambient temperature of 25°C. The moisture loss due to evaporation can be neglected, as the loss is small compared to the total water content in the pulp. The prepared pulp is then placed in the sensor tray. The homogeneity of spatial distribution of the pulp and reduction in the number of air pockets in the bulk of the pulp are achieved by manually rearranging the pulp in the tray. The interdigital sensor tray filled with paper pulp is then connected to the two channels of the RCL meter and measurements are made.Fig. 3. The top-down view of the interdigital sensor tray with the spatial periodicity of 40 mm, finger length of 160 mm and an approximate penetration depth of 13 mm.The interdigital sensor tray filled with paper pulp is connected to the two channels of the RCL meter. The RCL meter calculates the effective impedance between the two channels by computing the magnitude attenuation and phase shift between the input voltage and loop current. The measurements are made at frequencies in the range of 200 Hz to 100 kHz. The measurements made at the lower end of the frequency spectrum (below 200 Hz) have noise due to the AC power supply. The instrumentation limits the highest viable frequency to 100 kHz. Ten sets of measurements were taken at each frequency, and then averaged to reduce the noise. It is assumed that all sources of noise have zero mean distribution.III. Experimental ResultsExperiments were conducted to characterize the response of the sensor to the variation of moisture level in three-component pulp consisting of water, titanium dioxide and fibers. The titanium dioxide content of the pulp was varied from 0% to 7% in steps of 1%, and the moisture content is varied from 94% to 86%, and measurements were made using the setup described in Section II.Fig. 4(a) shows the dependence of admittance on the titanium dioxide concentration, moisture content, and frequency. The variation in admittance with moisture content is not well pronounced.Fig. 4(b) shows the dependence of phase on the titanium dioxide concentration, moisture content, and frequency. There are cross-overs in the phase plots at various frequencies and moisture levels. This is partly due to instrumentation errors and also due to the fact that the phase is highly sensitive to noise. Hence the phase shifts at two frequencies cannot be used with the frequency range under consideration to estimate the moisture content of the pulp as suggested in 9.Fig. 4. Measurements of paper pulp samples with 0% to 7% titanium dioxide concentration at frequencies from 200 Hz to 100 kHz.Fig. 4(c) shows the dependence of capacitance on the titanium dioxide concentration, moisture content, and excitation frequency. It can be seen that the variation is monotonous and strictly increasing with frequency and moisture content.Fig. 4(d) shows the dependence of conductance on the titanium dioxide concentration, moisture content, and frequency. The spatial separation of the curves is not adequate to mitigate the effect of any small inaccuracies in the measurement of conductance. This may explain the high error percentages reported in 13,14,20.IV. Data AnalysisThe variations in capacitance, conductance and other electrical parameters are influenced by all the three components of the pulp, namely, paper fiber, titanium dioxide, and moisture. Since two independent variables are involved here, it is not possible to estimate the fiber concentration using a single parameter as in 16. So we solve the inverse problem, by estimating any three of the electrical parameters as(1)where X, Y and Z are the electrical parameters estimated using fiber concentration p, titanium dioxide concentration t, moisture content w, and constants m11, m12, m13m33 and C1, C2, and C3.Once the constants are determined, the parameters X, Y and Z can be used to estimate the concentrations of water, titanium dioxide, and fiber in the pulp using (2), (3), and (4) respectively.(2)(3)(4)where,(5)(6)(7)(8)(9)(10)(11)(12)The key to the success of the estimation is in the choice of the parameters X, Y and Z; the constants m11, m12, m13m33; and C1, C2, and C3.Fig. 5, Fig. 6, and Fig. 7, respectively, compare the concentrations of fiber, titanium dioxide, and moisture as obtained using the method described above. The estimates were based on the measured phase, capacitance and conductance. These parameters were chosen manually.Fig. 5. Comparison of the estimated concentration of fiber in the pulp to the actual concentration.Fig. 6. Comparison of the estimated concentration of titanium dioxide in the pulp to the actual concentration.Fig. 7. Comparison of the estimated concentration of moisture in the pulp to the actual concentration.V. Blind TestsTo validate the estimation algorithms presented in Section IV, blind data tests were conducted. The algorithm was trained using the data from a single experiment. One of the data points obtained was omitted in the training data set. Hence, for the purpose of evaluation, the omitted data point serves as a blind data point. The entire data from the experiment is then provided to the estimation algorithms, and the estimated moisture content is compared to the actual moisture content. Fig. 8 shows the result of the validation tests performed.Fig. 8. Validation of estimation process described in Section IV.VI. ConclusionThe ability of the sensor to accurately measure the moisture content in pulp in the presence of calcium carbonate has been demonstrated. The estimation algorithm can be made more efficient by incorporating chemometric tools such as PLS and PCA. Apart from improving the algorithm, the effect of temperature variation, pulp variations, and pass line sensitivity will be studied as the next step.VII. AcknowledgmentThis work was supported by Center for Process Analytical Chemistry, NSF CAREER Grant 0093716, EEIC, NESBI scholarship, and McNealy scholarship. The authors would like to thank Dr. Mahendra Munidasa for his valuable guidance and inputs. Also, special thanks goes out to undergraduate students Alexei Zyuzin, Nick Semenyuk, and Chuck Wai-Mak for their assistance with experimental aspects of this research work.VIII. References1 J. V. R. Reddy and G. R. 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