On-line fouling/cleaning detection by measuring electric resistance––equipment development and application to milk fouling detection and chemical cleaning monitoring Xiao Dong Chen * , Dolly X.Y. Li, Sean X.Q. Lin, Necati € Ozkan Department of Chemical and Materials Engineering, The University of Auckland, Private Bag 92019, Auckland, New Zealand Received 29 March 2002; accepted 12 March 2003 Abstract An electrical resistance method, which has the potential to measure the extent of soft material fouling such as milk fouling on the surface of process equipment in situ and in real time, is described. An experimental fouling unit with the appropriate attachments has been devised and used to monitor the fouling build-up using the electrical resistance method. Reconstituted skim milks with solid contents of 10–30 wt.% were used to produce milk foulings, and these milk foulings were cleaned using a cleaning solution with 0.5 wt.% NaOH. Using the fouling unit, it was possible to measure the thermal resistance (essentially measuring heat flux) and electrical resistance simultaneously. As a result, the relationship between the electrical resistance and the thermal resistance during both fouling build-ups and cleaning processes was established. It has been shown that this technique is effective for measuring the extent of fouling, and it has the potential to be modified further so that it can be adopted in real process industries. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Fouling; Cleaning; Soft-deposit; Process monitoring 1. Introduction Monitoring fouling and cleaning can provide useful information for making operational decisions in food processing plants. Fouling is usually not visible from the outside of the industrial processing equipment, and can only be ascertained from significant effects, such as pressure drop measurement, which may not be sensitive enough to pick up small and local deposition. Fouling deposits in food and bioproduct processing plants contain significant proportions of liquid and they are often soft and fragile. As a result, they can be de- formed easily by contact, so that their thickness may not be accurately measured with conventional mechanical instruments. Furthermore, the tendency of biological fouling deposits to shrink or slump outside their natural environment makes it difficult to gauge them satis- factorily in any other location (Tuladhar, Paterson, Macleod, & Wilson, 2000). A number of early research methods were discontinuous or invasive in nature (e.g., gravimetric method and direct measurement of thick- ness). Continuous monitoring of the cleaning process may be done by measuring the contaminant in the ef- fluent flow (Tuladhar et al., 2000). A number of tech- niques have been developed over the past 25 years. Some techniques are simple and require minimum instrumen- tation. Others are sophisticated and provide direct evaluation of a number of relevant parameters (Tula- dhar et al., 2000). These devices are based on heat transfer measurement (Bott, 1995; Lalande & Rene, 1988), pressure drop (Corrieu, Lalande, & Ferret, 1979; Visser & Jeurnink, 1997), silicon sensor (Stenberg, Stemme, & Kittilsland, 1988), ultrasound method (Withers, 1996), microstrip monitoring technique (Root & Kaufman, 1992), photothermal deflection method (Fujimori, Asakura, & Suzuki, 1987), light transmission or reflectance (Tamachkiarowa & Flemming, 1999), ellipsometry (Karlsson, Wahlgren, & Tragardh, 1996), quartz crystal microbalance (Colberg et al., 1998), radiotracers (Scintillation detector) (Grant, Webb, & Jeon, 1996, 1997; Kabin, Saez, Grant, & Carbonell, 1996; Littlejohn, Saez, & Grant, 1998; Yan, Saez, & Eduardo, 1997), shear stress device (Bott, 1996; Fryer, 1989), pneumatic gauging technique (Gale, 1995; Journal of Food Engineering 61 (2004) 181–189 www.elsevier.com/locate/jfoodeng * Corresponding author. Tel.: +64-9-373-7599x7004; fax: +64-9-373- 7463. E-mail address: d.chen@auckland.ac.nz (X.D. Chen). 0260-8774/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0260-8774(03)00085-2