Fouling dynamics of SMUF on a plate heat exchanger

In this paper, we present our first experimental results related to fouling taking place at heat exchangers during heat treatment of milk. Instead of milk we used simulated milk ultrafiltrate (SMUF), which is an aqueous solution simulating the mineral composition of milk. We did the experiments in a simplified model geometry consisting of one heating plate (dimensions 32 mm by 40 mm) made of stainless steel in a flat flow channel (height 2.0 mm) in a closed flow loop. The 3D deposit structure was measured in real time with Optical Coherence Tomography (OCT) with ca. 4 μm resolution. Fouling was observed in a wide flow rate range 300 1500 ml/min (Reynolds number 250 1250). The largest amount of deposited layer was 70 g/m in 7.5 hours at flow rate 1500 ml/min. The thickness of the deposited layer was ca. 300 μm. The temperature of the heating element had to exceed 85 °C for fouling to happen. In some cases we observed detachment of deposit flocks, which was seen both in OCT images and in temporary decrease of the plate temperature (indicating improved heat transfer). The system seemed to be here in a dynamic equilibrium; reflocculation and floc detachment followed each other the plate temperature fluctuating in a range of a few degrees. The measured heat transfer from the heated plate to water flow was used to validate a CFD model that has been developed for this particular fouling process in a research study parallel to this experimental work. In the future, the model will be validated against experimental data for SMUF as well. INTRODUCTION The fouling of plant surfaces and the subsequent cleaning needed is a significant problem in various fields of the process industry (Bott 1995, Dürr and Thomason 2010). Filters, membranes and heat exchangers are especially sensitive to it, but the accumulation of deposits can take place on any surface when the conditions are favorable. Plate heat exchangers, e.g., are widely applied in dairy and other food processing industries, and are prone to fouling. The interest in studying fouling mechanisms has increased in recent years, since fouling is associated with increased costs of maintenance, downtime and decreasing energy efficiency, which is frequently compensated by oversizing of the process equipment. Fouling is a function controlled by a variety of parameters, including the geometry of the system, surface material and roughness, surface energy, interface temperature, deposit temperature, free stream velocity, and fluid characteristics. At present, the mechanisms of fouling are not yet fully understood. Monitoring the development of the conditioning film and the subsequent deposition process is thus very important in order to understand and ultimately control fouling. Fouling in dairy processes has been widely studied and the mechanisms of fouling from milk fluids is quite well understood (Visser and Jeurnink 1997). However, the behavior of the systems at ultrahigh temperature (UHT) processing, where the inorganic deposits also take place, is still not clearly understood (Sadeghinezhad et al., 2013). Particulate fouling is a serial process of transport of particles into the vicinity of the surface and the adherence on the surface. A Computational Fluid Dynamics (CFD) model, developed earlier for adhesion of calcium carbonate particles from dense particulate suspension (Ojaniemi et al. 2012), has recently been modified for dairy processes by including precipitation of calcium phosphate, and the precipitates adherence on a heated wall (Ojaniemi et. al. 2019, Puhakka et al. 2019). Such a model can be applied in predicting the likely profile of milk deposition, and in designing the plate shape, corrugation profile and surface materials in order to reduce the costs due to fouling. The experimental work described here was carried out parallel to the recent modelling efforts (Ojaniemi et. al. 2019, Puhakka et al. 2019). Measurements were done with simulated milk ultrafiltrate (SMUF), which is an aqueous solution simulating the mineral composition of milk, including the precipitating components of the CFD model. The motivation for the experimental work is two-fold, namely to make wellcontrolled measurements in order to quantify fouling of SMUF in real-time at high resolution, as well as to provide validation data for the modeling approach. EXPERIMENTAL SETUP AND MATERIALS We used a simplified model geometry consisting of one heating plate (dimensions 32 mm by 40 mm) made of stainless steel grade AISI 316L in a flat flow channel (height 2.0 mm) in a closed flow loop. Instead of milk we use simulated milk ultrafiltrate (SMUF). In what follows, we will describe the flow loop setup, the flow module where the fouling process takes place, and the materials used in this study. The flow loop The experimental setup used in this study is shown in Fig. 1. The flow is driven by gravity from a vessel through a flow module, where the fouling process takes place. The fluid level of the source vessel is 130 cm above the flow module, equating ca. 13 kPa of driving pressure. The gravity-driven setup allows steady flows without any disturbances created by conventional pumps. The volume of the SMUF solution is ca. 15 liters, and it is immersed into heat bath of ca. 15 liter of deionized water. During an experiment, the temperature of the heat bath is kept at 45 °C (calcium in the SMUF solution would precipitate at ca. 50°C). The flow discharges from the flow module through a control valve to a vessel standing on a scale. The control valve is used for adjusting the volumetric flow rate, which can reach 1500 mL/min (mean velocity 0.31 m/s inside the flow module) in the current setup. The Reynolds number for a shallow flow geometry between two parallel plates is defines as