CONCENTRATED BRINE PRODUCTION FROM SEA WATER BY ELECTRODIALYSIS USING ION EXCHANGE MEMBRANES REIICHI YAMANE, MUTSUMI ICHIKAWA, YUKIO M I Z U T A N I , AND YASUHARU ONOUE
Tokuyama Soda Co., Ltd., Mihagecho 1-1, Tohuyama, Yamaguchi, Japan
A demonstration plant which produces concentrated brine corresponding to 3000 tons of sodium chloride per year from sea water by electrodialysis using ion exchange membranes has been in operation for 2 years. Two tons of sodium chloride per unit cell for a year were produced as concentrated brine, with a concentration of 138 grams per liter, and an electric power consumption of 455 kw.-hr. per ton. This plant operated more than 7900 hours in a year and the ion exchange membranes were proved usable for more than 5 years.
MANY valuable
substances as well as sodium chloride are contained in sea water, but very few industries have succeeded in obtaining them. Because of their low concentrations, much energy is required for the removal of water. From olden times in Japan, table salt was obtained from sea water using solar energy. The process utilizing solar energy, however, had many disadvantages-Le., unstable production depending on weather and the requirement of enormous ground space. I n 1957, after the development of synthetic ion exchange resins discovered by Adams et al. (1935) and the studies of membrane phenomena proposed by Meyer and Siever (1936) and Teorell (1935), concentration of inorganic electrolytes using ion exchange membranes by electrodialysis was suggested by Xakazawa (1956). I n this method, instead of removing large quantities of water, only the required ion is separated by the ion exchange membranes by electrodialysis, and much less energy is required for concentration of sea water compared with the evaporation process. T o develop this idea for the industrial production of table salt from sea water, we had to overcome many problems: to produce ion exchange membranes superior in efficiency, durability, and other properties, and to establish a good electrodialysis process and stable operation. After most of these problems were solved, we constructed a demonstration plant in May 1965, which produced concentrated brine Corresponding to 3000 tons of table salt from sea water. For 2 years the plant operated smoothly, with results almost as scheduled. The present paper reports the design and operation of the plant, the problems during operation, and how the results obtained were utilized for the construction of an industrial plant. The demonstration plant was constructed on the site of the Kinkai Engyo Co., Ltd., the largest table salt maker in Japan, located at Okayama Prefecture on the Inland Sea. The product of the plant is concentrated brine containing about 140 grams of sodium chloride per liter, and its designed capacity is 3000 tons per year in terms of table salt.
~~
~~
Table I. Particulars of Plant
Capacity Electrodialyzer Number Type Dimensions" No. of unit cells Effective membrane area Ion exchange membrane Rectifier Floor area
3000 Tons per Year (100SC NaCl Equiualent) 3 Unit cell type, made of poly(vinyl chloride) 2.2 meters"^ 1.7 meters'' x 1.1meters'' 478 X 3 = 1434 1.15 sq. ni. Cation exchange, Keosepta CL-25T Anion exchange, Neosepta AVS-4T Si-type, 208 kw. 112 sq. m.
L = length, H = height, W = width.
Table I shows the particulars of the plant and Figure 1 shows the process flow sheet. The plant comprises three electrodialyzers, each of which has 478 unit cells, equipment for supplying sea water, acid, air-blowing, and electric equipment, and instruments. Ion exchange membranes used are Xeosepta CL-25T, cation exchange type, and Neosepta AVS-4T, anion exchange type (Tokuyama Soda Co., Ltd.,) (Mizutani et al., 1963), homogeneous ion exchange membranes reinforced with poly(viny1 chloride) cloth (Table 11). The distinctive characters of these ion exchange membranes are: As the diffusivities of salts and water in the membranes are low in spite of their low electric resistance, brine of high concentration can be produced by relatively low current density. As the anion exchange membrane, AVS-4T, is permselective for a univalent anion, the concentration of sulfate ion in the brine is very low. This enables the plant to run smoothly without deposit of calcium sulfate in the unit cells. Better flexibility protects the membranes from breakage during operation or handling.
The disadvantage of the ion exchange membranes is inferior dimensional stability accompanied by considerable change of temperature. However, this was overcome by VOL. 8 NO. 2 APRIL 1 9 6 9
159
RECTIFIER
0
i ..-..-..
,..-..-..
I
:ELECTRO,OIALYZEk
n
_I-
4-
0.9 8
/
Ctl25T
AVS-4T
2-1"Ph
TO EVAPORATOR
BLOWER
PUMP
Figure 2. Unit cell
FILTER
CONCENTRATED B R I N E
SEA WATER SUPPLY
TANK
Figure 1. Process flowsheet of plant
Table It. Characteristics of Neosepta
Type
Backing Thickness, mm. Water content" Exchange capacity6 Electric resistance' Transport numberd Na + K Ca + Mg-
c1 so,
Bursting strength, k g . / i . cm.
CL-25T Strongly Acidic Cation Exchange (Na-Form)
CLS-25T ShnglY Acidic Cation Exchange (Na-Form) Permselectiw for Uniualent Cation
A VS-IT Strongly
Basic
Anion Exchange (C1-Form) Pemelectiw for Univalent Anwn
YeS
Y e8
Yes
0.15-0.17 0.30-0.40 1.8 -2.0 2.7 -3.2
0.15-0.17 0.30-0.40 1.8 -2.0 2.7 -3.2
0.15-0.17 0.25-0.35 1.5 -2.0 3.7 -4.7
0.72 0.26
0.92 0.06
} 0.02
10.02
3-4
3-4
,
BU F F E R CHAlYBER
} 0.02 0.98 0.005 4-6
CELL
-
Equilibrcrted with 0.5N NaCl solution [grams HzO/gram Na-form dry membrane (or C1-form)]. b[Meq./gmmNa-form dry membrane lor C1-form)]. 'Measured by 1000-cp. AC, in 0.5N NaCl solution at 25" C. (ohms sq. em.). dMeasured by electrodialysis of sea water at current density o f 2 (ampereslsq. dm.), at 25" C.
SEA WATER -L AIR
devices on the unit cell and the electrodialyzer, described below. Anion and cation exchange membranes are attached to each other a t the round edge, and a tube of poly(viny1 chloride) is inserted a t the corner. This is termed the unit cell (Figure 2). Its effective area is 1.15 sq. meters. Between anion and cation exchange membranes a sheet of net is set in, and the concentrated brine which is electrodialyzed into the unit cell through the ion exchange membranes flows to the poly(viny1 chloride) tube along the strings of the net. The electrodialyzer is a unit-cell type (Figure 3), in which the cell described above is the minimum unit for
treatment and operation. This is one of the characteristic differences from the filter press type electrodialyzer which is usually used for desalination. A block assembled with 14 unit cells and 15 sheets of spacers piled on alternately and is a unit for treatment of the electrodialyzer. Since the unit cell is not fixed to the spacers but held between them, the dimensions of the unit cell can be changed in proportion to the change of the ion exchange membranes and wrinkles or breakage of the membranes can be prevented. The electrodialyzer is made of poly(viny1 chloride) and is of the water-box type, provided with cathode chambers a t opposite sides and the anode chamber a t the center. The main body separated by the anode chamber
a
160
I & E C PROCESS D E S I G N A N D D E V E L O P M E N T
Figure 3. Electrodialyrer
Table 111. Leakage Current in Electrodialyzer
Leakage Current. % In block Over unit cells Under unit cells Between walls of electrodialyzer and unit cells Between blocks At overflow At conduits Total
2.6 1.1 0.5 1.0
1.7 1.3 0.4
-
P
4.3
Figure 4. Spacer
is divided into 16 block chambers with 15 separating unit cells, and the blocks assembled with 14 unit cells are inserted in the block chambers. As the block can be taken in and out from the top of the electrodialyzer without overhaul, a block whose unit cell has decreased efficiency is replaced with little labor and short cessation of operations. This is one of the merits of the unit cell type electrodialyzer. The separating unit cell is the unit cell with a poly(viny1 chloride) frame 3 mm. thick, fitted with slits on the walls and bottom of the electrodialyzer. The separating unit cell is useful for diminution of the current leakage which bypasses at the space between the unit cells and the walls and the bottom of the electrodialyzer. The current leakage calculated for this electrodialyzer from the equation of Wilson (1960) is shown in Table 111. With the small electrodialyzer (effective membrane area 1 sq. dm.) the inlet and outlet pipes of the sea water chamber are long enough to prevent current leakage, current efficiency was 89 to 90%. Considering this value together with the calculated current leakage, the current efficiency of this plant was expected to be 84.7 to 85.770, identical with results described in Table IV. If separating unit cells were omitted, the current leakage was calculated to be more than 30%, and the separating unit cells had a remarkable effect.
The number of unit cells in a block was selected for ease of handling and the current leakage, which increased with the increasing numbers of unit cells. Sea water was supplied to every block chamber from the lower part of the wall of the electrodialyzer, flowed upward along the spacers, and overflowed to be discharged. The spacer is a framework with a number of bars of poly(viny1 chloride) placed in parallel (Figure 4). Its shape is not suitable for making the flow of sea water turbulent, but is fitting for the air blow process by which mud is removed continuously from the membranes in operation. The theoretical limiting current density for this spacer is as follows (Yamane et al., 1966):
From the relationship between current and cell voltage of this electrodialyzer, Zlim was defined as a current density from which cell voltage rose suddenly. Zlim was determined a t given concentration and flow velocity and then constants k and a! were calculated from this equation. The current density of this plant was suppressed under I,, calculated from operating concentration and flow velocity of sea water.
Table IV. Results for Two Years
Results
First Unit Operation Concentration of sea water Temperature of sea water in electrodialyzer Voltage per electrodialyzer Current per electrodialyzer Current efficiency Concentration of brine Output of NaCl Output of NaCl Electric power Electrodialysis General Total Rate of replacement of unit cell
Hr. /year Grams NaCl/liter QC. Volts Amperes %
Grams NaCl/liter Tons NaC1/ year, plant Kg. NaCl/ hr., unit cell Kw.-hr./ton Kw.-hr,/ton, 100% NaCl equivalent
Plan
7884 25.8
year 7929 24.8
22 110 200 85 143 3000
22 108 200 85 142 2984
0.265 365
377
75 -
78 -
440
%
0.262
...
455 0
Second year 7838 24.8 22 111
200 84
134 3079 0.274 378
75 453 2.6
--
VOL. 8 N O . 2 A P R l l 1 9 6 9
161
The anode is made of graphite and the cathode is made of iron. The electrode chamber is separated from the main body by the buffer chamber in which sea water flows. The electrodialyzer has a double bottom; the upper bottom provides many holes for the air blow process described below. Sea water is pumped from the sea and used for the barometric condenser, which is a part of the multiple effect evaporator for the evaporation of the concentrated brine to elevate the temperature if necessary. Then it is filtered through the synthetic fiber cloth. Filtered sea water is supplied to the first electrodialyzer, overflows from it, is supplied to the second electrodialyzer, and from the second to the third, and then discharged. Sea water for the electrode chambers and the buffer chambers is supplied in parallel. The rate of flow of sea water in the electrodialyzer was 6 cm. per second and the concentration of chloride in the discharged sea water was kept higher than 0.45 mole per liter. The flow rate and the concentration of sea water were designed according to the limiting current density defined by Equation 1. Equipment is provided to supply mineral acid to sea water intermittently, to prevent the formation of scale, CaCOJ, in the unit cells. Air is introduced to the space between the double bottom by the blower, and bubbles of air are blown up between the unit cells through the holes of the upper bottom for a minute every half hour. These bubbles vibrate the unit cells and flow of sea water and remove the mud accumulated on the unit cells and in the electrodialyzer (Ichikawa et al., 1966). Air blown into the electrode chamber also removes the products of electrode reaction. Concentrated brine is drawn from the unit cells, sent to the brine tank, and pumped to the evaporation process. Direct current is supplied to three electrodialyzers with parallel circuit from the silicon rectifier. The quantity of current can be controlled by the voltage of the rectifier.
PRODUCTION,
30c-
Tons/Month
200-
kATER,
"C
Results of operation for 2 years are summarized in Table IV and monthly results are shown in Figure 5 . The concentration of sodium chloride in sea water was expected to be 25.8 grams per liter, but was actually 24.8 grams per liter for 2 years. Because of this difference, the electric resistance of the electrodialyzer increased with decreased concentration of the sea water, and then the electric power increased. I n the rainy season during June and July, operation had to stop for several days because of the decrease in sea water concentration, attributed to a great amount of water from the neighboring river. The mean temperature of raw sea water for a year was 18"C., but when heated sea water from the barometric condenser of the evaporation plant was used in cold seasons, the mean temperature of electrodialyzed sea water was 22" C. As the power of electrodialysis is reduced with increasing temperature, sea water of high temperature is more profitable. The electric resistance of the electrodialyzer decreased with the increased temperature of the sea water and then the electric power decreased. The thermal stability of the ion exchange membranes, however, limited the temperature of sea water to under 30°C. From the operational and economic viewpoint, the use of sea water at constant temperature all through the year is desirable, but it is rare in industry. At this plant, too, the temperature of sea water changed periodically according to the season, and the conditions of operation had to be changed correspondingly. The operational conditions of this plant were determined by the fundamental rule that its capacity was used to the utmost-that is, the maximum electric current was supplied to the electrodialyzer within the limiting current density. Consequently, when the temperature of sea water was high, electric current was increased, and when the temperature of sea water was low, it was decreased. By this method of operation, output of the concentrated brine was changed with the season, but the concentration of product did not vary
0 J
I
2.0 -
$A
Results and Discussion
IO
--
I, ,
-------I ,
1955
,
,
,
,
,y-,
1966
Figure 5. Monthly results of plant 162
/
I & E C PROCESS D E S I G N A N D DEVELOPMENT
,
,
,
,
,
,
,
1967
,
,
,
,
,
Table V. Composition of Mud
Composition of Mud, Wt. 75
SiO, Fez03 A1203
TiO, H 2 0 and other Hz0-soluble substances
On CL-25T
On AVS-4T
31 14 12 0.7 42
33 19 13 1 34
much. Electric current varied between 180 and 220 amperes per cell, and the mean was 200 amperes per cell. At the Inland Sea, sea water contains suspended materials corresponding to 3 to 10 p.p.m. of turbidity, which deposit on the unit cells and in the electrodialyzer, and obstruct the flow of sea water by generating scale in the unit cells (Yamane et al., 1964). I n general, elimination of all the suspended material is very difficult and requires expensive treatment. Because of the characteristics of electrodialysis using ion exchange membranes, not more than 50% of the sodium chloride will be recovered from the treated sea water, a much larger quantity of sea water compared with the output must be treated, and complete filtration of sea water is not economical. At this plant, most of the large dust particles and about half of the suspended materials in sea water were removed by the filter, but the residue flowed into the electrodialyzer and some deposited on the unit cells and in the electrodialyzer. Most of the deposited materials were removed by the intermittent air blow. Table V shows analytical data on the mud deposited on the unit cells, and Figure 6 shows its quantity in normal operation. The mud deposited on the anion exchange membranes was almost identical in composition with the mud on the cation exchange membranes. The quantity of the former increased each month but that of the latter was constant after 3 months. Judging from these data, the electrodialyzers should be dismantled every 4 to 6 months and the mud cleaned out. As the result of the air blow process accompanied by simple filtration, the trouble from
t
4 2
=w
0
I
t / L
suspended materials seemed to be solved. However, in the summer of the second year, mussels grew abnormally along the coast of this area and spawn and larvae passed through the filter and settled on the unit cells and walls of the electrodialyzer. They grew and obstructed the flow of sea water and a t last the plant was obliged to stop. After the electrodialyzer and unit cells had been cleaned with mineral acid, they were almost regenerated, but the operation stopped for 2 weeks and the efficiency of unit cells did not entirely recover. On the basis of these experiences, in the design of the commercial plant, sea water was filtered by a sand filter and the site for entrance of sea water was transferred several hundred meters offshore. Concentrated brine contained 138 grams of sodium chloride per liter, higher than the concentration in the solar process-120 grams per liter. The concentration was higher in winter than in summer. Yamane et al. (1967) have reported that with increasing temperature of sea water the concentration decreases and with increasing current density it increases. Temperature of sea water and current density were high in summer. As the higher temperature had a greater influence on the concentration than the increased current density, it decreased in summer. Table VI shows typical compositions of the concentrated brine and the relative transport numbers calculated from Equations 2 and 3. For cation,
For anion,
(3) As AVS-4T is permselective for the univalent anion, the concentration of sulfate ion was very low and scale formation of calcium sulfate in the unit cells was completely avoided. The ratio of sodium ions to total cations was 70 to 75% in the brine, against 77% in sea water. As cations other than sodium ion are useless, the reduction of their concentration is a problem to be solved. Electric power required per ton of sodium chloride in the concentrated brine was 378 kw.-hr. for electrodialysis and 77 kw.-hr. for the pumps and miscellaneous, a total of 455 kw.-hr. I t did not change for 2 years. Concentrated brine corresponding to 3000 tons of sodium chloride was produced per year, just as scheduled, and converted into 2 tons of sodium chloride per unit cell per year. The first object of this demonstration plant was to examine the operative period: 7929 hours in the first year, and almost the same in the second year. This was
Table VI. Typical Compositions of Concentrated Brine and Relative Transport Numbers
Br
so4 Na
K Ca Figure 6. M u d deposited on membrane
T
Moles/ Liter
c1
Mg
3.20 0.008 0.015 2.37 0.07 0.08 0.32
1
... 0.009 1 1.4 1.5 1.1
VOL. 8 N O . 2 A P R I L 1 9 6 9
163
-
I FIRST YEAR
SECOND YEAR
Figure 7. Causes of cessation of operation
U
!E
w Qz
E
W
L,
satisfactory compared with the chemical industry in general. Figure 7 shows the causes of suspended operation. The main causes in the first year were decreased concentration of sea water caused by the heavy rain in summer and the frequent overhaul of the electrodialyzer due to the planned examination: observation in the electrodialyzer, measurement of the quantity of mud deposited on the unit cells, etc. I n the second year, overhaul caused by the abnormal breeding of mussels ranked first. As the main causes of suspension are to be prevented by employing a sand filter, which will take off the spawn and the larva, and by transferring the pumping site offshore where the concentration of sea water is not affected by heavy rain, the rate of operation of this process should be more than 90%. The second object was to confirm the life of the unit cell. As the cost of the unit cell is the main item in the production cost of brine, its life decides the success of this process. The current efficiency and the electric resistance of the unit cell changed very little for 2 years and the concentration of brine decreased about 570 per year (Figure 5 and Table I V ) . Replacement of the unit cell was zero in the first year and 2.5% in the second year. On the basis of these data, it seemed reasonable to assume that the unit cell could be used more than 5 years. These results showed that concentration of sea water by electrodialysis using the ion exchange membranes was successful as an industrial process. Using the above results as a basis of planning, we constructed two plants in the summer of 1967, with designed capacities of 3000 and 17,000 tons per year. At the new plants sea water is pumped about 300 meters from the coast and filtered by the sand filter. Some details of the electrodialyzer were changed. Above all, because CLS-25'I', which is permselective for Na' and K + , was used instead of C L - X T , the operational results were remarkably improved. 164
I & E C PROCESS D E S I G N A N D DEVELOPMENT
Table VII. Characteristics of Industrial Plants
A Designed capacity, tons (100% NaCl equivalent) per year No. of electrodialyzers No. of unit cells per electrodialyzer Effective membrane area, sq. m./unit cell Membranes Cation exchange Anion exchange Concentration of brine, grams NaCliliter Output Kg. iXaCl/hr. unit cell Electric power, kw.-hr.iton, 1 0 0 5 NaCl equivalent Electrodialysis General Total
3000 3 575
B 17,000 16 575
1.15 Neosepta Neosepta
1.15 CLS-25T AVS-4T
170 0.315
169 0.307
273 74
292 41 __ 333
347
The second column in Table I1 shows the characteristics of CLS-25l'. Its transport number oi univalent cationsratio of transported IUa and K - to total current-is higher than 90%, compared t o about 7 P ' C for CL-PS'I'. Table VI1 shows the results for a year since the start of operation. The concentration of sodium chloride in the concentrated brine was 160 to 170 grams per liter, 20"; higher than the value in Table I V , and the output per unit cell for a year increased by 20'G. The electric power required was 270 to 290 kw.-hr. per ton for electrodialysis, j 0 to 70 kw.-hr. per ton for pumping and other uses, a total of 330 to 330 kw.-hr. per ton. These figures were much better than the results of the demonstration plant. Nomenclature
C = concentration, equivalents/liter Illm= limiting current density, amperesisq. dm.
L k T t
= = = =
v = c y =
Meyer, K. H., Siever, J. F., Helu. Chim. Acta 19, 649, 665, 987 (1936). Mizutani, Y., Yamane, R., Ihara, H., Motomura, H., Bull. Chem. SOC.Japan 36, 361 (1963). Nakazawa, H., Japan Patent 224,611 (1956). Teorell, T., Proc. SOC.Exptl. Biol. 33, 282 (1935). Wilson, J. R., “Demineralization by Electrodialysis,” Butterworths, London, 1960. Yamane, R., Mizutani, Y., Ichikawa, M., Sata, T., Bull. SOC.Sea Water Sci. Japan 18, 73 (1964). Yamane, R., Sata, T., Mizutani, T., Bull. SOC.Sea Water Sei. Japan 20, 313 (1967). Yamane, R., Sata, T., Mizutani, Y., 17th Meeting, CITCE, Tokyo, 1966.
length of membrane along flow path, cm. constant relative transport number temperature of sea water, C. flow velocity of sea water, cm./sec. constant
SUBSCRIPTS Na C1 M A B
S
= sodium ion = chloride ion
= cation = anion = brine = sea water
Literature Cited
Adams, B. A . , Holmes, E. L., J . Soc. Chem. Ind. 54, 1T (1935). Ichikawa, M., Mizutani, Y., Yamane, R., Japan Patent 474,766 (1966).
RECEIVED for review January 15, 1968 ACCEPTED December 2, 1968 Studies on Ion Exchange Membranes, X X I X .
KOLBE-SCHMITT CARBONATION OF 2-NAPHTHOL Confirmation of the Mass Transfer Model and Process Optimization P.
G .
P H A D T A R E
A N D
1.
K .
D O R A I S W A M Y
National Chemical Laboratory, Poona 8 , India A mass transfer model for the Kolbe-Schmitt carbonation of 2-naphthol, proposed earlier by the authors, is confirmed by experimental data a t different temperatures and pressures and by determining the model constants from an independent series of mass transfer experiments. The data have been subjected to statistical analysis for optimizing the process conditions for maximum yield of BON acid.
I N an
earlier paper (Phadtare and Doraiswamy, 1965) an attempt was made to provide an engineering basis for the Kolbe-Schmitt carbonation of 2-naphthol by proposing a reaction model based on mass transfer control. However, as pointed out by Massoth (1966) and Doraiswamy (1966), more work was needed to delineate the reaction model unequivocally. Thus, additional kinetic data are presented and two probable models are fully examined. The mass transfer model (finally selected as the plausible one) is then further verified by estimating the model constants by an independent series of mass transfer experiments. A response surface is estimated to find the optimum yield of 2-hydroxy-3-carboxynaphthalene (BON acid). Confirmation of Mass Transfer Model
lMass Transfer Model. The main reactions that occur during the carbonation of sodium 2-naphtholate, the description of the conceptual models, and the assumptions
made have been described (Phadtare and Doraiswamy, 1965). A diagrammatic sketch of the model is reproduced in Figure 1. As the reaction progresses between S a naphtholate and COz, a product crust consisting of the Na salt of BON acid, free 2-naphthol, and a little tar forms on the surface of the Ka naphtholate particle suspended in kerosine oil. In view of the fact that 2-naphthol, one of the reaction products, and part of the tar are soluble in kerosine oil a t the reaction temperature, the product crust consists of the Wa salt of BON acid and undissolved tar. Since the total volume of the product crust is reduced because of the dissolution of 2-naphthol and part of the tar, it is likely that the void fraction-i.e., fraction occupied by the liquid-of the solid crust would be more than that of the unreacted core. The product layer covering the reactant core can thus be regarded as a dispersion of BON acid and tar in a medium of kerosine oil. The diffusion coefficient of C o r through this crust would then VOL. 8 N O . 2 APRIL 1969
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