Seawater salinity scale

Capital Costs A typical medium-scale RO seawater plant might produce 0.25 mVs (6 MGD). For a plant with an open sea intake, seawater salinity of 38 g/1, and conversion of 45 percent, the overall cost woiild be 26.5 miUiou (1996). A capital breakdown is given in Table 22-18. Capital charges are site specific, and are sensitive to the salinity of the feed. A plant of this size would likely contain six trains. For seawater RO, the Best estimate for the slopes of the family of lines in Fig. 22-55 is —0.6 for the equipment and 0.95 for the membranes. Capital charges, shown in TaBle 22-19, usually dominate the overall economics the numbers presented are only an example. Seawater economics are based on Shields and Moch, Am. Desalination Assn. Conf. Monterey CA (1996). 

This standard seawater has since proven problematic because it is based on real seawater, whose conductivity is influenced by concentration variations in the nonconservative ions and subtle fluctuations in the ratios of the major ions. To eliminate these issues, a practical salinity scale (PSS-78) was adopted by international agreement in 1978. As a... 

Salinity was first rigorously defined by Knudsen (1902, p. 28) as the weight in grams of the dissolved inorganic matter in one kilogram of seawater after all bromide and iodide have been replaced by the equivalent amount of chloride and all carbonate converted to oxide. In 1978, the JPOTS decided that a new definition was needed for salinity that was based more on a salinity/conductivity ratio and was termed the practical salinity scale. 

Salinity is presently determined by measuring the conductance of seawater by using a salinometer. The modem definition of salinity uses the practical salinity scale, which replaces the chlorinity-salinity relationship with a definition based on a conductivity ratio (Millero, 1996). A seawater sample of salinity S= 35 has a conductivity equal to that of a KCl solution containing a mass of 32.435 6g KCl in 1 kg of solution at 15 °C and 1 atm pressure. No units are necessary on the practical salinity scale however, in practice, one often sees parts per thousand, ppt, or the abbreviation psu. New salinometers using this method are capable of extremely high precision so that the salinity ratio can be determined to 1 part in 40 000. At a typical salinity near 35 this procedure enables salinities to be determined to an accuracy of 35.000 0.001. This is much better than most chemical titrations, which, at best, achieve routine accuracy of 0.5 parts per thousand. 

Millero, F. J., Feistel, R., Wright, D. G., McDougall, T. J., 2008. The Composition of Standard Seawater and the Definition of the Reference-Composition Salinity Scale. Deep-Sea Research, I, 55, 50-72. 

In Table 20.4, the abbreviation psu, short for practical salinity unit, indicates salinity expressed in the Practical Salinity Scale of 1978 (PSS-78) as a dimensionless quantity. The term psu is not an official unit (Unesco, 1985 Siedler, 1998) but is in widespread use and is particularly helpful to distinguish, say, a given salinity value from absolute salinity in g/kg. Before 1978, salinity was computed from chlorinity, CL by the Cox scale, 5= 1,80655 xCZ (Mamayev et al, 1991), The recommended numerical conversion factor between the PSS-78 salinity and the Cox salinity is 1, Cox salinity is usually expressed in parts per thousand, ppt, %o, or g/kg. None the less, it is lower by about 0.5% than the absolute salinity of seawater in grams of dissolved substance per kilogram of seawater, which in turn is not exactly known but can be estimated sufficiently well (Millero et al., 2008), see Section 20.2.1. 

Measurements made by calibration of electrodes with lUPAC aqueous RVS or PS standards to obtain pH(X) are perfectly valid. However, the interpretation of pH(X) in terms of the activity of hydrogen ion is complicated by the non zero residual liquid junction potential as well as by systematic differences between electrode pairs, principally attributable to the reference electrode. For 35%o salinity seawater (S = 0.035) calculated from pH(X) is typically 12% too low. Special seawater pH scales have been devised to overcome this problem ... 

The Practical Salinity Scale 1978 and the International Equation of State of Seawater 1980, Unesco Technical Papers in Marine Science No. 36, Unesco, Paris, 1981 sections No. 37, 38, 39, and 40 in this series give background papers and detailed tables. 

Table 8-1 lists the constants which are recommended for use on the mol/(kg-seawater) concentration scale and the total hydrogen ion concentration pH scale pH(T). The determinations by Roy et al. (1993 1994) and Dickson (1990) listed in Table 8-1 supersede the careful determinations of Hansson (1973), which covered a mine limited salinity range and that had a somewhat higher standard deviation. Careful stability crmstant measurements have also been made on the seawater pH(SWS) scale (Xi, Kz, Goyet and Poisson, 1989 K, Dickson and Riley, 1979) and on the pH(NBS) scale (K, Kz, Mehrbach etaL, 1973 K- Culberson and Pytkowicz, 1973). In addition, Dickson and Millero (1987) recalculated the Ki and Kz values of Hansson (1973) and Mehrbach et aL (1973) to the pH(SWS) scale and found no consistent difference between them. 

The definitions of 1902 and 1969 give identical results at a salinity of 35 %o and do not differ significantly for most applications. The definition of salinity was reviewed again when techniques to determine salinity from measurements of conductivity, temperature, and pressure were developed. The Practical Salinity Scale defined in 1978 is a complex function related to the ratio (K) of the electrical conductivity of a seawater sample to that of a potassium chloride (KCl) solution with a mass fraction in KCl of 0.0324356, at the same temperature and pressure. 

The iaterrelatioaship of nonalkaline scales (CaSO, CaSO /2H2O, CaSO 2H20) depeads oa temperature and the concentration of CaSO. To assure that no hemihydrate scale forms, MSF operators must mn their plants ia such a manner as to assure that the coaceatratioa of the total dissolved sohds does aot exceed 70,000 ppm at temperatures of 120°C. With average-salinity seawater, plants can operate at a concentration factor of 2, but in the Middle East where water salinity can be as high as 50,000 ppm, the concentration factor should not exceed 1.4. Under no circumstances should the total dissolved soHds exceed 70,000 ppm, ie, twice the concentration of normal seawater at 120°C. 

Cathodic Current Densities for Protecting Steel Examples of current density requirements for the protection of steel (to achieve a steel potential of —0-8 V vs. Ag/AgCl/seawater) are given in Tables 10.13 and 10.14. It should be realised that the current demand of a structure will be influenced by, inter alia, temperature, degree of aeration, flow rate, protective scales, burial status, presence of bacteria and salinity. 

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