Matthias Bohnet

Influence of the adhesion force crystal/heat exchanger surface on fouling mitigation

The accumulation of unwanted crystalline deposits (fouling) reduces the efficiency of heat exchangers considerably. In order to decrease the cost of fouling two strategies have been developed. The first fouling mitigation strategy is based on the modification of energy and geometry related characteristics of the heat transfer surface to realize an increased duration of the induction period. By means of a DSA (drop shape analysis) measurement device the interaction at the interface crystal/heat transfer surface is determined. The deployment of the fracture energy model and the interfacial defect model relates wetting characteristics to the adhesion phenomenon. Hence, a first estimation of the optimal choice of surface material is realized. Furthermore, the influence of surface topography on interfacial interactions has been analyzed. The second fouling mitigation strategy is based on the adjustment of the hydrodynamic flow conditions using a pulsation technique. Here, single strokes of higher velocity are superimposed on the stationary flow. These strokes shift the equilibrium of forces to an improved removal process. Fouling experiments have proved that pulsation is a powerful tool to mitigate the built-up of fouling layers on heat transfer surfaces.

Modification of molecular interactions at the interface crystal/heat transfer surface to minimize heat exchanger fouling

Abstract Recent studies with respect to mitigate heat exchanger fouling spend only minor attention to the possibility of modifying molecular interactions at the interface between heat transfer surface (substrate) and adjacent crystalline deposit (adhesive). New anti-fouling strategies deal with a defined modification of these interactions to reduce the corresponding adhesive strength favouring the removal process due to the wall shear stress. By means of drop shape analysis the interfacial free energy adhesive/substrate has been determined. On the basis of numerous fouling experiments the interfacial defect model has been identified to serve as a tool to estimate the optimal choice of surface material. For the description of the influence of interfacial energies between two materials on adhesion both van der Waals and hydrophobic interactions making allowance for the presence of polar media have been analyzed. The geometric and harmonic mean approach (van der Waals forces only) is chosen for the description of metallic surfaces. For polymeric materials where repulsive interactions have to be taken into account, the Lewis acid–base approach (also considering hydrophobic interactions) is favoured.

Influence of the interfacial free energy crystal/heat transfer surface on the induction period during fouling

Abstract The accumulation of unwanted crystalline deposits (fouling) reduces the efficiency of heat exchangers. In order to adjust the operating conditions with respect to fouling mitigation a model for the description of the entire fouling process has to be established. The main disadvantage of known models is the lack of a description of the induction period which is influenced by the nucleation rate and the adhesive strength between crystals and heat transfer surfaces. The deposition and removal process depends on the interfacial free energy crystal/heat transfer surface. For many industrial applications the only possibility to influence the interfacial free energy is to modify the surface energy characteristics of the heat transfer surface, i.e. its surface free energy. Based on the experimental investigation of the surface energy of several metallic and low energy materials their fouling performance when exposed to a liquid flow of a calcium sulphate solution was evaluated. The deployment of new surface materials such as DLC (diamond like carbon) coatings proved to be a strategy to increase the induction period.

Influence of the gas temperature on the separation efficiency of aerocyclones

Abstract The design of aerocyclones operating at high temperatures is still a difficult problem since practical experience shows that the separation efficiency really obtained is mostly smaller than that calculated. For this reason, the pressure drop and the grade efficiency has been measured over the temperature range 293–1123 K. Both quantities show a remarkable influence on the gas temperature. A new model into which a wall friction coefficient depending on the temperature is introduced and consideration taken of the re-entrainment of already separated particles together with the boundary layer flow at the cyclone cover plate, the cyclone wall and along the outside of the vortex finder leads to measured pressure drops and grade efficiency curves for high temperatures of a high accuracy.

Influence of the ratio of free hydrogen ions on crystallization fouling

Abstract Fouling and scaling of heat-transfer surfaces reduces the efficiency of heat exchangers. The design of heat exchangers usually takes into account the decrease of heat transfer by increasing the heat-transfer area. In addition to physical factors such as fluid velocity, heat flux and wall temperature, the pH value of the liquid has an important influence on the fouling behaviour. In many cases, a higher pH value is desirable to avoid acid corrosion during cooling processes. However, with increasing pH the number of free hydrogen ions decreases, keeping calcium ions in solution. The ratio of free hydrogen ions to calcium ions can be described as the saturation index. The saturation index has been calculated using the reaction equations for the dissociation of CaCO3. From these calculations, it can be seen that the saturation index is proportional to the deposited mass flow rate of the solids. Experiments have been carried out for the pH range of technical interest between pH 6–10. The asymptotic fouling resistance increases with pH value due to the higher strength of the fouling layer caused by increasing velocity of crystal growth for supersaturation. Taking into consideration the complexity of the fouling process, the results of fouling experiments carried out at different pH values are in good agreement with calculated data.

Modern design of aerocyclones

Unsolved problems in the field of hot gas cleaning led to a renewed interest in the performance of cyclone dust separators. Various calculation procedures exist for the calculation of cyclones. Most of these models need high calculation expenditure. For practical cyclone design a rough calculation of the separation efficiency and the energy consumption is often sufficient. This paper presents two possibilities to calculate the grade efficiency and the pressure drop. The first one is easy to handle but is based on simplified physical assumptions. The second one is a more complex calculation procedure which uses a detailed physical description of the flow field and considers turbulence, re-entrainment and the secondary flow. The results of the two calculation models are compared with experimentally obtained data for different gas temperatures.

Separation efficiency and pressure drop of cyclones at high temperatures

The design of aerocyclones operating at high temperatures is still a difficult problem, because practical experiences show that the separation efficiency really obtained is mostly smaller than calculated. For this reason, the pressure drop and grade efficiency have been measured in the temperature range from 293 to 1073 K. Both quantities show a remarkable influence of temperature. With regard to the calculation of the pressure drop, the proposed calculating procedure leads to very good results. The model for calculating the grade efficiency curve still shows differences between calculated and measured results and needs further improvement with respect to the boundary layer flow and the particle movement due to turbulence.

Improvement of process stability of microbiological quinoline degradation in a three-phase fluidized bed reactor

The biological degradation of quinoline by suspended and immobilized Comamonas acidovorans was studied under continuous and discontinuous operating conditions in a three‐phase fluidized bed reactor. C. acidovorans degrades quinoline into biomass and carbon dioxide. Quinoline and the intermediates of its metabolic pathway are found only by quinoline shockloads. The continuous degradation of quinoline by suspended biomass was only possible, if the dilution rate was less than the growth rate (μmax =0.42 h–1) and the concentration of a shockload was less than 1 kg/m3. A concentration greater than 1 kg/m3 led to an irreversible damage of the cells. Hence, two different carrier materials were used for immobilization by attachment, to increase the stability of the process. Using immobilization of biomass on carriers decouples the hydrodynamic retention time and the growth rate of the microorganisms. A comparison of the carrier material showed no differences with respect of activity and stability of the biofilm. The process stability of a three‐phase fluidized bed reactor was increased by immobilized biomass. The degradation of toxic shockloads was only possible with immobilized biomass. A dynamic model has been developed to describe the concentration profile of quinoline, 2‐hydroxyquinoline as metabolite and the suspended biomass. A comparison of the measured and calculated values showed good agreement.

Einfluß des pH-Wertes auf die Verschmutzung von Wärmetauschern

ZusammenfassungDie vorliegenden theoretischen und experimentellen Untersuchungen beschäftigen sich mit dem Einfluß des pH-Wertes auf das Foulingverhalten von Salzlösungen. Zur experimentellen Untersuchung des Einflusses des pH-Wertes wurde eine wässerige Calciumcarbonatlösung in dem für Kühlwassersysteme relevanten pH-Bereich von pH 6 bis pH 10 eingesetzt. Wäßrige Calciumcarbonat-Lösung zeigt einen deutlichen Anstieg des Foulingwiderstandes, wenn ein pH-Wert von 6 überschritten wird. Mit steigendem pH-Wert nimmt die Anzahl der freien Wasserstoffionen, die erforderlich sind, um die Carbonationen in Lösung zu halten, ab. Mit Hilfe der Reaktionsgleichungen und der entsprechenden Gleichgewichtskonstanten für die Dissoziation von CaCO3 wurde der Sättigungsindex berechnet, der das Verhältnis von freien Wasserstoffionen zu Carbonationen beschreibt. Es läßt sich zeigen, daß der Sättigungsindex proportional zur abgelagerten Feststoffmasse auf der Wärmetauscherfläche ist. Die Ablagerung läßt sich über die experimentell ermittelte Geschwindigkeitskonstante für die Einbaureaktion in das Kristallgitter berechnen. Foulingversuche bei veränderten pH-Werten bestätigen die Aussagen der Berechnung.AbstractFor the experiments a CaCO3/H2O solution was used with a pH-range between pH 6 and pH 10 which is relevant in cooling water systems. For pH-values above 6 aqueous solutions of CaCO3 cause an increase of the fouling resistance. With increasing pH-value the number of free hydrogen ions decreases, which keep the calcium ions in solution. The ratio of free hydrogen ions can be described by the saturation index which is calculated by the reaction equations and the equilibrium constants. It can be seen, that the saturation index is proportional to the deposition rate of the solid. The deposition can be calculated by the rate of reaction that is determined experimentally. The comparison of results of fouling experiments carried out with different pH-values shows good agreement with calculated data.

Untersuchungen der Kristallisation an wärmeübertragenden Wänden mit Hilfe der holografischen Interferometrie

ZusammenfassungDie holografische Interferometrie wurde eingesetzt, um Temperatur und Konzentrationsgrenzschichten während der Kristallisation aus Lösungen zu untersuchen. Der Meßaufbau wird beschrieben und die mit wäßriger Natriumsulfat-bzw. Kaliumnitratlösung gefundenen Meßergebnisse werden diskutiert. Die aus den Temperaturprofilen abgeleiteten Nusseltzahlen stimmen mit den aus Wärmeübergangsbeziehungen berechneten sehr gut überein. Es konnte jedoch keine signifikante Konzentrationsänderung gefunden werden. Untersuchungen beim Auflösen zeigten eine ausgeprägte Diffusionsgrenzschicht. Die Meßergebnisse lieferten Sherwoodzahlen, die um etwa 24% über Literaturangaben liegen. Demgegenüber konnte für beide Salzlösungen bei fortschreitender Kristallisation keine Diffusionsgrenzschicht nachgewiesen werden, so daß die Kristallschicht demnach einbaulimitiert wachsen müßte. Mikroskopische Aufnahmen der Oberfläche einer Kaliumnitratschicht zeigen eine stark zerklüftete Oberfläche mit Stufen von etwa 60 μm Höhe. Diese groben Strukturen stören die Ausbildung einer Grenzschicht, behindern aber auch den Lichtstrahl beim Durchtritt durch den Meßkanal und überdecken möglicherweise eine vorhandene Diffusionsgrenzschicht.AbstractThe holographic interferometry was used for investigations on temperature and concentration boundary layers during the crystallization of salts from aqueous solutions. Results are discussed for aqueous solutions of sodium sulfate and potassium nitrate. The Nusselt numbers derived from temperature profiles coincide very good with calculated results from heat transfer relations. However, no significant changes of concentration in the vicinity of the surface could be found. Investigations with a dissolving crystallization layer show a distinctive diffusion boundary layer. The measured results led to Sherwood numbers 24% higher than published. On the other hand no diffusion boundary layer could be detected for progressive crystallization. This means that the crystal layer grows is reaction controlled. Microscopical pictures of the crystal layer surface show a strongly rugged surface with steps of about 6o pm height. These coarse structures disturb the formation of a boundary layer and prevent the light beam which passes through the test channel. Therefore an existant diffusion boundary layer maybe covered up by the crystal layer.