Abstract: A flow reactor comprising: a cylindrical body defining a conduit extending from a first end to a second end; a conduit inlet for providing a flow of a liquid reagent into the conduit, the conduit inlet at or near the first end; a conduit outlet for providing a flow of a liquid content from the conduit, the conduit outlet at or near the second end; a rotating screw arranged within the conduit and extending in the conduit, the rotating screw arranged to rotate about an axis extending from the first end to the second end, to direct the liquid content from the conduit inlet to the conduit outlet; and one or more ultrasonic emitters arranged to emit ultrasound waves in the conduit. The flow reactor may be used for desulphurization of fuel oil.