So I've been fascinated with supercritical fluid extraction ever since the idea was first mentioned by Dr . VandenBout in my general chemistry class some four years ago. For the uninitiated, a supercritical fluid is a substance heated and compressed above its so-called "critical point," which is a coordinate on the pressure-temperature plane above and to the right of which the distinction between liquid and gas becomes meaningless. Theoretically, any substance can be made into a supercritical fluid, but of course some substances have more accessible supercritical domains than others. Carbon dioxide, for example, is the most commonly-used and -studied supercritical fluid because its critical pressure and temperature are accessible with relatively inexpensive apparatus.
The neat thing about supercritical fluids is that their capacity to solvate particular organic molecules can be tuned very selectively by precise adjustments of temperature and pressure. So they make useful solvents for industrial processes. In the case of CO2, an added "green" benefit is that the supercritical solvent is entirely benign, environmentally. Ever since I first learned about supercritical fluid extraction, I've been interested in the possibility of constructing a "garage-scale" supercritical fluid reactor. After doing some light reading on the subject in my old instrumental analysis book, I realized that, if a suitable pressure vessel could be found, performing supercritical fluid extraction of, say, natural products or pharmaceuticals could be readily conducted by the average shmoe in his garage using widely available materials. It is not even necessary to purchase or rent a high-pressure CO2 cylinder, as grocery-store dry ice can serve as the CO2 source, and can be conveniently measured out in the solid phase by weight or even volume. Simple calculations using the ideal gas equation give particular volumes and weights of dry ice to achieve particular pressures at particular temperatures. The dry ice is simply loaded into the pressure vessel, along with the material to be extracted, before sealing. The spreadsheet below gives all necessary physical constants and the results for an 8-quart pressure vessel:
DIY SCF Calculations
PV = nRT
SCP(CO2): 100 bar / 98.69233 atm / 1450.377 psi
SCT(CO2): 40 C / 313.15 K
Ves.Vol.: 8 qt / 7.570824 L
R 8.21E-02 L atm mol-1 K-1
MW(CO2): 44.01 g/mol
d(CO2[s]): 1.6 g cm-3
n=PV/RT
n = 29.08 mol => 1279.7 g => 799.8 mL
The big problem turns out to be the pressure vessel. My first thought was that a high-end kitchen pressure cooker might do the trick. NOT SO. A "high pressure" in the world of pressure cooking is 15 psi overpressure, which is about 2 atm. To access the supercritical fluid domain for CO2 requires nearly 50 times that pressure. A pressure cooker would explode (messily) long before the necessary pressure could be achieved. What's more, that pressure needs to be dynamically maintained. To recover solutes by supercritical fluid extraction, the SCF itself is slowly bled from the reactor and bubbled through an appropriate solvent, e.g. methanol. The CO2 blows off into the atmosphere and the goodies remain behind in solution. The reactor, however, needs to be designed to maintain constant pressure during this slow bleeding of the SCF. On a garage scale, this might be achieved by steadily elevating the vessel's temperature to compensate for bubbled-off SCF, but what effects the temperature ramp may have on substrate solubility are unknown to me. In "professional" SCF reactors, constant pressure is maintained by employing a syringe-type pressure mechanism in which reactor volume is continuously decreased during the extraction. Even if the "temperature ramp" method proposed above proved workable, the development of a useful garage-scale technique would still await the discovery or invention of a suitably accessible pressure vessel.
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