separation process ch 6-04-lle

M Azmi Bustam

Multistage countercurrentStage to Stage Calculations

How to determine N required to get to LN?

is the common operating point where all streams pass through.

Since end points are known,

x can be calculated, and

point located.

Or it can be located by graph.

M Azmi Bustam

Multistage countercurrentExample Stage to Stage Calculations

Pure isopropyl ether at 450 kg/h is being used to extract an aqueous solution of 150 kg/h with 30 wt% acetic acid by countercurrent multistage extraction. The exit acid concentration in the aqueous phase is 10 wt%. Calculate the number of stages required.

M Azmi Bustam


Pure isopropyl ether at 450 kg/h is being used to extract an aqueous solution of 150 kg/h with 30 wt% acetic acid by countercurrent multistage extraction. The exit acid concentration in the aqueous phase is 10 wt%. Calculate the number of stages required.

Known:

VN+1 = 450 kg/h, yAN+1 = 0, yCN+1 = 1.0, L0 = 150 kg/h, xA0 = 0.30, xB0

= 0.70, xC0 = 0, xAN = 0.10

Locate point M as per previous example

V1 gives yA1 = 0.07, yC1 = 0. 90

M

M Azmi Bustam


Pure isopropyl ether at 450 kg/h is being used to extract an aqueous solution of 150 kg/h with 30 wt% acetic acid by countercurrent multistage extraction. The exit acid concentration in the aqueous phase is 10 wt%. Calculate the number of stages required.Known: VN+1 = 450 kg/h, yAN+1 = 0, yCN+1 = 1.0, L0 = 150 kg/h, xA0 = 0.30, xB0 = 0.70, xC0 = 0, xAN = 0.10

• Operating point obtained by

intersection of lines L0V1 and LNVN+1

or calculated.

• Tie line from V1 locates L1

• Draw line L1 to to locate V2


• Draw line L2 to to locate V3


• So you have 2+ stages (make it 3)

M Azmi Bustam

Multistage countercurrentMinimum Solvent rate, VN+1

• When the line through operating point and a tie line is similar

• Infinite number of stages

• Use more than this value in actual operation

How to get this minimum?

• Draw tie line from L0 to intersect line LNVN+1

• This is min and is also location for (VN+1)min

M Azmi Bustam

Multistage countercurrentImmiscible Liquids

• The overall and component balances becomes:

• Becomes an operating line with slope ~ L’/V’

• Similar to distillation and absorption – stages can be stepped off the x-y phase diagram

1

1

1

1

0

0

1

1

1

1

0

0

1111

1111

y

yV

x

xL

y

yV

x

xL

y

yV

x

xL

y

yV

x

xL

n

n

n

n

N

N

N

N

M Azmi Bustam

Multistage countercurrentPacked Tower design

Similar to stripper

)A/ln(

A)A(m/yx

m/yxln

N NN

N

OL 1

11

10

Sa'K

LH

xOL

M Azmi Bustam

An effluent solution of 100 kgmoles/h containing 0.010 mole fraction of nicotine is to be extracted using kerosene. A kerosene stream of 200 kg/h containing 0.0005 mole fraction of nicotine is used in a counter current stage tower. The water and kerosene are essentially immiscible in each other. It is desired to reduce the concentration of the effluent solution to 0.0010 mole fraction of oil. The equilibrium data are given in the following table. Estimate the number of theoritical stages required for the separation. Compare it with kremser’s equation.

Equilibrium data for oil in water and hexane

Mole fraction of Nicotine in water, X’

Mole fraction of Nicotine in kerosene, Y’

0.001010 0.000806

0.002460 0.001959

0.005000 0.004540

0.007460 0.006820

0.009880 0.009040

0.202000 0.018500

M Azmi Bustam

Multistage countercurrentExample:

Pure solvent isopropyl ether at the rate of 600 kg/h is being used to extract an aqueous solution of 200 kg/h containing 30 wt% acetic acid by countercurrent multistage extraction. The desired exit acetic acid concentration is 4 wt%. Calculate the compositions and amounts of ether extract and aqueous raffinate.

To find M, determine xCM and xAM

Plot these points on the rect phase diagram

0.75 600200

600(1.0)200(0)

110

1100

N

CNNCCM VL

yVxLx

0.075 600200

600(0)200(0.30)

1

10

1100

N

ANNAAM VL

yVxLx

M Azmi Bustam

Palm oil mill effluent solution of 150 kg/h containing 0.010 weight fraction of oil in water is to be stripped using hexane. A hexane stream of 300 kg/h containing 0.0005 weight fraction of oil is used in a counter current stage tower. The water and hexane are essentially immiscible in each other. It is desired to reduce the concentration of the effluent solution to 0.0010 weight fraction of oil. The equilibrium data are given below. Estimate the number of theoritical stages required for the desired extraction.

Weight fraction of oil in water,

X’

Weight fraction of oil in hexane, Y’

0.001010 0.000806

0.002460 0.001959

0.005000 0.004540

0.007460 0.006820

0.009880 0.009040

separation process ch 6-04-lle

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