Question
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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
Answers
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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
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