Measuring moisture, fiber, and titanium dioxide in pulp with impedance spectroscopy

In a new measurement technique, impedance spectroscopy is used with fringing electric field sensors to measure the concentration of moisture, fiber, and titanium dioxide in pulp. This method can be integrated into a paper machine without disrupting the manufacturing process because the measurements are taken without contacting the pulp. Experimental results show consistent accuracy is when the moisture concentration is from 94% to 86% and the concentration of other constituents in the pulp is between 0% and 7%. The calibration procedure can be implemented with only a few experimental data points. Application: By accurately measuring the pulp moisture at the wet end of the paper machine, papermakers can exercise more precise control over the early stages of paper production. VOL. 4: NO. 2 TAPPI JOURNAL 23 24 TAPPI JOURNAL FEBRUARY 2005 PROCESS CONTROL EXPERIMENTAL SETUP The experiments were designed to emulate the operating conditions in a paper machine. In the wet end, the pulp is prepared so that the fibers form a homogeneous suspension in water.The moisture content of the pulp is above 98% at this stage of the manufacturing process. The pulp loses most of its moisture on the wire as a result of the vibrations from the rollers and the pressure differential caused by the aerofoils.By the end of the wire section, the pulp forms a paper web with a moisture content of about 70%. Our purpose was to measure the moisture content of the pulp while it is on the wire. Figure 2 is a photograph of the e x p e r i m e n t a l setup.The pulp was placed in the acrylic tray, and the sensor was placed beneath it.The sensor used was an interdigital sensor with a spatial periodicity of 40 mm, a finger length of 160 mm, and a penetration depth of 7 mm. A guard plane was placed underneath the sensor electrodes to shield it from external electric fields. Measured quantities of paper fiber, titanium dioxide, and water were mixed in a commercial blender.The pulp was cooled to 25°C and placed in the sensor tray.The geometry of the sensor and the electrical connections are shown in Fig. 3. The RCL meter generates 1 volt sinusoidal AC voltage in the frequency range of 50 Hz to 100 kHz. At each frequency, the meter calculates the effective impedance between the two channels by computing the magnitude of attenuation and phase shift between the input voltage and the loop current. This general-purpose instrument is good for studying the relative effects of different additives. In the future, this type of measurement could be made with multi-channel, custom-built circuits to gather data at more than one measurement point. The measurements were made at frequencies in the range of 200 Hz to 100 kHz. Ten sets of measurements were taken at each frequency and were averaged to reduce the noise. All sources of noise were assumed to have zero mean distribution. EXPERIMENTAL RESULTS Experiments were conducted to characterize the response of the sensor to the variation in moisture in a pulp consisting of water, titanium dioxide, and fiber. The titanium dioxide content of the pulp was varied from 0% to 7% in steps of 1%, and the moisture content was varied from 94% to 86%, as shown in Table I. Twenty-one electrical parameters, not all of which were independent, were measured or derived from measurements for each pulp sample at every frequency. Here, we consider only four parameters—the capacitance, the magnitude of complex admittance, the conductance, and the phase. Figure 4 shows the seven-dimensional scatter plot of the measurements of pulp samples with 0–7% TiO 2 at 100 kHz.The plot shows the variation of electrical parameters such as capacitance, admittance, conductance, and phase. It also shows the percentage concentrations of titanium dioxide, moisture, and fiber in the pulp with respect to each other. Each of these parameters is plotted against every other parameter in one of the rows of the scatter plot. For example, the first row shows the variation of capacitance against admittance, phase, conductance, and the percentage composition of the constituents of the pulp. The main advantage of representing the data this way is that the underlying correlations among measured parameters become explicit, and data trends in the experiment become more apparent. For example, in the data shown in Fig. 4, the data points at 92% and 93% moisture content are noisy. Figure 5 shows the five-dimensional scatter plot of the measurements of a pulp sample with 4% titanium dioxide, 89.28% moisture, and 6.72% fiber concentration at frequencies from 900 Hz to 100 kHz. The scatter plots in row and column (4, 5), (4, 6), and (4, 7) of Fig. 4 and (4, 5) of Fig. 5 show the dependence of conductance on the titanium dioxide I. Composition of the pulp samples. Sample TiO2, Moisture, Fiber, no. % % % 1 0 93.00 7.00 2 1 92.07 6.93 3 2 91.14 6.86 4 3 90.21 6.79 5 4 89.28 6.72 6 5 88.35 6.65 7 6 87.42 6.58 8 7 86.49 6.51 1. A fringing field dielectrometry sensor can be visualized as a parallel plate capacitor (a) with electrodes that open (b) to provide one-sided access (c) to the material being tested. 2. Photograph of the experimental