part ii: mass transfer of o 3 in water: fundamentals & applications l aboratory of p hysical and...
TRANSCRIPT
Part II: Mass Transfer of O3 in Water:
Fundamentals & Applications
L aboratory of P hysical and A nalytical C hemistry
REWARD 2004-2006
H. Vankerckhoven
Outline
What is mass transfer?
Quantitative measure: “kLa”
Factors affecting kLa
Types of gas-liquid contactors
Energy consumption in O3 contacting
Considerations regarding REWARD
L aboratory of P hysical and A nalytical C hemistry
Mass transfer = • absorption of a gas (O3) in a liquid (water)
• dispersion of an O3-containing gas in water
Mass transfer of O3 in water: an important process
Water treatment with O3:
L aboratory of P hysical and A nalytical C hemistry production
mass transfer
off-gas treatment
“kLa”: A quantitative measure for the mass transfer performance
Mass transfer rate:
L aboratory of P hysical and A nalytical C hemistry
driving force for mass transfer
kLa : • measure for mass transfer performance of a system
• characteristic for the contacting system used
• allows comparison between different systems
volumetric mass transfer coefficient (s-1 or hr-1)
VOOakN t,liq3max
liq3LO3
“kLa”: A quantitative measure for the mass transfer rate
kLa
L aboratory of P hysical and A nalytical C hemistry
a = gas-liquid interfacial area (cm-1), surface-to-volume ratio of gas bubbles
kL = liquid mass transfer coefficient (cm/s),
determined by hydrodynamic pattern
kLa: 0 to > 100.000 hr-1
• Improvement of kLa: increasing kL and/or a
Determination:• directly; chemical and physical methods
• indirectly; determining 2OLak
“kLa”: A quantitative measure for the mass transfer rate
kLa
L aboratory of P hysical and A nalytical C hemistry
a = gas-liquid interfacial area (cm-1), surface-to-volume ratio of gas bubbles
kL = liquid mass transfer coefficient (cm/s),
determined by hydrodynamic pattern
kLa: 0 to > 100.000 hr-1
• Improvement of kLa: increasing kL and/or a
Determination:• directly; chemical and physical methods
• indirectly; determining (kLa)O2
a = gas-liquid interfacial area (cm-1), surface-to-volume ratio of gas bubbles
Operating conditions and water quality affect kLa
Factors affecting kLa:L aboratory of P hysical and A nalytical C hemistry
Type of contactor +++
Energy dissipation +++
Temperature ++
Gas & water flow rates +++
pH /
Ionic strength +
Presence of organic compounds
- Nature + ~ ++
- Concentration +
Physical properties
- Surface tension -
- Viscosity -
- Density +
Contactor geometry
Operating conditions
Water quality
For every application, a suited gas- liquid contactor is available
Many different types commercially available:• different methods of operation
• different operating limits (water flow, ozone dose,…)
• characteristic kLa-range
L aboratory of P hysical and A nalytical C hemistry
Type kLa (hr-1)
Bubble column 5-50
Stirred vessel 70-720
Liquid jet reactor 70-1800
Venturi injector 350-10800
Static mixer 350-10800
Membrane contactor 72-7200…
Ozone transfer efficiency (OTE, in %) =
fraction of absorbed O3 from the gas phase
O3 transfer efficiency < 100 % !
L aboratory of P hysical and A nalytical C hemistry
100OQ
OQOQOTE
0,g30,g
exit,g3exit,g0,g30,g
OTE is generally < 100 %Need for additional O3 input
O3 contacting can be energy intensive
L aboratory of P hysical and A nalytical C hemistry
High performance contactors: high energy consumption
Exploitation costs:• O3 generation = low efficiency process
• Contacting: Operating energy (gas pressure, water pressure, electrical
power, …) Additional amount of O3 since OTE < 100 %
O3 treatment = energy intensive process
• Investment costs: purchase of a gas feed system, O3
generator, contactor, off-gas destruct, …
• Exploitation costs
O3 contacting in REWARD: various criteria to take into account
Ozone treatment in the REWARD project: importance of O3 contacting
Choice of an O3 contactor will depend on:
• Required minimum kLa (water temperature !)
• Investment costs: purchase of the contactor
• Exploitation costs: pumping, stirring,…
• Technical aspects (dimensions of the tank, water flow diagram in the dishwasher, safety issues, …)
L aboratory of P hysical and A nalytical C hemistry