Author Identifiers

Maryamsadat Amiraftabi

https://orcid.org/0000-0003-0611-2890

Date of Award

2021

Degree Type

Thesis

Degree Name

Doctor of Philosophy

School

School of Engineering

First Advisor

Mehdi Khiadani

Second Advisor

Hussein A. Mohammed

Abstract

Throughout past decades, the management of solid waste by producing methane gas, as a renewable source of energy, has featured as an important research objective. Anaerobic digesters are widely used in countries with environmental initiatives and green approaches, where biogas produced from a bioreactor is a carbon neutral source of energy. Biogas contains 70% methane, 30% CO2 and some other gases. The by-product of an anaerobic digester is solid sludge that can be used as either fertilizer or compost.

Anaerobic digestion biogas plants can benefit industries by adding value to solid organic waste, reducing fossil fuel usage, eliminating solid waste disposal costs, in addition to generating power. Setting up an anaerobic digestion biogas plant is a green investment for industries interested in environmentally friendly biological processes. A variety of organic solid waste including municipal, industrial, livestock, poultry, meat, and food waste can be digested in an anaerobic system.

To treat the large volume of waste generated by industries and urban sewerage systems, more efficient digesters and a continuous improvement of digestion processes are required. To accomplish these objectives, crucial factors including the size, design, and shape of a bioreactor, its working temperature, pH and the hydrodynamics of a system need to be studied. A considerable amount of literature has been published regarding the hydrodynamics of anaerobic digesters. Further, several studies have explored the factors thought to influence the hydrodynamics of anaerobic digesters. These studies have identified that the hydrodynamics of a system could be influenced by the rheological characteristics of sludge, as well as mixer type and shape. Inadequate and poor mixing in a digester can cause the failure of a reactor, non-uniform distribution of mass and heat, imbalanced microbial activity, as well as formation of sediment and scum. Although studies have successfully demonstrated that close-clearance mixers (screw, helical, anchor impellers) increase biogas production, the information about hydrodynamic characteristics and flow field generated by these types of agitators is inadequate. Although hydrodynamics and the rheology of sludge have been studied in the past, more research is required to address these gaps. The application of visual and measuring instruments could facilitate further research on sludge behaviour in an agitated anaerobic digester, but this type of study is not possible due to the opaque nature of real sludge.

The main objectives of this project are (i) to find a safe, cheap, clear and stable material that can emulate digested sludge rheological characteristics in a laboratory; (ii) to study and optimize the mixing performance of a dual helical ribbon as an efficient impeller to create an ideal mixing pattern (iii) to investigate the flow pattern and hydrodynamics of a shear thinning fluid in a batch gas-liquid reactor using a combination of a computational fluid dynamics (CFD) simulation and a population balance model (PBM).

Study 1 has analysed and compared the Zeta potential, pH resistance, flow curve, viscoelasticity, and thixotropy of four popular model fluids reported previously as ideal simulant of primary, activated, and digested sludge. The results of the correlational analysis indicate that xanthan gum is the best simulant to mimic the rheological characteristics of activated sludge that is sheared less than 100 S-1. There are similarities between the viscosity and flow curve of activated sludge and xanthan gum which can be described by its internal network and molecular structure. This study also compares rheological properties of 2% NaCMC solution and digested sludge containing 3.23% solid sheared between 10-300 S-1, concluding that they behave in an essentially identical manner. The findings from this study provide several contributions towards selecting and applying a clear and safe polymer that emulates the rheological behaviour of sludge.

Study 2 has evaluated the performance of a dual helical ribbon impeller in agitating shear thinning fluid. The effects of impeller rotational speed, gas flow rate, clearance to the bottom, and viscosity on power uptake and mixing time have been studied. This study suggests that determining optimum operating conditions can minimize power consumption and time required to achieve the maximum volume of uniformity in reactor. Although the study successfully reports a significant positive correlation between the rotational speed of the impeller and the performance of mixing, there is still a threshold limit for rotational speed. Experimental data shows that power consumption would increase with rotational speed however increasing the rotational speed beyond the certain level does not affect the mixing time significantly. This study suggests two practical equations to estimate power consumption and mixing time under specific operating conditions by applying an ANOVA method.

To cover some of the limitations related to the experimental study of hydrodynamics of gasliquid systems, a combination of computational fluid dynamics (CFD) simulation and population balance model (PBM) has been used in the third study. The main purpose of this work is to evaluate the impacts of using a dual helical ribbon on the hydrodynamics of a multiphase reactor. The governing equations and turbulent model of agitated bubbly flow have been solved through a standard k-e model and Eulerian-Eulerian (E-E) multiphase approach. Following grid sensitivity analyses, findings through simulation have been verified by PIV measuring tests. Further, the PBM model has been discretized into five bubble size groups.

The results show a positive relationship between rotational speed and bubble breakage. The comparative study indicates an increase in the likelihood of bubble channeling when the rotational speed is insufficient to break the gel-like structure of the liquid. By increasing rotational speed, the bubble hits the blades, breaks, and disperses, leading to improved interfacial area between phases. Further, rotating mechanical blades induce shear stress to bulk of liquid, resulting in a significant drop in viscosity and diminishing the stagnant regions.

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