by selectively converting C₂H₂ to ethylene in the presence of hydrogen. This step is essen- tial because C₂H₂ is a strong poison for polymerisation catalysts used in downstream polyethylene production. After hydrogenation, the treated C₂ stream enters the ethylene tower, where ethyl- ene is separated from ethane through cryogenic distillation. Since this separation requires high reflux ratios and signif - icant refrigeration duty, the ethylene tower becomes par- ticularly sensitive to distur- bances in feed composition.
Overhead vent
H accumulation in o verhead
Reduced condenser heat transfer
Acetylene Hydrogenation Unit (AHU)
Demethaniser (DM)
C stream (C H + C H + H )
Cracked gas
Deethaniser (DE)
Guard reactor
Lead reactor
Hydrogen slip (H 2 )
Ethylene tower (C splitter)
Figure 1 Propagation of hydrogen slippage from the AHU through the C₂ separation system and its influence on ethylene tower condenser performance and pressure stability
in hydrogen content can therefore affect condenser perfor- mance and disturb column pressure stability. Understanding how such upstream disturbances develop and propagate through the cold section is essential for effective troubleshooting of ethylene tower pressure fluc - tuations. The following sections examine this mechanism in greater detail based on practical operating observations from steam cracking plants. Typical C₂ separation system configuration The cryogenic recovery section of a steam cracking plant is designed to progressively separate lighter and heavier components from cracked gas before producing high-purity ethylene and other valuable products. While exact config - urations may vary depending on plant capacity and tech- nology licensor, many steam crackers employ a separation sequence that broadly follows the order: Demethaniser → Deethaniser → AHU → Ethylene tower Each of these sections performs a distinct function in pre- paring the C₂ stream for final ethylene purification. The demethaniser represents the first major cryogenic distillation step in the cold section. Its primary purpose is to remove hydrogen and methane from the cracked gas stream. These light components leave the column as over- head products, while C₂ hydrocarbons and heavier fractions are recovered in the bottoms stream and routed to down- stream separation units. Maintaining proper separation in the demethaniser is essential because carryover of methane or hydrogen into downstream units can affect the perfor- mance of subsequent cryogenic columns. The bottoms stream from the demethaniser is typically fed to the deethaniser, which separates C₂ hydrocarbons from heavier components such as propane and C₃+ fractions. The overhead stream from the deethaniser therefore contains mainly ethane, ethylene, acetylene, and trace quantities of lighter gases that may slip from the upstream section. Before entering the ethylene purification stage, this C₂ stream normally passes through the AHU. The objective of this unit is to remove C₂H₂ impurities from the C₂ stream
Under such conditions, even small quantities of light non-condensable gases entering the column feed can influence condenser performance and column pressure stability. Understanding the behaviour of upstream units, particularly the C₂H₂ hydrogenation reactors, is therefore essential for diagnosing disturbances observed in ethylene tower operation. The interaction between the AHU and the ethylene tower is illustrated schematically in Figure 1 , which shows how hydrogen slippage from the guard reactor outlet can propa- gate through the C₂ separation system and influence down - stream column behaviour. AHU hydrogen slippage The AHU plays a critical role in the cryogenic separation train of steam cracking plants. The primary objective of this unit is to selectively convert C₂H₂ present in the C₂ stream into ethylene before the stream enters the ethylene purifica - tion section. Even trace amounts of C₂H₂ must be removed because it can severely poison polymerisation catalysts used in downstream polyethylene production units. To ensure reliable C₂H₂ removal, hydrogenation reactors are typically installed in multiple fixed-bed configurations arranged in a lead-guard sequence. In many plants, three reactors are installed, with two operating in series and one maintained on standby mode to allow periodic changeover. The first reactor in the sequence acts as the lead reactor, where much of the hydrogenation reaction takes place. The second reactor serves as a guard reactor to ensure that residual C₂H₂ concentration remains within acceptable lim - its before the stream proceeds to the ethylene tower. At start-of-run (SOR) conditions, catalyst activity is high because the catalyst surface contains many available active sites. Under these conditions, C₂H₂ conversion is highly effi - cient, and catalyst selectivity toward ethylene formation is favourable. Hydrogen consumption follows a relatively sta- ble and predictable pattern, and hydrogen slippage from the reactor system is typically minimal. As the catalyst continues to operate over extended peri- ods, catalyst gradually deactivates. This deactivation results from the accumulation of various contaminants present in
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PTQ Q3 2026
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