NOVELTY - Producing auxiliary fuel stream containing low concentration of undesirable compounds from primary fuel stream involves isolating first vapor permeate stream and first retentate stream from primary stream by passing portion of primary stream through first membrane separation stage; condensing portion of vapor permeate stream in first separation stage partial condenser into first liquid stage stream and first vapor stage stream; passing vapor stage stream through reactor; and sending to polishing filter (506) having catalyst (507) that converts compounds into separable sulfides or oxides.    USE - For producing auxiliary fuel stream containing low concentration of undesirable compounds (claimed), used in fuel cell systems for vehicles such as hybrid or fuel cell powered automobile; and as logistic fuels or conventional fuels for diesel and kerosene cuts.    ADVANTAGE - The method and system allows efficient extraction of auxiliary fuel stream with low concentrations of sulfur compounds. The method is a simple process that provides the needed separation selectivity; can be implemented in a compact, inexpensive system; and can operate with a wide range of primary fuels from gasoline, kerosene, jet fuel and diesel. The process has ability to extract a low sulfur fraction of the fuel mixture while leaving behind all of the fuel's performance additives and sulfur compounds in the return primary fuel stream.    DETAILED DESCRIPTION - Producing an auxiliary fuel stream containing a low concentration of undesirable compounds from a primary fuel stream involves isolating a first vapor permeate stream and a first retentate stream from a primary fuel stream by passing a portion of the primary fuel stream through a first membrane separation stage; condensing a portion of the first vapor permeate stream in a first separation stage partial condenser into a first liquid stage stream and a first vapor stage stream; passing the first vapor stage stream through a reactor; and either sending the first vapor stage stream to a polishing filter (506), where polishing filter contains a catalyst (507) converting the undesirable compounds into separable sulfides or oxides; or condensing a portion of the first vapor stage stream in a second first stage partial condenser into a second liquid stage stream; isolating a second vapor permeate stream and a second retentate stream from the second liquid stage stream by passing the second liquid stage stream through a second separation stage; and mixing the first retentate stream and the second vapor permeate stream and returning the mixture to the primary fuel stream; or condensing a portion of the first vapor stage stream in a second first stage partial condenser into a second liquid stage stream; and isolating a second vapor permeate stream and a second stage retentate stream (542) from the second liquid stage stream by passing the second liquid stage stream through a second separation stage. INDEPENDENT CLAIMS are included for the following:    (1) system (590) for producing fuel stream containing low concentration of undesirable compounds from primary fuel stream comprising: first separation stage to separate portion of primary fuel stream into first vapor permeate stream and a first retentate stream; a first separation stage partial condenser connected to first vapor permeate stream condensing a portion of the first vapor permeate stream into a first liquid stage stream and a first vapor stage stream; a second separation stage partial condenser condensing a portion of the first vapor stage stream into a second liquid stage; a second separation stage for separating the second liquid stage stream into a second vapor permeate stream and a second retentate stream; and optionally mixer to mix first retentate stream and second vapor permeate stream and returning mixture to primary fuel stream;    (2) a system (S2) for producing auxiliary fuel stream containing low concentration of undesirable compounds from a primary fuel stream comprising: the first separation stage; a first separation stage partial condenser connected to first vapor permeate stream condensing a portion of first vapor permeate stream into a first liquid stage stream and a first vapor stage stream; second separation stage partial condenser condensing portion of the first vapor stage stream into a second liquid stage; and a polishing filter connected between first separation stage and the first separation stage partial condenser receiving the first stage permeate vapor, that contains a catalyst to convert the undesirable compounds to sulfides or oxides to increase the selectivity and absorption capacity of the polishing filter;    (3) producing auxiliary fuel stream containing low concentration of undesirable compounds from primary fuel stream involving isolating a first vapor permeate stream and first retentate stream from a primary fuel stream by passing a portion of primary fuel stream through a first membrane separation stage; condensing portion of first vapor permeate stream in first separation stage partial condenser into a first liquid stage stream and a first vapor stage stream; reacting vapor stage stream with reactant in a catalyst reactor to form isolatable compounds; isolating the isolatable compounds in a secondary isolation component; condensing a portion of first vapor stage stream in second first stage partial condenser into a second liquid stage stream; and isolating a second vapor permeate stream and a second retentate stream from second liquid stage stream by passing the second liquid stage stream through a second separation stage; and    (4) a system (S3) for producing auxiliary fuel stream containing low concentration of undesirable compounds from primary fuel stream comprising first separation stage to separate a portion of a primary fuel stream into a first vapor permeate stream and a first retentate stream; a first separation stage partial condenser connected to the first vapor permeate stream condensing a portion of the first vapor permeate stream into a first liquid stage stream and a first vapor stage stream; a reactive catalyst reactor receiving the first vapor stage stream; and a second separation stage partial condenser condensing a portion of the first vapor stage stream from the catalyst reactor into a second liquid stage.    DESCRIPTION OF DRAWING(S) - The figure shows schematic flow diagram of system for producing auxiliary fuel stream.    Tank (501)    Auxiliary fuel storage tank (502)    Stage one membrane (504)    Second stage membrane reactor (505)    Polishing filter (506)    Catalysts (507)    Separator (509)    Pump (511)    Heat exchanger (512)    Partial condenser (513)    Feed stream (533)    Stage one vapor permeate stream (534)    Second stage retentate stream (542)    Stage one retentate stream (550)    Second stage permeate stream (553)    Processing system (590)