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Recovery of heavy hydrocarbon upstream of a CFZ process

IP.com Disclosure Number: IPCOM000248364D
Publication Date: 2016-Nov-21
Document File: 4 page(s) / 539K

Publishing Venue

The IP.com Prior Art Database

Abstract

ExxonMobil invented the Controlled Freeze Zone (CFZ) process to circumvent this problem by deliberately allowing the freezing of CO2 in a specially-designed distillation tower. A clean methane stream (along with any nitrogen present in the raw gas) is produced overhead, while a liquid stream containing the remaining (heavier) components is discharged at the bottom of the tower. As such this process is synergistic with acid gas injection (AGI). Special consideration, however, needs to be given if the recovery of valuable NGLs is necessary. These components should be recovered either upstream or downstream of the CFZ process. If these components are recovered upstream of the CFZ process, then the resulting liquid stream generated at the bottom of the CFZ tower will be largely devoid of any hydrocarbons and can be directly integrated with AGI. The current disclosure presents a new technique that facilitates NGL recovery upstream of a CFZ process. The same technique can be extended to recover NGLs upstream of other gas treating technologies as well.

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Recovery of heavy hydrocarbon upstream of a CFZ process

Background:

Natural gas coming from underground reservoirs often contains a variety of hydrocarbon (such as methane, ethane, propane, etc.) and non-hydrocarbon compounds (such as CO2, H2S). Generation of a saleable methane-rich fuel that can be sold per specification requires such production stream to be treated. Additional revenue can be generated if heavier hydrocarbons such as ethane and propane are recovered and sold as natural gas liquids (NGL). However, recovery of these components becomes complex if there are significant amounts of CO2 and H2S present in the feed stream.

Solvent based gas treatment technologies can be used to separate small to moderate amounts of acid gases (CO2 and H2S) from the feed gas. The resulting gas stream can be further treated to recover NGLs and generate a methane-rich stream for sale. However, disposal of the concentrated acid gas stream becomes challenging. In many cases, this gas stream is sent to a sulfur recovery unit (SRU) to convert the toxic H2S into benign elemental sulfur, while the CO2 is vented. Sulfur recovery is a relatively expensive and complex process.  In some areas, additional elemental sulfur production is not desirable because there is no market for it or the market is saturated.  Thus, the sulfur must be stored, often in large, above-ground blocks.  Venting of CO2 is also coming under scrutiny, and is increasingly becoming restricted or no longer allowed in many parts of the world. An alternative to this is to re-inject the acid gas stream back into the ground  for sequestration or for use in for enhanced oil recovery. This route however, can be economically challenged when using a solvent based gas treatment technology, as the acid gas components are discharged as a low pressure, moisture-bearing vapor stream that will require significant and expensive compression before they can be injected back into the reservoir.

Gas processing based on cryogenic distillation avoids the use of solvents, minimizes acid gas removal (AGR) equipment, and discharges the acid gas components as a pressurized (e.g., 450-600 psig), dry liquid that can be easily boosted to re-injection pressure.  Further, since the liquefied acid gas has a high density; hydrostatic head can be used to great advantage in an injection well.  The reinjection advantages are obvious: the energy required to pump this liquid is much lower than that required to compress low-pressure acid gases to reservoir pressure, the equipment required to reach reinjection pressure is much simpler and less expensive, and finally the dry liquid is non-corrosive and thus has simpler metallurgical requirements. 

It is well known that if CO2 is present in the raw gas at concentrations greater than about 5%, it is very likely to freeze out as a solid in standard cryogenic gas processing units.  This rules out the use of conventional distillation at cryogenic conditions to process such feed...