Modeling Scale Formation and Optimizing Scale Inhibitor Dosages in Membrane Systems

摘要

Water chemistry design and treatment software currently in use for membrane systems is not adequate for the rigors of high recovery systems in many cases due to reliance upon simple indices for scale prediction, inhibitor selection, and inhibitor dosage estimation. Simple indices use total analytical values for calculating the driving force for scale formation and do not account for common ion effects and ion association. Their applicability is limited to lower TDS systems. This paper discusses the application of an industry standard ion association model calculation engine to predicting scale in membrane systems and optimizing control. The ion association model engine calculates the most likely distribution of species for a brine, and uses the free ion concentrations to calculate the driving force for scale for minerals such as calcite, gypsum, barite, celestite, and silica. Calcium phosphate based scales can also be modeled using this method. Models for optimizing scale inhibitor dosages, are also described, including the parameters critical to developing an effective dosage modulation model for scale inhibitors from laboratory data, field data, or a combination of both. The paper draws upon the concept of induction time as a basis for the mathematical models used to develop predictive models from actual data. The models are based upon the concept that threshold effect inhibitors do not prevent scale formation, they only delay the inevitable. The models are in agreement with current theories and practice and treat scale inhibitors as agents which extend the induction time before crystal formation and/or growth on existing active sites occurs in the case of calcium carbonate, calcium sulfate, and barium sulfate; and as dispersants which control particle size in the case of calcium phosphate and silica. Models presented are for commercially available inhibitors and have been used successfully to select treatments and optimize dosages in reverse osmosis systems, cooling water, and oil field applications.