honestly, i think a lot of people are over thinking the whole blasting process. i've done top down single blasts,bottom up single blasts, and multiple cycles using the machine as it came designed. if you are running super cold, as with dry ice, it quickly becomes a moot point on purity of product. if you freeze your column, you should be able to get waxes extracted to stick to the bottom of the spool and you pour off the good stuff. I can't emphasize enough that negative temps will make the biggest difference after good starting material.
Very interesting, i had wondered if chilling the butane/oil solution low enough would enable "winterizing" without the addition of any alcohol.