Page 127 - CW E-Magazine (2-4-2024)

Point of View

The dynamics of aromatics production

Olefins and aromatics are the two most important categories of petrochemical raw materials, underpinning the manufacture of an
array of products that include chemicals, plastics and rubbers. Nearly all the olefins and aromatics produced today comes from fossil
fuels – mainly crude oil & its refined fractions, natural gas, and coal – though a small, but growing amount comes from more sustainable
raw materials.

While a range of olefins and aromatics are produced – differing in molecular architecture, physical/chemical properties and
applications – a handful dominate. The most important olefins are ethylene, propylene and, to a lesser extent, butylenes, while the
most important aromatics – characterised by the presence of a six-membered carbon ring – are benzene (B), toluene (T) and xylenes (X),
collectively referred to as BTX.

The xylenes come in three chemical flavours – ortho-xylene (OX), para-xylene (PX) and the less common meta-xylene (MX). All three
are colourless liquids, and have varying uses. PX, is the most important of the aromatics, and is mainly used as a feedstock to manufacture
other industrial chemicals, notably purified terephthalic acid (PTA), which, in turn, is used to produce polyethylene terephthalate (PET) that
goes to make plastic bottles, polyester fibres and films (in decreasing order of importance). OX is the main raw material for the production
of phthalic anhydride – used for making plasticisers, and alkyd resins, amongst others. MX is the smallest volume xylene produced and
used mainly for producing purified isophthalic acid (PIA), which is used to modify the properties of PET. Benzene is a key raw material for
making a host of organic chemicals, though a few end-uses like ethylbenzene (EB), styrene, and phenol, dominate. It is a carcinogen and
not desired in fuel applications, such as motor gasoline wherein its limits is usually restricted by law. Toluene finds use to make a smaller
range of chemicals, of which toluene diisocyanate – a raw material for polyurethanes – is most important.

Sources of aromatics
An important aspect of aromatics production is that they are by-products – be it in the making of olefins in steam crackers (from
the pyrolysis gasoline or pygas); in the production of gasoline in a crude oil refinery (via a process called catalytic reforming); or in steel
making (by extraction from coke oven gases).

Steam crackers, designed and optimised to produce olefins in the main, are an important source of aromatics. They are contained
in the pygas produced – the amount and composition of which is determined by the feed into the cracker (e.g., ethane, naphtha).
Not surprisingly, the lighter the feed, the lesser the quantity of aromatics produced. Benzene is the largest aromatic component in pygas,
and in a typical naphtha cracker it constitutes close to 40% of the pygas, with toluene accounting for another 20% or so. Globally, steam
cracking accounted for about a third of all benzene produced in 2023.

Refiners resort to catalytic reforming to increase the octane rating of straight-run naphtha (part of the middle distillates produced in
a crude distillation unit, CDU). This is achieved by converting low- to moderate-octane molecules in naphtha to aromatics, so producing
a reformate that is the most important source of BTX for the petrochemical industry, besides being an important blend component of gasoline.
In other words, for the petrochemical industry to access aromatics, refiners must realise a better chemical value for it compared to the
octane value it can fetch in gasoline. This is a dynamic that plays out all the time, and is an important determinant of aromatics supply.
In 2023, about 42% of all benzene came from catalytic reforming.

In the steel making, coal (or coke) is used as a reducing agent in coke ovens, and the coke oven light oils contain aromatics. Again,
benzene is the single largest component in this oil, with a ~70% share by weight, followed by toluene (20%) and mixed xylenes & EB
(together ~10%). Globally, in 2023, this route accounted for about 6% of all benzene produced.

Inter-conversion technologies
In addition to the above three routes for aromatics, there are specific technologies available to convert one aromatic component
to another. The focus of most of these interconversion technologies is to upgrade the value that toluene (which is overproduced) can
yield by converting it to benzene and PX (both of which are under-produced). PX is by far the most sought after molecule amongst the
aromatics, largely for its use as a starting point for building the polyester value chain.

The interconversion technology options include selective toluene disproportionation (STDP), toluene transalkylation, and xylene

Chemical Weekly April 2, 2024 127

Contents Index to Advertisers Index to Products Advertised