Non-invasive temperature monitoring in freeze-drying: control of freezing as a case study

In this study a new technology, based on the use of an array of thin-film thermocouples, is proposed for the real-time and non-invasive monitoring of product temperature during a freeze drying cycle. This array of thin-film thermocouples was demonstrated to give quantitative accurate results without altering the freezing and drying behaviour of the monitored vials, making this sensor a good alternative to conventional wire thermocouples. Furthermore, unlike conventional thermocouples, the array of thin-film thermocouples can measure the entire temperature profile within the product being dried and can also measure the position of the interface of sublimation. This information was then used to estimate the heat and mass transfer coefficient of the system under investigation, showing a fairly good agreement with data obtained by conventional, time consuming procedures. Finally, it is shown how these parameters can be used to guide the lyophilisation professional during the cycle development.