Challenges with carbon disulphide removal in petrochemical naphtha

Meeting new specifications for removing carbon disulphide in petrochemical naphtha using dividing wall column technology.

Manish Bhargava, Anju Patil and Niyaz Ahmad
DWC Innovations

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Article Summary

In an era of unprecedented competition, refineries must continuously explore new opportunities in order to sustain and grow. Apart from energy integration of upstream and downstream operations, the other option is finding ways to divert available feedstock into more marketable products. Some of the following variations in existing facilities can help in improving profit margins:
• Focusing on refining and petrochemical integration
• Exploring feedstock flexibility options
• Exploring energy integration options
• Increasing petrochemical manufacturing capacity.

Decreasing conventional transportation fuel demand drives closer integration of refining and petrochemical assets, predicating minimal production of the naphtha-to-gasoline route in favour of higher value-addition strategies and instead upgrading naphtha to ethylene and other petrochemicals like benzene, toluene, and paraxylenes.

Global demand for ethylene, the most widely used monomer in the petrochemical industry, incentivises increased petrochemical naphtha production as an important ethylene feedstock. In parallel, many countries have seen their facilities invest heavily in new cracker projects.

Due to the expected growth in petrochemical demand, facilities are working on the integration of petrochemical assets in order to improve their refining margins and ensure participation in a growing market. Petrochemical naphtha can be converted into the following compounds with varied uses, including:
• Ethylene is commonly used to make different types of films and plastics. It can be found in cleaning agents such as detergents as well as lubricants
• Benzene is used to make nylons, which are helpful in the packaging industry
• Paraxylene is the raw material in large-scale synthesis of various polymers
• Propylene is used to produce polypropylene plastics for injection moulding and fibres and for manufacturing cumene.

Shifting to petrochemicals not only requires changes to existing configurations along with challenges faced complying with strict petrochemical specifications. Crude oil refining processes cater to crude feedstock with lesser impurities compared to petrochemical naphtha, so they are designed accordingly.

Petrochemical grade naphtha is typically composed of the lighter fraction of straight-run naphtha, wherein a cause of concern is the stringent compliance of carbon disulphide (CS2) in the naphtha. It was quickly recognised that CS2 is a potent poison to catalysts used in Ziegler-Natta petrochemical processes. CS2 is also known to induce fragility and imperfections in the polymeric chain, primarily in isoprene, an intermediate to rubber production.

The lighter naphtha fractions carry most of the CS2 due to the temperature range in which the CS2 boiling point lies. Moreover, CS2 remains chemically stable in the steam cracking process. Because of these negative consequences, controlling CS2 in petrochemical grade naphtha is a cause of concern and needs to be managed very wisely.

Sources contributing to the presence of CS2 in petrochemical naphtha include:
• Crude oil reserves
• Natural gas liquids (NGLs)
• Refinery processes
• Shale fracking solvents.

There are multiple reasons for CS2 entrainment in crude, including the use of formaldehyde additives to counter extraction difficulties caused by the presence of hydrogen sulphides and heavy metals amines. These bespoke additives tend to react with crude oil and form complex compounds, which eventually crack and release CS2. The presence of CS2 is sometimes directly related to the naphtha source.

Catalyst deactivation by CS2
It has been observed that H2S has the strongest catalyst poisoning effect and CS2 has the second strongest. Reactor temperature must be increased in the presence of a few ppm of CS2 in the feed to compensate for the catalyst deactivation. With CS2 present in the feed above the defined specifications, the catalyst activity shifts from  the top part of the catalyst bed to the bottom part. This is strong enough evidence to show CS2’s harmful impact on catalyst activity when considering that the top bed deactivates at only a few parts per million of CS2.

It is evident that CS2 acts as a strong inhibitor for the palladium-based catalyst. Many pygas units operating with a nickel-based catalyst have also observed that the catalyst is poisoned in the presence of CS2. A specific catalyst treatment is required to recover activity.

It is observed that the nickel-based catalyst can tolerate CS2 contamination in the range of 10-20 ppm with mild temperature elevation as compared to palladium (Pd) catalyst. But they tend to be poisoned with further increase in the CS2 level. However, Pd-based catalyst is more active than a nickel (Ni) catalyst and recovers its activity more easily without any specific treatment once feed conditions come back to their initial level (with CS2 in the specified range).

Moreover, Ni catalyst’s residual activity is minimised with CS2 poisoning, and it is impossible to recover the initial activity when operating with feed containing a high amount of CS2. Thus, while keeping up with growing petrochemical demand, technocrats face greater technological challenges because of the stricter specifications of petrochemical derivatives in comparison with transportation fuels.

Processes for producing petrochemical naphtha
When facilities were finding ways to diversify in view of the declining fuel market, petrochemical naphtha (PCN) production emerged as a lucrative option. PCN is light naphtha without CS2. Considering that it tends to concentrate in the light fraction of naphtha, it must be treated for CS2 removal.

Mainly seen in the world’s refineries, the naphtha splitter separates full-range naphtha into heavy naphtha which is routed to the CCR unit, while the light naphtha, primarily used as fuel, is mainly upgraded to a higher RON through the process of isomerisation. Chemically, light naphtha is the fraction which boils between 30°C and 90°C and consists of molecules with 5-6 carbon atoms, while the heavy naphtha boils between 90°C and 200°C and consists of molecules with 6-12 carbon atoms.

Petrochemical naphtha typically has an IBP of 40°C and an FBP of 130°C. With light naphtha being converted to PCN by processes that remove CS2, it caters to the making of ethylene. Steam cracking PCN converts to ethylene, the raw material for most plastics. Other ethylene feedstocks include ethane (C2H6) and propane (C3H8). Table 1 shows typical petrochemical grade naphtha specifications where the CS2 specification is as low as 1-2 ppm.

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