ERTC 2023
Recycling PVC production byproducts can unlock financial and environmental savings
Stefan Roeder Evonik Catalysts
They are suitable for hydrogenation units as part of fluid bed and fixed bed VCM syn- thesis reactors. The hydrogenation cata- lysts 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 commer- cial plants (see Figure 2 ).
It is no surprise that polyvinyl chloride (PVC) has near universal appeal, with a staggering 34 million tons produced annu- ally. It is both incredibly versatile and cost-effective to produce, hence its wide use across industries, including in build- ing and construction, and for packaging, vehicle parts, medical devices, and even electronics. However, while PVC has many appli- cations, it has not always been viewed favourably in terms of its environmen- tal compatibility because the byproducts of PVC production can be both toxic and costly to remove and discard. The good news is that a more economi- cally and environmentally friendly approach to PVC production is possible, thanks to hydrogenation technology. With hydrogenation technology, pro- ducers can prevent approximately 860 tons of toxic chlorinated byproducts in a 300 kta production of vinyl chloride mon- omer (VCM), the raw material for PVC production. During ethylene dichloride (EDC) cracking to VCM and hydrochloric acid (HCl), acety- lene (C₂H₂) traces are formed, which, when returned to the process in the HCI recy- cling stream, create lots of undesired chlo- rinated byproducts in the oxychlorination reactor. However, using fixed-bed catalysts for selective hydrogenation of C₂H₂ to eth- ylene (C₂H₄) in HCl recycle streams within the VCM process can both avoid undesired byproducts and return valuable raw mate- rial 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 technol- ogy, as the process is circular and has an immediate effect from when catalysts are introduced, ensuring the unwanted byprod- ucts are eliminated from the PVC produc- tion process. A benzene chlorination catalyst can ena- ble an additional recycle stream in the syn- thesis 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 ben- zene chlorination generates monochlo- robenzene, 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 arti- cle will go on to explore.
Chlorinated hydrocarbans are avoided and ethylene is returned as raw material
A
Selective actylene hydrogenation
Oxychlorination
H
Air
HCl (+ C H )
HCl (+ C H )
C H
EDC purication
EDC pyrolysis
VCM separation
Ethylene chlorination
VCM
Cl
EDC (+ C H )
Standard VCM process steps Recycle stream polishing
EDC (+ C H Cl)
Benzene chlorination
Cl
B Easy separation as monochlorobenzene in existing purication step
Aerolyst® BC 281
Figure 1 Evonik’s catalysts for the VCM process
Illustrative calculation for 300 kta VCM unit
with (%) 0.002 0.067 0.008 0.002
Hydrogenation unit
without (%)
Did you Know? With suitable hydrogenation catalysts such as Noblyst, C₂H₂
1,2 Di-cic 1,1,2 Trien 1,2 Di-trans CCl
0.010 0.101 0.012 0.035
860 tons of toxic byproducts avoided
Total low boilers
∆ = -0.079
0.079
0.158
Tetrachlorethylene
0.001 0.034
0.112 0.130
1,1,2,2 Tetrachlorethane
Yield increased by 0.3%
The series of palladium on silica crys- tal catalysts were designed specifically for the selective hydrogenation of acetylene- to-ethylene within the VCM production process, improving ethane dichloride selec- tivity and minimising byproduct formation in the oxychlorination step. Implementing the hydrogenation unit and avoiding acetylene reaching the oxychlorin- ation reactor prevents chlorinated byprod- uct formation. Acetylene will be chlorinated to low boiling compounds like di- or tri-chlo- roethane but also to high boilers like tet- rachloroethane and tetrachloroethene. In addition, the polymerisation of acetylene and ethylene or acetylene and acetylene, including chlorination, can take place, caus- ing 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 hydro- genation 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 envi- ronment, too. conversion rates of more than 98% can be achieved
Total high boilers
0.035
∆ = -0.207
0.242
Total byproducts
∆ = -0.286
1,2 EDC
98.921
∆ = +0.252
98.669
Figure 2 Noblyst E39 series benefit
Robust processes Numerous empirical studies have demon- strated that palladium (Pd) is the most suit- able 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 sup- port 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 recy- cle stream in VCM plants and actively sells catalysts in all major geographical areas, servicing more than 50 vinyl chloride facili- ties at present (see Figure 1 ). Evonik produces VCM hydrogenation catalysts based on proprietary knowledge.
Contact: stefan.roeder@evonik.com
PVC holds universal appeal. Used for multiple applications, including pipes
23
Powered by FlippingBook