Supersonic Separation Research Articles

Experimental and numerical investigation on the blade angle of axial-flow swirling generator and drainage structure for supersonic separators with diversion cone

The effect of axial-flow swirling generator and drainage structure with diversion cone on separation performance is very important to supersonic separators. But there is in the absence of experimental studies on separation characteristics of above-mentioned structure. In this paper, both experimental and numerical methods are utilized to investigate the influences of swirling generator and drainage structure. Good agreements are achieved between experimental data and high-order numerical simulation. The results demonstrate that the rotation strength decreases as the outlet angle of the swirler increases. The rotational flow causes the inconsistency of the radial distribution of the fluid in the nozzle and the inconsistency is more noticeable with the increase of rotation intensity. So the blade angle of swirling generator should be determined to find the balance between the expansion characteristic and swirling flow. Comparing two types of drainage structures, the internal extension structure has seriously damaged the supersonic flow in the nozzle, while the flush type drainage port has less influence on fluid. And the smaller outlet angle of drainage port can reduce the effect of drainage port on supersonic flow. When the pressure ratio is 1.4 for the flush type drainage structure, the optimal structural parameter is that the outlet angle of swirler is 55° and the inclination of the drainage structure is 22°. Under the optimum conditions, the ethanol removal rate is 57.06%.

Effects of fluid type and pressure order on performance of convergent–divergent nozzles: An efficiency model for supersonic separation

AbstractA deep analysis on the hydrodynamics of convergent–divergent nozzles is performed by changing the working conditions and fluid type. The nozzle geometry lowers the temperature of the flowing fluid, which contributes to condensation and phase change. The focus of this paper is to evaluate the nozzle performance and cooling capacity in terms of temperature, pressure, and gas type in a fixed geometry of Sajben Laval nozzle. The analysis has been conducted via a 2‐dimensional turbulent computational fluid dynamics simulation for illustrating the behavior of the fluid. A criterion for the nozzle performance is provided by prediction of exact shock wave position. An investigation on 6 different gas types demonstrates that heat capacity and thermal conductivity are the most rolling fluid properties of the nozzle performance. Furthermore, it is found that the shock wave position is unchanged during alteration of fluid type or pressure scale.A new model is provided for prediction of convergent–divergent nozzle performance in the supersonic conditions for dehydration of natural gas as a well‐known industrial application of the nozzles. The model is developed by the genetic algorithm as a multivariable optimization method.

Recovery of thermodynamic hydrate inhibitors methanol, ethanol and MEG with supersonic separators in offshore natural gas processing

In offshore natural gas (NG) production, hydrate formation is a big concern that can impact the production and even stop NG flow. In this context, the injection of Thermodynamic Hydrate Inhibitors (THIs) in wellheads is widely employed in order to avoid these undesirable problems on subsea flowlines to gas processing rigs. However, in the main three-phase high-pressure separator in the gas rig, THI losses for gas phase are significant, particularly when the adopted THI is volatile like methanol and ethanol. This work discloses a new supersonic separator (SS) THI recovery process – SS-THI-Recovery – that treats the gas effluent from three-phase high-pressure separator achieving four simultaneous results: (i) gas water dew-point adjustment (WDPA); (ii) gas hydrocarbon dew-point adjustment (HCDPA); (iii) production of C3+ (propane and heavier) liquids as LPG; and (iv) recovery of almost all THI which would be lost in the gas otherwise. The proposition employs a supersonic separator (SS) battery followed by an anti-hydrates separator (LTX), a liquid-liquid THI extraction step and auxiliary THI distillation. SS-THI-Recovery was evaluated with HYSYS 8.8 simulator using methanol, ethanol and monoethylene glycol (MEG) as THIs. Supposing that the THI in the gas phase would be totally lost along with the exported gas otherwise, with SS-THI-Recovery the losses of methanol, ethanol and MEG were reduced by 91.9%, 79.3% and 99.2%, respectively, and such recovery factors could be further improved by increasing water flow rate in liquid-liquid THI extraction. Such high THI recovery entails reduction of THI costs with make-up, storage and transportation. Additionally, SS-THI-Recovery process is simple, with low footprint, and of easy implementation even for non-volatile THIs like MEG. Furthermore, the produced NG is ready for commercialization, dismissing additional treatment steps, not counting the commercial value of LPG. Therefore, new SS-THI-Recovery process configures an innovative and profitable alternative for gas treatment and THI recovery on gas processing offshore platforms.

Condensation characteristics of CH4-CO2 mixture gas in a supersonic nozzle

In order to study the condensation process of CH4-CO2 mixture gas in the supersonic nozzle, the physical and mathematical model of CH4-CO2 mixture of supersonic condensation process in the nozzle was given out based on the gas-liquid two-phase flow control equation, droplet nucleation, growth model and CO2 droplet surface tension. The effects of inlet pressure, inlet temperature and inlet CO2 fraction on the flow and condensation parameters were investigated. The results confirm the feasibility of using the supersonic separator for the removal of CO2 from natural gas. With the increase of the inlet pressure and the decrease of the inlet temperature, the condensation position moves forward, the maximum nucleation rate decreases and the liquid fraction of CO2 increases. The increase of inlet CO2 fraction makes it easier to achieve spontaneous condensation but the liquefaction efficiency remains at 57%~58%. The liquefaction efficiency of CO2 in the supersonic nozzle can be improved by increasing the inlet pressure and decreasing the inlet temperature.

Structure improvements and numerical simulation of supersonic separators with diversion cone for separation and purification.

In the improved supersonic separator with the diversion cone, the reflow channel and the flush-type drain structure are adopted to overcome two shortcomings: the shock wave that easily appears in the diverging section of the nozzle and the swirling flow that occurs in subsonic conditions with poor efficiency, which makes the low-temperature section short and the cooling effect unsatisfactory. In this study, the distribution of the main parameters and the effects of the inlet temperature and outlet angle of the swirler were investigated by numerical simulation. The results indicated that the internal extension structure severely damaged the supersonic flow in the nozzle, while the flush type drainage port slightly influenced the fluid. The smaller outlet angle of the drainage port reduced its effect on the supersonic flow. Moreover, the improved device with the reflow enlarges the supersonic region and exhibits the better performance. In addition, it achieves a low temperature (221 K) and high centrifugal acceleration (2.2 × 107 m s−2). Moreover, the inlet temperature of 300–320 K and the outlet angle of 50°–60° are recommended for the improved supersonic separator based on the comprehensive consideration of good expansion characteristics and centrifugal separation performance.

Dehydration of low-pressure gas using supersonic separation: Experimental investigation and CFD analysis

Supersonic nozzles are recently applied for carrying out water separation from natural gas streams and dew pointing in early stages of gas processing. This paper represents an experimentally and numerically study on a novel low-pressure two-phase driven supersonic nozzle constructed based on a new annular design. The nozzle includes a set of tilted fixed blades at the entrance and a swirling stabilizer along with a convergence-divergence nozzle. The liquid phase is separated from the primary gas phase by decompression and compression happening accompanied with the centrifugal effect induced by the swirling of the gas stream. The phase change happens by gradual drops in temperature and pressure upstream of the shockwave position, and an abrupt change at the shockwave position followed by a subsequent gradual increase in temperature and pressure. The pressure, temperature, and moisture level of the gas are measured to investigate the performance of the supersonic separation unit.The computation is carried out by a 2D approach capable of two-phase heat and mass transfer modeling. For the first time, the analysis uses high order of discretization schemes in order to well capture the shockwaves in a low-pressure supersonic nozzle and find out their effects on separation. An assessment is carried out focusing on the effect of operational conditions on the nozzle performance. The experimental data for dehydration efficiencies are in good agreement with the simulation results within 3%.The shockwave position is found in the range of 0.3–0.5 of non-dimensional nozzle length. The positions of both shockwave and initiation of condensation are shifted toward the exit side when the nozzle pressure ratio decreases. Reducing the pressure ratio from 0.8 to 0.6 will enhance the dehydration efficiency by about 5%.

Numerical simulation of supersonic separator with axial or tangential outlet in reflow channel

In the separation process of the conventional supersonic separator, the shock wave easily appears in diverging section of the nozzle and the swirling flow occurs in subsonic conditions with poor efficiency, which make the low temperature section short and the cooling effect dissatisfied. In this paper, a novel supersonic separator with reflow channel that has axial outlet or tangential outlet has been designed. The fluid can be drawn into the nozzle by the differential pressure between the discharge clearance on the wall and the throat zone near the centerline. Axial and radial distribution of the main parameters and the effect of pressure ratio are investigated with Reynolds Stress Mode (RSM) turbulence model. The results show that compared with the device without reflow channel, the pressure recovery in the diffuser with reflow channel is faster, the centrifugal acceleration is higher and the temperature is lower, which manifest the new device has a better refrigeration and separation performance. The supersonic separator with axial reflow outlet can obtain the better performance with pressure ratio of 1.5 that has a low temperature (221 K) and high centrifugal acceleration (2.2 × 107 m/s2), while the high centrifugal acceleration (9.89 × 109 m/s2) and low temperature (223 K) can be achieved in the device with tangential reflow outlet when pressure ratio is 2.0. Moreover, the structure with axial reflow outlet enlarges the supersonic region in the novel device and has a better ability to resist flow fluctuation and backmixing under the same condition when pressure ratio is 1.5. As the pressure ratio increases, the position of shock wave shifts downstream and the centrifugal acceleration and the reflow impetus increase, while the minimum temperature decreases, which prove that the increasing pressure ratio can improve the fluid expansion characteristics.

Investigation of supersonic separation mechanism of CO2 in natural gas applying the Discrete Particle Method

In order to study the separation mechanism of CO2 droplets and natural gas in a supersonic separator, different structure of cyclones were designed and the flow characteristics of natural gas in the supersonic separator were numerically calculated. The mathematical model of the continuous phase (methane gas) and the dispersed phase (CO2 droplets) was established, the distribution of the gas flow field and the droplets paths in the supersonic separator were obtained using Discrete Particle Method, the droplet separation efficiency was defined, and the effect of the droplet sizes and speed on the separation efficiency were analyzed. The results show that there is an obvious restriction relation between the pressure and tangential velocity distribution caused by the swirl blade, a balanced point (optimized structure) is necessary to achieve the purpose of condensation and separation of CO2 droplets. And the separation efficiency increases with the increase of droplet size and the CO2 droplets can be separated completely when the droplet size is greater than 1.5μm. The initial axial velocity has no obvious effect on the separation efficiency of the supersonic separator.

Numerical investigation and improvement strategy of flow characteristics inside supersonic separator

ABSTRACTIn order to study the flow behavior and optimize separation performance, a three-dimensional numerical model of an improved supersonic separator was developed. The proposed model takes into account the compressible and strong swirling effect. Four widely-used turbulence models include Sparlart–Allmaras model, realizable k–ε model, shear-stress transport (SST) k–ω model, and Reynolds stress model (RSM) were validated and compared by the experimental date reported in the literature. The comparison results indicated that RSMs have great potential to predict the flow inside supersonic separator. Based on the established numerical model, the distribution of critical parameters such as temperature, pressure, and Mach number was obtained. The influence of the pressure loss ratio on the shockwave location occurred at the divergence section of the Laval nozzle was systematically studied. Analysis about the relationship between the pressure effect and shock wave location was carried out to explore the principal factors that limit the performance of the supersonic separator.

CFD modeling on non-equilibrium condensation process of H2S in CH4-H2S mixture expansion through supersonic nozzles

Supersonic separation is an alternative technology potentially applicable in bulk H2S removal from natural gas containing high concentration of H2S. The spontaneous condensation of H2S in the Laval nozzle is the precondition of the separation. An Eulerian-Eulerian two-phase model containing non-equilibrium condensation behavior is developed to predict the condensation characteristics of H2S in CH4-H2S binary gas during supersonic expansion. The effects of H2S feed concentration, operating parameters and nozzle geometry on the condensation process are investigated. The results show that H2S vapor nucleates at adequate level of supersaturation during the rapid expansion in supersonic nozzles. When the inlet mole fraction of H2S increases, the nucleation process occurs much earlier and more H2S can be condensed into liquid phase. Condensation process of H2S can be further enhanced by increasing the inlet pressure and decreasing the inlet temperature. The increment of expansion ratio can improve the condensation performance of supersonic nozzle as the flow is expanded further.

Offshore processing of CO2 rich natural gas with supersonic separator versus conventional routes

The supersonic separator (SS) was investigated for treating humid natural gas with 44%mol CO2 in offshore rigs and compared to the conventional Water Dew Point Adjustment (WDPA) via TEG Absorption, Hydrocarbon Dew Point Adjustment (HCDPA) via Joule-Thomson Expansion (JTE) and CO2 removal via Membrane Permeation (MP). SS was tested as a single-step operation for WDPA + HCDPA. To simulate SS and MP, two Unit Operation Extensions (UOE) were developed for simulator HYSYS 8.8 (AspenTech). MP-UOE uses an empirical approach calibrated with operation data, whereas SS-UOE is entirely funded on thermodynamics, not demanding calibration. MP-UOE and SS-UOE use the thermodynamic infrastructure of HYSYS: property packages and several proved multiphase flash algorithms. MP-UOE and SS-UOE performed accordingly the expected characteristics of the respective operations and were critical to accomplish this analysis as SS and MP are not available in simulators. In terms of final gas quality (WDP ≤ -45oC @1.01 bar, HCDP ≤ 0oC @45 bar, %CO2 ≤ 15%mol) the best process configuration was found to be a hybrid one: SS WDPA + HDPA and MP CO2 removal, with low footprint and low power demand (-6.9%) relative to conventional 3-step way. If used for CO2 removal, SS could abate CO2 from 44% to 21.85%mol. Albeit less effective than MP, SS CO2 removal is a noticeable option that produces fuel gas for power generation with %CO2 ≈ 20% as required by new turbo-shafts. Moreover, CO2 is withdrawn from SS as a pumpable liquid allowing a cut of 44% in the power demanded for CO2 separation and injection as EOR agent.

Speed of sound of multiphase and multi-reactive equilibrium streams: A numerical approach for natural gas applications

A method is presented for calculating the thermodynamic sound speed of multiphase multi-reactive streams. A rigorous formula for the thermodynamic sound speed is developed via a steady-state, unidimensional, horizontal, adiabatic, frictionless, multiphase and multi-reactive equilibrium plug-flow. The main theoretical point is a correspondence between a multiphase multi-reactive plug-flow element and an Equilibrium Closed System (ECS), which has only two equilibrium state coordinates. Momentum and energy flow balances are processed via the ECS framework allowing the sound speed derivation for complex streams. The method uses ECS thermodynamic properties provided by multiphase Flash(P,T) of HYSYS 8.8 simulator. Unit Operation Extensions (UOE) are developed for calculating the multiphase multi-reactive sound speed by HYSYS. HYSYS solves the multiphase multi-reactive equilibria, including liquid water separation, to feed the ECS sound speed formula with required properties. The sound speed is also investigated in the critical neighborhood via the Landau Model approach to prove that it does not exhibit ± ∞ singularities at the critical point, despite the critical lambda-shape ± ∞ singularities of C¯P and (T,P) derivatives of the density. Multiphase examples are solved by the sound speed UOEs for simultaneous adjustments of water and hydrocarbon dew points of natural gas with supersonic separator. Multi-reactive multiphase sound speeds are also predicted in supersonic reactors for natural gas pyrolysis (GTL) and for two-phase methanol oxidation to formaldehyde.

ENSURING WORK SAFETY DURING PREPARATION OF RAW GAS USING LOW TEMPERATURE SEPARATION UNIT

A host of complex gas, which is composed of an apparatus for implementing the process of low-temperature separation, is characterized by high pressures. So, gas coming from gas fields may reach values of pressure 12 MPa, which in the event of an accident, creates a risk of harm to the staff servicing this node. Thus, the necessary calculation of wall thickness of pipelines and gas equipment. Besides, the condensation of the heavy components of the gas stream is accompanied by a significant release of cold, which can cause adverse effects on personnel working with this equipment. Supersonic separation involves the expansion of pre-cleaned from mechanical impurities downhole products in a supersonic nozzle profile, which is accompanied by temperature decrease. When passing the critical section of the nozzle the gas flow reaches the speed of sound and later in the diffuser is cooled to the desired temperature. From the point of view of ensuring safety of the working personnel, the implementation method of low-temperature separation allows to avoid unwanted negative effects. This is because this technology does not involve the participation of employees, during the implementation of technological operations, as required, for example, the expander technology. The paper discusses promising from the point of view of modernization of existing installations of complex preparation of gas of low-temperature separation of a supersonic technology based on the use of isentropic expansion, followed by isentropic compression of the gas stream instead of the classic isoenthalpic cooling. For analysis of the nozzle was realized a mathematical model of the supersonic nozzle. Numerical simulation plays an important role in accelerating the development of the method necessary to optimize and improve the separation efficiency and reduce the cost of this method. To ensure trouble-free operation of the technology in question was used to calculate the thickness of the wall based on the loads acting on the profile, caused by the pressure of the medium inside the nozzle, and stresses in the metal from temperature changes.

Numerical simulation of CO 2 condensation process from CH 4 -CO 2 binary gas mixture in supersonic nozzles

Supersonic separation technology is a technique potentially applicable in separating CO2 from natural gas. An Euler-Euler two-fluid model is developed to predict the spontaneous condensation process of CO2 in CH4-CO2 binary gas system during supersonic expansion using the nucleation theory and droplet growth model. The proposed model is validated by the experimental data obtained from literature and the effects of inlet temperature, pressure and gas composition on the condensation process are analyzed. The simulation results show that the homogeneous nucleation of CO2 occurs intensively within a short period in the diverging part of the nozzle and the mass fraction of condensed phase increases continuously until the nozzle outlet. The onset of the condensation occurs earlier and more CO2 can be condensed out of the gas mixture at lower inlet temperature, higher inlet pressure and bigger CO2 feed concentration.

Optimization of dehydration process to improve stability and efficiency of supersonic separation

Supersonic gas-liquid separation is considered as one of the novel methods for natural gas conditioning that removes either water or heavy hydrocarbons from the natural gas without using chemical ingredients. The expansion of natural gas stream in a supersonic nozzle increases the fluid velocity to supersonic level and makes low pressure and temperature conditions in which nucleation of the liquid phase and condensation initiate. The process includes two isentropic processes with an intermediate shockwave. This research is the first serious attempt for controlling the supersonic separation units and applies the stability concept for actual conditions and disturbances in the gas pressure and composition during the lifecycle of a gas well. The article explores how the optimal performance of a supersonic nozzle can be saved with variations in the nozzle input gas pressure or composition. As the phase change and condensation are mainly occur at the shockwave position, the geometry configuration of supersonic nozzle is of high importance so that the drain location should be aligned with the shockwave location. An algorithm is employed to find a control function to compensate the backpressure of the nozzle within the feed pressure variation. It guarantees the equipment to be kept effectively in the optimal operating condition. In a case study, it is explained how the proposed solution enables the system to keep the dew point within ±2.5 °C drift from the desired criteria following a ±18% disturbance in the inlet pressure. Moreover, investigation on the effect of natural gas composition shows that the separation system is intrinsically robust to variations in the main natural gas composition.

Numerical simulation of carbon dioxide removal from natural gas using supersonic nozzles

Supersonic separation is a technology potentially applicable to natural gas decarbonation process. Preliminary research on the performance of supersonic nozzle in the removal of carbon dioxide from natural gas is presented in this study. Computational Fluid Dynamics (CFD) technique is used to simulate the flow behavior inside the supersonic nozzle. The CFD model is validated successfully by comparing its results to the data borrowed from the literature. The results indicate that the liquefaction of carbon dioxide can be achieved in the properly designed nozzle. Shock wave occurs in the divergent section of the nozzle with the increase of the back pressure, destroying the liquefaction process. In the supersonic separator, the shock wave should be kept outside of the nozzle.

NGL supersonic separator: modeling, improvement, and validation and adjustment of k-epsilon RNG modified for swirl flow turbulence model

The processing of natural gas requires the implementation of new technologies in a context of increasing demand around the world. The natural gas liquids (NGL) separation using supersonic devices is a novel and efficient way to reduce volume of installed equipment and operation costs using the effects of highly turbulent and circular flows. This research implemented Computational Fluid Dynamics (CFD) modeling to improve the efficiency of typical NGL recovery process using the supersonic approach. In this research, a novel turbulence modeling approach was implemented aiming to minimize the processing time, and the results obtained were validated with experimental data available. This research is based on the model called k-epsilon RNG modified for swirl flow, this model has not been used and validated previously in highly compressible, turbulent and circular flow systems. The efficiency of the process was improved by 11% in comparison to the efficiency reported in past studies, and the processing time for the modeling was reduced by 40% with the proposed and adjusted turbulence approach. During the validation of the model k-epsilon RNG modified for swirl flow the swirl factor, part of the turbulence model, was adjusted to an optimum value for compressible, turbulent and circular flow systems involved in supersonic NGL separation process, allowing accurate results to be obtained with lower processing time than with other typical and common approaches as RSM and LES.

Maximization of energy recovery inside supersonic separator in the presence of condensation and normal shock wave

Natural gases provide around a quarter of energy consumptions around the globe. Supersonic separators (3S) play multifaceted role in natural gas industry processing, especially for water and hydrocarbon dew point corrections. These states of the art devices have minimum energy requirement and favorable process economy compared to conventional facilities. Their relatively large pressure drops may limit their application in some situations. To maximize the energy recovery of the dew point correction facility, the pressure loss across the 3S unit should be minimized. The optimal structure of 3s unit (including shock wave location and diffuser angle) is selected using simultaneous combination of normal shock occurrence and condensation in the presence of nucleation and growth processes. The condense-free gas enters the non-isentropic normal shock wave. The simulation results indicate that the normal shock location, pressure recovery coefficient and onset position strongly vary up to a certain diffuser angle (β = 8°) with the maximum pressure recovery of 0.88 which leads to minimum potential energy loss. Computational fluid dynamic simulations show that separation of boundary layer does not happen for the computed optimal value of β and it is essentially constant when the inlet gas temperatures and pressures vary over a relatively broad range.

Trends in Supersonic Separator design development

Supersonic separator is a new technology with applications in hydrocarbon dew pointing and gas dehydration which can be used to condensate and separate water and heavy hydrocarbons from natural gas. Many researchers have studied the design, performance and efficiency, economic viability, and industrial applications of these separators. The purpose of this paper is to succinctly review recent progress in the design and application of supersonic separators and their limitations. This review has found that while several aspects of this study are well studied, considerable gaps within the published literature still exists in the areas such as turndown flexibility which is a critical requirement to cater for variation of mass flow and since almost all the available designs have a fixed geometry and therefore cannot be considered suitable for variable mass flow rate, which is a common situation in actual site. Hence, the focus needs to be more on designing a flexible geometry that can maintain a high separation efficiency regardless of inlet conditions and mass flow variations. This review is focusing only on the design and application of the supersonic separators without going through the experimental facilities, industrial platform, pilot plants as well as theoretical, analytical, and numerical modelling.

The Influence of Supersonic Separator’s Divergent Angle on Spontaneous Condensation and Separation Performance of Two-Phase Flow

The Influence of Supersonic Separator’s Divergent Angle on Spontaneous Condensation and Separation Performance of Two-Phase Flow