Chemistry of Clay racking Catalysts CHARLES L. THORIilS. JOHN HICKEY, AND GLEN STECKER Great Lakes Carbon Corporation, Morton Grove, Ill.
C
LAYS of the montmoril-
hfontmorillonite claj s are catalysts for the catalj tic In the comnioner clays, the cracking of petroleum. By acid treating the clays, their lonite type form the montmorillonite is usually catalytic activitj can be increased to a maximum, after inore complex as a result ( J f basis for the so-called "natuwhich further acid treatment decreases the activity. The isomorphous s u b s t i t u t i e i i s ral" catalysts for the crackacid treatment remo\ es metallic atoms (mostly alumi(7-9, 11). I n one such subsrii n g of petroleum. While tution, magnesium atonis lenum) from the central laj er of the montmorillonite crj snatural clays may have some tal. These metallic atoms are in octahedral coordination place a part of the aluininuni small activity, the clays used with oxygen atoms held by silicon atoms. It is postulated atoms in the central layer. .I commercially for cracking that removal of one of a pair of octahedral atoms remotes bivalent atom replacing a tripetroleum have been acid valent one leaves the crystal two hydroxyl groups and leaves the remaining metallic treated to produce a practiatom of the pair in tetrahedral coordination. This lattice negatively charged by cal activity. The source of change from octahedral to tetrahedral coordination an amount equivalent t o oiic this catalytic activity is of leaves the crystal lattice with a negatiie charge that is valence unit. Positive ioiii; both theoretical interest and balanced bj a hjdrogeii ion. It is postulated further that such as calcium, magnesium, industrial importance. this hydrogen ion is the source of the catal? tic actit i t 3 sodium, potassium, etc., lieDavidson, Ea-ing, a n d Evidence is presented to show that there is a correlation come associated with the Shute ($2) have postulated between the acidity of the catalysts and their catal?tic structure to neutralizc this that the catalytic activity actitity. The hj drocarbon reactions catal?Led are lattice charge. It is cusfor cracking petroleum is asthought to follow a carbonium ion mechanism. tomary to repwent these sociated x i t h the ovygen ions as being in the interatoms in these cla? s and is replanar space. lated to the fact that the disMore or less iron also replaces aluminum in the central layer. tance between t,wo oxygen atoms ( 2 . 5 5 X.)is close to the distance I n the ferrous state, the iron substitution causes the same changes between alternate carbon atoms in many hydrocarboris (2.54 observed with magnesium. I n the ferric state, iron substitution If one admits the arguments in favor of this postulate, one must for aluminum causes no change. also admit that there are a t least two facts which the postulate Besides the above, there is a t least one other important ijodoes not explain. First, t,hat there are inaiiy other similar st,rucmorphous substitution: tet'rahedrally coordinated aluniinuni can tures-e.g., the micas and talc-containing the same t,ype of silireplace tetrahedrally coordinated silicon in the silica layers. In con-oxygen structures and distances, which are not, act,ive as this substitution, a trivalent element replaces a tetravalent. one. cracking catalysts; secondly, that the postulate does not account This leaves a negative chargp on the lattice. This chiuge is a!w for the increase in cracking activity obtained by acid treatments. neutralized by metallic ions that become associated n-it,h the At the same time, Davidson et a/.called attention to t,he pyeslattice (?'-9>11). The effect is similar to that clcwxibed ahovc ence of hydrogen ions (acidity) in the montmorillonite catalysts in connection with magnesium. and postulated that t,his acidity was related to the ability of t'he Ross and Hendricks ( 1 3 ) have used a convenient n-riy of rcprecatalyst, to polymerize olefins by a Whitmore (17') mechanism. seiiting t,heee c l a p n i t h the various substitutions. One of the authors (15) has sholm the relation between the catalytic acit,ivity of several synthetic catalysts including syn[Base Exchangeable Ion?] thetic silica-alumina catalysts and the acidity of those cat,alyst's. t f Greiiall (6) has demonstrated t,he acidit,g of montmorillonit'e [Center Layer Ions jjrl'etrahedral Ions] O X :OII14 catalysh and has indicated that the acidity is related t o the catalgtic activity. This diagram separates the base escharigeable ions from tlie The present paper describes a new postulat,e to explain the center layer oct,ahedral ions and the tetrahedra! ions. The di:tsource of t,he acidity and the catalytic a d v i t y . This explanation gram is a convenient means for showing the various isomorphous is based on the at,omic structure of montmorillonite proposed by substitutions and the source of the change accounting for the b a w Hofmanii, Endell, and Wilm (10); the changes in composition that occur during acid treatment; the changes in catalytic activity which t,ake place during acid treatment (3); and on the post'uTABLE I. A s a r ~ ~ sOF r s CL.\V lated changes in the atomic structure that occur during acid [Results based on volatile-free t:J50" C . ) cia)-! treatment. hIoles '100 C .4n idealized atomic structure of montmorillonite as projected wt.%" on a plane is given in Figure 1 (10). I n this structure there is a 61.0 1.016 27.9 0.274 central layer containing aluminum atoms in octahedral coordina4.85 0.120 1.73 0.031 t,ion. Each aluminum atom is associated with four oxygen 0.01.5 2.42 0.028 atoms and two hydroxyl groups. Above and below the central 1.75 0 0033 .33 aluminum layer there are layers containing silicon in tetrahedral a These analyses h a r e been corrected for small amounts of crg-*ralline coordination with oxygen. All these layers unite to form a impurities identified and estimated h s x-ray diffraction. Quartz, 7 % and neutral plane or sheet,. There is an interplaner space which may kaolinite, 395, mere the major impurities. contain TTater, and t,heii the structure is repeated.
.
'
866
INDUSTRIAL AND ENGINEERING CHEMISTRY
May 1950
Two tetrahedral layers:
INTERPLANAR SPACE
si
4 0
2 OH
4 AI
4 0
2 OH
4 Si
6 0 Figure 1.
A10.7~
The data in Table I now permit the calculation of the atoms in the base exchange layer using simple ratios: sodium, 0.40; potassium 0.05; magnesium, 0.15; and calcium, 0.22. The above data permit the calculation of the molecular weight of the unit cell of the clay to be 748. This, with the data from the potassium hydroxide titration of the raw clay (Table I ) gives hydrogen ion equal to 0.3.
60. 4
867
Schematic Presentation of Idealized Montmorillonite Structure (10)
'exchangeable ions. It will be used below to illustrate the chemical reactions of the clay used in these studies. The analysis of the clap studied herein is given in Table I. This analysis has been fitted to a modified Ross and Hendricks formulation as shown in Figure 2.
In assigning various elements to the several positions, it must be remembered that the final structure must be electrically neutral. The above assignments fill this requirement. As will be shown later, the above assignments are consistent with the chemical behavior of the clay. It is one of the well-known characteristics of montmorillonite that the interplanar ions are base exchangeable (IS). These ions can be replaced by other ions without altering the fundamental montmorillonite structure. By treating the clay with cold, dilute acids, such ions can be replaced by hydrogen ions. (Since the hydrogen ions are small, there is no assurance that they remain in the interplanar layer. .4t present, their position is unknown. Even so, it may be convenient to show them as interplanar hydrogen ions for identification purposes.) Cold (room temperature) acid removed substantially all of the calcium, sodium, and potassium ions from the clay. I t seems clear that these ions came from the interplanar space. Cold acid also removed a part of the magnesium. It has been assumed that this, too, came from the interplanar space. It was on this basis that the split in Figure 2 between the magnesium ion in the interplanar space and magnesium in the central layer was made. Using the formulation in Figure 2, the action of cold acid on the clay can be represented by the following equation:
The following assumptions were used for constructing Figure 2: 1. The clay composition fits the pyrophyllite structure, modified as necessary to accommodate isomorphous substitutions. This assumption carries with it the implication that the two tetrahedral layers present actually contain four atoms each (a total of eight); that the octahedral layer actually contains four atoms.
Only base exchangeable ions are removed by cold acid treatment. (Since 17% of the magnesium is removed by cold acid then this much magnesium is in base exchange positions.) 3. All iron is ferric iron and is in the octahedral layer.
[ 1.49H+]
t
t
+
[AI3 08MgO 70Feop21[Si,~.410I ~ ] O ~ O [ O H0]40 ~ S a + -k O.O5K+ 0.15 Mg++ 0.22 Ca++
+
+
(3)
2.
The data in Table I were then used to obtain the atomic ratios: aluminum to silicon, 0.537; magnesium to silicon, 0.118; and iron to silicon, 0.030. Let A14 and Ala represent, respectively, the number of aluminum atoms in the tetrahedral and octahedral layers of a unit cell of the montmorillonite clay; and AI, the total number of aluminum atoms. Let Mg, Si, and Fe equal the total number of magnesium, silicon, and iron atoms in a unit cell of the clay. Then A10 = 4 - 0.83 Mg - Fe (1) Substituting Mg = 0.118 Si and Fe = 0.030 Si A16 = 4 - 0.098 Si - 0.030 Si = 4 - 0.128 Si AI; = 8 - Si and, A14 A 1 6 = .41 = 0.537 Si Then XI, = 0.537 Si - -416 and 8 - Si = 0.537 Si - Ala
+
Ale = 1.537 Si
-8
'Equating Equations 1 and 2: 1.537 Si
-8
=
4
- 0.098 Si - 0.030 Si
and solving gives: Si = 7.21, AI = 3.87, Mg = 0.85, and Fe = 0.22 Octahedral layer: .41,,08 Mgo.70 Feo.t2
(2)
t
+
w
H 6 0 4 Si
4 0 4 Si
6 0
Figure 3. Schematic Presentation of Idealized Cracking Catalyst Structure
Although cold acid under controlled conditions can almost completely remove the base exchangeable ions, practically no aluminum is removed: This suggests that cold acid does not attack the central layer. Hot acid (boiling) removes some aluminum indicating that the hot acid not only replaces base exchangeable ions but also attacks the central layer in the clay structure. The hot acid treatment can be regulated so that the aluminum can be
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
868
TABLE
.4cid Treatinn Conditions Initial .4cid acid ooncn., Temp. dosageb %
Sample No.
Ra w clay 1 2 3 4 5
RbOL
...
0.30 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50
...
10.0 3.7 5.2 6.9 8.4
Vol. 42, No. 5:
11. HYDROCHLORIC ACIDTREATMESTS O F COJfPOSITE CIAY 70Leached Out Yield,
%0
..
94 92 89 87 84 81 79 76 75
KO H Titration, Me. per G."
Catalytic Sotivity
Total matter dissolved, % of dry raw clay
Aluminum
..
..
43 35
0.41 0.51 0.53 0.73 0.84 0.89 0.97 1.04 1.04 0.99 1.04
B.p. B.p. B.p. B.P. B.p. 10.0 6 B.P. 11.4 7 12.9 B.P. 8 B.P. 13.3 9 72 B.P. 10 16.7 Determined on product heated t o 500' C. for 1 hour. b Expressed BS grams of HC1 pel gram of clay free of volatile matter. Basis: untreated clay and product dried at 110' C.
6 8 11 13 16 19 21 24 25 28
67 92 107 116 116 122 120 87
90
-
1
7 13 22 30 37 4 ?.J 53 58
68
by .4cid
97" of Total i n clay---------^--. Magneiilirn Cltlciulil Iron Sili?oi, , .
17 ZG 35
3 ,j 43 53 56 65
69 78
70 81 88 93
..
94
,.
95
95
..
...
4 12 20 29 37 44 51 58 66
0.3
...
2
0.2
o:i
. .
0.1
...
0.1
(1
partly removed. As the aluminum is progressively removed, the catalyst activity for cracking petroleum increases, reaches a maximum, and then declines (3). To account for the changes occurring a9 a result of hot acid treating, the following hypothesis is proposed: ( a ) One of the pair of octahedrally coordinated aluminum atoms together with the two hydroxyl groups is removed from the central layer. ( b ) The remaining aluminum atom changes from octahedral to tetrahedral coordination with the four remaining oxygen atoms. The trivalent aluminum tetrahedrally coordinated wit.h the four oxygen atoms leaves the lat.tice with a negative charge equivalent to one valence unit. (c) A hydrogen ion becomes associated with the latt,ice to neutralize this negat
t z I-
70
o 50
+ 611- ->
2hl"++
+ [Si,HslO,o
4
(3) 30
When all the aluminum has been removed, the prbduct should be a sheet made up of silica tetrahedra. Each silicon atom would be attached to one hydroxyl group and t o three oxygen atoms shared by other silicon atoms. This is represented in the above forniulation by [SigH~]020,although [Sis(OIl)a]Ojzmight be preferred.
I e 3 NUMBER OF METALLIC ATOMS REMAINING IN CENTER L A Y E R O F UNIT C E L L
4
Figure 4. Relation between Activity of Catalysts and Number of Metallic Atoms in Center Layer of Cnit Cell
INDUSTRIAL AND ENGINEERING CHEMISTRY
May 1950
are then attacked. Equation 6 shows aluminum or ferric iron renioval of the type indicated by Equation 4a.
TABLE111. CALCULATED COMPOSITION OF CATALYST SAMPLES Sample No.
[1.49H+]
‘6
[Ala OsMgO 7oFeo22][ i7 ZAO T~IO~O[OH]~+~H+ [ 2.49H +I
Raw clay
+
‘ B
+
1,413 osMgo.70Fe02~1[i7.21 Alo.7~lOzo[OH12 AI+++
+ 213~0
(6) 1
The magnesium in the central layer comes out in a similar way. [ 1.49 H + ]
t
869
t
+
-
2
[A13,0&kgO ~ o F ~ o . ~ ~ ~ [ S ~ ~ . ~ I A1.40 ~ O ,H~+~ ] O Z O [ O H ] ~ [1.49H]
T t
+
[AI, oaFe0.~&3i7 ~ , A ~ ~ . ~ ~ ] O Z O [ O I0.70 ~ ] ZMg++ .~O
+ 1.40H20 (7)
In Equation 7,removal of a magnesium atom also removes the hydrogen ion associated with it. ilt the s a v e time the adjacent aluminum atom is converted from octahedral to tetrahedral and the appropriate hydrogen ion becomes associated with the structure. The over-all result is no net change in the number of hydrogen ions associated with the structure, although the nature (ionizability) of the hydrogen ions may have changed appreciably-for example, the acid produced in Equation 7 may be a stronger acid than the starting acid. From the quantitative analytical data in Tables I and I1 the composition of each catalyst preparation was obtained. The hydrogen ion content has been calculated using the assumptions and postulates presented above. They have been combined and represented by the modified Ross and Hendricks formula. The “base exchange” position in the formula is used to show the hydrogen ions that the authors consider relatively strongly acidic and the source of the catalytic activity. (By so representing the hydrogen ions it is not implied that they are necessarily base exchangeable.) These formulas are presented in Table 111. The forniulas for the first six samples represent the replacement of the base exchange ions, the gradual removal of the central layer ions, and the conversion of remaining aluminum atoms to tetrahedral coordination with the resulting increase in the hydrogen ion up to 3.00. Beginning with sample 7 , the products are the results of reactions of the type given in Equation 5 . The catalytic hydrogen ions decrease owing to the removal of the tetrahedrally coordinated atoms in the central layer. The vacated positions are filled by hydrogen atoms that are weakly acidic and noncatalytic for cracking. These hydrogen atoms are shown with the tetrahedral layer atoms. In constructing the formulas in Table 111, it has been assumed that aluminum atoms in the tetrahedral layer have remained unattacked by the acid. Any removal of these atoms reduces the number of strongly acidic hydrogen ions. EXPERIMENTAL FACTS
From the work of others plus the data presented in this article, U
certain facts have been established. These are summarized below in a way that is independent of any of the t8heoryor hypotheses presented above. These facts are: 1. Raw clay has some catalytic activity (see Table 11). 2. Cold acid treatment removes most of the calcium, sodium, and potassium along with an appreciable part of the magnesium. None or only negligible proportions of the aluminum, iron, or silicon are removed (see Table 11). 3. The catalytic activity is not changed appreciably by treating the clay with cold acid (see Table 11). 4. Hot acid treatment removes aluminum, iron, as well as magnesium, not removed by cold acid. 5. As the aluminum, iron, and magnesium are removed by hot acid, the catalytic activity rises a t f i s t and then declines (Table I1 and Figure 4 ) . 6. When the acid treatment is severe enough to go past the maximum catalytic activity, the x-ray diffraction pattern changes.
3
4
5
6
8
9
10
The diffraction lines from samples 1 through 6 are unchanged. The lines from samples 8 through 10 are definitely broader. Saniple 7 may be sli htly broader. 7 . No signifcant amount of silica is removed by hot or cold acid (see Table 11). 8. The catalysts have acidic properties that can be titrated with alkali (Table 11). COMPARISON OF OBSERVATIONS WITH THEORY
Qualitatively and semiquantitatively the theory or hypothesis proposed above accounts for the observed facts. According to the composition of the raw clay as givenin Table 111, it contains acidic hydrogen and should have the catalytic activity which it exhibits. The theory states that the catalytic activity should reach a maximum when one half the metallic atoms are removed from the central layer of the lattice. Figure 4 illustrates this. The number of strongly acidic hydrogen ions also reaches a maximum when one half the metallic atoms in the central layer of the lattice have been removed (Table 111). The catalytic activity should be a function of calculated strongly acidic hydrogen ions. As is seen in Figure 5 this is approximately true. (The acidity in Figure 5 is expressed as milliequivalents per gram. The number of strongly acidic ions shown for each formula in Table I11 were converted to milliequivalents per gram using the molecular weight of the formula given in Table 111.) The observed acidities correlate very well with the observed catalytic activities as can be seen in Figure 6. Samples 9 and 10 have been omitted from Figure 6. As has been discussed above and shown in Table 111, samples 7 through 10 contain both strongly acidic hydrogen ions that can act as a cracking catalyst and weakly acidic ions that are not cracking catalysts. [It is known that
INDUSTRIAL A N D ENGINEERING CHEMISTRY
870 130 Lo
t; 110
>-
a &-
2 90 U
0
> 70
k
1
$ 50 30
*
0
1 2 3 4 5 6 CALCULATED ACIDITY, MILLIEPUIVALENTS PER GRAM
Figure .5. Relation between Activity of Catalysts and Calculated -4cidity of Catalysts
silica is not a cracking catalyst even though it shows titratable acidity (16).I The observed catalytic activity would reflect only the strongly acidic hydrogen; the alkali titration (observed acidity) n-ould not distinguish betyeen t,he t,wo, and would show the sum of the tn-o. This effect, seem t o shox up strongly enough in the values for samples 0 and 10 to justify their omission from the correlation in Figure 6. The fact that the observed acidity values for samples 9 and 10 do not correlate with ctLtalytic activity is regarded as a part'ial subst,antiation of t8hehypothesis. The calculated acidit,ies in Table I11 do not agree with the observed acidities in Tablc 11. The authors do not know the reason for.t.his. Further, the theory as presented in Table I11 indicates that the total acidity should increase from sample 7 through 10. The observed aciditmyvalues do not. The authors do not know the reason for this, although interplanar dehydration to form bonds seem a likrly reason. The catalysts were Si--0-Si heated to 500" C. for 1 hour before alkali titration. This heating might nell cause such an interplanar dehydrat,ion. The theory arid experimental facts are in sufficiently good agreement so that it can be stated with considerable assurance that the catalyt,ic activity of this type of catalyst for hydrocarbon reaction is associat,ed vith hydrogen ions in the catalyst. It also seems clear that the hydrogen ions arise as a result of central layer atoms in tetrahedral coordinat'ion n-ith silica tetrahedra. For some time it has been maintained by one of us (15) that catalytic cracking t,akes place by a carbonium ion mechanism ( 1 7 ) . This mechanism is t'hought to hold for the hydrocarbon reactions in the presence of the synthetic silica-aluminum catalysts as well as the clay catalysts. It is not necessary to go int,o the details of the mechanism here, but it is pertinent t.o point out that an acidic catalyst is an essential part of carbonium ion reactions. The present paper has proposed an origin of this acidic catalyst from montmorillonite clays. EXPERIMENTAL
CLAYSOKRCE.The montmorillonite clay on which t,his study was made was composited from a large deposit in the western United Stales. Cores were drilled systematically in a rect,angular grid pattern at 60-meter intervals. Samples from all cores were coniposited on the basis of in situ weights-Le., proportional to the bed thickness a t the core position. CATALYST ACTIVITY. Activity was determined following a procedure similar to that described by Shankland and Schmitkons (14) irith the following essential differences: (a) The catalysts were t,ested in powdered form instead of pellets. ( b ) Superfiltrol (a commercial, clay cracking catalyst) was used as the reference catalyst with an activity of 100. ( e ) 19 ml. of catalyst were used instead of 80 ml. ( d ) A liquid space hourlv velocity of 5.26 was used for 2 hours.
Vol. 42, No. 5
Acid Treatment of Clays. COLDACID TRIUTBIENT OF CLAY. Acid treatment a t room temperature was done in glass beakers; agitation was achieved by motor-driven glass stirring rod. The treatment vas carried on for 6 hours, after which the product was collected on a filter.. The clay treated in this way was too gummy for Soshlet extraction and had to be washed repeatedly with cold distilled .ilnt.ei by decantation until the filtrat,e was substantially free of the anion of the acid used. The product and filtrate were treated as described in t'he hot acid activation given below. IIOTilcrn TREATT.\IBST OF CLAY. All hot acid activation experimerits were carried out in glass fla$ks, either over a gentle flame, or submerged in a hot oil bath. The -60-mesh rlay was iritroduced to the hot acid, and mished donn the sides of the flask bvith remaining xater of dilution. Agitation was by ebullition, m d a reflux condenser was attached t o prevent loss of acid or Jwter. The standard length of treatment wa? 6.5 hours. \ 130 m
5 113 > 4
4 90 LL
0
>- 70
t 2_
54
50
30
04
06 08 OBSERVED ACIDITY OF CATALYSTS, MILLIEPUIVALENTS PER GRAM
IO
Figure 6. Kelation between Activity of Catalysts and Their Acidity as Determined by Potassium Hydroxide Titration
After acid treatment was completed, the product, was collected on a filter, washed several times with distilled water, then washed continuously in a Qoxhlet extractor for 23 to 21 hours. The combined filtrates were analyzed for elements leached from the clay. The activatmedclay was dried a t 110' C.:and brushed through a 60-mesh sieve. The treating loss, or conversely the yield, was calculated on the basis of 110" C. weights of the original and treated clays. Method of Determining Acidity. Ilinus 60-mesh clay was ignited a t 500" C. for 1 hour and cooled in a desiccator. Exactly 1.00 gram was quickly weighed into a 250-ml. glass-stoppered Erlenmeyer flask. and 50.0 ml. of 0.1 S potassium hydroxide solution were added. The flask was stoppered arid shaken mechanically for 30 minutes, after which the contents were filtered through Whatman No. 40 filter paper. The first 10 ml. of filtrate were discarded, then a 10.0-nil. aliquot, was titrat'ed to a methyl orange-xylene cyano1 end point with 0.1 S hydrochloric acid solution. Tile activity T ~ expressed S as milliequivalents of potassium hydroxide reacted per gram of clay. X-Ray Diffraction. The diffraction photographs were made using methods arid equipment that are ell known and need not be described here. The photographs were made on samples heated a t 500" C. for 1 hour and cooled in a desiccator. 4CKNOWLEDCMEYT
The authors are indebted to S. Mroeowski for making and analying the x-ray diffraction photographs, and to the Great Lakes Carbon Corporation for their permission to publish this work.
INDUSTRIAL AND ENGINEERING CHEMISTRY
May 1950
LITERATURE CITED
Baylis, W. S., U. S. Patent 1,776,990 (Sept. 30, 1930). Davidson, R. C., Ewing, F. J., and Shute, R. S., Nutl. Petroleum
News, 35, No. 27 R,318 (1943). Ewing, F . J., U. S. Patent 2,410,436 (Nov. 5, 1946). Ewing, F. J., Secor, R.B., and Warner, J. G., Ibid., 2,391,312 (Dec. 18, 1945) Glaeser, R., Compt. rend., 222, 1241 (1946). IND.ENG* C H E & f s i 2148 (1948); 413 1485 ( 1949) Grim, R. E., J. Geol., 50, 225 (1942). Hauser, E. A., Chem. Rev., 37, 287 (1945). Hofmann, U., and Bilke, VV., Kolloid-Z., 77, 238 (1936). I
Grenal’l
.
401
(10) (11) (12) (13)
87 1
Hofmann, U., Endell, K., and Wilm, D., 2. Krist., 86,340 (1933). Marshall, C. E., Ibid., 91, 433 (1935). Prutzman, P.W., U. S. Patent 1,397,113 (Nov. 15, 1921). Ross, C. S., and Hendricks, S. B., U . S . Geol. Survey, Profess.
Paper 205B (1945). (14) Shankland, R. U., and Schmitkons, G. E., Proc. Am. Petroleum Inst., 27, 111, 57 (1947). (15) Thomas, C. L., IND. ENG.CHEM.,41, 2564 (1949). (16) Van Horn, L., U. S. Patent 2,391,050 (Dec. 18, 1945). (17) Whitmore, F. C., IND.ENG.CHEM.,26, 94 (1934); J . Am. Chem. Soc., 54, 3274 (1932). RECEIVED June 2, 1949.
Calculation of Minimum Reflux in Distillation Columns Three methods for calculatR . N. SHIRAS volatility rates are valid. It Shell ~~~~l~~~~~~ company, sari ~ r a n c i s c o ,calif, is also suitable as an approxiing minimum reflux rates in distillation columns fracmate method for all column D. N. HANSON AND C. H. GIBSON design problems which nortionating multicomponent mixtures are presented and mally occur. The third Unicersity of Caltlfornia, Berkeley, CalV. evaluated. The first method method is an adaptation of is based on some overlooked the Thiele and Geddes plateequations originally published in 1932. It is very simple, by-plate method of calculating finite plate columns. I t is but is rigorous for only a restricted class of separations. completely general, since with it variable overflow and The second method is an extension and elaboration of a variable volatility ratios can be taken into account, and method recently published by Underwood. This ‘ @ functhere are no restrictions as to the type of separation. I t is tion” method is fairly rapid and is exact for all separations recommended as a tool in distillation research but is too for which the postulates of constant overflow and constant laborious for plant design work.
T
a?
HE calculation of minimum reflux is important both as an initial step in the design of distillation columns and as a correlating property in research on distillation calculations. In binary systems, methods for the rigorous calculation of this quantity have been well defined (5, 13) although some of the more useful equations appear to have gained less attention than they deserve. For multicomponent systems, both approximate niethods and an exact method have been proposed. The available rigorous method for multicomponent systems (2, 7 ) is quite tedious and can hardly be justified even in application to unusual design problems or to research in distillation calculations Of the proposed approximate methods (1, 4-6, 9, 11-18), the best is the method introduced by Underwood ( 1 1 , l a ) which is quite accurate and is, in addition, rapid in calculation. I t is thus adequate for most design purposes. In a recent paper Cnderwood has shown that this method is accurate not only for sharp separations between adjacent keys but also for cases where the separation is not sharp and where one or more components lie between the keys in volatility. Actually, Underwood’s equations are exact for any multicomponent system for which the assumptions of constant molal flows and constant relative volatility are valid. Further equations are presented here which facilitate the exact solution of a multicomponent system by Undei wood’s equations. When this elaborated Vnderwood method is not exact and a precise evaluation of minimum reflux is required, there is also a new rigorous method which, although time-consuming, is less tedious than that proposed by Jenny ( 7 ) and by Brown and Holcomb ( 8 ) . This new method consists of an adaptation of the finite plate method of Thiele and Geddes (IO) to the calculation for infinite plates (minimum reflux). For the calculation of minimum reflux, all multicomponent
separations can be classified under two categories: class 1, separations such that, with infinite plates, all components of the feed are present in both the top product and the bottom product; class 2, separations such that, with infinite plates, some of the components are completely in the top product or completely in the bottom product. Failure to recognize the distinction between these two classes has led to much confused thinking about the so-called pinch points. When there are infinite plates in a rectifying or stripping section, as one proceeds away from the section terminus toward the feed plate, a point will be reached where there are only infinitesimal changes in component concentrations and molal flows for successive plates. Some writers on distillation have called this the “pinch point”; the present authors believe that a better designation is the “point of infinitude.” With binary systems the point of infinitude for both sections occurs at the feedplatc. The same is true of all class 1 separations of which a binary separation is merely a special species. With class 2 separations, for one or both of the sections, the point of infinitude occurs away from the feed plate. If the top product contains all components of the feed, then the point of infinitude for the rectifying section will be at the feed plate. If not all the components are present in the top product, then the point of infinitude will be at an intermediate point of the rectifying section. In the top product the mathematical concentrations of the vanished heavy components are infinitesimals of the second order; the infinite plates between the top of the column and the point of infinitude build the concentrations of these. heavy components up to infinitesimals of the first order; the infinite plates between the point of infinitude and the feed plate build the concentrations for these components up to the
