NOVELTY - System comprises extracting rectifying tower (2), alkane water washing tower (8), stripping tower (9), olefin water washing tower (12), solvent recovery tower (14), hydrogenation reactor, feeding and discharging heat exchanger, feeding heater and separating device. The top part of the stripping is connected with the olefin water washing tower and bottom part of the stripping tower is connected with the upper part of the extracting rectifying tower. The dehydration step is performed in hydrogen feeding and discharging heat exchanger. The olefin water washing tower is a solvent recovery tower.    USE - System used for synthesizing isobutane (claimed).    ADVANTAGE - The system is cost-effective, recovers waste solvent and easily handles refineries.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for synthesizing isobutane, which comprises:    (A) entering unsaturated compound containing four carbons into extraction rectifying tower for performing extractive distillation process;    (B) entering the obtained material into alkane water washing tower for remove solvent and performing dehydration;    (C) extracting the obtained mixture by rectifying tower bottom and then collecting mono-olefin product, from the bottom;    (D) entering the mixture of saturated hydrocarbon and alkane from water washing tower to dehydration rectifying tower;    (E) heating the obtained material in hydrogenation reactor and then performing hydrogenation reaction of the heated mono-olefin and diolefin;    (F) removing the trace organic nitrogen and other impurity and then extracting solvent from the obtained material in extracting middle reboiler;    (G) cooling the obtained material to 40-60 degrees C and then recovering isobutane;    (H) cooling the obtained material to 40-75 degrees C and then refluxing the solvent in recycling tower and other path of solvent is extracted in rectifying tower; and    (I) entering the obtained material into hydrogenation separating device.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of a system.    Extracting rectifying tower (2)    Alkane water washing tower (8)    Stripping tower (9)    Olefin water washing tower (12)    Solvent recovery tower (14)