Calcium carbonate surface/bulk scaling mechanisms and kinetics in a once-through in-situ flow visualization rig

Scale management is usually a complex mixture of prediction, inhibition and sometimes removal strategies. Investigations into scale formation have largely been focused on precipitation in the bulk solution by assuming that surface scaling always results from pre-precipitated crystals in the bulk solution. Recent observations have shown that bulk and surface scaling do not share the same trends with respect to crystal growth kinetics and inhibition as such the relationship between these two scaling processes are being given attention in the literature. Despite much recent attention on scale formation on surface, there is still not a full mechanistic understanding of how scale layers build on component surfaces. Most of the previous studies have been carried out in a closed system where the saturation ratio was decreasing as a function of time. As such, the understanding of the precipitation/deposition system needs a suitable methodology to build an accurate surface deposition kinetic model. The objectives of this study are to develop a once-through flow rig system suitable to distinctively study bulk precipitation and surface scaling processes and also to improve on the understanding of surface scale deposition mechanisms and kinetics at constant supersaturation. The in-situ flow rig is designed to combine measurement of turbidity in the bulk and real-time visualization of scale build up on a solid surface. Calcium carbonate (CaCO¬3) surface and bulk scaling deposition were followed in-situ and in real-time in a newly developed flow rig that allows assessment and control of various scaling indices and parameters. The kinetics and mechanisms of CaCO3 surface scaling are evaluated from images taken with time using the image analysis protocols to determine the surface coverage, number of particles and average size of the crystal. Brines with different values of saturation ratio (SR) 15, 25, 45, 60, 70 and 90 were tested at 25˚C and 40˚C. The effects of SR, flow rates and temperature on scale deposition were studied. The flow rates used are 10ml/min, 20ml/min and 40ml/min. Also, the influence of SR and inhibitor concentration on surface inhibition efficiency of Polyphosphinocarboxylic acid (PPCA) were studied. The newly developed set-up allows for a mechanistic understanding of scale build up on the surfaces in flowing conditions at constant SR and helps to improve the understanding of both bulk precipitation and surface deposition scaling kinetics. Results show that at low SR, the residence time from the mixing point to sample was shorter than the induction time for bulk precipitation and, as a result, there are no crystals in the bulk solution as the flow passes the sample. Therefore, in contrast to popular thinking, the study has shown that crystals present on the surface are not always the result of a secondary deposition process occurring after the precipitation mechanism which occurs in the bulk solution. The determination of surface crystallization mechanisms and kinetics allow for the correct type and dosage of inhibitor to be selected. Addition of inhibitors at the bulk minimum inhibition concentration (MIC) actually aggravates surface scalin