(409f) Hybrid -Pervaporation-Distillation Processes – a Novel Heat-Integration Approach

Normally, homogenous azeotropic mixtures are separated using thermal separation techniques like extractive distillation, azeotrope distillation, pressure swing distillation described by Repke et al. (2004) or liquid-liquid extraction. A good overview of the possibilities and the application criteria of all these techniques are given by Widagdo et al. (1996). In the literature, it has been shown, that Hybrid-Pervaporation/Distillation processes represent a serious alternative to the existing processes. For example, it has been reported by Lipnizki et al. (1999) that especially for the separation of azeotropic and close-boiling mixtures, the combination of pervaporation and distillation can save energy costs. Nevertheless, until now, the possibilities of heat integration have not been analyzed for this type of hybrid process. In this contribution, a novel developed flow-sheet and a new laboratory plant are presented, the mathematical model used in the simulations is described and finally our new developed heat integration concepts are demonstrated.

The process is composed of a distillation unit and a pervaporation unit in which the homogeneous azeotropic binary mixture of acetonitrile and water has to be separated as an example.

To descript the process a model was developed. The mathematic steady state model for the column units considers the MESH-equations. For the equilibrium, the g-j concept is used. The model used for the Pervaporation module simulation is based on the idea developed by Wijmans and Baker (1993). In the future the dynamic column model described in Repke et al. (2006) will be used, to investigate the operation performance of the complete process.

With proposed heat integration concept, the heaters between the membrane-modules are not needed fig. 1. As shown in fig. 2, the heat transfer is realized directly in the membrane module to use the condensation energy of the distillate to increase the temperature in the module. There are two different types of new membrane module approaches presented. The first one has a ripped pipe to increase the turbulent flow over the membrane; the second approach has a corrugated active membrane surface to increase the surface and also to increase the turbulent flow. Both approach will save energy and increase the flux. This leads to a significant reduction of the membrane surface.   Table: Standard configuration vs. heat integration

In table two simulation runs of the hybrid process with and without heat integration are compared. The new developed heat integration concept leads to a simultaneous reduction of membrane area (-71%) and energy duty (-15%), which means a significant decrease in investment and operation costs.

For the evaluation of the proposed heat integration concept a laboratory plant was developed and built up. In a first step the plant is using for the PV model validation and the identification of required membrane process parameters. In a subsequent step the membrane will be linked with the distillation column and the practice of the whole concept should be demonstrated (see fig. 3).

With this plant different kinds of modules could be analyzed with or without a coupling to a high pressure distillation plant (see Repke 2006).

  References

[Lipnizki 1999]           Lipnizki, F. R.W. Field and P.-K. Ten, 1999, Journal of Membrane Science, vol. 153: Pervaporation-based hybrid process: a review of process design, applications and economics, 183-210

[Repke 2004]             Repke, J.-U., Klein, A. and F. Forner, 2004, Computer?Aided Chemical Engineering, vol. 18: Homogeneous azeotropic distillation in an energy- and mass-integrated pressure swing column system, 757-762

[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

[Widagdo 1996]        Widagdo, S. and W. D. Seider, 1996, AIChE Journal, vol. 42: Azeotropic Distillation, 96-130

[Wijmans 1993]         Wijmans, J.G. and R.W. Baker, 1993, Journal of Membrane Science, vol. 79: A simple predictive treatment of the permeation process in pervaporation, 101-113