Incremental
& breakthrough innovations in manufacture of polyester raw materials
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Polyesters,
chemically known as polyethylene terephthalate (PET), flow into the
production of goods that we use in our everyday lives. It can be found in
plastic containers for beverages, food & electronics, apparel, home
textiles, carpets & industrial fibre products, and audio & video
recording tapes. Its most important use is as a fibre, though the fastest
growing use is for making bottles.
The
manufacture of all forms of polyesters requires two raw materials: a diacid
and a diglycol. The most commonly used of the former is purified terephthalic
acid (PTA) and that of the latter is monoethylene glycol (MEG). Companies
that are dominant in polyesters prefer to be integrated to both these raw
materials, as it allows them to capture margins along a longer value chain –
important in a competitive business environment – and provides assured
supplies.
The
technologies for manufacture of PTA and MEG have seen significant changes –
some evolutionary and some revolutionary – and the process is ongoing.
PTA – meeting purity requirements
Worldwide
around 65% of PTA produced goes into polyester fibre, 27% to PET bottle resin
and the remaining 8% to film and other plastic end-uses.
The
broad contours of the technology to produce PTA have remained essentially the
same. What has been seen, however, are incremental improvements that are just
as important and have cumulatively added to build competitive advantage to
one or other producer. The innovation efforts have typically focussed on
improving efficiency of operation; reducing consumption of energy, water,
catalysts, solvents etc.; and reducing the carbon footprint.
The
conventional process for making PTA starts with p-xylene (PX) –
an aromatic typically produced by refiners from aromatic-rich streams or in
naphtha crackers – and is essentially a catalytic oxidation. The crude terephthalic
acid (CTA) first produced contains an impurity, 4-carboxybenzaldehyde
(4-CBA), which gives a slightly yellowish colour to PET. While this can be
camouflaged by addition of other colorants for some applications, the PET
industry demands a higher purity product, which is obtained by the deployment
of a second purification step that reduces the 4-CBA content to less than
40-ppm – a level deemed acceptable.
The
first stage air oxidation of PX to CTA is catalysed by metals (cobalt,
manganese) and bromide, and is accomplished in acetic acid. Despite the
recycle of acetic acid, PTA manufacture is one of the largest end-uses for
this chemical globally, and new plants require a significant quantity at
start-up (though top-up volumes thereafter are significantly lower).
Purification of crude
The
purification of CTA requires at least one chemical step in addition to
physical measures such as crystallisation and washing. The removal is not
trivial, and, indeed, has been the focus of research. Typically, it is
carried out by catalytic hydrogenation of CBA to its reduced form, p-toluic
acid, which is much easier to remove by physical methods.
The
last stage in the overall process is the removal of all by-products produced
in earlier stages, to yield commercially acceptable PTA.
Technology evolution
There
are several technology vendors who offer variations on the above, and their
aim is to reduce the complexity of the purification steps and reduce the
capex needed. BP, whose technology accounts for three-quarters of operating
capacity, uses its technology at its own PTA plants, besides licensing it to
others. Three avatars of the technology – representing improvements in plant
reliability & operating rates, economic size, capex, wastewater &
solid waste generation, and emission of greenhouse gases – were rolled out
between 2003 and 2015. The latest generation technology recovers enough heat
to power the entire site where PTA is made and has surplus electricity to
sell to the grid. Furthermore, it has 95% lower solid waste disposal, 75%
lower water discharge and 65% lower greenhouse gas emissions, as compared to
the conventional technology.
Carbon-neutral PTA?
In
September 2016, responding to signals from consumers for more sustainable
products, BP launched PTAir, a new low carbon and carbon-neutral
PTA brand through a combination of technology and carbon management
expertise. A feature of PTAir is its use of proprietary PX
and PTA technology, which supports a 29% lower global warming potential than
the average European PTA production. In addition, BP also launched PTAir
Neutral, the world’s first certified carbon neutral PTA. PTAir
Neutral offers customers the opportunity to purchase a
carbon-neutral product where associated CO2 equivalent
emissions are mitigated through the investment in carbon projects providing
equivalent CO2 benefits and delivering a ‘net zero’ position.
Biological routes in the works?
Biological
routes to PTA are in the research stage and typically use metabolically
engineered Escherichia coli system for biological
transformation of PX into PTA. The engineered E. coli strain
harbours a synthetic pathway optimized through manipulation of expression
levels of upstream and downstream modules. The upstream pathway converts PX
to p-toluic acid and the downstream pathway transforms to p-toluic
acid to PTA. The system might be a promising alternative for the large-scale
biotechnological production of PTA and could be the foundations for the
future development of sustainable approaches for PTA production.
MEG – in shades of green …..
MEG
is the third largest volume ethylene derivative and was first commercialised
in 1925 by what became Union Carbide, now Dow. Global demand for MEG is about
30-mt, with far more diverse uses than for PTA including for making PET,
anti-freeze and a range of speciality chemicals.
The
vast majority of MEG is produced by the catalytic oxidation of ethylene
(produced from steam cracking) to first produce ethylene oxide (EO), which is
then hydrolysed to MEG. The reaction, typically carried out using a silver
catalyst, has been optimised to give over 90% selectivity to EO, and is
available for licensing from several vendors including Dow Chemical,
Scientific Design and Shell Chemical. The second step – the hydrolysis of EO
to MEG – has also been optimised to minimise formation of by-products, viz.
diethylene glycol (9%) and triethylene glycol (1%) (though these have markets
of their own). Indeed, better selectivity has driven innovation in MEG
production. A variation on the theme, offered by Shell Chemical, uses CO2 as
reactant to produce ethylene carbonate from EO, which is then hydrolysed at
100% selectivity to MEG (with CO2 recycle).
A
small fraction of ‘green’ MEG is commercially produced today – including in
India – starting from natural ethanol (from sugarcane). This product is now a
constituent of PET bottles marketed as ‘partly renewable’ by consumer-facing
companies like Coca-Cola, Heinz, etc. as part of their sustainability agenda.
Volumes of the ‘green’ MEG are still small, but the commercial-scale
availability of second-generation ethanol derived from biomass could open up
opportunities for larger volumes, provided the economics hold in an era of
low oil and gas prices. Producers of this renewable MEG lament their
inability to get customers pay a premium for the product.
…. And black
A
novel route that has opened up more recently – all in China – produces MEG
from coal. This is a four-step process in which synthesis gas (a mixture of
hydrogen and carbon monoxide) is produced by gasification of coal; then
converted to dimethyl oxalate by reaction with an methyl nitrate, and then
reduced to MEG. There is full recycle of the nitrous oxide and the alcohol
(typically methanol) – the two reactants needed to make the methyl nitrate in
the fourth step. The ‘black’ MEG is so far only produced in China and while
currently there are just six plants operating, their number is expected to
rise to 18 by 2018, with a cumulative MEG capacity of 2.3-mtpa.
Wider
adoption of coal-based MEG outside of China seems unlikely. The technology is
carbon-intensive compared to petrochemical routes and is nearly twice as
capital-intensive. However, their advantage comes from lower operating rates
(thanks to cheap coal), but this could be eroded if a tax were to be slapped
on carbon. Early plants had problems meeting the quality requirement for PET,
but have improved with experience. Since 2016, operational capacity for
coal-based MEG has risen steadily, and fewer plants are offline, and by 2021
the technology could serve about 20% of China’s total MEG demand (estimated
at about 17-mt). This will make a significant difference to the overall
markets for MEG, which otherwise is expected to stay tight till 2018.
A
further development on the coal-based route for MEG involves a direct
conversion of syngas to MEG – eliminating the oxalate ester step – and is
jointly under development by Johnson Matthey Davy Technologies Ltd. and
Eastman. This new technology enables the production of MEG from a variety of
raw materials, including coal, natural gas, or biomass and is based on new,
proprietary catalysts and process design.
No technology is ever mature
PTA
and MEG continue to see innovation that aim to exploit new raw materials,
reduce costs, improve margins and lower environmental impacts. They are good
examples of the adage that ‘no chemical technology is ever mature,’ however
long the chemical may have been around.
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- Ravi Raghavan CHWKLY 171017
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