July 1950
INDUSTRIAL AND ENGINEERING CHEMISTRY
concentrations; higher original filling concentrations merely delay the equilibrium point a few replenishments. The equilibrium point is influenced mainly by the concentration of the replenishing solution and the v a t level a t which replenishment is done. Thus it seems possible that a certain saving in chemical could be effected by using a solution of one half of the usual strength in originally filling a vat and then replenishing with usual strength. Also if the vat inadvertently dropped below the desired SO% volume level it would not be necessary to use stronger than usual solutions to bring the concentrations back to the desired strength. This information on replenishing vats is somewhat speculative, particularly since it is not based on data derived from tests of actual replenishing schedules. Also the wide variation in stain hazard undcr different geographical and saw-milling conditions should greatly influence safe v a t levels. However, the information should be useful as a guide in handling mercurial solutions. Ethyl mercuric phosphate has been widely and successfully used for protecting green lumber against stain for many years. However, when this chemical is used it seems desirable to consider adsorption in devising replenishing schedules. The desired effect can be srcured either by replenishing vats with concentre tions higher than originally used or by replenishing a t frequent intervals with usual concentrations. In the latter case the indication is that the amouht of solution in the vat should never be less than 80% of the original volume. Frequent replenishment with usual concentrations would seem the most logical method of controlling concentrations in most milling operations. The importance of maintaining concentrations would be greatest during warm wet weather when stain is relatively difficult to control even with the best fungicides,
SUMMARY Laboratory and field tests showed that the fungicides commonly used for controlling stain, mold, and decay fungi on green
1355
lumber progressively decrease in effectiveness with repeated use. It is assumed that this is due t o adsorption of the toxic solutes t o the wood. Of the fungicides tested, solutions of ethyl mercuric phosphate were much reduced in effectiveness after sufficient wood has been dipped so that half of the solution has been taken up by the lumber. Sodium pentachlorophenate was but slightly weakened. Mixtures of either of these with borax, and of both together with borax were weakened to an intermediate degree. Only the weakening of the mercurial alone and the mercurial plus borax were considered of commercial significance under the conditions of the tests. Loss in strength of mercurial solutions probably can be reduced to a safe level by replenishing treating solutions with stronger solutions than originally used or by frequent replenishment of solutions with concentrations recommended for general use. The indications are that adequate strengths will be maintained if additions of recommended concentrations are made before the solution level in the treating vat drops below SO% of the original volume. LITERATURE CITED (1) Fritz, C. W., “Prevention of Sapwood Stain in White Pine in the
Seasoning Yard,” Forest Products Laboratories of Canada, mimeo. (Undated). (2) Glick, D. P., J . Baet., 45, 42-3 (1943). (3) Krause, R. L., doctorate dissertation, Univ. of Wisconsin, 1944. (4) Nelson, R. H., and Cassil, C. C., U. S. Dept. Agr. Circ. 610 (1941). (5) Plummer, B. (1941)
.
pa,Jr.,
and Bonde, R., Phytopathology, 31, 812-17
( 6 ) Vaughan, E. K., Ibid., 34, 175-84 (1944). (7) Verrall, A. F., J. Agr. Research, 78, 696-703 (1949). (8) White, R. P., Phytopathology, 14, 58 (1924). RECEIVED May 24,1919.
PETROLEUM HYDROCARBONS Separation and Analysis by Adsorption BEVERIDGE J. MAIR National Bureau of Standards, Washington. n. C. T h e work performed by the American Petroleum In&tute Research Project 6 during the past 15 years in cleveloping the frartionating process of adsorption as a powerful tool in its work on the separation and analysis of hydrocarbons is reviewed. The discussion includes: methods of operation; apparatus and materials; a theoretical analysis of tho process giving equations for computing the separation factor and the height equivalent to unit theoretical stage of separation; experimental results for the separation factor and for the height equivalent to unit theoretical stage of separation; and examples of the application of the process to the separation, analysis, and purification of hydrocarbonsof various typesand molecular weight ranges.
T
HIS paper summarizes the work done during the past 15 years by the American Petroleum Institute Research Project 6 on the development and use of the fractionating process of adsorption as a tool in its work on the separation and analysis of hydrocarbons (3, 6-11, 13, 16-18). The applications of adsorption to the separation of hydrocarbons may be enumerated as follows: separation of aromatics from paraffins and cyclopsrafis;
separation of mononuclear from polynuclear aromatics ; separation of olefins from p a r a b and cycloparaffins and from aromatics; separation, in some instances, of paraffins from cycloparaffins, of monocycloparafis from bicycloparaffins, and of individual members, including isomers, in the paraffin, cycloparaffin, and alkglbenzene classes from each other; separation of polar nonhydrocarbons, such as oxygen, sulfur, or nitrogen compounds from hydrocarbons. Adsorption procedures are applicable t o materials of low molecular weight, such as the gasoline fraction, and to materials of high molecular weight such as the gas-oil and lubricant fractions of petroleum. Recently the project developed a theory of the adsorption fractionating process based on the concepts used in the analysis of fractionating processes in generalfor example, the process of distillation (18). METHODS OF OPERATION
Two methods of operation have been used by the American Petroleum Institute Research Project 6. With the first method ( 6 ) the mixture of hydrocarbons to be separated is introduced at the top of a packed column and after the charge has entirely entered the adsorbent a desorbing liquid-that is, a liquid whiah is more strongly‘ adsorbed than any of the components of the
INDUSTRIAL A N D ENGINEERING CHEMISTRY
1356
Figure 1.
Separation of Z,Z,iE-Trirnethylpelltaneand Toluene (6)
original mixture-is added. The desorbing liquid forces tlie hydrocarbon portion down the column; during passage it is fractionated according t o the adsorbability of the various coniponents. The components issue from the bottom of the column in the order of their adsorbability, with the less strongly adsorbed 1.44 I
134
I32
-
, I
u IMETHANOL-
I
Vol. 42, No. 7
coniponenti issuing first. An important characteristic of this method of operation is that the fractionation takes place with only the components of the original charge present. The method is analogous to regular distillation. The results of the application of this method (6) to the separation of two synthetic mixtures of widely different concentration of an aromatic hydrocarbon (toluene) and a paraffin hydrocarbon (2,2,4trimethylpentane), with ethanol as the desorbing liquid, are shown in Figure 1. It is apparent that a sharp separation is obtained with this method. With the second method ( S ) , the mixture of hydrocarbons is introduced a t the top of a packed column, and arter the charge has entirely entered the adsorbent, a displacing liquid-that is, a liquid with an adsorbability about the same as or less than that of the least strongly adsorbed component of the original mixture -is added. This liquid mixes with the coniponents of the oiiginal charge, slowly desorbs them, and causes them to travel down the column. The components mixed with displacing liquid issue frorn the bottom of the column in the order of their adsorbability. ilfter the weakly adsorbed components have issued from the column a second more strongly adsorbed liquid may be used to displace the more strongly adsorbed components. Finally, a dpsorbing liquid is added to desorb the inost strongly adsorbed components and cause them to issue from the column. An important characteristic of this method of operation is that the fractionation
July 1950
INDUSTRIAL AND ENGINEERING CHEMISTRY
of the original mixture takes place in the presence of an added component. This method of operation is analogous t o azeotropic or extractive distillation. The result of the application of this method (8)to the separation of a synthetic mixture of paraffins, cycloparaffins, and aromatics is shown in Figure 2. Here npentane was used as the displacing liquid and methanol as the desorbing liquid. The separation of the mixture into two portions is apparent. The first portion contains only paraffin and cycloparaffin hydrocarbons together with n-pentane, and the second portion contains only aromatic hydrocarbons together with n-pentane and methanol.
n
1357
paraffins and cycloparaffins and for difficult separations, such as the separation of isomers. The columns are waterjacketed or electrically heated and insulated for operation above room temperature when this is desirable. Inert gas pressures are applied to the top of the columns t o increase and control the rate of filtration. For the steel columns pressures up t o 110 pounds per square inch gage may be used. As an adsorbent for the separation of hydrocarbons, silica gel is very satisfactory. Silica gel (Davison Chemical Corporation, No. 22-08) with a particle size such that about 60% of the material is between 200 and 325 mesh was used in all the experiments reported here. For desorbing liquids, alcohols are usually satisfactory. It is desirable that the viscosity of the desorbing liquid be a t least as great as that of the hydrocarbon portion (4) and that it be miscible with this portion. For this reason higher boiling alcohols are used with the higher boiling petroleum fractionsfor example, n-hexanol is used with the gas-oil fraction. THEORY O F THE METHOD
APPARATUS AND MATERIALS
The adsorption equipment used by the American Petroleum Institute Research Project 6 has included columns varying in size from small glass columns, 1.25 meters in length and 1 cm. in diameter (6), to tall stainless steel columns, 16 meters in length and 2 c,m. in diameter (9). A small glass column (6) and a tall steel column are shown in Figures 3 and 4, respectively. The small glass columns are used primarily for analytical purposes, such as the determination of the aromatic content of the gasoline or kerosene fraction of petroleum; the tall steel columns are used for the laboratory largescale separation of aromatics from
F i g u r e 3. Short (1.25Meter) Glass Adsorption Column and Receiver (6) All dimensions, mm.
The analogy between distillation and adsorption fractionation is illustrated for the separation of two substances, A and B , in figure 1 of (1%). In this illustration, the entire charge is assumed t o be contained in the vertical fractionating section of either a distillation or an adsorption column and the two phases are caused t o move in opposite directions. For the distillation process, the counterflow of the vapor and liquid phases is accomplished by placing a heater a t the bottom and a condenser at the top of the section, whereas for the adsorption process, the counterflow of the adsorbed and liquid phases is produced by introducing fresh adsorbent at the bottom and placing a suitable desorbing liquid a t the top of the section. As a result of the counterflow and the interchange of the molecules of A and B between the phases, there is a net transport of the more volatile (with distillation) or more strongly adsorbed (with adsorption) component A upward and a net transport of the less volatile (with distillation) or less strongly adsorbed (with adsorption) component B downward. I n actual practice, of course, the adsorption experiment is more easily performed by using a long column of adsorbent and having I 160-
I55
I
I
:i Ir
FRACTWATTICNOF GASCL By ADSORPTION
POLYWAR AROMAnCS
f
ALKYL
BENLENES
T
PARAFFW 7-
Figure 4. Tall (16Meter) Stainless Steel Adsorption Column with Head and Receiver ( 9 )
oLll 250 VOLUME
Figure 5.
OF FILTRATE IN M L
Analysis of an Aviation Gasoline (6)
500
I
I
I
1250
ISW
1750
"o
I
OF DESORBEM
2000
4 I-
2250
I 2500
VOLUME OF FILTRATE, ML.
Figure 6.
Separation of Gas-Oil Fraction of Oklahoma Petroleum ( 1 1 )
INDUSTRIAL AND ENGINEERING CHEMISTRY
1358
1 3
1 1.49
-
1.48
-
15.01-
I.%
f-
T
I
-
1.49
W
(A)
1.46-
a
-I
a
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i1.46- (B)
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+
iL
c a
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i I& LL 0
W
U
-I 3
e
7
g1.47-
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1
I1 T
1.48-
W
1.47-
Vol. 42, No. 7
1.45
-
J
x 1.45W
n
z
W X
n
1.44
-
z
5 0
1.43
r'
1.44W
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-
I-
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E1.43W
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1.42
1.42
z W
-
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w X
1.40
1.41
-
1.40
-
r
s V
u
-
1.39
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_j 40
VOLUME .OF FILTRATE IN M L .
Figure 7 .
5
2 1.37-
#
?
x, *.
1.36
0
10
VOLUME
I 21 OF
40
FILTRATE
0
IN ML.
10
VOLUME
OF
FILTRATE
IN ML
Separation of Three Mixtures Containing Paraffin, Olefin, and Aromatic Hydrocarbons ( 6 )
the molecules of -4 and B move downward over fresh adsorbent with a suitable desorbent following. The manner in which the composition of an equimolecular mixture of two substances A and B varies along the length of an adsorption fractionating column a t various stages of the process is shown in Figure 2 of ( l a ) . As soon as the flow of material has started, component A begins to accumulate at the top of the section and component B a t the bottom, while the middle portion remains, for some time, unchanged in composition. The net volumes of components A and B transported toward either end continue to increase as more material flows through the system until finally no material of the initial composition remains in the middle and a state of equilibrium has been established in the column. It has been shown (12) that the total net volume of component A , Z AvA, transported upward across a given cross section where the initial composition exists is given by the equation Z AVA = CrAVa (1) where U represents the total volume of material nhich has floFed
through the system and 4VA is the difference in volume fraction of component A between the adsorbed and liquid phases. It has been shown ( l e ) that the separation factor, cy, for the adsorption fractionating process is given by the equation o(
-1
4vA/vff(vf8- A v A )
(2)
where Vi and V b indicate the volume fractions of components A and B in the liquid phase. Since AVA may be evaluated from experimental observations using Equation 1, it follows that the separation factor may be obtained from Equation 2. When equilibrium has been established the relation between the concentrations of the components, A and B , along the length of the fractionating section L is given (12) by the equation lOg(vA/vB)~!- IOg(vA/vO)& = (l/Z)(lOgoC)L
(3)
If the ratio V A / V Bis known as a function of the length of fractionating section, a plot of log ( v A / V B ) tagainst length L gives, over the range of composition over which cy is fiubstantially constant, a straight line having a slope equal t o (l/z)(log a). Since a
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
July 1950
decreases from a value near 8 a t 10% benzene to near 2.3 at 90% benzene. For the systems benzene plus n-propylbenzene and benzene plus ethylbenzene, the decrease in separation factor with increase in concentration of benzene is slight. For equivolume mixtures for each of the pairs of components, the values of the separation factor are as follows: benzene plus ethylbenzene, 1.2; benzene plus n-propylbenzene, 1.4; benzene plus *hexane, 3.4; benzene plus cyclohexane, 3.0; ethanol plus n-heptane, 26.
1.43 -CYCLOHEXANE
1.42
1
1.41
HFAGHT EQUIVALENT T O U N I T THEORETICAL STAGE OF SEPARATION
F2 1.40
In experiments to determine the height equivalent to unit theoretical stage of separation, an amount of adsorbent greater than that required for the resolution of all the original material is used. The results of experiments to determine the height equivalent to unit theoretical stage of separation are given in (12). Plots of the logarithm of the ratio of the volume fractions with respect to the volume of filtrate are substantially linear for the systems benzene plus ethylbenzene and benzene plus n-propylbenzene but show some curvature for the system benzene plus n-hexane. Values for the height equivalent to unit theoretical stage of separation, 2 , are near 1 cm. for the 2-cm. diameter columns used.
I
X
w
0
139 W
: a
I
0
w
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1
K
N-HEXANE
7
1.37
4
ETHANOL>
1.36 CYCLOHEXANE
1.35
5
PLUS
10
IN
20
25
ML.
Separation of Mixture of n-Hexane a n d Cyclohexane ( 9 )
is known, this yields a value tor theoretical stage of separation.
2,
APPLICATIONS
0
N-HEXANE
15
VOLUME
Figure 8.
1359
the height equivalent to one
A substantially quantitative separation of aromatic hydrocarbons from paraffins and cycloparaffins, in fractions covering a wide boiling range, such as the entire gasoline, kerosene, or gasoil fraction, is readily effected by the process of adsorption. On a small scale the separation is used t o determine the aromatic content. The results of the analysis of a n aviation gasoline (6) are given in Figure 5 The aromatic content is computed from the volumes of the aromatic portion and the paraffin plus cycloparaffin portion from plots similar t o those shown in Figure 5. The
SEPARATION FACTOR
I n practice, the first method of operation described in the pred i n g section is used-that is, the charge of the two components to be fractionated is introduced at the top of a packed column and caused to pass downward over fresh adsorbent by the desorbing liquid which follows. The amount of adsorbent used is less than that required for the complete resolution of the charge, so that some solution of the original composition is obtained in the atrate. The net volume of material transported, ZAVA, is obtained from volume-composition diagrams for the filtrate and V i and VL are taken as equal to the volume fractions of components A and B in the original mixture. The total amount of material which has circulated through the system, U,is given by the equation U = Mu, (4) where 144 is the mass of fresh adsorbent used in the experiment and u, is the quantity of material resident in the adsorbed phase per unit mass of adsorbent] the material being measured in the liquid state a t 25" C. The total quantity of fresh adsorbent, M , is readily obtained. The quantity! u,, is obtained from experiments in which the adsorbent is equilibrated with the liquid, without physical contact of adsorbent and liquid, and with the transfer of material from liquid to adsorbent taking place through the vapor phase ( l a ) . Experimental results are given in (la) for the separation factor as a function of the composition for solutions of benzene plus nhexane, benzene plus cyclohexane, benzene plus n-propylbenzene, and benzene plus ethylbenzene. For the systems benzene plus nhexane and benzene plus cyclohexane, the separation factor decreases markedly with increase in the concentration of benzene. Thus, for the system benzene plus n-hexane, the separation factor
r ! 1 METHYLCYCLOHEXANE-
141
I
I
q 2 138
13' 1.36
1 t
,
PLUS, ZPf(-TRlMETHYLPENTANE METHYLCYCLOHEXANE
1.35 VOLUME
IN ML
Figure 9. Separation of Mixture of 2,2,4T r i m e t h y l p e n t a n e a n d Methylcyclohexane (9)
INDUSTRIAL AND ENGINEERING CHEMISTRY
1360
W
L
->
t 0
2 1.4
II.
w
a I. 3
I.! ISOPROPANOL
,
C93,4-TETRAMETHYLBENZENE
7
PLUS
N-BUTYXNZEhlE
IS
20
Figure 10.
paraffins may, in some cases, be separated from individual cyrloparaffins. Such separations require the use of greater amounts of adsorbent per unit volume of charge than the separation of aromatics from paraffins and cycloparaffins. The result's of the sinall scale separation of two equivolunic mixtures of a paraffin and a cycloparaffin (9) are shown in Figures 8 and 9. With one of these mixtures (n-hexane plus cyclohexane) the paraffin is the more strongly adsorbed, but with the other (2,2,4-trimethylpentane plus methylcyclohesane) the cycloparaffin is the more strongly adsorbed. Also, the fractionation of these components is unsymmetrical and, in both cases, the first portion of the filt'rate contains the least strongly adsorbed component in a substantially pure condition; t'he last part of the hydrocarbon portion of the filtrate consists of a mixture which is substantially nonfractionating. The occurrence of nonfractionating mixtures is not uncommon with hydrocarbon systems as has also been observed by Hirschler and ilmon (6). In terms of the theory previously given, the separation factor for a nonfractionating mixture is unit,y. The separation of isomeric alkylbenaencs (9) is illustrated by the results given in Figure 10 for an equivolume mixture of n-butylbenzene and 1,2,3,4tetramethylbenzene.In this case, the f i s t portion of the filtrate consists of n-butylbenaene in a substantially pure condition, and the last part of the hydrocarbon portion of the filtrate consists of a mixture which is substantially nonfractionating. The eeparat'ion of isomeric impurities from a paraffin and an aromatic and the preparat'ion of compounds of very high purity are illustrated in Figures 1and 5 of (16). With 2,2,3-trimethylbutane having originally a purity of about 99.6 mole %, a large portion of the charge was recovered with a purity of 99.98 mole yoby this single pass operation (16). Similarly with 1,4-diethylbenzene having originally a purity of 99.38 mole %, a large portion of the charge was recovered a t a purity of 99.94 mole % or better (16). Similar results have been obtained by other investigators (5, 14). LITERATURE CITED
1.37 0
Val. 42, No. 7
S
IO
VOLUME I N ML S e p a r a t i o n of Mixture C o n t a i n i n g TWO Isomeric A l k y l b e n z e n e s ( 9 )
aromatic content may be determined within about *0.15% by volume. The method has been adopted as a tentative standard for the analysis of the gasoline fraction of petroleum by the American Society for Testing Materials ( 1 ) . The result of a laboratory large scale separation of the gas-oil fraction of an Oklahoma petroleum ( 1 1 ) is given in Figure 6, which shows the sharp separation of the aromatic from the paraffin and cycloparaffin hydrocarbons and also the additional separation of the aromatic hydrocarbons into a portion consisting largely of mononuclear aromatics and a portion consisting largely of polynuclear aromatics. The small scale separation of olefin hydrocarbons from paraffins and aromatics (6) is illustrated by the results for three synthetic mixtures in Figure 7 . In this figure, A shows the separation of 2,2,4-trimethyl-l-pentene; B, that of cyclohexene; and C, that of both cyclohexene and 2,2,4-trimethyl-l-pcrltene from 2,2,4-trimethylpcntane and toluene. In C there is a subfrom cyclohexene. stantial separation of 2,2,4-trimethyl-l-pentene The use of adsorption procedures for the separation of olefin hydrocarbons is limited by their tendency t o be isomerized or polymerized by adsorbents. The Bureau of Mines has used the adsorption process for the separation and determination of olefins in the naphtha fraction from shale oil 1.2). The complete separation of the paraffin-cycloparaffin portion of a petroleum fraction into a paraffin portion and a cycloparaffin portion Fith silica gel as the adsorbent has not been effected, though changes in concentration may be produced and individual
(1) Am. Soc. Testing Materials, Standards on Petroleum Products, Committee D-2, Year Book, 1948, p. 469.
(2) Dinneen, G. U., Bailey, C. W., Smith, J. R., and Ball, J. S., A n a l . Chem., 19, 992 (1947). (3) Forziat.i, A. F., Willingham, C. B., &fair, B. J., and liossini, F. D., J . Research N a t l . Bur. Standards, 32, 11 (1944). (4) Gooding, R. M., and Hopkins, R. L., presented before the Division of Petroleum Chemistry, a t the 110th Meeting of the AMERICANCHEMICAL SocIEw, Chicago, 111. ( 5 ) Hirschler, A. E., and Amon, S., IND. ENG.CHEM.,39, 1585 (1947). (6) Mair, B. J., J . Research Natl. Bur. Standards, 34, 435 (1945). (7) Mair, B. J., and Forxiati, A. F., Ibid., 32, 151 (1944). (8) Ibid., p. 165. (9) Nair, B. J., Gaboriault, A . L., and Rossini, F. D., ISD.EXG. C H E X . . 39. 1072 (1947). (10) Mair, B. J., Schicktanz, S. T., and Rose, F. W., Jr., J . Research Natl. B u r . Standards, 15, 557 (1935). (11) Mair, B. J., Sweetman, A. J., and Rossini, F. D., ISD. Eso. CHEM.,41, 2224 (1949). (12) hlair,B. J.,TVesthaver,J. W., and Rossini, F. D., Ibzd., 42, 1286 (1950). (13) Mair, B. J., and White, J. D., J . Rescarch NatE. Bur. Standaids, 15, 51 (1935). (14) Melpolder, F. E., Woodbridge, J. E., and Headington, C. E., J . Am. Chem. SOC.,70, 935 (1948). (15) Rossini, F. D., Xlair, B. J., Forziati, A. F., Glasgow, A. R., Ji., and Willingham, C. B., O d Gas J . , 41, 106 (1942); Petroleum Refiner, 21, 73 (1942); Proc. Am. Petroleum Inst., 23, 111, 7 (1942). (16) Streiff, A. J., hfair, B. J., and Rosxini, F. D., 1x0. EKG.CHEM., 41, 2037 (1949). 117) , , White. J. D., and Rose, F. K., Jr., J . Research iVat2. Bur. Standards, 21, 151 (1938). (18) Willingham, C. B., Ibid., 22, 321 (1939). RECEIVED December 28, 1949. Presented at the Southwest Regional Meeting of the ERICA AN C H E m c A L SOCIETY, Oklahoma City, Okla., Dec. 9-10, 1949. This investigation was performed at the National Bureau of Standards as part of the work of the American Petroleum Institute Research Project 6 on the Analysis, Purification, and Properties of Hydrocarbons.
