You do not need an additional gas to obtain super-critical ethanol. To obtain super-critical ethanol in a flow system, you would compress liquid ethanol at room temperature to a pressure larger than 61 bar and feed it to a heated vessel to raise its temperature above in 240C. You will have supercritical ethanol, as long as the pressure is kept above 61 bar and the temperature above 240C.
In a batch reactor, you would load liquid ethanol (sufficient to raise the pressure to larger than 60 bar - estimate using n=pV/(ZRT) with Z =0.24). You need to ensure that the rest of the medium does not react with ethanol (i.e. an inert gas like nitrogen). The amount of this gas will be quite small in comparison to ethanol, and hence the properties of the mixture will not differ significantly from ethanol
If your purpose is getting supercritical ethanol for a supercritical drying (e.g. for preparing an aerogel), then indeed just reaching the critical pressure and temperature is not enough. Far better results are obtained if the critical point is significantly exceeded. In this condition, N2 flow is perfect, and this is what I used myself for getting excellent aerogels usin supercritical ethanol in N2 flowing at 200 bars. You'll find the corresponding papers in my RG profile.
Alain
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