I S D r S T R I A L A S D EXGINEERIlYG CHEMISTRY
March, 1929
separated 2.5 cm. (1 inch) was also carried out. The same mass velocity per gram of platinum was employed in all cases. Table I-Experimental Data (Preheat temperature, 470' C.; 9 to 10 per cent ammonia)
GAUZE ARRANGEMENT
0.636 liter per sq. cm. 2 1 . 6 cu. ft. per sq. ft.
1 , 6 1 0 liters per sq. cm. 5 2 . 4 cu. ft. per sq. ft.
2 . 4 4 liters per sq. cm. 80.0 cu. ft. per sq. it.
No. Effitests ciency
No. Effitests ciency
No. Effitests ciency
R Single Double close contact 1 . 3 cm. (0.5 inch) separated 2.5 cm. (1 inch) separated
70
70
3.51 liters per sq. cm. 1 1 4 . 5 cu. ft. per sq. ft.
I KO. Effi-
tests ciency
223
catalyst owing to the greater temperature stability of the ammonia-gas mixture oxidized by the second gauze. Apparently reactions 2, 3, and 4 tend to offset any advantage gained by speeding up reaction through increased gas temperature. The accurate measurement of the gas temperature between the gauze layers is a very difficult task and hence an estimate of this temperature was not made. In each instance where a double flat gauze was employed, the highest efficiencies were secured with a gas-flow rate of about 1.6 liters per square centimeter per minute (52 cubic feet per square foot) of total gauze in both layers.
70
5
90.2
8
90.0
6
89 7
3
81.9
Conclusions
s
91.5
7
95.5
7
94.3
6
91.1
14
90.6
5
95.3
l3 94
12
90.7
9
95.0
12
1--8 multi-layer platinum gauze catalyst has a greater capacity at a higher efficiency than a single gauze, irrespective of whether the layers are in direct contact or not.
93 2
91.3 6
90
o
Results
The quantitative results of the laboratory tests were not entirely those anticipated. K i t h a preheated gas mixture, the results of the three series of tests using two gauzes are essentially the same for a given ammonia content irrespective of the arrangement of the gauzes. It was exTected that with high gas-flow rates on a separated gauze the capacity and efficiency should be higher than for the close layer type of
capacities. 3-An intake gas flow of approximately 1.6 liters per square centimeter per minute (52 cubic feet per square foot) and preheated to 470" C. gives the highest oxidation efficiencies, irrespective of whether the layers are in close contact or not. This gives a catalyst contact time of approximately 0.00019 second.
A Complete Sludge-Washing Plant in a Single Unit' A. Anable THEDORRCOMPANY, 247
PARK
HE countercurrent principle is well known in chemical
T
engineering practice, particularly in connection with the washing of precipitates free from contaminating solutions or, when the solution is the material of value, for recovering the valuable solution from the residue or precipitate. Advanced forms of the application of mechanical means to the simplification of washing are countercurrent decantation plants, in which the material to be washed is continuously settled from weaker and weaker solutions in a series of continuous mechanical thickeners. The advantages of continuous washing plants are well known and need no elaboration. Suffice it to say that continuous countercurrent washing in conjunction with continuous agitation has shown marked advantages over older batch methods for the production of caustic soda, sulfate of alumina, phosphoric acid, barium sulfide, and blanc fixe, to mention only a few of the better known applications. Countercurrent Decantation in Caustic Soda Manufacture
The conventional type of plant of this sort is shown in flow sheet form in Figure 1, this particular flow sheet representing the manufacture of caustic soda by the lime-soda process. Causticizing is carried out on a continuous basis in three agitators, arranged in series and equipped with steam coils so that the milk of lime and soda ash may be kept a t an elevated temperature during the entire time of agitation required for effective causticisation. I n the first thickener the clear, hot, and strong caustic solution overflows continuously to evaporation or other further treatment, while the settled lime mud is moved to the center and pumped t o No. 2 thickener, and so on through the remaining thickeners, in each of which it 1
Received September 25, 1928.
4VE., N E W
YORK,
N. Y,
comes in contact with weaker and weaker solutions. As a result this lime is progressively impoverished in dissolved values and is eventually discharged from the last thickener virtually free from caustic solution. The wash water added in the last thickener cascades as an overflow product from one thickener to the next, always moving in a direction countercurrent to that of the solids. It finally overflows No. 2 thickener and is used for making up fresh milk of lime, after which it reenters the system via the agitators. Caustic soda is widely used in amounts from 1 ton u p s t plants manufacturing gases; producing phenol, creosote, mineral oils, etc.; reclaiming rubber from old stock; and making artificial silk, to mention only a few cases. Some of these companies purchase caustic in drums or tank cars, while others causticize soda ash with !ime by the batch method without taking any particular pains to secure a high chemical recovery through efficient sludge-washing. The accompanying table lists some of these operations which require a small amount of caustic soda and gives the use of the caustic in each case. I
MATERIALMANUFACTURED Gases, such as helium for dirigible balloons, oxygen for welding and hydrogen and nitrogen for manufacture of synthetic ammonia Phenol, creosote, mineral oils and tar oils Soda and sulfate pulp Reclaimed rubber Artificial silk
USEOF CAUSTIC SODA Circulated in gas scrubbers for absorption of carbon dioxide Purification of tar acid Digestion of wood chips Old rubber stock digested with caustic for destruction of fiber Various neutralization operations
Washing Thickener
T o meet the requirements. of such cases, where floor space is a t a premium but where sludge-washing is essential in order
224
INDUSTRIAL ALVDE,VGISEERI.VG CHE.IIISTRY
Vol. 21, s o . 3
to maintain a satisfactorily high recovery of chemical, a connected one with the other by means of seals in the t,rays. washing thickener has been developed and thoroughly demon- These seals consist of an upcast stationary hoot surrounding strated at several plants. In this washing thickener the the circular opening in the tray centers and a downcast re strong liquor is continuously decanted and the sludge washed volving cup attached to the thickener shaft. The sludge in a siugle unit and with only one tank instead of the four settled in the top compartment is raked to the center and, or five that usually are required. Although the tank is rela- when it builds up t o the correct depth, flows wider the edge tively high, since it is subdivided into several superimposed of the cup through the seal, is diluted with weak liquor, and settling coinpartmeritc, each one functioning 8% a thickener then flows down into the second compartment. Coming in i n itself, nevertheless a very great saving iii floor space is contact with wesker and weaker solutions, the sludge continsecured, as the diameter of the tank is the same as that of n ues by gravity through the rest, of the comparbrnents until it single tank used i n the usual foiir- or five-thickener series. is fin;ally removed from the bottom of the tank. I t is to he Thc washing thickener shown i n Figure 2 is a tray-type uoted that the sludge moves downward at all times from the tliickener, hut iri it the settling compariments forined by the top to the hott.om compartmeiit arid that there is in each superirnposed trays 0perat.e in series, not in parallel. When compartment a sufficiently deep bed of sludge to seal effecthe tray-type thickerier was introduced several gears :&go, t,lie tively one compartment from another so that the solutions various settling compartments operated in parallel, tlros ill- in the various compartments cannot mix. creasing the capilcit,? per unit of floor zpnce i n proportim tc Witb the exception of the strong solution whicli overflows the iiuinlier of trays added. for each additioiial ~:onip~rtmr~iitdirect from the top eompart,ment, tiif. solutions pass upward from one compartment to another in pipe lilies provided with I overflow-regulating boxes near the top of the tank. It will be appreciated that the column of solution in each of t.hese vcrtical overflow pipes is supported by a column in the thickener tank consisting of both solutiori and sludge. For this reason the overflow columns rise to differentheights in the regulating boxes, the lowest compartmeiit's ovcrflow column rising the highest, the second from the top the lowest, and intermediate onPs ranging themselves uniforinlp hetween these estremes. Siiiee iuash wa.t,er is added in tho lowest comport,ment and since the overflow from this is used as >\-ashin t,licnext compart.ment above and so on, this difference in elevation in overflow columns makes it possible far the solutioii to flow upward, countercurrently with respect to the sludge, hy gravity, or, t,o put it more accurately, by superelevation. - . . ~. If we take a specific case in which the coiiceiitration of solution in S o . 1 eompnrtment is 20" B6.; S o . 2, 7" R6.; P i w r r 2-Washlne-Type Thickener Xo. 3, 2" B6.: and No. 4, 0.6' I%., it will he easy t,o trace formed in this uiamier acted substantially tlie same as a through the operation of such a washing installation. When separate thickener. With the mechanicxi details of the tray the system is in equilibrium, the column of 0.6" 136, overflow type of thielcerier well estalilished, it has been a relatively from compartment 4 supports a slightly taller column of the simple matter to reroute the solution and sludge so that by fresh water being added as a wash in compartment 4. As series operation of the compartments the complete n-nshiiig fresh water is added in the weir box, the water column is operation can he completed within R single machine. increased in height, which upsets the equilibrium and causesthe I n this washing thickener the feed ent.ers the uppermost 0.6" Be. solution in the No. 4 overflow pipe t o rise and overflow compartment through a semi-submerged eylindricsl well, into the control box. This solution thereupon passes down as the strong solution oreflows into a peripheral launder a t the a wash into compartment 3, displacing an equal volume of top of the tank, and the washed sludge is pumped from the Z 0 BP. clear. solution from this compartment, which, after bottom of the lowest compartment, The compartments are rising and being collected in the control box, enters No. 2 as
I
~
IXDUSTRIAL A-J'D ENGINEERING CHEMISTRY
March, 1929
a wash, eventually overflowing as a 7" BB. solution for further use as required. Owing to the introduction of the wash solutions in the cups of the seals, the sludge in each is repulped with a solution weaker than that from which i t was settled, which is the distinguishing feature of countercurrent washing. Once the control box has been adjusted to provide for the proper superelevation of solutions between compartments, the machine is operated exactly as a single-compartment machine, although in effect it is equivalent to as many thickeners as it possesses compartments.
I
?
225
The mud discharged from the washing thickener is a waste product and is virtually free from caustic as it has been given three washes with an amount of water equivalent to that lost in the thickener discharge. Whatever slight loss of chemical occurs in the system is made up by the addition now and then of a small amount of soda ash. The layout may be materially simplified by using pulverized hydrated lime instead of quicklime. Hydrated lime may be fed directly into the agitators, thus eliminating the rotary lime slaker and the classifier. The gas-scrubbing operation is essentially a cyclic one in which the scrubbing agent is recirculated continuously in a closed system. In this system the small, compact recausticizirig plant operates as a booster, receiving as its feed spent liquor and returning to the system a recausticised solution entirely suited to the work in hand. Other Applications
90'C
AGITATORS
WASH WATER
EXCHANGER
I
1i
I
SYSTEM
U
Figure 3-Recausticizing and Gas-Washing Flow S h e e t Using Washing-Type Thickener
Obviously, the recausticizing flow sheet described herein is only one of the many possible applications of the washing thickener. In general, the field includes that of all non-corrosive pulps except those that are either so extremely light that they will not develop sufficient superelevation of solution in the overflow pipes, or else pulps which settle rapidly to a consistency a t which they no longer exhibit liquid, free-flowing tendencies. Test to Determine Washing Efficiency of Five-Compartm e n t Washing Thickener
A 4-day test has been made at a chemical plant where a washing thickener is used for separating a solution of alkaline iron cyanide salt from a precipitate and for washing this precipitate countercurrently for the removal of soluble salts. The feed to the washing thickener is at a dilution of 8 or 10 to 1, solution to solids by weight. The solution is subsequently evaporated, crystallized, and the crystals are used in the manufacture of dye blue. The washed precipitate, a mixture of calcium sulfate, calcium carbonate, and ferrous hydroxide, is of negligible value and is discarded to waste.
Application t o Recausticizing Operations
As stated previously, the washing-type thickener is especially adapted to relatively small recausticizing installations a t plants which use caustic soda solution for various neutralization operations, such as carbon dioxide absorption, cresylic acid production, etc. The flow sheet shown in Figure 3 represents a layout that has been developed for removing carbon dioxide from mixed gases in scrubbers. Burned lime is added to a rotary lime slaker by a mechanical feeder a t the rate required for causticizing the spent liquor from the absorption system, slaking being carried out with a portion of this spent liquor. The slurry from the rotary slaker is sent to a two-deck classifier, in which the grit, sand, and unburned core are settled, raked from the classifier and finally sprayed with wash water before being discarded as waste. The classifier overflow, a smooth, grit-free slurry, then joins the remainder of the spent liquor and passes through three reaction agitators arranged in series in which a detention period of 1 to 3 hours is provided in order to assure a high causticity of the resulting caustic soda solution. The discharge from the third agitator, a mixture of finely divided calcium carbonate and caustic solution, enters the washing thickener. The clear overflow from the top compartment passes through a heat exchanger on its way to the absorption system in order that it may be cooled before use as a scrubbing agent. The liquor discharged from the base of the scrubber, a mixture of caustic soda and soda ash, passes through the same heat exchanger in order that it may be raised to a high temperature before reentering the reaction agitators for recausticizing.
Feed Conditions TOTAL SOLUBLE SALT SALT Per cent Per cent 9.30 9.16 9.68 9.04 9.28 8.72 7.40 7.27
1 s t day 2nd day 3rd day 4th day
Averages
8.92
8.55
INSOLUBLE SALT Per cent 0.14 0.64 0.56 0.12 0.37
Lbs 49,640 413,100 462,740
Solids in feed (4-day total) Solution in feed (4-day total) Total feed
S t r e n g t h of S o l u t i o n in Various Washing C o m p a r t m e n t s (One set of samples only on 4th day) COMPARTMENT SALT Per cent 1 7.50 2 3.56 3
2.16
4 5
'
1.23 0.47
Discharge C o n d i t i o n s TOTAL SALTIN SOLUBLE SALTIN SOLN. DISCHARGED S O L N . DISCHARGED PUMPED WITH SLUDGE WITH SLUDGE Per cent Per cent Per cent 82.16 0.81 0.49 74.31 0.77 0.95 62.24 0.61 0.77 71.52 0.48 0.36 75.81 0.47 0.36 73.44 0.61 0.53 76.18 0.38 0.50 0.55 0.85 66.09 71.10 0.44 0.58 72.73 0.51 0.64 0.38 82.34 0.52
~ ~ O I S T U RIN E SLUDGE, AS
SAMPLB 1 2 3 4 5 6 7 S
9 10 11 Averages
74.2
0.653
Sludge dilution = 3 : l water t o solids by weight.
0.489
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
226
Calculations Feed Soluble salt in feed 462,740 X 0.0855 = 39,564 Ibs. 462,740 X 0.0892 = 41,276 Ibs. Total salt &feed Discharge Total solids in discharge 49,640 lbs. Total solution in discharge 49,640 X 3 = 148,920 lbs. Soluble salt lost in,discharge 148,920 X 0.005 = 745 Ibs. 148,920 X 0.00653 = Total salt lost in discharge 972 lbs. OverfLLw Soluble salt to evaporation and crystallization 39,564 745 = 38,819 Ibs. Total salt to evaporation and crystalllzation 41.276 972 = 40,304 lbs. Washing Ejiciency 38 819 On basis soluble salt 3~ X 100 = 98.12 per cent
--
On basis total salt
-X
100 = 97.65 per cent
The washing efficiencies cited above are admittedly not quite
VOl. 21, No. 3
so high as those that would be obtained in the standard fivethickener countercurrent decantation plant with sludge density regulation between each decantation step by means of diaphragm pumps, where washing efficiencies above 99 per cent are almost always secured. Yet a loss of a very small percentage in washing efficiency becomes of rather secondary importance when the governing factors are limited floor space and the consolidation of several washing steps in a single unit. The test results given above take on added significance when it is realized that 97.7 per cent of the soluble values are recovered in a feed of 8.4 gallons per minute by the use of about 7 gallons per minute of wash water, the entire washing taking place within a single closed tank which overflows a clear solution of a single strength to subsequent treatment and discharges a thoroughly washed waste product.
Nature and Constitution of Shellac I-Preliminary Investigation of the Action of Organic Solvents' Wm. Howlett Gardner2 and Willet F. Whitmore THESHELLAC RESEARCH BUREAUOF THE CNITED STATES SHELLAC IMPORTERS' ASSOCIATION, THe POLYTECHNIC INSTITUTE, BROOKLYN, N. Y.
H E L L A C has been
A qualitative study has been made of the solubility dye, e r y t h r o l a c c o i n , and of shellac in eighty-four organic solvents. This work odoriferous material present. known from the earliest time of recorded hip,clearly shows that the solution of shellac in these solThis soluble portion corntory, and it is difficult to devents is not simply a physical phenomena, but that posed about 7 per cent of the termine where it first became colloidal aspects are unmistakably present. Unqueslacs. They claim t o have commercially practical and in tionably there is a close connection between solubility been able to break down this and the chemical relationship between solvent and ether-soluble resin into %leucommon use. For centuries it has been found unequaled solute; and shellac seems to be most soluble in those ritic and monohydroxypalSolvents which approximate it in composition and mitic acids, C 1 5 H 2 8 ( O H ) 3 for numerous uses, but in structure. Some of the nitrogen bases are good solvents, C 0 O H a n d C15HSo (OH)spite of its great antiquity but the solvent action in this case seems to be a funcCOOH, respectively. They comparatively little is known tion of the basicity of the solvent rather than any also report the possible existof its ultimate nature and chemical relationship between the dispersed phase ence of dihydroxypalmitic constitution from the standand dispersion medium, especially since shellac is deacid, CllHZ9( 0H),C 0 0 H , point of c h e m i s t r y and physics. Although America cidedb' acidic. among the products. Gupta4 was unable to find these subhas been the largest consumer of shellac, this product has been the subject of little study stances, but reports that his extraction may not have been except by a few workers in Germany and Switzerland. The complete. He demonstrated the presence of palmitic acid in Shellac Research Bureau has therefore undertaken a study the decomposition products from some lacs. A detailed study of the resin insoluble in ether was made of its properties and constitution. This paper is the first of by Harries and Nagel.5 I n their work this resin was readily a series giving the results of this study. hydrolyzed, under the proper conditions, with 5 N aqueous Previous Investigations Dotassium hvdroxide in the cold. They isolated aleuritic From what has been published it appears that shellac is acid and another which they named shelloic acid, C ~ H W composed of 70 to 90 per cent resin admixed with other sub- (OH)2(COOH)2. Aleuritic acid they established to be hexastances. The resin apparently consists of complex linkages of decane-16, 9, lO-ol-l-acid,6 a trihydroxypalmitic acid. From a mixture of hydroxy-fatty acids containing 15 or 16 carbon the general behavior of these resin acids in forming under reduced pressure lactides resembling shellac, and their atJoms. Tschirch and his workers3 have found stick-lacs to consist method of obtaining same, they conclude that the original of a t least two distinct resins comprising 70 t o 80 per cent of shellac contained lactides of these and other yet unidentified the material, with varying mixtures of two dyes, sugars, water- resin acids.' They have also noted the curious effect that hydrogen soluble albumin, an odoriferous substance, several waxes, and also carbohydrate material derived from twigs and the chloride has upon this ether-insoluble resin,8 pointing out that body of the shellac insect, Trachardia lacca Kerr. Extrac- this reagent probably has some colloidal effect as well as tion of these stick-lacs with 10 liters of diethyl ether dissolved changing the state of aggregation of the shellac particles. one of these resins along with the small amount of yellow 1 Gupta, J . I n d i a n I n s f . Sci., 7, 142 (1924).
S
1 Received September 8, 1928. Presented before the Division of Paint and Varnish Chemistry at the 76th Meeting of the American Chemical Society, Swampscott, Mass., September 10 t o 14, 1928. This article is contribution No. 1 from the Shellac Research Bureau. 9 Research fellow. 8 Tschirch and Liidy, Hclu. Chim. Acta, 6, 994 (1923); Tschirch and Schaefer, Chem. Umschau Fettc, Oclc, Wachse, Harze, 81, 309 (1925).
Harries and Nagel, Ber., 66B,3822 (1922). Ibid., 60, 605 (1927); cf. Enderman, J . Franklin Inst., 164, 283 (1909); Bull. soc. chim., 5, 857 (1909). 7 Harries and Nagel, Chem. Umschau Fcfte, Oelc, Wachse, Harze, 31, 173 (1924); Wiss. Ver6fentlich. Siemens-Konzern, 3, 12 (1924). 8 Harries and Nagel, Kolloid-Z., 33, 247 (1923); Harries, Bcr., 668, 1048 (1923). 0
