Water shifts to on-site chlorine generation

On-site generation offers an economical solution to safety issues surrounding chlorine transportation

INNOVATIVE CHEMICAL engineering could help resolve a major disagreement between the water treatment industry and environmentalists over chlorine.

Chlorine has a stellar record in the disinfection of water, but in recent years its reputation has suffered a steady assault from activists opposed to its production and use. Perhaps their greatest objection is the possibility that thousands of people could be killed if a railcar or storage tank of the corrosive and highly poisonous gas were ruptured in a highly populated area.

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The point is reasonable. In 2005, a train carrying 90 tonnes of chlorine collided with a parked locomotive in Graniteville, South Carolina, US, releasing a toxic cloud that killed nine and drove thousands more away from their homes.

Chlorine has, however, saved many more lives than it has taken in the century since it was first used to ensure safe public water systems. It remains at the forefront of water disinfection today because nothing else comes close to offering the same combination of efficacy and economics.

In terms of cost, nothing can beat gas chlorination, which typically averages only $4/m gal of water treated, according to UK-based research firm Global Water Intelligence (GWI), in a recently issued report entitled Water Technology Markets 2010. Furthermore, chlorine gas not only disinfects water – it also remains at a residual level, preventing reinfection by viruses or bacteria during transport, storage, and distribution, GWI notes.

“For this reason, in the US, the EPA [Environmental Protection Agency] and individual state regulators set requirements that all municipal drinking water contain measurable residual chlorine. So even as other new disinfection methods, such as ozone and UV light, are gaining popularity, chlorine-based treatment remains indispensable in water treatment and potable water disinfection,” says GWI. “These merits, coupled with the regulatory challenges, have stimulated an emergence of a market for new types of chlorination equipment.”

HEAVY ON THE SALT
Each year, about 50m tonnes of chlorine are produced globally, almost entirely by the electrolysis of aqueous sodium chloride (table salt). The coproducts of this transformation, which is called the chlor-alkali process, are hydrogen and sodium hydroxide. Most of the chlorine produced is consumed in the manufacture of polyvinyl chloride (PVC) and other organic chemicals, but a significant quantity, about 2.5m tonnes in 2009, goes to water treatment, says GWI.

Safely transporting this chlorine to water treatment facilities, which often lie near population centers, is a sore point, particularly in this age of terrorism. One solution is to embed chlorine’s disinfectant activity in chemical products that are more easily transported, such as aqueous sodium hypochlorite (bleach) or chloramine.

On the other hand, what could be easier to transport than table salt? In the 1970s, a number of companies began to develop smaller equipment that could economically miniaturize the chlor-alkali process to manufacture chlorine gas or sodium hypochlorite where and when it was to be used, thus eliminating the problems of transportation and storage. Interest in these technologies has grown with the possibility of expanded regulation.

US-based GE Power & Water, for example, introduced its modular Cloromat membrane electrolysis system in 1973. It has since evolved to incorporate advances in materials and design. Cloromat can produce 30,000-300,000 liters/day of sodium hypochlorite solution in concentrations of 60-195g/liter of available chlorine. It can also be configured to produce both hydrochloric acid and sodium hydroxide.

“In the early days of Cloromat, the majority of installations were for on-site production of water or wastewater treatment,” says Michael Rees, product manager, Cloromat systems – water and process technologies at GE. “The shift in terms of users was then towards chemical [and] bleach producers who were looking to supply sodium hypochlorite (and/or HCl and NaOH) to the local market and also industrial customers.” More recently, he adds, “there has been an increasing interest from water authorities.”

Rees says Cloromat can pay for itself in two to five years.

“The economics and payback depend on several factors such as raw material costs (particularly salt and power), distance from the nearest supplier, as well as the ­competitiveness in the local market amongst local suppliers (both sodium hypochlorite and chlorine),” he explains. “It also depends on the size of the plant. In addition, ­intangible benefits such as improved safety by ­removing chlorine from a site are difficult to quantify.”

Other major equipment providers include MIOX and Severn Trent De Nora, both of which specialize exclusively in hypochlorite generators.

US-based MIOX supplies hypochlorite generators with capacities ranging from only 4.5kg/day of free available chlorine to more than 700kg/day. MIOX touts the ability of its technology to produce “mixed oxidants” as well as hypochlorite. Characterizing these mixed oxidants has been elusive, but the company says their presence is demonstrated by their superiority compared with hypochlorite. According to GWI, mixed oxidants provide more effective disinfection, elimination of biofilm, more durable chlorine residual, reduced formation of disinfection by-products and also improved taste and odor. “Mixed oxidants behave like chlorine dioxide or ozone, while offering a residual chlorine disinfectant to comply with EPA requirements,” says MIOX.

Severn Trent De Nora a joint venture between US-based water treatment firm Severn Trent Services and Italian electrolysis firm Gruppo De Nora, specializes in the electrolysis of seawater to generate hypochlorite.

AN OPPORTUNITY TO SERVE
Some chlorine producers might see competition in the trend toward on-site chlorine generation, but Dutch specialty chemical company AkzoNobel hopes to turn it into a new service business, which it calls “Remote-Controlled Chlorine Production.” Two and a half years in the making, the service, which is now available in Europe, targets chlorine users that do not want to run a chlorine plant. Instead, modular skid-mounted units, designed by Italy’s Uhdenora, are operated remotely by AkzoNobel personnel.

“We didn’t have the water treatment market directly in mind, but more in general the chlorine-consuming industry, which [was] not [yet] supplied via pipeline but by means of transporting the chlorine to its site,” says Ellen Holmen, business manager, remote-controlled chlorine production. “This industry covers everything from different kinds of organic and inorganic chemicals, pharmaceutical ingredients to sodium hypochlorite for, among others, water treatment. Our aim is to eliminate the chlorine transportation in general.”

The plant’s capacity is about 15,000 tonnes/year of chlorine. The standardization of size and design, as well as operational efficiency of remote, centralized operation, helps compensate for the loss of scale effects associated with larger chlor-alkali plants, Holmen notes.

“In the rest of the world, the same small unit is offered as a single plant which can supply the customer with chlorine, caustic and/or sodium hypochlorite for consumption directly on-site, perfectly suited also for water treatment applications as long as the volume suits the consumption on site,” says Holmen. “The latter is easier in the US than in Europe, as the size of the water treatment installations here are in general smaller, requiring less chemicals.”

GWI expects the market for chlorination chemicals to grow at 2-4%/year in North America and Europe, and 5-7%/year in the rest of the world. The firm estimates that the market for chlorine feed equipment, which includes electrolysis systems, chlorine dioxide systems and peripheral equipment, will total $250m (€204m) in 2010 and grow at a combined annual rate of 4.7% through 2016.

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