in rows one above the other in a building with removable sides. This arrangement permitted the air to circulate freely over the trays and carry away the water. About two weeks mere required to reduce the water content from 56 t'o 15 per cent. This remaining water was removed by heating the material on a shallow metal tray over a small stove ( 2 ) . Production by this method was so slow and required such a large instalKENNETH A. KOBE, CARL W. HAUUE, lation of drying sheds as to AND CARL J. CARLSON be commercially unfeasible. Controlled humidity drying Department of Chemical Engineering, also requires a large instalUniversity of Washington, Seattle, Wash. lation because the wet-bulb temperature must be kept below the transition temBy the use of submerged combustion, 46.1 per cent of perature. Both of these processes produce a fine anhydrous' sodium sulfate can be recovered from sodium powdery dust which does sulfate decahydrate by heating to 90" C. The evaporation not find a ready market. of a saturated solution of sodium sulfate can be carried The operation of a rotary out with no scale formation on the evaporator body. Scale kiln drier above the transiformation which occurs on the burner, and crystal size and tion temperature requires that a large fraction of the factors affecting it are discussed. A cycle of operations dehydrated product be is proposed, and the thermal requirements of this cycle are mixed with the decahydrate compared with those for a tube evaporator. feed so that, when the latter melts, the entire mass will have the consistency of sand and will not adhere to the sides of the kiln (Y). ITH the decline in the production of The success attained by evaporation methods on other hydrochloric acid from sodium chloride and salt solutions led to the use of multiple-effect evaporation sulfuric acid, the amount of by-product for the recovery of anhydrous sodium sulfate. It is necessary salt cake available for the chemical industry has decreased to that additional water be added to the decahydrate to form a such a n extent that the demand for sodium sulfate must be saturated solution, or a natural brine can be employed direct. met from other sources. This country has attempted to The operation of such a n evaporator system was studied by supply the deficiency by utilizing the natural deposits of Badger and Caldwell ( I ) . Under ordinary operating condisodium sulfate which occur in the western states (IS). These tions, crystal formation became so great in an hour that it deposits are of two kinds: Those in the Southwest are anhywas necessary to clean out the evaporator. By withdrawing drous sodium sulfate (thenardite) ; and those in the Korthwest the solution continuously from the evaporator, superheating, and Canada are sodium sulfate decahydrate (mirabilite). The and flashing it under the tubes, seed crystals formed and the deposits of thenardite have been worked t o the greater extent, vigorous circulation obtained made it possible to operate for for they produce directly a material comparable to salt a 10.5-hour period followed by a 1.5-hour boil-out period in cake (IO,11). The deposits of mirabilite have been worked which the sodium sulfate scale was dissolved from the evaporaonly on a small scale owing t o the necessity of dehydrating the tor body and tubes. Spray evaporators, in which the satudecahydrate before shipping. Sodium sulfate decahydrate is rated solution is sprayed countercurrent to a rising stream of composed of 44.1 per cent sodium sulfate and 55.9 per cent hot air, work satisfactorily but produce a fine, light powder water; the need for dehydration is apparent. which is undesirable for many uses. The difficulties surroundThe occurrence of large deposits of mirabilite in the State ing these various methods make it apparent that submerged of Washington has led to the study of dehydration so that combustion offers many advantages in the dehydration of the product may be more cheaply shipped by rail to the coast sodium sulfate decahydrate. where the sulfate pulp mills offer a market.
Evaporation by
Submerged Combustion 111. Sodium Sulfate Decahydrate'
vv
Dehydration of Sodium Sulfate Decahydrate
Advantages of Submerged Combustion
The dehydration of sodium sulfate decahydrate has been a rather difficult problem because of the character of the salt. At 32.4"C. the decahydrate melts, forming solid sodium sulfate and a saturated solution. Drying processes must operate below this transition temperature to avoid melting the salt. The first attempts were to expose the decahydrate to outdoor conditions. The hydrate +as spread on trays placed
Multiple-effect evaporation would be satisfactory were it not for the fact that sodium sulfate has an inverted solubility curve above 32.4' C. (Figure 1). This decrease in solubility of sodium sulfate with increase in temperature causes the salt to crystallize from its solution on surfaces which are a t a temperature above that of the solution. With tube evaporators there must be considerable surface a t a higher temperature so that heat may be conducted to the solution being
' For Parts I and 11,
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literature oitatioh 6.
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590
INDUSTRIAL AND ENGINEE R l \ G CHEMISTRY
evaporated. Thus we may expect the sodium sulfate to form rapidly a hard scale which will e v e n t u a l l y cause the eraporator to be shut down. AFwas prev i o u s l y pointed o u t (4, 8 ) , submerged combustion does not give theopportunity for scale formation. Combustion t a k e s p l a c e in a closed chamber beneath the surface of the liquid, and t,he bot product.s of coni b u s t,io ii hubble up through the solution. Heat transfer takes place directly between the gas and l i o o i d . so t h a t there is no large irietal surface on which sodium sulfate caii crystallizr. Even though some scale formation occurs 011 the burner, it is not necessary to shut down the entire apparatus; the burner is merely withdrawn and cleaned, vhile anot.lrer burner may be inserted into the liquid. The feed for the evaporator must be a solution of sodiuin sulfate, which is usually saturated, or a natural brine. For a submerged combustion system the solid decahydrate may be used as the feed. If a solution saturated v i t h sodiuni sulfate at 32.4" C. (the point of greatest solubility) is heat,ed, sodium sulfate will precipitate out as the temperature rises (Figure 2). If the solution is heated to 40' C., 2.8 per cent of the sodium sulfate in the solution saturated at 32.4" will be precipitated; if the temperature is raised to looo, 15.2 per cent of the salt separates. Thus, by merely heating the solution above the transition temperature, solid sodium sulfate separates and may be recovered. I n the usual operation the sodium sulfate solution is saturated at out.door temperature. Thus, if the temperature k helow 30" C., Figure 1 shows that there s i l l be no precipitation of salt due to solubility effect,, and it is usually necessary to evaporate water before the solution becomes saturated. If the solid decahydrate is heated, the recovery is much greater (Figure 2) than from a saturated solution. At 40' C., 38.9 per cent. of the sodium sulfate in the decahydrate remains as solid; if the temperature is raised to lOO", 46.7 per cent of the salt can be recovered 8s solid. Such a recovery may be obtained by using the solid decahydrateas the feed and allowing it to drop int.0 the saturated solution in the body of the evaporator. With a submerged combustion burner evaporating the solution at 90", the use of decahydrate as the feed will give a precipitation of 46 per cent of the sodium sulfate and a saturated solution containing the other 54 per cent, Thus, submerged comhnstion allows the recovery of sodium sulfate by 6wo methods: solubility-t.einperature effect, and evaporatiorr of a saturated solution. I
VOL. 28, NO. 5
Fifty pounds of decshydrate were plaoed in the evaporator and 13.5 pounds of hot water added t o give a liquid layer into which the burner could be introduced. As the tempereture of the solution increased, the decahydrate melted and the burner could he pushed down farther into the ovsporator. After ail decahydrate XT~E melted, the solid sodium suffste which had collected in the eone bottom was removed through the gate valve. The temperature of the solution m6e to 89.7' C. where evaporation took place. At 10-minute intervals the gate valve was opened and the precipitated salt withdrawn. The liquid sludge that was removed was fibered immediately t.hrough a Buehner funnel and the filtrate returned to the evaporator. The filtered salt contained about 10 per cent mbrr, although it is apparent that, by centrifuging the crystals, bhe moisture content could he reduced t o 5 per ccnt. The run ~ 3 - mdigcontinued after 2 hours and 10 minutes. Examination of the rvnporator showed no scale on the hody. The burner, hortver, sfrowed 8. fusion of salt forming a hollon cone over the end and leaving a hole barely large enough for the escape of the products of combust,ion. This is oafiily removed by B blow from a hammer. A more detailed investigationof this cone formstion was undertaken. A modified burner was oonstruoted so that changes in
the piping. d i s B screen, such as t,he screen fr& the top o r a laboratory Meeker burner or a number of layers of iron wire gauze. The screen separates the flame zone in D from the ga8air zone in B by incmssing t,he veioaity of the gas-air mixt.ure and p r e v e n t , i n g the
,
Submerged Combustion Evaporation The equipment used for submerged combustion evaporation was described (8).The apparatus used in this work is shown in Figure 3.
Fir the 8t;dv of cone formatioh x '/
