Green Engineering Principle #3
Design for Separation
Separation and purification operations should be designed to minimize energy consumption and materials use.
Contributed by Dr. Matthew J. Realff, Professor and David Wang Sr. Fellow, School of Chemical and Biomolecular Engineering, Georgia Tech
Industrial separation processes are very energy intensive and in most cases have not approached the thermodynamic limits of minimum work of separation . Historically, for liquid and condensable gas separation, multistage distillation has been the workhorse process, based on boiling point differences between the components to be separated. The energy consumption of oil refineries involved distilling crude oil into its various fractions and subsequent separation of thermally or catalytically cracked components in these various fraction. Moreover, many bulk organic chemicals involve distillation in their production, which adds significantly to their production CO2 footprints. Thus, avoiding distillation, making distillation more efficient, and searching for alternatives to distillation are very important aspects of implementing the third principle of green engineering.
How can distillation be avoided? One approach is to integrate the process reactions with the separations to avoid the generation of mixtures that have to be separated. For example, methyl acetate is produced along with water from methanol and acetic acid. This reaction is equilibrium limited and results in a mixture that has several binary azeotropes between the components. The traditional process involved the use of eight distillation columns, one liquid extraction and a decanter. This complex series of units was replaced by a single unit that integrated the reaction into the separation process. The key insight is that if one of the products, in this case methyl acetate, leaves in the vapor phase, and the other, water, in the liquid phase, the equilibrium conversion is avoided and the feeds can be completely converted to products. This results in a system that is 1/5th the capital and energy cost of the traditional process.
How can distillation be made more efficient? The energy consumption in distillation occurs in the reboiler where heat is used to create the vapor stream that travels up through the column and whose composition is enriched in the more volatile component by stripping it from the downward flowing liquid. This vapor is then condensed to a liquid product and a fraction returned to the column to contact the rising vapor from which the less volatile component is absorbed as the liquid travels down the column. This complex process creates the desirable composition gradient in the column, with the higher boiling component more concentrated at the reboiler, and the lower boiling component at the condenser. Typically, the heat of condensation is released is at a lower temperature than the heat of vaporization is absorbed, but what if the temperature of the vapor could be increase before it were condensed? In fact, vapor compression achieves precisely this outcome– where the work of compression is used to raise the vapor temperature and then reuse the heat matched to the reboiler. This improves the energy efficiency of the distillation column and can lead to separation processes for dilute products that would not be energy efficient otherwise.
What about alternative separation techniques to replace distillation? Distillation does not take advantage of the very specific differences in molecular size, or other physicochemical properties such as affinity for specific solvents or solid adsorbents and so there are more selective separation techniques that would potentially use less energy than distillation. Unfortunately, many of these techniques fail to scale up as effectively as distillation to the large flow rates required for industrial chemical production at world scale plants. Moreover, distillation enjoys the advantage of inertia – with a large installed industrial base and many years of experience in operation. These factors have hampered attempts to replace distillation with other more energy efficient technologies.
One technology that has broken the hold of distillation in a large scale application is reverse osmosis membrane separation for water desalination. Reverse osmosis uses mechanical pressure to overcome the osmotic pressure exerted by the salt solution and thereby push the water through a selective skin. The theoretical energy to de-mix water and salt is approximately 1 kWh/m3 of water, the current best membrane technologies have a real energy cost of 4.0 kWh/m3 and thermal “distillation” type technologies use on the order of 50 kWh/m3 . In this case the size difference between a hydrated salt ion and a single water molecule is a factor of 3 and the membrane matrix allow the water to cross but retain the salt ions. Membranes for this application are manufactured efficiently at very large scale and a large surface area can be packed into a relatively small volume, both of which factors enable the technology to not only be competitive from an energy perspective but also from an overall cost perspective.
To implement the third principle of green engineering, more applications of energy efficient, but often more capital intensive technologies, will have to be developed. A key enabler will be the combination of highly selective materials that can “grab” or “pass” certain molecules from those that are closely related in size or other properties, with manufacturing technologies, such as hollow fiber membranes, to lead to scalable energy efficient separation methods. Reverse osmosis membrane desalination is a leading example of such an approach and other applications are under active development.
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