Supersonic Separation Research Articles

Study on energy distribution characteristics of cyclone in Laval nozzle

Supersonic separator is a device for dehydrating and separating heavy hydrocarbons in natural gas. Laval nozzle and cyclone in the separator have a great influence on the separation efficiency. At present, there are few studies on the energy distribution characteristics of the cyclone, and the law of the influence of cyclone on subsonic and transonic flow fields is not clear. Based on numerical simulation, the paper studies the influence of gas-liquid density on the separation performance of the separator, studies the energy distribution characteristics of methane flow in Laval nozzle under subsonic and transonic velocities, and deduces the energy equation containing axial kinetic energy term and radial kinetic energy term in the nozzle. The results show that the gas-liquid density has some effects on the separation efficiency of the separator. The distribution of tangential velocity and tangential kinetic energy in the nozzle is similar before the cyclone along the axis, but after the cyclone, the distribution of tangential velocity and tangential kinetic energy is significantly different. The proposal of tangential kinetic energy has important guiding significance to the structure design of supersonic separator.

Swirl intensity as a control mechanism for methane purification in supersonic gas separators

Supersonic gas separator is proposed for methane purification. One-dimensional analysis is performed to examine the design aspects associated with the converging-diverging nozzle and the liquid separation chamber. The results indicate that at low swirl intensities, the separation chamber length is about 100 times of the nozzle throat. Increasing the swirl intensity would lower this length sharply and it will be less than 10 for swirl intensity of unity which is equivalent to a swirl angle of 45°. In addition, a sensitivity analysis of the separator performance to the inlet conditions like the temperature, pressure, and composition of the mixture is carried out. It is observed that increasing the inlet temperature will reduce the condensation rate and separation performance, while an increase in the inlet pressure and methane mole fraction will boost the flow condensation rate. Further, changes in the inlet condition will affect the critical droplets diameter and required separation length. In this regard, the separation length will increase with decreasing inlet temperature and increasing inlet pressure and methane mole fraction. The results reveal that the operation envelope of the supersonic separator is more sensitive to the variations in the inlet flow temperature where high swirl intensity is required at low inlet temperature in order to maintain the separation chamber length. Finally, the results support that, for a fixed geometry of the supersonic gas separator, in order to maintain the performance, single or combined variations in the inlet conditions can be compensated by changing the swirl intensity within a reasonable envelope.

Optimisation study of a supersonic separator considering nonequilibrium condensation behaviour

The supersonic separation offers an opportunity for natural gas processing. The problem is that the phase change of water vapour in the supersonic flow is not fully understood in the presence of shock waves in a supersonic separator. This study aims to evaluate the performance of the supersonic separation with the phase change process and shock waves. The condensing flow model is developed to accurately predict the energy conversion within the supersonic separator. The computational results show that the single-phase flow model over-estimates the vapour expansions by 12.43% higher Mach number than the condensing flow model. The liquid fraction of 8.2% is predicted by the condensing flow model during the phase change process in supersonic separators. The supersonic separator is optimised via combining the diverging part of the supersonic nozzle and constant cyclonic separation tube as a long diverging part of the newly designed nozzle. The optimised supersonic separator reduces the energy loss by eliminating the oblique and expansion waves in the newly designed nozzle, which improves the energy efficiency for natural gas processing.

On small-scale liquefaction of natural gas with supersonic separator: Energy and second law analyses

Under certain conditions supersonic separators promote superior natural gas conditioning over traditional routes. An instigating possibility consists in using supersonic separators to obtain liquefied natural gas that exhibits global growth. This work analyses natural gas liquefaction through a low-scale process using realistic non-isentropic supersonic separator whose design was proposed in the literature. Results are compared with other compact adiabatic expansion routes like the Joule-Thomson and turbo-expander, all starting from natural gas at (298.15 K, 80 bar) and reaching saturated liquefied natural gas at (115.4 K, 1.25 bar). The objective was to demystify the notion that 100% liquefaction per pass is possible with supersonic separator by obtaining the maximum liquefaction fraction per pass for the three compact routes and unveiling whether supersonic separators lead to a less power intense, technically, thermodynamically and economically superior short-scale liquefaction process. To this end, a HYSYS Unit Operation Extension was created to simulate supersonic separators with rigid-geometry, following expected physical behavior and satisfying the 2nd Law of Thermodynamics. Results include a 2nd Law analysis of processes leading to thermodynamic efficiency and liquefaction lost work. Supersonic separator route reached the highest Net Present Value of all routes but was somewhat outperformed by turbo-expander route in terms of power consumption, thermodynamic efficiency and lost power.

Investigation on the role of drainage structure in preventing the mass accumulation induced by shock/boundary-layer interactions in supersonic separators

The occurrence of shock wave in nozzle and the blockage of fluid at the so called "second throat" are closely related to the drainage structure of supersonic separator, which result from the thickening of the boundary layer owing to the interaction between boundary layer and shock waves. Two types of drainage structures are proposed to solve the above problems and promote the separation performance for gas purification. The CFD modeling is used to optimize and evaluate the different drainage structures. The results show that the cylindrical drainage and the cascade drainage can efficiently alleviate or remove the mass accumulation at the "second throat". The cascade drainage with reflux channel and oblique openings is superior to other drainage structures, due to the low flow velocity and high centrifugal acceleration in the drainage section. The flow velocity and temperature in the cascade drainage chamber with reflux channel are opposing for separation performance of the supersonic separator. It means that high flow rate in cascade drainage section will lead to lower temperature, but increase the number of droplets carried by airflow, and vice versa.

Supersonic separator’s dehumidification performance with specific structure: Experimental and numerical investigation

Supersonic separators are used in gas separation processes such as dehumidification of humid air due to high performance and its good pressure recovery. In the present study, a comprehensive numerical and experimental investigation on the hydrodynamic behavior of air as working fluid and dehumidification performance of supersonic separator have been accomplished. The effect of the operational parameters on shockwave’s position are examined. The outcomes show that by increasing the pressure level of supersonic separator, relative error between numerical and experimental results decreases from 20% to less than 10%. The effect of the operational parameters and humidity of inlet air on the dehumidification performance of supersonic separator have been investigated by proposing a new simplified method for predicting condensation. Results show that the improvement of dehumidification performance is about 5% for 2 bar increase of inlet pressure and 15% for 10 K decrease of inlet temperature. Moreover, with 0.05% increase in the molar percent of water vapor in the inlet air, the performance of supersonic separator is enhanced about 10%. In addition, decreasing the outlet pressure under specified operating conditions has no effect on performance of supersonic separator and only it could compensate the undesirable displacement of shockwave due to decreasing the inlet pressure.

Novel ethylene oxide production with improved sustainability: Loss prevention via supersonic separator and carbon capture

Sustainability must be always assured in process design. Not rarely, multiple sustainability criteria point oppositely, entailing a need for more systematic and coherent assessments. The Sustainable Process Systems Engineering method is introduced as a two-level hierarchical evaluation of process designs. The first level selects the best design via four-dimensional indicators (environment, efficiency, health-&-safety, and economic), while in the second level, sustainability hotspots of the best design are pinpointed to unveil possible improvements. The method is applied for sustainability assessment of two ethylene oxide processes: the conventional and a novel route employing supersonic separator to prevent ethylene oxide losses using liquid-water injection. Supersonic separator route reduces oxide losses by 83.33 kg/h, representing +0.9% greater ethylene oxide production, 95% less ethylene oxide losses, entailing 2.5% higher net value for 20 operation years despite 0.11% higher investment, and consequently exhibiting the best environmental, technical, health-&-safety and economic performances. Photochemical-oxidation and aquatic-ecotoxicity are environmental indicators with highest improvement due to supersonic separator inclusion. Ethylene oxidation reactor, carbon dioxide stripping-column and cooling-water tower are the main unit-operations with sustainability hotspots.

Low-pressure supersonic separator with finishing adsorption: Higher exergy efficiency in air pre-purification for cryogenic fractionation

Air fractionation in cryogenic plants demands a Pre-Purification Unit for air compression and removal of water, carbon dioxide and trace-species. The conventional Pre-Purification Unit – FULL-TSA – adopts temperature-swing adsorption (TSA) with an activated-alumina bed for dehumidification followed by a molecular-sieve bed removing carbon dioxide and trace-species. A novel scheme – SS-TSA – prescribes a Supersonic Separator abating ≈98.5% of moisture followed by a small single-bed molecular-sieve removing remaining impurities. SS-TSA-HI is a SS-TSA variant allocating compression heat for molecular-sieve regeneration. This work compares thermodynamic performances of FULL-TSA, SS-TSA and SS-TSA-HI via exergy analyses and discloses better utilization of resources. Exergy analyses are supported by HYSYS simulator running with unit-operation extensions for calculating the phase-equilibrium sound speed and simulating the supersonic separator. Since exergy rates depend on Reference Environmental Reservoirs, two Reference formalisms are employed at 1 atm and 25 °C. Reference#1 prescribes air with 60% relative humidity (the raw air feed), while Reference#2 adopts water-saturated air. It is shown that Reference#1 is appropriate for overall system analysis, while Reference#2 is better for sub-systems analysis as it dramatically lowers cooling-water exergy flows. SS-TSA has 61% less exergy losses than FULL-TSA in the purification step as the supersonic separator accomplishes bulk dehumidification drastically reducing steam and nitrogen for bed regeneration. Reference#1 exergy efficiencies of FULL-TSA, SS-TSA and SS-TSA-HI attained 57.9%, 60.0%, and 60.3%, respectively.

Numerical Simulation of the Recirculation Flow during the Supersonic Separation of Moving Bodies

The features of a flow generated during the injection of a supersonic small body (pellet) from the channel of a spherically blunted cylinder into a supersonic flow are studied. The cylinder’s diameter is 35 times larger than the diameter of the injected body. The numerical simulation is carried out using multilevel Cartesian grids with the local adaptation based on the wavelet analysis. The body’s motion is simulated by the free boundary method. The dynamics of the moving body’s interaction with the bow shock from the cylinder, the formation of the reverse flow region between the bodies, its evolution and disappearance, and the subsequent establishment of a stationary flow are studied. The decrease of the main body’s drag to 20% of the initial value is demonstrated.

Protected supersonic separator performance against variable CO2 content on natural gas processing: Energy and sustainability analyses

In offshore oil-and-gas fields with high CO2 content, oil-and-gas processing entails recycling CO2 to the field for enhanced oil recovery. Despite blocking fossil CO2 ingress into the biosphere and improving oil production, this entails increasing %CO2 in oil-and-gas with time, challenging the processing plant. Supersonic separator is a compact gas processing solution that can simultaneously adjust water and hydrocarbon dew-points. But, supersonic nozzles can only work if sonic-throat flow occurs. In this work a supersonic nozzle sized to a high-pressure 25%mol CO2 feed is submitted to varying %CO2 from 5%mol to 80%mol under constant feed temperature/pressure and molar flow rate. Varying %CO2 affects nozzle performance with three possible outcomes: (i) feed is insufficient to reach sonic-throat flow, entailing no supersonic processing; (ii) feed is at design condition (25%mol CO2) and nozzle works tightly; and (iii) sonic-throat flow is achieved with a fraction of the feed; i.e., there is no room to process the rest. These apparent shortcomings of supersonic gas processing are examined and a control-scheme based on manipulating feed-pressure is proposed to circumvent feed deficit/excess. With the proposed feed-pressure adjustment to protect supersonic separator operation, energy and sustainability assessments are conducted to evaluate environmental and gas-fired power generation impacts from the supersonic processing of feeds with varying %CO2. Feeds with 10%mol/15%mol/20%mol CO2 have highest sustainability-degrees.

Study on the influence of wall-mounted cyclone on the purification and separation performance of supersonic separator

To avoid the secondary nozzle caused by the large area occupied by the cyclone, in this paper, a new type of wall-mounted cyclone was proposed for the structural design of supersonic separator, which significantly reduces the space occupied by the cyclone and thus delays the generation of shock waves. The natural airflow field and droplets movement trajectory in supersonic separator were numerically calculated by RNG k-ε turbulence model and DPM model of Fluent software. The influence of different cyclone structures and different drain sizes on the separation performance of the device was studied systematically. The results show that the structure of the cyclone seriously affects the separation performance of the device. When the length of the cyclone is 0.02 m, the number of blades is 3, the thickness of the blades is 0.25 mm, the rotation angle of the blades is 60°, and the clearance of the drainage port is 2.5 mm, the separation efficiency of the liquid droplets with diameter of 2.5 μm reaches over 89 %, and the effect is good.

Experimental investigation and numerical analysis of separation performance for supersonic separator with novel drainage structure and reflux channel

Recently, a novel supersonic separator with reflux channel has been developed that has a better refrigeration and separation performance. However, the previous studies are limited to the investigation of numerical simulation, too little attention has been paid tothe experimentresearch. In the paper, an experimental platform for performance evaluation of the supersonic separator was established. The effects of reflux channel, drainage structure and operation conditions on separation performance were studied by experiment and 3d numerical simulation. The research results show that the consistency between experiment and numerical simulation can be increased by adopting material with small roughness and increasing pressure ratio. The reflux channel can effectively improve the flow field and separation performance. And the novel cylindrical drainage structures can weaken the interaction between shock wave and boundary layer,infavorof forming the lower and more balanced temperature field in the nozzle. The experimentalinvestigationand simulation analysis were utilized to confirm the optimum reflux structure: LDepth = 30 mm, Dinsert = 7 mm and Dreflow = 24 mm. The δsteam and ΔTd can reach 47.72% and 17.41 K, respectively for nozzle with reflux channel and flush drainage, which are 9.02% and 5.42 K higher than those for nozzle with flush drainage and without reflux channel. The circular opening drainage without reflux channel has the greatest δsteam and ΔTd which are 48.28% and 17.98 K, respectively.

Prediction of dehydration performance of supersonic separator based on a multi-fluid model with heterogeneous condensation

Supersonic separation is a novel technology. A multi-fluid slip model for swirling flow with homogenous/heterogenous condensation and evaporation processes in the supersonic separator was built to estimate the separation efficiency. This model solves the governing equations of compressible turbulent gas phase and dispersed homogenous/heterogenous liquid phase considering droplet coalescence and interphase force. Its prediction accuracy for condensation and swirling flows was validated. Then, the flow field, slip velocity and droplet trajectory inside the separators with different swirl strengths were investigated. The maximum values of radial slip velocity are 29.2 and 8.26 m/s for inlet foreign droplet radius of 1.0 and 0.4 µm. It means the larger foreign droplet has a better condensation rate. However, the residence time of larger foreign droplet in core flow is shorten. Thus, the inlet radius of foreign droplet has to be moderate for best separation efficiency. Finally, the dehydration performances of separator were evaluated. The optimal radius of inlet foreign droplet to maximize the dehumidification and efficiency was found. For the separator with swirl strength of 22%, the optimal radius is 0.85 µm at inlet pressure of 250 kPa, where the maximum dew point depression is 42.41 °C and the water removal rate is 87.82%.

Improvement of drainage structure and numerical investigation of droplets trajectories and separation efficiency for supersonic separators

As a new type of separation device, supersonic separator has received more and more attention in many areas. However, the interaction between boundary layer and shock wave in supersonic section results in the reverse pressure gradient and makes the boundary layer thicken, which form a so-called "second throat" that reduces the actual flow area and causes the fluid to be chocked. A novel cylinder drainage structure with larger drainage area was developed. When shock wave occurs, the high pressure will make more fluid enter into the openings on cylinder drainage structure, which is equivalent to increase the flow capacity. Therefore, the shock wave is weakened and the total pressure loss is reduced. Moreover, the prediction model of discrete phase coupling verified by experiments was adopted in order to predict the trajectory of droplets and separation efficiency for supersonic separator. The results show that with the increase of droplet size, the separation efficiency increases gradually and the particle diameter of 2–4 μm is sensitive to the separation. The droplets trajectory is more clear and regular in the nozzle with reflux channel and cylindrical drainage structure.

Overview of Selected Natural Gas Drying Methods

Abstract The gas produced from the deposit usually contains various types of pollution. They are the reason for limiting its use, often making its use impossible. Therefore, it requires appropriate treatment. One of the main pollutants in gas is water. Its occurrence causes a lot of problems, especially at the stage of gas transport, such as the formation of hydrates blocking pipelines and apparatus, causes the phenomenon of condensation and corrosion of pipelines, especially if there is carbon dioxide or hydrogen sulphide in the gas. The paper presents a number of methods that enable drying of gas after extraction. Each of them has different parameters that will be achieved for the gas after it is dried. Depending on the required degree of drying, the economics of the process and compatibility with other dependent processes, the individual methods are more or less used in gas engineering. The paper discusses methods of absorption in ethylene glycol solutions, adsorption methods using silica gel, molecular sieves or calcium chloride and low-temperature processes such as Twister® supersonic separator, IFPEX-1® process and DexProTM process.

Non-equilibrium condensation of water vapour in supersonic flows with shock waves

The fluid flow and heat and mass transfer in a supersonic separator are not understood well due to the complicated interaction of the supersonic flow, swirling flow, phase transition and shock waves. In the present study, we develop a wet steam model to investigate the flow structure inside a supersonic separator with the co-existence of non-equilibrium condensation and shock waves. A study of the effect of the inlet subcooling and inlet saturation on the condensation behaviour is conducted to evaluate the performance of the supersonic separation with a focus on the shock wave. The numerical result shows that the degree of supersaturation of the water vapour can reach a maximum value of 4.28 within the designed supersonic separator and generate a peak nucleation rate of approximately 1021 kg m−3 s−1. The occurrence of the shock wave changes the equilibrium thermodynamic state, which leads to the re-evaporation of the condensed droplet. Higher inlet subcooling and inlet saturation not only shift downstream the position of the shock wave, but also induce an earlier condensation and higher liquid fraction. For the present nozzle, when the inlet subcooling and inlet saturation are about 34 K and 0.28 respectively, the shock wave intersects the region of the intense nucleation process, the non-equilibrium condensation process is terminated due to the increase of the pressure and temperature downstream the shock wave. Stronger swirling flow results in non-uniform distribution of the static pressure and decreases the nucleation rate of water vapour. The high swirling flow with a maximum swirl velocity of 150 m/s weakens the liquid fraction by 25% compared to the no swirling flow. This indicates that it is important to balance the swirling flow and condensation process to achieve an efficient performance of the supersonic separator.

Numerical simulation of a supersonic separator

The current paper presents data on supersonic flow simulation process. The main goal of the study was to demonstrate supersonic separation technology with the help of computational fluid dynamics. In order to determine the quality of separation process itself a supersonic separator has been designed and numerically simulated. The study is mainly concentrated on investigation of the flow structure in the supersonic separator. As a result of the calculation, effective separation factor has been determined, separation process is qualitatively evaluated and described, flow simulation process is visually presented.

Investigating the effect of water nano-droplets injection into the convergent-divergent nozzle inlet on the wet steam flow using entropy generation analysis

In many industrial pieces of equipment, such as ejectors, thermo-compressors, supersonic separators, etc., in which the condensation phenomenon occurs spontaneously, steam enters the nozzle in a saturated condition. Liquid droplets are likely to be present in the saturated vapor. In the present paper, the effect of the existence of a number (9 cases) of liquid droplets (with a fixed size of 20 nm in all cases) on the saturation steam has been studied. The results demonstrate that with the increase in the number of droplets at the nozzle inlet; pressure, temperature, liquid mass fraction, mass-generated rate and entropy increase in the convergent part. However, the growth of droplets and Mach number decrease. Furthermore, due to steam condensation and latent heat transfers to steam, the degree of supercooling, the supersaturation ratio and the flow rate at the nozzle inlet are decreased, the intensity of condensation shock is reduced, and shock occurs with delays. With an increase in the number of droplets, condensation shock and the nucleation phenomenon will not occur. The amount of entropy production decreases with the surge in the number of droplets at first, and then begins to increase. The results demonstrate that in the presence of 3 × 10+14 droplets, compared to the absence of water droplets at the nozzle inlet, generated entropy, liquid mass fraction and the mass flow rate will decrease by 9.4%, 12.5%, and 1.7%, respectively.

Analysis for spiral vortex and effect of profile of nozzle and swirler on performance of supersonic separator

The supersonic separation technology has been widely concerned as a new technology in separating and processing liquid in many fields. It is well known that the shape of the nozzle is very important to the separation performance. However, most of the previous researches for structural profile of the nozzle ignored the the fact that diversion cone and swirler contributes to change the flow field. And the investigation of strong swirling flow in the nozzle is not enough and especially the diversion cone in nozzle increases the particularity and complexity of the swirling flow, which makes the rotation flow of the fluid deviate the motion of combined spiral vortex. In the paper, the structural profile of the nozzle and the swirler was studied considering the influence of diversion cone and swirler on the flow field. Meanwhile, the motion law of strong swirl caused by swirler in the nozzle was analyzed by 3D numerical simulation. The simulation results indicate that the presence of the diversion cone has a great influence on the nozzle profile and internal flow field. The optimum configuration was determined as follows: stable length of 0.5Din, linear convergent section, throat transition with circular arc, equal slope method for divergent section and diffuser with two stages. The flow fields in the nozzles with circular arc blade and power function blade are more uniform. For the circular arc blade, the attenuation of centrifugal acceleration is the slowest and the centrifugal acceleration at the nozzle outlet is the largest, which is more conducive to the separation of the condensate. Besides, the motion of the swirling fluid in the convergent section and throat does not have the characteristics of combined eddies on account of the viscosity loss and the diversion cone. Nevertheless, the presence of the diversion cone weakens the relative pulsation and stabilizes the flow field. The reflux channel between the drain cavity and the throat can solve the backmixing of fluid resulting from the low pressure in the forced spiral vortex region.

Numerical investigation of multiphase flow in supersonic separator considering inner body effect

AbstractThis paper presents research on the supersonic nozzle geometry with particular emphasis on the effect of the internal body of the nozzle that largely affects the separation efficiency. A numerical investigation of the supersonic nozzle with an internal solid body, which forms an annular flow inside the convergent–divergent nozzle is carried out. The present study revealed different hydrodynamic behaviors of the nozzle, exploring different shapes of the inner body, and the computational fluid dynamic simulations of the supersonic nozzle is utilized to find out the best geometrical design. Utilizing a coupled pressure–velocity scheme with high order of discretization of the governing equation yielded to find the shockwave positions in different conditions. The turbulent behavior of the fluid in the shockwave zone is well discussed and the phase‐change phenomena for the natural gas application are studied considering both water condensation and hydrocarbon condensation simultaneously. Different nozzle configuration elucidates the physical mechanisms of the supersonic flow inside the nozzle. Shockwave position, swirling velocity stability, and mass flow capacity are investigated. The lower the inner body radius, the less the change of shockwave position in the gas is found. Also, the higher stability of swirling velocity magnitude is found for the convergent–divergent inner body, which brings enhanced physical phase separation.