Hydrogen is required for various refinery processes such as Hydrocracking, DHT, NHT, etc.
In a real plant, there is usually a hydrogen rich gas header, and the pressure of the header is maintained. As consumers draw more or less hydrogen, the pressure in the header is altered. The Hydrogen Plant which supplies Hydrogen, then adjusts the supply so that the pressure in the header is maintained.
In process simulators, generally, we do not account for pressure balance in steady state simulations. We have to therefore have a different method of calculating the makeup hydrogen in hydro-processing flowsheet simulations.
The amount of hydrogen consumed during the reaction is the difference between the hydrogen supplied to the reactor in the feed and the hydrogen in the product streams (Hydrogen dissolved in the liquid stream and Hydrogen escaping in the gas stream).
The net make up hydrogen gas cannot be determined in a sequential modular simulation environment before calculating the amount of Hydrogen leaving with the products stream. However the amount of makeup H2 is required to initiate the calculations. Hence, a recycle operation is introduced to set the flow of makeup hydrogen. The recycle will assist the simulation model in ensuring the consumption of the hydrogen in the reactor and the hydrogen leaving with the product streams is equal to the makeup hydrogen amount. In absence if this, we will have a situation where the gas recycle will either be continuously depleting or accumulating, leading to erroneous results.