Systems, Surroundings and Balance Equations

Summary

Chemical Engineers use balance equations to describe, design, and troubleshoot products and processes. Balance equations describe the amount of stuff (e.g., mass, the concentration of a particular chemical component, energy, etc.) in a system that interacts with its surroundings.

Systems can be physical things like tanks or chemical reactors, or they can also be logical items such as a savings account or a stage in a multistage decision process. Likewise, stuff can be a physical thing such as energy or the mass of a material or a logical quantity such as information or financial value. Regardless of what the system and stuff are, the basic form of the balance equation remains the same. Thus, if we learn that basic form, we can describe, simulate or design all the products and processes, we are interested in.

At the end of this lecture module, students will be able to:

• Describe the four terms that appear in open and closed stuff balance equations

• Write balances for the mass or moles of material in an open and closed system

• Write balances for the energy of an open and closed system

• Write balances that describe the financial health or performance of a system