Process intensification encompasses the philosophy of green chemistry: waste reduction, improved safety, reduced energy usage, etc. (Anastas and Warner, 1998). Different processes might have different rate-controlling steps, for example, mass transfer, mixing, diffusion, etc. Often the controlling step is obvious, sometimes it is completely unknown, and sometimes there are different rate-controlling steps during the course of the reaction. It is of fundamental importance that some understanding of the rate-limiting steps is obtained. Oscillating columns offer moderate residence times with heat transfer coefficients better than the batch vessels. Plate heat exchanger-type reactors (Hex reactor from BHR) are good for clean, high-heat transfer duties. When performed in high-G or microreactors, diffusion-controlled reactions lead to several orders of magnitude increase in the overall rate of reaction. Spinning disc reactors offer very high surface-area-to-volume ratios, which can be used effectively to perform fast polymerization reactions and concentration of temperature-sensitive materials. Continuous reactors, including simple plug-flow pipe units, tubular models containing static or other mixing devices, and various jet devices, have been used to efficiently produce toxic materials for immediate consumption in downstream processing with little or no inventory. Unsafe and highly exothermic reactions could be performed in such designs safely under undiluted conditions.

The unit operations and processes that are modified are reactive distillation, reactive extraction, membrane separations, oscillating flows in reactors, membrane reactions, fuel cells, etc. (Stankiewicz and Moulijn, 2000). Examples of new equipments that are available due to PI include compact heat exchangers, structured packed columns, static mixers, high-G column, spinning...