Abstract: A process and system for producing bio or fossil fuels is provided. The process includes: feeding mixed hydrocarbon to a fixed-bed reactor system containing a zeolite catalyst; operating the fixed-bed reactor system for oligomerization of one or more C3-C8 hydrocarbons to form an oligomerized effluent; separating the oligomerized effluent into a C3-fraction, a gas stream, and a liquid stream in a gas/liquid separation unit; feeding at least a first portion of the liquid stream to a distillation unit; recycling at least a second portion of the liquid stream to the fixed-bed reactor system; separating the first portion of the liquid stream in the distillation unit into, a light fraction and a heavy fraction; and recycling the light fraction to the fixed-bed reactor system.