Reverse osmosis unit designs

Assessment of membrane damage was based on performance testing before and after chemical exposure. Testing was conducted in a small flat plate reverse osmosis unit designed to accommodate membrane discs of 45 mm diameter. Feed solution reservoir temperature was maintained at 25 1°C and the brine was continuously recirculated through a filter at the rate of 600 mL/min. Concentration polarization is considered negligible in this cell under these conditions. 

Water supplied to industry has to meet stringent specifications. For example, process water for the chemical and biotechnology industries is routinely purified beyond potable water standards. Boiler feed water for steam generation must contain a minimum of silica. Reverse osmosis units designed specifically for these purposes are in widespread use today. For example, reverse osmosis/distillation hybrid systems have been designed to separate organic liquids. For semiconductor manufacture, reverse osmosis is combined with ultrafiltration, ion exchange, and activated carbon adsorption to produce the extremely clean water required. 

A phenomenon that is particularly important in the design of reverse osmosis units is that of concentration polarization. This occurs on the feed-side (concentrated side) of the reverse osmosis membrane. Because the solute cannot permeate through the membrane, the concentration of the solute in the liquid adjacent to the surface of the membrane is greater than that in the bulk of the fluid. This difference causes mass transfer of solute by diffusion from the membrane surface back to the bulk liquid. The rate of diffusion back into the bulk fluid depends on the mass transfer coefficient for the boundary layer on feed-side. Concentration polarization is the ratio of the solute concentration at the membrane surface to the solute concentration in the bulk stream. Concentration polarization causes the flux of solvent to decrease since the osmotic pressure increases as the boundary layer concentration increases and the overall driving force (AP - An) decreases. 

It should be noted that the electric load of a desalination unit is a deferrable one (/. < . it does not have to met by the overall power system at a specific time), which is a significant advantage in the design of the autonomous power system, since reverse osmosis units can produce potable water during periods when the other electric loads are generally low and without having to oversize the whole power system. 

It is required to design a reverse osmosis unit to process 2500 mVh of seawater at 25°C containing 3.5 wt% dissolved salts, and produce purified water with 0.05 wt% dissolved salts. The pressure will be maintained at 135 atm on the residue side and 3.5 atm on the permeate side, and the temperature on both sides at 25°C. The dissolved salts may be assumed to be NaCl. With the proposed membrane, the salt permeance is 8.0 x 10 m/h and the water permeance is 0.085 kg/rn-.h.atrn. The density of the feed seawater is 1020 kg/m ( of the permeate, 997.5 kg/nv and of the residue (with an estimated salt content of 5 wt%), 1035 kg/rnc Assuming a perfect mixing model and neglecting the mass transfer resistances, determine the required membrane area and calculate the product flow rates and compositions. 

In Example 18.6, a reverse osmosis unit was designed to desalinate seawater based on provided data. It is required to rate an existing reverse osmosis unit, to determine its product rates and compositions. Data such as permeances, pressures, solution properties given in Example... 

The product water from a reverse osmosis unit will have a low pH and most probably a high concentration of carbon dioxide. The carbon dioxide can be removed and the pH of the product increased by use of a decarbonator. A de-carbonator is a packed column in which product water is introduced at the top while either forced or induced air is introduced at the bottom. The air and water flow countercurrently over and around the column packing. The carbon dioxide is stripped from the water and exits from the decarbonator at the top in the air stream. In a well-designed decarbonator, the carbon dioxide content can be reduced to about 5 mg/C in the water effluent. 

According to the foreseen design for the industrial plant of reverse osmosis, this would be formed by three blocks of membranes, each block containing two filtering stages the hrst working at a pressure of 15 bar and the second at about 23 bar. Each stage comprises several modules, each of which is formed by six membranes in series. The first unit shows a permeate recovery of 50%, which continues to increase until a value of 80% in the sixth unit. 

Membrane-based pretreatment, before reverse osmosis (RO), employing poly electrolytes, is used on wastewater, brackish water, and sea water plants [109-114]. It includes ultrafiltration or nanofiltration membrane units with feed pressure from 1-1.5 to 2.5-4 MPa. RO units need much higher feed pressure, 6-8 up to 10 MPa. It makes the RO technology economically expensive. Decreasing the high feed pressure in RO plants is the main direction for designers efforts. 

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