Wall layer formation in continuously operated tubular reactors for free- radical polymerizations

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Coupling of finite-volume-method and incompressible smoothed particle hydrodynamics method for multiphase flow

It is an intuitive way to use the advantages of two different simulation methods, such as the Finite-Volume (FV) and Smoothed Particle Hydrodynamics (SPH), to reduce the computational effort. Finite-Volume, like other grid-based methods, is advan- tageous for huge systems without fluid-fluid interfaces, whereas SPH is advantageous in the vicinity of fluid interfaces. We will present our first results for a combined simula- tion, including a moving SPH domain, in a simple Poiseuille flow and a more complex multiphase capillary rise scenario

Abluftoxidation in Monolith-Katalysatoren mit periodischem Wechsel der Strömungsrichtung

Mit steigenden gesetzlichen Anforderungen zur Reduzierung des Schadstoffgehalts gewinnen Verfahren zur oxidativen Umwandlung von Schadstoffen in Abluft zunehmend an Bedeutung. Typische Aufgaben der Abluftreinigung sind gekennzeichnet durch hohe Luftdurchsätze und niedrige Schadstoffkonzentrationen; die adiabate Temperaturerhöhung der Totaloxidation beträgt häufig weniger als 100°C. Das bedeutet, daß der Druckverlust in der Reinigungsanlage niedrig und der Warmerücktausch zwischen Zu- und Ablauf hoch sein muß, um die Betriebskosten niedrig zu halten. Im folgenden wird eine neuartige Betriebsweise der katalytischen Oxidation untersucht, die sich durch effektiven Warmerücktausch, niedrigen Druckverlust und geringes Bauvolumen auszeichnet. Sie beruht auf der Benutzung von Monolith-Katalysatoren in Verbindung mit der von Macros und Mitarb. entwickelten Prozeßführung mit periodischem Wechsel der Strömungsrichtung. Dabei wirkt jeweils der vordere und der hintere Teil des Katalysatorbettes als Regenerator-Wärmeaustauscher

Catalytic air purification : challenges and new solutions

Neue Verfahren der katalytischen Abluftreinigung mit im Reaktor integriertem regenerativem Wärmeaustausch erlauben die autotherme Abluftreinigung bis zu Schadstoffkonzentrationen entsprechend einer adiabaten Temperaturerhöhung von weniger als 20°C. Bei höheren Konzentrationen erlaubt ein Mittenabzug die nahezu vollständige Auskopplung der Verbrennungswärme auf dem höchsten im Reaktor vorliegenden Temperaturniveau. Infolge der Rückkopplung durch den integrierten Wärmeaustausch besitzt der Abluftreaktor ein ungewöhnliches Betriebsverhalten. Wie gezeigt wird, läßt es sich aber über eine Analogie zur Reaktionsführung mit integriertem Gegenstromwärmeaustausch mit einfachen Gleichungen abschätzen. Werden statt herkömmlicher Katalysatorschüttungen Monolithkatalysatoren eingesetzt, so lassen sich Stromungsdruckverluste und/oder Bauvolumen drastisch reduzieren. Auf die dafür maßgebenden Zusammenhänge wird eingegangen.The integration of regenerative heat exchange into the catalyst bed allows for the autothermal operation of catalytic air purification with a low content of combustible gas. Concentrations corresponding to an adiabatic temperature rise of less then 20 °C can be processed without an additional heat source; in case of higher concentratons a side stream withdrawal allows for the utilization of the total heat of combustion at the highest reactor temperature. The feedback of heat due to the integrated heat exchange gives rise to an unusual reactor behaviour. An analogy of fixed bed reactor operation with countercurrent heat exchange is used to derive simple equations for reactor design and operation. If conventional catalyst packings are replaced by monolithic catalysts, substantial reduction in pressure loss and/or packed bed volume can be obtained. The corresponding relations are briefly discussed

Catalytic cleaning of polluted air : reaction engineering problems and new solutions

Integration of regenerative heat exchange into a catalyst bed enables auto-thermal operation of catalytic purification of polluted air with a low content of combustible pollutants. Concentrations corresponding to an adiabatic temperature rise of less than 20°C can be handled without introduction of additional fuel. For higher concentrations, a technique involving side-stream withdrawal allows utilization of the total heat of combustion at the highest reactor temperature. Heat recovery by integrated heat exchange gives rise to an unusual behavior of the reactor. Based upon an analogy to fixed-bed reactors with integrated countercurrent heat exchange, simple equations are derived for reactor design and operation. Substantial reduction in pressure loss and in the volume of the packed bed can be obtained by replacing conventional catalyst packings by monolithic catalysts. The corresponding relationships are briefly discussed

Modeling the dynamics of partial wetting

The behavior of interfaces and contact lines arises from intermolecular interactions like Van der Waals forces. To consider this multi–phase behavior on the continuum scale, appropriate physical descriptions must be formulated. While the Continuum Surface Force model is well–engineered for the description of interfaces, there is still a lack of treatment of contact lines, which are represented by the intersection of a fluid–fluid interface and a solid boundary surface. In our approach we use the “non compensated Young force” to model contact line dynamics and therefore use an extension to the Navier–Stokes equations in analogy to the extension of a two–phase interface in the CSF model. Because particle–based descriptions are well–suited for changing and moving interfaces we use Smoothed Particle Hydrodynamics. In this way we are not only able to calculate the equilibrium state of a two–phase interface with a static contact angle, but also for instance able to simulate droplet shapes and their dynamical evolution with corresponding contact angles towards the equilibrium state, as well as different pore wetting behavior. Together with the capability to model density differences, this approach has a high potential to model recent challenges of two–phase transport in porous media. Especially with respect to moving contact lines this is a novelty and indispensable for problems, where the dynamic contact angle dominates the system behavior

Fixed bed reactors with periodic flow reversal : experimental results for catalytic combustion

The influence of design and operating parameters on the behavior of a fixed-bed reactor with periodic flow reversal has been studied in a laboratory set-up for the case of catalytic total oxidation. The results are in accordance with detailed model simulations published elsewhere. They show that the periodic operation is completely dominated by the regenerative heat exchange and that steady-state kinetics can be used. Like any other autothermal reactor, a fixed-bed reactor with periodic flow reversal has to be operated in the ignited steady state. It was shown that totally and partially ignited steady states may exist under the same operating conditions if several combustible components with different ignition temperature are present in the feed. Hot gas withdrawal from the middle of the packed bed proved to be a suitable method to utilize almost all of the heat of reaction at the highest temperature in the reactor and to prevent high temperature peaks at the respective exit valves. Together with an appropriate design of the fixed bed, composed of inert front and end sections with low effective axial conductivity and an active portion with large axial conductivity, hot gas withdrawal allows for an efficient control of the reactor under the conditions of both high and low feed concentrations

An application of the Cahn-Hilliard approach to smoothed particle hydrodynamics

The development of a methodology for the simulation of structure forming processes is highly desirable. The smoothed particle hydrodynamics (SPH) approach provides a respective framework for modeling the self-structuring of complex geometries. In this paper, we describe a diffusion-controlled phase separation process based on the Cahn-Hilliard approach using the SPH method. As a novelty for SPH method, we derive an approximation for a fourth-order derivative and validate it. Since boundary conditions strongly affect the solution of the phase separation model, we apply boundary conditions at free surfaces and solid walls. The results are in good agreement with the universal power law of coarsening and physical theory. It is possible to combine the presented model with existing SPH models

On single bubble mass transfer in a volatile liquid

We consider single bubble mass transfer of an non-condensible gas into a volatile liquid phase in indus- trial conditions, as observed for example in hydrocarbons liquid phase oxidation processes. Instantaneous bubble size, shape and velocity are measured using image processing with a particle tracking method. The mass transfer rate nitrogen into hot and pressurized liquid cyclohexane is deduced from the bubble volume decrease rate and is compared to literature correlations valid under isothermal conditions. Experiments are performed in a pressurized reactor for P = 20 bar, 30° ≤ T ≤ 150°C and bubble Reynolds number Re = O(10—100). The analysis of bubble rise dynamics shows that the gas-liquid system studied can be considered as a clean system. The mass transfer results are found to follow isothermal correlations predictions excepted for ambient temperature for which liquid evaporation in bubbles is shown to be coupled with mass transfer. This phenomena seems to be a consequence of having a high Lewis number