Process design for acid gas removal

Abstract

A membrane permeation system and process accommodates varying acid gas inlet concentrations over time while utilizing only the initially installed equipment and still maintaining the non-permeate gas specification. The system and process provide flexibility to operate efficiently over a wide range of inlet CO.sub.2 concentrations by adjustments to primary permeate, secondary permeate, and recycle gas operations. The glassy polymer membrane devices used in the system and process are selected so removal duty efficiency increases as acid gas concentration increase. Designing the system and process to handle about a 15% increase in acid gas concentrations over initial conditions effectively treats acid gas concentrations well above that 15% increase, thereby eliminating the need for additional equipment or for additional downstream amines and physical solvents.

Claims

1. A system comprising: a primary membrane unit arranged to receive an inlet natural gas stream containing an acid gas and housing a quantity YM.sub.B of first membrane devices; a compressor arranged to receive at least a portion of a permeate flow exiting the primary membrane unit; a bypass loop arranged to receive at least a portion of the permeate flow exiting the primary membrane unit; a secondary membrane unit arranged to receive a compressed permeate flow from the compressor and housing a quantity YN.sub.B of second membrane devices; and a recycle loop having a compressor arranged to receive a non-permeate flow exiting the secondary membrane unit; wherein M.sub.B+N.sub.B is a predetermined quantity of said membrane devices required for a first removal duty to reduce a first acid gas concentration C.sub.B of the inlet natural gas stream to an acid gas concentration C.sub.s of a final outlet natural gas stream; and wherein YM.sub.B+YN.sub.B is a quantity of said membrane devices required for a second higher removal duty to reduce a second acid gas concentration XC.sub.B of the inlet natural gas stream to the acid gas concentration C.sub.S of the final outlet natural gas stream; and wherein 1.1<X3.5; and wherein 1.1Y1.3.

2. A system according to claim 1 further comprising the first and second membrane devices including a spiral wound glassy polymer membrane device.

3. A system according to claim 1 further comprising the first and second membrane devices including a hollow fiber glassy polymer membrane device.

4. A system according to claim 1 further comprising the first and second membrane devices including a membrane selected from the group consisting of cellulose acetate, cellulose triacetate, polyimide, polyamide, polysulfone, and multi-layer composite.

5. A system according to claim 1 further comprising a thermal oxidizer arranged to receive the portion of the permeate flow exiting the primary membrane unit and a permeate flow exiting the secondary membrane unit.

6. A system according to claim 1 further comprising a low fuel gas system arranged to receive a portion of the non-permeate flow exiting the secondary membrane unit.

7. A system according to claim 1 wherein 1.1Y1.2 and wherein 1.15X3.5.

8. A system according to claim 1, wherein 1.15<Y1.3.

9. A membrane permeation process to achieve an outlet natural gas stream containing an acid gas concentration C.sub.S, the process comprising: passing an inlet natural gas stream through a primary membrane unit housing a quantity YM.sub.B of first membrane devices, the inlet natural gas stream to the primary unit containing a first acid gas concentration C.sub.B and, over time, a second different higher acid gas concentration XC.sub.B, where C.sub.B>C.sub.S and 1.1<X3.5; and passing at least a portion of a permeate flow exiting the primary membrane unit through a secondary membrane unit arranged in series with the primary membrane unit; the secondary membrane unit housing a quantity YN.sub.B of second membrane devices; wherein M.sub.B+N.sub.B is a predetermined quantity of said membrane devices required for a first removal duty to reduce the first acid gas concentration C.sub.B to the acid gas concentration C.sub.s; and wherein YM.sub.B+YN.sub.B is a quantity of said membrane devices required for a second higher removal duty to reduce the second acid gas concentration XC.sub.B to the acid gas concentration C.sub.s; and wherein 1.1Y1.3.

10. A process according to claim 9 wherein the first and second membrane devices include a spiral wound glassy polymer membrane device.

11. A process according to claim 9 wherein the first and second membrane devices include a hollow fiber glassy polymer membrane device.

12. A process according to claim 9 wherein the first and second membrane devices include a membrane selected from the group consisting of cellulose acetate, cellulose triacetate, polyimide, polyamide, polysulfone, and multi-layer composite.

13. A process according to claim 9 further comprising routing a portion of the permeate flow exiting the primary membrane unit and a permeate flow exiting the secondary membrane unit to a thermal oxidizer.

14. A process according to claim 9 further comprising routing a portion of the non-permeate flow exiting the secondary membrane unit to a low fuel gas system.

15. A process according to claim 9 further comprising: compressing at least a portion of a permeate flow exiting the primary membrane unit; and routing a portion of the permeate flow existing the primary membrane unit to a bypass loop.

16. A process according to claim 9 further comprising: compressing at least a portion of a non-permeate flow exiting the secondary membrane unit; and recycling the compressed non-permeate flow to the inlet natural gas stream.

17. A process according to claim 9, wherein 1.15Y1.3.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) FIG. 1 is a schematic of a preferred embodiment of a system and process for acid gas removal by membrane permeation. A flexible equipment and control system configuration allows maximal efficiency and continued use of the initially installed pretreatment, membrane and compression equipment to handle a very wide range of inlet gas compositions over the life of the project.

(2) FIG. 2 is a schematic of another preferred embodiment of the system and process, with the primary and secondary membrane units arranged in series as trains.

ELEMENTS AND NUMBERS USED IN THE DRAWINGS

(3) 10 System and process

(4) 15 Inlet natural gas stream

(5) 20 Primary membrane unit or train

(6) 23 Non-permeate stream (sales gas)

(7) 25 Permeate flow stream

(8) 27 Portion of permeate stream bypassing compression

(9) 30 Compressor or compression step

(10) 35 Compressed permeate stream

(11) 40 Secondary membrane unit or train

(12) 43 Non-permeate flow or stream

(13) 45 Permeate flow or stream

(14) 47 Fuel gas

(15) 50 Compressor or compression step

(16) 55 Compressed non-permeate stream

(17) 60 Thermal oxidizer (could also be a flare or some other downstream process)

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(18) Referring to FIGS. 1 and 2, embodiments of a system and process 10 for acid gas removal are arranged to receive an inlet natural gas stream 15 from one or more natural gas wells or fields. Although the inlet natural gas stream 15 increases in acid gas concentration over time, the system and process 10 accommodate this increase without additional equipment being added and without relying upon any increase in downstream amines or physical solvents. The glassy polymer membranes used in the system and process 10 are selected so removal duty efficiency increases as acid gas concentration increase. Designing the system and process to handle about a 15% increase in acid gas concentrations over initial conditions effectively treats acid gas concentrations well above that 15% increase, thereby eliminating the need for additional equipment or additional downstream amines or physical solvents.

(19) In one preferred embodiment, the glassy polymer membranes are arranged in a primary and a secondary membrane unit to handle acid gas concentrations more than twice that of the base (lowest expected) acid gas concentration at the same inlet gas flow rates. In another preferred embodiment, the glassy polymer membranes are arranged in a primary and a secondary membrane train to handle acid gas concentrations at the same inlet gas flow rates up to 3.5 times or more than that of the base acid gas concentration (e.g. 20% CO.sub.2 increasing to 70%). Also, as the acid gas concentration in the inlet gas increases, the existing membrane plant inlet flow capacity could simultaneously increase up to 20% to 25% without adding additional equipment.

(20) The system and process 10 use a primary membrane unit or train 20 arranged to receive the inlet gas stream 15 using one or more membrane steps that incorporate spiral wound or hollow fiber glassy polymer membranes and a secondary membrane unit or train 40 arranged in series to the primary unit or train 20 and uses one or more membrane steps that incorporate spiral wound or hollow fiber glassy polymer membranes. The secondary unit or train 40 receives all or a portion of the permeate stream 25 exiting the primary unit or train 20. Each unit or train 20, 40 incorporates spiral wound or hollow fiber glassy polymer membranes. When arranged as a train, the membranes within each train 20, 40 are in series with one another so that a portion of the permeate can be routed to an immediate downstream membrane unit within the train 20 or 40, and permeate pressures within the trains can be adjusted to match the current process requirements.

(21) As CO.sub.2 or H.sub.2S inlet concentrations increase, permeate flow 25 increases from the primary membrane unit or train 20. All or some portion of the permeate flow 25 can be compressed 30 and a compressed permeate stream 35 is routed to the secondary membrane unit or train 40, with another portion 27 bypassing compression 30 and being routed to a thermal oxidizer 60.

(22) All or a portion of the non-permeate flow 43 from the secondary membrane unit or train 40 can be compressed 50 in a recycle loop and blended with the inlet natural gas stream 15 flowing into the primary membrane unit or train 20. Another portion of the non-permeate flow 43 can be routed as a fuel gas 47. Similar to the bypass permeate flow 27 from the primary membrane unit or train 20, permeate flow 45 from the secondary membrane unit or train 40 is routed to the thermal oxidizer 60.

(23) Preferably, the glassy polymers used in each unit or train 20, 40 are the same type or kind of glassy polymer as those used in the other unit or train 40, 20, although the quantity or surface area of the membranes can differ between the two. Regardless of the quantity of glassy polymer or the surface area, the glassy polymer selected should be one that can provide increasing efficiency as CO.sub.2 or H.sub.2S acid gas inlet concentrations increase over time. Examples of glassy polymers include but are not limited to cellulose acetate, cellulose triacetate, polyimide, polyamide, polysulfone, and multi-layer composite membranes in either spiral wound or hollow fiber configurations.

(24) By way of example, a cellulose triacetate membrane can provide increasing efficiency as acid gas inlet concentrations increase over time. The CO.sub.2 removal capacity of this particular hollow fiber membrane gets increasingly higher as inlet CO.sub.2 concentration increase due to increasing partial pressures of CO.sub.2 in the inlet gas stream and CO.sub.2 solubility enhanced flux of the glassy polymer. Note that the same holds true for H.sub.2S, which typically tracks along with the rate of CO.sub.2 permeation.

(25) In an example case, at the same inlet flow rate the total CO.sub.2 removal duty increases 233% for an inlet gas stream that increases from 20% to 40% CO.sub.2 and up to 367% for an inlet gas stream that increases to 60% CO.sub.2 (see Table 1). Although acid gas concentration has doubled and tripled from base conditions, a preferred embodiment of the system and process need only be initially designed for 115% of base conditions to handle all cases, including 70% CO.sub.2. In other words, increasing the total amount of the glassy polymers used in the units 20, 40 in range of 1.05 to 1.3 relative to the total amount needed to handle base conditionsand even more preferably about in a range of about 1.1 to 1.2provides a removal duty in a range of 2 to 3.5 times that relative to the base case. This kind of increasing efficiency with rising CO.sub.2 concentration is not found or achievable in conventional amine or physical solvent systems. Additionally, operating costs at higher acid gas concentrations are significantly lower than traditional designs.

(26) TABLE-US-00001 TABLE 1 Example system and process efficiency with increasing CO.sub.2 concentrations. Operation at 650 psia, assuming 2500 MM inlet gas, and 6% CO.sub.2 outlet from membranes CO.sub.2 Removal CO.sub.2 Removal Duty Quantity of Duty as a % of Membranes as % Inlet CO.sub.2% (mmscfd) Base Case increase over Base 20 375 100 Base 100 27 550 147 110 30 625 167 113 35 750 200 115 40 875 233 113 60 1375 367 111

(27) In addition to the primary and secondary membrane units or trains 20, 40, other aspects of the system and process design 10 can be optimized, including the primary permeate compression 30, recycle compression 50, and control systems and piping configurations controlling the amount of permeate 25 bypassing the secondary membrane unit or train 40 and the amount of non-permeate 43 being recycled from this unit or train 40. Compression 30, 50 capacity could also be diverted to sales gas booster compression or to boost final outlet permeate gas pressure. This approach enables seamless operations as CO.sub.2 or H.sub.2S concentrations increase in the inlet stream 15.

(28) For example, low inlet CO.sub.2 gas requires a higher percentage of primary permeate gas 25 to be compressed 30 and sent to the secondary membrane unit or train 40 for additional hydrocarbon recovery. By contrast, when operating at higher inlet CO.sub.2 conditions, more primary permeate gas 25 can bypass compression 30 and the secondary membrane unit 40 yet still achieve hydrocarbon recovery goals. Bypassing the secondary membrane unit 40 allows the redeployment of some or all of the former compression 30 or 50 to other duties, such as sales gas booster compression or, in the case of enhanced oil recovery, the former compression 30 or 50 can be redeployed to boost final outlet permeate gas pressure to reduce downstream reinjection compression requirements. The flexibility of this design allows for changes to other equipment duties. For example, as CO.sub.2 or H.sub.2S increases in the inlet gas stream 15, the gas flow volumes from the primary and secondary membrane units or trains 20, 40 change significantly. This, in turn, requires changes in compression and flow control to optimize performance.

(29) Use of the primary permeate 25 compression 30 can be adjusted with the recycle compression 50 as the gas flows in and out of compression change, or gas flow to compression 30 may be reduced by means of flow balancing that bypasses the secondary permeate and sales gas service. Additionally, the initial quantity of membranes used can be reconfigured or adjusted between identical designed primary and secondary membrane units or trains 20, 40.

(30) The total quantity Q of serial arranged glassy polymer membrane devices used in system and process 10 is:
Q=Y(M.sub.B+N.sub.B), XC.sub.B.fwdarw.C.sub.s(Eq. 1) where M.sub.B and N.sub.B is a predetermined quantity of glassy polymer membrane devices in membrane unit or train 20 and 40, respectively, effective to reduce an expected minimum (base case) acid gas content C.sub.B of the inlet gas stream (e.g. 20%, 25% CO.sub.2) to a required non-permeate acid gas content specification C.sub.s (e.g. 5%, 6% CO.sub.2); and where Y(M.sub.B+N.sub.B) is effective to reduce an acid gas content XC.sub.B of the inlet natural gas stream to the required non-permeate acid gas content specification C.sub.S when XY and when X>Y; X is in a range of 1.0 to 3.5, Y is in a range of 1.05 to 1.3, more preferably in a range of 1.1 to 1.2.

(31) Note that Y affects the capital cost and X/Y is the increased efficiency relative to the capital cost. In the inventive system and process, Y<X through a wide range of inlet acid gas conditions to achieve the same non-permeate acid gas content specification. Because the rate of acid gas permeation (flux) in certain glassy polymer membranes increases with the rise in inlet CO.sub.2 gas composition so the system can remove more acid gas at higher inlet CO.sub.2 percentages, and unlike solvent systems which have fixed acid gas removal capacity, the Q glassy polymer devices can effectively treat, without additional equipment, acid gas content well above 15% and up to 350% over base conditions for a given inlet gas stream flow rate.

(32) The preferred embodiments of the system and process are provided as illustrative examples. The following claims define the scope of the invention and include the full range of equivalents to which the recited elements are entitled.