Sanitation of an RO system using hot water commonly involves incorporating a heat exchanger(s) into the traditional clean-in-place (CIP) system to gradually, at a controlled rate, heat and cool water circulating through the membrane system. Membrane manufacturers commonly stipulate a controlled heating and cooling rate (5 Degrees C per minute) to protect against irreversible damage to the membrane and ensure the system's long-term performance.
It is recommended that plants take this six-step approach to hot-water sanitation:
1. Perform ambient-temperature, low-pH chemical cleaning for scale removal or for other foulant as necessary.
2. Heat to 80 Degrees C at a rate not to exceed 5 Degrees C per minute (approximately half an hour).
3. Maintain feed pressure to RO system below 40 pounds per square inch gauge (psig).
4. Maintain cross flow so the pressure drop is less than 2 pounds per square inch drop (psid) per membrane.
5. Circulate water through the equipment at 80 Degrees C (half an hour).
6. Cool to 25 Degrees C at a rate not to exceed 5 Degrees C per minute (approximately half an hour).
Component design and system requirements
When designing a hot-water sanitation RO system, the designer must consider every water contact component to ensure temperature compatibility. Major components include membranes, membrane housings, pumps, pipe/fittings/ gaskets, valves, instruments and cleaning (CIP) tank.
Of these components, membranes are the most complex and costly. RO membranes used in hot-water systems are similar to ambient-temperature membranes in that they are cast in a polyamide material and constructed in a thin-film composite configuration. However, to withstand the elevated operating temperatures, they are manufactured with special adhesives, permeate tubes and connectors.
Hot-water sanitation membranes
Like ambient-temperature membranes, hot-water membranes exhibit different operating characteristics at higher temperatures. For example, flux rates increase while rejection rates decrease (See Fig. 1 and Fig. 2). Therefore, when considering continuous operation at elevated temperatures, plants also should consider dissolved solids loading to post-RO treatment ion exchange equipment.
Figure 1. Membrane Flux vs. Temperature
Figure 2. Membrane Rejection vs. Temperature
The original operating parameters of the membrane are essentially recoverable after the initial hot-water sanitation at 55 Degrees C or higher. However, after the first sanitation, membrane flux at ambient temperature is permanently reduced by 50 percent. Subsequent hot-water sanitations have no further appreciable effect on ambient temperature flux rates. To ensure minimum flow requirements are satisfied in the long term, hot-water systems are designed with significantly more membrane area than ambient-temperature systems with equivalent design flow rates.
System design options
Several hot-water sanitation membrane system designs currently are available. Among the key and minimal design criteria for a dependable system are:
Appropriate materials of construction.
Automated programmable logic controller/proportional integral derivative (PLC/PID) loop controlled heating and cooling processes.
Automated valving to minimize operator interface.
Fig. 3, Fig. 4 and Fig. 5 provide schematics for three basic system designs. In each design, a method to closely regulate feed pressure to the membranes during hot-water sanitation is employed by either a pressure-regulating valve or a variable-frequency drive for direct pump control. Fig. 4 and Fig. 5 provide designs with independent heating and cooling heat exchangers. Fig. 5 uses a common and dual-rated heat exchanger for both heating and cooling processes. In each case, the "hot" heat exchanger also serves to preheat RO feed water during normal operation.
It is assumed Fig. 3 and Fig. 4 use either sulfite chemical injection or ultraviolet (UV) technology for dechlorination. When using activated carbon (AC), as shown in Fig. 5, both the RO and AC could be hot-water sanitized during the same process.
Figure 3. Two-Heat-Exchanger System
with CIP/sanitation Tank, On-line/Off-line HXs
Figure 4. Two-Heat-Exchanger System with No Tank
Figure 5. One-Heat-Exchanger System
with Soft-water Tank, Feed-Water Cooling
Today, membrane technology has advanced to the point where it can ensure reliable, predictable performance for use in high-temperature processing applications. In addition, the systems themselves can be automated to perform dependably and safely. CP
Wise is the application engineering manager for engineered equipment at Osmonics Inc., Minnetonka, Minn. He can be reached at (952) 988-2277, or via e-mail at firstname.lastname@example.org.