LLOYD BERG , J. M . HARRISON,
ANDC. W. MONTGOMERY
G u lf R e s e a r c h & D e v e lo p m e n t C o m p a n y , P itts b u r g h , Pa.
B y m e a n s o f t h e h yd rogen b on d cla ssifica tio n o f liq u id s, th e sea rch for en tra in ers for effectin g a desired a zeotrop ic sep a ra tio n is con sid erab ly sim p lified . T h is sy stem is a p p lied to t h e se lec tio n o f en tra in ers s u ita b le fo r th e azeo tro p ic d eh y d ra tio n o f p y rid in e a n d it s h o m o lo g s. A n u m b er o f sa tisfa c to ry e n tra in ers are lis te d , a n d ex
p e r im e n ta l r e s u lts are giv en for th e d eh y d ra tio n o f p y rid in e, 2 -p ic o lin e , a n d 2,6- lu tid in e , u s in g to lu e n e , m e th y l iso b u ty l k e to n e , a n d propyl iso b u ty ra te , resp ectiv ely .
I
N T H E study of azeotropic distillation, very little information has been presented to enable an investigator to select a suitable entrainer for a desired separation. R ecently (3) a system was proposed for grouping all liquids into five classes, depending upon th e ir hydrogen-bond-form ing potentialities. The object of this paper is to m ake use of the generalizations derived from this classi
fication in th e selection of azeotropic system s, and specifically to determ ine suitable processes for the dehydration of pyridine and its homologs.
The high w ater-solubility of pyridine and its homologs m akes its recovery from water-insoluble m edia easy to accomplish by ex
traction. T he attractiveness of this m ethod is lessened, however, by th e difficulty of separating pyridine or its homologs from w ater. Pyridine, picolines, and lutidines form minimum-boiling azeotropes w ith w ater so th a t th ey cannot be separated by straig h t rectification alone. Furtherm ore, th e composition of these azeotropes is high enough in th e heterocyclic compound so th a t th ey contain too m uch pyridine or homolog to be discarded b u t an insufficient am ount to be treated as reasonably pure.
Previously reported work on the azeotropic dehydration of these heterocyclic compounds is lim ited (2, 5).
HYDROGEN BOND CLA SSIFICA TIO N O F LIQUIDS B ased on th e hydrogen bond classification of liquids, generaliza
tions concerning th e probable n atu re of th e azeotropes formed be
tween any two classes of liquids can be made. Briefly the classes are as follows (3):
Cl a s s I. Liquids capable of forming three-dim ensional net
works of strong hydrogen bonds.
Cl a s s I I . O ther liquids composed of molecules containing
bo th active hydrogen atom s an d donor atom s (oxygen, nitrogen, and fluorine).
Cl a ssI I I . Liquids composed of molecules containing donor
atom s b u t no active hydrogen atom s.
Cl a ssIV . Liquids composed of molecules containing active
hydrogen atom s b u t no donor atom s.
Cl a ssV. All other liquids—i.e., liquids having no hydrogen-
bond-form ing capabilities.
By this system of classification it is possible to predict the nature of th e deviations from R ao u lt’s law, and th e hydrogen bonding in a m ixture m akes it possible to judge approxim ately the extent of these deviations. W hen a system which shows posi
tive ( + ) deviations from R aoult’s law forms an azeotrope, it will be a m inim um -boiling azeotrope. W hen th e deviations are negative ( —), any azeotrope formed will be maxim um boiling.
On th e basis of this liquid classification, a system atic sum m ary of deviations for w ater (class I) and th e pyridine heterocyclic compounds (class I I I ) is th e following:
Cl a s s e s
I and V I and IV I and I I and II I and III I I I and II III and IV II I and III II I and V
Always ( + ) deviations;
solubility
Usually ( + ) deviations;
groups
De v i a t i o n s
frequently limited
very complicated
Always ( —) deviations
Quasi-ideal systems; always ( + ) deviations or ideal
ENTRAINER PR O P E R T IES
W ith th e understanding of azeotrope form ation afforded by th is classification, it is possible to design m ethods for separating
586 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 37, below the tem perature of the water-heterocyclic compound m ini
mum azeotrope—for example, below 92° C. for pyridine, below 95° C. for 2-picoline, and below 96° C. for 2,6-lutidine.
2. Form no minimnm azeotrope w ith pyridine or its homo
logs. Theoretically, it would be necessary only th a t th e m ini
m um azeotrope boil above the entrainer-w ater azeotrope. Ac
tually, however, when each possible pair in a three-liquid system forms a minimum azeotrope, a ternary azeotrope is alw ays form ed.
3. Form no ternary azeotrope w ith w ater and the heterocyclic compound. and V liquids. Pyridine and its homologs are class I I I liquids and form quasi-ideal solutions w ith other liquids of class II I . Mini- mum-boiling azeotropes will form only when th e boiling points of the two class I I I liquids are close together. Pyridine or its homo
logs will form no m inim um azeotropes w ith class IV liquids. Any azeotropes form ed will be maximum and will no t affect th e opera
tio n of this process. All class IV liquids forming azeotropes with w ater boiling below th e tem perature of th e heterocyclic com- pound-w ater azeotrope can be used. Class V liquids form quasi
ideal solutions w ith pyridine or its homologs and do no t form azeotropes unless they boil very close to th e heterocyclic com
pound.
Some suitable entrainers for dehydration of pyridine, 2-pico
line, an d 2,6-lutidine are listed in T able I. F o r use in a batch operation, alm ost an y entrainer having th e properties outlined above will be satisfactory. W hether the pyridine-w ater azeo
trope is m erely suppressed from th e distillate, or w hether it is destroyed by th e creation of the more volatile entrainer-w ater azeotrope, no difficulty is encountered in obtaining a pyridine- free overhead. E ven if a stable pyridine-w ater azeotrope does exist in th e column, th e separation is readily effected although th e boiling point difference betw een th e tw o azeotropes m ay be flTnfl.ll T he ease of separation appears to be largely determ ined by th e relationship of th e a ctiv ity coefficients (1).
P a rts E n -
June, 1945 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 587 ing column. T he overhead from the
above charge was a constant-boiling m ixture (boiling point, approxim ately 86° C.) containing all of th e w ater and toluene, while th e anhydrous pyridine rem ained as bottom s product. T he w ater was decanted from th e toluene;
th is completely separated th e three com
ponents of the original charge. If, instead of th e azeotropic composition, a w ater- rich m ixture containing 67% w ater and 33% pyridine were charged to th e azeo- trope column, then 704 p arts of toluene w ould be required to dehydrate th e same q u a n tity of pyridine as before. T h u s th e value of th e enriching column is evident.
T h e distillation of th e pyridine-w ater azeotrope and toluene is show n in Figure 2.
Ex a m p l e 2. A b atch rectification, was m ade of a charge consisting of 114 p a rts of 2-picoline, 86 p a rts of w ater, an d 300 p arts of m ethyl isobutyl ketone.
T h e overhead was th e w ater-ketone azeo
trope, boiling a t ab o u t 88° C .; th e bottom s product was anhydrous 2-pico
line. Phase separation of th e distillate gave w ater and substantially pure ketone. T his distillation is illustrated in Figure 3.
Example 3. A b atch rectification was m ade of a charge consisting of 104 p a rts of 2,6-lutidine, 75 p a rts of w ater, a n d 187 p a rts of propyl isobutyrate.
T he overhead was th e w ater-ester azeo
trope, boiling a t ab o u t 92.5° C.; the bottom s product was anhydrous 2',6- lutidine. Phase separation of th e dis
tillate gave w ater an d essentially pure ester. T his distillation is illustrated in Figure 4.
W hile 2-picoline an d th e lutidines are immiscible w ith h o t w ater, com
plete dehydration cannot be obtained by phase separation because, even when hot, considerable w ater is still dissolved in the heterocyclic compound.
ACKNOW LEDGMENT T he authors are grateful for th e sug
gestions of V. N . H u rd who reviewed th e m anuscript.
LITERA TUR E CITED
(1) Colburn, A. P., and Phillips, J. C., Trans. Am. Inst. Chem. Engrs., 40, 333-59 (1944).
(2) Downs, C. R. (to B arrett Co.), U. S.
P atent 1,290,124 (Jan. 7, 1919).
(3) Ewell, R. H., Harrison, j . M., and Berg, L., I n d . E n g . Chem.,- 36, 871-5 (1944).
(4) Hodgman, C. D., Handbook of Chem
istry and Physics, 28th ed., Cleve
land, Chemical Rubber Pub. Co., 1944.
(5) Huff, W. J. (to Koppers Co.), U. S.
P atent 1,416,206 (May 16, 1922).
(6) Lange, N. A., Handbook of Chemistry, 4th ed., Sandusky, Ohio, Handbook Publishers, Iqp., 1941.
V
F igu re 2. D istilla tio n Curve for D eh y d ra tio n o f P y rid in e U sin g T o lu e n e F igu re 3. D istilla tio n Curve for D eh y d ra tio n o f 2 -P ic o lin e U sin g
M eth y l Iso b u ty l K eto n e
F ig u re 4. D istilla tio n C urve for D eh y d ra tio n o f 2 ,6 -L u tid in e U sin g P rop yl Iso b u ty r a te
REFRACTIVEindex