CONTROL OF GAS-PHASE CHLOROBENZENE USING TIO 2 - MEDIATED PHOTOCATALYSIS AND PACKED-BED ABSORPTION C.RICHARDSON R. MARTIN T. FAN C. HALBERT VI

OUTLINE u INTRODUCTION u EXPERIMENTAL METHOD u RESULTS and DISCUSSION u CONCLUSIONS u FUTURE WORK u ACKNOWLEDGEMENTS

INTRODUCTION u WHAT IS PHOTOCATALYSIS? –A HETEROGENEOUS, ADVANCED OXIDATION PROCESS (AOP) INVOLVING THE GENERATION OF HIGHLY REACTIVE HYDROXL RADICALS u WHAT ARE THE PROCESS REQUIREMENTS? –A SEMICONDUCTOR AS A PHOTOCATALYST –UV LIGHT AS AN ENERGY SOURCE –WATER AS THE REACTION MEDIA

INTRODUCTION u WHY PHOTOCATALYSIS? –VOCs COMPRISE THE LARGEST CATEGORY OF TOXIC AIR POLLUTANTS –CONVENTIONAL TECHNOLOGIES ARE INEFFECTUAL AND UNECONOMICAL FOR CHLORINATED VOCs –PHOTOCATALYSIS CAN DEGRADE AND MINERALIZE MOST ORGANIC CONTAMINANTS IN WATER AND AIR –HIGH DESTRUCTION RATES ARE POSSIBLE –PROCESS OPERATES AT AMBIENT TEMPERATURE AND PRESSURE –SOLAR INSOLATION MAY BE A POTENTIAL ENERGY SOURCE

INTRODUCTION u REQUIREMENTS FOR A SEMICONDUCTOR –BAND GAP ENERGY HIGHER THAN 3.2 eV –STABLE, INEXPENSIVE, AND NON-TOXIC u ANATASE TITANIA (TiO 2 A BAND GAP ENERGY OF 3.2 eV IS THE MOST COMMONLY USED SEMICONDUCTOR FORM VS RUTILE TITANIA (TiO 2 3.O eV u PLATINIZED TiO 2 –BAND GAP ENERGY INCREASED FROM 3.2 eV TO 3.5 eV –PLATINUM ACTS AS AN ELECTRON TRAP DECREASING THE RECOMBINATION OF ELECTRONS AND POSITIVE HOLES WITH APPARENT HIGHER QUANTUM YIELD

MECHANISM OF PHOTOCATALYSIS

INTRODUCTION u ADVANTAGES OF SILICA-SUPPORTED TiO 2 OVER SLURRY TiO 2 PHOTOCATALYST –EASY SEPARATION FROM AQUEOUS PHASE –EXCELLENT FLUIDITY OF SILICA-GEL IN WATER –LARGER SPECIFIC SURFACE AREA OF SILICA-GEL –SILICA-GEL IS TRANSPARENT TO UV LIGHT –HIGHER QUANTUM YIELD POSSIBLE u ADVANTAGES OF ADDING AN INTERGAL COUNTERCURRENT PACKED-BED ABSORPTION UNIT WITH THE AOP –INCREASED VOC ABSORPTION DRIVING FORCE –INCREASED VOC DEGRADATION POTENTIAL

EXPERIMENTAL u CATALYST SYNTHESIS –PLATINIZATION OF TiO 2 SLURRY –PLATINIZED, SILICA-SUPPORTED CATALYST »CHEMICAL METHOD (SOL-GEL) »PHYSICAL METHOD u BENCH-SCALE SET-UP, QUANTIFICATION AND OPTIMIMIZATION OF ABSORPTION/PHOTOCATALYTIC UNIT u SAMPLE COLLECTION AND ANALYSIS

EXPERIMENTAL u PHOTOCATALYTIC/ABSORPTION –EPA METHOD ZERO AIR DEVICE –SAMPLE FEED AND STABLE CONCENTRATION DEVICE –PHOTOREACTOR »VOC SPARGER »UV LAMP AND FILTER »CATALYST FILTERING DEVICE »MAGNETIC STIRRER »RECIRCULATION PUMP –ABSORPTION COLUMN »RANDOM PACKING WITH DISTRIBUTORS »SAMPLING PORTS

EXPERIMENTAL u SAMPLE COLLECTION AND ANALYSIS –5 ML TREATED AIR SAMPLES TAKEN AT SPECIFIC TIMES AND LOCATIONS USING GAS-TIGHT GLASS SYRINGES WITH TEFLON-TIPPED PLUNGERS –CHLOROBENZENE CONCENTRATIONS ANALYZED WITH A HP 5880a GC WITH A FID DETECTOR –OFF-GAS CARBON DIOXIDE ANALYZED WITH ENVIRONMAX TM CO AND CO 2 MONITOR u PHOTOREACTOR OPERATION –450 W QUARTZ MERCURY-VAPOR LAMP AT 19 mW/cm 2 at 2.5 cm WITH 280 to 400 nm WAVELENGTH FILTER –AIRFLOW RATES RANGE FROM 0.76 to 3.8 slpm –H 2 O RECIRCULATION RATE CONSTANT AT 330 ml/min –Ch-SILICA-GEL CATALYST LOADING FROM 20 to 150 gm

BENCH-SCALE SET-UP

CLASSICAL DEGRADATION KINETIC MODEL

THREE -PHASE SLURRY REACTOR u LIQUID AND CATALYST PARTICLES ARE WELL-MIXED AS IN A CONTINOUSLY STIRRED TANK REACTOR (CSTR) u GAS BUBBLES RISE THROUGH THE LIQUID AS A PLUG WITHOUT COMPLETE MIXING u AS A RESULT, THE GLOBAL RATE CAN CHANGE WITH VERTICAL POSITION IN THE CSTR u FOR A GIVEN VERTICAL POSITION, THE OVERALL REACTION CONSISTS OF SEVERAL PROCESSES IN SERIES

OVERALL PROCESS STEPS u MASS TRANSFER FROM THE BULK CONCENTRATION IN THE GAS BUBBLE TO THE BUBBLE-LIQUID INTERFACE u MASS TRANSFER FROM THE BUBBLE INTERFACE TO THE BULK-LIQUID PHASE u MIXING AND DIFFUSION IN THE BULK LIQUID u MASS TRANSFER TO THE EXTERNAL SURFACE OF THE CATALYST PARTICLES u REACTION AT THE CATALYST PARTICLE

OVERALL REACTION u RESISTANCE IN STEP THREE IS SMALL u RATES OF REMAINING PROCESS STEPS ARE IDENTICAL AT STEADY-STATE WHICH ALLOWS THE ELIMINATION OF UNKNOWN INTERFACIAL CONCENTRATIONS u GLOBAL RATE EXPRESSED AS A SIMPLE FIRST-ORDER IRREVERSIBLE REACTION AT THE CATALYST SURFACE u EQUILIBRIUM EXISTS AT THE BUBBLE- LIQUID INTERFACE AND IS RELATED BY HENRY’S LAW

GLOBAL REACTION RATE

THREE-PHASE KINETIC MODEL

RESULTS

CONCLUSIONS u HIGHLY ACTIVE SUPPORTED PHOTOCATALYSTS WERE DEVELOPED AND VIABLE OPTIONS IDENTIFIED –SILICA-SUPPORTED, PLATINIZED TITANIA PHOTOCATALYST SHOWED A HIGHER DEGRADATION RATE THAN SLURRY TITANIA CATALYST –A SOL-GEL GENERATED SILICA-SUPPORTED CATALYST DEMONSTRATED EXCELLENT ABRASION RESISTANCE TO LOSS OF TITANIA FROM ITS SUPPORT WHILE MAINTAINING NO OBSERVED LOSS OF DEGRADATION POTENTIAL

CONCLUSIONS u THE ADDITION OF AN INTEGRAL ABSORPTION COLUMN ON THE TOP OF THE PHOTOREACTOR DID NOT SHOW ANY ASSISTANCE TO THE STEADY-STATE CHLOROBENZENE DEGRADTION RATES –THE ACTIVE OXIDATION SPECIES HYDROXYL RADICALS COULD ONLY EXIST ON THE CATALYST SURFACE AND DID NOT RESIDE WITHIN THE RECIRCULATED ABSORPTION COLUMN WATER

CONCLUSIONS u MODERATE DEGRADATION OF CHLOROBENZENE WAS ACHEIVED WITH THIS PHOTOCATALYTIC APPARATUS –RESIDENCE TIME IN THE PHOTOREACTOR HAD A SIGNIFICANT IMPACT ON DEGRADATION AND MINERALIZATION OF CHLOROBENZENE –DEGRADATION AND MINERALIZATION DECREASED AS INLET MASS LOADING INCREASED –MAXIMUM CHLOROBENZENE REDUCTION ACHIEVED WAS 50 PERCENT WITH 37 PERCENT MINERALIZATION TO CO 2 AT A LOADING OF 0.37 mg/min AT 1.3 SECONDS APPARENT RESIDENCE TIME –OVERALL REDUCTION WAS SLIGHLTY LOWER AT A LOWER PLATINIZATION RATIO OF TITANIA

CONCLUSIONS u A PREDICTIVE THREE-PHASE REACTOR MODEL USING FIRST-ORDER IRREVERSIBLE DEGRADATION KINETICS PROVIDED A MEANS TO QUANTIFY ABSORPTION AND DEGRADATION MASS- TRANSFER UNDER VARIED CONDITIONS –BOTH MASS-TRANSFER COEFFICIENTS INCREASED AS INLET MASS LOADING INCREASED –A LINEAR CORRELATION EXISTS BETWEEN INLET LOADING AND THE RESPECTIVE COEFFICIENT –A CONSTANT INTERCEPT FOR BOTH ABSORPTION AND DEGRADATION EXPERIMENTS WAS VERIFIED FOR CONSTANT AIR FLOW RATE EXPERIMENTS

FUTURE WORK u OPTIMIZE COMPOSITION OF SUPPORTED CATALYST (TiO 2 /SILICA-GEL WEIGHT RATIO, Pt/TiO 2 WEIGHT RATIO, AND OPTIMUM CATALYST DOSAGE) u INVESTIGATE DEGRADATION MECHANISM BY QUALITATIVE AND QUANTITATIVE ANALYSIS OF FORMED INTERMEDIATES u TEST DIFFERNT GAS-PHASE CHLORINATED COMPOUNDS TO COMPARE KINETIC PARAMETERS AND SUBSTANTIATE CONCLUSIONS OF THIS STUDY

FUTURE WORK u IMPROVE REACTOR DESIGN TO ACHIEVE LONGER RESIDENCE TIME u TEST VARIOUS COMPOUNDS THAT SHOW AN APPARENT SURFACE ADSORPTION FOR THE SILICA-GEL SUPPORTED CATALYST u ESTIMATE THE QUANTUM YIELD FOR THE THREE-PHASE PHOTOREACTOR AND INVESTIGATE MEANS TO OPTIMIZE QUANTUM EFFICIENCY

ACKNOWLEDGEMENTS u WASTE-MANAGEMENT and EDUCATION, RESEARCH CONSORTIUM (WERC) FOR FINANICIAL SUPPORT u DR. MICHAEL PRAIRIE, SANDIA NATIONAL LABORATORIES FOR TECHNICAL ADVICE DURING THE INITIAL PROJECT PHASE u MS. JENNIFER KNOWLTON, NEW MEXICO TECH ENVIRONMENTAL ENGINNERING UNDERGRADUATE FOR LABORATORY ASSISTANCE AND DATA REDUCTION