Imperial Chemical Industries originally coined the term Process Intensification (PI) in the early 1970s for the miniaturization of unit operations. While ICI itself recently disappeared (www.ChemicalProcessing.com/articles/2008/082.html), PI continues to progress.
After all, by using smaller equipment, PI promises to cut both energy use and capital expenditure, decrease plant profile (height) and footprint (area), and provide both environmental and safety benefits.
Earlier (www.ChemicalProcessing.com/articles/2006/176.html), we explored the use of PI for reactors. Now, let’s examine PI’s role in distillation, extraction and heat transfer.Still opportunities
Figure 1. Higee distillation columns dramatically reduce the height of a theoretical stage.
Distillation is the most popular separation method by far but also the most energy intensive unit of operation. Conventional columns are often massive and thus costly to build — with the current price of metals, use of 316L stainless steel, let alone exotic alloys, can make a new tower very expensive. So, unsurprisingly, PI has targeted distillation going all the way back to ICI’s pioneering work that led to today’s Higee column (Figure 1).
The Higee is a high gravity rotating contactor with a compact design. It can induce centrifugal forces of more than 1,000 times that of gravity. Higher “g” through centrifugal acceleration gives enhancements in mass transfer and throughput rates by one to two orders of magnitude and, consequently, drastically reduces column size for the same production objective. A torus-shaped rotor spins at approximately 500 rpm. Vapor traffic flows from the outside to the inside of the rotor while liquid traffic flows from inside to outside the rotor. Protensive, Newcastle upon Tyne, U.K., and GasTran Systems, Cleveland, Ohio, provide this technology, which also is known as the rotating packed bed. Currently there are approximately 30 units in operation worldwide; however, many are experimental columns at various universities. Typically Higee reduces the height of a theoretical stage to five centimeters to 12 centimeters from the 150 centimeters to 600 centimeters in a standard column. Packing can be PTFE-coated aluminum sponge for corrosion resistance or other materials.
Higee installations are often used to strip organics from wastewater; Dow Chemical relies on the technology to make hypochlorous acid. Future installations are expected to include various types of distillation applications and retrofits to existing columns.
Figure 2. This arrangement avoids the need for a second column with all its attendant costs. Source: Montz.
Another approach intensifies distillation by combining two columns into one, a so-called dividing wall column (Figure 2). This arrangement obviates a second separate column and its evaporator and condenser.
The unit features a vertical wall in the middle part of the column, creating a feed and draw-off section in this part of the column. The dividing wall, which is designed to be gas- and liquid-sealed, permits low energy separation of low and high boiling fractions in the feed section. The medium boiling fraction is concentrated in the draw-off part of the column.