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INDUSTRIAL A N D ENGINEERING CHEMISTRY
point, the separation was incomplete and the sulfide contaniinated. After removing the impurities it was necessary to resume the concentration of the sodium sulfide, carrying the temperature to about 222’ C. to produce the 62 per cent fused sodium sulfide of commerce. The economy of the practices here described in refinery operation is dependent upon a great many factors, not the
Vol. 18, No. 7
least of which is freight charges to possible consumers of the products; but a point not to be overlooked is the reduction in sewage pollution of small streams or larger bodies of water near city water supplies. The amount of cresylic acid of the grade described in Table 11, which may be recovered by the methods here outlined, is in the neighborhood of 0.00678 per cent by volume of the pressure-still distillate treated.
Absorption and Desorption of Ammonia in a CokePacked Column’ By T. K . SherwoodZand A. J. Kilgore MASSACHUSETTS INSTITUTE OB TECHNOLOGY, CAMBRIDGE, MASS.
HE important diffusion Wide possibilities are apData are presented on the absorption and desorption processes, in which a parent in the development of of ammonia gas in a column packed with 9 to 16 mm. the relationships between the gas or volatile material coke. The capacity coefficient of the column increased mechanisms and rates of abdiffuses to or from a liquid, from 1.45 to 4.39 grams ammonia diffusing per hour sorption and rectification, for are absorption, desorption, per cubic centimeter per atmosphere partial pressure d i f f e r e n t liquid-vapor sysr e c t i f i c a t i o n , and steam difference, while the gas velocity increased from 12.1 to tems, under various condi‘(stripping.” Absorption in41.3 kg. per minute per square meter of total cross sections. The development of volves the diffusion of the gas tion of the column. The liquor velocity was maintained s u c h r e l a t i o n s h i p s would or volatile material from the constant a t 26.3 kg. per minute per square meter of make available large amounts gaseous to the liquid phase, total column section. The average gas temperature of data on the performance of and may be accompanied by was approximately 32’ C., and the liquor temperature absorption apparatus for use t h e d i f f u s i o n of a small averages 21 to 25 a C. in the design of rectifying amount of vapor of the solA t the same gas and liquor velocities, the capacity vent, either from gas to liquid columns, and vice versa. coefficient of the column for absorption was found to be As a first step towards the or from liquid to gas. Dethe same as for desorption, within experimental error. establishment of these relasorption is the name given tionships under special conto the reverse process, the evolution of a gas or volatile material from a solution, which ditions, the experiments described belbw have been carried may also be accompanied by the diffusion of a small amount out, to determine the relation between the rates of diffusion of vapor of the solvent, to or from the liquid. Rectification of ammonia gas to and from a dilute aqueous ammonia soluinvolves the diffusion of both volatile material and vapor of tion in a coke-packed column. the solvent to and from the solution. More of the less Experimental volatile material diffuses from the gas to the liquid than from liquid to gas, and a greater amount of the more volatile maThe apparatus (Figure 1) consisted of an iron pipe, inside terial diffuses from liquid to gas than the reverse, so the net diameter packed for 107 of its length with 9effect is the diffusion of relatively large amounts of less vola- to 16-mm. household coke. The liquor feed, which was tile from the gas to liquid, and the evolution from the liquid Tvater in the absorption runs and dilute ammonia solution in of the heat equivalent of the more volatile. Stripping with the desorption runs, was fed at a rate to the top steam is a special case of rectification, in which the original of the columnand was distributed Over the packing by means gas contains none of the volatile material to be removed from of a tin cup, The rate of feed was maintained the liquid. constant in all runs a t 216 grams per minute, corresponding The similarity of the processes of absorption and to 26.3 kg. per minute per square meter of total cross-sectional tion has been pointed out before, but little attention has been of the column (5.39 lbs. per minute per square foot). given the possibility of using data on absorption and ret- The solution leaving the column was collected in one of two tification interchangeably in the design of absorption and ret- 5-gallon bottles, the gas space of each bottle being connected tifying columns. to the column by tubing in order to equalize the pressures in Received March 2, 1926. the bottles and column. Research Associate, Department of Chemical Engineering, MassaAir was supplied by a positive pressure blower, and measchusetts Institute of Technology, Cambridge. Mass.
T
O
1
Table I-Absorption PARTIAL PRESSURE Temp. of NHJ PRESSURE IN TOWERliquid Total NHs NHs Input Ingas Ingas AtbotAt atbottom difin gas output NHs NHa leaving entering tom top of tower leaving “a, G./1-in. fusing C. Run bottoms G./min. G./min. G./min. G./min. Mm. Hg Mm. Hg Mm. Hg Mm. Hg 7.78 26.4 799.2 790.4 25.5 6.65 22.35 4.82 1.9s 6.80 1 7 8 5 . 1 2 6 .7 8.32 29.0 794,O 6.75 22.76 4.90 1.92 6.82 2 8.32 31.25 788,O 783.0 28.0 6.65 1.60 6.50 22.72 4.90 3 7 8 2 . 1 2 8 .8 9 . 3 9 3 7 . 9 7 7 8 . 2 6 . 5 5 5.12 1.62 6.74 23.70 4 27.4 8.78 42,35 785.0 781.0 6.55 5.44 1.35 6.79 25.20 5 36.2 772.0 769.0 27.7 5.08 4.74 4.76 18.80 4.10 0.66 6 2.57 57.0 768,O 768.0 28.6 4.85 4.54 0.21 4,75 21.25 7 5.08 40.9 771.0 769.0 4.74 28.0 4.30 0.62 4.92 19.75 8 25.16 787.0 781.0 25.5 6.45 6.54 1.41 5.93 21.20 4.52 9 6.45 32.8 779.0 775.0 26.6 6.22 22.90 5.00 1.14 6.14 10 6.54 24.7 790.0 780.0 26.8 6.72 1.62 6.21 21.05 4.59 11
Air velocity Gas velocity Partial Lbs./rnin./ pressure K /min. over Ka fiq.m. Ka sq. f t . Lbs./hr./ total bottoms G./hr./ total area Mm. Hg cc./atmos. area cu.ft./atmos. 201 8.45 18.6 3.22 41.2 182 7.64 19.8 2.92 37.3 32.6 161 6.68 19.7 2.58 137 5.63 22.6 2.20 27.5 132 5.05 23.1 2.12 24.6 125 4.11 2.00 20.1 17.0 102 2.47 1.64 12.1 20.1 18.2 1.89 17.9 118 3.66 7.65 37.4 215 17.9 3.45 185 5.80 20.1 2.97 28.4 229 8.19 3.68 40.0 18.2
INDUSTRIAL A S D ENGINEERING CHEMISTRY
July, 1926
Run 12 13 14 15 16 17 18
NH3 G./l-in. bottoms 6.90 7.60 9.45 11.55 12.95 14.85 16.65
NHs diffusing G./min. 2.50 2.35 1.94 2.37 2.06 1,64 1.25
NH3 in gas leaving G./min. 2.41 2.31 2.02 2.25 2.06 1.68 1I27
Partial Pressure pressure a t top NH3in gas of tower leaving Mm. Hg Mm. Hg 776.8 10.1 773.3 10.2 774.9 10.8 771.6 12.7 769.4 13.1 767.8 14.1 770.3 15.35
Table 11-Desorption Temp. of liquid a t bottom NH3 of tower G./l-in. O C . feed 23.4 18.3 23.0 18.3 22.0 18.3 20.3 22.35 19.6 22.35 19.4 22.35 19.3 22.35
ured by means of a, 1.03-em. standard sharp-edged orifice. I n the absorption runs ammonia was supplied to the air line at the point indicated in Figure 1, from a cylinder of commercial anhydrous ammonia, and measured by a small calibrated glass flowmeter. I n the desorption runs the air, heated by the blower, was blown into the bottom of the column and served to strip the ammonia from the liquor, flowing down over the coke. A sample of the gas leaving the top of thc column was drawn off continuously throughout the duration of the run, and analyzed for ammonia by absorption in standard sulfuric acid. The gas sample for analysis was drawn successively through two wash bottles containing standard sulfuric acid, and through a wet gas flowmeter! the latter serving to measure the gas residue after the ammonia had been absorbed. The ammonia was determined by titrating the excess acid in the wash-bottle solutions with standard alkali. The bottoms were analyzed for ammonia by titration with standard acid, the sample being taken from the total drainings of the tower, as collected over the period of the run. I n the desorption runs the liquor feed was similarly analyzed for ammonia by titration with standard acid. Mercury thermometers were used to measure the temperatures of air and ammonia before mixing, the gas leaving the top of the column, the feed, and the liquor draining from the bottom. Pressures were observed by means of manometers placed a t the air and ammonia orifices, and a t points in the tower immediately above and below the coke. At these last two points piezometer rings were fitted to the column. The drainings from the column were collected and measured in glass bottles, calibrated in pounds of dilute ammonia solution. Results
The data collected are summarized in Tables I and 11,which include the calculated constant Ka. The calculated values of K a remesent the capacity coefficients of the columnAunderthe given conditions, in terms of the grams of ammonia diffusing per hour, per cubic centimeter
V
Temp. feed O C .
22 22 22 23 23 23 23
745
Air velocity Air velocity Partial presKg./min./ Ka Lbs./min. sure NHs Ka sq. m. Lbs./hr sq. f t . over feed Gm./hr./cc./ total cu. ft./ total Mm. Ha atmos. area atmos. area 12.8 3.42 39.1 213 8.00 12.8 3.13 34.8 195 7.14 12.8 2.63 30.4 164 6.25 16.8 2.21 27.7 138 5.67 16.8 1.96 24.8 122 5.08 16.8 1.65 103 3.82 18.7 14.4 16.8 1.45 90.4 2.94
of packed column, per atmosphere average difference between the partial pressures of ammonia in the gas and over the solution. The calculation of these values of Ka for both absorption and desorption runs offers the simplest basis for comparison of the data from the two sets of experiments. As a sample calculation, the value of Ka for run 12 is found as follows: Ammonia diffusing from liquor t o gas phase, based on liquor analyses = 2.50 X 60 = 150.0 grams per hour Volume of packed column = 10 22 X 0 785 X 107 = 8770 cc. Difference between partial pressures of ammonia a t the top lo = 0.00356 atmosphere3 of the column = (12 760 Difference between the partial pressures of ammonia a t the 5.2-0 bottom of t h e column = = 0.00685 atmosphere 760 Logarithmic mean difference in partial pressures in t h e gas and over the liquid = O ' 00s85-0~00355 = 0 ,0050 atmosphere 0.00685 2..71og (grams NHI per hour) 150 = 3.42 (cc.) (atmosphere) K a = 0.0050 X 8 1 7 0
'-
'
-
om
The accuracy of the experiments may be judged in a general way by the ammonia balances, as calculated from the analyses and rates of flow of the feed, bottoms, and gas entering and leaving the column. I n the absorption runs the ammonia in the entering gas should correspond to the ammonia in the bottoms plus that in the gas leaving the column. I n the desorption runs the ammonia in the feed should equal the ammonia in the exit gas plus t h a t in the bottoms. Tables 8 Equilibrium data from SherTOURNAL. 17. 1745 (1925). wood. THIS
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CONC PWKLD
a , Figure 1-Coke-Packed
Absorption Tower.
Layout of Apparatus
INDUSTRIAL A N D ENGINEERING CHEMISTRY
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I and I1 show that these balances check within 4 per cent except in run 9, where the discrepancy is about 10 per cent. I n each case the ammonia diffusing, as calculated from the rates of flow and analyses of the liquors, has been used in the calculation of the values of Ka, as these figures are thought to be more reliable than those based on the analyses and rates of flow of the gases. The calculated values of the constant Ka are shown in Figure 2, plotted against the mass velocity of the air through the column. This plot shows clearly that Ka for both absorption and desorption increases with gas velocity, and that a t a given air velocity the values of K a for absorption and desorption are, within experimental error, the same. The temperatures in the column were similar in the two sets of experiments, the average gas temperature being approximately 32" C. in most of the runs; while the average liquor temperature was 24" to 25" C. in the absorption runs and 21" to 22" C. in the desorption runs.
rate of diffusion of the gas. 'Decreasing the size of packing increases the area for absorption, but decreases the linear velocity of the liquor over the surface of the packing. When the packing is unsymmetrical, as in the case of crushed materials, small packing may be expected to retain more liquor in suspension in the column and hence to have more inactive area covered by stagnant liquor. Peters6 found that the efficiency of packed rectifying columns in general increased as the size of packing decreased. However, probably the most important factor in comparing the coke and quartz packings is the nature of the surface, as the liquor layers will be thicker and slower-moving on the rougher surface. The effects of the nature of the surface and size of packings on the rate of diffusion of gas will be largely affected by the ratio of the liquid film resistance to the total resistance to the diffusion of the gas. Whitman and Davis6 have recently reported tests on the absorption and also the rectification of carbon dioxide gas in a 15-plate bubble-cap column. I n the first case the carbon dioxide was absorbed from flue gas, using sodium carbonate solution. I n the second case the column was used for rectifying a carbon dioxidewater vapor mixture from the lye. The carbon dioxide-water vapor mixture was supplied from a reboiler, in which the carbon dioxide was boiled off from the lye in which it was originally absorbed. By extrapolation of the absorption data to the conditions of the rectifying tests, they were able to predict a value of Ka for the latter case, and found that the actual values of Ka for rectification were 2.4 times the predicted values. This they explain by the difference in the nature of the gas films in the two cases, as the inert gas mixed with the carbon dioxide in the flue gas was not present in the rectification tests. Thus their data check qualitatively the result of the tests with the coke column, that the rates of absorption and desorption are the same. The resistance to diffusion of the carbon dioxide offered by the liquid film in the lye is small compared with the resistance of the gas film, and this resistance of the liquid film is minimized by the type of apparatus employed-in which the diffusion takes place from a gas bubble rising through a liquid. Thus, in the bubble-cap column the resistance of the gas film is the controlling resistance to the diffusion of the carbon dioxide, and the over-all rate is sensitive to changes in the nature of the gas film. The difference found in the rates of absorption and rectification is probably much greater than would exist in cases of less soluble gases, especially where the resistance of the gas film, instead of that of the liquid film is minimized by the type of apparatus employed.
S DESORPTION
GAS VELOCITY
Figure 2
The upper dotted line in Figure 2 represents the results of Kowalke, Hougen, and Watson4 for the absorption of ammonia from a n air-ammonia mixture in a column packed with 3.18 to 4.45 cm. crushed quartz. The gas temperature was 27' C. and the rate of liquor flow 26.3 kg. per minute per meter. The conditions were therefore comparable in the two cases and the curves show the capacity constant Ka a t a given gas velocity to be considerably greater in the case of the quartz packing. A change in the size of packing has several effects on the 4
VOl. 18, No. 7
Chem. Met. Eng., 83, 443 (1925).
5
THIS JOURNAL, 16,402 (1923).
6
Ibrd., 18, 264 (1926).
Symposium on Raw Rubber, Philadelphia, Pa., September 6 to 10, 1926 At the Philadelphia meeting of the AMERICAN CHEMICAL SOCIETY, September 6 t o 10, 1926, the Rubber Division is t o hold a symposium on Raw Rubber. The discussion will include botanical, chemical, technical, and engineering problems, and t o deal properly with all these experts of international reputation have been assigned t o discuss the various aspects of the subject. The basic idea of the symposium is t o present new facts and ideas and not to review work already known and information which is common knowledge or which can readily be obtained elsewhere. I n other words, the speakers will confine themselves t o new information and data, which they will present in detail and discuss critically. The promise of new facts and original ideas from leading authorities indicates the importance of the symposium t o all rubber chemists and technologists. Eminent authorities have already promised t o discuss each of the following topics and arrangements are nearing completion for other prominent speakers in other problems of raw rubber.
I-Scientific developments in the plantation rubber industry 2-Wild rubbers, South American and African 3-Guayule rubber, its botany, chemistry, and technology 4-Synthetic or artificial rubber %Alternative materials for rubber 6-Rubber as a national asset 7-The physical structure of rubber 8-The examination and testiw of rubber 9-A comparison of the physical properties of raw rubber with thoqe of compounded vulcanized rubber
There is also t o be a banquet on Thursday evening, September 9, at which some of the leading executives of the American rubber industry are t o be present. The scientific interest in the subject is unquestioned, and since the general public has already become impressed with the importance of rubber, the symposium will undoubtedly make a wide appeal.
