Why is the pressure in the hydrogenation system the same as that of the cold high-boiling fraction?

Because at that time, the pressure at the inlet of the recycle hydrogen compressor was essentially the lowest pressure in the entire high-pressure system, so this was used as the standard :)

For a set of operating units, it is feasible to control both the reactor inlet pressure and the high-pressure side pressure, as the pressure drop in the reaction system is related to the processing capacity and scaling; with a constant processing capacity, the increase in the reaction system’s pressure drop is very slow. The newly built hydrogenation unit is equipped with a selector to control which one to use. Typically, reactors do not have pressure control; instead, only pressure drop is controlled, with high fraction being the control target. The main reasons for this are as follows: 1. Hydrogenation reactions all include a hydrogen off-gas discharge system, and the discharge point is located in the liquid separation tank at the inlet of the recycle hydrogen compressor. Controlling this pressure facilitates more stable operation of the reaction system. 2. As the pressure drop in the reaction system increases, if the reactor inlet pressure is used as the control parameter, it needs to be adjusted continuously. 3. Maintaining stable inlet pressure of the circulating hydrogen compressor facilitates the operation of the unit.

The cold high-pressure is the pressure at the inlet of the cycle hydrogen compressor; this pressure is easy to control, which facilitates stable operation of the reaction system.

The system pressure in our hydrocracking unit is determined by the pressure in the circulating hydrogen liquid separation tank; in other words, there is another tank after the cold high-pressure separator. If the cold high-pressure separator fed directly into the circulating hydrogen compressor, the safety factor would be reduced.