Tiny reactors aim for big role

The heart of chemical processing has always been the reactor, with the Continuous Stirred Tank Reactor (CSTR) long dominating continuous production. The CSTR was first used more than 300 years ago in the processing of gold ore, but maybe its time is up. PI is entering the scene in a big way; a number of new specialty reactors have appeared, bringing both technological advances and additional manufacturers into the market.

The term “Process Intensification” was originally coined at Imperial Chemical Industries in the U.K. in the 1970s. Put simply, PI involves the miniaturization of unit operations. This miniaturization should bring:

• reduced energy use;
• decreased capital expenditure;
• lower plant profile (height);
• smaller plant footprint (area);
• environmental advantages; and
• safety benefits.

When building a new plant, process equipment typically represents approximately 20% of the capital costs, with structural steel, piping, conduit, wire and instrumentation accounting for much of the balance. Smaller unit operations made possible by PI translate into a more-compact plant, lower weight, and less structural steel, piping, conduit and wire. The reduced weight of the equipment may even allow savings on concrete foundations. Overall, PI means less-expensive plants with smaller footprints. In addition, many process-intensified plants are amenable to construction on skids, which can lower costs even further.

Decreased costs aren’t enough, though, to guarantee acceptance of units so different from conventional ones. With reactors typically considered the heart of the plant, companies also want increased reactor performance. Here, the new PI reactors provide a number of advantages. They cut residence times, boost reaction rates, minimize side reactions, and reduce energy-intensive downstream processing steps such as distillation and extraction. In addition, the units can dramatically decrease the volumes of explosive, hazardous or toxic compounds in the process.

With many reactions, heat-transfer, mass-transfer or mixing limitations control the reaction rate rather than the fundamental kinetics, explains Protensive, Newcastle upon Tyne, U.K., a developer of PI units. An exothermic reaction may require a couple of hours to carry out in a batch reactor not because of any kinetic constraint, but because of the time necessary to remove the heat of reaction, adds the company. PI reactors offer a way to overcome such limitations.

Now let’s look at five commercially available PI reaction systems to see how they work and the benefits they offer. We’ll also touch upon two established PI technologies — reactive distillation and static mixing.

Spinning tube

Kreido Laboratories, Camarillo, Calif., offers the Spinning Tube in a Tube (STT) reactor. This unit induces so-called Couette Flow by mixing reactants in a narrow annular gap between a stationary stator and a rapidly rotating, concentric, internal rotor (Figure 1) so that the reactants move as a coherent thin film in a high shear field, says the company.

Figure 1. This design produces high shear along entire length of tube, accelerating reaction rate.

This very high shear field extends over the total length of the tube. Flow through the annular space is actually in the laminar range. This unit is very compact (Figure 2) and can easily perform gas/liquid and liquid/liquid reactions.

Figure 2. Compact unit can handle liquid/liquid and liquid/gas reactions.

The STT reactor accelerates the rates of chemical reactions by up to three orders of magnitude, increases conversions and yields, controls the quality of production in real-time, lowers costs, and dramatically decreases the time required for manufacturing scale-up, claims the company.