(94c) Design and Characterization of a Flexible Tray Column with Enhanced Operating Window and Turndown Ratio

Ensuring future competitiveness of production processes is an important task for the chemical industry. This requires the processes to be constantly and adequately adapted for the related challenges. These challenges result, among others, from the need to reduce greenhouse gases significantly. In particular, future energy and feedstock supply will increasingly rely on renewable rather than on fossil resources. Especially, an energy supply structure that heavily relies on intermittent renewable resources imposes new challenges for the operation of chemical production processes.

Adjusting the operation of the processes to the availability of energy streams is possible by enhancing the flexibility of processes and their unit operations In literature, various solution strategies to enhance process flexibility can be found, e.g. for chemical reactors. This also applies to downstream processing unit operations such as distillation columns. In fact, distillation processes consume a considerable amount of energy, so that energy- and separation-optimal operation needs to be ensured at all operating points. Technical solutions are required to cope with these increasing uncertainties.

In our work, we have developed a novel column design that allows for a more flexible operation of distillation processes. The internal volume of the column is segmented by installing vertical dividing walls throughout the total height of the column. Every segment in the column can be operated, started-up and shut-down independently. In fact, each segment can be operated at a distinctive separation-optimal operating point, which is the basis for the increased flexibility.

The segmentation of the column resulted in two specific tray layouts to achieve efficient contact between gas and liquid on each tray. The tray layouts differ in the flow direction of the liquid over the active area of the tray and therefore in the positioning of the weir and the downcomer. In the tray layout D_I, the liquid flows from an inner laying downcomer in radial direction over the active area to an outer weir. On the tray with the layout D_O, the flow direction of the liquid is vice versa and the liquid is entering the active area from an outer laying downcomer towards an inner weir. Both tray layouts are based on sieve trays for the active area.

Besides these distinctive tray layouts, the column is characterized by the capability of coupling the segments on each tray via the downcomer next to each other. The motivation behind this coupling is to accelerate start-up operation of single segments. If one segment is operated at steady-state, the start-up of the adjacent segment can be supported by liquid transferred from the downcomer of each tray. Thus, an inactive segment is started-up using a pre-defined concentration profile. In addition, we developed a dynamic column model to perform start-up simulations, analyzing this approach to start-up individual segments within one column shell. Results show that a significant reduction of the start-up time is possible, while the active segment is affected only for a short time. During start-up, only <10% of the liquid holdup from the active downcomer was used. Anyway, to validate these simulation results, it is necessary, to assess the liquid transfer from one active downcomer to the adjacent inactive ones in experimental studies. In addition, insights into the specific operation of single segments in this column design are yet to be discovered.

In this contribution, we present the results of the experimental characterization of the innovative tray design focusing on (a) characterization of fluid dynamics and operating limits and (b) strategies for a flexible operation of the new tray design in a d_column=600 mm test column. The column consists of four segments and can be operated with gas loads up to 2 Pa^0.5 and liquid flow rates up to 5.5 m³ per hour. Usually, for sieve trays, the liquid load is defined per meter weir length. For the particular tray design used in the flexible column, however, liquid load is specified per m² active area to ensure comparability between the tray layouts. This results in liquid loads between 25 m³âˆ™m^-2∙h^-1 and 105 m³âˆ™m^-2∙h^-1.

For the characterization of fluid dynamics, we determined dry and total pressure drops for different tray design parameters. Here, design parameters vary in terms of hole diameter and resulting opening ratio. We compare experimental results with existing correlations from literature. In addition, we assessed the weep point to define the lower operating boundary. Due to the different described tray layouts (D_I and D_O), flow regimes and liquid distribution on the trays is of special interest. Therefore, we present results from ultrasonic measurements of liquid heights on the trays. The ultrasonic sensors were distributed over the active area of the tray layouts to assess the liquid height at different locations.

Secondly, we operated the column under liquid transfer from one segment to an adjacent one to enable flexible operation. Therefore, we assessed maximum transfer rates. Gaining insights into operating stability was possible by pressure drop measurements on the trays and in the downcomers. Preforming the measurements at varying gas and liquid loads leads to the identification of superimposed operating boundaries during the operation with liquid transfer. In addition, the results help to determine switching points for the flexible operation. Switching points define the start-up or shut down of individual segments in the course of flexible operation.

Lastly, we operated two adjacent segments in parallel and assessed fluid dynamics experimentally. By operating two adjacent segments, we show that out design is robust and parallel operation of segments is based on comparable conditions.