A process control system can include a vessel, and at least one heat transfer property sensor that measures a heat transfer property of a substance at the vessel. The process control system can also include a monitoring device that receives an output of the heat transfer property sensor, and a process control device that is adjusted in response to the heat transfer property sensor output. A method of controlling a process can include measuring a thermal conductivity of a substance at a vessel, and adjusting the process in response to the measuring.