Methanol Plant
Theory of Distillation
Gerard B. Hawkins
Managing Director, CEO
Introduction
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Principles of Distillation
Distillation Theory
Flowsheet Description
Topping Column Details
Topping Column Operation
Refining Column Details
Refining Column Operation
Distillation Control
Distillation Problems
2
Principles of Distillation

The aim is to separate the product
from any impurities
• On a methanol plant this is the
separation of methanol from Water,
Alcohol's, Alkanes, etc.

This is achieved by
• Adding heat to boil liquid and passing
through a series of mixing zones

Here heat and mass transfer occurs
3
Typical Crude Analysis
Crude
300 ppm
~10 ppm
200 ppm
80 ppm
400 ppm
100 ppm
50-100 ppm

Ethanol
Acetone
Propanol's
Butanol's
Methyl Formate
Di Methyl Ether
Alkanes

+ Dissolved CO2, CH4






Refined Spec
10 ppm
20 ppm
4
Methanol Specifications
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Characteristics
Acetone
Ethanol
Acidity
Appearance
Hydrocarbons
Gravity
Minimum methanol
PFT
Water
Smell
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Grade AA
20 ppm
10 ppm
n/a
Clear and no sediment
miscibillity test clear in 20 mins
793 kg/m3
98.5%
60 minutes
0.1%
No characteristic smell.
5
Distillation Theory
Cool
80 % A
20 % B
Cool
70 % A
30 % B
70 % A / 30 % B
Cool
60 % A
40 % B
60 % A / 40 % B
Heat
50 % A / 50 % B
Heat
Heat
6
Principles of Distillation Processes within a
Column
Cooling
Top
Product
Liquid
Vapour
Liquid
Vapour
Liquid
Vapour
Liquid
Vapour
Bottom
Product
Heat
7
Distillation Column Schematic
weir
Liquid
Vapour
Vapour
Liquid
Down
comer
Sieve
Tray
Vapour
8
Distillation Theory
Mol frac. in Vap.
1
0.8
0.6
0.4
0.2
0
0
0.5
1
Mol frac. in Liq
9
Effect of Reflux on No. of Stages
Mol frac. in Vap.
1
0.8
0.6
Due to Reflux
0.4
Reduction in
Reflux
Feed Comp
0.2
0
0
0.5
1
Mol frac. in Liq
10
Distillation control
Methanol
70 te/hr 100 %
80 te/hr 100 %
Feed
80 te/hr Methanol
20 te/hr Water
Water 30 te/hr
High Methanol
20 te/hr Water
10 te/hr Methanol
20 te/hr Water
• Control Mass Balance !
11
GBHE Two Column Distillation Flowsheet
Refining Column
Topping Column
Vent Gases
CO2
Meth Form
DME
acetone
CO
CH4
N2
Crude Methanol
80 % Methanol
20 % water
+ impurities
81°C
0.86 barg
70°C
0.24 barg
89°C
2.1 barg
80°C
0.64 barg
82°C
104°C
125°C
1.29 barg
Refined
Methanol
99.99 %
Fusel Oil
Ethanol
Propanol
Butanol
Methanol
Ethanol
Water
(Prop, Butan)
Bottoms
Water
12
Topping Column Function


Remove Light Ends
Top product (vapour)
•
•
•
•
•
•
Di Methyl Ether
Methyl Formate
Acetone
Lower Hydrocarbons
Dissolved gasses (CO2 and CH4)
Low Alkanes <C10
Topping Column Products

Bottom product
•
•
•
•
Methanol
Ethanol
Water
Trace quantities higher hydrocarbons
propanol
 butanol

14
Topping Column Flowsheet
Vent
condenser
Primary
condenser
45°C
67°C
Reflux drum
70 °C
0.24 barg
Light ends
to fuel
Acetone
DME
MF
LIC
Feed
Preheater
Feed
Meth, Eth,
water, Acetone
DME MF
Topping
column
LIC
84 °C
0.64 barg
Reboiler
To refining column
Meth, Eth Water
15
Topping Column Details
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
42 trays
Normal feed Tray 34
• Also feed points at trays 27, 31, 38
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2 Reboilers duty, one typically
heated by reformed gas the other
by LP steam
Base level controlled by LIC
Varies flow to refining column
16
Condensing System


2 condensers
• Light ends column condenser
 85% of vapour condensed
• Vent Condenser
This is done to prevent re-absorption of lights
ends into reflux
Air
TIC
Vapour from
Light ends
column
To Vent
condenser
Light Ends Column
Condenser
67 °C
To Light Ends
Reflux Drum
17
Refining Column Function

To separate
• Methanol
• Ethanol
• High Alcohol's
• Water
18
Typical Refining Column Feed
Analysis
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Methanol
Water
Ethanol
Propanol's
Butanol's
Higher Alkanes >C10
80 %
 20 %
 300 ppm
 100 ppm
 80 ppm
 Trace Quantities

Refining Column Products

Top product
• Methanol 99.999 %

Bottom product
• Water (typically < 10 ppm
methanol)

Fusel Oil
• Ethanol
 Propanol, Butanol, higher
alkanes >C10
• Methanol Product Loss
Refining Column Flowsheet
Condenser
Purge
81°C
PIC
LIC
Reflux drum
81 °C
0.86 barg
89°C
Product
Methanol
Heavy Ends
column
Fusel Oil
Feed
Preheater
Feed
Meth, Eth,
water, Acetone
DME MF
Liquid Purge
LIC
130 °C
1.8 barg
Reboiler
Bottoms
Water
21
Refining Column Details
Typically 45-60 m high
72-93 trays
Normal feed Tray 26
Also feed points at trays 29, 23, 20
2 Reboilers
One heated by reformed gas
One heated by steam (trim heater)
Base level controlled by LIC
Varies bottoms water flow to drain7
Refining Column Reboiler
Duty as Function of Ethanol
in Crude
% design heat load
105
100
95
90
85
Fixed recovery efficiency
80
75
0
500
1,000
Ethanol in crude
1,500
2,000
23
Refining Column Composition
Profiles
Methanol / Water Fraction
1
0.9
0.8
0.7
0.6
Water
Methanol
Ethanol
0.5
0.4
0.3
0.2
0.1
0
Top
Theoretical Tray
Bottom
Distillation Column Problems Downcomer
Flooding
Symptoms - High PD, Poor Performance
Cause - Liquid rate too high
Holes in
Sieve Tray
New level
Downcomer
Backing up
Froth
Downcomer
Problems With Trays
Weeping
Jet Flooding
Tray
Liquid
Froth
Gas flow
Low vapour rate
Liquid drips down
Liquid
Vapour rate too high
Blows liquid on to tray
above
Effects of Various Components on
Methanol Specification Tests
Component
Test Affected
PFT
Water Miscibility
Odour
Saturated hydrocarbon,
None
Yes
-
Unsaturated hydrocarbon,
Large
-
-
Saturated aldehyde
Small
-
Yes
unsaturated aldehyde
V large
-
Yes
Keytones
None
-
Yes
Ether
None
-
-
esters
None
-
Yes
Iron Carbonyl
V large
-
-
PFT = permanganate fading time
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otherwise supplied to Users is believed to be
accurate and correct at time of going to press, and
is given in good faith, but it is for the User to satisfy
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and any implied warranty or condition (statutory or
otherwise) is excluded except to the extent that
exclusion is prevented by law. GBHE accepts no
liability for loss or damage resulting from reliance
on this information. Freedom under Patent,
Copyright and Designs cannot be assumed.