Technology from Europe:
France

Successful revamp of turbo-expander NGL recovery process

ConocoPhillips is operating a turbo-expander NGL recovery process on its 2/4 J platform in Ekofisk field, offshore Norway. This NGL recovery unit processes a gas obtained by blending associated gases from different upstream separators and compressors. Recovered NGLs are then mixed with the oil to maximize the oil production. The tri-ethylene glycol (TEG) unit, upstream of the NGL recovery process, initially designed with the cold finger regeneration process gave, from its initial start-up in 1998, insufficient water dew pointing, resulting in recurrent freezing problems in the NGL recovery process. These freezing problems resulted in reduced NGL recoveries and, thus, production losses. 

It was, therefore, decided to revamp the TEG regeneration process to get at least 99.95 wt % purity. Such a target could have been achieved by using additional stripping gas. But that would also have increased atmospheric BTX (benzene, toluene, xylenes) emissions through the vent gas, which was not acceptable. ConocoPhillips therefore decided to use the Drizo regeneration process developed and licensed by Prosernat, a French-based gas process house, and a subsidiary of IFP. The process is able, simultaneously, to reach lower water dew points and to decrease the environmental impact. The TEG unit is designed to treat 735 MMscfd of gas at 51 barg and 22°C inlet temperature. A TEG flowrate of 13 t/hr is being used. The scope of the revamp project covered implementing additional modules with the Drizo equipment. These were designed and manufactured by Prosernat to cope with the severe space limitations on the existing platform. 

The Drizo concept is based on use of a liquid hydrocarbon, which is vaporized to strip hot glycol from the thermal regenerator to remove the residual water. Use of a vaporized hydrocarbon solvent instead of gas to strip the glycol has several advantages: 1) the possibility to condense and recover the hydrocarbon solvent at the top of the still column. The solvent is therefore recycled in the process (the Drizo loop) and atmospheric emissions from the vent of the still column are minimized; 2) a better stripping efficiency results in higher glycol purities than with stripping gas. Under some conditions, the process can allow water dew points in the treated gas as low as 100°C (148°F). 

The Drizo process, in TEG NGL recovery process in Ekofisk field.

The hydrocarbon solvent used in the loop is usually based on a BTX mixture which is directly obtained from BTX present in the natural gas to be treated, and which is absorbed by the glycol. In most cases, the amount of BTX absorbed is high enough to more than compensate for losses in flash gas and from the still column condenser. The loop then enables some liquid hydrocarbon production. But, in some cases, like in the present project, where most of the heavier hydrocarbons are being removed from the feed gas before it enters the glycol contactor, some small solvent make-up may be necessary.

Seven main pieces of equipment needed to be implemented to shift to the Drizo regeneration process (see figure). To cope with severe space limitations, Prosernat developed a multi-module plug in concept, based on five small interconnecting modules. When assembled, these left only 12 tie-in points to be connected with existing equipment. After commissioning, the TEG regeneration could be shifted to Drizo operation without necessitating any production downtime. 

The new regeneration produces an above-target TEG purity of 99.97 wt %, corresponding to a water dew point of about -55°C in the treated gas. The former freezing problems are being avoided and NGL recovery has been maximized, since start-up in June 2001. As noted, some liquid hydrocarbon solvent make-up is, in this case, necessary to operate the new loop. But a fine tuning of the unit operation should lead to solvent self sufficiency.