Choosing Pumps for Biofuel Production

Pumps & Systems, April 2008

Biofuel production is one of the fastest growing sectors of the energy industry. To reduce our dependence on finite energy sources such as petroleum fuels and decrease the United States' CO₂ emissions, biofuel production is projected to increase by nearly 40 percent, from 6.5 billion gallons per year in 2007 to 9 billion gallons per year in 2008.

Pumps play an integral role in biofuel production. These primary drivers add energy to the pipeline and help achieve production goals. By employing the right pump, biofuel facilities are rewarded with optimized production, reduced maintenance costs and minimized power consumption.

Facilities that analyze the fluid viscosity and solids content, pipeline temperature and catalysts and denaturants will be rewarded with decades of reliable pump service and maximized biofuel production.

Biofuel Basics

Biofuel refers either to the production of ethanol or biodiesel. Ethanol, an alternative to gasoline, is alcohol that can be used as a fuel. Ethanol is blended in 46 percent of the United States' gasoline in the form of E10, a blend of 10 percent ethanol and gasoline¹, or as a replacement for methyl tertiary-butyl ether (MTBE), an additive that raises the oxygen content in gasoline to reduce emissions and prevent an engine from knocking.²

In 2006, the United States had 110 plants in operation with a capacity of 4.86 billion gallons of ethanol per year.  Some 145 additional plants worth $13 million to $200 million-each with capacities between 7 and 140 million gallons per year-are in the planning stages.

Feedstocks for ethanol production include corn, sorghum, switchgrass, sugarcane, sugar beets, potatoes and poplars. Corn, comprising 90 percent of all ethanol production, is most often used because it is a relatively low-cost source of starch that can easily be converted into sugar, fermented and distilled into ethanol. Switchgrass has one of the highest potentials for use as a biofuel crop in the United States, mainly because it grows well under a wide range of conditions, has a higher yield per acre and requires less energy to produce.²

Biodiesel is a domestic, renewable fuel for diesel engines derived from vegetable oils and animal fats, including used oils and fats. Biodiesel is not the same thing as raw vegetable oil; rather, it is produced by a chemical process that removes the glycerin and converts the oil into methyl esters.

The use of biofuel is not new. Rudolf Diesel (1858 -1913) of Germany experimented with many different types of fuel to feed his invention-a compression ignition engine. His first engines ran on peanut oil at the World Exposition in Paris more than 100 years ago. Biodiesel can be used in any concentration with petroleum-based diesel fuel with little or no modification to existing diesel engines. These blended fuels are referred to as "biodiesel blends," and include the percentage of biodiesel in the blend, such as B2 (2 percent), B5 (5 percent) or B20 (20 percent).

Feedstocks for biodiesel production include vegetable oil from soybean, canola (rapeseed), palm, corn, sunflower and cottonseed. 

Choosing the Right Pump

Any facility that is producing 50 million gallons or more of ethanol or biodiesel a year is working on a continuous production schedule. Facilities try to prevent unplanned shutdowns since they can cost tens of thousands of dollars a day.

To this end, energy companies rely heavily on the performance of their pumps. Companies should get two or three years out of their pumps before they have to engage in routine maintenance such as changing seals and bearings. The life of the pump should extend approximately 10 years.

Fluid Viscosity and Solids Content

The first stage of ethanol production consists of the feedstock going through a hammermill to create flour, which is mixed with process water in a cook tank to create a slurry. The slurry can be rated either by its viscosity-a fluid's resistance to flow-or solids content-a fluid's percentage by weight of nonvolatile material. Any of the cellulosic fibers used in ethanol production-such as wood byproducts from saw mills, switchgrass or sawgrass, potatoes and sugar cane-have a high solids content and viscosity in the range of 2,500- to 5,000-cst, roughly the consistency of warm molasses.

Since oils are used primarily for biodiesel production, viscosity is the most commonly used measurement. Oils used for biodiesel maintain some viscosity throughout the process, from 30- to 50-cst in its raw form, to about 5-cst at 40-deg C as 100 percent biodiesel.³ These generally clean liquids are easily handled with gear or positive displacement pumps for higher viscosity applications and centrifugal pumps for the lower viscosity applications.

The slurry maintains its viscosity and solids content from the beginning of the ethanol production process when the feedstock leaves the cook slurry tanks through the liquefaction and fermentation processes to the distillation tanks.

Fermentation converts the slurry into a mash, which is pumped into the distillation columns. Heat is added to separate the ethanol, leaving stillage, a mixture of non-fermentable solids and water. The stillage is pumped out of the bottom of the distillation columns to a centrifuge that separates the liquids and solids.

Slurries tend to be abrasive and can cause premature wear in pumps that rely on internal part contact-like progressive cavity and gear pumps. These pumps cannot run dry. If the pump is empty or if product dries up, the inside of the pump will wear prematurely from abrasion due to lack of lubrication. This is a common cause of downtime in biofuel facilities. Additionally, it takes a tremendous amount of torque to start up a progressive cavity pump when slurry has settled, causing energy consumption to increase.

To this end, companies should employ rotary PD pumps for applications in which slurry or oil is viscous during the primary production stages. Since these pumps do not have parts that contact each other, internal components are not susceptible to the same type of wear as positive cavity and gear pumps. Rotary PD pumps are also able to break up slurry and move it along the pipeline, minimizing additional energy consumption related to start-ups.  

Centrifugal pumps may be used to pump ethanol from the distillation column, through a molecular sieve, to storage tanks. Some of the thin stillage (about 5 to 10 percent solids in water) recovered from the separator is sent back to the cook tanks to reduce the amount of fresh water required for the process. The remnant is sent through an evaporation system where it is concentrated into syrup containing 25 to 50 percent solids.

A PD pump is recommended to draw the syrup off the evaporator tank and send it to be mixed with the dried grains recovered from the separator.

Turning Up the Heat

Operating temperatures fluctuate during biofuel production. As temperatures increase, metals will expand. Since most pump components are made of metal, temperature is a key consideration when selecting a pump.

Pumps with rotating parts that contact one another in the process fluid, like gear pumps, can bind up in extreme heat applications and are not recommended for the primary stages of biofuel production, which can reach temperatures of 190-deg F.

Rotary PD pumps are recommended for these applications. Traditionally, a rotary PD pump has a clearance of a couple thousandths of an inch between the rotor, the body and the end cover. If heat will cause the pump's components to expand, pumps should be equipped with "hot clearance" rotors, an increase to four or five thousandths of an inch. The clearance will tighten up as the metals expand. This design makes the rotary PD pump an ideal selection for high heat applications.

Making a Reaction

Denaturants, acids, caustics and catalysts play a vital role in biofuel production. These chemicals are added to the liquification tanks or pipelines to help facilitate a reaction, adjust pH and-in the case of ethanol-make the end product unfit for human consumption. Precision is important to ensure optimal fermentation results. Some of these chemicals are considered hazardous or environmentally unsafe and need to be monitored closely to seal off the process from fugitive emissions.

Companies will often add catalysts to the pipeline with a centrifugal pump. This process can be extremely tedious since it is necessary to have a control loop to monitor the rate of the pump. Fluctuations in discharge pressure will affect the output, making it difficult to provide a steady, accurate flow. Gear or rotary PD pumps have the tendency to slip with less viscous fluids, so they are not recommended for these applications.

Due to their precise metering accuracy, reciprocating hydraulic diaphragm or packed plunger pumps should be used for process applications that include pH adjustment of slurries, and for adding catalysts and denaturants.

For most ethanol production, companies recycle the feedwater after it has been precipitated out. Any possible effluent into the ground water needs to be Ph neutralized, a secondary application for acid and caustic additions. Though slightly less accurate, mechanically actuated diaphragm pumps are a good low cost pump to use in this application.

Conclusion

The number of biofuel production plants is increasing significantly and the popularity of alternative fuels will likely continue to grow for many years to come. For new energy plants to be successful and competitive, selecting the right pumps in the pipeline is paramount. By following these tips and making the appropriate pump selections, biofuel facilities will be rewarded with enhanced production, reduced maintenance costs and lowered power consumption.

References

1. American Coalition for Ethanol, http://www.ethanol.org/
2. U.S. Dept. of Energy, U.S. EPA
3. Brevard Biodiesel, http://www.brevardbiodiesel.org/