Cryogenic Unit Recovery of NGL’s

“Chillin” Extreme cooling of the inlet gas is necessary prior to entering the Demethanizer (cryogenic tower) How is this accomplished? Two inlet gas Routes: 1) 1st gas/gas exchanger 2) Side Exchanger 1. 1st Gas/Gas Exchanger. -Inlet gas travels through this exchanger on the tubeside promoting cooling of the gas before entering the 2nd Gas/Gas Exchanger. -Notice that another colder gas returning from the side exchanger is used to further cool our gas stream. 2. Side Exchanger -This inlet gas flow is cooled by the natural gas liquids within the demethanizer tower (cryogenic tower) operating at extremely cold temperatures (- 160 C). -Due to the very low operating temps of the tower we are able to cross exchange with the temperatures in that column. -Allows inlet gas to leave at a much lower Causes liquids in the side exchanger to be heated. (Thermodynamics anyone?)

Are You Kidding me Cold Inlet gas streams from the 1st Gas/Gas exchanger and Side -Exchanger rejoin before streaming through the 2nd Gas/Gas heat exchanger. 2nd Gas/Gas Exchanger will further cool the gas stream. Once again our gas stream is on the tubeside. - Notice that in both Gas/Gas heat exchangers that the residue gas from the top of the demethanizer tower provides the cooling in both of them. -Inlet gas comes out of the 2nd Gas/Gas heat exchanger at a very cold temperature causing NGL’s within the gas to condense and end up in the inlet separator.

Inlet Separator/Turbo-Expander The purpose of the inlet separator is to trap condensed liquids from the gas stream and prevents them from entering Turbo-Expander. NGL collected here are routed to the cryogenic tower The Turbo-Expander is designed to sharply drop the pressure of vapors which will cause an extreme temperature decrease of the gas stream. Cold gas stream enters the cryogenic tower The Turbo-Expander provides a secondary service as well: it is also performing work. Notice that as the turbo-expander turns it causes the compressor to turn. This provides rotational energy for the compressor.

Demethanizer/Cryogenic Tower The function of demethanizer tower is to heat up the inlet gas feed just enough to release methane and condense the remaining NGLs. A reboiler located at the bottom of the tower provides the heat needed to release methane from the rest of the NGLs.

Follow the Methane Residue (methane) gas leaves the top of tower -Used as coolant in gas/gas exchanges (shell side). This in turn heats up the methane. -Methane enters expander-compressor increasing the pressure of the methane allowing it to be sent down the sale’s pipeline. *Remember that the compressor is able to do work because the turboexpander provides rotational energy. No other drivers! Very efficient! -Methane leaving the compressor enters a condenser. Some methane liquifys and collects in the accumulator. The liquid methane returns to the tower as reflux for improved product. -The remaining methane travels to a recompressor further increasing the pressure of the gas. -The residue gas also travels through the reboiler providing heat to the demethanizer tower and also cooling itself down further. - Finally, Residue gas flows through a cooler before going to the sale’s pipeline.

NGL’s The NGL’s are going to leave the system too. NGL’s exit the bottom of the reboiler.  Because they were not vaporized by the column.  These natural gas liquids (demethanized product) and are an excellent revenue source.  Often times, the NGL’s continue on to a fractionation unit where they can be separated individual components such as ethane, propane, butane and even natural gasoline.