Aromatic Hydrocarbons

Benzene, Toluene and Xylene

Physical properties

Productions of aromatic compounds

i. Physical processes: solvent extraction, azeotropic/extractive distillation, fractional distillation, solid adsorption and crystalization

ii. Chemical treatments: dehydrogenation, dehydroisomerization, and dehydrocyclization (and hydrodesulphurisation)

Reactions of aromatic compounds – alkylation, halogenation, oxidation, nitration, sulfonation etc.

Overview

Benzene, Toluene and Xylene

History Background

Earlier – solvent mixture

1850 – as an individual compound

Mostly side product from coal carbonization to produce coke

The yield is rather small portion

Demand for WWI – production of TNT

Because of limited production from coal carbonization, other methods have been used – catalytic reforming, pyrolysis etc

During WWII – increase demand for benzene

7.7 million tonnes per annum – US capacity

Benzene, toluene and xylene from coal carbonization

Hydrocarbon Kg / tonne coal carbonized

Benzene 2 -8

Toluene 0.5 – 2

Xylene 0.1 – 0.5

Properties

IUPAC name: Benzene

Other names: Benzol, Cyclohexa-1,3,5-triene

Molecular formula: C6H6

Molar mass: 78.1121 g/mol

Appearance: Colorless liquid

Density: 0.8786 g/cm3, liquid

Melting point: 5.5 oC

Boiling point: 80.1 oC

Solubility in water: 1.79 g/L (25 oC)

Viscosity: 0.652 cP (20 oC)

Dipole moment: O D

Main hazards: Carcinogenic, toxic

Flash point: -11 oC

IUPAC name: Methylbenzene

Other names: Toluene, Phenylmethane, Toluol

Molecular formula: C7H8 (C6H5CH3)

Molar mass: 92.14 g/mol

Appearance: Clear colorless liquid

Density: 0.8669 g/cm3, liquid

Melting point: -93 oC

Boiling point: 110.6 oC

Solubility in water: 0.053 g/100 ml (20 - 25 oC)

Viscosity: 0.590 cP (20 oC)

Dipole moment: O.36 D

Hazards: Highly flammable

Flash point: 4 oC

IUPAC name: 1,2-dimethylbenzene 1,3-dimethylbenzene 1,4-dimethylbenzene

Common name: o-xylene m-xylene p-xylene

Other names: o-xylol, Orthoxylene m-xylol, Metaxylene p-xylol, Paraxylene

Molecular formula:

C8H10 (C6H4C2H6)

Molar mass: 106.16 g/mol

Appearance: Clear colorless liquid

Density: 0.88 g/cm3, liquid 0.86 g/cm3, liquid 0.86 g/cm3, liquid

Melting point: -25 oC -48 oC 13 oC

Boiling point: 144 oC 139 oC 138 oC

Solubility in water: Practically insoluble

Viscosity: 0.812 (20 oC) 0.62 (20 oC) 0.34 cP (20 oC)

Dipole moment: -

Hazards: Harmful

Flash point: 17 oC 25 oC 25 oC

Production of Aromatics

Physical Processes

Separation of AromaticsPhysical methods

1. Fractional distillation

2. Azeotropic distillation

3. Extractive distillation

4. Solvent extraction

5. Solid adsorption

6. Crystallization

Consideration

1. Close or identical boiling points

2. Reversible condition under reforming reactions

3. Solubility and miscibility

Fractional Distillation

Separate toluene

Benzene forms azeotropic mixture with naphthenes and paraffins

Products from catalytic reforming process is separated light fraction – boiling point at 180 – 250 oC

Further separation at above 225 oC to produce 90% toluene and 10% azeotropic mixture of benzene, naphthenes and paraffin

Aromatics separation plants:

Reformate moves up

Solvent moves down

Solvent extracted out to a stripper

Separate from alkanes and cycloalkanes

Further separation by fractional distillation

Aromatics Separation Plant

?

Separation of C8 Aromaticso-xylene separate by fractional distillation

Not possible for m- and p-xylene (preferred methods is crystallization or isomerization)

C8 stream

distillation

Crystallization or adsorption

p-xylene

o-xylene

Residual C8s

C8 stream

p-xylene separation

p-xylene

o-xylene

Isomerisation

The process requires specific solvent or azeotropic agent

As a volatile component in the mixture

Increase the rate of volatility – change the properties and encounter the separation problem

Example: separation of toluene

Azeotropic agents:

1.

2.

3.

4.

Increase the volatility of toluene

Distill off from benzene and xylene

See FIG. 3. and TABLE 3

Azeotropic D

istillation

?

In the presence of a solvent that reduced the volatility rate of the compounds to be separated

Lower the boiling point of the component

The added solvent must fit the properties:

1.

2.

3.

4.

5.

6.

See FIG. 4

Extractive Distillation

?

Examples:

Requires additional process for purification

1.Benzene treated with sulphuric acid; washed with caustic soda, water; separate polymers and sulphonate sludges in the re-run tower

2.Toluene treated with maleic anhydride, caustic soda, water; dried under calcium chloride

compound solvent

Xylene cresol

Benzene and toluene phenol

Extractive Distillation

Solvent has highly selectivity for aromatics against non-aromatics

Fit the following characters:

1.Form two phase system, able to separate at reasonable range of temperature

2.Non-corrosive

3.Non-reactive to the feed and the product

4.Thermally stable

Examples:

Udex Plant – aqueous diethylene glycol

Refinery of Humble Oil – SO2 extraction-double solvent extraction

Solvent Extraction

UDEX Plant

Separate benzene, toluene and xylene in a plant

Main component: an extractor, a water wash, a heater, a contact clay treater and distillation columns

Diethylene glycol-water increase the selectivity

More water will decrease the solubility

See FIG. 5, TABLE 4

Solvent Extraction

SO2 Extraction-Double Solvent Extraction

Toluene separation

First extraction: separate aromatics from non-aromatics by adding SO2

Second extraction: oil wash, then separate raffinate (SO2, oil wash and some non-aromatics) by extraction

SO2 and oil wash are reused

After removal of SO2 and oil wash to obtain 99.5% toluene

After acid wash to obtain 100% toluene

See FIG. 6

Solvent Extraction

Adsorption of organic compounds on compressed silica gel

Aromatics prefer to adsorb

Washing silica gel with xylene to remove aromatics from non-aromatics

The silica gel can be reused by washing with a paraffin solvent

Commercially known as Arosorb process

Aromatics and non-aromatics

xylenes

Xylenes + aromatics

Washing with paraffin

Solid Adsorption

Boiling points of isomers of xylene are very close each other:

Ethylbenzene: 136.2 oC

p-xylene: 138.2 oC

m-xylene: 139.1 oC

o-xylene: 144.4 oC

Separation by fractional distillation is impossible

Freezing temperatures of isomers of xylene

Ethylbenzene: -95.0 oC

p-xylene: 13.3 oC

m-xylene: -47.9 oC

o-xylene: -25.2 oC

Process: dried over activated alumina, cooled in two stages (first crystallisation of p-xylene at 13.3 oC in ‘waiting tank’, centrifuge, second crystallization)

Product: 95%, with 60-70% recovery

Crystallization

Production of Aromatics

Chemical Processes

An Overview1. Catalytic reforming

2. Dehydrogenation

3. Dehydroisomerisation

4. Pyrolysis Gasoline

5. Isomerisation

6. Hydrodealkylation

7. Disproportionation

8. Desulfuration

Also known as ‘hydrogen reforming’

Purposely to obtain main aromatic compounds (benzene, toluene, and xylenes) as much as possible

Three main types:

Dehydrogenation of cyclohexane and homologues

Dehydroisomerisation of methyl cyclopentane and homologues

Dehydrocyclisation of alkanes

Other related processes

Pyrolysis gasoline

Hydrodealkylation

Isomerisation

Desulfuration

Catalytic Reform

ing

Mostly the feedstocks are from naphthenes and homologues

‘Hydroforming’ process

Condition: ~500 oC, ~15 atm, dehydrogenation catalyst, a flow of hydrogen gas

Catalysts: molybdenum oxide, chromic oxide, Pt, Cu (mono-function catalyst)

Fisher and Welty

Feedstock: methyl cyclohexane,

Product: toluene

See attachment FIG. 1: Hydroformer

Dehydrogenation?

Combination of isomerisation and dehydrogenation processes

Mostly from substituted cyclopentanes

Examples: methyl cyclopentane, dimethyl cyclopentane etc.

Condition: ~500 oC, ~15 atm, hydrogen atmosphere

Catalyst: molybdenum oxide, chromic oxide, Pt, Cu

Dehydroisom

erisation

?

Hartley

Feedstock: mixture of methyl cyclopentane and cyclohexane

Product: benzene

Reason: the rate of isomerisation is slower than dehydrogenation, so that cyclohexane form benzene very quickly

Feedstock: dimethyl cyclopentanes, ethyl cyclopentane

Primary product: methyl cyclohexane (isomerisation)

Secondary product: toluene (dehydrogenation)

Dehydroisom

erisation

Greensfelder and Fuller

Feedstock: methyl cyclopentane

Condition: 490 oC, 10 – 20 atm, a flow of hydrogen

Catalyst: MoO3 on activated alumina

Product: benzene (no cyclohexane)

Platforming

Feedstocks: straight run gasoline containing methyl cyclopentane, cyclohexane, and other C6 and C8 naphthenes

Condition: 475 oC, hydrogen atmosphere, 720 psi

Catalyst: Pt on alumina

Reactor: pellitized Pt working at 250 – 275 oC

Product: mixture of benzene and other aromatics

See attachment: FIG. 2: Platformer

Dehydroisom

erisation

Fulton

Feedstocks: mixture of paraffins (13.4 %), isoparaffins (29.3 %), naphthenes (33.3 %), aromatics (6.1 %)

Condition: platforming

Product: 60 % isoparaffins and 40 % aromatics

Haensel and Berger

Feedstocks: mixture of C7 – C8 paraffins (28 %), C7 – C8 naphthenes (62 %), toluene (10 %)

Condition: platforming

Product: 53 - 56 % toluene

Edgar

Feedstock: naphthenes

Condition: 480 – 510 oC, 14 – 21 atm

Catalyst: Pt on alumina, and halides

Yield: 90 % aromatics HC

Dehydroisom

erisation

Advantages of the platforming process:

1. The naphthenes converted to aromatics

2. Isomerisation and cracking of the paraffins to improve the octane rating

3. The sulfur content is decreased

Caterole process

Introduce in England

Hydrocarbon stream heated at 600 - 700 oC with slightly above atmospheric pressure

Catalyst: copper or copper-iron

An example of the process

Feedstock: naphtha fraction (120 – 240 oC)

Condition: 700 oC, 20 – 50 psi, straight distillation

Product: benzene (92 – 94 %)

from another fraction to obtain toluene (94 – 96 %)

Can also produce xylenes, ethylbenzene, styrene, indene, naphthalenes, antracene etc

Dehydroisom

erisation

Ring closure of alkanes (paraffins)

Usually C6 or more

Proceed via olefins

A slower reaction than the dehydrogenation and dehydroisomerisation

Involve transformation of paraffins to olefins, ring closure to form naphthenes, and dehydrogenation

Condition: >500 oC

Therefore, inclusive reaction of cracking of alkanes

Dehydrocyclization

?

Hoog, Verheus and Zuiderweg

Feedstock: olefins

Condition: 450 – 550 oC

Catalyst: Chromic oxide, or other metal oxides and sulfides

Product: aromatics

Examples of paraffins conversion at 465 oC:

Paraffin hydrocarbon Aromatic hydrocarbon % aromatization

n-Hexane Benzene 19.5

2-Methylhexane Toluene 31

n-Heptane Toluene 36

2,5-Dimethylhexane Mostly p-xylene 52

3-Methylheptane Mainly o- and p-xylene 35

n-Octane Mainly o-xylene 46

n-Nonane Mainly methylethylbenzene 58

Dehydroisom

erisation

Shell Development Company

Feedstock: n-heptane

Condition: 490 oC, atmospheric pressure

Catalyst: 10 % Chromic oxide on alumina, cerium dioxide and potassium oxide

Product: toluene (80 % conversion), usually maintained at 40 %

Feedstock: n-nonane

Condition: 465 oC, atmospheric pressure

Catalyst: 70 % chromic oxide, 30 % alumina with potassium oxide

Product: 71 % aromatics, 6% olefins, 23 % paraffins

Aromatics was 80 % methylethylbenzene

Dehydroisom

erisation

?

?

Hurley

Feedstock: heptane (90 %), air (5%), HF (5 %)

Condition: 550 oC, atmospheric pressure,

Catalyst: ferric fluoride and manganous fluoride

Product: toluene (42 %)

Dunstand, Haque, Wheeler

Feedstock: C2 to C6 paraffins

Condition: 750 – 900 oC

Product: benzene (and other liquid aromatic products)

Dehydroisom

erisation

Frey and Hepp

Suggest the mechanism of the aromatization from ethane feedstock

Two steps; dehydrogenation (endothermic), aromatization (exothermic)

When the process through ethylene and butadiene

Dehydroisom

erisation

?

A fraction from cracking process with a boiling range between 20 – 200 oC

The stream contains aromatics, alkenes, dienes, alkanes and cycloalkanes

Most of the alkenes and dienes were removed by solvent extraction

Condition: 75 – 150 oC, 10 – 40 atm, fraction distillation

Product: C6 – C8 hydrocarbons, benzene, toluene, C8 aromatics

Typical products distribution (%):

Reformate Pyrolysis gasoline

Benzene 11 54

Toluene 55 31

C8 aromatics 34 15

Pyrolysis Gasoline

Involve 1,2-hydride or methyl shifts

A stage to produce more o- and p-xylenes

Feedstock: C8 aromatics

Condition: vapour phase, 500 oC, strong acidic condition

Catalyst: silica-alumina

Product: o-xylene (20 %), m-xylene (55 %), p-xylene (25 %)

Ethylbenzene not isomerised to xylenes unless the presence of catalytic reforming process

Isomerisation?

In term of market demand, benzene is higher than toluene

But toluene produced in excess

Transforms toluene to benzene

Feedstock: toluene

Catalytic condition: 550 – 650 oC, 35 – 70 atm

Non-catalytic condition: 650 – 750 oC, 20 – 67 atm

Yield: up to 99 %

Market From petroleum From coal tar

Benzene 65 10 80

Toluene 20 40 15

Xylenes 15 50 5

Hydrodealkylation

Converted toluene to benzene and o-, m-, and p-xylenesCondition: 480 oCCatalyst: zeolite (acidic catalyst)

Disproportionation

A process to avoid ‘poisoning’ to catalyst

Sulphurated compounds removed by this process

The feedstock pre-treated with cobalt oxide or molybdenum oxide at 400 oC

Sulphurated compounds turn into H2S – easier to remove

Desulfuration