(54d) Two-Phase-Flow and Stress on Internals during Pressure Relief Events on Distillation Columns: Experimental Investigation and Dynamic Simulation

Due to the large content of different substances consisting often of flammable solvents, distillation columns possess a high safety risk potential. Operation failure like the breakdown of different plant components (condenser etc.) can cause the process to leave the field of normal operation and finally cause an emergency pressure relief. The fluid dynamic behavior during the pressure relief event is not well understood. The assumptions in scope of design and operation concerning vapour flow, holdup and pressure drop correlations are made for stationary operations and are not necessarily valid during a pressure relief event.  Due to these facts and the unknown column dynamic in this field of operation, still today pressure relief devices especially pressure relief valves (PRV) are designed by guidelines based on experiences and short-cut methods like adaptation of the relief capacity to the overhead vapour rate or the reboiler vapour rate. The consequence of using these methods and to accept the high level of uncertainties is that the relief device has to be oversized to guarantee prevention of the worst case which is a container collapse. On the other hand the oversized relief device is causing a fast depressurization and higher pressure gradients inside the column which implies enhanced vapor flows. Normal tray column internals are designed for a stationary operation load of approximately 20mbar over a stage which can be easily exceeded during a pressure relief event and lead to a destruction of the internals. The total amount of product loss is also increasing due to the oversized relief device. The enhanced vapour flows are causing a transient level swell of the liquid phase which makes it more likely that a two-phase relief flow will occur which should be prevented if possible [1].

The objective is to develop a designing procedure which makes it possible to adapt the cross-section area of the relief valve according to the previous mentioned conditions. The relief area must be as large as necessary to guarantee a mass-flow capacity of the PRV high enough to prevent a container collapse but also as small as possible to avoid a two-phase relief flow and to prevent a destruction of column internals due to the high pressure gradients.

For this new approach methodic experimental analysis were carried out on a pilot scale column with a diameter of 100mm and a height of 4m to receive experimental data concerning response time, outflow (mass and state of phase) and load on column internals during a pressure relief event. The column was additionally equipped with a pressure relief device consisting of a safety valve and a phase separator for determination of the phase fractions during the pressure relief event. Different disturbances of normal operation mode were simulated, which caused an emergency pressure relief event. During the event the level swell of the two-phase region was documented and the relief mass flow and the relief time were determined. Special attention was paid to the phase condition of the relief flow (one-phase flow or two-phase flow).To consider the relevant aspect of stress on internals in the column during pressure relief event, an innovative measurement procedure for online measurement of tray load was developed, commissioned and successfully validated in a testing station. The concept is based on strain gauges, which measure the stress on the tray suspension during operation. By applying a theoretical strain model, conclusions can be drawn concerning the acting force on the column tray during operation. It is a novel approach to measure directly the stresses acting on the material in a distillation column. A comparable strategy was not found in literature. Figure 1 shows the results of the experimental investigation of a pressure relief event in the testing station with a single tray and a gas/vapour flow generated by a blower. During the first 200ms after the opening of the PRV (at 0 s) the pressure drop over the tray is higher than the stationary pressure drop. The maximum pressure drop of approx. 70mbar exceeds the design pressure drop of a standard tray (approx 20mbar) by far. A first dynamic simulation model describing the flow through the PRV was developed and validated with the experimental data (Figure 1).

Figure  SEQ Figure \* ARABIC 1 left: Comparison simulation-experiment, right: testing station

Using the obtained experimental data of the pilot plant concerning pressure gradient, level swell and relief time a rigorous dynamic column model including a PRV was implemented in gProms© to describe the pressure relief event [2]. The model consists of the column including all additional components (condenser, reboiler, controller etc.). The modeling of the PRV was done according to dynamic flow models of PRVs from literature. The PRV model comprises the option of calculating a one-phase or a two-phase relief flow. The opening dynamic of the valve itself (Popen Open Valve, Proportional Valve etc.) is also included in the model. The results show the option of integrating the detailed dynamic pressure relief model in the process simulation for an additional safety analysis of the column. Interaction between different plant components during a pressure relief event can be detected and the appropriate design of the relief capacity for different disturbances of column operation can be checked easily. The model can also be used in the design process. The design of plant components like trays and PRVs can be checked and altered if necessary in an early stage.

In the presentation an overview of the experimental investigation will be given. The online measurement procedure for measuring stress on column internals will be presented. The key features of the dynamic model will be explained and a discussion of simulation and experimental data will be done.

The authors would like to thank the Nafög-Foundation for sponsoring this research project.

[1] Ü.Can, M. Jimoh, J. Steinbach, G. Wozny, Simulation and experimental analysis of operational failures in a distillation column, Separation and Purification Technology 29 (2002) 163-170

[2] D. Staak, J.-U. Repke, G. Wozny: Outflow conditions and strains on internals during pressure relief events on distillation columns, Pres Conference 2006, Prag