1. Introduction to Material Balances

What is a chemical process?

A chemical process is a combination of steps and unit operations in which starting materials are converted into desired products.

What is a process flow sheet?

A process flow sheet is often used to represent the collection of steps and unit operations in a chemical process.

Common Process Units

What are Block Flow Diagrams (BFDs)?

• A Block Flow Diagram (BFD), also called a Process Block Diagram (PBD), is the simplest representation of a chemical process.

• Block flow diagrams can include flow rates, process conditions, temperatures, and pressures.

What are the conventions for drawing BFDs?

• BFDs are great for high-level information

• Pumps, compressors, and measurement systems and control (such as valves) are excluded from Block Flow Diagrams, which also often exclude heat exchangers.

What are Process Flow Diagrams (PFDs)?

• Process Flow Diagrams provide more detail than block flow diagrams, showing major equipment, piping, and stream information.

• They do include heat exchangers, pumps & compressors.

• They do not include measurements, controls, or valves.

What are open systems & closed systems?

• Open systems are where mass is allowed to cross system boundaries (i.e., mass can move from the system to the surroundings and vice versa)

• Closed systems are where mass cannot cross system boundaries (almost always, we can assume energy can cross boundaries)

What is a steady state?

• Steady state is a condition where system properties do not change with time.

• eg. concentration, temperature, mass of a system does not change

(Process parameters such as pressure, temperature, composition, and total mass in the system do not change with time)

What is an unsteady state?

• Unsteady state is a condition where system properties change with time.

• One or more key variables change over time

What is a fundamental rule regarding mass in a system?

• Mass can neither be created nor destroyed, but it can be changed in form, and the total mass must be conserved. (First Law of Thermodynamics)

• The total mass of the elements is unchanged, but the mass of individual chemical species in the system changes, such as CH₄ changing in a combustion reaction.

What is the general material balance equation?

• Input → stuff coming in

• Generation → stuff being produced by a chemical reaction

• Output → stuff leaving

• Consumption → stuff being used up by a chemical reaction

(Material balances can be applied as an overall balance and separately for each component.)

Different cases for general material balance equations?

What are some key information to know before solving material balance questions?

• Does the mass in the system change with time or is the accumulation term equal to zero?

• Each term in the equation must have the exact same unit of mass per unit time, such as kilograms per second.

• Calculations normally work in flow rate units such as kilograms per second.

Fast way to decide what the system is (batch, semi-batch, continuous, steady or transient state)