Thermodynamic Study On Low Temperature Multi-Effect Distillation In Dual-Purpose Power Plant Configuration

With the fast development of economy, the increase in population, and the overexploitation of underground water resources, fresh water shortage has become a critical constraint for sustainable development in China. Desalination is one effective option to solve problems of water shortage. As low-temperature multi-effect distillation has some technical advantages through the possibility of utilizing low temperature waste heat sources, reducing scaling and corrosion problems, low-temperature multi-effect distillation in a dual- purpose power plant configuration may become the main technology for future desalination plants.In this thesis, a mathematical simulation model is developed and a thermal analysis is conducted for a dual-purpose power plant, including low temperature multi-effect distiller, a thermal vapor compress or absorption vapor compressor and a steam turbine, under different working conditions. The cost of energy for water production and the effect of steam extraction for desalination from the turbine are accurately calculated. A low temperature multi-effect distillation experimental unit was built and its thermodynamic performance investigated, leading to an assessment of optimum design parameters and configuration. The main work can be summarized as follows:1. An analytical model of a multi-effect distiller and heat pump (thermal vapor compression and absorption vapor compression) is developed. The multi-effect distillation model includes the effects of temperature and salinity on the physical properties and thermodynamic losses, temperature losses in the demister and vapor transmission lines, brine point elevation and non-equilibrium allowances in the evaporators, and flash boxes. Mass, energy and momentum conservation equations are also applied to simulate thermal vapor compression and absorption vapor compression of the heat pump.2. The thermodynamic performance of multi-effect distillation and multi-effect of distillation combined with heat pump is analyzed. For different working conditions, the effect of heating steam temperature, the number of effects and the load of heating steam on performance of a multi-effect distiller is studied. The simple multi-effect distiller is quantitatively compared with the multi-effect distiller incorporating either thermal vapor compression or absorption vapor compression.3. The energy cost for water production and the thermodynamic performance of the steam turbine in a dual-purpose power plant is investigated. The off-design performance of the steam turbine was obtained by adopting well-established analytical methods. Based on the steam parameters, the thermodynamic performance of a condensing steam turbine and a combined heat-and-power turbine integrated with a desalination system are calculated by using the equivalent enthalpy drop theory and the cyclical function method respectively. The matrix model of the equivalent enthalpy drop theory and the cyclical function method are developed. These are readily computerized and have features of universal validity. The electrical equivalent of energy consumption as determined in this approach evaluates the energetic performance of the desalination process more validly than the GOR (Gained Output Ratio) that is widely used. By Genetic Algorithm, which is a powerful tool in solving global problems and now widely used in many applications, the parameters of water cost are optimized. These parameters include heating steam temperature, material price, the number of effects etc. The results indicate that the dual-purpose power plant can improve the efficiency of a power plant and effectively contribute to reducing water cost.4. A five-effect distillation experimental unit was designed and fabricated and the thermodynamic performance were measured and evaluated. In order to provide basic data for the design of the five-effect distillation experimental unit, a small-scale single effect experimental unit was built and the heat transfer performance was tested. The effect of flow conditions and fluctuations of horizontal falling film flow, flow density, heat flux, non-condensing gas and height of distributor on heat transfer of the outside tubes were studied. Based on the above data, the mechanical design and process operation parameters of the system were optimized, thus the best performance of the five-effect distillation experimental unit was achieved and maintained and its heat losses was minimized. The variation of temperature and pressure in each evaporator effect, and the overall behaviour of the system were tested and analyzed. The results of the analysis were found to be fairly close to the experimental observations, which justifies the expectation that the analytical model developed for the multi-effect distiller is reliable, and the overall design of the experimental system was correctly done.