Disclosed herein is a rotating packed bed, RPB, for mass transfer between a sorbent and a gas, the RPB comprising: a central chamber arranged to receive a flow of a sorbent that is a liquid; and a flow path for the sorbent between the central chamber and a region for mass transfer between a gas and the sorbent; wherein, in use, the flow of sorbent through the region for mass transfer is substantially in cross-flow with the flow of gas through the region for mass transfer. Advantageously, mass transfer between a liquid sorbent and a gas is improved.