T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
684
T h e kelp char made in t h e oil-jacketed retort did not prove as satisfactory as those originally prepared a t higher temperatures. I n t h e specimen distillation log t h a t follows, t h e column named “middle temperature,” indicates t h e temperature a t t h e center of the kelp charge, while “maximum temperature” refers t o t h e oil b a t h surrounding the retort. DISTILLATIONO F KELP Log of Run 7, May 17, 1918 CONDITIONS: Retort Dreviouslv heated to 220’ C. Stronelv heated a t first Middle Max. Total Time Temp. Temp. Distillate c. cc. REMARKS A.IM. c. 8:20 140 182 , Charged retort 175 Distillation commences but 8:35 104 soon stops 102 Minimum temperature 8:45 187 114 9:oo 211 2 15 118 Distillation recommences 9.02 ~~. 134 23 1 9:15 36.5 148 9.30 97 Clear, green 244 158 9:45 177.5 260 170 190.5 273 Becoming turbid. Fraction 1o:oo 104°-1700, 497 g. Flaky precipitate 285 191 10:15 180 184.5 10:30 191 296 183 10:45 201 305 213 319 188 1 1 :oo Fraction 170°-2130, 745 g. 225 333 185 11:15 340 173 23 1 11 :30 345 166 Reduced flame’ 11 :45 247 Tar coming over. Fraction 350 144 12:oo 256 213’-256’, 668 g.
-_
.. ...
... ...
P.M.
2:15 2:30 2:45
267 273 281 289 295 300 304 308 310 311 313
352 353 354 354 355 355 352 350 346 345 350
162 ’ 119 136 115 109 87 73 59 48 43 43.5
3:oo 3:30
317 320
352
34
12:15 12:30 12:45 1 :oo 1:15 1 :30 1:45 2:oo
...
Tar, 19 g. Water fraction 530 g. Tar, 50 g.
256O-295‘,
Tar, 55 g. T a r , 29 g. Shut off gas. Water fraction, including drip next morning, 373 g.
...
Tar, 71 g. Total weight of tar, 224. g. (1.4 per cent) 1 There are two kinds of tar oil: the first t h a t comes over is dark brown with a penetrating though faint odor resembling a n essential oil or certain ketones; the second is blacker and has a more unpleasant odor.
............... .. ................................ ............. ........... ..................... ........ ..................... ANALYSES OF WATER CUTS Fractios Degrees 104-170 170-213 2 13-256 256-295 295-320
Specific Gravity 1 .007 1.013 1.032 1 .os0 1.063
27.5 lbs. 107.6 lbs. 90.0 lbs 17.6 lbs. 9 . 9 lbs. 12,500 g. 3,042 g.
....
(24.3%) (11.7%)
LOG 7 Solids in 10 Cc. Grams 0.1699 0.2687 0.9159 1.5961 2.4833
SUMMARY
Sixteen distillations of dried kelp were made in a n oil-jacketed wood retort at temperatures not exceeding 320’ C. These yielded aqueous liquor, t a r , and a noninflammable gas. The residual charcoal was insufficiently heated either for a good extraction of potash or for use as a filtering medium. This work has demonstrated t h e necessity for distilling kelp a t a much higher temperature.
No. 7
By L. F. Hawley and H. N. Calderwood, Jr. FOREST PRODUCTS LABORATORY, U. S . FOREST SERVICE, MADISON, WISCONSIN Received February 21, 1920
Sometime ago t h e authors carried on efficiency studies a t several wood-distillation plants with t h e purpose of preparing a standard set of directions for t h e use of t h e refinery foremen and stillmen. T h e results of t h e work on t h e operation of t h e t a r still were of special interest. Since these results are directly applicable t o t h e tar-still operations of many other wood-distillation plants, i t was decided t o publish them. The operation carried out in t h e t a r still is for t h e purpose of recovering t h e acetic acid t h a t remains in t h e settled t a r after t h e pyroligneous acid has been removed. I n general, t h e method consists of heating t h e t a r by means of steam in closed coils until t h e water is driven off, and then, while keeping t h e closed coils still in use, blowing live steam through t h e charge.* It is obvious t h a t t h e most desirable conditions for running a t a r still are those under which t h e most acetic acid may be produced in t h e shortest time a n d with t h e greatest degree of concentration. The best conditions for this distillation with respect t o speed, length of time, and pressure of steam in t h e closed. coils, have never been determined, and t h e commercial practice varies greatly. E F F E C T OF S P E E D OF DISTILLATION
Two test runs were made with a charge of j I 0 gal. of t a r in a depth of 40 in. T h e conditions were t h e same in both runs, except t h a t the speed a t which t h e live steam was blown through t h e t a r was different. T h e results are shown in Tables I and 11. The tables TABLE I-SLOW
(64%) (36%)
12,
TAR-STILL OPERATION IN HARDWOOD DISTILLATION PLANTS
RESULTSO F R U N ? Weight of kelp distilled.. Weight of basket and kelp after heating. %re. Residual kelp (non-volatile) Loss by heating (total volatile) Weight of charge.. Total weight of liquid distillate.. Gaseous distillate..
Vot.
Time Start 12 : 50 1. . ., . ns
2 3 4 5 6 7 8 8 9 9 10 10
: 05
: 05 : 05 : 05 : 05 : 05 : 05 : 35 : 05 : 35 : 05 : 35
Total Speed Distillate Gal. per Hr. Gallons 40 10 .. 30 40 65 25 89 24 112 23 26 138 24 162 185 23 196 22 207 22 219 24 225 12 23 1 12 ~~
SPEED
In Frac-
tion Total Pressure Pounds Pounds in Coils 17.1 17.1 45 4.3 48.1 65.2 .. 19.35 27.6 92.8 46 13.31 112.0 41 9.68 19.2 124.0 43 6.25 12.0 134.0 43 4.62 10.0 35 3.00 139.9 5.9 42 144.0 2.12 4.1 40 145.8 2.08 1.8 40 1.87 147.4 1.6 41 149.0 1 . 6 . 1.69 45 1.61 149.8 0.8 45 1.52 150.6 0.8 Residual tar 0.28 per cent acid
Acid Per cent 20.6
show t h a t t h e speed has a slight effect on t h e concentration, t h a t is, t h a t a certain number of gallons of water in t h e form of steam going through t h e charge of t a r will remove more acetic acid when t h e steam 1 There is no mysterious effect of “steam distil1ation”jn this separation of acetic acid from tar. The steam is superheated all the time i t is in contact with the tar and acts like any neutral gas in its effect on the distillation. In fact, as far as the distillation is concerned, the same fractionation of acid and oil would be obtained by direct fire heat in a distillation without steam. The separation of acid from oil actually takes place after t h e condensation of the vapors when the acid is divided between the water and oil according t o its relative solubility in these two substances. This indicates the desirability of washing certain fractions of the oil with water to recover a further quantity of acetic acid.
July, 1920
T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING CHEMISTRY TABLE 11-FAST SPEED
Time Start 2 2 3 3 4
: 25 : 40 : 20 : 40 !
10
7 8 8 9
: : : :
40 10 40 10
Speed Total In Fraction Gal. Distillate Acid per Hr. Gallons Per cent Pounds 75 50 26 54 51 70 60 50 95 118 140 163 185 210 235 259 48 273 48 285 24 297 24
..
Total Pressure Pounds in Coils 45 ... 44 35.8 47 78.1 47 90.6 49 104.0 43 113.0 40 118.6 .. 123.6 43 127.2 38 46 130.8 134.1 44 137.1 47 46 139.2 141.0 46 46 142.6 per cent Emid
goes through slowly t h a n when t h e steam goes through quickly. A slow speed of distillation is, therefore, desirable from t h e standpoint of high concentration of distillate, a n d t h e speed should be kept as low as is consistent with t h e capacity of t h e t a r stills. Sometimes it might even be desirable t o build a n additional still i n order t o make slower distillation possible. E F F E C T O F STEAM PRESSURE
T h e effect of different pressures of steam in t h e closed coils was not determined in t h e commercial still, b u t a laboratory experiment was carried out t h a t showed this effect. Two charges of t h e same t a r were distilled with steam. T h e t a r was kept a t a temperature corresponding i n one case t o a Io-lb. steam pressure, i n t h e other case t o a jo-lb. steam pressure. T h e results of t h e two distillations are shown in Table 111. -15 Fraction No. Cc. 1 . _ . . . . 66 2 ...... 71 3 . . . . . . 78 4 . . . . . . 73 5. . . . . . 77 6 , . . . . . 81
TABLE111-EFFECT OF STEAM PRESSURE Lbs. Pressure----50 Lbs. Pressure--. Acid Acid in Per Fraction Total cent Grams Acid 3.5 2.3 2.3 2.8 2.0 4.3 2.0 1.6 5.9 1.6 1.2 7.1 1.1 0.8 7.9 0.8 0.7 8.6
Cc. 58 74 69 71 73 70
Acid Acid in Per Fraction cent Grams 6.8 3.9 5.1 3.8 2.5 1.7 1.3 0.9 0.3 0.45 0.25 0.2
Total Acid 3.9 7.7 9.4 10.3 10.6 10.8
It is plainly t o be seen from these results t h a t more acetic acid can be obtained with less steam when t h e t a r is kept a t a higher temperature, t h a t is, when t h e pressure i n t h e closed coils is kept higher. For instance, at a 50-lb. pressure four fractions, with a total volume of 2 7 2 cc., contain 10.3 g. of acid, b u t , a t a q - l b . pressure six fractions, with a total volume of 446 cc., contain only 8.6 g. of acid. END-POIST
OF DISTILLATION
After i t is known t h a t a slow speed a n d high steam pressure are desirable conditions for t a r distillation, t o insure a n efficient operation one other matter must be determined-the point a t which t o stop t h e operation. It may be seen from all three tables t h a t t h e concentration of acid i n t h e distillate decreases as t h e distillation progresses;’ i t finally reaches a point a t which t h e concentration is so small t h a t i t is no longer 1 There are two reasons for this: First, t h e proportion of acetic acid in t h e oil distilling over decreases as the distillation progresses, since t h e boiling point of the acid is lower than t h a t of the oil; and second, the proportion of oil to water decreases, since a t a constant temperature the vapor pressure of t h e residual t a r decreases as t h e lower boiling oils are removed and more water vapor is required t o carry over the same amount of oil.
68 5
profitable t o recover acetic acid from t h e distillate. T h e proper degree of concentration a t which t o s t o p t h e distillation is determined, on t h e one hand, b y t h e price of acetate, and, on t h e other, by t h e cost of t h e necessary fuel, labor, and apparatus. This computation must be made for each plant according t o local costs for these items, t h e main cost being for t h e fuel required t o make t h e steam for removing t h e acid from t h e t a r and for evaporating t h e acetate of lime solution. After t h e proper end-point for stopping t h e distillation has been decided upon, i t is necessary t o find a. method for determining when this point has been reached. Probably most t a r stills are operated on a time schedule. If t h e proper lengths of time are determined for reaching t h e end-point under certain cons t a n t conditions of distillation, and if those constant conditions could be maintained in subsequent runs, this method of operating t h e still would be satisfactory. However, variations in t h e original t a r are unavoidable, and accurate regulation of speed and pressure are very difficult t o maintain, so t h a t t h e end-point cannot readily be ascertained by t h e length of time of distillation. T h e end-point is most accurately determined by a n actual chemical analysis by titration of t h e acid contained in t h e distillate. This analysis may readily be made by t h e stillman if he is furnished with standard alkali a n d a graduated cylinder or pipette. I n some plants t h e dissolved t a r left in t h e copper still after t h e distillation of t h e crude pyroligneous acid is mixed with t h e settled t a r and distilled in t h e t a r still; in other plants t h e copper still has sufficient capacity so t h a t t h e dissolved t a r may be distilled by steam without removal for t h e recovery of t h e acetic acid. I n t h e latter case t h e same suggestions as t o speed, pressure, and end-point apply t o t h e dissolved t a r in t h e copper still as t o t h e settled t a r in t h e t a r still. DIRECTIONS F O R TAR-STILL
OPERATION
The following set of directions for running t h e t a r still a t one plant is given as a n example: ( I ) Distil with closed coils a t steam pressure of IO lbs. a t the start, increasing to 50 lbs., as rapidly as possible without boiling over, until very little distillate is coming over. ( 2 ) With steam in closed coils remaining a t jo lbs. or higher, if readily possible, blow in live steam a t a speed of 30 to 3j gal. per hour when the charge is 40 in. deep (equivalent to jro gal.), or a t a correspondingly different speed when the charge is difTerent. Continue this speed until 9.1cc. of distillate are required t o neutralize I O cc. of 0.5 N alkali. (3) Reduce the flow of live steam t o a t least one-half, and continue the distillation as long as not more than 9.4 cc. of distillate are required.
Such directions will, of course, vary with different plants a n d with varying conditions a t t h e same plant. With t h e information offered i n this paper, combined with a knowledge of local conditions, i t should not be difficult t o prepare a similar set of directions t h a t would help t o prevent loss of product on t h e one h a n d a n d loss of efficiency on t h e other.
