In a process for the thermal cracking of a heavy hydrocarbon in a cylindrical reactor, a portion of the content material in the reactor is taken out and injected back into the reactor according to a specific procedure to form a "wet wall" or liquid curtain on the inner peripheral wall of the reactor, thereby preventing coking in the reactor. The apparatus includes a radially outwardly extending interior annular recess around an upper portion of the inner wall thereof and forming a part of said inner wall and plural means for tangentially injecting the recycled liquid into said recess.