FLOW CELL STUDIES ON FOULING CAUSED BY PROTEIN -CALCIUM PHOSPHATE DEPOSITION IN TURBULENT FLOW

A comparative study of the calcium phosphate fouling process, with and without proteins, was carried out using both standard 316 2R stainless steel and 2R surfaces modified by TiN magnetron sputtering. Fouling behavior was assessed in a heat transfer flow cell operating in the turbulent flow regime. The fouling curves resulting from calcium phosphate deposition in the absence of proteins were substantially different from the ones obtained when protein was present. In this last case, two different fouling periods could be observed. The surface energy of the modified materials was found to affect the deposition parameters (rate of deposition and final amount of deposit) leading to higher amounts of deposit on higher energy surfaces in the absence of protein, while leading to less deposit in its presence. The standard 316 2R substrate proved to be less prone to fouling from protein-calcium phosphate solutions than the TiN modified surfaces. However, the same conclusion could not be drawn for calcium phosphate solutions

Ion implantation of stainless steel heater alloys for anti-fouling applications

Ion implantation of fluorine and silicon ions into stainless steel heater alloys inhibits the accumulation of CaSO4 deposits when used in an saturated aqueous solution of 1.6 g/l concentration. This anti-fouling action leads to an increase in the heat transfer coefficient by more than 100% under a heat flux of 200 kW/m2 and 200% under a heat flux of 100 kW/m2 when compared to unimplanted heater elements. Heat transfer data indicate that following a heating cycle of 4000 minutes a thick layer of CaSO4 deposit remain on unimplanted heater surfaces. Similar CaSO4 deposits also formed on the implanted alloys initially but did not remain after 1000 minutes causing a significant recovery in the heat transfer coefficient. Ion implanting these alloys leads to surface energy reduction and hence the anti-fouling action observed

Preliminary study of particulate fouling in a high temperature controlled experimental facility

Fouling is a highly complex process and numerical modeling of fouling has been an evasive task. One of the reasons for this is attributed to the lack of detailed experimental data. In-situ experiments performed at the power plants give a global picture of the overall deposition process in a qualitative manner. However, detailed understanding of the underlying mechanisms becomes difficult. This is due to the fact that too many parameters like varying particle composition and size, gas phase dynamics, chemical reactions etc. are lumped together in such experiments. On the other hand, controlled lab-scale experiments that have been reported are meager and those that have been published are performed either at low temperatures or at very high temperatures (>1000 °C). In order to understand the underlying mechanisms of particulate fouling and to provide experimental data for validation, a high temperature controlled fouling experimental facility has been built. The facility is a vertically oriented closed loop wind tunnel with which parameters like gas phase temperature, velocity and particle concentration can be controlled. The setup was tested for proper operation and preliminary experiments were performed on particulate fouling over a circular cylinder as function of gas phase velocity and temperature. It was found that the gas phase velocity and temperature has a major influence on particulate fouling. This is a preliminary study and will be extended in future

Effect of condensable species on particulate fouling

The flue gases emanating from the combustion of fuels or gasification process invariably comprises particulate matter and many chemical species in vapor form. The temperature of the flue gases gradually reduces when passing through different sections of heat exchanger like superheater, evaporator etc. If the temperature of the heat exchanger tube surface and the gas phase are favorable for condensation, the chemical species in the vapor form will condense on the particles and on the tube surface. The particle deposition behavior under these conditions is drastically different from the one observed in dry particulate fouling. In order to model the particle deposition under such circumstances, it is important to evaluate the criteria for particle adhesion to the surface. Impaction experiments of particles impacting a surface coated with a thin liquid film and particles which are coated with a liquid film impacting over a dry surface are performed to evaluate the limiting parameters under which a particle sticks to the surface without rebounding. The effects of liquid viscosity, liquid film thickness and interacting material properties are evaluated. The experimental results are compared to the results of existing models and a simple modeling approach for fouling is proposed. Controlled fouling experiments are performed for varying liquid films coated over a deposition tube under various process conditions to mimic the condensation effects on fouling. The results are compared with the detailed impaction experiments