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

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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:

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mass transfer

off-gas treatment

“kLa”: A quantitative measure for the mass transfer performance

Mass transfer rate:

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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

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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

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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 % !

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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

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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, …)

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