Brine and scale chemistry in MSF distillers

Abstract

The two mechanisms accounting for alkaline scale formation inside MSF units are reviewed in short. Previous studies addressing the problem were primarily concerned with the mass or composition of the formed scales. As the nature of the scale depends on the composition of the brine out of which it separates, it is only reasonable to consider both phases simultaneously. Brine samples were extracted from the condensers, brine heater and flash chambers of two types of 18-stage, MSF units operating in the Umm Al Nar desalination plant (Abu Dhabi, UAE) over a period of 4 years. These were analyzed for Ca 2+, Mg 2+, CO −2 3, HCO − 3, pH and the temperatures of the various cells recorded. During the cooling cycle of the brine, the first five parameters remained constant in cells (18-16) and (15-1). Between cells 16 and 15 the [Ca 2+], [Mg 2+], [CO 2− 3] and pH increased whereas the [HCO t- 3] decreased. These changes are due to mixing of the concentrated brine with make-up water. This conclusion was confirmed by calculating the material balance of all ions. The fact that the product [Ca 2+][CO 2− 3] of the brine is ca. 120–170 times larger than the solubility product of either aragonite or calcite, indicates delayed precipitation and excessive super-saturation with CaCO 3. The analysis of brine samples extracted from the flash chambers revealed a continuous, linear build-up of [Ca 2+ and [Mg 2+] as one moves from cell 1 to cell 18. The pH value increased noticeably in the first 3–4 cells and remained constant thereafter. The [CO 2− 3] and [HCO − 3] appeared to remain constant in all cells. The data were quantitatively treated by calculating cells' concentration factors and computing the expected amounts of the various ions in the different cells. Differences between these values and the experimentally determined quantities signified scaling. The results indicated the formation of Mg scales in cells 1–10 and of Ca scales in cells 1–16. The amounts of disappearing [CO 2− 3] exceeded that needed for CaCO 3 deposition. The rest, together with the loss of [HCO 2− 3] were not enough to account for Mg(OH) 2 precipitation. This latter was assumed to involve the participation of CaCO 3 present in super-saturation. Scale samples were collected from some of the condenser tubes and the flash chambers of the distillers. These were fully analyzed for Ca 2+, Mg 2+, CO 2−, OH −, Cl −, SO 2− 4, acid-insolubles, adsorbed humidity and organics. A scheme was proposed to relate the results of analyses to compounds of definite compositions with agreement better than ±3 %. The results indicated that in one type of distillers (IHI) Ca-rich scales formed both in the condensers and flash chamber. In the second type of distillers (Italimpianti) the scales were predominantly Mg-rich. This disparity in behaviour does not signify two scaling mechanisms. It is suggested that the nature of scaling is decided by distiller top-temperature, volume, material, deaeration, and depends on type of anti-scalant additive and ball cleaning. Attention is drawn to the need of basic studies on the effect of additives and metallic surfaces on the rate of HCO − 3 decomposition, as well as on the conditions governing super-saturation by CaCO 3 and Mg(OH) 2 and the kinetics and morphology of their crystallization.