Measuring Flow Velocity and Uniformity in a Model Batch Digester Using Electrical Resistance Tomography

Abstract

AbstractThe literature shows that the extent of delignification in batch digesters varies as a function of chip location in the vessel. This non‐uniformity may be exacerbated by a number of factors but is commonly attributed to poor and/or non‐uniform liquor flow through the reactor (which causes poor chemical and heat distribution to the chip mass during the cook). Electrical resistance tomography (ERT) was used to evaluate the uniformity of liquor flow in a laboratory model digester under scaled industrial conditions (a 1:15 geometrically scaled vessel, a vessel to particle diameter ratio of 93:1 to minimize wall effects, and close approximation of liquor superficial velocity and pore Reynolds number). Local interstitial flow velocities were measured using pixel‐pixel cross correlation techniques. It was possible to create uniform zones in the digester, but a stagnation point was observed in the centre of the vessel at the screen level. This point coincides with the location of highest kappa numbers (lowest degree of cooking) reported in industrial tests.