The deposition reaction of the mineral scale is a fundamental process in both natural and industrial processes, including the bio-mineralization in living tissue, the scale growth in oil and gas fields, and the mineral fouling in the water treatment process, etc. Barium sulfate is one of the most serious problems in scale deposition, due to its extremely low solubility, the potential to form difficult-to-remove scale and the tendency to co-precipitate with radium. Knowledge of barium sulfate crystallization, nucleation and solution thermodynamics has been established during the past decades. The next important objective is to understand the mechanism and kinetics of barium sulfate deposition. To reach this objective, we need to answer three questions:
(1) What is the mechanism of deposition kinetics in pipe flow? Crystal growth is the main process in scale deposition. In this work, the effects of temperature (50-150 ℃) and SI values (0.5 – 1.5) are tested in a dynamic loop apparatus. The deposition rate profile is also measured along with the tubing. From the experimental results, the deposition reaction is by and large founded to be diffusion controlled. The surface area change of the crystal layer also affects the deposition rate.
(2) How does scale deposition begin? In a different experiment with a microfluidic chip, crystals were observed, which represent the scale formation kinetics in the boundary layer. This result also explains the contradiction between previously reported induction times and the present deposition profiles: the predicted barite induction time is longer than the retention time inside the tubing in this work, which suggests that there should be no deposition. However, the deposition always happens near at the tubing entrance. The presence of the boundary layer gives enough time for crystal to form inside the tubing. The deposition begins with either heterogeneous nucleation on the tubing wall or particle formation in the boundary layer followed by attachment. Consequently, once the solution is supersaturated and the predicted induction time is shorter than operation time, the deposition will take place on the surface.
(3) What is the inhibition mechanism? Scale inhibitors including phosphonate, sulfonate and carboxylate polymers are tested in dynamic-loop experiments. In fresh tubing, induction-time theory can predict the required minimum inhibitor concentration (MIC). In tubing fully covered with barite, a Langmuirian adsorption mechanism can be used to model inhibitor performance. The result is that adsorption to crystals and nuclei is generally the mechanism of scale inhibition.
(3) What is the inhibition mechanism? Scale inhibitors including phosphonate, sulfonate, and carboxylate polymers are tested in dynamic-loop experiments. In fresh tubing and tubing partially coated with barite, a trace amount of inhibitor can stop the deposition process. The induction time model can be used to predict the required minimum inhibitor concentration (MIC). In tubing fully covered with barite, a Langmuir-type equation can be used to model the performance of inhibitor.