Demonstration of NIR inline monitoring for hops extraction and micronization of benzoic acid in supercritical CO2

The use of supercritical CO₂ as solvent for separation processes and chemical reactions is widespread. Many industrial extraction processes using CO₂ consume a huge amount of energy and hence are very cost-intensive. In the past, the duration of extraction was often longer than required for an optimum result. Therefore, an inline monitoring device for terminating CO₂ extraction processes at the right time has been desirable. In collaboration between KIT and SITEC-Sieber Engineering, Switzerland, a mobile near infrared (NIR) inline monitoring device has been developed to close this gap and to offer an opportunity to implement it into industrial extractions processes using CO₂ and to use the data for process optimization. NIR spectroscopy with fiber optics adaption has been proven to be a very good choice, because the CO₂ bands are well separated from all other bands of interest caused by the extracts (organics and water). The NIR inline monitoring device can easily be implemented into industrial plants to visualize the extraction progress and to terminate the process at the right time in order to save energy and money. Measurements on customer's demand are available for extraction and reaction processes in supercritical CO₂.This paper will actually focus on two applications. Firstly, in collaboration with NATECO₂, Germany, the NIR inline monitoring device has been implemented into a plant for hops extraction with supercritical CO₂. Extractions have been performed and successfully monitored for the varieties Hallertauer Magnum, Hallertauer Herkules, and French Strisselspalter at 60 °C and 25 MPa, 28 MPa, and 50 MPa, respectively. The detectability was 0.1 wt% of hops in CO₂.Secondly, the applicability of this NIR inline monitoring method on processes for the production of submicron particles in supercritical CO₂ has been demonstrated in a feasibility study. For this purpose, a RESS (Rapid Expansion of Supercritical Solution) high pressure plant (160 ml, max. 35 MPa, max. 120 °C) was built and the NIR inline monitoring device integrated. Two syringe pumps were coupled for an almost pulsation-free-feeding. Benzoic acid was used as a reference solid. In this feasibility study, the focus has not been to perform an optimized RESS process, but rather to monitor the loading of the CO₂ phase with benzoic acid continuously, as this information is crucial for the optimization of a RESS process and has not been available up to the present. The experiments have been carried out at 55 °C and 25 MPa as reference conditions. The performed calibration resulted in a detection limit of 0.1 mg benzoic acid per g CO₂. The loading of the CO₂ phase has been successfully and continuously monitored and can now be applied for other systems.A side effect of these investigations has been the finding that this NIR inline monitoring device can also be used for very precise solubility measurements and to visualize the establishment of thermodynamic phase equilibrium as a function of time, especially in cases of low solubility of a substance in CO₂. The NIR inline monitoring device can also be used for the inline monitoring of reactions producing data for kinetic modeling and process optimization.