Some physical properties of sea water in various concentrations

SOME

PHYSICAL

PROPERTIES

OF SEA WATER

IN VARIOUS

CO?4CENTRATIONS*

W.

H. EMERSON

ASD D. T. fA,MlESON

Naaiotxd Emginecriag Laboratory, Glasgow (Seotfand)

The vapour pressure of sea water and its concentrates has been measured IOOXI to 180-C using a single isotcniscope. and the thermal conducti\@

in the temperature range has been determined in the range 0’ to 7S-C by a hot-wtre compaxxtive method. Most of the measurements were made with a Qnthetic sea water containing no cakium sulphate, though some results obtained with natural ya water are also given. Concentrations up to five times that of natural sea \\ater were studied. The measured values of vapourFressure have been fitted by an equation which assumes that for a g&m composirion of safts the activity is a pure function of the total ionic strength. This yklds a srandard ckvtation as good as that obtained with mcasurcntents on distilkd water and the NEL Steam Tabks fu = 0.069 per cent). The smoothed \alucs of ha-mat conductivity are believed to be aaxrate to i3 per cult.

INI-RODL’CTION

Nationa Engineering Laboratory is currently engaged in determining the physical properties of sea water at the temperatures and concentrations which may be utilised in the future when problems of calcium sulphate scaling have been overcome and in filling the gaps in existing knowledge at lower temperatures. The present article gives the results of measurements of vapour pressure up to 180°C and five times concentration, and of thermal conductivity up to 75°C and five times concentration. For most of the tests a synthetic sea water from which calcium suiphate had been deliberately omitted was used. It had the composition given in Table I and is compared with a natural sea water in Tables II and III. The synthetic solution. represented sea water from which the calcium sulpbate had been removed by precipitation or pretreatment, as would be necessary for the operation of an evaporator at temperatures up to 180%. Some tests wtxe also performed using natural sea water obtained focal!y off the coast of Scotland. The salinity of this natural sea water differed from that in Table If, but the relative proportions of the constituent s&s may safely be presumed not to differ in any sigrkificant amount. It is estimated that if all the calcium sulphate in natural sea water remaind in solution at 180°C the boiing point elevation would be increased by O_OS”C(by Raouit’s The

l Paper -ted at the Seumd Euro1967, Athens, Greece. European Fe&ration

Symposium on Fresh Water from the Sea, May !3-I 2. of Chemical Engineering

213 Desaiination, 3 (1967) 21% 224

214

W.

H. EMERSON AND D.

TABLE COMPoSlTIoX OF

T. JAMlESON

I

*CA-FREE’ ARllFlClAL

~-

__----

---

26.9 6.83 2% 0.10 0.03 0.003

~tY~-7Hzo KBf H3BO3

NaF

TABLE ASALYGES OF NATURAL

AhI

II

ARTlFSC!AL

SEA WASit

34.32 19.M) 1.27 0.01 0.4 0.38 0.065 10.56 0.103 2649 0.026

Salinity Cl 2 Ca

rrgio~

TABLE XS OF RELATlvEco~P05IT10

NATURAL

33.0 18.99 1.26 G4 0.07 10.58 Y-756 0.03

l-II Ahr) ARlEICIAL

Narnraisea wrter

N&l

WATER

Compositim WI)

N&3 MgCI2.6&0

calcl M#?wt -.a- _Km sotlcr ::;:

=

0.555 6 0.021 06 o&56 94 0.020 0 0.f39 5 0.000 7 0.0034

!EA WATnt

‘ck-_#k* artl~ sea water 0.5573 @d&658 0_018i 0.0925* -0.0037

concentration has brm reduced by an ~IROIUW COP *nlcso4-responding to thatxequircdtopruzipitatcall thcCa++ asCaSO4 Lksakklion.

3 (1967) 213-224

PHYSICAL

PROPERTIES

OF SEA

WATER

215

law) and the vapour pressure reduced by 0.01 x IOsN/m? In fact a large but in4 determinate proportion of calcium sulpbate will be pfecipitated and the error resulting from its tot?1 exclusion is likely to be very small.

VAPOUR

PRESSURE

The vapour pressure of natural sea water and its concentrates (up to nine times) has been measured by Ham., Nakamura and Higashi (‘1, in the range of temperatures 0% to 175°C. Fabuss and Korosi (2) have obtained reSults with solutions of single salts at ionic strengths comparable with those of sea water concentrates up to five times normal salinity and at temperatures up to 16O’C; and they have confirmed an additivity rule first suggested by Robinson and Bower (3). The method adopted in this investigation was a direct one employing a single glass isoteniscope (Fig. I). The confining Iiquid *as mercury and the vapour pres-

f

W. H. E3fERSON

216

AND

D. T. JAMIESON

sure

in one limb of the mercury trap was balanced by an equal pressure of nitrogen in the other (except at temperatures near 100°C when the trap was used as a manometer). The nitrogen pressure was measured in either a 2m or an Sm mercury manometer. Both of these marometers were fitted with lhermocouples at 30 cm intervals and the larger was contained in a transparent duct through which a~ was continually drawn to maintain a uniform temperature. The levels in the mercury trap and in the smailer manometer were measured using a cathetometer, and the measured pressure was corrected for the vapour pressure of mercury at the temperature of the bath. The sample was continuously agitated to eliminate concentration gradients. The bath temperature was stabilized to within 0.003”C and was measured to a probable accuracy of O.Ol”C by a platinum resistance thermometer which was calibrakd againstthe triple, steam and sulphur points. The sample was demented in the the isoteniscopc (before the introduction of the mercury) by seven cycles of freezing and thawing. Each time the sample was frozen it was held for 30 minutes at approximately -80°C while the space above it was evacuated to 2 N,‘mz. TabIe IV shows results obtained with glassdistilted deionised water, compared with values obtained from the NEL Steam Tables 1964 f4]-

TABLE lbil%SUW-Ts

OF VAPOUR

f’c)

OF D&STILLED WATER

..-_-

(iWN/Xd) -

Difirmce twtwem weasurd tempefarure und rhor fr0m swam IaMes a; meDIurcd pressure

(per cetrf)* Cc) __-_-_-_-._--__.--,._ ---

zt 98

-~O.OCNX -0.0006 +0.0@10 -i-O.ooO2

E:t (Oh6) (0.021)

119 119 119 119

+0.0020 ;0.0014 iO.OO2S i-O.0026

$Ez; V.p; .

135 135 135 135

-kOXKkX 0.0000 ~~~

177 177 177

+0*0012 +0m12 io.oOi2

98

l

Pm

Di~erctue betwen J¶zeastb pressure and Ibat from steam rabks a: measm-cd rcmprarurc

Temperature

--J.--_.-_~

IV

-.---

-0.014 io.017 -0.030 -o.aK -0.033 -0.023 rgg.

(0.003) ww (O.oao) <0*@2a

-0.001 O.OOQ O.fXXb -0m9

(0.001)

-0.006 -o.a% -0.006

Standard deviation 0.069per cent. Dcdinasrion. 3 (1967) 213-224

PHYSICAL

PROPERTiES

OF SW

217

WATER

The results of measurements with saline solutions are given in Table V. The salinities for each sample given in column two may be seen to rise slightly with increasing temperature, though the total amounts of saIt and water in the isotcniscopz remained unchanged. This is a cons;lquence of migration of water from the liquid to the vapour phase- In the fifth column are given values of vapour pressure of pure water at the measured tempMature calculated from (4)

where x =

-2 -K.

t = 1.3869 x IO-%

y = z=

344.G- 2, i + 273.16, i = measured temperature 5.432 368, a= b -_ -22,oO5x x 103, The values of the activity,

where

IJO =

the vapour

p/p0

pressure

were

d =

“C,

found

1.1965 x 10’“,

e= f =

-4.4000 x 10-3, -5.7148 x 10m3, = 2.9370 x 105, IF p is in units of 10sN/mZ.

to fit an equation

of the form

of pure water at the same temperature,

and

h t=i -2.1609

x lo+, -3.5012 x l@-‘, and S = salinity in g(satt)/kg (water),

j

or

r=

h = i=

- lAO57 x lo-2, - 9.1673 x fOWd, and S = total ionic strength in g mot/kg.

(W

(2h)

The boiling-point elevation corresponding to each measured value of vapour pressure has been cakxdated by interpolation in the NEL Steam Tab& 1964 f4.I .and the resuhs are plotted against the bath temperature in Fig. 2. The baiting point eievations have aIso been calculated using Eq. 2 and the NEL Steam Tables, and plotted for comparison in Fig. 2 as a solid line. The broken line in Fig. 2 is cdcufated from the smoothed data for single-salt solutions obtained by Fabuss and Rorosi (2) and using the additivity ruIe of Robinson and Bower @I. Each line represents a SoIution of the salts NaCf, MgSQ,, KCI, MgCf, and Na,SO, in the same relative proportions as in the NEL synthetic sea water and having the same !I&: total ionic%trPngth as ail the salts in the sample. - The 2.. This by the held at

resuhs with ‘natural sea water are also plotted (denoted by%rcIes) in Fig. sea water was found to have an initial pH of 7.9 which was adjusted to 3.5 addition of hydr~hlo~c acid, to drive off carbon dioxide. The sample was 180°C for 14 hours, then cooled to room temperature and filtered. The chlor&salinath7n, 3 (1%7) 213-224

sulM1y

“,..

..We) .... .. ._.....,..

I

33,13 33,21 33.32 33S6 33,118

.. _.

Sample No, .-_-

2

33.13 33421 33.32

Trmprrurrtrc

(“C) ..__

Meuitireff pwwf re ( p) pm wulrr(job .. .

.

PIP”

I

.. _

.

P (per cent)* (lO~N/mz) -"I . _.___v.-

M”..

(“0

_,-_

measured pressure

.--...

0:61 a,70 0.81

.

fromf&y. I (IOfN/mz)

0.002 0.002 0,007 0.041 0.086

.

1.0098 I,9317 3.2320 6,0723 9,9219

z 0:61 0.70 0.81

(IOw/nq . .,. . ..,..

0,9918

-0SMl -0,w 0,ODl 0,002 OSlO9

it:::

0.98277 0.98270 0.98261 0.98215 0.98152

32169 6.0430 9.3940

E!

1.0275 1.9657 392890 6,1801 10.1ooo

0.98315 0.98281 0.98247 0.98223 0.98145

I,01 1.13 I,27 I.45 1.63

100.392 119.686 136.705 3:2316 6.0698 9.9134 1.0092 I,9951 3.2736 6.1503 9.5624

-0S.M -0.001 -o.ool O.OOl 0.004

I S9.998

180.315 0.9922 1.9tXI8 3.2162 6.0410 9.3850

3.2562 S,7622 9.44116 I.0017

I .8792

I.0477

I .9607

99.887 120.155 136.542 159.808 177.948

!%:4 696343 0.96287

I,94 2.16 2.41 2469 3.03

:%0

1.ow

1.0866 I,9493 3.3781 5.9795 Y.(IO55

0.030 0.012 0,020 -0.046 -0.011

0.96484

1.8806 3.2570 5.7608 9.4414

i%: s:7o43 9.4606

3

101.970 119.422 137.635 15H.705 179.031

0.93347 0893349 0.93312 0493323 0,93216

66.26 66.41 66,66 67.09 61.72

1.0792 1.9695 3.4194 6.1152 IO.lSo2

4

1.0074 I.8385 3.1907 5.7069 9.4616

2.16 2.39 2.71 3,w

101.776 114.748 138.059 159.583 180.S31

-0.045 O.oo8 -0.042 -0.021

115.97 116.24 ll6,66 Ill.44 118.61

I.9280 3.2183 8.8332 9,6tSO

5

0.93400 0.93324 0.93317 0.93321

1.9289 3.2181 5.8357 9.6170

%i 612536 10.3164

IZf.2SO 138.354 HO.463 1X1.240

1.IlAR 1.9251

116,27 116.67 117.49 118,69

3.01 3.29 3,66 4aO6 4,so

6

0.182 0.075 0.062 -0.082 -0,174 :s:': 9:7442

z!:: 3:oxM S,3260 8.7526

1.0(159 1.7322 3.0285 5.3301 8.7678

0.899o9 0.89980 0.89947 0.9ooo7 0.119980 102.800 il4.030 137.520 158.330 178.760

0.069 per ml.

165.68 166.03 16668 167.70 169.27 %lnndard deviation

PHYSLCAL PROPERTIES

OF SEA

WATER

219

46

i

EwalloN 2 --WLtulllDN FAOYfAm.3se uxasl II) + . F#*nUEdS1REO MLLRS.%WnETK SE4 WPTER

‘------z-~------

_-_._._“-_._1_‘-;~_--_

-2

140

‘20

:EHPEI?t.TUPE

Fig. 2. Boiling-point

170

rc)o

lC

elevation.

inity before and afttr acid dosing was 18.74 and 18.92 gjkg respectively and it was estimated that the total ionic strength before heating was 0.669 g moI/kg. The total ionic strength of the synthetic sea water at room temperature was 0.6455 g mol/kg. Table VI shows a comparison between the results of Ham, Nakamura and Higashi (2) and our own results as represented by Eq. 1 and 2b. We feel that the differences may result, to some extent, from the improvement in the standards of thermometry that has taken place over thirty-five years.

The thermal conductivity of sea water was determined between 10°C and 70°C by Nukiyama and Yoshizawa in 1934 (5). but the values obtained were lower (3 to 4 per cent) than would have been predicted from a knowledge of the thermat conductivity of sodium chloride solutions. We know of no measurements that have been made on concentrated sea water. Desaha~ion, 3 (1967) 213-224

W. If.

220

EMERSON

AND

TABLE CXMPARISBN

Cir;orindy

OF

MEASIJREHEbZ

Total ionic

OF

VAFOL’R

TernPerarure

strength W~J3)

-...-. _--

(IS ,mNW

__-_-.._.__..___.

D. T. JAMIESON VI

PRESSURE

WlTH

p~s.mr~(P,)

OF

HARA

ASD

Presure (p’) culeulnted frorn

measured by Hara et al (1)

OTHERS

P’-Pr*

NEL dara at Sante fotaf ionic strength (lWN/m’) (10JNjmz)

(lOW/rn~)

(‘0

THOSE

(I)

P* -P,,

p, (jet cenr)

-_-.___.._______~_..-_-__--_-.--.-

20.61

0.7364

9933 115.09 1so.34 175.22

0.%95 2.293 1 4.7503 8.8752

0.9699 27SI6 4.7090 8.7947

i-O.OOM -0.0115 -0.0413 -0.0805

f0.04 -0.50 -0.87 -0.9!-

58.65

2.0960

loo.02

0.9610

0.952d 21242 4.3805 s&M7

z-%

-1.65 -0.08

-0:0671

-1.55 -2.47

12d.16 149.24 176.11

2.1598 4.4476 uio46

The method adopted in this insestigaticn was the employing the hot-wire principle. The conductivity of a str&i_ehtplatinum wire of 0.075 mm diameter and end to a thicker platinum wire of 0.23 mm diameter

. .lQ4L13iTE

____.- GLCSS

_ _-f’~4llNUM

relative one described in (ti), cell shown in Frg. 3 consists 13 cm length, welded at e&her and concentrically sealed in 9

PLUG

TO

PLATINUM

SE4t

FILAMENT

IFISOLATED

Fig.

-0.1999

3. Construction

LEAD

OFccii.

iksdinatkw,

3 (1967) 213-224

PHYSICAL

PROPERTIES

OF SEA WATER

221

Pyrex glass tube of 5.1 mm diameter. The filament, which had a resistance of about 3.5 fi at 25”C, was connected in a Wheatstone bridge as shown in Fig. 4. The cell was immersed in a constant-temperature bath whose temperature could he controlled for the duration of the experiment to within +O.OOS”C!. The experimental technique consists essentially in passing various known currents through the filament and measuring the corresponding resistances when the temperatures are steady. The cell resistance was measured at 0 to O.oooO1Q, the usual allowance heiig made for cell lead resistance. Vaiues of thermal conductivity are carlculated from the equation .

(.

A =

m -

B' -

(3)

4

where A and B are constants depending on the celt dimensions, It.and 1, are the thermal conductivities of the liquid and glas”; respectively,

acd

m is the rate of change of filament temperature, T with power iriput, P,

in practice it is more convenient to obtain m from least-square analyses of tbe experimental values of the cell resistance and the power input in the equation R = R. + m,P,

where R. is the resistance of the cell with zero current. A minimum of seven different values of P were used_ The values of nr and m, are identical at the calibration temperature and values of m may be obtained from Muat other temperatures by multiplying by a factor which allows for the non-linearity of the temperature coefficient of the eIe&ical resistance of platinum. The values of A and B were obtained by calibrating the celI with water and toluene, whose thermal conductivities are accurately known.

GALVANOMETER

222

W. H- EhfERSON

AND

CL T. lAMIE!SON

Linearity af Eq. 4 was checked for each test and this was used as proof of the absence of convective currents within the test ~~11. Since the thernull conductivity is derived from the resistance of the platinum fiiament, it is esscntiai that the change from an ehxtrically insulating liquid to an electritrally conducting salt solution should not cause a measurable change in the overall cell resistance Acco&ing to ref. (2’) the change in cell resistance caused by the saIr solutions will be too small to be detected. In order to check this experimentally the zero cur&t resistance was determined with the cell filled with water, mineral oil, 33 per oznt sulphuric acid and with a saturated solution of sodium chloride. NO chans in the resistance of the cell was detected_ The thermal conductivity of distilled water and artificial sea water at about 1, 2, 3.5 and 5 times the concentration of natural sea water was measured and the results are shown in Table VII and Figs. 5 and 6. It was not expected that there would be any difference between the thermal conductivities of artificial and natural sea water

SEA WATER CCNCENTRATDN Fig.

5. Thamal

conductitity

(l-t33

0 g/i SAUNlTYl

of distilled water end ‘Ca-free* artiDesaha:bn,

sod watcli. 3 (1967) 213-224

PHYSICAL PROPERTIES OF SEA WATER

MERMAL cmmum Tenipmmuc (“0

TABLE

WI

ocse~

wmat

D&t&d water

nu

Jm-ts-t”C-t

Arti$i%zlsea water

Ixconc

Zxwnc.

Nururoisea water (saiiinty33.82 g/l)

3Jxconc.

Sxamc.

10-3x 5ss.7

10-3 x

551.5

25 50

607.4

Experimentat values SLnoatIKd vahcs

611

607.5

Experimental vab5 smoothed valua

641.5 654.3

641.8 640.8

637.1 637.3

627.6 632.1

Experimental

683.4 671.5

664.5 658.6 660.7 664.5

659.5 652.1 650-S 661

6%3 65518

values

75 smoothed

values

z8

223

10-3 x

E2

561 560.2

593.8 593.6

607.5

z.9

&

655.1 648.9 650.8

661 s

..

6645

ditilItxI

water 1 X sea water concentration 2 x sea water concentration 3.5 x sea water concentration 5 x sea water coneenttation (1 x z 32.0&l salinity o Nukiyama and Yoshinwz (Natural Sta Water I x cone.) I

2 3 4 5

Fig. 6. lhamal

conductivity

of distilled water and ‘Ca-fnc’ artifiul

sea water.

but measurements were made on a sampie of natural sea water (salinity 33.82 g/l) and Table VII.

these are shown in

Desahation. 3 (1966) 213-224

224

W. H. EMERSOS AND D. T. JAMtESCbN

From previous tests

with twenty-one different liquids covering virtually the w’nole range of thermal conductivities of liquids other than liquid metals, it was shown sf.ztistica!ly that the results were within -+3.3 per cent of the best known values fur a (;6 per cent probabiity and within rt6.6 per cent for a 95 per cent probabiiity. Since the ac~racy of the best-known values was in most instances only f3 to f 5 per cent, it is possible that the repeatability and accuracy of this method is better than that found from statistics. When the value reported is the average of three or more tests, then the value may be taken as accurate to within &3 per cent. The smoothed values reported in Table I may be taken as accurate. to within C 3 per cent. (6)

Tbe authors acknowledge the assistance in the experimental work given by Messrs. R. Morris and J. S. Tudhope. The investigation formed part of the research pr~powls of the DSIR Desalination Research Committee! and was carried out. at the National Engineering Laboratory, Ministry of TechnoIo_ey. This paper is pubiishcd by permission of the Ditior of the National Engintxridg Laboratory, is Crowa copyright and is reproduced by permission of the Controller of Her Britannic Majesty’s Stationery Office. REFEREFKXS 1.

z

AND IL HKXHI. The specific gravity and the vapour prcssurc of R. HARA, K. NAICAWJRA sea water at O’C to 17S’C. Tech. Rcpr. Tohoku Imp. Um+. (Japan), 10 (1932) 433-s& B. M. FABUSSmm A. KOR~SI.Vapour prcssum of binary solutians of NaCI, KC& NaBX and MgSO~ at concentrations and temperatures of intcnst in desalination proctsws. fksaiinatian,

conantratcd

l(Z) (1966)

139-148.

3.

R. A. Romsso~ ASD V. E BOWER, An additivity xuk for the vqour solutions. f. Rcs, &a~. Bur. Std. A. 69(a) (1965). 365-367.

4.

NATIONAL EMXGERIXG

LABDIUTORY.

XEL

Steam

Tabfes

1964,

pressurelowering of aqueous Her

MLljes:y’s

Stationmy

Otfice, Edinburgh (1964). Themtai conductivities of water. sea watff and some water 5. S. NUKIYAMAAND Y. \*OSKIZAWA. solutions. J. See. Mech. EngrJ. Japtxn. 37 (1934). 206 S424W 6. D. T. JAMIESN ASO J. S. TUDHOPE,A simple device for measuring the thermal conductivity of liquids with modcratc accumcy. J. IILW.Perrol., SO(W (1964). 150-153. The thamal conductivity of molten salts. Aurrrafkzn 1. App!. Se;., 12(l) (1961) 7. A. G. TL’RHBIXL,

30-H.

Des&tatiolt* 3 (1967) 213-224