Chemical engineering cheat sheet

Material balance

For a defined system: accumulation = input − output + generation − consumption. State the basis and time interval.

At steady state, accumulation is zero.

Mass and moles

Mass and amount of substance are related through molar mass. A balanced reaction constrains molar ratios, not equal masses.

n = m/M

Energy balance

Track energy entering, leaving and accumulating in the system. Sign conventions depend on how heat and work are defined.

A heater adds energy; a cooler removes it from the process stream.

Process conditions

Temperature, pressure, composition and flow rate describe different aspects of a stream. Units must match the chosen balance.

A mass flow rate and a volumetric flow rate require density to be related.

Steady state

At steady state, the amount stored within a chosen boundary does not change with time. Inputs and outputs need not individually be zero.

For a non-reacting component: input rate − output rate = accumulation rate.

Component balances

Write a separate balance for each useful component, then combine it with the total balance. A mass fraction is component mass divided by total mixture mass.

A stream of 100 kg/h at 20% salt by mass carries 20 kg/h of salt.

Reaction stoichiometry

Balanced coefficients relate molar amounts, not arbitrary masses. Identify the limiting reactant before calculating maximum product formation.

2H₂ + O₂ → 2H₂O uses two moles of hydrogen for each mole of oxygen.

Sensible heat

For a single phase over a range with approximately constant heat capacity, Q = mcΔT. Phase changes and reactions require additional energy terms.

For c in kJ/(kg·K), mass in kg and a temperature difference in K, Q is in kJ.

Worked example: evaporation

A 100 kg batch contains 30 kg dissolved non-volatile solid. Removing 20 kg of solvent without losing solid leaves 80 kg total, with solid mass fraction 30/80 = 0.375.

The final concentration is 37.5% by mass; the solid mass has not increased.

Common mistake: balance without a boundary

Define the system, time basis and assumptions before writing equations. Recycle streams inside a whole-process boundary are not additional external inputs.

Draw arrows crossing the selected boundary, then count only those in its external balance.

Check your understanding

A steady non-reacting tank receives 5 kg/min. What total output maintains constant stored mass?

5 kg/min, assuming there are no other inputs, outputs or losses.

Curriculum connections

Sources