How can we reduce the excess hydrogen content in a hydrotreater off gas stream (off gas from reactor effluent separator/flash drum, stripper off gas)?
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26/01/2017
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Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
The hydrogen content in the stripper off gas is a function of the flash in the separator that feeds the stripper, and that flash is a function of the composition, temperature and pressure of the feed to that drum. Changes in any of those three variables are not made to change the composition of the stripper off gas.
The flow rate of the separator off gas is adjusted to maintain a reasonable composition of the recycle hydrogen and is normally minimized to minimize (optimize) cost.
In other words, your question is like the tail wagging the dog.
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25/01/2017
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Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
If you decrease the purge gas rate (or reduce hydrogen make up depneding on how your controls are set up), it will allow other light hydrocarbons to build up in the system and thus lower the hydrogen concentration in the off gas. This reduces overall hydrogen usage but the other consequence is that hydrogen partial pressure is reduced, which can have negative implications on reactor performance and run length. Many refineries put hydrogen recovery systems like PSA's or cascading hydrotreater purge gas streams from high to low pressure units to allow maximum hydrogen partial pressure in critical units while minimizing losses of H2 to the fuel system.
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24/01/2017
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Morgan Rodwell, Fluor Canada Limited, morgan.rodwell@fluor.com
Unsure of what type of hydroprocessing unit you are dealing with. Assuming this is a high pressure unit (diesel, VGO or resid unit), and the losses are from the hydrogen dissolved in the product oil coming from a high pressure separator to a low pressure separator/stripper column.
There is little operationally you can do, because the solubility of H2 in the oil at HP separator conditions is fixed by thermodynamics. However, a revamp option that can be effective is to add a MP separator between the HP and LP separators. Simulation can help identify the ideal P/T conditions to maximize hydrogen recovery. This stream can be routed to a lower pressure naphtha hydrotreater as makeup, or to a PSA to purify the hydrogen before routing make to a makeup compressor.
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