Solvent extraction is the process of diffusing a solvent into oil-bearing cells resulting in a mixed solution of oil and solvent. It is one of the most popular methods of separating oils and fats from oil-bearing materials as opposed to other mechanical processes (such as expellers, hydraulic press, etc...) primarily due to the high percentage of oil recovered which can range upwards of 99%+ as opposed to the 85%+ of other methods. The common solvents of choice range from butane, hexane, and pentane among a handful of others, each of which is favored because of several factors: commercial economics, a low boiling point, and in the case of consumption grade product, a non-toxic impact. Distillation of the resulting oil mixture allows recovery of the alkane hydrocarbons (your "hex/but/propane") to be reused at minimal costs.
The extraction/collection tank collects the extracted oil and solvent at the end of the process. The larger the vessel the more trim (raw plant matter) you can process at once. It is not uncommon for a large collection vessel to be the repository of multiple material columns, provided it has enough volume to hold the volume of solvent initially. Once in the collection vessel, the solvent is warmed until it reverts to a gaseous form which can then be removed with a recovery pump such as the Haskel we carry. The extracted oil ("drop off") can then be poured into an external container for further vacuum purge if necessary.