Title

A novel discharge thermal energy combined desalination and power cycle utilising a vacuum spray flash evaporator

Date of Award

2016

Degree Type

Thesis

Degree Name

Doctor of Philosophy

School

School of Engineering

First Advisor

Dr Mehdi Khiadani

Second Advisor

Dr Kamel Hooman

Third Advisor

Dr Gordon Lucas

Abstract

Environmental destruction, global warming and limited potable water resources are problems often mentioned in opinion polls that ask the public to identify major problems to be solved, this century. Demand for freshwater continues to grow, but sufficient natural sources are not always available. One solution is to supply potable water from seawater using desalination technology. Most conventional desalination systems are very energy intensive and use energy supplied by fossil fuels. There is a new interest to utilise low grade energy and couple a desalinator with a thermal process such as oil, gas or utility plant to produce both freshwater and power.

The aim of this research is to design and evaluate a novel heat recovery system, which can cogenerate freshwater and power. Discharge thermal energy combined desalination (DTECD) with a power cycle utilises a waste heat stream as an energy source, which is available in some energy intensive plants such as ammonia or gas processing plant. DTECD includes two sub-systems: closed power cycle and open water cycle. At the core of the open water cycle is a vacuum spray flash evaporator (VSFE), which is based on low temperature thermal desalination system. The new VSFE system utilises a gas-liquid ejector to consume less energy and enable greater ease of operation compared to conventional vacuum desalinators, which use a vacuum pump or steam jet ejector.

The feasibility of the proposed DTECD process was evaluated by applying the ASPEN/HYSYS V8.0. Two configurations of process were modelled and the results were validated based on the waste heat of two industrial case studies: ammonia and gas processing plants. Subsequently, the performance of the proposed process was evaluated by using an exergy approach, thermodynamic modelling and a thermo-economic study. The bottlenecks of the DTECD process were improved by parametric optimisation and process modification. The results revealed that the overall exergy efficiency of the proposed heat recovery system is about 50% and this is an economic option to couple with a thermal plant.

Furthermore, the core VSFE system has been designed and manufactured for the experimental study of: (i) the performance of suggested downward gas-liquid ejector (eductor), (ii) the effects of temperature and salinity of motive fluid on overall performance of the eductor and desalination system. Numerical models of two-phase flow, based on the thermodynamic model and transfer phenomena were applied to the VSFE system. Experimental conditions were compared with a predictive model to find its accuracy. Computational fluid dynamic (CFD) method, which is embedded in ANSYS/Fluent 16.2 was undertaken to investigate the individual conversion laws inside the vessel. Based on the physics of VSFE, a discrete phase model (DPM) is needed to investigate the spray of saline water under vacuum conditions. The experimental data proved that the performance of the VSFE system is in good agreement with theoretical model and utilising a downward eductor with saline water as motive fluid is an economic option to generate the required vacuum. Also, the CFD results showed that the evaporation rate of the system follows the experimental data and the optimum geometry of the VSFE affects the overall performance. Finally, general aspects of working fluids in the proposed process and applications of wire mesh nozzle vs. spiral nozzle in desalinator are discussed.

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