An isothermal method is provided for separating aromatic hydrocarbons from a feed stream. The method includes flowing the feed stream through a first channel within a first wafer assembly that contains a rib member. Next, the feed stream is exposed to a first thin film polymer membrane. A stream permeates through the first thin film polymer membrane into a permeate zone. The permeate zone is heated via a heated fluid contained within the rib member. The permeate is produced from the first wafer assembly. The method may include having a plurality of tandem wafer assemblies arranged in series. An apparatus for separating aromatic components from a feed stream is also disclosed. In the preferred embodiment, the apparatus includes a series of tandem wafer assemblies specifically adapted for blending a transportation fuel.