In-line viscosity measurement in paper/pulp mills
March 2001
Sensors & Transducers
The measurement of viscosity of liquor when it is pumped into a black liquor recovery boiler has become more important during the last few years. Controlling the viscosity of the liquor has always been used to adjust the drop size of the liquor as it is sprayed into the boiler and this has traditionally been achieved by controlling the liquor temperature. Development of recovery boilers has allowed the incoming liquor dry solids content to be increased from 65-75% or even 80%. At these high levels the viscosity concentration curve becomes very steep and a change in the dry solids content will give a very large change in the viscosity, making temperature measurement insufficient to control the viscosity. A viscosity meter must be installed to provide a reliable control signal.
AFH Devers markets the Nametre viscometer to measure the viscosity of many different types of media including white, green and black liquor. There are no moving parts, the sensor is rugged and easy to clean and maintain. The viscometer uses a vibrating head that is in direct contact with the media. There is a choice of transducers available to suit the viscosity range of the media.
In one existing application for black liquor, the sensors are mounted directly in-line just before the liquor firing guns. Mounting the sensors in vertical lines gives more reliable results as evidence has shown that horizontal lines may not be completely filled and air pockets often form above the liquid causing a build-up of dried liquor on the sensor.
It is often difficult to recommend a viscosity level for efficient control purposes. The best method is to note the viscosity reading giving the best firing and reduction efficiencies of the boiler, and use this as a setpoint for the control loop. Each mill will have a liquor with its own specific characteristics depending upon the wood being pulped and therefore it is unlikely that a viscosity setpoint for one mill can be used as a setpoint for another mill.
The Nametre is able to provide fast in-line viscosity measurements for the white, green and black liquors, adhesives, polymers, syrups, resins, waxes - and many more.
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