Industrial chemistry and sustainability

From the lab to the plant

A reaction that works in a test tube is only the start. Turning it into a process that makes thousands of tonnes a year means balancing competing factors:

The best conditions are usually a compromise. For an exothermic equilibrium, a low temperature gives a better yield but a slower rate; high pressure may help the yield but costs energy and needs stronger, more expensive vessels.

The Haber process, revisited

You met the Haber process in Module 5 (and in Year 10). Here it is as an example of evaluating a process:

$\text{N}_2(g) + 3\text{H}_2(g) \rightleftharpoons 2\text{NH}_3(g) \quad \Delta H = -92 \text{ kJ mol}^{-1}$

• Conditions: about 400–450 °C, 100–250 atm and an iron catalyst. A lower temperature would favour ammonia at equilibrium but be far too slow; high pressure favours the side with fewer gas molecules.
• Yield: only about 15% converts in each pass, but ammonia is condensed out and unreacted gases are recycled, so the overall conversion is about 97%.
• Environmental issue: the hydrogen comes mainly from natural gas by steam reforming, $\text{CH}_4 + 2\text{H}_2\text{O} \rightarrow \text{CO}_2 + 4\text{H}_2$ (overall), so ammonia production emits roughly 1–2% of the world's CO₂. Making the hydrogen by electrolysis of water using renewable electricity ("green ammonia") avoids this, and several Australian projects are being planned around it.
• Social benefit: fertilisers made from ammonia grow the food for about half of the world's population.

Green chemistry

In 1998, Paul Anastas and John Warner set out twelve principles of green chemistry to design processes that reduce harm from the start, rather than cleaning up afterwards. The most useful for HSC are:

1. Prevent waste rather than treat it.
2. Maximise atom economy: design reactions so that as many reactant atoms as possible end up in the product. $\text{atom economy} = \dfrac{M(\text{desired product})}{\text{total } M(\text{all reactants})} \times 100$.
3. Use safer chemicals and solvents, such as water or supercritical CO₂ instead of chlorinated solvents.
4. Use less energy: run reactions near room temperature and pressure where possible.
5. Use renewable feedstocks such as plant sugars, starch or oils instead of oil and gas.
6. Use catalysts rather than reagents that are used up.
7. Design products to break down after use rather than persist in the environment.

A simple measure of waste is the E-factor: mass of waste ÷ mass of product. It is below 1 for many bulk chemicals but often 25–100 for pharmaceuticals, because they need many steps, solvents and purifications. Atom economy and percentage yield measure different things: a reaction can have a high yield but a poor atom economy if it produces a lot of by-product.

Case study: two routes to ethanol

Fermentation, $\text{C}_6\text{H}_{12}\text{O}_6(aq) \rightarrow 2\text{C}_2\text{H}_5\text{OH}(aq) + 2\text{CO}_2(g)$, uses a renewable feedstock at low temperature, but it is slow, is done in batches, gives only about 14% ethanol (yeast dies at higher concentrations) and needs energy-intensive distillation. Its atom economy is only 51%, because so much mass leaves as CO₂.

Hydration of ethene, $\text{C}_2\text{H}_4(g) + \text{H}_2\text{O}(g) \rightleftharpoons \text{C}_2\text{H}_5\text{OH}(g)$, has an atom economy of 100%, runs continuously and gives purer ethanol, but uses a fossil feedstock, high temperature and high pressure.

Which is "better" depends on what you value and on outside conditions: the price of oil, a price on carbon, droughts that make grain dear, and whether the feedstock is a waste product (as at Bomaderry) or a food crop. Plants that sell their by-products, such as spent grain solids for stock feed and captured CO₂ for industry, improve both their economics and their environmental case.

Bioplastics: designing for the end of life

Polylactic acid (PLA) is a condensation polymer made from lactic acid, which is produced by fermenting sugar or starch. It meets two green-chemistry principles: a renewable feedstock and a product designed to break down. Its ester links can be hydrolysed, so it composts to CO₂ and water. But this only happens quickly in industrial composting at about 58 °C with the right microbes; in a home compost bin, soil or the ocean it can last for years, and it contaminates PET recycling if put in the wrong bin. A sustainable product needs the right collection system as well as the right chemistry.