Fouling and cleaning

The cleaning process of fouled organic material on heat transfer surfaces is a complex interaction of interfacial interactions, mass transfer, heat transfer, fluid forces and chemical reactions, which is not fully understood at present. The main objective in this research project is to achieve a fundamental knowledge of the cause-effect relationships in the cleaning of immersed systems. Whey protein in the form of WPI gel are to be used as a representative model fouling system. The research project is divided into three parts. The first part comprises the investigation of all material properties which are relevant for a cleaning process, specifically strength, viscoelastic characteristics, diffusion coefficient, surface topography, adhesion and cohesion force. These material parameters will be used in the second part of the project. Here a fundamental data base for a continuum-mechanical FEM model of the fouling layer will be set up. The fluid phase will be simulated with a fluid-structure interaction model (FSI) model, so that the cleaning process of WPI can be simulated. To validate and improve the FSI model, two established methods, namely fluid dynamic gauging (FDG) and local phosphorescence detection (LPD), are used in the third part. The modeling approach will be developed further and based on this FSI model a tailored and system-specific cleaning strategy can be designed and validated.

Next to crystalline fouling, particle fouling is the most frequent cause of fouling formation problems and is the motivation for this research. In cooperation with the Chair of Modeling and Simulation (LeMoS) of the University of Rostock, the basics for the numerical and experimental determination of particulate fouling on structured surfaces are presented and physical mechanisms of deposition and removal with respect to heat transfer are identified. The central goal of this study is the development of methods for predicting particulate fouling. With this knowledge the evaluation of structured heat transfer surfaces can be extended beside the heat transfer performance and the pressure loss by fouling propensity.

In this project it could already be shown that on the one hand the particles are transported by the vortex structures out of the dimple, which leads to reduced or prevented fouling in the dimple. On the other hand higher wall shear stresses occur inside and behind the dimple resulting from the oscillating vortex structures. This provides a reduction of fouling in these areas through increased removal rates. Therefore, the use of dimpled structures is also advantageous under fouling prone conditions and this self-cleaning effect could thus be confirmed experimentally. Furthermore, the qualitative comparison of the determined fouling layers shows a high agreement between experiment and CFD simulation.

Due to geographic and climatic peculiarities, a large part of India has poor water quality. About 37.7 million Indians are affected by waterborne diseases annually and 1.5 million children are estimated to die of diarrhea alone (www.wateraid.org). Considering the high population density and low-income economic conditions, it is necessary to find inexpensive water treatment technology that meets the standards for safe drinking water in the country and, at the same time, is a reliable technology with reduced maintenance. Membrane filtration systems are advantageous due to their extremely high selectivity and easiness of operation, comparing with the conventional water treatment processes. Disadvantages include the need for relatively expensive equipment and the eventual contamination of membranes (fouling) that in turn increases energy consumption and cleaning frequency, and shortens the life time of the membranes. Biofouling and particulate fouling are the major problems affecting the performance of membrane water treatment processes. The mitigation of fouling is in the focus of the investigations here. The main objective of the project is to find an optimal technology and operation mode for effective membrane performance and longevity and to develop and test methods that may be integrated as pre-treatment processes to prolong membrane performance, decrease energy use and water consumption.

Coating of surfaces is a central strategy for prevention or mitigating fouling on heat transfer surfaces. An effective way to reduce fouling is the use of diamond-like carbon (DLC) coatings, especially if these coatings have a sufficient and equal amount of positive and negative charged functional groups (zwitterionic surfaces). The aim of this project is to gain a detailed understanding of the correlation between the properties of the surface and the fouling behavior, in particular adsorption of proteins and nucleation of calcium and phosphate salts. In a first phase of the project, several tests will be carried out (e.g. measurement of zeta potential, surface tension, adhesive force) to understand deposition mechanisms depending on different zwitterionic DLC-surfaces.

A promising way to design new, innovative and efficient processes is the consistent usage of micro components. Despite the advantages of microcomponents in continuous processes there are also a few challenges, e.g. the fouling and cleaning of micro structures. Therefore a cooperation of several academic and industrial research partner stacked together in one joint research project (Mi²Pro) to expand the knowledge of basic fouling mechanisms in micro components. Based on the systematic analysis of the impact of different parameters new methods for the detection of fouling in microchannels and suitable CIP-strategies should be developed to guarantee continuous operating of micro components in industrial environments.

The goal of the collaborative project together with the industrial partner Löhrke GmbH is to develop and establish a new pH based cleaning-in-place procedure (CIP) to remove fouling in heat exchangers and food processing systems. The innovative approach consists of CIP sequences passing acid and alkaline pH ranges at low temperatures in only one cleaning loop to accomplish the best possible cleaning efficacy. The automated process applied in a CIP pilot plant equipped with sensors to monitor the cleaning progress will allow a demand-based, efficient and economical way of cleaning – especially in the dairy industry – which also contributes to the product safety of food products. The work package of ICTV as participating research institution primary deals with the process development carrying out preliminary experimental investigation in lab scale. For example, the production of different model soilings (fouling layers) with reproducible qualities will be carried out. Furthermore, a test rig utilizing the applied fouling layers will be used to simulate the desired novel CIP procedure by passing though different pH ranges to remove the layers.

In cooling crystallization processes often crystal growth on the heat exchanger surfaces occurs, which can also be considered as fouling. The reasons for this are the increasing viscosity and lower solubility at lower temperatures near the surface. The energetic properties of surfaces, such as hydrophobicity, can be changed by coatings. A particularly targeted adjustment of the surface free energy can be achieved by doped amorphous carbon layers, the so-called DLC coatings (Diamond Like Carbon). Specific surface properties can be adjusted by doping the coating with non-metallic elements such as silicon, fluorine or oxygen. Therefor the coating can also influence the interactions between surface and liquids, as well as the crystal formation.

Biofouling is the unwanted adhesion of microorganisms on technical surfaces. Thereby microorganisms form a matrix of extracellular polymeric substances (EPS), which is both a water and nutrient reservoir and a protective barrier against environmental influences and biocides that hamper the cleaning. A fouling set-up was designed to cultivate biofilms with reproducible flow conditions on stainless steel surfaces, then subsequently the biofilms were removed by an integraded ultrasound sonotrode. Because microorganisms grow fast on already present biomass, a complete removal is intended. Anti-fouling layers, which change the free energy of the stainless steel surface and thus the interaction of proteins of cell surface and stainless steel surface, can reduce the biofilm formation.

Rising costs and environmental awareness, more stringent consumer demands for natural foods with longer shelf life as well as the high hygienic requirements in many areas of the manufacturing industry increasingly require more efficient cleaning processes, which are integral parts of the production processes themselves. Necessary for a targeted adjustment of cleaning processes is a deeper understanding of how contaminations react to cleaning procedures due to their nature, their interaction forces with the machine surface (substrate) and the cleaning agent. The aim of this project is the optimization of cleaning processes by characterizing the present contamination by dimensionless parameters. The combination of different laboratory measuring methods allows the prediction of the effective influence of the operating parameters (cleaning agent temperature, concentration and flow rate) for the removal of specific contaminations. In the first phase, the interactions between simple model contaminations and the cleaning agents were described. The second phase is focusing on the interaction between complex, industrial contaminations and various substrate materials as well as their surface treatment.

COMPREX® by Hammann GmbH, Germany, is an established cleaning technique for municipal water networks. COMPREX® uses plug flow with high flow velocities consisting of air and water plugs which are capable of mobilizing and flushing out various deposits inside the pipeline system. The goal of this collaborative project with is to transfer this technique and its application in heat exchangers to clean in situ efficiently and without the use of chemicals. In course of this project COMPREX® will be tested with different fouling model systems like crystalline CaSO4 deposits, biofilms and hydrogels in plate and frame heat exchangers. In order to conduct cleaning experiments systematically and to remove deposits from industrial heat exchangers, a cleaning test rig and a mobile Comprex® unit have been developed.

One promising potential to mitigate fouling is the coating of heat exchanger surfaces by a plasma assisted chemical vapour deposition (PACVD) process. Herewith thin amorphous DLC (Diamond Like Carbon) coatings can be applied to the heat exchanger surface. The solid’s surface free energy can be varied, when adding various elements like silicon or oxygen in the coating process. Changing the interactions between the depositing fouling mass and the surface can lead to decreasing adhesive forces. Up to now this technique can only be deployed on plate type heat exchangers, since the PACVD process cannot be applied to the inside of pipes until today. This research project deals with the realization of a production chain to apply the coating to the inner surface of pipes. At first flat stainless steel plates are coated by the PACVD process, then bended to a tube geometry and finally welded. The objective of the project is to identify how the different production steps affect the coating and what consequences in fouling behaviour result from each production step. Different welding and bending techniques were investigated in order to detect the best production process regarding fouling. One type of coating could be identified to retain its fouling mitigation properties over the applied production stresses during forming and welding.