Fouling phenomena in multi stage flash (MSF) distillers

Abstract

Fouling in multi stage flash (MSF) distillers has been occupying researchers for many years. A lot of work has been done and more is yet to come in order to fully understand the role of various components and their interaction including the effectiveness of scale control techniques. In this paper an attempt is made based primarily upon visual and reported observations of fouling in various parts along the flow path of brine solutions in MSF distillers. This analysis is aimed at proposing certain sequence of scale forming reaction steps and to suggest certain experiments that could verify the validity of the proposed reaction mechanism. The proposed reaction steps are shown in alphabetic order to denote the sequence that could prevail inside heat transfer tubes of recovery and heat input (brine heater) sections and water boxes and thereafter in flash chambers including demister pads of MSF distillers. To predict the fate of various species and pH values one must identify reaction steps. The common practice is that: pH values are measured of solutions drawn out of heat exchanger tubes of recovery section or brine heater. This leads to the evolution of carbon dioxide. In view of the above, variation in pH values which could support the proposed mechanism were never addressed. Certain experiments should, therefore, be devised where pH values of recirculating brine inside heat exchanger tubes could be measured on-line while still under pressure. It would then be probable (especially if particular species detection electrodes are installed) to measure variation in pH values due to hydronium ion generation and its consumption. Meanwhile, the presence of hydroxyl ions will be short lived primarily due to Mg(OH) 2 precipitation. The proposed steps support CaCO 3 precipitation ahead of Mg(OH) 2. The abundance of magnesium ions and the extreme low solubility of magnesium hydroxide will rapidly then lead to its formation. However, scale precipitation inside tubes are not only from initial scale formation under pressure inside the tubes but also due to nucleates recirculation from flash chambers back into heat gain exchanger tubes because of brine recycling. Recent analysis of variation in coloration of water boxes came as a strong support to this hypothesis. Reaction steps suggested by other workers will of water boxes came as a strong support to this hypothesis. Reaction steps suggested by other workers will be shown for the sake of comparison. It is worth to note that the end results of various reaction mechanisms are almost the same in the cited ones also the same to the proposed overall reaction.