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What Is Entrainment? | Process Engineering Glossary
What Is Entrainment?
Entrainment is the unwanted carryover of one phase into a stream that should ideally remain single-phase, such as liquid droplets carried along with a vapor stream, or gas bubbles carried along with a liquid stream. Entrainment typically occurs when disengagement space is insufficient or when velocity through a vessel or piping section is high enough to overcome the gravitational settling that would otherwise separate the phases.
Excessive entrainment can degrade downstream process performance in a variety of ways, from reducing distillation column separation efficiency to damaging compressors and other equipment not designed to handle the unexpected second phase.
Applications of Entrainment
Vapor-Liquid Separator Design
Separator vessels are specifically sized and equipped with demisters to limit entrainment to an acceptable level, since even well-designed gravity settling has practical limits on how fine a droplet size it can effectively remove.
Distillation Column Tray Performance
Entrainment between distillation column trays, where liquid from a lower tray is carried upward by rising vapor, reduces effective separation efficiency and is closely related to the column’s approach to flooding conditions.
Compressor Suction Protection
Preventing liquid entrainment into compressor suction lines is a critical design consideration, since compressors are generally not designed to handle liquid slugs without risking significant mechanical damage.
Benefits of Knowing Entrainment
Clarifies a common source of separation inefficiency. Understanding entrainment helps diagnose otherwise puzzling separation performance problems traced back to unwanted phase carryover.
Informs appropriate vessel and demister sizing. Recognizing entrainment risk directly supports sizing separator vessels and demister equipment appropriately for expected operating velocities.
Protects downstream equipment from damage. Managing entrainment appropriately helps protect compressors and other sensitive equipment from the damage unexpected liquid carryover can cause.
Limitations to Consider
Difficult to eliminate completely. Some degree of entrainment is difficult to eliminate entirely in real equipment, making the practical goal reducing it to an acceptable level rather than achieving zero carryover.
Correlations for predicting entrainment carry real uncertainty. Predictive correlations for entrainment rate involve meaningful uncertainty, particularly near the operating limits of a given vessel or column design.
Can increase unexpectedly during upset conditions. Entrainment that’s well controlled during normal steady-state operation can increase significantly during a process upset or surge condition that temporarily exceeds design velocity.
Entrainment FAQ
How does entrainment relate to vapor-liquid disengagement and demister design?
Entrainment is precisely the phenomenon that adequate vapor-liquid disengagement and a properly sized demister are designed to minimize, working together to keep carryover within acceptable limits.
Why is entrainment relevant to column flooding and weeping?
Entrainment between trays increases as a column approaches column flooding conditions, while weeping represents the opposite low-rate failure mode, together bounding a column’s practical operating range.
How does entrainment relate to annular flow and two-phase relief sizing?
Entrainment of liquid droplets within the gas core is a defining feature of annular flow, a consideration directly relevant to two-phase relief sizing and broader fluid mechanics-based analysis, including critical flow conditions.
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