In an apparatus for mass transfer between a liquid and a gas inside a rotor, the liquid is supplied to a center of the rotor and is driven outward by centrifugal force generated by rotation of the rotor, the gas surrounding the rotor is forced inward through the rotor by a pressure of the gas, counter to the liquid flow in the rotor, and the rotor has a plurality of passages lying in the plane of the rotor that begin at a center of the rotor and terminate at an outer circumference of the rotor. The passages are each filled with a packing that increases the area of contact between the liquid and the gas.