Economics of Demineralization by Electrodialysis

12 Economics of Demineralization by Electrodialysis R. E. LACEY, E. W. L A N G , a n d E. L. H U F F M A N

Southern Research Institute, Birmingham, Ala.

To d e t e r m i n e the a r e a s of research effort that will result in the largest reductions in the cost of

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mineralizing

w a t e r , the cost d a t a

of

Katz

deand

V o l c k m a n w e r e used to illustrate the a p p r o x i m a t e effects

of

conditions

membrane on

the

properties

total

cost

of

and

operation

demineralizing

w a t e r b y electrodialysis.

Reducing concentration

polarization

compartments

within

the

is

the

most p r o m i s i n g means of reducing the cost.

De-

v e l o p m e n t of lower-cost m e m b r a n e s w i t h

longer

o p e r a t i n g lives a n d l o w e r resistances w o u l d also reduce the cost, but not as much as might b e

ex-

pected.

If the cumulative effect of a l l of the in-

dividual

possibilities

for

cost

reduction

is

con-

s i d e r e d , the cost of d e m i n e r a l i z a t i o n m a y be red u c e d f r o m a b o u t $0.35 to a b o u t $0.20 p e r 1000 gallons.

y o guide research and development efforts on electrodialysis processes for demineralization of water it is desirable to know the effects of certain technical factors and unit costs, such as polarization phenomena and cost of membranes, on the total cost of demineralization by electrodialysis. C a r y ( I ) , Dankese ( 5 ) , K a t z ( 7 ) , Strobel (12), Volckman (2, 13), and W i l s o n (14) have published cost data and estimates of costs for demineralization by electrodialysis. Dankese and Katz presented estimates for large electrodialysis plants, based on extensive experience i n field tests. Cary and Strobel presented cost data based on the first six months of operating experience w i t h the 25,000gallon-per-day electrodialysis plant at Coalinga, Calif. V o l c k m a n and W i l s o n presented cost data on operating experience w i t h the 2,880,000-gallon-per-day plant at the Free State G e d u l d mine i n South A f r i c a . T h e cost data of Katz and V o l c k m a n were used as a basis for illustrating the approximate effects of membrane polarization, limiting current density, cost, operating life, and resistance of membranes, and temperature on the over-all costs of demineralization of water w i t h electrodialysis. T h e major objective was to pinpoint the problem areas needing greatest research effort and to show the probable effect of solving these problems upon the cost of demineralization. 168

Saline Water Conversion—II Advances in Chemistry; American Chemical Society: Washington, DC, 1963.

12. LACEY

ET AL.

Economics

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Electrodialysis

169

T h e data of Cary, Dankese, and Strobel were not used i n this study, because the data of Dankese have been superseded b y those of K a t z , and the data of Strobel and C a r y are for a small installation not typical of costs for larger installations. Certain revisions were made to place the cost data of K a t z and V o l c k m a n on a comparable basis. A l l data and estimates were revised to be i n accord w i t h the standard cost procedure of the Office of Saline W a t e r ( 8 ) , w i t h certain exceptions discussed later. The data of K a t z were revised to apply to a 2800-p.p.m. feed water; the data of V o l c k m a n for a 2800-p.p.m. feed water were revised to apply to a 2,000,000-gallon-per-day capacity [p.p.m. is used for parts per million of total dissolved solids ( T D S ) ] .


Basic Cost Data and Revisions T h e original cost data presented b y Katz and the revised data based on K a t z ' estimates are shown i n Table I. K a t z ' original estimate was based on a plant capacity of 2,000,000 gallons per day, an energy cost of 10 mills per kilowatt-hour, a membrane replacement rate of 1/5 per year, an amortization of 25 years w i t h 5 % interest, and the use of Ionics M a r k III equipment. T h e major revisions to place his costs on the basis of the standard procedure used by the Office of Saline Water are shown at the top of Table I. Katz presented data for a 1000-, 2000-, and 4000-p.p.m. feed water. Graphical interpolation was used to obtain the approximate capital and energy costs for a 2800-p.p.m. feed. T h e major anion in the feed water was chloride, w h i c h was also the major anion i n the water for the V o l c k m a n estimates. In K a t z ' original data the membranes cost $5 per sq. foot. This value was revised to $3 per sq. foot, the present cost for this thicker type of membrane ( 7 ) . The membrane replacement rate was not revised, because Ionics, Inc., guarantees the replacement rate to be no more than 1/5 per year. The other revisions are explained i n the footnotes to Table I. Volckman's cost data (Table II) were based on a plant capacity of 2,880,000 gallons per day, an energy cost of 5.2 mills per kw.-hr., a membrane replacement rate of 1 1/3 per year w i t h membranes that cost $0.20 per sq. foot, an amortization period of 15 years at 6.5% interest, and the use of the Free States G e d u l d electrodialysis presses. T h e major revisions were i n capital cost, type of membranes, and amortization schedule. It was assumed that a demineralizer of the "sheet flow" type used by V o l c k m a n would cost 2 0 % more if built in the U n i t e d States than if built in South A f r i c a . A n adjustment for reduced size of the plant was also made for the conventional process equipment by using the usual 0.6 power rule for capital cost applied to the 6 4 % of the total capital cost that is for conventional process equipment. The cost of the dialysis presses, exclusive of membranes, was not reduced. However, the number of membranes required for the first charge was reduced b y the ratio of throughputs. The energy consumption estimated by V o l c k m a n was 10.1 kw.-hr. per 1000 gallons, but from an examination of the operating data (13) it seemed more realistic and more conservative to use 12 kw.-hr. per 1000 gallons i n the revised estimates. The cost of energy was assumed to be 7 mills per kw.-hr. for the revised estimates. It was assumed that the membranes used in the electrodialysis demonstration plant at Webster, S. D . , would be used instead of those i n Volckman's plant. The membranes used at Webster cost about $1 per square foot. The membrane


170

ADVANCES

IN CHEMISTRY SERIES

T a b l e I.

Estimated Costs f o r

(Revised from data of Katz) Basis.

Feed water temperature 8 5 ° F. Demineralization range. Original, 4000 to 500 and 2000 to 500 p.p.m. T D S . Revised, 2800 to 500 p.p.m. T D S Anions predominantly chloride Electrical energy cost. Original, 10 mills per kw.-hr. Revised, 7 mills per kw.-hr.

Original 2000-p.p.m. feed Number of stacks Total capital cost

0

Electrical energy Chemicals Prefiltration" Membrane replacement Maintenance Labor Amortization


c

2,000,000 Gallons/Day Original 4000-p.p.m. feed

16 $850,000

24 $1,300,000

$0,134 0.025 0.030 0.060* 0.018 0.013 0.083 $0,363

$0,260 0.025 0.030 0.090& 0.027 0.017 0.126 $0,575

° Estimated by plotting capital costs listed by Katz for feed concentrations of 1000, 2000, and 4000 p.p.m., and reading capital cost for 2800p.p.m. feed from best line through points. In original estimate 55,000 sq. ft. of membrane used at $5.00/sq. ft. Adjusted capital cost figures reflect reduction in cost of original charge of membranes. 6

Energy data from 1000-, 2000-, and 4000-p.p.m. feeds plotted, and value for 2800 p.p.m. read from graph. Katz lists d.c. stack energy for 3000-p.p.m feed as 9.5 kw.-hr. per 1000 gal. From this and total energy cost as interpolated, pumping cost may be inferred to be $0.08 to $0.10/1000 gal. c

d

T o revise energy cost for 7-mill power:

($0,190) = $0,133.

• Cost of prefiltration will vary considerably with amount and nature

replacement rate at Webster is less than 1/5 mates shown in Table II a rate of 1/3

per year, but for the revised esti-

per year was assumed (the guaranteed

replacement rate). The original amortization conditions used by Volckman (15 years at 6.5% interest) reflect the high cost of money in South Africa.

The revised amorti-

zation conditions are those suggested by the standard procedure of the Office of Saline Water—20 years at 4% interest. One important assumption in both estimates is that the temperature of the feed water is fairly high ( 7 5 ° and 8 5 ° F ) .

If it is lower, the resistance of the


72.

LACEY

ET AL.

Economics

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171

Electrodialysis

D e m i n e r a l i z a t i o n b y Electrodialysis

Membrane replacement rates V 5 per year Membrane cost. $5 per sq. foot Amortization basis. Original, 25-year, 5% int. (7.1% per year). Revised, 20-year, 4% int. (7.4% per year). Maintenance cost other than membranes. 1.5% capital per year Operating days per year. Original, 365. Revised, 330.


Interpolated for 2800-p.p.m. feed

Revised Estimate (2800 P.P.M. Feed) with Membranes at $3.00/sq. ft. %2.50/sq. ft. %2.00/sq. ft.

20 $1,075,000

20 $965,000*

20 $938,000*

20 $910,000*

$0,190 0.025 0.030 0.072* 0.021 0.015 0.104* $0,457

$0,133* 0.025 0.030 0.043* 0.019* 0.015 0.098' $0,363

$0,133* 0.025 0.030 0.036* 0.018* 0.015 0.095/ $0,352

$0,133* 0.025 0.030 0.029* 0.018* 0.015 0.092* $0,342

of suspended solids. Standard cost for prefiltration of $0.03 was assumed for all estimates. I Cost of membranes in original estimate was $5/sq. ft.

* T o revise for cheaper membranes:

/ $ 3 00\ ( $5 QQ ) ($0*072) = $0,043.

L i-p . . , , , ., , * 10 revise maintenance tor changed capital cost: ($0,021) = $0,019. #

•

S

S

B

/ $ 965,000\ I $T075~000/

o o

g

FROM KATZ' DATA (5YR.LIFE)

*t040 z o N

< 030

a:


UJ

z 2

FROM VOLCKMAN'S DATA(3YR. LIFE)

UJ

Q 020 Ll_ O

I-

co o f 0.10

1.00 2.00 COST OF MEMBRANE, Jf/SQ.FT.

Figure 2.

3.00

Effect of cost of membranes on cost of demineralization

gives the minimum i n the curve for total costs. Because the size of the equipment and the area of membranes necessary become larger as current density is decreased, the membrane-replacement cost and the amortized capital cost vary i n versely w i t h current density. T h e power cost varies directly w i t h current density, but costs of labor, maintenance, and chemicals are not affected greatly. Because of the w a y i n w h i c h individual costs vary w i t h current density, there is a m i n i m u m total cost of demineralization, w h i c h determines the optimum current density. I n many electrodialysis plants the limiting current density is less than the optimum, a n d the cost of demineralization is higher than it w o u l d be i f polarization could be overcome. This paper does not discuss methods of overcoming or minimizing polarization. If the unit cost of power i n Figure 1 is increased, the optimum current density w i l l be lower than the one shown. Similarly, the unit cost of membranes, the resistance of the membranes, the average temperature of the feed, and other factors tend to shift the optimum current density, either higher or lower. F o r the feed water assumed i n Tables I a n d II little trouble was expected from polarization, and the current density used was close to the optimum. I n the calculations of effect of unit costs on total costs, the current densities used in the original estimates were not changed to optimize them for each n e w cost condition. F o r this reason the changes i n the total cost of demineralization w i t h changes i n unit costs or technical factors (Figures 2 to 5) should be v i e w e d as only approximations. Use of a different operating current density ( a n d therefore a different membrane replacement and amortization cost) w o u l d alter the slope of the various curves. However, the curves provide an approximate picture of the


12. LACBY

ET AL.

Economics

of

Electrodialysis

175

cost reduction that can be obtained b y solving technical problems, developing cheaper membranes, or developing membranes w i t h longer lives. Cost of Membranes The direct variation i n the total cost of demineralization w i t h the cost of membranes is shown i n Figure 2. D o u b l i n g the unit cost of membranes (from $1.50 to $3 per square foot) increases the total cost of demineralization only $0.02 per 1000 gallons [based on Katz data ( 6 ) ] . D o u b l i n g the unit cost for the data of Volckman (from $0.50 to $1) causes a $0.06 increase. 7


Operating Life of Membranes T h e effect of the operating life of the membranes is shown i n Figure 3. T h e present membrane costs of $1 to $3 per square foot were assumed for the Volckman a n d the K a t z data, respectively. In the Volckman unit doubling the life of the membranes to 6 years appreciably decreases cost, but increasing membrane life beyond 6 years decreases the cost of demineralization only b y small increments. If the unit cost of membranes is less than $1 a n d $3 per square foot, the advantage of increasing the membrane life is less. 060

LEGEND: 1 1 1 1 1 1 FEED WATER CONCENT RATION - 2800F?F>M.TC S PRODUCT WATER CONCENTRATION-50C)PPM .TDS THROUGHPUT- 2,000,000 GPD . r n c T o f pn\A/FR-ft nnn7/KWH

^ \ J

050

Economics of Demineralization by Electrodialysis

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