Role of chemical constituents in recycle brine on the performance of scale control additives in MSF plants

Abstract

To combat the scaling problems in multistage flash (MSF) desalination plants, either mineral acids or scale control additives (SCAs) are used. Scaling phenomena from acid is controlled by adjusting pH and total alkalinity and requires stoichiometric amounts while SCAs act non-stoichiometrically by threshold effect or crystal distortion effect. Antiscalant dose rates in brine are found to vary from plant to plant. Efficiency of SCA could be influenced by its chemical nature, plant configuration and design, heat transfer rates and its surface and antiscalant reaction with brine constituents. The study was carried out to understand the varied dose rate and the role of some of the brine chemical constituents on the efficiencies of antiscalants. Experiments conducted with a bench-top system by simulating the MSF conditions at 95°C with brine and performance of SCA were evaluated using the threshold effect technique. Performance or efficiency of two SCAs, viz., polyphosphonate based (PPN) and polymaleic acid based (PMA) were evaluated in the presence of traces of copper, chromium, nickel, molybdenum and iron, which are present in the brine as corrosion products. Effects of humic substances, particulates, and chlorine which is used as biocide, are also evaluated. Results of bench-top studies revealed that Cu, Fe, and Cr do have a detrimental effect on the performance of two SCAs (PPN and PMA), while Mo and Ni do not have an adverse effect. Humic substances and particulates also do not have significant effect. However, chlorination of SCA does reduce the performance of SCA.