59 Separation of 2,7-Dimethylnaphthalene from 2,6-Dimethylnaphthalene with Downloaded via UNIV OF CALIFORNIA SANTA BARBARA on July 14, 2018 at 22:28:41 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
Molecular Sieves JOHN A. HEDGE Sun Oil Co., Marcus Hook, Pa. 19061
Dimethylnaphthalene concentrate contains significant amounts of 2,6-dimethylnaphthalene bound in a binary eutectic with 2,7-dimethylnaphthalene. This eutectic cannot be broken by distillation or solvent crystallization. A practi cal method for separating this eutectic mixture of 2,7-dimethylnaphthalene and 2,6-dimethylnaphthalene has been achieved. Selective adsorption of 2,7-dimethylnaphthalene from a dimethylnaphthalene concentrate is obtained with sodium type Y molecular sieves. 2,6-Dimethylnaphthalene then can be crystallized from the unadsorbed raffinate frac tion. Separation factors of 6 to 8 are obtained, indicating the high selectivity of these particular molecular sieves for this adsorption. Previous work in this area achieved a separation factor of 2.7. A continuous method has been developed for adsorption and desorption of 2,7-dimethylnaphthalene. Toluene has been selected as the optimum desorbent. This process makes 2,7-dimethylnaphthalene potentially available.
T i J ~ o l e c u l a r sieves h a v e b e e n u s e d thus f a r to separate d i s t i n c t classes of o r g a n i c c o m p o u n d s . T h e s e p a r a t i o n of η-paraffins f r o m b r a n c h e d paraffins w i t h 5 A m o l e c u l a r sieves is w e l l k n o w n . Selective a d s o r p t i o n of aromatics f r o m m i x e d streams w i t h 1 0 X , 1 3 X , a n d t y p e Y sieves is per h a p s less w i d e l y k n o w n .
T h e use of 1 0 X m o l e c u l a r sieves to separate
m i x t u r e s of aromatics has b e e n d i s c l o s e d (1,2,3). separations aromatics
of m i x t u r e s of m o n o c y c l i c aromatics (3).
T h e s e patents disclose (1,
2)
and dicyclic
T h e s e w e r e t h e first i n d i c a t i o n s that m o l e c u l a r
c o u l d separate c o m p o u n d s w i t h i n a single c h e m i c a l class. 238
Flanigen and Sand; Molecular Sieve Zeolites-II Advances in Chemistry; American Chemical Society: Washington, DC, 1971.
sieves
59.
HEDGE
Separation
of
239
2,7-Dimethylnaphthalene
Discussion W e , at S u n O i l C o . , are greatly interested i n s e p a r a t i n g 2 , 6 - d i m e t h y l naphthalene
from 2,7-dimethylnaphthalene
(2,7-DMN).
These 2 d i -
m e t h y l n a p h t h a l e n e isomers are too c l o s e - b o i l i n g ( 2 6 2 . 0 ° C f o r 2 , 6 - D M N a n d 2 6 2 . 3 ° C f o r 2 , 7 - D M N ) to a l l o w s e p a r a t i o n via f r a c t i o n a l d i s t i l l a t i o n . F u r t h e r m o r e , b o t h isomers are s o l i d at r o o m t e m p e r a t u r e
(mp
111°-
1 1 2 ° C for 2 , 6 - D M N a n d 9 6 ° - 9 7 ° C for 2 , 7 - D M N ) . O n l y a small fraction of the n a t u r a l l y - o c c u r r i n g 2 , 6 - D M N i n a r e f i n e r y stream c a n b e c r y s t a l l i z e d before a b i n a r y eutectic c o n t a i n i n g 4 2 %
2 , 6 - D M N and 58%
2,7-
D M N begins to p r e c i p i t a t e . N a t u r a l l y , w e w e r e interested b y one of U n i o n s patent examples
(3)
w h i c h d e s c r i b e d use of 1 0 X m o l e c u l a r sieve to separate a 2 , 6 - D M N a n d 2 , 7 - D M N b i n a r y eutectic
(separation factor 2.7).
W e have
enhanced
this s e p a r a t i o n i n 3 w a y s . F i r s t , b y s u b s t i t u t i o n of N a - Y sieve f o r 1 0 X sieve, w e h a v e i m p r o v e d s e l e c t i v i t y f o r 2 , 7 - D M N a d s o r p t i o n . S e c o n d , a m u c h sharper separation is o b t a i n e d if a m i x e d refinery stream c o n t a i n i n g l i q u i d D M N isomers is u t i l i z e d i n s t e a d of a p u r e b i n a r y m i x t u r e . T h i r d , c a r e f u l c o n t r o l of w a t e r content of the sieve a l l o w s i m p r o v e d selectivity. Batch Separation Studies W e h a v e e v a l u a t e d several m o l e c u l a r sieves f o r the s e p a r a t i o n of 2 , 6 - D M N f r o m 2 , 7 - D M N i n the presence of l i q u i d isomers present i n a 2 5 7 ° - 2 6 5 ° C h e a r t - c u t of a n a r o m a t i c extract of c a t a l y t i c gas o i l . T h i s heart-cut c o n t a i n s : 12.5%
2 , 6 - D M N (free 2 , 6 - D M N removed)
16.9%
2,7-DMN
43.1%
Other D M N ' s
6.3%
1- a n d 2 - E t h y l n a p h t h a l e n e s
0.5%
Biphenyl
20.7%
Saturates a n d m o n o c y c l i c aromatics
P r i o r to e v a l u a t i o n , e a c h sieve w a s c o n d i t i o n e d c a r e f u l l y i n m o i s t a i r ( u s u a l l y at 1 2 5 ° C )
to c o n t r o l the w a t e r content of the sieve.
Batch
adsorptions w e r e r u n t h e n i n w h i c h 10.0 grams of heart-cut, 2.5 grams of isooctane, a n d 5.0 grams of sieve w e r e h e l d at 1 0 0 ° C f o r 2 h o u r s to ensure c o m p l e t e e q u i l i b r a t i o n b e t w e e n the raffinate a n d the
adsorbate.
T h i s 100 ° C t e m p e r a t u r e w a s c h o s e n because runs at a m b i e n t t e m p e r a t u r e were
considerably slower i n reaching
equilibrium.
The
unadsorbed
(raffinate) f r a c t i o n w a s t h e n filtered off, a n d the c o o l e d sieve w a s w a s h e d w i t h r o o m t e m p e r a t u r e isooctane to r e m o v e the r e m a i n d e r of the u n a d s o r b e d f r a c t i o n . T h e adsorbate w a s r e m o v e d w i t h r e f l u x i n g b e n z e n e . T h e results of these evaluations are s h o w n i n T a b l e I.
S o d i u m - Y sieve
Flanigen and Sand; Molecular Sieve Zeolites-II Advances in Chemistry; American Chemical Society: Washington, DC, 1971.
240
M O L E C U L A R
SIEVE
ZEOLITES
Π
Table I. Batch Runs, Selectivity of Various Molecular Sieves for Separation of 2 , 6 - D M N and 2 , 7 - D M N in Presence of 2 5 7 ° - 2 6 5 ° C H e a r t - C u t Isomers, 1 0 0 ° C , 2 H r s . Sieve Pretreatment Temperature in Air, °C
Type of Sieve Sodium form, type Y , powder (SK-40) Sodium form, type Y , 20-40 mesh* Sodium form, type Y , tablet A m m o n i u m form, type Y , powder R a r e e a r t h exchanged, t y p e Y , pellet ( S K - 5 0 0 ) Sodium form, type Y , 1/16" e x t r u d a t e L i n d e 1 3 - X 14 X 30 m e s h Linde 10-X 1/16' extrudate Potassium form, type L (SK-45) α 6
Sieve Capacity, Grams Hydrocarbon/ 100 Grams Sieve
Separation Factor, α
125
16.6
8.0
125
11.4
7.3
125
15.0
6.5
125
13.2
5.8
125
11.8
4.7
125 125
13.6 9.2
4.4 2.8
100
6.6
2.4
125
11.4
0.53
6
1/16" E x t r u d a t e ground to 20-40 mesh. 2 , 6 - D M N preferentially adsorbed.
is the most selective sieve s t u d i e d f o r a d s o r p t i o n of 2 , 7 - D M N , as s h o w n b y s e p a r a t i o n factor, a, w h e r e _ a
% 2 , 7 - D M N a d s o r b e d / % 2 , 7 - D M N unadsorbed
~~ % 2 , 6 - D M N a d s o r b e d / % 2 , 6 - D M N u n a d s o r b e d
F u r t h e r e x a m i n a t i o n of the d a t a i n T a b l e I reveals several i n t e r e s t i n g p o i n t s . Sieve g e o m e t r y affects selectivity s o m e w h a t as s h o w n b y the slight differences b e t w e e n p o w d e r e d , 2 0 - 4 0 m e s h size, a n d t a b l e t e d N a - Y sieve. B o t h basic s o d i u m a n d a m m o n i u m - Y sieves a n d a c i d i c rare e a r t h - Y sieves Table II.
Batch Runs, Comparison of Purified Binary
Type of Sieve
Charge Stock P u r i f i e d eutectic Heart-cut mixture P u r i f i e d eutectic Heart-cut mixture
Sodium Sodium Sodium Sodium
form, form, form, form,
type type type type
Y, Y, Y, Y,
powder powder tablet tablet
Flanigen and Sand; Molecular Sieve Zeolites-II Advances in Chemistry; American Chemical Society: Washington, DC, 1971.
59.
HEDGE
Separation
of
241
2,7-Dimethylnaphthalene
s h o w s e l e c t i v i t y f o r 2 , 7 - D M N . T y p e L sieves a d s o r b 2 , 6 - D M N i n p r e f e r ence to 2 , 7 - D M N , t h e reverse of a l l other sieves s h o w n i n T a b l e I. I n t u i t i v e l y , one w o u l d p r e d i c t that s e p a r a t i o n of a b i n a r y m i x t u r e s h o u l d b e s i m p l e r t h a n s e p a r a t i o n of 2 c o m p o n e n t s present i n a c o m p l e x m i x t u r e . H o w e v e r , this w a s n o t t h e case i n o u r w o r k . A s s h o w n i n T a b l e I I , a m u c h h i g h e r s e p a r a t i o n f a c t o r is o b t a i n e d w i t h a m i x e d stream t h a n w i t h a b i n a r y eutectic.
D e s p i t e the d i l u t i o n of 2 , 7 - D M N
present i n
adsorbate b y l i q u i d isomers, t h e w e i g h t of 2 , 7 - D M N f r e e d p e r 100 grams of sieve d r o p s o n l y s l i g h t l y . T h i s e n h a n c e d s e p a r a t i o n is i l l u s t r a t e d f u r ther b y t h e e q u i l i b r i u m d i a g r a m s f o r the b i n a r y eutectic ( F i g u r e 1) a n d for the heart-cut m i x t u r e ( F i g u r e 2 ). C o n t r o l of w a t e r content of this m o l e c u l a r sieve is q u i t e i m p o r t a n t . A n o p t i m u m H 0 content of 12.0% f o r N a - Y sieve is s h o w n i n T a b l e I I I . 2
T h i s w a t e r content w a s d e t e r m i n e d b y w e i g h t loss o n i g n i t i o n at 1 9 0 0 ° F . Sieves h a v i n g l o w e r w a t e r contents are less selective.
This
finding
im
plies that w a t e r present i n sieves is s t r u c t u r a l a n d does affect sieve p r o p erties.
Sieves w i t h h i g h w a t e r contents u p to 25 w t %
have reduced
c a p a c i t y f o r d i n u c l e a r aromatics. Sieves w e t w i t h m e t h a n o l h a v e g r e a t l y r e d u c e d s e l e c t i v i t y ; c o n d i t i o n i n g i n m o i s t a i r at 125 ° C restores o r i g i n a l selectivity. ο
WT % 2,7-DMN IN LIQUID PHASE
Figure 1. Separation of 2,6DMN and 2,7-DMN binary eu tectic with Na-Y sieve Eutectic vs. 2 5 7 ° - 2 6 5 ° C H e a r t - C u t Mixture, 1 0 0 ° C , 2 H r s . Adsorbed
%
2,6-DMN 23.7 4.4 28.8 4.8
Phase
%
Separation Factor, α
Wt. 2, 7-DMN Freed/100 Grams Sieve
76.3 38.0 71.2 37.0
3.1 8.0 2.8 6.8
5.8 5.2 4.8 4.4
2,7-DMN
Flanigen and Sand; Molecular Sieve Zeolites-II Advances in Chemistry; American Chemical Society: Washington, DC, 1971.
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MOLECULAR
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ZEOLITES
II
Selection of Desorbent, Batch Competition Studies T o d e v e l o p a c y c l i c s e p a r a t i o n process, o u r next step w a s to find a s u i t a b l e desorbent.
A s u i t a b l e desorbent m u s t a l l o w selective a d s o r p t i o n
a n d y e t m u s t desorb at a reasonable rate.
Three aromatic
desorbents
w e r e e v a l u a t e d : b e n z e n e , toluene, a n d o-xylene. B a t c h c o m p e t i t i o n experiments w i t h e q u a l w e i g h t s of desorbent a n d D M Ν heart-cut w e r e r u n to d e t e r m i n e the effect of desorbent o n selec t i v i t y f o r a d s o r p t i o n of 2 , 7 - D M N o v e r 2 , 6 - D M N .
T h e r e l a t i v e strength
w i t h w h i c h desorbent or a n y other c o m p o n e n t of a m i x t u r e is a d s o r b e d c a n b e s h o w n b y β factor, β F a c t o r is d e f i n e d as the r a t i o of c o m p o n e n t χ i n t h e a d s o r b e d phase over c o m p o n e n t χ i n the u n a d s o r b e d phase d i v i d e d
Figure 2. Separation of 2,6DMN and 2,7-DMN eutectic in 257°-265°C heart-cut with Na-Y sieve Table III. Batch Runs, Effect of Water Content on Molecular Sieve Selectivity in Adsorption of 2 , 7 - D M N from 2 5 7 ° - 2 6 5 ° C H e a r t - C u t Isomers, 1 0 0 ° C , 2 H r s .
Type of Sieve Sodium Sodium Sodium Sodium Sodium Sodium
form, type form, type form, type form, type form, type form, type
Y, Y, Y, Y, Y, Y,
Sieve Pretreatment Tempera ture in Air, °C powder 125 powder 400 tablet 25 tablet 125 tablet 175 tablet methanol washed
wt% HO
Sieve Capacity, G/100G Sieve
Separation Factor, α
12.00 2.30 23.34 12.13 6.00 10.02
16.6 20.6 4.4 15.0 21.2 15.0
8.0 3.2 4.1 6.5 5.2 3.5
2
a
° Based on weight of conditioned sieve.
Flanigen and Sand; Molecular Sieve Zeolites-II Advances in Chemistry; American Chemical Society: Washington, DC, 1971.
59.
Separation
HEDGE
of
243
2,7-Dimethylnaphthalene
b y w t 2 , 6 - D M N a d s o r b e d o v e r w t 2 , 6 - D M N u n a d s o r b e d . T h e β factor f o r 2 , 7 - D M N is e q u a l to a, as d e f i n e d earlier. ^ β Factor for component χ = n
r
W
t
W t χ adsorbed/Wt χ unadsorbed , r b e d / W t 2,6-DMN 2
>
6
D
M
N
a d s 0
unadsorbed T h e results of o u r c o m p e t i t i o n studies are s h o w n i n T a b l e I V .
The
most selective a d s o r p t i o n of 2 , 7 - D M N occurs i n the presence of o-xylene, the most w e a k l y a d s o r b e d desorbent.
The
least
selective a d s o r p t i o n
occurs i n the presence of b e n z e n e , the most s t r o n g l y a d s o r b e d desorbent. O t h e r D M N isomers are a d s o r b e d m o r e s t r o n g l y t h a n 2 , 6 - D M N . m a y e x p l a i n the h i g h e r separation factors o b t a i n e d w i t h the
This
heart-cut
mixture. Table IV. Competition of Benzene, Toluene, and o-Xylene with Dimethylnaphthalenes for Adsorption on Molecular Sieve, β Factors Obtained from Batch Studies"
2,7-DMN Other D M N ' s Benzene 1,6-DMN 1- a n d 2 - E t h y l n a p h t h a l e n e s 2,6-DMN Toluene o-Xylene M i x e d monocyclic aromatics a
Benzene
Toluene
2.7 2.0 1.8 1.4 1.7 1.0
4.7 2.2
— —
0.6
β F a c t o r 2 , 6 - D M N chosen equal to
o-Xylene
-
1.8 1.0 0.9 -
0.6
8.0 2.3 -
1.9 1.8 1.0 -
0.7 0.6
1.0.
T h e s e b a t c h c o m p e t i t i o n studies s h o w e d that b e n z e n e w o u l d cause the fastest d e s o r p t i o n w h i l e o-xylene w o u l d a l l o w the sharpest s e p a r a t i o n . S i n c e w e w e r e f a c e d w i t h this d i l e m m a , a l l 3 desorbents w e r e c a r r i e d i n t o o u r c o l u m n studies. Column
Separation
Studies
T h e e x p e r i m e n t a l a p p a r a t u s u s e d f o r o u r c o l u m n studies consisted of a 3 / 4 - i n c h i . d . b y 3 6 - i n c h glass c o l u m n filled w i t h N a - Y sieve c o n t a i n i n g a b o u t 12 w t
%
water.
T h e D M N heart-cut
containing
eutectic
2 , 6 - D M N a n d 2 , 7 - D M N is p u m p e d into the b o t t o m of the c o l u m n . A t the e n d of this D M N charge, desorbent is p u m p e d i n t o the b o t t o m of the c o l u m n . T h e desorbent pushes out a 2 , 6 - D M N e n r i c h e d raffinate a n d desorbs the 2 , 7 - D M N e n r i c h e d adsorbate. W h e n the last of t h e adsorbate is r e m o v e d f r o m the b o t t o m p o r t i o n of the sieve b e d , the c o l u m n is r e a d y
Flanigen and Sand; Molecular Sieve Zeolites-II Advances in Chemistry; American Chemical Society: Washington, DC, 1971.
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MOLECULAR
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II
f o r a n e w c y c l e . T h e i n c o m i n g D M N f e e d p u s h e d o u t t h e r e m a i n d e r of the 2 , 7 - D M N e n r i c h e d desorbate.
T h e effluent f r o m t h e c o l u m n w a s
t a k e n i n s m a l l cuts f o r o u r i n i t i a l studies. T h e s e cuts t h e n w e r e a n a l y z e d b y gas c h r o m a t o g r a p h y . T h e adsorbent c o l u m n is h e l d at a t e m p e r a t u r e just b e l o w t h e b o i l i n g p o i n t of t h e desorbent to speed d i f f u s i o n i n t o t h e sieve particles. O u r c o l u m n runs s h o w e d (as p r e d i c t e d b y t h e b a t c h studies) b e n z e n e is t h e most efficient s t r i p p i n g solvent.
that
o-Xylene was quite slow
to desorb 2 , 7 - D M N a n d w a s d r o p p e d f r o m c o n s i d e r a t i o n i n a n y c y c l i c process. Sharpness of separation is s h o w n b y p l o t t i n g t h e b r e a k t h r o u g h c u r v e of 2 , 7 - D M N i n t o t h e raffinate. B r e a k t h r o u g h curves f o r these 3 desorbents are s h o w n i n F i g u r e 3. B e n z e n e a l l o w s a r a p i d b r e a k t h r o u g h of 2 , 7 - D M N , as p r e d i c t e d .
S u r p r i s i n g l y , toluene is s u p e r i o r to o-xylene at t h e 8.0
^ Ο ο ο
