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Modeling the Performance and Hydrodynamic Behavior of a Countercurrent Multistage Fluidized Bed Adsorber
- Author / Creator
- Davarpanah, Morteza
Fluidized bed (FB) adsorbers provide excellent gas-solid contact at low pressure drops which allows for effective handling of large flow rates. However, the design, operation, and modeling of FB adsorbers could be challenging due to the complex processes involved. The goal of this research is to model the adsorptive and hydrodynamic behavior of multi-stage countercurrent FB adsorbers. The models proposed are versatile and responsive to changes in design and operating parameters including changes in the scale, weir height, temperature, humidity, adsorbent feed rate, air flow rate, initial concentration, the number of stages, and the type of adsorbent and adsorbate.
This research can be sectioned into two main parts. The performance of FB systems is studied in the first part. A two-phase model is developed to describe the adsorption of volatile organic compounds (VOCs) on beaded activated carbon (BAC) in a lab-scale fluidized bed adsorber. The model is then modified to capture the effect of adsorbent apparent densities (or heel buildup) and further refined to consider any changes in the FB scale. Next, the impact of humidity and temperature on the adsorption of VOC on BAC is investigated. Later, the intensification of the adsorption process in FB systems is discussed. Last in the first part, is a thorough study on the performance of the model using various formulas for the calculation of different parameters (bubble diameter, interphase mass transfer rate, etc.), where a generic set of formulas is proposed.
The hydrodynamics of FB systems are studied in the second part, where Computational Fluid Dynamics (CFD) is used to simulate one stage of the lab-scale FB. A presentation of the variables essential for the hydrodynamic study of the FB (solid volume fraction, air turbulent viscosity, etc.) is given in this section, followed by a comparison between the CFD simulation results and those of (semi)empirical formulas describing different hydrodynamic parameters (minimum fluidization velocity, bubble diameter, etc.).
The results indicate that the two-phase model shows good agreement with the experimental results obtained over a wide range of operating conditions (adsorbent feed rate, air flow rate, initial concentration, etc.). A sensitivity analysis of the two-phase model shows that internal diffusion within the adsorbent is rate-limiting for adsorption. It is also shown that the main characteristics of adsorbents (pore diameter, porosity, and adsorption capacity) can be correlated to their apparent densities to simulate the performance of BACs with different service lifetimes (degree of exhaustion as a result of heel buildup) in lab- and industrial-scale adsorbers using a two-phase model.
The effect of humidity on the adsorption of 1,2,4-trimethylbenzene (TMB) on BAC shows a drop in overall removal efficiency (ORE) starting at RH=75% which eventually plateaued at RHs close to 100% after decreasing ORE by 7.6 %. A small reduction (1.7%) in ORE is also observed when increasing the adsorption temperature from 22 to 50 ˚C in dry condition. The intensification simulations show that increasing the adsorbent feed rate is effective when there is a need for more adsorption sites (e.g. at high inlet concentrations). Reducing air flow rate at constant VOC load is always effective especially when there are enough adsorption sites available (e.g. high adsorbent feed rate, and low VOC loads and RHs). Similarly and under the same condition, increasing the number of stages can improve the FB performance. Also, the application of 3 adsorbers of 2 stages instead of 1 adsorber of 6 stages can improve the ORE by 34.5%.
The effect of different empirical equations on the performance of the model shows that although the best set of formulas depends on the adsorbent-adsorbate system, a generic set of formulas can be achieved with reasonably accurate predictions for a large dataset of FB experiments. According to the CFD simulations, the minimum fluidization velocity and bed voidage at minimum fluidization velocity are 0.194 m/s and 0.476, respectively. It is also shown that Yasui-Johnson’s formula for estimating the bubble diameter yields the closest overall results to those of the CFD.
In summary, different modeling and simulation methods are used to shed light on the adsorptive and hydrodynamic behavior of FB systems. The results of this research can pave the way for optimizing the design and operation of FB adsorbers, leading to cost savings and performance improvements.
- Graduation date
- Spring 2021
- Type of Item
- Doctor of Philosophy
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