Calcium Sulfate Crystallization in Presence of Fluorecent-Tagged Polyacrylate and Some Refinement of Scale Inhibition Mechanism

Abstract

Deposits of calcium sulfate scale on the surfaces of industrial equipment in distillation facilities, reverse osmosis desalination plants, in oil and gas industries lead to significant clogging of pipes and membranes and to a serious increase in production costs. For the mitigation of scale formation, the wide spectrum of antiscalants is applied. The present work is dedicated to the study of calcium sulfate deposition from supersaturated aqueous solutions in the presence of polyacrylic antiscalant with fluorescent marker (naphthalimide fragment) PAA-F1, which provides traceability of the scale inhibitor and a better understanding of its efficacy. A paradoxical phenomenon is being described here. Antiscalant causes a change in the crystal phase from bassanite to gypsum, significantly reduces the amount of deposit, but does not reveal the presence of its molecules either at the kink, step, or at selected edge sites of deposited crystals. Contrary to the predictions of the theory, it either stays in the aqueous phase, or forms its own separate phase Ca-PAA-F1. It is demonstrated that the antiscalant does not block the surfaces of calcium sulfate nuclei or crystal surfaces, but rather the foreign particles of nanodust with particle sizes around 1 nm, naturally occurring in any aqueous solution. As a result, the number of calcium nucleation sites is reduced, and the rate of scale formation decreases without any apparent antiscalant–scale interaction. On the grounds of the current experiment, the substoichiometric effect of scale inhibition obtained an alternative, quite stoichiometric explanation.

Enhancement of Polyacrylate Antiscalant Activity during Gypsum Deposit Formation with the Pretreatment of Aqueous Solutions with Spruce Wood Shavings

“…The fluorophore fragment represents 0.5% of the total mass of the PAA-F1 molecule, as shown in Figure 2. It was observed that PAA-F1 functions as an effective antiscalant for gypsum [16] and is superior to PAA due to its ability to be monitored by fluorimetry for concentration measurement.…”

“…The corresponding data are shown in Table 1. Our previous research [16] has revealed the unavoidable presence of a smaller particle fraction (1 nm ≤ size ≤ 100 nm), with concentrations reaching at least 10 7 units per 1 mL. Consequently, these nano/micro-impurities might act as crystal formation centers for gypsum.…”

“…In particular, the crystal size and gypsum crystal growth phenomena change in the presence of fibers in a manner similar to polymer additives. Meanwhile, the inflexible microfibers do not seem to act in the same way as the flexible polymers with a molecular mass of 2000 to 4000 Da and a molecular length of around 12 nm [16]. Unfortunately, the lack of important experimental details in [14,15], such as the sorption of calcium ions and particulate matter, does not allow the mechanism of action to be clarified.…”

“…Gypsum scale formation in artificially prepared supersaturated aqueous solutions was chosen as the model process following the previous publications on wood pulp fiber activity studies [2,14,15]. These experiments were carried out in comparison with a conventional polyacrylate antiscalant (PAA-F1) [16].…”

Homogeneous Organic Crystal Nucleation Rates in Solution from the Perspective of Chemical Reaction Kinetics

“…The reported small reduction in the nucleation rate for isonicotinamide crystallization from ethanol after removing particles with sizes > 0.45 µm [53] appears to be consistent with this proposition. It has also been reported that nano-sized (diameters ~1 nm) impurities are likely present in solutions with extremely high number densities [63], but these impurities would likewise not remove the need for assembling the complex transition state. They may perhaps appear as inclusions in nuclei formed in their neighborhood (note also that 1 nm is of a similar size as the reversible aggregate sizes).…”