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Modeling the supercritical desorption of orange essential oil from a silica-gel bed

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Brazilian Journal of Chemical Engineering
Print ISSN 0104-6632
MODELING THE SUPERCRITICAL DESORPTION OF ORANGE ESSENTIAL OIL FROM A SILICA-GEL BED
E.A.Silva
1
, L.Cardozo-Filho
2*
, F.Wolff
2
and M.A.A.Meireles
3
1
Departamento de Engenharia Química,Universidade do Oeste do Paraná, UNIOESTE, R. Faculdade, 2550; CEP 87020-900, Toledo, PR, Brazil; E-mail: edsonas@deq.uem.br
2
Departamento de Engenharia Química,Universidade Federal de Maringá, UEM, Av. Colombo, 5790, 87020-900, phone: 55 44 226-2727, Maringá - PR, Brazil; E-mail: cardozo@deq.uem.br;
3
LASEFI, Departamento de Engenharia de Alimentos, FEA, UNICAMP, Campinas - SP, Brazil; E-mail: meireles@fea.unicamp.br
(Received: March 29, 2000 ; Accepted: April 23, 2000)
Abstract
- One of the most important byproducts of the orange juice industry is the oil phase. This is a mixture of terpenes, alcohols, and aldehydes, dissolved in approximately 96% limonene. To satisfactorily use oil phase as an ingredient in the food and cosmetics industries separation of the limonene is required. One possibility is to use a fixed bed of silica gel to remove the light or aroma compounds from the limonene. The aroma substances are then extracted from the bed of silica gel using supercritical carbon dioxide. This work deals with the modeling of the desorption step of the process using mass balance equations coupled with the Langmuir equilibrium isotherm. Data taken from the literature for the overall extraction curves were used together with empirical correlations to calculate the concentration profile of solute in the supercritical phase at the bed outlet. The system of equations was solved by the finite volume technique. The overall extraction curves calculated were in good agreement with the experimental ones.
Keywords
: orange oil, oil phase, desorption, supercritical fluids, mass transfer modeling, solid matrices, carbon dioxide. Braz. J. Chem. Eng. vol.17 n.3 São Paulo Sept. 2000
Curriculum LattesHow to cite this article
Página 1de 20Brazilian Journal of Chemical Engineering -<B>Modeling the supercritical desorption of or...14/1/2003http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000300004&lng=en&...
INTRODUCTION
Brazil is the leading producer of concentrated orange juice on the international market. São Paulo state is the largest producer in Brazil. Concentrated frozen juice has been traditionally produced by concentration of cold-pressed orange juice under vacuum in multistage evaporators. The vapor that leaves the first evaporator contains the volatile compounds responsible for the characteristic orange flavor, a mixture of water, limonene and volatile substances with low molecular weight from a variety of chemical families, such as aldehydes, alcohols, terpenes, and the like. This mixture is sent directly to the system of essence recovery containing a fractionation column and a series of condensers and gas washers, from which two products are obtained: the oil phase and the aqueous phase. Both phases have chemicals that are partially responsible for the characteristic orange flavor. Therefore, both products can have important applications in the food, cosmetics, and pharmaceutics industries to aromatize a variety of goods. The oil phase has the most obvious applications because of its very large content of limonene. Indeed, in the citrus industry it is commercialized as limonene because its limonene content may amount to as much as 96% (Marques, 1997). Due to its high limonene content, the oil phase is subjected to oxidation that forms, for instance,
α
-terpineol, thus modifying the oil-phase flavor. Ultimately, the oxidation process considerably lowers the oil phase price. On the other hand, the oxygenated compounds that form the mixture are the main substances responsible for the characteristic orange flavor. Therefore, deterpenation of the oil-phase is of great interest to the citrus industry. Some studies have been done on a process of adsorption of the oxygenated compounds in a bed of silica gel, followed by desorption with organic solvents. The organic solvent present in the concentrated mixture of oxygenated compounds must then be removed. An alternative process was studied by Marques (1997). In the proposed process the organic solvent desorption step was substituted by a supercritical fluid (SCF) desorption step. The SCF desorption developed by Marques (1997) consisted of a low-density SCF (Step I), followed by a high-density SCF (Step II). Marques (1997) confirmed that the bed of silica gel retained no limonene. On the other hand, ethanol, linalool,
α
-terpineol, trans-2-hexenal, nonanal, decanal, citronellal, neral,
β
-sinensal,
α
-sinensal, and ethyl butyrate were completely retained by the silica gel. During Step I of the SCF desorption the following substances were partially removed: linalool, octanal, nonanal, decanal, neral, geranial, ethyl butyrate,
α
-pinene,
δ
-3-carene,
β
-mircene, and valencene. The other chemicals were partially removed only during SCF desorption Step II. The supercritical fluid extraction (SCFE) of chemical species from a solid substratum includes contacting the solid phase with the supercritical fluid. The solid phase can be a vegetable plant material or a bed of porous adsorbent material, as is the case in oil-phase deterpenation. In either situation, the solid is usually called a solid substratum or solid matrix. The substratum forms a fixed bed through which the supercritical fluid flows, solubilizing the solute. This type of process is semi-continuous because, although the fluid phase flows continuously, the solid phase is also continuously being depleted in the solute. It is generally accepted that the objectives of a mathematical modeling of any process are to identify and describe the major phenomena using an equation or set of equations capable of providing reasonable description of the overall process. Preferably, the chosen equation or set of equations should be kept as simple as possible with a limited number of parameters to allow the problem to be solved with the information available. All this, which is common knowledge, also applies to the modeling of processes carried out at high pressure, since in addition to the difficulties traditionally encountered in process modeling, here the experimental information available to the process engineer is still limited. SCFE from a solid substratum can be modeled using two approaches:
i
) modeling the process that Página 2de 20Brazilian Journal of Chemical Engineering -<B>Modeling the supercritical desorption of or...14/1/2003http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000300004&lng=en&...
occurs with the individual particles, followed by integration on the bed volume to obtain an overall solution, or
ii
) modeling the process using mean properties for the fixed bed. The second approach is very useful for cylindrical beds with concentration profiles predominantly in the axial direction and negligible variations in the radial direction. The balance equations are written for an element of the bed and the solution obtained by integrating along the length of the bed. Nevertheless, to get the concentration profile, in either case it is also necessary to carry out integration in variable time, because the overall process occurs in transient regime. A good picture of the physical problem is required in order to write the balance equations and to choose the appropriate simplifications. The mass-transfer as well as the thermodynamic limitations of the process must be known to provide a solution that can be used for process design. The mass transfer balance equations for the solid and fluid phases, and the rate of interfacial mass transfer must be combined with the phase equilibrium relationships in order to get the final solution to the problem. Therefore, the following information is required:
i
) a model to describe the species concentration profiles in the fluid phase located in the pores of the solid particles,
ii
) the mass transfer equation that describes the diffusion of the species in the solid phase,
iii
) a phenomenological equation to describe the interfacial mass transfer, which is in general written in terms of the resistance in the film surrounding the solid particles. The objective of this work was to model the desorption step of the deterpenation of the orange oil phase by the process proposed by Marques (1997). The method of finite volume was employed to solve the system of equations that describes the process. Overall extraction curves data from the literature (Marques, 1997) were used to get the required information to solve the balance equations.
MATHEMATICAL MODELING
The mathematical formulation of SCF desorption is similar to any desorption problem, as extensively discussed in the literature. To study the deterpenation of the orange oil-phase mixture using low-pressure desorption followed by SCF desorption, the model for diffusion in homogeneous solid applied to fixed bed extractors was chosen. To do so, it is necessary to know the mass transfer and the dispersion coefficients for the fluid phase, the effective diffusion coefficient for the solid phase, and the adsorption isotherm. Raghavan and Ruthven (1983) solved the problem of sorption from a homogenous solid for a bed formed of spherical particles, using the method of orthogonal collocation. Madras et al. (1994) solved a similar problem using the method of orthogonal collocation with finite elements. The hypotheses used for their model were the system was isothermal, the flow was unidirectional in the axial direction, dispersion was important only in the axial direction, the physical properties of the phases were constant, local equilibrium prevailed in the pores of the particles, and the isotherm was nonlinear. These assumptions adequately describe the SCF extraction from solid substratum (Brunner, 1994). Figure 1 shows the SCF extractor, formed of a bed of spherical porous particles, and indicates the mechanisms of mass transfer. The bed of porous particles contains oxygenated compounds inside the particles and in its void space. The solvent flows axially and extracts the oxygenated compounds from both places. The amount of oxygenated compounds extracted depends on the temperature and pressure of the system, as well as on the dynamics of the system.
Página 3de 20Brazilian Journal of Chemical Engineering -<B>Modeling the supercritical desorption of or...14/1/2003http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000300004&lng=en&...
The mass balance equations for the fluid phase for the bed element indicated in is with the following initial and boundary conditions: For the solid phase, assuming that local equilibrium is achieved, the differential mass balance is given by (1) in(2) in z = 1 (3) in z = 0 (4) (5) Página 4de 20Brazilian Journal of Chemical Engineering -<B>Modeling the supercritical desorption of or...14/1/2003http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000300004&lng=en&...
with the following initial and boundary conditions: where Y is the solute concentration in the bulk fluid phase, X is the solute concentration in the fluid phase inside the pores of the particles, z is the axial direction,
ρ
is the particle’s radii,
τ
is the dimensionless time,
ε
is the bed porosity, L is the bed length, q is the solute concentration in the solid phase, and Pe and Bi are the Peclet and Biot numbers, respectively. The Langmuir isotherm will be used to describe local equilibrium between fluid and solid phases as follows: where q
max
and b are the Langmuir parameters. The system of equations formed by Eq. 1 to Eq. 5 together with the initial and boundary conditions given by Eq. 2 to Eq. 4 and Eq. 6 to Eq. 8 was solved using the finite volumes technique (Maliska, 1995). Applying the method of finite volume using the Weighted Upstream Differenciation Scheme (WUDS) (Maliska, 1995) for the approximating functions in the fluid phase, Eq. 1 and in the bed internal elements in the axial direction, we have where , for (6) (7) (8) (9) (10) (11) Página 5de 20Brazilian Journal of Chemical Engineering -<B>Modeling the supercritical desorption of or...14/1/2003http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000300004&lng=en&...

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