ADSORPTION SEPARATION

RUATFELA DARLONG

16AG63R16

DEPARTMENT OF AGRICULTURAL AND FOOD ENGINEERING

INDIAN INSTITUTE OF TECHNOLOGY KHARAGPUR

Contents

• Introduction

• Applications of adsorption separation process in the food industry

• Types of adsorption

• Nature of adsorbents

• Principal adsorbents in general use

• Fixed bed adsorber

• Adsorption isotherms – Freundlich, Langmuir and BET isotherms

• Conclusions

• References

INTRODUCTION

• In adsorption processes, one or more components of a gas or liquid stream are adsorbed on the surface of a solid adsorbent and a separation is accomplished.

• Adsorption process involves two components :• Adsorbent

• Adsorbate.

WHY ADSORPTION OCCURS??

• Due to unbalanced forces

• Inside the adsorbent, forces acting between the particles are mutually balanced

• On the surface, the particles are not surrounded by atoms of their kind on all sides

• Hence they possess residual attractive forces.

Factors affecting adsorption:• Temperature• Pressure• Surface area

• Liquid phase adsorption : • Removal of moisture dissolved in gasoline• Decolorization of petroleum products and aqueous sugar solutions, • Removal of objectionable taste and odor from water,

• Gas phase adsorption : • Dehumidify air and other gases• Remove objectionable odors and impurities from industrial gases such as

carbon dioxide• Recover valuable solvent vapors from dilute mixtures with air and other

gases.

Applications of adsorption as a separating process in the food industry:

• Decolorization of edible oils

• Decolorization of sugar syrup

• Removal of bitter substances from fruit juices

• Removal of chlorine from drinking water

Liquid sugar before and after decolorization

TYPES OF ADSORPTION:• Physical adsorption:

• Van der waal’s adsorption

• Reversible

• Result of intermolecular forces of attraction

• Evolution of heat – exothermic.

• Chemisorption:

• Activated adsorption

• Result of chemical interaction

• Adhesive force is greater

• Irreversible

• Chemical change occurs

Nature of adsorbents:

• vary in size from 50 µm to 12 mm in diameter

• must not easily be carried away by flowing stream

• adequate strength and hardness

• large surface per unit weight (100 to over 2000 m2/g)

Principal adsorbents in general use:

• Fuller’s earths:• magnesium aluminum silicates• range from coarse granules to fine

powders• useful in decolorizing petroleum

products as well as vegetable and animal oils• adsorbent can be reused many times

• Activated clays: • bentonite or other clays treatment with

sulfuric or hydrochloric acid• clay is washed, dried and ground to a

fine powder• useful for decolorizing petroleum

products• discarded after a single application.

• Activated carbons:• Burned coconut shells, coal, and wood• Used in gas purification and water

purification

• Bauxite:• Naturally occurring hydrated alumina• Activated by heating at high temperatures• For decolorizing petroleum products • Reactivated by heating

• Bone char:• Obtained from heated, crushed, dried bones• Temperatures range: 600 to 900°C• Refining of sugar • Reused after washing and burning

• Silica gel: • Used principally as desiccant• Adsorption of vapors and gases in  gas mask

canisters

ADSORPTION MAY BE CARRIED OUT AS:

Batch process Semi-continuous process Continuous process

FIXED BED ADSORBERS

The fluid mixture to be treated is passed through a stationary bed of adsorbent.

As increasing amounts of fluid are passed through such a bed, the solid adsorbs increasing amounts of solute on its surface.

ADSORPTION ISOTHERMS

If the adsorbent and adsorbate are contacted long enough, an equilibrium will be established between the adsorbate in adsorbent and adsorbate in solution. The equilibrium relationship is described by isotherms.

1. FREUNDLICH ADSORPTION ISOTHERM: (1909)

where, x is the mass of gas adsorbed on mass m of adsorbent at pressure p,

k, n are constants whose values depend upon adsorbent and gas at particular temperature

Taking log on both sides of equation we get,

Limitation of Freundlich adsorption isotherm:• Relationship of adsorption with pressure at low values only

• Failed to predict value of adsorption at high pressure.

2. LANGMUIR ADSORPTION ISOTHERM: (1916) – Irving LangmuirAssumptions:

• Fixed number of vacant or adsorption sites

• Vacant sites are of equal size and shape

• Each site can hold maximum of one gaseous molecule

• Constant amount of heat energy is released during this process

• Dynamic equilibrium exists between adsorbed molecules and free molecules

• Adsorption is monolayer.

where, A (g) = unadsorbed gaseous molecule

B (s) = unoccupied metal surface.

AB = adsorbed gaseous molecule.Limitation of Langmuir adsorption isotherm:-

• valid at low pressure only

• assumption that no further adsorption occurs once the mono-molecular layer is full.

3. BET ADSORPTION ISOTHERM: (1938) • Brunauer, Emmett and Teller

• Multilayer adsorption

• Stated that the Langmuir theory can be applied to each layer.

where,

P = partial pressure of the adsorbate in the gas

P0 = vapor pressure of the pure adsorbate

Xm = monolayer value of x

K = constant.

CONCLUSIONS

• Adsorption is different from absorption

• Applications of adsorption include:

• Sugar syrup decolorized by bone char

• Water vapors adsorbed by silica gel

• Polyphenols and flavonoids separated to remove bitterness from citrus juice.

• Adsorption is affected by: Temperature, Pressure and Surface area.

REFERENCES:

• Robert E Treybal (2012). Mass transfer operations (3rd Edition), McGraw Hill Education (India) Edition, ISBN-13: 978-1-25-902915-8.

• Zeki Berk (2013). Food Process Engineering and Technology (2nd Edition), Elsevier, ISBN: 0124159230, 9780124159235