PRIORITY CLAIM OF EARLIER NATIONAL APPLICATION

[0001] This application claims priority to U.S. Application No. 13/399,802 filed February 17, 2012.

TECHNICAL FIELD

[0002] This document generally relates to methods and apparatuses for processing natural gas, and particularly relates to such methods and apparatuses that remove carbon dioxide from natural gas to form methane products.

BACKGROUND

[0003] Natural gas as sold in commerce is substantively different from natural gas that is extracted through wellheads. Processing of extracted natural gas to form commercial grade natural gas is in many respects less complicated than the processing and refining of crude oil, however, it is equally necessary before its use by end users. The natural gas used by consumers is composed almost entirely of methane. While natural gas as extracted from the earth contains a significant amount of methane, it is not nearly pure enough for commercial use. As extracted, natural gas typically exists in mixtures with other compounds including carbon dioxide and water.

[0004] Certain natural gas wells produce natural gas having high levels of carbon dioxide, such as levels above 30 mole percent (mol%). Natural gas with high levels of carbon dioxide can be difficult and/or expensive to process. Various fractionation methods, including cryogenic fractionation have been utilized to remove carbon dioxide from natural gas feedstocks. However, improvement both in process efficiency for carbon dioxide removal from natural gas feedstocks and in cost reduction for such processing are desirable for the production of methane rich, commercial grade natural gas.

[0005] Accordingly, it is desirable to provide methods and apparatuses for the processing of natural gas with enhanced carbon dioxide removal. In addition, it is desirable to provide methods and apparatuses that utilize selective permeation membranes for the production of methane products. Furthermore, other desirable features and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background.

BRIEF SUMMARY

[0006] Methods and apparatuses for processing natural gas are provided. In accordance with an exemplary embodiment, a method for processing a natural gas stream includes fractionating the natural gas stream to form an overhead stream and a bottoms stream. The overhead stream is then separated with a membrane to form a methane rich residual stream and a permeate stream.

[0007] In accordance with another exemplary embodiment, a method for producing a methane product includes passing a natural gas stream through a molecular sieve to remove water therefrom to form a dried natural gas stream. The dried natural gas stream is fractionated in a fractionation unit to form an overhead stream and a bottoms stream. The overhead stream is compressed in a compressor to form a compressed stream. Then the compressed stream is separated with a membrane to form a methane rich residual stream and a permeate stream.

[0008] Another exemplary embodiment provides an apparatus for processing a natural gas stream. The apparatus includes a fractionation unit configured to separate the natural gas stream into a bottoms stream and an overhead stream. Further, the apparatus includes a selective permeation membrane in fluid communication with the fractionation unit and configured to separate the overhead stream into a methane rich residual stream and a permeate stream.

BRIEF DESCRIPTION OF THE DRAWING

[0009] Exemplary embodiments will hereinafter be described in conjunction with the following drawing figure, wherein:

[0010] The FIGURE is simplified schematic representation of a natural gas processing apparatus arranged in accordance with an exemplary embodiment herein.

DETAILED DESCRIPTION

[0011] The following detailed description is merely exemplary in nature and is not intended to limit the natural gas processing methods and apparatuses claimed below. Furthermore, there is no intention to be bound by any theory presented in the preceding background or the following detailed description. Also, additional components, loops, and processes may be included in the apparatus but are not described herein for purposes of clarity. Stream compositions presented herein are merely illustrative of an embodiment and are not intended to limit the methods and apparatuses in any way.

[0012] The methods and apparatuses for processing natural gas described herein utilize a two stage carbon dioxide removal process. Specifically, a first stage removes carbon dioxide from the natural gas feedstock through fractionation. A second stage then takes the methane rich overhead stream resulting from fractionation and uses a membrane with selective permeation to remove carbon dioxide to form a carbon dioxide rich permeate stream, leaving behind a residual stream with a higher concentration of methane.

[0013] The FIGURE illustrates an exemplary embodiment of an apparatus 10 for processing natural gas with high levels of carbon dioxide. A feed stream 12 of natural gas with high levels of carbon dioxide is fed to a dehydration unit 16. The composition of the feed stream 12 depends on its source, and the apparatus 10 and methods described herein are not limited to use with a particular composition. However, in an exemplary

embodiment the feed stream 12 is comprised of 30 mol% to 40 mol% methane (CH ₄ ) and 60 mol% to 70 mol% carbon dioxide (C0 ₂ ). Other compounds may be present such as, for example, water.

[0014] An exemplary dehydration unit 16 uses molecular sieves to remove water from the feed stream 12 to form a dried feed stream 18. Molecular sieve dehydration units utilize adsorption and diffusion processes, rather than a thermal process, to separate water from the other vapors. As a result, molecular sieve dehydration units can be considerably more energy efficient. An exemplary molecular sieve dehydration unit utilizes two parallel columns with molecular sieves that preferentially adsorb water. As the feed stream vapor passes through the first dehydration column, water is continually adsorbed resulting in a dryer feed stream as it exits the first column. Over time, the first column will reach a saturation limit, at which time the flow of the feed stream is diverted to the second column and the molecular sieves in the first column are regenerated. In an exemplary embodiment, the feed stream 12 entering the dehydration unit 16 contains 0.0147 mol% water and the dried feed stream 18 exiting the dehydration unit 16 contains 0.0050 mol% water.

[0015] After the dehydration unit 16 forms the dried feed stream 18, the stream is delivered to a carbon dioxide fractionation unit 22 which separates an overhead stream 26 from a bottoms stream 28. While various processes may be used, cryogenic fractionation is particularly suited to the removal of carbon dioxide from a natural gas stream. In cryogenic fractionation, the stream 18 is compressed and cooled to a temperature sufficiently low to allow separation by distillation. Specifically, the carbon dioxide is condensed to a liquid and forms a liquid bottoms stream 28. The carbon dioxide rich bottoms stream 28 may then be removed from the fractionation unit 22.

[0016] An exemplary cryogenic fractionation unit 22 uses dual refrigerants for bulk removal of carbon dioxide. In an exemplary dual refrigerant unit, the refrigerant for an overhead condenser is a portion of the carbon dioxide bottoms stream 28. Specifically, the bottoms stream 28 may be compressed by a pump 30 to feed a recycle stream 32 of liquid carbon dioxide that is fed back to the fractionation unit 22. The liquid carbon dioxide is flashed to a relative low pressure where it chills and partially condenses the overhead vapor stream 26. The carbon dioxide used as refrigerant in the overhead condenser is then compressed, cooled, and returned back to the fractionation column where it is recovered in liquid form. The bottoms stream 28 leaving the fractionation unit 22 is pumped by pump 30 to pipeline pressure. The majority of any propane and heavier hydrocarbons in the natural gas stream 18 exit the column with the liquefied carbon dioxide 28. The bottoms stream 28 typically contains over 95 mol% carbon dioxide.

[0017] As shown in the FIGURE, after fractionation, the overhead stream 26 is fed to a compressor 34 which compresses the stream into a membrane feed stream 36. An exemplary overhead stream 26 exiting the fractionation unit 22 is comprised of less than

25 mol% carbon dioxide and more than 75 mol% methane. Typically, the overhead stream

26 has a pressure of 3447 kPa (500 psig) to 4137 kPa (600 psig) and is compressed to a pressure of 8274 kPa (1200 psig) by the compressor 34.

[0018] The compressed membrane feed stream 36 is then delivered to a module 38 holding a membrane 40 which separates a methane rich residual stream 42 from a carbon dioxide rich permeate stream 44. Specifically, the compressed membrane feed stream 36 flows into contact with the membrane 40 in the module 38. Carbon dioxide permeates through the membrane 40, leaving the methane.

[0019] The carbon dioxide permeable membrane 40 operates on the principle of selective permeation. Each gas component (i.e., the methane and the carbon dioxide) has a specific permeation rate. The rate of permeation is determined by the rate which a component dissolves into the membrane surface and the rate at which it diffuses through the membrane.

[0020] An exemplary membrane 40 is a nanoporous polybenzoxale (PBO) polymer modified inorganic membrane. Such a membrane 40 may have a pore size with a diameter in the range of 0.5 nm to 500 nm, such as 0.5 nm to 200 nm, or 0.5 nm to 50 nm. The inorganic membranes may be composed of silica, metal such as stainless steel, alumina such as alpha-alumina, gamma alumina and transition alumina, ceramics, or mixtures thereof. The selection of the material will depend on the conditions of separation as well as the type of nanoporous structure formed. An exemplary inorganic membrane 40 can have different geometries such as a disk, tube, hollow fiber, or others. An exemplary PBO polymer is insoluble in any organic solvents and is stable up to 500°C. An exemplary PBO polymer is derived from a PBO precursor polymer such as poly(hydroxyl imide), poly(hydroxyl amic acid), poly(hydroxyl amide), or a mixture thereof. An exemplary PBO precursor polymer is soluble in organic solvents such as NMP, DMAc, 1,3-dioxolane, and the like. The function of the PBO material in an exemplary membrane 40 is to enhance the membrane selectivity compared to the unmodified porous inorganic membrane.

[0021] As an example, a porous ceramic membrane disk having 180 nm pores and with dimension of 39.0 mm diameter and 2.0 mm thick obtained from ECO Ceramics BV can be used for the preparation of PBO modified nanoporous membrane. The membrane can be prepared by incorporating a layer of PBO polymer on the inside wall of the pores of the separation surface of the above porous ceramic membrane. An exemplary membrane preparation procedure includes: the above-mentioned commercial porous ceramic membrane disk having 180 nm pores is cleaned first by rinsing with 2-propanol and water to remove surface impurities and drying at 110°C for at least 24 hours in a vacuum oven. Then, one surface of the porous ceramic membrane is immersed in a PBO precursor solution for a certain time. The PBO precursor solution can be a solution of poly(hydroxyl imide), poly(hydroxyl amic acid), poly(hydroxyl amide), or a mixture thereof. After that, the excess solution on the surface of the ceramic membrane is removed and the surface is carefully cleaned. The resulting modified ceramic membrane is dried at room temperature under high vacuum followed by drying at 200°C under vacuum. The membrane is then heated to 400-450°C for a certain time to convert the PBO precursor polymer inside the pores of the ceramic membrane to high temperature stable PBO polymer. [0022] The components with higher permeation rates (e.g., carbon dioxide) will permeate faster through the membrane module than components with lower permeation rates (e.g., methane). Therefore, when the membrane feed stream 36 contacts the membrane 40, the carbon dioxide will permeate through the membrane at a faster rate than the methane. Thus, the membrane feed stream 36 is separated into the methane rich residual stream 42 on the interior of the membrane 40 and the carbon dioxide rich permeate stream 44 on the exterior of the membrane 40. The primary driving force of the selective permeation membrane separation is the differential partial pressure of the permeating component. Therefore, the pressure difference between the membrane feed stream 36 and permeate stream 44 and the concentration of the carbon dioxide determine the product purity and the amount of carbon dioxide membrane surface required.

[0023] In an exemplary embodiment, as formed by the membrane 40, the residual stream 42 comprises at least 90 mol% methane, such as more than 95 mol% methane. Further, an exemplary residual stream comprises less than 10 mol% carbon dioxide, such as 6 mol% or 2 mo 1% carbon dioxide.

[0024] In the FIGURE, the permeate stream 44 is fed to a recompression unit 48. The recompression unit 48 recompresses the permeate stream 44 to form a carbon dioxide recycle stream 52 at a pressure of 3792 kPa (550 psig) to 4137 kPa (600 psig). As shown, the recycle stream 52 is mixed with the dried feed stream 18 to form a combined feed 54 that is fed to the carbon dioxide fractionation unit 22.

[0025] In an exemplary embodiment, the stream 12 will include 60 to 70 mol% carbon dioxide, 30 to 40 mol% methane, and less than 5 mol% of other components which may include, for example, nitrogen, propane, water, and other alkanes, at a pressure of 7584 kPa (1100 psig) to 8963 kPa (1300 psig) and at a temperature of 15° to 25°C. Water content is reduced by 60-70% in the dehydration unit 16. Mixing with the recycle stream 52 further reduces water content by 5%, and reduces pressure by 50%. The overhead stream 26 includes 20-25 mol% carbon dioxide and 70-80 mol% methane, while the bottoms stream 28 includes 95-99 mol% carbon dioxide and less than 1 mol% methane. The overhead stream 26 is compressed to 8274 kPa (1200 psig) for interaction with the membrane 40. The residual stream 42 formed includes 96% methane and 2% carbon dioxide, while the permeate stream 44 is formed by 50-60 mol% carbon dioxide and 40-50 mol% methane. The exemplary embodiment is provided for illustration purposes only and is not meant to limit the various embodiments of the apparatus or methods contemplated herein.

[0026] In the method for processing the natural gas stream 12, carbon dioxide is separated and removed from methane in the natural gas. The method involves a two stage separation process. First, the feed stream 12 is fractionated in the fractionation unit 22 to form the carbon dioxide depleted overhead stream 26 and the carbon dioxide rich bottoms stream 28. The overhead stream 26 is then separated by the membrane 40 to form the methane rich residual stream or methane product stream 42 and the carbon dioxide rich permeate stream 44. The membrane 40 is able to efficiently form the residual stream with a methane composition of over 90 mol% methane, such as over 95 mol% methane, and with a carbon dioxide composition of less than 10 mol% carbon dioxide, such as 6 mol% carbon dioxide or 2 mol% carbon dioxide. Further, the membrane 40 forms the permeate stream 44 having a carbon dioxide composition of over 60 mol% carbon dioxide.

[0027] As indicated above, the present methods and apparatuses for processing natural gas produce a methane rich product from a natural gas stream having high levels of carbon dioxide. The methods and apparatuses utilize a two stage carbon dioxide separation process, including a first carbon dioxide fractionation stage and a second selective permeation membrane stage. As a result, carbon dioxide is removed from the natural gas stream in an efficient and cost effective manner.

[0028] Accordingly, apparatuses and methods for processing natural gas have been provided. While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the isomerization and deisohexanizer apparatuses or methods in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope as set forth in the appended claims and their legal equivalents.