Intrinsic Exposed Core Optical Fibre Sensors for Monitoring Heterogeneous Crystal Growth

Abstract

Unwanted surface (heterogeneous) crystallization, known as scale formation, can often be found in industrial processes that make use of water or oil and can cost the industries involved millions of dollars. Chemicals, known as scale inhibitors, can be added to process solutions in order to prevent or retard the formation of scale, however an accurate determination of the onset and growth of the scale is required to optimise the use of inhibitors. Current methods for monitoring scale are compromised due to inability of the techniques to discriminate between heterogeneous (surface) and homogeneous (bulk) crystallization. These two crystallization types propagate via different crystallization pathways and an ability to distinguish between the two crystal types is critical if the amount of the scale formed on a surface is to be determined. An intrinsic exposed core optical fibre sensor (IECOFS) has been tested for the detection and monitoring of heterogeneous crystallization. The exposed core of the optical fibre provides a surface for heterogeneous crystallization. The sensor works by the refraction of radiation out of the fibre by the presence of a medium of higher refractive index on the optical fibre core. A model describing the refraction of the radiation out of the fibre and out of the crystals has shown to accurately describe the IECOFS measurements. The sensor is capable of monitoring the average crystal growth process on the optical fibre sensor by the increase in interactions that accompanies the growth of the crystals. The IECOFS has shown to be insensitive to the homogeneous crystallization and is therefore able to distinguish between the two crystallization types. Furthermore, it was shown that the crystal growth kinetics between two different surface types under similar crystallization conditions do not differ significantly. Heterogeneous growth kinetic measurements have shown that the IECOFS can accurately monitor the heterogeneous process on a surface of interest, either in the presence, or not, of chemical scale inhibitors. The results obtained are important in the evaluation of the IECOFS as either an industrial or a lab based sensor for heterogeneous crystallization.