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Jan-2008

Coker naphtha hydrotreating

Highly exothermic olefins saturation and silica contamination can occur when hydrotreating coker naphtha.

Rasmus Breivik and Rasmus Egebjerg, Haldor Tops√łe
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Article Summary
In recent years, the application of residue upgrading technologies has grown in a market with increasing prices for light sweet crudes and a decreasing demand for heavy residual fuel oil. Carbon-rejection technologies such as delayed cokers have been the preferred choice, since crude quality declines with increasing metal content and Conradson carbon residue (CCR).

Installation of a new coker unit, resulting in an increase in cracked distillate, poses some additional challenges to downstream hydrotreaters. This is particularly the case for naphtha hydrotreaters, since the properties of coker naphtha are very different from those of straight-run (SR) naphtha.

Coker naphtha contaminants
Typically, coker naphtha contains 10–20 times more sulphur and higher amounts of olefins, nitrogen and silica than SR naphtha. Target levels of nitrogen in the reformer feed are around 0.1–0.5 wtppm to avoid ammonium chloride deposition. This implies that the coker naphtha hydrotreater needs to be operated at high severity to meet the required product specifications on nitrogen. However, it is practically impossible to increase operating severity with a higher temperature, since sulphur recombination takes place at high temperatures.

The origin of the silica can be traced back to the silicone oil added to the heavy residue feed to the coker. As a result of gas formation, silicone oil is added to the coker drums to suppress foaming. Excess quantities of silicone oil will crack or decompose to form modified silica gels and fragments. These are mostly distilled in the naphtha range and carried to the downstream hydrotreaters together with coker naphtha.

Unfortunately, silica poisoning of the catalyst in the downstream hydrotreaters reduces HDN activity and results in large catalyst volumes being required to ensure simultaneous removal of nitrogen and silica. The required space velocity to match the turnaround schedule of the reformer/coker unit will often be below 0.2hr-1 when using standard hydrotreating technology and catalyst.

Another important consideration for coker naphtha hydrotreating is control of the temperature increase from olefins saturation, since this reaction takes place readily and is highly exothermic. When olefin saturation is not properly controlled, this may lead to a premature shutdown, as excessive coke formation will take place due to gum formation/polymerisation in the top layer of the catalyst bed. In the future, it is expected that there will be a higher ratio of cracked components in the refinery slate.

For new grassroots units, a three-reactor layout is typical. In the first reactor, diolefins are mainly saturated at low temperature. In the second, silica is adsorbed on high-surface area catalysts known as silica guard. Simultaneously, most of the olefins are saturated, while a relatively large degree of HDS and a small degree of HDN take place. Finally, a reactor with high HDS and HDN activity catalyst ensures the sulphur and nitrogen specifications are met. Even when operating with only two reactors, these three steps must be carried out.

Controlled removal of conjugated olefins
Olefins formation is a result of high-temperature conversion reactions in the delayed coker. Upon contact with air, olefins and diolefins may form gum that complicates the transportation and processing of coker naphtha. Diolefins and, in particular, conjugated diolefins present in coker naphtha must be saturated in order to stabilise the feed. The stabilisation takes place in a separate reactor, since the required operating conditions are quite different from those of the HDS and HDN reactions. The saturation of diolefins is a fast reaction and can therefore be carried out at high LHSVs and at low temperatures. Conjugated diolefins polymerise at normal hydrotreating conditions, and the polymers cause fouling of the reactor, resulting in pressure drop build-up. The polymer formation potential of coker naphtha is about 300 times the potential for SR naphtha. A generally accepted way of controlled saturation of conjugated olefins is to use a hydrotreating catalyst operated at a low temperature (160–220ºC) in the presence of hydrogen.

With the growing market for coker technology, focus is put on the reliability of the coker unit, resulting in more coker units being designed with ISBL facilities to remove conjugated olefins. The benefit of making the removal of conjugated olefins independent of the coker naphtha hydrotreater is the possibility of sending the coker naphtha directly to storage if the coker naphtha hydrotreater offers an unforeseen shutdown. Many units are currently designed independently of the coker naphtha hydrotreater with once-through hydrogen, which facilitates the change of catalyst on-line to increase coker unit reliability.

Since the exothermic diolefin reactions produce large amounts of heat, it is necessary to operate at low temperatures using a selective catalyst to control the temperature increase. High selectivity means the catalyst should saturate diolefins only and not mono-olefins, which might lead to too high a temperature increase in the reactor. Topsøe has carried out studies of catalyst selectivity by processing a model feed containing 1, 3-hexadiene. The diolefin-to-olefin reaction selectivity was studied as a function of diolefin conversion, as shown in Figure 1a. As expected, the selectivity decreased with conversion, since the two reactions (saturation of 1,3-hexadiene to hexenes and saturation of hexenes to hexane) are consecutive.

The effect of pressure was also investigated. In Figure 1b, the selectivity is shown as a function of the reactor pressure for two catalysts. At all conditions, the hexadiene is completely converted, but the selectivity towards hexane increases with decreasing pressure. Furthermore, the two catalysts tested have different selectivity and slightly different responses to changes in pressure. This means that even though the saturation reaction is fast, there are a number of knobs that can be turned to control the reactions to ensure complete saturation of diolefins and high selectivity towards olefins are achieved. This knowledge is used to optimise the selection of catalyst and process conditions to maximise the run length.
 
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