Distillation

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The most widely used unit operation in the chemical industry for the separation of mixtures is distillation. In addition, distillation is also one of the most energy-intensive separation techniques in the chemical and petrochemical industries.

230 | 7 Energy Efficient Unit Operations and Processes 7.4.1

Basic Principles

The separation principle of distillation is based on different volatilities of components in a boiling mixture. The methods for decreasing the energy consumption of distillation generally focus on the reduction of the reboiler heat duty, because this represents the major energy requirement or on the recovery of the condensation heat at the condenser [1-3]. Electrical energy, which is necessary for pumping the process streams and cooling medium, is usually of minor importance compared with the heat demand [1].

The reflux ratio is the key parameter that influences the energy consumption of a distillation column. The lower the reflux ratio, the lower the energy consumption of the column. However, to achieve the required purity of the distillate, a minimum reflux ratio is required, which is based on the thermodynamic properties of the mixture [2, 3]. The reflux ratio and consequently the required heat input decrease as the number of theoretical stages in a column increases for a given separation. The boundaries are the minimum reflux ratio v^ with an infinite number of theoretical plates and the minimum number of theoretical plates at total reflux Nmin [2, 3], as shown in Figure 7.3.

Based on the assumption of constant mass flows, constant relative volatility a and a boiling feed flow, the minimum reflux ratio v^ can be determined by Fenske and Underwood [4, 5] where xD and xF are the molar fractions of the low boiling component in distillate and feed, respectively:

The design of distillation columns is usually a trade off between the operating costs and investment cost. Often reflux ratios between 10 to 30% above the minimum are chosen [1, 2].

Figure 7.3 Relationship between reflux ratio and number of plates.

However, beside the minimum reflux ratio based on thermodynamic principles the minimum reflux flow based on the fluid dynamics of the columns internals also needs to be considered. Therefore, in some cases an existing column has to be operated at a higher reflux than the minimum reflux ratio defined by the thermodynamics to achieve the required wetting of the column internals.

There are two main ways to increase the energy efficiency of distillation. In general, the first and the cheapest way is to improve operation and control. The second area of measures is improved column design for single and for multiple columns.

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