In general, the salt production in China's seaside is mainly made by the salt drying process. When salt is exposed, the seawater is first introduced into the evaporation pond. After the sun evaporates water to a certain extent, it is poured into the crystallization pond. When the sun continues, the seawater will become a saturated solution of salt, and then the salt will gradually precipitate.
However, the salt-making process using salt drying technology takes up a large area, has a long salt-making cycle, and is greatly affected by the weather. With the continuous increase of salt production, the area demand of salt fields is increasing. However, due to the limitation of the area where the salt is produced, the land resources around the salt field are tight, and it is impossible to expand the salt production.
Forward osmosis is a new membrane separation technology developed in recent years for wastewater treatment
and desalination. Forward osmosis refers to the process by which water is transferred spontaneously from a region of high water chemical potential (or lower osmotic pressure) to a region of low water chemical potential (or higher osmotic pressure) through a semi-permeable membrane. Two different osmotic solutions of water and saline are placed on both sides of the container separated by a semi-permeable membrane. When there is no external pressure, water will spontaneously diffuse from the pure water side to the saline side through the semi-permeable membrane, so that the saline side The liquid level rises until the pressure difference between the liquid level on both sides of the membrane is equal to the osmotic pressure difference on both sides of the membrane, which is the normal osmosis process. Compared with the pressure-driven membrane separation process, the forward osmosis process does not require external pressure, and is only driven by osmotic pressure. Therefore, the energy consumption during the forward osmosis operation is small, and the membrane pollution is relatively small, and it can be operated for a long time without frequent cleaning.
Ultrafiltration + nanofiltration + forward osmosis coupling process can realize the purification and concentration of brine. Through ultrafiltration + nanofiltration process, larger pollutants in brine and divalent ions in water are removed to achieve the purification of brine. This article mainly introduces the use of old brine as the draw solution, through forward osmosis concentration, to achieve the concentration of refined brine with low energy consumption, thereby reducing economic costs. This process mainly uses special separation membranes as the core, low operating cost and small footprint.
Test device diagram
As shown in the figure above, the old brine and brine respectively circulate on both sides of the forward osmosis membrane. Using the characteristic of selective permeability of the forward osmosis membrane, the osmotic pressure difference on both sides of the membrane is used as the driving force to move water from the brine side to the Halogen side, so as to achieve the purpose of brine concentration. In the experiment, the concentration process is carried out in two steps: first, the first-stage concentration of the nanofiltration produced water (brine-0) is performed using the diluted old brine (old brine-1) as the driving liquid, and the membrane flux will gradually increase during the concentration process. Decrease, when it reaches the predetermined value, stop concentrating to obtain brine-1; then use the new old brine (old brine-0) to perform secondary concentration of brine-1 to obtain old brine-1 and final product brine-2, as shown in the figure 4 shown.
Schematic diagram of two-stage concentration process of forward osmosis
In a two-month pilot test in a salt field in Shandong Province, the following conclusions were finally obtained by optimizing the test parameters:
Using the forward osmosis concentration process, using the refined brine as the feed liquid, and concentrating with 28 ° Be '(Baume degree) old brine, when the old brine: brine (volume ratio = 1: 1), the The brine is concentrated from 5.5 ° Be 'to 16.1 ° Be', and the average energy consumption per ton of water is 4.77kW.h when the average operating flux is 8.14LMH; when the old brine: brine (volume ratio = 1.5: 1), the brine is removed from 5.5 ° Be 'is concentrated to 17.7 ° Be', with an average operating flux of 9.01LMH, the energy consumption per ton of water is 4.32kW.h; when the old brine: brine (volume ratio = 1.5: 1), the brine is changed from 5.5 ° When Be 'is concentrated to 16.4 ° Be' and the average operating flux is 10.46LMH, the energy consumption per ton of water is 3.72kW.h. The electrodialysis condenses the brine to 19%, which translates to about 200 kW.h of energy per ton of salt. Therefore, the concentration of energy is about 1/4 of that of electrodialysis using forward osmosis technology.
Taken as a whole, the use of forward osmosis technology for concentration has lower energy consumption. Moreover, no additional driving force is needed. The forward osmosis membrane has good pollution resistance and long service life, so the operating cost is low.
However, the application of forward osmosis technology to industrialization is still constrained by many factors, the key of which is the choice of forward osmosis membrane and extraction solution. In this project, the retention rate of the forward osmosis membrane is relatively low, and the ion diffusion phenomenon is serious, which results in the degradation of the feed water quality. The calcium, magnesium, and sulfate ions need to be further removed before they can be used further. Therefore, the development of a forward osmosis membrane with high rejection and large flux is the key to the industrialization of forward osmosis technology. Related reading: Application of MBR integrated equipment in rural sewage treatment
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