Scale formation in flash chambers of high-temperature MSF distillers

Abstract

The general overhaul of a high-temperature (112°C) MSF distiller provided the opportunity for the collection and examination of scales formed on the bottom of the first nine flash chambers. These scales were inspected visually and subjected to wet chemical analysis as well as various spectral investigations. The scales of cells 1–3 were composed exclusively of brucite (Mg(OH) 2), having started as layers and developed with time to become coral-like structures. In cells 4 and 5 the scales acquired a hard glassy texture. In cell 4, the scales were a 50:50 mixture of brucite and anhydrite (CaSO 4). Calcium sulphate constituted the entire deposit in cell 5. The sulphate scale disappeared completely in subsequent cells while in cells 6–9 brucite reappeared with increasing amounts of calcite (CaCO 3). The two theories accounting for the formation of alkaline scales are reviewed in short. These are based on the thermal decomposition of the HCO − 3 ion of seawater to yield CO 2− 3 or OH −, which trigger the precipitation of CaCO 3 and Mg(OH) 2, respectively. Thermodynamic calculations of the free energy changes of the various probable reactions were carried out at 25°C and 100°C, and their implications are discussed. The scaling process cannot be explained purely by thermodynamics, and kinetics must be taken into consideration. A unified theory, that the primary, rate determining-step in HCO − 3 decomposition is the first order reaction — HCO − 3 = OH − + CO 2 — is proposed to explain the variation of scale composition between CaCO 3 and Mg(OH) 2. The resulting OH − can either neutralize a second HCO − 3 ion or react with a Mg 2+ ion, depending on the temperature. These reactions are assumed to have temperature-dependent activation energies. Calcium sulphate can exist in six different modifications. These are the α- and β-(soluble) and insoluble anhydrite (CaSO 4), the α- and β-hemihydrate (CaSO 4.½H 2O) and gypsum (CaSO 4.2H 2O). The free energy changes accompanying the formation of all compounds were computed at 25°C and 100°C. The implications of these figures are discussed, and the concept of scaling threshold is mentioned. A comparison is made between the conditions prevailing in the distiller under consideration and those described in the literature for CaSO 4 precipitation. Sulphate scaling in thermal desalination plants can be surmounted by lowering the operation temperature, by reducing the concentration factor, or by softening seawater. Antiscale agents specific to sulphate deposits have recently been developed and tested on a laboratory scale.