Principles of Distillation Distillation Theory Flowsheet Description Topping Column Details Topping Column Operation Refining Column Details Refining Column Operation Distillation Control Distillation Problems

1. Methanol Plant
Theory of Distillation
Gerard B. Hawkins
Managing Director, CEO

2. Introduction
 Principles of Distillation
 Distillation Theory
 Flowsheet Description
 Topping Column Details
 Topping Column Operation
 Refining Column Details
 Refining Column Operation
 Distillation Control
 Distillation Problems
2

3. 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

4. Typical Crude Analysis
4
 Ethanol
 Acetone
 Propanol's
 Butanol's
 Methyl Formate
 Di Methyl Ether
 Alkanes
 + Dissolved CO2, CH4
300 ppm
~10 ppm
200 ppm
80 ppm
400 ppm
100 ppm
50-100 ppm
Crude
10 ppm
20 ppm
Refined Spec

5. Methanol Specifications
5
 Characteristics
 Acetone
 Ethanol
 Acidity
 Appearance
 Hydrocarbons
 Gravity
 Minimum methanol
 PFT
 Water
 Smell
 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.

6. Distillation Theory
6Heat
50 % A / 50 % B
60 % A
40 % B
Heat
60 % A / 40 % B
70 % A
30 % B
Heat
70 % A / 30 % B
80 % A
20 % B
Cool
Cool
Cool

7. Principles of Distillation Processes within a
Column
7
Vapour
Vapour
Vapour
Vapour
Liquid
Liquid
Liquid
Liquid
Top
Product
Bottom
Product
Heat
Cooling

8. Distillation Column Schematic
8
Vapour
Liquid
Vapour
Vapour
Liquid
Sieve
Tray
weir
Down
comer

9. Distillation Theory
9
0
0.2
0.4
0.6
0.8
1
0 0.5 1
Mol frac. in Liq
Molfrac.inVap.

10. Effect of Reflux on No. of Stages
10
0
0.2
0.4
0.6
0.8
1
0 0.5 1
Mol frac. in Liq
Molfrac.inVap.
Due to Reflux
Reduction in
Reflux
Feed Comp

11. Distillation control
11
Feed
Methanol
Water
80 te/hr Methanol
20 te/hr Water
70 te/hr 100 %
30 te/hr
80 te/hr 100 %
10 te/hr Methanol
20 te/hr Water
High Methanol
• Control Mass Balance !
20 te/hr Water

12. GBHE Two Column Distillation Flowsheet
12
Vent Gases
CO2
Meth Form
DME
acetone
CO
CH4
N2
Crude Methanol
80 % Methanol
20 % water
+ impurities
Refined
Methanol
99.99 %
Fusel Oil
Ethanol
Propanol
Butanol
Bottoms
Water
Topping Column Refining Column
Methanol
Ethanol
Water
(Prop, Butan)
70°C
0.24 barg
125°C
1.29 barg
81°C
0.86 barg
80°C
0.64 barg
82°C
104°C
89°C
2.1 barg

13. 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

14. Topping Column Products
 Bottom product
• Methanol
• Ethanol
• Water
• Trace quantities higher hydrocarbons
 propanol
 butanol
14

15. Topping Column Flowsheet
15
Primary
condenser
Vent
condenser
Reflux drum
Reboiler
Feed
Meth, Eth,
water, Acetone
DME MF
To refining column
Meth, Eth Water
Light ends
to fuel
Acetone
DME
MF
Topping
columnFeed
Preheater
70 °C
0.24 barg
84 °C
0.64 barg
67°C 45°C
LIC
LIC

16. Topping Column Details
 42 trays
 Normal feed Tray 34
• Also feed points at trays 27, 31, 38
 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

17. 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
17
To Light Ends
Reflux Drum
Air
To Vent
condenser
Vapour from
Light ends
column
Light Ends Column
Condenser
67 °C
TIC

18. Refining Column Function
 To separate
• Methanol
• Ethanol
• High Alcohol's
• Water
18

19. Typical Refining Column Feed
Analysis
 Methanol
 Water
 Ethanol
 Propanol's
 Butanol's
 Higher Alkanes >C10
 80 %
 20 %
 300 ppm
 100 ppm
 80 ppm
 Trace Quantities

20. 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

21. Refining Column Flowsheet
21
ReboilerFeed
Meth, Eth,
water, Acetone
DME MF
Bottoms
Water
Heavy Ends
column
Feed
Preheater
81 °C
0.86 barg
130 °C
1.8 barg
LIC
Condenser
Reflux drum
81°C
PIC
LIC
89°C
Product
Methanol
Fusel Oil
Purge
Liquid Purge

22. 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

23. Refining Column Reboiler
Duty as Function of Ethanol
in Crude
23
0 500 1,000 1,500 2,000
75
80
85
90
95
100
105
Ethanol in crude
%designheatload
Fixed recovery efficiency

24. Refining Column Composition
Profiles
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Theoretical Tray
Methanol/WaterFraction
Water
Methanol
Ethanol
Top Bottom

25. Distillation Column Problems Downcomer
Flooding
New level
Downcomer
Backing up
Holes in
Sieve Tray
Downcomer
Froth
Symptoms - High PD, Poor Performance
Cause - Liquid rate too high

26. Problems With Trays
Liquid
Liquid
Gas flow
Froth
Tray
Weeping Jet Flooding
Low vapour rate
Liquid drips down
Vapour rate too high
Blows liquid on to tray
above

27. 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
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