the purest water? 5. Salt Water California does not have enough fresh water to s
ID: 104595 • Letter: T
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the purest water? 5. Salt Water California does not have enough fresh water to supply all the water we could use for agriculture, industry, home use, and recreation. Where could we get more? The obvious source is the ocean, but seawater contains 3.5% salt by weight. In the lab, we separated water from salt water by evaporating it. Why is this method not practical on a large scale? What other methods could be used to remove salt from water? Your assignment: look up desalinization. What processes are used for large-scale purification of seawater (such the new plant in San Diego)? How do they separate salt from water? What's the chemistry?Explanation / Answer
On a large scale evaporation is not possible as the amount of water to be evaporated is too large. Typical oceans are very deep.
Other methods used for purification of sea water are:
1. Vacuum distillation
In vacuum distillation, the mixture is boiled at low pressure to leave impurities behind. By reducing the pressure, the required boiling temperature is lowered.
2. Multi-stage flash distillation
Water is evaporated and separated from sea water through multi-stage flash distillation. Each subsequent flash process utilizes energy released from the condensation of the water vapor from the previous step and so on.
3. Multiple-effect distillation
Multiple-effect distillation: Incoming water is sprayed onto vertically or horizontally oriented pipes which are then heated to generate steam. The steam is then used to heat the next batch of incoming sea water.
4. Vapor-compression distillation
Vapor-compression evaporation involves using a mechanical compressor to compress the vapor present above the liquid. The compressed vapor is then used to provide the heat needed for the evaporation of the rest of the sea water. This method is adequate and cost effective at a small scale and only requires power.
5. Reverse osmosis (RO)
The method uses membranes to desalt saline water. The RO membrane processes use semipermeable membranes and applied pressure (on the membrane feed side) to preferentially induce water permeation through the membrane while rejecting salts. This method utilizes less energy, but frequent replacement of membranes.
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