Purchase tubing--from a lab supply store--that provides the desired molecular weight cutoff to retain caprylic acid inside the dialysis bag. The correct number for this cutoff depends on the molecular weights of the components you wish to separate from caprylic acid. Because permeability of the dialysis bag to different components depends not only on molecular weight but also on molecular shape, polarity, degree of permeability and ionic charge, viable cutoff numbers can be either twice the molecular weight of the solute expected to pass through tubing or half the molecular weight of the solute expected to remain inside the tubing (caprylic acid).
Fill an appropriately sized beaker with distilled water. Place the dialysis bag in the beaker.
Place a stir bar in the beaker and place the beaker on a magnetic stirrer, overnight. The next day, solutes with MW smaller than that of caprylic acid will have diffused out of the dialysis bag, into the surrounding water. Some distilled water will have diffused into the bag. Evaporate the distilled water with a rotovap, if necessary.