Propylene oxide (C3H6O, PO) is a major industrial product with a global production of more than 10 million tons per year [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 analyzer is the preferred solution to reduce chemical treatment, improve product quality, and increase profits.

NIRS system configuration for online analysis of water content in PO streams. All figures were adapted from GIT Labor-Fachzeitschrift article [2].
Figure 1. NIRS system configuration for online analysis of water content in PO streams. All figures were adapted from GIT Labor-Fachzeitschrift article [2].

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. Therefore, the hygroscopic nature of PO necessitates inline or online analysis of water content for the most precise results. Additionally, «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 1). 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. Gain more control over production with a Metrohm Process Analytics NIRS XDS system configured for applications in hazardous areas, capable of monitoring up to 9 process points with the multiplexer option.

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. 

Table 1. Typical water concentration range in PO according to ASTM guidelines
Components Range (mg/L)
Water 20-30
Validation of the NIR process data by laboratory Karl Fischer (KF) titration; Inset shows enclosed NIR calibration model for predicting water concentration in propylene oxide process streams. All figures were adapted from GIT Labor-Fachzeitschrift article [2].
Figure 2. Validation of the NIR process data by laboratory Karl Fischer (KF) titration; Inset shows enclosed NIR calibration model for predicting water concentration in propylene oxide process streams. All figures were adapted from GIT Labor-Fachzeitschrift article [2].

A reference method (i.e. KF titration) (Figure 2) is mandatory to build the 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, the samples had captured 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. 

Table 2. Dedicated solutions offered by Metrohm Process Analytics for sampling needs
Sampling Flow cells type Measurement Fiber type Connection Process measurement
 Flow cell Fixed pathlength
2 mm SS316
Transmission Single Swagelock  Online
Variable pathlength
0.5–12 mm
Transmission
Single Swagelock
Online
PTFE Flow-through cell Transmission
Single Swagelock
Online
  • Optimize product quality and increase profit with faster response time to process deviations 
  • Greater and faster return on investment (ROI) 
  • No manual sampling needed, thus less exposure of personnel to dangerous chemicals
  1. Kawabata, T.; Yamamoto, J.; Koike, H.; Yoshida, S. Trends and Views in the Development of Technologies for Propylene Oxide Production; Sumitomo Kagaku, 2019; pp 4–11.
  2. Kleimeier. Nahinfrarotspektroskopie Produktionsprozesse Unter Der Lupe. GIT Labor-Fachzeitschrift 2018, 36–38.
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