Measurement and Modeling of Supercritical Fluid Extraction from

Chapter 18

Measurement and Modeling of Supercritical Fluid Extraction from Polymeric Matrices Κ. M. Dooley, D. Launey, J. M. Becnel, and T. L. Caines

Downloaded by CORNELL UNIV on July 23, 2016 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0608.ch018

Department of Chemical Engineering, Louisiana State University, Baton Rouge, LA 70803-7303

Extraction of low-molecular weight solutes by supercritical fluids (SCFE) is often rate-limited by the interaction of the solute with a polymer. In typical applications, the effect of the SCF on the matrix through swelling is paramount. Both the equilibrium of the solute between polymer and SCF and its rate of mass transfer are affected by this polymer-solvent interaction. Experimental data for a situation where the SCF swells the polymer strongly (CO /ethylbenzene/polystyrene) are presented; for this system the extractions are neither close to equilibrium nor are they completely diffusion-limited. This results in unusual variations in the amounts of ethylbenzene extracted versus temperature and pressure, in particular a much stronger dependence on temperature than on density. We found roughly a 10 -fold increase in the experimental ethylbenzene diffusivity in the polymer phase when SCF-CO was present. A model used to estimate the solute's diffusivity in the swelled polymer is presented. 2

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Introduction Extraction of low-molecular weight solutes by supercritical fluids (SCFE) is often rate-limited by the interaction of the solute with a polymer. For example, in soil extractions the contaminants are often bound to organic polymers in the soil; these polymers are typically humic acids and polyphenols of molecular weights as high as 10,000. Also, polymer recycling is becoming more prevalent, and it is therefore necessary to open up new markets for recycled products. This will necessitate better purification methods for used polymers, and SCFE has shown promise in removing residual monomers, solvents, plasticizers and antioxidants from commercial polymers (1-5). In other polymer-related applications, SCFs can extract oligomers in the preparation of monodisperse polymers (6-7), and can separate mixed polyolefins by flotation, even those differing in density by only ~10 kg/m (8). We have undertaken a fundamental study of the factors controlling SCFE 3

0097-6156/95/0608-0269$12.00/0 © 1995 American Chemical Society

Hutchenson and Foster; Innovations in Supercritical Fluids ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

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of trace contaminants from polymeric materials, which includes high organiccontent soils. Phenomena of interest include (1) depression and broadening of the glass transition temperature (T ) region by the SCF if it swells the polymer, (2) the often opposite effects of increased temperature on the solute diffusivity in the polymer (typically increases exponentially) and equilibrium solubility in the SCF (typically goes through a maximum), and (3) the significant effect an SCF can have on the solute diffusivity if the SCF strongly swells the polymer, which is often the case at high pressure. g


Modeling Considerations for SCFE from Polymers Because of these phenomena, extractions from polymers, or materials containing polymers such as high organic-content soils, show unusual rate-limited behavior that often cannot be predicted by simpler desorption models. Such models would include local equilibrium theory (LET), which assumes equilibrium at all times and all spatial positions between the SCF and the solid (9), and models based upon rate-limiting intraparticle diffusion to the solid surface at a constant diffusivity (5). More complex models with analytical solutions combining intraparticle diffusion, equilibrium at the solid surface, and interfacial mass transfer have been presented (10-11), but these are based on the assumption of a linear equilibriuym relationship, which is sometimes a poor one where polymers are concerned. The problems these unusual phenomena pose in the design of an SCFE unit can be understood by examining Figure 1, which is from previous work in our group (72). The data are for the extraction of DDT from a high organic-content soil (4.8 wt% total organics, 1000 mg/kg DDT). The SCF solvents are dry C 0 , wet (-0.5 wt% water) C0 , and 5 wt% methanol/C0 . With dry C 0 the fraction of DDT which can be removed from the soil is fixed at -60%, i.e., the rate of extracion approaches zero at this point. With 5 wt% methanol, however, extraction is rapid and goes to completion. Wet C 0 exhibits a slower initial, but ultimately a faster extraction rate than does dry C0 . These data can be interpreted as follows. Because methanol at this concentration does not greatly affect the solubility of DDT in SCF-C0 (73), the methanol/C0 extraction data are more representative of an equilibrium solubilitycontrolled process. In other words, the methanol is enhancing the rate of mass transfer of DDT into the SCF, the non-methanol data are far from equilibrium, and so LET cannot apply to the non-methanol data. It can be seen that the times required to clean the soil using either dry or wet C 0 are considerably in excess of times characteristic of LET. However, the wet C 0 results suggest that water is a potentially effective cosolvent at longer times, when the soil has been depleted of most of its low molecular weight organic material. This is probably due to some polymer swelling by water. Conversely, it is possible to accurately model the data of Tan and Liou (14) for the extraction of ethyl acetate from activated carbon by LET, using a Langmuir isotherm. This result suggests that the primary reason for the unusual 2

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DOOLEYETAL.

behavior in Figure 1 is the presence of the organic polymeric phase in the soil, which is not present in activated carbon or other simple adsorbents. At first glance, the occurrence of phenomena (2)-(3) listed above would suggest extraction at as high an operating pressure as possible, and at a temperature much higher than is usually the case in SCFE. This operating strategy was essentially that adopted in a study of the extraction of plasticizers from polypropylene (5). However, aside from the economic impracticabilty of such a strategy, other results suggest lower temperature operation may be possible. In particular, the exact location of the modified T might be important in some extractions. Burgess and Jackson (3) observed maximum rates of extraction of CC1 from chlorinated polyisoprene using SCF-C0 at temperatures roughly the same as the modified T at any given pressure. These results suggest that depression of T through swelling by the SCF greatly affects either the solute equilibrium, or the solute's intraparticle diffusivity, or both. In our results we sometimes observed a similar phenomenon, in that as we approached the modified glass transition pressure at constant temperature the extraction rates were sometimes greater than was the case at higher pressures. But this was not universally true at all the temperatures we studied, and so does not represent a general principle.


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Experimental Apparatus and Procedures For the polymer extractions we used a bench-scale system (Figure 2) consisting of a cylindrical extraction cell (-6.4 mL) housed in a differential scanning calorimeter (DSC; Setaram C-80). The SCF entered the cell though a small tube just above the solid sample. An LC pump (Eldex) supplied the SCF-C0 , which was regulated by both a pressure controller (Dwyer Mercoid D) and a backpressure regulator (Tescom 26-1700). The effluent was sampled by a 16-port valve, then analyzed by GC (Varian 3400 with flame ionization). Therefore both the extraction rate and the enthalpy changes of the solid phase were measured. A constant 1.0 mL/min liquid C 0 feed rate was used to extract 3.0 wt% ethylbenzene (EB, Fisher, reagent) from thin circular disks of polystyrene (PS, Polyscience, MW = 20,000). The polymer was impregnated with EB in sealed tubes at 323 Κ for one day, under N . The pellet geometry was controlled by diemolding 335 mg EB/PS (-4.3 mm thickness, 10.8 mm diameter, which was the cell i.d.). This geometry was deemed most appropriate for the extraction cell in order to maintain a controlled shape, because the disk could expand axially but not radially. Therefore its surface area for mass transfer remained roughly constant. 2

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Results - Extraction of Ethylbenzene (EB) from Polystyrene (PS) The DSC measurements during extraction showed that the exothermic dissolution of C 0 into the polymer was rapid, followed by a long endotherm characterizing slow desorption of the solute ethylbenzene. These results showed that polymer swelling by SCF-C0 took place almost instantly considering the long time scale 2

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Measurement and Modeling of Supercritical Fluid Extraction from

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