NOVELTY - The membrane bioreactor comprises membrane modules having hollow fibers, a membrane tank for retaining the membrane modules partially submerged in a process liquid for formation of a biofilm on the outer surface of the hollow fiber wall by interaction of microorganisms with a process gas and for the production of a liquid product that mixes with the process liquid, a seal between contents of the membrane tank and ambient atmosphere, and a gas supply conduit for communicating the process gas with the hollow fiber lumens of the hollow fibers.    USE - The membrane bioreactor is useful for anaerobic conversion of gas containing carbon monoxide or a mixture of carbon dioxide and hydrogen, into liquid products including ethanol, n-butanol, hexanol, acetic acid and/or butyric acid (all claimed), where the liquid products are useful as liquid motor fuels or diesel motor fuels.    ADVANTAGE - The membrane bioreactor is suitable for large scale manufacturing processes and ensures effective and economical conversion of gas into liquid products.    DETAILED DESCRIPTION - The membrane bioreactor comprises membrane modules having hollow fibers, a membrane tank for retaining the membrane modules partially submerged in a process liquid for formation of a biofilm on the outer surface of the hollow fiber wall by interaction of microorganisms with a process gas and for the production of a liquid product that mixes with the process liquid, a seal between contents of the membrane tank and ambient atmosphere, a gas supply conduit for communicating the process gas with the hollow fiber lumens of the hollow fibers, an overflow conduit located to maintain the process liquid with a liquid surface at a predetermined level and operable to remove exhaust gas and the process liquid from the membrane tank, a gas/liquid separation tank operably connected to the overflow conduit to separate the exhaust gas from the process liquid, a pressure control valve operably connected to the gas/liquid separation tank to control pressure in the membrane tank, and a product recovery system operably connected to receive the process liquid from the gas/liquid separation tank to separate the liquid product from the process liquid and to return process liquid to the membrane tank. The hollow fibers have a gas permeable hollow fiber wall defining a hollow fiber lumen and an outer surface. The membrane tank retains the membrane modules in a common horizontal plane across which the hollow fibers extend vertically when partially submerged in the process liquid. The gas supply conduit is operably connected to the hollow fibers to supply the process gas containing carbon monoxide or a mixture of carbon dioxide and hydrogen. The membrane tank retains membrane modules in the process liquid for the formation of the biofilm containing microorganisms and for the production of the liquid product. The membrane modules are one-headed membrane modules comprising the hollow fibers having free ends and having a gas inlet chamber to operably connect the gas supply conduit with the hollow fiber lumens. The membrane tank is arranged to maintain a liquid surface and the free ends of the hollow fibers are above the liquid surface. The free ends of the hollow fibers are closed-ended and the membrane tank is arranged to maintain the hollow fibers completely submerged in the process liquid. The membrane tank is adapted for complete filling by the process liquid. The bioreactor further comprises a liquid outlet in communication with the membrane tank to withdraw the process liquid, a pressure reducing valve operably connected to the liquid outlet to receive the process liquid and reduce to the pressure of the process liquid to diffuse dissolved process gas from the process liquid, a gas/liquid separation tank operably connected to the pressure reducing valve to separate the diffused process gas from the process liquid. The membrane module is a two-headed membrane module having a first potted end spaced apart from a second potted end. The first potted end is operably connected to one end of the hollow fibers and the second potted end is operably connected to the other end of the hollow fibers to allow the process gas to flow through the hollow fiber lumens from the first potted end to the second potted end. The hollow fibers have a length equal to 1.015-1.15 times the distance between the first potted end and the second potted end to produce slack in the fibers. The liquid outlet is operably connected to the membrane tank to receive the process liquid containing the liquid product. The bioreactor contains axially stacked membrane modules, and a channel communicates process liquid from one membrane module to an adjacent membrane module. The membrane modules further comprise a gas inlet chamber operably connected to the first potted end and a gas outlet chamber operably connected to the second potted end. The gas outlet chamber of membrane module communicates with the gas inlet chamber of an adjacent membrane module. The seal comprises a hydrostatic tower for retaining a column of the process liquid above the membrane tank to provide a liquid seal between the tank and the ambient atmosphere and to pressurize the process liquid that surrounds the membrane modules. The membrane modules comprise two-headed membrane modules having a first potted end spaced apart from a second potted end. The two-headed membrane modules are operably connected so that the process gas flows in parallel through the hollow fiber lumens of the two-headed membrane modules and are open for contact of the outer surface of the hollow fibers with the process liquid across the volume of the membrane tank. An INDEPENDENT CLAIM is included for a bioreaction method.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of a bioreactor system with gas and liquid circulation.    Feed gas stream (10)    Flow meter (11)    Exhaust gas stream (12)    Feed gas distribution chamber (13)    Membrane unit. (15)