A review of in-line and on-line measurement techniques to monitor industrial mixing processes

[Display omitted] •Review of in-line and on-line sensors for industrial mixing processes.•Reviewed techniques are suitable for process control and within Industry 4.0.•Liquid-liquid, gas-liquid, solid-liquid, solid-gas-liquid, solids mixing included.•Pharmaceutical, food, polymers, rubber, cement and concrete industries included.•Spectroscopic and tomographic techniques reviewed in addition to many others. Mixing is a ubiquitous operation in process engineering. It is not only used for combining materials, but also for promoting heat and mass transfer, increasing aeration, suspending solids, and modifying material structure. Measurement techniques have the potential to optimise industrial mixing processes and improve product quality by monitoring critical process parameters. Real-time sensing techniques that do not require manual material sampling are of particular interest owing to their automatic data acquisition capability. This makes them suitable for use in control systems for process automation and eventually as connected sensors in Industry 4.0. This review article focuses on these measurement techniques, defined as in- and on-line, along with their capability for implementation in industrial processes. The applications reviewed include liquid–liquid, gas–liquid, solid–liquid, solid–gas–liquid, in addition to solids blending. A technique selection section discussing the decision-making process when choosing a sensor and a summary table including the advantages, disadvantages, applications and limitations of each technique are provided. The article concludes by discussing the future of monitoring techniques for mixing processes.