Special Separation

Processes

Ali Ahmadpour

Chemical Eng. Dept.

Ferdowsi University of Mashhad

2

Contents

Introduction

Separation properties

Separation process classifications

Separation techniques

• from solids

• from liquids

• from gases/vapors

References

3

Separation properties

The extent of separation depends on the differences

in molecular, thermodynamic, and transport

properties of the species in the different phases.

1. Molecular properties

- Molecular weight

- Van der Waals volume or area

- Molecular shape (acentric factor)

- Dipole moment

2. Thermodynamic and transport properties

- Vapor pressure

- Solubility

- Polarizability

- Dielectric constant

- Electric charge

- Radius of gyration

- Adsorptivity

- Diffusivity

4

Separation process classification

1. Based on phase creation or addition

- Condensation

- Flash vaporization

- Distillation

- Absorption

- Crystallization

2. Based on barrier

- Osmosis

- Reverse osmosis

- Dialysis

- Microfiltration

- Liquid-liquid extraction

- Drying

- Evaporation

- Leaching

- Foam fractionation

- Ultrafiltration

- Pervaporation

- Gas permeation

- Liquid membrane

5

Cont.

3. Based on solid agent

- Adsorption

- Chromatography

- Ion exchange

4. Based on force field or gradient

- Centrifugation

- Thermal diffusion

- Electrolysis

- Electrodialysis

- Electrophoresis

6

Another separation process

classification

1. Equilibration separation processes

- Evaporation

- Distillation

- Absorption

- Adsorption

- Extraction

- Crystallization

- Flotation

- Chromatography

- Gel filtration

- Ion exchange

- Drying

- Leaching

- Clathration

- Osmosis

- Foam fractionation

- Magnetic separation

- Freeze drying

- Desublimation

7

Cont.

2. Rate-governed separation processes

- Gaseous diffusion

- Sweep diffusion

- Thermal diffusion

- Mass spectrometry

- Dialysis

- Electrodialysis

- Gas permeation

- Electrophoresis

- Electrolysis

- Ultracentrifuge

- Reverse osmosis

- Ultrafiltration

- Molecular distillation

8

Cont..

3. Mechanical separation processes

- Filtration

- Settling

- Sedimentation

- Centrifuge filtration

- Cyclone

- Electrostatic precipitation

9

Important factors in the selection

of feasible separation operation

A. Feed conditions

1) Composition

2) Flow rate

3) Temperature

4) Pressure

5) Phase state (solid, liquid, gas)

B. Product conditions

1) Required purities of products

2) Temperatures

3) Pressures

4) Phase states

10

Cont.

C. Property differences that may be exploited

1) Molecular

2) Thermodynamic

3) Transport

D. Characteristics of separation operation

1) Ease of scale-up

2) Ease of staging

3) Temp., pressure, and phase-state requirement

4) Physical size limitations

5) Energy requirement

11

Some considerations

To apply any separation technique, the

chemical nature of raw material,

organization and structure of components,

and physical properties are vital

consideration.

12

Physical properties and separation

processes

Physical property

Size (distribution), shape

Density

Viscosity

Surface properties

Thermal properties

Diffusion

Solubility

Separation technique

Screening, air classification

Centrifugation, sedimentation

Liquid extraction

Floatation

Evaporation, drying

Extraction, membrane separation

Solvent extraction

13

Separation techniques

All separations (from small molecules toparticulate) rely on differences in physicalor chemical properties.

Separation rates are very important

affected by the size of driving forces

Rate= Flux ×Area= (D.F./Resistance) ×Area

When a 2nd phase is involved, mass transferbecome important (higher M.T. surface).

Some separation involved both heat andmass transfer.

14

Cont.

Separation processes:

Batch/Continuous

Single/Multiple stages

Co-current/Counter-current

Steady state/Unsteady state

Equilibrium data (solubility, vapor/liquid, water sorption,partition) are very important.

Operation conditions, flow rates, temp. and time indicatehow close process operates to equilibrium.

All reaction rates are temp. dependent.

15

Separation from solids

Solid-Solid separations (based on particle size,

shape, density, aerodynamic properties, electrostaticcharge, degree of hydration, photometric, magnetic,…)

Separation from the solid matrix (Compression

or solvent extraction, Dehydration, Blanching or airremoval)

16

Separation from liquids

One important physical property is fluid viscosity.

Liquid-Solid separations Conventional Filtration suspended particles from liquids based on

particle size (>50,000Å)

Microfiltration removal of very fine particles (500-50,000Å)

Ultrafiltration separation of macromolecules (Proteins or starch)

Settling water and effluent treatment (based on particle size anddensity)

Centrifugation separation of insoluble solids (<1%)

General liquid separation processes Extraction recovering oils and oil-soluble components

Process with phase change within the liquid (Crystallization, evaporation)

Use of solid phase in the form of resins or beads (ion exchange)

17

Cont.

Crystallization: Fractionation separation of fats

Freeze concentration: crystallization of ice from liquids (fruit juicesor beverages)

Evaporative crystallization: freezing in scraped surface H.E

manufacture of salt and sugar

Evaporation (heating process): Dehydration (roller drying, band drying, spray drying, freeze

drying) solid product with moisture content below 10%

Flash cooling: heating the liquid followed by sudden pressurereduction separation of flavor

18

Separation from gases and vapors

Filtration (bag filters): Recovering solids suspended in

gas

Cyclones: Separation of powders from gases based on

particle size and density

Wet scrubber: Separation of suspended solids from gases

based on solubility

Electrostatic precipitator: Separation of fine solids

from gases based on the particle electric charge

Centrifugation: Removing of water droplets, rust and oil

19

References

Separation Processes, King, 1982.

Handbook of separation process technology,

Rousseau, 1987.

Handbook of separation techniques for

chemical engineers, Schweitzer, 1988.

Separation Process Principles, Seader and

Henley, 2006.