A feedstock processing system extracts a product from a solid using a CTSE system comprising a plurality of continuous stirred tank extraction stages arranged in fluid communication with each other in series such that effluent from one stage flows to a next stage in the series. One of the stages has an inlet to allow a measured amount of liquid solvent and the solid to be introduced to the continuous stirred tank extraction stage. The stage mixes the solid with the introduced solvent to form a homogeneous slurry to enable the product associated with the solid to be extracted with the solvent. A solid-liquid separator is arranged in fluid communication with the continuous stirred tank extraction stages, and receives an effluent from one of the stages and separates the liquid solvent containing the product from the solid to form a product-containing liquid and a product-depleted solid.