RO and Vacuum

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I am wondering, does anyone know if a positive type vacuum pump has been tried with the RO process ?

One reason why I wonder about this is that vacuum, while drawing water molecules through a membrane might allow a water molecule to be less excited.

What is known about water molecules is that when heated or cooled from about 4 degrees Celsius, they expand. What I am wondering is if anyone knows the effect of 40 to 80 bars of pressure on the quantum state of a water molecule compared to it's having a vacuum with the same potential acting on it.

And if so, does temperature's effect change because of vacuum's effect ?

4 Answers

  1. very interesting topic James.

    1 Comment

    1. Thanks Jamie. The link is something you might find interesting. A guy named Tarig suggested a different thought and it is something that could be useful. https://water.tallyfox.com/discussions/electro-magnetic-desalination#comment-3452

  2. One last thought/ If the feed pump were moved to the discharge side of the RO system, a recirculating line could be used so that only the one pump would be needed. Instead of the pump discharging into the fresh water system, it would discharge into the inlet side of the RO system until the primary discharge valve is opened and the recirculating valve is closed. It's suction would be taken off of the RO system and a fresh water tank when the system is being started. And since the water would be circulating in a loop, it's operating pressure might be less than normal.

  3. Hi James, it's a very interesting thought. Unfortunately I have no input on the behavior of water molecules, nor the membrane in the conditions you describe. Gut feel is the membrane (and water) would behave the same - it's all about the pressure differential, moving the reference line up or down doesn't change that. The comment I wanted to make is the practicalities of such a method. Although applying vacuum on the permeate side of the membrane, will result in less pressure requirement in the feed - it is only a maximum of 1 bar reduction (compared to the ~60 bar feed). Therefore any effect can only be minor. Also the added complexity of plant operation, maintenace etc. Increased energy costs, due to 1) vacuum pumps are typically less efficient than multistage centrifical pumps, and 2) reduction in energy recovery. Not trying to be negative, excellent thinking - but I think the practical side of this will win out over any minor increases in system performance. But please keep us posted if you do pursue this. Hope this helps. Anthony

    1 Comment

    1. Anthony, Thank You very much for your wonderful answer. You did manage to let me know exactly what I was wondering about. Again, Thank You. What you have alluded to is that if the multistage feed pump were placed on the discharge side of a membrane, then an energy savings of about 1.5% might be possible. It took me a moment to realize this. And while 1.5% is a small number, on larger RO systems, 1.5% could be a significant cost savings. Myself, I have worked with feed pumps before and do know you are right about their being efficient. And if the work they did on the intake side increased, the discharge pressure would decrease. And the flow rate should be able to be maintained. It should only change how they perform work. Adding; if this is right, then with RO systems powered by petrol, 15 liters of every 1,000 liters might be saved. And a primer pump may be necessary to ensure air is out of the system and the membrane is properly saturated. Still, for long term cost savings, am hoping someone can look into it. Have posted this where I know one university student will see it. Maybe he will let his professor know about this.

  4. One thing I thought I would mention, one concern with vacuum is the membrane sealing itself. It may be possible that once the membrane is saturated with water, that water's ability to adhere would keep channels open for the water molecules to move through. When water is "compressed" (water is not considered compressible) or pressurized that it might become excited and expand. And when something is under or in a vacuum, normally that element or molecule expands or becomes more excited. While it may be counter-intuitive, water may conserve itself due to an absence of pressure effecting it. If so, then water molecules might be in the lowest state (it's densest form). If so, this may also increase the cohesion of water molecules which could help to increase the efficiency of the process.