How minimum inhibitor concentration (MIC) and sub-MIC concentrations affect bulk precipitation and surface scaling rates

Conventional testing of scale inhibitors (SI) usually focuses only on the inhibition of bulk depositional processes, e.g. in jar tests of inhibition efficiency. In previous work, we have extended the study of inhibition efficiency to examine both bulk and surface depositional processes and their inhibition using scale inhibitors. Where the minimum inhibitor concentration (MIC) is well exceeded, both bulk and surface scaling are prevented. However, we have demonstrated that the presence of an inhibitor, at levels marginally below the MIC, actually enhances surface scale growth over a range of temperatures typically encountered in the production system. Where this precise concentration lies with respect to the MIC is not generally known for scaling systems and it is therefore difficult to predict where potential problems with sub-MIC scale inhibitor levels would occur.

An experimental programme has been conducted to assess the efficiency of PPCA in inhibiting barium sulphate scale formation in the bulk solution and on a metal surface over a range of concentrations and test temperatures. The concentration range tested was chosen (a) to allow identification of the MIC at which both surface and bulk scale formation is controlled and, (b) to establish where potential problems with enhanced surface scaling arise at sub-MIC levels. The results obtained from this study have allowed trends in surface and bulk inhibition to be established and the region of sub-MIC concentrations where surface scaling is enhanced to be identified for three temperatures. We summarise our results using some simple schematic models of the bulk and surface scaling regimes and the corresponding inhibition process.