The use of PPCA in scale-inhibitor precipitation squeezes: Solubility, inhibition efficiency, and molecular-weight effects

Phosphino polycarboxylic acid (PPCA) is an industry standard polymeric scale inhibitor (SI) that is often applied as a precipitation- squeeze treatment. In this phase-separated process, PPCA forms a sparingly soluble complex with calcium ions. We show that the solubility of the precipitated complex is rather different than the stock ("as supplied") PPCA. We examined important properties such as its inhibition efficiency (IE) and its response in assay [by inductively coupled plasma (ICP) spectroscopy or wet-chemical methods], which are all understood in terms of the molecular-weight distributions (MWDs) of the various species involved in the precipitation squeeze. Our findings on the MWD effects have relevance not just in determining the concentration of the polymer in the return curve (PPCA), but also in its ability to prevent scale formation (i.e., its IE in the PPCA return curve), and they have implications for how the process should be modeled correctly. We study the MWDs of PPCA in great detail along with the solubility behavior of the PPCA-Ca complex, which plays an important role in precipitationsqueeze treatments. We describe several novel findings on the solubility of the (PPCA-Ca) complex system. Specifically, the precipitated complex was isolated and its solubility was determined experimentally in various brine compositions and as a function of temperature. MWDs were determined for the various fractions of PPCA. The solubility of the precipitated PPCACa complex becomes lower as it is exposed to successive fresh supernatant brine, and the behavior is very unlike that expected from a "solubility product" model. However, by being able to carry out MWD experiments on the various PPCA species, a fairly complete understanding is being generated. This has led to the proposal of the "stripping" model of dissolution for the precipitated complex of PPCA. We believe that these results are the most detailed to be published in the literature on the PPCA system and that they are of particular significance and application for all polymeric SI precipitation- squeeze treatments. The relevance of these results for field precipitation-squeeze treatments is also discussed.