What is the difference between pre-heater hydrogen mixing and post-heater hydrogen mixing?

What was said on the 2nd floor isn’t quite correct – how can mixing hydrogen in front of the furnace reduce coking? The hydrogen-rich oil used after the furnace doesn’t enter the furnace at all. The biggest challenge with mixing hydrogen in front of the furnace is the issue of gas-liquid two-phase flow distribution within the furnace tubes of large-capacity units; the greater the load on the unit, the more pronounced this problem becomes. Therefore, thorough calculation is required when designing hydrogen mixing in front of the furnace to achieve optimal results; generally, reaction heating furnaces use a horizontal-tube double-sided radiation furnace design. It has the following characteristics: ① High operational flexibility, as horizontal pipes are more likely to achieve annular or mist flow patterns compared to vertical pipes, making the heating furnace better suited for operation under various working conditions ； ②The pressure drop is low; since the average heat intensity of two-sided radiation in horizontal pipes is 1.5 times that of one-sided radiation, the horizontal length of the furnace tubes is only 0.66 times that in the case of one-sided radiation. Therefore, when the flow velocity inside the tubes is the same, the pressure drop is only 66% of that in the case of one-sided radiation ； ③The equipment investment is low; the furnace tubes of the hydrogenation reaction feed heating furnace are made of TP321 or TP347 material. These furnace tubes account for over 40% of the total investment in the furnace. Therefore, reducing the length of the furnace tubes results in less weight and lower costs. The hydrogen mixing process in front of the furnace must address the design of material flow distribution within the heating furnace and the prevention of coking in the furnace tubes during its design. The advantages of hybrid hydrogen mixing 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 temperature at the outlet 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; therefore, it is possible to **increase the wall temperature of the heating furnace tubes, thereby 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, even 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.

Hydrogen mixing in front of the furnace yields **more than** that behind the furnace

4th floor: Your view is incorrect: 1. Currently, hydrogen mixing in front of the furnace is widely used in hydrogenation units. 2. Post-furnace hydrogen mixing, with the feed oil not entering the heating furnace, is only possible when heat exchange meets the required standards; adjusting the temperature of the feed mixture relies on heat exchange sources, which poses significant design challenges.

Post-furnace hydrogen mixing is actually called post-furnace oil mixing

The hydrogen feed heater is a key piece of equipment in the plant; the process medium flowing through its tubes is high-temperature, high-pressure hydrogen along with oil and gas mixtures, resulting in extremely harsh operating conditions. Hydrogen mixing in the hydrocracking reactor heating furnace is divided into pre-furnace hydrogen mixing and post-furnace hydrogen mixing, and each of these two methods has its own characteristics. In a furnace with pure liquid-phase flow, the selection of flow velocity is primarily based on reducing pressure drop and preventing coking. The high flow rate can lead to a turbulent state, reducing the temperature difference in the oil film and preventing localized overheating ； It also serves a scouring function, enabling the coke layer to fall off more quickly, which is beneficial for preventing coking. However, the pressure drop is proportional to the square of the flow rate; excessive flow rates not only increase the electrical power consumption of the pump but may also raise the pressure requirements for the equipment and fittings upstream of the heating furnace, thereby increasing the initial investment. Conversely, by reducing the flow rate, especially to a level where laminar flow is achieved, coking becomes almost inevitable. Furthermore, when there are two or more flow paths, low flow rates can easily cause deflection. Once deviation occurs, a vicious cycle of coking – increased resistance – and further deviation can quickly lead to the burnout of the furnace tube. For reheating furnaces with in-furnace hydrogen mixing, when selecting the flow velocity in the gas-liquid two-phase flow furnace tube, the flow pattern should also be taken into consideration. The flow patterns of two-phase flow are generally classified into wavy flow, foam flow, long-foam flow, liquid choking flow, annular mist flow, and mist flow. The flow pattern depends mainly on the flow velocity, in addition to the components of the gas-liquid phases and their physical properties. To avoid coking, the flow rate (mixing flow rate) must be high enough to ensure the formation of a annular mist flow and a misty flow. Given the high tendency for coking of oils in the heating furnace due to hydrogen mixing in front of the furnace, the design should ensure that a circular mist flow can still be achieved at a processing capacity of 70%. Of course, in practical operation, if the processing volume is even lower, measures such as increasing the hydrogen-to-oil ratio can be taken to ensure a high flow rate. The biggest challenge in hydrogen mixing in front of the furnace is the issue of gas-liquid two-phase flow distribution within the furnace tubes of high-capacity units; the greater the load on the unit, the more pronounced this problem becomes. Therefore, thorough calculation is required when designing hydrogen mixing in front of the furnace to achieve optimal results; generally, reaction heating furnaces use a horizontal-tube double-sided radiation furnace design. It has the following characteristics: ① High operational flexibility, as horizontal pipes are more likely to achieve annular or mist flow patterns compared to vertical pipes, making the heating furnace better suited for operation under various working conditions ； ②The pressure drop is low; since the average heat intensity of two-sided radiation in horizontal pipes is 1.5 times that of one-sided radiation, the horizontal length of the furnace tubes is only 0.66 times that in the case of one-sided radiation. Therefore, when the flow velocity inside the tubes is the same, the pressure drop is only 66% of that in the case of one-sided radiation ； ③The equipment investment is low; the furnace tubes of the hydrogenation reaction feed heating furnace are made of TP321 or TP347 material. These furnace tubes account for over 40% of the total investment in the furnace. Therefore, reducing the length of the furnace tubes results in less weight and lower costs. The hydrogen mixing process in front of the furnace must address the design of material flow distribution within the heating furnace and the prevention of coking in the furnace tubes during its design. The advantages of hybrid hydrogen mixing 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 temperature at the outlet 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; therefore, it is possible to **increase the wall temperature of the heating furnace tubes, thereby 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, even 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.