MASS TRANSFER AND KINETIC MODELLING OF
Brazilian Journalof ChemicalEngineeringISSN 0104-6632Printed in Brazilwww.scielo.br/bjceVol. 34, No. 03, pp. 799 – 810, July – September, MASS TRANSFER AND KINETIC MODELLING OFSUPERCRITICAL CO2 EXTRACTION OF FRESHTEA LEAVES (Camellia sinensis L.)Pravin Vasantrao Gadkari1, 2 and Manohar Balaraman12*1Department of Food Engineering,Central Food Technological Research Institute, Mysore-570 020, India2Academy of Scientific and Innovative Research, CSIR-CFTRI, Mysore, IndiaE-mail: manoharb@cftri.res.in; Phone: 91 821 2514874; Fax: 91 821 2517233.(Submitted: August 25, 2015; Revised: December 25, 2015; Accepted: April 29, 2016)Abstract – Supercritical carbon dioxide extraction was employed to extract solids from fresh tea leaves (Camelliasinensis L.) at various pressures(15 to 35 MPa) and temperatures (313 to 333K) with addition of ethanol as a polaritymodifier. The diffusion model and Langmuir model fit well to experimental data and the correlation coefficients weregreater than 0.94. Caffeine solubility was determined in supercritical CO2 and the Gordillo model was employed tocorrelate the experimental solubility values. The Gordillo model fit well to the experimental values with a correlationcoefficient 0.91 and 8.91% average absolute relative deviation. Total phenol content of spent materials varied from 57to 85.2 mg of gallic acid equivalent per g spent material, total flavonoid content varied from 50.4 to 58.2 mg of rutinequivalent per g spent material and the IC50 value (antioxidant content) varied from 27.20 to 38.11 µg of extract permL. There was significant reduction in polyphenol, flavonoid and antioxidant content in the extract when supercriticalCO2 extraction was carried out at a higher pressure of 35 MPa.Keywords: Supercritical CO2; Fresh tea leaves; Mass transfer; Caffeine; Polyphenols; AntioxidantsINTRODUCTIONGreen tea is one of the highly preferable raw materialsfor the food and pharmaceutical industry due to the presenceof many biological active molecules such as polyphenols,caffeine, theanine and specially catechins which impartthe flavor, taste and health benefits to human beings(Parket al., 2012). The catechins possess anti-cancer, antiinflammatory, anti-microbial and anti-obesity properties.Green tea contains caffeine (3 to 4% w/w), which is a kindof alkaloid and some clinical studies show that it possesshealth benefits to cure Alzeimer’s disease and cancertreatment (Eskelinen and Kivipelto, 2010; Gadkari andBalaraman, 2013; Kang et al., 2010).* To whom correspondence should be addressedThere are several techniques used to extract thebioactives from the tea matrix such as conventional solventextraction, pressure assisted solvent extraction, ultrasoundassisted extraction, microwave assisted extraction andsupercritical fluid extraction (Gadkari et al., 2014;Ghoreishi and Heidari, 2012, 2013; Pan et al., 2003; Parket al., 2012; Xia et al., 2006). Several studies reportedthat supercritical CO2 could be employed for selectiveextraction of bioactives from herbaceous materials (Langand Wai, 2001). Due to the higher initial capital investment,supercritical fluid extraction (SFE) is less preferred thanother techniques of extraction, but it has a unique qualityof selective separation of bioactive compounds with slightmanipulation of pressure and temperature (Gadkari et
800Pravin Vasantrao Gadkari and Manohar Balaramanal., 2015). Green tea has been extracted using differentsolvent systems and different methods for improving theextract quality (Gadkari and Balaraman, 2015; Gadkariet al., 2014). Green tea contains polar compounds butmuch less non-polar components and hence polar solventssuch as water, ethanol, methanol, etc. are more preferredfor extraction. The process of extraction of componentsfrom the matrix involves dissolution of a solid componentwithin the matrix into the fluid and then diffusion of thesolid from the matrix controlled by external mass transferprocesses(Roy et al., 1996).The main advantages of supercritical CO2 extractioncompared to other methods are: residue-free extracts, lowtemperature of processing, less number of unit operations,less thermal degradation, higher mass transfer ratesand many more. A supercritical fluid (SCF) has liquidlike densities, gas-like viscosities and diffusivity, andzero surface tension which cause superior mass transfercharacteristics and solvent effectiveness with density control(Ghoreishi and Heidari, 2012). In the 21st century, SCFextraction and encapsulation processes have been hailedby researchers as a solution for separation of importantbiomolecules and developing effective delivery systemsfor them (Gadkari and Balaraman, 2015). Supercriticalfluids have high potential in downstream processing, butreliable and versatile mathematical models are needed tounderstand mass transfer for their use in process designand economic feasibility studies (Brennecke and Eckert,1989). There are many applications of SCF reported inthe literature (Brunner, 2005; Herrero et al., 2006; King,2014; Knez et al., 2014).Hacer and Gurub(2010) studiedsupercritical CO2 extraction of caffeine from Turkish teastalk and fiber and found that with addition of ethanolduring the extraction process, the yield of caffeine couldbe increased from 62.5 to 63.1% (w/w) and the extractiontime reduced from 7 to 2 h. Caffeine was extracted fromguaraná seeds and mate tea leaves with solubility values ofcaffeine ranging from 6.01 10-4 to 1.11 10-5 mole fractionat a CO2 pressure 10 MPa and temperature from 313 to 343K (Saldana et al., 2002). To date, there are no reports on thekinetics of extraction of fresh tea leaves using supercriticalCO2 and analysis of spent material.The main objective of study was to understand the masstransfer processing during supercritical CO2 extraction offresh tea leaves with application of diffusion models anda solubility model to identify the dominant mass transfermechanism. Also, the spent material was extracted andanalyzed for its polyphenol, flavonoid content and in vitroantioxidant activity.Diffusion and Kinetic models for extractionThere are many models available for understanding themass transfer process during extraction of solute from asolid matrix and these models are based on mass transferintegration with the following assumptions;1. Particles are considered as spherical with radius (R).2. Solute free solvent is entering the system.3. Extraction of solute (extractable solids) happens in asingle step.4. At the interface, thermodynamic equilibrium isestablished.During the extraction solute diffuses to the surfaceand internal diffusion is modelled using either aneffective diffusion coefficient or solid-phase mass transfercoefficient (Campos et al., 2005). If several componentsexist in the sample matrix, the fitting has been made bytaking into account just a single one, called the solute. Theextraction system is considered as a fixed bed comprisedof two phases:(i) Solid (static): tea leaf matrix which holds the solute.(ii) Fluid (mobile): supercritical CO2 polar solvent.The solvent flow rate and physical properties areconstant during the extraction process. Pressure losses,temperature gradients and heat of dissolution are neglectedin the bed. Superficial velocity was calculated from thesupercritical fluid flow rate by neglecting the extractedsolute flow rate. To understand the mass transfer processduring extraction,the following models are employed.Fick’s Diffusion modelSupercritical CO2 extraction is a diffusion-basedprocess in which the solute is leached out from the leafmatrix into the solvent phase. The law states that the fluxis proportional to concentration gradient and diffusion of asolute occurs in the direction of decreasing concentration.The general form of Fick’s diffusion equation is given asfollows (Aguerre et al., 1985): 2 C CDe 2 . (1) r t(1)where, C is the concentration of solute at the time t andat the radial position r within the (planar) leaf matrix; theinitial and boundary conditions are as below: c 0; tat the surface : F0 0, r 1, c 0;at the center : F0 0, r 0,at the start; F0 0, 0 r 1, c 1.Brazilian Journal of Chemical Engineering
Mass transfer and kinetic modelling of supercritical CO2 extraction of fresh tea leaves (Camellia sinensis L.)The solution of Eq. 1 can be given as follows (Aguerreet al., 1985; Gadkari and Balaraman, 2015).C* 6 nπ n 1whereC* 2(C C ) ( C0 C )2 D t exp nπ e2 R (2)andC is the concentration ofthe Fourier number (F0), defined as De t r 2 , is greater than 0.1, all terms other than n 1 can be neglected and Eq. 2 issimplified as follows:62 D t exp π e2 2πR (3)From the slope of the plot of ln C* versus time, one candetermine the value of the effective diffusivity De.Langmuir modelTo evaluate the mass transfer process, two simplemodels, i.e., the exponential and Langmuir model havebeen repeatedly used by researchers (Manohar andKadimi, 2012; Murthy and Manohar, 2014). The Langmuirmodel is one of the well-known models used to explain theextraction kinetics. Though the adsorption model is usuallyemployed for studying the extraction process of oil seedmaterials, it can also be used for the extraction process of aleafy matrix. The material is soaked in fluid (supercriticalCO2 EtOH) in an extraction vessel and after some timesolute diffuses from the internal matrix and gets adsorbedon the surface, which further travels to the separator vesselin the solvent. The Langmuir extraction model is presentedin the following form,Y coefficient is governed by an Arrhenius equation as follows(Al-Jabari, 2003),(5) E K L K 0L exp RT where E is the activation energy (kJ/mol), K 0L is the preexponential coefficient, and R is the universal gas constant.solute at infinity, C is the average concentration of solutein the solvent phase, C0 is the concentration of solute attime (t 0), and D e is the effective diffusivity (m2/s). WhenC* 801Yf . t(KL t )(4)where Y is % extraction yield (w/w), Yf and KL areconstants (Yf is the yield at infinite time).The temperature dependence of the adsorptionThe Gordillo model for caffeine solubilityThe Gordillo model is an empirical model used forcorrelating the solubility of a solute in supercritical CO2.The model gives the relationship between pressure,temperature of extraction and their influence on thesolubility of the solute. Gordilloet al. (1999) proposed amodification of the original equation presented by Yu et al.(1994) in order to correlate experimental solubility data ofPenicillin G. The Gordillo model is represented as follows:(6)ln y 2 D0 D1 P D 2 P 2 D3 PT D 4 T D5 T 2where D0 to D5 are model coefficients, y 2 is the molefraction caffeine solubility, P is the pressure and T is thetemperature.In order to provide a reliable criterion to comparethe accuracy of the model, the average absolute relativedeviation (AARD) was calculated from:100 N yexp y predAARD, % yN i 1exp(7)where y exp is the experimental solubility, y pred isthe predicted solubility and N is the total number ofexperiments.MATERIALS AND METHODSFresh tea leaves (Camellia sinensis) were suppliedby M/s. Dollar tea estate, Ooty, India. After picking, theleaves were transferred to a freezer (-253 K) within 5 h.Food grade carbon dioxide (99 % pure) was procuredfrom M/s Kiran Corporation, Mysore, India. Aluminumchloride and folin-ciocalteu reagent were procured fromSRL Chemicals (Mumbai, India). DPPH* (2,2-diphenyl-1picrylhydrazyl), gallic acid, rutin, and trolox were procuredfrom Sigma-Aldrich Company Ltd., Germany. Absoluteethanol, HPLC grade methanol, acetonitrile and acetic acidBrazilian Journal of Chemical Engineering Vol. 34, No. 03, pp. 799 – 810, July – September, 2017
802Pravin Vasantrao Gadkari and Manohar Balaramanwere purchased from Merck Chemicals, Mumbai, India.Total polyphenol contentExtraction of Fresh leaves under Supercritical CO2Total polyphenol content (TPC) was determinedusing a well established spectroscopic method with slightmodification, the ISO14502-1 method as described inearlier studies (Gadkari et al., 2014). It is a colorimetricassay in which the polyphenols present in the extract reactwith Folin–Ciocalteu reagent to produce a blue colouredcomplex. The absorbance of the complex formed wasdetermined at 765 nm for further calculation purposes.Gallic acid was used as the polyphenol standard and thestandard calibration curve was obtained in the range 0 to40 µg of gallic acid. The results were expressed as mg ofgallic acid equivalent per g of spent material.Supercritical CO2 is a non-polar solvent and reportedby many researchers to be a powerful tool for extractionof non-volatiles in their natural form (Brunner, 2005;Campos et al., 2005; Manohar and Kadimi, 2012). Ethanolwas added (1.2 % w/w) as a polarity modifier duringsupercritical CO2 extraction to enhance the solubility ofpolar compounds into the solvent (Gadkari et al., 2015).The polarity modifier was pumped into the extractorvessel using a high pressure pump (Milton RoyTM duplexpump, USA). The leaves were extracted in a pilot scalesupercritical fluid extraction unit (NOVA Swiss WERKEAG, Switzerland) designed for working pressure up to 100MPa, temperature up to 373 K. The frozen tea leaves werecrushed in the presence of dry ice to an average particlesize less than 1.5 mm in an analytical mill (model A10,IKA, Germany) prior to supercritical extraction. 100 g ofcrushed tea leaves were loaded in the extractor vessel withinjection of a polarity modifier into the extractor vesselwhere CO2 is continuously circulated through a closedloop system. Each fraction was collected separately atvarious time intervals up to 9 h extraction and weighed onan analytical balance (AT-201, Metller, USA).Caffeine solubility measurementThere are different methods for measuring solubilityof the solute (caffeine) in supercritical CO2, i.e., static,dynamic and recirculation methods. Most researchers usea dynamic method for determination of solute solubilityin supercritical CO2 due to the simplicity of the method(Ismadji and Bhatia, 2003). It was assumed that thesaturation of solute in the CO2 was attained at lowersuperficial velocities (2.9 10 5 and 4.6 10 5 m/s). Thesolubility of caffeine was determined from slope valuesobtained by fitting a second-order polynomial equation tothe curve where the X-axis represents the kg of caffeineand the Y-axis represents the kg of CO2 used (Campos etal., 2005). The solublity values were further converted tomolefractions prior to the fit of the Gordillo model.Extraction of spent materialAfter supercritical CO2 extraction, the spent materialobtained after each experiment was extracted using wateras a solvent (material: solvent, 1:50) at 353 K for 40 minin a hot water bath (Labbe et al., 2006). The extracts werebrought to room temperature (300 K) under running waterand then filtered through a 0.22µm syringe filter. To preventthe oxidation of extracts due to light and temperature,theextracts were stored in amber coloured glass vials inarefrigerated condition (277 K) until analysis.Total Flavonoid contentThe total flavonoid content of the extract was quantifiedusing the method described in earlier studies (Gadkari etal., 2015). It is a colour-producing spectrophotometricassay where aluminum chloride forms acid stablecomplexes with the C-4 keto group and either the C-3or C-5 hydroxyl group of flavones and flavonols to formcoloured complexes. Rutin was used as a standard and thecalibration curve was plotted with different concentrationsfrom 0 to 1000 µg. Finally, the total flavonoid content wasexpressed as mg rutin equivalent per g of spent material.DPPH assay (IC50 value)The antioxidant activity was determined using theDPPH assay with a slight modification in methodologyand the results were presented in terms of the IC50 value(amount of extract required to achieve 50 % of inhibitionagainst DPPH radical) (Kutti Gounder and Lingamallu,2012). 1 mL of extract or trolox or ethanol as blank (0100 µg/mL) was mixed with 1 mL of 0.4 mM of DPPHsolution (prepared in ethanol). The mixture was vortexedfor a minute and allowed to stand in the dark for 30 min.Finally, the absorbance of the mixture was observed at517 nm using a UV-Visible spectrophotometer (UV-1800,Shimadzu, Japan). The DPPH scavenging activity wascalculated using Eq. 8, ADPPH inhibition, % 1 S AB 100 (8). ( 8 ) where AS is the absorbance of the sample andabsorbance of the blank.A B is theHPLC analysis of extractable solidsThe samples were dissolved in HPLC grade methanolBrazilian Journal of Chemical Engineering
Mass transfer and kinetic modelling of supercritical CO2 extraction of fresh tea leaves (Camellia sinensis L.)and then filtered through a 0.22 µm syringe filter. Theseparation of caffeine and individual compounds wascarried out on a Shimadzu LC-10A system (Tokyo, Japan.)equipped with a reverse phase C18 (15 μ-Diamonsil)column (250 mm 4.6 mm) and a PDA detector set torange from 200 to 600 nm. The peak integration anddata collection was carried out with Class 10 software(Shimadzu, Tokyo, Japan). The mobile phase was preparedand degassed under vacuum as per our earlier studies(Gadkari et al., 2014). The identification and quantificationof individual compounds were done using authenticatedanalytical standards.Statistical analysisdominates over the solvent density effect, which can leadto an increase in % extract yieldat higher temperatures(Park et al., 2012). Also, the same effect was observed atthe pressures of 25 MPa and 35 MPa where a similar trendwas observed from Fig. 1b and Fig. 1c.Diffusion and Langmuir models for extractionThe Fick’s 2nd law model has been employed forseveraldecades for understanding the mass transfer process duringextraction of herbaceous material. Eq. 1, which is the basicform, was further resolved into Eq. 3 for F0 0.1. The F0number calculated after obtaining the diffusion coefficientfor each experiment is presented in Table 2. The diffusioncoefficients obtained from the slope of the curveAll experiments were carried out in duplicate andvalues were expressed with their means. The regressionanalysis for each model was carried out using the Excelprogram (MS Office 2013).RESULTS AND DISCUSSIONInfluence of Extraction pressure and temperature onextraction yieldIn order to study the effect of extraction pressure andtemperature, the experiments were carried out at pressuresfrom 15 MPa to 35 MPa and temperatures from 313 K to333 K (Table 1). Figure 1a shows that, when extraction wascarried out at a pressure 15 MPa with varying temperature(313 K to 323 K), the % extract yield increases slightlyfrom 2.59 % to 2.66%.But when the temperature changedto 333 K, the % extract yield increased to 3.76 %. Thesudden increase in % extract yield may be attributed totemperature dominancy where the temperature effect803C* (C C ) ( C0 C )versus time are presented in Figure 2 and the modelregressed well with correlation coefficients 0.94.Diffusion coefficients varied from 3.50x10-11 to 6.71x10-11m2/s depending on the extraction pressure and temperature.At lower pressure and temperature, i.e., 15 MPa, 313K, ahigher diffusion rate was found due to the higher density ofsolvent at lower temperature, which increases the yield ofextractable solids. The matrix of the leaf is not very strong,so the extraction completed within 9 h is comparativelyless than that of conventional solvent extraction (Gadkariet al., 2015). These extracts when subjected to HPLCshowed a major peak of caffeine; more than 85% (w/w)of the caffeine was extracted with very little amount ofchlorophyll (not quantified). The effective diffusivity ofmango ginger (Curcuma amada Roxb.) extract variedfrom 0.669 10-12 to 18.50 10-12 m2/s with extractionTable 1. Experimental conditions for the supercritical CO2 extraction of fresh tea leavesPressure(MPa)Tempera
During the extraction solute diffuses to the surface and internal diffusion is modelled using either an effective diffusion coefficient or solid-phase mass transfer coefficient (Campos et al., 2005). If several components exist in the sample matrix, the fitting has been made by taking into account j
Write 3 examples of kinetic energy. (answer in your journal, label as kinetic energy) Factors that affect Kinetic Energy Mass Velocity (speed) The heavier an object is, the more kinetic energy it has. The more speed an object has, the more kinetic energy it has
Basic Heat and Mass Transfer complements Heat Transfer,whichispublished concurrently. Basic Heat and Mass Transfer was developed by omitting some of the more advanced heat transfer material fromHeat Transfer and adding a chapter on mass transfer. As a result, Basic Heat and Mass Transfer contains the following chapters and appendixes: 1.
112 2 22 && . (2.26)2 The kinetic energy can also be written as the kinetic energy of the center of mass plus the kinetic energy about the center of mass. We define r R position of the center of mass, and rr r rr r 12 vector between m1 and m2. The kinetic energy of the cent
Kinetic energy Kinetic energy is the energy of motion. Kinetic energy depends on the mass of the object as well as the speed of that object. Just think of a large object moving at a very high speed. You would say that the object has a lot of energy. Since the object is moving, it has kinetic energy. The form
expression for the kinetic energy of a system of particles that will be used in the following lectures. A typical particle, i, will have a mass m i, an absolute velocity v i, and a kinetic energy T i (1/2)m iv i ·v i (1/2)m iv i2. The total kinetic energy of the system, T , is simply the sum of the kinetic energies for each particle, n n 1 .
Calculating kinetic energy If we know the mass of an object and its velocity we can determine the amount of kinetic energy. kinetic energy 1/2 (mass of object)(velocity of object)2 or KE 1/2 mv2 or KE 0.
A. There is no kinetic friction because the blocks are moving at constant speed. B. The magnitude of the kinetic friction between the blocks and the kinetic friction between M 1 and the tabletop are the same because the coefficient of kinetic friction is the same. C. The kinetic friction force that acts on M points to the left. D. Block M
The Kinetic Theory of Gases . and temperature was later found to have a basis in an atomic or molecular model of gases called "the kinetic theory of gases" that was developed by Maxwell in the late 1800s. The kinetic theory of gases is a model in which molecules move freely with kinetic . (or "me
