WO2005086963A2

WO2005086963A2 - System and method for removing a contaminant vapor from a fluid

Info

Publication number: WO2005086963A2
Application number: PCT/US2005/008242
Authority: WO
Inventors: Thomas R. Riesterer; Kovvali A. Sarma; Wayne E. Sudos
Original assignee: Baker Hughes Incorporated
Priority date: 2004-03-11
Filing date: 2005-03-11
Publication date: 2005-09-22
Also published as: WO2005086963A3

Abstract

A system for reducing the concentration of a contaminant vapor in a gas stream comprises a vessel having an inlet header for injecting a sour gas into the vessel. A pump injects a scavenging fluid into the vessel. A porous media in the vessel causes the contaminant vapor to contact the scavenging fluid for at least a predetermined time.

Description

SYSTEM AND METHOD FOR REMOVING A CONTAMINANT VAPOR FROM A FLUID BACKGROUND OF THE INVENTION

Cross Reference to Related Applications This application claims the benefit of U.S. Provisional Application No. 60/552,295, filed March 11, 2004.

Field of the Invention This invention relates generally to the removal of contaminants from a fluid and more particularly to a system and process for removing contaminant vapors, such as, for example, hydrogen sulfide, from a fluid stream while not substantially impeding the flow of the fluid stream.

Description of the Related Art A number of contaminant vapors, also called fugitive vapors, may be present in, or released from, various industrial and waste fluid streams. In addition, such fugitive vapors are commonly emitted from settling ponds and landfills. Many such fugitive vapors are considered toxic and their removal is necessary for health and safety reasons. For example, the toxicity of hydrogen sulfide in fluid streams such as hydrocarbon liquid streams and hydrocarbon gas streams (also known as sour gas) is well known in the industry and considerable expense and efforts are expended annually to reduce its content to a safe level. Some regulations, for example, require pipeline transported gas to contain no more than 4 parts-per-million (ppm) hydrogen sulfide. If the quantity of H₂S in the fluid stream exceeds such limits, it must be removed using such techniques as (I) absorption in an alkaline solvent, (ii) reaction of the H₂S on a solid particle, (iii) reaction with a non-regenerative scavenger such as triazine. In large production facilities, it is generally more economical to install a regenerative system for treating sour gas streams. These systems typically employ a compound used in an absorption tower to contact the produced fluids and selectively absorb the hydrogen sulfide and possibly other toxic materials such as carbon dioxide and mercaptans. The absorption compound is then regenerated and reused in the system. Typical hydrogen sulfide absorption materials used in such installations include alkanolamines, PEG, hindered amines, and the like. Commonly, the process can involve the bubbling of a sour gas stream upward through a vertical column filled with a scavenging liquid. For a sour liquid/gas stream, in another example, the process may inject a liquid scavenger into a flow moving in such a manner to allow mixing and contact time for the scavenging process. Such processes commonly require sufficient flow energy and upstream pressure such that the pressure drop through the treatment system can be adequately accounted for. Some wellhead gas pressures are relatively low such that the back pressure of the previously described processes greatly reduce the amount of flow of the gas stream through such devices. This renders these processes unusable for such low pressure applications. The present invention addresses the above-noted problems and provides a system and process for removing fugitive vapors, such as hydrogen sulfide, from a fluid stream while not substantially impeding the flow of the fluid stream.

SUMMARY OF THE INVENTION In one aspect the present invention, a system for reducing the concentration of a contaminant vapor in a gas stream, comprises a vessel having an inlet header for injecting a sour gas into the vessel. A pump injects a scavenging fluid into the vessel. A porous media in the vessel causes the contaminant vapor to contact the scavenging fluid for at least a predetermined time. In another aspect, a method for reducing a hydrogen sulfide(H₂S) concentration in a gas stream, comprises injecting a sour gas through an inlet header into a vessel. A scavenging fluid is pumped into the vessel. A porous media in the vessel causes the sour gas to contact the scavenging fluid for at least a predetermined time. Examples of the more important features of the invention thus have been summarized rather broadly in order that the detailed description thereof that follows may be better understood, and in order that the contributions to the art may be appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form the subject of the claims appended hereto.

BRIEF DESCRIPTION OF THE DRAWINGS

For detailed understanding of the present invention, references should be made to the following detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, in which like elements have been given like numerals and wherein: Figure 1 is a depiction of a sour gas scavenging system attached to a wellhead; Figure 2 is a depiction of a sour gas scavenging system; Figure 3 is a depiction of a sour gas scavenging system attached to a storage tank; and Figure 4 is a depiction of a sour gas scavenging system attached to a truck mounted storage tank.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The description herein is directed to particular embodiments of the present invention that are intended as examples, and not limitations, of the present invention. Fig. 1 shows a common low pressure well 30 with wellhead 31 wherein the production gas 31 is at a pressure in the range of 10 to 15 psig. The production gas may contain unacceptable levels of contaminant gas such as hydrogen sulfide (H₂S) that must be removed for safe shipment and/or handling. Such production gas is commonly referred to as sour gas. The relatively low wellhead pressure precludes the use of conventional systems for removing the H₂S. A low pressure drop sour gas scavenging system 20 of the present invention may be used to remove sufficient amounts of the H₂S to conform to acceptable levels for transportation and/or handling without causing a substantial impediment to flow. In one embodiment of the present invention, see Fig. 2, a low pressure drop sour gas scavenging system 20 comprises a substantially horizontal vessel 1 having an inlet line 2 attached thereto. Vessel 1 may be any suitable cross-sectional shape to contain the internal fluid pressure in vessel 1. The vessel cross sectional area is application dependent in order to provide a relatively low pressure drop. Such sizing calculations are known in the art and the vessel dimensions may be determined for a particular flow rate without undue experimentation. Inlet line 2 is coupled to inlet header 10 that extends the length of vessel 1. Inlet header 10 has multiple radially oriented apertures 9 along the length of inlet header 10 to allow inlet sour gas 11 to exit inlet header 10 and enter the interior of vessel 1. Apertures 9 may be any suitable shape including, but not limited to circular holes, elliptical holes, axial slots, and circumferential slots. Only one inlet header is shown, however, multiple inlet headers may used to increase the dispersion of inlet sour gas in vessel 1. Running substantially parallel to inlet header 10 is scavenging fluid header 4 that runs substantially the length of vessel 1. Scavenging fluid header 4 has multiple spray nozzles 5 disposed along the length of the header 4 for spraying droplets of scavenging fluid 14 into vessel 1. The absorption zone 13 of vessel 1 contains porous media 6 that separates inlet header 10 and scavenging fluid header 4 from outlet header 3. In one embodiment, porous media 6 is an expanded plastic mesh material having a void fraction greater than 50%. Alternatively, any structured or unstructured porous media known in the art having a void fraction greater than 50% may be used in the present invention. Examples include, but are not limited to, Raschig rings, Berl saddles, and Pall rings. Scavenging fluid 14 substantially saturates porous media 6 such that sour gas 11 is forced into contact with scavenging fluid 14 as it flows from inlet header 10 through porous media 6 to outlet header 3. One skilled in the art will appreciate that the expanded mesh material provides substantially greater surface area for contact between sour gas 11 and the scavenging fluid 14 than would simple interaction of droplets of scavenging fluid 14 intermixed with sour gas 11. The larger surface area increases the efficiency of the scavenging process compared to one without the porous media. Note that the inlet and outlet flow piping is exemplary only. Any number of inlet and outlet flow configurations will be apparent to one skilled in the art and the present invention is intended to cover all such configurations. For example, outlet header 3 may be oriented in a substantially vertical manner to prevent substantial droplet entrainment in the exiting outlet flow 12. In one embodiment, scavenging fluid 14 is a triazine compound. The scavenging fluid 14 passes downward, due to gravity, through porous media 6 and falls into sump 7 at the bottom of vessel 1. From there, the fluid is re-circulated by pump 19 through lines 8 and 9 back to scavenging fluid header 4. Controller 22 takes H₂S readings from sensor 21 located in output flow line 3 and adjusts the flow of scavenging fluid 14 by adjusting the output of pump 19 to maintain a predetermined H₂S content in output flow 12. Controller 22 may have a processor and memory for storing absorption models based on different flow constituents. Controller 22 may also store sensor 21 readings for future retrieval and analysis. Alternatively, the pump may be manually adjusted based on the H₂S concentration of outlet flow 12. Alternatively, other scavenging fluids known in the art, such as those known for removing acid gases, may be used. Examples of such fluids include, but are not limited to, alkanolamines (such as monoethanol amine, ethanol diamine, and methyl diethanolamine) and amine-adehyde condensates. Alternatively, the system described herein may be used to remove contaminant acid gases such as, for example, carbon dioxide (CO₂) and sulfur dioxide (SO₂) from the input gas stream. The thickness of porous media 6 is determined, using techniques known in the art, such that there is at least a three second contact time between the sour gas and the scavenging fluid in the porous media. This contact time provides an outlet flow having an H₂S concentration of less than 4 ppm for inlet gas flow rates up to one million standard cubic feet per day. The relatively low flow restriction of the system causes a pressure drop of no more than 5 psi at the rated flow. Alternatively, the flow restriction causes a pressure drop of approximately 1 inch of water at the rated flow rate. In another embodiment, see Fig. 3, a hydrocarbon storage tank 35 has a hydrocarbon liquid 36 with a gas 37 in a space above liquid 36. The gas 37 may contain contaminant vapors such as, for example, H₂S. In order to safely transport the gas from tank 35, it is passed through a sour gas scavenging system 20 as previously described. In some cases, the vapor pressure of gas 37 is too low to force all of the gas through the connecting piping and system 20 and a suctioning device, such as blower 38, is used to suction gas 37 from tank 35. The gas 37 is treated in system 20 and the outlet gas 39 is transported to another location. In yet another embodiment, hydrocarbon storage tank 41 is transported on truck 40 and has liquid hydrocarbon 42 and gas 43 contained therein. Gas 43 may contain unacceptable concentrations of contaminant gas such as, for example, H₂S. When offloading gas 43, it is passed through sour gas scavenging system 20, as previously described, and the resultant sweet gas 35 is transported to a suitable location. As with the stationary tank of Fig. 3, a suctioning device (not shown) may be used to assist in suctioning the gas 43 from tank 41. While described above for particular exemplary applications, the present invention is intended for use in any application requiring the removal of contaminant gas from a gas stream. The following are exemplary applications. Production Tank Vents: where the production tank vents contain contaminant gases such as, for example, H₂S which needs to be removed for meeting regulatory compliance standards. Casing Vents: the casing gas from wells needs to be vented for recovery of hydrocarbons. For various reasons, the casing gas can contain contaminant gases such as, for example, H₂S which needs to be removed for meeting compliance and to continue production. Tank truck venting while loading and in transit: tank trucks carrying sour fluids (sour water or sour hydrocarbons) will have contaminant gases such as, for example, H₂S in the headspace. The H₂S from the headspace needs to be treated for safe transport and loading and unloading operations. Pig receiver venting: When pigs (disks used for cleaning the pipelines) are employed, they are usually collected in a place at the end of the pipeline for recovering them. If sour fluids are present, in the pig receiver section, the fluids have to be bled to atmosphere before the pig can be recovered. De-sand pits: Any settling areas for sand sediments (large settling tanks) used for heavy crude recovery. The tanks can have contaminant gas headspace. Vacuum Truck operations: large vacuum trucks pull vacuum on fluid transport trucks to pull fluids out of the trucks. The large vacuum trucks are vented to atmosphere. If the fluids contain contaminant gas, the vacuum truck vents need to be treated ro remove such contaminants. Sour Gas Dehydration operations: During regeneration of glycol, H₂S is released in the vapor phase. The H₂S must be treated before the vapor phase can be vented. Acid related work over fluids: H₂S can be generated during workover if iron sulfide is present in the fluid. Sour fuel gas operations: In some applications, natural gas is used as fuel gas to satisfy on-location energy demands. The gas volumes that are treated can be small. The H₂S from gas needs to be removed before the natural gas can be used for the energy generation. Instrument gas: In some applications, natural gas is used as instrument gas. The gas volumes that are treated can be small. The H₂S from natural gas needs to be removed before the natural gas can be used as instrument gas. The foregoing description is directed to particular embodiments of the present invention for the purpose of illustration and explanation. It will be apparent, however, to one skilled in the art that many modifications and changes to the embodiment set forth above are possible. It is intended that the following claims be interpreted to embrace all such modifications and changes.

Claims

1. A system for reducing the concentration of a contaminant gas in a gas stream, comprising: a. a vessel; b. an inlet header injecting the gas stream into the vessel; c. a pump injecting a scavenging fluid into the vessel; and d. a porous media causing the contaminant gas to contact the scavenging fluid for at least a predetermined time.

2. The system of claim 1, wherein the porous media has a void fraction greater than 50%.

3. The system of claim 1, further comprising an exit header for removing a scavenged outlet gas.

4. The system of claim 3, wherein the contaminant gas is hydrogen sulfide(H₂S) and the scavenged outlet gas has an H₂S concentration not greater than 4 part per million(ppm).

5. The system of claim 1, wherein the scavenging fluid comprises a triazine compound.

6. The system of claim 1, wherein the porous media comprises an expanded plastic mesh material.

7. The system of claim 4, further comprising a sensor disposed in said exit header for detecting H₂S in said scavenged outlet gas.

8. The system of claim 7, further comprising a controller, said controller cooperatively coupled to said pump and the sensor, said controller adjusting said pump injection of scavenging fluid to said vessel to maintain the H₂S concentration in said scavenged outlet gas to not greater than 4 ppm.

9. The system of claim 1, wherein the predetermined time is at least three seconds.

10. The system of claim 1, further comprising a suctioning device for assisting removal of the gas stream from a hydrocarbon storage tank and inputting said gas to the vessel.

1 1. A method for reducing a contaminant gas concentration in a gas stream, comprising: a. injecting the gas stream through an inlet header into a vessel; b. pumping a scavenging fluid into said vessel; and c. causing said the gas stream to contact said scavenging fluid in a porous media in said vessel for at least a predetermined time.

12. The method of claim 1 1, wherein the porous media has a void fraction greater than 50%.

13. The method of claim 11, further comprising removing a scavenged outlet gas through an exit header.

14. The method of claim 13, wherein the contaminant gas is hydrogen sulfide (H₂S) and the scavenged outlet gas has an H₂S concentration not greater than 4 part per million(ppm).

15. The method of claim 11, wherein the scavenging fluid comprises a triazine compound.

16. The method of claim 11, wherein the porous media comprises an expanded plastic mesh material.

17. The method of claim 14, further comprising detecting H₂S in said scavenged outlet gas with a sensor disposed in said exit header.

18. The method of claim 17, further comprising controlling the pumping of scavenging fluid to said vessel to maintain the H₂S concentration in the scavenged outlet gas to not greater than 4 ppm.

19. The method of claim 11, wherein the predetermined time is at least three seconds.

20. The method of claim 11, further comprising suctioning the gas stream from a hydrocarbon storage tank and inputting the gas stream to the vessel.