Structural biocomposites found in nature often have a well-defined organization on the nanometer scale. For mineralized materials, interactions between organic and inorganic phases are important for controlling crystal size, morphology, and spatial arrangement, which is a requirement when structural biomaterials are designed. In this paper, we studied influence of low concentrations of alginate on calcium carbonate crystallization by seeded and unseeded experiments, at controlled activity-based supersaturations. Crystal growth and nucleation were characterized by scanning electron microscopy (SEM), calcium concentration measurements, and crystal volume distribution measurements through the crystallization experiments. Alginate concentrations as low as 10 ppm were found to have a significant effect on growth of vaterite seeds, resulting in decreased growth rates and extensive agglomeration, compared to the case without alginate. For increased alginate concentrations (100 and 200 ppm), vaterite seed growth rates were decreased further. The decreased growth rates were probably caused by adsorption of alginate onto the active growth sites of the crystal surface. Alginate with 65% G-units (HighG) reduced the growth rate more than alginate with 43% G-units (LowG), which may be accounted for by the greater G-block length, and thus higher affinity to calcium, in HighG alginate. The unseeded experiments showed that mainly small vaterite crystals nucleated with 100 ppm alginate present, after an induction time of 50-80 min, while large calcite crystals were formed after some time by transformation from vaterite. The decreased crystal growth rates and higher nucleation rates caused by increased concentrations of alginate explain how small size mineral particles can be formed in alginate gel networks to form nanostructured composite materials.
The effect of initial supersaturation and temperature on crystal morphology below and above the threshold for the onset of spherulitic growth has been compared for calcium carbonate, L-glutamic acid, and an aromatic amine derivative in aqueous systems. For the vaterite polymorph of calcium carbonate is has also been documented how spherulites (polycrystalline particles) develop with time in a solvent mixture of water and monoethylene glycol. For the three studied substances, the temperature was found to have different effects on the compactness of the crystals. The influence of supersaturation on the onset of spherulitic growth and particle morphology, however, bears striking similarities for the investigated substances. While monocrystalline particles are found at low supersaturation, polycrystalline particles form with increasing degree of branching when supersaturation is increased.
Two UK gas/condensate fields are being developed as a single integrated subsea production system. Mono Ethylene Glycol (MEG) is used as a thermodynamic hydrate inhibitor. The main processes for recycling of MEG are regeneration and reclamation. Operational problems within closed MEG system are carbonate (CaCO 3) scaling deposits at MEG injection point, pipeline, and salt removal system as well as accumulation of corrosion products. This paper presents the results of prediction of precipitation of solids and corrosion in the closed MEG loop system and presents solutions for these challenges. The key points discussed in this paper are (i) The choice of acid to be used to neutralise excess alkalinity in the Lean MEG; (ii) The upper limit value of pH in order to keep a low corrosion rate and prevent scaling risk; (iii) The risk and consequences of HCl overdosage; (iv) The location of the injection point of this acid; (v) Calculation of the acid flowrate; (vi) The choice of the alkalinity source required to precipitate the divalent cations in the rich MEG pre-treatment; (vii) Calculation of the flowrate of this alkalinity source; and (viii) Monitoring of alkalinity and acid injection. The modeling methodology used as a basis for this study is a purely thermodynamic approach. The equilibrium calculations are done with the MultiScale software with the glycol add-in.
The transportation of natural gas in long subsea pipelines is a challenge when it comes to hydrate prevention, corrosion and mineral scaling. When monoethylene glycol (MEG) is injected into carbon steel pipelines to prevent formation of gas hydrates, the solubility of the corrosion products is altered. Understanding the kinetics of FeCO3 precipitation may make it possible to avoid deposition in the gas liquid separation process and improve solids removal in MEG recovery units. In this work, the growth kinetics of iron carbonate (siderite) has been studied in seeded batch experiments in MEG‐water solutions with 0 and 40 wt% MEG at 50 and 70 °C. Precautions were taken to keep anaerobic conditions and avoid oxidation of ferrous ions. The growth rate (G) was measured as function of supersaturation (S) and fitted to the equation: G = kr(S‐1)g. The growth order (g) was approximately 2 independent of the MEG concentration at the two temperatures. The growth rate constant (kr) was in the range of 6 × 10−11 to 1 × 10−10 m/s. Temperature increase from 50 to 70°C had no measurable effect on the growth rate while in the presence of 40 wt% MEG the growth rate constant decreased.
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