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Rich-Oil Demethanizer Calculator

GPSA Ch. 16

Rich-Oil Demethanizer Calculator
Sizes the demethanizer column in lean oil absorption plants. Strips dissolved methane and ethane from rich absorption oil to produce NGL-laden oil for fractionation. Calculates column diameter, tray count, reboiler duty, overhead gas rate, and stripping efficiency per GPSA Ch. 16.
GPSA Ch. 16 ASME VIII
Calculation Mode:
Design: Size rich-oil demethanizer column from rich oil flow, composition, and methane spec.

Rich Oil Feed

GPM
°F
g/mol
SCF/gal

Dissolved Gas Composition (mol%)

mol%
mol%
mol%
mol%

Column Parameters

psig
°F
mol%
%
in
%

Typical Rich-Oil Demethanizer Parameters

ParameterTypical Range
Column Pressure75–200 psig
Reboiler Temperature250–400°F
Theoretical Stages6–14
Tray Efficiency45–65%
C₁ in Bottoms Spec0.5–2.0 mol%
Dissolved Gas Loading3–15 SCF/gal

Engineering Basis

Column Diameter (Souders-Brown):

Uflood = CSB √[(ρL − ρV) / ρV]

Where CSB depends on tray spacing. Design velocity = Uflood × flood fraction. Column cross-section sized for overhead vapor traffic.

Function: The rich-oil demethanizer strips dissolved methane and ethane from absorber rich oil. Overhead vapors (primarily C₁/C₂) are returned to the absorber or fuel system. Bottoms (demethanized rich oil) proceed to the oil purification still for NGL recovery.

Design Guidelines

Heat Source Selection: Hot oil loops are most common for demethanizer reboilers. Steam heating is used where available. Direct-fired heaters require careful temperature control to prevent oil degradation above 450°F.
Oil Degradation: Keep reboiler temperature below 400°F for most absorption oils. Higher temperatures cause thermal cracking and increased oil loss.