Computational fluid dynamics (CFD) has been used to analyze the flow fields generated by FDG in the quasi-stagnant configuration to allow that technology to give simultaneous measurements of deposit thickness and strength. Stress field predictions were generated by solving the governing equations using the numerical solver <i>Fastflo™</i>, and validated by comparison with experimental hydrostatic pressure measurements. Moreover, an analytical approximation for the shear stresses imposed during the gauging experiments has been shown to yield reasonably good agreement with the CFD predictions. Particular predictions of note were that the surface shear stress is largest in the area directly underneath the rim of the gauging nozzle, and that shear stresses characteristic of industrial cleaning-in-place systems could be generated in these quasi-stagnant systems by suitable selection of the hydrostatic suction head and the clearance between the gauging nozzle and the surface. This enhanced FDG tool was employed to study the removal characteristics of tomato paste soils and weak calcium sulphate scale deposits. The critical shearing yield stress for the tomato paste was found to be strongly dependent on the extent of baking (ageing), approaching an asymptote as the material was transformed from a soft, malleable paste into a hard, brittle semi-solid. It was also found that after extended deposition and ageing, calcium sulphate layers could not be removed by these gauging flows, implying that liquid velocities customarily used in industry would not be able to dislodge such deposits. Polystyrene co-polymer fouling layers were fabricated in the laboratory. The kinetics and mechanisms of cleaning them from stainless steel surfaces using NaOH, MEK and TPU solvents were successfully investigated using FDG. NaOH was found to be a non-solvent across the range of experimental conditions, in the sense that the polymer films simply swelled to constant thickness after an induction period, without dissolving. In contrast, the polymer films were removed completely in MEK and TPU, more rapidly in the former case, in terms of swelling and dissolution rates, and overall cleaning time. The removal characteristics of different polymer films in MEK and TPU were shown to differ fundamentally, and an improved 'campaign' strategy for polymerization reactors is thereby identified. Experiments on pilot plant samples immersed in MEK and TPU indicated that the results from the laboratory samples are applicable to pilot plant scale.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:597594 |
| Date | January 2005 |
| Creators | Chew, Yong Min John |
| Publisher | University of Cambridge |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://www.repository.cam.ac.uk/handle/1810/251948 |
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