Strategies to control sediment and coke in a hydrocracker

Fouling control techniques enabled an ebullated-bed resid hydrocracker to operate at high severity while the rate of fouling of critical areas was


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Article Summary

Global supply of good-quality crude is decreasing, while 
at the same time the 
requirement for clean and ultra-low- sulphur-containing products is increasing. Ebullated-bed resid hydrocracking processes, known as LC Finer or H-Oil, have gained increasing interest during the last decade due to their capability of producing light and middle distillates that are high in quality and low in sulphur. Major economic drivers for ebullated-bed processes are run length, maintenance cost and, most of all, conversion. Often, a compromise has to be made between operations and conversion due to fouling and the limitations/specifications of sediments in the heavy fuel oil produced from the hydrocracking process. Controlling fouling at optimal conversion can provide significant operational and performance benefit.

The limiting factor is the fouling of the fractionation section, especially the bottom stream areas, and specifically the atmospheric and vacuum column bottoms and vacuum column furnace, as observed by Putek, Sherwood and McNamara.1-3 The bottom stream heat exchanger run lengths and maintenance requirements set limits to the overall unit conversion and economic efficiency. Fouling also appears in the high- and mid-
pressure separators downstream of the ebullated-bed reactors, in the vacuum fractionator heater, and in the atmospheric and vacuum fractionators. In many cases, fouling limits unit operation.

The deposit generation mechanism from hydrocracked residues is mainly due to lost solubility of asphaltenic fragments generated by thermal cracking at reactor temperatures. These cracked asphaltenes may recombine easily to more insoluble coke precursors. When these molecules exceed their maximum solubility in the residuum matrix, they phase-separate into non-solvated particles and grow rapidly via polymerisation reactions into coke particles of sufficient size and agglomerating potential to become a fouling problem.

This mechanism is slowed and partly controlled by catalytic hydrogenation, which saturates the free radical fragments of the cracked asphaltenes — at least at the reaction section where thermal cracking reactions are faster — and catalyst is present together with hydrogen.3 These reactions enable much higher yields compared to thermal processing, such as in visbreaking or delayed coking.

Conversion via thermal cracking leads to increased asphaltene decomposition, which is moderated due to hydrogen saturation reactions that inhibit sediment formation, as observed by Bartholdy and Andersen.4 However, typically, the top 10% volume of the reactor has no catalyst present, and in that area reactions are purely thermal, leading to uncontrolled conversion of coke precursors to coke, as there is no moderating or inhibiting effect from the hydrogenation reactions that occur in the lower reactor section.

As a result, the increase in temperature necessary to increase conversion above certain limits leads to increased sediments 
and coke generation, creating 
difficult-to-control foulants that deposit in critical plant areas and/or cause sediment formation in the heavy fuel oil. The fouling tendency can be shown to increase exponentially with conversion increase (see Figure 1). Therefore, the value benefit of the conversion increase can be lost to fouling and sediments. However, this fouling-to-conversion relationship can also show that there is no major advantage in decreasing severity whenever fouling rates are acceptable or controllable. To do so would simply result in lost conversion without a corresponding value improvement in terms of sediment deposition control and run lengths. This relationship brings about the concept of optimising conversion as a function of the rate of fouling and fuel oil sediment formation.

Fouling problems, their monitoring and control
Problem areas

The sections most prone to fouling are the atmospheric column, vacuum column and preheat exchangers.  At very high conversion, the reactor and separator may also suffer from high coke generation. Extensive fouling of the separators and columns can lead to unplanned shutdowns, downtime and lost production.

The same trends for conversion severity are valid for sediment generation in the fuel oil. Below certain limits of conversion the fuel will be stable, while above certain limits the tendency to generate sediment with time cannot be controlled.

From the above considerations, it is clear how setting the proper operating conditions is important, as this enables the best trade-off between conversion maximisation and production of stable fuel and acceptable rates of fouling. Optimal plant management requires continuous control for resid product stability. The stability is related to the tendency to produce fouling deposits and generate sediment.

Optimal severity depends on the properties of the feed being processed. The feed changes whenever the refinery feed quality, residual feed make-up or the plant feed rate changes.1  Feed composition-related factors that may influence severity/conversion are the stability reserve of asphaltenes in the vacuum resid (often reported as p-value), the content of asphaltenes and the intrinsic solubility of these asphaltenes. Low stability reserve, high asphaltene content and poor solubility will all contribute to an increased tendency to generate coke and unstable  residuum product and fouling deposits.

The amount of metallic contaminants (especially sodium and iron) is another factor that can impact process performance by affecting catalyst performance and, in some cases, by increasing coking tendency, favouring dehydrogenation and conversion of coke precursors — those less soluble thermally cracked asphaltenes — into coke.

To avoid deposit generation, the LC Finer catalyst plays a major role.3 The type of catalyst utilised in the process can have a great effect. In ebullated-bed reactors, the catalyst is changed continuously to maintain catalyst activity and to remove metals from residue oil. The effect of the catalyst operations on the process and fouling can be monitored easily with sophisticated monitoring techniques described below. The effect of the catalyst- changing sequence and the catalyst’s age are directly observed with these techniques.

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