CHAPTER 18EVAPORATION AND FREEZING

Evaporation and freezing can be used to convert water (1) to a pure vapor thatcan be condensed or (2) to a pure solid that can be separated from a saline motherliquor and melted. Both processes leave saline residues containing essentially allthe solute originally in the feed water.

EVAPORATORS

Evaporators are widely used in many water treatment operations, such as prepa-ration of boiler feed water, concentration of diluted liquor, evaporation of sea-water to produce fresh water, and concentration of waste liquors to reduce vol-ume for further processing or disposal.

The typical evaporator is like a fire-tube boiler, with the flame replaced bysteam or process vapor as the heat source; but, there are significant differences:

1. The evaporator has a much smaller temperature gradient across the heat trans-fer surfaces.

2. It usually holds less liquid.3. Evaporator tubes are usually made of special metals (titanium) or alloys (stain-

less steel), whereas boiler tubes are made of steel.

To transfer heat efficiently at low temperature gradients, the evaporator sur-faces must be kept free of deposits, which have an insulating effect. Correct chem-ical treatment and scheduled cleaning are important.

Every liquid exerts a vapor pressure, the magnitude of which is a measure ofits volatility. High vapor pressure liquids evaporate readily, while those with lowvapor pressures evaporate more slowly, requiring an increase in temperature tospeed the rate. The kinetic energy of all molecules increases with increasing tem-perature. The rate of evaporation depends on the nature of the substance, theamount of heat energy applied to the liquid, and surface effects.

When a liquid reaches the temperature at which its vapor pressure equalsatmospheric pressure, boiling occurs. This is the rapid evaporation from all partsof the liquid mass, with bubbles of vapor forming in the interior and rising to thesurface. The pressure within these bubbles equals the vapor pressure of the liquidat that temperature, so the boiling point depends on the external pressure. Forexample, at sea level, pure water boils at 2120F (10O0C) where its vapor pressure

NaCI solution strength, %

FIG. 18.1 Effect of solution concentration on atmos-pheric boiling point using NaCl as an example. Forany specific concentration, the solution boiling pointsat several pressures plotted against the boiling pointsof water at these same pressures produce a straightline.

exceeded. This results in precipitation, usually as scale on the heat transfer sur-faces. Where water is being evaporated, the scale may consist of salts of calcium,magnesium, and silica. This scale severely reduces the heat transfer rate, slowingevaporation and reducing thermal efficiency.

It takes a lot of heat to evaporate water. Raising the temperature of 1 Ib ofwater I0F requires 1 Btu; to change that 1 Ib into vapor at atmospheric pressurerequires 970 Btu. (It takes 1 cal to raise 1 g of water I0C, 539 cal to vaporize 1 gat atmospheric pressure.) The high energy requirement for evaporation makes itimportant that the heat balance of a plant be controlled for maximum use ofenergy. A typical evaporator usually receives heat from live steam or from steambled from a turbine.

is 1 atm (1 bar), or 14.7 lb/in2 (1.0 kg/cm2). If the external pressure is reduced, asoccurs at elevations above sea level or under vacuum, water boils at a lowertemperature.

EFFECT OF SAL T CONCENTRA TION

Since evaporators may process liquids other than pure water, factors other thanatmospheric pressure must also be considered. Soluble salts in the solutiondecrease the vapor pressure, elevating the boiling point. Therefore, as diluteliquor evaporates and becomes more concentrated, its boiling point rises. Figure18.1 shows the boiling point elevation as the concentration of a salt increases inaqueous solution.

As water is evaporated from a solution and the liquid becomes more concen-trated, it is possible to concentrate to the point where the solubility of the salts is

Boi

ling

poin

t, 0F

EVAPORATOR DESIGN

There is a large variety of designs of evaporators, although the majority work onthe principle of steam passing on the outside of a series of tubes with water orwater solution, either confined or recirculated, flowing as a thin film over theinside of the tubular heating surface. The various types of evaporators are clas-sified according to the way the water is vaporized:

1. Boiling type: Evaporators which heat water to the boiling point and evapo-rate it by applying an external heat source.

2. Flash type: Evaporators which superheat water by an external heat sourceand flash it into vapor.

3. Compression type: Evaporators which add energy to water vapor by compres-sion and return this to the evaporator body as the heat source for boiling.

In the submerged tube boiling-type evaporators (Figure 18.2), steam enters atubular element, boils water, and discharges water vapor from the evaporatorshell. The heating elements are usually bundles of tubes of various configurations.

Vapor to condenser. This can be a separatesur face condenser to produce disti l late or itcan be built into the heat cyc le on a stageheater operat ing at a lower temperaturethan the steam supply.

Feedwater inSteam in

Drains(Condensate) Blowoff to waste

FIG. 18.2 Simple submerged-tube evaporator.

These may be completely submerged in the water, partially submerged, orarranged so that only a film of water flows across the surfaces. In each design, thespace for vapor generation must be ample to avoid steam blanketing and to pre-vent fouling with baked on sludge.

As in a boiler, bleed-off regulates the solids concentration of the boiling liquid.Vapor-purifying devices trap entrained water droplets. This is particularly impor-tant where the aim of evaporation, as in most water treatment systems, is to pro-duce high-quality distillate. The vapor purifiers are comparable to those in boil-ers. In addition to the conventional designs, bubble cap purifiers are sometimesused. These return part of the distillate to continually wash fresh vapor.

A boiling-type evaporator with proper disengaging area should produce dis-tillate with less than 1 mg/L total dissolved solids. The quality is affected bythe dissolved solids content of boiling water which may be entrained inthe vapor discharge. Lower solids levels are attainable with more sophisticatedvapor purifiers and conservatively designed evaporator elements. Vapor qual-ity is affected by the CO2 liberated from the bicarbonate alkalinity, just as in boileroperation.

CONDENSATION

Purified vapor leaving the evaporator is condensed in several ways:

1. In older utility systems where evaporators were used to provide high-qualitymakeup, the vapor was discharged through the deaerating heater and con-densed by the boiler feedwater (Figure 18.3). (Modern utilities use deminer-alizers instead of evaporators to process makeup.)

FIG. 18.3 Typical utility-type evaporator operation.

2. Vapor may be condensed by a surface condenser if the purified liquid phase isto be kept separated for some reason. Each pound of vapor becomes 1 Ib ofdistillate in the condenser shell.

3. The vapor may be fed to the tube element of the second evaporator body, andthe vapor from this second unit fed to a third, producing a multiple-effectevaporator (Figure 18.4). In this type of multiple-effect evaporation, vapor

FIG. 18.4 Multiple-effect evaporator with condenser.

from the last unit is liquefied in a condenser. Each pound of fresh steam fedto the first stage theoretically produces 1 Ib of condensate from each stage. Inpractice, however, a triple-effect evaporator produces about 3.1 Ib of total con-densate per pound of steam instead of 4.0, the total condensate including thatproduced by the fresh steam applied to the first effect.

Turbine condensateat 1850F (850C)

(1510C)

Deaeratingheater

Condensate at 3040F

Boiler feedwaterat 2440F

(1180C)Treatedmakeup

Trap

Makeupvapor

Steam from turbineat 75 psia

Steam from turbine

at 28 psia

(20 kg/cm2)

Blowoff

0.8* Vapor 0.7 * Vapor0.6* Vapor

Feed

1 ̂ Condensate

Blowoff

1* Steam

2.1* Distillate

3.1* Total condensate

FIG. 18.5 Eight-effect kraft pulping liquor evaporator. This flow sheet applies to the installation shown inFig. 18.6. (Courtesy of HPD, Inc., Naperville, III.)

No. 1 EFFECT No. 2 EFFECT No. 3 EFFECT NO.4 EFFECT No. 5 EFFECT No. 6 EFFECT No. 7 EFFECT No. 8 EFFECT

CONDENSER

WEAK LIQUOR FEED

PULP MILL

HDTVELL

No. 3 FEEDFLASH TANK

No. 2 FEEDFLASH TANK

No.1 FEEDFLASH TANK

FRDM SQAPSKIMMERTD SQAPSKIMMER

TO BOILER

PRODUCT

STEAM

Multiple-effect evaporators are used principally for chemical process opera-tions. Typical of such evaporators are the units found in the pulp industry for theconcentration of sulfate black liquor (Figure 18.5).

The primary aim of evaporation in the power plant is to produce boilermakeup of high quality. The chemical treatment program must be designed toproduce high-purity vapor while helping to maintain clean heat transfer surfaces.As in boiler water conditioning, the goals of chemical treatment are control ofcarryover, prevention of deposits, and elimination of corrosion. The makeup tomost utility evaporators is pretreated to remove hardness, reduce alkalinity, andeliminate dissolved oxygen. The chemical treatment applied to the evaporatorshould be controlled to maintain the limits shown in Table 18.1.

Where there is no pretreatment of the evaporator makeup, then the chemicalprogram should be the same as used for internal treatment of a low-pressureboiler, as shown in Table 18.2. Where the feed to the evaporator is brackish orseawater, it is often difficult to maintain a scale-free system even with a goodinternal treatment program. In such cases, the operation is programmed so thatscale is allowed to build up on the heating tubes for a planned period, then thetemperature of the system is suddenly dropped, creating a thermal shock thatcracks the scale from the tube surface for removal from the bottom of the evap-

Reading

TDS, mg/LSiO2, mg/LHydroxide, mg/L

CaCO3Dispersant and

antifoamPO4, mg/LPHSO3, mg/L

Range Where and how maintained

1 500 max In evaporator, by blowdown100 max In evaporator, by blowdown150-250 In evaporator, by blowdown or by

chemical feedTrace In evaporator, by selecting proper

treatment combinationO

8.2-8.6 In vapor, amine treatmentIf the evaporator is used intermittently, a residual SO3 of

30-60 mg/L should be maintained to protect theevaporator shell from corrosion

TABLE 18.1 Typical Evaporator Controls Following Pretreatment

Reading

TDS, mg/LSiO2, mg/LHydroxide, mg/L

CaCO3Dispersant and antifoamPO4, mg/LSO3PH

Range

2500 max100 max150-200

Trace30-6030-608.2-8.6

Where and how maintained

In evaporator, by blowdownIn evaporator, by blowdownIn evaporator, by blowdown or by NaOH feed

In evaporator, by chemical feedIn evaporator, by phosphate-organic treatmentIn evaporator, by chemical feedIn vapor, by amine treatment

Note: Where there is no pretreatment of evaporator makeup, chemicals should be fed for internaltreatment just as for a low-pressure boiler, as shown by Table 18.2.

TABLE 18.2 Evaporator Controls—Internal Treatment Only

orator. Other evaporators are manufactured with bowed tubes which flex withtemperature change, also resulting in scale-cracking and shedding.

MULTIPLE-EFFECT UNITS

Although multiple-effect evaporators are usually used for process operations, theyhave a definite tie-in to the utility system. In the pulp industry, five-, six-, or evenseven-effect evaporators are used to concentrate water from the pulp washers forrecovery of cooking chemicals. The black liquor may be concentrated to approx-imately 65% total solids, of which about half are organic materials. In this con-dition, the black liquor can be fired to a black liquor recovery furnace; the organicmaterial supports its own combustion and smelts the cooking liquor salts to arecoverable form. Figure 18.6 shows a typical large installation of such a black

FIG. 18.6 Black liquor evaporator in a kraft pulp mill. Some of the condensatemay be recovered for reuse, but much of it is contaminated with sulfur com-pounds. (Courtesy of HPD Corporation.)

liquor evaporator. Fresh steam fed to the first effect produces condensate that canbe reused as boiler feed water. However, the condensate produced in subsequentstages is too contaminated by volatiles for such use, but may be used for brownstock washing, stock dilution, or other purposes. Some of this condensate is sofoul that it must be stripped before it can be put into the sewer for treatment inthe waste treatment plant. The vapors stripped from the foul condensate may besent to the lime kiln where the organics responsible for the foul odors are burned.

The bauxite industry also uses multiple-effect evaporators for concentration ofsodium aluminate liquors, producing more condensate than required for boilermakeup. Again the condensate is usually too contaminated to be directly usableas boiler feed water.

A final example is the beet sugar industry, where syrups are concentrated byevaporation, again producing an excess of condensate over boiler makeuprequirements. These condensates are frequently contaminated with sugar, whichis very detrimental to boiler operation, and ammonia, which is corrosive to sys-tems that contain auxiliary equipment fabricated of copper alloys.

The advantage of multiple-effect in increasing yield per unit of energy can alsobe built into two quite different designs of evaporators which bear little resem-blance to the standard multiple-effect evaporator. The first of these is the vaporcompression still, which was developed initially for seawater evaporation aboardship (Figure 18.7); the second is the multistage flash evaporator, which has

FIG. 18.7 Schematic of vapor compression still.

become popular for the production of potable water from brackish or seawater(Figure 18.8). Both of these designs work on low temperature differentials andmust be kept free of deposits to maintain efficient heat transfer. Flash evaporatorshaving a capacity of 7.5 mgd (20 m3/min) have been installed for municipal water

FIG. 18.8 Schematic of multistage flash evaporator.

supply in the Middle East, where energy costs are favorable for such an installa-tion (Figure 18.9). Where seawater is being used as the feed, chemical treatmentfor prevention of calcium carbonate, calcium sulfate, and magnesium hydroxidescales is required. The treatment includes (a) reduction of alkalinity to minimizesupersaturation of calcium and magnesium compounds, (b) application of scale-control agents, such as acrylates, polyphosphates, or combinations of these, (c)oxygen scavengers or other types of corrosion inhibitors, and (d) antifoams toprotect the quality of the distillate. With this treatment it is possible to concen-

Flosh chambers

Preheat exchangers-

Steam inat 60 psi

Slowdown to waste

Feed in

Recycle

Bleedoff

Product

Slowdown to waste

Feed in

Recovery heatexchanger

Vapor

Motor-drivencompressor

Product out

FIG. 18.9 Three flash evaporator modules, each with a capacity of 2.5 mgd (6.7 m3/min), pro-ducing potable water for Al Khobar, Saudi Arabia. (Courtesy of Aqua-Chem Inc.)

Vertical tube forced circulation(single pass)

FIG. 18.10 A type of evaporator design used forconcentration of radioactive wastes from nuclearfuel reprocessing. (Courtesy of Unitech Division,Ecodyne Corporation.)

Circulation pump

Weak feed in

Distillatereflux in

Vapor out

Steam in

Heatingelement

Condensateout

Concentratewaste discharge

Vapor body

Liquid level

FIG. 18.11 Compact evaporator designed for concentration andrecovery of plating solutions from rinsewater. (Courtesy of IndustrialFilter & Pump Manufacturing Company.)

trate seawater about 1.6 times. Large installations may find pretreatment of thesea water of value. Such pretreatment has been ruled out by the high cost of chem-icals and the extra equipment, but the increasing cost of fuel may offset this in thefuture. Special alloys must be used throughout to counteract the corrosive effectsof the concentrated seawater.

FIG. 18.12 Compression still designed to operate as a crystallizer as well as an evaporator intreating cooling tower blowdown and other wastes in a zero-discharge utility station. (Courtesyof Resources Conservation Company.)

Mixed wastes,(blowdown, etc.) Acid

feed

Feed Feedtank

Feedpump

Condensateto polishingdemineralizer forboiler makeup

Heatexchanger

Condensatepump

Condensatetank

Recirculationpump

Concentrate,to pond

Steamcompressor

Deaerator

Falling filmevaporatorVent

* Not determined

Evaporators are finding application in the concentration of wastes to minimizevolume, simplify ultimate destruction, or both. Examples include the treatmentof radwaste from nuclear power plant operations (Figure 18.10) and concentra-tion of plating wastes (Figure 18.11).

In special cases where the EPA permit requires "zero discharge," a modifieddesign of compression still is being used to concentrate combined wastes, such ascooling tower blowdown and flue gas scrubber effluent, to yield a saturated solu-tion containing a crystal phase, so that the salinity can be removed as solids (Fig-ure 18.12). Table 18.3 shows the concentration achieved at one utility stationusing this scheme. Treatment of the liquor to prevent scaling is essential.

FREEZING

As water begins to freeze in a container, the dendrites of ice that first form on theheat-extraction surface consist of fairly pure H2O. The remaining water has con-centrated the solute originally present in the makeup water. Before the advent ofthe home refrigerator, commercial ice plants manufactured ice in cans, and it was

TABLE 18.3 Chemical Characteristics of "Zero-Discharge'* Evaporator System

Description

pH, initialafter acid

ConductivityTDS, mg/LTS, mg/LSS, mg/L

Feed: tower andscrubber

8.66.26400400084004400

Liquor: evaporatorconcentrate

6.6-6.8

ND*257,500324,90067,400

Product:distillate

6.8-7.4

10-1577

Nil

FIG. 18.13 Schematic of mine water desalination by freezing. This system is tied into the mine'sair-conditioning load, improving the economics of the process. (Courtesy of CBI Industries, Inc.)

V-4Feedprecooler

EMFreezer

.NH3out

Feedstream

V-3

Feed mixingtank

P-12 P^2

V-6Receiver PJ

Reject Reject

P-7

F-4F-3 Surgetank

V-10

Rejectsolids andconcentrate

Rejectsolids andconcentrate(blowdown)

Glycohcooling

F-1,F-2,F-3, F-4Precipitateremovalfilters

Productwater

V-9Gravitywashcolumn

E-5Melter

Warmwater

common practice to suck out the "core" of unfrozen water, containing most ofthe original dissolved solids, when this residue had concentrated about 10-fold.This improved the quality and strength of the finished cake of ice. If the core werenot removed, the final solidification of the cake would include salt crystals mixedwith ice crystals.

Various schemes have been proposed for producing pure H2O as a solid, freeof solute originally present in the feed water. These ideas are being seriously pur-sued because they hold some promise of energy savings over distillation pro-cesses; the evaporation of 1 Ib of water requires about 1000 Btu (539 cal/g), com-pared with only 144 Btu (80 cal/g) to freeze it. Figure 18.13 is a schematic diagramof a pilot plant being used to desalinate mine drainage in South Africa by freezing.The feed system contains 9500 mg/L TDS, and product water contains 500 mg/L, with a water recovery rate of 90%. Product quality can be improved at theexpense of a lower production rate.

Although freezing as an economical process of water desalination may be farfrom commercial use, its use as an energy storage scheme is being practiced to alimited extent. Water is frozen during nighttime off-peak electrical periods, whenthe cost of electricity is reduced; during the day, the ice is melted by cooling airto supplement the mechanical air-conditioning load in large buildings, hospitals,or other such facilities.

Finally, the potential use of natural ice as a source of potable water and as ameans of cooling in arid regions has been seriously studied and proposed.Whether this is ever put into use will depend on future costs of energy.

Table of ContentsPart II. Unit Operations of Water Treatment8. Coagulation and Flocculation9. Solids/Liquids Separation10. Precipitation11. Emulsion Breaking12. Ion Exchange13. Neutralization14. Degasification15. Membrane Separation16. Aeration17. Adsorption18. Evaporation and FreezingEvaporatorsEffect of Salt ConcentrationEvaporator DesignCondensationMultiple-Effect UnitsFreezing

19. Oxidation-Reduction20. Corrosion Control21. Deposit Control22. Control of Microbial Activity23. Biological Digestion

Index