Apparatus and process for the vaporization of a heavy hydrocarbon feedstock with steam. The apparatus comprises a first tubular element (2) and a second tubular element (3) surrounding the first tubular element (2) to form an annular space (8), means (4) for introducing a heavy hydrocarbon feedstock into the first tubular element (2), means (7) for introducing superheated steam into the annular space (8), the first and second tubular elements (2, 3) being provided with open ends for the supply of the superheated steam as an annulus around a core of the heavy hydrocarbon feedstock, a frusto-conically shaped element (10) connected to the open end of the second tubular element (3). To avoid coke formation in the apparatus the frusto-conically shaped element (10) diverges in a direction away from the second tubular element (3) and has an apex angle of at most 20 degrees.