How High-temperature RO Can Assist Industrial Wastewater Recycling And Energy Conservation
By By Rich Franks, PE, Joshua de la Cruz, Craig Bartels, PhD and Gerard Van Gils, PhD
The standard reverse osmosis (RO) spiral-wound element is used in numerous separation applications, ranging from reclamation of municipal wastewaters to desalination of seawater. Because these elements are constructed of plastic materials, they come with specific limitations on feedwater quality, pH, pressure, and temperature. Specifically, the standard spiral element is limited to a maximum temperature of 45°C. For the most part, this temperature limitation has restricted the direct use of RO for treating some high-temperature streams such as:
- Hot condensate
- Boiler-water blowdown
- Laundry wastewater
- Annealing baths
- High-temperature mining wastewater
- Produced water in the oil and gas industry
Exceeding the 45°C limit of the RO element may not necessarily cause immediate, catastrophic failure of the element. However, some specific components of the element undergo a physical change at high temperature that will adversely affect performance. Specifically, the polyamide chemistry of the RO membranes undergoes a permanent annealing process as temperature exceeds 45°C. The change leads to lower flow through the membrane, which results in greater feed pressures to maintain flow from the RO system.
Another plastic component of the element that is effected by high temperatures is the permeate carrier. The permeate carrier is a plastic sheet inserted between the membrane leaves on the back side, or permeate side, of the membrane. The permeate carrier sheet is comprised of parallel channels that direct the permeated water toward the central permeate core tube and out of the element. These channels are less than a millimeter in width. When temperature exceeds 45°C, the permeate carrier loses rigidity. When pressure is applied, the permeate channels will then collapse, restricting the water flow. It is because of the potential collapse of the permeate channel that temperature limitations are coupled with pressure limitations. At a maximum temperature of 45°C, the element is limited to a maximum pressure of 70 bar (1,000 pounds per square inch gauge [psig]). At lower temperature, the highest pressure can be increased such that an element can withstand 1,200 psig when the temperature is reduced to 10°C.
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