Elisabetta Brunazzi

Spectral and wavelet analysis of the flow pattern transition with impeller clearance variations in a stirred vessel

Abstract The double- to single-loop pattern transition in stirred vessels stemming from a change in the off-bottom clearance of a Rushton turbine has been investigated by laser doppler anemometry. Time-resolved data showed the transition occurring within a range of clearance values and allowed the distinction of three types of flow: the double-loop regime, the single-loop regime and an unstable one termed “transitional state”. Experiments of up to 3– 4 h duration showed that both the onset and the lifetimes of these types of flow were random; however, in the transitional state, the flow varied between the two circulation patterns in a periodic manner, with a frequency linearly related to the impeller rotational speed. The results have important implications for mixing process and vessel design as well as CFD predictions of the flows which are discussed.

Mixing of Two Miscible Liquids in T-shaped Microdevices

Numerical simulations were performed to study the flow fields and mixing characteristics of liquid flows converging in a T-shaped micromixer, when the two inlet fluids are both water or water and ethanol. We showed that at smaller Reynolds number, Re < 100, mixing is controlled by transverse diffusion, and therefore by the residence times of each fluids. Accordingly, mixing ethanol and water is slightly easier than mixing water with water, due to the fact that, as ethanol is slightly more viscous than water and therefore it is slower, the residence time of water-ethanol mixtures is larger than that of the water-water case. On the other hand, at larger Reynolds number, mixing water and ethanol may take considerably longer, as the onset of engulfment is retarded and occurs at larger Reynolds number, namely increasing from Re ≅ 140 in the water-water case to Re ≅ 230 in the water-ethanol case. This is due to the fact that a water-ethanol mixture has a viscosity that is up to almost three times larger than that of water; therefore, at the confluence of the T-mixer, the water and the ethanol streams are separated by a quite viscous layer of a water-ethanol mixture, that hampers any vortex formation, thus retarding mixing.

Conventional and Complex Knitted Mesh Mist Eliminators

Knitted wire mesh mist eliminators have a widespread application in many industrial plants as they assure an optimum cost/performance for many applications compared with other separation devices. Complex mesh pads allow the performance and the range of applications of conventional wire mesh pads to be extended. In recent years, increasing research effort has been dedicated to the experimental investigation of both common and complex mesh pads and to the development of reliable design models that are essential for the design and optimization of complex separation units.

DME synthesis via catalytic distillation: Experiments and simulation

Abstract This paper regards the field of the chemical engineering that is commonly identified as Process Intensification (PI) . The main objective of PI is to improve processes and products to obtain technologies more safe and economic. ENI and the University of Pisa are partners in the European project INtegrating SEparation and Reactive Technologies (INSERT) that considers the integration of the two key steps common to conversion processes (reaction and separation) , to develop new configurations with advanced performances respect to the conventional ones. It has been chosen to apply Catalytic Distillation, the most promising application of the intensification principles, to the synthesis of dimethyl ether (DME) from methanol. This is one of the seven industrial case studies being investigated to test and validate the INSERT methodology.

Modelling and experimental validation of H2S emissions in geothermal power plants

A commercial computer software, used to model and simulate geothermal power plants, has been investigated by comparing simulation outputs with field data of existing plants. Although most simulation results were in good agreement with the field data, the program failed when predicting the H2S distribution between the plant streams. The computer program has, therefore, been partly modified on the basis of pilot-scale experiments directed at investigating the H2S behaviour in the two main plant sections, i.e. the direct contact condenser and the cooling tower. The modified version fits the field data well and is a useful tool for assessing methods to reduce the environmental impact of releasing H2S into the atmosphere.

CALCULATION PROCEDURE FOR FLOODING IN PACKED COLUMNS USING A CHANNEL MODEL

The combined effect of a channel-based approach for dry pressure drop and the Buchanan equation for wet pressure drop in packed beds has been numerically evaluated within the flooding region. The flooding point is an important design parameter since it establishes the maximum hydrodynamic capacity at which a packed column can operate. Upon analyzing the aforementioned approach, it was found that the usual practice of fixing a “reasonable” wet pressure drop at the flooding point (e.g., 1025 Pa/m) may not yield the correct flooding velocity of the gas, particularly at higher liquid loads. In fact, numerical evaluations of the aforementioned model showed a rather “retrograde” non-monotonic behavior of pressure drop with respect to the f factor of the gas near flooding at different liquid loads. A calculation procedure was therefore devised in this work to correctly compute the flooding point for a given liquid load when using the aforementioned modeling approach. Interestingly, it was found that the correct flooding velocity can be directly computed from liquid holdup below the gas loading point. To illustrate the use of the procedure, maximum capacity calculations were performed for a well-known random packing, a conventional structured packing, and a novel catalytic structured packing.

A simple method to compute hydrogen chloride abatement in geothermal power plants

Abstract One of the most important problems affecting geothermal fields is the abatement of hydrogen chloride contained in the vapor phase. If the chloride concentration exceeds a few ppmw, steam scrubbing must be provided in order to prevent corrosion of the gathering system and turbine failure. In some fields at Larderello, one of the most important geothermal areas in Italy, steam scrubbing is performed by injecting a caustic solution directly into the steam pipeline. In particular, the abatement system depends on absorption with chemical reaction of hydrogen chloride by a sodium hydroxide solution. This paper describes some of the Larderello power plant abatement systems and presents analyses of the different solutions adopted for this purpose. Finally, some simplified models for computing abatement efficiency in sprays, pipelines, static mixers, cyclones and vane type demisters—the equipment generally used in these plants—are proposed. The proposed models are able to predict the data measured in these power plants with good accuracy, and so they can be regarded as useful tools for designing new abatement systems or optimizing the existing ones.

Mixing performance of arrow-shaped micro-devices.

The process of liquid laminar mixing in arrow-shaped micro-devices is studied by direct numerical simulations. Two different CFD codes, i.e. Fluent (based on finite volume method) and Nek5000 (based on spectral element method) have been used to investigate the flow and concentration fields. Unexpectedly we observe that within the engulfment regime, the degree of mixing first increases and then diminishes as the inlet flow rate is increased. Such reduction in the degree of mixing, not observed in T-shaped mixers, can be imputed to the presence of a strong vortical structure at the center of the mixing channel. This result is important for control operations, as it shows that on one hand arrow type mixers are characterized by higher degree of mixing with respect to T-shaped mixers, but on the other hand they present a narrower range of optimal conditions.

Mass Transfer Study on Catalytic Structured Packings for Reactive Separations

This paper deals with a study on mass transfer in modular catalytic structured packing Katapak-SP with the purpose of determining the effective mass transfer area. Information has been gathered by using the chemical absorption method and monitoring the composition profiles along the packed bed. Liquid load ranged between 5 and 27 m 3 /(m 2 h). A procedure was developed taking into account the hybrid structure of the packing. The use of correlations for the volumetric mass transfer coefficient available in the literature and of data obtained from targeted experiments allowed chosing the proper operating conditions. A simple model to calculate the effective area is presented. The model is based on the packing geometry and the liquid flow distribution within the hybrid structure of the packing.

Effect of Composition-dependent Viscosity of Liquids on the Performance of Micro-mixers

The process of laminar mixing in a T-shaped micro-device is studied by direct numerical simulation for a model binary mixture, composed of two liquids having the same density and the same viscosity, yet presenting a strong fluidity of mixing effect, i.e. the viscosity of the mixture is a function of its composition. In particular, we consider the case where the viscosity of the mixture is up to three times larger than that of the pure liquids, with the maximum viscosity corresponding to either a 50 %-50 % (type 1 mixture), or a 25 %-75 % composition (type 2 mixture), to better emulate the behavior of real mixtures. The results are compared to the case with no fluidity of mixing effects (type 0 mixture), which has been largely investigated previously. In this latter case, the inlet streams remain separated up to a critical Reynolds number, corresponding to a strong increase of the degree of mixing. This transition is also characterized by a symmetry breaking, from a vortex flow regime, with a double mirror symmetry, to an engulfment flow regime, with a point central symmetry. When the fluid mixture has a larger viscosity than that of its pure components, a viscous layer forms at the confluence of the inlet flows, which tends to keep the two streams separated. Therefore, in this case, one would expect that the onset of the engulfment regime should be shifted to larger Reynolds numbers, in comparison with type 0 mixtures, with no sudden increase of the degree of mixing. Although this is what happens for symmetric, type 1 mixtures, for type 2 mixtures we unexpectedly find that, due to the lack of symmetry of the mixture rheology, the transition from vortex to engulfment regime, although occurring at larger Re, occurs suddenly, corresponding to a sharp increase of the degree of mixing.