【機(jī)械類(lèi)畢業(yè)論文中英文對(duì)照文獻(xiàn)翻譯】緊湊型管柱狀氣液旋流分離器技術(shù)目前的發(fā)展?fàn)顩r

【機(jī)械類(lèi)畢業(yè)論文中英文對(duì)照文獻(xiàn)翻譯】緊湊型管柱狀氣液旋流分離器技術(shù)目前的發(fā)展?fàn)顩r,機(jī)械類(lèi)畢業(yè)論文中英文對(duì)照文獻(xiàn)翻譯,機(jī)械類(lèi),畢業(yè)論文,中英文,對(duì)照,對(duì)比,比照,文獻(xiàn),翻譯,緊湊型,柱狀,旋流分離器,技術(shù),目前,發(fā)展,狀況,狀態(tài) DISTINGUISHED AUTHOR SERIES 58 JULY 1998 ? STATE OF THE ART OF GAS/LIQUID CYLINDRICAL-CYCLONE COMPACT-SEPARATOR TECHNOLOGY Ovadia Shoham, SPE, U. of Tulsa, and Gene E. Kouba, SPE, Chevron Petroleum Technology Co. SUMMARY The petroleum industry has relied mainly on conventional, vessel- type separators to process wellhead production of oil/water/gas flow. However, economic and operational pressures continue to force the industry to seek less expensive and more efficient separa- tion alternatives in the form of compact separators, especially for offshore applications. Compared with vessel-type separators, com- pact separators, such as the gas/liquid cylindrical cyclone (GLCC), are simple, low-cost, low-weight separators that require little main- tenance and are easy to install and operate. However, the inability to predict GLCC performance adequately has inhibited its wide- spread deployment. Current R however, we have not dis- cussed them here because little or no performance information is available. These include a variable inlet-slot area and the config- urations of the gas and liquid outlets. SIMULATION In the past, performance predictions of GLCC separators have been carried out on the basis of experience, rules of thumb, and empirical correlations. These methods are limited in their ability to be extrapolated to different flow conditions and untried appli- cations. Currently, efforts are under way to develop mechanistic models for the GLCC and conduct computational fluid dynamic (CFD) simulations. Mechanistic modeling offers a practical approach to GLCC design and performance prediction. Simplifying assumptions are used, but, ideally, the models still capture enough of the fundamental physics of the problem to allow interpolation and extrapolation to different fluid-flow conditions. CFD simulations predict details of the com- plex hydrodynamic-flow behavior in the GLCC, including flow field, holdup distribution, and trajectories of discrete particles of the dis- persed phase. While well-suited for local simulation of single-phase or dilute dispersion flows, current CFD simulators cannot yet handle the complexities of the full range of multiphase flow. Furthermore, CFD models of large piping systems that include the GLCC typical- ly are too unwieldy to be practical for design purposes. Because mechanistic models are greatly simplified, they are not as detailed, rigorous, or accurate as CFD models. However, mech- anistic modeling has many advantages: speed of setup and compu- tation, ability to model an entire system, and suitability for PC operation. Consequently, these models are more accessible to engi- neers as a design tool than are CFD models. Mechanistic Modeling. The ultimate aim of modeling work to date has been to predict the operating envelope for the GLCC with respect to liquid carry-over in the gas stream and gas carry-under in the liquid stream. Each fluid-flow path has its own particular set of calculations. The starting point for either calculation path is the global distribution of gas and liquid in the GLCC, namely, the equilibrium liquid level. Equilibrium Liquid Level. The equilibrium liquid level in the GLCC is determined by the pressure drop between the gas and liq- uid outlets. Because the frictional losses in the GLCC are low, the equilibrium liquid level is a reasonable indication of the amount of liquid in the GLCC. The model is based on a pressure balance on the gas and liquid legs. Ref. 2 gives details of this model. Vortex Shape and Location. The shape and location of the vortex are important for prediction of both liquid carry-over and gas carry- under. The vortex model assumes rigid-body rotation (i.e., a linear tangential-velocity profile in the radial direction). 2 Coupling the cal- culations for equilibrium liquid level and vortex shape makes deter- mination of the location of the vortex and the height of the vortex crown possible. This model of the global distribution of gas and liq- uid provides the groundwork for the performance models. Liquid Carry-Over. Liquid carry-over in the gas stream is largely dependent on the flow pattern in the upper part of the GLCC. Flooding may occur in the GLCC at high liquid levels and low gas rates, produc- ing bubbly flow. The unstable liquid oscillations, characteristic of churn flow at moderate gas rates, may splash liquid into the gas outlet. Liquid can also be carried out in droplets at the onset of annular mist flow at high gas rates. At very high gas rates, the centrifugal force of the swirling gas pushes the liquid to the wall of the pipe, where it may form an upward-spiraling continuous ribbon of liquid. At present, the onset of liquid carry-over is predicted for low to moderately high gas rates. The key to onset of liquid carry-over has been to predict accurately the maximum liquid holdup (volume frac- tion) occurring in the upper part of the GLCC under zero-net-liquid- flow conditions and its effect on the pressure balance between the gas and liquid legs. Fig. 2 compares model predictions with experimen- tal results in plots of the maximum liquid holdup in the upper GLCC region (i.e., zero-net-liquid-flow holdup, y L0 , vs. the superficial gas velocity, v gs , in the GLCC). 2 Additional data collected for a range of liquid viscosities from 1 to 10 cp showed negligible effect on the zero-net-liquid-flow holdup. 6 Once the maximum liquid holdup Fig. 2—Zero-net-liquid-flow holdup in air/water system. 2 Fig. 3—Operational envelope for liquid carry-over in a 3-in. GLCC operated with air and water. 2 v gs , ft/sec y L0 v gs , ft/sec v Ls , ft/sec ? JULY 1998 61 allowed in the upper part of the GLCC is known for a given gas rate, the pressure-balance calculation is used to determine the liquid rate required to achieve this holdup and initiate liquid carry-over. Fig. 3 compares the experimental and predicted operational envelopes for a 3-in. laboratory GLCC in a loop configuration, operated with air and water at low pressures. 2 The operational envelopes are presented in terms of superficial liquid velocity, v Ls , vs. superficial gas velocity, v gs , in the GLCC. The agreement of model predictions with the data is very good. Comparison with data from Ref. 6 showed that the model seems to capture the phys- ical phenomena and predict well the reduction of the operational envelope with increasing liquid viscosity. Future improvements to liquid-carry-over modeling will include expansion to different operational conditions (e.g., high gas rates) as well as prediction of the quantity of liquid carry-over and dynamic responses to flow-rate surges. Gas Carry-Under. Three mechanisms have been identified as possi- ble contributors to gas carry-under in the liquid stream: (1) shallow bubble trajectories prevent small bubbles from escaping to the gas-core filament, (2) rotational-flow instability causes helical whipping and breaking of the gas-core filament near the liquid exit, and (3) liquid- rate surges can produce a concentrated cloud of bubbles that hinders bubble migration to the gas core. Currently, attempts to predict gas carry-under have focused only on the first mechanism, discussed next. Bubble-Trajectory Analysis. This analysis is carried out by assuming successive steady-state force-balance calculations on a bubble. The forces acting on the bubbles are centrifugal, buoyancy, and drag. Recent work compared bubble trajectories predicted by the mechanistic model and CFD simulations for the same flow con- ditions. 9 Fig. 4, where x/d and r/R are the dimensionless axial and radial coordinates below the GLCC inlet, respectively, provides an example of such a comparison. The figure shows good agreement with respect to the trend and absolute value. Bubble-trajectory analysis 10 was used to predict the onset of gas carry-under and separation efficiency for different sized bubbles in a manner similar to the liquid/liquid analysis carried out for hydro- cyclones. 11 The minimum diameter of the bubble that always migrates from the GLCC wall to the gas core and thus is separated (i.e., d 100 ) was predicted. Fig. 5 shows the effect of the ratio of the tangential velocity at the inlet slot to the axial velocity in the GLCC (namely, v tis /v z ) on d 100 . The continuous line represents the regres- sion curve of the simulation results. For these conditions, d 100 decreases with increasing v tis /v z ratio up to about 100 and remains approximately constant for larger values of this ratio. The region from the bottom of the vortex to the liquid exit is where small bubbles are separated and captured by the gas-core fil- ament. Because vortex height is a strong function of tangential-inlet velocity and bubble-trajectory length diminishes with vortex height, an optimum tangential-inlet velocity must exist that mini- mizes gas carry-under. A tangential-inlet velocity that is too low produces insufficient centrifugal and buoyancy forces, whereas the available length for bubble trajectory is too short with a tangential- inlet velocity that is too high. As yet, a general scheme to determine optimum velocity has not been presented. Work is now in progress to develop the methodology to predict overall separation efficiency in a GLCC. This requires two addition- Fig. 4—Bubble-trajectory comparison of mechanistic model and CFX simulations with v Ls = 0.25 ft/sec, v gs = 10 ft/sec, v tis /v z = 34, d= 3 in., and d b = 20 μm. 9 Fig. 5—Effect of tangential-/axial-velocity ratio on d 100 for a 3-in. GLCC operated with air and water at atmospheric conditions. 10 v Ls = 0.05 ft/sec v Ls = 0.1 ft/sec v Ls = 0.5 ft/sec 100 80 60 40 20 0 d 100 , μ m v tis /v z 62 JULY 1998 ? al pieces of information: the amount of gas entrained and the bub- ble-size distribution. Coupling these to the bubble-capture efficien- cy ultimately will enable prediction of overall separation efficiency. CFD Simulation. Verifying mechanistic models with real data is not always practical or possible. CFD simulations are used to validate and improve the mechanistic models. CFD simulations for the GLCC can be lumped into two broad categories: single- phase flow with particle tracking and two-phase flow. Single-Phase Flow and Particle Tracking. The simplest and most widely used approximation for CFD simulation of two-phase flow is to consider single-phase flow populated with particles (bub- bles) that neither interact with each other nor influence the flow. This, in effect, is simply solving for a single-phase-flow field and superimposing particle-trajectory tracking. CFD and bubble-trajectory analysis were used to investigate the sensitivity of gas separation to bubble-size distribution. 12,13 Two- and three-dimensional (2D and 3D) simulations 14 were carried out with CFX, a commercially available CFD code. 15 The authors con- cluded that the axisymmetric simulations (2D) gave good results compared with the 3D simulations. Fig. 6 compares single-phase CFD simulations with experimental data. 16 Both the data and CFD simulations demonstrated that the tangential-velocity distribution is dominated by a forced vortex, confirming this assumption in the mechanistic models. Furthermore, the CFD simulations also veri- fied the mechanistic model with respect to axial decay of tangen- tial-velocity distribution (5 to 7% L/d decay). The simulations in Ref. 14 also predicted the existence of an axial-flow-reversal region where the flow is downward near the wall and upward in the center core. The bubble-capture radius, R cap , is defined as the radial location where the axial-velocity component is zero as the flow reverses from downward to upward. Bubbles that migrate into the capture-radius area are separated and pushed upward into the upper part of the GLCC. Fig. 7 shows the capture radius as a function of the tangential-/axial-velocity ratio, v tis /v z , and axial location below the inlet. The results indicate a rapid decline of the capture radius as the velocity ratio decreases below 10. The cap- ture radius and the reversal in the axial-velocity profile recently have been incorporated into the mechanistic model. 9 Two-Phase Flow. Actual two-phase-flow CFD simulation is still in its infancy. Such simulations should predict the influence of the dis- persed phase on the flow of the continuous phase and the interface between the two phases. Recent two-phase-flow CFD simulation work has proceeded on two fronts: with CFX 14,17 and through development of a dedicated internal code. 17 The two-phase simula- tions provided details of the velocity field and gas-void-fraction dis- tribution. The simulations also provided the free interface between the gas and liquid phases (vortex), which compared favorably with experimental data. Fig. 8, which shows the gas-void-fraction distri- bution in the GLCC, gives an example of the results obtained. The figure reveals that the gas-void-fraction values at the top and bottom of the GLCC are nearly unity and nearly zero, respectively, indicat- ing efficient separation. For the first time, results have predicted the gas-core-filament diameter accurately and provided insight into the mechanism for its formation (continuous entrainment and radial migration of small gas bubbles into the gas core).* Fig. 6—Axisymmetric-tangential-velocity prediction vs. data for a 7.5-in. GLCC operated with air and water at atmospheric con- ditions. 14 Fig. 7—Variation of capture radius with tangential-/axial-veloci- ty ratio. 14 v tis /v z R cap /R 6 in. 12 in. Fig. 8—Void-fraction distribution for a 7.5-in. GLCC operated with air and water at atmospheric conditions. 14 α=0.98 α=0.00 V t , ft/sec *Unpublished results, F.M. Erdal, U. of Tulsa, Tulsa, Oklahoma (1998). ? JULY 1998 63 APPLICATIONS A variety of GLCC applications have requirements that may vary from partial to complete gas/liquid separation. Recent technologi- cal development has helped increase deployment of GLCC separa- tor systems in the industry. Successful Applications. The GLCC modeling effort to date has resulted in successful deployment of the GLCC in a variety of selected applications, as discussed next. Multiphase Measurement Loop. Most of GLCC’s deployed to date (approaching 100) have been configured in a multiphase metering loop. Fig. 9 is a schematic of the GLCC in a multiphase metering loop, first introduced by Liu and Kouba, 18 and Fig. 10 shows a GLCC field prototype operated by Chevron in Oklahoma. This type of measurement-loop configuration affords several advantages over either conventional separation with single-phase measurement or nonseparating multiphase meters. The loop con- figuration is somewhat self-regulating, which can reduce or even eliminate the need for active level control. The compactness of the GLCC allows the measurement loop to weigh less, occupy less space, and maintain less hydrocarbon inventory than a conven- tional test separator. The advantages of a GLCC metering loop over a nonseparating three-phase meter include much improved meter- ing accuracy of individual phases over a wider range of flow rates and significantly lower cost. For flow conditions where gas carry-under cannot be prevented, a three-phase metering system is required on the liquid leg. In gen- eral, the accuracy of a multiphase meter on the liquid leg benefits significantly from removal of some of the gas. Most multiphase meters have an upper limit on the gas volume fraction allowed through the meter to maintain their accuracy specifications. Apart from improved accuracy, partial gas separation provides the addi- tional benefit of a smaller, less expensive multiphase meter. For multiphase meters (whose price scales directly with size), the cost savings of using a smaller meter in conjunction with a GLCC can be four times the cost of the GLCC. Partial Processing (Separation). A compact GLCC is often very appropriate for applications where only partial separation of gas from liquid is required. One such application is the partial separa- tion of raw gas from high-pressure wells to use for gas lift of low- pressure wells. The GLCC was a central feature in an offshore raw- gas-lift system designed by Chevron that allowed elimination of gas compressor and lift-gas pipelines. 19 Compact Separation Systems. Compact separation systems are a key element in reducing cost of production operations through reduction of size and weight. Furthermore, separating a significant Fig. 10—Chevron-operated GLCC field prototype. Fig. 9—GLCC in a multiphase metering loop configuration. 64 JULY 1998 ? portion of the gas reduces fluctuations in the liquid flow and may result in improved performance of other downstream separation devices, such as a wellhead desanding hydrocyclone. Chevron is investigating the series combination of a GLCC with a free-water- knockout hydrocyclone and a deoiling hydrocyclone in an effort to improve discharge-water quality. The GLCC was used to control gas/liquid ratio of a two-phase- flow mixture entering a multiphase pump to improve pumping effi- ciency. 20 Another study showed several combinations of GLCC and jet pumps that could be used to extract energy from high-pressure multiphase wells to enhance production from low-pressure wells. 21 Enhancement of Existing Separators. Cyclone separation already has proved useful in internal separation devices for large horizontal separators. The GLCC may also function as a useful external preseparation device to enhance performance of existing horizontal separators (Fig. 11). By separating part of the gas, the separator level might be raised to increase residence time without encountering the mist-flow regime in the vessel. Petrobrás Brazil has retrofitted an existing separator in one of its fields with a GLCC preseparator. 1 Another company is evaluating enhancement of their existing test separators with GLCC preseparation. Commercial GLCC Products. Most GLCC’s to date have been field fabricated for relatively straightforward applications. Applications of and demand for GLCC’s are growing rapidly. Several vendor compa- nies are in the process of incorporating the GLCC into their com- pact-separator product line. Also, as mentioned before, a commercial multiphase metering system that uses a GLCC and a second-stage horizontal separator is now available. Greater commercialization will be needed to meet the growing industry demand. Future Applications. Current successful GLCC applications lend confidence to future potential GLCC configurations. This requires enhancement of the existing models and is currently under way. The following are two of the most compelling applications. Subsea Production. The biggest impact to the petroleum industry from GLCC technology may be in subsea separation applications. Conclusions in Ref. 22 state that “wellhead separation and pumping is the most thermodynamically efficient method for wellstream transfer over long distances, particularly from deep water.” In a recent study, Prado et al. 23 argued that this is applicable to shallow and moderately deep waters. Undoubtedly, development of marginal offshore fields will depend on development of efficient and economical technologies. Subsea applications require a high degree of confidence in separator design and performance while demanding that the equipment be sim- ple, compact, robust, and economical. Here again, the virtues of the GLCC should place it in good standing among competing technologies. Production Separation. Vertical separators with tangential inlets are fairly common in the oil field. These predecessors of the GLCC are often big and bulky, with perpendicular low-velocity tangential pipe inlets. The tangential velocities are usually so low that gravi- tational, centrifugal, and buoyancy forces contribute approximate- ly equally to separation. Technological developments in both GLCC hardware and software should reduce the size and improve the performance of vertical separators. One challenge in optimiz