Integrate Material and Energy Flows

Design of products, processes, and systems must include integration and interconnectivity with available energy and materials flows.

Contributed by Dr. Concepcíon Jiménez-González, Director, New Product Development Clear Skin, Stiefel Skin Health, a GSK company

When we spend years working on a new synthesis to produce the next big reaction or molecule, it is very easy to keep our focus in our little world of lab reactors, chromatographic columns, HPLC and all the other cherished equipment. Perhaps we get so familiar with one part of the process that we now know the most intricate details of the reactor and the reaction, and have our ideas on how to maximize its efficiency.

However, when transferred into a larger scale, a chemical process is a system of interrelated units, inputs, outputs, and recycle streams. Even though it may be easier to study separate units as we are getting to know their inner workings, chemical process is not really a set of separated parts, but a complete unit where the individual components are intimately interrelated – similar to our bodies, with overarching systemic functions in addition to the individual organs.

The principle of Integrating Material and Energy Flows reminds us to treat processes as an entire system, and use the inter-relationships of the parts to our advantage. Chemical Engineers would recognize this as the application of Process Integration. Process integration is best understood as a holistic, systematic framework to optimize the mass and energy required for a given process.

Of HEN and MEN

Mass and energy are of course intimately related in any production process, since for instance a chemical process can be described as the way by which we convert mass and energy into a more valuable product. In a chemical process, we start with raw materials (mass) and then require steam (energy) to carry out chemical and physical transformations. As a result, we typically will have several hot streams that need to be cooled and several cool streams that need to be heated. To achieve this, one could cool all the hot streams with say, cold water or refrigerants and then heat all the cold streams with stream; but by doing this the total energy requirements would be maximized. To minimize the amount of utilities that need to be used, and therefore minimize costs and environmental impacts, we use process integration – specifically energy integration.

Energy integration can be simply described as using hot streams to heat cold streams, and vice versa, before any additional utilities are used, with the result of reducing the overall use of utilities. The simpler example is a heat exchanger – but this could be very complex, depending on the system, and one can need an entire network of those heat exchangers in the plant for effective energy integration. These networks are known as Heat Exchange Networks – or HEN. There are many examples of successful heat integration in a wide variety of industries, such as refineries, petrochemical, chemical, food and drink, pulp and paper, and metallurgical. The net results of heat integration applied successfully are cost savings, increased throughput, and reductions in emissions and environmental impacts.

Heat Exchange Network

Mass Exchange Networks, or MEN, are the parallel concept for using process streams to integrate mass. MEN are very similar to HEN, but instead of exchanging energy using process streams, we exchange mass. For instance, very often we need to separate unreacted species from our desired product. We could use an additional material (say, a solvent) to remove the unreacted material and send it back to the reactor. However, that comes with additional cost and environmental impacts. By using MEN, we take advantage of lean streams (streams with low concentrations of a given material) to separate and recover mass from rich streams. The idea is to use as much of the lean streams to recover and potentially recycle the materials, before we have to use an external agent, thus reducing environmental and life cycle impacts in the process.

Of course, designing HEN and MEN is easier said than done – the mass and energy transfer is governed by thermodynamic and equilibrium laws, and thus will require your friendly neighborhood chemical engineers to do some calculations and graphs to produce the design– but that is where the fun begins.

If you want to get into some of the details on how to integrate mass and energy, the papers below may help.

• El-Halwagi, M. M. Process integration. In Process Systems Engineering, Vol. 7, Academic Press, San Diego, CA, 2006.
• Dunn, R. F., El-Halwagi, M. M. Process integration technology review: background and applications in the chemical process industry. J. Chem. Technol. Biotechnol., 2003, 78, 1011–1021.
• Smith, R. State of the art in process integration. Appl. Therm. Eng., 2000, 20, 1337–1345.
• Jimenez-Gonzalez and Constable, Green Chemistry and Engineering – A practical design approach. John Wiley and Sons, 2010.

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