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Thermodynamics, Transport, and Fluid Mechanics
Low Voltage Electrical Demulsification of Oily Wastewater Hui Zhang, Scott C. Bukosky, and William D. Ristenpart Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01219 • Publication Date (Web): 01 Jun 2018 Downloaded from http://pubs.acs.org on June 1, 2018
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Industrial & Engineering Chemistry Research
Low Voltage Electrical Demulsification of Oily Wastewater Hui Zhang,1,2,* Scott C. Bukosky,2,* William D. Ristenpart2,† 1
College of Biosystems Engineering and Food Science, Fuli Institute of Food Science, Zhejiang
Key Laboratory for Agro-Food Processing, Zhejiang R & D Center for Food Technology and Equipment, Zhejiang University, Hangzhou 310058, China 2
Department of Chemical Engineering, University of California Davis, Davis, CA 95616, USA
ABSTRACT Many industrial processes generate “oily wastewaters,” characterized by low volume fractions of micron-scale, oil-in-water droplets that are difficult to separate by mechanical or chemical means. High DC voltages are traditionally applied for the electrical demulsification of water-inoil emulsions. In this work, we demonstrate that oil-in-NaOH contaminated wastewater emulsions respond to low voltage, low frequency oscillatory fields by aggregating near the electrodes. Optical microscopy shows that droplets initially separate upon the application of a ~10 Hz oscillatory field, but slowly form aggregates over longer time scales of several minutes. The rate of aggregation varies non-monotonically with the applied field strength, exhibiting a peak near 3 Vpp and decreasing at higher strengths. Finally, we demonstrate that a combination of low frequency fields with a small DC offset induces coalescence to break the emulsion. These results point toward a low energy, non-chemical method for recovering oils from oily wastewaters.
* These authors contributed equally to this work.
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INTRODUCTION “Oily wastewaters” are generated in a variety of industrial processes, including petroleum refining, metals manufacturing and machining, and food processing.1–5 Often it is necessary to remove the emulsified oil for environmental or regulatory reasons; in some instances, the oil itself is valuable and recovery is therefore desirable (e.g., recovery of emulsified olive oil from olive processing wastewater). Typically, the wastewater undergoes a primary treatment to separate the bulk oil via physical means that exploit the oil/water density difference, e.g., gravity separation or flotation followed by skimming. Nonetheless, sufficiently small emulsified oil droplets will remain in the aqueous phase, and the oil-in-water emulsion must then be broken with a secondary treatment. The most widely used secondary treatment for oily wastewaters involves the addition of various chemicals to induce colloidal destabilization of the dispersed droplets (as with ferric or aluminum salts), or chemical degradation of emulsifying agents present in the solution.1,5 The addition of large quantities of metallic salts or other compounds is undesirable from a green chemistry perspective. Accordingly, many physical separation techniques have been explored, including techniques based on heating, centrifugation, ultrafiltration, membrane, and cellulosic absorption.1–5 These techniques avoid the necessity of added chemicals, but do present scale-up challenges associated with large energy requirements (for heating or centrifugation) or fouling (for filtration or absorption methods). A potentially more sustainable class of demulsification techniques involves applied electric fields,5–9 which have been commonly used for more than a century to separate emulsified water droplets from petroleum and various food oils.
Demulsification occurs because
application of the field causes the droplets to polarize, and the interaction between the induced dipoles10 causes the drops to align in chains parallel to the applied field such that they eventually 2 ACS Paragon Plus Environment
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Industrial & Engineering Chemistry Research
coalesce into larger droplets, increasing their sedimentation velocity and ultimate separation.11 This approach does not work for oil-in-water emulsions, however, because the continuous aqueous phase is too conductive to allow field strengths sufficiently high enough to induce dipolar attraction. Instead, the standard electrical demulsification approach for oil-in-water emulsions involves “electrocoagulation” in DC fields, where electrochemical reactions help alter the pH and thus destabilize the oil droplets.12 The required current densities (and comparable energy usage), however, are correspondingly large. A desirable alternative electric approach involves low voltage oscillatory fields, which have the benefit of avoiding electrochemical reactions and requiring comparatively less energy. In fact, colloids in aqueous electrolyte solutions have long been observed to exhibit complex behavior near electrodes in response to low voltage AC fields. Early work by Trau et al.13,14 demonstrated that particles adjacent to the electrode aggregate laterally toward one another yielding planar clusters. They interpreted this behavior in terms of electrohydrodynamic (EHD) fluid flow, where the presence of the particle disrupts the electric field near the electrode and creates a tangential field component, driving an EHD fluid flow directed radially inward toward the particle.13,14 Adjacent particles become mutually entrained in their respective flows, resulting in aggregation. Because both the charge in the polarization layer and the perturbation due to the particle scale with the applied field strength E, the resulting EHD flow scales as E squared.14–16 This basic mechanism has been further investigated by several groups, all of whom interpreted the aggregation to be a consequence of electrically induced fluid flows near the electrode.15–24 Importantly, electrically induced aggregation has been observed with a variety of other systems,23–30 including oil-in-water emulsions.8,9,31
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A particularly confusing aspect, however, has been the role of the electrolyte (salt) in the aqueous phase. In some electrolytes particles aggregate laterally to form planar clusters, while in other electrolytes the same particles instead separate.20,21,32–34 The delineation between aggregation and separation was previously thought to depend solely on the identity of the electrolyte, but recent work has established that solid colloidal particles suspended in so-called ‘separating electrolytes’ will nonetheless aggregate at sufficiently low applied frequencies (
