BIOFUEL BOOM: Peristaltic Pumps Find Home in Ethanol Production

There are many critical fluid applications in the production of biofuels for which these pumps are ideal

Editor's Note: Despite their many advantages, peristaltic pumps represent a modest percentage of the positive displacement market in the U.S. This is primarily because they're relatively new in the U.S. However, as plant managers are pressured to reduce the life cycle costs of pumps, their benefits are becoming more widely known, making peristaltic hose and tubing pumps the fastest growing pumps in North America. Here's a look at how these pumps can be used in ethanol production.

By Chuck Treutel

Sulfuric acid, lime slurry, ethanol, methanol, glycerin, spent yeast, and waste slurries — these are just a sample of the difficult chemicals and materials pumped throughout the biofuels industry. They are also the materials that can shut down processes due to pump failures.

When faced with the challenge of installing a mechanical pump in the heart of a biofuels production process to meter or transfer aggressive chemicals or abrasive slurries, engineers need to juggle the following three complex problems.

1. Finding a pump that withstands abrasive and aggressive chemicals and runs reliably.

2. Finding a pump that accurately meters to optimize chemical usage and product yield.

3. Finding a pump that is quick and simple to maintain and operate.

More and more process engineers are turning to peristaltic pumps to solve all three problems, reducing life cycle costs and driving gains in process efficiency. Over the past 50 years, peristaltic pumping technologies have become the fastest growing segment of the pumping market. Whether it's for the handling of abrasive slurries, such as spent yeast, lime slurry, or mash stillage, or for the precise metering of enzymes, such as alpha amylase of glucoamylase or acid or caustic for pH adjustment, peristaltic pumps deliver a significant advantage from both a performance and financial perspective. Before considering the issues inherent to pumping applications in biofuel production, it is beneficial to recognize and understand the differences in the various types of pumps.

Positive Displacement Pumps

Unlike constant-speed solids-handling centrifugal pumps, which predominantly find use in transferring fluids, positive displacement pumps (PD pumps) were created to meter or transfer hard-to-handle fluids such as corrosive, viscous, shear sensitive, or abrasive slurries at various speeds without pressure-induced flow drop. Within the realm of PD pumps, there are reciprocating and rotary pump technologies: diaphragm pumps, which are the most commonly used reciprocating pumps, and progressive cavity pumps, which are commonly used rotating pumps. Other PD technologies include gear pumps, piston pumps, and rotary lobe pumps. Hose and tubing pumps are a classification of peristaltic rotary-style PD pumps that take their name from the biological process of peristalsis: muscular contractions that move mixed phase fluids (solids, liquids, gases) throughout the digestive system.

Although PD pump technologies differ, all positive displacement pumps incorporate moving parts that come into contact with the material being processed — a reality that is critical to life cycle costs when pumping an abrasive fluid. For example, diaphragm pumps (typically air-operated or electro-hydraulic) use a reciprocating diaphragm to induce flow between two internal ball check valves. Abrasive fluids inevitably cause erosion or clogging of the valves, requiring frequent rebuilds of the pump's wetted end. Progressive cavity pumps move fluid along the successive cavities formed between the meshing of a fixed stator and rotating rotor. Erosion from abrasive fluids widens clearances between the rotor and stator, causing internal slip that requires the user to speed up the pump in order to maintain capacity, which in turn accelerates wear until the rotor and stator need to be replaced — normally four stators for every rotor replacement. With many PD pumps, abrasive fluids can cause problems beyond the normal wetted end of the pump. For example, on a progressive cavity pump, it is only a matter of time until universal joint seals (gear or pin-type) fail, allowing abrasive slurry to erode the numerous parts within each joint, including the ends of the connecting rod.

The negative ramifications of wear on common PD pumps from abrasion cannot be overstated. When repairing a progressive cavity pump, for instance, it is necessary to disassemble the entire apparatus and replace the stator and, sometimes, the rotor — a repair cost that often represents more than 75 percent of the initial purchase price of the pump. This does not take into account the lack of productivity that results from significant downtime since the complexity of a progressive cavity pump normally requires the pump to be removed from the installation site for service in a maintenance shop.

Peristaltic Pumps

Peristaltic pumps are unique because there are no seals, valves, or moving parts in the product stream. The pump's operation is elegantly simple: a hose or tubing element positioned along a stationary pump housing is compressed from the outside by shoes or spring-loaded rollers that are mounted to a rotor. Fluid is pushed toward the discharge as the rotor or roller slides the shoes along the hose or tubing element, while the restitution of the hose element behind the shoe allows more fluid to be drawn into the pump. This design means the fluid is completely contained within the hose or tubing element; the rotor remains outside the pumpage zone and never actually touches the product being moved. The complete closure of the hose element gives the pump its positive displacement action, preventing flow drop or erosion from backflow and also eliminating the need for check-valves.

A hose or tubing element has a serviceable life before fatigue requires replacement — it is predominantly dependent on the pump speed and compression forces on the hose element but not influenced by the abrasiveness of the fluid that is pumped. Peristaltic pumps can deliver flows up to 400 gpm against 240 psi discharge pressure and will typically deliver thousands of hours of hose life. Reputable peristaltic pump manufacturers will machine their hoses to maintain tight tolerances and utilize adjustable shoes to set the perfect compression force for specific process conditions. Such steps optimize hose longevity, maintain flow stability over the life of the hose, eliminate the potential for abrasive wear from slip, and ensure repeatable performance from hose to hose.

The stark contrast in maintenance and installation simplicity versus other PD pumps shows why peristaltic pumps offer the lowest cost of ownership in abrasive and corrosive applications. Although the initial capital cost of a peristaltic pump can be higher than other positive displacement pumps, the subsequent costs associated with repair, downtime, and ancillary items quickly tip the life cycle cost calculation in favor of the peristaltic pump. Hose element replacement on even the largest hose pump model takes about one hour and is performed at the installation site. To replace a hose element, simply remove the flanges from the pump and jog the motor to expel the old hose and feed in a new one. Replacement hose elements costs are approximately 5 percent of the initial pump price. In comparison, progressive cavity pumps with wetted end replacement parts are not only laborious to replace but also cost 75 percent of the pump's original price.

Peristaltic pumps also do not require the ancillary equipment commonly used with a progressive cavity pump in abrasive applications such as double mechanical seals, seal water flush systems, run dry protection systems — hose pumps can run dry without damage— and in-line check valves. For ancillary equipment, a hose pump may require a pulsation dampener in installations with long pipe runs and very high fluid velocities; however, pulsation is normally eliminated without a dampener through minor pipe changes or the use of flexible lines.

Biofuel Production Applications

There are myriad critical fluid applications in the production of biofuels for which peristaltic pumps are ideal. Because they have a non-slip positive displacement design, they give repeatable flow per revolution along the entire speed range regardless of discharge pressure. This makes them inherently excellent metering pumps.

In ethanol production, properly controlling the pH of the hot slurry phase and secondary liquification stage is essential. Also key is the metering of the correct amount of alpha amylase enzyme and glucoamylase enzyme. Peristaltic pumps provide repeatable, accurate metering to within ۪.5 percent, and the ease of maintenance makes them ideal candidates for these critical metering applications. In slurry applications including pumping lime slurry or stillage, hose pumps are fully reversible and self-priming plus they can run dry without damage. The stillage and lime typically contain a high amount of abrasive solids that, as mentioned earlier, can be difficult to pump with other pump types. Utilizing hose pumps to move stillage out of the fermenter tanks and on to the centrifuges for additional processes is an ideal solution. These pumps also add the benefit of being able to blow out blockages or drain process lines, ridding them of high settling solids between batch runs.

Peristaltic pump usage is not limited to abrasive applications. The low operating speeds of these pumps make them naturally low shear and perfect for enzyme addition, yeast, starch, and polymers. In addition, the ability to pump mixed phase fluids efficiently and to run dry makes hose pumps ideal for draining tanks or pumping "off-gassing" fluids such as sodium hypochlorite. Corrosive and caustic fluids used in pH control are also easy to handle because there is no metallic contact — the fluid is contained within the hose or tube element.

Chuck Treutel is marketing manager at Watson-Marlow Bredel Pumps. Additional information is available at www.watson-marlow.com.