26 a tube 27 extends from the space 28 within the filter unit 13 above the top of the pipe 23 to the space 29 within the top of the reactor 11 above the slurry 16 an opening 31 in the tube 27 connects the two spaces 28 29 in operation gaseous reactants are introduced to the reactor vessel 11 via the inlet 14 and the plate 15 the reactants form bubbles in the slurry 16 which pass upwards past the filter unit 13 the slurry 16 consists of a liquid phase of the reaction products and a catalyst in finely divided form the gaseous reactants react as they contact the catalyst thus adding to the products in the slurry at the same time the products pass through the filter element 22 to form a product filtrate 32 which is free of catalyst any gaseous products and unreacted reactants can be vented through the outlet 17 and subsequently treated and or recycled the product filtrate 32 leaves the filter unit 13 via the constant level device 23 and outlet 24 and is collected in the collector 25 for regulated continuous or periodic removal the difference in level between the slurry 16 and the product filtrate 32 determined by the constant level device results in a.