EP--74435 A A hydrocarbon cracking pocess comprises passing a mixture of e the hydocabon with superheated steam through a reactor conduit extending through a radiation block structure, heating the mixtue while passing heating gas cocurrently through the structure and quenching the hot reaction product in a downstream heat exchanger. - Appts. comprises a superheated steam generator, a stea/ hydrocarbon mixer, a cracking reactor conduit arranged in a radiation block structure with space for heating gas to flo around the conduit, and a heat exchanger. - Also claied are (a) a mixer for cocurrent mixing of the stea and hydocarbon in which one of the inlets is transverse to the flow direction and is aerodynamically shaped to be rounded on the side facing the other inlet and more pointed towards the outlet, and (b) a superheated stea generator coprising a conduit ranged in a radiation block structure in such a way that cooler steam is preferentially heated. - Arrangement provides an improVed temperature profile though the reaction zone, giving better yield of products such as olefins. The coke yield can also be reduced and cleaning is much simpler and quicker than in conventional cracking plant. EPAB- EP--74435 B A hydrocarbon cracking pocess comprises passing a mixture of e the hydocabon with superheated steam through a reactor conduit extending through a radiation block structure, heating the mixtue while passing heating gas cocurrently through the structure and quenching the hot reaction product in a downstream heat exchanger. - Appts. comprises a superheated steam geneator, a stea/ hydrocarbon mixer, a cracking reactor conduit arranged in a radiation block structure with space for heating gas to flo around the conduit, and a heat exchanger. - Also claied are (a) a mixer for cocurrent mixing of the stea and hydocarbon in which one of the inlets is transverse to the flow direction and is aerodynamically shaped to be rounded on the side facing the other inlet and more pointed towards the otlet, and (b) a superheated stea generator coprising a conduit ranged in a radiation block structure in such a way that cooler steam is preferentially heated. - Arrangement provides an improVed temperature profile though the reaction zone, giving better yield of products such as olefins. The coke yield can also be reduced and cleaning is much simpler and quicker than in conventional cracking plant. (40pp) - EP--74435 B A process for cracking hydrocarbon in which a hydrocarbon/superheated steam mixture is passed through a reactor zone and heated to a temperature sufficient to crack the hydrocarbon and subsequently, the cracked hydrocarbon is passed from the reaction zone into a heat exchanger for quenching the cracked reaction product, characterised in that in the reaction zone, the hydrocarbon/superheated steam mixture is flowed through a reactor conduit extending through a radiation block structure which defines a passage to accommodate the reactor conduit and free spaces between the reactor conduit and the interior surface of the radiation block structure to allow heating gases to flow through the radiation block structure in a direction cocurrent with the flow of the hydrocarbon through the reactor conduit. (28pp) - EP--74435 B A hydrocarbon cracking pocess comprises passing a mixture of e the hydocabon with superheated steam through a reactor conduit extending through a radiation block structure, heating the mixtue while passing heating gas cocurrently through the structure and quenching the hot reaction product in a downstream heat exchanger. - Appts. comprises a superheated steam generator, a stea/ hydrocarbon mixer, a cracking reactor conduit arranged in a radiation block structure with space for heating gas to flo around the conduit, and a heat exchanger. - Also claied are (a) a mixer for cocurrent mixing of the stea and hydocarbon in which one of the inlets is transverse to the flow direction and is aerodynamically shaped to be rounded on the side facing the other inlet and more pointed towards the outlet, and (b) a superheated stea generator coprising a conduit ranged in a radiation block structure in such a way that cooler steam is preferentially heated. - Arrangement provides an improVed temperature profile though the reaction zone, giving better yield of products such as olefins. The coke yield can also be reduced and cleaning is much simpler and quicker than in conventional cracking plant. USAB- US4426278 A Hydrocarbon compsn. is cracked by (a) passing steam through a conduit enclosed by a radiation block structure which is a gas passage surrounding the conduit; (b) flowing heating gases through the passage to superheat the steam to 1000-1500 deg. C; (c) mixing the hydrocarbons with steam; (d) passing the mixt. through a reaction conduit which extends through and is enclosed by a second similar reaction block structure; (e) flowing heated gases through the second passage in contact with the conduit cocurrently with the hydrocarbon steam mixt. flow to heat the mixt. to 700-1000 deg. C; (f) causing the heated hydrocarbon compsn. to crack in the conduit; and (g) passing hot reaction prod. from the conduit into a heat exchanger for quenching. - Pref. the hydrocarbon is a 5C or less hydrocarbon, and its residence time in the conduit is 0.06-0.15 secs. The process is esp. used in the cracking of naphtha, and the short residence time is maintained to prevent undesirable side reactions. (8pp)r