A Containment Building and Scrubber Design for Reduction of Risk in a Process with a Highly Hazardous Chemic

Phosgene, also known as carbonyl dichloride, is a strategically-important industrial
intermediate, used for the large-scale production of aromatic diisocyanates such as MDI
and TDI, which in turn provide the basis of innovative polyurethane materials.
Phosgene is a highly hazardous chemical and can, even in small amounts, cause acute
damage to the respiratory tract following inhalation. Higher doses of inhaled phosgene
can be life-threatening. At Bayer, as pure intermediate for use in manufacture, phosgene
is neither transported or stored, but it is processed and immediately consumed at its site
of production. In doing so, Bayer follows the highest global safety practices known for an
industrial chemical. Bayer has more than 100 years of experience of working with
phosgene and has been safely using it on a large scale for more than half a century.
Each Bayer plant working with phosgene complies with both core principles of its global,
phosgene-specific safety concept. Firstly, there are primary measures (prevention
measures) which ensure that no phosgene can leave the equipment, apparatus and
pipework of the unit. In the very rare event of a minor leakage, levels of phosgene do not
exceed the prescribed regulatory limits at the plant or site perimeters. Secondly, the
phosgene-producing isocyanate industry, including Bayer, has voluntarily committed
itself to install so-called secondary measures that go beyond standard procedures of the
chemical industry (additional mitigation measure). These measures serve to protect and
mitigate against the unlikely event of a large-scale leakage at a plant. At Bayer, usually
three forms of secondary measures are available, each of which has been tried-and-tested
and is considered of equal value and effectiveness: containment with caustic scrubber,
fully jacketed technologies, steam ammonia curtain. The choice of method is dependent,
amongst other factors, on the process design, the process conditions, a specific process
risk assessment, the location and the size of the plant as well as local regulatory
requirements.
The scrubber system is designed to handle both contaminated ventilation air from
containment and the discharges from process relief devices that would have significant
phosgene gas concentrations. The phosgene is neutralized to chloride and carbonate salts
with a caustic scrubbing solution. The design basis for the scrubber packing is a very low
outlet concentration, so that atmospheric dispersion modeling would not be necessary to
prove acceptable levels of the toxic gas at ground level. The limiting design case for the
scrubber packing is to treat exhaust air from the containment building with the maximum
achievable phosgene concentration regarding the worst case leak scenario. The height of
packing in the scrubber was scaled up from HTU data determined in a pilot plant at Bayer
AG in Germany. An Aspen Plus Rate-based model was also used to model the gas to
liquid interfacial mass transfer coupled with the very rapid neutralization reaction in the
liquid phase of the scrubber. During normal operation, when there is not an emergency,
ventilation air bypasses the scrubber to minimize carbonate formation in the circulating
caustic solution. In case phosgene is detected inside the containment the redundant
switching system is automatically activated. Structured packing, while expensive and
vulnerable to fouling, saves height and capital costs for the vessel and support structure
installation; packing height for 250Y packing is calculated to be 67% of the bed height
required for a distillation-quality discrete packing, such as #40 IMTP SS packing.