Nov-2023

Recycling PVC production byproducts can unlock financial and environmental savings (ERTC 2023)

It is no surprise that polyvinyl chloride (PVC) has near universal appeal, with a staggering 34 million tons produced annually.

Stefan Roeder
Evonik Catalysts

Article Summary

It is both incredibly versatile and cost-effective to produce, hence its wide use across industries, including in building and construction, and for packaging, vehicle parts, medical devices, and even electronics.

However, while PVC has many applications, it has not always been viewed favourably in terms of its environmental compatibility because the byproducts of PVC production can be both toxic and costly to remove and discard.

The good news is that a more economically and environmentally friendly approach to PVC production is possible, thanks to hydrogenation technology.
With hydrogenation technology, producers can prevent approximately 860 tons of toxic chlorinated byproducts in a 300 kta production of vinyl chloride monomer (VCM), the raw material for PVC production.

During ethylene dichloride (EDC) cracking to VCM and hydrochloric acid (HCl), acetylene (C₂H₂) traces are formed, which, when returned to the process in the HCI recycling stream, create lots of undesired chlorinated byproducts in the oxychlorination reactor. However, using fixed-bed catalysts for selective hydrogenation of C₂H₂ to ethylene (C₂H₄) in HCl recycle streams within the VCM process can both avoid undesired byproducts and return valuable raw material to the process.

Return on investment
With suitable hydrogenation catalysts such as Noblyst®, C₂H₂ conversion rates of more than 98% can be achieved, together with increased EDC purity and reduced fouling rates of the EDC cracking furnace. These catalysts also limit unwelcome chlorinated hydrocarbons from forming, resulting in a lower cost for separation and disposal of these byproducts.

What is more, the return-on-investment period can be as little as two years when introducing such a hydrogenation technology, as the process is circular and has an immediate effect from when catalysts are introduced, ensuring the unwanted byproducts are eliminated from the PVC production process.

A benzene chlorination catalyst can enable an additional recycle stream in the synthesis of VCM. Recycled EDC (boiling point 84°C) and benzene (81°C) are hard to split through distillation, leading to high fouling rates in the EDC cracker.

Utilising a catalyst such as Aerolyst® BC281 in a trickle bed operation for benzene chlorination generates monochlorobenzene, which boils at 123°C and so can be easily split from EDC. This process can complement the waste reduction by the hydrogenation catalyst, which this article will go on to explore.

Robust processes
Numerous empirical studies have demonstrated that palladium (Pd) is the most suitable precious metal for both activity and selectivity when choosing the most active component in the hydrogenation of C₂H₂. Compared to other precious metals, it has the major benefit of being able to adsorb large amounts of hydrogen. A suitable support material for the active Pd must be selected carefully, with a very non-porous acid resistant SiO₂ granulate with low BET surface area the most promising option.

For the past 40 years, Evonik has been producing hydrogenation catalysts for acetylene-to-ethylene within the HCl recycle stream in VCM plants and actively sells catalysts in all major geographical areas, servicing more than 50 vinyl chloride facilities at present (see Figure 1).

Evonik produces VCM hydrogenation catalysts based on proprietary knowledge. They are suitable for hydrogenation units as part of fluid bed and fixed bed VCM synthesis reactors. The hydrogenation catalysts have been successfully used with the highest performance in all existing VCM process technologies, such as Vinnolit, OxyVinyls, INEOS, Mitsui, and Solvay.
 
Catalyst Noblyst E39 series
A frontrunner in providing catalysts tailored to the hydrogenation process is Evonik’s Noblyst E39 catalysts, developed by Evonik in cooperation with Vinnolit GmbH & Co. KG and tested and used in its commercial plants (see Figure 2).

The series of palladium on silica crystal catalysts were designed specifically for the selective hydrogenation of acetylene-to-ethylene within the VCM production process, improving ethane dichloride selectivity and minimising byproduct formation in the oxychlorination step.

Implementing the hydrogenation unit and avoiding acetylene reaching the oxychlorination reactor prevents chlorinated byproduct formation. Acetylene will be chlorinated to low boiling compounds like di- or tri-chloroethane but also to high boilers like tetrachloroethane and tetrachloroethene. In addition, the polymerisation of acetylene and ethylene or acetylene and acetylene, including chlorination, can take place, causing chlorinated tar formation.

By reducing said formation, the EDC quality increased substantially. Alongside this, the EDC yield also increased by about 0.3%. In turn, the increased yield can potentially save producers using a hydrogenation reactor significant operational and capital expenditure costs.

Catalyst for cleaner production
While PVC has many applications and offers numerous benefits, it has not always enjoyed the best reputation when it comes to environmental compatibility. However, as has been set out, there are ways to produce the versatile material in a more economical, resourceful and sustainable way that not only saves money but is kinder to the environment, too.

This short article originally appeared in the 2023 ERTC Newspaper, which you can VIEW HERE