PTQ Q3 2026 Issue

activity, reactor temperature, or feed composition can cause hydrogen spikes to appear periodically in the reactor outlet stream. When these spikes enter the ethylene tower feed, hydrogen may accumulate and then gradually be removed through the vent system, producing cyclic patterns of accu- mulation and removal. This cyclic behaviour often manifests itself as periodic pressure fluctuations in the ethylene tower, sometimes accompanied by variations in condenser duty and reflux flow. Since these disturbances appear in the column and refrigeration system, the true upstream origin of the problem may not be immediately recognised during troubleshooting. Understanding how hydrogen slippage from the AHU influences condenser performance is therefore an impor - tant step in identifying the root cause of certain ethylene tower pressure instability events. Why the root cause is often misidentified Despite the clear mechanism through which hydrogen slip- page from the AHU can affect ethylene tower stability, this root cause is often overlooked during troubleshooting. In many operating plants, when pressure fluctuations or con - denser duty variations are observed in the ethylene tower, the initial focus of investigation remains on the column itself or on the refrigeration system serving the condenser. One common reason for this misinterpretation is the per- ception that hydrogen concentrations in the C₂ stream are extremely small and therefore unlikely to influence the oper - ation of a large cryogenic distillation column. Since hydrogen slip is often measured at low concentrations, it is frequently assumed that such quantities cannot significantly affect con - denser performance. However, since hydrogen behaves as a non-condensable component under the operating condi- tions of the ethylene tower condenser, even relatively small increases in hydrogen concentration can influence heat transfer behaviour and disturb column pressure stability. Another factor that contributes to misidentification of the problem is the uncertainty associated with hydrogen analyser readings. Hydrogen analysers installed at the outlet of the guard reactor sometimes show fluctuations or spikes that may be attributed to sampling system issues or cali- bration drift. As a result, operating teams may occasionally treat unexpected analyser readings with caution or suspect instrumentation malfunction rather than immediately inter- preting the signal as a genuine process change. Reactor changeover practices can also play a role in delaying recognition of the issue. In many plants, hydro- genation reactors are changed over according to predefined performance and cryogenic column stability is essential for improving troubleshooting effectiveness in steam cracker cold sections Recognising the connection between hydrogenation reactor

operating schedules. If hydrogen slippage begins to increase slightly before the scheduled changeover point, operators may choose to continue operating the reactor to maximise catalyst utilisation. During this extended operating period near EOR conditions, hydrogen slippage may increase fur- ther and begin to influence downstream units. In some cases, the disturbance resolves itself shortly after the reactor changeover is eventually performed. When a fresh reactor is placed into service, hydrogen consumption returns to normal levels and hydrogen slip decreases. As a result, the ethylene tower pressure fluctuations may disap - pear without any direct intervention on the column itself. Since the instability appears to resolve automatically, the relationship between hydrogen slippage and ethylene tower behaviour may not be fully recognised. Consequently, the same disturbance may recur during future operating cycles without the underlying cause being clearly identified. Recognising the connection between hydrogenation reac- tor performance and cryogenic column stability is therefore essential for improving troubleshooting effectiveness in steam cracker cold sections. Practical monitoring and early detection Preventing ethylene tower instability caused by hydrogen slippage requires careful monitoring of both reactor per- formance and downstream separation behaviour. Since hydrogen slippage may develop gradually or appear inter- mittently, early detection depends largely on identifying subtle trends in process data rather than waiting for major operational disturbances to occur. One of the most important indicators is the hydrogen concentration at the guard reactor outlet of the AHU. This measurement provides a direct indication of hydrogen slip from the reactor system. Operating teams should monitor not only absolute hydrogen concentration values but also gradual increases or intermittent spikes that may appear during later stages of catalyst life. Even relatively small increases in hydrogen concentration can be significant because hydrogen behaves as a non-condensable compo- nent in the ethylene tower condenser. Analyser reliability is another important considera- tion. Hydrogen analysers should be periodically validated through calibration checks and comparison with laboratory analysis, where possible. When analyser readings begin to show persistent increases rather than isolated spikes, these trends should be investigated carefully rather than immedi- ately attributed to instrumentation problems. Monitoring reactor performance parameters can also pro- vide early warning signals of catalyst ageing. Changes in hydrogen consumption patterns, reactor temperature pro- files, or C₂H₂ conversion trends may indicate declining cat - alyst activity. Observing these trends alongside hydrogen analyser readings can help operators identify approaching EOR conditions before hydrogen slippage becomes severe. Another important operational practice involves reviewing the reactor changeover philosophy. Instead of relying solely on fixed operating schedules, reactor changeover decisions should consider catalyst performance indicators such as hydrogen slip behaviour and selectivity trends. Performing

PTQ Q3 2026