Structure/Performance relationships of high-temperature MSF scale control additives

Abstract

Low molecular weight polymers (MW w = 500–10,000) of maleic, fumaric, itaconic, methacrylic, acrylic, and vinylsulfonic acid were prepared and evaluated as high-temperature alkaline scale inhibitors. Suppression of calcium carbonate and magnesium hydroxide scale was measured in a laboratory single-stage flash, evaporator (120°C) and the results compared with the performance of monomeric inhibitors such as amino and hydroxyphosphonates and phosphonocarboxylates. Some critical properties which contribute to scale control were also investigated - including the ability of these additives to inhibit crystal growth, to disperse or suspend alkaline precipitates, and to remain soluble in concentrated seawater over a range of temperatures. Key findings of these studies included the following: 1. Low molecular weight maleic-based polymers provided the most effective all-round scale control. 2. Magnesium hydroxide deposition was more difficult to control than that of calcium carbonate, because Mg(OH) 2 crystal growth inhibition and crystallite dispersion required much larger amounts of inhibitor. Monomeric antiscalants and polyvinylsulfonic acids were essentially ineffective against magnesium hydroxide. 3. Poor compatibility or temperature-dependent solubility in seawater rendered some additives ineffective. For example, hydroxyethylidene diphosphonic acid and polytaconic acid (MW w∼7,000) formed insoluble calcium and sodium complexes which precipitated from seawater. The relationship between the alkaline scale control performance of the various additives and their chemical structure, molecular weights and degree of ionization is discussed.