Hydrodynamic cavitation for effluent treatment

: Using vortex-based cavitation devices

Abstract

Hydrodynamic cavitation is the formation, growth and collapse of vapour/gas-filled cavities. The collapsing cavities result in intense shear and formation of hydroxyl radicals thereby leading to physicochemical transformations. These physicochemical effects can be utilised for a variety of industrially relevant applications. The promise of this technological platform arises from the absence of additives, realisation of the phenomenon by fluidic devices often without moving parts and the potential to retrofit and scale-up. In this thesis, application of hydrodynamic cavitation for effluent treatment is investigated. Despite the promise, hydrodynamic cavitation based effluent treatment is limited in industrial practice. This could be due to problems arising from the use of conventional - linear flow devices, namely venturi tubes or orifice plates that are prone to erosion and clogging. Recently a vortex-based cavitation device has been disclosed, this device employs rotational flow to harness hydrodynamic cavitation. The device does not use restrictions and shields cavities from the reactor wall – thereby lowering propensity to erosion and clogging, which are problems typically encountered in conventional hydrodynamic cavitation devices.

There have been efforts to demonstrate typical applications of the vortex based hydrodynamic cavitation device, however, systematic characterisation is lacking. The lacunae for systematic characterisation were identified as, lack of i) clear guidelines to identify inception, ii) reaction engineering models to design these devices for continuous effluent treatment, and iii) understanding on the influence of scale on performance. An attempt to address the identified lacunae is made in this work as discussed here.

Chapter 1 provides a background, applications, parameters influencing hydrodynamic cavitation devices and the motivation for the thesis work. Chapter 2 discusses the use of cavitation noise for identifying inception in cavitation devices. This was achieved by the use of smartphone recording of cavitation noise and post-processing of recorded data using a MATLAB code. This can be used as an unambiguous and non-invasive approach to identify cavitating conditions. Chapter 3 reviews existing reaction engineering models and proposes a model suitable for cavitation devices for effluent treatment. A per-pass modelling methodology was developed for effluent treatment, containing simulated effluent with solvents. The developed model can be used to compare cavitation devices, provides a basis for translating batch data to design continuous cavitation based wastewater treatment and provides a way towards linking sophisticated models of cavitation.

Chapter 4 proposes a multi-layer model by coupling computational fluid dynamics (CFD) and cavity dynamics models to the per-pass modelling methodology, which was used to simulate the performance of vortex-based hydrodynamic cavitation devices and in general any hydrodynamic cavitation device. The multi-layer modelling approach provides a sound basis for further work on optimisation of cavitation reactors. Chapter 5 discusses the evaluation of performance of four different scales (flow capacities – 1.3, 5, 19.5 and 248 LPM at ΔP = 280 kPa) of vortex-based cavitation device using an aromatic pollutant. Correlations to explain obtained data based on per-pass degradation coefficient is proposed and discussion regarding the scale-up of vortex-based devices is made.

This thesis carries out a systematic characterisation of a novel vortex-based cavitation device for effluent treatment. The important aspects of the performed work are: i) it is the first to systematically propose a per-pass model for degradation of simple pollutants such as acetone/isopropyl alcohol/ethyl acetate and complex pollutants – dichloroaniline in water and ii) to investigate the influence of scale of cavitation device (device was scaled-up 200 times). The models developed and results obtained in this work will be useful for expanding applications of hydrodynamic cavitation and vortex based devices for effluent treatment.

Date of Award	Dec 2020
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	Queen's University Belfast
Supervisor	Vivek Ranade (Supervisor) & Peter Robertson (Supervisor)