Distillation. Absorption - Pilot columns

The industrial study of a distillation or absorption is normally carried out by following the successive steps below:

• choice of parameters (pressure or temperature at column head) ;

• Establishment of a thermodynamic model for calculating liquid-vapor equilibria and heats of mixing;

• simulation of column operation by calculation, to define the theoretical number of plates and distillation operating conditions (heat and mass flows in particular) according to the desired result;

• definition and sizing of the material exchange device, i.e. choice of plates or linings as a function of process parameters such as pressure drop;

• efficiency calculation, or estimation with reference to existing columns; choice of packing height or number of actual trays to be used;

• validation of the choice by a pilot test.

Commercially available software (such as PROSIM and ASPEN) enables rigorous simulation of column operation. Developments in thermodynamics have led to models that enable precise calculation of liquid-vapor equilibria if the parameters are established from experimental data (ebulliometry). If the thermodynamic model has been obtained by a group contribution method (i.e. by calculation alone), the accuracy of the model is likely to be lower.

Efficiency calculation is the weak point, especially when dealing with complex, highly non-ideal mixtures.

A pilot column test provides an overall validation of the thermodynamic model chosen and the efficiency selected for the column. It may reveal a gross error in the choice of thermodynamic model.

A pilot distillation test is desirable in the following cases:

• relative volatility (see article ) of the key constituents is less than 3 and the activity coefficients have been obtained by a group contribution method, or the relative volatility has been defined by a TBP distillation (true boiling point, cf. ) on a laboratory column;

• the relative volatility of the key constituents is less than 1.25 and the basic experimental liquid-vapour equilibria have not been verified on an existing plant;

• the aim is to obtain a high-purity product (greater than 99.95%).