Fractionation of liquid charges in continuous column with Supercritical Fluid. Tiziana Capolupo

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1 Fractionation of liquid charges in continuous column with Supercritical Fluid Tiziana Capolupo

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3 Unione Europea FONDO SOCIALE EUROPEO Programma Operativo Nazionale 2000/2006 Ricerca Scientifica, Sviluppo Tecnologico, Alta Formazione Regioni dell Obiettivo 1 Misura III.4 Formazione superiore ed universitaria UNIVERSITÀ DEGLI STUDI DI SALERNO Department of Chemical and Food Engineering Ph.D. Course in Chemical Engineering (IV Cycle-New Series) Fractionation of liquid charges in continuous column with Supercritical Fluid Supervisor Prof. Libero Sesti Osséo Ph.D. student Tiziana Capolupo Scientific Referees Prof. Ernesto Reverchon Prof. Ignacio Gracia Ph.D. Course Coordinator Prof. Ernesto Reverchon

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5 List of publication Libero Sesti Osséo, Tiziana Capolupo Phase equilibria measurement of system carbon dioxide triolein and their correlation, using the Soave-Riedlich-Kwong EoS, sendt to Journal of supercritical fluid. Libero Sesti Osséo, T. Capolupo Continuous fractionation of fried oil by supercritical CO 2 and hexane, sendt to 10 th European meeting of supercritical fluid, December 2005 Strasbourg (France). Libero Sesti Osséo, Tiziana Capolupo, Giuseppe Caputo, On the modelling of fractionation of fried oil with supercritical carbon dioxide: a first step, 6th International Symposium on Supercritical Fluids, June 2004, Trieste (Italy) III

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7 Contents Abstract...1 I. Introduction...3 II. State of the art...7 II.1 Actual utilization... 7 II.2 Alternative regeneration with supercritical fluid fractionation.. 8 II.3 Modelling of coutercurrent fractionation... 9 II.4 Thermodynamic of sub- and super-critical systems III. Aim of the work...11 IV. Materials and methods...13 IV.1 Materials IV.2 Laboratory Fractionation Apparatus IV.3 Chromatography IV.4 Equilibrium cell IV.5 Model and software V. Thermodinamic of non ideal mixture: SRK-EoS...19 V.1 Thermodynamic literature data of non ideal fluids V.2 Lumping in pseudocomponents VI. Equilibrium modelling and simulation results...27 VI.1 Simulation of equilibrium POL-CO VI.1.1 Critical point and acentric factor prediction of POL VI.1.2 Evaluation of EoS constant a and b VI.1.3 Simulation of thermodynamic equilibrium VI.1.4 Non linear regression Introducing in SRK-EoS: VI.2 Equilibrium TG-CO 2 : comparison of experimentation and simulation VI.2.1 Phase equilibrium measurements VI.2.2 Simulation of phase equilibria VII. Extrapolation of quaternary equilibrium...41 VIII. Mass transfer...47 IX. Modelling of coutercurrent extraction...51 V

8 VI IX.1 To solve multicomponent separation without calculator routine IX.1.1 Two empirical methods McCabe Thiele and Ponchon Savarit...52 IX.2 Multicomponent separation with calculator routine X. Column simulation XI. Conclusions References Appendix I: Quaternary equilibrium implementation 76 Appendix II : simulation of fractionation column... 89

9 Index of figure Figure. IV.1 Chemical group of POL Figure. IV.2 Chemical group of POL Figure IV.4. HPLC chromatography for bottom fraction of fried oil...16 Figure VI.1. P-T diagram for the possible types of binary mixtures...30 Figure VI.2. P-T diagram for the system triglyceride-co Figure VI.3. P-x,y diagram for the system TG-CO Figure VI.4. P-x,y diagram for the system T-CO Figure VI.5. P-x,y diagram for the system POL-CO Figure VI.6. Molar phase composition of the system triolein-co 2 at 313, 333 and 353 K Figure VI.7. Distribution coefficient for the system triolein-co 2 : ( ) K triolein 313 K, ( ) K triolein 333 K, ( ) K triolein 353 K; ( )K CO2 313 K, (o) K CO2 333 K, ( ) K CO2 353 K...37 Figure VI.8. Comparison of SRK-EoS results with respect to experimental data for the system triolein-co 2 at 313, 333 and 353 K; our experimental points; Simulation SRK-EoS...38 Figure. VII.1 Phase equilibrium in a quaternary system...42 Fig. VII.2 Phase equilibrium in a quaternary system...43 Fig. VII.3 Phase equilibrium in the quaternary system...44 Figure IX.1 Ponchon-Savarit diagram for a specific case (fatty acid etil ester FAEE)...53 Figure IX.2 Flow scheme of a coutercurrent extraction column...54 Figure IX.4. Schematic representation of the extraction column...56 Figure X.1. PRO II shielded Figure X.2. Percentage of POL, TG and LMWC on the bottom fraction obtained by simulation...61 Fig. X.3. Percentage of POL, TG and LMWC on the head fraction obtained by simulation...62 Fig. X.4. Percentage of POL, TG and LMWC on the bottom fraction obtained by simulation, considering POL as a lighter compound we obtain...63 Figure X.5. Percentage of POL, TG and LMWC on the head fraction obtained by simulation, considering POL as a lighter compound we obtain...64 VII

10 Fig X.6 Difference between experimentation of fractionation column and simulation with two type of definition of heavy compound POL...65 VIII

11 Index of table Table V.1. Parameters a, b, u and w for equation V Table VI.1. fried oil properties...28 Table VI.2. Binary interaction parameter...34 Table VI.3. Source of data...34 Table VI.4. Experimental mole fraction x, y of the system CO2 triolein at 313, 333 and 353 K...35 Table VI.5. Extrapoled parameter for SRK- EoS for system CO2 triolein37 Tab VI.6. Critical properties...40 IX

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13 Abstract The countercurrent extraction with supercritical fluids as solvents, especially carbon dioxide, offers new opportunities for the solution of separation problems. The supercritical fluid can separate various types of organic compounds on the base of their polarity and molecular weight Rizvi et al., (1988). Major advantages of this purification process might be low temperature separation and good selectivity. Selectivity variation of the supercritical fluid requires only a change in the density of the fluid via manipulating pressure and temperature. Besides, the supercritical CO 2 offers many other favourable features due to the fact that it is a clean, inexpensive, non-flammable and non-toxic solvent. Knowledge of vegetable oil solubility and phase equilibria in supercritical CO 2 and of its pressure and temperature dependence is important for the success of application of the supercritical fluid technologies. SCF technique has been successfully applied to separate lipid components from various different food products. The purification and fractionation of used frying oil by supercritical fluid extraction has given good results I. Gracia et al., (2003). Supercritical CO 2 is a promising solvent for extraction and fractionation of used vegetable oils, as we test in laboratory, since these processes can be carried out at low temperature and give a final product similar to original oil. This purification could be achieved to recuperate triglycerides (TG) and to remove the undesirable oxidized low molecular weight compounds (LMWC) and polymerised triglycerides (TGD). Study of present work consist in a preliminary study of fried oil components, in a description of thermodynamic properties of principal component, than in a phase equilibrium description of CO 2 -fried oil system at supercritical condition, that continue with the study of mass balance, amount separation and plant scale-up. In a preliminary study VLE equilibrium data has been estimated with a predictive method Brunner, (1994), this uses the classic procedure of determination of the constants of binary interaction to leave from the points 1

14 of P-x-y equilibrium. Knowledge of phase equilibria is a necessary factor to describe the Supercritical Fractionation (SCF) and to optimize the design of separation processes. In this work the simulation will be supported by experimental equilibrium data and by experimental data of fractionation laboratory apparatus. 2

15 I. Introduction A large proportion of fats and oils in the world is used for the preparation of fried foods. During deep fat frying a significant quantity of oil is heated for a long period. Oils undergo a complex series of changes and reactions during frying. D.Lgs. 5/2/1997, n.22 of Italian law shall bring into force the competent authority and the private producer to regenerate refusal rather than to send in rubbish dump or recycle in other application. Than the regeneration of fried oil it is destined to grow. In order to improve the quality of used oils, procedures were designed to regenerate the oils, instead, economic considerations and the need to produce fried foods of uniformly desirable quality have stimulated an interest in purification of frying oil. Some factors, namely moisture, heat and oxygen cause different physical and chemical changes such as hydrolysis, oxidation, and polymerization that consequently decrease oil quality. Oils typically contain straight chain fatty acids with or more carbons which are often unsaturated, having from one to six double bonds. Fried oils consist mainly of triglycerides, triacylglycerols of fatty acids. Triglycerides are characterised by their carbon number, which is total number of carbon atoms in the three constituent fatty acids. Triolein (glyceroltrioleate) with carbon number 54 is their typical constituent, because oleic acid is the most abundant acid in vegetable oils. Next to oleic acid are other fatty acids with eighteen or sixteen carbon atoms in the molecule. Fried oils as mixtures are characterised by presence of undesirable compounds, fractionation consist in the triglycerides extraction, and separation of other compounds in heavy and light fraction Tessa, (1993). enes, formation and decomposition of hydroperoxides, formation of low molecular carbonyl compounds, hydrolysis of triglycerides, and polymerization via complex free radical processes at elevated temperatures around 180 C Marquez-Ruiz et al., (1996). Free fatty acid content, peroxide value, polar compounds, fatty acid composition, iodine value, induction period, colour, and viscosity of the used oils after certain periods of frying change. Used frying oils are generally discarded because oxidized lipid

16 Chapter I degrades the quality of fried foods and poor quality oils might affect the health of people. In fact biological effects of used frying oil on human species are not negligible. Because of such large consumption of frying oils, the effect of high temperatures on the oils is of major concern both for product quality and nutrition, taking into account that dietary fat source deeply influences several biochemical parameters, mainly of mitochondrial membranes. More than ever the virgin olive oil possesses specific features for modulating the damages occurred by endogenous and exogenous oxidative stress being particularly rich in antioxidant molecules. Several components like antioxidant vitamins could be lost due to oxidation and some others with toxic effects could appear. The extent of modifications following a shorttime deep fat frying procedure: vitamin E and phenolic compound as well as total antioxidant capacity decreased, while polar compounds increased. The intake of such an altered oil mainly affected the hydroperoxide contents of mitochondrial membranes which were enhanced after the dietary treatments Battino et al., (2002). The effects on the man are: a slight depression in growth; diminished feed efficiency; increased liver, kidney and heart sizes; increased fatty tissues of liver, kidney and heart organs, liver enzymes such as thiokinase and succinyldehydrogenase had lower activity, the evidence of carcinogenicity ( in highly abused frying oil). The discarded used-frying oil still has a large portion of triglycerides, but there are also volatile compounds, that includes hydrocarbons, aldehydes and ketones, and polymers, that includes mostly triglycerides dimers. With regard to the origin of these oils, three types of activities with distinctive characteristics may be noted: - Frying industries. Many companies use continuous frying systems, with continuous replenishment and rejection of a very small volume of oil. Therefore only small- or mediumsized enterprises should be considered, which use discontinuous systems where significant quantities of reused oil are produced as waste to be disposed of every day. It is a type of activity which is easy to control as regards disposal of these cooking oils. - Large catering or community restaurant enterprises. These produce the largest proportion of reused oil waste. They are also centres which are easy to control as regards production of waste oil. In addition, many are chains with a large number of establishments in the same town, which facilitates their control. - Small restaurants or bars. These also produce a fair proportion of the total cooking oil waste, although their individual production volume is much smaller than that from large catering companies or chains. In these establishments, the recycling culture is less common (varies according to country) and should be especially encouraged by means of official campaigns directed towards these establishments. 4

17 Introduction Influence of fatty acid composition and initial quality of the oil was specifically noted in the formation of polymerization compounds. Crude oils presente very low contents of (TGD) Tri Glyceride Dimers, which were not even detectable in olive oils. As is known, polymerization take place at high temperatures and depends on the unsaturation degree of oils Dobarganes et al.,(1984), which explains why lower levels were found in olive oils as compared to sunflower and soybean oils. Several attempts have been previously made to purify the used frying oil without good results, supercritical fluid extraction might be a valid alternative. The conventional methods for fractionation and isolation of these components include vacuum distillation, urea crystallization, hexane extraction, or conventional crystallization having the disadvantages of requiring high-temperature processing resulting in degradation or decomposition of the thermally labile compounds or employing flammable or toxic solvents having adverse health effects, respectively. Super Critical Fractionation SCF can be considered a sustainable chemical technology ( green chemistry ). Supercritical CO 2 is an environmentally compatible alternative to many conventional industrial solvents. When appropriate operating conditions are set, a recovery of about 97% of TG fed into the column can be obtained, with a composition very similar to the fresh frying oil. The design of supercritical CO 2 fractionation column requires the knowledge of phase equilibrium data, to estimate the driving forced for the material transfer between the phases. Local phase equilibria, mass transfer rate and material balances must be simultaneously solved and integrated along the column. Unfortunately, the fried oil consists of many components and equilibrium data for a part of them are not available. In fact, whereas equilibrium data between CO 2 and the classical components of vegetable oils (LMWC; TG) are available in literature, the data between CO 2 and heavier (DPTG) components has not yet been investigated. An industrial application of oil regeneration with supercritical fluid fractionation is thinkable for regenerate oil of refuse collected, or to re-use fried oil in food industries and catering chains. 5

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19 II. State of the art In this paragraph are described uses and regeneration methods of fried oil. In reference to supercritical fractionation, an accurate state of the art is developed for thermodynamic of non-ideal system (as sub- and super-critical system). In the last decade, supercritical fluids has been proposed as environmentally compatible media for chemical and related processes. Many new processes and products have been developed, using the inherent physical and chemical properties of supercritical fluids. The prerequisites for this success however, are a knowledge of physical-chemical properties and phenomena in supercritical mixtures and the availability of other chemical engineering data. This requires an effective exchange of knowledge between a large number of branches of science. In the following, which should give an overview of recent results on fundamentals of supercritical extraction and their applications is performed. II.1 ACTUAL UTILIZATION Used cooking oils constitute a waste generated from industries and restaurants, which has greatly increased in recent years, leading to recycling. The main reuses at present are: o in animal feed; o in the manufacture of soaps; o as biodegradable (although not recyclable) lubricants ; o for production of bio-diesel; o for combustion (raw fuel in industrial plants or incinerations). The use of recycled cooking oils mixed in the animal feed is a diffused practice that is not recommended for the future. Particularly after the recent accidents involving dioxin contamination, many studies have suspected that cooked oil are responsible. Other toxic substances can also be present in recycled cooking oils. Accumulative and persistent contaminants undergo a bio-magnification effect, increasing their concentration along the food chain. Some health risks arise from the use of recycled cooking oils in animal feed, 7

20 Chapter II such as undesirable levels of contaminants, particularly PAHs, PCBs, dioxins and related substances. Analysis of the risks for consumer health involved in the reuse of waste cooking oils in the food chain, lead to the prospective to prohibit the use of unrecycled used cooking oils in food or animal feed PE n (2001). Fried oils are used also as renewable sources for production of Biodiesel (fatty acid alkyl esters). This is a cleaner-burning diesel replacement fuel made from natural, renewable sources such as new and used vegetable oils and animal fats. Just like petroleum diesel, biodiesel operates in compression-ignition engines. Biodiesel fuel can be made from new or used vegetable oils and animal fats, which are non-toxic, biodegradable, renewable resources. Fats and oils are chemically reacted with an alcohol (methanol is the usual choice) to produce chemical compounds known as fatty acid methyl esters. Biodiesel is the name given to these esters when they're intended for use as fuel. Glycerol (used in pharmaceuticals and cosmetics, among other markets) is produced as a co-product. Another fried oil utilization is the combustion at high temperature in furnaces for cement production or to support thermally the incineration. II.2 Alternative regeneration with supercritical fluid fractionation The increasing commercial interest of food and biochemical industries in polyunsaturated fatty acids from oils has led to extensive studies on the fractionation of natural oils. Supercritical CO 2 combines the advantages of moderate working temperatures with a non-flammable and non-toxic extraction solvent. Decomposition or degradation of thermally labile compounds as is known from conventional separation methods, e.g. vacuum distillation, can be avoided. Extraction and fractionation using supercritical carbon dioxide are studied in last years. The supercritical CO 2 fractionation of fried oil are tested in a batch extractor Yoon et al., (2000), there were obtained promising results when a two-stage operation was performed. A similar batch process has been recently reported in a patent, where ethyl alcohol has to be used as cosolvent. On supercritical fractionation there is also a license of Carlo De Vittori, for a separation system applied to industry SEPAREX Chimie fine di Champigneulles De Vittori, (1999). The license consist in fractionation of liquid feeds (oils, molten fats) by continuous processes with supercritical CO 2 in order to obtain refined oils (color and odor removal), or highly purified products (polyunsaturated fatty acids, polar lipids, ) for use in pharmaceutical, cosmetic or diet applications. 8

21 State of art The SCF technology applied to used cooking oil in a continuous fractionation column, not yet tested at any scale, could thus provide results that can be analysed in view of an industrial applications. The fractionation of the oil in a continuous packed tower are studied Gracia et al., (2002), where supercritical CO 2 is used as solvent to fractionate the fried oil in: a part, that can be reused and a part, that must be treated as a toxic residue. The products have been characterized, in a first approach, in three fractions, distinguishing by High Pressure Size Exclusion Chromatography: Polymeric compounds (POL), Triglycerides (TG) and Low Molecular Weight Compounds (LMWC). The actual trend of experiments in this field is divided in two view: selective fractionation, to obtain specific component, Brunner et al., (1998), Brunner, Riha, (1999)], Da Ponte et al., (2000). massive fractionation, to obtain different molecular weight group: -Purification of used frying oil by supercritical fluid continuous extraction Brunner et al., (1997); Sesti Osséo et al., (2001); Poletto et al., (2000); Sesti Osséo, E. Reverchon, 1997); Reverchon, L. Sesti Osséo, (1994). -Purification of used frying oil by two stage supercritical fluid extraction Yoon, (2000) II.3 Modelling of coutercurrent fractionation Fractionation by supercritical fluid is a classical multicomponent continuous unit operation were enrichment of some component is performed. So, in principle, the model should be well known and thus not to complicated. Material balances coupled with mass transfer equation and equilibrium information should be simultaneously solved. Modelling of SCF countercurrent fractionation can be analyzed in different manner: -distillation model Chrisocoou et al.,(1997); Brunner et al.,(1998), with a simpler method; -adsorption model Da Ponte et al., (2000). Recent years have seen a rising interest in separating complex fluid mixture by Super Critical Fractionation SCF. Here, the separation problem shifts away from gaining a single product from a feed mixture towards fractionating a binary (or multicomponent) mixture into two pure substances (or fractions). This type of extraction is termed fractional extraction fulfilling 9

22 Chapter II the same task as fractional distillation. Fractional extraction of fluid is usually performed in continuous countercurrent columns with extract reflux Pratt, (1991). Countercurrent extraction with supercritical fluids has been investigated for quite a long time. But studies were dedicated mainly to experimental investigations, and only few studies performed some basic modelling and simulation of the process Riha, Brunner, (1994). Based on phase equilibrium data, Staby and Mollerup, (1993) outlined the scheme of a complex SCF process. The basic information to apply both models are not available in literature, because the fried oil composition is not completely characterized, especially concerning the high molecular weight compounds generated during frying. In addition, the interaction parameters among the chemical compounds constituting the fried oil are not yet characterized and reported in literature. Moreover, the fluid-dynamic behaviour of the supercritical column is not yet well described. For all reasons above, the model must be build up, we start from parameter definition to have a correct estimation of equilibrium. II.4 Thermodynamic of sub- and super-critical systems In order to evaluate SCF performance, relevant phase equilibrium data is needed. Up to now the main focus in literature has been to measure and correlate the mixture equilibrium characteristics with evaluation of binary interaction. For our oil mixture, three binary interaction are relevant: CO 2 - LMWC, CO 2 -TG, CO 2 -TGD. Because binary interaction between TG, TGD and LMWC aren t relevant. Phase equilibria of fat compounds with supercritical CO 2 have been measured by various authors Ashour, Hammam, (1993)]; Zou, et al., (1990). Ternary and multi component data are more scarce Nilsson, (1991). Distribution coefficient between the phases formed and separation factor of different components in complex mixtures were determined for fish oil Staby, (1993). Since the available data basis is not sufficient to evaluate phase composition. It can be said that up to now is possible to predict solubility or phase equilibrium behaviour with a simulation technique, but it is necessary an experimental validation, because as well for cubic interaction parameters as for solubility behaviour by Chrastil equation parameters a, b and k have to be fitted to experimental data Chrastil, (1982). This is the reason why a lot of publications deals with determination of solubility and phase behaviour. For fatty compounds few experimental data are available. In this work, is presented binary thermodynamic data for triolein Sesti Ossèo et al., (2005), Brunner et al., (1999), LMWC T. Fornari, et al., (2001), and triolein-dimer (calculated) in carbon dioxide. These data are needed for construction of equilibrium correlation of the mixture, and for evaluate phase separation. 10

23 III. Aim of the work The study of fractionation column using supercritical fluid have the objective of define a column simulation method, useful to limit experimental measuring and to project other column. The aim were to obtain an interpretation of experimental results. The separation process of countercurrent gas extraction can be modelled using the common basic equation: mass balance, energy balance, equilibrium distribution coefficients and rate equations of mass transfer. The resulting system of differential equation can be solved, but the solution is not easily accessible in the case of gas extraction system, since pure component properties, properties of mixtures, equilibrium relation and mass transfer correlations are so far not available in most case, especially not for multicomponent mixtures. The sequence of step to obtain the description of fractionation are: simulation of equilibrium for a first prediction experimentation in a view cell for equilibrium determination determination of mass transfer between phases determination of hydrodynamic of column simulation of column The start point of this work are definition of equilibrium for witch we use Cubic Equation of State ( Cubic- ) to correlate equilibrium data of mixture. Motivation to use phase equilibrium correlations is to reduce the experimental effort and to combine phase equilibrium measurement with separation design. Incorporating phase equilibrium correlations in separation analysis demands reliable information about the influence of composition on partition coefficients. The cubic consider the complex mixture interaction as the sum of binary interaction between mixture compounds. Than with the knowledge of binary phase equilibria, it will be possible to predict the thermodynamic equilibrium of system. The system has been lumped as CO 2 and tree pseudo-components of fried oil of major interest (LMWC, TG and POL). Because we assumed from phase equilibrium measurements that separation due to the chain length is possible. Therefore, all components of the mixture were lumped into a set

24 Chapter III of tree pseudocomponents as a function of their chain length and group characteristic. 12

25 IV. Materials and methods The fractionation tower is the main apparatus of our research and the good result obtained from experimentation of fried oil extracted with SCCO 2, is the basis of our studies to define a model for this new type of liquid mixture separation. This choice of non ideal mixture (fried oil) causes the long study of thermodynamic definition of mixture. IV.1 MATERIALS Exhausted oil was obtained as residual oil after a frying for 6 hours at 150 C in an industrial frying machine. The properties of the oil analyzed with the methods described in the next section, are indicated in Table 1, together with those corresponding to the fresh oil used in the food industry. Table IV.1. Fresh and fried oil composition POL % TG % LMWC % Acidity % PV (Meq O 2 /kg) Fresh Oil Fried Oil With POL: Polymer, TG: Triglyceride, LMWC: Low Molecular Weight Compounds, PV: Peroxide Value. Carbon dioxide (purity 99 %) was supplied by S.O.N. (Naples, Italy). All analytical reagents were bought by Sigma-Aldrich, Italy. Analyses: The Acidity and Peroxide Value parameters of the oil were determined according to standard methods specified in European Commission Regulation 2568/91. First definition of POL (POL1) as a triolein dimer can be resumed with group contribution method as follow.

26 Chapter IV Figure. IV.1 Chemical group of POL1 Also another hypothesis for chemical structure of POL (POL2) was made during study of column to explain best the fractionation problem. POL is defined as a triolein with another olein attached on double bond CH=CH, this can be resumed with group contribution method as follow. Figure. IV.2 Chemical group of POL2 IV.2 LABORATORY FRACTIONATION APPARATUS A scheme of the experimental apparatus is shown in Figure 1. The column, is 1920 mm long and has an i.d. of 17.5 mm. It consists of 5 cylindrical sections (Autoclave Engineers), 305 mm long and with a O.D. of 25.4 mm, connected to each other and to the process lines by 6 similar 4-port elements. All parts are made in AISI 316 stainless steel and are designed to withstand up to a maximum internal pressure of 70 Mpa at 30 C. The column is packed with stainless steel packings 5 mm nominal size with

27 Materials and methods m-1 specific surface and 0.9 voidage. The temperature along the column is controlled by five PID controllers (Watlow Model 965). The solvent is fed to the column by a high-pressure diaphragm pump (Milton Roy Model Milroyal B), that can deliver CO 2 flow rates up to 12 kg/h and that is provided with a cooled head. P-1 Oil pump P-2 CO 2 pump TC Temperature controller FC Flow controller Figure IV.3. Experimental Apparatus. PI Pressure indicator, TI Temperature indicator FI Flow indicator SP sample point. The CO 2 temperature at the column inlet is controlled by a PID controller. Similar temperature-controlling techniques are also adopted for the liquid feed. The oil mixture is withdrawn directly from a reservoir, and fed to the column by a piston pump (Milton Roy Model Minipump). The stream exiting from the top of the column is heated to C before being depressurized to 2 Mpa by a micrometering valve, and then it is fed to a separator kept at a temperature around 0 C by another controller. A second 15

28 Chapter IV separator collects the eventual part of the most volatile extract. Before the vent, a rotameter and a dry test meter measure the CO 2 flow rate and the total quantity of solvent used, respectively. The oil fractions are then collected at the bottom of the tower and at the bottom of the first and the second separator. Extract and raffinate samples were weighted and analyzed according to the analytical procedures. The product flow rates were calculated by weighing the samples collected at the top and bottom of the column at fixed time intervals. IV.3 CHROMATOGRAPHY Since the analysis of the exact distribution of the different compounds in the oil is out of the scope of this work, we decided to evaluate the oil composition on the basis of the molecular weight of the three main components of the oil: LMWC, TG, POL. Molecular weights of these compounds were determined by High Pressure Size Exclusion Chromatography (HPSEC). We used a HP 1100 chemstation equipped with a 20 µ L injector loop, a UV detector set at 230 nm, and two PLGel columns (Perkin-Elmer, U. K.) 30 cm x 0.75 cm. mau tim e, m in Figure IV.4. HPLC chromatography for bottom fraction of fried oil. In Figure IV.4 is reported a HPLC chromatography for BFR, that show at time of min TG peak and at time min DPTG peak. IV.4 EQUILIBRIUM CELL Phase equilibrium measurements for the binary system triolein CO 2 have been carried out at pressures ranging from 10 to 65 MPa; temperature has been set at 313, 333 and 353 K. The apparatus used is a high pressure cell (NWAGmblt, Lorrach, Germany), consisting of a horizontal cylinder

29 Materials and methods confined by two sapphire windows that allow the visual observation of the mixture behaviour, where the liquid solute and solvent are filled in. This apparatus, sketched in figure 1, is used for phase equilibria measurements according to the synthetic method. The volume of the horizontal cylinder (inner diameter of 36 mm) can be set positioning a piston, activated by an hydraulic system, from a maximum volume of 61 cm 3 down to a minimum volume of 32 cm 3. The piston position is known throw a measurement of an electrical resistance of a Weatstone Bridge. The temperature of the cell volume can be set and controlled, though a temperature controller actuated by an electrical heating. In order to mix the fluid a stirrer is available. S1 CO 2 vessel feed V1, V2, V3, V4 valves S2 triolein vessel feed HT-1, HT-2 electrical heaters S3 purge vessel C-1 equilibrium cell PI1, PI2, PI3 pressure indicator MIX-1 stirrer TI1 temperature indicator P-1 CO 2 pump TIC1 temperature indicator controller Figure IV.5. High pressure view cell The temperature measure is obtained by two resistances, having a length of 60 mm, lodged in opportune cavities of the metallic body of the cell. The cell is supply of a system of agitation, of two incomes, for feed of the liquid phase and the gas phase, of an outlet for the drainage of the liquid phase, of two escapes for the instrumentations of measure and control (pressure and temperature). The thermocouple measure the temperature of bulk in the cell, the measure comes in the controller in order to act on resistances HT1 and 17

30 Chapter IV HT2 figure 3. The feeding of the CO 2 is obtained by one manual pump (P2) and a piston pump (P1). All lines are supplies of valves (V2, V3, V4) for the isolation of the cell. IV.5 MODEL AND SOFTWARE Calculation methods with equations of state took already place extensively in the past. They have been used -SRK with Van der Waals mixing rule for equilibrium evaluation, that have demonstrated little error for fatty compounds. To evaluation of parameter critical point of pure species are need, TG and TGD cannot have an experimental determination for degradation problems, therefore a method of contribution group are used, Fedors evaluation method of critical point are used. MATHCAD 2001i Professional, is used for implementation of critical point and parameter calculation. Concerning data of a triglyceride-dimer and gases equilibrium, these are obtained with a simulation method that use contribution group method to obtain binary interaction parameter. PRO II SIMSCI 7.1 has been used to evaluate the composition. This data are validated with an experimental apparatus for evaluate equilibrium (Static analytical method for phase equilibrium measurements). MATHEMATICA 5.0 is used for non linear fitting of Cubic, to obtain binary interaction parameter from point of Pxy equilibrium curve. PRO II SIMSCI 7.1 has been used also to evaluate data of fractionation. 18

31 V. Thermodinamic of non ideal mixture: SRK-EoS The behaviour of pure compressed gases near and over the critical region is relatively well known.the pvt behaviour can be calculated with different equations. On one hand empirical equations were published using lot of parameters for describing phase behaviour over a wide range of pressure and temperature, very accurate. For calculating pvt behaviour of pure substances equation of Bender Bender, (1970), Sievers, et al., (1984) is often used, this equation includes 20 parameters and therefore lot of experimental data are necessary to fit those. The density behaviour of carbon dioxide calculated by equation of Bender is shown in Fig. 3. FigureV.1. Density behaviour of CO 2 calculated with equation of Bender Another possibility is related to the use of cubic equations of state developed by van der Waals equation van der Waals, (1873) take into account attractive and repulsive forces into account. That have the following generalized formula.

32 Chapter V RT a( T ) p = (V.1) 2 2 V b V + ubv + ωb With parameters a, b, u, and w defined in following table. Table V.1. Parameters a, b, u and w for equation V.1 a b u ω van der Waals RK SRK PR R Tc Pc R Tc Pc R Tc Pc R T P c c 2.5 (1 + fω(1 T (1 + fω(1 T 0.5 r 0.5 r 2 )), 2 )), 2 fω = ω 0.176ω 2 fω = ω ω RTc P RTc P RTc P Redlich and Kwong Redlich,. Kwong, (1943) published their (RK-EoS) modifying the attractive term by an explicit universal temperature dependence. Various modifications have been purposed and the most accepted one was published by Soave Soave, (1972) SRK-EoS taking individual temperature dependence of substances into account. Another well known comes from Peng and Robinson Peng et al., (1976) PR-EoS, which should solve the weak point of calculating liquid densities. All cubic equations can be written by following form using different values for u and w as given in Table 2. It is obvious that for SRK-EoS and PR-EoS parameter a is a function of temperature and for these two equations beside critical data p c and T c the acentric factor ϖ has to be known. For describing mixtures by cubic the parameters a and b are calculated by mixing rules and by this way the mixture is reduced to a hypothetical pure substance and therefore phase behaviour can be described as for pure substances. The interaction forces between different molecules are taken into account by interaction parameters k ij which have to be fit to experimental data. The quality of calculation depends on quality of mixing rules and this is the reason why research interest is focused in this direction. Beside normal quadratic mixing rules there are different approaches to fit experimental data with higher accuracy Wong et al., (1992); Fischer et al., (1995). On the other side there are few developments for new or modified cubic Bertucco et al., RTc 8P c c c c

33 Thermodynamic of non ideal fluid (1995). Calculating phase equilibria between solids and compressed gases problems arise when critical data (pc, Tc) and acentric factor ω of solids are not available, because the results with the existing methods differ too much Ambrose, (1980), Constantinou et al., (1995) and therefore results of cubic are not sufficient. For description of solubility behaviour of substances in compressed gases different equations were developed including the density of the fluid Robin et al., (1953), Mitra et al., (1991). As representative equation of Chrastil, Chrastil, (1982) is given where a, b and k are the adjustable parameters which have to be fit to experimental data. c = ρ k exp (a / T + b) (V.2) A modification was published by Adachi et al. Adachi et al., (1983) by introducing a density dependency of parameter k. The modelling of non ideal vapor-liquid equilibria are correlated with van der Waals Equation of State and its modification. Equations of state are applicable to wide ranges of temperature and pressure conditions. They can be used to calculate all of the related thermodynamic properties such as K-value (y/x), enthalpy, entropy and density. The reference state for both liquid and vapour is the ideal gas and deviations from ideal behaviour are determined by calculation of the fugacity coefficients for both phases. For cubic equations in particular, critical and pseudocritical conditions can also be predicted quite accurately. We use the Soave-Redlich-Kwong equation of state (SRK), that is a modification of the Redlich-Kwong equation of state (which is based on the van der Waals equation) and was published by Georgi Soave (1972). Soave replaced the term, (a/t 0.5 ), in the Redlich-Kwong equation with a more general temperature dependent term, a(t). His modified expression is: p = p rep -p attr (V.3) The simulated data on a pseudo binary basis were correlated by SRK- EoS p a fω = 0.48 b RT = V b = = a ( T ) V ( V + b ) 2 R T P c R T P c 2 c c (1 ω fω (1 T ω 2 1 / 2 r )) 2 21

34 Chapter V Parameters a and b for the mixture, (a: parameter of attractive energy, b: co-volume), and by PR-EoS p RT a ( T ) = (V.4) V b V ( V + b ) R T a = ω 2 2 c, i 2 1/ (1 ( ω i i ) (1 Tr, i Pc, i c, i )) 2 (V.5) R Tc, i b = (V.6) P Where a and b are referred to pure species i. To evaluate a and b for the mixture we use the Van der Waals mixing rule b = ω = acentric factor for component i k i Ci ij, P a(t) = i Ci 0.5 i i = critical temperatu re and pressure of component i = ( RT = i b = RT T Ci a = a α a α Ci x b i i i i = 1+ m (1 T j i i Ci j / P Ci i ) Ci x x ( a a ) Ci i 1/ 2 j / P ) Ci (1 k 2 m = ω 0.176ω binary interactio n constant for components i and ij i ) Introduction of the alpha term was an attempt to improve the vapour pressure prediction for the pure components. The combination formula for calculation of a(t) with the introduction of the term k ij was intended to improve prediction of the mixture properties. Using the Soave formulation for prediction of mixture properties involves two steps. First, the i-component acentric factor, ω i, is "tuned" such that the vapor pressure is accurately predicted. Secondly, the term k ij is determined j 22

35 Thermodynamic of non ideal fluid from experimental data for the ij-binary system such that the phase equilibria is matched. This equation give accurate predictions for non-polar mixtures of hydrocarbons. It does not give accurate predictions for polar components. We use VLE calculations, that represent equilibrium between one vapour and one liquid phase, because we see only one liquid and one vapour phase in our experimentation in view cell. The modelling of non ideal vapour-liquid equilibria are correlated with van der Waals Equation of State. The vapor liquid equilibrium for binary, ternary and multicomponent systems of fats and oils related compounds with mixing and combining rules. We use the Van der Waals mixing rule: a b m m = = i i j j x x a i i j j x x b ij ij and the following combining rules, a = ( a a b ij ij ( b = ii ii 2 ) 1/ 2 jj + b jj (1 k ) (1 l ij ij ) ) where k ij and l ij are the binary interaction parameters, computed by fitting experimental vapor liquid equilibrium data to model equations. Supercritical fluids are involved in numerous aspects of natural or industrial situations related to energy production or transfer. According to classical thermodynamic theory, a fluid is in a supercritical state when it is at a pressure or temperature exceeding its critical value; the value of the pressure, p, temperature, T, or molar volume, v, divided by its corresponding critical value (subscript c) is called the reduced (subscript r) value. What truly characterizes the supercritical state is the impossibility of a two-phase region. Indeed, when P r > 1 or T r > 1, in the (p,t) plane there is no longer the possibility of a two-phase (i.e. gas/liquid) region and instead there is only a single-phase region. The general term for the substance is fluid (neither a gas nor a liquid) and it is in a supercritical state. Classical thermodynamics provides guiding rules (that are a consequence of the definition based on P r and T r to differentiate between the subcritical and supercritical states. For example, it is well known that both the surface tension and the latent heat, being manifestations of the two-phase regime, become null at and past the critical point. In the supercritical regime, 23

36 Chapter V solubility effects become important and the heat of solvation becomes the relevant thermodynamic quantity for fluid interpenetration. A special case is the appellation of evaporation constant or evaporation rate used indiscriminately under subcritical and supercritical conditions. Under true supercritical conditions there cannot be evaporation since the latent heat is null, and a surface cannot exist. Therefore, the term emission rate and emission constant which are of more general meaning will be used here instead, with the understanding that in the subcritical regime the meaning is that of the classical terminology ( evaporation ) which will be interchangeably employed. Another thermodynamic fact is that the heat capacity at constant pressure becomes very large in the transcritical regime, this being an indication of the critical point being a singularity. It is also known that the critical point of a mixture is a function of the mixture fraction, and that the critical locus (the ensemble of the critical points as a function of the mixture fraction) may be a non-monotonic and convoluted curve, according to the species in the mixture. V.1 Thermodynamic literature data of non ideal fluids The phase behaviour of many materials from natural sources with CO 2 has already been measured, data of binary systems of SC-CO 2 with oleic acid are in literature Weber et al., (1999), but data of binary systems SC-CO 2 with triglycerides and triglycerides-dimer are scarce. For developing extraction with supercritical gases the knowledge of solubility and phase behaviour is essential. The solubility in CO 2 SC is sensitive in particular to free fatty acids, mono- and diglycerides, which may be present in oils and which are more soluble in CO 2 than triglycerides. Solubility of free oleic acid, rich in fried oil, was observed to be up to several times larger than the solubility of triglycerides contained in the oil. Potential application of supercritical CO 2 extraction to fractionation of fried olive oils is based on this solubility difference. Solubility of mono- and diglycerides is between that of free fatty acids and that of triglycerides. The most soluble compounds in the mixture of mono-, di-, and triglycerides of fatty acids with eighteen carbons in the molecule are monoglycerides. V.2 Lumping in pseudocomponents The different characteristics of oil components affect the introduction of a system for collecting and classifying used cooking oils. With regard to the classification of waste, it is difficult to discuss how this should be done. The main issue, which is of decisive importance, is that the collected oils may vary greatly in terms of composition and level of degradation, depending on the type of cooking or frying to which they have been subjected and the amount of reuse or abuse. In theory, it would be interesting to be able to separate or classify collected oils according to the level of degradation factor, as this would facilitate the optimisation of subsequent recycling 24

37 Thermodynamic of non ideal fluid operations. In practice, it is difficult to classify waste oils separately according to their level of degradation, as the collection process will certainly mix different qualities. Despite this, it could be attempted by establishing the main components, which would permit classification according to degradation. Information of complex system, witch shall be treated by gas extraction, must be prepared so that the information needed can be provided at the minimum necessary effort. One way is to transform the complex system into a system with as few as possible components. For the purpose of gas extraction a complex system often can be handled as a pseudo-binary system, for fractionation as a pseudo-ternary or quaternary system. Often a pseudo-binary treatment of complex system is not sufficient. An entrainer, a modifying compound, or compounds of medium volatility, witch have similar effect on phase behaviour as an entrainer, should be treated as individual components. Equilibrium distribution of identified compounds of interest may be determined by analyzing the concentration of these individual compounds in the different phases. If phase equilibrium calculation shall be carried out, pseudocomponents for the complex mixture must be chosen, so that parameters necessary for the calculation procedures can be obtained. A difficulty with handling data on complex mixture is that data dependent on the individual treatment of the mixture, with respect to the concept of pseudocomponents. Pseudo components are often chosen after chromatographic analysis of the complex mixture. Depending of separation efficiency of this analysis and on component identification, different pseudocomponents can be obtained. Therefore, the results of experiments on complex mixtures are somewhat difficult to compare. Conclusion on agreement or disagreement of result can only be drawn after extensive comparison of the original complex mixture, the analytical procedures, and the mixture of the arbitrarily chosen pseudocomponents. Numerous components were lumped into tree pseudocomponents on the basis of the chain length representing more than 95% wt of all components. Looking at tree pseudocomponents on a chain length basis is a good compromise in the case of fried oil at this moment. Than we made two hypothesis: in first case the fried oil is lumped in three pseudocomponents: LMWC - Low Molecular Weight Compound (principal component: Oleic Acid C 18 H 34 O 2, PM: 282 g/mol), TG - Triolein C 57 H 104 O 6, PM: 885 g/mol, POL1 - Triolein-Dimer (principal component: Triolein dimer C 104 H 206 O 12, PM: 1768 g/mol); 25

38 Chapter V in second hypothesis the fried oil is lumped in: LMWC - Low Molecular Weight Compound (principal component: Oleic Acid C 18 H 34 O 2, PM: 282 g/mol); TG - Triolein C 57 H 104 O 6, PM: 885 g/mol; POL2 - Lighter Triglicerid-Dimer (principal component: dimer C 104 H 206 O 12, PM: 112 g/mol). 26

39 VI. Equilibrium modelling and simulation results A model with tree pseudocomponents will allow us to focus attention on the two product-streams separed. At the same time it is possible, with definition of vapour-liquid equilibrium, to monitor the interaction between the major component groups with CO 2. However, following processes might look at a separation as a function of saturation. The combination between the equation of state and the van der Waals mixing rule proved to be very reliable in this case, because the pseudocomponent interact very slowly between itself. We obtain from a non linear fit the k ij constant at T and P fixed for tree binary mixture Oleic acid-co, Triolein-CO 2 and Triolein dimer-co 2, the other binary interaction are not relevant. In fact from an equilibrium curve Pxy (simulated or experimental) we obtain for a fixed pressure the x-y value, with T c, P c and ω we can calculate the constant a and b of EoS, than k ij, the only incognita of EoS, can be calculated with a non linear regression. With the knowledge of binary phase equilibria, it will be possible to predict the performance of the separation of fat mixtures by gas extraction, and in our case, it will be possible to predict the performance of the fractionation of fried oil mixtures by SC-CO 2 extraction. The problem is that the database of experimentally determined phase-equilibria of high boiling molecules with gases is very small compared to the need for these data. The phase equilibrium of sub-critical mixture: fried oil CO 2, has been carried out from data of pure substances and binary interaction parameter. A short bibliography is available for fatty compounds and SC-CO 2 binary interaction parameter. In fact for pseudocomponents of fried oil are available data for binary mixtures: Oleic acid-co 2 and Triolein-CO 2 (in a short range of pressures and temperatures), but the phase behaviour of Triolein dimer- CO 2 are not detected. We simulate all tree binary mixture with the PR and SRK equation of state,: Oleic acid-co 2 simulation give result in agreement with experimental data; Triolein-CO 2 simulation give result in agreement with experimental

40 Chapter VI data in range detected and we have extended study to higher pressure that result in agreement with our experimental data (cap. 7); Triolein dimer-co 2 simulation don t have experimental result to confront. Table VI.1. fried oil properties VI.1 Simulation of equilibrium POL-CO 2 VI.1.1 Critical point and acentric factor prediction of POL The critical constants are not available for fat compounds, because unstable to high temperatures, therefore an experimental determination is not possible. For organic compounds the critical temperature Tc, pressure Pc and acentric factor of components is estimated in satisfactory way from correlation of Ambrose (1980), Reid (1987) and Lee and Kesler, (1975) that is function of constants T sum of the group contributions constants. All group contribution factors needed for the method of Ambrose were taken from the literature Ambrose, (1979). The evaluation of Tc, Pc and acentric factor for triolein dimer is explained in sequent pages. 28