Maldistribution of flow is a serious problem in hydrocracking, hydrotreating, and many other commercial processes. In hydroprocessing units, including those which rely on fixed-bed and ebullating bed catalysts, maldistribution diminishes catalyst utilization and can cause dangerous temperature excursions. Refiners have increased their ability to identify maldistribution by increasing thermocouple coverage. They calculate so-called radial temperature distribution – the difference between the highest and lowest thermocouple reading in a horizontal plane – and use it as an indication of flow maldistribution. A high radial dT is a symptom of maldistribution, but it does not show how bad the problem really is. Step-test methodology goes further. It identifies possible blockage of flow. It entails dropping reactor-bed temperatures, usually by increasing quench flow, and then measuring dead time, time-to-steady state, and gain. It is applicable both to bed temperatures and reactor skin temperatures. Periodic step tests measure whether or not a problem is growing over time. In this presentation, two examples are discussed. The first case is hypothetical. It illustrates the concept. The second is a real-world case in which step testing was conducted after a serious temperature excursion to define a procedure for moving forward safely.

Step tests to diagnose maldistribution of flow in hydroprocessing units

ACS Fall 2020 Virtual Meeting

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