Methanol Plant Theory of Distillation

Gerard B. Hawkins Managing Director, CEO

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

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

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

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.

Distillation Theory

6 Heat

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

Principles of Distillation Processes within a Column

7

Vapour

Vapour

Vapour

Vapour

Liquid

Liquid

Liquid

Liquid

Top Product

Bottom Product

Heat

Cooling

Distillation Column Schematic

8

Vapour

Liquid

Vapour

Vapour

Liquid

Sieve Tray

weir

Down comer

Distillation Theory

9

0

0.2

0.4

0.6

0.8

1

0 0.5 1

Mol frac. in Liq

Mo

l fra

c. in

Vap

.

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

Mo

l fra

c. in

Vap

.

Due to Reflux

Reduction in Reflux

Feed Comp

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

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

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

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 column Feed

Preheater

70 °C 0.24 barg

84 °C 0.64 barg

67°C 45°C

LIC

LIC

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

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

Refining Column Function

To separate • Methanol • Ethanol • High Alcohol's • Water

18

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

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

21

Reboiler Feed 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

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

23

0 500 1,000 1,500 2,00075

80

85

90

95

100

105

Ethanol in crude

% d

esig

n he

at lo

ad

Fixed recovery efficiency

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

Met

hano

l / W

ater

Fra

ctio

n

Water Methanol Ethanol

Top Bottom

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

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

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

Process Information Disclaimer Information contained in this publication or as 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 itself of the suitability of the Product for its own particular purpose. GBHE gives no warranty as to the fitness of the Product for any particular purpose 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.