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Dealing with hydrogenation catalyst poisons

For stable operation of a pyrolysis gasoline unit, it is crucial to know the effects of poisons and inhibitors on catalyst performance and how to counteract them.

ANKE DERKING, Shell Global Solutions International
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Article Summary
The hydrogenation of so-called pyrolysis gasoline is an important part of most liquid feed steam cracker operations. Feeding a liquid cracker with a naphtha or hydrowax diet, the share of pyrolysis gasoline can be as 
high as 20% of the cracker product mix.1

The purpose of hydrogenation of pyrolysis gasoline is to eliminate highly reactive species deemed to polymerise and potentially cause fouling and plugging, such as alkynes, diolefins, styrene and olefin compounds, as well as to remove sulphur, while minimising losses 
of aromatics.

This is commonly achieved by hydrogenation in two subsequent stages. The first stage is a trickle phase operation employing palladium or sulphided nickel catalyst to remove alkynes as well as diolefinic and styrenic species. The second stage is a vapour phase operation employing sulphided nickel molybdenum (NiMo) and cobalt molybdenum (CoMo) catalysts to remove mainly olefins and convert sulphur compounds to hydrogen sulphide.

Targeting reliable and stable operation, the performance of the catalyst is essential. Knowing the feed composition, operating parameters, and start of run catalyst activity, one can model and predict the catalyst’s performance, inclusive of cycle length, for given end of run criteria. This includes adjustments of the model for known feed poisons and inhibitors. If catalyst activity is permanently reduced by a feed contaminant, one classifies such a contaminant as a poison. If catalyst activity is restored after contact with the specific feed contaminant has been stopped, one classifies such a contaminant as a temporary poison or inhibitor.

However, a great advantage is the flexibility to use a broad range of cracker feedstocks. Inherent with the benefit of such flexibility, the composition of the feed and the catalyst poisons and inhibitors in the feed change by nature and quantity.

For stable operation of a pyrolysis gasoline unit, it is of high value to know the effects that various poisons and inhibitors have on the performance of the catalyst as well as to know if and what measures can be taken to counteract and to restore the catalyst’s performance.

Feed contaminants
While there are numerous literature sources available on catalyst poisons and inhibitors,2,3,4 we focus in this study on a few, which are sulphur (as CS2), nitrogen (as ethylamine and pyridine), silicon (as hexamethyldisiloxane) and lead (as tetraethyl lead).

Most feed sulphur contaminants originate from cracker feeds where they are commonly found in the range 50-250 ppm. However, some sulphur also originates in intentionally added components such as dimethyl disulphide, which is deemed to convert to hydrogen sulphide (H2S) in the radiant coils and to passivate the nickel sites on the tube surface and reduce coking rates (see Figure 1).1
While mercaptans and organic disulphides are mainly cracked to H2S in the radiant coils and later washed out in the caustic column, thiophenic, benzothiophenic, and other complex sulphur compounds are reasonably stable and end up in the pyrolysis gasoline fraction. The same happens to carbon disulphide (CS2), which mostly originates from the cracker feed passing the radiant section without conversion.

The possibility of CS2 formation in the radiant section has to be considered as well. Up to the 1950s, CS2 was indeed produced from the elements at about 800°C.4 However, the partial pressures of sulphur and carbon in the radiant section are mostly regarded as too low and thus formation of CS2 can only contribute marginally to the overall balance. It has been observed that in many imported feeds the level of CS2 has recently increased remarkably, causing trouble in the downstream pyrolysis gasoline hydrogenation reactors.5

Nitrogen containing contaminants also originate from the cracker feed and from extraction solvents such as N-formylmorpholine, dimethylformamide or N-methyl-2-pyrrolidone. Amines are cracked mostly to ammonia and hydrogen cyanide, while cyclic structures such as pyridine and pyrrole are stable and make their way into the pyrolysis gasoline.6

Silicon is of some interest as additives containing silicon are frequently added in the quench tower to reduce foaming and support water organic separation. As a matter of practice, some of the silicon additives arrive with the pyrolysis gasoline feed. Potentially, silicon can accumulate on the catalyst surface and form glass structures, for instance under conditions in which the catalyst is regenerated, which deactivates the catalyst significantly or totally.7

Lead is a metal less frequently found than nickel, vanadium or mercury, but it is commonly found in liquid feedstocks and mostly as an organolead component.8

Experimental set-up
A bench scale testing unit was employed to determine the impact of various contaminants on catalyst activity. The unit was fully automated and utilised a process control system. Liquid samples were taken with an automatic sampler. The testing unit consisted of two independent reactor systems, allowing for two tests to be performed in parallel. Two separate feed tanks allowed for easy switching between straight pygas and pygas spiked with the desired catalyst poison.

The adjusted testing conditions mimic the typical conditions of a first stage pygas unit, but no recycle flow of hydrogenated product was applied. The weighted average bed temperature, adjusted to achieve a defined start of run activity of fresh catalyst, was kept unchanged after the switch to ‘poisoned’ feed. At the end of each run, initial conditions were adjusted to monitor the loss of activity (point of return). Hot hydrogen strip was performed to evaluate the possibility of regeneration of the catalyst after exposure to contaminants.

The catalyst used for these experiments was a CRI first stage nickel catalyst. The shape of the catalyst was a 2.5mm trilobe extrudate. The catalyst was loaded in the pre-
sulphided form and in situ reduction was performed at 750°F (400°C) to make it ready for use. To ensure proper radial distribution and to minimise wall effects, the catalyst was diluted with an inert material. A fresh catalyst sample was used for every test. The results of the poisoning experiments are benchmarked with a reference test of fresh catalyst without poisons.

The main criterion to evaluate the catalyst’s performance was the change of hydrogenation activity of styrene. Liquid samples were analysed to determine the content of slipped contaminant. The spent catalyst samples were tested for accumulation of poisons.

The health and safety risks of the planned poisons were assessed prior to the beginning of the experiments. A risk assessment was conducted for every poison.
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