Natural Gas Decarbonation with Supersonic Separators
The supersonic separator (SS) is used in the NG industry for water dew-point adjustment (WDPA) via water removal and hydrocarbon dew-point adjustment via propane and heavier hydrocarbons (C3+) removal from raw NG. New SS applications contemplate CO, removal from CO,-rich NG after WDPA and HCDPA, otherwise water and C3+ would condense, hampering the fall of temperature necessary for CO, liquefaction, which normally occurs at colder conditions.
A supersonic separator (SS) achieves separation by expanding and accelerating a compressible fluid to supersonic speeds through a Laval nozzle, which comprises a converging section, a throat and a diverging section (Teixeira et al., 2018). Besides the Laval nozzle, SS encompasses a static swirling device in the converging section, a liquid collector after the supersonic section, and an ending diffuser, which is a continuation of the Laval diverging section. The thermodynamic transitions in SS are described in terms of the Mach number, Ma = v/c, where c is soimd speed and v is flow velocity. The flow accelerates from subsonic to Ma = 1 at the throat and then becomes supersonic (Ma > 1) in the diverging section. The acceleration converts enthalpy into kinetic energy, reducing the fluid temperature along the flow path (de Medeiros et al., 2017), liquefying condensable components like water, C3+ and even CO, at special conditions. A centrifugal field is imposed by swirling vanes, pulling liquid droplets towards the walls, where collecting vanes capture them. Figure 7 sketches an SS where increasing arrows indicate axial acceleration; only the Laval and the ending diffuser are shown, where the former corresponds to the converging-diverging nozzle upstream of the liquid collector.
Downstream, the throat the flow is supersonic and temperature and pressure are low', turning the flow unstable as the discharge pressure is higher. This eventually produces a normal shock front, a flow discontinuity where axial velocity suddenly drops to subsonic (Ma < 1) accompanied by sudden increases of temperature, pressure and entropy. For successfril SS operation, the formed liquid should be collected before the shock front, otherwise separation is lost as everything would re-vaporize through the shock. Thus, in the supersonic section, enthalpy to kinetic energy conversion changes w'ater and C3+ to low enthalpy liquid mist, centrifugally collected by separating vanes, and removed before the shock (Arinelli et al., 2017). The shock phenomenon can be explained as follows: Assuming adiabatic SS flow', there is an analogous subsonic flow w'ith the same mass, momentum and energy flow' rates, but hotter and with greater entropy rate, which is globally stable by the 2nd Law' of Thermodynamics and is accessible via an irreversible, adiabatic, sudden collapse of supersonic flow' at a specific location in the diverging section. This phenomenon is the normal shock front. As any metastable collapse, normal shock is easily provoked by irreversibilities (e.g., fr iction) so that, as the flow' accelerates beyond Ma = 1, the shock is more likely to occur. For NG processing, supersonic flow' with high pressure recovery is unlikely to
Figure 7. Sketch of a supersonic separator.
exist above Ma = 2 as it progressively loses stability against higher discharge pressure. At the shock, entropy is adiabatically created, at rates that increase with shock irreversibility, which increases with Ma immediately upstream of the shock, reducing the SS backpressure.
Critical parameters for SS control are SS head-loss, minimum temperature attained and gas velocity (v). Minimization of head-loss produces a pressurized lean gas (Arinelli et al., 2017), while temperature and Ma must be rigorously controlled. In the case of CO, removal via SS, if too low temperatures (too high Ma) are achieved. CO, freeze-out occurs, potentially clogging the SS (de Medeiros et al., 2019). SS geometry must be calculated in order to guarantee Ma = 1 at the throat: inadequate sizing would degrade SS performance with loss of lean gas and insufficient WDPA and HCDPA (de Medeiros et al„ 2019).
Benefits and shortcomings
A benefit of using SS with CO,-rich NG is that it can simultaneously perform WDPA and HCDPA (de Medeiros et al., 2019). SS can also remot e CO, from NG, provided the NG feed has been submitted to WDPA and HCDPA in order to avoid water and HC condensations (Arinelli et al., 2017). For CO, removal from NG, the SS feed should be at higher pressures and lower temperatures relatively to ordinary NG WDPA and HCDPA SS applications. An advantage of SS compared to other CO, capture technologies is that SS retains performance when CO, content rises, being applicable to CO,-rich streams (Arinelli et al., 2019).
SS is also modular, with a low footprint and no rotating parts (reducing maintenance routines). Separation occurs very rapidly, resulting in a compact equipment, especially beneficial for FPSOs (Hammer et al.. 2014). A further beneficial aspect is that, despite the SS head-loss duiing separation, the CO,-lean NG is obtained at high pressures as well as the condensed CO„ reducing EOR compression costs (Arinelli et al., 2017).
The main shortcoming of SS derives from its tight pressure, temperature and velocity ranges for operability. NG must be pre-conditioned and design should be appropriate to avoid the flow crossing the solid-vapor-liquid equilibrium (SVLE) freeze-out border that traverses the VLE envelope for CO,-rich NG feeds. In fact, current SS research shows a limited scope of applicability for CO, removal. For example, considering an SS for CO, removal from a CO,-rich NG feed (after WDPA and HCDPA) with 45%mol CO„ to avoid freeze-out, SS reaches a minimum CO, content of 21%molin the treated NG. Thus, SS is indicated for bulk CO, removal from CO,-rich NG, with a lower operating range in comparison to to CD (Arinelli et al., 2017). Lastly, SS technology is still in early stages of development and operational problems have not yet to be repotted.
Proof of concept approaches, aiming at determining application niches, outlining benefits and shortcomings, are the bulk of SS research. Hammer et al. (2014) studied the liability of using SS for CO, removal from gas turbine flue-gas in FPSOs, which is the dowmnost step in the CO,-rich NG to energy supply-chain, showing successful capture of CO, in three different systems. A more complete sequence of developments on SS process simulation is related to the Brazilian pre-salt context (Machado et al., 2012: Arinelli et al., 2017; de Medeiros et al. 2017; Teixeira et al., 2018; Brigagao et al., 2019; Arinelli et al., 2019; de Medeiros et al., 2019), bearing more than 45% of CO, in the gas, with two pending Brazilian patents (Teixeira et al.. 2017; Brigagao et al., 2017). Arinelli et al. (2017) developed and used a Unit Operation Extension (UOE) simulating SS (SS-UOE) for the HYSYS process simulator (ASPENTECH Inc.), which simulates SS considering phase-equilibria sound speed (to precisely calculate Ma) determination based on rigorous thermodynamics for predicting phase behavior inside the equipment (de Medeiros et al. 2017). SS-UOE enables the definition of geometrical and operational parameters for NG applications. Another HYSYS UOE (PEC-UOE) was developed by de Medeiros et al. (2017) to determine phase-equilibrium c for any SS flow condition; i.e.. single-phase gas, two-phase gas and liquid and three-phase gas-liquid-water. A diversity of SS configurations was tested and compared.
exhibiting better results than conventional processes, not only for WDPA/HCDPA of raw NG, but also for CO, removal. SS outperformed traditional MP CO, removal because the separated CO, exits the SS as a high-pressure liquid, a situation especially suited for its utilization as an EOR agent, while the CO,-rich permeate from MP is at low pressure and demands high power consumption for compression to become usefiil in EOR (Arinelli et al., 2017; de Medeiros et ah, 2017).
Teixeira et ah (2018) modelled the utilization of SS in different scenarios, as in the recovery of thermodynamic hydrate inhibitors (THI) methanol, ethanol and monoethylene glycol (MEG) from raw NG streams, used to prevent hydrate formation in subsea pipelines. The SS operation, while aiming at THI removal from raw NG feeds, also entailed several concomitant benefits, such as WDPA and HCDPA of the NG stream, besides producing lean NG ready for exportation and crude LPG (C3+) ejected as liquid from SS. Brigagao et ah (2019) developed a process employing SS for air pre-purification in CD. This could further improve the technologies horizon by using it in a hybrid process as a CO, bulk removal and impurity removal unit. De Medeiros et ah (2017) simulated multiphase sound speed, both in multi- reactive and non-reactive multiphase mediums, for SS and for supersonic reactors (SR).
Another mainstream simulation of SS operation employs Computational Fluid Dynamics (CFD), as in Wen et ah (2012), where, trading off CFD and rigorous phase-equilibria calculations, thermodynamics is oversimplified, often leading to unrealistic operation conditions. For example, Wen et ah (2012) used a commercial CFD software employing SS to obtain LNG from a dehydrated NG, obtaining very low pre-shock temperatures of-116.5 °C, questioned in Arinelli et ah (2017). The rigorous thermodynamic approach in Arinelli et ah (2017) and de Medeiros et ah (2017) allows the correct determination of SS flow properties and associated phenomena, such as the prediction of the SYLE CO, freeze-out borders, among other special phase behaviors. Attention has been turned towards improving SS geometry so as to obtain easier removal of the solids/liquids formed and more appropriate flow characteristics. Wang et ah (2018) improved a conventional Laval nozzle to improve flow pattern inside the SS; better swirling is obtained in the divergent section, improving separation.