Journal
CANADIAN JOURNAL OF CHEMICAL ENGINEERING
Volume 89, Issue 5, Pages 985-995Publisher
WILEY
DOI: 10.1002/cjce.20503
Keywords
mixing; cavern formation; pulp fibre suspensions; yield stress; impeller off-wall clearance; electrical resistance tomography
Categories
Funding
- NSERC
- FPInnovations (Paprican Division)
- Dynamic Mixing Group at UBC
- AbitibiBowater
- Canadian Forest Products Ltd
- Catalyst Paper Corporation
- Domtar Inc.
- Entech/Emerson Process Management
- Tolko Industries Ltd
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For a side-entering impeller of diameter D, the clearance from the off-wall (E) is characterised by E/D and it affects the mixing quality attained in a pulp stock chest. For agitation of pulp suspensions, mixing can be characterised by the size of cavern produced by the impeller. In this work, we have investigated cavern size as a function of the impeller off-wall clearance in a laboratory-scale cylindrical stock chest. Hardwood pulp suspensions of C-m = 2%, 3%, and 4% (fibre mass concentration) were agitated using an axial flow impeller with E/D varied from 0.14 to 0.68. Cavern size was measured using electrical resistance tomography (ERT) in batch operation and dynamic mixing tests in continuous operation, with cavern size increasing with increasing E/D. At E/D = 0.14, throttling of the impeller suction occurred which reduced cavern size. Computational fluid dynamic (CFD) simulations for steady state operation under-predicted the cavern size, but correctly captured the trend in cavern size variation with E/D. The measured cavern volumes compared well with predictions of an axial force model that accounted for interaction between the cavern and the vessel walls only when impeller throttling was absent.
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