Fenske Equation Calculator

Fenske Equation Formula

Worked Example

Separating to 95 % and 5 % with α = 2.5:

1. N_min = ln[(x_D/(1 − x_D)) × ((1 − x_B)/x_B)] / ln α
2. N_min = ln[(0.95/0.05) × (0.95/0.05)] ÷ ln 2.5 = 6.426866226

GATE tip

Fenske gives N_min, Underwood gives R_min, and Gilliland links them to the actual stages at the chosen reflux.

More Mass Transfer Calculators

• Minimum Reflux Ratio Calculator

  Σ αxF/(α − θ) = 1 − q; R_min + 1 = Σ αxD/(α − θ)

  saturated liquid feed (q = 1): R_min = [x_D/x_F − α(1 − x_D)/(1 − x_F)]/(α − 1)

  Example: A saturated-liquid feed at 50 % with α = 2.5 and 95 % distillate: Minimum reflux ratio R_min = 1.1.

  Open the minimum reflux ratio calculator →

• Kremser Equation Calculator

  N = ln[((y_N+1 − mx₀)/(y₁ − mx₀))(1 − 1/A) + 1/A] / ln A

  A = L/(mV); N = (y_N+1 − y₁)/(y₁ − mx₀) when A = 1

  Example: Absorbing a gas from 5 % to 0.5 % with pure solvent, m = 1.2 and L/V = 1.8: Number of ideal stages N = 3.41902.

  Open the kremser equation calculator →

• Fick’s Law Calculator

  N_A = (D_AB P / RTz) ln[(P − p_A2)/(P − p_A1)]

  equimolar: N_A = D_AB (p_A1 − p_A2)/(RTz)

  Example: Water evaporating at 25 °C through 10 mm of still air (D = 0.26 cm²/s): Molar flux N_A = 0.00337961 mol/(m²·s).

  Open the fick’s law calculator →

• Evaporator Calculator

  S λ_s = F c_p (T₁ − T_F) + V λ

  P = F x_F/x_P, V = F − P, economy = V/S, A = Q/(U ΔT)

  Example: 10,000 kg/h of 5 % solution fed at 25 °C, concentrated to 25 % at 100 °C with 120 °C steam: Steam required S = 9562.22 kg/h.

  Open the evaporator calculator →

Frequently asked questions

Sources

• The International System of Units (SI), 9th edition (SI Brochure), Bureau International des Poids et Mesures .

Last reviewed: 2026-10-15. Factors are stored exactly and computed with 50-digit decimal arithmetic. See how we verify conversion factors.