Advances in Water Treatment and Enviromental Management

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Chapter 12GAS LIQUID MIXING AND MASSTRANSFER IN THE WATERPROCESSING AND CHEMICALINDUSTRIES: A BRIEF COMPARISONOF THE TECHNOLOGIESA J Green and M J Whitton (BHR Group Ltd, Cranfield, UK)1. IntroductionThe aim of this paper is to briefly review, compare and contrast some of thetechnologies employed in the aerobic treatment of wastewater, against thoseused by the chemical/pharmaceutical industries for gas liquid mass transfer.The paper does not profess to provide a detailed analysis of the pros and consof the technologies, but more suggest areas where transfer from one industryto another may be of value.The paper limits its scope to devices employing gas in liquid dispersions, ratherthan liquid in gas dispersions or liquid films. Thus such devices as packedcolumns, plate columns, trickling filters etc. are not covered.2. Gas Liquid Mass TransferThe rate of mass transfer of a specified component, e.g. oxygen, from a gas to aliquid can be described by:-whereJ = mass transfer rate (kg s-1 )K L = liquid side mass transfer coefficient (m/s)V L = volume of dispersion (m 3 )a = interfacial area/unit volume (m -1 )C L * = Interfacial concentration of the specified component in the liquid(kg m -3 )C L = concentration of the specified component in the bulk of the liquid(kg/m 3 )(1)©1991 Elsevier Science Publishers Ltd, EnglandWater Treatment—Proceedings of the 1 st International Conference, pp. 117–128117
118 WATER TREATMENTFor the aeration of water, the mass transfer rate must be sufficient to satisfy theoxygen demand, both chemical and biological, over the required treatment period.Equation 1 identifies the main controlling variables in the process:1. The mass transfer coefficient (K ). This is affected to some degree by theLlevel of turbulence in the liquid,but more significantly by the liquid physicaland chemical properties. Thus K will depend on the effluent composition.L2. Interfacial area (a). This will depend on the degree of turbulence in thesystem and the fluid properties. The physical and chemical properties ofthe liquid medium influence bubble coalescence behaviour and hencebubble size distribution.3. Concentration driving force ().For the calculation of k a the interfaciallconcentration is assumed to be the liquid concentration which is inequilibrium with the bulk gas concentration. The concentration drivingforce can be increased either by increasing the percentage of oxygen inthe gas (eg use of pure oxygen rather than air) or by pressurising the gas(eg by increasing tank depth).To cope with the uncertainties of 1 and 2, many manufacturers quote a so called“a-factor”, which is the ratio of oxygen transfer rate in the actual effluent to that inclean water (where most development work has been carried out). K and a areLdifficult to measure independently so the vast majority of literature data are reportedas the product k a.lIn addition to the primary purpose of supplying oxygen, mixing systems often haveto satisfy a number of additional functions.(i) Good liquid mixing to prevent ‘dead zones’,(ii) Suspend solids,(iii) Strip out Carbon Dioxide produced by the bacteria.3. The Differences between Water and Chemical Process RequirementsRequirementsBefore looking at the techniques employed in the two industries, it is worth identifyingsome significant differences between the requirements. These, in general terms,are as follows:1. The value of the product. Water is probably the ultimate “low value”product. As such, running and capital cost are the key considerations inthe design. For higher value chemical, and particularly pharmaceutical,products cost is of lower importance. If a system can be designed, forexample, to increase product yield, this is very likely to overwhelm anyincrease in development or capital/running costs.
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ADVANCES INWATER TREATMENTANDENVIRO
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ELSEVIER SCIENCE PUBLISHERS LTDCrow
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PART VI—RIVERS AND RIVER MANAGEME
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FOREWORDThe quality of the environm
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PART IPolicy matters
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ENVIRONMENTAL STEWARDSHIP: THE ROLE
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18 WATER TREATMENTTable 1: Comparis
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EUROPEAN DRINKING WATER STANDARDS 2
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Chapter 3COST ESTIMATION OF DIFFERE
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COST OF DIFFERENT WATER QUALITY PRO
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32 WATER TREATMENTThe Act enabled t
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34 WATER TREATMENTSome of the descr
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36 WATER TREATMENTTable III: Parame
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38 WATER TREATMENT8.3 Deficiencies
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40 WATER TREATMENTAPPENDIXSCHEDULE
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Chapter 5CONTROL OF DANGEROUSSUBSTA
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CONTROL OF DANGEROUS SUBSTANCES IN
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Chapter 6THE GREATER LYON EMERGENCY
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GREATER LYON EMERGENCY WATER INSTAL
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Chapter 7EXPERT SYSTEMS FOR THEEXPL
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EXPERT SYSTEMS FOR PURIFICATION STA
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Page 140: PART IVControl and measurementtechn
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COLLECTION EFFICIENCY OF LIQUID/LIQ
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Chapter 17UNDERSTANDING THE FOULING
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FOULING PHENOMENON IN CROSS-FLOW MI
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Chapter 18ADSORPTION OFTRIHALOMETHA
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ADSORPTION OF TRIHALOMETHANES ON TO
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Chapter 19DEVELOPMENTS IN IONEXCHAN
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DEVELOPMENTS IN ION EXCHANGE DENITR
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200 WATER TREATMENTL m = molar liqu
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202 WATER TREATMENTliquid-liquid ex
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204 WATER TREATMENTThe investigatio
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206 WATER TREATMENTAn additional 14
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208 WATER TREATMENTFig. 7b Concentr
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210 WATER TREATMENTTABLE 1Design pa
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212 WATER TREATMENTTABLE 2Performan
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214 WATER TREATMENTdosing of Calgon
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216 WATER TREATMENTGreen D W & Molo
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Chapter 21GREEN ENGINEERING: THE CA
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GREEN ENGINEERING: THE CASE OF LAND
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Chapter 22PREVENTION OF WATERPOLLUT
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NON-POINT DISCHARGES FROM URBAN ARE
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Chapter 23THE RIVER FANE FLOWREGULA
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RIVER FANE FLOW REGULATION SCHEME 2
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Chapter 24RIBBLE ESTUARY WATER QUAL
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RIBBLE ESTUARY WATER QUALITY IMPROV
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256 WATER TREATMENTThe long term ef
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258 WATER TREATMENTIn the bottom of
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260 WATER TREATMENT7.1 Efficiency o
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262 WATER TREATMENT8. THE RESULTS8.
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264 WATER TREATMENTSS and AramN lev
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266 WATER TREATMENT8.5 RainfallRain
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268 WATER TREATMENTsimultaneously.
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Advances in Water Treatment and Enviromental Management

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