INDUSTRIAL A N D ENGINEERING CHEMISTRY
1310
RESULTS WITH MIXTURES OF PIGMENTS Impurities in Pigment Solution. Colorless substances in the pigment solutions affected the adsorbability of each pigment of a niivture just as they affected the adsorbability of a single pigment For example, adsorption of two xanthophylls from petroleum ether containing a strongly adsorbed impurity, followed by adsorption of more of the pigment solution without impurity, often yielded as many as three or four colored zoneq. The effect of impurities upon band formation during the adTorption of mixtures of pigments was complicated by variation of the sequence in which the bands occur in the adsorption column ( 4 ) . When a mixture of lutein and chlorophyll (I, was dissolved in petroleum ethrr containing a little (0.2%) n-propyl alcohol and adsorbed in a column of powdered sugar, the lutein formed a yellow band above the green chlorophyll. When this column t(‘a.s washed with petroleum ether or with petroleum ether plus benzene, the lutein band moved rapidly through the green band. Addition of a Polar Substance to Developing Solvent. When certain xanthophylls in petioleum ether solution were adsorbed with chlorophyll b in a column of powdered sugar and the chromatogram was then developed with petroleurn ether plus about 0.5% propyl alcohol, the xanthophylls were precipitated a3 illustrated in Figure 7 . With violaxanthin and chlorophyll b, this formation of two yellow bands is illustrated by Figure 8. Upon continued development of the chromatogram, a single yellow band was formed above the chloiophyll b. Khen a mixture of the principal xanthophyll from EugEena v a s adsorbed with chlorophyll b, followed by development of the chromatogram with petroleum ether plus 0.5% propyl alcohol, a series of bands nas formed as illustrated in Figure 9. Here the initial yellow band was above the chlorophyll b, and the final yellon band was below the chloiophy!l b
These double zoning effects have frequently been observed when the chloroplast pigments of various plants are separated in columns of powdered sugar. I n order to avoid erioneous conclusions regarding the number of pigments, the chromatograms must be developed extensively with the petroleum ether plus propyl alcohol mixture. SUMMARY In adsorption columns, each adsorbed pigment usually yields a single colored zone. I n the presence of certain colorless contaminants, however, a single pigment may yield two adsorption zones. The formation of these adsorption zones depends upon many conditions, such as the order of the addition of the solution5 to the column and the solubility and the adsorbability of the pigments. LITERATURE CITED (1) Claesson, S . , Arkic. Kemi Mineral. GeoZ., 23A, So. 1 (1946). (2) Strain, H. H., in R. E. Burk and 0. Grummitt’s “Frontiers in Colloid Chemist.ry,” 5’01. VHI, Xew York, Interscience Publishers, 1950. (3) Strain, H. H., “Chromatographic Adsorption Analysis,” New York, Interscience Publishers, 1942. (4) Strain, H. H., IND. ENG.CHEM.,ANAL.ED.,18, 605 (1946). (5) Strain, H. H., Manning, W. If.,and Hardin, G., Bio2. Bull., 86, 169 (1944). (6) Tswett, RII., Ber. deut. botan. Ges., 24, 316 (1906). (7) Zechmeiater, L., and Cholnoky, L. v., “Die chromatographische Adsorptionsmethode. Grundlagen, hlethodik, Bnwendungen,” 2nd ed., Berlin, J. Springer, 1938; English tr. by A. L. Bacharach and li‘. A. Robinson, London, Chapman and Hall, 1941. RECEIVED S o v e i n h e r 22, 1949.
HENRY J. HIBSHMAN,Esso Laboratories, Standard Oil Development
-
T
Vol. 42, No. 7
Company, Elizabeth, N. J.
Normal paraffins with five, six, and seven carbon atoms are selectively adsorbed a t room temperature from iso-octane by activated carbon. Countercurrent flow of carbon and hydrocarbon is beneficial, indicating t h a t a n analogy exists between such solid adsorption and conventional extraction and distillation processes. The carbon may be regenerated by thermal treatment a t about 900” F., steaming a t about 580” F., or washing with other hydrocarbons. I n t h e latter case, with proper choice of wash solvent it is not necessary to remove the wash solvent from the carbon before reuse, since selectivity is essentially unimpaired by the presence of the wash liquid. A flow plan for continuous separation using countercurrent flow of liquids and solids is presented.
HE use of solid adsorbents for separating hydrocarbons as discussed in this paper is a specific application of an old and extensive art. Although no attempt is made to present a complete review of the art, for orientation purposes a few facts from recent reviews on the general (S), analytical (9),knd industrial (5)aspects of adsorption seem relevant. Adsorption of gases on charcoal was reported as early as 1773 and was first -used commercially t o decolorize sugar about 1800. In spite of early work on separation of complex mixtures of similar components, chromatographic and related adsorption separations did not receive much attention until after 1930. Since then
many applications of this type of adsorption have been made, including the separation of complex mixture8 of difficultly separable hydrocarbons. Various adsorbents such as silica gel and activated carbon have been used to effect analytical and industrial separation of hydrocarbon mixtures. Silica gel ( 7 ) and several synthetic ( 2 ) and natural ( 1 ) zeolites exhibit selectivity for separating certain paraffins. I t is the narrow field of separating iso- and normal paraffins in the gasoline boiling range usiiig activated carbon Tyith which this paper is concerned. There is an excellent review ( 4 ) of the structure, properties, and preparation of active carbons. Accordingly, the following dis-
INDUSTRIAL AND ENGINEERING CHEMISTRY
July 1950
UNADRORBED PRODUCT YIELD: VOL. 94 ON FEED
Figure 1. Fixed-Bed Yield to Quality Relationship of feed stock. 1 to 1 n-heptane-iso-octane mixture Adsorbent. 48-inch bed containing 273 grams of 20- to 48-mesh carbon
cussion is limited primarily to separation results obtained with a commercial carbon product. This work, which is largely empirical, was carried out during the recent war in an effort to develop processes for improving the octane number of aviation fuels.
FIXED-BED ADSORPTION Preliminary investigation of various adsorbents indicated that the ability of activated carbon to adsorb n-heptane selectively in preference to iso-octane is not a common property of solidsfor example, of the following solids Activated carbon Silica gel Activated alumina Magnesol Super Filtrol Porocel iilundum
Fuller's earth Diatomaceous earth Kaolin Infusorial earth Various catalysts Magnesia
Only Columbia activated carbon showed any selectivity. Throughout this paper, carbon is understood to be Columbia G activated carbon sold by the Carbide and Carbon Chemicals Corporation; iso-octane refers to 2,2,4-triniethylpentane. Several different treating techniques have been used in testing adsorbents and investigating adsorption variables. Batch contacting in an agitated vessel as well as liquid and gas phase operation in fixed beds has been used. Because batch contacting effected less than 10% enrichment of the 1 to 1 mixture of iso-octane and n-heptane used as a standard test mixture, only results from fixed-bed operations are considered in the following discussion of operating variables. Two runs utilizing a simulated moving bed with liquid phase operation were also made under conditions found to be optimum with fixed beds. In studying the relative merits of liquid and gas phase operation i t became apparent that each has a particular advantage. The advantage of vapor phase operation is that with a given amount of carbon in a bed of fixed height, a considerably purer product is obtained in the initial portion of the run. This effect is shown in Figure 1. The first 8% yield of unadsorbed hydrocarbon was essentially pure iso-octane, whereas with liquid phase operation the product purity did not exceed about 85% for such a yield. During the latter portion of the runs there is a slight advantage for the liquid phase process. However, an advantage of the liquid phase operation is that the amount of carbon required to obtain a given yield of unadsorbed product based on feed stock, referred to as treat, is considerably smaller, As Figure 2 shows, about 80% higher carbon treat is
1311
required in the vapor phase at 10% yield and 50% higher a t 50% yield. At high yields, where the greatest practical interest lies, the liquid phase process not only is more selective but requires less carbon and is accordingly the preferred process. However, a t high levels of unadsorbed product purity the vapor phase process does show considerable yield advantage for fixed-bed operations. Because product quality is of prime importance, these data are replotted in Figure 3 to show the lower treats required by the liquid phase operations to attain a given product quality. These effects are apparently a function of the differences in hydrocarbon holdup of the carbon under the different operating conditions. Less weight of hydrocarbon is required to fill the voids initiallv when it is in the vapor state than in the liquid state-approximately 44 and 80 ml. of hydrocarbon per 100 grams of adsorbent, respectively. There may be other contributing factors, such as the effect of temperature level on the carbon selectivity. The vapor phase process is always carried out a t a higher temperature. In this process it was expected that as in the case of extraction with solvents ( 6 ) , liquid phase operation would permit enrichment of wider boiling naphtha fractions than vapor phase operation. For this reason and because of the smaller treat required and the somewhat better selectivity a t high yields discussed above, all subsequent work was limited to liquid phase operation. It was demonstrated that considerably less branching than is present in iso-octane is sufficient t o permit a separation to be made. For example, in percolating a mixture containing 34 volume % isopentane in n-pentane through a 48-inch bed of 20to 48-mesh charcoal, accumulative yields of 2.5, 10, 25, and 50 nil. per 100 grams of carbon were found t o contain 55, 49, 43, and 38 volume % isopentane. respectively. eo0
n
I160 d
s 0
t
le0
20
i: eo
a I-
I
I
I
I
10 PO 30 40 UNAOSORBED PRODUCT YIELD: VOL.%
0
I
I SO ON FEE0
60
Figure 2. Fixed Bed Yield to Treat Relationship of feed stock. 1 to 1 n-heptane-iso-octane mixture Adsorbent. 48-inch bed containing 273 grams of 20- t o 4e-mesh carbon
In order to gain some knowledge of the maximum width of cut over which the process might be effective for separating normal paraffins from isoparaffins, percolations were made with nhexane and n-pentane mixed with iso-octane. The results summarized in Table I show little separation of n-pentane and isooctane, but a significant separation of n-hexane and iso-octane.
Table I. Effect of Number of Carbon Atoms in n-Paraffin Operating conditions. Percolation through 20- to 48-mesh carbon at 80" F Feed 1 to 1 blend of iso-octane and indicated normal paraffin Normal Paraffin
c6
5
25
CS
c7
Filtrate Composition, Vol. yo Iso-octane
Accumulative Yield, M1./100 G. C 53 55
66 63
81 70
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
1312
I
so
I
I
Vol. 42, No. 7
current action is needed to obtain higher yield-purity rehtioiisliips. The effects of temperature and contxct time were studiotl a,ntl :ire shown in Table 111. There appears to be an optiinum at &out 1 volume/volume/hour and 32" F. Although the effect3 of these variables are considerable at, l o ~ yields, r they are much less significant a t high yields. (It is noten.orthy that whcri temperature was increased in v:tpo~' phase operation, the selcctivity remained high up t o 250" F. but disappeared almost entirely when the temperature yas iiicreascd to 340' F., although the capacity of the carbon WLF not greatly different at the two temperatures.) Several additional variables liavc: I)ccii investigated to a liini tcd extent. However, tlecause they do not appear to be of major importance, they are only briefly mcritioned here. E'nrticle sieo below 4- to 14-mesh \Tag found to p1:i.y n minor role, and the coininercial 20- to 48-nicsh carbon shoned t,he best results. I n i-ked-bed operat,ion little advautagc rc~ultedfrom the uso of 1)ccls of greater than W i n c h depth, :i fact again indicating tlic need for countercurrent, action to obtain the ultimate yieldpurity relationship.
I
60 70 80 90 si ISO-OCTANE IN UNADSORBED PRODUCT
Figure 3. Relationship of Product Purity to Treat Feed stock. 1 t o 1 n-heptane-iso-octane mixture Adsorbent. 48-inch bed containing 273 grams of 20- to 48-mesh carbon
.Utliougli the effects of degree of branching and molecular n-eight have not been exhaustively investigakd, these data indicate that the separation is probably limited to fairly narrow boiling fract,ions when a liquid phase operation is used. Drawing an analogy with the field of extaractionand estractiw distillation, it seemed possiblc that :I phenorncnon similar to azeot,rope formation might occur in the presence of the solid. In this case, it might be possiblc to separate a mixture of OILC composition but not another. In order to investigate this possibility, several different ratios of n-heptme to iso-octane were treated. As Table I1 shom, the carbon absorbs the n-heptane with good select'ivity, regardless of the level of the normal paraffin concentration in the feed. However, an azeotropelike phenomenon has recently been reported ( 7 ) to occur with ~ c v e r a l mixtures of other saturated hydrocarbons on activated carboll. Such rcsults make prediction of separations on complex refinery stocks difficult. This is also coniplict~ted by the fact that aroma.tics are more strongly adsorbed t,han n-heptane.
COUNTERCURRENT ADSORPTION It iippcws from thcsc fixed-bcd test8 that the adsorption techniyue using activated carbon \vi11 separate certain paraffin hydrocarbons according to molocular struct'ure. Ilowever, commercial application ~ o u l doi)viously be involved if it w!re tlt:sired to produce high purity products in a fixed-bed opcration, /)OCLLUFC of the necessity of regenerating the carbon so frequently. The use of a moving or fluidizcd bed would greatly simplify such :I. process. In addition, the use of countercurrent and reflux techniques (as discussed in conncction with Figure B), easily :ipplied in such a process, should gi,r!atly enhance the cfficicncy ,iccordingly, several runs wrw matlr with the 1 to 1iso-octanen-heptane feed to determine the etfcct of countercurrent action on the separation. The opcration TKIS accomplished by feeding to the top of a column of eight. 6-inch adsorption tubes, mounted one above the other in such n inanncr that after a predctermincd
:I 2
I
\ I \ I
I
I
I
I
B
s
Table 11. Effect of Feed Composition Operating conditions.
Percolation through i8-incli bed of 20- t o 48-nieuh carbon a t 80' 1'. ___% Iso-octane in Feed _ _ 2s 50 80
icciiinulative Yield, X1./100 G. C 5 25 50
THEORETICAL
>
Filtrate Composition, Vol. YoIso-octane 38 3s 31
81 70
63
9-1 YO
87
a V
In the more conventional use of carbon to remove small amounts of impurities that differ greatly in molecular weight or chemical composition from the rest of the gas or liquid being treated, the quantity of feed stock that can be purified is roughly inversely proportional to the per cent of the impurity in the feed. However, in this work the filtrate had the same composition as the feed after about the same amount of filtrate had been collected, regardless of the feed composition. This is interpreted as an indication that a less selective equilibrium is established rapidly between the feed and carbon, beyond which further adsorption of a given component will not occur, and that counter-
'Ol
I FILTRATE VIELDS:mh
I
I
IO 20 50 40 ACCUMULATIVE CC.1100 GRAYS CARBON 20 27.2 33.3 V 0 L . U BASED ON FEED
I
so 38.4
Figure 4. Liquid Phase Variables Fixed and moving bed operation Feed stock. 1 t o 1 n-heptane-iso-octane mixture Operating conditions. Percolation through 48-inch adsorbent bed containing 273 grams of 20- t o 48-mesh carbon a t 1 v./v./hour and 80" F.
INDUSTRIAL AND ENGINEERING CHEMISTRY
July 1950
Table 111. Effect of Temperature and Contact Time Operating conditions, Percolation of 1 to 1 iso-octane-n-heptane mixture through 48-inch bed of 20- to 48-mesh carbon 0.1 Yield, bfl./lOO G. C 5 25
V./V./Hour" 1.0 1.4
0.7
5.0
Filtrate Composition, Vol. 70Iso-octane
Temp., F . 108 .. ,. 81 .. .. 108 .. ,. 70 .. .. 50 - 108 .. ,. 67 .. .. 93 .. .. 5 32 76 . .. 32 25 67 .. 32 50 70 69 78 86 81 80 5 65 66 69 73 68 25 80 60 62 63 64 60 80 50 Filtration rate, volumes of feed per volume of adsorbent used per hour.
-
. ..
a
Y
amount of liquid had been collected a t the bottom, a tube containing fresh carbon could be inserted at the bottom and one removed from the top at intervals to simulate a continuous countercurrent operation. This procedure of continuous liquid feeding and ineerting a fresh tube of carbon after each predetermined incremental volume of liquid was collected, unlike a fixed-bed operation, could be continued indefinitely and effective separation maintained. However, i t was stopped after the composition of the collected liquid reached a constant value. The composition of this liquid was different from that obtained using a fied bed with t h e same ratio of charcoal to feed stock. That this should be so necessarily follows from consideration of the fact t h a t fresh feed is continually fed into the top of the column, and unless the charcoal is moved upward, no mechanism is provided for returning any components, either adsorbed or liquid, t o contact upper portions of the column liquid (this is an essential feature of true countercurrent action and is accomplished by changing the location of the hydrocarbon rather than the adsorbent in those analytical percolations where a large excess of adsorbent is used in a fixed bed and the charge is followed down the column with a suitable desorbent, 8). The resultant data are compared in Figure 4 with fixed-bed results and the ultimate is predicted for countercurrent operation based on fixed-bed results. It is apparent from the close approach of the countercurrent results to the ultimate theoretically possible t h a t the countercurrent action was effective. The yield-purity relationship is definitely limited in such an operation to low yields of pure iso-octane, inasmuch as the carbon removed must of necessity be in equilibrium with the feed stock and therefore contain considerable amounts of iso-octane. The ultimate curve is calculated, assuming that the carbon leaving the tower top is in equilibrium with the feed stock and accordingly will contain the same ratio and amounts of iso-octane and n-heptane as are contained on a fixed bed of charcoal after it has operated sufficiently long for the effluent coming out the bottom to have the same composition as the feed. It is further assumed t h a t the separation efficiency of the countercurrent operation is perfect, so that the effluent from the countercurrent operation will be pure isooctane a t all yields less than that equivalent to the n-heptane in excess of the iso-octane going out with the charcoal a t the top of the column (calculated from the integrated area between filtrate composition and the 50% composition line on plots such as shown on
13L3
Figure 1 carried out to yields when the effluent composition was the same as the feed stock). In view of this demonstrated analogy to other countercurrent two-phase separation processes such a8 extraction, distillation, and absorption, i t seems reasonable t o expect that the use of both a stripping and enriching section on either side of the feed point with the introduction of reflux at the top would result in much more favorable yield-purity relationships. T h e results would then be dependent upon the height of carbon bed and the reflux used in a manner analogous to the effect of plates and reflux ratio on distillation processes.
DESORPTION In the practical application of either a fixed-bed or fluid process, a simple means of regenerating the carbon is important. Although normally gaseous hydrocarbons can be thermally desorbed a t sufficiently low temperatures t o avoid cracking, it was found that a t atmospheric pressure significant amounts of both n-heptane and iso-octane remain on the carbon at temperatures where cracking becomes appreciable. This is apparent from the desorption curves and gas make shown in Figure 5. It appears from these data that temperatures would have to be kept below 700" F., or possibly even lower, to avoid undesirable cracking. The data in Table IV show that while the selectivity of spent carbon was essentially restored by heating t o 910' F., heating to only 700" F. was relatively ineffective. Steaming a t elevated temperatures was effective, although selectivity was not completely restored-for example, as Table I V shows, using 14
Table IV. Effect of Desorption Temperature on Selectivity Test conditions. Percolation at SOo F. of 1 to 1 mixture of iso-octanen-heptane through 48-inch bed of 20- to 48-mesh carbon (273 grams) Desorption Conditions Heat Steam, Fresh QIOa F . 700° F . 600' F . 580° F.5 Carbon Yield, Filtrate Composition,,Vol. % Iso-octane M1./100 G . C 5 79 60 63 74 81 25 65 .. 56 67 68 50 59 54 61 60 0 14 grams of steam passed through bed containing 273 grams of carbon held at 580° F.
TEYPLR ATURE-.
f.
Figure 5. Desorption by Heating Left.
Right.
Atmospheric pressure n-Heptane from activated carbon Iso-octane f r o m activated carbon
1314
INDUSTRIAL AND ENGINEERING CHEMISTRY rCARBONPL"Sy~-
- - ----
I
I
7
Vol. 42, No. I
presence of the isopentane remaining after the washing operation. The isopentane was simply pushed out of the carbon ahead of the heavier feed stock. An advantage in product purity a t a given vield is shown for using isopentane over n-pentane as a wash. k t h o u g h the adsorbent life was not thoroughly investigated, it was shown that with n-pentane washing, the second cycle was better than the first; this indicated that life is not limited to a once-through basis. PROCESS FLOW PLAN
I Figure 6.
I
I
C o n t i n u o u s Process Flow P l a n for Separation of Iso- a n d Normal Paraffins
grams of steam a t 580 O F. on a spent carbon bed containing 273 grams of carbon resulted in good desorption. However, thirtytwo times this quantity of steam a t 212" F. was ineffective in producing complete regeneration. It is indicated, therefore, that a fairly high temperature level for steam regeneration is necessary, although the exact temperature has not been established.
Table V. Conditions.
Desorption by Washing
80" F. washing of carbon saturated with n-heptane in 30-inch bed of 20- to 48-mesh carbon Wash Solvent n-Pentane Isopentane Vol. %, n-Heptane Removed
Wash Solvent, Vol. 70 (Based on Adsorbed n-Heptane)
zin _360 __
540 1270
Table VI.
90 ..
87
97 98
93 94 99.5
Effect of Presence of Wash Solvent o n Selectivity
Test conditions. Percolation a t
80' F. of 1 t o 1 mixture of iso-octanen-heptane throu h 48-inch bed containing 273 grams of carbon saturated hydrocarbon (all data on a pentane-free basis) with Carbon. Cycle No. n-Pentane Isopentane Dry Saturated Saturated Yield Filtrate Composition, Vol. 70Iso-octane M1./100'G. C
&
30 50
I
I
L
I
68
61 57
66 57
69 63
63
In addition to purely thermal or steam stripping, a third possibility for regenerating the spent carbon is a liquid phase washing operation. Both normal and isopentanes are effective in removing la-heptane, as shown in Table V. I t was also found, as shown in Table VI, that the selectivity of the carbon for separating iso-octane and n-heptane was not greatly impaired by the
By combining the essential features of these absorption and desorption steps in an appropriate manner, several different continuous separation processes can be visualized. A flow plan for one such process utilizing fluidized solids technique is presented in Figure 6. Although the components of other feed stocks might possibly be separated by such a process, for illustrative purposes this flow plan is set up to separate a I to 1 mixture of iso-octane-n-heptane. The feed stock is introduced a t the center of a countercurrent adsorption system represented by separate upper and lower sections. The carbon introduced a t the top falls through the system countercurrently to the feed and liquid reflux streams introduced at' the center and bottom, respectively. The system may be considered analogous to distillation, with the vapor phase of distillation replaced by the hydrocarbon liquid phase and the liquid distillation reflux replaced by the phase adsorbed on the solid. The carbon withdrawn a t the bottom of the adsorber contains n-heptane of 90% purity which is removed from the carbon by washing countercurrently with pentane. The pentane is then removed from the n-heptane by distillation. A portion of the nheptane is removed as product and the remainder returned to the bottom of the absorption syst'em for stripping purpose8 (as reboiler vapors would function in a distillation). The pentane remaining on the carbon after the washing is not removed before entering the adsorption st'ep and consequently appears with the filtrate, from which i t is separated by distillation. The resultant filtrate is 90% pure iso-octane. It is concluded that, solid adsorption using activated charcoal is a potential means for separating difficultly separable isoand normal paraffins, but that development of mnre selective adsorbents effective over wider boiling ranges is desirable. The techniques developed are not limited to separation of paraffins or to the use of charcoal as an adsorbent.
LITERATURE CITED (1) Barrer, B. M., J . Chem. Soc., 1948, 127.
( 2 ) Barrer, B. M., and Ibbitson, D. A,, Trans. Faraday Soc., 40, 195 (1944).
(3) Claesson, S., Arkiv. Kemi Mineral. GeoE., 23A, KO.1 (1946). (4) Emmett, P. H., Chem. Revs., 43, 69 (1948). (5) Harris, B. L.. IND. EXG.CHEM.,41, 15 (1949). (6) Hibshman, H. J., Ibid., 41, 1366, 1369 (1949). (7) Hirschler, A. E., and Amon, S., Ibid.,39, 1685 (1947). (8) Mair, B. J., Westhaver, J. W,, and Rossini, F. D., Ibid., 42, 1279 (1950). (9) Strain, H. H., IND. ENG.CHEM.,ANAL.ED.,21, 75 (1949). RECXIVED November 25, 1949.
