Mass Transfer Part (1)

1. INTRODUCTION TO MASS TRANSFER 1.1

Introduction A number of unit operations are carried out in Chemical Engineering applications which do not involve chemical reactions.

These operations are

carried out for separating either a component by mechanical means like screening, filtration and salting or increasing its concentration in a mixture. The latter is called as mass transfer operations. Frequently, these mass transfer operations are used for the separation of a product from the by-products formed and or also from the unreacted raw materials. The separation technique plays a vital role in fixing the cost of final product. 1.2

Classification of mass transfer operations It is classified as gas-liquid, liquid-liquid and fluid- solid operations.

1.2.1

Gas-Liquid Absorption: Transfer of a solute from a gas mixture to a solvent. (e.g.) (i) removal of ammonia from gas from by-product coke ovens using water. ii) removal of H2S from naturally occurring hydrocarbon gases by alkali solutions. Desorption: Reverse of absorption. Removal of a solute in a solution using a gas. (e.g.) removal of NH3 from NH3-water solution using air. Humidification: Transfer of a liquid to a gas phase containing one or more components by contacting dry gas with pure liquid. Dehumidification: Transfer of a vapor component from a gas-vapor mixture to a liquid phase by contacting them. (e.g.) transferring water vapor from air-water vapor mixture to liquid water. Distillation: It is a method of separating the components in a liquid mixture by the distribution of substances between a gas and a liquid phase. The method of separation depends on their relative volatility and applied to cases where all the components are present in both the phases. Here a new phase is created from the original solution itself. (e.g.) separation of petroleum fractions by the application of heat. Separation of high boiling water insoluble mixtures using steam.

1.2.2

Liquid-Liquid Extraction: Liquid – Liquid extraction involves the separation of a component (solute) from a liquid mixture using another insoluble or partially miscible solvent. The separation depends on the distribution of solute between the two phases based on its physico-chemical characteristics. The two phase are solvent rich phase (extract) and residual liquid phase (raffinate). (e.g.) (i) separation of acetic acid from acid-water mixture using isopropyl ether as solvent. (ii) Separation of dioxane from water – dioxane solution using benzene as solvent.

1.2.3

Solid-Liquid/Gas Leaching: This is a process of separating a soluble solute from a solid mixture by contacting it with a solvent. (e.g.) (i) Separation of oil from oil seeds using hexane. (ii) Separation of sugar from sugar beets using hot water. (iii) Removal of copper from its ore using sulfuric acid. Adsorption: Adsorption involves contact of solid with either a liquid or a gaseous mixture in which a specific substance from the mixture concentrates on the solid surface. (e.g.) (i) Removal of color from solutions using activated carbon. (ii) Removal of moisture from air by silica gel. Desorption: It is the reverse of adsorption operation. Drying: Drying refers to the removal of moisture from a substance. . (e.g.) (i)Removal of water from a cloth, wood or paper. (ii)Removal of water from solution (Manufacture of spray dried milk.) Crystallization: The process of forming solid particles within a homogenous phase is called crystallization. (e.g.) (i) The homogenous phase could be a vapor as in the formation of snow (ii) the formation of crystals of sugar from a concentrated sugar solution.