Post-furnace hydrogen mixing

Hydrocracking: The key to mixing hydrogen after the furnace is to have an adequate hydrogen circulation rate (hydrogen-to-oil ratio) to carry away heat, without causing the temperature at the exit of the hydrogen heater to become too high. Generally, the hydrogen-to-oil ratio in hydrocracking is greater than 800; therefore, the amount of recycled hydrogen is sufficient to meet the requirements. The advantages of mixing hydrogen behind the furnace are: ① The hydrogen is relatively pure and does not cause coking; thus, it is possible to **increase the wall temperature of the heating furnace tubes, reducing the size of the furnace and saving steel ； ②Hydrogen is distributed more evenly. In a heating furnace with multiple feed streams, as long as the resistance in each stream is equal, uniform distribution can be achieved automatically without the need for control valves, thus saving on investment ； ③Heating furnaces are easy to design; for some heat exchangers, the material used can be reduced as appropriate to save on costs. Hydrorefining: The reaction section generally features two major categories with four types of heat exchange processes. One category involves the mixing of feed oil and hydrogen after passing through a heater, and includes single-phase heat exchange, hydrogen mixing after the heater, and single-phase heat exchange again, along with oil mixing after the heater ； A category consists of those where the feed oil and hydrogen are mixed before the heating furnace, including single-phase heat exchange, hydrogen mixing before the furnace, and mixed-phase heat exchange, as well as hydrogen mixing before the furnace. For the first category, the characteristics of post-furnace hydrogen mixing and post-furnace oil mixing are that the heating furnace heats only hydrogen or crude oil, resulting in simple engineering design. Its disadvantages are as follows: in high-pressure heat exchangers, the feed oil and hydrogen exchange heat with the reaction effluent separately, resulting in a complex heat exchange process; since both feed streams are in a single phase, the heat transfer coefficient is low, the temperature difference for heat transfer within the heat exchanger is small, which requires a larger heat exchange area and thus higher investment costs ； Regarding heating furnaces, heating the oil alone tends to cause coking, while heating hydrogen alone is generally suitable for hydrocracking units where the hydrogen-to-oil ratio is high. In hydrorefining units, where the hydrogen-to-oil ratio is low, heating hydrogen alone in the furnace would result in an excessively high temperature at the furnace outlet, which is highly unreasonable.