2.500 Desalination and Water Purification

Transcription

1 MIT OpenCourseWare hp://ocw.mi.edu Desalinaion and Waer Purificaion Spring 2009 For informaion abou ciing hese maerials or our Terms of Use, visi: hp://ocw.mi.edu/erms.

2 Inroducion To Physical Oceanography Rober H. Sewar Deparmen of Oceanography Texas A & M Universiy Copyrigh 2007 Sepember 2007 Ediion

3 Chaper 6 Temperaure, Saliniy, and Densiy Hea fluxes, evaporaion, rain, river inflow, and freezing and meling of sea ice all influence he disribuion of emperaure and saliniy a he ocean s surface. Changes in emperaure and saliniy can increase or decrease he densiy of waer a he surface, which can lead o convecion. If waer from he surface sinks ino he deeper ocean, i reains a disincive relaionship beween emperaure and saliniy which helps oceanographers rack he movemen of deep waer. In addiion, emperaure, saliniy, and pressure are used o calculae densiy. The disribuion of densiy inside he ocean is direcly relaed o he disribuion of horizonal pressure gradiens and ocean currens. For all hese reasons, we need o know he disribuion of emperaure, saliniy, and densiy in he ocean. Before discussing he disribuion of emperaure and saliniy, le s firs define wha we mean by he erms, especially saliniy. 6.1 Definiion of Saliniy A he simples level, saliniy is he oal amoun of dissolved maerial in grams in one kilogram of sea waer. Thus saliniy is a dimensionless quaniy. I has no unis. The variabiliy of dissolved sal is very small, and we mus be very careful o define saliniy in ways ha are accurae and pracical. To beer undersand he need for accuracy, look a figure 6.1. Noice ha he range of saliniy for mos of he ocean s waer is from o pars per housand, which is 200 pars per million. The variabiliy in he deep Norh Pacific is even smaller, abou 20 pars per million. If we wan o classify waer wih differen saliniy, we need definiions and insrumens accurae o abou one par per million. Noice ha he range of emperaure is much larger, abou 1 C, and emperaure is easier o measure. Wriing a pracical definiion of saliniy ha has useful accuracy is difficul (see Lewis, 1980, for he deails), and various definiions have been used. 73

4 74 CHAPTER 6. TEMPERATURE, SALINITY, AND DENSITY World Ocean 0 o 1 o 2 o Poenial Temperaure (Celsius) 4 o 3 o Saliniy Figure 6.1 Hisogram of emperaure and saliniy of ocean waer colder han 4 C. Heigh is proporional o volume. Heigh of highes peak corresponds o a volume of 26 million cubic kilomeers per bivariae class of 0.1 C and Afer Worhingon (1981: 47). A Simple Definiion Originally saliniy was defined o be he Toalamoun of dissolved maerial in grams in one kilogram of sea waer. This is no useful because he dissolved maerial is almos impossible o measure in pracice. For example, how do we measure volaile maerial like gasses? Nor can we evaporae sea-waer o dryness because chlorides are los in he las sages of drying (Sverdrup, Johnson, and Fleming, 1942: 50). A More Complee Definiion To avoid hese difficulies, he Inernaional Council for he Exploraion of he Sea se up a commission in 1889 which recommended ha saliniy be defined as he Toal amoun of solid maerials in grams dissolved in one kilogram of sea waer when all he carbonae has been convered o oxide, he bromine and iodine replaced by chlorine and all organic maer compleely oxidized. The definiion was published in This is useful bu difficul o use rouinely. Saliniy Based on Chloriniy Because he above definiion was difficul o implemen in pracice, because saliniy is direcly proporional o he amoun of chlorine in sea waer, and because chlorine can be measured accuraely by a simple chemical analysis, saliniy S was redefined using chloriniy: S = Cl (6.1) where chloriniy Cl is defined as he mass of silver required o precipiae compleely he halogens in kg of he sea-waer sample. As more and more accurae measuremens were made, (6.1) urned ou o be oo inaccurae. In 1964 unesco and oher inernaional organizaions appoined a Join Panel on Oceanographic Tables and Sandards o produce a

5 6.1. DEFINITION OF SALINITY 75 more accurae definiion. The Join Panel recommended in 1966 (Wooser, Lee, and Dierich, 1969) ha saliniy and chloriniy be relaed using: S = Cl (6.2) This is he same as (6.1) for S = 35. Saliniy Based on Conduciviy A he same ime (6.2) was adoped, oceanographers had began using conduciviy meers o measure saliniy. The meers were very precise and relaively easy o use compared wih he chemical echniques used o measure chloriniy. As a resul, he Join Panel also recommended ha saliniy be relaed o conduciviy of sea waer using: S = R R R R R15 5 (6.3a) R 15 = C(S, 15, 0)/C(35, 15, 0) (6.3b) where C(S, 15, 0) is he conduciviy of he sea-waer sample a 15 C and amospheric pressure, having a saliniy S derived from (6.4), and C(35, 15, 0) is he conduciviy of sandard Copenhagen sea waer. Millero (1996) poins ou ha (6.3) is no a new definiion of saliniy, i merely gives chloriniy as a funcion of conduciviy of seawaer relaive o sandard seawaer. Pracical Saliniy Scale of 1978 By he early 1970s, accurae conduciviy meers could be deployed from ships o measure conduciviy a deph. The need o re-evaluae he saliniy scale led he Join Panel o recommend in 1981 (jpos, 1981; Lewis, 1980) ha saliniy be defined using only conduciviy, breaking he link wih chloriniy. All waer samples wih he same conduciviy raio have he same saliniy. The Pracical Saliniy Scale of 1978 is now he official definiion: 1/2 3/2 S = K K K K 2 5/ K 15 K 15 = C(S, 15, 0)/C(KCl, 15, 0) 2 S 42 (6.4a) (6.4b) where C(S, 15, 0) is he conduciviy of he sea-waer sample a a emperaure of 15 C and sandard amospheric pressure, and C(KCl, 15, 0) is he conduciviy of he sandard poassium chloride (KCl) soluion a a emperaure of 15 C and sandard amospheric pressure. The sandard KCl soluion conains a mass of grams of KCl in a mass of kg of soluion.

6 76 CHAPTER 6. TEMPERATURE, SALINITY, AND DENSITY Lewis (1980) gives equaions for calculaing saliniy a oher emperaures: S = R 1/ R R 3/ R R 5/2 + ΔS R = C(S,, 0)/C(KCl,, 0) [ ]( ( 15) ΔS = R 1/ R ( 15) ) R 3/ R R 5/2 2 C 35 C An exension of (6.4) gives saliniy a any pressure (see Millero 1996: 72). Table 6.1 Major Consiuens of Sea Waer Ion Aoms 55.3% Chlorine 55.3% Chlorine 30.8% Sodium 30.8% Sodium 7.7% Sulfae 3.7% Magnesium 3.7% Magnesium 2.6% Sulfur 1.2% Calcium 1.2% Calcium 1.1% Poassium 1.1% Poassium Commens The various definiions of saliniy work well because he raios of he various ions in sea waer are nearly independen of saliniy and locaion in he ocean (able 6.1). Only very fresh waers, such as are found in esuaries, have significanly differen raios. The resul is based on Dimar s (1884) chemical analysis of 77 samples of sea waer colleced by he Challenger Expediion and furher sudies by Carri and Carpener (1959). The imporance of his resul canno be over emphasized, as upon i depends he validiy of he chloriniy: saliniy: densiy relaionships and, hence, he accuracy of all conclusions based on he disribuion of densiy where he laer is deermined by chemical or indirec physical mehods such as elecrical conduciviy... Sverdrup, Johnson, Fleming (1942). The relaionship beween conduciviy and saliniy has an accuracy of around ±0.003 in saliniy. The very small error is caused by variaions in consiuens such as SiO 2 which cause small changes in densiy bu no change in conduciviy. Insrumens for measuring saliniy are calibraed using Normal Sandard Seawaer (P-series). The sandard waer is made from large samples of waer from he norh Alanic, carefully dilued o S = 35, which is disribued in 275 ml sealed glass ampoules. Each is labeled for is conduciviy raio and saliniy according o he Pracical Saliniy Scale 1978 and disribued worldwide by Ocean Scienific Inernaional in England since Each sample is carefully calibraed using he sandard KCl soluion.