Aromatic hydrocarbons especially benzene, toluene, xylene, ethyl benzene 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. Originally, the aromatics were produced from coal tar distillation, which is the by-product of destructive distillation (carbonisation).
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. Various sources of aromatics is mention in Table M-VII 7.1.
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-xyleneIsohthalic acid. Typical yield of benzene, toluene, xylene in kg per tonne of coal carbonised is about 2.8, 0.5-2, and 0.1-0.5 kg.
Table M-VII 7.1: Various Sources of Aromatics
(Coke oven plant)
From coke oven plant during carbonisation, 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 tonne of coal.
Steam cracking of
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.
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, isomerisation of paraffins and naphthenes, dehydrocyclisation of paraffins to aromatics, 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.
Dearomatisation of naphtha
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 and disproportionation
Hydrodealkylation: It involves production of benzene by dealkylation of toluene either by catalytic or thermal process.
Catalytic process: Hydeal, Deltol
Thermal process: HAD (ARCO), THDC Gulf Oil
Disproportionation: It involves conversion of toluene into benzene and xylenes
This process consists of conversion of C8 stream into valuable o- and p- xylene having isomerisation and isomer separation stage.
This process uses metallosilicate zeolite catalyst to promote
dehydrogenation of paraffins followed by oligomerisation and
dehydrocyclisation of paraffins followed by oligomerisation.
This process uses a shape selective catalyst to convert C2 and C3paraffins to aromatics.
It is similar to conventional catalytic reforming processes and L-type zeolite
This process consists of conversion of C8 stream into valuable o- and p-
xylene having isomerisation and isomer separation stage.
This process uses a metallosilicate zeolite catalyst to promote dehydrogenation of paraffins followed by oligomerisation and dehydrocyclisation of paraffins followed by oligomerisation.
This process uses a shape selective catalyst to convert C2 and C3 paraffins to aromatics.
It is similar to conventional catalytic reforming processes and L-type zeolite catalyst.
A typical catalytic reforming process includes following three sections:
Basic steps in catalytic reforming involve feed preparation, temperature control, reaction in reformer and product recovery, various types of catalytic reformer are – semi regeneration, nonregeneration cyclic moving bed two types of reformer reactors are in use radial flow and axial flow. Details of this has been covered in Module VI Lecture 6.
Reactions in Catalytic Reforming Process
Number of reactions takes place in catalytic reforming. Dehydrogenation is one of the major reactions. these reactions are discussed in detail in module VI lecture 6Some of the major reactions are
Methyl Cyclohexane → Toluene + H2
MCP → Benzene + H2
n-Hexane → Neohexane
Dehydrocyclisation of paraffins, i-paraffins to aromatics
n-heptance → Toluene + H2
Btx from Petroleum
Major Units of Aromatic Complex
Process steps in aromatic production: Figure M-VII 7.1 gives the details description of aromatics complex. The various steps involved in aromatic production are given below:
Separation of Aromatics: As non aromatics and some of the aromatics have close boiling points, various methods used for their separation are
Various process variables in the catalytic reforming for the production of aromatics are
Details of these parameters are discussed in Module VI Lecture 6