PTQ Q3 2026 Issue

Hydrogen slippage from acetylene hydrogenation units

A hidden cause of ethylene tower pressure instability originates upstream of the column, leading to periodic hydrogen spikes entering the cryogenic separation system

Vikram Mohite Petrochemical Process Consultant

E thylene towers in steam cracker cold sections operate under highly sensitive cryogenic conditions, where even small disturbances in feed composition or con- denser performance can affect column stability. In many plants, intermittent pressure fluctuations in the ethylene tower are often attributed to refrigeration system distur- bances or condenser control issues. Operating experience, however, shows that certain dis- turbances originate upstream of the column itself. One frequently overlooked cause is hydrogen slippage from the acetylene hydrogenation unit (AHU) during late catalyst life. As catalyst selectivity declines, hydrogen slip from the guard reactor outlet may increase. If reactor changeover is delayed, intermittent hydrogen spikes can enter the cryo- genic separation system and reach the ethylene tower. Since hydrogen behaves as a non-condensable compo- nent under condenser operating conditions, its presence can disturb condenser heat transfer and lead to cyclic pressure fluctuations. The following discussion explains the mecha - nism of hydrogen slippage, how it propagates through the cold section, and why its impact on ethylene tower oper- ation is often misinterpreted during plant troubleshooting. Ethylene tower sensitivity in steam cracker cold sections Steam cracking plants rely on complex cryogenic separation systems to recover and purify valuable products such as hydrogen, methane, ethylene, and propylene from cracked gas streams. Within this cold section, the ethylene tower (C₂ splitter) is one of the most critical and operationally sensitive distillation columns. The primary function of the ethylene tower is to separate ethylene from ethane and produce pol- ymer-grade ethylene that meets stringent purity specifica - tions required by downstream polymerisation units. This separation is inherently challenging because the relative volatility between ethylene and ethane is relatively low under typical operating conditions. As a result, the eth- ylene tower normally operates with high reflux ratios, sig - nificant refrigeration duty, and tightly controlled operating conditions. These characteristics make the column highly sensitive to even small disturbances in feed composition, condenser performance, or internal vapour-liquid equilib- rium. Minor deviations in operating parameters can quickly

propagate through the column, affecting pressure stability, reflux generation, and overall separation performance. In many operating plants, engineers occasionally encoun- ter intermittent or cyclic pressure fluctuations in the ethyl - ene tower. These disturbances may appear in several ways, including gradual oscillations in column pressure, variations in reflux flow rate, fluctuations in condenser duty, or sudden increases in overhead vapour flow that may even result in intermittent flaring events. Since these symptoms are visible directly at the column, troubleshooting efforts often focus initially on the ethylene tower itself. The most common assumption in such situations is that the disturbance originates in the refrigeration system, par- ticularly the propylene refrigeration (C3R) circuit supplying the ethylene tower condenser. The condenser typically represents one of the largest consumers of C3R in the cold section, and even small variations in condenser duty can influence the entire refrigeration loop. Consequently, potential causes such as refrigeration control valve instabil- ity, moisture ingress into the refrigeration circuit, condenser fouling, or level fluctuations in refrigerant drums are often investigated first. While such issues can certainly affect column perfor- mance, operating experience in several steam cracking facilities suggests that the true origin of ethylene tower instability may originate upstream of the column itself. Changes occurring in upstream reaction systems or sepa- ration columns can gradually propagate through the cry- ogenic system before eventually manifesting as pressure fluctuations in the ethylene tower. One such mechanism involves hydrogen slippage from the AHU. The hydrogenation reactors are designed to selectively convert acetylene (C₂H₂) impurities to ethylene before the C₂ stream enters the ethylene tower. Under nor - mal operating conditions, hydrogen consumption within the reactors keeps hydrogen concentrations in the outlet stream extremely low. However, as catalyst activity declines toward the end of its operating cycle, hydrogen slippage from the reactor outlet may begin to increase. Although hydrogen concentrations may still appear small in absolute terms, hydrogen behaves as a non-condensa- ble component under the cryogenic operating conditions of the ethylene tower condenser. Even modest increases

PTQ Q3 2026