Water desalination by indirect freezing

Desahation

WATER

- Elsevicr Publishing

DESALINATtON

Company,

Amsterdam

BY INDIRECT

- Printed in The Netherlands

FREEZING

H. hf. CURRAN Xfecb~nicuf En-gineering Dr,~arfmetzf, The Cat/x&: I US.A.)

(Received

February 20, 19691

ffnivcrsit_v of Americ.;,

Washi~~gtott. D-C.

’

SUMMARY

The results of an experimenta investigation of the effects of container geometry on the recovery of product water from indirectly frozen sait water are presented. Saft water was frozen in containers having circtdar or rectangular crosssection, then allowed to melt and drain until the residual ice was potable. Thin rectangular cross-sections were found to be more effective than circular cross-sections. The product water recovery was found to increase with increasktg ice height up to 60 cm, beyond which the effect of height was negligible.

c 5 d

-

concentrated brine mass fraction sak mass fraction in concentrated dilute brine mass fraction salt mass fraction in diiute brine

E

-

container

f K m n

-

P

-

P Ss T c A

-

C-

P 0

-

brine

surface effectiveness ice mass fraction constant of proportionality salt mass fraction in input to freezer instantaneous

salt mass fraction

at cumulative

drained

mass fraction

C + D

product water mass fraction salt mass fraction in product water supply salt water mass fraction salt mass fraction in supply salt water temperature thickness of rectangular cross-section diameter of circular cross-section density time

Desahafion,

7 (1970) 273-284

274

ti. N. CURRAN

lNTRODUCllON

Freezing processes for water desalination are based on the physical principle that ice crystals obtained by freezing an aqueous salt solution are pure water in the solid phase. Because of the commensurate densities of the ice crystals and the residual brine these processes invofve t&e two basic qxx&ons of formation of ke crystals and sepamtion of these crystals from the miduat brine. fn general, these are consecutive operations, but th:zy may be efEcted simultaneously under certain conditions (t,2). Two basic freezing methods may be utilized for water desalination: (I) Indirect freezing in which the enthalpy change! required for the partial freezing of salt water is effated by transfer of heat through a solid barrier; and (2) direct, or evaporative, freezing in which the enthalpy change required for the partial f&e&g of salt water is effected by partial evaporation of the water, or by the evaporation of an immiscible refrigezaat in contact with the salt water. The distinction between direct and indirect freezing processes is thus primariIy with respect to the method of effecting the entha?py change required to convert the salt water into a mixture of ice crystafs and unfrozen brine in the crystallizer. Research on indirect freezing processes has shown that substantial fractions of potable water can be recovered from salt water ice at atmospheric pressure and without washing;, simply by allowing the ice Co melt and drain simultaneously until the residual ice is potable (3). Although desalination processes requiring only indirect freezing followed by melting and draining, all at atmospheric pressure, have the advantage of being Iess complex than other freezing processes, they have the disadvantages of requiring greater processing times* and larger temperature di%rentiafs. Further research may indicate, however, that processes of this type are feasible, if not for large--ate plants, at least for smaller special-purpose plantsThe primary objective of the research reported here was the determination of the effects of container geometry on the melting-draining operation in the recovery of potable water from indirectly frozen saft water. This work may be considered as an extension of the experiments of Thompson and Netson (3). In their experiments sea water samples were frozen in cylindrical Lucite containers (12.3 cm diameter, 50 cm height) and subsequently allowed to melt and drain. Analysis of the water drained from the sfowfy melting ice showed a constantly diminishing salt concentration, with the fintil 30% to 40% of the original mass being potabl, ANhLYTlCAL

CONSIDERATlONS

The maximum l

theoretical mass fraction of pure water avaifabie in a unit

Tk ~~t~n~-~~iaing timeem be dccrtxstd by use of a centrifuge <4>, but this inmascs the

compiexity of tlie process.

Redk*tii&,

7 (1970)273-284

DESALINATION

BY INDIRECT

275

FREEZING

mass of frozen salt solution is equal to the ice mass fraction, I. This is a function of the initial salt mass fraction*, IN, and the equilibrium temperature, T, (5) , T L T, c To where T, is the freezing temperature

(1)

of pure water, T, is the initial freeting tem-

.

-c

\

DRAINED

MASS

FRAU

Fig. 1. Typical salinity curve obtained solution.

ION

by gradual

melting and draining of frozen salt

* In this paper all salinitiesare expressedas salt mass fractions. Thus. sea water with a salinity of 35.000 ppm has a salt mass fraction of 0.035.

Desalination, 7 ( 1970) 273-284

Hs Sl. CURRAS

276

pcratore comsponding to m, and K is a axstat of proportionality related to the freezing cunc ifar lea uz~er the Kelvin scale value of K is 52_41j_ figa I ilhswiws, on semi-tgrttirhmic coordinates: the t>pical form of the salinity cunz cxperimenlaJfy obtained by affowing gradual mefting and draining of a frozen salt &ution. This curve *epresents the instantaneous salinity_ expressed as a salt mais fraabn. of the draining solution as a fusion of the cmnuhtive drained mad fraction. based en a unit aas of solution. The drained solution maybe divided into three mass ftzctions: c’c:onanrrat,d brine with average salinity: c. D - dihirle brine with a~era_er salinity d. P - product water with average saiinity ppTbc dkzinction between the cmxxmtrated and dilute brines is made at the point where the instantaneous salinity of the draining solution is equal to the salinity, J, of the supply salt water- The distinction bcrwcen dilute brine and product water co&d be ma.& theoreticaff_v, at an instantaneous salin& n, such that the average salinity of the residuzd mass fraction is equaf so a salinity, p, defined as acceptabk for the product water_* H~WGTZ,becaze of variations in the dr&ing CuNe from batch to batch the pr%ise vahie of n for a panhhr batch cannot be predicted a priori. In desalination applications an average value of P would be ~blished empiricailyon the basisoftest resului and small batch-t&batch variations in p s-oufd be accepted. Also- in such appiicuions the product fraction, P, could be melted rapidly without draining Assuming that a unit mass of solution with saiinity m is frozen, then melted with concurrent draining the mass balancrr equations are: Solution:

(2)

C*D+P=i salt: Cc + Dd i

Pp = m

(3)

Assuming sequential processing of equal units of salt l.vater the dilute brine fraction. D. from one unit may be utilized to dilute the supply water for the next unit prior to the freezing operation. The maximum value of P is I -m and the minimum value of C is m, corresponding to complete separation into -2ter and salt The mass balance equations for dilution of the supply water are:

and

Desabmiion, 7 (I 970) 2i%284

Sr i

(5)

Dd = m,

From Eqs. (2) and (3X S=CiP

(6)

and ss = cc f

(7)

Pp

Eqs. (6) and (7) indicate that the seqr:en~ial process separates the supply mass S into a concentrated brine fraction, C, and a product fraction, P. The dilute brine fraction_ D, is simply recycled.

Samples of sodium chloride soiution were frozen in Lucite containers in a Frigidaire freezer having inside dimensions of I 17 cm long, 68 cm high. and 48 cm wide, This unit fiad an operating range of -27 ‘C to - I2 ‘C. The containers were placed standin_e upright on the bottom of the freezer Cpxcept one container which was placed diagonally because of its hei@-&?_The direction of freezing was primarily inward from the iateraf surfaces of the containers. Nine Lucite containers were used. five with circular cross-section and four with rectangular cross-section. The dimensions of these containers are shown in Fig 2. These conrakers

r -

Fig.

r

permitted

determination

of the effect of three ice dimens-

?HEPMOCOUPtE

cr-ional dimclu ions tin nxillis-eters) of tucite containers.Al1 63 cm high. CXCJZ@the one on rhe right. which was 122cm high.

were approximately

containcn

ions on the values of the product fraction, P, namely, height, diameter of circular cross+ectiort, thickness of rectangutar cross-section. Desalinafion,

7 (I 970) 273-284

278

H. hf. CURRAN

Fig. 2 also shows the location of thermocouples used to monitor temperatures inside and on the surfaces of the containers at approximately mid-height of the ice. The thermocouples were connected to a millivolt recorder and to a precision potentiometer. The general procedure was to place in the freezer a container filled to a desired level* with sodium chloride solution having a salt mass faction, m, of 0.030. After the solution had been frozen to a suitable temperature the container was removed from the freezer and mounted vertically on a supporting frame. The drain plug was then removed from the bottom of the container and the ice was allowed to melt and drain simultaneously at room temperature, approximately 25°C. Small samples of the draining solution were taken at suitable intervals and anaIyzed for salinity by use of a conductivity meter and by titration with silver nitrate solution. The values of the product fraction, P, and of the concentrated brine fraction,. C, were then calculated. ExPERIMENrrAL RESULTS

Containers wifh circular cross-secrion Fig. 3 shows results obtained using circufar cylindrical containers. The upper SYMBOL DIAM..mm f 35

f

-9

HEIGHT,cm 27

l

3s

38

3s

60

x

63

60

0

76

b0

P

99

60

0

102

10

0

123

1s

ta*r.31

.e

TEMPERAT~JRE,

*c

3. Product mass fraction, P. for circular cylindricalcontainers, p = 0.0005. upper curve shows the ict mass fraction, I, based on EZq.(I), with m = 0.030. Fig-

e Appt-bximately

2 to 7% less than the final ice height, depending

The

on the final ice fraction.

Desahation, 7 (1970) 273-m

DESALINATION

BY INDIRECX

279

FREEZING

curve is the theoretical ice mass fraction calculated from Eq. (1). The experimental curves indicate that P tends to increase as the freezing temperature decreases. The three lowest curves show that P increases with increasing ice heights in the 35 mm I.D. cylindrical container_ The third, fourth, and fifth curves from rhe bottom show that for diameters up to 76 mm P increases with increasing diameter for fixed ice height. The number of tests with diameters greater than 76 mm was not sufficient to indicate a significant continuation of this effect. However, such continuation is indicated by the sixth curve which is based on data given in Table III of reference (3) for sea water with In = 0.030 in 123 mm I.D. Lucite cylindrical containers. with recratgular cross-section Fig. 4 shows the results obtained with rectangular prismatic containers. The lowest curve is for the 13 mm thick container with ice height of 27 cm. As indicated by the data points along the second curve the value of P appears to be Corttainers

1

SYMBOL

i

THiCKNESS.mm

HEIGHT,

+

13

A

13

to

x

13

120

tm

27

0

2s

60

0

38

60

13

13

60

l

2s

k8.at.r

60 I during

usad melting

3 P *a-3 .2 .1 -2s

-20

--Is TEMPERATURE,°C

40

-S

Fig. 4. Product mass fraction, P, for rectangular prismatic containers.p upper curve shows the ice mass Fraction, I, based on Eq- (I), with m = 0.030.

=

0.0005. The

relatively independent of the thickness and of heights greater than 60 cm. In two tests the melting rate was approximately doubled by use of an electric heating coil wrapped around the container. The data points indicate this had no significant effect on the value of P. As noted below the accelerated melting did have a significant effect on the value of C, however. Desalination, 7 (1970)

273-284

250.

H. M. CURRAN

brine fiyactions, C a& U

&e.

The concentrated brine frktion, C, iS plotted versus the produtit_fraction, P, in Fig. 5. The datri for rectangular cross-sections are shown.in the upper pati of Fig_ 5, and the data for circular cross-sections in the lower part. The two upper data

::I;. .6 C

n .A I L

+

* 1. -!

&%

i

AX 00

-2

:

y---L 1

'\A

$

o-'

I

I.3 -

Cl RCULAR 1

I 0

t -2 i:

t

.-4

. P

.6

.a

1.0

_

Fig_ 5. -Concentrated brine fraction, C, wrsus product fnction &l30, and p = O.tJCMk5. (See Figs_ 3 and 3 For identification of symbols.)

P, for s =

0.035, NI =

points on the upper part of Fig. 5 are for the tests mentioned above in tihich mklting was kcelcrrited by means of a heating cdiI. Akhough Fonsiderabk scatter is evident, the generai trend of C decrea&q$ -&ith.in&easing P is evident. This trend is approximated by thk c&e drawn through the data points (same curve on both parts of Fig_ 5). The point A c.orresp&ds to compkte. separation into water and s&k .-For ni = 0.03, th&co&dinatks Of A are P =’ 0.97; k‘ =. 0.03, and -from Eq. (2) the vahk;of D is 0. .I ,. l%g. 6 sh&vs the C W. P curve.from. Fig; 5 and the correspkding D.-W. P, curve comptited .by use of E& (2). For the theoretical case in which D L .O; C is a.: ‘. ~..

Des&ration,

7 (19%) 273-284

DESALINATION

BY INDIRECT

281

FREEZING

linear function of P. This limiting function is also shown in Fig. 6. The approximate C and D curves in Fig. 6 correspond only to melting and draining under the specitied test conditions. Other melting conditions would alter the curves as indicated, for example, by the data points for the two accelerated melting tests.

Fig. 6. Approximate concentrated brine fraction, C. and dilute brine fraction, D, vs. p:oduct fnction, P, for s = 0.035, 01 = 0.030, and p = O.ooO5. (See Fig. 4 for identification of symbols.) Freezing

time

Fig. 7 shows typical freezing curves for freezing with natural convection in the freezer. These are, of course, specific to the equipment used. Much higher freezing rates, obtainable with other equipment, would be necessary for practical applications to water desalination. (Tests currently in progress with salt ice formed rapidly by evaporative freezing, then packed into cyIindrica1 Lucite containers for melting and draining, indicate that the product fraction, P, is probably not significantly dependent on the rate of ice formation or on the shape of the container used during freezing). &felting-draining

time

For salt ice in the temperature range - IO” to - 20°C the time required for melting and draining to a residual salinity of p = O.GOOS does not appear to be significantly dependent on the ice temperature. Fig. 8 shows average meltiogdraining times for Lucite containers with either circular or rectangular cross-section and ice heights of 60 cm. Container

surface

elpkctiseness for

The container the mass of product time.

meitin&draining

surface effectiveness for melting-draining water per unit of lateral surface area-hour

may be defined as of melting-draining

Desalinatiotr, 7 (1970) 273-284

H. hf. CURRAN

282

a-

RECT.

o-CIRC.

.

TEMPERATURE.

*C

Fig. 7, Time required to freeze sodium chloride solution (m - 0.03, ice height approx. prismatic (I.6 mm wall) Lucite containers in -25°C freezer with natural air circulation, 60 cm) using circular cylindrical (3.2 mm wall) and rectanylar

Fig. 8. Avenge time required to melt and drain salt ice in circular cylindrkat (X2 mm walfj and rectangular prismatic (1.6 mm wall) Lucite containers top = O.ooo5 for ia tempe;raturrr -2 to -ZO”C, 60 cm ice height, ana 25°C room temperature.

Desahalion,

7 (197Oj 273-284

DESALINATION

BY INDIRECT

Thus, for circular E = (.I41 _- A2M KAO and, for rectangular

FREEZING

283

cylinders APP = --, 40

prisms (rteglecting

(8) narrow edges), (9)

where A is the diameter of a circular cross-section, t is the thickness of a rectangular cross-section, p is the ice density, and 0 is the melting-draining time. Using p = 0.95, interpolated data from Figs. 3 and 4 for P at - 15°C and melting times from Fig, 8 in Eqs. (8) and (9). the curves in Fig. 9 were obtained.

),A.

mm

Fig. 9. Container surface effectiveness for mefting-dtaining, 40 cm ice height, and 25°C room temperature_ These

curves

indicate

that

rectangular

cross-sections

E, for - 15°C ice temperature,

provide

a higher

surface

effectiveness

for mehing-draining than circular cross-sections*. Since the effectiveness must be zero for zero thickness the fitted curve indicates that a maximum for rectangular cross-sections probably exists for thicknesses in the vicinity of 10 mm.

CONCLUSIONS

Substantial product water fractions, above 0.4, can be obtained from salt water by indirect freezing fohowed by simultaneous mefting anddraining. Containers with thin rectangular cross-section are more effective in utilization of l As noted in Fig. 2 t-,e circuiar cylindrical and rectangular prismatic containers used in the experimentsdid not have equal wall thicknesses, so this conclusion strictly applies only to thii condition. However. the thermal conductivitics of salt ice and Lucite are such that the effect of the difference in wail thicknesses is relatively small.

Desalination,

7 (1970) 273-284

fi. M. CURRAN

284

surface area than containers with circular cross-section. The recovery of product water appears to be independent of ice height for heights greater than 60 cm. The recovery of product water increases as ice temperature decreases in the range -2 to - 20°C. FUTURE

1NWSTlGATlONS

Future investigations are expected to include both direct and indirect rapid freezing techniques. rapid melting, adaptation to continuous processing, and effect of subcooling the salt solution. ACENO\VLEDGblEtiT

The assistance of graduate assistants Houshang Lolachi, Bong-Soo Chung and Sung Tung in the experimenml work is gratefuIiy acknowledged. This research was supported by an institutional grant from the National Science Foundation. REFERENCES 1. hf. LECOINTE, Rertte Gencruf de Therntiqtte. 442) (1965) 629. 2. 1. L. Dsc~u. h;nlrc:echnik. 19(9) (1967) 273. 3. T. C. Ttio~psos AND K. H. NELSON, Refr;g. Eng.. 62 (1951) 4-L -5_ H. M. CURRAS. Proc. Ninth Intern. Congress of Refrigerarbrt. Paris. 6.12 (1955). 5. H. M. CURRAN. .-?dwrrces itt Chetttisrr_v Series NLL 27, Am. Chem. Sot., Washingtcrr:.

1960.

D-C..

p.56.

DesaiitmGott, 7 (I 970) 273-284