Chapter 3b - Development of a Separation Process

8/18/2019 Chapter 3b - Development of a Separation Process

1/18

EP426Chemical Process Design and Optimization

Chapter 3b - Separation train synthesis.

Development of a separation process


2/18

Common Industrial Separation Methods

SeparationMethod

Phase ofthe feed

Separationagent

Developed oradded phase

Separationprinciple

Equilibriumflash

L and/or V Pressurereduction or

heat transfer

V or L differencein volatility

Distillation L and/or V Heat transferor shaft work

V or L difference

in volatilityGas

AbsorptionV Liquid

absorbentL difference

in volatility

Stripping L Vapor strippingagent

V differencein volatility

ExtractiveDistillation

L and/or V Liquid solventand heattransfer

V and L differencein volatility

AzeotropicDistillation

L and/or V Liquidentrainer andheat transfer

V and L differencein volatility


3/18

Development of a separation process

It requires the selection of:1. Methods & Equipment

2. Separation Factor (Energy or Mass Agent )3. Optimal arrangement (Sequencing)

4. Operating parameters (Temp. & Pressure)


4/18

1. Selection of separation methodequipment

Largely depends of feed condition:

• Vapor: partial condensation, distillation,absorption, adsorption, gas permeation

(membranes)

• Liquid: distillation, stripping, LL extraction,supercritical extraction, crystallization, adsorption,and dialysis or reverse osmosis (membranes)

• Solid: if wet drying, if dry leaching


5/18

2. Separation factor (Energy or Mass)

The separation factor, SF , defines the degree of separation achievable between two key componentsof the feed.

This factor, for the separation of component 1 fromcomponent 2 between phases A and B, for a

singlestage

of

contacting, is defined as:

C = Composition variable

Phases rich in components 1 and 2. =1

/2

1/2

Note: SF is selected according to their ease of recovery for recycle and to achieve relatively large values.


6/18


SF is generally limited by thermodynamic equilibrium, ESA.

For example, in the case of distillation,

Let:

• Mole fractions as the composition variable;

• Phase A be the vapour and phase B be the liquid;

The limiting value of SF is given in terms of vapour-liquidequilibrium ratios (K-values) as:

VandLidealfor

/

/

2

12,1

2

1

22

11s

s

P

P

K

K

x y

x y SF


7/18

• For vapor-liquid separation operations (extractivedistillation) that use an MSA

s L

s L

P

P

SF 22

11

2,1

• For Liquid-liquid extraction, the SF is referred to as therelative selectivity, b , where:

I I

II II

SF

21

212,1

/

/

b


Note: Both case is when the MSA is used to create liquid phases which MSA will causes the

formation of a non-ideal liquid solution )


8/18

• in each column is > 1.05.

• The reboiler duty is not excessive.

• The pressure does not cause the mixture to approach the TC

• Column pressure drop is tolerable.

• The overhead vapor can be at least partially condensed at the columnpressure to provide reflux without excessive refrigerationrequirements.

• The bottoms temperature is not high that chemical decomposition.

• Azeotropes do not prevent the desired separation.

Use a sequence of ordinary distillation (OD) columns provided:

3. Sequencing of OD Columns


9/18

Number of Sequences for Ordinary Distillation, Ns is

determined by

)!1(!

)]!1(2[

P P

P N s

# of Product, P # of Separators Ns

2 1 1

3 2 2

4 3 5

5 4 14

6 5 42

7 6 132

8 7 429



10/18


Example 2 : Sequences for 4-component separation


11/18


12/18


Example 2 : Sequences for 4-component separation


13/18

Identifying the Best Sequences

1. Remove thermally unstable, corrosive, or chemically reactive components

early in the sequence.2. Remove final products one-by-one as distillates (the direct sequence).

3. Sequence separation points to remove, early in the sequence, thosecomponents of greatest molar percentage in the feed.

4. Sequence separation points in the order of decreasing relative volatility so

that the most difficult splits are made in the absence of other components.5. Sequence separation points to leave last those separations that give the

highest purity products.

6. Sequence separation points that favor near equimolar amounts of distillateand bottoms in each column. The reboiler duty is not excessive.

Objective: to reduce the number of OD sequences.

What to-do?: List out the guidelines to meet the objective:


14/18

4. Pressure Temperature operatingcondition


15/18

Exercise 1

Q. Ordinary distillation is to be used to separate theordered mixture of C2, C2

=,C3,1-C4=,nC4. Determine

number of possible sequence.


16/18

Exercise 2

Q. Consider the separation as below. Use heuristicsto determine a good sequence of ordinary distillationunits.


17/18

Exercise 3

Q . Design best possible sequence of ordinary

distillation columns to meet the given specifications.


18/18

End

Documents

Chapter 3b - Development of a Separation Process