Supercritical Extraction of Organic Components from Aqueous Slurries

Chapter 23

Supercritical Extraction of Organic Components from Aqueous Slurries 1

Aydin Akgerman and Sang-Do Yeo

Downloaded by UNIV OF CINCINNATI on May 30, 2016 | http://pubs.acs.org Publication Date: December 17, 1992 | doi: 10.1021/bk-1992-0514.ch023

Chemical Engineering Department, Texas A & M University, College Station, TX 77843

Continuous operation of an extractor for solids extraction by supercritical fluids is hindered by the difficulty of feeding the solids to the extractor at high pressures. Supercritical extraction of organic compundsfromsolid matrices is performed by slurrying the solid with water. This introduces a three phase, supercriticalfluid- water - solid, equilibrium. Activated carbon, alumina and soil are used as the solid phases with carbon dioxide as the supercritical phase. Equilibrium distribution of naphthalene and phenol between the three phases is determined employing a single stage extractor as a function of pressure at different temperatures. A thermodynamic model, which uses two two-phase equilibrium partitioning to predict the three-phase equilibrium, is also developed.

The interest in applications of supercritical extraction in environmental remediation is increasing rapidly (1). Contaminated soil, river and lake sediments, and industry sludges are all solid matrices of major environmental concern and supercritical extraction may be a feasible alternative for remediation of these matrices. In continuous operation of an extractor to handle solid materials, the solids have to be introduced to the vessel continuously and extracted with the solvent. Since supercritical extraction is a moderately high pressure process, feeding of solids continuously from ambient pressure to the extractor is not a trivial task. Therefore, most supercritical extractors for solids are operated in the semi-batch mode where the solids are loaded to the extractor, the extraction vessel pressurized, the supercritical fluid circulated over the solid bed continuously until the extraction is completed, the vessel de- pressurized, and the solids removed from the extractor. This mode of operation is both expensive and not suitable for high capacity processes. One approach 1

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Kiran and Brennecke; Supercritical Fluid Engineering Science ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

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to solve the materials handling problem is to slurry the solid with a liquid phase, such as water, and use slurry pumps for delivery. For environmental applications water may have an additional advantage of replacing selectively the organic contaminant on the solid phase, and enhancing the extraction extent and rates. Extraction of an aqueous slurry introduces a three phase (solid-water-supercriticalfluid)equilibrium where the fourni component is distributed between these three phases. To the best of our knowledge, no data has been reported on equilibrium behavior of such a system. The objective of this study was to determine the equilibrium distribution of an organic compound in the three phase system and develop the thermodynamics for prediction of this distribution. Four different systems were studied; phenol distribution in activated carbon/water/supercritical C0 and soil/water/supercritical C0 systems, and naphthalene distribution in alumina/water/supercritical C0 and souVwater/supercritical C0 systems. The solid phase was loaded with the organic component (phenol or naphthalene), slurried with water, brought to equilibrium with supercritical C0 in a single equilibrium stage extractor, and the distribution of the organic in the three phases determined. 2

2

2

2

2

For the three phase system consisting of the supercritical phase (F), the water phase (L), and the solid phase (S), the criteria for equilibrium dictates that flF=flL~flS

0)

where /y'e are the fugacity of component i in phase j. The objective of the thermodynamic modeling was to predict the loading of the organic component on the solid phase in this three phase system from equilibrium information on two two-phase systems, Le. supercriticalfluid-waterequilibrium partitioning and liquid-solid equilibrium partitioning (adsorption equilibrium) of the organic component. In Equation 1, the first two terms can be obtained from a supercritical fluid/water equilibrium for which

fiL^ttLXlP

(3)

where the molefractionof component i in the water and supercritical phases are x, and y , respectively. The molefractionsand the fugacity coefficients in the two phase system can be calculated by using the model proposed by Yeo and Akgerman (2). This model uses aflashcalculation procedure combined with the Peng-Robinson equation of state and composition dependent mixing rules. It enables calculation of the fugacity coefficient and the distribution coefficient of each component in a multicomponent system. t

Therighttwo terms in Equation 1 can be obtainedfromequilibrium data on a liquid-solid system, i.e the adsorption equilibrium. It is not easy to obtain adsorption equilibrium data when the liquid phase is water and the adsorbate (the organic solute)


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is a hydrophobic organic compound only scarcely soluble in water (such as naphthalene, solubility ~20 ppm), because it is very difficult, if not impossible, to control the concentration of the hydrophobic adsorbate in water at different concentration levels to obtain reliable data for the adsorption isotherm. Therefore, it is assumed that the fugacity of the solute in the solid phase is independent of the solvent it is adsorbed from (3,4). Thus fis(Xi.T)\ .


aq

« hsiyJ^scF

®

this assumption also implies that the fugacity of the adsorbed species is independent of the system pressure since liquids are incompressible compared to supercritical fluids. Employing this assumption, in order to determine the fugacity of naphthalene on the solid phase, naphthalene was dissolved in cyclohexane at different concentration levels and the amount adsorbed on the solid phase was determined at these levels to construct the adsorption isotherm. The fugacity of the adsorbate on the adsorbent can then be calculated from hs-XiYiiXiVFf'V)

(5)

In this equation, the activity coefficient γ, can be calculated by the UNIFAC group contribution method (5) as a function of mole fraction and temperature. The vapor pressure of the component can be calculated by available correlations (6). The liquid phase molefractionx, of the adsorbate in the liquid-solid system is related to the solid phase concentration, or the solids loading, through the adsorption isotherm. For most dilute systems, the Freundlich isotherm gives a reasonable fit to the data, although other model equations for adsorption isotherms can also be used. (6)

N

S = FC

where S is the loading on the solid phase (mol/g), C is the liquid phase concentration (mol/1), and F and Ν are empirical constants. After the fugacity of the component in the supercritical fluid-liquid system is calculated as a function of temperature and pressure through Equations 2 and 3, and the relationship between the fugacity of the component in the liquid phase and the solid phase loading is established using Equations 5 and 6, the solid phase loading can be predicted as a function of temperature and pressure of the extraction system. EXPERIMENTAL METHODS. In this study radiolabeled compounds were used because of accuracy in analysis. Radioactively labeled C phenol and C naphthalene were purchase from Sigma Chemical Co., activated carbon (Darco G-60, -100 mesh powder, lot no. 00429TV) and activated alumina (neutral, Brockmann I, -150 mesh, lot no. 01820DX) were purchasedfromAldrich Chemical Co., and EPA Standard Soil was obtained from EnviResponse Co. Phenol was loaded on activated carbon and soil by preparing 150 ml of 5.18 wt% radiolabeled phenol solution and contacting it with 10 g of activated carbon or soil 14

14


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to form a slurry. The slurry was agitated in a constant temperature bath for 7 days until it reached equilibrium. It was observed that about 95% of the equilibrium adsorption was established in one day, yet about a week was necessary to reach equilibrium. The equilibrium concentration of the water phase was measured in a Beckman Liquid Scintillation Counter (model 3801) and the solid loading determined from material balance closure. The slurry was then placed in the extractor for extraction with supercritical C 0 . Thus in these cases we started with a slurry already at equilibrium in terms of solute partitioning between the two phases, solid and water.


2

In order to load naphthalene on alumina or soil, it was precipitated from methanol. 0.03 g of radiolabeled naphthalene was dissolved in 40 ml of methanol and 30 g of solid phase (alumina or soil) was added to the solution. Methanol was evaporated from the mixture by agitating in a fume hood. After all the methanol evaporated, a solid sample was taken and directly added into scintillation cocktail, sonicated, and counted for naphthalene using a direct solids analysis technique we developed (7). It was observed that naphthalene loss due to sublimation during methanol evaporation was less than 2%. This 30 g solid phase was then slurried with 150 ml water and immediately loaded into the extractor. In both cases, the amount of total adsorbed phenol or naphthalene, that was placed in the extractor, was less than the minimum amount of these species needed to saturate the supercritical phase at the experimental conditions (8, 9). The experimental assembly consisted of a single stage extractor where the slurry was pressurized with C 0 to the desired pressure, agitated for 24 hours followed by a phase separation period of rninimum 2 hours, the aqueous and the supercritical phases were sampled under pressure, the samples were expanded into scintillation cocktail, and the amount of organic in these phases were determined. Details of the experimental set-up and procedure are given elsewhere (10, 11). Thus only two phases, the supercritical phase and the water phase were analyzed, and the amount of organic on the solid phase was determined by difference from material balance closure. We have shown that under these conditions an organic rich fourth phase does not form (10, 11). 2

R E S U L T S A N D DISCUSSION. Phenol partitioning between supercritical COa/water/activated carbon at 295 Κ and 320 Κ as a function of pressure are presented in Figures 1 and 2. Although the experimental temperature of 295 Κ is in reality subcritical for C 0 , nevertheless it is called the supercritical phase from a practical point of view, i.e. conditions for supercritical extraction are more or less optimized at 0.9

Supercritical Extraction of Organic Components from Aqueous Slurries

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