| Seawater desalination technology is the hot spot of research at present, the desalination of seawater with conventional way has not only consume great amount of the fuel, but aggravates the energy resource shortage, and emissions sulfur oxide compound, as well as the contaminant of compounds of nitrogen and oxygen, the greenhouse gases etc., therefore, in this case, using the renewable clean energy, such as solar energy etc. For desalination of seawater, will become the important direction for development of the technology of seawater desalination.In view of the existing limitations such as low solar thermal utilization, low system unit efficiency on the current main stream desalination of seawater technology(e.g. solar multi-effect evaporation, solar multi-stage flash, solar reverse osmosis, etc.). The paper has formed a kind technology for heating-vacuum membrane distillation-condensation of new type of solar vacuum membrane distillation seawater desalination system; first, the experiment research on the vacuum membrane distillation seawater desalination system has carried out by used the electric heating to provide heat energy, and verified the effectiveness of this technology system and obtained the optimal operation conditions affected the system performance; second, the main components of system's main subsystem solar energy heating system, vacuum membrane distillation system and cooling system were selected and trial-produced, and the performance of the system's major subsystem has carried on the theoretical analysis and numerical simulation by used the relevant theories of heat transfer and fluid mechanics, and the device of solar vacuum membrane distillation seawater desalination system has been designed and trial-produced; finally, in order to verify the feasibility of the technology route and the validity and reliability of device of system, an experiment of natural seawater desalination was carried out on the system device in the actual environment conditions. The conclusion achieved in the study is as follows:Firstly, to test the effectiveness of heating-vacuum membrane distillation-condensation technology route of seawater desalination system, first of all, experiment studied on the vacuum membrane distillation seawater desalination system by used the electric heating to provide heat energy, it was obtained that feed liquid temperature, vacuum, feed flow rate, feed liquid mass fraction were the main operation conditions influenced the system performance, and gained the optimal operation conditions of system as follows: the maximum permeate flux 10.894 kg/m2/h can be obtained at feed temperature was 70 ?, vacuum degree was 0.095 MPa, feed flow rate was 90 L/h, with the simulated seawater as the feed liquid with mass fraction of 35 g/kg; Second, an experiment of natural seawater desalination was carried on at the optimal operation conditions, with the combination of the quality obtained by atomic absorption and ion chromatographic determination, gained the average permeate flux of 11.171 kg/m2/h, total fresh water production of 16 kg, the conductivity of total fresh water was 0.00503 m S/cm, and the rejection rate was of 99.99%, the desalination effect was good. Thus, it can provide the research basis and technical support with the design and development of heating-vacuum membrane distillation-condensation technology route of solar vacuum membrane distillation seawater desalination system.Secondly, the device of system's main subsystem solar energy heating system, vacuum membrane distillation system and cooling system were designed and trial-produced on the basis of experiment research of vacuum membrane distillation seawater desalination system. The main conclusion of this section has the following points:(1) In the solar heating system, the theoretical research and experimental analysis were carried respectively on the solar collector and solar cooker, it was determined that the number of the vacuum tube solar collector of 12, the hot water tank dimension of solar cooker and solar radiation intensity and liquid flow in the pipe was the main factor affecting the solar collector and solar cooker temperature rise; when the liquid flow was 20 L/h, the temperature of the outlet water of the solar collector and solar cooker was highest; the temperature of the outlet water of the solar collector and solar cooker was getting higher with the increasing of solar intensity of radiation under the constant condition of the liquid flow. The temperature rising performance of solar collector coupled solar cooker was also analyzed, the result was showed that the solar heating system after coupled could better use solar energy, the outlet temperature of system was higher than a single device, at the same time, two devices coupling use could adapt to different environmental conditions.(2) In the vacuum membrane distillation system, the use of heat and mass transfer and fluid mechanics theory, the liquid flow regime in the evaporator, analysis of energy the evaporation process in the evaporator and vapor-liquid separation mechanism of the vapor evaporation process were analyzed, and the use of FLUENT software VOF model and a heat transfer model was carried on the numerical simulation for the evaporator. The result was showed that the increase of feed temperature accelerated the movement of water molecules in the evaporator, the water molecules and the liquid separated at the same time, the greater the increase in buoyancy, the greater the evaporation capacity; the increase of feed flow rate made the liquid of disturbance, led to the flow of the liquid membrane into a multi-channel flow, reduced the evaporation capacity of liquid; the increase of the vacuum degree made the surface tension of the liquid smaller, which was beneficial to the liquid film spreading, thereby increased the evaporation area, and the drag force of water vapor got smaller, the volume fraction of vapor in the evaporator was increased; shorten half the length of the evaporator, the steam volume fraction was reduced, the hot water evaporation was not fully and discharge, which led to the decrease of the evaporation capacity; the increase of evaporator angle changed the material liquid velocity on the inclined plate evaporator, led to the liquid film from film-like to multi-channel flow, and water vapor density was lighter than air, when the evaporator angle was changed, the flow of the water vapor also changed, also caused the change of the evaporation rate; and the height of the support plate was too high, caused the temperature and heat of the steam entering the film hole in the process of water evaporation loss, thus resulted in the decrease of water vapor volume fraction, and the height of the support plate was too low, the water vapor in the evaporator easy reached saturation, thus resulted in a decrease of water vapor volume fraction.(3) In the cooling system, the mathematical model of heat conduction of the spiral heat exchange tube wall was established by used the theory of heat transfer, etc., the length of the spiral heat exchange tube and the specification of the heat exchanger determined, and the heat transfer process of vapor into the spiral exchanger tube was carried on numerical modeling by used the finite element software. The result showed that when the length of the spiral heat exchange tube of 9.42 m, the heat exchanger specification of length 200 mm, width 200 mm, and height 400 mm, spiral heat exchange tube material of stainless steel, the heat transfer coefficient was larger; at the vapor temperature of 70 ?, and the flow rate of 20 L/h in the tube, the temperature of water vapor into spiral heat exchange tube dropped rapidly and the temperature of the vapor from the inner wall of the tube to the outside of the tube wall also gradually reduced, while the heat exchanger in the environment, due to the reason that such as the solar radiation absorbed heat and heated seawater in the heat exchanger, so that the water temperature to rise.Thirdly, to verify the feasibility of the system technology route and the effectiveness of the device, the experimental research was carried on solar vacuum membrane distillation seawater desalination system device which was designed and trial-produced. The main conclusion of this section has the following points:(1) Under the test mode of the electric heating and evaporator, with the simulate seawater as feed liquid, examined the influence on the performance of vacuum membrane distillation evaporator under the four conditions i.e. the feed liquid temperature, vacuum, feed liquid flow rate and feed liquid mass fraction, thus obtained the optimal operation conditions as follows: the maximum permeate flux 5.899 kg/m2/h could be obtained at feed flow rate of 20 L/h, vacuum degree of 0.095 MPa, feed temperature of 90 ?, and the seawater with mass fraction of 35 g/kg as the feed liquid for studying the desalination.(2) Under the test mode of the solar heating system and evaporator, took the analogy seawater with mass fraction of 35 g/kg as the feed liquid, feed flow rate of 20 L/h and vacuum degree of 0.095 MPa, examined the effect on the influence of evaporator system with the heating provided by the solar heating system instead of the electric-heating, the result indicated that: under the three days similar environmental: the maximum temperature of the seawater heated by the separate solar collectors was 64 ?, and that by the separate solar cooker was 76 ?, and 82 ? by the solar collector coupling solar cooker; at the same time, the maximum permeate flux was 1.004 kg/m2/h gained for the separate solar collector combined vacuum membrane distillation evaporator, and that of 1.231 kg/m2/h by the separate solar cooker combined vacuum membrane distillation evaporator; and 1.716 kg/m2/h for the solar collector coupled solar cooker combined vacuum membrane distillation evaporator.(3) Under the test mode of the solar collector coupled solar cooker and evaporator, with the natural seawater as the feed liquid, feed flow rate of 20 L/h and vacuum degree of 0.095 MPa, the solar collector coupled solar cooker was used to provide the heat energy. In order to certify the stability of the system, the two typical summer days 7:00 to 18:00 of August 22 and 29 were chose to test. the quality of the fresh water determined by combination of atomic absorption and ion chromatography determination, with the result showed that: On August 22, the fresh water of 72 kg could be produced, which with the electrical conductivity of water was between 0.00276 m S/cm and 0.06730 m S/cm, the rejection rate was between 99.67% and 99.99%; and the average salt rejection was over 90%, fresh water quality indexes can meet the national drinking water health standards(GB5749-2006) and the water quality standard of drinking water purification(CJ94-2005); On August 29, the fresh water of 69 kg could be produced, with the electrical conductivity of water was between 0.00312 m S/cm and 0.07210 m S/cm, the rejection rate was between 99.65% and 99.99%, and the average salt rejection was over 90%, fresh water quality indexes can meet the national drinking water health standards(GB5749-2006) and the water quality standard of drinking water purification(CJ94-2005); the system has a good effect for seawater desalination and could be operated stable.To sum up, the device of solar desalination system are advanced, valid and reliable which have designed and trial-produced, and with capable of service the remote area with the brackish water and the solitary island. |
PDF Full Text Request