ANHYDROUS SODIUM HYDROXIDE Production by Partial-Pressure Evaporation DONALD F. OTHMER AND JOSEPH J. JACOBS, JR.'
Polytechnic Institute of Brooklyn, N. Y ,
The separation of the 0.5-1 pound of chemically combined and solvent water from each pound of caustic soda after the solution has been evaporated to as high a concentration as possible is normally accomplished in caustic fusion pots which are high in capital cost per ton of throughput and expensive in operating costs, particularly i n heat requirements. The method of removing solvent water and the water in the several hydrates (3), depending on t h e partial-pressure evaporation of this water in the presence of a water-insoluble liquid such as kerosene, has been conducted on a large enough pilot-plant scale to demonstrate its commercial advantages over other systems which have been used or proposed. I t may be conducted continuously in equipment t h a t may probably be constructed of ordinary steel with satisfactory life due to the protective action of the kerosene always present. The product is a finely divided anhydrous crystalline material with free-flowing properties due to the rounded surfaces. I t has more rapid chemical reactivity than, and some other advantages over, present commercial forms. The heat cost indicated is only one third t h a t required by the usual fusion process, and the time in the continuous process is minutes rather than days. The removal of the residual kerosene from the caustic is readily accomplished.
T
HE manufacture of sodium hydroxide or caustic soda is one of the largest of the heavy chemical industries. I n 1937 about 900,000 tons were sold, with a total sales value of approximately $32,000,000 ( 2 ) . Sodium hydroxide is made in dilute solution by either of two methods. The first and oldest is to react soda ash with lime. This is the so-called chemical process. The second is to electrolyze salt brine solution. Weak caustic liquor from either the chemical process or the cells (electrolysis) is concentrated in multiple-effect evaporators, generally with three effects. The concentration of caustic liquor in the last effect is brought to about 45' Bi.., and this may be further increased in special single-effect evaporat'ors. I n order to prepare solid anhydrous caustic soda from this approximately 50 per cent solution of sodium hydroxide (often as high as 70 per cent), the clarified caustic liquor is drawn to caustic pots. These are large open hemispherical pots, 10 feet or more in diameter and about 2 inches thick, and made of a special dense gray cast iron. They are supported on a carefully laid brick setting and heated by direct fire, care being taken in the laying of the brick that the tlamedoes not play upon the pot. The caustic liquor is then 4
Present address, .4utoxyyen, Inc., S e w York. N .
nater were diatilled over into
finally decided to use a kerosene with a boiling range of np-
Y.
154
l 280
l
l
l
I
I
I
/
FIGURE1. BOILINGPOINTOF CONCENTRATED C.4U6TIC SODA SOLUTION6
INDUSTRIAL AND ENGINEERING CHEMISTRY
FEBRUARY, 1940
proximately 356" to 482" 17. This material satisfies the condition of nonreactivity with 'the caustic soda (although the unsaturates in t h r fresh kerosene did react to discolor the caustic, repeated use of the kerosene gave a white caustic). This kerosene also has a wide enough range of boiling points and consequently a wide enough range of vapor pressures to satisfy the conditions of the process. There is a large change in boiling point (or vapor pressure) between 50 per cent solutions and anhydrous caustic (Figure 1) which may be attributed to normal boiling point elevation and the fact that sodium hydroxide forms hydrates. As a matter of fact, all the hydrates, from the septahydrate to the monohydrate, can be crystallized from solutions of varying concentration. It is necessary to remove the water of hydration and supply what chemical heat this requires as well as to remove the water of solution. This wide variance in the vapor pressure of water over caustic solutions of changing concentration presents a problem in the selection of a diluent. If one is chosen which will have a vapor pressure such that it will evaporate with a substantial amount of water a t the initial 50 per cent concentration (say a ratio of 1 part of diluent to 1 of water), this same compound will be so volatile that it will evaporate with only a minute amount of water when the concentration of caustic is 90 per cent. This, obviously, will require a large expenditure of heat for removing the last water because i t will be necessary to evaporate large quantities of diluent per unit of water. If, on the other hand, a pure compound is chosen which will exert such a low relative vapor pressure that it will evaporate with a sufficient amount of water a t high concentrations, it will exert practically no vapor pressure a t the lovier concentrations of caustic and consequently will degenerate the process practically to a normal evaporation. I n the selection of a substance such as kerosene, a combination of the desired properties is obtained. There is a mixture of many compounds in kerosene which are able t o exert suitable vapor pressures over the entire range required, as the progressive evaporation of water proceeds.
Experimental Procedure Many experimental runs were made, but only a few, representative of the different) operations studied, will be described here. The first, experiment A , \vas a duplication of the laboratory method of Kokatnur. Five-hundred grams of a 50 per cent caustic solution and I500 cc. of Bayol D-1 (kerosene with a boiling range of 410-527' F.) were charged into a 1-gallon lagged nickel pot fitted with a condenser. The pot was then heated with a Bunsen burner until the first drop of liquid came over at 284' F. Thereafter, the time and quantity of kerosene and of water were measured at 10-minute intervals: Time .Win. 0
10 20 30
3 60 70
so
90
Pot Temp. O F. 284 302 320 338 365 392 415 433 446 473
Kerosene CC.
...
60 90 75 110 145 120 215 185 205
Water
cc. .. 45 55 35 40 35 24 20 14 2
Vol. Ratio, Kerosene-Water
..
1.3 1.6 2.1 2.9 4.1 5.0 9.8 13.2 102.5
Two hundred and seventy cc. of water were collected in 93 minutes, or 0.384 pound per hour. More water was removed than was added; this indicated that some water was present in the solid caustic charged. Two hundred and ten cc. of oil were filtered on a Biichner funnel from the caustic sludge. Filtration was easy, and the small crystals of sodium hydroxide were washed free of kerosene with petroleum ether. The second run, B , was made in the apparatus shown in Figure 2. A column with a side arm was fastened in the vapor
15
outlet of the same nickel pot and packed with laboratory Lessing rings. The vapors were condensed, and the mixture of kerosene and water was separated in the glass decanter shown in Figure 2. The bottom layer (water) was siphoned off and collected continuously, and the top layer (kerosene) was run back into the column and returned to the pot:
Time
Min. 10 15 20 25 30 35 a
-Temp.-Pot Column a
F.
302 306 313 320 333 336 347 35s
Watera
F.
Cc.
183 210 211 212 212 212 212 212
..
,.
..
..
.. ..
25 43
Time .Win. 40 50 60 70 75 SO 90 115
-Temp.Pot Column F. F. 368 212 386 212 401 210 419 210 419 210 419 210 425 212 454 212
The decanter was being filled for the first 25 minutes.
aster ratio was not measured.
Water4
.
cc 60 85 110 125 140 150 175 200
The kerosene-
Pilot Plant A pilot plant (Figure 3) was next built to allow for quantitative measurements and continuous operation. It was built for continuous operation with a packed column. The vapor pressure of water is greater over a 50 per cent solution than over any higher concentration; therefore at this concentration the greatest amount of water was present in the distillate. Thus if 50 per cent n caustic solution is fed into the top of the column, the vapors entering t h e condenser should be in equilibrium with that solution and contain a minimum amount of kerosene. At the bottom of the column, n-here the highest con cen t r a tion i s reached, the kerosene in the vapors would be greatest; and as the vapor progresses up the column, i t comes in contact with t h e p r o g r e s s i v e 1y more dilute solution of caustic having a higher a n d higher v a p o r p r e s s u r e of mater. I n this FIGURE 2. SEMICONTINUOUS DEmanner a succeedHYDRATION APPARATUS ingly larger quantity of water would be picked up until a t the top of the column the ratio of kerosene to water would be at a minimum. POT. A 30-gallon still pot fabricated from 20 per cent nickelclad steel was available. It had an all-iron clip valve for the bottom outlet, a mercury manometer connection, a nickel propeller-type agitator powered by a 0.75-horsepower motor and two 5000-watt nickel-sheathed immersion heaters. One heater was connected through a 4000-watt water-cooled Cenco rheostat, an ammeter, and a voltmeter to a double-throw switch giving either 208 or 110 volts. The other heater was connected directly to the 208-volt line giving 5200 watts.
156
I ~ D C S T l i I A LASD EKGIKEERING CHESTISTRY S T I L LH E A D . The column was a 3-inch nickel tube, 6 feet long, with flanges and steel backing rings top and bottom and a nickel inlet pipe 6 inches from the top for feeding cauitic solution and r e t u i ning k e r o f e n e . The column vas packed n i t h Diecea ithrouah :/,-inch, 'on 17,inch screen) supplied by the Carhorundum Company. Carborundum is inert to caustic, and is efficient for rectification-i. e., a good vapor-liquid c o n t a c t o r -but has a low throughput. Both column a n d pot were covered with 1.5 inches of 85 per cent magnesia insulation. A vertical shella n d - t u b e con-
CAUSTIC STORAGE
THERMO. WELL
ROSENE RETURN
an outlet near the top for kerosene and a pipe extending to the bottom for decanting water were used. CAUSTICFEED. A 20-gallon iron pot with a bottom outlet was connected so as to discharge to the line returning kerosene from the decant>erto the top of the column, A feed-measuring apparatus was devised which would also revent the formation of sodium carbonate in the valve or oc! controlling the flow from this tank. This was a graduated glass tube discharging to a rubber tube fi,ted with a pinch clamp. The valved inlet line from the supply tank and a drying bulb were introduced through a rubber stopper at the top. The stopcock was set and the pinch clamp opened to allov caustic solution t,o feed into the column. The pinch clamp was then closed, and the time needed to collect a definite amount of caustic solution in the graduated tube recorded. The sodium hydroxide drying bulb was necessary to remove carbon dioxide and water from the air drawn into the tube. An open discharge of the kerosene from the decanter was desirable in order to allow measurement of kerosene flow b? means of a graduate and stop watch.
MATERIALS.Bayol D-1. the kerosene of the laboratory experiments, was too expensive to use in larger quantities so Texaco Crystallite was employed. The distillation data on Crystallite kerosene are as follows: C'C.
Dietd. 0 10
20 30 40 50
B; P.,
F.
356 386 396 407 415 42i
(~C.
Ilistcl.
:1
P.
F.
60 486 70 448 80 464 90 486 98 522 3Iean B . P. 431' F.
;iccording to Perry (j),the average molecular weight corresponds to a 13-carbon hydrocarbon and is 170, the average latent heat is 100 B. t. u. per pound, and the \ a p o r pressure may be obtained from the Cox chait. Since the latent heat is important, i t n a s also experimentally determined for an average sample of the kerosene which came over wit11 water during an actual run. The determined latent heat of the kerosene x a s 96 R.t. u. per pound.
VOL. 32. NO. 2
This checks the above figure fairly well, arid the higher value was used. The physical properties of the kerosene are thus: Boiling range, a F. Average boiling point, F. .$yerage mol. weight Specific gravity Latent heat, R . t . u./Ib.
336-522 43 1 170 0.8150 100
A~P~ALYSIS OF CAUSTICSODA. The caustic soda dissolved to give t,he solution to be concentrated was technical flake caustic, manufactured by the Hooker Electrochemical Company and having a determined analysis of 95.6 per cent sodium hydroxide and 2.3 per cent sodium carbonate. The balance was probably salt, water, and other impurities. A method of analysis had to be devised to determine whether the material from the still was entirely anhydrous. The kerosene could be filtered off and the caustic washed with a solvent and dried, but t'here were two objections: ( a ) There could be no assurance that all the solvent was dried off; (b) the caustic would probably pick up water and carbon dioxide from the air. Consequently it was analyzed in the presence of kerosene : Approximately 20 grams of caustic, wet with kerosene, were dissolved in a 100-cc. volumetric flask whose stem was calibrated to 0.1 cc. The sample \vas then dissolved in enough water t,o bring the interface up to exactly 100 cc., and the amount of kerosene was measured. An aliquot portion of the water layer was analyzed for caustic and carbonate. Any difference in the total caustic present may be assumed t o be water. For samples of solid cawtic containing very small amounts of kerosene, this was extracted in a Yoxhlet apparatus and dried in a stream of air, free of water and carbon dioxide. The loss in weight indicated the amount of kerosene. ESPERIMESTAL WORKW I T H PILOT PLAXT. After preliminary runs the still was charged with 42 pounds of caustic soda, 24.8 pounds of water, and 18 gallons of kerosene, and heated to give a batch dehydration (Table I). Readings were taken every 10 minutes over a 10-hour period. I n the last half hour only 10 cc. of water were withdrawn from the decant,er. The caustic was analyzed and contained about 0.3 per cent water. This could be considered anhydrous since 0.3 per cent i$ probably within t'he limits of experimental error.
TAB LE I. BATCH DEHYDRATIOX lime
--Temp.Tank
Mill.
F.
0 "0 40 60 80 100
120 140 160 180 200 220 240 260 280 :300 320 340 360 a 80 400 420 440 460 480 ,500 ,520 240
.>60
010
294 329 338 344 349 3.54 360 36H 372 380 38.5 390 394 39i 40 1 40 1 40 1 40 I 401 403 40J 403 40.5 403 do.-> 407 118 428 428 128
IN THE
Ilermene
Top 0 F. 210 210
208 208 208 208 "08
208 208 208 239 280 288 294 302
PILOTPLAXT
Water
Kerosene-Water Vol. Ratio,
Pc p e r min 36 32 24 2 .j 22 20 2.j 23 22
3i 33 36..; 31
31 2 8 , .5 27 24 24
...
1.1 0 97 0.65 0.80 0.71 0.70 0.94 0.95 0,92
302 :302
302 302 302 309 302 302 302 302 302 302 311 356 374
I1 ~~
45 45 42 ._
88
11 10. .i
in R
4 . ;3 4.1 4.3 4.2 4 8
The advantages of operating a plant continuously are selfe\ itlent. In the batch run it was evident from the keroseneTyater ratio that the heat requirements for this process of partial-pressure evaporation were low by comparison wlth present practice, I t seemed possible that a continuous operation would reduce the heat requirements even further. From the batch run it was evident that the pot temperature should he kept a t about 423" F . in order to ensure anhydrous caustic. .Ifter several shorter runs caustic soda \Vas dehydrated continuously during a 17-hour run nhich was long enough to give data indicative of a true continuous operation. The data are shown in Table 11. The weight ratio of kerosene and water v-as continuou4y maintained a t approximately 0.7 part of kerosene per 1 part of water TABLE 11. Time .Win. 0
i.. n 20 30 40 .5 0 60
i0 80 90 100 120 140 160 180 200 230 260 290 320 350 380 410 440 470
.,no
,530 .?GO 590 620 650
D.4TA O S A C:ONTIXTJOUS 0 P E R . I T I O S
Tank Temp." F. 41.7 415 41,j 415 415 415 416 417 41,
Kerosene
---PI.. i
7 7
7
8
30% Caustic Water Soh. per minute----
....
+
z6
Vol. Ratlo
KeroseneWaterb
11 11
11 11 10
41i
417 418 418 418 419 420 421 421 421 421 42 1 420 42 1 421 421 421 42 1 421 421 421 421
1.57
INDUSTRIAL AND ESGINEERING CHEMISTRY
FEBRUARY, 1940
1 1 1 1 1
01 0 0 0 3
1 1 1 1
1 1 0 1
i n 8 8 8 8 8 8 8 8 8 8 8
8 8 8 8 8 8 8 8 8
8 8 8 8
8 8
8 2 8 , i 3 10 19 IO
9 8 i 9 9 9 9 q
9
10
to 10 10 10
12 12 12 12 12 12 12
IS
12 1"
1 1 1 1 1 1
0 0 0 0 0 0 0 96 0 90
1 0 0 0 0 0
1 8 5 8 88 9 0 88 0 88 0.88 0.88 0 88 n 88
a T h e temperature a t the t o p of t h e colunin "as 211' F. i n every case. h I n order t o obtain the weight ratio of kerosene t o water, nlultlpl?. t h e volume ratio b y 0.8150.
EFFICIESCY OF CoLnrv. An experiment waa performed upon the packed colurnn to determine its rectification efficiency when using isopropanol and water. The &foot column was found to be equivalent to 1.8 theoretical plate5 or an H. E. T. P. (height equivalent to a theoretical plate) of 3.1 inches for isopropanol and water. This indicated that a relatively small nurnbei of theoretical plates for the caustic soda-keroqene systeni had accomplished the necessary yapor-liquid contacting to give the desired result. BUBBLE-CAP C o L n r s . Several attempt. to use a brorize bubble-cap column TTere made: although there was no plugging, the efficiency was not so good as with the packed column. This was probably due to the fact that the heaters were too low in capacity to operate the 10-inch coluiiin a t a high enough vapor velocity to keep the layers of caustic and kerosene from separating on the plates.
Centrifuging and Solvent Recower! KEROSENEREMOVAL. I t is obvious that to produce a commercial product the hydrocarbon must be removed cotiipletely from the anhydrous caustic unless the uses, quch as
those in the petroleum and some other industries, would not lie affected by a trace of kerosene. I n order to do this the kerosene would have to be filtered or centrifuged from the caustic; then the kerosene retained on the crystals would be washed with a lowboiling solvent, and the caustic finally dried. It was found necessary to have a t least 2 pounds of kerosene per pound af caustic in order to obtain a workable sludge. Less kerosene gave an uneven cake and a sludge which would probably lie difficult to handle. Nore kerosene woiild be objectionable, not only from the standpoint of the larger volumes of material to be handled, but also because the caustic, which is so much more dense than the kerosene, set'tles out rapidly and thus prevents the use of a workable thin slurry. It was found possible in a 10-inch centrifugal to reduce the kerosene retained to 1.8 per cent in 22 minutes, and it is believed t'hat even lower retentions are possible in a longer cycle with the greater force of an industrial machine. Experiments were made on washing in :t centrifuge which indicated that about 100 pounds of solvent, such as a light naphtha cut,,mould be ample to wash a ton of caustic down t o a kerosene retention of the order of 0.1 per cent. All of the kerosene from the centrifuge prior to washing would return directly to the system, probably through a heat exchanger using the hot caustic sludge as a heating medium. The wash from a ton of caustic would be a mixture of less than 36 pounds (1.8 per cent x 2000) of kerosene in 100 pounds of solvent. Obviously an enclosed centrifuge designed to minimize solvent losses would have to be used in industrial operation. SOLVENTRECOVERY. .liter the resitlual kerosene is washed from the cake, the solvent-kerosene mixture will be separated in a fractionating column. I n order to ascertain the ease of this operation, the vapor composition curve was determined by the Othmer method (4);it showed that the separation requires only a few plates and will be economical in heat requirements. These operations constitut'e the sequence necessary in plant production. For convenience in studying their interrelation, a flow sheet is given in Figure 4.
Physical Characteristics of Caustic. Although no identification of a definite crystal class is possible when samples of the caustic prodricetl are examined under a microscope, the fact. that polarization colors are in evidence proves the existence of a crystalline structure. The crystals are imperfectly foriiiecl. -1photograph of t,hese crystals a t 200 magnification is shown in Figure 5 . A screen analysis of the crystals was rnatle as rapidly as possible, since the caustic is very hygroscopic. The material on the S o . 16 screen seemed to be an agglomeration of the caustic cryst>alsclue to moisture: n-I.
Screen S o . 6 12 16 30
::
yo
Retained 0 0 . .i 1 , .i
i.0 16.0
li.0
io
wt., yc Retained 37.0
I00 140
18.0 8.0
"00
3.0
T h r o u g h 200
2.0
Screen h-o,
White crystals were obtained in runs in the sniall nickel pot; but a slightly gray product came from the pilot plant runs because of the impurities in kerosene which re-use would have eliminated, particularly those from the rubberized
INDUSTRIAL AND ENGINEERING CHEMISTRY
158
returning from the decanter are within 10" F. of the vapor leaving the top of the column (210" F.); ( b ) as shown in the continuous runs, 0.7 pound of kerosene is distilled per pound of water; (c) 2 pounds of kerosene leave the bottom with 1 pound of caustic; ( d ) 80 per cent of the heat of the kerosene leaving in the sludge may be recovered by heat interchanging; and ( e ) the specific heat of solid caustic is 0.5 and of kerosene 0.54; then the sensible heat requirements per pound of sodium hydroxide handled are :
DEHYDRATING
Top:
CENTRIFUGE
DRYER
ANHYDROUS CAUSTIC
FIGURE 4. PROPOSED FLOW SHEET 1. Solvent 2. Caustic soda solution
VOL. 32, NO. 2
3. 4.
Kerosene Solvent a n d kerosene
gaskets, packings, etc., employed in the assembly. Because of their rounded edges these crystals flow readily. So far as is known. this is the first time a finely divided crystalline caiistic in an anhydrous form has been prepared.
Heat Requirements The total heat input is made up of the following: 1. Sensible heat for raising the water and kerosene vaporized to the boiling point of the mixture, and the caustic and kerosene discharged to the pot or discharge temperature 2. The heat of solution or chemical heat necessary to disengage (vater and caustic 3. The latent heat of the kerosene and water evaporated 4. Heat losses or radiation from the unit Calculations haye been made to indicate, in so far as possible, the quantitative distribution of the heat supplied. The run of interest' from a possible industrial-scale operation is the continuous run shown in Table 11, anti the heat calculations for that run are reproduced here. It is somewhat difficult to work with the data on heats of solution of caustic soda and water (the equivalent of the chemical heat above mentioned, but its opposite in sign) and also with the data on the specific heat of solutions and of solid caustic because of incompleteness and disagreement among various writers. It is believetl, however, that these calculations may be made sufficiently close for most engineering purposes. CHEMICAL HEAT. Assume the specific heat of sodium hydroxide to be 0.5 throughout the range. Assume the average temperature a t which the chemical heat of solution is added to be 375" F.: Heat content above 32" F. of 1 lb. NaOH at 375" F. (0.5 X 343 X 1) = 172 B. t. u. Heat content above 32" F. of 1 lb. water at 375" F. (from steam table) = 348B. t. u. Sensible heat of 2 lb. 50% NaOH = 520 B. t. u. From the Solvay Company's booklet
(e),
Heat content of 2 lb. 50% NaOH soln. at 375" F. (2 X 358) = 716 B. t. u. Heat of solution = 716 - 520 = 196 B. t. u. for 2 lb. 50% soln. or 1 lb. solid NaOH SENSIBLEAND LATEKTHEATS. If we assume in a commercial operation: (a) The caustic feed and the kerosene
Kerosene (0.7) (10) (0.54) = 3.8 B. t. u. = 5.0 B. t. u. Caustic (1) (10) (0.5) Water (1) (10) (1) = 10.0 B. t. u. Total sensible heat for top = 18.8B.t . u.
Bottom: Caustic (1) (423 - 210) (0.5) = 106.5 B. t. U. Kerosene (2) (423 - 210) (20%) (0.54) = 46.0 B. t. u. Total sensible heat for bottom = 152.5 B. t . u. = __ 18.8 B. t. u. Total sensible heat for top Total sensible heat = 171.3 B. t. u. The latent heat requirements are as follows: For 1 lb. water a t 210" F. = 971 X 1 = 971 B . t . u . For 0.7 Ib. kerosene at 210' F. = 100 X 0.7 = 2 B . t. u. 1041 B. t. u. The t,otal heat for dehydration is:
+
Total heat per lb. NaOH produced = 196 171.3 1408.3B. t. u. Per ton = 1408.3 X 2000 = 2,816,600 B. t. u.
+ 1041 =
To calculate t,he solvent recovery per ton of caustic, assume a reflux ratio of 1 to 1, 100 pounds of naphtha (mean boiling point, 250" F.), and 36 pounds of kerosene. Then the sensible heat is: Kaphtha = (100) (0.5) (250 - 70) = 9,000 B. t . u. Kerosene = (36) (0.54) (431 - 70) = 7,000B. t. u. 16,000 B. t . u.
.It 1:1 reflux there are 200 pounds naphtha vaporized; and, at a latent heat, of 110 B. t. u. per pound, this amounts to: (200) (110) = 22,000 B. t. U . Total = 38,000 B. t. u. -Issume that drying is a t 170" F. and the qolvent to be evaporated is 36 pounds per ton: Caustic Solvent Latent heat of solvent
=
= =
(2000)(0.5)(170 - 70) (36)(0.5)(170- 70)
=
100,000 B. t. U. 1,800 B. t. U.
(36)(110) = 3,960 B. t . u. Total heat for drying = 105,760 B. t. u.
The total heat requirements, then, per ton of caustic are: Dehydration Solvent recovery
2,816,600 B. t. u. 38,000 B. t. u. Drying = 105,760 B. t. U. 2,960,360 B. t. u. u. +23% radiation = ~639,640 _ B. t. _ Total = 3,600,000 B. t. u. = =
It is evident and has been experimentally shown that the amount of heat required by this process t o dehydrate a stronger caustic, such as 70 per cent, is almost directly proportional to the amount of water to be removed. I n the case of 70 per cent caustic there is only three sevenths as much water to be removed as in the case of 50 per cent caustic; there would thus be required 1,600,000 B. t. u. per ton for finishing caustic from 70 per cent solution. DISCCSSIO?;.Runs were made with the dehydration unit to determine the actual heat losses when no caustic was present but with the temperatures maintained approximately
FEBRUARI-, 1940
INDUSTRIAL AND ENGINEERIKG CHErtlISTRY
the same as those of a regular run. Thus a figure for the amount of radiation from this rather poorly insulated experimental unit was o b t a i n d ; and when this value was subtracted from the total heat input as shown by the electrical readings, the amount of useful heat (i. e., latent, sensible, and chemical) was obtained. These determinations gave values in accord with those above. But it was felt that the more accurate method of determining useful heat is from these calculated values because the useful heat determined was the difference of two large quantities: and one of these, the radiation value, was known to be only approximate. I n plant practice it is possible to hold radiation down to 20 per cent from equipment of this sort, and these figures are believed to be representative of possible operating conditions.
Materials of Construction The choice of materials of construction is important because of the effect upon capital investment; some qualitative observations have been made which may indicate the desired materials. The dehydrating still is the unit most likely to be subjected to chemical action of caustic. Other units handle solid caustic, with or without kerosene; this probably justifies the specification for steel or iron as a material of construction for the centrifuge, dryer, and solvent recovery still although nickel-clad steel might be used for the first two. The dehydrating still had a nickel surface wherever caustic was present except for the iron discharge valve, and no attack was noticeable on nickel or iron. When the packed column was replaced by the bubble-cap plates, made of cart aluminum bronze, there \vas still no corrosion. Except for a slight pitting of one of the caps on the top plate (in contact with 50 per cent liquid caustic), there was no sign of corrosion other than a brightening of the metal surface. The iron pipe connecting the pot to the column was scrubbed clean but no
FIGURE5 . PHOTOUICROGRlPH O F C.4USTIC i x 200)
SODA
CRYSTALS
corrosion was evident. These facts were noted after approximately 50 hours of operation and 250 hours of contact with caustic in the presence of kerosene. During one of the frequent dismantlings of the column, a n iron nail was inserted on each plate. After 40 hours of operation these nails showed practically no effect of the caustic. This seems to indicate that iron or steel could be used for the dehydrating still. Cold 50 per cent caustic is usually handled in iron and steel equipment. Corrosion of iron in fusion pots is usually attributed not so much to the high concentrations of caustic as to the high temperatures involved. It has been stated that the major part of the corrosion on the iron pots takes place when the last few per cent of water are being removed-that
159
is, a t the highest temperature. I n the case of the dehydrating still the maximum temperature (423" F.) is so much lower than the final temperature in the pots (900-1000" F.) that this temperature effect is greatly reduced. Also, kerosene in this process must have a n inhibitory effect on the attack of metals; otherwise the bronze would have been badly attacked. Since kerosene is in excess, the caustic soda droplets are suspended in a kerosene medium, and the kerosene a t all times forms a film or layer as a protective coating for the metal and consequently prevents contact with the caustic. More conclusive data are necessary before a full-scale plant can be built completely of steel, but the observations are sufficient to warrant the construction of a steel semiplant.
Comparison w i t h Present Practice It is almost impossible for one outside the industry to break down accurately the cost factors in dehydration of' caustic. The only definite fact is that there is a differential in price between 50 per cent liquid and fused caustic of $7 per. ton a t the plant; this figure probably does not represent the true cost of processing. However, various scattered facts have been culled from the literature, men in the field, etc. HEATR E Q c I R E m N T s . One source estimates the amount of coal now used for fusion as a t least 1000 pounds per ton of anhydrous caustic, produced from 50 per cent solution. Another source gives the requirements for dehydrating a ton of caustic from 70 per cent solution as about 400 pounds. Angel (1) calculates a value of 5,000,000 B. t. u. per ton from 50 per cent liquid as a minimum which actually has never been approached. With the partial pressure evaporation process, some 350 pounds of coal would be necessary per ton of caustic dehydrated from 50 per cent solution, assuming usual boiler efficiencies, or about one third the coal requirements of the caustic pots. As mentioned above, the heat requirements of' the partialpressure evaporation are approximately proportional to the amount of water to be removed; and when 70 per cent is to be handled, there is thus approximately three sevenths as much water as when 50 per cent is to be handled. On this basis there ail1 be required about three sevenths as much coal or 150 pounds per ton of caustic soda if 70 per cent rather than 50 per cent caustic is to be finished. This amount of coal is also not far from one third of the amount required in cdehydrating 70 per cent caustic by other methods. The temperature in the still pot (423" F.) indicates that if steam is used as the heating medium, i t would have to be under a t least 425 pounds per square inch pressure; and the use of a high-temperature heating medium such as Dowtherm might be considered more economical in most plants. There is also the possibility of using a partial vacuum on the tlehydrating still which not only reduces the boiling temperature to one within the usual steam range, but improves the kerosene-water ratio and thus further reduces the heat requirements. PURITY. All the samples analyzed showed less than 1 per cent water, and most of them had less than 0.4 per cent. There seems to be no doubt that for commercial purposes a n anhydrous caustic is produced; and the caustic produced should contain neither more nor less impurities than the 50 per cent solution from which i t is made. The only other source of impurity is the kerosene, and repeated use gives a white caustic. The retention of kerosene on the caustic has been
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shown to be no more than 0.8 per cent and is probably of the order of 0.1 per cent; this would be reduced even more in a plant where efficient washing devices could be used. Samples show no charring when placed in a flame and thus indicate a lack of kerosene on the caustic. I n producing fused caustic in the pots, some purification IS undoubtedly affected by the sulfur treatment, but there are conflicting opinions as to whether the final product has more or less impurities than the original 50 per cent liquid. I n any case, about 5 per cent of the batch in a caustic fusion pot contains a high percentage of iron and has a dark color. These caustic bottoms must be reworked or sold at a low price; with this crystalline caustic no such dregs are obtained. PHYSIC.4L CHARBCTERISTICS. Anhydrous caustic soda is marketed in the following forms-fused, flakes, pellet, powder, and in one case as cubes. The various special forms are for isolated uses and usually command a premium above the base price for fused. Fused caustic is difficult to handle and hard to dissolve and usually entails high handling costs. Flake dissolves more easily but commands n higher price. Pellets are used for laboratory and analytical purposes. The caustic produced by this process is a fine free-flowing crystalline material. Its apparent density is approximately the same as flake caustic and consequently the packaging costs would be about the same. It could be shipped in a Dry-Flow tank car because of its fluidity, or in a standard tank car from which it could be dissolved at the buyer's plant. By virtue of its crystalline structure and fine state of division, this caustic dissolves rapidly, and consequently handling costs for the consumer will be lower. Other workers using this caustic report that it reacts more rapidly in alkali fusions,
\ OL. 32.
so. 2
saponifications, and other similar reactions, and that it has a higher melting point, which indicate possibly a different physical constitution. I t would be ideal for use as a laboratory reagent.
Conclusions Anhydrous caustic soda was prepared continuously from 50 per cent solution on a small pilot-plant scale utilizing the partial-pressure eraporatiori procsss of Kokatnur. Kerosene was used as a diluent in the evaporation: and that remaining on the solid particles was removed by cenh-ifuging and washing with a solvent. This solvent was then purified by distillation for re-use. The total heat cost wax calculated a t about 330 pounds of coal per ton of caustic from 50 per cent solution (only about 150 pounds from 70 per cent solution) or one third the present cost in the usual caustic fusion. Because of the protective film act'iori of the kerosene, practically no corrosion of equipment was noted and it appears that production equipment might be fabricated of steel. The material produced was of high purity; it was a fine powder which was of a crystalline nature, free flowing, and apparently of more rapid chemical reactivity because of its greater surface area.
Literature Cited Angel, Gosta, Chem. & M e t . End.. 34, 683 (1927). A4nonymous,Ibid., 46, 108 (1939). Kokatnur, V. R., patents pending. Othmer, D. F., IND.ESG. CHEM.,Anal. Ed.. 4, 232 (1932). Perry, J. H., Chemical Engineers' Handbook, pp. 1111, 1116, 1117, Kew York, McGraw-Hill Book Co., 1934. Solvay Co., Bull. 6 (1938).
THIODIGLYCOL Unit Process and Operations Involved in Its Synthesis from Ethylene Oxide and Hydrogen Sulfide DONALD F. OTHMER AND DONALD Q. KERX
T
HIODIGLYCOL was prepared first by hIeyer (2) in 1885 b y a somewhat expensive method, which has generally been used since in its manufacture as an intermediate in the production of mustard gas (2,2'-dichlorodiethyl sulfide). Cheaper methods of preparation might introduce new and more peaceful uses for this material which contains the potentially important carbon-sulfur-carbon linkage. Chichibabin (1) synthesized thiodiglycol from ethylene oxide and hydrogen sulfide by the sealed-tube method; Kenitzescu and Scarlatescu (3) combined these gases in molecular proportions to form the product continuously in yields of 90 per cent, but under what appear to have been conditions far below the optimum: 2 (CHz)zO
+ HzS +(HOCHz.CH2)zS
The reaction is thus a condensation of two cheap and readily available gases; i t gives immediately, quantitatively, and irreversibly a pure liquid of high boiling point (168" C. a t 14 mm.). Nenitzescu and Scarlatescu assumed that the reaction took place in two steps, that either activated carbon or a
Polytechnic Institute of Brooklyn, N. Y.
previously prepared amount of thiodiglycol was necessary as a catalyst for the reaction, and that the reaction was essentially accomplished in the gas phase. Preliminary work indicated that all of these assumptions were wrong; it appeared that the reaction was conducted most readily in the liquid phase of previously produced thiodiglycol. If the reaction occurs in the liquid phase, it is obvious that a first step must be the absorption of the two gases by the liquid, and this part of the mechanism might be understood by applying the methods of Whitman (6) and others. The chemical reaction in the liquid then might be studied as a second step by the application of well established kinetic methods involving the familiar equations for the reactions of different order, the equation of Arrhenius, etc. (see, for example, such standard works as Taylor and Hinshelwood, 5 ) . A simple method of studying the rate of reaction of two gases is the reading of the change of their total pressure in a closed reaction chamber. Since the volume is constant, the total amount of gases a t any instant is directly measured by the pressure. This simple reading of pressure a t various
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