xcept for the phenomenon of scale formation, the
E desalting of seawater by evaporation would be an easy way to solve many of the growing shortages of
Carbon dioxide is nature's way of controlling many chemical changes in the oceans; therefore, it may be the best
way to combat scale in future d
desalination plants
42
INDUSTRIAL A N D ENGINEERING CHEMISTRY
fresh water throughout the world. Even so, production is now about 100 million gal. a day, and by 1975 daily production may exceed 400 million gal. This amazing prospect for growth shows the incentive to develop the lowest cost processes for saline water conversion. By far the most important process is multistage flash, and by 1975 more than 90% of the world's production of fresh water from the sea may come from large multistage flash plants, similar to the one illustrated in Figure 1. For many years the submerged tube evaporator provided fresh water for merchant and naval ships. During World War 11, however, the character of marine warfare and the rigid requirements for evaporators on submarines and small naval vessels stimulated the development of more efficient and more compact machines. As a result, the vapor compression and the multieffect submerged-tube evaporators were developed and widely used. Yet, the most serious problem was the formation of scale which impaired the efficiency of the evaporator and reduced production. Concurrently with the development of better evaporators, chemical and mechanical methods for controlling scale were devised, but they were successful only within certain limits and the cost of these methods was of secondary importance. Prior to the 1950's the economics of water production did not seriously concern the engineer, except for the initial cost of the evaporator as a competitive piece of equipment. Not until large plants began to supply water for overseas domestic and industrial uses was it necessary to consider the total economics and design more efficient plants, which in turn required effective scale prevention methods. The conventionalsubmerged tube and the newer vapor compression evaporators met the needs of shipboard uses, but because of serious scale problems, were not adaptable to large land-based installations. The flash evaporator being less susceptible to scaling was regarded as the better process, and an intensive program of research and development began. The major stimulus for this activity was the Office of Saline Water (OSW) established by Congress in 1952 to develop economical
a
Figure 1. ConccpLuaI de+
of 50-million-g.p.d. rnunisrngc j?mh plont VOL 59
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processes for saline water conversion for industrial, municipal, and other uses. By 1965 great strides had h-en made in flash evaporation technology, and the cost of producing fresh water was on its way down. flash Evaporator
The chemistry of seawater has had a profound influence in determining the course of developments in seawater evaporators. The early work of Partridge (4) and later of Langelier (2) characterized the chemical changes which occur when seawater is heated. The mechanism of scale formation and its effect on the evaporator design have been described by Badger (7). The flash evaporator is less apt to scale because the boiling of seawater occurs in the stages rather than on the heat transfer surfacesin the evaporator. The flashing process is relatively simple and depends upon the temperature and pressure conditions inside the evaporator. When heated seawater is pumped into a chamber (stage) in which the pressure is less than the vapor pressure of the water, vaporization (flashing) occurs. The vapor rises and condenses into pure water on the stage condensers. A modern day flash plant has many such stages, and its operation may be understood hy the sketch of Figure 2, which is the basic cycle. It is called the once-through process. Seawater enters the plant and flows through the condensers (heat recovery section) where it is heated regeneratively by the condensing vapors. After passing through the brine heater (heat input section), the seawater enters the first flashing stage. The brine cascades from stage to stage flashes producing more and more pure water. Finally, a cool concentrated brine is returned to the sea. In the oncethrough process of Figure 2 there is no appreciable scale formation as long as the maximum temperature remains below 160' F., but the production of fresh water under
these conditions is small, being on the order of 5% of the total seawater passing through the plant. This low rate of production must be improved if the cost of water is to be reduced. The answer to increased production is to be found in the thermodynamics of the flashing process (5). According to the basic equation, the difference between the temperature of the seawater out of the brine heater and the temperature of the discharged brine to the sea establishes the rate of production. This is called the flashing range. Therefore, it was clear that higher temperatures were required. Soon many flash plants were operating above 160' F. and production increased, but scale began to form. To overcome this, small amounts of starches, certain phosphates, and acid salts were added to the incoming seawater. These chemicals permitted scale free operation up to 190' F. with increased production. Unfortunately, the chemical treatment costs began to mount because in a once-through system all of the chemical treatment was lost in the discharged brine. To minimize this loss, a large part of the brine was recirculated back through the plant and only enough new seawater was added to maintain a constant rate of feed to the heat input section. Thus, the recirculation cycle was born. It is illustrated by the schematic of Figure 3. Now, the only chemical treatment required is for the new (makeup) seawater entering the system. However, the plant design has become more complex. There is a recirculation pump, and a heat rejection section has been added to maintain thermal balance. This additional equipment, of course, increases the capital costs. In essence the recirculating brine plus the treated seawater makeup become the feed to the flash plant, but the recycle stream contains more salt and also more scale forming chemicals. The flash process has reached a
FiWe 2. Thc basic q c h of a modem abyparh plant 44
INDUSTRIAL A N D ENGINEERING CHEMISTRY
dilemm-the solution of one created several new ones. At cussion. of the chemistry of seawater will help in understanding scale formation and the methods to prevent it. Chemis.lry of Seawater
Seawater is a complex solution containing an average of 3l[& dissolved saits. Aboui 90% of the mineral content is sodium chloride and 5% is magnesium and calcium ions, which are the chief scale forming elements: The alkalinity of seawater is due to the bicarbonate ion which. is present 'to the &tent of 0.4%. I t is the most important ion in alkaline scale formation-i.e., the calcium carbonate and magnesium hydroxide scales. The sulfate ion, having about,twenty tipes the concentration of the bicarbonate ion, is mponsible for the tenacious calcium sulfate scale which &ally forms at evaporation temperatures above 250' F. I t was recognized early that scale d e w i t s were related to the inverse solubility phenomenon of these chemical compounds. In other words, the solubility decreases with increasing temperatures. Moreover, the chemical changes which occur in heating seawater play a vital part. The reactions are shown in Figure 4. In Equation 1 of Figure 4 heat breaks down the bicarbonate ions into carbon dioxide which escapes as a gas. The carbonate ion combines with calcium to produce .the insoluble calcium carbonate (Equation 2 of Figure 4). If higher temperatures i r e involved, the carbonate ion breaks down into more carbon dioxide (Equation 3 of Figure 4) and the hydkixyl ion is formed. This produces magnesium hydroxide scale (Equation 4 of Figure 4). As temperatures rise above 250° F. the remaining calcium ions combine with sulfate ions to produce calcium sulfate scale (Equation 5 of Figure 4). The one point to be
Figurc 3. T h re&datimi
2 HC08-
.+
e COaf (9) + COS- + H a 0
+ Ca-
1
(1)
+ 250'
+ SO,"
+
F.
Cas04 (5) (PPt.)
CaCOn (ppt.) (2) (Alkaline scale) Vote: In seawater at ambient temperature under equiibrium conditions, all dissolved COI is primarily in the orm of HCOP-. Molecular COZ and COBexist only ^*.-"
..
aycle addad to th baric cycle of rhePashpLxt VOL 59
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Figure 5. Recirculation qcle with acid trtatm811l
noted here, and it will be emphasized later on, is that the release of carbon dioxide is necessary in the formation of the akaline scales; it also may play an important part in calcium sulfate scale formation. As long as evaporation temperatures remain below the 250’ F. or 260’ F. limit, acid, especially sulfuric, is the most effective and the lowest cost method of scale pnvention. Other techniques have been developed, but they are either more expensive or require elaborate equipment. %die Pmvenlion by Suifurlc Acid
The acidification of seawater to prevent scale has been so successful that it is now recommended for almost all large multistage flash plants. By using sulfuric acid, the OSW 1-million-gal.-per-day (g.p.d.) plant at San Diego raised the maximum operating temperature from 200°F. to 250’ F. and production increased to 1.4 million g.p.d. (40%). This was tantamount to a breakthrough because for the first time in saline water conversion history, a significant reduction in the cost of desalted water had been achieved at a modest increase in operating costs and virtually none in the capital cost. The procedure for adding acid to the seawater feed is important and is shown in the flow sheet of Figure 5. Only the makeup of feed is treated, as at A . If the full amount of acid is added and completely mixed with the seawater, the reaction may be written as follows: Ha01
+ 2 H C 0 a - e 2 CO, T + 2 H,O + SO,4
Sulfuric acid breaks down the bicarbonates into carbon dioxide gas and water, and there is an increase in the sulfate ion concentration in the treated feed. In practice, enough acid is added to lower the pH of the feed from 8.2 to about 4.4 to ensure complete reaction with the bicarbonates. When the acidified feed enters the free space of the deaerator, carbon dioxide escapes and the pH returns to a neutral value near 7.0. It is im46
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
portant that all liberated CO, be purged from the feed water to avoid the corrosive acidic conditions or the reforming of the carbonate ion upstream. After the feed has passed from the deaerator, it is mixed with recirculated feed at B and then moves into the stage condensers of the heat recovery section. While there are many advantages in using sulfuric acid, there are also penalties. I t is not generally el€e.ctive above 250’ F.; it is a corrosive liquid and dangerous to handle. In its concentrated form it has almost twice the specific gravity of water; consequently it tends to “stream” in water and rapid and complete m k i g is apt to be difficult. For plants of 5, 10, or 50 million gal. daily, the mixing problems can be staggering. As a case in point, about 1 gal. of acid must be homogenized with approximately 12,000 gal. of seawater between the point of addition and the deaerator. From a logistics point of view, approximately 11/8 tons of acid are required to produce 1 million gal. of water. Thus, if by 1975 the total daily output exceeds 400 million gal., a year’s consumption of sulfuric acid would be about 165,000 tons. For these reasons, other effective methods for scale control are being developed. k o k Prevention by Carbon Dioxide The use of sulfuric acid is really a paradox. Being a strong acid it is used to remove a weak one-i.e., carbonic acid (COS H,O) from seawater. In the oceans, carbon dioxide or rather the carbonic acid derivatives are necessary to maintain the proper pH conditions and stabilize the calcium and magnesium ions. Therefore, it would appear that removing CO, from seawater is con-
+
Al!THOR Edgar A . C a d d l a d e r is Chuf of Chemual Enginem’ng Division, Oj’ice of Engineering, A z e q for I&national Development (AID), Washingtm, D. C. This article hos been mode available by the Compressed Air and Gas Institute, Clcueland, Ohio.
.. .. .,
.
. .
.
FigUra 6. trary to the natural method for holding the carbonates in solution. The point is clear: Use more carbon dioxide to prevent scale. Tidball and Woodbury (5) reported that when full strength seawater was heated in a closed system, no scale formed in the evaporator below 300° F. But, it is known that calcium carbonate scale forms at temperatures over 160° F. in some of the stage condensers and in the brine heater. Under the latter condition it is obvious that the bicarbonate ion decomposed because the system pressure was too low to prevent it. The reaction may be written:
+
+
2 HCOsGO, GOa HzO P = 4.7 Ib./sq. in. (vapor pressure of HnO at 160”F.) Carbon dioxide and calcium carbonate formed. However, at 300’ F. higher pressures are encountered (P = 67 Ib./sq. in.) and the carbon dioxide, being under greater pressure, remained bound up in the bicarbonate ion. Now, if carbon dioxide were added to a pressurized system, the equilibrium would shift to the left in the above equation and more bicarbonate would be formed. Then, the alkaline scales would not precipitate. The additional carbon dioxide and the increased pressure are the key elements in this process. At temperatures over 300’ F. the highly soluble bicarbonate ion may also aid in reducing the likelihood of calcium sulfate scale in spite of its decreasing solubility. In the preferred method of this process, a controlled amount of carbon dioxide is pumped into the stage condensers by the compressor as shown in Figure 6. The pH of the feed water drops from 8.2 to some value, say 6.8 to 7.2. This will add sufficient carbon dioxide to inhibit alkaline scales and also avoid acidic corrosive conditions. The feedwater then flows though the brine heater and a t e m the carbon dioxide release section. This is the heart of the process. When the seawater enters the free space of this chamber, there will be a vigorous evolution of carbon dioxide gas from bicar-
bonate breakdown. An abundance of carbonate ions will form to precipitate as calcium carbonate (see Equations l and 2 of Figure 4). Actually, there probably will be an equilibrium condition existing wherein calcium carbonate, magnesium hydroxide, and calcium sulfate will copmipitate from the hot seawater. These solids can be removed from the modified stage. The brine then flowing downstream will contain less calcium so that in the recycled stream there is less calcium in the feed, and the potential for calcium sulfate scale is consequently reduced. Carbon dioxide from the release section is recycled to the compressor for muse. The precipitated calcium carbonate can be heated to provide C o nmakeup. The technical and economic advantages of this process are: Scale free operation up to and above 300’ F., greater thermal economy, and longer plant life. These will add up to reduced water costs. The capital cost will be somewhat greater owing to the compressor and the carbon dioxide release section. Fmsh Waka from h e Sea
In the next few years the large multistage flash plants will produce cheaper water These plants will be better designed, have greater thermal economy, and last longer. There will be problem with scale, but if carbon dioxide is a natural way to control chemical changes in the oceans, it may be the best way to combat scale in future desalination plants. REFERENCES
