The invention relates to a spray drying system and a method for its operation that provides a composition of dry. or essentially solvent free, particles from a solution of an agent. The system comprises a generally vertical tube reactor arranged for counter-current removal of solvent from a process flow fed with aerosol droplets of solution descending from the top said reactor with an ascending gas flow. The reactor includes a perforated process tube for transportation of the process flow from the outlet of an aerosol generating device to a dry particle collecting device. A membrane sleeve essentially surrounds the peripheral area of said process tube and separates the descending process stream from the ascending gas stream. Vaporized solvent is transported from the process stream to the ascending gas stream. A reactor housing is sealingly covering said process tube and membrane sleeve and is provided with means for introducing and/or removing the process fluids.