376982 Multiple-Effect Membrane Distillation for Deep Concentrating Brine By Using PTFE Hollow Fiber Membrane
For the pollution and uneven distribution of fresh water resource, water shortage has become a serious problem in the world. Desalination of seawater and brackish water is attracting increasing attention and has been developed rapidly. At present, commercially available desalination technologies are multi-stage flash (MSF), multiple-effect distillation (MED) and reverse osmosis (RO). Due to technical limits and production cost, those desalination technologies have low water recovery which usually ranges from 30% to 50%, and the concentrated brine with high salinity and many harmful chemicals is also generated. The brine by-produced is usually discharged directly into the sea and even a bay, which has negative impacts on the ecological environment. The concentrated brine can also been used to produce sea salt. In the other side, the seasalt-related industry in China is facing the problem of insufficient area of saltpan. Therefore, it is important and necessary to deeply concentrate the brine drained from desalination plants.
Nowadays, membrane distillation (MD) for desalination is receiving more and more attention. Because MD process is not seriously affected by concentration polarization and feed salinity, a high-salt concentration can be reached in MD process. However, the thermal efficiency of the traditional membrane distillation is extremely low. When the performance ratio (PR) is used to evaluate the thermal efficiency of evaporation-based process, the PR of traditional MD process is usually in the range from 0.3-1 which is much lower than that of MSF and MED. In order to solve this problem, multiple-effect membrane distillation (MEMD) with the function of heat recovery is developed. In MEMD process, the heat of condensation can be recovered in AGMD module with function of internal heat recovery. So the PR of MEMD can reach up to 7 to 20.With its unique advantages, it is ideal for the desalination of seawater and further concentration of the high salinity solutions.
A problem with MEMD is the wetting of hydrophobic membrane. When the hydrophobic membrane is wetted, the hydrophobic membrane can't prevent the feed solution from entering into its dry micropores, which leads to the leakage and contamination of the permeate product. So the membrane material used in MEMD process must have good hydrophobicity. The common membrane materials for MEMD process include polypropylene (PP), polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE). Compared to PP and PVDF, the hydrophobicity, chemical resistance, thermal stability and mechanical strength of PTFE is more outstanding, thus PTFE porous membrane is an attractive membrane for MEMD process.
In this study, MEMD process by using AGMD module of PTFE hollow fiber membrane was developed to deeply concentrate the brine drained from the conventional desalination plant. The inner diameter and outer diameter of the PTFE porous hollow fiber were 0.45mm and 1.0mm, and the porosity was 60%. The performance of MEMD was evaluated by the water vapor flux (J), performance ratio (PR) and thermal efficiency (ŋ). Experiments were conducted to investigate the performance of PTFE-MEMD process by using the brine (4.8°Bé) from a MED desalination plant in Tianjin, China. The brine could be directly concentrated to a content of 12°Bé, furthermore, a modified de-calcium technology was used to remove >90% of calcium ions, since the precipitation of insoluble calcium salt on membrane surface leaded to severely decaying of MEMD performance when the salt concentration of brine was over 12°Bé. The decalcified brine could be concentrated from 12°Bé to 24°Bé. In the MEMD process to concentrate a brine of 4.8°Bé to a concentrate of 24°Bé, the value of J decreased from 6.8L/(m2•h) to 2.6 L/(m2•h), the value of ƞ decreased from 0.91 to 0.53, and the value of PR decreased from 12.8 to 4.5. When the concentration of brine was reached up to 24°Bé, the electrical conductivity of the permeate was still less than 150μs/cm. Then a long-term test on the operational stability of the MEMD process was investigated with the deep concentration of the de-calcium brine of 12°Bé for 80 days, in which the PTFE module remained in good condition. All these results demonstrated that MEMD process with PTFE hollow fiber membrane can be efficiently used to deeply concentrate the brine from desalination plants. Compared with the traditional salt making technologies with the brine from desalination plants, high-quality fresh water can be obtained and most land occupation is saved with the MEMD process. Moreover, the concentrated brine achieved in the MEMD process is highly purified, from which high purity solid sodium chloride, high purity magnesium salts and other sources could be obtained.
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