Petrochemical Engineering-IUNIT-IV

AROMATIC PRODUCTION

Introduction Aromatic hydrocarbons especially Benzene (80.10C),

Toluene (110.60C), Xylene [m-xylene (139.10C), p- Xylene (138.40C), o-Xylene (144.40C)], Ethyl benzene (136.20C) are major feedstock for a large number of intermediates which are used in the production of synthetic fibers, resins, synthetic rubber, explosives, pesticides, detergent, dyes, intermediates, etc.

Styrene, linear alkyl benzene and cumene are the major consumer of benzene.

Benzene also finds application in the manufacture of a large number of aromatic intermediates and pesticides.

As per CMAI, demand for benzene is forecast to grow at an average annual rate of 2.8% per year through 2020 resulting in nearly 57 million tonnes of demand by 2020.

Major application of toluene is as solvent. Other uses are in the manufacture of benzoic acid, chloro derivatives, nitro toluenes, toluene sulphonic acid, toluene sulphonamide, benzaldehyde, etc.

Xylenes are another important aromatics.Amongst the xylenes, about 80% of the

production is of p-xylene. Finds application in the manufacture of Terephthalic acid/DMT.

o-Xylene used in the manufacture of phthalic anhydride and m-xylene Isophthalic acid.

Various Sources of Aromatics

Process Description Coal Carbonization

From coke oven plant during carbonization, light oil is obtained as by product which contains about 2-8 kg, 0.5-2 kg, 0.1-0.5 kg of benzene, toluene and xylene respectively per ton of coal.

Steam cracking ofhydrocarbons

Steam cracking of naphtha and light hydrocarbon like ethane and propane produce liquid product (pyrolysis gasoline) rich in aromatics containing about 65% aromatics about 50% of which is benzene. About 30-35% of benzene produced worldwide is from pyrolysis gasoline.

Process Description

Catalytic Reforming

Catalytic reforming is a major conversion process, which converts low octane naphtha to high-octane gasoline and produce aromatics rich in BTX.Major reactions involved are dehydrogenation of naphthalenes to aromatics, dehydrocyclisation of paraffins to aromatics, isomerisation of paraffins and naphthenes, and hydrocracking of paraffins.

BP-UOP Cyclar Process

In this process, BTX is produced by dearomatisation of propane and butane.The process consists of reaction system, continuous regeneration of catalyst, and product recovery. Catalyst is a proprietary zeolite incorporated with a non noble metal promoter.

Process Description

Dearomatisation ofnaphtha

Process consists of extraction of aromatics from high aromatic naphtha feed without prior reforming. The process is useful for naphtha having high aromatics.

Hydro dealkylation anddisproportionation

Hydrodealkylation: It involves production of benzene by dealkylation of toluene either by catalytic or thermal process.Catalytic process: Hydeal, DeltolThermal process: HAD (ARCO), THDC Gulf OilDisproportionation: It involves conversion of toluene into benzene and xylenes.

Isomerization process

This process consists of conversion of C8 stream into valuable o- and p-xylene having isomerisation and isomer separation stage.

Process Description

Mitsubishi's Z-formingProcess

This process uses a metallosilicate zeolite catalyst to promotedehydrogenation of paraffins followed by oligomerisation and dehydrocyclisation of paraffins followed by oligomerisation.

KTI Pyroforming

This process uses a shape selective catalyst to convert C2 and C3 paraffins to aromatics.

Cheveron'sAromax process

It is similar to conventional catalytic reforming processes and L-type zeolite catalyst.

Petroleum feedstock for aromatic hydrocarbons

Catalytic reforming and pyrolysis of gasoline are the two major sources of aromatics.

aromatic content of the crude oils varies from source to source of the crude.

Pyrolysis gasoline may contain around 50-70% of aromatics.

Aromatic content of various crude oilCrude Paraffins Naphthenes Aromatics

Assam Mix, Vol% 32-40 52-43 16-17

Gujart(North),Vol%

52.50 42.00 5.30

Gujart (Ankleshwar),Vol%

70.80 25.00 4.20

Bombay High, Vol%

53.70 25.00 21.30

Iranian, Wt% 57.50 31.20 11.40

Aromatic hydrocarbon productionCatalytic reforming and pyrolysis gasoline are the major

sources for aromatics.Some of the other aromatic conversion processes commonly

used include toluene disproportionation to produce benzene and xylene, hydrodealkyaltion of toluene to produce benzene.

The main processing scheme in catalytic reforming involves: Catalytic reforming of naphtha or steam reforming of naphtha

for production of pyrolysis gasoline. Solvent extraction for separation of non-aromatics from

aromatics. Pre-treatment of pyrolysis gasoline, which includes two-stage

selective hydrogenation. Separation of benzene, toluene and C8 fractions. Further separation of C8 hydrocarbons –Xylene and ethyl

benzene which includes separation of ethyl benzene by superfractionaction and separation of p-xylene by either crystallization or by selective adsorption.

Pyrolysis Gasoline as aromatic feedstock

During Naphtha, pyrolysis gasoline having boiling point range 20-2000C is obtained as byproduct which contains a high proportion of aromatic hydrocarbon.

Typical aromatic product distribution from pyrolysis gasoline may be about 5-7% aromatics of which roughly 50% is benzene, 30% toluene and 20% m-xylene, including ethyl benzene.

Before aromatic separation pyrolysis gasoline need two-stage hydrogenation treatment process because they contain dienic and sulphur compounds

In the first stage selective hydrogenation of di-olefins takes place under mild conditions in the presence of nickel or palladium catalyst at temperature and pressure of 80-1600C and 10 atm and here the aromatic hydrocarbons are not affected.

Second step is the selective hydrogenation of olefins and hydrodesulphurization of C5-C6 cut carried out at temperature and pressure of 280-3500C and 15 atm in the presence of composite sulphide of molybdenum and cobalt on alumina.

After hydrogenation process, the pyrolysis gasoline is distilled to give fractions containing benzene, toluene, and xylene.

The objective of first stage hydrogenation is:To eliminate unsaturated hydrocarbons such as diolefins and

alkenyl aromatics.To provide hydrogenated pyrolysis gasoline with better stability

properties.To avoid gum formation in the second stage.

Diolefin + H2 OlefinsAlkenyl aromatic + H2 Alkyl aromatic

Catalyst is palladium which is sensitive to a number of impurities

The second stage hydrogenation aims to eliminating the olefins and sulphur compounds.

Olefin + Hydrogen ParaffinsCycloolefin + Hydrogen Naphthene

Sulphur compounds + Hydrogen Paraffins + H2SHere some of the contaminants which acts as poisons are

organic chlorides, existing gases, inorganic chlorides and other miner salts, As, Sb, Pb, Si etc.,

Aromatic Separation from reformate and pyrolysis gasoline

Catalytically reformed Naphtha is a mixture of benzene, toluene, xylene and close boiling non-atomic components especially alkanes and cycloalkanes.

Separation of aromatics from non-aromatics presents difficulties in product isolation because of their close boiling points.

Here also the separation of xylene(1390C) and ethyl benzene (1360C) difficult due to their close boiling point.

The main processing steps involved are:Separation of aromatics from non-aromatics using solvent

extraction.Separation of the aromatics from each other. The following options are available for the separation of aromatics Solvent extraction for separation of aromatics form non-

aromatics. Superfractionaction for separation of ethyl benzene from xylene. Separation of p-xylene from m-xylene using adsorption or

crystallization process.

1. Liquid-liquid extractor 2. Extractive distillation column 3. Solvent recovery column

4. Water wash column 5. Clay treatment 6. Benzene recovery column

7. Toluene recovery column 8. Xylene & Ethyl benzene column 9. Ethyl benzene recovery column

Selection of the solvent

Solvent selectivityRecoverability of solvent Density difference between dispersed and

continuous phaseViscosityMelting pointInterfacial tensionCorrosionToxicityThermal and chemical stabilityLow miscibility with feed solution

These are some of the solvents are used for separation of aromatics from non-aromatics:

N-Methyl Pyrollidene (2040C)N-Fromylmoropholine (2440C)Dimethylformamide (1530C)Sulpholane (2880C)Tetraethylene Glycol (2870C)Triethylene Glycol (3280C)