Prevention of shearing of droplets to aerosol sizes

Abstract: This invention pertains to addition of polyisobutylene to a predominantly gaseous stream in a process for preventing the shearing of hydrocarbon droplets in the stream to aerosol sizes in predominantly gaseous streams. It is difficult to remove droplets of aerosol sizes from gaseous streams with demisters, scrubbers, filters or the like. Generally, aerosol size droplets are created when a predominantly gaseous stream containing some liquid is passed at relatively high velocity through an orifice, valve, bend, tee, choke or the like, or subjected to compression by a turbine. The addition of polyisobutylene would improve the separation efficiency of the demisters, scrubbers, filters or the like. In addition, when the demister pads or compressor has its blades coated with polyisobutylene, drag on the droplets is reduced, resulting in less shearing of the droplets to aerosol sizes. This in turn would result in minimum dissemination of droplets and less wear on the compressor's blades.

Description: This invention pertains to a process for preventing the shearing of droplets to aerosol sizes in predominantly gaseous streams. It is difficult to remove droplets of aerosol sizes from gaseous streams with demisters, scrubbers, filters or the like. Generally, aerosol size droplets are created when a predominantly gaseous stream containing some liquid is passed at relatively high velocity through an orifice, valve, bend, tee, choke or the like, or subjected to compression. U.S. patent specification No. 3,996,023 discloses a polymer dissolved in a liquid hydrocarbon fuel to reduce the tendency of the fuel to particulate dissemination when the fuel is subjected to shock. Column 4, line 2 mentions the polymer may be derived from isobutylene. European patent specification EP-78998A discloses a liquid hydrocarbon fuel containing dissolved polyisoprene to reduce mist formation. Czech patent specification CS 225800-B discloses the treating of cutting oil with a polyisobutylene agent to decrease the formation of aerosols. U.S. patent specification No. 4,758,354 discloses, column 1, lines 52-55, that certain linear polymers such as high molecular weight alpha-monoolefinic polymers are noted for their effectiveness as drag-reducing agents and as anti-misting agents. Column 3, line 39 of this patent specification mentions polyisobutylene. In summary, the foregoing prior art references disclose methods to treat liquid hydrocarbons with compositions which may include polyisobutylene. European patent specification EP-112053 discloses the use of a mineral oil based scrubbing oil containing polyiso-butylene for purifying exhaust gases or waste air (page 1, line 5, page 3, line 25 and page 7, claim 1) from a manufacturing process, for example rolling of metal. EP 0 246 826 A2 discloses a method whereby liquid is injected into a hydrocarbon-rich gas stream upstream a chiller to enhance coalescence of small droplets during flow of the stream through the chiller. The liquid is injected downstream other processing equipment, such as compressors. A method has now been discovered for facilitating the removal of a hydrocarbon mist from a gaseous stream. The method comprises mixing the mist in the gaseous stream with an agent functional to substantially prevent shearing of the mist to smaller particle sizes which are difficult to remove from the gaseous stream in separation apparatus. The method according to the invention is defined in claim 1. The agent is polyisobutylene which is mixed with a diluent such as diesel oil. More preferably, the agent is sprayed into the gaseous stream in a direction countercurrent to the flow of the gaseous stream. Most preferably, the agent is sprayed through an elongated spray nozzle, or the like, having orifices pointed substantially'in one direction to facilitate mixing through counter-current spraying. Shearing of the mist to smaller particle sizes occurs in compression as well as in flow of the mist through orifices. Typically, the separation apparatus is a demister followed by a coalescing filter or a scrubber. A preferred use for the invention is with a production stream containing predominantly natural gas, some oil, moisture and hydrogen sulphide which is treated to remove substantially all of the oil, then treated with said agent to prevent shearing of the remaining oil to smaller particle sizes in the following shearing steps wherein the gas is subjected to steps such as compression, scrubbing or filtering to remove the remaining oil, and then sent with the hydrogen sulphide to a process for oxidizing the sulphide to elemental sulphur. Another preferred use of the invention is in the treatment of a production stream from an offshore well containing predominantly natural gas with some oil which is treated, first to remove most of the oil, and then mixing the remaining stream with an agent such as polyisobutylene to prevent shearing of oil droplets to aerosol sizes. Then the production stream is filtered or scrubbed to remove the remaining oil so that a purified natural gas stream remains in the pipeline. The scrubber/filter surfaces are preferably coated with polyisobutylene which acts as a drag reducer to the impinging droplets thereby minimizing their dissemination and increasing their coalescence and drainage. Another preferred use of the invention is in the treatment of fuel gas upstream of compressors to assist in separating the liquid hydrocarbon through scrubbers or filters. These and further aspects of the method according to the present invention are disclosed in the accompanying claims, abstract and drawings. In the drawings: Fig. 1 is a schematic view of a gas compression facility at Denver City. Fig. 2 is a schematic view of a gas plant at Yellow Hammer. Fig. 3 is a schematic view of an offshore well facility at the Auger Platform. Fig. 4 is a schematic view of an atomization apparatus. Fig. 5 is a schematic view of an alternative atomization apparatus. Fig. 6 is a description of the mechanism by which droplet size is kept intact. Figs. 7-9 show frequency of distribution in relation to droplet size. The present invention is directed to separating a hydrocarbon mist from a gaseous stream. This is very difficult where some or all of the mist is of aerosol size (less than one micron), which passes through a conventional filter or scrubber. Accordingly, it is necessary to prevent the mist from being degraded to aerosol sizes. Degradation primarily occurs in the shearing steps of two operations: passing the mist through an orifice at high velocity or subjecting the mist to compression, which entails flow of the gas through compression stages at high velocity. Hence, the present invention adds an agent to the mist which substantially prevents the occurrence of degradation during the shearing steps or during the impingement of droplets on surfaces. Also, the agent will coat the surfaces which it contacts, and condensate droplets will bond to the agent located on the surfaces and then further droplets will collect to form larger droplets which will then drain. The agent is a high molecular weight polymer, preferably polyisobutylene which is mixed with a diluent such as diesel oil, crude oil, condensate, toluene, xylene, paraffinic oil, or the like. The agent is preferably used at a concentration of 2-7 %v liquid hydrocarbon to active ingredient in liquid hydrocarbon. The agent preferably is sprayed in a mist form into the gaseous stream in a direction countercurrent to the flow of the gaseous stream. More preferably, the agent is sprayed through an elongated spray nozzle, or the like, having orifices pointed upstream. The chemical would collide with the hydrocarbon droplets and prevent them from further shearing into smaller particle sizes before their being separated in a demister, coalescing filter or a scrubber. A preferred use for the invention is with a production stream containing predominantly gas, some oil and hydrogen sulphide which is treated to remove substantially all of the oil, then treated with said agent to prevent shearing of the remaining oil to smaller particle sizes in the following shearing steps that the gas is subjected to such as compression, scrubbing or filtering to remove the remaining oil, and then sent with the hydrogen sulphide to a process for oxidizing the sulphide to elemental sulphur. Fig. 1 shows this type of arrangement at the Denver City gas compression facilities. Production gas stream 1, for example 100 MMSCFD (= about 37,300 million Nm 3 , i.e. normal cubic metres, where normal means that the volume is measured at 0 °C and at a pressure of 1 bar) is passed to a gas/liquid separator and then to an inlet scrubber 3 to get rid of a substantial part of its oil and water. The stream coming out of inlet scrubber 3 is at a temperature of approximately 29.5 °C and a pressure of approximately 100 N/cm 2 . An injection via line 4 of polyisobutylene is made into the outlet stream of the inlet scrubber 3. The gas stream 1 is then passed through knockout vessel 5 which has internal demisting vanes for further removing oil, then to a fin fan cooler 6, and then through a compressor 7. Coating of the demister vanes with the polyisobutylene functions to reduce the drag on the droplets, thereby reducing their dissemination and increasing the potential for droplets coalescing. At this stage the stream 1 is at a temperature of approximately 37.8 °C and a pressure of approximately 200 N/cm 2 . Then the stream 1 is passed through filter vessel 8, being preferably of about five micron size, and finally to a coalescer filter 9. It is in compressor 7 that mists would be degraded to aerosol sizes but for the present invention. Coating of the vanes of the compressor with polyisobutylene functions to reduce the drag on droplets passing through the compressor, thereby reducing the formation of aerosol droplets. Stream 9A is passed to a unit (not shown) for chemical treatment to oxidize sulphides (hydrogen sulphide) contained in the stream to elemental sulphur. In the Denver City process, described hereinabove, oil carryover is about 1590-3180 litres of oil per day without the polyisobutylene chemical of this invention in use. With the polyisobutylene in use in the Denver City process at a concentration of 20 to 100 ppm, the oil carryover was reduced to non-detectable quantities. Another application of the invention is shown in Fig. 2 in the treatment of a gas plant stream, in the Yellow Hammer Gas Plant. Gas production stream 10, for example from 160 to 230 MMSCFD (= about 5968-8580 million Nm 3 /day), is first passed to a gas/liquid separator 11 where a substantial majority of the liquid in stream 10 is separated. The stream exiting the separator is at high pressure, for example about 660 N/cm 2 . Chemical injection into this stream, preferably polyisobutylene, is made via line 12. The gas production stream is then admitted to a high pressure scrubber 13. The stream exiting scrubber 13 is at a temperature of, for example, 37.8 °C and is admitted to a coalescer filter 14. The filter of this unit preferably has a fineness of 0.3 to 1 micron. Polyisobutylene coating the interior surfaces of scrubber 13 and filter 14 functions to reduce drag of droplets passing therethrough, thereby reducing the formation of aerosol size droplets (which are generally difficult to separate) and enhancing their coalescence and drainage. Stream 14A exiting filter 14 is relatively free of droplets and is passed to aqueous MDA treatment (methyl diethanol amine) plus a Claus sulphur plant plus triethylene glycol gas dehydration. In the Yellow Hammer process, described hereinabove, with use of the polyisobutylene chemical at a concentration of 20-100 ppm, oil carryover was reduced from 1590-3180 litres per day to less than 31.8 litres per day. Still another use of the invention is in the treatment of a production stream from an offshore well containing predominantly natural gas with some oil which is treated, first to remove most of the oil, and then mixing the remaining oil with an agent such as polyisobutylene to prevent shearing of the oil droplets to aerosol sizes. Then the production stream is filtered or scrubbed to remove the remaining oil before it is dehydrated in the glycol contactor. Figure 3 shows this type of arrangement, at the Auger Platform. High pressure gas from wells is brought via line 15, to a high pressure gas separator 16. High pressure oil from wells is brought via line 17 to a high pressure oil separator 18 and via line 19 to high pressure oil separator 20. Stream 15 has a lower oil to gas ratio than stream 17. Intermediate pressure production 21 from wells is combined with bottom streams from vessels 16, 18 and 19 and brought to intermediate pressure separator 22. A stream 23 from the wells goes to a test separator 24 where it is learned how best to treat the stream in the process. Stream 24A, an oil-rich stream from test separator 24, is recycled to an upstream separator such as intermediate pressure separator 22. The chemical agent of the present invention, polyisobutylene, is injected via line 25 into the overhead stream from vessel 18. Overhead streams from vessels 16 (25.6 MMSCFPD = about 955 million Nm 3 /day), 18 (59.4 MMSCFPD = about 2215 million Nm 3 /day), 20 (28.9 MMSCFPD = about 1078 million Nm 3 /day) and 24 are then combined into a stream at about 1200 N/cm 2 and sent to cooler 26. (The flowrates and pressures listed herein are intended as illustrative and not as limiting.) From the cooler a stream is sent to cyclone separator 27. A stream (147 MMSCFPD = about 5483 million Nm 3 /day), from the cyclone separator is combined with a stream (50 MMSCFD = about 1865 million Nm 3 /day), from a compressor (described hereinafter) at a pressure of about 1100 N/cm 2 . The combined stream from the compressor goes to a high pressure gas filter separator 28. A stream from vessel 28 is sent to glycol contactor 29. Glycol from the contactor is sent to glycol reconcentration 29B, and an overhead stream from the contactor along with streams from vessels 27, 28 and compressor 30 form dry gas 29A sent to sales. Referring back now to the initial separation steps of this embodiment, a stream 31 from low pressure production is combined with a bottom stream 32 from the intermediate pressure separator 22, and the resulting stream is taken to a low pressure separator 33, and an overhead stream (14 MMSCFPD = about 522 million Nm 3 /day) is taken from separator 33 and combined via line 34 with polyisobutylene which is then injected into discharge scrubber 35. An oil-rich bottom stream 34A is sent to an upstream separator such as the low pressure separator 33. An overhead stream (26 MMSCFPD = about 970 million Nm 3 /day) is taken from intermediate pressure separator 22, and combined with polyisobutylene via line 36, and the resulting stream is taken to second stage discharge scrubber 37. A bottom stream is taken from discharge scrubber 35 to first stage suction scrubber 38 and then to compressor 30A. A discharge stream from compressor 30A at about 440 N/cm 2 is merged with the overhead stream from intermediate pressure separator 22, which is taken to second stage scrubber 37 as above noted. An overhead stream is taken from scrubber 37 and passed to first stage suction scrubber 39 and then to compressor 30. As already noted, a discharge stream (46.9 MMSCFPD = about 1450 million Nm 3 /day) is taken from compressor 30 and merged with a stream from cyclone separator 27. Oil-rich bottom streams 35A, 38A, 37A and 39A are preferably recycled to an upstream separator such as intermediate pressure separator 22. The polyisobutylene admitted via lines 34 and 36 assist in removing the liquid hydrocarbon which generally ends up in 30 and 30A. Fig. 4 shows how the chemical, preferably polyisobutylene, is atomized for introduction into the stream being treated. The polyisobutylene together with a diluent as described herein, is injected into line 40 by chemical pump 41. A check valve 42 prevents the backflow of the chemical as high pressure gas, about 33-66 N/cm 2 higher than in line 43, is injected into line 40 via line 44 to atomize and deliver it to line 43 in a mist form. The polyisobutylene, diluent, and gas pass through a second check valve 45, into a nozzle 46, and then into stream 43 via nozzles 47 which are pointed upstream to ensure good mixing. In Fig. 5 a second embodiment of an atomizer nozzle is shown. Pump 50 injects a mixture of polyisobutylene and diluent via line 51, through check valve 52 and nozzle 55, and into line 56. Again orifices in the nozzle are pointed upstream to ensure good mixing. Fig. 6 compares the shearing effects on two droplets of oil 61 and 62, one of which is in the presence of polyisobutylene. The polyisobutylene coats droplet 61 which distorts under shear but resumes its shape after the shearing stops. Uncoated droplet 62 splits into two parts when it is subjected shear. Figs. 7-9 show plots of frequency of distribution versus particle size derived from treatment of solutions of polyisobutylene in crude oil condensate. Two types of polyisobutylene were used: PIB1 and PIB2 with two different molecular weights, the latter being the highest. The solutions were tested by air-blast atomization at air velocities of 190 m/s, 230 m/s, and 270 m/s. In Fig. 7 the air velocity is 190 m/s, the fluid stress is 5400 pa. In Fig. 8 the air velocity is 230 m/s, the fluid stress is 12400 pa. In Fig. 9 the air velocity is 270 m/s and the fluid stress is 20800 pa. For all three figures curve A is for oil, curve B is for 0.125 g per litre of PIB1 in oil. Curve C is for 0.25 g per litre of PIB1 in oil. Curve D is for 0.5 g per litre of PIB1 in oil. Curve E is for 0.5 g per litre of PIB2 in oil, and curve F is for 1.0 g per litre of PIB1 in oil. From this data it can be seen that PIB1 can effectively reduce misting of condensate even at the lowest polyisobutylene concentration tested. The use of a higher molecular weight polyisobutylene, PIB2, produced significantly better mist control than PIB1 at low air velocity (190 m/s) but the advantages of PIB2 relatively to PIB1 diminished considerably at high air velocity (270 m/s).

Claims: A method facilitating the removal of a hydrocarbon mist from a gaseous stream which is subjected to shearing steps of operations such as compression or passing the mist through an orifice, the method comprising: mixing the mist in the gaseous stream with an agent, and passing the gaseous stream to a separation apparatus, characterized in that the agent is functional to substantially prevent shearing of the mist to smaller particle sizes which are difficult to remove from the gaseous stream in separation apparatus, the agent is polyisobutylene, and the mist is mixed with the agent before the mist is subjected to the shearing steps. The method of claim 1 wherein the agent is mixed with a diluent in the form of diesel oil. The method of claim 2 wherein the gaseous stream is a stream of natural gas. The method of claim 1 wherein the agent is sprayed into the gaseous stream in a direction countercurrent to the flow of the gaseous stream for maximum contact. The method of claim 4 wherein the agent is sprayed through an elongated spray nozzle having orifices pointed in said direction. The method of claim 4 wherein the agent coats the surface of a scrubber, demister or coalescer which contacts the gaseous stream. The method of claim 5 wherein the agent helps coalesce droplets in the gaseous stream by reducing drag on the surface of the scrubber, demister or coalescer. The method of claim 1 wherein the agent is delivered with a mechanical atomizer. The method of claim 1 wherein shearing of the mist to smaller particle sizes occurs in compression of the gaseous stream. The method of claim 1 wherein shearing of the mist to smaller particle sizes occurs in flow of the mist through orifices. The method of claim 1 wherein the separation apparatus is a coalescing filter. The method of claim 1 wherein the separation apparatus is a scrubber. The method of claim 1 wherein a production stream containing predominantly gas, some oil and hydrogen sulphide is sent to separation to remove substantially all of the oil, then treated with said agent to prevent shearing of the remaining oil to smaller particle sizes, and then scrubbed or filtered to remove the remaining oil before it is sent to treat the hydrogen sulphide in a process for oxidizing the sulphide to elemental sulphur. The method of claim 1 wherein a gas plant stream containing predominantly gas and some liquid hydrocarbons is passed to a separator wherein a substantial majority of the liquid hydrocarbon is separated from the gas, adding said agent to the gas, and then removing substantially all of the liquid hydrocarbon from the gas, and passing the gas to treatment by other processes which are normally sensitive to the presence of hydrocarbons. The method of claim 1 wherein a production stream from a well containing oil and gas is passed to a separator to remove most of the oil, adding said agent to the gas and then removing substantially all of the oil from the gas and passing the gas to a glycol contactor for drying. The method of claim 1 wherein the agent polyisobutylene acts as a drag reducer for the impinging droplets on surfaces. The method of claim 1 wherein said agent is sprayed upstream of compressor stations to prevent the fouling and erosion of the compressor blades. The method of claim 1 wherein a production stream containing predominantly natural gas with some crude oil and/or condensate is treated.

Inventors: KHATIB ZARA IBRAHIM, KHATIB, ZARA IBRAHIM
Applicants: SHELL INT RESEARCH, SHELL INTERNATIONALE RESEARCHMAATSCHAPPIJ B.V.
Application year: 1997
Main classification: B01D47
IPC codes: C10L3, C10G5, B01D51