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Desalination in pilot scale column crystallizers
Desalination,21 (1977) 83-97 @ ElsevierScientificPublishingCompany, Amsterdam - Printed in The Netherlands
DESALINATION
IN PILOT SCALE COLUMN
c. BATES, R P. GL.ADWIN The City University, Lu_&n
AND
L.
CRYSTALLIZERS
McGRATH
(U.K.)
(ReceivedSeptember 10,1976)
SUMMARY
Results ob”tined from the continuous operation of a two inch-diameter column crystallizer and scale-up considerations involved in the design of a four inch column are presented. Implications of a mathematical model for continuous column crystallizers are discussed, including countercurrent flow of reflux liquid phase and crystal phase, leading to a reduction in salt concentration in liquid
adhering to the crystal. The factors discussed include speed of crystal transport and superimposition of oscillatory motion, column length and attitude, feed location in coIumn and the effect of product to feed ratio. Variations in crystal rate are considered and diffusion and mass transfer coefficients calculated. A value for the optimum crystal production rate is derived. SYMBOLS
a
-
A c D F H K L L’
-
Le 4 Y Y’
-
G
-
l
Area available for mass transfer between the adhering and reflux liquids per unit volume of the column (L-l). Cross sectional area of the column (L’). Crystal production rate (M T’) Coefficient of diffusion (Lz7Y1). Feed flow rate (M l?). Separation factor for the enriching section, defined by Eq. (3). Mass transfer coefficient (L-T’).
Reflux liquid stream rate (M l?). Adhering liquid stream rate (M r’). Enriched stream rate (L3 l?). Stripped stream rate (L3 T-l). Impurity content of reflux Iiquid (M M-I). . Impurity content of adhering liquid (M M-I). Impurity content of enriched stream (M M-l).
Presente$at the Fifth InternationalSymposium on Fresh Water from the Sea, Alghero, May
i6-20,1976.
84
Y,
C. BATES, R. P. GLADWIN
-
YP u, y.P Z 2, -
AND
L. MCGRATH
Impurity content of feed stream (M M-r). Dehned by Eq. (2) (M M-r). Impurity content of stripped stream (M M-r). Impurity content of reflux liquid at the feed point (M M-l). Distance in the column measured from the freezing section (L). Distance of feed point from the freezing section (L).
Greek symbols - .Ratio of adhering liquid stream to crystal rate. Q & - Impurity content of solid phase (M M-l).
5
-
P
-
Volume fraction of free liquid. Density of liquid phase (M LW3).
INTRODUCTION
Of the many experimental freeze-desalination plants, design has been based on vacuum freezing (I-6)
or on the evaporation
of an immiscible hydrocarbon
(I,
7-10) or fluorocarbon (8, II, 22) from the brine. The ice slurry is then separated and transported to a melter, some of the melt water being used to wash the remaining crystals. The use of continuous column crystallization for the purpose of desalination has previously been described (13), th e method having the advantage over other freezing processes in that crystallization, washing and melting occur in a single piece of equipment. The size of ice crystal produced is dependent.on the rotational speed of the conveyor (14) as well as the cooling rate. Thus the control of the washing process by producing suitably sized crystals is more easily facilitated than in other types of crystallizer. This process is somewhat analogous to distillation using a packed column. In its simplest form, the equipment consists of a cylindrical tube in which an Archimedean screw rotates. At one end of the tube is a freezing section with continuous refrigerant circulation, to promote nucleation of crystals in the feed solution. Crystals formed are carried by rotation of the screw to the other end of the tube which is heated_ Countercurrent contact of the two phases allows concentration and temperature gradients to be established, one phase being the reflux or wash liquid and the other is a disperse retlux liquid_ Mass the composition of purification section
solid associated with liquid of different composition to the transfer occurs between the two phases progressively changing the solid and the associated impurity as they pass through the to the melting section.
THEORY
A mathematical model for continuous column crystallizers was developed
DESALINATION
IN PILOT
SCALE
COLUMN
85
CRYSTALLISERS
by Henry and Powers (25) based on the anaIogy with packed extraction columns. Since, during a continuous operation, a crystal phase attached, to an adhering liquid phase is in countercurrent contact with a reflux liquid phase, the factors to be considered are crystal production rate, mass transfer of impurity across the adhering liquid/reflux liquid boundary and the diffusion of impurity in the reflux liquid from the “impure” end of the column towards the melting section. The following assumptions are made: (a) Internal flow rates, L, L’ and C and the factors affecting diffusion and mass transfer are independent of position in column. (b) Column is assumed to be at steady state. (c) Impurity E associated with the crystal is constant. (d) Radial variations in each phase are negligible. (e) All transport properties are constant. (f) The adhering liquid rate is proportional to the crystal rate and the ratio is independent of the position in the column. By consideration of an elemental description of the enriching section and the overall enve!ope of that section, one obtains a differential equation whose solution is y-
where
G _ -eexp-
%i - Y, .
(“HZ,)
(1)
_
and a(1 f
1
a)C2 - aLeC (3) KaAp These equations allow a prediction of the concentration of impurity in the column for any value of 2, C or Le once the constants 5, A, p and a have been determined and Y,, Y,, D and Ka estimated. As an extension to Powers’ equations, if H in Eq. (3) is differentiated with respect to C, it is possible to evaluate the maximum theoretical crystal rate, above which separation decreases. Le. 1 dH = (C _ LeF - W-Q + E dC For maximum
and
-= dH dC
or minimum
value
0
a(1 + a) (C” - 2CLJ a Lz -f- KaAp KaAp which may be solved for C provided C # Le. Dz;Ap =
86
C. BATE&R. -P;‘GLADWIN AND L:.MCGRATH-
ExPERlMENTAL
WORK
50 mm (2”) column crystallizer (a) Design aspects In the early continuous experiments, although ice crystal-prqduction would be high at the o&et, after 1 hour’s operation, the transfiortation’of crystals would decrease towards zero. This problem was overcome .when the inner walls of the freezing section were polished to a mirror finish,. thus facititating continuous crystal transportation. Several transportation screws were designed; with the experiments described the column bad a total volume of 1830 ml, an available vOlume of 1020 ml and a length to diameter ratio of 18.6.
Stripping
i
LS.YS
q-
section
i
CS C.Y’
F. Y, IEnriching section
t
I
C---Freezing
jacket
-
Purificutitm sectibn
c---
Heating jacket
L.Y
c
7
1
I
c
Le.Ye
Fig. 1. Column crystallizershowing internal and external flows.
T
Scnw
speed
R.P.M.
Fig. 2. Effat ofscrew speed onenriched pmductpurity
forcrystaktransporteddownwards.
DESALMATION
(b)
IN PILOT’ SCALE COLUMN
CRYSTALLISERS
87
Continuous operation
The crystallizer. was optimised for continuous operation by studying the effects of changing cohimn variables. The object of these experiments was to determine the maximum possible’ separation and to analyse the mechanisms affecting the separation. All runs employed a feed solution containing 3.5 % W/W sodium chloride. Scre,v speed. For crystals transported downwards, Fig. 2 shows the effect of increasing skew speed from 53 to 140 rpm on the purity of the enriched product after 3 h operation. Z, = 0.5 cm. Feed rate (F) = 1200 g/h. Crystal rate (C) = 620 g/h. Enriched product rate (L,) = 540 g/h. Oscillations. Produced by mechanically
moving transportation screw 104 cycles/min through 6 mm or pulsing the liquid in the column by use of a displacement pump 95 cycles/min through 2 mm. Fig. 3 shows the concentration profiles obtained for a screw speed of 60 rpm with crystals transported downwards, after 3 h operation. Z, = 0.5 cm. F = 1200 g/h C = 620 g/h L, = 540 g/h.
+ Mzchonical pulse.
104 osc /min thro’6mm pulse 95c6c/min thro’ 2mm A Zero oscilkxtions
Q Liquid
Pogition in column: distance feed point (cm I
from
Fig.’ 3. Effect of oscillations on concentrationprofile of impurity in column.
C:; B&ES.
88 m-
'0
; z
AND’L.:MCGRATH
Thcareticql_values
+-I-I+
35‘
R.. P.-.GLADWIN
0: ~Experin+ntal 1. : values CrystOlS 30-
-Th&nti&l
vakfes
x Experime~~ol .’
values ~ccrystals
--Estimakd
L *
s
*
I
I
VaIUQ-
I
I
t
*
t
(302 0.04Qo6 008 0.10 012 OJ4 0.16O.l8 0.20
0
Crystal
rote
(9 /set)
Fig. 4. Effect of crystal production on enriched product purity. Screw speed 100 t-pm.
0
10
20
30
40
50
Feed point distance fmm freezing jacket (cm) Fig. 5. Effect of column length on enriched product purity.
Cohen attitude. The effect of column attitude on the concentration profile at various crystal rates for a screw speed of 100 rpm is shown in Fig. 4 (after 3. h operation). 2, = 2.5 cm. F = 1210 g/h. L, = 480 g/h. Enriching section length = 60.5 cm.
DESALINATION
_i 9 g -*
.20
I
IN PILOT:SCALE
.cOLUMN
CRYSTALLISERS
89
I
18-
Feed point distance from freezing &&et (cm) Fig. 6. Effect of feed position on enriched product purity. Crystals transported upwards. Screw speed 100 rpm.
Fig. 7. ..EfFect of reflux ratio on separation transported downwards.
obtained
in column
at 100. rpm. Crystals
C, BATES_ R. :Pi GLADWIN.
90
AND: Li MCGRATH
Column. length, Fig. -5 shows. .the. effect of column .length ck the eniiched product
purity
for a screw kneed of 100 &I;
2, = 2.5 cm, F = 1210 g/h for crystals transuorted @wards. L, = 480 g/h. Feed position. In Fig. 6, the effect of moving the feed position a=+ay from the freezing section is demonstrated for 100 rpm screw speed ivith cr$tals transported upwards, again after 3 h operation. P = 1210 g/h. L, = 480 g/h. Enriching section_ ltingth = 60.5 cm. Products ratio. The effect of increasing the L,IF ratio on the concentration
Screw -._
, r
Fig. 8.
drive Refrigeration return Cunc. brine retun
Fieezirg section f
3ocmS
Freez;nS section jacket Fuifiation colunn Wig section -
24 cmz SO cm5. 20 clns.
4- diameter column and associated equipment.
DmATXON
IN
PILOT
Concentration
SCALE
COLUMN
CRYSTALLISERS
91
profiles
60 r
CI
'0 9~
speed
60
R.P.M.
;
a. P t
(Z-2,)
Fig_ 9. Concentration
ems.
profiks.
9
$2 20 *
E
p” = Y
le16-
0 0
1OORPM BORPM
Lc -=C
Fig. 10. Effect of refhu ratio on separation obtained in the column at 60.100 rem varying the ratio of Le to C,
of NaCI in the enriched stream is givea in Fig. 7 for 100 rpm screw speed, with crystals transported downwards after 3 h operation. Z, 5 2.5 cm.
C. BATES. R. P. GLADWIN
92 F =
AND
L. MCGtiTH
1800 g/h.
Enriching section length 60.5 cm. IO0 mm (4”) column crystallizer (a) Design Modifications to the 4” diameter column previously described (13) were based on the optimisation runs from the 2” column. The original Archimedean screw conveyor was replaced by one of increased pitch, thereby increasing the working volume from 3.8 to 9.0 litres. The stainless steel purification section was, also, replaced; a thick-walled perspex tube of the same dimensions allowed the build-up of crystals to be observed. To ensure continuous crystal transport the inner surface of the freezing section was honed to a mirror finish.
The column was assembled with the freezing section at the top, crystals being transported down through the p+ication section to be melted at the base. Fig. 8 shows the 4” diameter column and its associated equipment.
(b) Operation Initial investigations were concerned with the effects of variations in refrigerant temperature, consequently, crystal production rate, ratio of product offtake to feed for the case where L, 4 C and L, - C and variation of the concentration of sodium chloride in the free liquid with increasing feed and crystal production
rates. Samples were only taken when the column was packed with crystals, this being indicated by an increase in pressure at the base.
I
\‘9
F&d
OS.38
5716 6710
Screw
speed
0536 DS37
ZF=
10
20 L
30
40
60
POST in column;
60
70
Fig. 11. Vaiiation of the concentration rate and crystal production rate.
- 25% -26-C
-26-C -2e’c 100 Rl?M.
g/hr. (av)
O.S.cms
80
distance
FWrigenmt ten
7660 7763
Le - 1491
0
g/hr.
rate
0~32
from
of NaCl
90
loo feed
110 point.
120
130
(ems)
in the free liquid with increasing feed
DESALINATION
IN PILOT SCALE COLUMN
CRYSTALLISERS
93
All the runs were carried out with a feed stock containing 3.5 % W/W sodium chloride varying the screw speed between 60 or 100 rpm, Fig. 9 showing the variation in concentration down the column with crystal rate. Effect of change in product removal rate on the salt content of the product is illustrated in Fig. 10 for the case where more product is removed than crystals produced and vice versa. The variation in the sodium chloride content of the free liquid with increasing feed rate and subsequent lowering of refrigerant temperature is displayed in Fig. 1l_ DISCUSSION
From the previous analysis, for crystals transported downwards in the 2” column with a screw speed of 60 rpm, the diffusion coefficient (D) was calculated to be 0.16 cm’ s-l and the overall mass transfer coefficient (Ku) 5.56 x 10-4 s-‘. These values allow the derivation of an optimum crystal rate (C_). In the experimental work the calculated C,,,,, = 0.330 g/s was never attained since the freezing section would block at high crystal rates. Consideration of Eq. (1) shows that a low value of If, low 2, and a high 2 will cause Y to be diminished when Y, and YPare constant. Now, in Eq. (3), since c, A, p and a are constants for a fixed screw speed, then for constant L,, the value of H is governed by C, D and Ka. As C is increased l/(C - L,) decreases but c&C)/KaAp] increases. [@Cl + W2For low values of C, the diffusion factor is dominant and H decreases whereas, when C becomes large, the effect of C2 in the mass transfer term causes it to assume greater importance. Thus, an optimum value of crystal rate will be obtained above which the separation factor H decreases_ The effects of diffusion and mass transfer are less complicated; the value of H is raised as D increases and falls as KQ increases. Thus, for good separation in the column, diffusion should be minimised whilst mass transfer is increased. The effect of the column variables may now be more easily understood. Screlv speed. In Fig. 2 for negative slope, increase in screw speed enhances the washing action more than it increases the diffusion effect. At zero slope, mass transfer and diffusion are balanced whereas for positive slol~s the diffusion term is dominant. Oscillations. These are intended to improve the mass transfer of impurity from the adhering liquid to the refiux liquid. However, the diffusion term is increased to a greater extent, thus destroying the separation in the column. Column attitude. For crystals transported upwards, the melt water produced in the heating section simply percolates back down the column whereas, for the converse, the reflux liquid is forced through the ice mass. Hence, in the latter case, the overall mass transfer coefficient is higher atid separation is enhanced. Column length. As shown in Eq. (I), where 2 increases, Y decreases for all other factors cc&ant. Thus, a longer column gives improved separations; how-
C. BATES; R. P. GLADWIN
94
-ANIYL.
MCGRATH
ever, continued increase in Z will eventually lead to_little improvement inieparation. Feedposition. As the feed position is moved away from the frcrzing~section, ZF increases, and it is demonstrated by Eq. (1.) that a-larger value of Z, causes an increase in Y when the other factors are-constant. Thus, as Z,-approaches-the freezing section, the enriched .pro_duct.purity will increase to an- optimum value at Z, = 0. Producfs ratio. Consideration of the enriched product shows that as L, is increased then the volume of reflux liquid must decrease for a fixed crystal rate. Consequently, the washing action will be adversely affected and the quality of the enriched stream must decrease. A high purity product. will only be obtained for a low flow rate of that product, it being necessary to arrive at a balance between product volume and purity. Crystal rate. The effects of crystal rate have already been explained; its values may be used to determine diffusion and mass transfer coefficients. Eq. (1) may be rewritten In(Y-
YP) = -
(” izF)
+ ln(Y+ -
YP)
which is the equation to a straight line of slope (- l/H). By plotting In( Y - Y,) versus (Z - Z,) for different crystal rates, the values of W and hence D and Ka may be evaluated. The average values of D and Ka allow H to be recalculated and hence theoretical values of Y may be calculated for known values of Z. The fit between experimental and theoretical concentration profiles in Figs.
Run 54 -
Theoretical
x
Experimental Screw Crystals
(2 -fF)
plot points
speed 60 R.P.M. transported downwards
cm
Fig. 12. Effect of position iricoiumnonconcentiatiohof NaClin free liquid:C,=.O.l8Og/s.
DESALINATION
IN PILOT SCALE COLUMN
CRYSTALLISERS
95
Run s5 -. Theoretical plot X
Experiment421 points Screw
speed 60 R.P.M.
Crystals
transported downwards
x
lolyy , .::,, 5
X
10
x-x
20
30
40
50
60
70
(Z-ZFlcm
Fig. 13. Effect of position in column on concentrationof NaCl in free liquid. C = d.2OOgh
Run 56 Theoretical plo’t x Experimental points
SCRW Crystals
sped 60 R.P.M. transported downwards
Fig. 14. Effect of position in column onconcentration of NaCl in free liquid. C = O.ZlOg/s.
12-14 is thus seen to be good and the values of D and Ku may be used to calculate the’ concentration profiles for o&r values of crystal rate. Such a rigorous treatment of results obtained. from the la&r ‘diameter column has not yet been attempted since the attaining of steady state conditions proved more diffikult than in-the smaller column. Modific$.ions to the original design hake given more encouraging results. It was found that too low a Crystal
96
CBATES.
R; P. GLADWIN
AND
L; MCGRATH
rate gave .poor separation, -and increases in the rate de&eased the impurity~content of the free liquid in the column since more.reflux
was .passing up-the column -and
the &ys~tils were packed- more -closely together.. In all cases the be&‘Separations were obtained ~when the column was completely packed with crystals, ninnmg with a base pressure of 3-4 psi. As the feed to the column was increased, it became necessary to use-lower refrigerant temperatures to maintain the steady production of crystals. Thelimiting feed rate was reached when crystals were being carried over in the waste stream from the freezing section, and the capacity of the refrigerant system was at-maxi-mum. It was.also found that when product was removed at a greater rate than the crystals were produced, the impurity content of the product stream in&ased, due to diffusion of sodium chloride down the purification section. The lowest refrigerant temperature used has been -28”C, this requiring a feed rate of over 5500 g/h to prevent blockage of the freezing section with ice. CONCLUSIONS
when the 4” column can be operated reliably under steady state conditions it will be possible to establish scale-up relationships with the smaller equipment. However, the initial problem of ice nucleation encountered with the 4” column has been overcome; it is now possible to achieve a high crystal production rate yet still transport crystals out of the freezing section before blockage occurs. As with the 2” column, it has been essential to maintain the highly polished surface in the freezing section to prevent crystai growth on the freezer walls. ACKNOWLEDGEMENTS
The authors wish to thank the Science Research Council for Studentships awarded to two of us (C. B. and R. P. G.). Thanks are also due to Mr. D. R. Durkin for assistance in the preparation of this Paper.
REFERENCES 1. A. J. BARDUHN,Desdinution, 5 (1968) 173-84. 2. R. R BRIDGE, K. A. SMITH, S. JOHNKINAND W. W. RINNE,AI ChE. Symp. Ser., 67 (1970) 212-6. 3. M. PACHTER, DesaIination, 2 (1970) 358-67. 4. R; E. PECK, U.S. Pu?. 3,714,791, 1973. 5. J. SCILWARTZ AND R. F. FROBSIXIN, Desdinatiott, 4 (1968) 629_ 6. K. S. SPIEGLER, Principles of Desalination, Academic Press, New York, N.Y.; 1966. 7. ANON..Chem. Pior. &g_, 53 (1972) 3. 8. A. J. BAEXDUHN, C&m. &. Progr., 63 (1967) 98403. 9. W. H. D&N, M. J. S. SMITH, J_ T. KL_&SCHKA,R. FORGAN, M. R. Drmy, C; I-L RUMARY
AND R. W. DAWKIX Proc. 4th intern.
sept_ 9-13. 3 (1973) 291-311,
Symp.
on Fresh Watei fioin
t& Sea, Heidelberg,
DESALINATION
IN PILOT SCALE COLUMN
CRYSTALLISERS
97
10. G. LAWJZS, New Sci., 46 (1970) 628. 11.. J. H. FRAZERAND W. E. GIBSON, J. Am. Water Wks. Ass., 64 (1972) 7m. 12. W. JOEINSON, Proc. Fourth Intern. Symp. on Fresh Wtierfrom the Sea, Heidelberg, Sept. 9-I 3, 19?3,3 (1973) 371-82. 13. M. D. HOBSON AND L. MCGRATH. Proc. 4th Intern. Symp. on Fresh Water from the Sea, Heidelberg, Sept. 9-13, 1973, 3 (1973) 357-69. 14. P. BoLsAlT& C’hem. Eng Sci., 24 (1969) 1813-25. 15. J. D. HENRY, JR. AND J. E. POWERS,Am. Inst. Chem. Eng. J., 16 (1970) 1055-63.
DESALINATION
IN PILOT SCALE COLUMN
c. BATES, R P. GL.ADWIN The City University, Lu_&n
AND
L.
CRYSTALLIZERS
McGRATH
(U.K.)
(ReceivedSeptember 10,1976)
SUMMARY
Results ob”tined from the continuous operation of a two inch-diameter column crystallizer and scale-up considerations involved in the design of a four inch column are presented. Implications of a mathematical model for continuous column crystallizers are discussed, including countercurrent flow of reflux liquid phase and crystal phase, leading to a reduction in salt concentration in liquid
adhering to the crystal. The factors discussed include speed of crystal transport and superimposition of oscillatory motion, column length and attitude, feed location in coIumn and the effect of product to feed ratio. Variations in crystal rate are considered and diffusion and mass transfer coefficients calculated. A value for the optimum crystal production rate is derived. SYMBOLS
a
-
A c D F H K L L’
-
Le 4 Y Y’
-
G
-
l
Area available for mass transfer between the adhering and reflux liquids per unit volume of the column (L-l). Cross sectional area of the column (L’). Crystal production rate (M T’) Coefficient of diffusion (Lz7Y1). Feed flow rate (M l?). Separation factor for the enriching section, defined by Eq. (3). Mass transfer coefficient (L-T’).
Reflux liquid stream rate (M l?). Adhering liquid stream rate (M r’). Enriched stream rate (L3 l?). Stripped stream rate (L3 T-l). Impurity content of reflux Iiquid (M M-I). . Impurity content of adhering liquid (M M-I). Impurity content of enriched stream (M M-l).
Presente$at the Fifth InternationalSymposium on Fresh Water from the Sea, Alghero, May
i6-20,1976.
84
Y,
C. BATES, R. P. GLADWIN
-
YP u, y.P Z 2, -
AND
L. MCGRATH
Impurity content of feed stream (M M-r). Dehned by Eq. (2) (M M-r). Impurity content of stripped stream (M M-r). Impurity content of reflux liquid at the feed point (M M-l). Distance in the column measured from the freezing section (L). Distance of feed point from the freezing section (L).
Greek symbols - .Ratio of adhering liquid stream to crystal rate. Q & - Impurity content of solid phase (M M-l).
5
-
P
-
Volume fraction of free liquid. Density of liquid phase (M LW3).
INTRODUCTION
Of the many experimental freeze-desalination plants, design has been based on vacuum freezing (I-6)
or on the evaporation
of an immiscible hydrocarbon
(I,
7-10) or fluorocarbon (8, II, 22) from the brine. The ice slurry is then separated and transported to a melter, some of the melt water being used to wash the remaining crystals. The use of continuous column crystallization for the purpose of desalination has previously been described (13), th e method having the advantage over other freezing processes in that crystallization, washing and melting occur in a single piece of equipment. The size of ice crystal produced is dependent.on the rotational speed of the conveyor (14) as well as the cooling rate. Thus the control of the washing process by producing suitably sized crystals is more easily facilitated than in other types of crystallizer. This process is somewhat analogous to distillation using a packed column. In its simplest form, the equipment consists of a cylindrical tube in which an Archimedean screw rotates. At one end of the tube is a freezing section with continuous refrigerant circulation, to promote nucleation of crystals in the feed solution. Crystals formed are carried by rotation of the screw to the other end of the tube which is heated_ Countercurrent contact of the two phases allows concentration and temperature gradients to be established, one phase being the reflux or wash liquid and the other is a disperse retlux liquid_ Mass the composition of purification section
solid associated with liquid of different composition to the transfer occurs between the two phases progressively changing the solid and the associated impurity as they pass through the to the melting section.
THEORY
A mathematical model for continuous column crystallizers was developed
DESALINATION
IN PILOT
SCALE
COLUMN
85
CRYSTALLISERS
by Henry and Powers (25) based on the anaIogy with packed extraction columns. Since, during a continuous operation, a crystal phase attached, to an adhering liquid phase is in countercurrent contact with a reflux liquid phase, the factors to be considered are crystal production rate, mass transfer of impurity across the adhering liquid/reflux liquid boundary and the diffusion of impurity in the reflux liquid from the “impure” end of the column towards the melting section. The following assumptions are made: (a) Internal flow rates, L, L’ and C and the factors affecting diffusion and mass transfer are independent of position in column. (b) Column is assumed to be at steady state. (c) Impurity E associated with the crystal is constant. (d) Radial variations in each phase are negligible. (e) All transport properties are constant. (f) The adhering liquid rate is proportional to the crystal rate and the ratio is independent of the position in the column. By consideration of an elemental description of the enriching section and the overall enve!ope of that section, one obtains a differential equation whose solution is y-
where
G _ -eexp-
%i - Y, .
(“HZ,)
(1)
_
and a(1 f
1
a)C2 - aLeC (3) KaAp These equations allow a prediction of the concentration of impurity in the column for any value of 2, C or Le once the constants 5, A, p and a have been determined and Y,, Y,, D and Ka estimated. As an extension to Powers’ equations, if H in Eq. (3) is differentiated with respect to C, it is possible to evaluate the maximum theoretical crystal rate, above which separation decreases. Le. 1 dH = (C _ LeF - W-Q + E dC For maximum
and
-= dH dC
or minimum
value
0
a(1 + a) (C” - 2CLJ a Lz -f- KaAp KaAp which may be solved for C provided C # Le. Dz;Ap =
86
C. BATE&R. -P;‘GLADWIN AND L:.MCGRATH-
ExPERlMENTAL
WORK
50 mm (2”) column crystallizer (a) Design aspects In the early continuous experiments, although ice crystal-prqduction would be high at the o&et, after 1 hour’s operation, the transfiortation’of crystals would decrease towards zero. This problem was overcome .when the inner walls of the freezing section were polished to a mirror finish,. thus facititating continuous crystal transportation. Several transportation screws were designed; with the experiments described the column bad a total volume of 1830 ml, an available vOlume of 1020 ml and a length to diameter ratio of 18.6.
Stripping
i
LS.YS
q-
section
i
CS C.Y’
F. Y, IEnriching section
t
I
C---Freezing
jacket
-
Purificutitm sectibn
c---
Heating jacket
L.Y
c
7
1
I
c
Le.Ye
Fig. 1. Column crystallizershowing internal and external flows.
T
Scnw
speed
R.P.M.
Fig. 2. Effat ofscrew speed onenriched pmductpurity
forcrystaktransporteddownwards.
DESALMATION
(b)
IN PILOT’ SCALE COLUMN
CRYSTALLISERS
87
Continuous operation
The crystallizer. was optimised for continuous operation by studying the effects of changing cohimn variables. The object of these experiments was to determine the maximum possible’ separation and to analyse the mechanisms affecting the separation. All runs employed a feed solution containing 3.5 % W/W sodium chloride. Scre,v speed. For crystals transported downwards, Fig. 2 shows the effect of increasing skew speed from 53 to 140 rpm on the purity of the enriched product after 3 h operation. Z, = 0.5 cm. Feed rate (F) = 1200 g/h. Crystal rate (C) = 620 g/h. Enriched product rate (L,) = 540 g/h. Oscillations. Produced by mechanically
moving transportation screw 104 cycles/min through 6 mm or pulsing the liquid in the column by use of a displacement pump 95 cycles/min through 2 mm. Fig. 3 shows the concentration profiles obtained for a screw speed of 60 rpm with crystals transported downwards, after 3 h operation. Z, = 0.5 cm. F = 1200 g/h C = 620 g/h L, = 540 g/h.
+ Mzchonical pulse.
104 osc /min thro’6mm pulse 95c6c/min thro’ 2mm A Zero oscilkxtions
Q Liquid
Pogition in column: distance feed point (cm I
from
Fig.’ 3. Effect of oscillations on concentrationprofile of impurity in column.
C:; B&ES.
88 m-
'0
; z
AND’L.:MCGRATH
Thcareticql_values
+-I-I+
35‘
R.. P.-.GLADWIN
0: ~Experin+ntal 1. : values CrystOlS 30-
-Th&nti&l
vakfes
x Experime~~ol .’
values ~ccrystals
--Estimakd
L *
s
*
I
I
VaIUQ-
I
I
t
*
t
(302 0.04Qo6 008 0.10 012 OJ4 0.16O.l8 0.20
0
Crystal
rote
(9 /set)
Fig. 4. Effect of crystal production on enriched product purity. Screw speed 100 t-pm.
0
10
20
30
40
50
Feed point distance fmm freezing jacket (cm) Fig. 5. Effect of column length on enriched product purity.
Cohen attitude. The effect of column attitude on the concentration profile at various crystal rates for a screw speed of 100 rpm is shown in Fig. 4 (after 3. h operation). 2, = 2.5 cm. F = 1210 g/h. L, = 480 g/h. Enriching section length = 60.5 cm.
DESALINATION
_i 9 g -*
.20
I
IN PILOT:SCALE
.cOLUMN
CRYSTALLISERS
89
I
18-
Feed point distance from freezing &&et (cm) Fig. 6. Effect of feed position on enriched product purity. Crystals transported upwards. Screw speed 100 rpm.
Fig. 7. ..EfFect of reflux ratio on separation transported downwards.
obtained
in column
at 100. rpm. Crystals
C, BATES_ R. :Pi GLADWIN.
90
AND: Li MCGRATH
Column. length, Fig. -5 shows. .the. effect of column .length ck the eniiched product
purity
for a screw kneed of 100 &I;
2, = 2.5 cm, F = 1210 g/h for crystals transuorted @wards. L, = 480 g/h. Feed position. In Fig. 6, the effect of moving the feed position a=+ay from the freezing section is demonstrated for 100 rpm screw speed ivith cr$tals transported upwards, again after 3 h operation. P = 1210 g/h. L, = 480 g/h. Enriching section_ ltingth = 60.5 cm. Products ratio. The effect of increasing the L,IF ratio on the concentration
Screw -._
, r
Fig. 8.
drive Refrigeration return Cunc. brine retun
Fieezirg section f
3ocmS
Freez;nS section jacket Fuifiation colunn Wig section -
24 cmz SO cm5. 20 clns.
4- diameter column and associated equipment.
DmATXON
IN
PILOT
Concentration
SCALE
COLUMN
CRYSTALLISERS
91
profiles
60 r
CI
'0 9~
speed
60
R.P.M.
;
a. P t
(Z-2,)
Fig_ 9. Concentration
ems.
profiks.
9
$2 20 *
E
p” = Y
le16-
0 0
1OORPM BORPM
Lc -=
Fig. 10. Effect of refhu ratio on separation obtained in the column at 60.100 rem varying the ratio of Le to C,
of NaCI in the enriched stream is givea in Fig. 7 for 100 rpm screw speed, with crystals transported downwards after 3 h operation. Z, 5 2.5 cm.
C. BATES. R. P. GLADWIN
92 F =
AND
L. MCGtiTH
1800 g/h.
Enriching section length 60.5 cm. IO0 mm (4”) column crystallizer (a) Design Modifications to the 4” diameter column previously described (13) were based on the optimisation runs from the 2” column. The original Archimedean screw conveyor was replaced by one of increased pitch, thereby increasing the working volume from 3.8 to 9.0 litres. The stainless steel purification section was, also, replaced; a thick-walled perspex tube of the same dimensions allowed the build-up of crystals to be observed. To ensure continuous crystal transport the inner surface of the freezing section was honed to a mirror finish.
The column was assembled with the freezing section at the top, crystals being transported down through the p+ication section to be melted at the base. Fig. 8 shows the 4” diameter column and its associated equipment.
(b) Operation Initial investigations were concerned with the effects of variations in refrigerant temperature, consequently, crystal production rate, ratio of product offtake to feed for the case where L, 4 C and L, - C and variation of the concentration of sodium chloride in the free liquid with increasing feed and crystal production
rates. Samples were only taken when the column was packed with crystals, this being indicated by an increase in pressure at the base.
I
\‘9
F&d
OS.38
5716 6710
Screw
speed
0536 DS37
ZF=
10
20 L
30
40
60
POST in column;
60
70
Fig. 11. Vaiiation of the concentration rate and crystal production rate.
- 25% -26-C
-26-C -2e’c 100 Rl?M.
g/hr. (av)
O.S.cms
80
distance
FWrigenmt ten
7660 7763
Le - 1491
0
g/hr.
rate
0~32
from
of NaCl
90
loo feed
110 point.
120
130
(ems)
in the free liquid with increasing feed
DESALINATION
IN PILOT SCALE COLUMN
CRYSTALLISERS
93
All the runs were carried out with a feed stock containing 3.5 % W/W sodium chloride varying the screw speed between 60 or 100 rpm, Fig. 9 showing the variation in concentration down the column with crystal rate. Effect of change in product removal rate on the salt content of the product is illustrated in Fig. 10 for the case where more product is removed than crystals produced and vice versa. The variation in the sodium chloride content of the free liquid with increasing feed rate and subsequent lowering of refrigerant temperature is displayed in Fig. 1l_ DISCUSSION
From the previous analysis, for crystals transported downwards in the 2” column with a screw speed of 60 rpm, the diffusion coefficient (D) was calculated to be 0.16 cm’ s-l and the overall mass transfer coefficient (Ku) 5.56 x 10-4 s-‘. These values allow the derivation of an optimum crystal rate (C_). In the experimental work the calculated C,,,,, = 0.330 g/s was never attained since the freezing section would block at high crystal rates. Consideration of Eq. (1) shows that a low value of If, low 2, and a high 2 will cause Y to be diminished when Y, and YPare constant. Now, in Eq. (3), since c, A, p and a are constants for a fixed screw speed, then for constant L,, the value of H is governed by C, D and Ka. As C is increased l/(C - L,) decreases but c&C)/KaAp] increases. [@Cl + W2For low values of C, the diffusion factor is dominant and H decreases whereas, when C becomes large, the effect of C2 in the mass transfer term causes it to assume greater importance. Thus, an optimum value of crystal rate will be obtained above which the separation factor H decreases_ The effects of diffusion and mass transfer are less complicated; the value of H is raised as D increases and falls as KQ increases. Thus, for good separation in the column, diffusion should be minimised whilst mass transfer is increased. The effect of the column variables may now be more easily understood. Screlv speed. In Fig. 2 for negative slope, increase in screw speed enhances the washing action more than it increases the diffusion effect. At zero slope, mass transfer and diffusion are balanced whereas for positive slol~s the diffusion term is dominant. Oscillations. These are intended to improve the mass transfer of impurity from the adhering liquid to the refiux liquid. However, the diffusion term is increased to a greater extent, thus destroying the separation in the column. Column attitude. For crystals transported upwards, the melt water produced in the heating section simply percolates back down the column whereas, for the converse, the reflux liquid is forced through the ice mass. Hence, in the latter case, the overall mass transfer coefficient is higher atid separation is enhanced. Column length. As shown in Eq. (I), where 2 increases, Y decreases for all other factors cc&ant. Thus, a longer column gives improved separations; how-
C. BATES; R. P. GLADWIN
94
-ANIYL.
MCGRATH
ever, continued increase in Z will eventually lead to_little improvement inieparation. Feedposition. As the feed position is moved away from the frcrzing~section, ZF increases, and it is demonstrated by Eq. (1.) that a-larger value of Z, causes an increase in Y when the other factors are-constant. Thus, as Z,-approaches-the freezing section, the enriched .pro_duct.purity will increase to an- optimum value at Z, = 0. Producfs ratio. Consideration of the enriched product shows that as L, is increased then the volume of reflux liquid must decrease for a fixed crystal rate. Consequently, the washing action will be adversely affected and the quality of the enriched stream must decrease. A high purity product. will only be obtained for a low flow rate of that product, it being necessary to arrive at a balance between product volume and purity. Crystal rate. The effects of crystal rate have already been explained; its values may be used to determine diffusion and mass transfer coefficients. Eq. (1) may be rewritten In(Y-
YP) = -
(” izF)
+ ln(Y+ -
YP)
which is the equation to a straight line of slope (- l/H). By plotting In( Y - Y,) versus (Z - Z,) for different crystal rates, the values of W and hence D and Ka may be evaluated. The average values of D and Ka allow H to be recalculated and hence theoretical values of Y may be calculated for known values of Z. The fit between experimental and theoretical concentration profiles in Figs.
Run 54 -
Theoretical
x
Experimental Screw Crystals
(2 -fF)
plot points
speed 60 R.P.M. transported downwards
cm
Fig. 12. Effect of position iricoiumnonconcentiatiohof NaClin free liquid:C,=.O.l8Og/s.
DESALINATION
IN PILOT SCALE COLUMN
CRYSTALLISERS
95
Run s5 -. Theoretical plot X
Experiment421 points Screw
speed 60 R.P.M.
Crystals
transported downwards
x
lolyy , .::,, 5
X
10
x-x
20
30
40
50
60
70
(Z-ZFlcm
Fig. 13. Effect of position in column on concentrationof NaCl in free liquid. C = d.2OOgh
Run 56 Theoretical plo’t x Experimental points
SCRW Crystals
sped 60 R.P.M. transported downwards
Fig. 14. Effect of position in column onconcentration of NaCl in free liquid. C = O.ZlOg/s.
12-14 is thus seen to be good and the values of D and Ku may be used to calculate the’ concentration profiles for o&r values of crystal rate. Such a rigorous treatment of results obtained. from the la&r ‘diameter column has not yet been attempted since the attaining of steady state conditions proved more diffikult than in-the smaller column. Modific$.ions to the original design hake given more encouraging results. It was found that too low a Crystal
96
CBATES.
R; P. GLADWIN
AND
L; MCGRATH
rate gave .poor separation, -and increases in the rate de&eased the impurity~content of the free liquid in the column since more.reflux
was .passing up-the column -and
the &ys~tils were packed- more -closely together.. In all cases the be&‘Separations were obtained ~when the column was completely packed with crystals, ninnmg with a base pressure of 3-4 psi. As the feed to the column was increased, it became necessary to use-lower refrigerant temperatures to maintain the steady production of crystals. Thelimiting feed rate was reached when crystals were being carried over in the waste stream from the freezing section, and the capacity of the refrigerant system was at-maxi-mum. It was.also found that when product was removed at a greater rate than the crystals were produced, the impurity content of the product stream in&ased, due to diffusion of sodium chloride down the purification section. The lowest refrigerant temperature used has been -28”C, this requiring a feed rate of over 5500 g/h to prevent blockage of the freezing section with ice. CONCLUSIONS
when the 4” column can be operated reliably under steady state conditions it will be possible to establish scale-up relationships with the smaller equipment. However, the initial problem of ice nucleation encountered with the 4” column has been overcome; it is now possible to achieve a high crystal production rate yet still transport crystals out of the freezing section before blockage occurs. As with the 2” column, it has been essential to maintain the highly polished surface in the freezing section to prevent crystai growth on the freezer walls. ACKNOWLEDGEMENTS
The authors wish to thank the Science Research Council for Studentships awarded to two of us (C. B. and R. P. G.). Thanks are also due to Mr. D. R. Durkin for assistance in the preparation of this Paper.
REFERENCES 1. A. J. BARDUHN,Desdinution, 5 (1968) 173-84. 2. R. R BRIDGE, K. A. SMITH, S. JOHNKINAND W. W. RINNE,AI ChE. Symp. Ser., 67 (1970) 212-6. 3. M. PACHTER, DesaIination, 2 (1970) 358-67. 4. R; E. PECK, U.S. Pu?. 3,714,791, 1973. 5. J. SCILWARTZ AND R. F. FROBSIXIN, Desdinatiott, 4 (1968) 629_ 6. K. S. SPIEGLER, Principles of Desalination, Academic Press, New York, N.Y.; 1966. 7. ANON..Chem. Pior. &g_, 53 (1972) 3. 8. A. J. BAEXDUHN, C&m. &. Progr., 63 (1967) 98403. 9. W. H. D&N, M. J. S. SMITH, J_ T. KL_&SCHKA,R. FORGAN, M. R. Drmy, C; I-L RUMARY
AND R. W. DAWKIX Proc. 4th intern.
sept_ 9-13. 3 (1973) 291-311,
Symp.
on Fresh Watei fioin
t& Sea, Heidelberg,
DESALINATION
IN PILOT SCALE COLUMN
CRYSTALLISERS
97
10. G. LAWJZS, New Sci., 46 (1970) 628. 11.. J. H. FRAZERAND W. E. GIBSON, J. Am. Water Wks. Ass., 64 (1972) 7m. 12. W. JOEINSON, Proc. Fourth Intern. Symp. on Fresh Wtierfrom the Sea, Heidelberg, Sept. 9-I 3, 19?3,3 (1973) 371-82. 13. M. D. HOBSON AND L. MCGRATH. Proc. 4th Intern. Symp. on Fresh Water from the Sea, Heidelberg, Sept. 9-13, 1973, 3 (1973) 357-69. 14. P. BoLsAlT& C’hem. Eng Sci., 24 (1969) 1813-25. 15. J. D. HENRY, JR. AND J. E. POWERS,Am. Inst. Chem. Eng. J., 16 (1970) 1055-63.