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Natural Gas Decarbonation with Membrane Permeation

A more recent technology, relatively to CA and PA, called Membrane Permeation (MP) employs semi-permeable porous or non-porous barriers (i.e., membranes) to separate components of gas or liquid mixtures through a variety of material-dependent mechanisms (D'Allesandro et al., 2010). Its compactness finds an application niche in CO,-rich NG decarbonation at ultra-deepwater platforms. The

“game-changing” attribute of MP results from its small footprint, an appealing factor on the topside of FPSOs (Araujo et al., 2017). Additionally, MP can have high CO, selectivity, low energy requirements (D’Allesandro et al., 2010) and modularity, which allows easy scale-up, high flexibility and resilience to feed-changing conditions (Reis et al., 2018). MP is also a relatively simple separation system without moving parts easing control, operation and scale-up, resulting in more than 200 MP plants in the NG industry—mostly installed by UOP Honeywell and Schlumberger in offshore rigs (Yeo et al., 2012).

Available materials for membrane production are organic (acetates, polysulfone and other polymers), inorganic (carbon, zeolite, ceramic or metallics) and mixed matrix (composite organic inorganic compounds) (Al-Mamoori et al., 2017). Separation costs are impacted by material-dependent properties (e.g., membrane porosity, permeability, selectivity, pressure range and chemical impurities resistance) and overall operational ranges. Currently, polymeric membranes are the most common type for providing reasonable separation results at reduced costs and are easier to synthesize (Adewole et al.,

2013). Polymeric membranes can be cast into flat sheet, hollow-fiber or spiral-wound configurations (Zhang et al., 2013), facilitating their wide use in CO, separation applications. Separation of CO, from CH4 and heavier hydrocarbon components in MP produces a CO,-rich permeate and a lean-СО, retentate (upgraded NG), illustrated in Figure 4.

Five possible material-dependent separation mechanisms can occur in MP: (a) Knudsen diffusion, (b) molecular sieving, (c) solution-diffusion separation, (d) surface diffusion and (e) capillary condensation (Olajire, 2010). The main separation mechanisms in NG applications comprise molecular sieving—separation through molecular size—and solution-diffusion separation, in which CO, dissolves into the membrane material and then diffuses through it due to a concentration gradient (Ismaila et al., 2005). Independently of the mechanism, the main driving force is the trans-membrane difference of fugacities of the permeating species, resulting in flux differentiation of species. To assure a sufficiently high driving force, feed streams are usually at high pressure, requiring compression prior to MP, implying in high power demand (Yeo et al., 2012), despite the small heat requirement.

One of the most important parameters in designing MP is the permeability-selectivity tradeoff—the more permeable a membrane is, the less selective it is (Robeson, 1991). Highly permeable membrane able to process high flowrates of CO,-rich NG would result in high methane loss to the permeate. On the other hand, a more selective membrane would require a larger membrane area (Al-Mamoori et al.,

2017). A carefirl choice of membrane material is essential in obtaining the highest permeability while maintaining selectivity. Although the permeate (captured CO,) is at low pressure, the retentate (treated NG) faces a small pressure loss (Araujo and de Medeiros, 2017), reducing the compression effort to pipeline dispatch pressure.

Upgrading CO,-nch natural gas with membrane permeation

Figure 4. Upgrading CO,-nch natural gas with membrane permeation.

Benefits and shortcomings

Offshore processing of CO,-rich NG uses EOR, since the early production stage, to destine and monetize the high flow rate of separated CO,. Crude oil recovery factor varies from 2 bbl/tCO, (close to U.S. operational data) to 4.35 bbl/tCO, (Azzolina et al., 2016). Early injection in the reservoir results in longterm enrichment of CO, in the produced gas because only approximately 60% of the injected volume is stored in the reservoir formation (Gazalpour et al., 2005) while the remainder returns to the gas processing plant. In parallel, along the operation lifetime, the production curve declines, intensifying the impact of CO, injection in NG composition. This scenario requires resilience and flexibility, offered by MP due to its modularity and compactness. The addition of a new MP module would suffice as a means to adapt the processing plant, while absoiption-based technologies would require revamping. The small size of MP makes it the leading technology in offshore CO, removal applications where spatial requirements are a paramount constraint (Reis et al.. 2017), although the need to recompress the captured CO, remains a drawback, similarly to CAand PA (Araujo et al., 2017). A second benefit of MP is that it does not require solvents, eliminating the need for handling and storage of chemicals. For applications on ultra-deepwater FPSOs, solvent makeup and waste management challenge logistics: Chemicals need to be collected, stored as waste, and transported onshore for treatment and disposal (EC, 2019). Besides, with a wide variety of materials to choose from, a high CO, selectivity can be achieved with very small unwanted hydrocarbon losses (Yeo et al., 2012).

A shortcoming of MP is its sensitivity to chemical impurities or membrane poisoning, mainly in polymeric NG membranes (Yeo et al., 2012). Raw NG components, such as sulfur-containing molecules, har e extremely adverse effects on membrane surface with excess exposure, leading to changes in porosity or even inhibiting separation (Adewole et al., 2013). Plasticization alters initially glassy polymers by disrupting chain packing and enhancing inter-segmental mobility of polymeric chains. NG applications require adjustment of hydrocarbon dew-point (HCDP) of the gas feed (refer to Figure 4) as condensing hydrocarbons could plasticize membranes and degrade selectivity (Flao et al., 2008). In CO,-rich NG processing, CO, and other hydrocarbons induce plasticization (Zhang et al., 2019), resulting in increased permeability at the expense of extreme reduction in selectivity, to the point that separating only CO, becomes impossible (Yeo et al., 2012). Pressure dependent plasticization occurs by dissolution of penetrants into the membrane matrix, resulting in the destruction of the membrane surface and increased chain flexibility (Sulemau et al., 2016). The higher the pressure the higher the plasticization of the membrane, making this process extremely relevant in NG applications.

Membrane material selection faces tradeoffs: High resistance to plasticization, low permeability and high selectivity (Adewole et al., 2013). Depending on the material, several pre-treatments are required in order to facilitate stable separation and reduced membrane replacements. A second shortcoming of polymeric membranes is then physical aging (Olajire, 2010). Over time, polymeric membranes lose separation performance with net loss of CO, recovery, more pressure differential needed and lower selectivity. Consequently, continuous use of polymeric membranes results in membrane plasticization, poisoning or poor selectivity/penneability, with loss of performance before their lifespan limit is reached (Suleman et al., 2016).

Recent focus

The mam development points in MP are: (a) new materials (mixed matrix membranes, МММ), (b) membrane surface treatments, (c) additives against plasticization and (d) membrane stability (chemical/thennal). MMMs are produced by embedding a filler material inside a polymeric matrix, being the most active membrane research area (Vinoba et al.. 2017). Sautaniello and Golemme (2018) developed an МММ from Hyflon*AD60X and SAPO-34, which takes advantage of the unusually high plasticization resistance of Hyflon*AD60X, while maintaining the good separation properties of SAPO-34. Results showed a twofold reduction in permeability but an increase in selectivity by a factor of 9 due to the tortuous paths CH4 must follow in order to permeate (Sautaniello and Golemme, 2018).

Gamali et al. (2018) and Agahei et al. (2018) added finned silica into a Pebax (amide 6-n-ethylene oxide) matrix, achieving improved selectivity and permeability of CO, at high pressures (= 3.5 MPa).

Another area of investigation is doping existing matrices to improve performance. Adewole et al. (2013) reviewed membrane modifications and repotted the main changes: Crosslinking with thermal treatment, crosslinking with diamino compounds, polymer sulfonation, thermal rearrangement, polymer blending and dual-layer hollow fiber spinning. Thermal rearrangement and crosslinking are the most common modifications reported in literature. Crosslinking creates intermolecular connections through chemical bonds, resulting in membranes being more resistant to swelling and plasticization, while thermal rearrangement seeks stability at higher temperatures (Adewole et al., 2013). Wang and Hu (2018) evaluated PHA rearrangement at = 450 °C, noticing improved permeability for CO,.

Natural Gas Decarbonation with Cryogenic Distillation

When processing CO,-rich NG (CO, > 40%mol), CA and MP become less cost-effective (Maqsood et al.,

2014), and cryogenic distillation (CD) finds its application niche. Low temperature distillation is a well- established process in the gas industry (e.g., O, and N, production). CO, can be obtained in CD as solid, liquid or cold vapor at high pressures, saving compression costs in transportation and storage. Whereas solid CO, may be the desired product, solid generation inside a CD column presents negative effects in process efficiency (Maqsood et al., 2014). Figure 5 shows a typical CD process and AT phase envelopes for binary CO,/CH4 mixtures, where the dotted line represents the CO, freeze-out barrier-—to the left of the barrier dry-ice will be present. Operational temperatures are usually below -40 °C varying from -45 °C to -80 °C, as in the Shute Creek plant of ExxonMobil (EPA, 2018), although values lower than

Cryogenic distillation process and pressure-temperature phase diagram for CO, and CH binary mixtures

Figure 5. Cryogenic distillation process and pressure-temperature phase diagram for CO, and CH4 binary mixtures.

-70 °C are not recommended. At too high temperatures, separation is not totally achieved, and, at too low temperatures, diy-ice forms in the distillation column—CO, freeze-out—resulting in clogging.

Higher pressures increase CH4 losses, but reduce solid formation, avoiding freeze-out problems, and result in higher quality product (both in the lean and pure CO, streams). On the other hand, lower pressures reduce process efficiency, decrease quality and require higher separation time (Maqsood et al.,

2014). Usual operating pressures for commercial CD range from 31 to 70 bar, as in the Total pilot plant in Southern France.

Benefits and shortcomings

CD products have high purity (> 95%), while in PA, CA and MP the CO, stream contains CH4 and impurities, and traces of the solvent used (PA and CA). Additionally, CD allows CO, recovery at high pressure, does not need water makeup nor recurring consumable costs, and has practically no corrosion (Olajire, 2010). On the other hand, high power consumption for refrigeration is needed to promote separation (Maqsood et al., 2014). Besides its high cost and difficult control, CD has a high footprint. To avoid efficiency loss and solids in the columns, a dehydration unit is also required, and if liquified NG (LNG) is targeted, a demethanizer column is also needed (Maqsood et al., 2014).

Recent focus

Process optimization is the focus, mainly dealing with heat integration. CD has high power consumption due to the refrigeration cycles required in order to meet cooling needs, and the high operating pressure. Sim et al. (2019) proposed the addition of two inter-reboilers into a conventional three-column CD process, resulting in a total duty of 1.066 GJ/'tCO, compared to the 1.231 GJ/tCO, of a conventional CD plant. Ali et al. (2018) used mixed-integer optimization to determine the best arrangement for a novel packed bed CD, concluding that power consumption and separation time could be greatly reduced with then arrangement. However, the CO,-lean gas did not achieve methane specification, requiring further system-wide optimization.

Three commercial technologies for CD are currently available: Ryan-Holmes process (Figure 5), CryoCell® and Controlled Freeze Zone™. Hart and Gnanendran (2009) discussed field trials of the CryoCell®, identifying scenarios where CD shows superior performance relatively to traditional CA. Kelley et al. (2011) present advantages of using a single-step CD process (Controlled Freeze Zone™) for reducing the footprint and separation tune. A potential niche for CD technologies applied to CO, separation is its possible integration into LNG (Liquefied Natural Gas) production. This integration uses the necessary refrigeration to transform NG into LNG to remove CO,, minimizing the energy intensity of the production chain.

Xiaojun et al. (2015) studied integrating Pressurized LNG (PLNG) process with CO, removal to reduce footprints of both processes. Smaller equipment sizes and specific power were achieved but higher overall energy consumption for high CO, content was reported. The lack of toxic solvents, corrosion and waste streams could greatly improve environmental performance of NG processing and increase CD acceptance. CD economic performance is expected to improve with increasing CO, content in NG.

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