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Topics: Viscosity, Heat transfer, Shear stress Pages: 57 (10516 words) Published: May 24, 2014
Journal of Food Engineering 109 (2012) 49–61

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Journal of Food Engineering
journal homepage: www.elsevier.com/locate/jfoodeng

A spinning disc study of fouling of cold heat transfer surfaces by gel formation from model food fat solutions
Jen-Yi Huang, Y.M. John Chew, D. Ian Wilson ⇑
Department of Chemical Engineering and Biotechnology, University of Cambridge, New Museums Site, Pembroke Street, Cambridge CB2 3RA, UK Department of Chemical Engineering, University of Bath, Claverton Down, Bath BA2 7AY, UK

a r t i c l e

i n f o

Article history:
Received 10 August 2011
Received in revised form 19 September
2011
Accepted 29 September 2011
Available online 8 October 2011
Keywords:
Crystallisation
Fats
Fouling
Freezing
Gel

a b s t r a c t
The formation of immobile gels on heat transfer surfaces (‘coring’) caused by cooling fat solutions below their cloud point was studied using a novel spinning disc apparatus (SDA). The SDA features a cooled, removable heat transfer surface with well defined heat and mass transfer characteristics. Measurements of heat flux were combined with computational fluid dynamics simulations to yield reliable estimates of the surface temperature and shear stress. Fouling studies were performed with model solutions of 5 wt.% tripalmitin in a paraffin oil operating in the ‘cold start’ mode, wherein the experiment starts with the surface colder than the steady state, simulating one mode of operating a standard ‘cold finger’ experiment. Local heat flux measurements allowed the thermal fouling resistance to be monitored: deposit mass coverage and composition were also measured. The cold surface promotes the rapid formation of an initial gel layer, followed by a period of linear fouling, and finally falling rate fouling behaviour. The linear fouling rate was relatively insensitive to temperature and shear rate, while the fouling rate in the falling rate regime was found to depend on the temperature driving force for crystallisation kinetics. The solids fraction within the deposit layer increased over the duration of a 12 h fouling test, indicating rapid ageing. The rheological properties of the deposits were highly sensitive to solids fraction. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction
The accumulation of unwanted solids on process surfaces is a persistent problem. These materials usually have low thermal conductivity, creating significant resistance to heat transfer. This fouling deposition can also cause partial or complete blockage of piping and process equipment, reducing flow rate together with increasing pressure drop. The formation of deposits from liquid fats on cold surfaces can affect both heat exchangers and distribution lines in the food sector. This ‘coring’ of distribution lines can impair product quality by contamination and by harbouring micro-organisms, as well as reducing the performance of trace heating or cooling designed to maintain the fat in a particular state. Coring is an example of crystallisation fouling, which Epstein (1983) defined as deposition resulting from solubility differences, such that solids are generated from solution at the heat transfer

Abbreviations: CFD, computational fluid dynamics; DSC, differential scanning calorimeter; NTU, number of turbidity units; PPP, tripalmitin; SDA, scanning disc apparatus.
⇑ Corresponding author at: Department of Chemical Engineering and Biotech nology, University of Cambridge, New Museums Site, Pembroke Street, Cambridge CB2 3RA, UK. Tel.: +44 1223 334791; fax: +44 1223 334976.

E-mail addresses: ian_wilson@cheng.cam.ac.uk,diw11@cam.ac.uk(D.IanWilson). 0260-8774/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2011.09.034

surface. The fouling layer can form via heterogeneous growth on the surface, as in water scaling, or by crystallisation of the higher melting point components in the solution at the cold...

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