What is calcium carbonate scale and calcium sulphate scale in desalination plant?

Scale formation represents a major operational problem encountered in thermal desalination plants. Scale may form because of the composition of the make-up, but mostly develops as a result of further change occurring during evaporation. Scale formation is mainly caused by crystallization of alkaline scales, e.g., CaCO3 and Mg(OH)2 and non-alkaline scale, e.g., CaSO4. The formation of CaCO3 scale strongly depends on temperature, pH, and the release rate of CO2 as well as on the concentrations of HCO3-, CO32-, Ca2+, and Mg2+ ions. Scaling in industrial processes is affected by the following factors: (i) bulk variables and composition, i.e. CaCO3 precipitation potential, pH buffering capacity, chloride and sulfate concentrations and concentration of dissolved oxygen, (ii) thermal effect, i.e. heat flux, surface temperature and bulk temperature, (iii) flow field, i.e. velocity of flow and solid/liquid interface conditions and (iv) substrate properties, i.e. materials properties and surface conditions.

In previous works, Al-Rawajfeh et al. [1-3] have modeled the CO2 release rates in multiple-effect distillers (MED)distillers. This model did not account for the deposition of alkaline scale and its effect on CO2 release rates. Calcium carbonate and magnesium hydroxide were assumed to precipitate at negligible rates. Recently, Al-Rawajfeh [4,5] developed a model to simulate the simultaneous release of CO2 with the deposition of CaCO3 and investigated their mutual release-deposition relationship in MED [4] and in the flash chambers in MSF distillers [5]. The influence of CO2 injection on the carbonate chemistry and the scale formation were also studied [6]. The model begin to calculate the CaCO3-Mg(OH)2 (alkaline) scale in the brine chambers, because part of the scale is deposited there and will be reduced from the total scale precipitate or reduce the ions available to precipitate CaSO4 scale inside the tubes when it is recycled with the make-up. Details on the CO2 release and alkaline scale modeling can be found in previous works [1-6].

REFERENCES

[1] Al-Rawajfeh, A. E., Glade, H., Ulrich, J., CO2 release in multiple-effect distillers Controlled by mass transfer with chemical reaction. Desalination, vol. 156, PP. 109-123, 2003.

[2] Al-Rawajfeh, A. E., Glade, H., Qiblawey, H. M., Ulrich, J., Simulation of CO2 release in multiple-effect distillers. Desalination, vol. 166, PP. 41-52, 2004.

[3] Al-Rawajfeh, A. E., Glade, H., Ulrich, J., Scaling in multiple-effect distillers: the role of CO2 release. Desalination, vol. 182, PP. 209-219, 2005.

[4] Al-Rawajfeh, A.E., MoDELLing of Alkaline Scale Formation in Falling-Film Horizontal-Tubes Multiple-Effect Distillers. Desalination, vol. 205, PP. 124-139, 2007.

[5] Al-Rawajfeh, A.E., Simultaneous desorption-crystallization of CO2- CaCO3 in multistage flash (MSF) distillers. Chem. Eng. Proc., Proc. Inten., vol. 47, PP. 2262-2269, 2008.

[6] Al-Rawajfeh, A.E., Al-Amaireh, M. N., The influence of CO2 injection on the carbonate chemistry and scaling in multiple-effect distillers. Desalination & Water Treat., vol. 7, PP. 191-197, 2009.