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The Al-Jubail SWRO plant has a unique feature of using two entirely different types of RO membranes. The plant has 15 trains, producing 20 MIGD product water of WHO recommended quality. Eleven of these trains use polyamide hollow fine fiber (HFF) membranes and the remaining four utilize polyamide spiral-wound (SW) membranes. Initially, the HFF permeators were installed in trains A, B, C and D. Since the production of these permeators has been discontinued, an alternate solution had to be found. Hence the SW membranes were selected. The SW vessels have been arranged in such a way that three vessels are side-port connected in series. The vessels were specially fabricated so that the existing racks, feed/product headers and manifolds could be utilized as they were already fabricated and installed. This study compared the performance parameters like salt rejection, feed pressure, and flux decline of these two types of membranes. The seawater temperature in the Arabian Gulf varies from 15°C to 37°C and the TDS from 42,000 mg/L to 45,000 mg/L. Among 15 trains (11 HFF + 4 SW), one train was selected from the SW group and another train from the HFF group for comparison purposes. The performance of the membranes changed with a wide fluctuation of feed water temperature. As per the plant design, the product water flow rate remains constant in spite of variations in feed water temperature. The software at the DCS reduces the feed pressure if temperature increases or vice versa.

Salt carry-over due to excessive foaming in a multistage flash (MSF) distiller and detrimental foaming in the deaerator section have been reported in the Saline Water Conversion Corporation (SWCC) of Saudi Arabia and other desalination plants. The application of a suitable antifoaming agent has prevented distillate contamination and improved release of dissolved oxygen in the deaerator section, thus improving the operating efficiency of MSF plants. The developer of a polyoxypropylene glycol-based antifoaming agent collaborated with SWCC in carrying out trial runs to show that their product was as effective and economical as the one currently being used (a polyglycol blend in a hydrocarbon solvent) in SWCC plants. Trial tests were carried out at laboratory, pilot and commercial plant levels. Miscibility of this agent in water was determined initially in the laboratory, followed by field trial runs in an MSF pilot plant and the commercial plants of Al-Jubail I and II. Foam control, non-interference with antiscalant performance, distillate purity and stability at a low dose rate over a wide range of temperatures were some of the important criteria evaluated during testing of this antifoaming agent. During MSF plant runs, distillate conductivity and dissolved oxygen level in the feed were regularly monitored and found within the acceptable limits. Compatibility of this agent with antiscalant was also assessed. Heat transfer and plant performance data were found satisfactory confirming that this agent did not exhibit any adverse effect on the antiscalant used in Al-Jubail plants. Results of laboratory and field trial runs of this antifoaming agent are discussed.

The Saline Water Conversion Corporation (SWCC) has been actively involved in the investigation of scale formation in order to improve the performance of its distillers. A number of optimization tests have been carried out resulting in successful operation of MSF distillers at low antiscalant dose rates. The additives which were tested include polymaleic, polyphosphonate and polycarboxylates. The present study reports the results of the tests which have been carried out to evaluate the effectiveness of polyphosphonate in controlling scale formation on the heat transfer surfaces at a top brine temperature (TBT) of 90.6°C and a dose rate of 0.8 ppm, which is considered low compared to previous practices. These tests were firstly performed on a 20 t/d MSF pilot plant. The test results at the pilot plant showed stable overall heat transfer coefficients and fouling factors through out the test period, thus, indicated insignificant scaling of the heat transfer surfaces. Based on satisfactory results at the pilot plant, the test was performed in Al-Jubail Phase II at a TBT of 90.6°C and dose rate of 0.8 ppm for a total test duration of 93 days. The overall heat transfer coefficient of the brine heater was almost constant at a value of 2700 W/m 2 K, thus consistently greater than the design value (2258 W/m 2 K). The unit performance ratio and distillate production rate were also fairly close to the design value. Chemical analysis results showed that product water conductivity and loss of total alkalinity of brine blowdown were maintained within acceptable limits. The brine heater and flash chambers were inspected at the end of the test. The brine heater tubes were found clean and also no sign of fresh scaling was noted on the demister pads, which confirmed the capability of polyphosphonate in controlling alkaline scale formation at TBT of 90.6°C and a dose rate of as low as 0.8 ppm.