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L 40:Fluid- Fluid/Solid

Reactions (Non catalytic)

Prof. K.K.PantDepartment of Chemical Engineering

IIT Delhi.

kkpant@chemical.iitd.ac.in

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General rate expression

The absorption of A from gas is larger when reaction occurs within the

liquid film than for straight mass transfer.

Thus for the same concentrations at the two boundaries of the liquid filmwe have

E >1

Can be correlated with HATTA MODULUS/ NUMBER (MH)

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Role of the Hatta Number, MH

To identify whether reaction is fast or slow, we focus on unit

surface of gas-liquid interface, assume that gas-phaseresistance is negligible, and we define a film conversion

parameter

If MH >>1, al l react ion o ccurs in th e f i lm, and surface area is the con trol l ing rate

factor.On the other hand, if MH

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For the special case of negligible gas-phase resistance, for

example, if pure reactant is used, A in the gas phase, (kAg-

>high) then

= KAl CAiE , E = x0/x

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Case C: Fast Reaction; Low CBThe plane of reaction for case

A now spreads into a zone of reaction in which A and B are bothpresent. However, reaction is fast enough so that this reaction

zone remains totally within the liquid film.

Thus, no A enters the main body of liquid to react there.

Since the last resistance term in the general rate equation, (I/k fl

)is negligible (large k), the rate form for this case is

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Case G: Slow Reaction with Respect to Mass Transfer. all reaction occurs

in the main body of the liquid; however, the film still provides a

resistance to the transfer of A into the main body of liquid.Thus,

three resistances enter into the rate expression, Thus

Case H: Inf in i tely Slow React ion .

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Factors to Consider in Selecting a Contactor

(a) Contacting pattern.

Towers approximate plug G plug L.

Bubble tanks approximate plug G / mixed L.

Agitated tanks approximate mixed G / mixed L.

Towers have the largest mass transfer driving force and in this respect have an

advantage over tanks.

Agitated tanks have the smallest driving force.

(b) kg and kL. For l iqu id d roplets in gas kg is high, kL, is low.

For gas bubbles rising in liquid, kg is low, kLis high.

(c) If the resistance is in the gas and or liquid films : Use a large interfacial

area "a," thus most agitated contactors and most columns. If the L film

dominates, stay away from spray contactors.

If the G film dominates stay away from bubble contactors.

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