Over the past 30 years, reverse osmosis technology has made great progress, and its application fields have been continuously expanded. But in terms of its function, it is mainly used for desalination to prepare pure water.
The traditional industrial pure water preparation mainly removes inorganic salts and other components in water by distillation or ion exchange. Regeneration of ion exchange resin consumes a large amount of acid and alkali, and discharges waste acid and alkali, which pollutes the environment. Reverse osmosis desalination makes this process energy-saving and simple. Due to the small footprint of reverse osmosis equipment, high desalination rate and high water utilization rate, the application in the field of water desalination is expanding day by day. However, because some systems have high requirements on water quality, reverse osmosis alone cannot meet the requirements for desalination, so various methods for preparing pure water are often used in combination, such as reverse osmosis-ion exchange, electrodialysis-reverse penetration etc. Here, through examples, several issues that should be paid attention to in the production of industrial pure water by reverse osmosis technology are discussed.
1 Example introduction
The use of reverse osmosis technology in the preparation of pure water has become increasingly popular. Table 1 lists three examples of the combination of reverse osmosis technology and other desalination technologies to prepare industrial pure water.
(1) The pure water production line of Tianjin Sanfeng Automobile Co., Ltd. originally used the electrodialysis -ion exchange process. In order to extend the regeneration period of the ion exchange system and improve the water quality, reverse osmosis is added after electrodialysis. The process flow is shown in the figure. 1 Reverse osmosis concentrated water is not discharged back to reuse before electrodialysis. The electrodialyzer has 4 stages and 8 sections, 200 pairs of membranes, the conductivity of the product water is 35-40μs/cm, and the desalination rate is about 93%. After reverse osmosis treatment, the conductivity of the produced water is 4μs/cm. The desalinated water enters a mixed bed with a diameter of 500mm, and the final conductivity of the produced water is 0.5μs/cm. The produced water is used to prepare cathodic electrophoretic paint and supplement electrophoretic coating. Rinsing water for workpieces. After introducing the reverse osmosis system, the mixed bed will be regenerated every 25 days.
(2) Beijing Bida Pure Water Company uses its own well water to prepare bottled drinking pure water. The total hardness of the well water is about 630mg/L, and the total amount of Ca2+ and Mg2+ ions is 200mg/L, among which bicarbonate, sulfate, chloride and nitrate are the main ones, so it is not suitable for direct drinking. In order to prepare bottled drinking pure water, two-stage reverse osmosis is adopted, and the process flow is shown in Figure 2.
For the sake of preparing bottled drinking pure water, the two-stage reverse osmosis treatment process can not only effectively remove salt, but also effectively remove bacteria, viruses and some organic substances in the water. 8 modules are used, 8 modules are connected in series in the first stage, and 4 modules are connected in series in the second stage.
(3) Baoding Swan Chemical Fiber Group uses the soft water produced by the factory's soft water station. The total hardness of the water is 7.6mg/L, and the total dissolved solids are 300mg/L. The main products are bicarbonate and sulfate of low-priced metal ions. , The water production capacity of this system is relatively large, and the purity of the produced water is required to be high, so the reverse osmosis-ion exchange process is adopted, and the flow chart is shown in Figure 3.
The system removes , and then further removes the remaining ions through ion exchange. Since the raw water is demineralized water, the cost is high, so the reverse osmosis water yield is limited to 85% in the design. In order to prolong the regeneration cycle of the ion exchanger, the desalination rate can reach 96%, which is relatively harsh for the reverse osmosis system. For this reason, the reverse osmosis components adopt a 4/2/1 arrangement, thereby ensuring that water is fully recovered on the basis of high desalination rate, and the discharge is minimized.
2 questions for discussion
(1) In the preparation of pure water, in order to ensure that the inorganic salts do not scale on the membrane surface during the desalination process of the membrane module, it is necessary to maintain a high tangential flow rate of the feed liquid on the membrane surface, so it is necessary to maintain a high The input amount of feed liquid is relatively small, so the water yield of a single module is generally not higher than 15%, so in the case of large processing capacity, the reverse osmosis modules are connected in series. For example, 8 modules are connected in series in the first stage of reverse osmosis in Bida Plant, and 7 modules are connected in series in the first stage of Swan Factory.
It can be seen from Table 2 that the salt content of the last component feed liquid is nearly double that of the first component feed liquid salt, so the raw water needs to be acidified, otherwise CaSO4 will be deposited. At the same time, as the number of stages increases, the salt content of the product water also increases, so the total salt removal rate of the system will be lower than that of a single component.
With the increase of the number of strings in series, it can be seen that the water production of the modules gradually decreases. The water production of the eighth module is only 63% of that of the first module, and the water production only accounts for 10% of the system water production. This is due to the The salt content of the raw material liquid increases, on the other hand, the pressure of the raw material liquid also drops a lot, so if necessary, a booster pump can be added to the system to improve the working state of the multi-stage series intermediate parts.
(2) In the design of reverse osmosis, the temperature of the feed liquid is easily overlooked. The water production rate of the reverse osmosis membrane is strongly affected by the temperature. If the temperature changes by 1°C, the water permeability will change by about 3%. If the water yield is less than the design, the raw water should be heated if necessary.
(3) The membrane method can generally remove . It is suitable for the pre-stage of ion exchange in the deep water desalination system, which can effectively reduce the workload of the ion exchange system. If membrane desalination is used as the second stage, since the salt content in raw water is relatively low, the desalination rate is generally lower than the given desalination rate of the module. For example, the Sanfeng plant uses electrodialysis for the first stage of desalination, and reverse osmosis for the second stage. The module desalination rate is only 90%, so the second stage uses the ion exchange method instead of the membrane method as much as possible.
(4) Judging from the project cost of the three examples, the water production costs per m3 of the pure water systems of the Sanfeng, Bida and Swan plants are 185,000 yuan, 160,000 yuan, and 45,000 yuan respectively. It can be seen that the water treatment capacity is large, and the corresponding The project cost is reduced. As far as the membrane method itself is concerned, the cost of electrodialysis is relatively low due to the low cost of domestic products, while the cost of reverse osmosis membranes is relatively high because it is mainly supplied from abroad.
