(52c) On the Inability of Weeping Correlations to Predict the Behavior at Zero Liquid Load: New Experimental Data for Trays, Correlation Development, and Dynamic Weeping

Distillation columns and especially tray columns are still among the most important and widely used thermal separation units in chemical industry. In most chemical production facilities, they are operated at steady-state and ensure, for example, the purification of valuable reaction products at high throughput. The actual design and sizing of distillation columns adheres to specific boundary conditions given by the production process or plant (e.g. separation task, production capacity, operational flexibility). Due to today’s improved understanding fluid dynamics in distillation columns and advancements in process modeling and simulation, occurring phenomena within tray columns and their feasible operating window are often sufficiently predictable as long as the plant is operated as intended. If the required operation window is left, especially in short time horizons, safety-relevant column conditions could occur. Normally, these operation conditions appear during malfunctions result in dynamic processes inside distillation columns especially regarding fluid dynamics and relevant safety scenarios take place outside well described operation regions of distillation columns. To prevent bursting of the column and release of hazardous substances, it is hence required to install safety equipment, whose selection and sizing is based on worst case predictions (e.g. total reboiler failure) and resulting mass and heat flows. It is thus desirable to investigate the dynamic weeping behavior for such a scenario and generate suitable correlations and model structures to predict the expected re-evaporation and the resulting pressure increase more accurately. This way, a better understanding of process dynamics inside tray distillation columns can be obtained via experiment-based correlation development and a safer plant operation can be realized through better understanding of the process as well as dynamic simulations. In addition, investment cost for safety equipment can be reduced by avoiding oversizing.

Knowledge or correlations describing the fluid dynamics for these scenarios are primarily available for rather simple tray configurations in the open literature. The existing literature contains a large quantity of sieve tray data, as for example Lockett et. al investigated experimentally the weeping inside a sieve tray column and developed a weeping correlation based on their steady-state experimental data [1]. Wijn developed a weeping model, with the focus on the description of the clear liquid height under low gas loads, for trays and validated it on available test data [2]. However, the applied gas load factors described as hole load factor in Wijn only varied on the range of 0.09-0.90 m/s and thus relevant shutoff scenarios are not fully captured. Especially for gas loads < 0.20 m/s and high liquid loads > 14 m³/m²h, the qualitative weeping behavior for trays is insufficiently described. Experiments of safety-relevant scenarios inside of distillation columns need to be carried out in order to develop correlations for a better understanding of the plant behavior in the non-designated and safety-relevant operation window. It is furthermore desirable to have a model-based description of the occurring fluid dynamic phenomena in distillation columns outside their operational limits, such as very low or vanishing gas load factors and resulting weeping behavior, available. Therefore, this contribution discusses a typical distillation safety scenario for the separation of wide boiling multicomponent mixtures using tray columns and highlights experimental investigations to enable the development of suitable weeping correlations for trays. Combined with a relatively high liquid holdup on the trays compared to structured packings, an evaporator malfunction leads to dynamic weeping of large quantities of the liquid hold-up on the trays and in the downcomers. Coming from steady-state operation, especially liquid rich on light boiling components is weeping from the upper section of the column into the column sump or reboiler resulting in a potentially very fast evaporation of the light boiler. As a consequence, the pressure inside of the column can increase rapidly and strongly, due to re-evaporation.

Steady-state weeping experiments were carried out for weeping correlation development and to improve the process knowledge regarding trays. Therefore, our experimental setup consists of a DN 300 column made of glass and holds a mounting structure for exchangeable trays, liquid downcomers, as well as exchangeable weirs. A liquid cycle is established using a gear pump, feeding liquid from a separate collector tank to the top of the column. The liquid is then collected after passing the column and transported back to the storage tank, whereby a lower collector tray ensures separate flow measurements of liquid weeping through the trays and liquid exiting via the downcomer using magnetic-inductive flow meters. The gas load is applied by using a speed-controlled fan and gas distributor at the bottom of the column, while also providing the option for abrupt shutoff of the gas flow. The setup is fully automatized using ABB Freelance and operable in a range of 0.0-1.0 Pa^0.5 and 1.3-35.5 m³/m²h for gas load and liquid load, respectively. Weeping rates from 0 up to 100 % as well as tray pressure drop are reliably tracked. As a test system, water/air is used at ambient temperature and pressure conditions (both measured). Experimental results shown in this contribution focus on the application of standard trays for two different weir heights.

The experimental results from our own investigations for trays are presented and discussed. We used our data to develop a new weeping correlation, which captures the weeping behavior over a large range of gas load factors as well as liquid loads. Therein, special focus is laid on the mathematical structure to ensure usability within dynamic tray column models and robust convergence behavior. Finally, the structure and applicability of this new correlation will be critically discussed in comparison the gained experimental data and validation runs, as well as applicability for safety assessments and dynamic process modelling.

Based on our experiments, we present a comparison of the measured weeping rates with existing literature correlations. These results show a different quantitative and qualitative weeping behavior for specific trays. Most importantly, no weeping experiments are published so far which are able to correctly capture the behavior in the vicinity of zero weir load. This inability of the published data respectively correlations make them virtually inapplicable to the safety-relevant scenarios. In addition, dynamic weeping experiments for different initial conditions will be presented.

[1] Lockett, M. J., and S. Banik. "Weeping from sieve trays." Industrial & Engineering Chemistry Process Design and Development 25.2 (1986): 561-569.

[2] E.F. Wijn, On the lower operating range of sieve and valve trays, Chemical Engineering Journal, Volume 70, Issue 2, 1998, Pages 143-155, ISSN 1385-8947, https://doi.org/10.1016/S0923-0467(98)00089-X.