(565e) A Short-Cut Model for Distillation in Rotating Packed Beds for Conceptional Process Design

A Short-Cut Model for Distillation in Rotating Packed Beds for Conceptional Process Design

Authors: Daniel Sudhoff*, Mathias Leimbrink, Philip Lutze

Keywords:

Rotating Packed Bed, Distillation, centrifugally-enhanced separation, short-cut model

* Dipl.-Ing. Daniel Sudhoff

TU Dortmund University

Phone: +49 (0) 231/ 755 6002

Email: Daniel.sudhoff@bci.tu-dortmund.de

Web: www.fvt.bci.tu-dortmund.de

To meet future challenges in the chemical and bio-chemical industries additional requirements for chemical production plants arise. That includes, besides others, quick market entry by saving time and cost to operation, increased flexibility with respect to the capacity by the possibility to adjust process parts, increased flexibility with respect to changes in the composition of the feed and a dynamic operation by additional control levers. These requirements not only apply for reaction but also for separation processes and call for new and innovative separation apparatuses.

A potential device to meet these requirements is the rotating packed bed (RPB) that offers intense mass and heat transfer and a flexible operation by the adjustable rotational speed at a compact and space economic design. By the rotation of a cylindrically shaped packing at a large number of revolutions per minute the RPB creates high centrifugal forces in a centrifugal field. The phase contact between a liquid and a vapor occurs inside the rotating packing and therefore inside the centrifugal field. Due to the high applied forces to the liquid and the intense mixing of the vapor very high mass transfer rates can be achieved. Additionally, packings with a large surface area can be applied due to the high centrifugal forces applied to the liquid. Both the large surface area and the intense mixing enable high separation efficiency at a small volume and therefore relatively compact devices. The additional degree of freedom, the rotational speed, which has a significant influence on the separation efficiency, gives the opportunity of a quick change in separation performance of the RPB and therefore of a quick response to changes in the feed streams or in the requirements for the product streams. These advantages make the RPB a potential candidate to meet the above mentioned requirements for chemical production plants.

Although having first been described already in the 1980s by Colin Ramshaw [1] and the RPB's potential advantages over columns RPBs have not found their way into the toolbox for the conceptional process design yet. Except for the application as a reactor for the production of nano-particles and for special absorption and stripping processes mainly in Asia, RPBs for distillation have not been studied systematically. Although approaches for the prediction of the separation efficiency of RPBs in analogy to those in packed columns have been proposed in some of these research activities, a reliable mathematical description is still missing for the process design. Due to this lack the RPB technology is often not considered in the conceptional design of new processes, although it might be advantageous for certain applications. The scope of this work is set into this context.

The aim of this investigation is to develop a short-cut model for the description of the separation efficiency of RPBs for distillation which is simple but accurate enough to a certain degree and allows quantifying the separation efficiency and the potential failure also while extrapolating. These requirements are due to the use as a model at an early stage of the conceptional process design to evaluate the feasibility of the RPB for distillation for a given problem without a large effort. Based on this a decision on whether the RPB should be investigated further for the given problem can be done. To derive the short-cut model a comprehensive overview of existing approaches for the description of the separation performance of RPBs for distillation has been created from literature. These approaches have been evaluated on the bases of general applicability for the special geometry and force fields inside an RPB, of applicability for a single or for general RPBs and sizes and of its potential for extrapolation in a given range. Additionally, the complexity of the approaches has been evaluated in order to use them as a short-cut model. Based on this evaluation a short-cut model has been derived using the reduced literature approaches with own modifications by the identification of important process and design parameters. The short-cut model has than been adapted to an RPB pilot plant facility consisting of three consecutive rotors. For this purpose finite distillation experiments varying the important operating parameters have been carried out in a pilot plant multi-stage counter-current rotating packed bed (MSCC-RPB) that has been presented in earlier works [2]. Doing so the short-cut model has been tested and validated against own experimental data. The developed short-cut model fulfills the requirements for the conceptional process design and enables the process engineer to consider RPBs for distillation in an early process design stage.

[1] Ramshaw, C.: “Higee” Distillation – An Example of Process Intensification, Chem. Eng. 1983, 13, p. 13-14.

[2] Sudhoff, D.; Lutze, P.; Górak, A.: Multi-Stage Counter-Current Rotating Packed Bed for Distillation; presentation at AIChE Spring Meeting 2013, San Antonio, USA.