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1 EFFECT OF THE PHASE BEHAVIOUR OF SOLVENT- ANTISOLVENT SYSTEMS IN GAS-ANTISOLVENT CRYSTALLISATION A. Weber 1) *, L. V. Yelash 2), T. Kraska 3), R. Küel 4) 1) Goshen BoTech Inc., 83-2 Wolun-r, Waboo-eup, Nayangu-s, Kyongg-do, Republc of Korea , eal: tel.: , fax: ) Insttute of Physcs, Johannes-Gutenberg-Unverstät of Manz, Staudnger Weg 7, D Manz, Gerany 3) Departent of Physcal Chestry, Unversty Cologne, Luxeburger Str. 116, D Köln, Gerany 4) Fraunhofer-Insttute for Envronental-, Safety- and Energy-Technology, Osterfelder Str. 3, D Oberhausen, Gerany Crystallsaton processes usng supercrtcal CO 2 as an antsolvent have been shown to be feasble for edu and large scale applcatons n the past years. However, the developent of concrete applcatons stll requres a lot of experental and theoretcal work based on detaled experence n the feld. Here we dscuss soe effects based on ternary phase equlbra aed at crystallsng paracetaol. A calculaton ethod for the proper choce of the solvent s proposed, based on a global phase dagra ethod. INTRODUCTION Gas-Antsolvent Crystallsaton (GAS) was ntroduced n the 1980 s as a laboratory-scale ethod and has ganed a lot of nterest n the context wth the producton of crystallne solds havng adustable orphologcal propertes [1]. The prncple of ths technque s based upon the pressursaton of a lqud soluton wth a gas, whch dssolves n the lqud phase thus causng the precptaton of the solute fro the soluton. In recent years, results of plot-scale applcatons have shown that ths process s readly applcable also on a larger scale. The developent of applcatons, however, stll requres a lot of experental work and theoretcal analyss n order to obtan the requred process data. Frst coparatve studes of gas antsolvent crystallsaton were ntated by Kordkowsk et al. They easured the volue expanson of lqud solvents as a result of the uptake of the antsolvent for dfferent solvent-antsolvent systes and deduced a close relatonshp wth solute solubltes [2]. The studes of de la Fuente Badlla et al. lead to a ore descrptve expresson of the volue behavour of the lqud phase based on a therodynac descrpton [3,4] capable of predctng an optu pressure range for the crystallsaton. An even ore thorough analyss has been carred out by Kkc et al. on well deterned systes contanng naphthalene and phenanthrene dssolved n toluene and carbon doxde [5,6]. The proposed odel s capable of predctng solublty curves of the solutes wth good accuracy and can be appled ether for S-L-V or S-L1-L2-V phase behavour occurrng at dfferent syste teperatures. In practce, t would be favourable to derve a ethod to predct the phase behavour as well as the sold solublty of a less known ternary syste wthout runnng great nubers of experents n order to fnd sutable solvents or ether solvent xtures.

2 METHODS Ternary phase equlbru calculaton The phase equlbru between solvent and antsolvent s one key factor to nfluence the gas antsolvent crystallzaton process snce t fxes the axu attanable supersaturaton by the equlbru concentraton of each of the coponents of the syste. It can be descrbed by cubc equatons of state and depcted n phase dagras. Wthn ths work, the Peng-Robnson equaton (1) was used for odellng the phase behavour of the nvestgated substances [7]. RT p = V b V a( T) ( V + b) b( V b) (1) Here, V s the olar volue, R s the gas constant, T the teperature and p the pressure. The paraeters a and b are specfc paraeters of the substance whch can be calculated fro the crtcal data T c and p c. For xtures of substances, the bnary attracton paraeters were calculated by the Lorentz-Berthelot cobnng rules. ( k ) a = x x a a 1 (2) ( l ) b = x x b b 1 (3) The Peng-Robnson equaton s not able to descrbe the fugacty of sold coponents. Hence, the fugacty of a vrtual sub-cooled lqud state s calculated for the sold phase and corrected by eans of a Clapeyron-type approach. ln f s = ln f l H + R tr 1 Ttr 1 T (4) Here f s s the fugacty of the sold, H tr the enthalpy of fuson at the trple pont and T tr the trple pont teperature. Acceptng a sall devaton, the noral eltng pont teperature T and heat of fuson H can be used nstead of the values at the trple pont. For calculatng the sold-lqud-gas equlbru of a three-coponent syste contanng a sold, a lqud and a gaseous substance, a set of nonlnear equatons has to be solved sultaneously as descrbed n ore detal elsewhere [8, 9] or deduced by other authors [5, 10]. In order to be able to predct the propertes wth the odel, all avalable substance-related paraeters have been taken fro lterature data. Snce crtcal paraeters and acentrc factors are ust avalable for a few sold substances only, they had to be deterned by group contrbuton ethods [11]. The nteracton paraeters k and l n the xng rules were also taken fro publshed data of bnary systes [12,13,14]. The nteracton paraeters between solds and solvents were deterned fro the solublty at abent condtons. Hence the only paraeter left to ft easured solublty data was the nteracton paraeter between sold and gas. For paracetaol T s K and H has a value of 27.1 kj/ol [15,16]. The solublty data was easured on a dynac solublty setup descrbed n detal elsewhere [8]. Vewcell experents were carred out on a vewcell wth varable volue (NWA Lörrach, Gerany).

3 In addton to the analyss of the phase behavour based on px-dagras the global phase dagra ethod can be eployed to estate the usefulness of a specfc solvent-antsolvent xture for the GAS process. In global phase dagras ratos of the olecular nteracton paraeters such as the van der Waals attracton paraeter a are plotted rather than therodynac varables. In such dagras the regons of dfferent type of phase behavour are separated by therodynac boundary curves. The boundary curve of nterest n the context of ths work s the trcrtcal curve. It separates phase dagra types wth and wthout a lqudlqud-gas three phase coexstence usually located close to the crtcal pont of the ore volatle substance, whch s here the antsolvent CO 2. RESULTS Based upon the above assuptons, systes lke paracetaol/ethanol/co 2 and ascorbc acd/ethanol/co 2 could be descrbed reasonably well, despte these systes contan polar substances. It was possble to keep constant values for the nteracton paraeters over the regarded teperature range. Furtherore, the odel was able to predct a phase splt of the syste paracetaol/ethanol/co 2, whch could be verfed n vew cell experents. Fgure 1a) shows the solublty of paracetaol n the ternary syste dependng on the CO 2 pressure. Whle at 313K the solublty curve s not an unabguous functon for pressures above 7.5 MPa, the bup at 323 K and above 10 MPa clearly ndcates a lqud-lqud scblty regon. The behavour of the syste becoes clearer by lookng at the denstes of the correspondng phases. Fgure 2 shows an enlarged plot of the solublty curve at 313K above 7.0 MPa and the calculated densty of the flud phases. The dscontnuty of the densty functon for the lqud phase acknowledges a phase splt at a pressure above 7.5 MPa. The correspondng phases can be regarded as lqud, due to ther denstes between 800 and 900 g/d 3 at the desgnated teperature and pressure, and show a dfferent solublty of paracetaol. Gas-phase and l 2 -phase eet n a bnary crtcal pont at a pressure of 8.2 MPa showng a densty of 450 g/d 3. The phase behavour of ths ternary syste resebles the type IV behavour of a correspondng quas-bnary syste. a) b) Fgure 1: Solublty curves of paracetaol n ethanol/co2 at 313K (sold lne) and 323K (broken lne) (a), n acetone/co 2 at 313 K (b).

4 Fgure 2: Enlarged plot of the solublty curve of paracetaol n ethanol/co 2 at 313K ncludng the calculated denstes of the dfferent flud phases. The sold black curve denotes the solublty n the lqud phases, the broken grey curve shows the solublty n the copressed gas phase. The dotted curves denote the lqud densty, the broken black curve the gas -densty. Fgure 3: Lqud-lqud dexng of a paracetaol soluton n ethanol when t s pressursed wth CO 2. Crystallsaton only takes place n the upper, CO 2 -rch lqud phase. The splttng of the lqud phase leads to undesred effects n practcal crystallzaton as descrbed by Reverchon et al. and akes t possble to attan safe processng condtons [18]. Hence, an alternatve solvent for paracetaol was sought usng odel calculatons. The syste paracetaol/acetone/co 2 showed a ore balanced solublty curve and no phase splt at even uch hgher teperatures (Fgure 1b). The effect of lqud-lqud dexng can also be observed n hgh-pressure vewcell experents. A saturated soluton of paracetaol n ethanol (c=150 kg/ 3 ) splts nto two lqud phases when pressursed wth CO 2 at pressures above 7.6 MPa (Fgure 2). The sae experent wth a saturated soluton of paracetaol n acetone (c=75 kg/ 3 ) does not show any lqud-lqud dexng.

5 Fgure 4: Part of the global phase dagra for CO 2 /ethanol (bullet) and CO 2 /acetone (daond). The sold lne s the trcrtcal lne for the co-volue rato of the CO 2 /ethanol syste. The dashed lne s the trcrtcal lne for the co-volue rato of the CO 2 / acetone. The axes are defned as λ a12, whch s related to the cross-attracton, and ς a22 a11 whch s a reduced dfference between the nteracton paraeters of the pure coponents. Global Phase Dagra calculatons Fgure 4 shows the locaton of the two CO 2 /solvent systes n a global phase dagra. One can see that the CO 2 /ethanol syste s very close to the trcrtcal boundary whch eans that t can change ts phase behavour by sall perturbatons leadng to a lqud-lqud scblty. The CO 2 /acetone syste, however, s n sgnfcant dstance to the trcrtcal boundary. Therefore, t s unlkely that the addton of a solute wll affect the type of phase behavour and furtherore lqud-lqud scblty s not expected. Such systes are sutable for the GAS process. CONCLUSIONS The phase behavour of a ternary gas-antsolvent syste has been odelled based on the cubc Peng-Robnson equaton of state suppleented by a Clapeyron-type approach for the sold phase. The results show that for certan systes a lqud-lqud phase splt appears after addng a sold substance to the solvent-antsolvent syste. For further analyss we calculated the trcrtcal boundary for each bnary solvent syste, whch lts the appearance of the lqudlqud phase splt nterferng wth the solute precptaton. As a crteron for the sutablty of a solvent-antsolvent syste for the gas antsolvent process, the dstance to the correspondng trcrtcal curve has been analysed. Ths crteron goes beyond a possble nspecton of a pressure-ole fracton phase dagra and gves ore relable estates of the solvent sutablty for GAS. The closer a solvent-antsolvent syste s located to a trcrtcal boundary the ore lkely the quasbnary syste s oved across ths boundary by addng the solute. In case of suffcent dstance to the trcrtcal curve the addton of the solute less lkely leads to a change n phase behavour.

6 ACKNOWLEDGEMENT Fnancal support by the Geran Scence Foundaton (DFG) s acknowledged. REFERENCES : [1] REVERCHON, E., J. Supercrtcal Fluds 20, 1999, 1. [2] KORDIKOWSKI A., SCHENK, A. P., VAN NIELEN, R. M., PETERS C. J., J. Supercrtcal Fluds, 8, 1995, 205. [3] DE LA FUENTE BADILLA J. C., PETERS C. J., DE SWAAN ARONS J., J. Supercrtcal Fluds 17, 2000, 13. [4] DE LA FUENTE BADILLA J. C., SHARIATI A., PETERS C. J., Proc. 6 th Int. Syposu on Supercrtcal Fluds, Versalles, Apr [5] KIKIC I., BERTUCCO A., LORA M., Ind. Eng. Che. Res. 36, 1997, [6] BERTUCCO A., LORA M., KIKIC I., AIChE Journal 44, 1998, [7] PENG D. Y., ROBINSON D. B., Ind. Eng. Che. Funda. 15, 1976, 59. [8] WEBER A., KÜMMEL R., KRASKA T., n Supercrtcal Fluds as Solvent and Reacton Meda, G. Brunner (ed.), Elsever, Asterda, n prnt. [9] WEBER A., YELASH L. V., KRASKA T., J. Supercrtcal Fluds, subtted. [10] SHARIATI A., PETERS C. J., J. Supercrtcal Fluds 23, 2002, 195 [11] POLING B. E., PRAUSNITZ J. M., O'CONNELL J. P., The Propertes of Gases and lquds, 5th ed., McGraw-Hll, New York, [12] DAY C.-Y., CHANG C. J., CHEN C.-Y., J. Che. Eng. Data 41, 1996, 839. [13] STRATMANN A., PhD thess, Unversty of Bochu [14] LÜDECKE D., LÜDECKE C., Therodynak, Sprnger-Verlag Berln Hedelberg 2000 [15] MANZO R. H., AHUMANDA A. A., J. Phar. Sc., 79, 1990, [16] GRANBERG R. A., RASMUSON A. C., J. Che. Eng. Data 44, 1999, [17] WUBBOLTS F. E., PhD thess, TU Delft [18] REVERCHON E., DELLA PORTA G., FALIVENE M.G., J. Supercrtcal Fluds 17, 2000, 239.