Apparatus for conducting a three-phase reaction, in which gaseous reactants are reacted in the presence of a finely divided solid catalyst in suspension in a liquid comprising, at least in part, the reaction products of the gaseous reactants, thereby forming a slurry, the apparatus comprising: a reactor vessel including a reactant inlet and arranged, in use, to accommodate the slurry with a gas space above; a filtration system including a primary filter which is arranged to be in contact with the slurry in use, the primary filter having an outlet for primary filtrate comprising fine catalyst particles which has been subjected to a primary filtration; a gas demister arranged to remove entrained material from gas in the gas space above the slurry and to produce a demister gas stream comprising gas and fine catalyst particles; a cyclone arranged to receive the demisted gas stream and to remove the entrained fine catalyst particles from the demisted gas stream thereby producing a substantially particle-free gaseous product stream and a particle-containing liquid product stream, the cyclone has a substantially particle-free gaseous product outlet for the substantially particle-free gaseous product stream and a particle-containing liquid product outlet for the particle-containing liquid product stream, a filtrate cleaning unit arranged to: receive the primary filtrate from the primary filter and the particle-containing liquid product stream from the cyclone, and remove the fine catalyst particles from the primary filtrate stream and the particle-containing liquid product stream to produce a substantially particle-free liquid product stream and a slurry stream including the fine catalyst particles, the filtrate cleaning unit has a product outlet for the substantially particle free liquid product and a slurry outlet for the slurry stream including the fine catalyst particles; wherein the effective cross-sectional area of the demister is smaller than the effective cross-sectional area of the reactor above the slurry level, whereby the velocity of the gas is increased as it passes through the demister, in use, and the gas demister is a vane pack mist eliminator which has a series of corrugated plates, arranged generally vertically whereby gas flows vertically upwardly through the vane pack mist eliminator. a reactor vessel including a reactant inlet and arranged, in use, to accommodate the slurry with a gas space above; a filtration system including a primary filter which is arranged to be in contact with the slurry in use, the primary filter having an outlet for primary filtrate comprising fine catalyst particles which has been subjected to a primary filtration; a gas demister arranged to remove entrained material from gas in the gas space above the slurry and to produce a demister gas stream comprising gas and fine catalyst particles; a cyclone arranged to receive the demisted gas stream and to remove the entrained fine catalyst particles from the demisted gas stream thereby producing a substantially particle-free gaseous product stream and a particle-containing liquid product stream, the cyclone has a substantially particle-free gaseous product outlet for the substantially particle-free gaseous product stream and a particle-containing liquid product outlet for the particle-containing liquid product stream, a filtrate cleaning unit arranged to: receive the primary filtrate from the primary filter and the particle-containing liquid product stream from the cyclone, and remove the fine catalyst particles from the primary filtrate stream and the particle-containing liquid product stream to produce a substantially particle-free liquid product stream and a slurry stream including the fine catalyst particles, the filtrate cleaning unit has a product outlet for the substantially particle free liquid product and a slurry outlet for the slurry stream including the fine catalyst particles; wherein the effective cross-sectional area of the demister is smaller than the effective cross-sectional area of the reactor above the slurry level, whereby the velocity of the gas is increased as it passes through the demister, in use, and the gas demister is a vane pack mist eliminator which has a series of corrugated plates, arranged generally vertically whereby gas flows vertically upwardly through the vane pack mist eliminator.