Optimizing NGL treatment for sulfur and methanol reduction at Pembina NGL Corporation’s Redwater fractionation plant—Part 2
Part 1 of this article can be found in the April issue of Gas Processing & LNG.
Pembina’s asset system. Pembina NGL Corp. provides sweet and sour gas gathering, compression, condensate stabilization, and both shallow-cut and deep-cut gas processing services, with a total capacity of approximately 6 Bft3d. The company operates a plant site in Redwater, Alberta, Canada. This plant receives raw natural gas liquid (NGL) from field pipelines that is then fractionated into ethane, propane, butane and C5+ condensate. The final products are transported by pipeline, rail and truck, depending on the commodity.
The main contaminants that Pembina is concerned with are sulfur (mercaptans), water and methanol in the final products of butane and propane. Mercaptans and water are naturally occurring compounds, while the methanol is a result of intentional upstream injections to reduce the formation of hydrates in pipelines. Currently, there are three fractionation trains designated as RFS1, RFS2 and RFS3, with a fourth train (RFS4) under construction.
Methanol solubility in propane and butane. For Pembina, the most cost-effective method of reducing the entrained methanol content in the C3/C4 streams was to use a supplementary water extraction or waterwash unit. This method would rely on the affinity of methanol for water due to its polar nature.
While it is theoretically possible to develop a liquid–liquid extraction equation of state and computational model, most commercial packages available are of limited utility for polar interactions such as those found with water and methanol.4 Instead, here, a ternary diagram for the propane-water-methanol system or butane-water-methanol system was recreated with extrapolated tie lines for the areas of interest in water extractions (typically < 3 wt% water and < 1,000 ppmw methanol) using original data from an earlier study.5
The method is straightforward and requires the initial inlet feed composition of methanol, water and hydrocarbon concentrations. Then, an assumed water extraction level is added. For example, a 1.5 vol% (3 wt%) of water was injected and the stream composition with the added water was calculated. This is the mixture composition designated as M1 and can be found on the ternary system diagrams in FIGS. 5 and 6.
FIG. 5. Ternary propane-methanol-water phase diagram at 20°C.
FIG. 6. Zoomed-in view of ternary phase diagram for liquid-liquid extraction area of interest.
This is located in the two-phase region and falls on a tie line. At the two ends of the tie line, where the phase equilibrium curve intersects, there will be a hydrocarbon-rich phase designated as H1 and an aqueous-rich phase at the other end of the tie line designated as A1. This process can then be repeated, starting with the H1 composition and then adding a second water injection—1.5 vol% (3 wt%)—and repeating the process to obtain the corresponding new tie line end points of H2 and A2. Theses results are summarized in TABLE 4.
Using proprietary, highly-efficient liquid-liquid coalescersb. Pembina experienced issues on the propane line in the RSF2 train due to entrained water and methanol exceeding the design capacity of the mol sieve. This resulted in the combined problems of operating at reduced throughput while requiring more frequent regeneration of the mol sieves than specified in the original design. This issue was exacerbated by using a closed-loop design (using finished product to regenerate the offline bed and circulating into the online bed). A rental skid with a prefilter and high-efficiency liquid-liquid coalescersb was brought out to the site and used with a simple waterwash utilizing a mixing valve and no recirculation. The use of the waterwash with the co-author’s company’s coalescerb unit reduced the stream’s net methanol/water content. Utilizing this unit in conjunction with the existing mol sieves, the two ensured the product remained on-spec with the following exceptions: excursions in methanol content from suppliers, degraded mol sieve desiccant or Merox unit excursions (plant trips/upsets). Pembina subsequently purchased the rental unit for the RSF2 train and added a similar second unit on the propane line in the RSF3 train. A third coalescer unit has also been ordered for the future RSF4 train.
The high-efficiency liquid-liquid coalescerb separates aqueous phases, including caustic and methanol, from hydrocarbons using polymeric and fluoropolymer media. The inlet streams contain emulsions with small aqueous drops that are captured by the fibers in the coalescer cartridge and then forced into close proximity to allow for the merging or coalescing of the drops. The fiber media is designed to have larger spacing or micron pore sizes near the outlet of the cartridge to accommodate the coalescences of ever larger drops being formed. High-efficiency liquid–liquid coalescers have been proven to separate liquids with low interfacial tensions6 and consequentially small drop sizes. A prefilter with cartridge-type elements has been found to be an effective way of extending the coalescer cartridge life by removing solid contaminants and can assist the separation process by starting the coalescence process. A horizontal coalescer system7 consisting of two stages (pre-filter and coalescer) is depicted in FIG. 7.
FIG. 7. Horizontal liquid–liquid fiber-bed coalescer system with pre-filter.
Coalescer performance. High-efficiency cartridge liquid–liquid coalescer systems can reduce the discontinuous phase-down to ppm levels in the outlet and can process difficult emulsions with interfacial tensions as low as 0.5 dyne/cm. They can typically process streams with up to 10% inlet discontinuous phase concentrations. These types of coalescers can provide water with an outlet concentration of < 15 ppmv of free aqueous phase, which can be measured by the AquaGlo method (ASTM D3240). The use of fluoropolymers for the coalescer fibers and support layers allows for excellent chemical resistance.
Rental units. The use of rental pre-filter and cartridge liquid-liquid coalescer skids provides flexibility for users to avoid having to use their capital budget and allows a way for them to trial the separation equipment before optioning to purchase. Typically, the time frame for rental unit deployment is days to weeks compared to months and years for permanent equipment. An important feature of rental units is that they are built to refinery codes, and if desired, there is a rent-to-buy option that allows for seamless continuation of operation. Examples of the co-author’s company’s liquid-liquid coalescerb rental skid and pre-filter rental skid are provided in FIGS. 8 and 9, respectively.8
FIG. 8. View of the co-author’s company’s high-efficiency horizontal liquid-liquid coalescerb rental unit.
FIG. 9. View of the co-author’s company’s horizontal pre-filter rental unitc.
Comparison of field data to theoretical calculations. Pembina sampled the propane stream on the RFS1 train where the high-efficiency liquid-liquid coalescer rental unitb was first introduced. The methanol concentration in the propane was measured at various points throughout the process, including before the Merox treater, before the coalescer, after the coalescer and after the molecular sieve dryer. These data, along with the theoretical separation based on the phase envelope analysis, are presented in FIG. 10.
FIG. 10. Methanol concentration in the propane at various locations in the RFS1 train using high-efficiency liquid-liquid coalescersb.
A plot of the actual and modeled separation efficiency of the high-efficiency coalescer unit is presented in FIG. 11, based on the feed methanol concentrations.
FIG. 11. Comparison of the model and actual separation efficiency of the high-efficiency coalescersb by feed methanol concentrations in the RFS1 propane train.
This modeling assumed that there would be 15 ppm of aqueous carryover from the high-efficiency liquid-liquid coalescers. The measured concentrations of methanol downstream of the coalescers were found to be fairly close to the theoretical calculations, with most points having within 10% higher values. This showed that this method is a useful way to predict methanol removal by waterwash and coalescer separators.
Process improvements. The combination of separating Merox caustic streams, feed methanol control and the addition of the co-author’s company’s filtration units resulted in the finished C3 streams being within the sulfur and methanol specification limits and addressed one of the main off-spec risks for C4 product. In the event of an excursion or circumstance resulting in off-spec product, Pembina can perform any of the following:
- The product can be recycled to the inlet of the fractionation train for reprocessing
- Propane only: The product can be blended in the lower price commodity distribution (HD-5) for sale
- Propane only: The worst case is to convert railcars designated as HD-2 propane into the lower-spec commodity HD-5 propane.
The options listed above can lead to frac rate reductions due to re-processing or impacts to rail loading operations and delays to shipments of HD-2 propane to the Prince Rupert Terminal in British Columbia, Canada. All carry financial and reputational losses, so it was important that the process changes, in addition to the coalescer package, were vetted and the limitations understood before implementation in three fractionation trains.
Based on the rental testing and long-term operation, the use of the co-author’s company’s skid has allowed Pembina to produce HD-2 propane, meeting the required methanol spec limitation with a few exceptions noted above. The high-efficiency liquid–liquid coalescersb were also found to compare favorably vs. the use of the water knockout—mesh pad in the waterwash system with a much smaller footprint and reduction in capital spending. The process was found to be reliable enough to sustain the shipment of HD-2 propane, which is priced at approximately 1.2 times the sales price of HD-5 propane, resulting in a payback within 1 mos of operation and enabling a lucrative branch of Pembina’s NGL business. In addition to the ability to meet specifications, the unit has also limited the reduction in throughput caused by maintaining the lower methanol target, which has resulted in less waste energy/greenhouse gas emissions.
Takeaways. A number of process improvements were made at Pembina’s Redwater plant to meet sulfur and methanol specifications and increase plant efficiency. Field tests to segregate the caustic stream so that the propane and butane lines were isolated proved that the methanol was concentrating in the propane phase and that the caustic was a conduit to transfer methanol over to the butane phase. Ideally, the propane and butane should have independent caustic treating which, when possible, can be done by connecting separate trains with pipework.
To reduce the methanol concentration in the butane, the waterwash system—with knockout and a mesh separator—was evaluated and modifications were made to increase its performance. By reducing the pressure drop across the static mixer, reducing the re-circulating aqueous stream and increasing the water level in the knockout tank, the methanol removal efficiency was increased from 64%–69% up to 81%–84%. These changes were sufficient to meet the 50-ppm methanol target in butane for the RFS2 train, but not for the RFS3 train.
Based on available ternary equilibria data, a method for predicting how much methanol can be extracted from butane or propane for a given water injection has been described here with the use of high-efficiency liquid–liquid coalescers. The predicted methanol reductions were compared to plant data and found to be within 15% of these values.
An improved method for waterwash has been described here that used water injection followed by a high shear mix valve and then high-efficiency liquid–liquid coalescers. This method has been found to be very effective and gave results close to the theoretical phase equilibria data prediction. The use of the high-efficiency liquid–liquid coalescers also had the advantage of occupying about a tenth of the footprint of the water knockout with mesh pad and did not require a recirculating aqueous stream. The use of rental prefilter and coalescer units was effective in letting Pembina gain experience with this approach before committing to permanent installations.
LITERATURE CITED
4 Riaz, M., M. A. Yussaf, G. M. Kontogeorgis, G. H. Stenby, W. Yan and E. Solbraa, “Distribution of MEG and methanol in well-defined water and hydrocarbon systems: Experimental measuring and modeling using the CAP EOS,” Fluid Phase Equilibria, 2013.
5 Noda, K., K. Sato, K. Nagatsuka and K. Ishida, “Ternary liquid-liquid equilibria for the systems of aqueous methanol solutions and propane or n-Butane,” Journal of Chemical Engineering of Japan, 1975.
6 Wines, T. H. and R. L. Brown, “Difficult liquid-liquid separations: High-performance, polymer-fiber coalescers break up hard-to-handle emulsions and dispersions,” Chemical Engineering, December 1997.
7 Wines, T. H. and S. Mokhatab, “High-efficiency coalescers for gas processing operations,” Petroleum Technology Quarterly, Q4, 2017.
8 Wines, T. H., A. Gorin, J. Rios and J. Trucko, “Improve kerosene mercaptan sweetening with fluoropolymer-cartridge liquid-liquid coalescers,” Hydrocarbon Processing, September 2021.
NOTES
b Pall’s liquid-liquid coalescers
c Pall’s Duplex horizontal pre-filter rental unit
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