Reboiler Mist Eliminators in Ethylene, C3 Splitter Heat Pumps and Refrigeration Chillers

A reboiler is not a generic vessel with an eliminator bolted on. Between an LNG pre-cooling kettle, an ethylene cold-section reboiler, a C3 splitter heat-pump reboiler and a refrigeration chiller evaporator, the same physical rule applies but the binding constraint changes completely. This page sets out what differs, so that an element recommended for one service is not transplanted into another on the strength of a similar duty.

The rule that does not change

In every one of these services, vapour is generated inside a shell and has to leave through a dome. Three consequences follow in all four cases:

• The cross-section is set by volumetric vapour flow, not by heat duty. Two units with the same duty but different boiling pressures need very different dome areas.
• The element area is limited by the shell chord at its installed height, so the level band is a capacity parameter, not just an operating range.
• Liquid must drain back into the pool without passing counter-current through the gas it was just removed from, otherwise a correctly sized element re-entrains what it captured.

What changes is which of these bites first, and what the failure looks like when it does.

The four services side by side

Ethylene cold section: fouling decides the element, not the cut point

In ethylene separation the reboilers sit in a cold section where trace heavies, polymer and tar can deposit on any surface that liquid passes through. A wire mesh has an enormous internal surface and a tortuous path, which makes it an excellent coalescer and a poor choice where deposition is expected: once the bed begins to load with polymer, the drainage channels close, the pressure drop rises and the element starts to re-entrain.

Practical consequences:

• Prefer vane geometry where fouling is credible. Open channels drain and can be cleaned; the coarser cut point is acceptable because the downstream compressor protection is usually provided by a second stage.
• If mesh is required for the cut point, design for irrigation from the start. A wash header above the pad, and a drain path that does not rely on the pad being clean, are part of the element specification rather than an afterthought.
• Watch the level, not the pad. Polymer deposition is often a symptom of a level that is too low, which raises the local boiling film temperature and cooks the heavies onto the tubes and the element.

C3 splitter heat pump: small approach means large vapour flow per degree

A heat-pump reboiler is doing double duty: it boils the column bottoms and it condenses the compressor discharge in the same shell-and-tube body. The economic point of the arrangement is the very small temperature approach, and that has two consequences for the eliminator. First, the required heat transfer area is large, so the bundle and the shell are large. Second, the vapour is generated at a low temperature difference, so the vapour volumetric flow per unit of heat transferred is high, and the dome area requirement is correspondingly high.

The failure mode is severe, because the vapour goes directly to a compressor inlet. A small liquid flow that would be tolerable in a process condenser is not tolerable here:

• Size the element for the highest vapour rate in the operating envelope, including the high-pressure ratio condition at start of run when the approach is largest.
• Provide drainage that works at part load. A heat-pump reboiler at 50 per cent load produces much less vapour, so the element velocity falls far below design; the drainage must still function, or liquid accumulates on the element and is released in bursts when the load returns.
• Do not accept a cut point specification without a liquid load figure. Liquid mass flow per unit area is what floods a vane pack, and it is often omitted from the datasheet while the droplet cut is stated precisely.

Refrigeration evaporators: the density is whatever the suction condition says

In a refrigeration chiller the allowable velocity can differ by a factor of two or three between refrigerant selections at the same cooling duty, because the vapour density at the evaporating temperature differs by that much. A design transplanted from an ammonia chiller to a propane chiller, or from a high-temperature to a low-temperature application, will be wrong at the dome even if the duty is identical.

• Compute the dome area at the lowest evaporating temperature in the envelope, not at the nominal rating point.
• Provide oil management before the element. Oil carried from the compressor accumulates in the evaporator, raises the effective boiling point locally and fouls the element; a drain and a return path are part of the separation design.
• Consider icing at the element edges. Where moisture ingress is possible, the coldest metal in the dome is the element support, and ice there both blocks drainage and sheds into the compressor.

What to put in the datasheet, whatever the service

An element quoted against the first two lines only is a guess. The four below it are what turns a guess into a specification.

Reference

The LNG kettle case and the capacity factors quoted on this site follow Knoepfler et al., LNG2023, paper 3.2.1, whose literature also cites work on C3 splitter heat-pump reboilers as a route to lower energy use and lower CO2 in the same equipment class.

Frequently Asked Questions