A fluid catalytic cracking reactor vessel, comprising: a) a lower internal reactor riser, wherein a portion of the lower internal reactor riser is located within the fluid catalytic cracking reactor vessel, and wherein the lower end of the lower internal reactor riser is attached to the shell of the fluid catalytic cracking reactor vessel; and b) an upper internal reactor riser located within the fluid catalytic cracking reactor vessel, wherein the lower end of the upper internal reactor riser terminates in a conical section which is connected to a cylindrical sleeve; wherein the upper section of the upper internal reactor riser is not in fluid connection with the dilute phase section of the reactor vessel, and at least two riser outlet ports are mechanically connected to the upper section of the upper internal reactor riser; wherein the upper end of the lower internal reactor riser is in fluid connection with the upper internal reactor riser and the dilute phase section of the reactor vessel; the lower internal reactor riser is not mechanically connected to the upper internal reactor riser; the largest diameter of the conical section of the upper internal reactor riser is larger than the diameter of the upper end of the lower internal reactor riser; the cylindrical sleeve of the upper internal reactor riser has a diameter larger than the diameter of the upper end of the lower internal reactor riser; at least a portion of the cylindrical sleeve of the upper internal reactor riser overlaps with at least a portion of the upper end of the lower internal reactor riser; and there is no mechanical means located in the region of the overlapping portion of the cylindrical sleeve of the upper internal reactor riser and the upper end of the lower internal reactor riser for restricting the eccentricity between the cylindrical sleeve and the upper end of the lower internal reactor riser; wherein the length of the overlapping portion of the cylindrical sleeve of the upper internal reactor riser and the upper end of the lower internal reactor riser is from 6 to 36 inches (15-24 to 91.44 cm) when the fluid catalytic cracking reactor vessel is in the operating (hot) condition; and wherein the acute angle between the conical section of the upper internal reactor riser and the axis of the upper internal reactor riser is from 5° to 25°; and wherein a transition length as measured from the top of the lower internal reactor riser to the bottom of the conical section is from 6 to 36 inches (15.24 to 91.44 cm) when the fluid catalytic cracking reactor vessel is in the operating (hot) condition; and wherein there is an annular gap between the lower internal reactor riser and the cylindrical sleeve of the upper internal reactor riser of from 1 to 4 inches (2.54 to 10.16 cm). a) a lower internal reactor riser, wherein a portion of the lower internal reactor riser is located within the fluid catalytic cracking reactor vessel, and wherein the lower end of the lower internal reactor riser is attached to the shell of the fluid catalytic cracking reactor vessel; and b) an upper internal reactor riser located within the fluid catalytic cracking reactor vessel, wherein the lower end of the upper internal reactor riser terminates in a conical section which is connected to a cylindrical sleeve; wherein the upper section of the upper internal reactor riser is not in fluid connection with the dilute phase section of the reactor vessel, and at least two riser outlet ports are mechanically connected to the upper section of the upper internal reactor riser;