Modern society is becoming ever more demanding in the quality of the products it uses and for health and environmental reasons a better removal of some components is required. This better puriRcation of many products sometimes requires operating at very low pressure. Fortunately, the development of industrial equipment to obtain better and lower vacuum has been maintained. The cost of carrying out these operations is of course more expensive than at around atmospheric pressure. The increase in cost is inversely proportional to the absolute operating pressure. Distillation at low pressure is used for special cases with one or more of the following characteristics: 1. heat sensitive products; 2. liquid feeds or liquid residue with high viscosity; 3. liquids with fouling and/or foaming tendencies; 4. low operating pressure (medium and high vacuum); 5. low residence time. The applications may be classiRed into four groups: distillation or evaporation of sensitive organic chemicals; concentration of foods, chemicals, polymers and biological compounds; recovery of organic solvents; desolventing, devolatilization and Rnishing of polymer solutions. Table 4 shows the levels of vacuum used and also lists representative equipment. Some advantages of vacuum and molecular distillation are: low residence time; high selectivity due to the higher values for relative volatility; cheaper heating requirements.
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