Complex flow regimes are known to arise in certain phenomenon where the interaction of gas-liquid phases takes place over packed beds of solids such as those observed in trickle bed reactors. There are four major flow regimes that are known to occur in downward cocurrent flow trickle bed reactors (TBR), namely: trickle, pulse, bubble and mist. In this work, the focus was on meso-scale experimental and computational investigations of the interplay between flow regimes and the various parameters that affect them.

The major objectives of this work are to characterize the flow regimes that occur in an industrial trickle bed reactor (TBR) and to determine their transition boundaries. Trickle bed reactors may employ catalyst particles of different sizes and shapes that may be configured in the bed under a number of arrangements. It is also required to study and understand the different bed parameters that may affect the transition of the flow regimes.

The present work is therefore divided into three major parts:

 1.      Experimental Observations of 2-D Trickle Bed Hydrodynamics

A two dimensional experimental setup is developed consisting of a Hele Shaw cell packed with catalyst particles to simulate the trickle bed reactor. An inlet manifold is provided to inject the liquid and gaseous phases under the effect of gravity cocurrently. Flow rates are measured by flow meters and the flow regimes developed are captured by a high speed camera. It is proposed that the transition boundaries between the flow regimes may shift under different bed geometries. The following parameters are studied and their effect on the transition boundaries is discussed by developing flow maps.

A 2-D experimental setup was developed with an average pore diameter close to the values encountered in industrial trickle beds to reproduce and capture the flow regimes photographically. Experiments were conducted in a Hele Shaw cell to study the hydrodynamic behavior or air-water system and flow regimes development at room temperature and atmospheric pressure. Cylindrical particles of constant diameter and thickness were used to simulate the catalyst bed.

A parametric study was done for the development of flow regimes and the transition between them when the geometry and arrangement of the particles within the catalyst bed was varied. Liquid and gas flow velocities were also varied to capture the different flow regimes. Real time images of the multiphase flow were obtained using a high speed camera, which were then used to characterize the transition between the different flow regimes.

 2.      Development of a 2-D computational model to study the TBR hydrodynamics and comparison with experimental observations

A numerical model is developed using the commercial software ANSYS Fluent to study the multiphase flow in the packed bed. Operating conditions and boundary conditions are to be specified to be as similar as the experiments as possible such that the flow regimes observed experimentally are also captured through computations. The model is to be verified by comparing it against experimental results.

The visualization and characterization of flow regimes is supported by computational modeling, which employs a “discrete particle” approach together with “volume of fluid (VoF)” multiphase flow modeling. Different air and water inlet velocities corresponding to the experimental studies are employed in order to visualize flow regimes in TBR.

The table provided summarizes the air and water inlet velocities and their flow regimes obtained at these velocities. In accordance with the experimental results it was observed that changes in the fluid inlet velocities can induce transition of the flow regimes within the catalyst bed. An interesting phenomenon that was observed experimentally and was also captured numerically was the simultaneous presence of more than one flow regime in catalyst bed albeit at different positions.  

We can observe the transition of the flow regimes on the basis of these fluid velocities. At the higher end of the gas velocities and when liquid velocities are sufficiently low, the reactor slips into what is known as the “spray regime” in which the liquid phase is present in the form of isolated droplets or mist. Capturing this flow regime was the most challenging part of this experiment due to the high velocities of the liquid droplets as they move downwards being “washed away” by the dominant gas phase assisted by the gravity. This was confirmed by the numerical study where capturing the downward flow of the liquid droplet confirmed the high velocities involved.

Table 1 Water and gas inlet velocity and corresponding flow regimes for various cases

When operating at the lower end of both the water and air velocities, the trickle flow regime is observed. This trickling flow regime is indicated by the liquid trickles over the packing beds while the gas phase is the continuous phase occupying the remaining interstitial space in TBR. Similar behavior was also observed in the experimental observation. A “web” of the liquid can be observed in which the liquid bridges tend to form and break at low frequencies.

As the water inlet velocity is increased further, pulse flow regime starts to set in the TBR. This is indicated by alternate gas-rich and liquid-rich regions flowing downward which are the salient feature of the pulse flow regimes. The alternate pathways are easier to visualize in the parallel (square) arrangement as compared to the staggered (triangular) arrangement. Another point of interest is that two flow regimes occur simultaneously in this case: pulse and trickle. This condition may not be beneficial for real reactor where trickle flow is desired.

Bubble flow regime is observed in TBR when water inlet velocity is considerably higher than gas inlet velocity. The two phase flow undergoes pulse flow regime in initial transient period. Once steady state condition is reached, however, the development of bubbles of gas phase can be seen. The bubbles formed are relatively small in size and restricted to specific regions within the catalyst bed. The liquid occupies almost the entire bed area and can be observed as a continuous phase. In addition, it is found that gas bubble size decrease as the liquid inlet velocity increase. Overall, the 2D experiments as well as the mathematical model can capture the four possible flow regimes in TBR which were also observed in the experimental observations.