In a cryogenic gas plant, once the feed has been chilled cold enough to condense liquids, one column does the crucial separation: it drives the methane back up into the sales gas while keeping the valuable heavier components down in the liquid product. That column is the demethanizer, and how it is operated largely determines how much liquid the plant recovers. This guide explains the demethanizer as a cold distillation tower, its temperature and pressure profile, the roles of reflux at the top and reboiler duty at the bottom, and how column control ties into the plant's overall recovery targets.
Demethanizer Column in one line: A demethanizer is the cold distillation column at the core of a cryogenic gas plant that separates methane from the heavier natural gas liquids. The very cold feed enters the tower, and the column drives light methane up and out the top into the residue or sales gas while the heavier components - ethane and heavier, depending on the plant's mode - collect at the bottom as the NGL product. Its temperature and pressure profile, top reflux, and bottom reboiler duty set how cleanly that split is made.
A demethanizer is a distillation column, which means it separates components by their difference in volatility, but it operates at cryogenic temperatures because methane is so light and volatile that only extreme cold keeps the heavier components in the liquid phase where they can be captured. Cold feed streams from the plant's chilling equipment, including the turboexpander outlet, enter the column, and inside the tower vapor rises while liquid falls across trays or packing. As they contact, the lightest component, methane, is stripped upward into the overhead vapor, while ethane and the heavier hydrocarbons are washed downward into the bottoms.
The overhead product is the residue gas, largely methane, that becomes the plant's sales gas after recompression. The bottoms product is the recovered NGL - a mixed liquid that, depending on how the plant is run, contains ethane and everything heavier, or rejects ethane and keeps propane and heavier. The name demethanizer captures the job precisely: its purpose is to remove methane from the liquids so the bottoms product is not contaminated with light gas that belongs in the sales stream, while at the same time not letting the valuable heavier components escape overhead where they would be lost to the residue gas.
A demethanizer runs a steep temperature profile, coldest at the top and warmest at the bottom. The top of the column is extremely cold, which is what condenses and holds back the heavier components from leaving with the methane overhead, while the bottom is heated to boil methane out of the liquid product. The reflux at the top - cold liquid returned to the column - provides the contacting liquid that scrubs heavier components out of the rising vapor, improving recovery by preventing ethane and propane from slipping out the top. The colder and better-refluxed the top, the more of the valuable liquids the column keeps.
At the bottom, the reboiler adds heat to strip residual methane out of the NGL product, and its duty is a key control handle. Too little reboiling leaves methane in the bottoms, which can put the liquid product off specification and cause problems downstream; too much wastes energy and can boil heavier components back up the column. Because the column sits between the cold top and the hot reboiler, operators balance the two: the pressure at which the column runs, the top temperature and reflux, and the reboiler duty together determine both the sharpness of the methane split and how much liquid is recovered. Adjusting these is how a plant trades between recovery and energy use and between ethane recovery and rejection.
The demethanizer does not operate in isolation - its behavior is coupled to the plant's recovery target and to the equipment feeding it, especially the turboexpander that supplies its cold. When a plant is in high ethane-recovery mode, the column is run colder and with more reflux to keep ethane down in the liquid; when it is in ethane-rejection mode, it is operated so that ethane leaves overhead with the methane and only propane and heavier are recovered. Deciding between these modes is often an economic call driven by the relative value of ethane as a liquid versus as gas, and the column's operating point is what executes that decision.
A cloud SCADA or DCS platform such as Merobix supports this by bringing the column's temperature profile, pressure, reflux, and reboiler duty into a live, trended view alongside the feed conditions and product streams, so operators can see how a change at one point propagates through the tower. Watching the top temperature and the overhead composition tells them whether valuable liquids are slipping into the residue gas; watching the bottoms and reboiler tells them whether methane is contaminating the product. With those signals continuously visible, an operator can keep the demethanizer at the operating point that hits the plant's recovery target without drifting off product spec, and can catch an upset - a warming top or a swing in feed - before it costs recovery.
The overhead product from the top is residue gas, mostly methane, which becomes the plant's sales gas after recompression. The bottoms product from the bottom is the recovered natural gas liquid, containing ethane and heavier or propane and heavier depending on whether the plant is in recovery or rejection mode. The column's job is to make that methane-versus-NGL split cleanly.
Because methane is extremely light and volatile, only cryogenic temperatures keep the heavier components in the liquid phase where they can be captured while methane rises to the top. The cold top condenses and holds back ethane and heavier so they do not escape with the methane overhead. Without that extreme cold, valuable liquids would leave with the sales gas and be lost.
The reboiler adds heat at the bottom of the column to strip residual methane out of the liquid NGL product so it meets specification. Its duty is a key control handle: too little leaves methane in the bottoms and puts the product off spec, while too much wastes energy and can boil heavier components back up the column. Operators balance reboiler duty against top temperature and reflux to control the split.
Merobix reads your field devices into a cloud SCADA - the real thing behind these terms, live in days from any browser.