Going with the Flow

Like a fluid line of credit at the bank, a steady and unhindered flow of water is critical for the efficient operation of an ethanol plant of any size. And just as interest on that credit varies depending on the unique circumstances of the individual plant, water, depending on its source, contains its own unique set of properties.
By Frank Zaworski | January 03, 2009
No two ethanol plants in the world, whether a mile or a continent apart, have exactly the same water with exactly the same characteristics. This uniqueness of a plant's industrial water creates challenges for plant operation and opportunities for those with technological solutions. "In terms of water quality, the most demanding water user in an ethanol plant is the energy center," says Mike Mowbray, marketing manager for U.S. Water Services in Plymouth, Minn. (www.uswaterservices.com). "In terms of quantity, the biggest user is usually the cooling system. Both systems require sophisticated control in order to maintain the efficiencies needed to make a modern ethanol plant profitable."

U.S. Water Systems has helped design and start water treatment systems in about 80 dry-grind ethanol plants in the United States. The company provides water management solutions from sophisticated pre-treatment for groundwater or grey water to zero-liquid discharge systems.

Mowbray says U.S. Water Services is familiar with the site-specific water challenges all ethanol plants face, and the company is continuously creating and refining technologies and services to meet any plant's needs.

Texas-Sized Problem"When an ethanol plant in Texas (name withheld), serviced by U.S. Water Services, started experiencing problems with its reverse osmosis (RO) system, conventional, on-site methods of cleaning membranes failed, and the reason soon became clear," Mowbray says. When the city changed the flow of the water to accommodate the ethanol plant, it stirred up red clay that had settled in the distribution piping. The fine clay silt, so fine it could pass through a 1 micron filter, found its way to the ethanol plant's RO system and fouled the membranes.

Mowbray says attempts were made to clean out the red clay using the same methods as those used to remove organic compounds, but only limited success was achieved.

With conventional methods failing at every turn, unconventional methods had to be considered, Mowbray says. Replacing the 180 membranes would be too expensive, especially if the situation persisted.

The Texas plant turned to U.S. Water Services' RO cleaning station in Cambridge, Minn. This RO station is unique in the water services industry, Mowbray says, utilizing a variable frequency drive, along with complex valve and control schemes, allowing water to flow through the membrane in either direction, adjusting the pressure over a wide range in order to achieve the best results. "Membranes, by design, allow water to flow in a single direction," Mowbray says. "Reversing the water flow can provide enough force to kick a majority of the debris free from the membrane, something that is impossible to do on-site, and is rarely seen in other off-line cleaning systems."

Finally, U.S. Water Services' membrane cleaning station has the ability to test the performance of each cleaned membrane at actual operational flux rates to ensure that the membrane meets industry specifications before being sent back to the customer.

On its first test run, the RO cleaning station proved successful, Mowbray says. The red clay silt was removed from a substantial number of the membranes. The performance was verified, and the membranes were sent back to the customer, ready to reinstall.

"The U.S. Water Services membrane cleaning station is the newest and most sophisticated system in the industry," Mowbray says. "It was designed and built by our engineering group using knowledge gained from extensive experience with our customers' reverse osmosis applications. Not every plant will need to clean membranes off-site. However, considering the critical nature of a properly functioning water treatment plant, and the cost of replacing membranes, it is good to have the option of enhanced cleaning when needed."

More Solutions
Boiler water treatment: The heat recovery steam generators (HRSG) built into most new ethanol plants are designed to convert the waste heat coming out of the thermal oxidizers (TO) into steam instead of burning oil or gas in a fire box. In order for the HRSG to be effective, according to U.S. Water Services engineers, it must be built to maximize the capture of heat energy from the waste gas stream. This leads to designs with small diameter steam tubes that are packed very tightly in the steam generating section. Some of the tubes have fins on the fire side in order to increase heat transfer as much as possible.

The modern HRSG is like a sports car, Mowbray says. "It packs a lot of performance in a small package, but it demands respect and attention to get the most out of it."

From a water treatment point of view, Mowbray says, the steam generator is prone to scale and deposits. As a result, it is critical that the highest quality water is used for make-up to these systems. This requires either a good demineralizer system or a RO unit followed by polishing softeners.

RO or softened water alone typically does not provide the level of water quality needed in the HRSG, Mowbray says. Once water of the proper quality is available, it is equally important to treat the water with corrosion inhibitors and a dispersant polymer program. "The use of older style precipitating chemistries will not provide the level of boiler tube cleanliness needed in a HRSG," he says. "Lastly, this high-tech chemistry should be controlled with automated chemical and blowdown controllers."

Since the steam generating tubes are narrow, and the heat flux is high, scale forming in the hot section of the system can quickly plug and blow tubes, Mowbray says. This will lead to the shutting down or de-rating of a plant to repair the problem. The design of many HRSG systems makes repair a difficult undertaking. Plugging or replacing individual tubes is virtually impossible. The only recourse is to replace the entire generating bank.

Another reason to pay such close attention to boiler water treatment is simple economics, Mowbray says. Waterside tube scale is an insulator that blocks the efficient transfer of heat from the thermal oxidizer waste gas to the boiler water. As little as a sixty-fourth of an inch (thinner than an eggshell) of calcium carbonate or iron oxide scale will reduce heat transfer efficiency by as much as 15 percent. Scaling in HRSG tubes can result in increased energy costs that directly affect plant profitability. Good water treatment is cheap insurance, he says.

Unfortunately, make-up water is not the only water source that can cause problems in your ethanol plant boiler. As with any production facility, contaminants can, and usually do, come back into the steam generator through the return condensate. In an ethanol plant, the contaminant is typically mash. When mash is in the boiler water, conductivity goes up, the water turns black, and the boiler foams and causes the drum level to bounce. The boiler water will also carry over into the steam due to the foaming. This has been known to plug steam control valves, eventually shutting down the plant.

Cooling water treatment: The cooling water system requires the largest amount of water in an ethanol plant's subsystems. A typical 40 MMgy to 50 MMgy ethanol plant has to remove 100 million to 120 million British thermal units per hour, according to U.S. Water Services. Just as in the steam generating system, a small amount of scale on heat exchanger surfaces will significantly reduce heat transfer efficiency, possibly resulting in having to run chillers more often. If serious enough, it can affect a plant's ability to produce the maximum amount of ethanol.

The quality of incoming water will be the major factor in cooling tower management. A cooling tower releases heat to the atmosphere by evaporating water. The dissolved solids from the well water are left behind in the cooling water. As more well water is added, and more cooling water is evaporated, the dissolved solids become more concentrated. The
various constituents in the well water each have their own limits of solubility. Eventually, the cooling water will cycle up to a point where some of the dissolved solids will precipitate.

Typically, they will do this first at the point of highest temperature in the cooling system, the heat transfer surface of the hottest heat exchanger.

There are various methods used to increase the cycles of concentration in a cooling system. The first, as with the boilers, is to take the dissolved solids out of the cooling tower make-up water before adding it to the system. According to Mowbray, many plants use extra RO treated water to supplement well water to the tower. "This is not a free fix, however, as the RO also has to be treated to prevent it from scaling, and there are significant operating costs," he says. "However, depending on the initial plant water quality, this may be the best option."

The other method is to treat the cooling water with chemicals that will prevent scale from forming at higher concentrations of dissolved solids than would normally be possible.

Cooling systems are also treated to prevent corrosion and bacteria growth. Serious corrosion can cause equipment failure. Even minor corrosion can cause problems, since the rust will move with the water flow until it comes to an area of slower flow. Then the rust particles can settle out and cause plugging.

Bacteria can cause corrosion and system plugging as well, Mowbray says. But the biggest concern with bacteria is health related. Legionnaire's disease still pops up occasionally in industrial facilities, and cooling towers need to be treated to prevent them from becoming a breeding ground for this dangerous bacterium, he says. A variety of biocides are available to the industry. The choice of which to use depends on the system being treated, and on local discharge restrictions. The Occupational Safety and Health Administration and the Cooling Tower Institute recommend a combination of continuously fed oxidizing biocide (usually chlorine gas or sodium hypochlorite bleach, fed with sodium bromide) and a nonoxidizing biocide slug fed on a weekly basis.

"Water quality and water treatment can have a huge impact on the profitability of a plant," Mowbray says. "It is important to get a competent water treatment professional involved with your plant early in the design process. Waiting until a couple of weeks before start-up to choose a water treatment chemical supplier is a recipe for trouble."

Frank Zaworski is an Ethanol Producer Magazine freelance writer. Reach him at fzaworski@gmail.com.