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 tower 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 alkane water washing tower is a hydrogen feeding and discharging heat exchanger. The 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 removing solvent and performing dehydration;    (C) extracting the olefin and solvent mixture by rectifying tower 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;    (I) combining the solvent recovery tower and feeding and discharging heat exchanger to solvent recovery tower for exchanging heat;    (J) entering the obtained material into hydrogenation separating device; and    (K) cooling the obtained material to 40-75 degrees C to obtain isobutane.    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)