Industrial processes
Most of the reactions that you see in a school science laboratory are single, irreversible processes that occur relatively quickly.
The production of chemicals in industry rarely relies on such simple processes. Reactions required for these processes are often reversible, resulting in the possibility of low yields. They may have high activation energies and require the use of specialist catalysts. The production may involve a series of reactions, all reliant on the success of the previous step.
There may be significant safety issues involving the chemicals used or the conditions required for the reaction to be successful. Finally, the products may require separation from each other and/or any remaining reactants. Some of the terms and processes used in the chemical industry are listed below, with examples. Understanding these will help you in the activities that follow.
This is where the reactions occur. It could be a blast furnace used in the production of iron, an electrolytic cell in the case of aluminium or a fermenting tank in the production of wine.
Reproduced with the permission of ALCOA.
Reaction conditions describe the environment in which the reaction is taking place. They can include the temperature, pressure, the concentrations of any solutions used, the type of solvent used, whether a catalyst is present, whether it is in the dark or light and even whether the reaction is occurring in air or in an inert gas environment. The conditions used will have a significant effect on the reaction rate and even the type of reaction occurring. They can also dramatically change the position of a system in a state of equilibrium.
A batch process is where a certain amount of chemical is processed at one time in a closed system. After a certain length of time, the chemicals are removed from the reaction vessel, and are then replaced by more reactants. An example of a batch process is the production of wine.
A continuous process is where reactants are continuously fed into the reaction vessel and the products are removed as they are formed. Most industrial chemical processes use this technique.
The collision theory is used to explain the rates of reactions in the chemical industry. A fast rate of the reaction is normally preferable, but sometimes the best conditions required for a high equilibrium yield may not be the ones that will encourage a fast reaction. In these situations, the chemical engineer will have to find a suitable compromise in the reaction conditions in order to produce the best overall result. Alternatively, in some situations a reaction that is too fast may prove dangerous so conditions must be chosen to limit the rate of the reaction.
A chemical reaction will often produce substances other than the main product. These are called by-products, and need to be considered when designing an industrial process.
Questions to be asked include the following.
- Are the by-products a pollutant?
- Can they be used in another process, or sold as a commercial product?
- How can they be separated from the main product?
- Will the disposal of the by-products be environmentally or economically viable?
There will also be the issue of unused reactants, especially in reversible reactions that may not produce a 100% yield. Unused reactants can cause some of the issues mentioned above. The chemical process is often designed to recycle as much of the unreacted chemicals as possible.
- Why will a state of chemical equilibrium never be achieved in a continuous production process?
- The baking of loaves of bread is a batch process. Can you think of any foods that are more likely to be produced using a continuous process?
- Using Le Châtelier's principle, explain why the continuous production method will normally result in the production of a higher yield of products.
