Low-Hanging Fruit

Relatively inexpensive modifications to the cooling tower and RO can save energy and water.
By Randy McDaniel | November 15, 2010
It is no surprise that plenty of research and focus is spent on squeezing as much ethanol as possible out of every bushel of corn. Although not as glamorous, there are ways to squeeze a little more water and energy out of the plant as well. Two projects provide conservation opportunities that require minimal investment with immediate return.

Project 1: Cooling Tower Blowdown
I am going to bypass the obvious choice to discuss ways to increase cycles of concentration to reduce the amount of blowdown required. All respectable water treatment companies should already be working diligently to maximize cycles of concentration while preventing the adverse impacts of scale and corrosion, as this can deliver significant water and chemical savings. Instead, this discussion deals with the location of the cooling tower blowdown. Are you blowing down relatively hot or cold water?

In some plant configurations, cyclonic filtration is utilized on the cooling tower supply. This makes sense as it reduces the suspended solids loading to the plant, thus reducing the risk of heat exchanger fouling. Sludge build up in the cyclone filter is removed via blowdown. Some plants have utilized this as the primary source of cooling tower blowdown. The downside is relatively cold water gets utilized for cooling tower blowdown.

The cooling tower supply is designed to be at least 10 degrees Fahrenheit cooler than the cooling tower return. However, it is commonly at least 13 F cooler. Assuming the plant requires about 75 gpm of cooling tower blowdown to maintain the appropriate cycles of concentration, blowing down the cooling tower supply that is 13 F cooler wastes 11.71 million Btu per day. The math used to figure that waste appears below and can be applied to your situation.

Since about 1,000 Btu are removed in a cooling tower per pound of water evaporated, this project will conserve 11,709 pounds of water for evaporation each day. This equates to 1,404 gallons per day. Over 365 days, this conserves 512,460 gallons of water for evaporation. Assuming four cycles of concentration, the reduced evaporation also reduces the required blowdown by 468 gallons per day since Blowdown = Evaporation / (Cycles - 1). The cooling tower make-up requirement is reduced by 1,872 gallons per day since Make-up = Evaporation + Blowdown. As a result, this project actually conserves a total of 683,280 gallons of plant water per year. Beyond the water conservation, there is a reduced thermal load on the cooling tower, which may reduce electrical use by the cooling tower fans.
The goal should be to use the hottest water possible for blowdown to conserve the most water. This project typically requires minimal piping and control changes.

There is a note of caution when implementing this project. Since the plant will be blowing down warmer water, there is the potential to have an impact on the water discharge permit. This is especially true if the discharge has little retention time prior to being directly discharged.

Project 2: Free Cooling RO Preheater
Reverse osmosis (RO) membranes are designed to operate at 77 F. Designers account for machines to operate below 77 F by over-sizing machines or installing cold water membranes. It is normal for an RO to operate with about 55 F water. This project evaluates the opportunity to pre-heat the RO feedwater closer to 77 F, which is very common as the benefits are numerous. The twist discussed here is to utilize cooling tower return as the heat source using a plate and frame heat exchanger.

In order to evaluate the benefits, it is necessary to establish a few assumptions:
>Current RO feedwater usage is 500,000 gallons per day and some of the RO permeate is used in the cooling tower to increase cycles of concentration. In this example, cooling tower adds about 100 gpm RO permeate.
>Current RO feedwater temperature is 55 F.
>Proposed RO feedwater temperature of 70 F.

In this example, there are about 62.55 million Btu available from the cooling tower to be transferred to the RO feedwater. These Btu reflect a "free cooling" opportunity for the cooling tower and a direct load reduction on the tower. As seen in Project 1, reducing the Btu load on the tower can conserve water. One must subtract the Btu increase, however, from the RO permeate added back to this cooling tower to help increase the cycles of concentration and determine the total available free cooling Btu. In this case, it is 18.01 million Btu. As a result, the total available free cooling is 44.54 million Btu.

Since about 1,000 Btu are removed in a cooling tower per pound of water evaporated, this project will conserve 44,540 pounds of water for evaporation each day. This equates to 5,340 gallons per day. Over 365 days, this conserves 1,949,100 gallons of water for evaporation. Assuming four cycles of concentration, the reduced evaporation also reduces the required blowdown by 1,780 gallons per day since Blowdown = Evaporation / (Cycles - 1). The cooling tower make-up requirement is reduced by 7,120 gallons per day since Make-up = Evaporation + Blowdown. As a result, this project actually conserves a total of 2,598,800 gallons of plant water per year.

Although this project conserves more water, it also requires a slightly higher capital investment. In order to make it work, a heat exchanger is required in addition to piping and plumbing. There are plenty of other benefits associated with this project.

Since the RO operates at a higher permeate flow rate as it approaches 77 F, an existing RO will operate for fewer hours to produce the same quantity of water. This reduces the electrical demand for the high pressure RO pump, along with mechanical wear and tear on the machine. Should a new RO be used, the designer can select a smaller unit instead of oversizing. As a result, the initial capital investment is smaller.

Additionally, there is a benefit to having higher temperature RO permeate, assuming it is used for boiler make-up. It should also be stated that if operating an ethanol plant with an RO where the permeate is not used for make-up to the boiler or heat recovery steam generator (HRSG), there may be an even bigger opportunity for savings in the form of increasing boiler cycles of concentration. Increasing the RO permeate from 55-70 F using Btu from the cooling tower means fewer Btu are necessary from natural gas. Assuming the boiler make-up rate is 120 gpm and the boiler efficiency is 80 percent, 27.02 million Btu can be conserved daily. The math is shown below.

Assuming natural gas cost is about $5 per 1 MMBtu, this also conserves $135.11 daily for natural gas or $49,314 annually. The natural gas reduction for a HRSG may not be as easily calculated due to the nature of the HRSG. If supplemental natural gas is used in the HRSG in order to provide steam load, the natural gas savings would be similar. If the HRSG is only generating steam from the waste heat, a large portion of the Btu may end up exiting the stack.

Increasing ethanol yield in fermentation by 0.1 percent is as thrilling as watching Peyton Manning hit Reggie Wayne on an 80-yard touchdown pass against the Patriots. Neither of these projects will deliver that kind of excitement, but they are fairly simple and inexpensive to implement. Call it the linesman approach. Delivering incremental and continuous improvements in the utility is my goal because saving electricity, water and fuel is good for business and the longevity of the industry. With a thorough understanding of the plant operation, it is possible to maximize every gallon of water, every kilowatt of electricity, every therm of fuel and make the plant work to the last Btu.

Author: Randy McDaniel
Area Manager, Weas Engineering
(317) 867-4477.
randy.mcdaniel@weasengineering.com