Available safety margins of time and antiscalant dose rate

Saline water Conversion Corporation (SWCC), Research & Development Center (RDC) was encountered with a request to establish available safety margins of time and antiscalant dose rate. This request came to RDC from operating plants in order to set limits of available time and for clear identification of operating procedures in case of antiscalant dose rate interruption. This paper will review past works of scale control in SWCC. Results obtained during the past couple of decades (1981-2000 AD) are cited as background information. Such results include scale control techniques of ball cleaning and antiscalant dose rate optimization and costs. The paper will also describe some preliminary ground work carried out in laboratory and on the 20 kl/d pilot plant MSF distiller available at SWCC– RDC in Al-Jubail. Laboratory work will show threshold scale formation potentials as a function of time using change in solution alkalinity. In these tests various antiscalant types of polyphosphonate and polymaleic and polycarboxylic acids were dosed into natural seawater and artificially concentrated brine solutions. On the other hand, operating results of the 20 kl/d MSF pilot plant will be shown at low down to very low antiscalant dose rates. In addition, operating results on pilot plant MSF distiller for limited duration will be discussed while suspending either antiscalant dosing or on-line rubber ball cleaning scale control technique.

The current dose rates of 1–3 ppm for MSF operation at TBT of 90–110°C is quite safe. Moreover, the above levels are found to be somewhat excessive. On the other hand, it was established that failure of one and up to three days of ball cleaning system in commercial plants is tolerable provided seawater turbidity remains normal.