What is vacuum distillation
A lowering of the boiling point is achieved by lowering the pressure. In vacuum distillation, the pressure is set so that the temperature from the bubble to the condenser is below the maximum permissible temperature. This initially guarantees that no chemical changes in the form of decomposition can occur. The temperature profile within the bladder requires special consideration. Because here, despite the low column temperature, higher temperatures can occur at the bottom of the bubble. The cause is the static pressure level of the liquid layer. By lowering the pressure, however, there is also the need to keep the resistances occurring in the column as low as possible. This means that the total pressure loss that occurs must be significantly smaller compared to normal tray or packed columns. The result is that other column shapes must be selected with the same good separating effect. First a solution was found in the so-called rotary columns. The liquid is finely distributed by a rotor located inside the column and brought into contact with the steam in small droplets. The large surface that forms has a positive influence on the separating effect. However, the liquid fractions that are entrained despite the reduction in vapor density have a negative effect. Investigations have shown that the formation of very fine droplets should be avoided as far as possible in vacuum distillation and that the amounts of liquid in the column should remain very small. In accordance with this requirement, a number of special designs were developed, which will not be discussed further here. In addition to separating sensitive mixtures, vacuum distillation can also be used for mixtures whose equilibrium curve in the Mc Cabe-Thiele diagram is very close to the diagonal. In many cases, the lowering of the pressure shifts the equilibrium curve to such an extent that adequate separation is achieved. Mixtures with an azeotropic point can also be separated by distillation in certain areas by vacuum distillation.
Carrier steam distillation
A further method of gentle distillation can be derived from the finding that insoluble two-component mixtures boil at a given system pressure at a temperature lower than the boiling point of the lowest-boiling component of the mixture. If a liquid is added to such a mixture to be separated, which liquid forms an insoluble two-component system with the component to be recovered, the boiling point of the total mixture drops and a separation by distillation can be achieved at a low temperature. The advantage over vacuum distillation is that it can be distilled at normal pressure (atmospheric pressure) and the boiling temperatures are still lower. This also eliminates the difficulties mentioned with vacuum distillation, and it is possible to work with the usual distillation columns. The added liquid is separated off from the component to be recovered by a simple settling process through liquid separation. In the literature, carrier steam distillation is often referred to as steam distillation. This fact is justified by the fact that water forms such an insoluble mixture of two substances with a large number of organic compounds and is therefore used as a liquid additive.
The extractive distillation presupposes that the auxiliaries which are to be added to a mixture which is very difficult to distill or which are azeotropic are known precisely in terms of their effect on improving the distillation. The principle of extractive distillation is that the auxiliary and the components of the solution form one or two binary azeotropic mixtures (often also a ternary mixture). The adjuvant reduces the partial pressure of one component far more than that of the second component. This practically creates a new two-component system. On the one hand we have substance A, which consists of two components (excipient and one component of the solution), and on the other hand, substance B, which contains either only the second component of the solution or the excipient and second component of the solution. The separation by distillation is then carried out into substances A and B, the further separation of which must take place depending on the auxiliary substance. An example of an extractive distillation is the benzene-cyclohexane system with and without the auxiliary aniline. This clearly shows the changing effect of the auxiliary aniline. In order to achieve such an effect, very large amounts of excipient are first required. They are often a multiple of the amount of the original solution. In practical use, care should be taken to ensure that the excipient only forms an azeotrope, if possible, with the component to which less stringent requirements are placed, since the removal of the excipient gives rise to new separation problems.
Multi-fuel distillation will only be dealt with here to the extent that it is necessary for a first approximate solution. An exact calculation is very extensive and usually requires computer-aided simulations. So-called key components S are introduced as auxiliary variables in order to obtain a sewing solution. This reduces a multi-component system to a two-component system (Si, Sa). The choice of which component is called Si and which is called Sa depends on the requirements of the task at hand. The component is selected that is either still to be obtained in the distillate or still in the sump. The z. B. lower-boiling component than the key component Si will then be in the distillate with certainty. The components that lie between Si and Sa will occur both in the distillate and in the sump. When determining the system design, it is also important to know whether fractions are to be obtained in the distillation in which the individual components are enriched to different degrees (petroleum distillation) or whether the mixture is to be broken down into the pure components. In the case of fractionation, a single column is usually sufficient, from which so-called side fractions are withdrawn at the appropriate points. In the second case of the pure recovery of the individual components, however, several columns expediently connected in series are required. In the case of a mixture consisting of n components, n - 1 columns are required for their purification. There are very many circuit options.
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