Reverse Osmosis equipment (RO) in power generation facilities is primarily used in the boiler water pre-treatment area. Power boilers tend to operate at extremely high pressures (>700 psig), so boiler feed water must be extremely pure. Therefore, Reverse Osmosis equipment systems for boiler pre-treatment are almost always followed by some type of demineralization polishing equipment, designed to reduce feed water dissolved solids, especially silica, to trace levels.
The Reverse Osmosis design performance (permeate water quality and quantity) will dictate how the polishing demineralization equipment will be sized and any problems with the Reverse Osmosis equipment operation can have drastic effects on demineralizer performance. Poor demineralizer performance in turn can cause operating costs to increase sharply, through increased regenerations and acid and caustic usage.
INVERSE OSMOSIS
Further downstream, the impact of Reverse Osmosis design on the power generation boilers can be severe, ultimately leading to plant de-rating, and even boiler and turbine damage.
Understanding Reverse Osmosis Design Fundamentals In order to understand how Reverse Osmosis equipment works, one must look into the physics of osmotic pressure and semipermeable membranes.
A semipermeable membrane allows the passage of specific molecules through it. If a concentrated aqueous solution exists on one side of a semipermeable membrane, pure water molecules tend to spontaneously diffuse from the more dilute side of the membrane to the more concentrated side. This is called Osmosis.
As water molecules continue to flow across the membrane, the amount of water increases on the concentrated side of the membrane, as does its pressure, called the head pressure. Once this head pressure increases to a given level such that further water flow can no longer occur across the membrane, the system is said to be in equilibrium. The pressure at this point is called the Osmotic Pressure.
Osmotic pressure is proportional to the dissolved solids concentration in the more concentrated solution.
According to the Van't Hoff equation for the calculation of osmotic pressure:
(symbol P)...
PV = nRT = (g/m)RT or
P = (g/m)RT/V, where
R = universal gas constant, 0.0821 Litreoatm/(moloK)
T = absolute temperature, K (degrees Kelvin)
g = solute weight, grams
V = volume of solution, Litres
m = molecular weight of solute, if non-ionic
n = moles
P = osmotic pressure, atmospheres
Using this equation, and applying it to an aqueous solution of 1,000 mg/L. of dissolved ionic solids, as CaCO3, we arrive at an osmotic pressure of 7.2 psi [50 kPa] at 77° F. In general terms, the osmotic pressure averages about 1 psi [6.9 kPa] for every 100 mg/L. of dissolved solids.
By applying a pressure on the concentrated side of this membrane, we can cause this process to reverse. Pure water molecules (and dissolved gas molecules) can be forced to flow from the concentrated side to the dilute side.
This is the entire Reverse Osmosis design or "RO" design process in a nutshell. Water purification occurs when water molecules are forced to flow from a concentrated solution through a semipermeable membrane to the dilute side in the Reverse Osmosis equipment.
To overcome the osmotic pressure, and force water molecules to reverse flow, one must apply a pressure. The Net Driving Pressure needed is defined as:
NDP = Feed Pressure + Permeate O. P. (usually negligible) - Permeate Pressure - Feed O. P.
O. P. = Osmotic Pressure
The flow through a Reverse Osmosis equipment membrane is proportional to the NDP.
In order to obtain reasonable permeate flow rates, and to minimize membrane fouling, the applied feed pressure must be very much greater than the calculated P. It is generally in the range of 200 - 450 psi [1.4 - 3.2 MPa]. This high pressure requires specific Reverse Osmosis design considerations.
Reverse Osmosis Design For the Power Generation Industry - Designing Reverse Osmosis Equipment INVERSE OSMOSIS