Fouling and Cleaning of Modified Stainless Steel Plate Heat Exchangers Processing Milk Products (2003)

Fouling of heat exchangers in dairy industries is still quite a severe problem both technically and economically. Altering the surface properties of the heating surfaces would be a way of solving this issue. Modified steel surfaces were tested in an Alfa Laval V2 plate heat exchanger throughout dairy product sterilization. The behavior was analyzed for 8 different surface treatments, such as coatings (Diamond Like Carbon [DLC], Silica, SiOX, Ni-P-PTFE, Excalibur, Xylan) and ion implantation (SiF, MoS2). All fouling and cleaning experiments were carried out in standard and well-controlled operating conditions. After fouling, no significant difference could be seen between all the modified steels and the reference by statistical variance analysis. Cleaning efficiency of Ni-PPTFE appeared significantly the best. It could be suggested that the free surface energy plays a predominant role and the roughness a minor role in the level of fouling and cleaning efficiency. INTRODUCTION Plate Heat Exchangers (PHEs) are very common in dairy industries due to their ease of maintenance and cleaning, their compact designs and their excellent heat transfer coefficient characteristics required for thermal sterilization/pasteurization purposes (Shah and al., 1988). However, fouling of plate heat exchanger is a severe problem both technically and economically. It causes significant increases in capital and operating costs (Sandu and Lund, 1983). For example, in the dairy industry, the yearly estimated total fouling costs in the Netherlands are approximated to exceed 40 million US$ (Visser and Jeurnink, 1997). Frequent cleaning of the plant is needed for both microbiological reasons and to restore PHE heat transfer characteristics, i.e. to remove the additional heat resistance of the fouling layer and to reduce the pressure drop in the process plant. Complex Cleaning-In-Place (CIP) techniques have been developed empirically (Romney, 1990 ; Bird & Fryer, 1991). Two types of CIP treatments are mainly found in milk processing (Timperley and Smeulders, 1987): 1Two-stage cleaning, using alkali, commonly sodium hydroxide and an acid wash of nitric or phosphoric acid. 2Single-stage cleaning, using formulated detergents containing wetting and other surface agents as well as chelating compounds. Selecting the correct cleaning strategy requires an understanding of milk fouling, which has been studied for a number of years. The composition of the milk deposit is now well known (Fryer et al., 1996). The key roles are played by proteins, especially β-Lactoglobulin. Lalande et al. (1985) showed that the heat denaturation of this βLactoglobulin protein governs the milk deposit formation on the heat transfer area when the temperature is below 90 °C. This has been confirmed by many investigators (Jeurnink, 1996 ; De Jong, 1997 ; Visser et al., 1997). A number of authors have modeled milk fouling in PHEs based on a simple representation of the process hydrodynamics. Delplace, Leuliet and Tissier (1994) performed experiments in a PHE with complex flow arrangements consisting of 13 plates. The overall heat transfer coefficient and pressure drop were measured as a function of time. The temperature profiles for each channel were determined based on numerical simulation. These profiles explained the uneven deposit formation in each channel. Delplace and Leuliet (1995) studied the milk fouling of several PHE flow arrangements by measuring both the heat transfer coefficient and dry mass of deposit. A predictive model of deposit mass in each channel was proposed based on temperature profiles and protein denaturation. Investigation of anti-fouling techniques has received much attention in the past and a variety of chemical and mechanical methods have been suggested to reduce the formation of a deposit on heat transfer surfaces. However, mechanical methods, such as pipe inserts or circulating sponge balls, are costly and usually limited to the inside of pipe-like geometries. For PHEs, only chemical mitigation is possible. Unfortunately, using chemical additives may cause undesired product contamination or have adverse effects on the environment. The use of magnetic, electric, radiation or catalytic treatments (Donaldson and Grimes, 1988) have not yet given consistent results. Altering the surface properties of the heating surfaces would be a way of finding a solution. Recently several new techniques such as direct ion implantation, magnetron sputtering, dynamic mixing, plasma enhanced vapor deposition and auto-catalytic Ni-P-PTFE coating (Zhao and 1 Beuf et al.: Fouling and Cleaning of Plate Heat Exchangers Published by ECI Digital Archives, 2003 al., 2002) have been considered as being of interest in fouling reduction. However, very few studies have been successful in applying such novel surface treatments. Muller-Steinhagen and Zhao (1997) investigated the influence of SiF4 ion implanted stainless steel, which significantly reduces CaSO4 scale formation during pool boiling. The aims of this work are to increase the productivity by lengthening the processing time and substantially reducing the time needed for cleaning the heat exchangers, thereby minimizing resource consumption and the use of chemicals. Different surface treatments including Diamond Like Carbon [DLC], Silica, SiOX, Ni-P-PTFE, Excalibur, Xylan and ion implantation (SiF, MoS2) were tested. Fouling and cleaning references on non-modified steel plates of a V2 plate heat exchangers were established to allow comparisons with the fouling and cleaning behavior of the modified steels. The decrease in the overall heat transfer coefficient and the increase in the pressure drop were used for comparison, being a direct consequence of the fouling of the heated surfaces. The quantity of denatured β-lactoglobulin after the heat treatment was calculated. MATERIALS 1Pilot plant Fouling experiments were carried out on the pilot plant presented in Fig. 1. It consists of 2 stirred tanks, 2 test heat exchangers (PHE1 and PHE2) connected to 2 heating units and a cooler (PHE3). Preheater Cooler Heater P1 P2 Stirred tanks