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Strong Acid System - Absorption and Stripping
September 15, 2003

Introduction

In the chemical industry, it is necessary to selectively remove a constituent (e.g. SO₃) from a gas mixture by dissolving it in a liquid (e.g. H₂SO₄), or to free a liquid (e.g. weak H₂SO₄) of dissolved (e.g. SO₂) gases by contact with an inert gas (e.g. Air).  These unit operations of chemical engineering are known as absorption and stripping, respectively.

            Driving forces and processes are in opposite direction in the two cases, but exactly the same forces are present.

Equilibrium Factors

In a particular system, for a given concentration of a constituent in the liquid, there is an equilibrium concentration in the vapor above it.

• If the actual vapor concentration is higher than the equilibrium concentration, absorption occurs.

• If the actual vapor concentration is lower than the equilibrium concentration, stripping occurs.

• At equilibrium, there is no mass transfer.

The driving force is the difference between the actual concentration and the equilibrium concentration.  The rate of mass transfer is directly proportional to the distance from the equilibrium.

The solubility of gases in liquid increases at higher pressure and lower temperature, hence the driving forces for absorption increase under these conditions and the driving forces for stripping decrease.

Absorption

Absorption is a mass transfer process that applies to the removal of solute gases in the presence of inerts (i.e. oxygen and nitrogen).  In a sulphuric acid plant, the primary absorption process is the absorption of sulphur trioxide (SO₃) into concentrated sulphuric acid (H₂SO₄).

The absorption process can be represented by the two-film theory which states that the overall mass transfer coefficient is related to the mass transfer coefficients in the gas and liquid phase.

In the absorption of SO₃ into H₂SO₄, the process is said to be gas-phase controlled.  This means that the rate of absorption of SO₃ is more a function of the gas properties than the properties of the liquid.  Once the SO₃ reaches the liquid, the rate at which SO₃ passes through the liquid and reacts to form H₂SO₄ is so fast that the process is limit by what is happening in the gas.

Stripping

Stripping is the opposite of absorption and involves the removal of dissolved gases in the liquid by an inert gas.  In a sulphuric acid plant, the primary stripping process is the removal of dissolved SO₂ from weak and concentrated sulphuric acid streams.

In these stripping operations, the process is liquid phase controlled rather than gas-phase controlled as is the case with the absorption of SO₃ into H₂SO₄.

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