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Electrostatic precipitators (ESPs) belong to the most efficient devices for control of particle emissions in the process industry and environmental technologies in general. The operating principle of ESPs, relying on particle precipitation by electrical charging, leads to an enhanced turbulent motion by electrohydrodynamic (EHD) effects within the gaseous working fluid passing through the ESP. This enhanced turbulence is generally considered detrimental to the separation efficiency, because the additional shear forces acting on the collection electrodes promote re-entrainment of already precipitated particles. Further, the enhanced turbulent cross mixing diminishes the transport of charged particles by electric forces directed towards the collection electrodes. In other industrial applications however, many process units that are based on heat or mass transport would benefit from enhanced turbulent mixing. These include heat exchangers for energy recuperation or evaporation and absorption units such as flue gas scrubbers. A combination of these with an ESP possibly allows for versatile new applications in the process industry.
The motivation of this thesis is to study the effects of electrohydrodynamically generated motions onto the turbulent cross mixing within gaseous flows over a wide parameter space including variations in geometry and operating conditions. In order to do so, the influence of EHD effects on the three major transport properties, i.e. thermal energy, mass and momentum, were experimentally investigated. Based on analogies caused by similar transport mechanisms, the experimental results were compared despite different methods being used. In order to help with the comparison a simplified eddy viscosity model is applied.
The study for each transport property was conducted with a different setup of a tube type ESP. These setups include axial and radial pressure drop measurements to quantify the momentum transport, a self-built local heat transfer sensor to determine heat transfer coefficients and a wetted wall-column to study the enhancement of mass transport phenomena. Experiments on momentum and heat transport include a large variation of different geometries and operating variables such as discharge electrode design, tube diameter, flow velocity, as well as magnitude and polarity of operating voltage. In addition, selected measurements with aerosol particles were conducted, to gain additional insights on the effects of EHD induced turbulence generation by particle bound space charge compared to ion bound space charge in clean gas flows.
The results do not only provide transport coefficients for the hands-on application by plant designers, but also feature numerous experimental data that can be used as reference values for ongoing research in the numerical simulation of electrohydrodynamically enhanced flow and therefore significantly increase the available amount of data provided by current literature.
Vortrag “ICESP XIV. International Conference on Electrostatic Precipitation 2016”, Wroclaw, Polen, 19.-23.09.2016
Back corona and dust resistivity are well known topics in electrostatic precipitation. Back corona results from high dust resistivity and occurs when the field strength in the dust layer surpasses a critical value E crit, which is typically found to be in the order of 15 to 30 kV/cm. Besides the resistivity ρ, the current density is the main parameter: E = i ρ < E crit. Hence, possible actions against back corona include a reduction of ρ by dust conditioning, or a reduction of i, for example by pulsed corona operation.
Much work has been devoted to study the dependence of ρ on dust composition, temperature, humidity, adsorption layers and dust layer porosity, and a variety of different set-ups for dust resistivity measurements has been proposed. Even though some authors report a dependence of ρ on current density or field strength resp., dust resistivity is mostly seen as a material property.
However, in measurements on product dusts in the upper resistivity range, we found that dusts show extremely non-ohmic properties:
Most strikingly, resistivity may vary by several orders of magnitude with time.
Also, the experimental arrangement can change the resistivity results by orders of magnitude: When resistivity is measured with the dust layer exposed to a corona discharge (imitating the situation in a real ESP), ρ depends strongly on the layer thickness. Meanwhile the same dust does not show the strong layer thickness dependence when electrodes are placed on both sides of the dust layer, and also time effects are much less prominent.
Additional findings include that dusts that have been exposed to resistivity measurements show a high level of electrostatic charging afterwards.
An interpretation of these results may be found from the theory of semiconductors and electret materials. Accordingly, highly resistive dust layers do not contain mobile electrons. Current transport occurs only after charge carriers (free electrons or holes) have been injected from the electrodes or from a gas discharge adjacent to the surface.
When the injection process is non-symmetric (e.g. with corona discharge), the current transport through the dust layer can be dominated by charge carriers of one polarity, depending on the polarity of the corona. Some of the effects observed in experiment, namely the layer thickness effect and the current density influence, can indeed be ascribed to unipolar injection and space charge limited conduction.
The time effects can mainly be ascribed to the “deep trapping” of the charge carriers, leading to a high level of immobile space charge. As overall space charge is limited, this reduces the level of mobile space charge available for current transport. The space charge also leads to a deformation of the electric field inside the dust layer, and hence influences injection via the Schottky (thermionic field emission) mechanism.
By integrating the mechanisms of charge transport in dielectrics (injection, drift, immobilization, recombination) into the OpenFOAM® simulation package, rather realistic simulations of time-dependent resistivity are possible. The simulations also are helpful to identify the mechanisms responsible for certain types of resistivity behavior.
This work refers to water in oil emulsion formulation, characterization and destabilization. These topics were studied in order to generate knowledge that could be used as a base for the treatment of highly stable W/O emulsions. Two types of emulsions were studied: synthetics and naturals; the last ones come from the crude oil production process of different Venezuelan regions. In this book, chapters 1 and 2 give an introduction to the research area and the theoretical background related with it, respectively. In chapter 3, the effect of surfactant content, water content, HLB value, alcohol content, salinity, emulsion volume, and mixing properties on water in paraffin emulsion stability was studied. Emulsion stability was determined by the amount of water and oil separated after 30 days. After finishing the variables scans, the most appropriate formulation conditions were established, and a formulation protocol was defined. Emulsion density and apparent viscosity were measured, and viscosity was modeled by the Power Law considering both shear rate and water content. Chapter 4 deals with emulsion characterization. Although some particle sizing devices have gained a lot of popularity, none of them is able to distinguish between drops and solids, and few are able to handle viscous oily samples. In this chapter the use of optical microscopy, enhanced by the use of digital video capabilities and image analysis software, to characterize oil production wastes is discussed. Several thousands of particles were counted and their projected areas were measured. A discussion is given about the different corrections required in order to extract the most reliable information from the image, overcoming some of the drawbacks of this kind of measurements. A new technique the so-called cross-linked method was developed to offer unique features like the determination of the fraction of droplets that exist in the form of non-coalesced agglomerates. When cross-linked with the results obtained by standard ASTM procedures, it is calculated the amount of disperse phase that exists in the form of submicron droplets or large free water drops that are usually not sampled for microscope slides. Chapters 5 and 6 deal with emulsion destabilization. In chapter 5, a variety of materials were used to investigate their applicability as so-called collector materials, to improve the destabilization and separation of water in oil emulsions stabilized by a non-ionic surfactant. The emulsion destabilization degree was determined by the amount of water and oil separated after centrifugation. The recovery of both phases was strongly dependent on the nature of the material, material/emulsion ratio, particle size, and contact time. By varying conditions, it was possible to increase the water separation from 0 (without material) up to 95%. In chapter 6, the effect of a DC electric field on W/O emulsions was evaluated. Electric field magnitude, water content, cell size, electrode design, and electrode coating were considered. Emulsion destabilization was improved by combining electric field, collector material and centrifugation. A comparison is given about the emulsion characteristics in terms of microscopy between both, synthetic and crude oil emulsions.