An experimental and numerical study of the kinetics of barium sulphate in flowing systems

Abstract

The formation and deposition of mineral scales, such as barium sulphate (BaSO4) and calcium carbonate (CaCO3), is a common problem in many industrial and life science processes. This is caused by chemical incompatibility due to either the mixing of incompatible aqueous solutions or due to changes of the physical conditions, usually temperature and pressure. Many laboratory studies have been conducted using techniques broadly classified into batch and flowing tests to understand the reaction and mechanisms. In this study, we focused on the dynamic (kinetic) deposition of barium sulphate arising from the mixing of 2 incompatible brines. The mechanism of barium sulphate (barite) deposition is often assumed to be a one-step reaction in which the ions in the bulk fluid directly deposit onto a surface. However, there is strong evidence in the literature that barium sulphate may deposit through an intermediary nanocrystalline phase which we refer to as BaSO4(aq) in this work. This initial nucleation species or nanocrystalline material (BaSO4(aq)) may remain suspended in the aqueous system and hence may be transported through the system before it ultimately deposits on a surface In this work, we have formulated a barite formation/deposition model which includes both of these mechanisms noted above, i.e. (i) barite formation in solution of a nanocrystalline precursor which may be transported and deposited at an interface and (ii) the direct kinetic deposition of barite from the free ions in solution. The kinetic approach is most important in flowing conditions, since the residence time in a given part of the macroscopic system (e.g. in a pipe or duct) may be shorter than the time required to reach the full equilibrium state of the system. A CFD study is carried out by solving the Stokes equations to accurately model the local residence time, species transport, and calculate the hydraulic and mass transfer layers. Geometry alteration due depositing barite is also an important phenomenon to consider and model in a flowing system. This is rarely done in mineral deposit calculations, especially with a full kinetic deposition model, but it is included in our model. The geometry change affects both hydraulic and mass transport layers in the vicinity of the depositing surface and may often change the deposition regime in terms of the balance of dominant mechanism which applies. The effect of geometry change on the local residence time is investigated through performing a ramping up of the flow rate and explicitly deforming the geometry as the deposition occurs. We also performed and report experiments on two levels to gain information on the kinetics. First, we studied the kinetics of incompatible brines using batch tests. Second, we developed a laboratory experimental flow cell that enabled us to (i) use different flow geometries through 3D printing, (ii) visualise the deposition process as it happens, and (iii) understand the rates of the reactions by analysing the effluent from the system. We used three different categories of geometries including a (i) simple flow channel, (ii) simple constrictions with different configurations to enforce different mixing regimes, and (iii) more complicated geometry with different constriction sizes. This allowed us to investigate the hypothesis developed in the modelling work. The visual findings from laboratory experiments show the deposition growth happens in the normal direction of the flow, and as the local residence time reduces, the deposition tends to move further down the line. This is true in all three different geometries investigated, showing the concept of the diffusion penetration length. Our results from the modelling and experimental work, show that in the laminar flow regime, the extent of deposition on a surface is limited by the diffusion penetration length (δ) referred to above. This means that there will be more deposits at lower flow rates, where the diffusion penetration length is larger. In this case, since the diffusion penetration length is relatively larger, the deposition mechanism will be kinetics-limited. As the deposition reduces the flow path cross-section area near the inlet vicinity, the velocity increases. Thus, the hydraulic layer becomes smaller, resulting in a smaller diffusion penetration length, which causes the deposition location to move towards the end of the flow path, where the velocity is still lower. In this case, since the diffusion penetration length is relatively smaller, the deposition process will be more transport-limited. The results of this study have the potential to contribute to the development of more effective strategies for preventing scaling in a wide range of industrial processes.