Fouling Studies of Food Fat

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1 Fouling Studies of Food Fat Jen-Yi Huang 1, Y. M. John Chew 2, D. Ian Wilson 1* 1 Department of Chemical Engineering and Biotechnology, University of Cambridge, New Museums Site, Pembroke Street, Cambridge, CB2 3RA, UK 2 Department of Chemical Engineering, University of Bath, Building 9 West, Claverton Down, Bath, BA2 7AY, UK ABSTRACT Coring of fat melts in distribution lines occurs when flow-line temperatures reach the solution cloud point: crystallisation yields a viscous gel which can harden to a semi-solid deposit over time. The operation of a novel spinning disc apparatus (SDA) for studying freezing fouling is reported here. The SDA features cooled, removable heat transfer surfaces with well defined heat and mass transfer characteristics. Heat transfer measurements were combined with computational fluid dynamics (CFD) simulations to yield reliable estimates of the surface temperature and shear stress. Its application to food fat fouling is demonstrated by tests on a model solution of tripalmitin (PPP) in a non-crystallising paraffin solvent. Local heat flux measurements allow thermal fouling resistance to be monitored and compared with quantitative fouling models. Falling rate and asymptotic behaviour was observed, and the gels formed were found to change in composition over time. The SDA therefore affords new insights into freezing fouling behaviour. Keywords: Fouling; Crystallization; Spinning disc; Fat INTRODUCTION Fouling is the accumulation of unwanted solids on a process surfaces which impede heat transfer and increase the resistance to fluid flow. Crystallisation fouling is a broad class that is conveniently grouped into scaling and freezing according to the solubility behaviour: freezing fouling is where a deposit layer is formed on a subcooled surface via crystallisation of a pure liquid or the higher melting constituents of a multi-component solution. Freezing fouling caused by food fats can be a major problem in distribution systems and arises when flow-line surface temperatures lie below the cloud point of components in the fat. The cloud point lies below the equilibrium temperature: nucleation is followed by rapid growth and orientation of crystallites, ending up with aggregation of the crystals (Walstra et al., 2001). A number of models have been reported for crystallisation fouling (Bansal et al., 2008). However, relatively little work has been reported on food fat fouling. A closely related analogue, studied extensively, is wax formation and deposition in crude oil pipelines. Fernandez-Torres et al. (2001) constructed a fouling regime map of binary fat/solvent solutions and presented a model for deposit growth in laminar pipe flows. Fitzgerald et al. (2004) studied freezing fouling experimentally using model solutions of tripalmitin, PPP, in a non-crystallising paraffin solvent. Rapid initial fouling rates were observed under conditions of strong subcooling. They reported porous deposits consisting of a matrix of PPP platelets.

A novel apparatus is used here to study freezing fouling with model solutions similar to those used by Fitzgerald et al. The spinning disc apparatus (SDA, Nigo et al.

The geometry and flow conditions mean that mass and heat transfer fluxes to the surface are uniform over the disc surface (Grant et al., 1996; Huang, 2010), making experiments simpler to interpret.

2 A novel apparatus is used here to study freezing fouling with model solutions similar to those used by Fitzgerald et al. The spinning disc apparatus (SDA, Nigo et al., 2009) is based on a cooled, rotating, vertical cylinder and operates in the laminar flow regime. The geometry and flow conditions mean that mass and heat transfer fluxes to the surface are uniform over the disc surface (Grant et al., 1996; Huang, 2010), making experiments simpler to interpret. Spinning disc devices have been used to study heat-induced fouling (Rosmaninho and Melo, 2006) and cleaning (Grant et al., 1996), but chilled systems are rare. In addition, computational fluid dynamics simulations of the SDA were generated and their results compared with data from heat transfer tests, allowing confidence bounds to be generated for estimates of surface-fluid shear stress and temperature fields. This will allow a systematic investigation of freezing fouling processes. The gels formed in the fouling experiments were characterized to gain better understanding of their composition. MATERIALS & METHODS Spinning disc apparatus plate (a) (b) Figure 1. (a) Schematic of the spinning disc apparatus. (b) Construction of the fouling cell plate. Dimensions are in millimeters (not to scale). A detailed description of the SDA, shown schematically in Figure 1, is given in Nigo et al. (2009). Briefly, the apparatus consists of a rotating cylindrical can connected to a water bath circulating coolant (a water/glycol mixture). The can is positioned within a jacketed vessel holding the fat solution. Warm water is circulated through the water bath to regulate the bulk solution temperature. A stirrer located at the bottom of the jacketed vessel provides stirring and maintains temperature uniformity in the bulk solution. Figure 1(b) shows a schematic of the arrangement of the fouling cell plate employed in heat transfer studies. A micro-foil heat flux sensor (Rhopoint, UK, type 27160) was attached between a brass block and a removable 316 stainless-steel disc to measure the local heat flux. A description of tests characterising the heat transfer performance of SDA is given in Huang et al. (2010). Numerical Simulation The temperature and shear stress levels at the SDA surface are key parameters in freezing fouling. Computational fluid dynamics (CFD) simulations of the SDA can be performed with reasonable accuracy as the device is operated in the laminar flow regime. The commercial finite element method (FEM) software COMSOL Multiphysics (version 3.5, Chemical Engineering Module) was used for the estimation of the heat

3 transfer behaviour between the paraffin liquid and the can base plate. The flow and temperature field could be simulated by solving the continuity, the Navier-Stokes and the steady state energy equation for a Newtonian liquid. The flow was modelled as being axisymmetric, incompressible, and steady state. Physical properties, such as density, thermal conductivity and specific heat capacity, were assumed not to change significantly with temperature and are assumed uniform throughout. The physical configuration is cylindrically symmetric, implying that only one plane of the system need be considered. This study extends the numerical simulation of Nigo et al. (2009) using similar boundary conditions. Fat Deposition Studies Model solutions of PPP, the highest melting component in palm oil, in a non-crystallizing paraffin solvent were employed in this study. The PPP (>85% purity; Sigma Chemicals, UK) was dissolved in paraffin (density at 25 C, 870 kg/m 3 ; BDH Chemicals, UK) to give solution concentrations of 5 wt%. The solution cloud point was measured as 29.8 C with a ODEON turbidity meter (NEOTEK-PONSEL, France). Deposition experiments were performed with a coolant temperature, T cw, of 9.8 C; bulk temperature, T b, 60 C; and rotational speed, ω, 5.4 rad s -1. Fouling tests were stopped after 1, 3, 6, 9 and 12 hr, at which point the deposit formed was then scraped off the surface using a plastic spatula into pre-weighed clean plastic containers, weighed and stored for further analysis. Deposit Analysis The deposits contain both PPP solids and PPP solution entrained in the deposit matrix. GC analysis (HP Agilent 6890 with a dimethylpolysiloxane column) yielded the total PPP content of the deposit. The GC was calibrated and programmed using the BS EN 14105:2003 method. The solids content of the deposit was determined by filtration. About 3 g of deposit was filtered under vacuum, established by a peristaltic pump, through a 0.2 μm polytetrafluoroethylene membrane filter paper (Cole-Parmer, USA). The filtered solids were washed with hexane (analytical reagent grade, Fisher Scientific, UK) at ambient temperature to remove any entrained paraffin and then with acetone to remove traces of hexane, and allowed to dry at room temperature for an hour before weighing. RESULTS & DISCUSSSION CFD Simulations The CFD simulations yield predictions of velocity and temperature distributions in the bulk liquid in the heat transfer experiments. The coolant flow is not modelled: its contribution is expressed as a film heat transfer coefficient. Figure 2 shows the stream functions (contour lines) and the temperature profiles (coloured background) obtained for the conditions used in the deposition experiments. Two vortices are evident in the bulk liquid: an upper one driven by the rotation of the disc, and a lower one induced by the magnetic stirrer acting in the opposite direction. The relative importance of resistances in the SDA was assessed by varying the disc rotational speed.

The heat flux through the rotating disc at any point is given by Newton s law of cooling; q U Tb Tcw hb Tb Tss,out (1) and the overall heat transfer coefficient, U, is given by; 1 1 1 Relse Rcw Rw U

Once Relse is established, the difference between the Relse and the static resistance, Rw, which is a constant, is Rcw. The results are plotted in Figure 3.

transfer is on the bulk (fat) side. 1/U 1/hb Rcw Rw -1 Thermal resistance [m K W ] 0.030 2 0.025 0.020 0.015 0.010 0.005 0.000 0 5 10 15 20 25 30 35 40 45 Rer Figure 2.

4 The heat flux through the rotating disc at any point is given by Newton s law of cooling; q U Tb Tcw hb Tb Tss,out (1) and the overall heat transfer coefficient, U, is given by; Relse Rcw Rw U hb hb (2) U is obtained from the SDA experiments and hb is extracted from the simulation. Relse can then be estimated from (1/U 1/hb). Once Relse is established, the difference between the Relse and the static resistance, Rw, which is a constant, is Rcw. The results are plotted in Figure 3. The film resistance 1/hb decreases with increasing rotational Reynolds number, Rer, and is consistently higher than Rcw at all rotation speeds, indicating that the dominant resistance to heat transfer is on the bulk (fat) side. 1/U 1/hb Rcw Rw -1 Thermal resistance [m K W ] Rer Figure 2. Flow patterns and temperature profiles in the SDA for disc rotation speed of 5.4 rad s-1, Rer = 27. Black arrows are velocity vectors. Color indicates temperature, dark red = 60 C, dark blue = 9.8 C. Figure 3. Effect of Reynolds number on the thermal resistances in the SDA: circle 1/U, triangle 1/hb from simulation, square Rcw and line Rw. Temperatures as in Figure 2. The initial, clean, surface temperature could be estimated using: T T q b cw Ts Tb Tb hb R R 1 cw w hb 1 hb (3) Figure 4(a) shows the steady state average surface temperature at various coolant temperatures and disc rotational speeds. The plots show that the surface temperature is more sensitive to coolant temperature. (b) (a) Figure 4. (a) Temperature and (b) average shear stress on the disc surface, extracted from CFD simulations for various coolant temperature and can rotational speeds.

5 The shear stress imposed on the disc surface can also be calculated from the simulated velocity field (Figure 4(b)). The CFD calculations indicate that the average shear stress is of the order of Pa for the initial clean surface. Since surface temperature and shear stress are key parameters in freezing fouling, Figure 4 established the range of surface conditions that can be used in deposition studies. Fouling Experiments The experimental protocol involved in these studies involves immersing a cooled surface (with temperature close to that of the coolant) into the warm solution: steady state heat transfer is achieved after a couple of minutes. This cold start mode can induce the formation of a gel on the plate if the degree of subcooling is large, and this is evident in the fouling resistance, R f, - time plot in Figure 5. R f is calculated from R f 1 1 (4) U ( t) U ( t) The figure shows an initial step increase in R f due to the cold start, followed by falling rate behaviour and approaches an asymptote. This is consistent with surface-controlled growth: as deposit accumulates, the deposit-solution interface temperature increases and supercooling (the driving force for growth) decreases. The deposit mass data exhibit similar behaviour, indicating that the heat flux measurements provide a reliable monitor of deposit growth. clean Fouling resistance (m 2 K W -1 ) Fouling resistance Mass of deposit Deposition time (hr) Mass of deposit (g) wt% in gel deposit Deposition time (hr) Overall PPP PPP solids Figure 5. Thermal resistance and mass measured during deposition. Conditions: 5 wt% PPP solution, T cw = 9.8 C, T b = 60 C, and ω = 5.4 rad s -1. Figure 6. Variation in deposit composition with time for fouling runs in Figure 5. The associated composition data in Figure 6 show that the PPP concentration of the deposit (obtained by analysing the deposit obtained from interrupted experiments: the individual R f -t profiles showed excellent agreement) increased with time. As time proceeds, the deposit solids content increases, which could be caused by deposit ageing, and also by the change in surface conditions: as the surface temperature increases, the viscosity of the liquid phase decreases and the solids content required to form a gel strong enough to resist deformation by the surface shear stress will increase. These competing aspects of temperature have been reported for wax formation in crude oils by Jennings and Weispfennig (2005). Decoupling these factors is the subject of ongoing work.

6 CONCLUSIONS Experimental investigations of the SDA heat transfer performance were combined with CFD simulation studies to generate maps of the temperature and shear stress conditions on the SDA surface. The results confirmed that the thermal resistance across the disc was small and heat transfer was controlled by the bulk fluid. Application of the SDA to study freezing fouling was demonstrated with a model solution of PPP in a noncrystallising solvent. Fouling resistance-time profiles could be generated reliably and reproducibly, allowing fouling rate laws to be studied in depth. Deposit analyses showed that the nature of the layer changes with time, confirming the complexity of the fat freezing phenomenon. NOMENCLATURE Roman h b film heat transfer coefficient in bulk fluid (W m -2 K -1 ) q heat flux (W m -2 ) R cw thermal resistance of coolant side (m 2 K W -1 ) R else thermal resistance of components apart from bulk (m 2 K W -1 ) R f, Rf fouling resistance, asymptotic value (m 2 K W -1 ) t time (s) T b bulk fluid temperature ( C) T cw coolant temperature ( C) T s disc surface temperature ( C) U, U clean overall heat transfer coefficient, clean condition (W m -2 K -1 ) Greek τ fouling time constant (s) ω disc rotation speed (rad s -1 ) REFERENCES Bansal, B., Chen, X. D. and Müller-Steinhagen, H. (2008). Analysis of classical deposition rate law for crystallisation fouling. Chem. Eng. Proc.: Process Intensification, 47, Fernandez-Torres, M.J., Fitzgerald, A.M., Paterson, W.R. and Wilson, D.I. (2001). A theoretical study of freezing fouling: limiting behaviour based on a heat and mass transfer analysis. Chem. Eng. Process., 40, Fitzgerald, A. M., Barnes, J., Smart, I. and Wilson, D. I. (2004). A model experimental study of coring by palm oil fats in distribution lines. Food Bioprod. Process., 82, Grant, C. S., Perka, A. T., Thomas, W. D. and Caton, R. (1996). Cleaning of solid behenic acid residue from stainless-steel surfaces. AlChE J., 42, Huang, J.-Y. (2010). Crystallisation and Gelation Behaviour of Food Fats on Cold Surfaces. CPGS Dissertation, University of Cambridge, UK. Huang, J.-Y., Chew, Y. M. J. and Wilson, D. I., (2010), Experimental studies of food fat fouling using a novel spinning disc apparatus, Fouling and Cleaning in Food Processing 2010, Cambridge, UK, Jennings, D. W. and Weispfennig, K. (2005). Effects of shear and temperature on wax deposition: Coldfinger investigation with a Gulf of Mexico crude oil. Energy Fuels, 19, Nigo, R.Y., Chew, Y.M.J., Houghton, N.E., Paterson, W.R. and Wilson, D.I. (2009). Experimental studies of freezing fouling of model food fat solutions using a novel spinning disc apparatus. Energy & Fuels, 23, Rosmaninho, R. and Melo, L. F. (2006). The effect of citrate on calcium phosphate deposition from simulated milk ultrafiltrate (SMUF) solution. J. Food Eng., 73, Walstra, P., Kloek, W. and van Vliet, T., 2001, Fat crystals networks, in Crystallization processes in fats and lipid systems, K. Sato and N. Garti, (Marcel Dekker, New York, USA) pp

THE EFFECT OF START-UP ON FAT (AND WAX) FOULING TESTS

THE EFFECT OF START-UP ON FAT (AND WAX) FOULING TESTS Proceedings of International Conference on Heat Exchanger Fouling and Cleaning - 2013 (Peer-reviewed) June 09-14, 2013, Budapest, Hungary Editors: M.R. Malayeri, H. Müller-Steinhagen and A.P. Watkinson