Page 135 - CW E-Magazine (15-10-2024)

Point of View

Eternal vigilance key to maintain incident-free safety

record of phosgene industry

Phosgene is an important chemical that finds wide use, both for the manufacture of some large volume chemicals, as well as for
several small volume ones. The products made from it go to making polymers, agrochemicals, pharmaceuticals, aroma chemicals,
etc. What makes phosgene stand-out, however, is it hazard. Even at small doses, phosgene is deadly and can cause severe damage
to the lungs. But that is not to say that the risks cannot be managed. And they are.

Today, almost all phosgene produced is consumed on site, and there is little or no storage. This mitigates – but does not eliminate –
a significant portion of the risks associated with its use. To further better safety, a wide body of learning is available from some of
the major chemical companies that have decades of experience in handling this chemical. What is noteworthy is that these companies
are open to sharing their experiences with companies less expert. This was evident at the 12th India International Phosgene Safety
Conference recently held in Vadodara (a detailed report will appear in the October 22 issue). The conference was organised by
the Indian Phosgene Council (IPC), which is probably the only industry body focused on just one chemical, under the aegis of the
Indian Chemical Council. The IPC has as its members all major phosgene producers in India (Atul, GNFC, Paushak, and UPL) and the
international majors (BASF, Covestro and Huntsman) who do not produce phosgene in India but do handle isocyanates.
Industrially, phosgene is produced by passing purified carbon monoxide and chlorine gas through a bed of porous activated carbon,
which serves as a catalyst.
Listed as a Schedule 3 substance under the Chemical Weapons Convention, phosgene is considered too dangerous to transport
in bulk and is produced/consumed within the same plant, as part of an ‘on demand’ process. This involves maintaining equivalent
rates of production and consumption, which keeps the amount of phosgene in the system at any time low, reducing risks in the event
of an accident.

Isocyanates and polycarbonate
The most important category of products commercially produced using phosgene are the isocyanates, of which the two most
important are toluene diisocyanate (TDI) and methylene diphenyl diisocyanate (MDI). While TDI is used to make flexible polyurethane
(PU) foams, packaging, and textiles, MDI, is more versatile, and used in the production of rigid PU foams (for building/industrial insulation
and refrigeration), reaction injection-moulding, thermoplastic resins, high-performance elastomers, and spandex fibres, as well as in
coatings, adhesives, and sealants. Globally, MDI production accounted for ~55% of global phosgene demand (in 2021), with ~27%
going to make TDI.

Another large-volume outlet for phosgene is the manufacture of polycarbonate (PC) resin, which is obtained by reacting bisphenol-A (BPA)
with phosgene. This polymer finds use in industrial equipment parts and electronic components and accounted for ~13% of global
phosgene consumption in 2021.
But the phosgene route to PC entails a number of drawbacks in environmental terms, including the toxicity of phosgene, the use of
the low-boiling-point solvent methylene chloride to which exposure must be restricted, and the large quantity of wastewater containing
methylene chloride which must be treated. A further incentive to eliminate use of phosgene is the economic penalty incurred because
the chlorine content of the phosgene is wasted and converted to sodium chloride. Caustic soda is consumed in this conversion and
the disposal of waste salt solutions presents an ecological problem by itself.

Today, several PC producers including Sabic, Covestro, Asahi and Mitsubishi Chemical/Mitsubishi Gas Chemical have independently
developed non-phosgene PC production technologies, based on the trans-esterification of diphenyl carbonate with BPA and using these
processes (though not exclusively) for PC manufacturing. The non-phosgene routes have gained market share in recent times, and
given their lower plant construction and feedstock costs, it is expected they will be even more widely adopted. As a consequence,
the main driver for future phosgene demand increase – expected at a CAGR of ~4-5% – is expected to be production of isocyanates,
not PC.

Global markets
Global phosgene production has risen from ~4-mt in 2000, to about 12-mt in 2020 and is expected to reach ~20-mt by 2030.

Chemical Weekly October 15, 2024 135

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