Squeeze More From Your Process

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.

Dividing wall columns can be used wherever multi-component mixtures must be split into high purity individual components. They are well suited for obtaining pure medium boiling fractions (sometimes called heart cuts). For instance, separating a three component mixture into its pure components in conventional systems requires at least two main columns and a side column. In contrast, a single dividing wall column can handle this task — and cut installation costs by 20% to 30% and operating costs by around 25%. This approach also reportedly significantly simplifies process control and reduces maintenance work.

Dividing wall columns have been used commercially to produce chemicals for more than 20 years. One major supplier is Julius Montz GmbH, Hilden, Germany, which now has more than 60 installations.

Academics continue to explore other PI variants for distillation. For instance, the Indian Institute of Technology, Kanpur, India, is developing novel units. In one Higee variant, the overhead condenser, reflux drum and the rectifying section are installed in one common vessel while the reboiler, column sump and striping section are combined in a second common vessel. Thus a complete distillation column looks like two horizontal vessels with dished or hemispherical heads. This reportedly provides a dramatic reduction in installed costs. No commercial installations have been built to date.

In addition, researchers at Carnegie-Mellon University, Pittsburgh are proposing a multi-effect distillation method. The multi-effect approach has been applied to evaporation for years but to my knowledge it’s never been used for distillation, although patents abound. This concept can dramatically reduce the amount of energy required to produce a gallon of product. The University has had a number of inquires but no projects are currently in the development stage.