Figure 6. Microchannels permit use of much more active catalysts, which greatly boosts throughput.
This structure is said to allow use of much more active catalysts than conventional systems, greatly boosting the throughput per unit volume. A catalyst can be tethered to the reaction wall or coated inside the channels. Overall system volumes reportedly can be reduced by 10 to 100 fold compared to conventional hardware. Figure 7 shows a large-scale, prototype microchannel reactor that will begin operating in 2007.
Figure 7. Large-scale demonstration unit is slated for operation in the first quarter of 2007.
Some of the applications under development include:
- hydrogen production using steam methane reforming;
- high intensity oxidation and partial-oxidation reactions with improved process selectivity and yield;
- high-performance emulsification processes; and
- synthetic-fuel production and methanol synthesis in compact units suitable for land or offshore installation.
Steam reforming highlights the power of the approach. About 95% of the hydrogen produced today in the U.S. is made by reforming a methane source such as natural gas using high-temperature (700°C to 1,000°C) steam. Refineries are major producers of hydrogen, using it primarily for their hydrotreaters and hydrocrackers. In the reforming process, methane endothermically reacts with steam under 325-bar pressure in the presence of a catalyst to produce hydrogen, carbon monoxide and a relatively small amount of carbon dioxide.
With the microchannel technology, hot combustion gases push the reaction forward, with the hot gases flowing in channel layers alternating with the reactor channel layers. The hydrogen is then purified in a pressure swing adsorption unit. Plant size is reduced by 90% compared to conventional reformers, and the approach boasts a 30% savings in capital cost, higher thermal efficiency and lower emissions, according to the company.
Scale-up is easily accomplished by adding more layers of channels. Once a plant is in operation, its capacity can be increased simply by installing additional layers of channels.
Cambridge Reactor Design, Cottenham, U.K., offers the Oscillating Flow Reactor, which takes advantage of the companys Oscillating Flow Mixing (OFM) technology (Figure 8).
Figure 8. Baffle geometry coupled with intensity of oscillation produced by pistons control mixing behavior.
OFM combines fluid oscillations with baffle inserts to provide highly effective mixing in tube reactors. Mixing behavior is controlled dynamically by oscillation intensity or geometrically by baffle design. While the technology can be applied to batch operations, it is said to be particularly suited to continuous processing.
The standard reactor consists of an oscillator base and a reactor tube top section (Figure 9). A nutating cam mechanism driven by an electric motor and linear actuator controls the amplitude and frequency of operation. A pair of pistons driven off the two cams provides oscillations in an inverted U arrangement of reactor tubes. All of the variations are achieved by electronic control of the motors.
Figure 9. Standard design includes an oscillator base and a reactor tube top section.
Process tubes are added via top plate extensions, each of which takes two reactor tubes. In this manner, the unit can be operated as four-pass, six-pass, etc., as required to increase reactor volume or residence time.
Typical applications include biodiesel, suspension polymerizations and liquid/liquid dispersions.
PI doesnt necessarily have to involve cutting-edge mechanical developments. Another, long-established form of PI is reactive distillation, which combines a reactor and distillation column. The technique typically is used with reversible, liquid-phase reactions. For many such reactions including esterifications, transesterifications, hydrolyses, acetalizations and aminations byproduct formation limits the amount of product made. Reactive distillation allows removal of the byproduct, thus shifting reaction equilibrium and leading to more product. Other types of reactions that could benefit from reactive distillation include: alkylation/transalkylation/dealkylation, isomerization and chlorination.
Reaction components are fed countercurrent into a distillation column. Then, the product and byproduct can be separated by distillation. Some reactions require placement of catalysts inside the column e.g., via structured packing coated with the appropriate catalyst, trays containing pillows filled with catalyst particles or pillows filled with catalyst particles rolled into bales.
Figure 10 shows an esterification reaction for a high-boiling carboxylic acid being added at the top of the reactive distillation column and a lower-boiling point alcohol being added at the bottom. Byproduct water comes off the top of the column and the product ester comes off the bottom.
Figure 10. This unit produces an ester as bottoms product, while byproduct water goes overhead.
We expect installations using reactive distillation to continue to grow at a moderate pace in this decade.
Another traditional form of PI also garnering increasing interest relies on the use of so-called static or motionless mixers as reactors either as single units or in bundles with a jacket.
Chemical reactions in the laminar fluid-flow range (below a Reynolds number of 2,000) are possible with a Continuous Flow Reactor (CFR) developed by R. C. Costello & Associates (Figure 11).
Figure 11. Internal elements fold streams billions of times, providing intense mixing.
The Model CFR-T consists of a 3/8-in.-diameter Type-316 stainless steel tube with mixing elements that divide the flow at the beginning of each element. Subsequent stretching and folding produces a radial motion of the high-velocity core regions outward toward the wall of the reactor, which has an inside diameter of about 0.20 in. Without the elements, flow clearly would be laminar but with them intense mixing occurs, enabling liquid/liquid and gas/liquid reactions to occur.
A bench-scale device is offered with 34 mixing elements, which means the liquid contents is split and folded 234 (or more than 17 billion) times after passing through the CFR.
For multiple units in series, BHR Group, Cranfield, U.K., offers the FlexReactor, which looks similar to a heat exchanger with static mixers in each tube. The unit can be re-piped with simple U-tube connections to have multiple passes in series or parallel operation. Heating or cooling is easily achieved with the FlexReactor.
A small revolution
The unrelenting pressure for less expensive plants and better process performance favors the increasing use of PI reactors. Prospects for acceptance are improving as the technologies prove themselves more widely in commercial applications and more vendors promote the PI approach. So, its clear that these small units promise to have a big impact.
Rocky Costello is president of R. C. Costello & Associates, Inc., Redondo Beach, Calif. E-mail him at email@example.com.