Two Step Nitrogen and Methane Separation Process

Abstract

A system and method for removing nitrogen and producing a high pressure methane product stream from natural gas feed streams is disclosed. A system for also producing natural gas liquids in conjunction with nitrogen removal from natural gas feed streams is also disclosed. The system and method of the invention are particularly suitable for use with feed streams in excess of 50 MMSCFD and up to 750 MMSCFD and containing up to 75 ppm carbon dioxide. Typical power requirements for compressing the methane product stream to produce a suitably high pressure stream for sale are reduced according to the system and method of the invention.

Description

BACKGROUND OF THE INVENTION[0001]1. Field of the Invention[0002]This invention relates to a system and method for separating nitrogen from methane and other components from natural gas streams. The invention also relates to a system and method for integrating natural gas liquids (NGL) extraction with nitrogen removal. The system and method of the invention are particularly suitable for use in recovering and processing feed streams typically in excess of 50 MMSCFD.[0003]2. Description of Related Art[0004]Nitrogen contamination is a frequently encountered problem in the production of natural gas from underground reservoirs. The nitrogen may be naturally occurring or may have been injected into the reservoir as part of an enhanced recovery operation. Transporting pipelines typically do not accept natural gas containing more than 4 mole percent inerts, such as nitrogen. As a result, the natural gas feed stream is generally processed to remove such inerts for sale and transportation of t...

AI Technical Summary

Benefits of technology

[0007]The system and method disclosed herein facilitate the economically efficient removal of nitrogen from methane in a two step process. The system and method are particularly suitable for NRU feed gas flow rates in excess of 50 MMSCFD and are capable of processing NRU feed gas flow rates of up to around 750 MMSCFD. The system and method are also capable of processing NRU feed gas containing concentrations of carbon dioxide up to approximately 75 ppm for typical nitrogen levels between 20-50%

[0008]According to one embodiment of the invention, a system and method are disclosed for processing an NRU feed gas stream containing primarily nitrogen and methane through two fractionating columns to produce a processed natural gas stream suitable for sale to a transporting pipeline. The first stage column is designed to remove methane and heavier hydrocarbon components from nitrogen, while the second stage column is designed to remove nitrogen from the remaining methane. The overhead stream from the first stage column feeds the second stage column. The NRU feed gas and the first stage overhead stream are not cooled to traditional targeted temperatures of −200 to −245 degrees F. The bottoms streams from the first and second fractionating columns are at varying pressures after further processing and are separately fed to a series of compressors to achieve a processed gas product stream of sufficient pressure for sale, typically at least 615 psia. The higher temperatures in the feeds to the fractionating columns allows the bulk of the methane to be separated from the NRU feed stream while reducing the overall compression required for the process by up to 40% when compared to traditional NRU processes.

[0010]There are several advantages to the system and method disclosed herein not previously achievable by those of ordinary skill in the art using existing technologies. These advantages include, for example, an ability to process higher flow rate NRU feed streams from around 50 MMSCFD up to around 750 MMSCFD, NRU feed streams containing up to 75 ppm carbon dioxide, reduction in overall compression requirements, and integration of NGL extraction. Although the present system and method has the disadvantage of higher capital costs associated with additional equipment, compared to prior single column NRU processes, the costs of such are sufficiently offset by the savings in operating expenses, such as those from the reduced compression requirements, and the ability to efficiently produce a suitable processed natural gas stream and valuable NGL stream.

Problems solved by technology

In traditional nitrogen separation systems, the separation of NGL components is more difficult in streams containing more than 5% nitrogen because nitrogen has a stripping effect, absorbing ethane and heavier components.

Although the present system and method has the disadvantage of higher capital costs associated with additional equipment, compared to prior single column NRU processes, the costs of such are sufficiently offset by the savings in operating expenses, such as those from the reduced compression requirements, and the ability to efficiently produce a suitable processed natural gas stream and valuable NGL stream.

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