Kinetics and Reaction Engineering

So last week I dealt with separations, which one of the most important parts of chemical engineering and is extremely important throughout industry. Separations isn't the only part of major chemical processes though. If you ever see a chemical plant, you'll probably see lots of tall separation towers, but if you look closely there will be much smaller buildings that are also extremely important to chemical processes. These are chemical reactors. Though are often pretty small compared to separation towers that can easily be hundreds of feet tall, they're no less important. It's very rare in the chemical industry that the desired product is already in the form required to take to market. Most often, plants take in raw materials and use the reactors to create the desired products or sometimes, reactions occur and the products themselves must be further reacted because they are highly toxic. Probably the most common example of this is combustion, the process of taking in various carbon molecules and burning them in the presence of oxygen to produce carbon dioxide and water: . This reaction is involved in everything. Every time you burn wood for a fire, it's this reaction that's occurring, though normally the carbon molecule will contain several hydrogen molecules as well and carbon dioxide won't be the only product. Now, this reaction assumes perfect combustion and it's great for teaching the basics of reactions in chemistry classes, but often carbon dioxide and water are not the only products. Many combustion reactions are called "incomplete" and this means they produce products other than just carbon dioxide and water and these other products can often be very dangerous gases. For example, incomplete combustion can create methane (CH4 and various sulfur and nitrogen molecules). The problem with these chemicals is that they are very powerful greenhouse gases - must more dangerous than just carbon dioxide. There are ways of getting rid of these gases though before they're released to the atmosphere. Everyday cars produce billions of tons of carbon dioxide and yet relatively little extremely dangerous greenhouse gases and this is because of the catalytic converter in the vehicle. Catalysis is a way of reducing the amount of energy needed for a reaction to occur and the catalytic converter in a car allows for the highly toxic greenhouse gases to react and form much less dangerous products. See the picture below for a basic diagram of this process.

Combustion is far from the only type of reaction we see in our daily lives. Reactions are everywhere and it can be very useful to know about them. Cooking is probably one of the most common forms of chemical reactions we produce everyday. Most cooking relies on breaking up large protein molecules into smaller and more digestible amino acids. The heating also denatures bacteria and kills parasites that might be present in the food. Now there's a particular reaction that's extremely important for making delicious meats and other foods: the Maillard reaction, commonly called caramelization. This is the reaction that produces that brown-colored skin you see on grilled meats and vegetables. Basically we take amino acids, and sugars present in the raw ingredients and when we add heat, a compound called GBD is formed and this is the chemical responsible for that delicious dark-brown skin we see on grilled food. 

Now I've touched on some basic reactions that we see in our day to day lives but now I'm going to discuss some of the actual important technical details (I apologize in advance if this is too boring). As I've said, Kinetics is the study of chemical reactions and every reaction out there varies massively from others. Some are slow and some are extremely fast. If you add NaCl (table salt) to water, it nearly instantly dissociates into it's component ions of sodium and chlorine. This is because this reaction has a very low activation energy and activation energy is the energy needed for a reaction to move forward. There are plenty of extremely slow chemical reactions though. The combination of hydrogen and oxygen gases to form water is a very slow reaction and can take months to form even a single water molecule. There are several reasons that this reaction is very slow. It takes a great deal of energy to cause this reaction, and it also is very rare for two gas molecules to hit one another at the perfect angle, Gases are made up of more than trillions of tiny molecules bouncing around at extremely fast speeds, but they very rarely hit one another in atmospheric conditions and even when they do hit one another, they may not have the proper orientation to react and create a new chemical and even when they do hit at the right orientation, they may lack enough energy to react. Now there we can speed up slow reactions by heating them up or by adding a catalyst. The heat helps provide more energy and thus means more molecules have sufficient energy to react and a catalyst (like platinum) can help lower the needed activation energy by helping orient the molecules properly. The catalyst basically holds one molecule in a specific orientation such that it can react much easier.

Chemical engineering relies on kinetics and reactions every day around the world in productions plants making all kinds of different products that we use. Now that doesn't mean not everyone can create their own chemical reactions at home. There are plenty of fun chemical reaction experiments you can do at home like instantly freezing water, turning pennies into gold (not actually gold, but gold colored), or waterproofing sand and here's a link of 20 different fun experiments you can try at home. Enjoy and make sure you follow any safety precautions needed!

Image sources

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