Reducing scale formation in brine distn. appts.

MEMORY TO BE COMPLETED VOLUNTARILY BY THE APPLICANT

This invention relates to the removal of scale and, more specifically, relates to improved apparatus for substantially reducing or eliminating the speed and degree of scale formation of calcium carbonate and magnesium hydroxide in water distillation facilities.

Extensive research has been directed towards the elimination of alkaline scale formation during the distillation of water containing dissolved magnesium and / or calcium salts, such as brackish water or seawater. In all types of water treatment plants Seawater, the heating of seawater at a temperature of up to approximately 82 ° G, produces scale which consists predominantly of calcium carbonate (CaCO3). At temperatures of approximately 93 ° G, the magnesium hydroxide scale predominates. / - /, / ~ Mg (0E) 2_7 · At a temperature between approximately 82 and 93 ° G, any type of scale or a mixture thereof can be found. The calcium carbonate and magnesium hydroxide scale are collectively called alkaline scale,

Since the scale is a. Thermal insulation, even a thin layer of scale on thermal transfer surfaces or other components of the process installation, significantly reduces the thermal transfer capacity of the components. The accumulation of scale on evaporator tubes can result in a significant reduction in flow capacity, increases in energy supply per unit flow capacity or both. The accumulation of husk results in frequent interruptions, the cost of which is substantial, such as the direct cost of cleaning pipes with scale.

Various means have been used in an attempt to:

reduce or eliminate scale formation in seawater distillation units, with limited success. In attempts to destroy the total alkalinity (sum of the bicarbonate ions - 2 - (HCO), carbonate (CO) and hydroxyl (OH)) in the brine of the evaporators, acids, usually mineral acids, have been added to the feeding currents of the distillation units. However, effective control of scale formation by continuous addition of acids is difficult to achieve due to the highly corrosive nature of the mineral acids and the sensitivity of the scale forming reactions to the used acid concentrations. It requires precise control of the acid dosage since the introduction of excess acid into the water feed stream of. Sea translates into corrosion of the installation, while the introduction of insufficient acid translates into rapid formation of scale. The continuous addition of acid is a. expensive process and, unless the carbon dioxide formed in the chemical reaction between the alkalinity of the seawater and the acid, such as by deaeration, is substantially completely eliminated, fast rates of corrosion of the components of the evaporator are obtained. base of copper and steel alloy. This is impractical in many seawater distillation facilities. unless the carbon dioxide formed in the chemical reaction between the alkalinity of the seawater and the acid, such as by deaeration, is substantially completely eliminated, fast corrosion rates of the alloy-based evaporator components are obtained. copper and steel This is impractical in many seawater distillation facilities. unless the carbon dioxide formed in the chemical reaction between the alkalinity of the seawater and the acid, such as by deaeration, is substantially completely eliminated, fast corrosion rates of the alloy-based evaporator components are obtained. copper and steel This is impractical in many seawater distillation facilities.

The use of inorganic or organic chemical additives to control the scale, in order to avoid the formation of alkaline scale, has had only a limited success and is often not effective at relatively high brine temperatures. Therefore, and while many commercially available scale control additives provide

good control of the formation of calcium carbonate scale, said chemical compounds have. have been only marginally effective at controlling the formation of magnesium hydroxide scale at relatively high temperatures.

It has long been recognized that the release of carbon dioxide from the brine as a result of boiling promotes the decomposition of bicarbonate and carbonate ions from the brine, ultimately resulting in the formation of insoluble magnesium hydroxide ions and scale of magnesium hydroxide. high temperatures. Thus, attempts have been made to inhibit the formation of hydroxyl ions by the addition of carbon dioxide under pressure to the brine which is heated under boiling conditions, such as in multi-stage instantaneous evaporators, in order to prevent the formation of hydroxyl ions. inhibit the hydroxyl ion-forming reaction. ·

Although in the past it has had some success on a pilot scale in instantaneous multi-stage evaporator units where the brine is heated under pressure and vaporized instantaneously in a separate chamber of the heating stage, this technique has not been successful in other types of evaporating units where the boiling brine enters ;, directly in contact with the thermal transfer surfaces. In addition, the presence of free carbon dioxide (chemically uncombined) promotes corrosion and this has made the addition of carbon dioxide commercially impracticable in all types of evaporation apparatus.

US Patent No. 3,218,241 to Checkovich (16 November 1965), the disclosure of which is incorporated herein for reference purposes only, discloses a method for controlling training.

of cascarilla in systems of recovery of running water by l vaporization in multiple stages (MSF) maintaining the concentration- [t

of carbon dioxide, in the pressure brine to be heated, at a level sufficient to inhibit the hydrolysis of bicarbonate ions to carbonate ions. Checkovich's patent states that this can be achieved by recycling the carbon dioxide released during the distillation of the seawater fed.

Checkovich's patent discloses that scale formation can be delayed by the addition of vitreous phosphates or other chemical compounds that have chelating or wetting properties. The addition of acids at high temperatures may be necessary.

However, the method described by Checkovich requires a relatively low pH (e.g., 7.5 or less) and extremely high free carbon dioxide concentrations (e.g., 4-15 ppm). Said conditions achieve the elimination of scale only at rates of the relatively high corrosion rates of the steel and copper alloy components normally used in the processing facilities. As a result, this solution to the problem of control of alkaline scale formation has had little or no commercial success.

In short, the problem of control of the formation of alkaline scale in commercial plants for the distillation of seawater, has been solved in various ways. Plants operating at relatively low temperatures (i.e., below about 882c) may employ the addition of polyphosphates or other chemical control additives from the scale to control the formation of calcium carbonate scale. The operation above about 88QC, has required the continuous addition of acid to the feedwater followed by deaeration to destroy the alkalinity of the feedwater and separate the carbon dioxide from the system.

- 5 Alternatively, the continuous addition of an additive! The chemical control of the scale has been combined with mechanical means (for example, rubber sponge balls of the Taprogge type) to separate the soft scale from the heat transfer surfaces, or with partial destruction of the alkalinity of the feed water by continuous addition of acid followed, by deaeration for removal of carbon dioxide.

!

The above systems have generally required a periodic interruption with circulation of acid to remove the accumulated scale. The frequency of interruptions has been inversely related to the effectiveness of the chemical compound and / or mechanical treatment used. In general, an attempt has been made to reach a balance between the cost, complexity and corrosion risks of the pre-treatment method of the scale used and the cost, loss of production and corrosion associated with interruption and periodic cleaning with acid.

It is an object of the invention to solve one or more of the problems described above.

According to the present invention, in the brine of a water distillation unit or the like, at least one chemical control additive of the scale, with threshold effect, high temperature, and carbon dioxide, is dissolved, so that the concentration of free carbon dioxide (not combined) in the brine is kept at a very low level (for example, less than about 1 ppm), at the same time that, in the case of operation with seawater, the pH of the brine is maintained between 8.7 and 9.3 and the alkalinity P of the brine is maintained between approximately 2C and 90 ppm, expressed as CaCOg. Said J <

- β treatment does not reduce the total alkalinity of the brine, but effectively reduces the concentrations of hydroxyl ions and carbonate in the brine in relation to those existing when the chemical additive is used on its own and, therefore, substantially eliminates the formation of scale by significantly reducing the conversion of bicarbonate ion to carbonate ion and, finally, to hydroxyl ion.

The invention is especially effective in evaporator systems where the brine is boiled by contact with hot heat transfer surfaces, such as in steam seawater distillation systems. The dissolution of carbon dioxide in brine that has lost part of the carbon dioxide as a result of an evaporative process. reduces the effective loss of carbon dioxide, reducing this the upward displacement of the concentration of hydroxyl ions by inhibiting the decomposition of bicarbonate ion, to such an extent that the presence of the controlling chemical additive of the scale eliminates practically the complete Formation of calcium carbonate scale and magnesium hydroxide. In this way, the two main causes of alkaline scale formation are neutralized.

Therefore, the chemical control additives currently available in the shell become effective at the time of a practically complete formation of alkaline scale.

This synergistic result is obtained without a significant increase in the corrosion rate since the total alkalinity of the brine is not reduced and any reduction in the pH of the brine is not sufficient to significantly increase the corrosion rates. of building materials

- 7 I i

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I normally used in distillation facilities.

Therefore, it is possible to maintain the maximum production capacity during long operating periods and the substantial reduction in the frequency of interruptions. Distillate and brine are not significantly more corrosive than in the case of chemical threshold treatment only and are much less corrosive than those found with carbon dioxide treatment alone.

Other objects and advantages of the invention will be apparent from the following detailed description taken in conjunction with the drawings and appended claims. Figure 1 is a schematic diagram, partially in section, of a compression distillation apparatus of and incorporating the present invention.

Fig. 2 is a fragmentary view of a carbon dioxide injection manifold of the apparatus of Fig. 1, said view taken generally along line 2-2 of Fig. 1.

Figure 3 is a schematic diagram, partially in section, of a preferred embodiment of a distillate apparatus. steam compression fuse incorporating the present invention.

The present invention provides a method and apparatus for substantially reducing or eliminating the formation of alkaline scale in any of the various types of processing facilities where water containing dissolved salts of magnesium and / or calcium, which are precipitable as scale, is contacted. in the presence of bicarbonate decomposition products, with the surfaces of the processing facility, during an evaporative process. The invention is especially advantageous in seawater desalination plants, but may be useful in various other types of processing facilities, being applicable to the production of bottled distilled water or water for pharmaceutical use, or in the concentration of cooling tower purges by evaporation, for example.

The reference in this invention to sea water is to be understood as including brackish water. The term "brine" refers to water that contains dissolved magnesium and / or calcium salts and that has been subjected to evaporation. As used herein, the term evaporation represents a process by which a volume of water loses carbon dioxide simultaneously with a loss of pure water.

Calcium carbonate scale results from the thermal decomposition (at relatively low temperatures) of bicarbonates and reaction of the resulting carbonates with calcium present in seawater, as follows:

2HC0 ~ CO_twot + C0₃^two + H_two0 (i) +2 -2 ca + co₃ £ caco₃; (two)

The carbonate ions appear in the brine at pH values ​​of 8.3 or greater.

The heating of seawater at elevated temperatures causes the reaction of the carbonates with water to release more carbon dioxide and form hydroxyl ions, which precipitate as magnesium hydroxide, as follows:

C0 ~^two + H_two0 g C0_twot + 20H ~ (3)

Mg⁺² + 20H ~ £ Mg (OH)_twoJ, (4)

The boiling of seawater promotes the formation of scale by the reactions indicated above. The degree in the '

When the above reactions occur, the boiling temperature increases as the retention time increases and as the concentration factors increase. The carbon dioxide released from the brine by boiling is conventionally separated from the evaporator systems by venting to the atmosphere with other non-condensable gases, such as nitrogen and oxygen.

According to the invention, and contrary to the prior practice, it has been found that the formation of scale on thermal transfer surfaces and other components of processing facilities can be substantially eliminated or minimized by dissolving carbon dioxide in brine which is has undergone or is undergoing an evaporative process and, thus, contains a substantial amount of carbonate ions resulting from the decomposition of the bicarbonates present in the feed water, in case the carbon dioxide is added to the water. the brine simultaneously or after the evaporation of a portion thereof and in the case that the brine also contains a chemical additive^onewith a high-temperature, selected high-temperature, scale-cutter, under certain conditions later indicated.

The added carbon dioxide is preferably administered to the brine by recycling at least a portion of the carbon dioxide released from the brine by boiling.

The carbon dioxide is dissolved in the brine by the means detailed below, so that the concentration of free carbon dioxide (chemically uncombined) in the brine is kept below about 1 ppm, preferably below 0, 5 ppm approximately. This is essential since the presence of free carbon dioxide increases the tendency to separate the dissolved carbon dioxide from the brine during boiling. The separated carbon dioxide tends to move to the steam spaces and distillate collection spaces, thus increasing the degrees of corrosion.

The presence of carbon dioxide dissolved in boiling brine from the evaporator displaces the equilibrium of equations (i), (2) and (3) above to the left, thus inhibiting the decomposition of bicarbonate ions. Indeed, the carbonate ions act as a sponge "with respect to carbon dioxide, avoiding losses of it in free form. Therefore, the tendency of magnesium to precipitate as magnesium hydroxide is reduced or substantially eliminated. In addition, the presence of the chemical controlling additive of the scale interferes with the precipitation reaction (2) above, generally increasing the solubility of calcium carbonate during long periods of high temperatures, thus substantially avoiding or minimizing the formation of calcium carbonate. calcium carbonate.

There is a true synergism between the chemical additive and the dissolved carbon dioxide since, although the presence of the chemical compound only eliminates or reduces the formation of calcium carbonate, it inhibits the rate of magnesium hydroxide scale formation only to a degree limited, which may be insufficient in many plants. Treatment with only carbon dioxide to the extent necessary to control the formation of alkaline scale, as suggested in US Pat. No. 3,218,241 of Checkovich, drastically decreases the pH of the brine and increases the concentrations of carbon dioxide. free, which translates into intolerable corrosion voltages.

According to this invention, a high proportion (eg, more than about 70-75%) of the reduction of the rate of alkaline scale formation is achieved by the scale controlling chemical additive, but the addition of carbon dioxide in combination with The chemical additive, in order to establish alkalinities P and pH values ​​of the brine within specific ranges, performs a practically complete elimination of scale formation.

A feature of the system using the method of the present invention is that the total alkalinity of the brine, measured as the total concentration of hydroxyl ions, carbonate and bicarbonate, is decreased by adding carbon dioxide to the brine containing the chemical threshold additive. · '.

However, the concentrations of hydroxyl ions and carbonate are reduced by inhibiting the decomposition of the bicarbonate ion to carbonate ion by the addition of carbon dioxide. The elimination of the bicarbonate ion decomposition is not necessary. .

The total alkalinity, as previously defined, conventionally refers to the alkalinity M and is expressed as ppm CaCO 2 equivalents as determined by titration with a conventional mineral acid-using a methyl orange indicator at pH 4.5 and room temperature. (Although in certain conventional texts reference is made to an end point of pH 4.3, in the seawater distillation technique it is conventional to use a pH endpoint of 4.5). The alkalinity P of the brine is expressed as ppm equivalents of CaCOg as determined by titration with a conventional mineral acid · using a phenolphthalein indicator at pH 8.3 and room temperature.

The conventional procedures for determining the P and M alkalinities of the brines are described in Powell, Water Conditioning Por Industry, p. 473-475 (McGrav-Hill 1954), the details of which are incorporated herein for reference purposes only.

In the practice of this invention in relation to the distillation of seawater, the alkalinity P of the brine is maintained between about 20 and 90 ppm and preferably between about 30 and about 80 ppm and the pH of the brine is maintained between 8, 7 and 9.3 approximately, by addition of a high-temperature, threshold-effect chemical controlling additive in combination with carbon dioxide. The alkalinity M of the brine is not decreased.

When applying the invention to distilleries operating with tap water, the addition of caustic dioxide to brine treated with the chemical threshold additive may cause little or no reduction in alkalinity P, but a significant increase in the alkalinity M together with a small reduction of the pH (for example, 0.2 to 0.5 units approximately) in comparison with the brine treated with only the chemical additive of threshold effect.

The selection of alkalinity values ​​P and pp within the indicated ranges will depend on operational variables such as type of evaporator, construction materials of the evaporator, operating temperature, degree of dilution of the brine with feeding in multi-stage vaporizing units, etc. .

It has been found that the operation according to the invention is generally translated into a value (2P-M) which is maintained by i

below zero ί i

I

Although it is preferable to inject the gaseous carbon dioxide directly into the brine, other means can be used to provide carbon dioxide in the brine. For example, excellent results have been obtained by dissolving gaseous carbon dioxide in a stream of water, such as a minor portion of the feed seawater stream, and then directing the stream of treated water to the evaporator brine. Thus, even though a smaller amount of the carbon dioxide feed may be treated before evaporation, the brine as a whole is to be considered as being treated with carbon dioxide substantially simultaneously with evaporation or after evaporation .

Carbon dioxide can be obtained from any source, such as bottled carbon dioxide, flue gas from which contaminants have been separated, etc. No embp; go, tf⁴ It is preferable to recycle the carbon dioxide that is released from the brine during boiling or flash. However, only limited amounts of non-condensable gases separated from the system should be recycled since the use of large quantities of non-condensable gases other than carbon dioxide, such as oxygen and nitrogen, will interfere with the evaporator's thermal transfer to form a mantle. above the thermal transfer surfaces.

A sufficient amount of carbon dioxide must be introduced into the brine to interfere (and thereby substantially inhibit) with the decomposition of the bicarbonate ion, but not sufficient to translate it into the presence of substantial amounts of free carbon dioxide (chemically not combined). ). In practice, it has been found that a rate of introduction of carbon dioxide of approximately 0.199 kg / hr, based on a water feed rate of

10 sea of ​​approximately 2,615.6 liters / hr. for a distillate flow rate of approximately 1135 liters / hr. in a steam compression distillation unit (VC), with a chemical additive dosage rate equal to or lower than that recommended by the manufacturer, it will be satisfactory in reducing the P alkalinity to 86 ppm from a value of 127 ppm . The pH is reduced to 9.25 from 9.5, without recycling of ventilation gases. The free carbon dioxide in the brine should be less than about 1 ppm and preferably less than 0.5 ppm. Although the introduction of carbon dioxide into the brine may tend to decrease the pH of the brine, it has been found that pH reductions of only 0 are obtained, 2 to 1 unit approximately by the addition of the carbon dioxide necessary to bring the alkalinity P of the brine to a value within the desired amount of 20-90 ppm. The pH of the brine treated with the additive, chemical and carbon dioxide should be about 8.7 to 9.3, measured at room temperature.

The efficiency of the carbon dioxide injection system is of course a factor in determining the efficiency at which carbon dioxide is absorbed by the brine.

The method of the invention is particularly applicable to seawater distillation processes which operate at brine temperatures of 882c or more, as is normally found in instant multi-stage (MSF) distillation distillation apparatuses. However, the method is applied with excellent results to the types of steam compression distillation (VC) facilities that, although they can operate at significantly lower brine temperatures compared to the MSP units, can experience environments

I relatively severe with respect to the formation of scale.

As already indicated, the invention resides in the dissolution, in the brine, of carbon dioxide in combination with one or more high temperature chemical controllers of the scale, which provide a threshold effect of inhibition of the scale. , maintaining a high concentration of carbonate ions in solution in the presence of calcium and magnesium ions. Chemical additives that only have wetting and sequestering properties are not suitable.

It is believed that the inhibition of scale formation according to the invention is a result, at least in part, of the modification of the surface of the scale crystals by the chemical additive as the scale is in the forming process, which reduces the rate of crystallization. The microscopic studies carried out have revealed a substantial crystalline distortion when chemical controlling compounds of threshold effect are present.

Various suitable additives can be found in commerce, including polymers and copolymers of maleic acid, polyphosphonates, phosphonic acid derivatives, aminophosphonic acid derivatives, polyacrylic and polymethacrylic acid derivatives and polyester polyols.

However, polyphosphates are not suitable for use in connection with this invention since, at temperatures above about 882c, they are inverted to the orthophosphate and can precipitate with the calcium or iron present in the brine. They also fail to control the formation of magnesium hydroxide scale.

I

The mineral acids are not suitable for use in this invention.

A preferred chemical additive is a hydrolyzed polymaleic anhydride sold by Ciba-Geigy Corporation '(Aresley, New York) under the trademark BELGARD EV or EVN. This material is described in US Pat. No. 3,810,834, the description of which is incorporated herein for reference purposes only.

Another very effective chemical treatment agent is sold by American Cyanamid Company under the registered designation P8o, and appears to be composed of a copolymer of maleic acid and sodium allylsulfonate. Other suitable chemical treatment agents include the compounds sold by Monsanto Company under the trademark DEQUEST. The commercial product DEQUEST 2010 (comprising i-hydroxy-ethylidene-1-diphosphonic acid) is an example. Also, the product of Pfizer, Inc. designated FLOCON (eg FLOCON 247) and those of Mechanical Equipment Cp. (New Orleans, Louisiana) designated M209 and M235 are also suitable.

The products BELGARD, Mechanical Equipment Co. M209, and DEQUEST previously identified are preferred. Those skilled in the art will recognize that the behavior of the various suitable chemical additives will vary; however, the method of the present invention will improve the performance or performance of all polymers of the type described above.

The dosing rates of the previously identified chemical controllers of the husk can j

I can easily be determined empirically and it has been established that dosage rates lower than the speeds recommended by the respective additive manufacturers will provide acceptable results when used in combination, with added carbon dioxide according to the method of this invention.

The following detailed examples are intended to illustrate the practice of the present invention, but the scope of the same, should not be considered as limited by them.

Example 1 - Steam compression distillation

Figure 1 illustrates a type of vaporized, seawater evaporator that has been modified to employ the method of this invention. Although the apparatus of Figure 1 will be described in detail, it should be understood that the method of the invention is not limited to the use of the specific apparatus of Figure i, but can be incorporated into a wide variety of distillation equipment or other evaporation equipment. , as detailed below. · - '

The steam compression distillation unit of Figure 1 is a modified version of the units described in the US Patents of Pottharst, Jr. Nos. 3,748,234, 4,002,538 and 4,260,401, the descriptions of which are incorporated herein by reference only. reference purposes. The distillation unit includes a shell and tube evaporator, generally represented by 10, and an associated vapor compressor 12. In said system, the water existing on the side of the tubes of a calender or tube bundle 14, which comprises a plurality of tubes 15, is vaporized by thermal exchange with condensing water vapor from the side of the evaporator housing 10.

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I next to the casing of the evaporator 10, where most of the steam is condensed by contact with the tubes 15. The distillate (condensate) is separated through a line 24 by a distillation pump 25. The distillate flows to through a meter 26 and a heat exchanger, generally represented by 32, to the storage point.

The filtered feed area is pumped by a feed pump 34 through a line 35 having an integral mixing device 36 to the heat exchanger 32. The feed water is heated by heat-exchange with hot distillate and purge and fed to a deaerator 42.

At some point prior to injection into the mixing device 36, the feed water is treated by the addition of a scale controlling chemical additive, described above, at 44, by means of a pump and dosing tank (shown schematically in FIG. ) as is well known in the art.

The feed water is preferably heated to a few degrees below its boiling point in the heat exchanger 32.

In the deaerator 42, the non-condensable gases, including nitrogen, oxygen and a small amount of carbon dioxide, are separated from the feed water by washing, preferably against water vapor. The deaerator 42 is preferably a filled tower and the feed water is injected into the tower 42 by a spray head 46 or other water distributing device.

The vented water vapor of the calender 14 and containing carbon dioxide released in the decomposition of the bicarbonate ions present in the feed water is injected to the bottom of the deaeration tower at 47 and is provided

- 19 preferably to the deaerator through a line 48 leading from a vent valve 49 on the side of the carcass of the evaporator 10. The injection of water vapor to the bottom of the deaerator tower 42 facilitates the release of non-exhaust gases. of the feedwater descending and preheating the feed water to a temperature substantially equal to or preferably higher than the temperature of the water circulating in the evaporator 10. The contact of the ventilation current in line 48 with seawater supply relatively cold concentrates carbon dioxide present in line 48 of the gas stream of a line 50 leading to deaerator 42.

The non-condensable gases and a minor amount of water vapor flow from the deaerator 42 through line 50 to the vent valve 51. A smaller portion of the non-condensable gases is vented from line 50 to through the vent valve 51 and the larger portion (for example ','

80% approximately) of the non-condensable gases, flows through a valve 52 and line 60 to a carbon dioxide injection means, represented generally by 61, as can best be seen in Figure 2. The carbon dioxide injector 61 comprises a perforated tube 62 attached to one end of line 60 and located below the level of the brine inside the evaporator 10.

In another embodiment, a minor portion of the seawater feed stream is diverted from line 35 by line 63 (shown in dashed line) to a small tower 64 in which the feedwater absorbs carbon dioxide. - if

not supplied from the aerator 42 by line 65. The gases i

Non-condensable gases are vented from tower 64 via line 66 and water charged with carbon dioxide is supplied to the carbon dioxide injector by a pump 67 and line 68.

In any of the embodiments, the deaerated feed water flows through a line 70 from the bottom of the deaerator 42 to the bottom portion of the evaporator 10.

A slight pressure drop occurs from the deaerator 42 to the evaporator 10 to allow fluid flow. The liquid level control 72 of the deaerator controls a valve 74 of line 70 so that the liquid level of deaerator 42 remains essentially constant (alternatively, a liquid seal circuit (not shown) may be employed which may include a orifice renstrictor of the flow, between the deaerator and the evaporator to limit the steam flow from the deaerator to the evaporator).

By controlling the valve 52 of the gas line 60 in combination with the ventilation valve 51, the speed of injection of carbon dioxide and small amounts of other non-condensable gases into the brine of the evaporator 10 is controlled.

The pressure inside the evaporator 10 is maintained at a slightly higher than atmospheric pressure (e.g., about 1.1 kg / cm) so that the brine boils at about 102,620 (an elevation of the boiling point of about 0.94).^twoC) condensing the steam in the tubes 14 to 2062c approximately, for a net T of approximately

3.5^SC.

The parts of the evaporator 10 in contact with the brine are generally constructed of. aluminum or brass! an alloy of copper-nickel (90 / l0). !

A series of distillation tests of ι are carried out

seawater using a steam compression distillation apparatus such as that shown in figures '1 and 2, Mechanical Equipment Co.' (MECO) Model M3B, with a flow rate of 1,135.5 liters / hour (nominal distilled flow rate), with chemical additive controlling the scale, with and without carbon dioxide injection. (The source of carbon dioxide is the ventilated gas from the deaerator 42);

The alkalinity P of the cold feed water is practically 0 and the alkalinity M of the feed remains practically unchanged during the test at approximately 110-120 ppm. The pH of the feed varies between approximately 7.8 and 8.2. Table I gives the results of the tests in terms of operating time, factors of average brine concentration, fouling factors (R ^), pH of the brine and alkalinity "P of the brine, and copper content in li purge and in the distillate.

Table I indicates that experiments 5 and 6, which use carbon dioxide injection and a chemical control agent for the scale, so that the alkalinities P of the purges are relatively low, exhibit fouling factors. hours of operation, of the orifice of the mitaa of those exhibited by those experiments without carbon dioxide injection, without a substantial increase in the copper content in the purge and in the distillate. (These contents in copper are indicative of the corrosion rate).

TABLE I

<td>fxperimen</td><td>Time of</td><td>Factor of</td><td>Average pH</td><td>Alcalini</td><td>Factor of</td><td>Cu in</td><td>Cu in</td><td>Treatment</td>

<td>Tone.</td><td>operation (hours)</td><td>average concentration</td><td>purge</td><td rowspan="2">d ad P (ppm CaCO ^) _ 131</td><td>dirty. R / 100 hrs ^ K_d(to) . V ·</td><td>purge - ppm</td><td>Distillate - ppm</td><td>in evaporated /^b&gt;</td>

<td>one</td><td>190.5</td><td>1,8</td><td>9.85</td><td>2,83X1O ~⁴ 0, .201xl0^-4</td><td>0.1-0.2</td><td>(d)</td><td>12ppm MECO M-209 Without injection ^Ctwo</td>

<td>two</td><td>144 </td><td>1,8</td><td>9.90</td><td>120</td><td>2.25X1O^-4 0.211X10^-4</td><td>0.1-0.2</td><td>(d)</td><td>12ppm MECO M-209 Without co-injection_two .</td>

<td>3</td><td>192</td><td>1.9</td><td>9.60</td><td>133</td><td>1,93X1O^-4 O, 136xlO ~⁴</td><td>(d)</td><td>(d)</td><td>Belgard EVN 12ppm Without injection ^C0two</td>

<td>4</td><td>100</td><td>1.9</td><td>9.55</td><td>134</td><td>-4 -4 1,20x10 0,162X10 -4 -4</td><td>0.2</td><td>(d)</td><td>6ppm MECO M-209 + 3ppm MECO H-235 Without injection. C02</td>

<td>5</td><td>164</td><td>1,8</td><td>9.50</td><td>95</td><td>1.15x10 0.095x10 -4</td><td>0.2</td><td>0.1</td><td>6ppm MECO H-209 + 3¿. ppir, HECO H-235 injection CO ^</td>

<td>6</td><td>163</td><td>1.9</td><td>9.20</td><td>76</td><td>0.66x10 0.054</td><td>0.2</td><td>0.2</td><td>6ppm MECO M-209 + 3 i ppm</td>

f MECO H-235 greater injection • Co_two(c) (a) (milligrams / sec.cm .se.) (b) All treatment concentrations are based on feedwater flow rates.

(c) The injection speed of C0_two in experiment No. 6 (compared to No. 5) was achieved by adjusting valves 52 and 51, figure 1.

(d) No tests were performed for Cu.

Example 2 - Steam compression distillation with addition of carbon dioxide via the feed side stream.

Figure 3 is a schematic representation of another embodiment of the steam compression distillation system of Figure 1, illustrating preferred means for dissolving carbon dioxide in a feedwater side stream for final brine delivery. The elements common to both figures 1 and 3 are designated by the same reference numerals. From figure 3, some elements of figure 1 have been omitted, for reasons of clarity, such as the compressor 12, several valves and the distillation and purge lines that go through the heat exchanger 32).

In the system of Figure 3, seawater gives a cold feed (for example, 27).^twoC), after the addition of chemical additive at point 44, it is divided into a side stream (line 80) and a main power stream (line 35). The pH of the feed is about 8.3. The respective flow rates in lines 80 and 35 can be, for example, approximately 3.78 liters per minute (gpm) and approx. 45.36 liters / minute. The feed water-line 35 is pumped by the pump 34 through the heat exchanger 32 where it is heated by heat exchange with the distillation and purge lines (not shown) and directed to the spray head 46 of the deaerator 42

Sidestream 80 is divided into point 82 into two streams 84 and 86. Stream 84, at a rate of about 3.02 liters / minute, is directed to an eductor 90.

Line 86, at a speed of approximately 0.75 liters / min.

., i is directed through a valve 92 to the spray head

94 of a contact column 96 of carbon dioxide / water of | sea. (Alternatively, the spray head 94 can be replaced by a water inlet (not shown)).

A suitable eductor is the 12.7mm Penberthy Model LM eductor.

The eductor 90 also communicates with a vent line 50 from the deaerator 42 carrying the non-condensable gases (oxygen and nitrogen), carbon dioxide and a smaller amount of water vapor.

In the eductor 90, the feedwater of stream 84 is pressurized and mixed with the vent gases of line 50 and injected into a submerged line 100 terminating within the column 96 below the surface of an amount of sea water 102 present there.

After injection into seawater 102, the water vapor and most of the carbon dioxide from line 100 dissolve in sea water 102. The non-condensable gases and a smaller amount of carbon dioxide are not dissolve and collect in a vapor space 103 of column 96 above the surface of water 102. ¡¡

The cold sea water introduced into the column by the spray head 94 absorbs most of the carbon dioxide present in the vapor space 103. The carbon dioxide and undissolved non-condensable are vented from the vapor space 103 by a valve of 104 ventilation and are discharged to

the atmosphere through the line 106. (The valve 104 is illus- ί

optionally a float-type valve but may comprise) a hand-held needle-type valve or other suitable valve type). i

The feed water enriched in carbon dioxide is pumped (the pump is not shown) from the column 96 through line 110 to a sprayer 112 disposed below the level of brine in the evaporator 10. (The evaporator 10 it is fed with deaerated seawater through line 70 as described in connection with the system of figure 1 above).

Example 3 - Multi-stage and multi-effect evaporators

As indicated above, the method of the invention is applicable to forms of seawater distillation apparatus other than the steam compression apparatus of Figure 1. Examples of distillation units to which the method can be applied include instantaneous multistage vaporization units and multi-effect evaporators of vertical or horizontal tubes.

Those skilled in the art are familiar with various hypos of multi-stage and multi-effect distillation systems. US Pat. No. 3,218,241 to Checkovich describes, for example, two modalities of instantaneous evaporation units of multiple stages, both one-step (without recycling) and- with recycle, used for the desalination of seawater-. ,. '

In general, multi-stage and multi-effect evaporators comprise a series of prior and subsequent stages, each stage comprising a brine retention space and a vapor condensing space. The brine retention space is defined as the portion of each stage in which brine is boiled (or vaporized instantaneously) or said brine is approximately boiled (or vaporized instantaneously). The respective subsequent stages operate at lower pressures than the previous stages. I

When applying the method of the invention to instantaneous multi-stage evaporators, it is convenient to introduce recycled carbon dioxide in the brine of the final stage instead of in previous stages so that once introduced, the carbon dioxide absorbed It will not be boiled until after it has been pumped through the tubes of the heat exchanger. In this way, carbon dioxide is preserved for its main task of reducing the formation of scale in the condensers of the stages (spaces of condensation of steam). But nevertheless,

Since each subsequent stage of an instantaneous multi-stage evaporator operates at a low pressure level compared to its previous stage, each stage condenser can be vented in the brine section of its immediately subsequent stage and thereby replace most of the of the carbon dioxide released by the brine in the previous step. In this way, it is possible to avoid extreme chemical conditions in the brine that tend to promote the formation of magnesium hydroxide scale throughout the apparatus.

of distillation. I

However, in some cases, it may be necessary to vent the carbon dioxide from a one-stage condenser to the brine from the second or third stage later with the! In order to provide a sufficient pressure drop for the injection of carbon dioxide to be practical since, in some units - of instantaneous multi-stage vaporization, the pressure drop between the immediately following stages is relatively small. This problem is aggravated since the carbon dioxide is preferably injected at a substantial brine depth.

Another technique in instantaneous multi-stage evaporators is to collect carbon dioxide-rich streams from the stage banks and inject the carbon dioxide collected in the brine at a certain point in the unit of lower pressure.

In the case of evaporators of multiple effects, of vertical or horizontal tubes, the currents rich in.

c ς * carbon dioxide should be vented from the vapor condensation sections of the respective effects and mixed in the brine for subsequent effects.

Experts in this field will appreciate that the method and apparatus described above allow the realization of the desalination of seawater or other evaporative operations with brackish water, with elimination or substantial reduction of the formation of alkaline scale without substantial increase in the characteristics of distillation corrosion and brine. In this way, the production capacity can be maintained during substantial operative periods without a decrease in production due to the formation of scale on surfaces of tubes or other surfaces.

From the above detailed description, it will be apparent to those skilled in the art that the present invention resides in the limited reduction of the alkalinity P and pH of the brine treated with chemical additive by the addition of dioxide of |

- 28 10 carbon to it, thereby greatly enhancing the scale inhibitor behavior of the chemical additive. This effect is achieved without the undesirable side effect of a significant increase in the corrosive tendencies of the brine, since the concentration of free carbon dioxide in the brine is limited.

The desired ranges of pH and alkalinity P herein are convenient with the chemical additives currently available in the trade and identified herein. In the event that in the future chemical additives capable of inhibiting the formation of alkaline scale for higher alkalinity P values ​​and higher pH levels are deoiled, it is contemplated that the objects of the invention may be obtained with such chemical additives by means here described, that is, the alkalinity P and pH of the brine treated by such chemical compounds by the addition of carbon dioxide.

With this, the alkaline scale inhibiting characteristics of such chemical additives will be enhanced.

Having sufficiently described the nature of the invention, as well as the manner in which it is carried out in practice, it should be noted that the aforementioned provisions are subject to modifications of detail insofar as they do not alter its fundamental principle.