Mixing and Dynamics of Agitated Pulp Stock Chests

The range of mixing applications in pulp and paper manufacture is extensive. These include examples from liquid - liquid to gas - solid - liquid mixing, from the blending of complex fluids to the mixing of reactive chemicals. Prof. Bennington's work over the years covered many of these applications. In particular, in the last 10 years his group focused extensively on macro-scale mixing operations accomplished in agitated pulp stock chests.

Agitated pulp chests are the most common mixer used in pulp and paper manufacture. They serve a number of purposes, including blending of pulp suspensions, chemical contacting, consistency control, modification of fibre properties and attenuation of process disturbances. Although stock chests are extensively used in industry, only limited information has been published on pulp agitation and their design is based mainly on past experience using proprietary information based largely on empirical correlations. The dynamic behavior of pulp stock chests has received even less study, restricted to model studies based on ideally mixed tanks. However, measurements made in the mill and on laboratory-scale chests show that the dynamic behavior of stock mixing chests is far from ideal and the design criteria used in these installations are insufficient in many cases.

One significant difficulty in the design process is specification of the pulp suspension rheology. Pulp fibre suspensions are non-Newtonian and display a yield stress, with their flow behaviour significantly dependant on the suspension mass concentration. In past work the suspension rheology has been characterized using 'factors' which are estimated based on pulp type and past experience or determined after the fact.

The main objective of our work has been to develop enhanced understanding of the mixing process occurring in these agitated chests and to move towards an actual design procedure with tangible physical interpretation. To achieve this goal, a broad number of current installations' strategies were surveyed and the characteristics of these designs evaluated based on pilot-scale and industrial-scale dynamic measurements. We developed methods to identify non-idealities in stock chests and used them to evaluate the mixing quality achieved. The latter is closely related to the physical properties of the suspension. Hence, significant efforts have been directed towards developing better models of the suspension rheology.

A number of advanced experimental techniques including electrical resistance tomography, ultrasonic velocity profiling and tracer injection tests have been used at the laboratory and industrial scales to better understand the underlying phenomena behind the non-ideal flows measured dynamically. In addition, computational fluid dynamics simulations have been implemented at both laboratory and industrial scales allowing us to examine issues of chest scale-up and assess the performance of current installations.

Finally, a design method has been proposed based on the desired cavern size formed in the vessel by matching the momentum generated by the impeller(s) with the yield stress of the suspension. This procedure has been compared with other design methods...

...at this point, Prof. Bennington died very suddenly at the age of 53. Feb 14, 2010.