The metering of enzymes can include alphaamylase of glucoamylase or acid or causticfor pH adjustment.

BIOFUEL BOOM: Peristaltic Pumps Find Home in Ethanol Production

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

Editor's Note: Despite their many advantages, peristaltic pumps representa modest percentage of the positive displacement market in the U.S.This is primarily because they're relatively new in the U.S. However, asplant managers are pressured to reduce the life cycle costs of pumps,their benefits are becoming more widely known, making peristaltic hoseand tubing pumps the fastest growing pumps in North America. Here's alook 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 asample of the difficult chemicals and materials pumped throughout the biofuels industry. They are also thematerials 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 tometer or transfer aggressive chemicals or abrasive slurries, engineers need to juggle the following threecomplex 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 lifecycle costs and driving gains in process efficiency. Over the past 50 years, peristaltic pumping technologieshave become the fastest growing segment of the pumping market. Whether it's for the handling of abrasiveslurries, such as spent yeast, lime slurry, or mash stillage, or for the precise metering of enzymes, such asalpha amylase of glucoamylase or acid or caustic for pH adjustment, peristaltic pumps deliver a significantadvantage from both a performance and financial perspective. Before considering the issues inherent topumping applications in biofuel production, it is beneficial to recognize and understand the differences in thevarious types of pumps.

Positive Displacement Pumps

Unlike constant-speed solids-handling centrifugal pumps, whichpredominantly find use in transferring fluids, positive displacementpumps (PD pumps) were created to meter or transfer hard-to-handlefluids such as corrosive, viscous, shear sensitive, or abrasive slurries atvarious speeds without pressure-induced flow drop. Within the realm ofPD pumps, there are reciprocating and rotary pump technologies:diaphragm pumps, which are the most commonly used reciprocatingpumps, and progressive cavity pumps, which are commonly usedrotating pumps. Other PD technologies include gear pumps, pistonpumps, and rotary lobe pumps. Hose and tubing pumps are aclassification of peristaltic rotary-style PD pumps that take their namefrom the biological process of peristalsis: muscular contractions thatmove mixed phase fluids (solids, liquids, gases) throughout the digestivesystem.

Although PD pump technologies differ, all positive displacement pumpsincorporate moving parts that come into contact with the material beingprocessed — a reality that is critical to life cycle costs when pumping anabrasive fluid. For example, diaphragm pumps (typically air-operated orelectro-hydraulic) use a reciprocating diaphragm to induce flow betweentwo internal ball check valves. Abrasive fluids inevitably cause erosion orclogging of the valves, requiring frequent rebuilds of the pump's wettedend. Progressive cavity pumps move fluid along the successive cavitiesformed between the meshing of a fixed stator and rotating rotor.Erosion from abrasive fluids widens clearances between the rotor andstator, causing internal slip that requires the user to speed up thepump in order to maintain capacity, which in turn accelerates wear until the rotor and stator need to bereplaced — normally four stators for every rotor replacement. With many PD pumps, abrasive fluids can causeproblems beyond the normal wetted end of the pump. For example, on a progressive cavity pump, it is only amatter of time until universal joint seals (gear or pin-type) fail, allowing abrasive slurry to erode the numerousparts 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 repairinga progressive cavity pump, for instance, it is necessary to disassemble the entire apparatus and replace thestator and, sometimes, the rotor — a repair cost that often represents more than 75 percent of the initialpurchase price of the pump. This does not take into account the lack of productivity that results fromsignificant downtime since the complexity of a progressive cavity pump normally requires the pump to beremoved from the installation site for service in a maintenance shop.

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In lime slurry applications, hose pumps arefully reversible and self-priming.

Peristaltic Pumps

Peristaltic pumps are unique because there are no seals, valves, ormoving parts in the product stream. The pump's operation is elegantlysimple: a hose or tubing element positioned along a stationary pumphousing is compressed from the outside by shoes or spring-loadedrollers that are mounted to a rotor. Fluid is pushed toward thedischarge as the rotor or roller slides the shoes along the hose ortubing element, while the restitution of the hose element behind theshoe allows more fluid to be drawn into the pump. This design meansthe fluid is completely contained within the hose or tubing element; therotor remains outside the pumpage zone and never actually touches theproduct being moved. The complete closure of the hose element givesthe pump its positive displacement action, preventing flow drop orerosion from backflow and also eliminating the need for check-valves.

A hose or tubing element has a serviceable life before fatigue requiresreplacement — it is predominantly dependent on the pump speed and compression forces on the hoseelement but not influenced by the abrasiveness of the fluid that is pumped. Peristaltic pumps can deliver flowsup 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 utilizeadjustable shoes to set the perfect compression force for specific process conditions. Such steps optimize hoselongevity, 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 peristalticpumps offer the lowest cost of ownership in abrasive and corrosive applications. Although the initial capitalcost of a peristaltic pump can be higher than other positive displacement pumps, the subsequent costsassociated with repair, downtime, and ancillary items quickly tip the life cycle cost calculation in favor of theperistaltic pump. Hose element replacement on even the largest hose pump model takes about one hour andis performed at the installation site. To replace a hose element, simply remove the flanges from the pump andjog the motor to expel the old hose and feed in a new one. Replacement hose elements costs areapproximately 5 percent of the initial pump price. In comparison, progressive cavity pumps with wetted endreplacement 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 pumpin 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 pumpmay 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 theentire 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 isessential. Also key is the metering of the correct amount of alpha amylase enzyme and glucoamylase enzyme.

them ideal candidates for these critical metering applications. In slurry applications including pumping limeslurry or stillage, hose pumps are fully reversible and self-priming plus they can run dry without damage. Thestillage and lime typically contain a high amount of abrasive solids that, as mentioned earlier, can be difficult topump with other pump types. Utilizing hose pumps to move stillage out of the fermenter tanks and on to thecentrifuges for additional processes is an ideal solution. These pumps also add the benefit of being able toblow 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 makethem naturally low shear and perfect for enzyme addition, yeast, starch, and polymers. In addition, the abilityto 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 easyto 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.

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