Crystallization as a Separations Process - American Chemical Society

Chapter 21

Observation of High-Pressure Crystallization of Benzene from Benzene-Cyclohexane Mixture 1

Hideo Narahara, Kenji Yamada, Ken Toyokura, MasatoMoritoki ,and Nobuhiko Nishiguchi Downloaded by UNIV OF CALIFORNIA SAN DIEGO on July 18, 2016 | http://pubs.acs.org Publication Date: September 21, 1990 | doi: 10.1021/bk-1990-0438.ch021

1

Department of Applied Chemistry, Waseda University, 3-4-1, Ohkubo, Shinjuku-ku, Tokyo 169, Japan

High pressure crystallization characterized by merits of purer product and high yield was studied by the labora tory equipment of the high pressure crystallizer on whose wall two optical glasses were set and crystal lization of benzene from benzene-cyclohexane mixture was observed under high supersaturation. Between 10 and 50 MPa of supersaturation, benzene crystals appeared on the glass surface and the fringe of the crystals moved on it. Moving rate of the fringe of crystals was corre lated as power function of relative supersaturated pressure, and the power number was between 1.67 and 1.85. When high supersaturation between 20 and 60 MPa was kept, fine crystals were observed to be floated in inlet flow of the melt solution; and the purity of the obtained crystal was over 99.9 mol%. High pressure c r y s t a l l i z a t i o n , where supersaturation i s made by compression t o several hundreds megapascals, has been studied f o r i n d u s t r i a l purposes recently. Less energy consumption f o r product, purer c r y s t a l and higher y i e l d o f product, have been reported as several merits o f t h i s c r y s t a l l i z a t i o n i n comparison with conventional c r y s t a l l i z a t i o n ( J J . But fundamental phenomena o f t h i s c r y s t a l l i z a t i o n are not c l e a r yet. In t h i s study, the phenomena o f high pressure c r y s t a l l i z a t i o n were observed; and the moving rate o f the f r i n g e o f c r y s t a l grown on glass wall o f the c r y s t a l l i z e r was measured. The benzene concentration o f the c r y s t a l s obtained by high pressure c r y s t a l l i z a t i o n was determined a f t e r the uncrystallized melt was removed from the c r y s t a l p a r t i c l e s by replacement o f water. 1

Current address: Kobe Steel, Ltd., 1-3-18, Wakinohama-cho, Chuo-ku, Kobe 651, Japan 0097-6156/90/043g-O281$06.00/0 © 1990 American Chemical Society

Myerson and Toyokura; Crystallization as a Separations Process ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

282

CRYSTALLIZATION AS A SEPARATIONS PROCESS

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on July 18, 2016 | http://pubs.acs.org Publication Date: September 21, 1990 | doi: 10.1021/bk-1990-0438.ch021

EXPERIMENTAL EQUIPMENT A laboratory equipment used f o r t h i s study consisted of pressing, o p t i c a l and separating c e l l s as shown i n Figure 1. The pressing and separating c e l l s were c y l i n d r i c a l high pressure vessels of twenty m i l l i l i t e r s , and the l i q u i d i n them could be compressed to 500 megapascals by piston i n the v e s s e l . The o p t i c a l c e l l consisted of a c y l i n d e r and two sapphire glasses, which were set on the walls o f i t . The pressing c e l l and the o p t i c a l c e l l were put i n a thermostat bath i n order to keep the melt at a desired temperature. These three c e l l s were connected i n series by high pressure pipes. Individual thermocouples and pressure gauges were equipped on each c e l l . EXPERIMENTAL METHODS 1) Observation of high pressure c r y s t a l l i z a t i o n A benzene-cyclohexane mixture was fed i n t o the o p t i c a l c e l l through the pressing c e l l and then compressed t o the equilibrium pressure. When the temperature of melt i n the c e l l became constant, the melt mixture was quickly compressed to the desired pressure. The c r y s t a l l i z a t i o n took place was observed through a video camera system. C r y s t a l s grew along the wall surface, and the moving rate of c r y s t a l fringe on glass surface was measured. The benzene concentration of feed melt was 70.0, 80.0 and 90.0 mole percent for t e s t s at 283°K. Experimental data f o r mixtures, whose i n i t i a l concentration was 80.0 mole percent of benzene, were a l s o obtained a t 278, 283 and 293°K. These t e s t s were c a r r i e d out with and without seeding. Seed c r y s t a l s used i n these t e s t s were prepared through the following method: the nucleation was occured under low supersaturation i n the o p t i c a l c e l l at f i r s t and nuclei qrew up to f i n e c r y s y a l s . Then, the pressure was released to make Some n u c l e i melted i n unsaturated conditions. After some fines had disappeared, the melt mixture was compressed again; and remained fines continued to grow. These melting and growing operations were c a r r i e d out several times; and r e l a t i v e l y large c r y s t a l s were obtained and used as seed c r y s t a l s . 2) Tests o f the p u r i t y of grown c r y s t a l s In these t e s t s , seed c r y s t a l s made by the same way as described above, were placed i n a supersaturated s o l u t i o n i n the o p t i c a l c e l l . The seed c r y s t a l s grew mainly on the wall surface o f the o p t i c a l c e l l under r e l a t i v e l y low supersaturation. Under high supersaturated conditions, c r y s t a l growth was recognized both i n the moving melt and on the surface of the w a l l . While c r y s t a l l i z a t i o n took place i n the c e l l , the piston of the pressing c e l l moved, and the melt mixture flew i n t o the o p t i c a l c e l l . When the movement of the piston stopped, i t was considered that the c r y s t a l l i z a t i o n was over, and the c r y s t a l s i n the o p t i c a l c e l l were washed by the water almost saturated by mixing with benzene on a preliminary treatment, and u n c r y s t a l l i z e d melt among c r y s t a l p a r t i c l e s was a l s o replaced by the water. Then benzene c r y s t a l s and the water were cooled down to about 263°K, and the pressure i n the c e l l were released t o atmospheric pressure. The benzene c r y s t a l s were taken out from o p t i c a l c e l l and the benzene c r y s t a l s were d r i e d on a f i l t e r paper completely. The wiped benzene c r y s t a l s were melted and t h e i r composition was analyzed by gas chromatography.



NARAHARAET AL.

Figure 1.

High-Pressure Crystallization ofBenzene

Schematic diagram of experimental apparatus


284



RESULTS AND DISCUSSION 1) Observation o f high pressure c r y s t a l l i z a t i o n Various phenomena o f c r y s t a l l i z a t i o n were observed i n the o p t i c a l c e l l and the shapes o f c r y s t a l s were modeled as shown i n Figure 2. In these t e s t s , 170 runs were c a r r i e d out and the experimental r e s u l t s are p l o t t e d i n Figure 3. The marks o f • , +, O, A, x and V i n Figure 3 indicate the phenomena shown i n Figure 2. In some t e s t runs that the melt mixture i n the o p t i c a l c e l l was compressed t o a desired pressure and kept under the same pressure f o r f i f t e e n minutes, no c r y s t a l appeared. As shown i n Figure 3, these t e s t r e s u l t s were often obtained f o r the system with 80.0 mole percent benzene a t 283°K, under the supersaturation o f 20 megapascales. But i n some other t e s t s using d i f f e r e n t melt compositions, some nucleation and c r y s t a l growth were observed under almost same operat i o n a l conditions. In r e l a t i v e l y low supersaturation, d e n d r i t i c c r y s t a l s were grown on the glass o f o p t i c a l c e l l . The shape o f c r y s t a l s i s expressed by the modeling sketch which was l i k e skeleton o f l e a f as shown i n F i g ure 2-b). This c r y s t a l appeared under operating conditions shown as the mark + i n Figure 3. This phenomenon was often observed under r e l a t i v e l y low supersaturated conditions, and a l s o a t high temperatures when melts with high concentration o f benzene were used. When the operational supersaturation i s s l i g h t l y increased, t r i a n g l e c r y s t a l s with a skeleton i n the c e n t r a l part as shown i n Figure 2-c), appeared. These operational conditions are shown by the mark O i n Figure 3. Triangle shaped c r y s t a l s as shown i n Figure 2-d), a l s o appeared i n some t e s t s . These c r y s t a l s appeared under the operational conditions o f the mark A i n Figure 3. When a r e l a t i v e l y low supersaturat i o n was imposed, most c r y s t a l s grew on the w a l l . In these t e s t s , the heat o f compression i s supposed t o be released through the wall of the o p t i c a l c e l l and supersaturation o f the melt on the wall i s considered t o become higher. Isothermal high pressure c r y s t a l l i zation operation had the trend that benzene c r y s t a l appeared on the w a l l surface, due t o the temperature gradient across the w a l l . When a higher supersaturation was imposed, two d i f f e r e n t phenomena as shown i n Figure 2-e) and 2-f) were found. Phenomenon i n Figure 2-e) showed that t r i a n g l e c r y s t a l s grew on the glass surface and some f i n e s were suspended i n the melt that flowed i n t o the o p t i c a l c e l l . This phenomenon was found on t e s t s o f mark x i n Figure 3. Another phenomenon o f Figure 2-f) was that vigorous nucleation occured and much f i n e s were found t o be suspended i n the melt. A f t e r the nucleation, another shape o f c r y s t a l i n Figure 2-f) appeared on the o p t i c a l glass and grew quickly. This phenomenon was found f o r operational conditions marked by v i n Figure 3. These phenomena d i d not occure repeatedly, but the trend that nucleation i s apt t o occure under r e l a t i v e l y higher supersaturation, was confirmed. The t e s t r e s u l t s with seed c r y s t a l s were s i m i l a r t o those without seed. The c r y s t a l s grew mainly on the wall with o r without seeding; and t h i s phenomenon i s considered t o be caused by the high supersaturation o f the melt on the w a l l . The supersaturation caused by compression o f the melt was supposed t o spread uniformly throughout the melt, and homogeneous c r y s t a l l i z a t i o n was expected. But i n these c r y s t a l l i z a t i o n t e s t s ,



21. NARAHARA ET AL.


285

Figure 2. Classification of crystal type, a) No crystallization, b) Dendritic crystal grown on the glass of optical unit that was observed under relatively low supersaturation. c) Triangle crystal with a skeleton in the central part of it on the glass surface, d) Triangle crystal grown on the glass surface, e) Triangle crystal grown on the glass surface with fine crystals in the meltflowedinto the optical unit, f) Vigorous nucleation in the melt under relatively high supersaturation.



286


nucleation and growing phenomena were d i f f e r e n t under r e l a t i v e l y low supersaturation; and heat t r a n s f e r was supposed t o a f f e c t the c r y s t a l l i z a t i o n . When high supersaturation was suddenly imposed t o the melt, homogeneous c r y s t a l l i z a t i o n was assumed t o occur since the e f f e c t of heat t r a n s f e r was considered t o be small. When high pressure c r y s t a l l i z a t i o n i s considered i n a i n d u s t r i a l scale, large c r y s t a l l i z e r might be used and the e f f e c t of heat t r a n s f e r i s assumed t o be l e s s on c r y s t a l l i z a t i o n than on a laboratory scale c r y s t a l l i z a t i o n . In general high pressure i n d u s t r i a l c r y s t a l l i z a t i o n , the opera t i o n i s near adiabatic; and since supersaturation i s generally very large, homogeneous nucleation i n the melt may be favored. The shape o f moving boundary o f c r y s t a l s was d i f f e r e n t i f the composition of melt mixture was changed. When pure benzene was compressed, the shape of moving boundary of c r y s t a l was roundish. The f r i n g e shape made by 90.0 mole percent benzene was s l i g h t l y sharp, and the c r y s t a l s formed from the melt 80.0 and 70.0 mole percent benzene had sharper moving boundary. But the d e t a i l s of these d i f ference are not c l e a r and future studies are expected. The distance of movement of the f r i n g e of a c r y s t a l formed on the sapphire glass was recorded by video f i l m , and the moving r a t e of the f r i n g e was c a l c u l a t e d from the time required f o r the f r i n g e of the c r y s t a l t o pass the distance between two p a r t i c u l a r points on the o p t i c a l glass, ( A 1 / A 8 ) . This moving rate of the f r i n g e was defined as the c r y s t a l l i z a t i o n r a t e i n t h i s study. Data o f c r y s t a l l i z a t i o n rate were a l i t t l e b i t scattered; and t h i s was supposed t o be caused by complicated phenomena of both t r a n s f e r of c r y s t a l l i z a t i o n heat and change o f composition of the melt due t o c r y s t a l l i z a t i o n of benzene. At the i n i t i a l stage of c r y s t a l l i z a t i o n , the e f f e c t s of c r y s t a l l i z a t i o n heat and the changing of composition are small, and therefore the c r y s t a l l i z a t i o n rate would be f a s t . As the c r y s t a l l i z a t i o n continues, the temperature of the s l u r r y of suspended c r y s t a l s increases and the benzene concentrat i o n decreases, and so c r y s t a l l i z a t i o n rate decreases. Therefore, the maximum c r y s t a l l i z a t i o n r a t e under the same operational condit i o n s i s obtained a t the i n i t i a l stage of operation. In Figure 4, the maximum c r y s t a l l i z a t i o n r a t e s are p l o t t e d against a, r e l a t i v e supersaturated pressure f o r t e s t s where nucleation took place without seed c r y s t a l s . The r e l a t i v e supersaturated pressure, a i s defined by Equation 1,

(1) e where P i s the operational pressure, and P i s the equilibrium pressure. The c r y s t a l l i z a t i o n rates obtained from the growth of seed c r y s t a l s a t 283 °K were p l o t t e d against the r e l a t i v e supersaturated pressure i n Figure 5. The same c o r r e l a t i o n was made i n Figure 6 f o r the temperatures 278,283 and 293°K with 80.0 mole percent melt. As may be seen Figures 4, 5 and 6, the c r y s t a l l i z a t i o n rate increased with r e l a t i v e supersaturated pressure, and the power number of the c o r r e l a t i v e l i n e s between these two variables were almost same, as shown by Equation 2. e

A 1 A e

1.77

=

1.06 x

a


(2)

21. NARAHARAETAL.


287

u o

CO CO


T3

cu a 3 CO

70mol%

80mol%

90mol%

I

278°K

283 °K

293°K

I I

C r y s t a l l i z a t i o n at 283°K

I

Feed melt of 80.0 mol% of benzene

•

a :No c r y s t a l l i z a t i o n

+

b :Dendritic c r y s t a l

O

c :Triangle shaped c r y s t a l with skeleton

A

d :Triangle shaped c r y s t a l

X

e :Triangle shaped c r y s t a l with fine c r y s t a l s i n the melt mixture

V

f :Vigorous nucleation i n the melt mixture

Figure 3.

Dependence of c r y s t a l types on operational condition

o 70mol°/ 10

0

h A80mo% l ° 90mot%

10

h

-2

10

10

a Figure 4.

[-]

Correlation between r e l a t i v e supersaturated

pressure

and c r y s t a l l i z a t i o n rate (unseeded, 283 K)




288



21. NARAHARAETAL.


289

I t was found that c r y s t a l l i z a t i o n rates do not depend on the presence of seed c r y s t a l s , the composition o f feed melt and the operational temperature. The maximum c r y s t a l l i z a t i o n r a t e observed i n t h i s study was 13.73 [mm/sec]. However, the growth rate o | cooled type c r y s t a l l i z a t i o n i n the same system was about 3.0 x 10 [mm/sec].(2) The maximum supersaturation corresponding t o 1.0 o f r e l a t i v e supersaturat i o n i n Figure 4, i s 60 megapascals, and 60 megapascals i s estimated to correspond t o 15°K from equilibrium data i n the paper (_3) reported by the author. The supersaturation corresponding t o the maximum growth rate i n a cooled type c r y s t a l l i z e r , was about 3°K. This phenomenon that the supersaturation made by compression t o high pressure becomes higher value quickly, i s supposed t o come from incompressibil i t y o f the l i q u i d mixture. The incompressibility o f the mixture makes large uniform supersaturation before nucleation and higher c r y s t a l growth rate possible. 2) Tests o f the p u r i t y o f grown c r y s t a l s The experimental conditions and r e s u l t s o f the analysis o f the p u r i t y of separated benzene c r y s t a l s are shown i n Table 1. In t e s t s o f No. 1-1 t o 1-4, and 3-1 and 3-2, the melt was compressed t o the pressure shown i n Table 1 and kept on the same value, without seed c r y s t a l s . Nucleation occured on the wall and c r y s t a l s grew there. In t e s t s o f No.2-1, 2-2 and 3-3, seed c r y s t a l s were made as described above; they grew both i n s i d e the o p t i c a l c e l l and on the w a l l . In these t e s t s , since the melt around benzene c r y s t a l s was replaced by the water, the c r y s t a l s were taken out without serious destruction. The shapes of benzene c r y s t a l s were d e n d r i t i c , and p u r i t y o f i t was over 99.9 mole percent, independent from the operational conditions and the feed compositions as shown i n Table 1. Therefore, c r y s t a l s obtained by high pressure c r y s t a l l i z a t i o n i s considered t o be very pure due t o the complete removement o f mother l i q u i d from c r y s t a l surface.

Table 1. Experimental c o n d i t i o n s and t e s t s r e s u l t s of p u r i t y o f benzene c r y s t a l

Run No.

Feed Cone.

[•oil] 1-1 1-2 1-3 1-4 2-1 2-2 3-1 3-2 3-3

90.0 90.0 90.0 90.0 90.0 90.0 80.0 80.0 80.0

Super Press. [MPa] 29.4 39.2 49.0 58.8 49.0 53.9 31.4 50.9 90.2

Purity [molZ] 99.94 99.95 99.99 99.99 99.99 99.98 99.99 99.99 99.98


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CONCLUSION The experiments under high supersaturation effected by compression at high pressure were c a r r i e d out. C r y s t a l l i z a t i o n rates defined as the moving r a t e of the f r i n g e o f a c r y s t a l on the sapphire glass were very f a s t . The power number i n the c o r r e l a t i o n of c r y s t a l l i z a t i o n rate against r e l a t i v e supersaturated pressure d i d not depend on feed composition, supersaturation and operational temperature. When r e l a t i v e l y higher supersaturation was given, nucleation i n i n l e t flow o f melt was observed. The p u r i t y o f c r y s t a l s formed by high pressure c r y s t a l l i z a t i o n was over 99.9 mole percent even when c r y s t a l s were obtained under high supersaturation. From these r e s u l t s , the high pressure c r y s t a l l i z a t i o n i s concluded t o be a quick process f o r separation o f pure products from t h e i r mixture. LEGEND OF SYMBOLS P operational pressure [MPa] P equilibrium pressure [MPa] A l distance which f r i n g e o f c r y s t a l moved between two p a r t i c u l a r points on o p t i c a l glass [mm] A8 time required f o r the fringe o f the c r y s t a l t o pass distance between two p a r t i c u l a r points on the o p t i c a l glass [sec]

LITERATURE CITED (1) Toyokura K. : "Kagaku Kogaku Ronbunshu" 12, (5), 622 (1986) (2) Toyokura K. : "Industrial Crystallization '87" edited by J.Nyvlt and Zacek, Elsevier, 561 (1989) (3) Toyokura K. : "Industrial Crystallization '87" edited by J.Nyvlt and Zacek, Elsevier, 485 (1989) RECEIVED June 27, 1990


Crystallization as a Separations Process - American Chemical Society

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