ilec Technology Withstands Severe Treatment at Reverse Osmosis Facility Site Information Location: Freeport, Texas, USA Purpose: Evaluate ilec technology under severe handling conditions Comparative Performance: No o-ring leakage or other operational issues with ilec technology Texas Operations, a division of The Dow Chemical Company, is home to many chemical processes. FILMTEC elements with ilec technology achieved leak-free operation in a reverse osmosis facility at this site. Introduction Siemens Water Technologies owns and operates a reverse osmosis (RO) water treatment facility located in the Plant B sector of The Dow Chemical Company s Texas Operations in Freeport. The water is purchased by Dow for use as boiler feed water in its petrochemical operations, where products include polyethylene, ethylene glycol, polyurethanes, epoxy resins, caustic soda and vinyl chloride. The high pressure drop in the first stage of the first-pass RO system, together with frequent handling of elements for off-site cleaning, causes problems with the plant s element interconnectors. The pressure drop leads to excessive movement of the elements during the frequent start-up and shutdown required by flushing and off-site cleaning. The frequent reinstallation provides opportunities for o-rings to become pinched and damaged. FILMTEC elements with ilec technology were installed into one pressure vessel in the first stage of the first-pass RO system, replacing the elements with conventional interconnectors. The objective was to verify leak-free operation over an extended period by monitoring the quality of the permeate from the vessel using the ilec elements.
Benefits of ilec Technology ilec technology eliminates the need for multiple sliding seals between adjacent membrane elements, reducing the number of sealing surfaces per connection to a single, axially compressed o-ring. The result is a seal that is lubricant- and maintenance-free. Long-term permeate quality is improved through prevention of o-ring leaks that occur as o-rings gradually become worn and abraded. In contrast to standard interconnectors, the possibility that o-rings will be pinched or damaged during installation, requiring the debugging of leaks upon start-up, is also eliminated. The interlocking endcaps also remove the permeate-side flow restriction associated with the small flow diameter of conventional interconnectors, reducing energy costs. Figure 1 compares the radial o-ring seal used in the industry-standard sliding coupler with the stationary, axial seal provided by ilec technology. Figure 1. Comparison of radial o-ring seal (standard sliding couplers, left) with the stationary, axial seal provided by ilec technology. Arrows show the direction of o-ring compression. Through permanent attachment of the endcap to the element center-tube, ilec elements are particularly well-suited to rough or frequent handling. The ilec element comprises a rugged protective envelope for the FILMTEC membrane inside. Finally, elements using ilec technology are installed in minutes with a level of effort comparable to that associated with standard interconnectors. Special snap and alignment features provide tactile, audible, and visual feedback to inform the installer that a positive connection has been made between adjacent elements. Optional tools are available to make the handling of elements even easier (Figure 2). Figure 2. A lightweight strap wrench (left) and element pulling tool (right) ease loading and unloading of elements using ilec technology.
Plant Design The plant is a double-pass RO system. The permeate from a group of 11 first-pass trains is fed to seven second-pass trains. The pressure vessel configuration is identical for all 18 trains, namely 4:2. The vessels hold six elements each. Both passes contain FILMTEC BW30-365FR elements. A schematic of the facility is shown in Figure 3. The design capacity of each first-pass train is approximately 27 m 3 /h (120 gpm) at 80 percent recovery. The capacity of each second-pass train is 32 m 3 /h (140 gpm) permeate, also at 80 percent recovery. The Dow facility requires 227 m 3 /h (1000 gpm) of permeate with a maximum conductivity of 5 μs/cm and less than 20 ppb silica. The feed is surface water taken from the Brazos River. It is chlorinated, clarified, filtered, softened, and then dechlorinated prior to treatment by RO. The silt density index (SDI) of the water entering the RO system is typically between 3 and 4, with excursions above 5. The RO plant provides permeate with conductivity varying seasonally between 3 and 5 μs/cm. This range is reflective of the broad seasonal swings in feed water salinity (260 to 660 ppm total dissolved solids) and temperature (10 to 30 C, 50 to 86 F). Figure 3. Flow schematic for Siemens Plant B, Freeport, Texas. The RO system, and particularly the first pass, is prone to both biological and colloidal fouling. It is in continuous operation and only idle during flushing or element removal for off-site cleaning. A Siemens proprietary flushing sequence is conducted up to twice weekly. Additionally, the membrane elements are regularly removed for off-site cleaning, up to four times per year. The frequency of the flushing and off-site cleaning is dependent on the pressure drop. During the summer months, the rate of fouling and the frequency of off-site cleaning is significantly increased. The SDI may at times exceed 5, which results in rapid fouling of the first stage of pass 1. The frequent flushes and off-site cleanings required to deal with these excursions constitute a severe test of both seal and membrane element integrity.
Cost Resulting from O-ring Damage Siemens estimated the frequency of o-ring leaks at the plant to be as high as one per train per month, with a typical frequency of eight leaks per month for the entire first-pass system. These leaks are identified and located, first through monitoring of the vessel permeate conductivity and then by conductivity probing. They are repaired by replacing the o-rings, which requires shutting down the train, opening the affected vessel, and removing and reinstalling the elements. It takes approximately two manhours per pressure vessel to address o-ring leakage. At a fully burdened labor rate of $25 per hour and a leak frequency of eight per month, the annual cost to the plant is $4,800. The five-year net present value (NPV) of this annual expenditure is $18,200, given a discount rate of 10 percent. Dividing by the number of elements in the first pass, the o-ring related expense is $46 per element. This represents a significant increase in membrane life-cycle cost. There are additional costs in time and lost production associated with o-ring leakage. Depending on the severity of the salt passage increase, the train may have to be shut down. In severe cases, the permeate is diverted because it does not meet the permeate quality requirements for boiler feed water. Plant Performance The RO plant began operation in 1993. In March 2002, elements from a pressure vessel in the first stage of the first pass were replaced with FILMTEC BW30-365FR elements with ilec technology. The ilec elements have been operating with no o-ring leaks or other operational issues. Figure 4 shows how the first-stage permeate quality did not change in response to repeated removal and reinstallation of the elements, despite the wear and tear of handling and operation. The ilec elements ran at full performance, meeting Dow specifications without o-ring problems. Figure 4. Effect of repeated element removal and reinstallation on permeate quality.
Conclusions In the two years since installation, ilec technology has delivered on the promise of providing trouble-free operation despite high feed-side pressure drop, frequent starts and stops, and frequent handling. For More Information For more information about ilec technology, call Dow Water Solutions: North America: 1-800-447-4369 Latin America: 55-11-5188 9222 Europe: +800-3-694-6367 Asia/Pacific: +800 7776 7776 www.dowwatersolutions.com Notice: No freedom from any patent owned by Seller or others is to be inferred. Because use conditions and applicable laws may differ from one location to another and may change with time, Customer is responsible for determining whether products and the information in this document are appropriate for Customer s use and for ensuring that Customer s workplace and disposal practices are in compliance with applicable laws and other government enactments. Seller assumes no obligation or liability for the information in this document. NO WARRANTIES ARE GIVEN; ALL IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE ARE EXPRESSLY EXCLUDED. www.dowwatersolutions.com/ilec TM Trademark of The Dow Chemical Company ( Dow ) or an affiliated company of Dow 609-00468-0109