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If we can increase hydrogen partial pressure in our hydrocracker, what will be the gain in conversion and how do we achieve this?

Jan-2022

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Jake Gotham, InSite Technical Services, jake.gotham@insitetechnical.com

I’ll answer your second question first.

Without wishing to state the obvious, the partial pressure is determined by the operating pressure and recycle gas purity:
1. Operating pressure. The operating pressure is set to keep a safe margin below the design pressure in all equipment in the high pressure loop.

- The design pressure is obviously fixed in the original design, though sometimes there is scope to rerate equipment to take advantage of increases in allowable stress in more recent revisions of design codes. I’m neither a mechanical engineer nor an expert in this particular aspect, but I believe this is possible in some jurisdictions, but not in others, and it will also depend on the records kept of the original material, fabrication and inspection. Even if it is possible, it will be a significant undertaking and will only gain a few percent on the design pressure.

- The margin between the operating pressure and the design pressure is largely set by the choice of relief valve type. If you have a conventional relief valve, you may be able to increase operating pressure if you switch to a pilot operated valve, but keeping the separator below design pressure at settle-out (when the recycle compressor trips, the separator pressure increases as the higher pressure in the reactor and furnace ‘settles out’ into the effluent exchangers and separator) reduces the potential advantage.

- Another factor in the margin between operating and design pressure is the assumptions made about the pressure drop through the reactor and exchangers. It may be possible to revisit these assumptions with the benefit of several operating cycles of experience, but the management of change would need to consider what happens in the future if more fouling did occur.

2. Purity of the recycle gas.

- The biggest influence on the recycle gas purity is the makeup gas purity. If you have a PSA on the hydrogen plant, your makeup purity will be 99.999% pure. You won’t get much better than that. But if you have an old hydrogen plant without a PSA, or a large amount of your makeup comes from a platformer, the purity will be lower. I have never seen a cost/benefit analysis for retrofitting a PSA to an old hydrogen plant, but it would be interesting to see if it can justify itself in terms of increased hydrocracker cycle length and performance. If you are using platformer hydrogen, can the refinery hydrogen balance accept routing it elsewhere and using only PSA hydrogen in the hydrocracker? Is there spare capacity in the PSA to route the platformer hydrogen through the PSA?

- If the unit doesn’t have a recycle gas scrubber, retrofitting one will improve purity and partial pressure.

- A smaller influence on recycle gas purity is the cold high pressure separator temperature. Lower temperature improves the purity of the recycle gas.

- Monitor recycle gas composition with GC analysis at least weekly. A build-up of methane should be investigated and purged from the system.

- As Eric has already mentioned, increased recycle gas rate will also improve purity, but the scope is unlikely to be great.

The summary of the above is that is only usually possible to make small improvements in partial pressure. Such small changes are unlikely to have a measurable effect on conversion. Partial pressure more strongly influences catalyst deactivation rate, so you might still gain some benefit in terms of cycle length and maximising partial pressure is always a worthwhile objective.

Feb-2022
Eric Streit, KBC (A Yokogawa Company), eric.streit@kbc.global

Increasing hydrogen partial pressure in a hydrocracker will increase conversion for the same reactor temperatures (and thus approximately the same cycle length). Alternatively, increasing hydrogen partial pressure at constant conversion requires lower reactor temperatures and thus increases the cycle length of the catalyst. The higher partial pressure favours the desired reactions (cracking) and supresses the undesired reactions (coking).

There are a number of ways to increase partial pressure. The most obvious is to increase the hydrogen recycle rate. However, for many units this is not possible. Also, increasing the hydrogen recycle rate increases energy consumption and has diminishing returns, so it is not always the right answer. Lowering separator temperature and/or improving amine scrubber performance to improve recycle gas purity are other good ways to increase the hydrogen partial pressure at the same recycle gas rate. Increasing system pressure will also increase hydrogen partial pressure, as will lowering throughput.

Developing a good kinetic model of the unit can help to determine the optimum operating conditions, balancing product yields, cycle length, and energy consumption. The kinetic model will also help with quantifying the net effect of increasing hydrogen partial pressure. It is impossible to say how much conversion could be gained by increasing hydrogen partial pressure without a thorough examination of the system. The increase is going to be highly dependent on the current recycle rate and purity, feed type, liquid hourly space velocity, current cycle length and other factors, and can only be evaluated through unit modelling and/or step tests.

Jan-2022