The Use of Walnut Shell Filtration with Enhanced Synthetic Media for the Reduction and/or Elimination of Upstream Produced Water Treatment Equipment

Transcription

1 Siemens Water Solutions The Use of Walnut Shell Filtration with Enhanced Synthetic Media for the Reduction and/or Elimination of Upstream Produced Water Treatment Equipment White Paper January 2016 Researchers set out to find a substitute for black walnut shells for oil/water separation due in part to the seasonal availability of black walnuts shells. Additionally, a media with improved performance was needed. Testing of a new synthetic media was performed on synthetic produced water and in field pilot studies. These tests have found that the new media can manage significantly higher inlet oil concentrations and have a higher specific load of oil than traditional walnut shells. 1

2 Introduction Walnut shell media filtration has traditionally been used in upstream and downstream produced water treatment applications onshore; walnut shell filtration has rarely been used offshore. This difference in use has been driven by two factors: the need for better treatment for onshore disposal; and the footprint required for the walnut shell filters using traditional design flux. On an offshore platform or Floating Production Storage and Offloading Vessel (FPSO), any additional footprint required for treatment is very expensive. Traditional hydrocyclones and flotation equipment have been refined to minimize the area required. At traditional design flux, walnut shell filters typically require a larger footprint atop a platform or FPSO than is readily available, especially for equipment that is not needed to reach the less-stringent discharge limits that have been used offshore in the past. Recent developments, including the incorporation of internalized backwash systems as well as increased processing capacity using reduced footprint, has made media filtration using walnut shells more appealing to both onshore and offshore users and operators. In addition, there has been a trend to use tertiary treatment technologies downstream of secondary equipment such as flotation cells, due to the need to reduce hydrocarbon discharge to the environment for both regulatory and public relations reasons. One limitation in using walnut shell media, however, is that walnut shells, as well as other nut shell media, are produced by, and are subject to, an agricultural season. Due to the fact that nut shell media is subject to growing seasons and uncontrollable environmental factors, crop yields can be sparse, resulting in dramatic market fluctuations. A new synthetic media has been developed PerforMedia oil removal media as a direct replacement of nut shells. This media is manufactured from materials that are not subject to annual agricultural limitations. Through experimental and field testing, the following benefits have been observed in using this synthetic media: Ability to handle greater than four to five times the amount of oil before requiring a backwash, compared to standard nut shell media Ability to maintain feed concentrations of greater than 500 mg/l while producing an effluent well under 10 ppm Applicability to a wide range of oil API from and elevated temperatures as high as 90 C. With these realized benefits, this paper will highlight the performance and ability to remove and/or eliminate treatment equipment upstream of media filtration, such as gas flotation and its combination with standard nut shell filtration. It will also highlight the potential reduction of treatment chemicals used to enhance gas flotation and the reduction or elimination of waste sludge into recoverable crude. Tests and Pilots Lab Testing Laboratory testing was performed using synthetic produced water. The produced water was made by injecting crude oil into a pressurized water stream. A globe valve and static mixer were used to shear the oil into droplets to simulate produced water. The produced water was then routed to the top of a packed column, and treated water exited the bottom. After the bed had been exhausted, found by monitoring effluent oil concentrations, the media was backwashed using up-flow water and an internal gas lift pump. 2

3 The same produced water that the unit was treating was used for backwash. Following backwash, the produced water was routed to the top of the bed for filtration. The lab test configuration is presented in Figure 1. Figure 1: Laboratory testing process flow diagram Results and Discussion Laboratory testing was conducted to compare the oil loading of PerforMedia media to walnut shells, presented in Figure 2. At the same flux (13.5 gpm/ft 2 or 22 m 3 /h/m 2 ), the synthetic media was able to load approximately four times more oil before break through than walnut shells. This increased oil loading capacity will decrease the required backwash frequency, allowing for either longer loading between backwashing or the ability to withstand higher oil concentrations. Figure 2: Media comparison of laboratory results 3

4 Testing was performed using different oils, a range of 13 to 32 API, at different fluxes. Figure 3 presents the results of the testing. Figure 3: Oil loading based on API of oil and flux The testing indicates that as the flux is increased, the oil loading decreases. The decrease in loading capacity is likely due to the increase in velocity as the flux is increased. As the velocity through the interstitial space increases, larger droplets can be carried through the media bed. Also, as the API of oil decreases, so does the oil loading. This is likely due to higher density particles requiring less velocity to be carried through the bed. Walnut shells are effective at removing small free oil droplets. For an effective alternative, the new media must be able to remove small free oil droplets too. A particle analyzer was used to compare the influent and effluent particle distribution. Figure 4 presents oil droplet analysis data from laboratory testing. The influent contained about 75% of the oil particles greater than 15 microns. The effluent contained nearly zero oil droplets remaining larger than 10 microns, showing that PerforMedia oil removal media is effective at removing oil droplets greater than 15 microns. 4

5 Figure 4: Oil droplet size distribution In order to reduce capital cost, the tertiary treatment system must be made smaller, or other equipment must be removed from the treatment system. Since PerforMedia media operates at fluxes similar to walnut shells, the tertiary system has a similar footprint to walnut shells. A test was performed to see if PerforMedia media could produce effluent quality equal to tertiary systems with primary effluent as feed. The test unit was operated with heavy oil, API 13, at 500 mg/l feed oil concentration. The synthetic media was able to produce effluent less than 10 mg/l for three consecutive runs, presented in Figure 5. Figure 5: Laboratory testing of high influent concentrations 5

6 Based on all laboratory testing results, this alternative media appears to be effective at handling oil inlet loading typical of both secondary and tertiary treatment applications. South America Pilot Testing Upon completion of laboratory testing, the pilot unit was taken to the field to validate the effectiveness on real world produced water. Pilot testing was performed in South America on heavy crude oil, API 11. The pilot unit contained two media vessels that could be used either in stand-alone or parallel mode, allowing for the ability to perform side by comparisons between media or manage larger feed flows. The unit was tested using PerforMedia oil removal media in one of the vessels. The principal of operation and more detailed description are discussed below: Principal of Operation The pilot unit was a continuous flow system. A single feed source was the inlet to two vessels that could be operated in parallel or stand-alone. The untreated feed flowed from the top down in order to maintain a packed bed. The free oil droplets and suspended solids in the water were captured by the media. Once the media bed was saturated, a breakthrough (oil/solids concentration increase in the effluent) occured. At the breakthrough point, a backwash was performed to regenerate the media. The backwash was accomplished by using a gas lift pump to agitate and turn over the media bed. A backwash was initiated by a pressure differential across the bed, breakthrough, or on timed intervals. The backwash used the same internal distribution system as the outlet for forward flow. Pilot Vessel Description The pilot consisted of two vessels that could be operated in parallel, each 24 inches ( m) in diameter, with one vessel containing PerforMedia media. Each vessel contained a header at the bottom, a gas lift pump located at the center of vessel, a gas distribution nozzle that drove the gas lift pump and a top screen to retain media inside the vessel. The pilot unit was used upstream of the flotation units and prior to chemical addition, as noted in Figure 6. Figure 6: Site process flow diagram 6

7 Oil Concentraiton (mg/l) White paper Synthetic Media January 2016 The unit was operated 24 hours per day for approximately 17 days, and encountered temperatures ranging from C and oil feed concentrations greater than 1200 mg/l. Results and Discussion Over the course of this pilot test, three fluxes were tested. The unit was run at a flux of 10, 8, and 5 gpm/ft 2 (24.5, 20, 12.2 m 3 /h/m 2 ). The initial test was performed at a flux of 10 gpm/ft 2 (24.5 m 3 /h/m 2 ) since a majority of laboratory testing with lower API was done around this flux. Figures 7 and 8 below show the oil and total suspended solids removal during a test South American Pilot Study PerforMedia Oil Removal 10 gpm/ft 2 (24.5 m 3 /h/m 2 ) Hours of Operation Feed Oil PerforMedia Effluent Oil Figure 7: Oil removal at 10gpm/ft 2 (24.5 m 3 /h/m 2 ) 7

8 Oil Concentration (mg/l) TSS (mg/l) White paper Synthetic Media January 2016 South American Pilot Study PerforMedia Total Suspended Solids Removal 10 gpm/ft 2 (24.5 m 3 /h/m 2 ) Hours of Operation Feed TSS PerforMedia Effluent TSS Figure 8: Solids removal at 10gpm/ft 2 (24.5 m 3 /h/m 2 ) As noted in the above figures, the removal efficiencies were not as high as expected and this API of oil was not included in the laboratory testing. It was decided to further reduce the flux to determine the effect. Figures 9 and 10 below show the oil and total suspended solids removal while running at a flux of 5 gpm/ft 2 (12.2 m 3 /h/m 2 ) South American Pilot Study PerforMedia Oil Removal 5 gpm/ft 2 (12.2 m 3 /h/m 2 ) Hours of Operation Feed PerforMedia Effluent Figure 9: Oil removal at 5 gpm/ft 2 (12.2 m 3 /h/m 2 ) 8

9 Oil Concentration (mg/l) TSS (mg/l) White paper Synthetic Media January 2016 South American Pilot Study PerforMedia Total Suspended Solids Removal 5 gpm/ft 2 (12.2 m 3 /h/m 2 ) Hours of Operation Feed PerforMedia Effluent Figure 10: Solids removal at 5 gpm/ft 2 (12.2 m 3 /h/m 2 ) The reduction in the flux from 10 gpm/ft 2 (24.5 m 3 /h/m 2 ) to 5 gpm/ft 2 (12.2 m 3 /h/m 2 ) made a noticeable difference in the removal efficiencies. It appeared that at this oil API, solids loading, and temperature, the media required a lower flux. It was decided to increase the flux to 8 gpm/ft 2 (20 m 3 /h/m 2 ) and note the effects. Figures 11 and 12 below show the oil and total suspended solids removal while running at a flux of 8 gpm/ft 2 (20 m 3 /h/m 2 ) South American Pilot Study PerforMedia Oil Removal 8 gpm/ft 2 (20 m 3 /h/m 2 ) Hours of Operation Feed PerforMedia Effluent Figure 11: Oil removal at 8 gpm/ft 2 (20 m 3 /h/m 2 ) 9

10 Percent Removal (%) White paper Synthetic Media January 2016 Figure 12: Solids removal at 8 gpm/ft 2 (20 m 3 /h/m 2 ) As noted in the above figures, the increase in flux caused the removal efficiency to deteriorate. Figure 13 shows the comparison between the three different fluxes and percent removals South America PerforMedia Pilot Flux vs Percent Removal Oil TSS Flux (gpm/ft 2 ) Figure 13: South American pilot study flux comparison 10

11 As noted in the above figure, the optimal flux for this API oil is between 5 gpm/ft 2 (12.2 m 3 /h/m 2 ) and 8 gpm/ft 2 (20 m 3 /h/m 2 ). Since PerforMedia media was found to be effective for secondary treatment without the use of flotation and flotation chemicals, the oil can be recovered from the backwash. In addition, the chemical costs will be reduced, as well as flotation waste disposal or flotation waste handling costs. North America Pilot Testing A second pilot test was conducted at a Western USA refinery in a downstream application where the oil API was around 35. The testing was conducted as secondary treatment prior to chemicals and flotation, as seen in the process flow diagram in Figure 14. Figure 14: Site process flow diagram Principal of Operation A smaller pilot unit with a single, 6 -diameter vessel was used for this pilot study. It operated in the same fashion as the larger pilot that was used in the South America pilot study referenced above. A backwash was initiated by a pressure differential across the bed, breakthrough, or on timed intervals. The backwash used the same internal distribution system as the outlet for forward flow. Pilot Vessel Description The pilot consisted of a single, 6 (0.15 m)-diameter vessel that contained PerforMedia TM oil removal media. The vessel contained a header at the bottom, a gas lift pump located at the center of vessel, a gas distribution nozzle that drove the gas lift pump and a top screen to retain media inside the vessel. The pilot unit was used upstream of the flotation units and prior to chemical addition as noted in Figure

12 Results and Discussion The feed oil concentration in this application was very low, averaging only 37.3 mg/l during the entire pilot study. The oil removal efficiency of PerforMedia media increased as the feed oil increased. The PerforMedia media pilot was tested at a flux of 10 gpm/ft 2 for 45 days and at a flux of 15 gpm/ft 2 for 11 days. The pilot was operated in parallel with two IGF units to directly compare their performance. The results from a portion of the 10 gpm/ft 2 flux testing are shown below in Table 1. Table 1 PerforMedia Media vs IGF - 10 gpm/ft 2 Feed PerforMedia Effluent IGF Effluent Total O&G (mg/l) TSS (mg/l) Range Average % Removal 82% 59% Range n/a Average n/a % Removal 70% n/a During the 10 gpm/ft 2 flux testing, PerforMedia media had an average oil removal efficiency of 82%. The IGF s were only able to remove 59% of oil, even when assisted by chemical addition. The PerforMedia media pilot was also able to remove 70% of all TSS. Toward the end of the study, the flux of the pilot unit was increased to 15 gpm/ft 2. Results from a portion of this test condition are shown below in Table 2. Table 2 PerforMedia Media vs IGF - 15 gpm/ft 2 Feed PerforMedia Effluent IGF Effluent Total O&G (mg/l) Range Average % Removal 81% 38% Range n/a TSS (mg/l) Average n/a % Removal 80% n/a During the 15 gpm/ft 2 flux testing, PerforMedia media had an average oil removal efficiency of 81% (only 1% lower than the 10 gpm/ft 2 test) and the IGF s were only able to remove 38% of the oil, even when assisted by chemical addition. The PerforMedia media pilot also was able to remove 80% of all TSS. 12

13 The PerforMedia oil removal media pilot was able to remove over 80% of the influent feed oil during both the 10 gpm/ft 2 and 15 gpm/ft 2 flux test conditions. There was very low oil in the API effluent and it is likely that a filter system using PerforMedia oil removal media could displace both primary and secondary treatment in this application since the synthetic media can handle feed oil concentrations greater than of 500 mg/l. It is also likely that as the feed oil concentration is increased the oil removal efficiency will also increase. Conclusions PerforMedia media can tolerate higher influent oil concentrations and produce equivalent effluent concentrations as nut shells. PerforMedia media has a higher specific load as compared to nut shells, thus reducing backwash frequency. PerforMedia media used in a secondary treatment application produces equivalent effluent quality to traditional secondary and tertiary treatment processes within a single step. When used in a secondary treatment application, chemical costs can be reduced and oil can be recovered from backwash. Chemical contamination of the remaining crude within the produced water can be reduced or eliminated. PerforMedia media is manufactured from materials that are not subject to agricultural limitations. Data is consistent with a trend observed in previous testing in which lower API oil required a lower flux. With higher API oil, higher flux may be used. Particle analysis has shown that PerforMedia media is effective at removing oil droplets larger than 15 microns. Depending upon variables such as temperature and flux, removal efficiencies can be increased or decreased. In applications governed by stringent discharge limits, further downstream treatment may still be required based upon feed concentrations. Future Outlook Further studies/pilot unit usage to collect operational experience and close knowledge gaps. Gather more details about cleaned water quality and flow rates Check performance in the presence of Enhanced Oil Recovery polymers 13

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