Deisobutanizer / Butane Splitter Calculator
Sizes deisobutanizer (butane splitter) fractionation columns for the close-boiling iC₄/nC₄ separation. This is one of the most difficult separations in NGL processing due to the very low relative volatility (α ≈ 1.1–1.3), requiring high tray counts and high reflux ratios. Calculates minimum stages, minimum reflux ratio, actual trays, column diameter, and reboiler/condenser duties per GPSA Ch. 16 and GPA 2140.

GPSA Ch. 16 GPA 2140

Calculation Mode:

Design Rate

Design: Size butane splitter from feed composition and product specifications.

Feed Conditions

Feed Rate (Mixed Butanes)

BPD

Feed Temperature

°F

Operating Pressure

psig

Feed Composition (mol%)

Propane (C₃) Light end from debutanizer

iso-Butane (iC₄) — Light Key

n-Butane (nC₄) — Heavy Key

iso-Pentane (iC₅)

n-Pentane (nC₅)

Pentane+ (C₅+)

Product Specifications

Max nC₄ in Overhead

mol%

Max iC₄ in Bottoms

mol%

Condenser Type

Column Design Parameters

Reflux Ratio Multiplier (R/R_min)

Overall Tray Efficiency

Tray Spacing

Design Flood Fraction

Typical Butane Splitter Operating Conditions

Parameter	Range
Pressure	60–100 psig
Overhead Temp	90–130°F
Bottoms Temp	140–180°F
Reflux Ratio	5–15
Trays	60–120

Engineering Basis

Fenske Equation (Minimum Stages):

N_min = ln[(x_LK,D / x_HK,D)(x_HK,B / x_LK,B)] / ln(α)

Where x_LK,D = light key (iC₄) mole fraction in distillate, x_HK,B = heavy key (nC₄) mole fraction in bottoms, α = average relative volatility of iC₄ to nC₄ (typically 1.1–1.3).

Gilliland Correlation:

(N − N_min) / (N + 1) = f[(R − R_min) / (R + 1)]

Correlates actual stages N to actual reflux ratio R given minimum stages N_min and minimum reflux R_min from the Underwood equation.

Underwood Equation: Determines minimum reflux ratio R_min from feed composition, feed condition (q), and relative volatilities. Combined with Gilliland correlation to find actual stages at the design reflux multiplier.

Column Diameter: Sized from vapor/liquid traffic using Fair’s flooding correlation at the design flood fraction.

📚 Learn the Theory

Understand deisobutanizer design, close-boiling iC₄/nC₄ separation challenges, and structured packing alternatives

Read Engineering Guide →

Results

Actual Trays

trays

N_min (Fenske) -

R_min (Underwood) -

R_actual -

N_theoretical -

Feed Tray -

Rectifying / Stripping -

Column Dimensions

Diameter -

Height -

Tray Spacing -

% Flood -

Shell Thickness -

Weight -

Duties & Utilities

Condenser Duty -

Reboiler Duty -

CW Flow -

T_condenser -

T_reboiler -

iC₄ (Overhead) -

nC₄ (Bottoms) -

Design Guidelines

Close-Boiling Separation: The iC₄/nC₄ separation has α ≈ 1.1–1.3, requiring 60–120 trays and reflux ratios of 5–15. This is one of the most energy-intensive columns in NGL processing.

Structured Packing: Due to the high tray count, structured packing is often considered to reduce column height by 30–50% compared to trayed columns.

iC₄ Applications: Isobutane overhead product is primarily used as alkylation unit feed, refrigerant (R-600a), and aerosol propellant.

Deisobutanizer / Butane Splitter Calculator