عنوان مقاله [English]
نویسندگان [English]چکیده [English]
Reverse osmosis (RO) is a type of pressurized filtration in which the filter is a semi-permeable membrane. The system is a complete technology and commonly uses as a practical desalination method. Nowadays, RO is a developing technology with53% world share in fresh water production. In previous studies parametric analysis was carried out based on mathematical or software modeling. In this study, field data was used to determine the impact of each of the parameters, considering environmental factors and operating conditions.
In this model, according to the widely used solution-diffusion model, the water flux through the membrane is given by:
Where C1 is a conversion factor, Kw the water permeability, A the membrane area, ?P the difference between the feed and permeate sides of the membrane pressure and ?? the osmotic pressure difference across the membrane.
By extending the proposed equation, mathematical modeling of reverse osmosis desalination system was developed. In this modeling, the permeate flux was assigned as the dependent variable and other parameters (such as temperature, pressure and feed concentration) as the independent variable. Finally diagrams of pressure, temperature and feed concentration in relationship with permeate flux were drawn using MATLAB 2012. The Reverse Osmosis Systems Analysis (ROSA) was supplied by Dow-Filmtec Company. This software was designed to evaluate the efficacy of RO system with RO membranes and Nanofilteration. So, after mathematical modeling, software modeling with ROSA was performed with the same conditions and assumptions. According to the modeling, results were more consistent in temperature of 15 to 30 degrees. Raising and decreasing the temperature increased the difference between the results. These differences may be due to the distance from the standard conditions and low impact of raising and decreasing the temperature in mathematical equations. Because in software modeling assigning the pressure as constant was not possible, the influence of pressure on other parameters was unknown. The empirical and field studies have shown that inlet pressure increased by increasing the rate of efficiency and leads to a linear increase of permeate flux and specific energy rate. With increasing the temperature, permeate flux had a non-linear increase. However, with constant flux, increase in temperature caused a decrease in brine. While, the lower feed water temperature resulted in the better quality of permeate. In addition, the operation of reverse osmosis systems requires large amounts of energy at low temperatures to reduce water permeability. Quality of feed water has a direct impact on the permeate quality. As the amount of dissolved solids in the feed water is higher, the amount of salt in the permeate water will increase. The arrangement of membranes was also evaluated. Water quality in series was better than parallel mode; however, more pressure was needed. Finally, as a case study, the desalination system with 20 cubic meters per hour capacity in Nina oil plan was intended. According to the desired permeate water quality and to achieve maximum efficiency, the single-stage device was designed with four parallel pressure vessels. Six membranes were placed in a pressure vessel. Life of each membrane according to feed pressure and recovery was assumed as 5 years, approximately. Also to the design, the efficiency of 65% was expected for this system. One of the main challenges for desalination system is the brine which has a high volume and concentration. Due to the analysis of the brine, it can be used for green space of factory. The pre-treatment system included different filters such as sand, carbon, 20 micron and 5 micron filters as well as UV lamp and antiscalant injection. Also, caustic soda in the post-treatment was used to adjust the pH.
The results of this study are extracted as fallowing: (1) The pressure and temperature are always inconsistent factors in maintaining water quality and quantity of permeate. Rising feed pressure increases permeate quality, but on the other hand, needs stronger pump and increases costs. Increasing temperature increases the efficiency while reduces the quality of permeate. Hence, the pressure and temperature should be optimized. (2) Feed concentrate has a direct impact on permeate quality. The important factor in reducing the salt in the feed water, and eventually increasing water quality, is using a suitable pretreatment system. (3) Designing as an important factor in the efficiency, cost and water consumption, requires engineering experience and vision. To increase the efficiency, two-stage or water-returning system can be designed. Also, the membrane arrangement affects the efficiency and quality of permeate water.