INDUSTRIAL .4-VD ESGINEERING CHEMISTRY
470
Conclusion
I n conclusion, it is desired to emphasize again the four important factors which enter into the production of pressure vessels for safe service under severe working conditions. (1) An appreciation of the working conditions to which vessels are to be subjected. (2) A balanced design such as will prevent areas of stress intensification which may give trouble in operation. (3) Assurance that the metal has not been damaged in the construction of the vessel and that fabrication strains have been relieved.
VOl. 20, No. 5
(4) An adequate acceptance test which will guarantee to the purchaser that the vessels are able to do the work expected of them.
For the more hazardous processes in which pressure vessels are used a periodic proof test should be given such vessels, or even the whole equipment involved. These proof tests can be made with liquids, using pressures sufficiently far above the working conditions so that, should a failure occur there will be no danger of disaster or even an accident of any consequence.
Prediction of Boiling Points of Concentrated Caustic-Salt Solutions' F. W. Adams2 and C. W. Richards3 MASSACHUSETTS INSTITUTE OF TECHNOLOGY, DEPARTMENT OF CHEMICAL ENGINEERING, CAMBRIDGE, MASS.
The effect of pressure on the boiling-point raisings of N T H E calculation and Several methods have been sodium hydroxide solutions saturated with sodium design of e v a p o r a t i v e proposed for the prediction chloride has been determined a t various concentraequipment handling conof the change of the boiling tions. A method using Duhring's rule is proposed centrated solutions a knowlpoint of a solution with presfor the prediction of the boiling point of concentrated sure. Of these, one particuedge of the boiling point of solutions of this type. It has been shown t o give relarly deserves mention. This the solution is of major imsults checking observed data with a maximum deviation is t h e a p p l i c a t i o n of Duhportance. The data relative of 2.1' C. a t 50 mm. pressure. ring's6 rule, where the boiling t o s o l u t i o n s of s a l t and caustic soda available in the temperature of the solution is literature cover a rather narrow range of pressures, usually plotted against the temperature of the pure solvent a t the around atmospheric, but are quite complete as regards same pressure. I n the range for which this rule holds, a concentration. I n the multiple-effect operation of evapo- series of straight lines is obtained for various concentrations. rators pressures below atmospheric are usually encoun- Thus it is only necessary to have the boiling points of a solutered. Where the boiling-point raising is high, the ca- tion at two pressures to calculate its boiling point a t any other pacity of the system is lowered. This is particularly true in pressure. I n order to facilitate evaporator calculations, and at the the evaporation of electrolytic caustic liquors, which in addition to sodium hydroxide contain an appreciable amount of same time to test the value of this method of predicting boiling points, this investigation was conducted. sodium chloride. The data in the literature for these solutions are rather Experimental Procedure meager, comprising 70 boiling p o i n t s at The boiling points of caustic solutions saturated with salt atmospheric p r e s - were d e t e r m i n e d b y sure as given by von Pink' for various presG 60 sures. A caustic soluA n t r ~ p o f f , supple~ 50 mented by data pre- tion was placed in a 4 sented by Badger6 1500-cc., round-bottom f 40 for use in evapo- flask i m m e r s e d i n a r a t o r calculations. bath of oil. Connec+. g 30 Badger, however, tions through a conk mistrusts the preci- denser, receiver, and 4 20 sion of his data and trap were made to a 4 fir of those of von An- vacuumpump. Acon- e? 8o Concenfiofed 1 4 /o tropoff. The results s t a n t p r e s s u r e was of this investigation maintained b s bleeding 70so b'o 7 i /do o! ',a Gempemfure of Pure Solvent 'C a mercury ' / d , l ~ ~ l & ' & l & ' & ' & prove ' & that in the air through -Pressure was Figure 2-Validity of Diihring's Rule for Pressure n m Mercury commercial range of column. Concentrated Solutions Figure 1-Boiling-Point Raising of NaOH caustic evaporation measured on a mercurv Solutions of Various Concentrations Satuthis mistrust is un- manometer, and absofute pressure was calculated from the barated w i t h NaCl warranted. These rometer reading. The boiling point was read on a mercury results were, however, obtained prior to the publication of thermometer immersed in the surface of the caustic. PrecauBadger's data, in order to fill the wide gap then existing in tions were taken to insure saturation of the solution with salt our knowledge of the boiling points of caustic-salt solutions by always maintaining a small amount of crystals in suspension. A delicate capillary leading to the bottom of the solution bled a t reduced pressures. in a minute quantity of air sufficient to prevent bumping. 1 Received December 12, 1927.
I
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Director, Bangor Station, School of Chemical Engineering Practice. Assistant director. 2. Elckttochcm., 80, 457 (1924). Trans. A m . Inst. Chem. Eng., 18, 231 (1926).
1
"Neue Grundgesetze der rationellen Physik und Chemie," Leipzig,
1878. 1 Unpublished report of M. I. T. School of Chemical Engineering Practice, September, 1926.
INDUSTRIAL AND ENGINEERING CHEMISTRY
May, 1928
The temperature of the oil bath was controlled to maintain boiling within the flask and at the same time prevent superheating of the liquor. Below a pressure of 100 mm. difliculty was experienced with superheating of the most concentrated solutions, where the maximum error was 1'C. The caustic concentration of the solution was determined by titration with standard acid. Where no information Qn the solubility of sodium chloride was available in the literature the sample was titrated for chloride, using Mohr's method. The caustic concentration is expressed as the ratio of water to caustic-i. e., grams H20per gram NaOH. This method of expressing concentration is convenient in the calculations of caustic evaporation, since the amount of caustic remains constant, the salt and water decreasing during the process. Discussion of Results
The boiling points of caustic-salt solutions a t various pressures are presented in Table I. From these data boilingpoint raisings have been calculated and are presented in Figure 1. For the lines, the water-caustic ratios, in grams H20per gram NaOH, are given in Table I. Points of Caustic Soda Solutions Saturated with Sodium Chloride ABSO- BOILING BOILINGBOILING- A B S O - B O I L I N G B O I L I N G LUTE POINT POINT POINT LUTE POINT POINTBOILING-
Table I-Boiling
PRESSURE
OF
OF
RAIS-
WATER
' C.
IN'
WATER-CAUSTIC R A T I O ( C U R V E A)
0.714
Mm. Hg 51 99 152 231 325 425 538
' C. 96.7 110.6 120.0 130.0 138.9 146.7 163.3 87.8 105.6 120.0 131.1 137.8 141.1
=
73.3 83.3 97.8 106.1 113.3 120.0 124.4
=
1.62
30.0 30.3 31.6 31.0 31.4 31.5 31.7
43.3 53.0 66.2 75.1 81.9 88.5 92.7
WATER-CAUSTIC R A T I O
0.992
43.9 43.4 44.0 44.6 44.9 45.1
43.9 62.2 76.0 86.5 92.9 96.0
WATER-CAUSTIC R A T I O (CURVE c)
66 107 198 290 384 497 582
=
2.16
(CURVE D) I
48 99 143 307 470 594
56.7 70.0 78.3 97.2 107.8 114.4
-
37.4 51.4 59.0 76.4 87.1 93.2
SURE
OF
OF
e
C.
POINT
c.
c.
WATaR 0
WATER-CAUSTIC RATIO
3,68
(CURVE F)
58.2 59.2 59.6 60.3 61.1 62.2 62.7
38.5 51.4 60.4 69.7 77.8 84.5 90.6
WATER-CAUSTIC R A T I O (CURVE B)
68 165 302 460 586 658
' C.
PRES-
Mm. Hg
19.3 18.6 19.3 20.8 20.7 21.2
:2: 330 459 582
:!: gg: 92.2 100.6 107.2
WATER-CAUSTIC R A T I O (CURVE 0 )
23 48 86 119
35, 47.8 58.3 65.6
450 567 674
104.2 97.8 108.3
:ii
7763;
i:::
::;:;
ii:2
78.2 86.4 92.7
14.0 14.2 14.5 = 6.32
24. 37.4 48.5 55.2
10.4 9.8 10.4
!::! i::; 92.0 85.9 i;:: 96.7 11.6 E:l i;:;
WATER-CAUSTIC RATIO = 8.26 ( 2 0 . 6 % NaCI) (CURVE R)
61 119 234 353
50.6 63.3 78.9 89'4
41.8 55.2 70.0 79'8
760
111.1
100.0
::::
i!; lt::i ;:: igi:;
9'6
E::
11.1
WATER-CAUSTIC R A T I O = 3.04 (CURVE E )
48 81 117 208 332 447 596
51.7 61.1 6S.9 82.2 93.3 101.7 109.4
37.4 47.3 54.8 67.3 78.3 85.8 93.3
14.3 13.8 14.1 14.9 15.0 15.9 16.1
To test the validity of Diihring's rule for caustic solutions, data already available in the literature were assembled in Table 11. Two series of data were found for concentrations up to a water-caustic ratio of about 1.5; one8 giving vapor pressures at 100' c . ; the other,Qboiling points a t atmospheric pressure. More complete data are available a t a watercaustic ratio of 1.10.10 Figure 2 has been constructed plotting the temperature of the solution against the temperature of the pure solvent a t the same pressure. For the lines, the water-caustic ratios, in grams HzO per gram NaOH, are given in Table 11. It will be noticed that a series of sensiTammann, Mbm. ocad. Pbfersbourg, (7) 36 (1887). Gerlach, 2. anal. Chem., 26, 413 (1887). 10 Baker and Waite, Chcm. Mef. Eng., 26, 1137 (1921). 8
9
bly parallel lines is '60 obtained. A similar % , line, Q, for sodium chloride w i t h an 140 H20-NaCl ratio of 3.467.8 is included. f0 The boiling points of ~ 2 0 the caust,ic-salt solutions are similarly s//o plotted in Figure 3, k,oo three of the lines for pure caustic being included for comparison. These lines are all very close to parallel. Thus, knowing the slope of Diihring's line for any c o n c e n t r a t i o n of 4 p u r e c a u s t i c , it is o n l y necessary to Temprrcrture of P u r e So/vent 'C. have t h e boiling point of a caustic- Figure 3-Validity of Duhring's Rule for Concentrated Caustic-Salt Solutions salt solution, or caustic solution, a t one pressure to calculate its boiling point a t any other pressure. The precision of this method for predicting boiling points is shown in Table 111. A line parallel to line N was drawn on Figure 3 through the boiling point a t atmospheric pressure of a caustic-salt solution having a water-caustic ratio of 0.992. Boiling temperatures were calculated and tabulated. The observedboiling points of the solution read from Figure 1 are also tabulated for comparison. It will be noticed that this method checks within 0.7" C. at pressures above 200 mm., while the maximum deviation a t the lowest pressure of 50 mm. is only 2.1' C. If data on a solution having a concentration closer to that of the solution in question are available as a reference, the precision of the method is increased. Thus, using the slope of line L as a reference, a maximum deviation at 50 mm. pressure of 0.1 C. is found. 0
O
8.8
:::
471
Table 11-Boiling *'O 'R
PRES-
Points of Caustic-Soda Solutions VAPOR PRES-
BOILINGBOILING S U R E O F BOILING BOILING WATER POINT POINT BOILING- WATER POINT POINTBOILINGABOVE OF OF POINT ABOVE OF OF POINT SOLN. SOLN. WATER RAISING SOLN. SOLN. WATER RAISING Mm.Hg "C. O C . OC. Mm.Hg OC. O C . OC.
S U R E OF
NATER-CAUSTIC RATIO (CURVE I.)
166 202 255 312 373 456 540 646 747
101.5 106.3 111.5 116.5 121.0 126.2 130.6 135.5 139.5
WATER-CAUSTIC R A T I O (CURVE M)
255 760
100.0 128.3
=
62.3 66.7 72.0 76.8 81.2 86.3 90.7 95.5 99.5 72.0 100.0
WATER-CAUST1C (CURVE N)
1.10
39.2 39.6 39.5 39.7 39.8 39.9 39.9 40.0 40.0 = 1.60
=
WATER-CAUSTIC RATIO = 3.00 (CURVE 0)
505 760
100.0 111.5
88.9 100.0
11.1 11.5
WATER-CAUSTIC RATIO = 5.M (CURVE P)
620 760
100.0 106.2
94.4 100.0
5.6 6.2
SODIUM CHLORIDE S O L U T I O N (HIO/NaC1 = 3.46) (CURVE Q)
619 760
100.0 105.8
94.4 100.0
5.6 5.8
28.0 28.3 2.00
367 100.0 80.8 19.2 760 120.6 20.6 Table 111-comparison of Calculated and Observed Boiling Points
.
BOILING P O I N T OF
PRESSURE
Mm. Hg 50 100 200 300 400 500 600 700 760
WATER
c.
38.1 51.6 66.4 76.9 82.9 88.7 93.5 97.7 100.0
CALCULATED
BOILING
P O I N T OF
SOLUTION
c.
79.5 93.6 109.5 119.3 126.8 132.8 138.0 142.6 145.1
OBSERVED
BOILING
P O I N T OF
SOLUTION
c.
81.6 95.2 110.2 119 9 127.2 133.2 138.3 142.9 145.1
