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Chemical Scroll Pumps Are Ideal for Lab Applications.

Chemical scroll pumps are superior to oil-sealed rotary vane pumps and other dry pumps for pumping the condensable vapors of water, organic solvents, and corrosive gases.

Laboratory applications, such as gel drying, freeze drying, rotary evaporation, and ultracentrifugation, have traditionally been carded out using oil-sealed rotary vane pumps. To protect the pumps from organic-solvent vapors or aggressive chemicals (such as acids and alkalis.), accessories like cold traps, condensers, chemical traps, oil circulation systems, and oil mist filters have been used. Because these accessories do not completely capture the vapors, solvents and corrosive chemicals can accumulate in the pump, leading to performance problems, such as poor vacuum, oil degradation, increased pump wear, frequent maintenance of the pump and its accessories, increased chemical waste, and numerous pump rebuilds.

Chemical scroll pumps, by contrast, provide an effective way of pumping the vapors of condensable and reactive chemicals used in common laboratory applications. Because chemical scroll pumps use no vacuum-side oil or grease, chemical reactions between process gases and lubrications are eliminated. In addition, by using air or inert-gas ballast purging, concentrations of condensable components in process gases can be controlled to levels that prevent condensation in the pumping mechanism. For these and other reasons, the chemical scroll-pumping mechanism is a promising new technology that can provide more consistent vacuum performance, higher vapor pumping capacities, and greater chemical resistance to the organic solvents and aggressive compounds found in laboratory applications.

The new XDS-C chemical scroll pump from BOC Edwards, Wilmington, Mass. (800-848-9800), is not to be confused with a conventional scroll pump, although they both rely on the same basic pumping mechanism. Scroll pumps consist of an orbiting scroll that meshes with one or more stationary scrolls. The movement of the orbiting scroll traps gas molecules entering through the inlet of the pump in successive crescent-shaped volumes that shrink in size. Gas is compressed as it's swept toward the exhaust opening at the center of the pump.

The inlet of the XDS-C chemical scroll pump is situated away from the periphery of the orbiting scroll. This eliminates particle rejection through the pump inlet. Unlike a conventional scroll pump, the motor shaft of a chemical scroll pump does not penetrate the scroll mechanism, so there is no need for shaft seals. This eliminates air leaks inside the pump and chemical or gas leaks from the pump.

The scroll of the XDS-C uses a proprietary, single-piece tip-seal material that floats on top of the scroll wall, thereby accommodating wear while maintaining a seal against the opposing scroll channel. Unlike a conventional scroll pump, the bearings of the XDS-C are sealed inside a bearing shield and maintained at atmospheric pressure. The bearings are thus isolated from the process gases in the pump so that bearing lubrication is not subject to attack by the process gases and condensed chemicals.

A gas ballast purge can be admitted to the final stage of a chemical scroll pump. This purge can be used to dilute process gases to prevent condensation of process vapors in the pump. It can also be used to introduce an inert gas to reduce the concentration of a flammable gas component, thereby eliminating the chances of an explosion. If there are toxic reactants or byproducts in the process gas, gas ballast purging can reduce concentrations below threshold limit values. A gas ballast purge can also be used to prevent back-migration of reactive chemicals and moisture to the pump. This is critical when the process gases are treated by wet scrubber systems.

Process gases cool down in the exhaust of the pump, which may lead to the condensation of some vapors in the exhaust port. For this reason, the exhaust port of the XDS-C is angled downwards to prevent condensate from returning to the pump. The condensed chemicals can be recovered by using a trap after the exhaust port.

Because of the absence of vacuum-pump oil, the condensation pressures of vapors passing through a chemical scroll pump are much higher than in an oil-sealed rotary vane pump (OSRVP). This means that condensable components remain in the gas phase even when partial pressures are two orders of magnitude higher than those in OSRVPs. This gives the chemical scroll pump a tremendous advantage in pumping condensable vapors. In the first place, by keeping the chemicals in the vapor phase, the corrosion reaction rates can be slowed by a few orders of magnitude. This is because the corrosion of the metal surfaces and elastomer seals of a pump occurs primarily when chemicals are condensed inside the pump, as happens with OSRVPs.

Secondly, since there is no accumulation of chemicals in a chemical scroll pump, ionization of the chemicals being pumped does not occur. This further reduces the extent of corrosion. These factors, together with the fact that the XDS-C is constructed of chemically resistant materials, make the chemical scroll pump a better choice than OSRVPs for handling corrosive chemicals.

Although other dry pumps have no vacuum-side pump oil, grease lubricant at the bearings can capture the condensable vapors and corrosive chemicals. This can cause materials to accumulate in the bearing region, leading to bearing failures.

Gel drying is used after separating biological macromolecules, such as proteins, DNA, RNA, and enzymes in a gel medium like agarose or polyacrylamide. In evaluations of chemical scroll pumps, a polyacrylamide gel (approximately 20 x 18 x 0.07 cm) containing 10% acetic acid and 25% ethanol was dried using a 6.5-cfm chemical scroll pump. The total gel-drying time was about 40 min. The highest pressure during the process was 545 torr. At the end of the gel-drying cycle, the pressure dropped to 80 torr--corresponding to air intake or a leak in the gel dryer. At the highest gel-drying pressure, the solvent evaporation rate was estimated to be about 4 gmol/min. At 30 [degrees] C, the solvent evaporation rate was about 25% lower (3 gmol/min).

During the gel drying of the acrylamide gels, there was no visible condensation at the exhaust port of the chemical scroll pump. The pump's ultimate pressure was checked immediately after the gel-drying cycle, and found to be about the same as the ultimate pump pressure before the gel-drying cycle. This reconfirmed the absence of solvent condensation in the pump. Thus, at the chemical scroll-pump operating conditions (temperature and pressure) and at the gas ballast purge setting of 3 slm of air, the solvent vapors were in sub-saturated conditions, preventing the condensation and accumulation of the solvents in the pump. The chemical scroll pump was used on the gel dryer for about 25 gel-drying cycles. There was no significant change in the ultimate pressure of the chemical scroll pump, and no evidence of any corrosion at the pump inlet, outlet ports, or elastomers.

One of the pump issues in ultracentrifugation involves the breakage of tubes containing the media solution (water, glycerin, sucrose, or cesium chloride). This often leads to a considerable amount of solution being splashed in the centrifuge chamber. A slug of sample solution enters the pump, degrading the vacuum instantaneously. This is a particularly serious problem when the centrifugation protocol calls for a long centrifugation time (1-2 days) during which centrifuge operation is generally unsupervised. Experiments were done to determine the recovery of a chemical scroll pump after spills of various gradient media to the pump.

The chemical scroll pump was operated without the gas ballast purge prior to the introduction of a liquid slug in the system to simulate normal ultracentrifuge system operation. It was found that upon introducing slugs of water or a 10% glycerin or 20% sucrose solution, the pressure increased instantaneously to as high as 100 torr. After about one minute, the gas ballast purge of the pump was turned on to start a flow of about 15 slm of air. It took about five minutes for the pump inlet pressure to drop to 1 torr. The chemical pump's ultimate pressure was reached within 30 to 60 sec.

After passing about 10 slugs of water and glycerin of volumes ranging from 5 to 50 ml, the pump's ultimate pressure was still recovered. There was no appreciable change in the pump's noise and motor current, implying that there was no change in the pumping load due to material accumulation in the pump. If a media spill occurred with an OSRVP, its pumping oil would need to be changed immediately to avoid oil degradation, pump heating, and pump rotor and stator wear. When a sucrose media solution is used, transfer of the sucrose solution to an OSRVP can result in a pump seizure.

In experiments with a 20% sucrose solution, the pump maintained the ultimate pressure even after five runs. The pump current increased by about 10% after passing six slugs of sucrose solution through the chemical pump, with a cumulative sucrose solution volume of 100 mL. There was a slight increase in pump noise. OSRVPs have failed with a single slug of sucrose solution passing through them. Failure analysis indicated the presence of a highly viscous liquid in the OSRVPs.

The chemical scroll pump has shown a much higher resistance to the slugs of solution used in ultracentrifuges, with practically no maintenance required upon transport of ultracentrifuge media solutions of water and glycerin. Additionally, the chemical scroll pump can withstand several slugs of sucrose solution before exhibiting an increase in the load on the motor with a consequent increase in pump noise.

Freeze drying involves removing water from a product by sublimation and desorption. A laboratory freeze dryer consists of a manifold having ports for flasks containing frozen solutions, a condenser or cold head to trap water removed from the products, a vacuum pump, and a cooling system to supply refrigerant to the cold head. The temperature of the cold head in the condenser can range from -40 to - 100 [degrees] C.

A typical freeze-drying cycle proceeds in two steps: primary drying and secondary drying. In the primary drying phase, the chamber pressure is reduced and frozen solution (from loose or unbound solvent) sublimes. The solvent vapor condenses on the surface of the cold head. The solvent that is not captured by the cold head passes to the pump. Primary drying involves the highest solvent load. The vapor load is higher when volatile solvents are used in the solutions. In the secondary drying step, there is desorption of bound water from the sample. Secondary drying has a much lower solvent vapor load, and is usually performed at pressures close to ultimate vacuum of the freeze-drying system.

As discussed earlier, due to the absence of pump oil and lubrication greases, the condensation pressures of solvents in chemical scroll pumps are a few orders of magnitude higher than those in OSRVPs. This translates to a high solvent pumping capacity for chemical scroll pumps as compared to OSRVPs. The pumping capacity is particularly beneficial in applications, such as freeze drying and rotary evaporation, that involve pumping vapors of organic solvents that are soluble in pump oil. The amount of condensable vapors the vacuum pump must deal with varies with the temperature of the cold head and vapor pressure and concentration of various chemicals in the frozen solution. This solvent causes the oil and seals of an OSRVP to change progressively, even under normal operation. Under failure conditions, such as low or no refrigerant flow to the cold head, the chemicals captured by the cold head can melt and vaporize, leading to the transfer of a significant amount of solvent to the OSRVP as a vapor and in some cases as a liquid. This can cause the pump to fail and may also create unsafe conditions if the solvent is flammable.

There are several dry vacuum pumping mechanisms available that vary in their pumping capacities, ultimate vacuums, condensable vapor-pumping capacities, and extent of contamination generated by the pumps. Industrial dry pumps based on Roots, claw, or screw mechanisms have capacities of more than 25 cfm and typically require utilities, such as water and nitrogen, for normal operation. These pumps are overkill for laboratory applications. Diaphragm pumps, on other hand, do not meet the requirements of many laboratory applications because of their vacuum levels or lower pumping capacities. Multiple diaphragm pumps are required to increase the pumping capacity or improve the vacuum level. Dry rotary vane pumps are not suitable for pumping condensable vapors, and are used only to pump clean, dry gas loads.

As discussed earlier, the conventional scroll pump is only suitable for clean applications not involving condensable and corrosive vapors. Only the chemical scroll pump meets the pumping-capacity, ultimate-vacuum, condensable vapor-pumping, and chemical-resistance requirements of laboratory applications.

Yau is with the department of biological chemistry at the Univ. of California, Davis, and Patil is with BOC Edwards Vacuum Technology, Santa Clara, Calif.
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Comment:Chemical Scroll Pumps Are Ideal for Lab Applications.
Author:Yau, Peter M.; Patil, Atul N.
Publication:R & D
Geographic Code:1USA
Date:Jun 1, 2000
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