Peristaltic Pumps Provide Option for Ethanol Producers

By Chuck Treutel | August 27, 2007
Sulfuric acid, ethanol, spent yeast and waste slurries are just samples of the difficult chemicals and materials pumped throughout an ethanol plant. They're also the materials that can shut down processes due to pump failure.

Numerous pumps are used to meter or transfer aggressive chemicals or abrasive slurries throughout the ethanol production process. When faced with the challenge of installing a mechanical pump in the heart of the process, engineers juggle three complex problems. The first problem is finding a pump that can run reliably while withstanding abrasive and aggressive chemicals. The second is finding a pump that meters accurately, optimizing chemical usage and product yield. The third issue is finding a pump that is quick and simple to maintain and operate. 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 pump technologies have become the fastest growing segment of the pumping market. Whether it's handling abrasive slurries like spent yeast and mash stillage, or precisely metering alpha amylase and glucoamylase enzymes or caustic for pH adjustment, peristaltic pumps deliver a significant advantage from a performance and financial perspective. Before the issues inherent to pumping in ethanol production are considered, it is beneficial to recognize and understand the differences in the various types of pumps.

An Overview of Positive Displacement Pumps
Unlike constant-speed, solids-handling centrifugal pumps, which are predominantly used in transferring fluids, positive displacement (PD) pumps were created to meter or transfer hard-to-handle fluids like corrosive, viscous, shear-sensitive or abrasive slurries at various speeds without a pressure-induced drop in flow. PD pumps include reciprocating and rotary pump technologies. Diaphragm pumps are the most common reciprocating pumps, and progressive cavity pumps are the most common rotating pumps. Other PD technologies include gear, piston 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 and gases) throughout the digestive system.

Although PD pump technologies differ, all PD pumps incorporate moving parts that come in 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 or electro-hydraulic operated) 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 stator and 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. This accelerates wear until the rotor and stator need to be replaced. Four stators are normally required for every rotor replacement. With many PD pumps, abrasive fluids can cause problems beyond the wetted end of the pump. For example, on a progressive cavity pump, it is only a matter of time until universal joint (gear- or pin-type) seals fail, allowing abrasive slurry to erode the numerous parts within each joint, including the ends of the connecting rod.

The ramifications of abrasion wear on common PD pumps cannot be overstated. For instance, when repairing a progressive cavity pump it is necessary to disassemble the entire apparatus, replace the stator and sometimes the rotor—a repair cost that often represents over 75 percent of the initial purchase price of the pump. This doesn't include the lack of productivity that results from significant downtime, since the complexity of a progressive cavity pump normally requires being serviced in a maintenance shop.

The Peristaltic Pump Difference
Peristaltic pumps are unique in that 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 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. 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 pumping zone and never touches the product. The complete closure of the hose element gives the pump its PD action, preventing flow drop or erosion from backflow and eliminating the need for check valves.

A hose or tubing element has a serviceable life before fatigue requires replacement. Its life is dependent on the pump speed and compression forces on the hose element, but not by the abrasiveness of the fluid that is pumped. Peristaltic pumps can deliver flows up to 400 gallons per minute against 240 pounds per square inch 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 slippage 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. While the initial capital cost of a peristaltic pump can be higher than other PD 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 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 element costs are approximately 5 percent of the initial pump 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 and in-line check valves. 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 liquefaction 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 0.5 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, self-priming and can run dry without damage. The stillage and lime typically contain a high amount of abrasive solids that can be difficult to pump with other pump types. Utilizing hose pumps to move stillage out of the fermenter tanks and 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. Their low operating speeds make them naturally low shear and perfect for the addition of enzymes and yeast. The ability to run dry and pump mixed-phase fluids efficiently make 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.

Despite all of these advantages, peristaltic pumps represent a modest percentage of the PD market in the United States. This is primarily because peristaltic technology is relatively new in the U.S. market. However, with the pressure on plant managers to reduce life-cycle costs of their pumps, the functionality and benefits of peristaltic pumps are becoming more widely known, making peristaltic hose and tubing pumps the fastest growing pump type in North America.

Chuck Treutel is the marketing manager for Watson-Marlow Bredel Pumps. He is a registered professional engineer in Wisconsin and has been engineering, selling and marketing Watson-Marlow Bredel pumps for the past 20 years. Reach Treutel at [email protected] or (800) 282-8823.

The claims and statements made in this article belong exclusively to the author(s) and do not necessarily reflect the views of Ethanol Producer Magazine or its advertisers. All questions pertaining to this article should be directed to the author(s).