FRED. D. 1 3 ~ 0 ~ 1 7 ~ OX FRACTIOSAL DISTILLATION. 4 9 V.-The Comnpnratixe Value of Uifereizt Xethods of Fyactiorznl Distillation. By FREDERICK D. BROWN E.Sc. WHERE fractional distillation is carried out on a large scale carious forms of appratus of a somewhat complicated character are generally employed ; though differing much in detail they are all clesigued to subject tlie mixed vapours to one or both of two well defined processes, which for the purposes of this paper may be termed respectively L V I L S ~ ~ ~ and cooZi?rg. In the process of washing the mixed vapours issuing from the still are made to pass through sevei-a1 layers of liquid obtained by their own partial condensation they are thus washed by these successive layers and it is supposed that the vapour of the liquid of highest boiling point is partially removed by this process which results there-fore in a distillate containing more of the liquid of lo\T-er boiling point than would be obtained by simple distillation.There is a t first sight however no reason t o suppose that a mixture of vapours should be materially altered in composition by a liquid having about the same composition and the same temperature as itself. It would appear that the first step iii the construction of these forms of apparatus resulted from an attempt to utilise the latent heat! of the vapour given off by a weak spirit to distil R stronger one ; the latent heat of the vapour of this second spirit might then be used t o distil a third and so on thus effecting a series of distillations in the same apparatus with the same fuel.I f this series of distillations actually took place that is to say if the ~apours rking from the still really voi. XXXTIl. E 5 '3 FRED. D. BROTT" THE COJIPARBTIVE VALUE OF condensed in the first layer of liquid thus causing that liquid to boil and emit vapour which condensed in the second layer causing that to boil in its turn the advantages of the process would be undeniable ; but since apparently the vapours only pass through the layers of liquid, and may therefore rcmnin nearly unaltered the real valne of this method of distillation can only be determined experimentally. In the process of c007i72y the mixed vaponrs are partially condensed, cither by allowing radiation to talic place o r by passing them through a coil kept at a given temperature ; the liquid of highest boiling point suffers of course the most condensation and runs back into the still, :t better distillate being t h u s obtained.Now a possible explanation of' the silccess of the fimt process i s that the successive layers of liquid, by obstructing the passage of the vnpour @re it more time to cool by radiation and that thus the t w o processes are really one and the same. With a \-iew t9 leawing whether these two processes of zoasliiug and cooZivg really differ in their effects I liave made the experiments now to be described. Two distillations were firbt made one with a form of apparatus advo-cated by Linneman (AwnccZen 160 Ins) Le Be1 and Henninger (Conyf. w?id. 79 480) and others designed to wash the 1-apours and termed ;;t dephlegmator the other merely with a long ascending tube suitable for partially condensing them.The liquids employed were carbon clisulphide and benzene selected for reasons given in a former paper (this Journal 1879 547). The composition of the several fractions of the distillates was determined by observing their densities in the manner described in the same paper. All the percentages given below represent the number of molecules of CS in a hundred moleeules of the mixture assuming of course that the weights of the molecu~es of CS2 and C,H are to one another as 76 is to 78. Distillation with De23?ilegrncttor.-547.5 grams of a mixture having a density of 1.07672 a t 19*SOo and containing therefore 62.54 per cent. of C& were clist,illed from a flask having a capacity of about a litre, and fitted with a dephlegmator having the form and dimensions shown in Fig.1; the distillate waq collected in the receiver described in the before-mentioned paper (this receiver was used also in all the subse. quent distillations). The barometric pressure during the distillation was 7513.7 min. (corrected and reduced to 0"). The following table gives tbe details of the disiAlation : DIFFEItEST XETHODS OF FRACTIONAL DISTILLATION. 5 1 Percentage of CSd. 8'7 *71 a . 5 1 81 +29 78.62 7 2 -113 66.04 49 -00 34.39 14 *30 2 *84 0 *15 --Nuinher of fraction. 1 . 11. . 111. IV . v VI . VII VIII . 19 . x XI. Telvpcrature of distillation. 0 0 61 -5-52-5 52 -5-54 -0 54 '0 -55'0 55 *0-56 ' 5 56 '5-60 '0 60 '0-63 *O ci3 -0-70 -0 70 -0-75 '0 '75 '0-75 *5 78 '5-80 '0 50 .o-80.1 %'eight of fraction.-52.34 105 *41 63 -18 81 -15 2-8 '37 51 -70 35 -31 22.4% 27 *6S 30.69 31 *05 Density at 19 *SO". 1 '19562 1 *17857 1 -16230 1 '14912 1.11878 1 '09150 1 '02430 0 '9'7525 0.916'77 0 '88733 0 '88072 Distillutioiz with Long Tube.-The tube used in this experiment i m s that shown in Fig. 2. Two thermometers were placed in this tube, one (B) at the bottom the other ( G ) at the top ; i t was thus possible to observe any change which might take place in the temperature of FIG. 2. lG the mixed vapours as they passed up the tube. 499.3 grams of a mixture having a density of 1.07660 at 19-80' and containing tliere-fore 62-51 per cent.of CS were distilled; the barometric pressure during the distillation was 771.3 mm. The results are given in the following ta,ble : 52 FRED. D. BROWN THE COXK'ARATIVE VALUE OF Number O f fraction. I . i . IT a . 111. . . . . . . IT v . . . . . * . VI . . . . . . . TI1 ,. VIII . . . IX . . . . . . . x . . . . . . . . Tenipcrntnre of distillation. 0. 0 0 50 *5 -51 *5 51 -5 -52 '2: 5 2 '23-53 *5 53 *5 -55.5 55 *5 -58 '0 58.0 -61'0 61.0 -71.5 71.5 -77 - 5 77 -5 -79 9 ' 7 79.7 -80'0 Weight Of fraction. Density n t 19 .80° 41 .OO 65 .04 7 3 . 5 6 66.69 51 *77 31 '46 51 *81 31 -00 25.10 28 *82 1 .l!)528 1 * 19047 1 'lS6Oa 1 -15631 1 .13432 1 .lo869 1.03583 0.94180 0 0 88158 Percent age of cs,.--87 -65 86 *07 84'01 80.11 75 *54 69.97 52 -14 23.27 5 -36 0 -50 I n order to compare the results of these two distilhtions they must be reduced to some common form ; for although the original mixtures contained the same amount of CS2 in both cases tho weights of the corresponding fractions were not equivalent. The distillations were therefore expressxl graphically (see diagram) in the following simple manner :-Let W = the total weight distilled wl w2 wj . . . = the weights of the several fractions ; then - loo lL'l ___ loo 7'2 he. will represent the weights of the fractions expressed in hundrecltlis of the total weight W. Now let yl yI? y3 . .. = the percentages of CS in the serernl fractions ; during the distillation of a fraction the vapour passing over varies i n composition its mean value being represented by the composition of the whole fraction = y. If we suppose that the vaponr has this mean composition when exactly half the fraction has passed over then when the accumulated distillate equals 1 -the vapoiir passing over will contain yl per cent. of CS2 ; when the dis-tillate equals ____ + ' 1 r2 the vapour passing over will contain w ' w 50 1 0 w - - I 100zcl 50 l.L' W W I00 (?/+ + 9 ' 3 . ) 50 71'7 - + I_- - x3 the w W y2 per cent. of CS,; when it equals vapoiir will coz:tnin y3 per cent. of CS2 and so on. W e have therefore the necessary data for determining the points of a curve expressing the variation in compositioii of the vapour as the distillate passes over or in other words tlic curve gives u s the com-position of each hundredth of the clistillute.I have inc*ludecl in tlic diagram tlie ciirvcs representing the two first distilhi ions recorded in the paiier pre\kms!J- referred to (thi I r n 490 80 70 60 9 4w 30 f20 70 0 I0 20 3u 60 70 80 700 Harrison & Sans Lith. 3 Martins Lane.w. UIFFEREXT METHODS OF FRACTIOXAL DISTILLATIOS. 5s Journal 1879 Trans. 547)) which were made with mixtures containing practically the same amounts of CS2 viz. 61.95 per cent. and 61-76 per cent. The following table gives t'he values of tel y1 ; x2 yz &c., for each of the four distillations a representing the distillation from the retort b that; from the flask with T-piece c that with dephleg-mator d that with long cooling tubc :-a.2. 2 . 9 14.3 28.9 43 .o 55 -9 65.4 72.8 78 .9 84-9 89 *7 Y3 *3 95 -8 Ye --82 .9 81 '2 78.8 75.4 70 ' 3 64 '1 xi -4 4.5 -8 30 .6 13 *1 5 - 3 0 -9 2. 3 . 6 14 -7 29.7 42 .9 53.1 62 .0 69 -9 76.6 83 .0 83 *7 93 .7 98.1 6. Y. 2. 4.8 19 .8 34 *6 47.8 57 *8 63 *1 73 *3 78 '8 83.4 88% 94 -5 -c. d. 2. 4 . 1 14 *8 28.8 48 '8 54 -7 63 '0 71.4 79.7 85 -3 90 -7 --Y. 87 *7 86.1 84.0 80.1 7 5 . 5 70.0 52 .I 23.3 5 -4 0 *5 ---On examining the four curves obtained by joining each series of points of which the above numbers are the co-ordinates we see, Srstly that the T-piece although generally employed merely to immerse the stem of the thermameter in the vapour was not without considerable influence on the distillation ; secondly that both the dephlegmator and the cooling tube produced much better results than the retort and the flask with T-piece; thirdly that the distillation with the cooling tube was rather better than that with the dephleg-mator.Of these results the last alone calls f o r further examination. Strictly speaking the comparison of the distillations c and tl shows that under certain conditions better results can be obtained with a long tube than can be obtained under certain other conditions with a dephlegmator. Those who have had experience witli these forms of apparatus will have observed that tlie rapidity with which the process of distillation is conducted largely influences the result ; the amount of cooling surface also cannot be a mattcr of indifference.In order therefore to compare accurately the two methods and to see whether the dephlegmator has any special value the sevei.al distillations should be effected in the same time and with the same extent of c u 01 i ng surface 54 FRED. D. BROWN THE COJIPARATI’STE VALUE OF FIG. 3. With this object i n vicw I constructed the dephlegmator represented in Fig. 3 ; it consists of a glass tube 500 mm. long and having an internal diameter of 20 mm. This tube carries nine small discs of wire ganze (40 holes to the inch) a piece of brass tube cd 2**> mm. in bore and 45 mm.long is passed through the centre of each disc and soldered to the gauze, SO that about 8 mm. of the tube project above it. The lower end of each picce of tube is deeply notched with a triangular file and sol-dered into a small brass cup d (tlie heads of upholsterer’s brass nails serve admirably for tliese cups). The lower end of the glass tube is fitjted with a cork iato the flask or still to the upper end is adapted an ordinary T-piece. The tubes cd serve to carry away the liquid. from the top of the gauge discs the cup a t the bottom answering as a trap to prevent tlie va-pour passing up the tube instead of going through the gauze. It is essential when No. 40 gauze is used that these tubes should be at least 45 mm. long as the adhesion of the I liquid to the gauze is so considerable that the pressure of the vapour underneath is sufficient to force the liquid out of a shorher tube; with filler gauze a still longer tube mnst of course be employed.IQ the figure the discs of copper gauze are drawn fitted into small rings of cork ; this not only hdds them more firmly but also causes the whole of the condensed liquid to accumulate on their upper sur-faces instead of running down the side of the glass tube; the cork would of course be unsuitable for liquids of high boiling point. This dephlegmetor was constructed only with a view to the ready re-moval of the discs &c. so that the same tube might be used with and without them. ”hi simple construction the absence of fragility and the fact that under no circumstances does it get overloaded with con-Gensed liquid are however advantages which render it preferable to those ordinarily used.I now attempted to carry out two distillations with the same liquid in the same length of time one with the gauze discs the other with-out. I foumd however that this could not be done ewn when a gauge was fitted to the Bunsen burlier used and the gas maintained at a constant pressure by means of a screw clamp. Three distillations mere therefore made with the discs and two without; these as will be men wexe suflicient t o show whether the two methods were identica DIFFEREXT 3IETHODY OF FRACTIOSXL DISTILL4TIOS. 3.3 Number of fraction. T 11 III . . . . . . . . I V I&. or not. In aach case 1000 C.C.of a mixture of benzene and carLon disulphide containing about 62.7 per cent. of C8 was employed. Tlit: density of the benzene a t 19.80* was 0.88049 that of the carbon clisul-phide was 1.26629; they were both carefully purified and their densities w'ere so nearly the same as tl~ose of the liquids previons!y used viz. 0.88034 and 1.26642 that tlie same iable of densities and percentages was employed. The distilltitioil was continued until four fractions of 200 C.C. each had passed over ; it was then stopped and the densities of the four fractioiis and of the residue were observecl ; the results were as follows :-Time occupied in distilling. 23 30 44 97 -Temperature 01 dijtillatim. Density corr. to 19 -80'. I Number of fraction. I .. . . . . . . . . 11 111 1 c lies. Percentagc of cs,. Time occupied iii distilling. -5 1 4 5-1, 52 95 -~~ 11. Distillatimi with yawe discs. 2llixtui.e coiitairu'ng 62-70 ye^ cent. 01' C'S:. Teinpersture of distiU:ttioii. Density Cory. to 19 *SO". Time occupied in distilliiig . I- -Temperature of clist illation. 1.21939 1 .a0705 1,14022 0.94429 0-88019 Percentage of CS:. Suniber of fraction. 1 I 1 111 I v . . . . . . . . Re&. . 85 40 60 73 -Density corr. to 19 -80". 1 228877 1.18359 1.14599 0.92097 0.8305 4 Percentage of CS2. 03 a 83 .8 E 77.987 15 .85 00 '0 56 FRED. D. BROJTN THE COJIPARATIVE VALUE O F Number of fraction. I I1 I11 IV Res.IV. lJistillatlon with same tube without gauze. Dfixtzcre containing 62.74 per cent. of CS,. Tiiiic occupied in distilling. -____ 45 45 40 36 -Density corr. t o 19.80'. Tcmpemture of distillation. Percentage of of CS2. 0 0 - -52.6 58 -6 --54 '9 54.9 -60.8 60.8 -77.2 - -I I Density con. to i9'80". 1.19066 1 86'79 1 '17413 83.63 1 -13420 75 *55 1.01833 47 '31 0 -88756 1 2.93 Percentage of CS,. Number of fraction. 1 * 20960 1.20032 1.14'887 0 *93691 0 -88053 ~ I I1 111 I V Bes. 90 '30 88' 58 78 '57 21.56 0 .oo Time occupied in distilliiig. 225 300 285 225 -Temperature of ~ distillutiou. -58.7 50'7 -54.6 54.6 - 4 6 '1 66.1 -80.5 - -On inspectiiig these numbers we see in the first place that they afford convincing proof of the statement previously made that the rate of distillation is an element of considerable importance showing that the slower the distillation the more carbon disnlpliide does the dis-tillate contain and that this is equally the case whether the vaponrs are washed or not.I n comparing together the percentages of the various fractions it must of course be remembered that only the first fractions of each distillate are strictly comparable for after these had passed over the liquid in the still had no longer the same composition i n each successive distillation. This explains f o r instance why the third fraction of the first distillation contained more CS than that of the third though i t clistilled in a shorter time.I n the first case a less quantity of CS had been previouslyi*ernoved; a t the beginning of the third fraction therefore the liquid in the still was much richer in CS2. These distillations further prove conclusively that the processes of ir:cnshi~ig and of cooliiig are not identical. The fifth distillation was made under the most advantageous circumstances that is it was con-ducted a t the slowest possible rate yet the resclt was decidedly inferior to that of the second in which tlie vapour was washed and ahicli was completed in a fourth of the time. Tlie same remark applies to the fifth and third and to the fourth and first distillations. Since in e q i d times with eqnsl surfaces the radiation must be the same the effect of radiation iii the fifth case must 1iai.e been greater than in the second ; the better result in tlie second case must there DIFFERENT METHODS OF FRACTIONAL DISTILLAITIOS.5 7 fore have been dne to the presence of the gauze discs. A certain amount of secondary distillation does therefore take place in the clephlegmator and the question whether the apparatus has anj- special value is ans-ivered in the afErmative. Notwithstanding this resnlt we are not yet in a position to stfate that the dephlegmator is the best apparatus for fractional distill a t ' ion, and tbat because we have by no means exhausted the resources of the process of cooZ;ng. The mixed vapours in passing up a tiibe may ob-1-iously be subjected t o more and more radiation if the tube be gradu-ally increased in length until a t last no vapour mill reach the top of the tuhe ; the best possible result will be obtained when the vapour just creeps over the top before complete condeiisation takes place, having then reached its lowest possible temperature.It would of course be impracticable to effect distillations in this way but the object. in view is readily attained by passing the vapours through a tube or still head maintained a t a given temperature as is done every day when distillations are carried out on a manufacturing scale. If the temperature of the still head be the lowest possible compatible with the passage of vapour into the condenser the result will be the best which the process admits of. The next stepin this investigation was therefore to make a suitable apparatus f o r the above purposes the general finally adopted is shown in Fig.4. The still A conimunicates with a coil C ter-minating in a vertical portion open a t the top, into which a thermometer is fitted witJh a coFk in the usual manner; a lateral tube B serves t o conduct the vapour from the coil into the condenser. The whole of the coil C including the junction with the tube B is enclosed in a cylindrical copper box 300 mm. long by 110 mm. in diameter. The two ends of this cylinder are tied to-gether by means of the copper tube aa which not only strengthens the cylinder but being open a t the top serves for the introduction of a second therinometer which indicates its tem-perature. This copper box is filled lip to the level I with a liquid or mixture of liquids boiling at the required temperature the heat being furnished by a ring burner 217~.The vapour given off by this boiling liquid passes LIP the tube dd to the condenser F whence it returns in a liquid form by the tube kk to t'he bottom of the box E. character of the one FIG. 4 58 FRED. D. BR0TT-X THE COMPARATIVE VALUE OF The small tube m serves as a communication with the atmosphere. As there is considerable trouble in changing from one liquid to another when a different temperature is required the apparatus is cornpletcly closed except a t 112 by connecting with an air chamber, iiz which any constant pressure can be conveniently maintained ; the boiling point of the liquid in E can be altered a t will. With a view to the use of the apparstns in this manner the copper box E and the tubes connected with i t were tested a t a pressure of 100 Ib.on the square inch. . I n employing this apparatus the ring burner bb is first lighted, when the liquid in E begins to boil the teniperature as indicated by tbe thermometer in aa (this. tube is filled with water or petroleum) soon becomes const,ant and at the same time the readin$ of the ther-mometer placed in the top of the coil ceases to vary; heat is then applied t o ths still A and the distillation carried out as rapidly as can be done without altering the reading of the thermometer in the coil. For the first dist,illation made with this apparatus methylated spirit was put in the box E and the pressure maintained was such that ebullition took place at about 57".About 800 grams of a mixture of benzene and carbon disulphide were put in the still A and the distil-lation collected i n 10 fractions. The mixture i n the still a t the beginning of the distillation con-tained 45.00 per cent. of CS ; a t the end it contained 26.56 per cent. The barometric pressure daring the distillation was 750.7 mm. The thermometer H was placed in an the therinonieter F in the t3p of the coil. The dehails of the distillation were as follows :-No. of fraction. I I1 . I . . JV v . . . . V I VII VIII IX x in . . . . Readiilg of B m-11 en frac t ion remored. 57 -50 57 -50 57 -50 57.50 57.52 57 -51 57 5 3 57 *53 57 -53 57 -53 --Reading of F2 whcn fraction removed.57 -10 57 '10 57.1C 57 -10 57 * I 2 57.10 57 . I 2 57 '30 58 *OO 57 -90 Density of frao-tion at 19SO". 1 '13523 1'13185 1 .I3348 1 -13528 1 -1364.9 1 *136,36 1 -13733 1.13819 1.13358 1-13333 Percentage of CS2 in fraction. 7 5 9 3 75.02 75.36 75 -74 7 5 -99 76 *01 76 *17 76 *35 73 *38 75 -33 The slight rise in the reading of I? towards t'he end of the distilla-tion was due to the vapour passing too rapidly through the coil ; it did not seem however to have any influence on the composition of the distillate DIFFEREXT METHODS OF FRAACTIONAL DISTILL-ITIOS'. 5:) A second distillation was made with 600 grams of a mixture con-taining 11.47 per cent. of CS ; tlie barometric pressure was 753.1 mm. At the end of the experiment the liquid remaining in the still con-tained only 4.33 per cent.of CS ; the details were as follows :-Dcnsity of frac-tion at 19.80". -98826 -98516 *98498 -98527 No. of fraction. LI_-I . . . . I1 I11 I V . . Percentage of CSL. - ~ ~ _ _ _ I 3s -44 37'48 37 '42 37.5% 70 -29 70.35 70.57 70.56 69 -94 69.98 70 23 70.24 These two distillations show that the composition of the distillate is independent of the amount OP carbon disulphide in the still but varies with the temperature of the coil or still head. Two distillations were now made with the copper cylinder E filled with pure carbon disulphide the boiling point of this liquid being evidently the lowest temperature a t which any vapour given off by a mixture of benzene and carbon disulphide can pass into the con-denser.I n the first of these distillations a mixture containing 42-61) per cent. of CS was employed while the residue in the still when the experiment was stopped contained 32.37 per cent. of CS?; the amount of CS in the various fractions of the distillate was found to vary between 9'7.32 per cent. and 98.72 per cent. The distillate did not gradually increase or diminish in density the higher and lower Fercentages following each other indiscriminately. I n the second clis-tillation for which the liquid remaining from tlie first \vas used the liquid in the still contairied at the beginning 32-37 per cent. and at the end 25.21 per cent. of CS2 ; in this case the amount of CS2 in the distillate varied from 97-48 per cent to 99-19 per cent.These two experiments show that when the stiil head is maintained at a temperature equal to the boiling point of the most volatile com-pound in a mixture that compound alone or nearly alone passes over into the condenser; it is therefore possible by the use of this appa-ratus to effect an almost complete separation of the various constituents of a mixture of volatile substances in one dishillation. I n order to show the practical value of the method I may mention that I have frequent1.y used it t o separate benzene from the toluene and other substances which ordinary commercial benzne contains in large qnant i ties. As an examplc the following rectification of some crude benzene will suffice. A sample of 200 C.C.of this liquid was distilled in an ordinary flask ; i t The benzene thus obtained has always been nearly pure GO ML'XR AND SLhTElC INFLUENCE OF WATER began t o distil a t 8.5" ; a t 90" 120 C.C. had accumulated in the receiver, a t '3.5" 153 c.c. at 100" 168 c.c. a t 105" 170 c.c. and a t 110" 192 C.C. 1100 C.C. of this very impure liquid were distilled with the apparatus above described maintained a t a temperature of 81° ; this was rather too high to give a really good result ; nevertheless 500 C.C. of rectified benzene were obtained of which a sample of 'LO0 C.C. distilled as 101-lows :-Distillation commenced a t 80.0" ; a t 80.4' 155 c.c. a t 80.8" 176 c.c. a t 81.0" 180 c.c. arid a t 83.0" 191 C.G. had accumulated in the receiver. When 500 C.C. of this rectified benzene had passed over the experiment was stopped and a sample of the residue distilled as follocw :-Distillation commenced at 91" ; a t 95" 80 c.c. a t 100" 130 c.c., a t 105" 158 c.c. a t 110" 186 C.C. had passed into the receiver ; it is therefore seen from how impure a liquid pure benzene niay be directly obtained. As regards the results contaiiied in the above paper it niay be con-tended that they present but little novelty ; it was liowevei* necessary That tbe effects obtained by the different niethods should be exactly defined and numerically stated before a complete explanation of iractional distillation could be advanced