Squeeze More From Your Process

Process intensification can enhance distillation, heat transfer and other operations

By Rocky C. Costello, R. C. Costello & Associates, Inc.

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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.

Essentially a stream to be distilled is split and fed to two columns similar except that one is at high pressure while the other at low pressure. The high pressure column’s condenser serves the low pressure column’s reboiler. Thus the two columns are thermally linked.

Boosting output

Sometimes reaction kinetics limit production. Removing a byproduct can drive the reaction to make more of the desired material. Reactive distillation leverages this by using distillation within the vessel to remove the byproduct.

Reactive distillation is used with reversible liquid-phase reactions, such as:
Unfortunately, here, as for many reversible reactions, the equilibrium point lies far to the left and little methyl acetate product is made:
However, if the byproduct water is removed, more of the product (C) will be made because of Le Chatlier's Principle: 
So, the reaction mixture is heated; then product and byproduct are separated by distillation. In this case, water is continuously removed to force the reaction to the right. When done in a distillation column, the water byproduct comes off the bottom of the column while the methyl acetate product comes off the top.

Some reactions require the use of catalysts inside the distillation column. Methods for introducing the catalyst include: structured packing coated with the appropriate catalyst, trays containing pillows filled with catalyst particles or pillows filled with catalyst particles rolled into bales, and pillows installed between the layers of structured packing.

Figure 3. Reactive distillation flowsheet for methyl acetate (right) is far simpler than that of conventional process (left). Source: Eastman Chemical.

Eastman Chemical, Kingsport, Tenn., pioneered one of the first major applications of reactive distillation, to significantly simplify the production of methyl acetate (Figure 3). This unit first went into operation in 1983.

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