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Distillation Columns

DistillationColumn is a ProcessPI equipment unit for the preliminary design of a simple distillation column: one feed, a total condenser, a partial reboiler, a distillate and a bottoms product.

It runs the Fenske-Underwood-Gilliland-Kirkbride shortcut (any number of components), checks a binary design by McCabe-Thiele stepping, and then sizes a tray column: overall efficiency, actual trays, diameter, tray-section height and the condenser and reboiler duties.

from processpi.components import Benzene, Toluene
from processpi.equipment import DistillationColumn
from processpi.streams import MaterialStream
from processpi.units import MolarFlowRate, Pressure, Temperature

feed = MaterialStream(
    "Feed",
    composition={"Benzene": 0.4, "Toluene": 0.6},
    molar_flow=MolarFlowRate(100, "kmol/h"),
    temperature=Temperature(95, "C"),
    pressure=Pressure(1.01325, "bar"),
)
column = DistillationColumn(
    name="T-101",
    feed=feed,
    components=[Benzene(), Toluene()],
    light_key="Benzene",
    heavy_key="Toluene",
    distillate_lk_fraction=0.97,
    bottoms_lk_fraction=0.02,
)
result = column.design()
print(result.summary())

==========================
DISTILLATION COLUMN: T-101
==========================
Method              : Fenske-Underwood-Gilliland-Kirkbride (Gilliland: molokanov)
Keys                : light Benzene, heavy Toluene
Pressure            : 1.0132 bar

Material balance
----------------
Component    Feed z  Distillate x   Bottoms x  alpha (to HK)
Benzene      0.4000        0.9700      0.0200         2.4701
Toluene      0.6000        0.0300      0.9800         1.0000
Feed F              : 100.000 kmol/h  (q = 1.001, subcooled liquid feed)
Distillate D        : 40.000 kmol/h
Bottoms B           : 60.000 kmol/h
Key recoveries      : 97.00 % LK to distillate, 98.00 % HK to bottoms

Stages and reflux
-----------------
Minimum stages      : 8.15 (Fenske, reboiler included)
Minimum reflux      : 1.565 (Underwood)
Reflux ratio        : 2.034 (1.3 x R_min, R/Rmin = 1.30)
Theoretical stages  : 17.32 (Gilliland, reboiler included)
Rectifying/stripping: 8.66 / 8.66 (Kirkbride)
Feed stage          : 10 from the top (theoretical)
McCabe-Thiele check : 17 stages (16.64 fractional), feed on stage 8 (ideal VLE at column pressure)

Trays
-----
Tray efficiency     : 54.8 % (O'Connell)
Actual trays        : 30
Feed tray           : 18 from the top

Temperatures and duties
-----------------------
Condenser           : 80.73 C
Top stage           : 81.65 C
Reboiler            : 109.74 C
Condenser duty      : 1041.6 kW
Reboiler duty       : 1124.5 kW

Column sizing (tray column)
---------------------------
Top     F_LV 0.039, C_sb 0.103 m/s, u_flood 1.788 m/s, D 0.996 m
Bottom  F_LV 0.092, C_sb 0.094 m/s, u_flood 1.539 m/s, D 1.116 m
Column diameter     : 1.116 m (governed by the bottom section)
Tray spacing        : 0.60 m, design at 80 % of flooding
Tray section height : 18.00 m (without top and bottom allowances)

Assumptions
-----------
- Constant molar overflow, total condenser, partial reboiler counted as a theoretical stage.
- Column pressure taken as uniform top to bottom.
- Ideal vapour-liquid equilibrium (Raoult's law) from the components' DIPPR vapour pressures.
- Duties from latent heats only (saturated reflux, sensible heat of products neglected).
- Surface tension 0.020 N/m (no component data); give `surface_tension` to correct the flooding velocity.
- Tray spacing 0.6 m (default).
- Design at 80 % of flooding (default).
- Downcomer area 12 % of column area (default).

Every number is also in result.data (or result["key"]), as ProcessPI unit objects where the quantity has a unit: for example result["diameter"], result["condenser_duty"].to("kW"), result["distillate"]["x"].

Specifications

Spec Meaning
light_key, heavy_key Key components, by name. Names match loosely: "chlorobenzene", "ChloroBenzene" and "Chloro Benzene" are the same component.
light_key_recovery, heavy_key_recovery Fraction of the feed light key that leaves in the distillate, and of the heavy key in the bottoms.
distillate_lk_fraction, bottoms_lk_fraction Binary feeds only: light-key mole fraction in each product, instead of recoveries.
components ProcessPI components; equilibrium from Raoult's law with their vapour pressures. Needed for temperatures, duties, efficiency and diameter.
relative_volatility Instead of components: constant volatilities (any reference). Gives the stage and reflux design only.
pressure Column pressure; default is the feed pressure.
q Feed thermal condition. By default it comes from the feed temperature (subcooled or two-phase feed); a superheated feed needs q.
reflux_ratio or reflux_ratio_factor Operating L/D, or R/Rmin (default 1.3).
gilliland_correlation "molokanov" (default) or "eduljee".
tray_efficiency Overall efficiency; overrides O'Connell.
tray_spacing Default 0.6 m.
flood_fraction Design fraction of the flooding velocity; default 0.80 (warning above 0.85).
downcomer_area_fraction Default 0.12.
surface_tension N/m; default 0.020, the reference value of Fair's correlation.
flooding_k_factor Tray-geometry factor on the flooding velocity; default 1.0.

In a flowsheet

DistillationColumn is an Equipment with the inlet port feed and the outlet ports distillate and bottoms. simulate() runs the design and writes the products (composition, molar and mass flow, pressure, and the condenser and reboiler temperatures) to the outlet streams.

from processpi.integration.flowsheet import Flowsheet

distillate, bottoms = MaterialStream("Distillate"), MaterialStream("Bottoms")
column = DistillationColumn(
    name="T-101", components=[Benzene(), Toluene()],
    light_key="Benzene", heavy_key="Toluene",
    distillate_lk_fraction=0.97, bottoms_lk_fraction=0.02,
)
fs = Flowsheet("BTX")
fs.connect(feed, column, "feed")
column.connect_outlet("distillate", distillate)
column.connect_outlet("bottoms", bottoms)
fs.run()

Methods and sources

Step Method Source
Minimum stages, component split at total reflux Fenske, with the geometric mean of the top and bottom volatilities OpenExamPrep PE Chemical 11.4
Minimum reflux Underwood; one root per volatility gap between the keys, so components between the keys distribute same
Theoretical stages Gilliland, Molokanov et al. (1972) form; Eduljee form optional M. A. Soliman, J. King Saud Univ. Eng. Sci. 21(1) (2009) 1-5, eq. (1); OpenExamPrep (Eduljee)
Feed stage Kirkbride OpenExamPrep
Overall efficiency O'Connell, 0.503 (μL α)-0.226 M. Duss, R. Taylor, Chem. Eng. Prog., July 2018, eq. (2)
Flooding velocity Fair, Csb from the Lygeros-Magoulas fit, (σ/0.02)0.2 correction C. A. P. Souza et al., Ind. Eng. Chem. Res. 64 (2025) 2256, eqs. (1)-(3), (5), Table 1

The OpenExamPrep worked example (ethane/propane/n-butane/n-pentane) is reproduced in tests/test_distillation_shortcut.py and tests/test_distillation_column.py.

Limits

  • Ideal VLE only: right for similar molecules (aromatics, light hydrocarbons), wrong for non-ideal mixtures such as ethanol/water. Use relative_volatility for those, with values you trust.
  • Components whose vapour-pressure constants are inconsistent with their critical point are refused with an error rather than used.
  • Constant molar overflow and a uniform column pressure; duties from latent heats only.
  • Tray columns only; packed columns are not covered yet.
  • The tray-section height leaves out the top disengagement space and the bottom sump.