How is Physics applied in the manufacturing of Newater?

Sea Water Desalination Reverse Osmosis Units

The laws of physics require approximately 1 psi driving force for each 100 mg/l of solids dissolved in the water to be treated. Seawater has about 35,000 mg/l of dissolved solids so to obtain the first drop of permeate water a pressure of 350 psi is required. Reverse osmosis units on the other hand normally treat water where the dissolved solids content is in the area of 6,000 mg/l or less, thus, the required pressure is 60 psi. To insure an adequate flow of water from the respective devices, seawater units usually operate in the area of 1,000 psi and RO units in a range of 150 to 250 psi.

Reverse Osmosis

When two water (or other solvent) volumes are separated by a semi-permeable membrane, water will flow from the volume of low solute concentration, to the volume of high solute concentration. Whenever a solute movement is blocked by the membrane it will transfer momentum to it and, therefore, generate pressure on it. Since the velocity is the same as that of a free molecule, the pressure will be the same as the pressure of an ideal gas of the same molecular concentration. Hence, the osmotic pressure p, is given by van't Hoff formula , which is identical to the pressure formula of an ideal gas:
p = cRT
where c is the molar solute concentration, R is the gas constant, and T is the absolute temperature. The osmotic pressure depends only on its molar concentration, as the formula states.

Figure-1 shows connected vessels separated by a semi-permeable membrane. If there is only water in the device, the level will be the same at both arms. When solute molecules are added to one arm, water will start to flow into it, so that its level will go up at this arm, and down at the other arm. The system will stabilize when the osmotic pressure is balanced by the hydrostatic pressure generated by the difference h in the water levels.

cRT = rh, where r is the water specific gravity.

The conservation laws of energy and momentum require that whenever particles collide with a moving wall, they will change direction and increase or decrease their speed. Thus, they transfer both momentum and energy to the wall .

The conservation law of momentum requires that solute molecules, that generate osmotic pressure on the semi-permeable membrane, must generate, via the volume of the solution, the same osmotic pressure on all the solution boundaries, including its free surface. In this respect the osmotic pressure acts according to Pascal law. Pascal's law states that for all points at the same absolute height in a connected body of an incompressible fluid at rest, the fluid pressure is the same, even if additional pressure is applied on the fluid at some place.



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