CHAPTER 18 EVAPORATION AND FREEZING

Transcription

1 CHAPTER 18 EVAPORATION AND FREEZING Evaporation and freezing can be used to convert water (1) to a pure vapor that can be condensed or (2) to a pure solid that can be separated from a saline mother liquor and melted. Both processes leave saline residues containing essentially all the solute originally in the feed water. EVAPORATORS Evaporators are widely used in many water treatment operations, such as preparation of boiler feed water, concentration of diluted liquor, evaporation of seawater to produce fresh water, and concentration of waste liquors to reduce volume for further processing or disposal. The typical evaporator is like a fire-tube boiler, with the flame replaced by steam or process vapor as the heat source; but, there are significant differences: 1. The evaporator has a much smaller temperature gradient across the heat transfer surfaces. 2. It usually holds less liquid. 3. Evaporator tubes are usually made of special metals (titanium) or alloys (stainless steel), whereas boiler tubes are made of steel. To transfer heat efficiently at low temperature gradients, the evaporator surfaces must be kept free of deposits, which have an insulating effect. Correct chemical treatment and scheduled cleaning are important. Every liquid exerts a vapor pressure, the magnitude of which is a measure of its volatility. High vapor pressure liquids evaporate readily, while those with low vapor pressures evaporate more slowly, requiring an increase in temperature to speed the rate. The kinetic energy of all molecules increases with increasing temperature. The rate of evaporation depends on the nature of the substance, the amount of heat energy applied to the liquid, and surface effects. When a liquid reaches the temperature at which its vapor pressure equals atmospheric pressure, boiling occurs. This is the rapid evaporation from all parts of the liquid mass, with bubbles of vapor forming in the interior and rising to the surface. The pressure within these bubbles equals the vapor pressure of the liquid at that temperature, so the boiling point depends on the external pressure. For example, at sea level, pure water boils at F (10O 0 C) where its vapor pressure

2 is 1 atm (1 bar), or 14.7 lb/in 2 (1.0 kg/cm 2 ). If the external pressure is reduced, as occurs at elevations above sea level or under vacuum, water boils at a lower temperature. EFFECT OF SAL T CONCENTRA TION Since evaporators may process liquids other than pure water, factors other than atmospheric pressure must also be considered. Soluble salts in the solution decrease the vapor pressure, elevating the boiling point. Therefore, as dilute liquor evaporates and becomes more concentrated, its boiling point rises. Figure 18.1 shows the boiling point elevation as the concentration of a salt increases in aqueous solution. As water is evaporated from a solution and the liquid becomes more concentrated, it is possible to concentrate to the point where the solubility of the salts is Boiling point, 0 F NaCI solution strength, % FIG Effect of solution concentration on atmospheric boiling point using NaCl as an example. For any specific concentration, the solution boiling points at several pressures plotted against the boiling points of water at these same pressures produce a straight line. exceeded. This results in precipitation, usually as scale on the heat transfer surfaces. Where water is being evaporated, the scale may consist of salts of calcium, magnesium, and silica. This scale severely reduces the heat transfer rate, slowing evaporation and reducing thermal efficiency. It takes a lot of heat to evaporate water. Raising the temperature of 1 Ib of water I 0 F requires 1 Btu; to change that 1 Ib into vapor at atmospheric pressure requires 970 Btu. (It takes 1 cal to raise 1 g of water I 0 C, 539 cal to vaporize 1 g at atmospheric pressure.) The high energy requirement for evaporation makes it important that the heat balance of a plant be controlled for maximum use of energy. A typical evaporator usually receives heat from live steam or from steam bled from a turbine.

3 EVAPORATOR DESIGN There is a large variety of designs of evaporators, although the majority work on the principle of steam passing on the outside of a series of tubes with water or water solution, either confined or recirculated, flowing as a thin film over the inside of the tubular heating surface. The various types of evaporators are classified according to the way the water is vaporized: 1. Boiling type: Evaporators which heat water to the boiling point and evaporate it by applying an external heat source. 2. Flash type: Evaporators which superheat water by an external heat source and flash it into vapor. 3. Compression type: Evaporators which add energy to water vapor by compression 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 a tubular element, boils water, and discharges water vapor from the evaporator shell. The heating elements are usually bundles of tubes of various configurations. Feedwater in Steam in Vapor to condenser. This can be a separate surface condenser to produce distillate or it can be built into the heat cycle on a stage heater operating at a lower temperature than the steam supply. Drains (Condensate) Blowoff to waste FIG Simple submerged-tube evaporator. These may be completely submerged in the water, partially submerged, or arranged so that only a film of water flows across the surfaces. In each design, the space for vapor generation must be ample to avoid steam blanketing and to prevent 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 important where the aim of evaporation, as in most water treatment systems, is to produce high-quality distillate. The vapor purifiers are comparable to those in boilers. In addition to the conventional designs, bubble cap purifiers are sometimes used. These return part of the distillate to continually wash fresh vapor. A boiling-type evaporator with proper disengaging area should produce distillate with less than 1 mg/l total dissolved solids. The quality is affected by the dissolved solids content of boiling water which may be entrained in the vapor discharge. Lower solids levels are attainable with more sophisticated vapor purifiers and conservatively designed evaporator elements. Vapor quality is affected by the CO 2 liberated from the bicarbonate alkalinity, just as in boiler operation.

4 CONDENSATION Purified vapor leaving the evaporator is condensed in several ways: 1. In older utility systems where evaporators were used to provide high-quality makeup, the vapor was discharged through the deaerating heater and condensed by the boiler feedwater (Figure 18.3). (Modern utilities use demineralizers instead of evaporators to process makeup.) Steam from turbine at 28 psia (20 kg/cm 2 ) Steam from turbine at 75 psia Blowoff Treated makeup Makeup vapor Trap Condensate at F Boiler feedwater at F (118 0 C) Deaerating heater (151 0 C) Turbine condensate at F (85 0 C) FIG Typical utility-type evaporator operation. 2. Vapor may be condensed by a surface condenser if the purified liquid phase is to be kept separated for some reason. Each pound of vapor becomes 1 Ib of distillate in the condenser shell. 3. The vapor may be fed to the tube element of the second evaporator body, and the vapor from this second unit fed to a third, producing a multiple-effect evaporator (Figure 18.4). In this type of multiple-effect evaporation, vapor 0.8* Vapor 0.7 * Vapor 0.6* Vapor 1* Steam Feed Blowoff 1 ^Condensate 3.1* Total condensate FIG Multiple-effect evaporator with condenser. 2.1* Distillate from the last unit is liquefied in a condenser. Each pound of fresh steam fed to the first stage theoretically produces 1 Ib of condensate from each stage. In practice, however, a triple-effect evaporator produces about 3.1 Ib of total condensate per pound of steam instead of 4.0, the total condensate including that produced by the fresh steam applied to the first effect.

5 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 STEAM HDTVELL TO BOILER FRDM SQAP SKIMMER TD SQAP SKIMMER No.1 FEED FLASH TANK No. 2 FEED FLASH TANK No. 3 FEED FLASH TANK WEAK LIQUOR FEED PULP MILL PRODUCT FIG Fig Eight-effect kraft pulping liquor evaporator. Thisflowsheet applies to the installation shown in (Courtesy of HPD, Inc., Naperville, III.)

6 TABLE 18.1 Typical Evaporator Controls Following Pretreatment Reading TDS, mg/l SiO 2, mg/l Hydroxide, mg/l CaCO 3 Dispersant and antifoam PO 4, mg/l SO 3 PH Range 2500 max 100 max Trace Where and how maintained In evaporator, by blowdown In evaporator, by blowdown In evaporator, by blowdown or by NaOH feed In evaporator, by chemical feed In evaporator, by phosphate-organic treatment In evaporator, by chemical feed In vapor, by amine treatment Note: Where there is no pretreatment of evaporator makeup, chemicals should be fed for internal treatment just as for a low-pressure boiler, as shown by Table TABLE 18.2 Evaporator Controls Internal Treatment Only Reading TDS, mg/l SiO 2, mg/l Hydroxide, mg/l CaCO 3 Dispersant and antifoam PO 4, mg/l PH SO 3, mg/l Range Where and how maintained 1500 max In evaporator, by blowdown 100 max In evaporator, by blowdown In evaporator, by blowdown or by chemical feed Trace In evaporator, by selecting proper treatment combination O In vapor, amine treatment If the evaporator is used intermittently, a residual SO 3 of mg/l should be maintained to protect the evaporator shell from corrosion Multiple-effect evaporators are used principally for chemical process operations. Typical of such evaporators are the units found in the pulp industry for the concentration of sulfate black liquor (Figure 18.5). The primary aim of evaporation in the power plant is to produce boiler makeup of high quality. The chemical treatment program must be designed to produce high-purity vapor while helping to maintain clean heat transfer surfaces. As in boiler water conditioning, the goals of chemical treatment are control of carryover, prevention of deposits, and elimination of corrosion. The makeup to most utility evaporators is pretreated to remove hardness, reduce alkalinity, and eliminate dissolved oxygen. The chemical treatment applied to the evaporator should be controlled to maintain the limits shown in Table Where there is no pretreatment of the evaporator makeup, then the chemical program should be the same as used for internal treatment of a low-pressure boiler, as shown in Table Where the feed to the evaporator is brackish or seawater, it is often difficult to maintain a scale-free system even with a good internal treatment program. In such cases, the operation is programmed so that scale is allowed to build up on the heating tubes for a planned period, then the temperature of the system is suddenly dropped, creating a thermal shock that cracks the scale from the tube surface for removal from the bottom of the evap-

7 orator. Other evaporators are manufactured with bowed tubes which flex with temperature change, also resulting in scale-cracking and shedding. MULTIPLE-EFFECT UNITS Although multiple-effect evaporators are usually used for process operations, they have a definite tie-in to the utility system. In the pulp industry, five-, six-, or even seven-effect evaporators are used to concentrate water from the pulp washers for recovery of cooking chemicals. The black liquor may be concentrated to approximately 65% total solids, of which about half are organic materials. In this condition, the black liquor can be fired to a black liquor recovery furnace; the organic material supports its own combustion and smelts the cooking liquor salts to a recoverable form. Figure 18.6 shows a typical large installation of such a black FIG Black liquor evaporator in a kraft pulp mill. Some of the condensate may be recovered for reuse, but much of it is contaminated with sulfur compounds. (Courtesy of HPD Corporation.) liquor evaporator. Fresh steam fed to the first effect produces condensate that can be reused as boiler feed water. However, the condensate produced in subsequent stages is too contaminated by volatiles for such use, but may be used for brown stock washing, stock dilution, or other purposes. Some of this condensate is so foul that it must be stripped before it can be put into the sewer for treatment in the waste treatment plant. The vapors stripped from the foul condensate may be sent to the lime kiln where the organics responsible for the foul odors are burned.

8 The bauxite industry also uses multiple-effect evaporators for concentration of sodium aluminate liquors, producing more condensate than required for boiler makeup. Again the condensate is usually too contaminated to be directly usable as boiler feed water. A final example is the beet sugar industry, where syrups are concentrated by evaporation, again producing an excess of condensate over boiler makeup requirements. These condensates are frequently contaminated with sugar, which is very detrimental to boiler operation, and ammonia, which is corrosive to systems that contain auxiliary equipment fabricated of copper alloys. The advantage of multiple-effect in increasing yield per unit of energy can also be built into two quite different designs of evaporators which bear little resemblance to the standard multiple-effect evaporator. The first of these is the vapor compression still, which was developed initially for seawater evaporation aboard ship (Figure 18.7); the second is the multistage flash evaporator, which has Vapor Slowdown to waste Motor-driven compressor Feed in Recovery heat exchanger Product out FIG 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 and must be kept free of deposits to maintain efficient heat transfer. Flash evaporators having a capacity of 7.5 mgd (20 m 3 /min) have been installed for municipal water Preheat exchangers- Slowdown to waste Steam in at 60 psi Feed in Recycle Bleedoff FIG Flosh chambers Schematic of multistage flash evaporator. Product supply in the Middle East, where energy costs are favorable for such an installation (Figure 18.9). Where seawater is being used as the feed, chemical treatment for prevention of calcium carbonate, calcium sulfate, and magnesium hydroxide scales is required. The treatment includes (a) reduction of alkalinity to minimize supersaturation of calcium and magnesium compounds, (b) application of scalecontrol agents, such as acrylates, polyphosphates, or combinations of these, (c) oxygen scavengers or other types of corrosion inhibitors, and (d) antifoams to protect the quality of the distillate. With this treatment it is possible to concen-

9 FIG Three flash evaporator modules, each with a capacity of 2.5 mgd (6.7 m 3 /min), producing potable water for Al Khobar, Saudi Arabia. (Courtesy of Aqua-Chem Inc.) Vapor out Vapor body Distillate reflux in Liquid level Steam in Heating element Condensate out Weak feed in Circulation pump Concentrate waste discharge Vertical tube forced circulation (single pass) FIG A type of evaporator design used for concentration of radioactive wastes from nuclear fuel reprocessing. (Courtesy of Unitech Division, Ecodyne Corporation.)

10 FIG Compact evaporator designed for concentration and recovery of plating solutions from rinsewater. (Courtesy of Industrial Filter & Pump Manufacturing Company.) trate seawater about 1.6 times. Large installations may find pretreatment of the sea water of value. Such pretreatment has been ruled out by the high cost of chemicals and the extra equipment, but the increasing cost of fuel may offset this in the future. Special alloys must be used throughout to counteract the corrosive effects of the concentrated seawater. Vent Falling film evaporator Mixed wastes, (blowdown, etc.) Feed Feed tank Acid feed Feed pump Condensate to polishing demineralizer for boiler makeup Heat exchanger Condensate pump Deaerator Condensate tank Recirculation pump Concentrate, to pond Steam compressor FIG Compression still designed to operate as a crystallizer as well as an evaporator in treating cooling tower blowdown and other wastes in a zero-discharge utility station. (Courtesy of Resources Conservation Company.)

11 TABLE 18.3 Chemical Characteristics of "Zero-Discharge'* Evaporator System Description Feed: tower and scrubber Liquor: evaporator concentrate Product: distillate ph, initial after acid Conductivity TDS, mg/l TS, mg/l SS, mg/l ND* 257, ,900 67, Nil * Not determined Evaporators are finding application in the concentration of wastes to minimize volume, simplify ultimate destruction, or both. Examples include the treatment of radwaste from nuclear power plant operations (Figure 18.10) and concentration of plating wastes (Figure 18.11). In special cases where the EPA permit requires "zero discharge," a modified design of compression still is being used to concentrate combined wastes, such as cooling tower blowdown and flue gas scrubber effluent, to yield a saturated solution containing a crystal phase, so that the salinity can be removed as solids (Figure 18.12). Table 18.3 shows the concentration achieved at one utility station using 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 the heat-extraction surface consist of fairly pure H 2 O. The remaining water has concentrated the solute originally present in the makeup water. Before the advent of the home refrigerator, commercial ice plants manufactured ice in cans, and it was V-4 Feed precooler EM Freezer.NH3 out F-1,F-2,F-3, F-4 Precipitate removal filters Product water V-9 Gravity wash column E-5 Melter Glycoh cooling Warm water V-3 Feed mixing tank Feed stream V-6 Receiver P-12 P^2 Reject solids and concentrate (blowdown) PJ Reject solids and concentrate F-3 F-4 V-10 Surge tank Reject Reject FIG Schematic of mine water desalination by freezing. This system is tied into the mine's air-conditioning load, improving the economics of the process. (Courtesy of CBI Industries, Inc.) P-7

12 common practice to suck out the "core" of unfrozen water, containing most of the 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 were not removed, the final solidification of the cake would include salt crystals mixed with ice crystals. Various schemes have been proposed for producing pure H 2 O as a solid, free of solute originally present in the feed water. These ideas are being seriously pursued because they hold some promise of energy savings over distillation processes; the evaporation of 1 Ib of water requires about 1000 Btu (539 cal/g), compared with only 144 Btu (80 cal/g) to freeze it. Figure is a schematic diagram of 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 the expense of a lower production rate. Although freezing as an economical process of water desalination may be far from commercial use, its use as an energy storage scheme is being practiced to a limited extent. Water is frozen during nighttime off-peak electrical periods, when the cost of electricity is reduced; during the day, the ice is melted by cooling air to 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 a means 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.