Solubility Measurement and Modelling of Urea in Supercritical CO 2 and CO 2 + Ethanol Mixtures

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1 Solubility Measuremet ad Modellig of Urea i Supercritical CO 2 ad CO 2 + Ethaol Mixtures O. J. Catchpole*, S. J. Tallo,. J. Dyer, J.-S. La, B. Jese, O. K. Rasmusse, J. B. Grey Idustrial Research Limited, O Box , Lower Hutt, New Zealad, , o.catchpole@irl.cri.z The solubility of urea i supercritical CO 2 ad CO 2 + ethaol was measured over the pressure ad temperature rages bar ad K respectively, ad ethaol cocetratios 0 25 % by mass (urea free basis). The solubility i CO 2 was measured by a oce-through packed bed gravimetric method at a laboratory ad pilot scale. The solubility i CO 2 + ethaol was measured usig two differet gravimetric methods: atisolvet precipitatio ad gravimetric packed bed. The solubility of urea i pure CO 2 is very low, ragig from mole fractio at 100 bar, 333 K to at 300 bar, 353 K. The solubility icreases expoetially with a liear icrease i ethaol cotet. The highest solubility measured was ~ mole fractio at 333 K, 150 bar ad 26 % ethaol. At high ethaol cotets, the solubility was almost idepedet of pressure at a fixed temperature. The solubility was correlated by the eg-robiso equatio of state with various mixig rules for the repulsive ad attractive parameters. It was ot possible to fid pairs of iteractio parameters that modelled both the solubility i pure CO 2 ; ad CO 2 + ethaol. The solubility of urea i ethaol, ad ethaol i CO 2 was also modelled to obtai iteractio parameters for the model. Ethaol appears to icrease the solubility above that predicted usig iteractio parameters derived from urea/pure CO 2 ad ethaol/co 2 biaries. This icrease i solubility may be due to favourable iteractios betwee ethaol ad urea, such as hydroge bodig. INTRODUCTION Urea is a widely used agrochemical that is highly water soluble, ad poorly soluble i opolar orgaic solvets. The solubility of urea i supercritical CO 2 ad CO 2 + ethaol does ot appear to have bee measured before by other groups, although it has bee used as a model compoud for atisolvet precipitatio [1] usig CO 2 ; for the i situ urea complexig of fatty acid esters [2]; ad extractio of fatty acid esters from urea [3] usig CO 2. Recet studies have show that polar compouds with suitable fuctioal groups are capable of formig dimers i solutio, ad/or formig complexes with alcohol co-solvets i CO 2 [4,5]. Urea is expected to have low solubility i supercritical CO 2, but to be able to form hydroge boded complexes with ethaol as a co-solvet due to the hydroge bod acceptor carboyl group, ad hydroge bod doator amie groups. I this work, the solubility of urea is measured i pure CO 2 by a packed bed cotiuous flow method; ad i CO 2 + ethaol by packed bed ad atisolvet precipitatio methods. The solubility of urea i CO 2 ad CO 2 + ethaol was modelled usig the eg-robiso equatio of state [6]

2 EXERIMENTAL Urea was obtaied from Ravesdow NZ Ltd. Absolute ethaol was obtaied from Scharlau. Carbo dioxide was obtaied from BOC (NZ) Ltd. The solubility of urea was measured i a laboratory scale plat show i figure 1. A packed bed of urea of kow mass was placed ito the 500ml extractio vessel EXV1. The extractio apparatus was pressurised to the FM4 FM3 FM G1 CO2 GC1 HE1 V3 WT1 G2 FM DC1 HE2 B1 B2 WT1 FM1 T T Figure 1 Schematic of solubility apparatus G4 G3 EXV1 FM2 G5 SV2 V2 V1 RV1 SV1 operatig pressure usig airdrive compressor DC1. CO 2 the passed dowwards through the packed bed cotaied withi extractio vessel EXV1, before passig through a back-pressure regulator RV1, ad ito separatio vessel SV1. The CO 2, ow gaseous, passed through coriolis flow meter FM1 ad water trap WT1, where it was mixed with make-up liquid CO 2 from cylider OC1. The liquefied mixture passed through subcooler heat exchagers HE1 before beig recycled back to DC1. After a sufficiet mass of CO 2 had passed through the packed bed to esure a measurable amout of urea was extracted, the experimet was stopped ad EXV1 veted, iitially back to the cyliders ad the to atmosphere. The packed bed was weighed, stored i a sealed vessel overight, ad the reweighed. The urea collected i separatio vessel SV1 was removed with the aid of ethaol, which was forced through the extractio vessel (after the bed had bee removed), coectig pipework, ad valve RV1. The mass loss of the bed was compared with the mass recovered after evaporatio of ethaol to determie the mass balace, ad solubility. The experimets were carried out over log time periods ad high circulatio rates to miimise the weighig errors associated with the measuremet of lowlysoluble materials. Whe ethaol was used as a co-solvet, the apparatus was used i a oce-through flow mode. Whe the extractio vessel was pressurized as above, ethaol pump 1 was started at the same time as flow bega through the packed bed. Samples of urea dissolved i ethaol were recovered from the separator SV1 at regular time itervals, ad the ethaol removed uder vacuum to obtai the solubility. The separator was operated at a pressures of bar, to maximise the recovery of ethaol. Agai, the mass collected i the separator was compared with the mass loss from the bed (after heatig to remove ay residual ethaol) for mass balace purposes. Atisolvet experimets were performed by modifyig the apparatus to eable the CO 2 ad ethaol/urea mixture to cotact each other iside the extractio vessel. Ethaol solutios cotaiig a kow cocetratio of urea were pumped ito the extractio vessel usig 1 through the iside pipe of a cocetric pipe arragemet. CO 2 passed through the outer pipe, ad cotactig took place at the outlet of the ethaol/urea pipe. recipitated urea was collected i the extractio basket, which was heated ad weighed at the ed of the experimet. Ethaol ad o-precipitated urea that was recovered i the 1

3 separator SV1 were separated uder vacuum to eable a mass balace to be performed. The apparatus was further modified for low ethaol packed bed solubility measuremets. A secod pump (ot show i figure 1) was added. This pump ijected ethaol ito the CO 2 after the extractio bed ad before RV1 to esure complete collectio of urea. MODELLING The eg-robiso equatio of state with two iteractio parameters was used to model the phase equilibrium. The equatios for the solubility of a solid i a ear-critical fluid have bee preseted i detail elsewhere [7]: here the mixig rules are preseted for completeess. a b m m = i= 1 j= 1 = i= 1 j= 1 a b a i j b 0.5 j (1 k (1 l ij ij ) ) The model requires iteractio parameters, k ij ad l ij, for the biary systems CO 2 /ethaol; CO 2 /urea; ad ethaol/urea. The eg-robiso equatio was also used to obtai these parameters by obtaiig a best fit betwee literature data for CO 2 /ethaol [8,9] ad ethaol/urea [10,11]; ad the data obtaied i this work for CO 2 /urea. The sublimatio pressure of urea was obtaied from [10]. Urea critical parameters were estimated usig the Joback group cotributio method [12] RESULTS AND DISCUSSION I SOLUBILITY OF UREA IN SUERCRITICAL CO 2 The solubility of urea i pure supercritical CO 2 was measured i the apparatus show i figure 1 usig recycle of solvet. The solubility of urea i CO 2 was very low, reachig a maximum mole fractio of at 353 K ad 300 bar. The solubility was highly temperature depedet, which reflects the relatively high vapour pressure of the solid. At low pressures, the solubility was ot highly depedet o pressure, whereas at high temperature, the solubility showed some ressure, bar pressure depedece. The solubility i pure CO 2 is show i figure 2. The eg-robiso EOS with iteractio parameters, k ij = ad l ij = , was able to fit the data well. Mole fractio urea Experimetal, 313 K Experimetal, 333 K Experimetal, 353 K eg-robiso, 313 K eg-robiso, 333 K eg-robiso, 353 K Figure 2 Solubility of urea i CO 2 II SOLUBILITY OF UREA IN SUERCRITICAL CO 2 + ETHANOL The solubility i CO 2 + ethaol showed completely differet behaviour to the solubility i pure CO 2. The solubility icreased expoetially with ethaol at fixed temperature

4 ad pressure. The solubility 10 0 results for 150 bar, 313 K are show for fixed bed ad 10-1 eg-robiso atisolvet-type measuremets, Atisolvet acked bed, series 1 over a broad rage of ethaol acked bed, series cocetratios i figure 3. The solubility of urea i pure ethaol 10-3 at 150 bar ad 313 K is also show, alog with the predicted 10-4 solubilities usig the eg- Robiso equatio of state with 10-5 iteractio parameters from CO 2 /ethaol, CO 2 /urea ad 10-6 ethaol/urea biary systems There is good agreemet ercetage ethaol (urea-free basis) betwee the experimetal methods, but poor agreemet Figure 3 Solubility of urea i CO 2 + ethaol at 313 K ad 150 bar with the EOS, except at the extremes of the rages, i.e. at zero percet ethaol (ot show o the graph) ad 100 % ethaol. There is some scatter i the data at low ethaol Mole fractio urea cocetratios, due to the difficulties i performig the experimets. The solubility of urea i CO 2 + ethaol is show at 150 bar, 313 ad 333 K; ad 300 bar, 313 ad 333 K i figure 4. There is some scatter i the data at high ethaol cocetratios however, there is o discerable pressure depedece, ad very little temperature depedece. This is a uusual fidig, as there is pressure depedece i the solubility of urea i pure CO 2. The solubility of most solids measured i CO 2 + cosolvet show some pressure depedecy. At 100 % ethaol, there is temperature depedece i the data, which matches the temperature depedece of solubility at atmospheric pressure [10,11]. The eg-robiso EOS predicts that there is a slight Mole fractio urea 1e+0 1e-1 1e-2 1e-3 1e-4 1e bar, 313 K 150 bar, 333 K 300 bar, 313 K 300 bar, 333 K 1e Mole fractio ethaol (urea-free basis) Figure 4 Solubility of urea i CO2 + ethaol at K ad pressures of 150 ad 300 bar decrease i solubility of urea i pure ethaol as the pressure is icreased. This predictio is observed i the experimetal measuremets.

5 III MODELLING OF THE SOLUBILITY OF UREA IN CO2 + ETHANOL Iteractio parameters derived from biary systems could ot model the solubility whe usig the eg-robiso equatio of state. The fit of the model is highly depedet o the iteractio parameter betwee CO 2 ad urea. The best-fit iteractio parameter whe fitted agaist the CO 2 + ethaol experimetal data was substatially larger tha that obtaied from the biary system, suggestig that there is a much stroger iteractio tha that obtaied from dispersio forces aloe. Hydroge bodig iteractios have bee determied experimetally for ethaol ad other -alcohols i supercritical CO 2 [13]. There have bee a umber of reports showig that hydroge bodig occurs betwee ethaol (as a co-solvet) ad solutes that have hydroge door ad/or acceptor capabilities, such as fatty acids [4,5]. Additioally, the solutes are able to exist as redicted mole fractio urea Atmospheric pressure data High pressure data arity plot Measured mole fractio urea Figure 5 arity plot of predicted versus measured solubility of urea i ethaol at atmospheric ad high pressure (150 ad 300 bar) dimers, eve at very low cosolvet cocetratios [4,5]. We are uable to determie from our experimets whether hydroge bodig ad dimerizatio occurs. However, the structure of urea suggests that hydroge bodig should be possible whe ethaol is preset as a co-solvet. Hydroge bodig would be miimal or o-existet for pure CO 2, givig rise to a low iteractio parameter for the attractive term i the eg Robiso EOS. However, i pure ethaol, it is likely that urea forms strog iteractios, ad that iteractio parameters determied from solubility data are for urea complexes i ethaol. The eg-robiso EOS gives a surprisigly good fit to the literature data at atmospheric pressure, ad high pressure data measured i this work. A parity plot is show i figure 5. The fit of the EOS dimiishes with icreasig temperature. It is kow that the stregth of hydroge bods also dimiishes with temperature. I future work, we ited to model the solubility assumig that complex formatio has occurred, accordig of the models of Lemert ad Johsto [14] ad Tig et al [15]. CONCLUSIONS The solubility of urea was measured i pure CO 2, CO 2 + ethaol, ad pure ethaol at pressures i the rage bar, ad temperatures of K. The solubility of urea i pure CO 2 was low, ad followed solubility behaviour ormally observed i supercritical fluids. The solubility i CO 2 + ethaol was highly depedet o the ethaol cocetratio i the supercritical fluid mixture, ad weakly depedet o pressure ad temperature withi the

6 rages ivestigated. At ethaol mole fractios i the rage , there was o pressure ad temperature depedece. It is postulated that the lack of ifluece of temperature ad pressure is due to strog iteractios betwee the co-solvet, ethaol, ad urea. The eg- Robiso EOS was able to model the solubility of urea i pure CO 2 ad i pure ethaol, but ot i the mixture whe usig iteractio parameters derived from the relevat biary systems. Further work is required to improve the fit of the model by icorporatig iteractio parameters to accout for the formatio of complexes betwee urea ad ethaol; ad dimers of urea. REFERENCES [1] WEBER, A.; BEUTIN, M.; TSCHERNJAEW, J.; KÜMMEL, R., roc. 2 d It. Meetig o High ressure Chem. Eg. CD Rom, 2001 [2] HIROSHI, U. et al. J , 1985 [3] KULÅS, E.; BREVIK, H. WO 01/10809, 2001 [4] YAMAMOTO, M., IWAI, Y., NAKAJIMA, T., ARAI, Y., J. hys. Chem. A., 41, 2002, [5] IWAI, Y., UNO, M., NAGANO, H., ARAI, Y. J. Supercrit. Fluids, 28, 2004, [6] ENG, D.-Y.; ROBINSON, D. R., Id. Eg. Chem. Fudam., 15, 1976, [7] KWAK, T. Y.; MANSOORI, G. A., Chem. Eg. Sci., 41, 1986, [8] DAY, C.-Y., CHANG, C. J. CHEN, C. Y., J. Chem. Eg. Data, 41, 1996, ad 44, 1999, 365 [9] SUZUKI, K.; SUE, H.; ITOU, M.; SMITH, R. L.; INOMATA, H.; ARAI, K.; SAITO, S. J. Chem. Eg. Data, 35, 1990, [10] MESSEN, J. H.; ETERSEN, H. Urea, Ullma s Ecyclopedia of Idustrial Chemistry, Vol A27, VCH Verlagsgesellschaft, 1996, [11] STEHEN, H.; STEHEN, T. Solubilities of Iorgaic ad Orgaic Compouds, vol 1: Biary Systems, part 2, ergammo ress, Oxford, 1963, [12] REID, R. C., RAUSNITZ, J. M., OLING, B. E., The roperties of Gases ad Liquids, 4 th editio, McGraw-Hill, 1987, [13] BULGAREVICH, D. S., SAKO, T., SUGETA, T., OTAKE, K., TAKEBAYASHI, Y., KAMIZAWA, C., HORIKAWA, Y., KATO, M., Id. Eg. Chem. Res., 41, 2002, [14] LEMERT, R. M., JOHNSTON, K.. Id. Eg. Chem. Res., 30, 1991, [15] TING, S. S. T., TOMASKO, D. L., MACNAUGHTON, S. J., FOSTER, N. R. Id. Eg. Chem. Res., 32, 1993,