Determination of drainage resistance coefficients under conditions of known shear rate.

Application: Design of paper machine formers, drainage rates on existing paper machines, headbox slurry characterization.

The drainage rate of papermaking stock determines the design and operation of the wet end of a paper machine. For single and twin wire machines with various elements, the effectiveness of the draining process cannot be quantified unless there are reliable data for the drainage resistance of a given pulp under conditions that resemble the industrial process.

Wildfong and coworkers at the University of Maine (2000) recently reported the development of a drainage apparatus capable of determining the drainage resistance coefficients of headbox samples at very short times and drainage rates. However, in this device the pulp does not experience shear during drainage. This paper presents a new method to measure the drainage rate under known shear conditions. To generate a quantifiable shear rate over the forming fabric, an inverted cone is rotated inside a drainage jar. Electrodes continuously measure the volume of stock in the drainage jar so that drainage velocity can be estimated.

For this work, we compared the drainage resistance for three different paper grade headbox samples containing a wide range of pulp types and fines content. For one headbox sample we determined the effect of shear rate on drainage resistance. This report also covers the influence of the various constituents in lightweight coated paper on drainage rate. It characterizes forming fabrics in terms of their resistance to flow in a flow loop.

The laboratory results show that the drainage resistance coefficients, when plotted as a function of basis weight, are different for each headbox sample and the furnish constituents. In all cases the drainage resistance increases with increasing basis weight. This increase in drainage resistance may be related to pore plugging, which alters the effective void fraction of the forming mat. The drainage resistance is also different at various shear rates. For low shear rates the drainage resistance increases with increasing shear rate. At higher shear rates however, the drainage resistance decreases with increasing shear rate.

Paradis, Genco, Bousfield, and Haesler are with the Department of Chemical Engineering, University of Maine, Orono, Maine, USA; Wildfong is with Johnson Foils Corp., Springfield, Massachusetts, USA. Email Paradis at [e-mail].