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.