Osmosis, the simple experiment of a column of wine capped at the bottom with a membrane from a pig’s bladder and immersed into a vessel of pure water has intrigued many for 200 years. Although Priest Nollett expected that the column of wine would force flow downward through the membrane into the water, water flowed uphill through the membrane into the wine. A large pressure (osmotic pressure) applied to the wine was required to stop the flow of water into the wine.
We now know that solutes (sugar) in the wine reduced the vapor pressure of the wine. Water flowed from the higher water vapor pressure region to the reduced water vapor pressure in the wine. (Raoult’s Law). Small pores in the membrane allowed the flow of water molecules but not the flow of the larger sugar molecules.
From the calculations of Bess Ruff, (https://www.wikihow.com/Calculate-Vapor-Pressure) we get
Water sugar sugar/water at 25C
1 liter 1 liter ~1 liter
55.1 moles 3.08moles 58.18 moles
pv=23.8 mm hg vapor pressure pv = 22.54 in hg = 55.1/58.18 *23.8
The driving force is the small vapor pressure difference. To stop the flow, we can increase the pressure on the sugar water solution. The required pressure increase on the sugar solution using van’t Hoffs equation is. Osmotic pressure = π = Nb/Volume * R * T
= 3.08moles/1 liter * 8.3145 J/mol-K * 298 K = 75.3 atmospheres.
Thus 75.3 atmospheres of pressure applied to the sugar water solution will increase water vapor pressure by (23.8-22.54) 1.26 mm hg. Further increase of applied pressure on the sugar water solution will cause pure water to flow from the sugar water. This is called Reverse Osmosis.
Forward osmosis is also used to recover pure water from the sugar solution. More than 3.08 moles of a recoverable solute are added to the pure water to reduce the water vapor pressure in the pure water side. Pure water flow then occurs from the sugar water solution to the pure water side. An extra process must be used to remove the solute from the pure water. The Oasys company uses a thermalytic salt that vaporizes at a low temperature for the recoverable solute. Thus, forward osmosis process adds a recoverable solute to the pure water to draw clean water from the sugar water solution.
Water vapor pressure increases with temperature. Membrane distillation uses waste heat to increase temperature and water vapor pressure in the sugar water solution. Pure water then flows from the sugar water solution to the pure water side of the membrane. Since the membrane is usually not a good insulator, continual cooling of the pure water side is required.
An added complication for each of these water purification methods occurs if they are used for generating fresh water from sea water.. Salt molecules are not much larger than water molecules. The membrane pore size becomes much more critical than the sugar water separation example. With small pore size, the membrane is also more easily to plugged. © Larry Howlett HTMD Engineering 2020
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