Free calculators for process engineering
17 chemical process calculations: reactors, kinetics, mass balances, heat, vapour pressure and distillation. Formula shown, with practice problems.
Reactions and reactors
Conversion, yield and selectivity
Yield = conversion × selectivity. Falling selectivity often points to a temperature that is too high or an ageing catalyst.
Space time and conversion: CSTR and PFR
First-order reaction, constant density, isothermal. For the same volume a PFR always converts more than a CSTR.
Reactor volume for a target conversion
First order, isothermal, constant density. Above 90 % conversion a single CSTR becomes very large: several tanks in series are used instead.
Arrhenius: effect of temperature on rate
Rule of thumb: with Eₐ ≈ 50 to 55 kJ/mol the rate roughly doubles every 10 °C. Careful with exothermic reactions: heat release follows the same curve.
Activation energy from two runs
k₁ and k₂ in the same unit. Only two points: for a robust value, plot ln k against 1/T over at least three temperatures.
Batch cycle time and productivity
Dead times (charging, discharge, cleaning) often weigh as much as the reaction itself: that is where capacity is gained.
Mass balances and blending
Mixing two streams
Balance on mass, not volume: volumes do not always add up. Also works for blending two tanks (kg instead of kg/h).
Diluting a strong acid or base
Always add acid to water, slowly and with mixing (strong heat of dilution). Work in mass: volumes do not add up.
Evaporator: water removal
Solids do not evaporate: the balance sets the water to remove. Typical economy: 0.8 to 0.9 single effect, about 0.8 × n for n effects.
Process gas: density and actual flow
Ideal gas (Z = 1): good at low pressure; above about 10 bar correct with the compressibility factor Z. Air: M = 28.96; methane: 16.04; nitrogen: 28.01.
Heat and energy
Reaction heat and adiabatic temperature rise
ΔTad = temperature rise if all cooling is lost. For a batch use the batch kmol and kg. Above about 50 K, carry out a runaway study.
Jacketed reactor: cooling water and area
Well-mixed reactor (uniform temperature). Typical U for a water-cooled jacket: 300 to 600 W/m²·K, lower with fouling or a glass-lined vessel.
Heating a batch: energy and steam
Sensible heat only; losses and vessel mass ignored (add 10 to 20 %). Lv ≈ 2133 kJ/kg at 3 bar abs, 2015 kJ/kg at 10 bar abs.
Vapour-liquid equilibrium and distillation
Vapour pressure (Antoine equation)
Constants in mmHg and °C (standard chemical engineering tables); do not extrapolate outside the range shown. Where Psat equals the vessel pressure, the liquid boils.
Relative volatility and K-values (Raoult)
Ideal mixture (Raoult), fair for similar compounds (benzene-toluene, hexane-heptane). Methanol-water and ethanol-water are non-ideal: real α differs.
Minimum number of stages (Fenske)
Total reflux, constant α, binary mixture; Nmin includes the reboiler. In real operation about 2 to 2.5 times Nmin are needed.
Minimum reflux (Underwood, binary)
Saturated-liquid feed (q = 1), constant α, constant molar overflow. Usual factor f: 1.2 to 1.5.
Learning and checking tools. The ranges shown are general guidance: always follow your plant's procedures, permits and set points. Nothing is sent anywhere: the calculations run in your browser.