As our February cover story "The Tide is Turning," highlighted, chemical makers increasingly are focusing on water-related issues. In particular, concern over availability is spurring
leading operating companies to implement a host of novel strategies and technologies to optimize water use.
For instance, as part of its contribution to World Water Day on March 21st, Dow Chemical, Midland, Mich., outlined how it's optimizing water use at two of its major facilities — at Brazosport, Texas, and Terneuzen, the Netherlands.
Brazosport, which covers more than 5,000 acres near the mouth of the Brazos River and which Dow claims is the largest integrated petrochemical facility in the world, is in an area that experienced severe drought conditions in 2011. This prompted the company to launch a number of initiatives aimed at conserving water — including establishing a water strategy director and steering team, a community water symposium and a contest for employees for water-conservation-project ideas.
As a result of all these efforts, Dow identified and implemented:
• Chlorine once-through cooling-water recycle. This project recycles the once-through cooling water for the rectifiers at one of the chlorine plants, as well as the once-through cooling water for an air compressor station to the site's clarified water system for general re-use, and represents about 1,300 gpm in savings.
• Eliminating one-pass fire-water monitor cooling at a power station. Installation of piping allowed use of seawater for cooling, saving 200 gpm of continuous freshwater use and 200–600 gpm during startups and shutdowns;
• Demineralization plant resin change. This project significantly improved the operation of a resin bed, and also included modifications to a reservoir to maintain the gains in efficiency;
• Supplemental cooling automation at another power plant. The idea initially was to change from water cooling to air cooling for a section of the power plant that required intermittent cooling. However, space was insufficient to install the required fin-fan exchangers. So, instead, existing equipment was slightly modified and process control was implemented to allow for the automation of the cooling water valve;
• Improved cooling-tower chemistry. Dow worked with its water treatment chemical provider to modify the water treatment chemistry on 25 cooling towers to reduce makeup water by 400 gpm;
• Soft water recycle. Installation of piping, valves, flow meters and other instrumentation enabled recycling soft water from a propylene oxide plant to the site river water header when only two trains are running — saving 3,000 gpm;
• Improved maintenance of many older river water lines. Identifying and fixing leaks as part of a maintenance strategy reduced water draw by another 1,000 gpm; and
• Other efforts. Eliminating slab washing and watering landscaping saved another 3,500 gpm.
In addition to the almost 10,000 gpm saved by these projects, temporary water-conservation measures further reduced consumption by 3,500 gpm.
At Dow's Terneuzen facility, which is another massive chemical complex and the Dutch city's heaviest industrial water user, one of the main challenges has been the plant's location — at a seaport where for decades fresh water has had to be piped in from a distance of over 120 km (Figure 1).
Over the years, Dow has worked with local desalination plant operator Evides to re-engineer its plant with reverse osmosis membranes, low-pressure feed pumps and improved process automation. In 2010, the city's wastewater treatment plant was equipped with a membrane bioreactor (MBR). The MBR serves various objectives — for example, it adds 20% treatment capacity to the municipal plant while dramatically improving the quality of effluent sent to Evides' facility.
As a result, Dow and its partners now are twice reusing the community's treated wastewater through an innovative wastewater recycling program, thereby using every liter of water three times instead of once.
Today, Evides purifies 10,000 m3/d of municipal household wastewater for Dow, which uses it to generate steam and feed its manufacturing plants. Steam condensate serves in cooling towers until the water finally evaporates into the atmosphere. Compared with conventional desalination of seawater for the same use, recycling Terneuzen's wastewater cuts energy use by 95% — the equivalent of reducing its carbon dioxide emissions by 60,000 tons each year, says the company.
Not only is membrane separation inherently more energy efficient than desalination but using municipal effluent instead of seawater as the raw water source requires less driving force (pump pressure) to remove salt from the water because it has a lower salt content. In addition, the lower-salt-content effluent pares use and cost of chemical treatment of the membrane systems in half.