A feed nozzle assembly for introducing a mixture of a gas and a liquid hydrocarbon droplets into a vessel in the form of a substantially sheet-like spray, which feed nozzle assembly comprises a nozzle body provided at its upstream part with means to supply the gas and means to supply the liquid hydrocarbon to said nozzle body and at the downstream end with a closed outlet end which outlet end is provided with an elongated opening wherein in the opening an insert is positioned such that a continuous loop opening results for discharge of the mixture of gas and liquid droplets.