Green engineering principles and applications

The word ‘chemical’ has always gained negative attention in the eyes of the public even when it enhances their lives. This is because of the side-effects these chemicals have on the environment and their health due to pollution and toxicity.

The chemical industry basically produces 4 kinds of chemicals:

1. Commodity chemicals: Chemicals that are used by other chemical industries before becoming a consumer product. These are produced at huge scale. For example, petrochemicals produced in a refinery such as olefins and aromatics go on to become polymers.
2. Fine chemicals: Starting materials for speciality chemicals. These are of very high purity and are produced in limited quantities. Hence these are low-volume, high-value products.
3. Speciality chemicals: Pharmaceuticals, Dyestuff and pigments, flavours and fragrances, speciality polymers, catalysts and enzymes, food additives. These are consumer products.
4. Renewable energy: Biofuels such as ethanol, biogas.

These chemicals are produced using some basic unit operations and unit processes. Unit operations involve physical separation of products that are obtained from unit processes, whereas unit processes involve chemical conversion of substances.

Unit process + unit operation = an entire chemical process

Examples of unit processes:

• Sulfonation
• Nitration
• Hydrogenation
• Hydrolysis

Examples of unit operations:

• Fluid flow operations: e.g. fluid mixing
• Heat transfer operations: e.g. evaporation
• Mass transfer operations: e.g. distillation, extraction
• Thermodynamic operations: e.g. refrigeration
• Mechanical operations: e.g. crushing of solids, sedimentation

A good example of how unit operations and unit processes overlap is evaporation. It is both a heat transfer as well as a mass transfer operation as it involves the transfer of both heat and mass.

Chemical processes are capable of eliminating the pollution and toxicity that is caused by it. They are their own problems and they are their own solutions. Paul Anastas and Julie Zimmerman developed the 12 principles of green engineering and can be found in Env. Sci. and Tech., 37, 5, 94A-101A, 2003 or at ACS. These are similar to the 12 principles of green engineering because Chemistry and Chemical Engineering are interdependent on each other.

Green chemistry is the design of chemical products and processes that reduce or eliminate the use and generation of hazardous substances. Whereas, green engineering is the development and commercialization of industrial processes that are economically feasible and reduce the risk to human health and the environment.

A green chemical engineer care about these three things:

1. Efficiency: Processes should be efficient with respect to all resources.
2. Safety: Processes should be inherently safe to carry out throughout its production cycle.
3. Financial feasibility: The process should be profitable.

Let’s look at some examples.

1 Integrate Material and Energy Flows: Design of products, processes, and systems must include integration and interconnectivity with available energy and materials flows.

Optimization of heat is crucial for energy savings. Energy savings mean fuel savings. Fuel savings mean less greenhouse gas emissions. Pinch Technology provides such a thermodynamically based optimization methodology for energy saving in processes.

2 Renewable Rather Than Depleting: Material and energy inputs should be renewable rather than depleting.

Some unit processes/operations require heating. But what if energy comes from a green source? Energy required to heat a process is called as ‘Process heat’. It is often sourced from fossil fuels. A greener option in this case would then be a solar collector that can collect heat to be supplied to the process, termed as solar process heat.Further reading: Seven Ways to Optimize Your Process Heat System

Cogeneration or combined heat and power (CHP) is another technique to save energy. In this, the heat engine or power station simultaneously generate electricity and useful heat.

Some unit processes/operations require cooling. A major coolant in the chemical industry is water. It is used in large cooling towers. It has to be treated and reused since it can be contaminated with chemicals it comes in contact with. These chemicals could also be entrained by surrounded air and cause airborne emission problems. For this purpose, drift eliminators are used –  an air pollution control measure.

3 Inherent Rather Than Circumstantial: Designers need to strive to ensure that all materials and energy inputs and outputs are as inherently nonhazardous as possible.

A part of my Masters thesis involved catalytic transfer hydrogenation. It is the transfer of hydrogen atoms from a donor reagent to a substrate under catalysis. It is supposedly the safest way to carry out a hydrogenation reaction. Hydrogenation reaction is one of the deadliest of all reactions since it involves hydrogen gas. Hydrogen gas is very light and diffuses into the air very quickly. It is highly flammable too. Not a nice combination. It not only catches fire but spreads wildly. Transfer hydrogenation on the other hand doesn’t require hydrogen gas, it just needs the donor reagent. I think the reason it is not so good to scale-up is because it would need change in existing infrastructure and that comes at a cost.

Another example of safer engineering is eco-friendly coolants. Transformers made it possible for electricity to reach long distances without huge losses. Polychlorinated biphenyls is a banned coolant fluid that was used to cool these transformers. Efforts have been made to produce greener coolants. In 2013, Cargill won the Presidential Green Chemistry Challenge Award from the U.S. Environmental Protection Agency (EPA) for developing  Envirotemp™ FR3™, a eco-friendly coolant. It can be used in high voltage electrical transformers.

4 Meet Need, Minimize Excess: Design for unnecessary capacity or capability (e.g., “one size fits all”) solutions should be considered a design flaw.

Effluent treatment plants (ETP) often are of the ‘one size fits all’ kind. We call them Common Effluent Treatment plants (CETP). Instead, each plant can have its own ETP. Individual ETPs are more efficient than CETPs.

5 Design for Separation: Separation and purification operations should be designed to minimize energy consumption and materials use.

Separation and purification operations allow us to recycle materials. To make these operations as energy efficient as possible is therefore necessary. As we know that Pharmaceuticals industry produces highly pure products. Moving bed bioreactor (MBBR) systems are a type of biological treatment that may be utilized in pharmaceutical wastewater applications.Further reading:  Water and Energy Conservation in the Pharmaceutical Industry

6 Prevention Instead of Treatment: It is better to prevent waste than to treat or clean up waste after it is formed.

Ultrasonication is another part of my Masters thesis. I basically bombarded a flask of chemicals by ultrasound. In this way, I was able to carry out the reaction without a solvent and at room temperature. This was opposed to previous attempts to carry out the same experiments that used solvents and higher temperature and pressure. As far as industrial applications of ultrasonication are concerned, a company called Industrial Sonomechanics has created industrial scale ultrasonic reactors.

7 Maximize Efficiency: Products, processes, and systems should be designed to maximize mass, energy, space, and time efficiency.

Microreactors, reactors with dimensions of about 5-100 ml, can run reactions that are not possible to run at large scale. These reactions are often explosive or hazardous in nature. Such a technology when scaled up can lead to material-efficient, energy-efficient as well as safe way to carry out reactions.Further reading:Excellent review: New trends for design towards sustainability in chemical engineering: Green engineering, J. Garc´ıa-Serna et al. / Chemical Engineering Journal 133 (2007) 7–30