∗ separated by a band 22 ) is the catalyst of choice 2 , IDeaS, Innovation, Design and Sus- 2011 Society of Chemical Industry acts as a photocatalyst due to c , an electron is promoted from the 2 2 TiO 2 Titanium dioxide (TiO , is absorbed by TiO 4 g E Correspondence to: Peter K.J. Robertson tainability Research Institute, Robert Gordon University,AB10 Schoolhill, 1FR, UK. Aberdeen, E-mail: [email protected] IDeaS, Innovation, Design and Sustainability Research Institute, Robert Gordon University, Aberdeen, UK st configurations being reported. The key considerations effluents. Furthermore they have also been applied to the ∗ st. Photocatalytic reactors have the potential versatility to valence band to the conductionelectron band. This in generates the a conductance reducing valence band band and an (Fig. 1).may oxidizing hole recombine The in with excited the energy. the conduction Alternatively valence they band may band electrons undergo be holes reactions trapped generating in with surface heat electron states donating and or accepting species scale studies. The centralreactors problem is of the provision scale-up of ofcatalyst sufficient photocatalytic high and specific the surface uniform areaarea. of distribution of illumination across this MECHANISM OF HETEROGENEOUS PHOTOCATALYSIS As a process for wateradvantages purification, over photocatalysis many has existing numerous technologies.result The in technique the can mineralization ofthem pollutants to rather an than alternative transferring carbon phase, adsorption. such Furthermore, as photocatalysis is doesthe not the require use case with of activated or hazardous materials ozone. such as hypochlorite, peroxide as it isphotocatalytically inexpensive, active. non-toxic, chemically stable and is highly its electronic structure, characterizedvalence band by and empty an conduction band electronically filled gap. If a photon ofenergy, energy greater than or equal to the bandgap to fuel products have also been reported. This paper considers tive mass transfer of pollutants to the photocatalyst surface eactors for waste and potable water treatment plants has also m; suspended catalyst mass transport; rate control Semiconductor 1–6 and for the inactivation 8 and for the remediation of oil : 1002–1017 www.soci.org viruses 86 6,7 15–18 2011; Semiconductor photocatalysis has also been 1–5 9,10 photocatalyst; reactor; fluidized bed; immobilized fil nitrogen fixation

19–21

11–14 2011 Society of Chemical Industry In order to demonstrate the viability of semiconductor Heterogeneous photocatalysis for the remediation of polluted c of cancer cells. applied to thegas, photo-splitting of water to produce hydrogen photocatalysishasbeenappliedtoadiversearrayofenvironmental problems including air, potable, andversatile process wastewater has also been treatment. utilized for This the destruction oforganisms micro- such as bacteria, photocatalysis for environmental remediation,an reactor equally design critical is development factor. aims Effective to reactor scale designto up laboratory research industrially bench and scale feasiblereactors processes applications. is, Scaling however,needing up consideration a photocatalytic to complexefficient yield process. a process These technically factors withand and include photocatalyst, economically pollutant distribution many mass of transfer, factors irradiation pollutant reaction kinetics, characteristics. and Theillumination is issue particularly important as of this essentially determines the effective amount of photocatalyst water that may bedeployed treated photocatalyst. per A effective unit wide area range of have of been photocatalytic developed reactors and used in both basic research and pilot water streams falls intobasic two chemical distinct transformations areas. on First,materials. semiconductor These the photocatalyst investigations focus have is concentratednation on on of the basic exami- photocatalytic processesmaterial including photocatalytic science, surface interactions onmechanisms, photocatalysts, reaction and kineticsmolecular that level. impact on the processes on a J Chem Technol Biotechnol INTRODUCTION The application of semiconductorengineering photocatalysis and in science the is fields anhas of important grown area significantly of in research, thenumbers which of last publications three appearing decades every year. with increasing Abstract Research in the fieldwith of an photocatalytic reactors extensive in rangefor the of photocatalytic past suspended three reactors, and decades however, has fixed remain been film an the photocataly areabe same; of applied effec extensive to and the diversetreatment remediation activity of of potable a waters. The range scale-up of of water photocatalytic andthe r main gaseous photocatalytic reactor configurations that have been reported to date. Keywords: remediation: a review Cathy McCullagh, Nathan Skillen, Morgan Adams and Peter K.J. Robertson Photocatalytic reactors for environmental (wileyonlinelibrary.com) DOI 10.1002/jctb.2650 Review Received: 16 December 2010 Revised: 24 March 2011 Accepted: 12 April 2011 Published online in Wiley Online Library: 18 May 2011 and effective deployments and illumination of the photocataly been demonstrated. Systems for the reduction of carbon dioxide spills.

1002 1003 40 . There et al reported 43,44 48 . slurry, and a 2 This study was et al 46 Nan Chong . 2 39 )(8) who demonstrated the o I / I 47 . wileyonlinelibrary.com/jctb ln ( 42 et al =− . The authors proposed that these 1 λ − represents the light intensity obtained o I ms (ANE) 4 − 10 ANE depends on the particle size and is inversely × 41 ] as a function of time. Pramauro − The photocatalyst film thickness may affect the 38 . 45 is the outgoing light intensity after interaction with I catalysts have been investigated in both slurry/suspension 2 It has been reported that the ratio between backward reflected An alternative approach is to attach the catalyst to a transparent An early slurry based reactor effected the complete miner- the complete degradationherbicide, of within monuron 1 h (Fig.investigated 2(a)), of were photocatalysis. a simulated The persistent solar reaction irradiation, conditions TiO are, however, problemswhich associated include the with dependence immobilizedthe on systems, photocatalyst mass transfer surface ofphotocatalyst and pollutant surface ensuring to of effectivemolecules. access both to activating the photons and reacting proportional to the particle size where: complete dehalogenation of chloroformincrease in but [Cl also illustrated an and incident photon flow iswithin strongly influenced the by the reactor. geometry coefficient The for slurries Apparent is a Napierian parameterabsorption rate. that Extinctance is related (ANE) to the photon novel nanoparticles could delivercatalyst a separation true on engineering an industrial solution scale.it to It was was difficult also to reported effect that irradiation ofin all the the photocatalyst slurry particles in theparticles unit in due the to body shielding ofwalls. from the Therefore the unit depth light by of source particles light closer of was penetration restricted. to into the the reactor slurry reactor Suspended liquid reactors Slurry/suspension systems TiO in a blank setup.cannot be The separated from scattering, absorption which makes ofof kinetic experiments analysis light more challenging. within slurry systems stationary support over whichIn such the a contaminated system waterof it passes. all is possible the to achieve photocatalyst effective deployed illumination in the reactor. batch reactor. Severalafter intermediate 30 min products irradiation; these were wereirradiation not determined time. detected The at complete the degradation end of and3,4-dichloropropionamide(Fig. the dechlorination of 2(b)),anotherpersistentherbicide, have reported the use of H-titanate nanofibresphotoreactor. in They an studied annular slurry congo red asshowed a that model one compound and of theof key the advantages catalyst. was Using the Kynch’s theorythat settling the batch velocity H-titanate settling trials nanofibre revealed photocatalysts resultedvelocity in of a 8 settling where catalyst suspension and alization of chloroform to chloride and CO internal mass transferphotocatalyst as material it in may theinterface. not Each proximity of be these of factors possible will the result inof to support–catalyst a reduction decomposition access in in the rates the immobilizedcompared film with photocatalyst slurry processes. units when systems and immobilized systems. Thephotocatalyst main slurries advantage of is using thethe immobilized system. larger The separation surface of the areaparticles nanometre catalyst compared is, however, with expensivecommercialization and of is a this major type drawback of in system. the further supported by Kormann ) 2 − (2) (4) (5) (6) (7) 2 2011 Society of Chemical Industry 3,23,24 c )(1) vb . + 2 h − + 2 O cb OH 36 − + + (e 2 • ,ads 2 • 2 •− 28–31 O 2 2 OH Oxidized products (3) TiO OH O HO H • 34 → → → → → → → surface. The electron and holes : 1002–1017 −− −− −− −− −− −− −− 2 − − − − − − − 86 27 38 • ν + cb 2 h ,ads H − 25,26 2,ads − e 32,33 + 2011; HO + O 2 + 35 + OH + 37 •− 2 Reactant • 2 tr TiO + 2 O O + 2 − e vb H HO + ,ads h • OH Processes that occur on photo-excitation of TiO through a reaction of the valence band holes with adsorbed 2 swirl flow reactor, corrugated plate reactor. fluidised bed reactor, coated fibre optic cable reactor, falling film reactor, thin film fixed bed sloping plate reactor, packed bed photoreactor, photocatalytic Taylor vortex reactor, annular photoreactor, Table 1 shows a comprehensive yet not exhaustive overview of Examples of these various reactors will be considered in the Figure 1 illustrates the basic processes involved in Photocatalytic reactors for environmental remediation: a review www.soci.org (Equation (4)). Thermodynamically the redox potential of the TiO photoreactor type, reactant phase, experimentalemployed, targets, and catalyst industrial applications. following sections. photocatalysis. formed are highly charged and cancan induce redox ultimately reactions, result which in theHydroxyl mineralization radicals of are aqueous believed pollutants. toTiO be generated on the surface of water, hydroxide orphotogenerated conductance surface band electrons titanol reactacceptors with groups electron such (Equation (2)). as The oxygen which generates superoxide (O J Chem Technol Biotechnol • • • • • • • • TYPES OF REACTOR CONFIGURATIONS A wide variety of reactor configurations haveliterature been over reported the in past the 30 years including: • electron/hole pair shouldperoxide, enable primarily the via production the(Equations (1)–(7)). of reduction of hydrogen adsorbed oxygen Figure 1. that are adsorbed on the TiO

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1004 1005 53 55 slurrycatalyst slurry may be 3 2 slurry reactor. 2 CH 2 2 O H C Employing photocatalyst for the removal of oil and NH C 54 C NH wileyonlinelibrary.com/jctb 3 charge.TheTiO 57 n of trichloroethylene, with a 2 N CH O (Fig. 3). 56 C Cl 3 H Depending on the level of contaminant reported the comparison of a standard flat plate 56,57 Cl 56 . Cl (B) (A) et al Chemical structure of the herbicides Monuron and 3, A study of the adsorption and photocatalytic degradation of the Adams The reactor set up utilized three connecting drum reactors. present the effluentfor would prolonged pass treatment. Onin from average each one the drum ‘drum’ sample fortime of would to approximately approximately reside 10 3 another min. min In the with event an of the overall final sample reaction still reactor with a novel drum reactor safira HEXL dye, has been reported using a TiO Figure 2. 4-dichloropropionamide. concentration. Furthermore, the additionenhanced the of reaction rate electron significantly. acceptors The authors concluded that dyesurface adsorption was to critical the for photocatalyst processwasalsopHdependentwithanimproveddegradationrate efficient photocatalytic degradation. The observednearthepointofzeroTiO This overcomes one ofseparation of the catalyst from major the remediated drawbacks waste of stream. slurry systems, The paddles positionedwere placed on to the allowsolution the for inside removal exposure of ofthe to pellets main the from UV stock. the reactor illumination reaction drum before returning to filtration with a bubble column reactor and TiO an efficient process for the destruction of phenoxyacetic acid. removal efficiency of 97% reported. has also been employedcolumn within reactor an for internally the circulating degradatio bubble gas hydrocarbon contaminants from water.was The constructed flat from plate polymethylmethacrylate design (PMMA)a and small-scale was laboratoryreactor unit, was however, a proposedwas ‘concertina’ for a designed scale single up. passwastewater/effluents, The which continuous utilized rotating drum flow paddles to reactor ensure system an even for concept distribution of the catalyst treatment of 51 sus- 2011 Society of Chemical Industry 2 c .Inthe 1 − using a TiO was obtained with 2 49 concentration, and higher 2 compared the effectiveness 35 batch suspension photoreactor. : 1002–1017 2 86 was determined as part of this investi- 2 2011; investigated the kinetics of degradation of 50 investigated the photodegradation of 3- 52 . et al Further work by Li Puma and Yue San Photocatalytic reactors for environmental remediation: a review www.soci.org photocatalyst levels caused a shielding effect ofLi the incident Puma light. and Yue pension photocatalytic reactor. It was reported that dechlorination of the herbicide was dependent on TiO aminophenol (3-AP) in a TiO single component experimentsto the allow kinetics the were authorsway to too would determine complex be whether preferential the toversa. photolytic the During path- photocatalytic pathway the or multi-componentthat vice experiments the overall they oxidation kinetics observed werein controlled by excess, the as reactant the substratefound present to in degrade smaller at concentrations acomponent was much experiments. Overall slower the rate authors thanthe entire demonstrated that course that of in photocatalytic the oxidation of single- and single-component multi-component systems of chlorophenols can besatisfactorily predicted using simple kinetic modelsthe that design could and be modelling usefulThis of in large-scale work photocatalytic was reactors. followedflow laminar by falling studies film of slurry a photocatalytic reactor pilot-scale (LFFSIW), continuous- J Chem Technol Biotechnol which utilized commercially availablesive UV investigation lamps. of A thisof comprehen- unit parameters, was including reportedincident the looking irradiation wavelength source, at the and areagents, influence the range intensity concentrations of of additional of both reactants oxidizing the and and the photocatalyst, irradiation time. Li Puma andphoton Yue based also processes, studied and determined six that different the UVC photocatal- ysis/UVCphotolysis/UVCperoxidationprocesswassuperiortoUVA photocatalysis, UVC photocatalysis/UVC photolysis,catalysis/UVA peroxidation, UVA UVC photolysis/UVC photo- peroxidation and UVC photolysis systems. The results of the experimentsat conducted different incident radiation intensities clearly indicated that low- wattage UVC lamps were preferablebecause enhancements to of high-wattage reactor conversions UVC due lamps to higherpower lamp were offset by an increase in electricity costs. was reported by Pathirana and Maithreepala the UVC-photocatalysis-photolysis-peroxidationand process the (28%) lowest value(1.7%). The was mineralization efficiency with of the the photocatalyticphotolytic-photocatalytic UVA-photocatalysis and processes the was process in the rangeabove 58–65%, that far of the14–18%. photolytic process which was in the range of of a range ofactor. photooxidation They processes investigated UVA-photocatalysis, in UVA-photocatalysis- aperoxidation, falling UVC-photolysis, film UVC-photolysis-peroxidation, pilot UVC- re- photocatalysis-photolysis andperoxidation. They selected salicylic acid UVC-photocatalysis-photolysis- as a model compoundall for experiments. The highest conversion ofwhich salicylic was acid was obtained 58%, withcess. Conversely the the highest UVC-photolysis-peroxidation conversion to pro- CO single component and multi-componentnols. systems They of studied chlorophe- thelong simultaneous wavelength ultra-violet use light to of investigate the short,simultaneous integration photocatalysis of medium, and and photolysis. Theof optimal loading photocatalyst for theoxidation geometry of of 2-CP their to batch CO reactor via the gation, with mineralization ratesup achieving a to three-fold a increase maximum photocatalyst loading of 0.5 mg L They reported that this process followed pseudo-first-orderics, kinet- with the apparent rate constant depending on the initial 3-AP . 62 . et al et al : 1002–1017 86 2011; investigated the effects 64 . slurry reactors 2 et al bined with 30 g catalyst and Degussa did not observe either intra or extra 60 found no diffusion limitations within their J Chem Technol Biotechnol 63 (A) Temporal absorption spectral pattern displaying the degra- found a loss in reactor efficiency when the optical thickness 61 . (B) (A) Mass transfer within slurry reactorsof has not attention received a primarily greatrecognized because deal as being such major problemsslurry impediments reactor. Chen have to and the Ray not application been of a Peralta Muniz Moreira Figure 4. dation of MB overcatalyst (pellet a form) 60 min only,P25 UV time (powder) com period effect and onwith MB (B) permission effect degradation. from (Reproduced Elsevier). of from UV reference only, 58 30 g Mass transfer limitations within TiO reactor when pseudo-first-order kinetics were appliedthe along Weiz with and Prater criterion.rate How of mass transport reaction influences within the to slurry obtain reactors relevant kinetic is information very aboutfor the important reaction photoreactor in but order also design. Ballari of the suspension was greater than a calculatedlarge optimum level. catalyst For loading, restrictions on mass andinside radiation the transport catalytic particle have been reported by Mehrotra particle diffusive limitations when working with areactor. suspended Considering solid the optical thickness of the suspension Martin et al 59 www.soci.org C McCullagh pellets was 2 compared with catalyst present, 2 2 O of TiO 2 2011 Society of Chemical Industry 1 − described the degra- c The addition of air and 58 . 56 et al 50% abundance reduction in > In this study the unit achieved a 50% 59 Schematic of photocatalytic drum reactor. (Reproduced from Further evaluation of the drum reactor has been carried A subsequent paper McCullagh dation of methylene blue (MB)reactor in (Fig. (4)): a 98% slurry degradation continuous oftion flow utilizing MB a over drum high 60 loading min weight of of illumina- 30 g L therefore utilizing 600 g for the entire system.was The investigated flat plate with design regard to optimizingmaximum conditions destruction. to It provide was concluded that aincreases lower retention angled plate times of compoundssuccessful and catalyst–pollutant thus interface. the chance of hydrogen peroxide to increase destruction proved80% effective, degradation with over 135 min recorded for H wileyonlinelibrary.com/jctb being of high hydrocarbon concentrationrecirculated the into effluent the would system. be Asdeemed an more attempt environmentally to develop efficient,used a pelletized reactor to catalyst reduce was with downstream the filtration of processing powdered catalyst. While restrictions both designs proved effectiveintheremovalofhydrocarbonsthedrumreactorachieved associated 90% removal in less than 10a min. This short high level period of of reductionthree over time consecutive is drums each attributed with 200 to g samples of TiO passing through Figure 3. reference 58 with permission from Elsevier). reported. The investigation of different loading weights concluded that a maximum weight of 18098% g degradation of was catalyst recorded was for attainable. 30 g While 60 catalyst min, the (pellet use form) of over Degussa P25 asmore a photocatalyst efficient, was significantly with 90%photocatalysis of (Fig. 4(B)). the MB decomposed within 20 min out in theinterceptor remediation tank. of oily waste water (OWW) from an 40% for air alone. tetradecane. Following a further 90 min illuminationOWW period in of the the reactor all volatileidentified organic compounds were (VOC) initially almostorganic completely carbon removed. TOC Additionally wasin investigated total TOC showing for a OWW 35% samplesresults reduction passed highlighted twice through demonstrate thephotocatalytic reactor. a reactor The with high large scale efficiency applicationstechniquetobecoupledwithcurrentwatertreatmentprocesses. as for a polishing a novel reduction in abundance90 for min photocatalysis both along with decane and dodecane after

1006 1007 and 2 could 3 particles O 2 2 photocatalyst reported that 2 65 The design of this reactor 67 wileyonlinelibrary.com/jctb bed location. (Reproduced from concluded that this process required 75 . (minimum fluidization velocity) which focused on the decomposition of acetic et al 70 mf investigated the use of combined TiO –NOsystem. 70 2 in an attempt to overcome the drawbacks associated . 3 catalyst. Son Schematic of flat plate fluidized bed reactor displaying the O et al in the photocatalytic reaction of NO. Utilizing an annular 2 2 75 reached 2.5, where a 70% conversion was achieved. Based . 0 concentrations. A two-dimensional fluidized bed reactor was used by Lim Son 2 mf et al acid and ammonia utilizing a three-phase photocatalytic system. with fluidized reactors for photocatalysis. Paz adequate residence time and an effectivebubbles gas of velocity an appropriate to size produce to ensureand contact between the UV TiO light upon their results Lim and the reactant gaslight (NO) transmission which, whenreaction. to coupled A the with effective seriesmeans unit, of to facilitated alteringgas increase the concentration, conditions photocatalytic photocatalytic residence were timeand efficiency, of assessed irradiation gas, including reaction as intensity. temperature, publication initial a A surrounded particular the pointtransmission superficial of interaction. gas interest Resultsincreasing in displayed velocity superficial in this and gas Fig.in light 7 velocity show increased the that lightapproximately reactor. transmission 1.3 The U light intensity significantly increased at flow-photoreactor the authorsenabled reported that efficient this contact configuration between a P25 TiO circulated fluidized bed reactor (CFDB). Figure 6. illumination directionreference and 66 with permission catalyst from Elsevier) utilised a loop seal whichby allowed the particles air that stream wereSeveral other carried to factors up were flow investigated in backwavelength, this initial paper down TCE including concentration, and UV circulation rate, re-enter and O the system. eliminate this problemAl–TiO and as such many researchers use an fluidization of small particles such as‘drifting’ P-25 from was the challenging due primary to operationthe area catalyst, in the however, unit. with Combining larger particles such as Al H with Al equated to a sharpthe increase UV in light NO transmission conversion. throughThe Below the photocatalytic catalyst this conversion bed value of was NOU minimal. continued to increase until 2011 Society of Chemical Industry c mol (g µ [2 1 − min 1 − used a flat plate fluidized 66 : 1002–1017 m reference 33 with permission from 86 , and effective mixing. catalyst loading, irradiation rates of 1 1 − s − 2011; 1 mol TCE (g catalyst) − µ Fluidized bed reactors are capable of utilizing 8 . ]. These results are significant in that they are 1 32,64–74 − Einsteins cm Schematic of fluidized bed reactor utilized for the destruction min 7 − 1 − ) 10 2 low pressure drop, high throughput, and high photocatalyst surfacecatalyst–reactant area, interaction. which allows for increased In 1992 Dibble and Raupp The advantages of this style of reactor include: × Photocatalytic reactors for environmental remediation: a review www.soci.org an upward stream ofstationary fluid phase to (gas be or brought toallowing liquid) a for to suspended photocatalytic or transformations allow ‘fluidized’ to state particles occur (Fig. in (5)). a • • • Fluidized bed reactors The applications of fluidized bedreported with reactors around 1000 have papers published been on this extensively topicpast over 6 the years. TiO 1 J Chem Technol Biotechnol of photocatalyst irradiation,total photocatalyst suspension loadings, volume, flow andthe rates, changes reactor. in They illumination found lengthwere significant of likely concentration to gradients cause that the limitations non-uniformity in of mass the transport irradiationthese resulting area. concentration The from gradients authors are state difficulteradicated that to if avoid fully but developed could turbulentreactor. be flow They operated concluded within that the overcome mass transport using problems a could be 1 g L Figure 5. of microcystin-LR. (Reproduced fro Elsevier). comparable with results produced infor a liquid–solid the oxidation slurry system ofmagnitude TCE; increase. TCE specifically is demonstrating aby an standard many, evaluation order including test more of recently carried Lim out and Kim who investigated a bed reactor (Fig. (6))quantum efficiency to range of photoxidize 2–13%rate was trichloroethylene achieved with peaking (TCE). a reaction at A 0 . et al was half as : 1002–1017 2 86 2011; coating to overcome any fluidization 2 pellets to a fluidized state has also J Chem Technol Biotechnol also investigated the use of vibration to 2 catalyst for toluene removal. 2 74 The reactor consisted of a reaction chamber . 76 et al loading weights, and the use of glass beads (GB) to 2 Voronstov A fluidized bed system utilizing an upward stream of air oidage on light transmission with measuring light at (a) 96 mm, (b) 53 mm which contained amobile foraminated photocatalysts member along supportingfor with a an agitation bed aeration of of generated device photocatalytic gas to particles. allow bubblesagitation of The photocatalysts. through The reactor aeration configuration a wasfor utilized device the perforated treatment of shelf wasteThe water reactor concept in allowing was designed a to reduce flow or completely throughthe remove style need process. for moving parts, thus allowing for a reactor concept been reported. (AC) particles with a TiO which brought TiO improve fluidization of granular photocatalysts forsition the of decompo- gaseous acetone.were investigated A together variety with the of vibrofluidizedenable bed fixed efficient system bed to comparison. constructions Thethe vibrofluidized most bed effective system with was being an achieved. The 8.7% high efficiency increase of ininterestingly, the vibrofluidized attributed quantum bed to efficiency was, the externalwith the vibrations ‘periodic used light phenomenon’. together Thisfrom phenomenon resulted the eccentric photocatalystwhich movement consequently within enabled the increasedlight. reactor absorption of scattered problem and toresearch investigated promote the impact of an altered relativevaried evenly humidity TiO (RH), distributed catalyst. The replace AC along with and withoutof the activated use carbon of solely a reflector.however, is The effective upon use in saturation thedecreasedsignificantly. of removal Itwasestablishedthat30%RHwasoptimal of the toluene, ACfor particles efficient toluene toluene removal removal. Interestinglyresult increasing in RH increased did rates not competitive of adsorption toluene removal between suggestingat water there higher and is levels toluene ofaccredited molecules RH. to both The the use degradation of of activatedhowever, carbon toluene and saturation photocatalysis, vapours resulted was inusing AC. certain The results restrictions showed that when theof solely combination of toluene AC removal andextend the toluene removal photocatalytic duration. In removal comparingof the the could two effectiveness catalyst types significantly it was found that the GB/TiO effective as AC/TiO (B) . . 2 2 2 2 2 3 2 71 O and 2 2 –Al www.soci.org C McCullagh 2 65 .Theseproblems production rate of 2 were monitored as 2011 Society of Chemical Industry 2 2 c has led to an increased 2 production was achieved 2 and CO 2 when compared with increased ure after methanol destruction. who investigated the removal of 2 for CO 32 obtained with the packed bed reactor. . 1 1 − − et al ; the conversion of acetic acid to CO . In a previous study that used a FeTiO 2 min 2 min 95% being reached with 10 mol% Al–TiO 3 was found to be an effective photocatalyst 3 − > 3 − reported the comparison of a fluidized TiO O 2 72 . –Al mol cm however, found that replacing the Fe with Al produces 2 mol cm et al 7 70 7 − Results observed for the effect of (A) superficial gas velocity and (B) v combined particles and a fluidized reactor. ., − 2 (A) 10 10 with the use of Al–TiO et al × × 3 Nelson The drawbacks of fluidizing pure TiO 0 0 . . and (c) 10 mm. (Reproduced from reference 75 with permission from Elsevier) 1 number of papersdemonstrated reporting by results Kuo using combined catalyst, A similar trend wasa observed for conversion ammonia rate decomposition, with Overall TiO levels with pure TiO a catalyst withammonia decomposition. increased They concluded stability, fromthe their thus efficiency results of that providing removal ofAl–TiO enhancing VOC is increased by both the use of can often result in the powderedmain catalyst area ‘drifting’ of away operation. from the The investigation used activated carbon for the fluidized reactor compared with a CO wileyonlinelibrary.com/jctb system for methanol oxidationconcludedthatfluidizationresultedinfasterratesofphotocatalytic with a packed beddecomposition reactor. than They achieved on the2 packed bed unit. A rate of Son material it was reported that thetransformed anatase into structure a of rutile the catalyst struct compared with 70% inthe the suppression steady of reactor. the A more point of undesirable interest products is i.e. NO instead of solelyreached approximately TiO 90% after 600 min with 10 mol% Al–TiO Decomposition was enhanced when carried out in the three-phase fluidized bed reactor, witha significant steady improvement reactor. over Production use of in N Figure 7. which is in agreement with the results obtained by Paz. a means of measuring aceticIn acid terms and of ammonia acetic decomposition. acidincreased decomposition efficiency the over fluidized that bedalong showed of with a increased conventional efficiency steady reactor with the addition of Al–TiO NO It was reported thatin the use the of both processhowever, static to only mixing effective and reduce vibration with photocatalyst Degussa P25 separation and rates not TiO was, toluene vapours fromlimitations a included continuous thephotoreactor ‘loss gas due or stream. to trapping’ the The fine of structure specific powder of within TiO a

1008 1009 1 − for the 1 − h 2 − for the coated 1 − h , and a small reactor 2 2 − within the reactor. The rotating disk reactor for 2 2 − wileyonlinelibrary.com/jctb Wcm µ containing 21 U-shaped lamps of 3 composite ceramic ball photocatalyst m 2 4 of reactor volume was achieved compared − developed a TiO 3 10 81 . × investigated two fixed bed photocatalytic reactors, 65 et al dichloroacetic acid solutions using the packed bed . 80 . 1 − Schematic representation of rotating disc photo reactor. (Repro- et al Dionysiou Feitz 90% 4-chlorobenzoic acid after 6 h irradiation. The light intensity a packed bedillumination. reactor and They a assessed coated the mesh processing reactor, rate using solar for 2 mg L the decomposition of organicused pollutants in a water commercial (Fig. (8)). TiO They unit were only 40% lower than suspension systems.proposed They therefore that this systemcontaminated was water. particularly effective for treating material. LiCl tracer studies performed under different disk angular velocities, between 5 andthe 20 rotating disk rpm, photocatalytic reactor demonstrated RDPR was thata similar continuous to mixing that stirred of tank in reactor. They reported> the destruction of distribution within the reactor was alsovary determined from and found about to 30 to 1500 packed bed reactor with a rate of 20 mg m RDPR has aeffluent number filtration of as advantages: theof it catalyst the eliminates flow is created the immobilized, enablesof effective the need mixing, the 3-D while for thin the nature film formation allowsgas more phase effective to oxygen the transport photocatalyst from the surface. The photonic efficiency diameter 0.45 cm coatedincrease with in the reactorannular efficiency P25 photocatalytic was photocatalyst, reactor. achieved a Furthermoreefficiency compared a 695% was 259% obtained with for increase an thereactor. in new unit compared with a slurry phenol solutions, and calculated a rate of 140 mg m Figure 8. duced from reference 81 with permission from Elsevier) volume ratio while eliminatingabsorption the and scattering potential by lightenabled reaction loss a matrix. large through This surface configuration ofrelatively photocatalyst small to reactor be volume. deployed Between 70 withinin and a surface 100 area fold per increase m to tank volume ratio. Photonicof efficiencies for 100 the mg decomposition mesh reactor. The lower activityreactor was obtained believed to with be due the to insufficient photocatalyst coatedcontact, surface mesh low levels of available attached TiO with a conventionaldegradation annular reactor of configuration. specialwas The investigated brilliant photo- and 90% blue, photocatalyticachieved after a 100 destruction min of irradiation. model This the study was dye the followed dye up development with of a pollutant, tube light reactor whichincrease had a in 100–150-fold surface areacompared to per a unit conventional annular volume reactorfold design of and increase a fluid over 10–20- being anvolume treated immersion reactor. of In 3 a study of a reactor = 2011 Society of Chemical Industry c Relatively proposed 0.0065 for slope while 79 = 34 ◦ and found that ◦ 0.010, 0.17, and 0.08 supported on a glass = 2 particles via radial refraction slurry reactor. particles onto quartz fibres and 2 2 2 : 1002–1017 86 reported the photodegradation of angle. Ray and Beenackers 78 investigated TiO ◦ 2011; 77 The system was conceived to allow for remote light immobilized on a flat glass plate as a support. They 34 2 They reported a limited range of slopes 22–25 This study was followed with an investigation into the Nogueira and Jardim i. the fluid thickness film which flowed over the plate; and Photocatalytic reactors for environmental remediation: a review www.soci.org 89% was degraded at 25 methylene blue usingwith solar TiO irradiation on a fixed bed reactor matrix for the2,4-dichlorophenol, photo-decomposition and of 2,4,5-trichlorophenol. phenol,followed The 4-chloropheol, first-order degradation kinetics withsurface of the the reaction semiconductor. occurring The1 irradiation on filter source the was simulating an solaremploying AM- irradiation. a A fibre-optic fixedHoffmann. cable bed (OFR) reactor was system reported by Pill and ii. the light intensity that reached the system. investigated the slope of the plate andphotodegradation found was that influenced methylene by blue two factors: application of the OFR systemof to pentachlorophenol, the photocatalytic oxalate degradation and dichloroacetate. J Chem Technol Biotechnol Immobilized liquid reactors Fixed bed Al-Ekabi and Serpone with a reduceddesign. foot To ensure print constant agitation suggestingsettling of a the velocity particles of more the the terminal energyany particles must efficient upward not flow exceed of themore the than velocity 10 of liquid ms. through Theof reactor the pellets retained perforated which the both shelf advantage allowand, of by via for the agitation, reduced present use a downstream number processing ofby faces illumination. capable of excitation light was transmitted to the TiO of light out of0.011 the for fibre. the A oxidation maximum ofbe 4-chlorophenol quantum compared was efficiency to achieved. a of maximum This ø quantum can 4-chlorophenol efficiency oxidation of in ø a TiO a distributive type fixed bed reactor systemglass that tubes employed hollow as lightparticles. conductors for The distribution to reactor photocatalyst configuration increased the surface to distribution to photocatalysts, to effectivelyyields through determine effective quantum light flux measurement. Furthermoreallowed OFR for reactor reuseinput angles, to and to assess minimize potential heating different anddelamination. photocatalyst coatings They anchored TiO and light high apparent quantum efficiencies ofwere achieved ø for PCP, OXmineralization and DCA, reported. respectively, It with was complete had the concluded advantages that of the aefficiency fixed-bed OFR of unit system together a with thenot slurry kinetic reactor. only The thephotocatalyst OFR distribution within configuration but a enhanced also particularstandard reaction the fixed-bed volume designs. uniformity compared These oftransport to characteristics limitations for activated reduced photochemical conversion mass efficiencyallowed and higher processing capacities. Furthermore, potential light loss via absorptionminimized. or The OFR scattering system could by beflow used the in operation batch reaction or for continuous medium bothtransmission was liquid cable and also allowed orphotocatalyst. for gas remote phase light reactions. delivery The to the 95.8% of the model compound was degraded at 22 . 2 91,92 et al Long 90 : 1002–1017 photocatalyst 86 .Theyfurther 1 2 − h 2011; 3 The optical fibres were coated mixed photocatalyst. The OFR for the degradation of methylene 93 2 88 . coating on a tubular photocatalytic 2 –SiO 2 et al J Chem Technol Biotechnol The work focused on the effects of flow-rate 89 . 2 reported the use of a corrugated plate reactor 38 . (Fig. (9)). There was no limitation on mass transfer et al to fuels by a visible light activated catalyst has been 26 2 An annular photocatalytic reactor, assimilated to a plug flow Zhang A sol-gel prepared TiO An internally illuminated monolith reactor (IIMR) has been The photocatalytic oxidation of a non-ionic surfactant was reactorwithre-circulationmodeandabatchphotocatalyticreactor was investigated by Lin blue and phenol.effective photocatalytic The activity for sol-gel thecompounds decomposition film of in organic water synthesised andreactor demonstrated the for authors water proposedof purification. the phenanthrene, use During of 67.6% a this conversion destruction 180 to min CO was photoreaction observed with 40.1% reactor, with afibre fixed glass bed support was of reportedacetone Degussa for the P25 remediation immobilized of gaseous onto a reaction times were investigated for the photodegradation ofRed Acid using an immobilized Aeroxideflow Degussa rates P25 affected the catalyst. efficiency of Slower the system, withtimes mineralization varying from 35 h to 60 h depending on flow rate. coated individual channel walls.with this Photonic reactor efficiencies were obtained but below greater those than obtained that fora reported a reactor for slurry configuration an reactor with annular side photoreactor light and fibres immersed in a TiO They reported apparent quantum yields of10% 5% for formic for acid. oxalic acid and studied the remediation of Acid Red 18, an azo dye. The IIMR had sidechannels light of emitting a fibres ceramic incorporated within monolith the containing TiO slurry. The authors reportedcatalyst exposed that to illumination the and this IIMRother is photocatalytic had its reactors. key a advantage over larger area of Immobilized gas reactors The use of the Optical Fibreof Reactor (OFR) CO for the photoreduction reported in the investigation of multi-phase photocatalysis. observed either internally orconditions investigated. externally under the experimental on the decomposition ofconcluded that the the non-ionic optimum photodegradation surfactant. ofwas The the observed surfactant authors with a flow-rate of 11.98 dm reactor operated onin the two principle beams of when(Fig. first incident 10). in Part light contact of with being thefibre the split light surface and penetrates of creates the the layeroff excitation, fibre of while the catalyst the fibre onfibre. the other and beam This transmitted is allowed along reflected the the light length to of gradually the spread optical through the with a gel-derived TiO reported by Nguyen and Wu. carried out inphotocatalyst bed. a labyrinth flow reactor with an immobilized configuration whichchlorophenol was as developedconfiguration model and with pollutant. a assessed flatcorrugated They plate using plate reactor compared reactor 4- andmass was a the transfer reported slurry rates to new 600% reactor. higherThe The be than authors 150% that of suggested faster a thatcorrugated with flat the plate configuration reactor. enhanced was performanceilluminated a photocatalyst of surface result the area perwith of unit effective the volume, coupled delivery relativelyphotocatalyst surface. of larger both photons and reactants to the 2 86 . the 82,82 1 − et al www.soci.org C McCullagh The activity 85 film coated on the 2011 Society of Chemical Industry 2 c levels in the reaction 2 reported the use of a sealed rtmaterialsyieldedcomparable adsorbed on a quartz tube for to the surface. The wires were 83 2 for the degradation of oxalic acid . 2 2 et al They investigated the photo-degradation reported the use of a stirred tank reactor with reported the use of a photoreactor with TiO 87 . 81,82,84 27 . reported the use of an external lamp, annular et al 68 et al . immobilized on a stainless steel mesh and on a et al 2 coated mesh or cloth has been reported. 2 McMurray A tubular photocatalytic reactor for water purification using a Lim Mehrvar A novel photocatalytic reaction system, composed of solution immobilized Degussa P25 TiO photocatalytic reactor with TiO and formic acid. Themass rate transfer limited with of propeller speeds degradation greater than 1000 of rpm. both acids was not the degradation of phenanthrene and pyrenestream. from They reported a that dilute above a water feed velocity of 7 cm min wileyonlinelibrary.com/jctb Phenol, trichloroethylene, and bisphenolpollutants A to were examine used the as effectivenesscomplete of model degradation the of photoreactor. each The pollutantwas within observed, 2 with h the reaction authors time concludingof that the the reactor performance was dependent on the aeration of the system. coatedtellerettepacking.Thetellerettepackingswereconstructed from stainless steelpromote welding the adhesion wire of which TiO was ‘roughened’ to A similar study by Hamill calculated for the experimentanticipate this at value 4 rpm to was improve 2.7%. with The process optimization. authors inner surface of the tube has been developed by Zhang process was rate controlled andlimitations. not influenced by mass transfer ceramic cylindrical tube with a Pt-loaded TiO rotating photocatalytic reactor (RPC) of similar configuration to the Dionysiou RDPR. of TiO solution, which increaseddestruction of the organic compounds photocatalyticsystem in activity for water. The the for benefitcarbons photo-degradation was of the that of it this did aqueous notvolatilization require volatile of air the bubbling, organic contaminants which to resulted the in atmosophere. of dichlorobutene and examined thecombinations effects of of mass transfer pollutants. and effectively They degrade a reported range of substrates that and thatrate the the was degradation dependent on RPC rotation speed. They could also reportedconfiguration that this could be applied topollutants. both volatile and non-volatile photocatalytic activities the fibre glass clothThe was Teflon the most membrane stable. enhanced the O fibre-glass cloth using isopropanolinvestigated.Althoughbothsuppo as a model compound was then wound in a spiral orwound ‘spring like’ wire structure was to be‘spring’ cut formed. The into adjusted smaller toto lengths allow form and the the eachselected tellerete individual ends and shape. manufactured to The on thesufficient tellerette be basis light type dispersion that brought into they packings the would together effective interior were permit of photocatalytic the reaction bed to ratestransfer maintain with limitations. no The significantlight mass penetration photoreactor throughout allowed itsmass interior, substantial transfer and limitations had UV no duringof significant the 1,2-dioxane. It photocatalytic was degradation concludedcoefficients that of at interest, the the reaction range rate of atwould attenuation various radial positions not bephotocatalyst activity significantly was massmagnitude. increased transfer by limited at least unless one the and order gas of spacesTiO that were divided by a thin Teflon film and

1010 1011 (Fig. 11(a) 2 reaching 44%, was dependent 2 2 loading resulted in a 2 loading. These results were wileyonlinelibrary.com/jctb 2 photoreactor was attributed to loadings of 7.7, 9.4, 16.3 and 2 2 and formaldehyde selectivity further 2 and formaldehyde selectivity. The methanol 2 developed a Taylor vortex photocatalytic reactor into a Pyrex tube with internal illumination from content within the reactor. The authors concluded 2 28 2 loading where TiO photocatalyst was simply coated on the inside of the (P25)-water/ethanol (1 : 1) solution followed by drying. 2 2 2 supported carbon foam; carbon foam was immersed in 2 compared with 7% forthe carbon a foam wall supported TiO coated reactor. The efficiency of and (b)). The Taylor–Couettefluids in flow between is a the pair movement of of viscous coaxial cylinders which experience reactor wall. In this case the highestwas methanol conversion 22%, reached whereafter any increase in TiO supported the findings in the paper with CO the ability to increaseand the hence exposed increase surface theincreased of TiO surface the to carbon reactor foam ratio, which allowed screening effect of excess particles, resulting in noin further methanol increase conversion. CO The photoreactor was establishedsupported by TiO packing the carbon foam TiO impressive compared with those obtained in a photoreactor where the TiO aTiO an 8 W centralconversion UV-A and CO light. Results were based upon methanol 28.5%(w/w) achieved methanol75%, conversions of respectively. 52, Awas 57, achieved maximum 61 for methanol a and 666%(w/w) conversion TiO of 81% that the reactors airvery to low surface pressure ratio drops allow and its its use in ability practical to applications. perform at Suspension vs. immobilized reactors: a comparison Dutta and Ray that created unsteady Taylor–Couette flowcylinders between two through co-axial re-circulation ofreator fluids to from the the inner body cylinder of the wall, coated with TiO on TiO conversion with the carbon foam supported TiO 1 − h 2011 Society of Chemical Industry 1 –acac, − c 2 on optical was found 2 mol g –SiO 1 µ 2 − catalyst. These .Theproducts awing (B) of the annular photoreactor. (Reproduced from reference 26 with permission from h 2 2 1 supported on a and the presence 279 . − 2 2 –SiO –SiO 2 2 –acetyl acetone (acac) 2 mol g µ was found when the catalyst –SiO 575 1 2 . − : 1002–1017 86 -cat h and Cu–Fe/TiO 1 − 2 2011; mol g µ in comparison with pure TiO 2 860 . –SiO 2 Schematic representation (A) and sectional dr This investigation focused on TiO 94 . Photocatalytic reactors for environmental remediation: a review www.soci.org Elsevier) fibres was used.concentrator) In produced a comparison, production natural rate of sunlight 0 (from a solar Immobilized vapour reactors The development ofsupportforanimmobilizedsystemwasinvestigatedbyHaijiesmaili a cost effectiveet al and easy-to-use catalyst of Cu and Fevisible metals which spectrum. shifted The the benefitslight light of absorption distribution into this throughout the systemin the included traditional uniform packed reactor,catalyst, bed a which designs, the feature and authors notcommercial concluded the and enhanced industrial seen visible scales. applicability light at driven Cu(0.5%(w/w))–Fe(0.5%(w/w))/TiO J Chem Technol Biotechnol with the catalyst Cu(0.5%(w/w))–Fe(0.5%(w/w))/TiO length of theinvestigation; reactor. Cu–Fe/TiO Two photocatalysts were utilized in this Figure 9. was used.and Ethylene was evolutionthe only was highest production seen selective rate over of in 0 Fe- production and Cu-containing catalysts; obtained in this study included ethylene andtraces methane, of along ethane with and methanol. The productillumination formation demonstrated under UVA selectivity towardsused, the photocatalyst withwith the each exception ofproduction, the of 1 photocatalysts. methane, The highest which rate was of methane evolved three-dimensional carbon foammethanol for in the aimpregnation oxidation vapour technique of phase was gaseous flow-through adopted photoreactor. for An production of the results were attributeduse to of the TiO increased surface area from the significantly higher than the rate for the TiO when the catalyst Cu(0.5%(w/w))–Fe(0.5%(w/w))/TiO . . 3 1 1 1 − − − et al day 3 was coated : 1002–1017 2 86 of the laboratory 2 2011; examined the mechanisms of 101 . 98–100 et al J Chem Technol Biotechnol reported the investigation of a pilot scale thin proposed a scaled up multi-annular photocat- 102 101 . , which gave a throughput of 2.35–9.32 m 1 et al − , a significant increase on the 81 cm 2 instead of a semiconductor photocatalyst. The TGAPF system 2 O These two examples of scaled up pilot photoreactors show Shu and Chang In this study Imoberdorf Examples where industrial scale photoreactors have been 2 alytic reactor for theof remediation four of concentric cylindrical air borosilicate pollution. glassillumination tubes This (Fig. source, consisted 13). a The Philips TL18Wcentral UV axis lamp, was of placed the on177 system. cm the the With total the surface available area5209 available cm reaction for length radical production of was that there is the potential for increasing capacity, although it The UV lamp source was a 5000 Wat medium 253.7 pressure mercury nm lamp and wasThis configuration positioned achieved a centrally degradation in rate of the 0.26dye min to quartz per 99% L housing. of of original concentration,compared e.g. 100 with L in the 26.9 min.reactor degradation When the rate TGAPF reactor of wasbatch the 233 reactor times recirculated taking more 6267 efficient, batch min withconcentration to the of degrade azo dye. the same volume and radiative transfer, rate kinetics, andthat mass transfer. the They proposed processwith should a be reactor operated free under kineticwas from control. possible mass It to was transfer make found that limitations based accurate it predictions purely of reactor onengineering, behaviour, and the radiation transport chemical theory. Thisof reaction took the no adjustable account fundamentals, or unknown reactor parameters of photoreactor design. test reactor. Using a sol-gel process a thin layer of TiO ents sampled from a wastewaterrequired comparable treatment irradiation times plant, to the however, slurry system they tothe reach bacterial detection limit. Industrial applications The potential industrialcatalysis applications are for wide semiconductor andeffluent diverse photo- to ranging potable from water. treating Theactor oil reality and designs of gas research are, based however,reactors photore- are that ultimately very feasible few inan industrial terms laboratory environment of scale the industrial test volume scaleproduction is and up. in rate the In order of of waste hundredsof of effluent litres cubic per meters, day. i.e. Typically laboratory millions photoreactors haveof a capacity between 1 mL and36 1 W L, with and a 500 UV W. illumination Transforming source between a 1 L capacity reactor to a 1 m on two walls offlowing the through reactor, each annulus which of were the unit. in contact with the gas employed on a largeSolar scale photoreactors have are the those advantagelight of of not solar sources requiring activated artificial but designs. depend do on the require solar insolence. huge amounts of space andPilot scale also studies Imoberdorf with a simulated wastea water total prepared volume in3000 a of mL tank and 100 the reservoir L. dye was with pumped Theto through at 6.5 TGAPF a L rate min of reactor 1.5 L had min a capacity of capacity unit is not a simplebetween transformation, with materials, the volume, relationship penetration catalyst presenting complex loading, challenges. turbidity, and UV was tested using acid orange 10 at a concentration of 20 mg L gap annular plug flow (TGAPF) andbatch photoreactor and reactor recirculated for theH degradation of azo dye waste water using Results www.soci.org C McCullagh 96 Mass trans- 95 film caused by 2 2011 Society of Chemical Industry c slurry system. 2 Reproduced from reference 93 with coated 15 pores-per-inch alumina aqueous suspensions and methy- 2 Escherichia coli The authors investigated the effect of increasing cata- Schematic of optical fibre photo reactor displaying light 3 and 2 mm) to gain an understanding of the photocat- 97 . 1 = A pilot reactor utilizing a TiO Three different reactor configurations, slurry reactor, wall reac- The performance of a suspended catalyst system, an immobi- d alytic activity. The authors concludedthe that photocatalytic large activity. beads enhanced lene blue photodegradation (Fig.for 12). Titania the was slurry inwall reactor, suspension reactor, immobilized and on immobilizedreactor. the on reactor the wall packing for in the the fixed bed for the degradation ofindicated 1,8-diazobicyclo[5.4.0]undec-7-ene (DBU) that thesystem was foam more monolith efficientsuggesting than immobilized that the the photocatalyst slurry immobilized photocatalystwater system purification. reactor could be scaled up for tor, and fixed-bed reactor, weredisinfection compared of for the photocatalytic the heat treatment reduced thefor surface the micro-organisms area and of therefore reduced catalyst the available activity. photocatalytic When the immobilized reactor was investigated for efflu- wileyonlinelibrary.com/jctb inner centrifugal instabilitydifferentially with respect when to the outer thethe cylinder. They effect investigated inner of Reynolds cylinderon photodegradation number rotates and (Fig. 11(c)) comparedslurry and the reactor. results catalyst They with loading notedincreased that that the for increasing rate a the of Reynolds photodegradationorange of number II, their demonstrating model pollutant thatthe overall external reaction rate. mass transfer controlled lized catalyst where theimmobilized wall system of packed the with reactorcompared coated for was glass the coated, beads photodegradation of have and formic been an acid. transmission along coated fibres. ( Figure 10. permission from Elsevier) lyst layer thickness and compared it withof increasing concentration catalyst in thephoto-oxidation were slurry in system. good agreement Themobilized for results both system. slurry for For and methylene im- thethis blue was photocatalytic not the disinfection, case. however, The increased density of TiO reticulated foam monolith incorporatedcentrally in deployed the UV space between lampwas a and compared the with internal a wall Degussa of P25 the TiO reactor fer limitations were observed in thecatalyst immobilized coated system on with the the reactorovercame wall. However this aerating the mass system transferpacked bed reactor problem. was investigated The for two performance( different sized of beads the

1012 1013 wileyonlinelibrary.com/jctb 6 mm glass Raschig rings placed × f time-dependent Taylor vortex flow ing Degussa P25 TiO2 onto the 15 cm long glass tube ocatalytic reactor. (C) Progress o 29 with permission from Elsevier) 2011 Society of Chemical Industry c A fixed-bed reactor, immobilizing Degussa P25 TiO2 onto 6 mm m reference 97 with permission from Elsevier) ions of Degussa P25 TiO2. (B) A wall reactor, immobiliz 111. (Reproduced from references 28 and = : 1002–1017 Rec 86 2011; (C) (A) (B) (A) A slurry reactor, using suspens (A) Schematic of Taylor vortex reactor and (B) image of Taylor vortex phot Photocatalytic reactors for environmental remediation: a review www.soci.org around critical Reynolds number, into the annular reactor volume. (Reproduced fro that constitutes the inner-tube wall of the reactor. (C) J Chem Technol Biotechnol Figure 12. Figure 11. . et al : 1002–1017 86 2011; 50% of the organic pollutants in the waste J Chem Technol Biotechnol 100 increased surfaceand area reactant allowing interactiontreatment increased and of has photocatalyst downstream wastewater. proved processing procedures, effective particularly Limited withto respect in light catalyst the penetration separation,commercialization however, and and scale restrict up. these concepts for surface area tomations. pollutant Research ratios has for demonstratedeffective that for photocatalytic both these gas transfor- and systems liquid phase are photocatalysis.enabled This the has use of highlyP25, efficient which powders benefit from such the as ‘periodicated Degussa by light fluidized phenomenon’ systems. Drawbacks cre- to the system includeloss the or ‘drifting’ of particles within the systemprocessing and restrictions. downstream advantages of no downstream processing restrictionsseparation such as and filtration andand allow continuous operation flow in phase.this both There system including are batch difficulty in restrictions the regarding illuminationsupport of containing the entire the catalyst andaffected by mass catalyst transfer thickness. Furthermore, limitations in order to achieve aneffectivesurfaceareaofphotocatalystrelativetotheeffluent being treated, scaled up units require a significant ‘footprint’. rect parameters the individualmany concepts comparisons of are systems describe effective. the While lystreactorset-upashaving immobilized cata- decreasedefficiencyorrestrictions due to mass transfer limitations, it wasstrictions reported were that if overcome such results re- were comparableof with slurry those and fluidization systems. The following conclusions could be drawn following a consid- As has been demonstrated here, a vast array of photoreactor eration of the current state of the art in1. this field. Slurry and suspended systems offer the advantage of 2. Fluidized bed reactors also present excellent photocatalyst 3. Fixed bed designs utilize immobilized catalysts which have the 4. Comparisons of the systems demonstrate that given the cor- concepts have been reported, allcharacteristics displaying varying in engineering termsphotocatalyst deployment, of and activation. efficiencyare These all for characteristics critical, pollutant however,does the not transport, future solely ofthe rely photoreactor development in technology of thein more design rate effective of limited photocatalysts, systems. thecan The particularly reactor development achieve of itself, greater photocatalysts but conversionenergies, that and in efficiencies ultimately at visible lowera light irradiation absorbing critical materials will componentversatile be in technology. ensuring For wideneed to industrial meet scale the challenge applications of adoption capacity, ruggedness, photoreactors reliability of and this CONCLUSIONS The research detailedphotocatalytic in this reactor review designcations. highlights along Suspended the with liquid diversity their reactors,immobilized in potential immobilized gas reactors liquid appli- and reactors, immobilizedconsidered vapour and, reactors where were appropriate,address compared the in advantages and an disadvantages attempt of individual to designs. water were degraded within 8–11the h photocatalytic irradiation. process The efficiency wasinitial of pollutant found concentration, to the be timesurprisingly, the dependent of solar illumination, on UV light and, the flux not density. Wolfsburg, Germany. www.soci.org C McCullagh (Fig. 14). The (reproduced from  99,103 2011 Society of Chemical Industry  c r showing the inner structure Schematic view of a DSSR reacto Schematic representation of a pilot-scale multi-annular photo- Thedoubleskinsheetreactor(DSSR)comprisesaflattransparent Reference 103 with permission from ASME). wileyonlinelibrary.com/jctb is worth notingrequire that a 5000 W the UV source Shu whichassociated is operating and expensive costs. and has Chang significant TGAPF reactor does box framework constructed from PLEXIGLAS Figure 14. of the transparent structured box made of PLEXIGLAS catalytic reactor: (A) UVglasstubes.(Reproducedfromreference101withpermissionfromElsevier) lamp, (B) distribution heads, and (C) borosilicate Figure 13. photocatalystwasdeployedasasuspensioninthewastewaterand the slurry was then pumped through the channels ofthe the unit. degradation After process thefrom the photocatalyst suspension had either to byDSSR filtering be has or removed been by demonstrated sedimentation.diffuse to The portion utilize of both the solarinvestigated for direct radiation. the and treatment A of the industrial pilot wastewater effluent scale in DSSR, has been

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