The cycle time, and, in particular, the duration of the quenching phase, is known to play an important role in the rate of damage accumulation in a coke drum. A shorter cycle is desirable because it allows for increased production but this comes at the cost of more frequent repairs and a shorter overall life. Therefore, a trade-off decision needs to be made in order to balance these two effects and maximize profitability.
This paper provides guidance for making these sorts of decisions by looking at several different coke drum operating strategies from the perspective of a high level financial analysis, taking into account many cost/benefit factors in order to determine the optimal strategy that maximizes profitability with various levels of confidence. It is shown that the most critical factor is predicting the rate of repairs as the drum ages. This depends on many factors and can only be determined probabilistically due to the many uncertainties involved. One of the most important factors driving the repair rate is the frequency at which fatigue cracks initiate and grow. A probabilistic fatigue model is used to describe this, taking into account the large surface area of the coke drum, which provides more potential nucleation sites for fatigue crack initiation.
It is shown that there is, indeed, an optimal, intermediate cycle time for certain conditions. For other situations running with the shortest possible cycle time is shown to be the best choice.