Use of a direct heating device with a reheater in a dehydrogenation unit

A method for increasing the efficiency and/or expanding the capacity of a new or existing dehydrogenation unit for dehydrogenating of alkyl aromatic hydrocarbons to alkenyl aromatic hydrocarbons, comprising:

• providing at least one dehydrogenation reactor and a feed stream comprising said alkyl hydrocarbons;
• adding at least one reheater and at least one direct heating unit (DHU) to a new or existing dehydrogenation unit, whereby the DHU and reheater are positioned before or after at least one reactor, and wherein the DHU and reheater are in series arrangement with respect to each other;
• diverting between 0.5% and 85% of a reactor effluent from the reactor to the DHU for heating and feeding the remainder of the reactor effluent to the reheater for heating;
• and
• feeding the heated streams from the DHU and the reheater to a subsequent reactor, wherein there is an energy savings for operating the new or existing dehydrogenation unit as compared to operating a dehydrogenation unit without an added DHIJ and added reheater;
• wherein the energy savings is a usage of 0.1% to 50% less energy when at least one DHU and at least one reheater are used to heat a process stream.