Investigation of High-Pressure Phase Equilibrium with the Observation of the Cloud Point

Transcription

1 Investgaton of Hgh-Pressure Phase Equlbrum wth the Observaton of the Cloud Pont Introducton In the last 2 decades supercrtcal solvents have had a more sgnfcant role as the solvents n chemcal reactons. In fact a sgnfcant number of hgh-pressure reactons n the gas phase (e.g. synthess of ammona, methanol or polyethylene) are performed under supercrtcal condtons. The large-scale manufacture of these products has been executed for several decades, thus the necessary chemcal and mechancal engneerng experence was obtaned to permt the safe desgn and operaton of hgh-pressure equpment. The effect of a supercrtcal flud (the solvent, the reactants, or the whole system s above crtcal pont) on chemcal reactons has been studed snce the begnnng of the 1970s. The number of publcatons dealng wth ths subect ncreased dramatcally n the 1980s, and addtonally to extracton and chromatography a new feld of applcaton of supercrtcal fluds evolved. If a chemcal/bochemcal reacton s carred out n a supercrtcal solvent (generally carbon doxde or water), t clear advantages. One possblty s that the reacton, whch was prevously carred out n halogenated or other toxc organc solvents, can be performed n supercrtcal solvents wth the same or better selectvty and reacton rate. Ths way VOC (volatle organc compound) emssons can be reduced sgnfcantly, whch s an mportant ssue both envronmentally, and economcally. Loss of VOC durng the processng can be sgnfcant, however only the n case of expensve solvents s the economy of solvent recovery the man drvng force of substtuton. Ofter only a new technology can meet the ncreasngly strck emsson lmts. Another reason of applcaton s f durng the downstream processng the whole manufacturng procedure can be more economcal due to the use of supercrtcal solvents. The benefts of supercrtcal solvents are that the propertes of the solvent (solublty, dffuson coeffcent, mass transfer coeffcent, etc ) can be modfed wth two ndependent and easlycontrollable parameters, the pressure and temperature. Thus, the reacton can be optmzed to reach the maxmal selectvty and/or reacton rate. In the case of carbon doxde (P C =73.8 bar, T C =31 C) the quantty and qualty of the co-solvent, whch s typcally added to the system n 1-10 %, s normally ncluded n the varable parameters. Another mportant advantage over the conventonal solvents s that the prevously soluble substance precptates after depressurzaton. In the case of carbon doxde the solvent s gaseous under standard condtons, thus f the product s not gaseous (typcally lqud or sold), then the solvent leaves the substance. In the case of water the separaton of the product and the solvent s also smple because the sub and supercrtcal water (P C =220 bar, T C =374 C) s nonpolar, whle at atmospherc condtons t s a polar solvent. Thus the components soluble n supercrtcal water have a poorer solublty n water at room temperature and atmospherc pressure, and the substances soluble n water under normal crcumstances are slghtly soluble n supercrtcal water. The maor drawback of usng supercrtcal solvents s the relatvely hgh pressure appled durng the process ( bar). In certan ndustres ths pressure range s generally accepted and frequently appled (e.g. the polymer ndustry). Currently n other sectors such as fne chemcal and pharmaceutcal ndustry t s rarely used. However, hgh-pressure devces can be found n every

2 analytcal laboratory, e.g. the operatng range of a standard HPLC (hgh performance lqud chromatograph) covers the bar pressure range appled n processes usng supercrtcal fluds. Fundamentals Lquefacton of gases under pressure occurs only below crtcal temperature, whch depends only on the qualty of the substance. The pressure, whch should be appled at the crtcal temperature to condense the gas, s called the crtcal pressure. If the substance s heated above crtcal temperature and compressed wth hgher pressure than the crtcal pressure, the materal remans homogeneous. Ths s called the supercrtcal flud state, and s a state n whch the propertes of the materal are between the propertes of the lqud and gaseous flud states. Fg. 1. Carbon doxde phase dagram Fgure 1 shows the P-T phase dagram of carbon doxde. On the dagram the sold, lqud and gas phases can be seen that are separated by the meltng, bolng and sublmaton curves. The crtcal pont s ndcated by C. The area representng the supercrtcal flud state s bounded by dashed lnes. Crossng the dashed lnes no change of phase occurs, the changng of physcal propertes s contnuous. Ths can be also observed on the densty curves whch change contnuously. The Aspects of Selecton of Supercrtcal Solvents The selecton of the solvent plays an mportant role n every aspect of chemcal operatons, lke extracton, reactons, crystallzaton or analytcal processes. The condtons (pressure and

3 temperature) are determned by the crtcal parameters of the solvent. Supercrtcal flud extracton can be feasbly appled to numerous substances that are sold or lqud at room temperature and pressure. The general selecton crtera for the tradtonal solvents are also vald n ths case (e.g. good solvent power, easly avalable, cheap, not toxc, not harmful to the product and envronment and not flammable). Carbon doxde s favoured as a solvent n supercrtcal operatons for the followng benefcal propertes: It s not harmful to health, thus t s well applcable for the producton of medcnes, food and consumer goods. It has a hgh densty, thus solublty of non-polar materals s suffcently hgh. Carbon doxde does not react wth the treated materal. It has low crtcal temperature (31 C), thermal degradaton can be mnmzed. It s non-flammable and non-corrosve. It s avalable n large quanttes and n a purty requred n the food ndustry. CO 2 s envronmentally frendly. After extracton t can be removed from the product wthout any resdue. The solvent power can be nfluenced by modfyng the pressure and temperature of the supercrtcal solvent. However generally only non-polar molecules can be dssolved n the non-polar carbon doxde. The solvent power can be mproved by applyng mxed solvent or co-solvent, ethanol for example. The crtcal pont of the mxture can be modfed sgnfcantly by small quanttes of co-solvent (Fgure 2). Table 1 shows the effect of small amount of ethanol n carbon doxde for the crtcal pont of the mxture. Table 1: The effect of ethanol on the crtcal pont n carbon doxde ethanol [mol%] content T C [ C] P C [bar]

4 Fg. 2: The crtcal ponts of the ethanol-carbon doxde mxture. Every coherent temperaturepressure ponts belong to dfferent compostons. The crtcal ponts of the pure components are marked wth ponts. Cloud pont When the soluton becomes oversaturated due to pressure reducton t becomes vsbly cloudy and foggy, known as the cloud pont. The cloud pont s charactersed by the composton, pressure and temperature. The pressure and temperature n the cloud pont at gven composton s obvously lower than the crtcal pressure and temperature. Ths phenomenon can be observed n Fgure 3, where the phase dagram of the CO 2 -ethanol system can be seen at 100 C (component 1 s CO 2 ). The purpose of the laboratory exercse s to observe the pressure values when the phase separaton occurs at dfferent temperatures and at two dfferent compostons.

5 Fg. 3: Isothermal phase dagram of CO2-ethanol system at 100 C. The sothermal change at constant concentraton s sgned by the arrow. Cloud pont occurs when enterng the two-phase area. The measurement Durng the experment the cloud pont values of mxtures of carbon doxde and a chosen solvent (e.g. ethanol, acetone or methanol) wll be observed at several dfferent temperatures and two dfferent concentratons. The measured cloud pont pressures aganst composton values have to be plotted n a common dagram wth lterature data and wth equlbrum data calculated by equaton of state at constant temperatures. A schematc dagram of the hgh pressure equpment used durng the measurement s shown n Fgure 4. The front wall of the cell s a pressure-resstant sapphre wndow. The back wall s a movable pston. Durng sothermal condtons the pressure n the nner space s controlled wth changng the volume of the cell. The densty of the homogeneous mxture has to be also determned.

6 Fg. 4: A schematc drawng of the hgh-pressure optcal cell used for measurement. After wrtng the test, the group wll be dvded nto two (group /I and group /II), so that everyone wll have the opportunty to use the devce. Before startng measurements, group /I determne the exact steps of weghng wth the help of the measurement team leader. The steps are the followng: Selecton of the solvent. Select the frst weghng concentraton usng the sothermal phase dagrams avalable (condtons: cloud pont pressure should be well-measurable n the C or C temperature range, the soluton has to be dluted. Choce of temperature range s your task.). Determne the weght of the solvent to be weghed (mnmal volume of the cell 44 ml, the pressure s chosen by you, but t must be n the homogeneous phase, the densty belongng to the chosen pressure has to be read off). Meanwhle group /II become acquanted wth the cell and observe the crtcal pont of carbon doxde. Whle group /I after fnshng the calculatons perform ther task, group /II calculates the maxmum dluton and the possble composton. The steps are the followng: The startng pont s the data calculated by group /I.

7 Determne the cell volume f the planned amount of substance s heated to 45 C and the pressure s 5 bar above the cloud pont pressure. The soluton can be dluted up to 60 ml at constant pressure wth pure carbon doxde enterng the cell, based on ths the dluton and the second planned composton can be determned. The tasks of group /I: Weght the calculated amount of solvent wth ppette, but the weght of the solvent has to be determned. Close the cell, thermostat the cell at the planned temperature and refllng the cell at the planned volume at the planned pressure. The amount of loaded CO 2 s measured. For ths purpose a cooled CO 2 syrnge pump s avalable. Refll the dosng pump wth carbon doxde and cool so that the carbon doxde s n lqud phase n the pston (the data can be read off on the phase dagram of the pure carbon doxde) The lqud has sgnfcantly lower compressblty than the supercrtcal flud therefore the measurement s more accurate f carbon doxde s n lqud phase n the pston. Set the desred pressure on the dosng pump. Ths pressure should be bar hgher than the planned pressure of the cell. Ensure a steady state has been reached nsde the pump; steady-state has been acheved f the pressure and volume values on the dsplay of the pump no longer change. Then wrte the volume, the temperature and the pressure of the pump and then start fllng the cell. At frst feed some carbon doxde the cell and move the pston forward (mnmum volume of the cell). Fll up the cell to the planned pressure whle contnuously montorng the phases (and recordng the observatons) then set the prescrbed volume (at constant pressure). Then you shall see homogeneous phase n the cell. Wrte down the volumes from the dosng pump. The mass of the weghed carbon doxde can be calculated from the dsplacement of the pston of the dosng pump (volume reducton) and the densty of carbon doxde at the set pressure and temperature n the pston. Measure the pressure of cloud pont of the soluton at 3 dfferent temperatures. Each group member should measure at least 3 ponts. Observe the phenomenon of cloud pont whle ncreasng the volume of the cell, wrte down the pressure, temperature and volume measured at phase separaton. After phase separaton start slowly decreasng the cell volume and wrte down the pressure, temperature and volume values when the two phases become homogeneous agan (the densty of the soluton wll be calculated from these data). Choose the best equaton of state and nteracton parameters for the temperature values planned by group /II from the sotherms llustrated by group /II wth the help of the PE2000 program. Prepare the complete phase dagram wth the use of GPEC program. Get acquanted wth the operaton of the program and observe whether the calculated model predcts the expected phase behavour.

8 Calculate the composton n mole fracton (the more volatle compound s carbon doxde) of the soluton, whch group/ii measures n the cell after dluton (check the orgnal composton calculaton!). Read the cloud pressure values calculated by the model for each measured data (composton and accurate measured temperature). The tasks of group /II: Gve the data of the gven mxture (n every temperature values the equlbrum compostons of lqud and vapour phasesn mole fracton of the more volatle compound, the carbon doxde) receved from lterature to the PE2000 program. Wth the nteracton parameters from the lterature let the model calculate the sotherms. Illustrate all data and curves from model results n a sngle dagram and save t as an mage. The dagram has to be attached to the report. Select the best fttng model and ts parameters n the temperature range appled n the measurements of group /I. Calculate the concentraton of the soluton measured by group /I based on the weghng data (mass fracton and mole fracton of the more volatle component). Create the complete phase dagram wth the chosen model and model parameters n the GPEC program and read the cloud pressure values calculated by the model at the measurement ponts (exact temperature values and composton) appled by group /I. Also read the calculated densty belongng to the homogenous phase (pressure, composton, temperature). When group /I carred out the measurements, perform the dluton of the soluton (accordng to plan, but modfcaton mght be needed accordng to the actual weghng of group /I). Measure the amount of loaded CO 2.For ths purpose a thermostated syrnge carbon doxde dosng pump s avalable. Fll the dosng pump wth carbon doxde s necessary and thermostat so that the carbon doxde s n lqud phase n the pston (data can be read from the phase dagram of pure carbon doxde). The lqud has sgnfcantly lower compressblty than the supercrtcal flud therefore the measurement s more accurate f carbon doxde s n lqud phase n the pston. Set the desred pressure on the dosng pump. Ths pressure has to be 10 to 50 bar hgher than the planned pressure n the vew cell. Adust the fllng system nto steady state tll the carbon doxde nlet valve of the cell. The steadystate s set, f the pressure and volume values on the dsplay of the pump no longer change. Make sure that pressure n the vew cell s lower than the pressure n the syrnge pump! Wrte the volume, the temperature and the pressure of the pump and then start fllng the cell. The soluton n the cell shall be always homogeneous. Set the requred volume whle loadng carbon doxde to the cell (constant pressure). Wrte the volume read on the dosng pump. The mass of the weghed carbon doxde can be calculated from the dsplacement of the pston of the dosng pump (volume reducton) and the densty of carbon doxde at the set pressure and temperature n the pston. Measure the pressure of cloud pont of the soluton at 3 dfferent temperatures. Each group member should measure at least 3 ponts. Observe the phenomenon of cloud pont whle

9 Calculaton ncreasng the volume of the cell, wrte down the pressure, temperature and volume measured durng phase separaton. After phase separaton start decreasng the cell volume and wrte down the pressure, temperature and volume values when the two phases become homogeneous agan (the densty of the soluton wll be calculated from these data). For the calculatons two software packages, the Phase Equlbra 2000 (PE2000) and the Global Phase Equlbrum Calculatons (GPEC) programs wll be used. The PE program s sutable for the combned llustraton of the theoretcal curves calculated wth the chosen equaton of state and the expermental data (data of the equlbrum compostons of lqud and gas phases at gven temperature and pressure). At frst, expermental data from lterature and the functon calculated wth the nteracton parameters on the gven temperature of state equaton from lterature. Evaluate the goodness of ft. Let the GPEC program calculate the phase equlbrum curve wth the nteracton parameters consdered good n a wde pressure and temperature range. The GPEC program s sutable to read off the cloud pressure calculated by the model at the known composton and temperature, whch wll be needed to prepare the record. Models used [Kemény S., Thury É., Deák A.: Állapotegyenletek fázsegyensúlyok számítására, BME, 1991]: At the calculaton of equatons of state the ntal condton s always that n equlbrum, the chemcal potentals of the components n each phase are equal. The chemcal potental s not a measurable parameter; t s calculated from the fugactes. The fugacty depends on the pressure, the temperature and the mole fracton. For the calculatons two (or one of them) equatons of state wll be used. Both of them are based on dense gas model. These are the Peng-Robnson and Soave- Redlch-Kwong models whch wll be appled wth usng the van der Waals mxng rule. They are n prncple sutable for calculaton of vapour phase mxtures of non-assocatng molecules and lqud phase mxtures of non-polar molecules n a relatvely wde pressure range. The nteracton parameters (k, l ) are mostly temperature dependent. The Peng-Robnson equaton of state: R T a P 2 V b V a a z z b b z z where P s the pressure (Pa), R s the unversal gas constant, T s the temperature (K), V s molar the volume (m 3 /mol), a and b are coeffcents, and are components, Z s the mole fracton of component. The Soave-Redlch-Kwong equaton of state (Redlch-Kwong equaton of state modfed by Soave, where a depends on T):

10 R T a P V b V V b a b a ( T ) z z P Tr 0,7 log P b z z C 2 2 C R T a( T ) 0,4274 P C ,481,57 0,176 1 T Where addton to the above ω s the asymmetry factor, T r s the reduced temperature (rato of the actual and the crtcal temperature), the crtcal pont s ndcated by the ndex C. The mxng rules can be the Van der Waals mxng rules: a b b a b 2 a 1 k 1 l The nteracton parameters k and l are determned by the functon ftted on the expermental phase equlbrum measurement data. Report It has to contan: r 2 the comparson of the measured and calculated data and the evaluaton of the dfferences dagram created wth the PE2000 program (the own measurement ponts has to be also marked) If necessary the detals of the measurement and report wll be dscussed wth the leader of the group durng the measurement. The report can be submtted on paper or electroncally (sz-edt@mal.bme.hu, pdf format). In case of electronc submsson the measurement sheet contanng the orgnal measurement data has to be submtted on paper. Please ndcate on the submtted paper t that the report was submtted electroncally (also gve your e-mal address the report was sent). Measurng table: Measurng the crtcal pont of carbon doxde (approxmately 6 rows) P (bar) T ( C) Observaton 1. Measurement Mass of weghed solvent (m): Mass of weghed carbon doxde (m CO2 ):

11 Pump T: P: Intal volume: Volume at the end of fllng: Densty of carbon doxde: Composton of weghed soluton: Mass fracton: Mole fracton: P (bar) T ( C) V (ml) Observaton Calculated pressure (bar) Measured densty (n case of homogenous phase!) Calculated densty Number of rows: 6 per person 2. Measurement Before dluton P: T: Mass of carbon doxde added to the mxture (m CO2 ): Pump T: P: Intal volume: Volume at the end of fllng: Densty of carbon doxde: Composton of the dluted soluton Mass fracton: Mole fracton: P (bar) T ( C) V (ml) Observaton Calculated pressure (bar) Measured densty (n case of homogenous phase!) Calculated densty Number of rows: 6 per person The densty of carbon doxde should be retreved from the database of NIST Chemstry Webbook ( (flud propertes; select the unts of measurement, temperature and pressure range, then press for data). Wrtten by Edt Székely, 2012/2013 I semester