Concentration and Viscosity Measurements in Industrial Processes

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Two-component system with discontinuity:A successfully realized customer application is the measurement of sulfuric acid concentrations between 90% and 110% in the process. Figure 1 (see picture gallery) shows the acid’s density and sound velocity development from 0% to 110%. Both the density and sound velocity show different minima and maxima as well as a discontinuity. Therefore, a conversion into concentration based on one parameter is only partially possible. For example, sulfuric acids with concentrations from 0% to 90% are successfully measured with a density sensor, while a sound velocity sensor accurately determines sulfuric acid concentrations from 90% to 100%. By developing a specific concentration formula and combining a density sensor with a sound velocity sensor, a highly accurate concentration determination from 90% to 110% was realized.

Two-component system with reversal points:A customer request for a full-range concentration measurement of acetic acid (0% to 100%) was also successfully met. Both the density and the sound velocity reach a maximum, so the concentration measurement is only partially possible (figure 2, see picture gallery). By combining a density sensor with a sound velocity sensor and developing a specific concentration formula, the concentration of acetic acid can be determined across the entire range.

Concentration determination of three-component mixtures:Through catalytic oxidation, methanol is turned into formaldehyde and water. To monitor this reaction, methanol and formaldehyde concentrations are determined. The concentration of three-component mixtures can only be determined by means of two independent physical parameters such as density and sound velocity. An essential factor is the different behavior of the components in the solution when its concentration changes. A change to the formaldehyde in the solution has a much greater influence on density than on sound velocity. The 90° intersections featured in the diagram are an ideal prerequisite for the use of density and sound velocity as physical parameters (figure 3, see picture gallery). Employing the density and sound velocity sensor, the three-component mixture is successfully measured and/or monitored.

Viscosity determination of suspensions such as slurry:Determinations of dynamic viscosity are employed in the production of ceramic slurry, in which water, kaolin, feldspar, quartz and other additives are mixed and processed over the course of several days into a homogeneous suspension. By incrementally adding water and a liquefier, the substance can be brought to the desired viscosity. The picture below shows the change in viscosity after the addition of a liquefier. The resulting slurry is then either pressed or poured into molds. The challenge in this application is to measure the viscosity of the entire suspension, not just the liquid alone. This requirement is fully met by a rotational viscometer. Further essential factors for selecting a sensor are the abrasive character of the sample and a customer’s wish for a measurement directly in the tank. Extensive preliminary investigations in the laboratory are required to determine the rheological behavior of the suspension (figure 4, see picture gallery). The Anton Paar viscometer’s characteristics as well as rheological expertise and extensive process know-how are the basic requirements for solving tasks of this kind.

* The author is Head of Product Management for Process Instrumentation, Anton Paar, Graz/Austria.

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Concentration/Viscosity/Density Measurement