PTQ Q3 2022 Issue

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

I n 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 mar - ketable products. Some of the following variations in exist - ing 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 pet - rochemicals like benzene, toluene, and paraxylenes. Global demand for ethylene, the most widely used mono - mer in the petrochemical industry, incentivises increased petrochemical naphtha production as an important ethyl - ene 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 mo ulding and fibres and for manufacturing cumene. Challenges Shifting to petrochemicals not only requires changes to existing configurations along with challenges faced com - plying 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 petro - chemical processes. CS2 is also known to induce fragility and imperfections in the polymeric chain, primarily in iso - prene, 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 naph - tha 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 addi - tives tend to react with crude oil and form complex com - pounds, which eventually crack and release CS2. The pres - ence of CS2 is sometimes directly related to the naphtha source. Catalyst deactivation by CS 2 I t 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 deac - tivation. 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

PTQ Q3 2022