The global market for sodium hypochlorite is nearly 2 billion metric tons (4.4 trillion lbs) on a 100% chlorine basis, growing at about 2% annually. North America alone consumes more than 15 trillion pounds (6.8 million MT) of sodium hypochlorite (bleach) on a 10% chlorine basis. About two thirds of that sum is used in the form of laundry detergent. The remainder goes into disinfectant applications – most notably water and waste water treatment. Bleach is a rapidly growing alternative to chlorine in water treatment and has increasingly become an essential component in large scale wastewater treatment and drinking water purification the world-over.
Chlorine and bleach have long competed in water purification, paper bleaching and other applications. Legislation for the safe handling of chlorine is strongly influencing users to make the switch to sodium hypochlorite. In the US, the practice of shipping chlorine by rail has now come under serious scrutiny by the Department of Homeland Security. Bleach, in contrast, is a common liquid that poses considerably less concern. In fact, the Clorox Company announced recently that it would stop making bleach from shipped chlorine. Instead the company will purchase high-strength bleach (15%) and dilute it down to household strength (6%). Similarly, responding to public concern over the shipping of chlorine, Olin has now developed a fleet of 300 rail cars specifically designed to carry concentrated bleach as a less hazardous alternative.
The movement away from bulk chlorine rail shipments is causing an upsurge in the regional production distribution and storage of concentrated sodium hypochlorite. The Clorox announcement alone will affect operations at seven of its nine facilities in the US. Clorox is the leading global sodium hypochlorite bleach provider to the household laundry and bleaching sector with 27 bleach plants in nineteen countries.
Derek Stone, Sales Manager with ZCL Composites outlines a similar opportunity with K2 Pure, “With increasing regulations and safety concerns, the transportation of chlorine in any form has become a difficult and dangerous proposition. K2 Pure has approached this hurdle by creating multiple “Brine to Bleach” Sodium Hypochlorite production facilities throughout North America, thus eliminating the need to transport bulk chlorine. ZCL Composites recognized this issue as an opportunity to provide various bulk storage tanks for their process either in high temperature chemical processing using Dual Laminate Tanks or in ambient storage of standard industrial 12% Sodium Hypochlorite solution FRP Tanks using premium Ashland Derakane Epoxy Vinyl Ester resins. For K2 Pure’s California plants we provided four 50,000 USG Sodium Hypochlorite tanks manufactured in our ASME RTP-1 Accredited Xerxes Anaheim, CA facility. These were all designed and built using Derakane 411 Epoxy Vinyl Ester Resin with BPO/DMA throughout and a 20 mil Nexus Corrosion Barrier. Since FRP storage tanks were already working perfectly at several other locations there was no reason to look beyond FRP for their material choice.”
These production and distribution sites will require hundreds of sodium hypochlorite storage tanks. Few materials of tank construction can withstand the highly aggressive nature of sodium hypochlorite. Improper selection of tank storage construction materials can result in catastrophic failures. As a general rule no metals (with the exception of titanium) should ever be allowed to come into contact with this chemical.
Titanium storage tanks are probably the best choice for storage of sodium hypochlorite. Service life is estimated at 30 years or more. However, the cost of titanium is prohibitive unless there is a requirement for virtually unlimited maintenance-free service. Normally titanium tanks are only used for pressure reactors or small process tanks and even then only if time for repairs cannot be tolerated.
Rubber-lined steel tanks have been used successfully for bleach storage using chlorobutyl linings – typically ¼ inch (6.35 mm) in thickness. These linings require heat curing and a skilled applicator. Unfortunately, depending on the type of rubber and the skill of the applicator the service life is normally only 3-6 years at which time the liner may require total replacement. Furthermore, if liner failure is not recognized in time, the steel tank will be chemically attacked by the sodium hypochlorite resulting in iron contamination of the product and structural damage to the tank. For these reasons, rubber-line storage tanks are not common in bleach service.
Polyethylene tanks can be of linear or crosslinked polyethylene construction. Usually the tanks are vertical cylindrical construction with flat bottom and domed top. They are very competitively priced. However, these tanks typically have a service life of only 4-7 years when placed outdoors and exposed to direct sunlight. If the tanks are painted frequently or located indoors, service life may be extended to 6-9 years. Polyethylene tanks should always be used in a location (inside or outside) where they can easily be replaced upon failure. Mike Covey, VP Business Development for Plas-Tanks Industries, a high quality ASME RTP-1 supplier of FRP storage tanks and process vessels to the wastewater treatment industry since 1976 summed it up as follows, “We’ve replaced dozens of polyethylene tanks that failed prematurely in hypo service – some in as little as five years. Our FRP tanks, on the other hand last more than twice as long in the very same service.”
The Buffalo Sewer Authority’s (BSA) Bird Island Wastewater Treatment plant was one of those facilities. Tom Caulfield, Administrator of Capital Improvements and Development relates his experience, “We replaced three 5000 USG HDPE tanks about 2 years ago with FRP tanks from Plas-Tanks. The polyethylene tanks showed signs of deterioration after only 8 years service with 15% sodium hypochlorite. They were leaking badly when we took them out of service. Various metal structures in the storage area were corroding rapidly and the air was becoming inhospitable. The tanks had become so brittle that we were able to easily cut them up with a Sawzall. The new FRP tanks from Plas-Tanks made with HETRON FR992 Epoxy Vinyl Ester resin from Ashland continue to look excellent and are serving us well.”
OxyChem reports in their Sodium Hypochlorite Handbook that although some sodium hypochlorite users have had success with polyethylene tanks, many suppliers will not certify their equipment for bleach storage. None of the major US sodium hypochlorite producers or users (OxyChem, K2 Pure, Olin, Clorox and Odyssey Mfg) utilizes polyethylene to store bulk sodium hypochlorite in their facilities. All of these producers, however, have a large number of FRP storage tanks.
It is interesting to note that in Europe following a number of environmental stress cracking induced failures, the most widely used German tank design standard – Merkblatt DVS 2205 – omitted sodium hypochlorite as a suitable application for tanks in polypropylene and HDPE. Moreover, the relatively new and less stringent European Standard BS EN 12573 – 2000 also omits sodium hypochlorite from its listings for polyethylene tanks
One major source of failure is the mechanical PVC fittings on the tank. Both bulkhead and bolted fittings are flat which adds stress to the curved sidewall of the tank leading to embrittlement, stress cracking and failure. Premature tank failure typically occurs on the tank wall near the fittings, especially the larger ones. Over time the fittings also crack and the tank leaks. Corrosion-resistant Titanium bulkhead fittings with titanium bolts should be used below the liquid level with Viton full faced gaskets between the backing flange and the inside tank wall. These outlet fittings add considerable cost to the polyethylene storage tank which tends to negate their “cost advantage” over more durable materials of construction.
There is a perception in the market that FRP storage tanks are more expensive than polyethylene. While initial procurement costs may be less for smaller polyethylene tanks in mild service with standard fittings, this assumption does not hold true for more aggressive service, moderate volumes, user specified customization, and the ability to maintain equipment for longer periods of time without failures. Diamond Fiberglass specializes in the wastewater treatment market with a variety of products including fiberglass sodium hypochlorite storage tanks. Few companies in this market know this application better. Paul Cohen, Vice President of Diamond put it this way, “FRP can be economically more attractive both at the time of purchase and also in the long run. FRP storage tanks provide greater design flexibility, larger storage capacities and considerably lower maintenance costs when compared to polyethylene. These benefits, in addition to the competitive pricing of FRP tanks, result in the highest long term value for our customers, particularly for an aggressive chemical service such as sodium hypochlorite.“
FRP storage tanks for sodium hypochlorite are common and when designed properly can be one of the best choices for product storage. The success or failure of this type of tank in bleach service depends upon a large number of variables including resin type, reinforcement type, fabrication technique, storage temperature and the characteristics of the solution. A well-designed and properly constructed FRP tank can last for 20 years or more with only routine inspection and minor repair of the corrosion barrier as required. While FRP tanks are the industry standard for bleach storage, it is still advisable to deal exclusively with fabricators having experience with sodium hypochlorite. Paul Cohen of Diamond Fiberglass goes on to say, “Qualifying fabricators is critical for purchasers of sodium hypochlorite tanks. Buyer’s should visit multiple fabrication shops, review their quality assurance programs, understand the procedures for properly fabricating bleach tanks (which are different from most tanks) and then review a list of reference projects that demonstrate the fabricator’s capabilities. Competent fabricators will have no objection to this level of qualification.”
Odyssey Manufacturing reports that FRP tanker trucks are very successful for hauling sodium hypochlorite when the entire container is made of FRP with the correct construction methods. The industry trend in Canada and the United States has been the replacement of steel lined tankers with FRP tankers over the past ten years due to the long life of the FRP tanker. The FRP trailer has over 30 years of use and it has been proven to be the best choice for sodium hypochlorite if constructed correctly.
As confirmed by a 2004 Dow Chemical company industry survey, FRP based on premium epoxy vinyl ester resin is the most common material of construction for bulk sodium hypochlorite storage and transport, backed-up with case histories of more than 20 years service life. Well-specified and properly constructed FRP tanks were found to last 20-30 years or more with only routine corrosion barrier inspections (typically every two years) with minor repairs as required. Ashland Performance Materials has conducted exhaustive research over the past several decades to determine the best FRP solutions for Sodium Hypochlorite Service. This research culminates with the most recent paper given at the WEFTEC 2010 Conference in New Orleans, “Best Available Technology for Sodium Hypochlorite Storage Tanks” by Michael Stevens and Paul Cohen.
Based upon ASTM C-581 laboratory corrosion testing, FRP coupon testing in the field and a portfolio of case histories, Ashland has determined that storage tanks and equipment exposed to 9% or higher sodium hypochlorite solutions should utilize a corrosion liner fabricated with DERAKANE 510A-40 Epoxy Vinyl Ester resin or HETRON FR992 Epoxy Vinyl Ester resin. Maximum durability will be obtained with a BPO/DMA cure system. No cobalt should be present in the corrosion liner. Cobalt/MEKP cure systems can, however be used in the structural portion of the equipment. The corrosion liner should contain a minimum of 1 layer of C-glass veil, (2 layers are preferred), backed by 5 mm of chopped strand boron-free glass reinforcement. Service life will be maximized if the corrosion barrier is post cured. A properly designed and fabricated FRP tank can be expected to last over 20 years, even in this aggressive environment.
Service life on any sodium hypochlorite tank can be further extended by keeping the product stabilized. The stability of sodium hypochlorite solutions is adversely affected by heat, light, pH and metal contamination. The GE Water Purification handbook reports that the rate of decomposition of 10% and 15% bleach solutions doubles with every 10 degree F rise in the storage temperature. Sunlight reduces the half-life of a 10% – 15% hypochlorite solution by a factor of 3 to 5. If the pH of a stored solution drops below 11, decomposition is even more rapid. As little as 0.5 ppm of iron causes rapid deterioration of 10% – 15% solutions. Care should be taken to avoid all of these conditions in order to maximize storage stability and storage tank service life.