How do we use the particle "en" as a pronoun?
The particles "y" and "en" don't have a 1 on 1 translation in english. The are used to refer to a place (y) or subject (en) when it has already been mentioned. For example: Parlez vous des dernieres nouvelles? (are you talking about the latest news?) Oui, on en parle. (Yes, we're talking about them)
What is the use of integration in real life?
Integration as you know is defined as the area under the curve that your are analyzing. In real life, integration helps understand how much material has been added to a system or how much material has converted into the desired product.
What is the core knowledge that a chemical engineer uses on a daily basis?
Understanding the material and energy balance of your system is what is the most important. Mastering the material balance is what makes the process itself: how does matter change, why, what does it take to increase the change rate? Understanding the energy balance helps you manage the resources you have available to produce the desired product.