Propylene oxide (C₃H₆O, PO) is a major industrial product with a global production of more than 11 million tons [1]. PO is mostly produced to make polyether polyols, propylene glycol, propylene glycol ether solvents, and other products.

There are several production processes available, however the majority of PO is still co-produced along with styrene monomer (approximately one-third of PO production worldwide). Other PO production routes include the chlorohydrin process, epoxidation of propylene with hydrogen peroxide, epoxidation of propylene with organic peroxides, and even epoxidation using molten salts.

This Process Application Note presents a method to closely monitor in «real time» low levels of moisture in PO safely, reliably, and optimally. Due to the hazardous and hygroscopic nature of PO, a single explosion-proof inline process analyzer is the preferred solution to reduce chemical treatment, improve product quality, and increase profits.

PO is a highly hazardous, flammable, and hygroscopic substance, and therefore needs to be handled with extreme caution. Tight control over moisture and other impurities in the final product (as well as along the manufacturing process at critical points) is necessary to overcome unwanted side reactions or poor yields.

Manual laboratory methods can be quite cumbersome and can introduce bias depending on the analyst. Additionally, the hygroscopic nature of PO necessitates inline or online analysis of water content for the most precise results. «Real-time» analysis is a requisite for high throughput PO production because this gives short response times in case of process changes or increased water content in the final product.

Safe analysis of low moisture content in PO is possible with reagent-free techniques such as near-infrared spectroscopy (NIRS). Suitable NIRS process analyzers are available for use in hazardous environments with robust stainless steel flow cells (Figure 1a). Metrohm NIRS process analyzers enable comparison of «real-time» spectral data from the process to the primary method (titration) to create a simple, yet indispensable model for the PO production process.

a)

b)

Wavelength range used: 1850–1950 nm. Stainless steel flow cells were used for online measurements. Explosion-proof process analyzers are recommended for hazardous areas such as these. 

A reference method (i.e., Karl Fischer titration) (Figure 1b) is mandatory to build the NIRS prediction models.

Measurements performed in the laboratory showed higher water content values than those predicted via online NIRS. By the time the KF titration was performed in the lab, the PO samples had absorbed atmospheric moisture and were no longer completely representative of the actual process conditions. Therefore, online KF titration was used to build more accurate prediction models.

WP-023 Karl Fischer titration and near-infrared spectroscopy in perfect synergy

8.000.5325 Water Content Analysis

1. Propylene Oxide Market Size, Growth, Share & Forecast, 2032. https://www.chemanalyst.com/industry-report/propylene-oxide-po-market-755 (accessed 2023-09-28).
2. Kleimeier. Nahinfrarotspektroskopie Produktionsprozesse Unter Der Lupe. GIT Labor-Fachzeitschrift 2018, 36–38.