Boron removal from geothermal water by using air gap / vacuum membrane distillation processes

Energy is increasingly becoming the most important need in the world. Today, the reduction of these resources increases the importance of alternative resources. Geothermal energy has an important place in alternatives according to fossil fuels. The environmental problems encountered in geothermal energy are usually contamination of surface and ground waters with toxic heavy metals. New methods are needed to remove and reduce the harms of pollutants, such as boron in geothermal water content, that have serious environmental impacts to acceptable levels. In this thesis, it is aimed to utilize the heat of waste heat discharged from geothermal power plants and the hot geothermal waters discharged and to reduce the levels of the pipes by the help of membrane distillation processes. Although membranes used in membrane distillation are considered to play only the role of liquid-gas balance and not involved in mass transfer, recent experiments and theoretical investigations show that some membrane materials affect membrane performance. Membrane distillation performance is based not only on the vapor / liquid balance of the respective components in the feed solution, but also on the thermodynamic effects (the relationship between these components and the membrane materials) and the kinetic effects (the membrane diffusion of these components). For membrane distillation, generally porous and hydrophobic membranes made of different materials (polytetrafluoroethylene, PTFE, polypropylene, PP, polyvinylidene fluoride, PVDF) are used. In this thesis, performance results are compared using air gap membrane distillation process and vacuum membrane distillation process. Electrospinning is a process in which fibers under varying diameters from microns to nanometers are produced under high electric fields. The electrospinning layer has a very high specific surface area, high strength / weight ratio, high porosity. Together with these, the properties of the nanofibers can be improved by changing the layer thickness, fiber diameter and porosity and electroporation parameters, material selection and final healing methods. In this thesis, performance results of membrane distillation system were investigated using membranes produced by using 15% PVDF with electrospinning method using 0.45 - 0.22 PVDF and 0.22-0.45 PP commercial membranes. For the production of hydrophobic membranes by electrospinning, experiments were carried out with the polymer feed rate being fixed at 12 ml / h, the tip collector distance being 15 cm and the voltage being 24 Kv. DMAC and acetone solvents were added in one ratio. These electrospinning productions are limited to 2 hours. xxii In the HBMD configuration, the cooling chamber is separated by a membrane plate to form an air gap. In this configuration, vapor molecules passing through the membrane pores are forced to pass through the air space and then concentrate on the plate. The concentrating part is taken from the air space by gravity effect. In the cooling reservoir, cooling water is used to dissipate the latent heat generated during condensation of the vapor in the liquid. In the VMD, the pressure of the feed solution in direct contact with the membrane surface is maintained at a lower level than the pressure minimum input pressure (LEP), and on the support layer side of the membrane, the permeate pressure is usually adjusted by a vacuum pump under the equilibrium vapor pressure. In this thesis, The system is optimized with a commercial PVDF membrane with a pore size of 0.45 μm before proceeding to salt water and real geothermal water experiments. Experiments carried out in this context; Effect of feeding-filtrate water temperature difference, Different vacuum pressures (0.1, 0.08, 0.06, 0.04, 0.02 bar) and temperature difference experiments.In the VMD system, the feed water temperature was kept constant and the coolant temperatures were tried as 20-30-40-50 0C (ΔT = 50-40-30-20 0C). At this time all other parameters of the system are fixed. The experiment was carried out at a vacuum pressure of 0.1 bar. In this experiment, a mass of 0.1% saline solution was used. As the cooling water temperature increases, in other words, when the tempera ture difference between the two flows decreases, the flow decreases. As the temperature difference increased, the feed water temperature was kept constant at 70 ° C as the flow increased, while in the following experiments it was decided that the temperature of the cooling water was 20 ° C. In this thesis, after the optimization studies it was decided that the feed water temperature was 70 degrees Celsius and the cooling water was 20 degrees Celsius. 0.6-0.4-0.2 pressures for these experiments were tested in a vacuum membrane distillation system. Commercial MD membranes were compared with air-gap membrane distillation (HBMD) and vacuum membrane distillation (VMD) methods at the beginning of the experiments. The primary purpose of the thesis is; in particular, to examine the effect of the operating mode on the total performance of the MD process (flux and hold / removal) and to form the experimental matrix according to the obtained results. The secondary purpose of the thesis is to produce nanofiber membranes which can be alternative to porous commercial membranes produced by classical phase transformation method by electrospinning method and to be tried in MD process. In the vacuum membrane distillation which gives the best performance result, the membranes produced by the electrospinning method are investigated and their performances are evaluated in synthetic brine and real geothermal waters. In this thesis, the operating performances of air gap membrane distillation process (HBMD) and vacuum membrane distillation process (VMD) are compared. Synthetic brine and real geothermal water were used for performance comparison. Pressure-flow, temperature difference-flux, pressure-salt retention and pressure-boron removal results were investigated in terms of membrane types and mode of operation in the context of synthetic brine and actual geothermal water studies. In the scope of the thesis, 7 mm air gap was studied and it is aimed to compare this process with vacuum membrane process. xxiii When the boron removal from actual geothermal waters operated under this thesis is examined, it is seen that the permeate water boron amounts in all membranes and operating systems do not exceed the 0.5 mg / l limit and there is no problem with discharge standards.