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Sulphuric Acid on the WebTM Technical Manual DKL Engineering, Inc.

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Handbook of Sulphuric Acid Manufacturing
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Strong Acid System - Absorption and Stripping
September 15, 2003

Introduction
Equilibrium Factors
Absorption
Stripping
Associated Links

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.

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.