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How to confirm the run away reactions in Diesel and Vaccum Gas Oil hydrotreater units reactors? Is it localized phenomena at particular location or gets spread on wide area of the catalyst?

As the 1st bed hydrotreater bed exotherm for our unit already stays at above 40degC during normal operation, I would like to understand how to confirm the run away phenomenon and take necessary actions like actuation of EDPS. Kindly guide.

Jun-2023

Answers

Nasir Hussain, PARCO OIl refinery Pakistan, nasir.mughal3010@gmail.com

Temperature excursion or temperature runaway is the abnormal process condition at which catalyst bed temperatures go high beyond the normal operating limit on account of uncontrolled hydrotreating reactions. During a temperature runaway condition, highly exothermic cracking reactions may take place. Temperature runaway is a very serious, unsafe situation in hydroprocessing units since extremely high temperatures can be generated within a short period of time. Consequently, equipment may exceed the design temperature and can cause catastrophic damage to the catalyst and reactor.

1. The low-flow areas in the reactor due to the maldistribution of flow streams can exhibit very unstable temperature control and channelling in the catalyst bed. This can be seen by a temperature hot spot in a catalyst bed and in extreme cases generating a local temperature runaway which can go well over 500 oC. This situation may arise on account of feed flow rate below turndown, reactor failures, and improper catalyst loading. Because of the potential problems due to the maldistribution of flow in the catalyst bed, it is extremely important that the catalyst be loaded properly with a dense loading.

2. Loss of quench gas will result in abrupt temperature increment across the catalyst bed. Quench hydrogen gas in hydroprocessing reactor is utilized to control the temperature of the next catalyst bed by mixing the previous bed effluent with fresh gas and dispersing evenly over the top of the catalyst bed.

3. A significant reduction in space velocity over the catalyst is discussed in point 1.

4. A fresh sulphided catalyst is highly active and it is the most critical time when very special care should be taken to control the reactor temperatures. Any careless action by the Boardman may result in temperature upset across the reactor.

5. Uncontrolled firing at the furnace will rise the reactor feed temperature and will cause thermal excursion in the reactor.

6. An unexpected drop in liquid or gas feed rate will increase the temperature of reactor feed because both feeds were also working as heat sink in the hydrotreating reaction environment.

7. Sudden change in feed quality. A high concentration of Olefins or a high flow rate of cracked unsaturated feedstock will result in high reaction rates and high heat generation across the hydrotreating reactor.

8. The presence of higher concentrations of CO and CO2 in the reactor feed may result in thermal excursion due to highly exothermic methanation reactions. It should not be built up in the reactor environment. Further, their presence will also affect the reactions.

For hydroprocessing reactions triangle requires hydrogen, hydrocarbon feed, and catalyst. Breaking the link in the triangle by stopping any of the components will stop the hydrotreating reactions. As it is not possible to remove the catalyst, so it should be removing either hydrogen or hydrocarbon immediately to stop the temperature excursion situation. The quickest way to stop the runaways is to remove the feed from the unit and to cool the catalyst with recycle gas.

Sep-2023
Marcio Wagner da Silva, Petrobras, marciows@petrobras.com.br

This is one of the key points during the operation of hydrotreating units, especially those processing chemically unstable feeds like VGO or delayed coking gas oils and can reach a great part of the catalytic bed. The temperature runaway in hydrotreating reactors is a phenomena where the catalytic bed presents a sudden and uncontrolled temperature overshooting produced by exothermic reactions which can be provoked by the reasons below:

1 - Sudden flow rated reduction of the feed: This can led to a hot points in the catalytic bed and, in extreme cases, damage the catalyst due to sintering of the active phase;

2 - Inefficient control of the fired heater: The fired heater control needs to respond adequately in case of overheating of the reacting section;

3 - Change in the feed composition: A significant raise in the composition of chemical unstable compounds of the hydrotreating unit can lead to a temperature raising of the catalytic bed once the rate of highly exothermic reactions raises significantly. It's necessary to keep the feed composition as stable as possible;

4 - Failure or deficient quench gas flow: The quench injection is responsible for keep under control the temperature of the catalytic bed as well as supply additional hydrogen to the hydrotreating reactions, for this reason it's necessary to ensure that this system is well designed and operated as the design requirements aiming to minimize hot points in the catalytic bed which can led to temperature runaway of the reactors;

5 - Sudden Change in the Capacity of Recycle Compressor: This can led to a drastic reduction in the quench flow rate to the reactors and produce hot points in the catalytic bed, the change in the capacity needs to carried out in a smooth way to avoid sudden variations in gas flow rate through the catalytic bed;

6 - Methanation Reactions: This phenomena is a concern especially in processing units operating under high severity (hydrocracking units, for example) and is related with the dragging of CO2 and CO to the reactors which combined with the operating conditions (temperature and pressure) can favour methanation reactions which are highly exothermic and will produce temperature runaway in the catalytic bed;

The main characteristic of the temperature runaway of the catalytic bed is a sudden and abnormal raise in the reactor temperature. For this reason an adequate temperature monitoring of the catalytic beds is fundamental to identify the temperature runaway and allow mitigation actions in an adequate moment. One of the main side effect of the temperature runaway is the significant raise of coke laydown rate and the sintering of active phase of the catalyst which can produce a raise in the pressure drop in the reaction section, this can indicate that you have a problem with temperature runaway in the hydroprocessing unit. In summary, check if your processing unit is presenting uniform temperature distribution through the catalytic bed and if the pressure drop is raising under abnormal rate and if you is facing with some of the 6 reasons of temperature runaway above.

Jun-2023