Investigation of electrochemical combustion plant for rural water disinfection and industrial organic effluent removal

Cronje, Martin (2004-04)

Thesis (MScIng)--University of Stellenbosch, 2004.

Thesis

ENGLISH ABSTRACT: Recent years have seen the development of various treatment methods for the purification of industrial waste waters due to the increased demand for reduced pollutant effluents. Aqueous waste streams containing toxic organic compounds are of special interest, since conventional treatment methods such as biological waste treatment can not always be used. Other popular treatment methods are often ineffective. Catalytic oxidation of organic wastes has been investigated since the 1960s with varying degrees of success. A major problem associated with this method is the high temperatures and pressures required to improve the activation energies involved. Electrochemical oxidation has become a popular method in the literature of treating these wastes, since the applied voltage determines the activation energy, and therefore the process can often be performed at ambient conditions. This thesis investigates the capability of a unique reactor system in the treatment of these wastes. The reactor utilises proton-exchange membrane technology to eliminate the requirement of conductivity in treated waste streams; thus the membrane serves as a solid electrolyte. The reactor system has therefore been referred to as a solid-polymer-electrolyte reactor. Novel metal oxide anodes are responsible for the oxidation of the organic molecules. These metal oxide catalysts show promise in the treatment of a wide variety of organic wastes. A SnO2 catalyst doped with ZrO2 is used as anode in this study. Dopants are added to the catalyst to improve properties such as catalytic activity and conductivity. Kinetic data was obtained on a wide range of values for the chosen experimental parameters (current density and flow rate). Phenol, an organic molecule often referred to in the literature as model contaminant due to its resistance to oxidation,was also used as contaminant in this study. The use of the reactor system in the disinfection of water containing selected pathogens, were included in the experimental work. This kinetic data served in the development of a simple model of the process, and provided the basis for a full analysis regarding potential scale-up and economic feasibility. A requirement of the study was the accurate determination of the various oxidation breakdown products of phenol. This led to the refinement of an HPLC analytical method in order to quantitatively determine these products. The full analysis showed that the current reactor system would not be economically viable — mainly due to very long reactor lengths required for the complete removal of all organic material. Both mass transfer and charge transfer at the chosen experimental conditions influenced the electrochemical oxidation of phenol. High pressure drops, causing low flow rates in the reactor, accounted for this because of the narrow flow channels required in the reactor. Some catalyst deactivation was also suspected to affect the overall reaction, but the full extent of the deactivation was not investigated thoroughly. There is still room for improvement in the electrochemical oxidation of organic wastes. The design of the flow channels, a factor that was not investigated, can significantly improve efficiency. Another aspect that was not investigated was the catalyst type. The catalyst has been identified in the literature as the main contributing factor to the success of the oxidation reaction. A wide variety of metal oxide catalysts are currently being researched and may improve the kinetics of the process even further. Further improvement needs to be made on the membrane/electrode assembly to improve current density distribution. Every improvement of the process in terms of the reactor design and catalyst will impact on the economics of the process, thus making the process more competitive with current treatment technologies.

AFRIKAANSE OPSOMMING: In die afgelope paar dekades, is daar ’n wye verskeidenheid metodes ontwikkel wat gebruik kan word om industri¨ele afvoer strome te behandel. Hierdie ontwikkeling het plaasgevind as gevolg van die verhoogde eis aan skoner afvoerstrome. Wateragtige afvoerstrome wat organiese verbindings bevat, is van besonderse belang omdat hierdie tipe strome soms besonders moeilik kan wees om te behandel. Gebruiklike metodes is in die meeste gevalle ongeskik vir behandelings-doeleindes. Katalitiese oksidasie is sedert die 1960’s gebruik, maar hierdie prosesse benodig dikwels ho¨e drukke en temperature om suksesvol te wees. Elektrochemiese oksidasie het intussen ’n populˆere behandelingsmetode geword, aangesien die aktiveringsenergie vir die oksidasieproses hoofsaaklik afhanklik is van die aangewende potensiaal en dus kan die proses by atmosferiese toestande gebruik word. In hierdie tesis word die geskiktheid van ’n unieke reaktorstelsel vir water-suiwering ondersoek. Die reaktor gebruik ’n proton-uitruilings-membraan om die behoefte vir konduktiwiteit in die water uit te skakel. Die membraan dien dus as ’n tipe soliede elektroliet en as gevolg hiervan word na die reaktorstelsel verwys as ’n soliede-polimeer-elektroliet reaktor. Nuwe metaal-oksied anodes word in die reaktor gebruik aangesien hulle belowende resultate toon in die oksidasie van organiese verbindings. In die navorsing, is ’n SnO2 katalis wat klein hoeveelhede ZrO2 bevat gebruik. Oksiede soos ZrO2 word dikwels gebruik om die aktiwiteit en konduktiwiteit van hierdie kataliste te bevorder. Kinetiese data is oor ’n wye bereik van parameter waardes ingesamel. Die hoof parameters in die eksperimentele werk was stroom digtheid en vloeitempo. Fenol, ‘n komponent wat volgens die literatuur in hierdie tipe van werk gebruik word, isas die besoedelende komponent gekies. Die doeltreffendheid van die reaktor in die ontsmetting van water, wat met ’n verskeidenheid skadelike mikro-organismes besmet is, is ook getoets. ‘n Eenvoudinge model is opgestel m.b.v. die kinetiese data, waarna ’n volledige analise met betrekking tot grootskaalse bedryf en ekonomiese uitvoerbaarheid gedoen is. ‘n Vereiste van die studie was om die konsentrasie van die afbreek-produkte van die oksidasie akkuraat vas te stel. As gevolg hiervan is ‘n ho¨e-druk-vloeistofchromatografie analitiese metode verfyn. Die analise het getoon dat die reaktorstelsel nie ekonomies sou wees nie. Een van die hoofredes hiervoor is die onrealistiese reaktorlengtes wat benodig sou word. Resultate het getoon dat die reaksie deur beide massa-oordrag en lading-oordrag be¨ınvloed word. Ho¨e drukvalle in die reaktor wat gelei het tot lae vloeitempo’s was hiervoor verantwoordelik. Die deaktivering van die katalis be¨ınvloed waarskynlik die reaksie, maar die deaktiveringsverskynsel is nie ten volle ondersoek nie. Die reaktorstelsel kan verder verbeter word deur verskeie elemente van die reaktor te ondersoek. Die ontwerp van die vloeikanale in die reaktor is nie ondersoek nie en kan die werksverrigting van die reaktor verhoog. Uit die literatuur is gevind dat die tipe metaaloksied wat as katalis gebruik word, die reaksie direk be¨ınvloed. Dus kan navorsing wat tans op die kataliste gedoen word nuwe kataliste na vore bring wat meer doeltreffend sal wees. Laastens, is die huidige membraan/elektrode samestelling nog oneffektief en kan die reaktor-opstelling dus nog verbeter word. Elke verbetering wat op die bogenoemde faktore van die reaktor ontwerp verkry word, sal die ekomoniese uitvoerbaarheid van die proses be¨ınvloed. So, sal die proses al meer kompeterend met huidige behandelingsmetodes word.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/16292
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