The HTI membrane is a rugged composite. An ultrathin (10 Âµ) FO layer is sandwiched with a microfilter and a fabric backing, which provides strength. With pores measuring 5 Ã (0.0005 Âµ) in diameter, the FO layer successfully filters E. coli bacteria, anthrax spores, viruses, heavy metals, suspended particles and other health threats.
Forward osmosis is normally a very slow process since it is not driven by hydraulic pressure. The HTI membrane allows water to pass at a much higher rate (as high as 1 L/hr in some of the personal hydration systems) because the FO layer is so thin. Also, since it is made from cotton-derived, cellulose-ester plastics, the FO layer is extremely hydrophilic. This allows water to pass through easily, accelerating the filtration process.
Hydration Technologies&rsquo X-Pack is the first FO filter to be commercialized. The success of the membrane is due to the combination of ultra-small and highly consistent porosity with a high pass-through rate.
The rotor/stator agitator on the triple-shaft mixer creates more intense shear than that created by a dual-shaft mixer. It also provides more versatility. Each agitator is independently controlled, and each offers a range of capabilities that complements the other two agitators, which significantly extends the overall range of the mixer. (See sidebar: &ldquoTriple-Shaft Mixer Anatomy.&rdquo)
Figure 2. Water spontaneously diffuses into HTI's X-pack
when a generic sport-drink powder is in contact with the
inner, clean side of the membrane. It requires no energy
and no moving parts.
Ross&rsquo triple-shaft mixer is a closed-batch system; most applications include vacuum and thermal control with hot oil, water or steam circulating in a jacket surrounding the mix vessel. The vacuum proved unnecessary for HTI&rsquos application, but thermal control was important because the cellulose acetate polymer is heat-sensitive. When it overheats, it melts and forms a smeary plastic. Since the system is closed, very little solvent was lost due to evaporation during the mixing cycle.
Triple-Shaft Mixer Anatomy
The Ross VersaMix is a triple-shaft mixer that includes a slow-speed anchor agitator and two high-speed devices: a rotor/stator high-shear mixer and a high-speed disperser (Figure 3). The anchor sweeps the vessel wall and bottom, constantly removing material that would otherwise impede heat transfer. As it turns, it also stimulates radial and axial mass flow to promote thorough batch turnover.
The rotor/stator high-shear mixer applies intense shear and generates moderate flow. The high-speed disperser, shown here with a sawtooth blade, applies moderate shear and generates substantially greater flow. All three agitators are independently controlled with electronic variable-speed drives and monitored with instrumentation.
For demanding applications, the triple-shaft mixer offers a broad range of capabilities. This includes the ability to apply extremely intense shear with the rotor/stator mixer, while maintaining vigorous flow with the other two agitators.
With independent controls, the two high-speed agitators are often used selectively during a series of mixing phases. This allows the operator to choose the right combination of agitators and tip speeds for each phase, depending on key process variables, such as shear requirements, heat thresholds and batch viscosity.
|Figure 3: This unit combines a three-wing anchor with scraper, a two-blade, high-speed disperser, and a rotor/stator high-shear mixer. It also features a dished cover with angled ports for adding solvent.|
Success tastes great
The triple-shaft mixer quickly breaks down the cellulose acetate polymer and disperses it into the casting solution, along with the other ingredients.
The rotor/stator mixer is especially well-suited for the first phase of the HTI application. The process takes a long time in a dual-shaft mixer because polymer pellets bounce off the teeth of a high-speed disperser blade. The rotor/stator mixer, on the other hand, draws them into the high-shear zone and doesn&rsquot let them escape until they are broken apart. Meanwhile, the disperser and anchor agitators maintain energetic flow, which ensures a high degree of batch homogeneity.
Also vital to the HTI application, vigorous flow helps carry heat away from the high-shear zone and disperse it evenly throughout the vessel. This helps protect the heat-sensitive polymer while allowing for an increased shear rate.
The rotor/stator mixer can be fitted with a variety of stators, which provides a great deal of flexibility in matching the mixer configuration to the application (April, p. 31).
&ldquoWith a dual-shaft mixer, the initial phase produced âgel balls&rsquo that resisted wetting out and dispersion,&rdquo Peterson says. &ldquoBut with the triple-shaft mixer -- thanks to the intense shear that the rotor/stator mixer applies -- the gel balls dissipate very quickly. At that point in the cycle, we turn off the high-shear mixer and proceed with only the high-speed disperser and the anchor agitator.&rdquo The batch was complete in two hours, compared with the six-hour cycle that a dual-shaft mixer required.
Today, HTI operates with a 40-gal. VersaMix in full-scale production, while a 2-gal. model supports a vigorous R&D program. Anticipating the need for multiple process lines before long, the company plans to follow up its success on the battlefield with applications in other industries, including homeland security, international relief and development, and the maritime industries.
Doug Cohen is vice president of technical services for Charles Ross & Son Co., Hauppauge, N.Y. E-mail him at firstname.lastname@example.org.