About R.O. w/membrane desalination

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I was wondering if someone can answer a question for me.

If the high pressure pump is doing 42 % of the work and 40% of the sea water is being desalinated, has anyone tried placing the high pressure pump on the fresh water side and have it draw water through the membrane ?

The only reason I am asking this is because water has as much force pushing against a pipe's or membrane's walls (inside of it's pores) as it does moving forward. And if so, then drawing water through the membrane might require less energy than pumping it through.

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7 Answers

  1. One thing about water is that if it has 40 bars of pressure applied to it, it is also applying 40 bars of pressure to the sides of the pores. Yet under such pressure, water is slowed. Just look at how cold water is at the bottom of the sea. Placing water under vacuum could have the same effect but with a much lower expenditure of energy. I think Newton would say something like for every action their is an equal but opposite reaction. Force is one way, lack of force is the opposite way. And to create 1 bar of negative pressure by restricting the intake would seem to take less energy. And then to create 1/2 the force of what a system is using would take 1/2 of the amperage or Kwh of energy. Equal but opposing minus adhesion. Who knows, if not tried with the consideration that the force water exerts outward or on the sides of pores, pipes, etc. is not accounted for...

  2. Vacuum Driven RO

    Hi James:

    Vacuum is limited by nature to one bar only, while in seawater desalination we need to operate at 60 to 70 bars. 

    So running SWRO system under vacuum vs. under pressure will be impossible.

     

  3. How to facilitate the penetration of fresh water through the membranes

    Dear James,

    I agree with Derek and Gerrit, they are absolutely correct.

    But if you are interested in a way to increase the efficiency of desalination process you should apply hydro-dynamic salted water pre-treatment. This is treatment that alters the structure of water by disrupting the water clusters to smaller ones. Thus, pure water more easily permeates through the pores of the membranes (please look at the attachment here).

    This is water pre-treatment that would completely change desalination technologies.

  4. Yes, it's possible!!

    Yes, drawing water through the membrane require less energy than pumping it through with near 100 % of salt rejection.

    One research study shows the hybrid membrane achieves high water flux of about 10 kg/m2·hr at a feed temperature of 60-70°C and a vacuum of about 5 Torr with salt rejection up to 99.9%.

  5. It won't work

    I believe someone has already answered the question. There are suction limitations to a pump. Theoretical suction lift is 32 feet negative and practical 20 feet negative. The negative is to show creation of vacuum and therefore to attain desalination there has to be positive pressure. This is the reason why it won't work. 

    Desalination requires high pressure and therefore it is always recommended to use reciprocating pumps since they are highly efficient. The efficiency of positive displacement / reciprocating pumps tends towards 90%, in some cases even higher. So if you are operating in such high efficiency areas how much more would you save by pulling the water through. The level of energy used in suction or in discharge will not change much, probably a difference of 2-3% age points on the efficiency curve.

  6. Dear James, As Derek already wrote, you need a high pressure to press the water through the membrane, around 60 bar. Suction gives a maximum pressure of -1 bar.

  7. desalination membrane operation

    Hi James,

    There are a few issues here. Remember that you have a waste stream that goes from the concentrate side of the membrane to drain and without primary pumps pushing the water you have no cross flow velocity to keep the membrane functional with that feed water flowing to drain under reasonable velocity.

     As well the amount of vacuum required to pull water through a membrane is so high it would vaporize the water on the suction side of the pump causing aviation and wiping out the pump. The trans membrane pressure to desalinate is substantial I don't know of a pump that will pull that much vacuum at the flow rates required.

    There will be issues relating to membrane mechanical stability with high vacuum on the low pressure side of the membrane - they aren't made that way. 

    Hope that helps.

    Derek