Transfer Unit-Absorption-Lecture Slides, Slides of Technical Writing

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Process Mass & MomentumProcess

Mass & MomentumTransfer

AbsorptionAbsorption Dr Muhammad Tayyeb Javed

DCME-PIEASDCME PIEAS

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Transfer Unit „^ Changes

in^ compositions

of^ the

liquid

and^

vapour

phases

occur differentially in a packed column rather than in stepwisefashion as in trayed column. „ In a packed absorption column, the solute diffuses through thegas phase to the liquid surface where it dissolves and is thentransferred to the bulk of the liquid

(^

)

m^ A^

L^ Ai

AL

AL dC

k a C

C^

AdZ =^

−^

(^ A^ L )

m dC^ dZ Ai^ AL

k a

L C^ C

= (^) −

„^ The

left^

side^

to^ the

equation

is^ the

rate^

of^ change

of

concentration with height for unit driving force „ A measure of the efficiency of the column „ High value of it is associated with a high efficiency „ Reciprocal of this is called height of transfer unit

2

PMMT^

Dr. M. T. Javed, DCME, PIEAS

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Transfer Unit^ H i^ ht

f th^ li

id^ fil t^

f^ it i

H^ L

k^

C ′

If the driving force is taken over the gas-film

G^

G H^ G

′ Pk a =

Height of the liquid-film transfer unit is

L^ m

L^

T H^ L

′ k a C =

only, the height of a gas-film transfer unit

G^ m

G H^ G

Pk a

4

PMMT^

Dr. M. T. Javed, DCME, PIEAS

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Transfer Units I^ thi^

l^ i^

it^ i^

d^ th t

th^ t^

t l^

b^ f

l^

f

„^ In this analysis, it is assumed that the total number of moles of gasliquid remain the same „^ True in absorption only when a small change in concentration takesplace

Th^ h i

ht^ f

t^

f^

it (HTU) i

place^

The height of a transfer unit (HTU) is ameasure of the separation effectivenessof the particular packing for a particularseparation process SEMMELBAUER

presented

the

following

equations

to^ evaluate Hand^ G^

H^ for^ RaschingL^

Rings

separation process Hand^ H^ forG L^

Rasching

Rings

and Berl Saddle

β^ =

for β^ =

for Rasching rings β^ =^

21 for Berl Saddle

5

PMMT^

Dr. M. T. Javed, DCME, PIEAS

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PMMTMuhammad Zaman, DCME, PIEAS

(^7) docsity.com

Importance of Liquid & Gas Flow rates& the Slope of Equilibrium Curve& the Slope of Equilibrium Curve For dilute concentrations Lower the value of mG

///L, the lower will be ym^ m^

, ande

hence the exit liquid 0 7 to 0 8 is the optimum range

8

hence^

the exit liquid. 0.7 to 0.8 is the optimum range. PMMT^

Dr. M. T. Javed, DCME, PIEAS

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Types of Packings^ „^ Requires good phase distributions^ „^ Packing requires

g^ q ‰^ Large surface area ‰^ Low pressure drop ‰^ Promote uniform liquid distribution on the packings ‰^ Promote uniform gas distribution across the cross section „ Two broad classes of packings ‰^ Regular geometry: stacked rings, grids and structured packingsR^

d^

ki^

i^

ddl^

d^

i t^

h

‰^ Random

packings:

rings,

saddles

and^

proprietary

shapes,

which^

are^ dumped

into^

the^ column

and^ take^ up

a^ random

arrangement

10

PMMT^

Dr. M. T. Javed, DCME, PIEAS

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Types of Packings^ „^

Grids have open structure„ Grids

have open structure „^ Used for high gas rates „^ Where low pressure drop is essential „^ e.g. cooling towers

11

PMMT^

Dr. M. T. Javed, DCME, PIEAS

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Random Packing general

Free^

area^

and^ liquid

distribution is improved

generaluse

Raschig Ring

Pall Ring

Berl Saddle

Intalox Saddle

Metal Hypac

Super Intalox

13

Improvedliquiddistribution

Improved BerlSaddle,

easy manufactures

Improved typeof Pall ring

Improved type ofIntalox Saddle

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PMMTMuhammad Zaman, DCME, PIEAS

(^14) docsity.com

Packing Size^ „^ Largest size up to 50 mm^ „^ Small sizes are appreciably more expensive than thel^

t^ i largest sizes „ Above 50 mm the lower cost per cubic meter doesnot normally compensate for the lower mass transfernot normally compensate for the lower mass transferefficiency „ Use of too large a size in a small column can cause

li^ id di

ib^

i

poor liquid distribution

16

PMMT^

Dr. M. T. Javed, DCME, PIEAS

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