PhD Thesis Colloquium: Mr. Gadi Venkat Arunchaitanya (06/02/23)
Thesis title:
A fundamental investigation of discrete liquid flow, gas and fines in a random packed bed, along with applications
Faculty advisor(s):
Prof. Govind S Gupta
When?
06th February, 2022 (Monday), 11:00 AM (India Standard Time)
Where
KPA Auditorium, Dept of Materials Engineering
Abstract
The liquid flow through a packed bed is quite common in many chemical engineering applications such as distillation, stripping, catalysis, and in metallurgical processes like blast furnace, heap leaching, etc. Many researchers have modelled the liquid flow in a packed bed using continuum hypothesis for both wetting and non-wetting conditions. However, from the physical observation made by various researchers, in a non-wetting or in a low liquid flow rate (heap leaching) system, it is found that the liquid flows not as a continuous stream but as a mixture of discrete rivulet(s) or droplet(s) or combination of both. The liquid flow in any direction, in a packed bed, depends on the local void shape and size. The deterministic approach to find out the void shape and size locally, is a big challenge which is lacking currently. Moreover, the effect of gas flow on the particles and liquid phase of the packed bed, taking into account the random nature of the bed and the discrete nature of the liquid, is lacking. The liquid flow in the presence of gas flow in a bed of particles is a common phenomenon in various metallurgical and chemical engineering processes. In the dripping zone of the blast furnace, where iron ore is converted to molten iron/slag (liquid), the liquid descends the coke bed in the form of droplets and rivulets. The hot blast of air injected laterally, along with the pulverized coal, from the tuyere region flows counter and cross-current to the liquid flow. A cavity, known as a raceway, is formed at the tuyere region due to the gas injection.
In the current study, the Discrete Liquid Flow (DLF) theory is used to model the liquid flow in a randomly packed bed where the shape and size of the void changes with location, The model considers the discrete nature of the flow taking into account the local non-uniformities of the packing. Random packing has been created using the Discrete Element Method (DEM). The void size and shape are determined using a novel graph-based algorithm in the random 2D bed to study the liquid flow. The liquid flow behaviour has been studied in various conditions, like changing the packing size and bed height. This study confirms that the bed topology plays an important role in dictating the liquid flow behavior in a randomly packed bed.
The study is extended to heap leaching process, where the liquid flows as droplets and rivulets due to very low liquid flow rates. The liquid flow behaviour is studied in terms of tortuosity, liquid distribution, breakthrough time, contact angle etc. The study shows that heap leaching processes can be modelled more accurately using DLF theory by avoiding the uncertain experimental parameters (like bed permeability etc). The study shows that bed topology, the local velocity of the liquid, tortuosity, and liquid dispersion affect the distribution of the liquid and the amount of liquid in a heap packing.
Understanding the interaction of gas phase and solid particles of the packing is important before studying the interaction of gas-fines-particle-liquid phases in a packed bed. The study of solid flow has been carried out in presence of gas flow under the counter-current condition in the shaft based processes. Shaft based processes are used in many disciplines such as in metallurgical and chemical engineering. However, their performance suffers due to the lack in understanding of solid and gas flow thus the heat and mass transfer inside the shaft. A slow moving packed bed, inside the reactor, has been considered. Particles are discharged from the bottom and gas is injected laterally. The gas flow has been modelled using continuum-based fluid flow equations. The effect of various parameters (like particle size, gas flow rate, solid discharge rate etc.,) on the residence time of solid particles has been studied in detail. It is found that gas flow is not symmetric inside the reactor due to asymmetric distribution of voids.
Finally, the flow behaviour of the liquid phase in a random packed bed is studied, taking into account the movement of particles due to the lateral gas and fines injection. Gas and fines phase are modelled using continuum-based momentum conservation equations. The study shows that the cavity/raceway size increases due to the liquid injection. The deviation of the liquid path due to the gas and fines drag is also captured. It is found that the deviation of the liquid path is higher for the larger particle sizes.