Evaporative crystallizations

Increasing e concentration by evaporation or distillation is a common method of increasing supersaturation and inducing crystallization. Since solvent is removed over a finite period of time, it is inherently a semibatch operation. Semicontinuous or continuous operation is also possible. The evaporation or distillation can be run at atmospheric pressure, or at reduced pressure when substrate stability is not compatible with the required atmospheric distillation temperature. 

One of the primary advantages of evaporative procedures is that they can often be combined with other process operations to reduce equipment requirements and/or time cycles. In addition, it is possible in some cases to complete the crystallization without the addition of a second solvent, thereby avoiding the costs of separation and recovery. Some of the process advantages that may be realized are as follows  

These operational advantages must be evaluated against the disadvantages that are discussed in the sections to follow. These disadvantages may include 

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The reactor effluent, containing 1—2% hydrazine, ammonia, sodium chloride, and water, is preheated and sent to the ammonia recovery system, which consists of two columns. In the first column, ammonia goes overhead under pressure and recycles to the anhydrous ammonia storage tank. In the second column, some water and final traces of ammonia are removed overhead. The bottoms from this column, consisting of water, sodium chloride, and hydrazine, are sent to an evaporating crystallizer where sodium chloride (and the slight excess of sodium hydroxide) is removed from the system as a soHd. Vapors from the crystallizer flow to the hydrate column where water is removed overhead. The bottom stream from this column is close to the hydrazine—water azeotrope composition. Standard materials of constmction may be used for handling chlorine, caustic, and sodium hypochlorite. For all surfaces in contact with hydrazine, however, the preferred material of constmction is 304 L stainless steel. 

Fig. 9. Schematic of KNO2 from NH2 and KCl A, KCl—HNO2 reactor B, NOCl oxidizer C, acid eliminator D, gas stripper E, water stripper F, H2O—HNO2 fractionator G, evaporator—crystallizer H, centrifuge I, NO—NO2 absorber , NH2 burner K, CI2 fractionator and L, NO2 fractionator.

An evaporator—crystallizer is used to reverse the sodium bisulfite formation reaction and release the sulfur dioxide as a vapor. The regenerated sodium sulfite, which crystallizes out of solution, is redissolved and returned to the absorber. The absorber overhead gas can be vented to the atmosphere. A concentrated sulfur dioxide stream is produced as a by-product of this process. 

Titanium Sulfates. Solutions of titanous sulfate [10343-61-0] ate readily made by reduction of titanium(IV) sulfate ia sulfuric acid solutioa by electrolytic or chemical means, eg, by reduction with ziac, ziac amalgam, or chromium (IT) chloride. The reaction is the basis of the most used titrimetric procedure for the determination of titanium. Titanous sulfate solutions are violet and, unless protected, can slowly oxidize ia coatact with the atmosphere. If all the titanium has been reduced to the trivalent form and the solution is then evaporated, crystals of an acid sulfate 3 Ti2(S0 2 [10343-61-0] ate produced. This purple salt, stable ia air at aormal temperatures, dissolves ia water to give a stable violet solutioa. Whea heated ia air, it decomposes to Ti02, water, sulfuric acid, and sulfur dioxide. 

Barium nitrate is prepared by reaction of BaCO and nitric acid, filtration and evaporative crystallization, or by dissolving sodium nitrate in a saturated solution of barium chloride, with subsequent precipitation of barium nitrate. The precipitate is centrifuged, washed, and dried. Barium nitrate is used in pyrotechnic green flares, tracer buUets, primers, and in detonators. These make use of its property of easy decomposition as well as its characteristic green flame. A small amount is used as a source of barium oxide in enamels. 

In the United States boric acid is produced by United States Borax Chemical Corp. in a 103,000 2 3 ric ton per year plant by reacting cmshed kernite ore with sulfuric acid. Coarse gangue is removed in rake classifiers and fine gangue is removed in thickeners. Boric acid is crystallised from strong hquor, nearly saturated in sodium sulfate, in continuous evaporative crystallizers, and the crystals are washed in a multistage countercurrent wash circuit. 

Recovery Process. Boron values are recovered from brine of Seades Lake by North American Chemicals Corp. In one process the brine is heated to remove some water and burkeite. The remaining brine is cooled to remove potassium chloride. This cooled brine is then transferred to another crystallizer where borax pentahydrate, Na2B40y 5H20, precipitates (18). In a separate process, boron is removed by Hquid—Hquid extraction followed by stripping with dilute sulfuric acid (19). Evaporator-crystallizers are used to recover boric acid [10043-35-3] H BO. In a third process, borax is recovered by refrigerating a carbonated brine. 

Recovery Process. Figure 5 shows a typical scheme for processing sodium chlodde. There are two main processes. One is to flood solar ponds with brine and evaporate the water leaving sodium chlodde crystallized on the pond floor. The other is to artificially evaporate the brine in evaporative crystallizers. Industrial salt is made from solar ponds, whereas food-grade salt, prepared for human consumption, is mosdy produced in the crystallizers. 

Enei y. In recent years the concern for energy conservation has resulted in many innovative process improvements to make the manufacture of citric acid more efficient. Eor example, heat produced by the exotherm of the neutralization of citric acid with lime is used in another part of the process where heat is requited, such as the evaporation/crystallization step. 

Evaporative crystalli rs generate supersaturation by removing solvent, thereby increasing solute concentration. These crystallizers may be operated under vacuum, and, ia such circumstances, it is necessary to have a vacuum pump or ejector as a part of the unit. If the boiling poiat elevation of the system is low (that is, the difference between the boiling poiat of a solution ia the crystallizer and the condensation temperature of pure solvent at the system pressure), mechanical recompression of the vapor obtained from solvent evaporation can be used to produce a heat source to drive the operation. 

FK . 18-64 Forced -circulation (evaporative) crystallizer, (Swenson Process Equipment, Inc.)... 

Ethylenedioxy-2l-acetoxypregn-4-en-3-one A solution containing 3,3 20,20-bisethylenedioxypregn-5-en-21-ol acetate (120 mg) and /7-toluene-sulfonic acid hydrate (12 mg) in dry acetone (3 ml) is allowed to stand at 22° for 14 hr. Sodium bicarbonate solution and ether are added and the organic layer is separated, washed with water, dried and evaporated. Crystallization of the residue from hexane yields 81 mg (75%) of 20-monoketal, mp 140-141°. 

Sowul, L. and Epstein, M.A.F., 1981 Crystallization kinetics of sucrose in a CMSMPR evaporative crystallizer. Industrial and Engineering Chemistry Process Design and Development, 29(2), 197-203. 

Virtanen, J. 1984. Automatic control of batch evaporative crystallization. In Industrial Crystallization 84. The Hague September 1984. Eds. S.J. Jancic and E.J. de Jong. Elsevier Science. 

An extension of the filter-driers are the reactor-filter-driers, as illustrated in Fig. 7.2-9 by an apparatus known under trademark name Nutrex" and developed by Rosenmund. Equipment of this kind is still more versatile and operation safer. In one vertical position the device can act as a filter, a granulator, and an apparatus for all operations with filter cakes (i.e. re-slurrying, smoothing, and squeezing). In the reverse position, it can operate as a reactor, extractor, evaporator, crystallizer, drier, etc. There are many other companies offering reactor-filter-driers, e.g. SEN, Giovanola, Schenk, and Cogeim. 

Choong KL and Smith R (2004) Novel Strategies for Optimization of Batch, Semi-batch and Heating/Cooling Evaporative Crystallization, Chem Eng Sci, 59 329. 

Evaporative crystallization is not preferred if the product needs to be of high purity. In addition to evaporation concentrating the solute, it also concentrates impurities. Such impurities might form crystals to contaminate the product or might be present in the residual liquid occluded within the solid product. 

The feed to the crystallizer is saturated at 60°C (C = 2.867 kg sucrose-kg H20 1). Compare cooling and evaporative crystallization for the separation of sucrose from water. 

A flowsheet for the Wellman-Lord process is shown in Figure 25.26. Again the gas stream with S02 enters a scrubber into which is sprayed a sodium sulfite solution. This then goes to an evaporator/crystallizer to crystallize out the resulting sodium bisulfite, which converts the sodium bisulfite back to sodium sulfate, releasing the S02. The crystals are dissolved in water and recycled to the scrubber. The effect of the Wellman-Lord process is to produce a concentrated S02 stream from a dilute S02 stream. The resulting concentrated S02 still needs to be treated. 

Evaporation. An extreme case of the use of evaporative crystallization is to use evaporation to simply concentrate the contamination as a concentrated waste stream. This will generally only be useful if the wastewater is low in volume and the waste contamination is nonvolatile. The relatively pure evaporated water might still require treatment after condensation if it is to be disposed of. The concentrated waste can then be recycled or sent for further treatment or disposal. As with evaporative crystallization, the cost of energy for such operations can be prohibitively expensive, unless the heat required for evaporation can be supplied by heat recovery, or the heat available in the evaporated water can be recovered. 

Distillation, sublimation, evaporation Crystallization, gas absorption, leaching Liquid extraction... 

Evaporative-cooling crystallizers, 8 135 Evaporative crystallizers, 8 134 Evaporative efficiency, 9 97 Evaporative emissions, 10 58-59 standards, 12 417... 

Multiple parallel SCWO reactors are used to process the accumulated hydrolysate held in the SCWO feed tank. Liquid effluent from the SCWO system containing inorganic salts is processed in an evaporator/crystallizer, where salts are concentrated into salt cakes for disposal and clean water is recycled. The gaseous effluent from the SCWO, containing primarily carbon dioxide and oxygen, is scrubbed, monitored, and filtered through activated carbon before being released to the atmosphere. 

The products of hydrolysate oxidation are C02, H20, and salts N2 and excess 02 are also present. At the exit of the reactor, recycled water recovered from the downstream evaporator/crystallizer unit is injected to quench the reactor products to the subcritical temperature of 3159C, which results in essentially all of the product salts redissolving. 

The slurry effluent is analyzed for residual organics, and if it meets total organic carbon (TOC) specifications, it is pumped to an evaporator/crystallizer system, where water is evaporated and the salts crystallized for off-site disposal. These inorganic salts are readily stabilized and are suitable for disposal in existing permitted landfills. The recovered water is either reused in the process, sent to a unit that produces deionized water, or used for making caustic solution. If the effluent does not meet TOC specifications, it is routed to an off-specification effluent tank and then returned as part of the SCWO reactor feed. 

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