Case Study: Sound Water Management Reduces the Environmental Footprint of Two Ethanol Plants

By Tony Stanich and Jason Van't Hul | June 02, 2008
Balancing environmental sustainability with maximized production and minimized investment can be a difficult task. Recent studies show significant improvements in energy usage in dry-grind ethanol plants. However, more needs to be told about the work being done to reduce water usage as well. An ethanol plant producing 100 MMgy of ethanol will displace nearly 2.8 million barrels of crude oil per year, but it also consumes 300 million to 600 million gallons of water. This equates to a water ratio of three to six gallons of water used per gallon of ethanol produced. As discussed in EPM's March 2007 article "Water Efficiency is a Result of Sound Water Management," process water accounts for only one-third of the required water in the plant. The other two-thirds come from utility systems—the bulk from the cooling system.

The article emphasized the necessity of a sound water management process to reduce the plant's water usage. The article reviewed key inputs, outputs, critical activities, resources needed and the critical constraints the process must manage. The water management process it discussed could be applied to any plant design regardless of the feedstock or site requirements. Required inputs include understanding what type of ethanol production process design is being built, the resulting plant water balance, the ion constituents (quality) of the incoming water sources, and the water discharge constraints (National Pollutant Discharge Elimination System permit).

Environmental discharge, capital and operational costs all become critical constraints that the process must take into consideration. Resources needed include analytical laboratories, historically accurate modeling services, pilot trial capabilities and on-site expertise to implement the solution. All of these result in an output including specific water pretreatment equipment, effluent discharge estimations, and operating parameters of the water-based processing equipment including the largest user, the cooling tower.

The overwhelming response to the March 2007 article combined with the heightened debate on the environmental impact of ethanol production led Nalco Co. and two Midwestern ethanol plants to develop a case study for the industry. One of the major goals of this process implementation is to reduce the plant's water ratio. Many steps within the process have an effect on this goal. The case study mainly focuses on the effects of cooling water treatment modeling, chemistry and control, and briefly discusses the effects of pretreatment equipment.

The case study highlights two new 110 MMgy plants where Nalco, in conjunction with the customer, their engineering firm and equipment supplier, worked to reduce the water ratio (gallons of water used per gallon of ethanol produced) from a no-treatment ratio of 26.73 for Plant No. 1 (Chart 1) and 6.26 for Plant No. 2 (Chart 2) to 2.78 and 2.67, respectively. Additional constraints on the team included environmental discharge issues that required the design and implementation of a zero-liquid discharge system. This necessitated the reduction of water usage because every unnecessary drop of water brought into the plant places additional load on the zero-liquid discharge system.

The customer, engineering and equipment design team implemented a similar water management process as discussed and modeled the pretreatment equipment for the plant to substantially reduce the water ratio in each case. High ion content suppressing cooling tower cycles substantially elevated the water ratio in each plant (moderate levels of treatment would have been necessary to meet process requirements). A few questions need answering. First, why does the cooling tower need so much water in the first place? Second, what are cycles and what is their impact to the production process?

Chart 1

SOURCE: NALCO CO.

Chart 2

SOURCE: NALCO CO.

First, the cooling tower utilizes significant amounts of water due to the enormous levels of heat generated in the process that must be removed. The two primary places heat needs to be removed are within the fermentation process and in distillation. Cooling water is used to cool incoming mash and maintain temperature throughout fermentation. In distillation, vacuum is maintained on the columns via the 190 condensers. Heat is transferred to the water and cascades through the cooling tower at which point the heat is released via evaporation. In the case of the 110 MMgy design, the resulting evaporation amounts to 2.33 gallons per gallon of ethanol, or 256 MMgy for each plant. This number remains fixed in both plants due to their identical designs.

Cycles are determined by the amount of ion concentration that can build up in the cooling tower before deposition, corrosion or fouling begins. These types of issues can cause substantial process inefficiencies and equipment failures. Concentration is caused by water being evaporated (tower evaporation) at a faster rate then it is being replaced (tower makeup) with lower levels of the problem ions. Concentration, and thus cycles, can be extended by the mechanical removal of the problem ions or through the addition of chemistries. These chemistries modify either the pH of the water or interact directly with the precipitate, but both techniques aim to avoid any scale formation throughout the cooling system.

In order to more effectively visualize the process, think of a pan filled with hard tap water and then boiled (evaporated) to dryness. A residual will be left on the pan. This residual is the dissolved ions left after the water is evaporated. If the pan is filled to the level prior to boiling (bringing in fresh makeup) and no water is allowed out of the pan (tower blow-down), the ions would have concentrated in the pan twice. In other words, the pan would now have two cycles. The more often the boiling pan example is repeated, cycles will continue to increase until dissolved ions precipitate out of solution. This is the same phenomenon happening in the cooling tower. However, as mentioned earlier, the tower has added chemicals and is constantly blown down before precipitation and then deposition occurs.

To reduce the water use in the cooling system, makeup water must be reduced, and because evaporation is fixed, blowdown must be reduced as well. However, as illustrated in the boiling pan example, a precipitation and potential deposit condition could be created. Therefore, to accomplish water reduction and avoid any deposit formation, a balance must be reached between treatment equipment needed to remove the ions, cycles of concentration, chemical limits of the inhibitors, pH of the water, and the resulting capital and operational costs of each.

The pretreatment equipment solution (cold lime softening and reverse osmosis) in both plants reduced the ion content to produce cooling tower makeup water to safely achieve design specification of four cycles of concentration. In most normal water scenarios, three to four cycles are generally targeted in order to project a worst-case scenario for sizing of the pretreatment equipment. However, in this instance, all parties realized that more could be done.

Nalco applied its patented 3D TRASAR optimizer technology and determined that it was possible to substantially increase the cycles in both facilities. The optimizer takes into account all of the relevant variables and creates a recommendation by integrating industry accepted solubility models with thousands of chemical performance results to predict the success of the program. The optimizer can also estimate impact to assets through predicted corrosion rates, effect on heat exchange efficiencies through predicted scaling and microbial potential. The modeling technique is iterative in nature as one variable can cause the entire model to break down and violate the constraints discussed above.

After running the 3D TRASAR optimizer, the customer agreed to safely increase their tower cycles at Plant No. 1 from a design of four to an optimized 6.2 and at Plant No. 2 from a design of four to an optimized eight cycles. Refer to the green column in Charts 1 and 2 to see the optimized impact. Implementation of the water management process set these operational parameters. However, to realize these gains, excellent monitoring and control is required. This will be delivered through Nalco's patented 3D TRASAR chemistry and controller. The result? This Midwestern ethanol producer will reduce its projected annual water usage by 36 million gallons at Plant No. 1 and 73 million gallons at Plant No. 2 for a total of more than 100 MMgy.

Tony Stanich is Nalco Co.'s global marketing manager. Reach him at astanichiii@nalco.com or (630) 305-1901. Jason Van't Hull is an industry technical consultant with Nalco Co. Reach him at jvanthul@nalco.com or (605) 330-1280.