Post-furnace hydrogen mixing process in hydrocracking

Qilu Petrochemical has pre-furnace hydrogen mixing and post-furnace hydrogen mixing. This post was last edited by chengkang on 2009-4-13 at 13:39.]

The post-furnace hydrogen-blended heater is used to heat recycled hydrogen; once it reaches the desired temperature, it is mixed with the feed oil for heat exchange

What was mentioned upstairs refers to the oil mixing process after the furnace; in the hydrogen mixing process after the furnace, it is the oil that is heated by the furnace, and only after that is it mixed with hydrogen before entering the reactor.

For the hydrocracking and hydrorefining reaction sections, two processes are generally used: one involves single-phase heat exchange with oil mixing after the furnace, while the other involves mixed-phase heat exchange with hydrogen mixing before the furnace. The advantage of the post-furnace oil mixing process is that only hydrogen flows through the heating furnace, so there is no risk of coking in the furnace tubes; however, it has the following disadvantages: a) Oil and hydrogen exchange heat separately with the reaction products, resulting in a low heat transfer coefficient ； The heat exchange process is complex, and the temperature difference for heat transfer in the heat exchangers is small, which results in a large heat transfer area being required for high-pressure heat exchangers and higher investment costs. b) Only single-phase hydrogen flows through the mixing furnace tubes behind the furnace; the temperature at the outlet of these media is high, the surface temperature of the furnace tubes is also high, and accordingly the temperature of the flue gas is high. c) Since only hydrogen flows through the furnace tubes, a failure of the circulating hydrogen compressor can result in a \"flow interruption\" in the heating furnace. Hydrogen mixing in front of the furnace not only eliminates the disadvantages of oil mixing behind the furnace but also simplifies the process. The two-phase heat exchange of the hydrogen mixing materials results in a high heat transfer coefficient, which reduces the required heat exchange area and thus saves on investment costs.

Currently, newly built large-scale hydrocracking units almost all employ post-furnace hydrogen mixing, such as those at Hainan Refining & Chemical, Qingdao Petrochemical, CNOOC Huizhou Petrochemical, CNPC Qinzhou Petrochemical, and Dushanzi Petrochemical. The hydrogen mixing process in front of the furnace must address the distribution of materials within the heating furnace and prevent coking of the furnace tubes during design. The advantages of mixing hydrogen in front of the furnace are a simple heat exchange process and heater design, a high heat transfer coefficient, a small heat exchange area, and the difficulty of interrupting the flow in the heating furnace in case of an accident. The key to hydrogen mixing behind the furnace is to have a sufficient hydrogen circulation rate (hydrogen-to-oil ratio) to carry away heat without causing the hydrogen heater temperature to rise too high. Generally, the hydrogen-to-oil ratio in hydrocracking is greater than 800, so the amount of recycled hydrogen is sufficient to meet the requirements. The advantages of mixing hydrogen behind the furnace are as follows: 1. The hydrogen is relatively pure and does not cause coking, which allows for an **increase in the wall temperature of the heating furnace tubes, resulting in a smaller size of the furnace and thus savings in steel usage ； 2. The hydrogen distribution is relatively even; in heating furnaces 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 ； 3. The heating furnace is easy to design; for some heat exchangers, the material used can be reduced as appropriate to save on costs.

What was said upstairs is incorrect, right? Hainan Refining and Chemical uses hydrogen mixing in front of the furnace, Qingdao Refining and Chemical uses hydrogen mixing both in front of and behind the furnace, while Huizhou Oil Refinery employs a technology for mixing oil behind the furnace.

Each has its advantages and disadvantages; mixing hydrogen in front of the furnace offers more benefits than drawbacks for residue hydrogenation； For hydrocracking, post-furnace hydrogen mixing is more suitable.

To be precise, hydrogen mixing before the furnace and oil mixing after the furnace. Hydrogen mixing before the furnace: hydrogen is fed into the heating furnace together with the feed oil ； Mixing of oils after the furnace: only hydrogen is heated, and then it is fed into the reactor along with the feed oil.