Material columns are one of the most integral parts of any extraction system and commonly found between the transfer vessel (solvent tanks) and the extraction tank or collection plate. Inside a column the raw trim (plant matter) is packed down and a chilled solved, such as butane/hexane/propane/etc... is pushed through. Keeping the plant matter chilled within a range of -20°C to -50°C can produce a higher extraction rate. Tri Clamp screen gaskets placed at both ends of the column retain the plant material inside the column where the solvent now infused with extract can be collected.
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.