Tiny reactors aim for big role
Process Intensification (PI) has promised many things but has it fulfilled its promises? When looking at reactor technology, the answer is a definite Yes.
Scaleup involves holding shear constant:
D1πω1/d1 = D2πω2/d2
where D is the diameter of the rotor in millimeters, ω is the revolutions per second of the rotor, and d is the gap millimeters. If d1 = d2 then, for any change in diameter, the new ω can be calculated.
Spinning disk
Protensive makes the Spinning Disk Reactor (SDR), which provides plug flow and intense mixing while resisting fouling. The SDR relies on high centrifugal acceleration over a disk surface to overcome interfacial mass-transfer limitations that thwart conventional processes (Figure 3).
Figure 3. This small Spinning Disk Reactor can serve as a complete mini-development plant.
The generation of very thin films, typically fractions of a millimeter down to a few microns thick, through controlled flow rate and disc speed or RPM can deliver surface-to-volume ratios tailored to processing requirements, ranging from 1,000s of m2/m3 for high viscosity materials such as polymer melts, down to 100,000s m2/m3 for low viscosity systems typical of a wide range of chemical synthesis routes.
The SDR boasts an overall heat transfer coefficient typically five to 10 times greater than achieved by most heat-transfer devices, says the company, enabling small discs with low process fluid inventory to handle significant thermal duties. Figure 4 shows a laboratory unit.
Figure 4. High centrifugal acceleration overcomes conventional interfacial mass-transfer limitations.
Fast exothermic reactions can be conducted in the thin turbulent film on a spinning disc reactor using much higher temperatures than could be contemplated in stirred tanks. This is because the superior heat-transfer performance of the unit carefully controls temperatures, and completes reactions in a residence time of just 12 sec.
The reaction to produce CaCO3 via CO2 absorption is completed within 1 sec., says the company. The high surface-to-volume ratio can be used both to allow rapid transport between gas and liquids for simple operations such as stripping liquids of volatiles, scrubbing gases or for more-complex gas/liquid reactions.
For crystallization and precipitation, the vapor-stripping characteristics of the SDR, arising from the thin turbulent liquid film on the disk surface, combined with the reactors plug flow characteristics, are said to allow excellent control over particle size selection and attainment of a relatively narrow particle-size distribution.
The reactor also reportedly can remove solvents or monomers left trapped in bundled polymer chains after polymer production to very low levels difficult to achieve in traditional equipment even with the use of vacuum and temperature.
Controlled cavitation
Hydro Dynamics, Inc., Rome, Ga., harnesses cavitation in its ShockWave Power Reactor (SPR) to provide increased mass transfer and scale-free heating. Basically, the shockwaves and resulting microscopic bubbles cause intense mixing as well as a cleaning action. Because heating takes place in the material, not by conduction through metal, there are no hot and cold spots.
The heart of the SPR technology is a specialized spinning rotor with cavities. The spinning action generates hydrodynamic cavitation within the cavities away from the metal surfaces. This cavitation is controlled by RPM thus there is no damage to the equipment. Eight different parameters determine optimum hole location, depth, angle, layout, etc. The SPR looks like a pump from a casual observation (Figure 5); however, that is where the similarity ends.
Figure 5. Purposely generated cavitation enhances mass transfer and produces uniform heating.
To see a demonstration of the action of cavitation, including what happens after a small amount of gas has been added, go to http://www.hydrodynamics.com/technology_review.htm.
The SPR can provide: reduced reaction time, uniform temperature with no solid scale build-up, fewer side reactions, and improved yield and quality.
The reactor suits both batch and continuous processes, and can provide up to 150-million-gal/yr processing capacity in a single unit. In addition, the unit can be easily retrofitted into existing operations.
The device already is used in numerous commercial applications, including the mixing of consumer products, food pasteurization/homogenization, gel and gum hydration, scale-free heating of chemicals, and concentration of solvents.
The unit also can serve as a superior gas/liquid mixer, handling gas-to-liquid volume ratios as high as 5 to 1, says the company.
Microchannel reactors
Velocys, Plain City, Ohio, uses microchannel technology to dramatically reduce heat and mass transport distances commonly found in conventional systems, thus increasing the rate of heat and mass transfer and, in turn, greatly accelerating reaction rates. Further, as the efficiency of converting feedstock material to products is strongly governed by the ability to control these chemical reactions which depends upon the ability to control reaction temperature, which in turn is governed by the ability to move heat quickly the technology often can increase product yield.
Velocys chemical processors feature parallel arrays of microchannels, with typical dimensions in the 0.010-in. to 0.200-in. range (Figure 6).
