PTQ Q3 2026 Issue

the feed stream, including both temporary poisons and per- manent metal deposits. Over time, these deposits reduce the number of active catalytic sites available for the hydro- genation reaction. As catalyst activity declines, several changes in reactor behaviour begin to appear. One of the earliest signs is a change in hydrogen consumption patterns within the reac- tor system. The catalyst may no longer utilise hydrogen as effectively as during earlier stages of operation. At the same time, catalyst selectivity may begin to deteriorate, resulting in an increased tendency for secondary reactions, such as the hydrogenation of ethylene to ethane. As catalyst ageing progresses further, measurable hydro- gen slippage may begin to appear at the outlet of the guard reactor. Hydrogen slip from the guard reactor outlet is often used by operating teams as an important operational indica- tor that the reactor system is approaching end-of-run (EOR) conditions. Under these circumstances, reactor changeover is typically planned to restore hydrogenation performance. In practice, however, reactor changeover may not always occur immediately when hydrogen slippage begins to increase. Several operational considerations can influence this decision. Plant operators may choose to extend reactor operation to maximise catalyst utilisation, or there may be uncertainty regarding the accuracy of analyser readings. Production planning constraints may also delay reactor changeover. During this extended operating period near EOR con- ditions, hydrogen slippage can increase further and may not remain constant. Variations in feed composition, reac- tor temperature, or catalyst activity distribution within the reactor bed can lead to intermittent hydrogen spikes in the reactor outlet stream. These intermittent hydrogen spikes pass downstream into the cryogenic separation system along with the C₂ hydrocarbon stream. Although the absolute concentration of hydrogen may still appear relatively small, its presence can have a disproportionate impact on downstream cryo- genic separation units. This effect becomes particularly important in the eth- ylene tower, where the presence of hydrogen in the feed stream can influence condenser performance and column pressure stability. Understanding this interaction between hydrogenation reactor behaviour and cryogenic distillation operation is therefore essential for diagnosing certain types of ethylene tower pressure disturbances observed in steam cracking plants. Hydrogen slippage impact on ethylene tower operations When AHU hydrogen slippage increases, the excess hydro- gen eventually becomes part of the feed entering the eth- ylene tower. Under normal operating conditions, hydrogen concentration in the C₂ stream is extremely small and has a negligible influence on column performance. However, when hydrogen slippage increases, particularly during late catalyst life, the behaviour of the ethylene tower can change noticeably. Hydrogen behaves as a non-condensable component

under the cryogenic operating conditions of the ethylene tower condenser. While ethylene and ethane readily con- dense at the operating temperature and pressure of the condenser, hydrogen remains primarily in the vapour phase. As the C₂ stream containing hydrogen enters the tower, the hydrogen tends to migrate toward the overhead vapour region of the column. Most ethylene towers are equipped with an overhead vent system designed to remove small quantities of light gases that accumulate in the overhead section. This vent system typically operates through a control valve that maintains column pressure within the desired operating range. Under normal conditions, the vent capacity is sufficient to remove trace amounts of hydrogen or methane that may be present in the system. However, when hydrogen slippage increases from the hydrogenation reactors, the amount of hydrogen entering the column can exceed the removal capacity of the vent sys- tem. In such cases, hydrogen begins to accumulate intermit- tently in the overhead vapour region of the ethylene tower. The presence of hydrogen in the condenser region affects the heat transfer performance of the condenser. Since hydrogen does not condense under these operating con- ditions, it occupies a portion of the vapour phase in the condenser. This effectively reduces the partial pressure of When hydrogen slippage increases from the hydrogenation reactors, the amount of hydrogen entering the column can exceed the removal capacity of the vent system condensable hydrocarbons such as ethylene and ethane in the vapour mixture. As a result, the effective condensation rate decreases, and the amount of liquid reflux generated by the condenser becomes unstable. As condenser performance fluctuates, the ethylene tower begins to experience variations in its internal vapour-liquid equilibrium. These variations may appear in several operat- ing parameters, including fluctuations in column pressure, variations in reflux flow rate, and changes in condenser duty. Since the condenser is responsible for providing reflux to the column, any instability in condenser operation quickly affects the internal mass transfer conditions within the tower. Another important factor is the interaction between the ethylene tower condenser and the C3R system. The ethyl- ene tower condenser typically represents one of the largest consumers of refrigeration duty in the cold section. When condenser heat transfer fluctuates due to hydrogen accu - mulation, the refrigeration system may respond with corre- sponding variations in refrigerant flow or compressor load. These responses can amplify the apparent instability of the system. A further complication arises from the fact that hydrogen slippage from the hydrogenation reactors may occur inter- mittently rather than continuously. Variations in catalyst

PTQ Q3 2026