(264c) Homogenous Azeotropic Pressure Swing Distillation – a Discussion of the Inverted and the Regular Batch-Process

The separation of a homogeneous azeotropic mixture is a common separation task in the chemical industry. Especially batch distillation is a very popular unit operation in chemical engineering. The discontinuous operation competes with the continuous operation but is still important for fine chemicals, seasonal demands of products and for pharmaceutical industries and food industry. For the separation of a binary homogeneous azeotropic mixture or closed boiling systems using pressure swing distillation (PSD) is one of the simplest solutions for this task. But other alternative processes such as azeotropic distillation or extractive distillation have a great industrial relevance whereas the pressure swing distillation is only seldom applied [Phimister 2000]. One of the main advantages of the PSD is the abdication of recycling an additional substance (entrainer) compared to the other alternatives.

In PSD the dependency of the azeotropic concentration on the system pressure is used to overcome the azeotrope. In the literature, there are only a few theoretical results of the pressure swing distillation for the separation of binary homogeneous azeotropic mixtures and experimental data are needed [Phimister 2000]. Only for the continuous operation a few experimental data are available [Repke 2006].

The objective of this contribution is on the theoretical and experimental analysis of batch PSD. For the regular and the inverted process a rigorous model, which comprise a full set of dynamic balance equations and also describes the start-up from cold and empty initial condition, was formulated in the software package gPROMSÔ.

As an example the binary azeotropic mixture acetonitrile/water was considered and experimental runs for both batch distillation processes starting from cold and empty state with low and high system pressure were carried out. In Figure 1 a comparison between the simulation and experimental results is shown, in this case for the high pressure inverted batch PSD process. The experiments were used for model validation and to discuss the special operational behaviour of the regular and inverted batch PSD, respectively.

In the investigation different separation tasks (various feed compositions) have been used to analyse the regular and the inverted batch PSD. In the scope of this discussion the ideal theoretical volumetric ratio between the amount of distillate and bottom product, which can easily be estimated using the overall component and mass balance under some assumption is considered. The ratio depends on the feed composition and the pressure difference between the low and the high pressure operation runs and can indicate which batch mode should be preferred.

The advantages and limitations of the regular and inverted batch PSD will be discussed in the presentation.

[Phimister 2000]      Phimister, J.R. & Seider, W.D., 2000, Semicontinuous Pressure Swing Distillation, Ind. Eng. Chem. Res. 39, pp. 120-130

[Repke 2006]          Repke, J.-U.; Forner, F.; Klein, A. Separation of Homogeneous Azeotropic Mixtures by Pressure Swing Distillation ? Analysis of the Operation Performance. Chem. Eng. Techn. 28, 2006