[机械模具数控自动化专业毕业设计外文文献及翻译]【期刊】紧凑型管柱状气液旋流分离器技术目前的发展状况-外文文献

DISTINGUISHED AUTHOR SERIES58 JULY 1998 STATE OF THE ART OF GAS/LIQUIDCYLINDRICAL-CYCLONECOMPACT-SEPARATOR TECHNOLOGYOvadia Shoham, SPE, U. of Tulsa, and Gene E. Kouba, SPE, Chevron Petroleum Technology Co.SUMMARYThe petroleum industry has relied mainly on conventional, vessel-type separators to process wellhead production of oil/water/gasflow. However, economic and operational pressures continue toforce the industry to seek less expensive and more efficient separa-tion alternatives in the form of compact separators, especially foroffshore 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 inabilityto predict GLCC performance adequately has inhibited its wide-spread deployment. Current R&D is aimed at creating the neces-sary performance-prediction tools for proper design and operationof GLCC separators. This paper presents the status of the develop-ment of the GLCC, the state of the art with respect to its simulationand design, and current successful and potential applications. INTRODUCTIONThe GLCC (Fig. 1) consists of a vertical pipe with a tangentialinclined inlet and outlets for gas and liquid. The tangential flowfrom the inlet to the body of the GLCC creates a swirl that producescentrifugal and buoyancy forces on the fluids that are an order ofmagnitude higher than the force of gravity. The combination ofgravitational, centrifugal, and buoyancy forces separates the gasand liquid. The liquid is pushed radially outward and downwardtoward the liquid exit, while the gas is driven inward and upwardtoward the gas outlet. The low-cost, low-weight, compact GLCCseparator offers an attractive alternative to the conventional vessel-type separator. A size comparison between the GLCC and conven-tional vessel-type vertical and horizontal separators has been con-ducted recently for a typical field application with oil and gas flowrates of 100,000 B/D and 70,000 Mscf/D, respectively, at 100 psig.1For this case, the required GLCC inner diameter and height are 5and 20 ft, respectively. These dimensions are approximately one-half of the corresponding dimensions of the required conventionalvertical separator (9 x 35 ft) and about one-quarter of the dimen-sions of a conventional horizontal separator (19 x 75 ft). The operational envelope of a GLCC is defined by two limitingphenomena: liquid carry-over in the gas stream and gas carry-underin the liquid stream. The onset of liquid carry-over is identified by thefirst trace of liquid in the gas stream. Similarly, the first observablebubbles in the liquid underflow mark the onset of gas carry-under.The difficulty in developing accurate performance predictionsarises largely from the variety of complex flow patterns that canoccur in the GLCC. The flow patterns above the inlet can includebubble, slug, churn, mist, and liquid ribbon. Below the inlet, theflow generally consists of a liquid vortex with a gas-core filament.At liquid levels far below the inlet, the liquid flows from the inletto the vortex in a swirling film.This difficulty in predicting the hydrodynamic performance ofthe GLCC has been the single largest obstruction to broader use ofthe GLCC. Even without tried and tested performance predictions,several successful applications of GLCCs have been reported.2Thedevelopment of reliable performance-prediction tools will improveGLCCs through hardware modifications and, ultimately, will gov-ern the speed and extent to which GLCC technology is deployed inexisting and new field applications. HARDWARE DEVELOPMENTSFew systematic studies of design configurations of different GLCCmechanical features have been conducted. Recent laboratory obser-vations and computer simulations indicate that hardware modifica-tions to the GLCC can have a profound effect on GLCC perfor-mance. Ref. 3 discusses these in some detail. The following is a sum-mary and update of the most important hardware improvements.Inlet Design. The inlet section determines the incoming gas/liq-uid distribution and the initial tangential-inlet velocity in theGLCC. Because GLCC performance is strongly dependent on thetangential-inlet velocity, the inlet has been the single mostredesigned component of the GLCC.Inclined Inlet. Conventional vertical separators typically use aperpendicular inlet. Recent studies on the GLCC have demonstrat-ed that an inclined inlet improves GLCC performance by reducingliquid carry-over in the gas stream through two mechanisms.4First, the downward inclination of the inlet promotes stratificationand provides preliminary separation at the inlet nozzle. Second, thedownward inclination causes the liquid stream to spiral below theinlet after one revolution, preventing the liquid from blocking theflow of gas into the upper part of the GLCC.Inlet Nozzle.The nozzle is the last element of the inlet that influ-ences flow distribution and the tangential velocity entering theGLCC body. The tangential inlet nozzle is the most expensive part ofthe GLCC to fabricate. Several nozzle configurations have been test-ed, aimed at optimizing hydrodynamic performance cost-effectively.The optimum configuration for hydrodynamic performance is a thin,rectangular tangential wall slot, which is difficult to fabricate. On theother hand, the concentric-circular tangential inlet is easy to fabricatebut exhibits lower performance. A preliminary experimental com-parison of three different inlet-slot configurations (rectangular, con-centric-circular, and crescent) with the same cross-sectional areafound that the concentric-circular nozzle (reduced pipe) configura-Copyright 1998 Society of Petroleum EngineersThis is paper SPE 39600. Distinguished Author Series articles are general, descriptive repre-sentations that summarize the state of the art in an area of technology by describing recentdevelopments for readers who are not specialists in the topics discussed. Written by individu-als recognized as experts in the area, these articles provide key references to more definitivework and present specific details only to illustrate the technology. Purpose: to inform the gen-eral readership of recent advances in various areas of petroleum engineering. A softboundanthology, SPE Distinguished Author Series: Dec. 1981Dec. 1983, is available from SPEsCustomer Service Dept. JULY 1998 59tion had the poorest performance, while the crescent nozzle (tan-gential flat plate) performed closest to the rectangular slot.5Dual Inlet. Dual inclined inlets provide preseparation of the inletstream into a liquid-rich stream (lower inlet) and gas-rich stream(upper inlet). Testing of the dual inlet indicated a significant improve-ment in liquid-carry-over performance at low to moderate gas rates(slug flow dissipating to stratified flow at the inlet) with less dis-cernible improvement at high gas rates (annular flow at the inlet).6GLCC-Body Configuration. Despite the simple design of theGLCC, several possible modifications to the body configurationcan influence performance. Inlet Location. For GLCCs without active liquid-level control, itis important to locate the inlet section just above the liquid level.Most testing to date indicates that the optimum liquid level in thesingle-inlet GLCC is approximately 1 to 3 L/d below the inlet.Liquid levels farther below the inlet than 3 L/d result in significantdecay in the tangential-inlet velocity, which compromises theGLCC performance. If the liquid level is above the inlet, gas mustblow through the liquid and is more likely to cause carry-over.Optimum Aspect Ratio. The aspect ratio is the ratio of GLCClength to diameter. The dimensions of the GLCC influence perfor-mance and cost. For a given diameter, the length of the GLCCabove the inlet provides liquid-surge capacity, while the lengthbelow the inlet determines residence time for separating bubblesfrom the liquid. In addition, centrifugal and buoyancy forces areinversely proportional to diameter and tangential-velocity decay isdirectly proportional to length. Because of the complexity of thisphenomenon, a fundamental set of criteria to determine optimumaspect ratio has been proposed only recently.1Cyclone-Body Taper. An investigation on diverging, converging, andcylindrical cyclones concluded that cylindrical walls are slightly supe-rior to either converging or diverging walls for gas/liquid separation.7Liquid-Level Controls. Active liquid-level control in a GLCC for awide range of flow conditions is not straightforward owing to itscompact size. Several different liquid-level-control strategies arebeing investigated, including flow control on the gas leg, flow con-trol on the liquid leg, and flow control on both legs. Also beingconsidered are combinations of backpressure control on the gas legand liquid-level control on the liquid leg. Other issues of concerninclude power requirements, robustness, and cost. Several alternatives for GLCC liquid-level control have beenimplemented. For example, a commercial multiphase measure-ment system has used conventional control equipment successful-ly to maintain a tight control on liquid level by controlling the gas-outflow rate of the GLCC. Another project explored low-poweralternatives to conventional level controls that exploit hydrostatic-head difference in the GLCC to operate the controls.8A recentstudy examined GLCC performance with a passive control systemthat uses only the flow energy and no external energy.5Crucial future work is to develop robust, active liquid-level-con-trol strategies. Because of the smaller residence time of the compactseparator and the stringent response time requirement of the controlvalve, this is not a simple extension of the control technology avail-able for large vessel-type separators. The strategies should enable theGLCC to handle slugging, surging, and a wide range of flow rates,from essentially full-gas-flow to full-liquid-flow conditions. Integrated Separation System. Great economic incentivesexist for the industry to move away from conventional gravity-based separators to compact separation systems. Depending onthe application, the GLCC can be used for full or partial sepa-ration. Partial gas separation allows downstream equipment tobe smaller (and therefore less expensive) and perform moreefficiently. The GLCC has been particularly effective whencombined with multiphase meters, desanders, and liquid/liquidhydrocyclones. Configured either alone or in combination withother equipment, the GLCC can reduce cost and weight signif-icantly. This is particularly important in designing or retro-fitting offshore platforms, where savings in platform-construc-tion costs may be many times greater than the cost of the sep-aration equipment. Fig. 1GLCC configuration.60 JULY 1998 Another GLCC combination is two GLCCs used in series. A the-oretical work successfully predicted that a second-stage GLCC onthe gas leg would increase the liquid-carry-over performance of thetwo-stage GLCC to the onset of the mist-flow boundary, which wasanticipated as the theoretical performance limit.4It was found thatat very low liquid loading, the centrifugal forces on the liquidwould allow the operational envelope to push well beyond the fullydeveloped annular-mist-flow boundary. A commercial measurement system has been developed that usesa second-stage horizontal pipe separator to remove any small gasbubbles that may have passed along with the liquid underflow fromthe GLCC. This has allowed extension of the system operationbeyond the “normal” operating range of the GLCC for completegas/liquid separation.Miscellaneous Hardware Improvements. Several other potentialimprovements have been considered; however, we have not dis-cussed them here because little or no performance information isavailable. These include a variable inlet-slot area and the config-urations of the gas and liquid outlets.SIMULATIONIn the past, performance predictions of GLCC separators havebeen carried out on the basis of experience, rules of thumb, andempirical correlations. These methods are limited in their abilityto be extrapolated to different flow conditions and untried appli-cations. Currently, efforts are under way to develop mechanisticmodels for the GLCC and conduct computational fluid dynamic(CFD) simulations. Mechanistic modeling offers a practical approach to GLCC designand performance prediction. Simplifying assumptions are used, but,ideally, the models still capture enough of the fundamental physics ofthe problem to allow interpolation and extrapolation to differentfluid-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-phaseor dilute dispersion flows, current CFD simulators cannot yet handlethe 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 notas 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 PCoperation. 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 datehas been to predict the operating envelope for the GLCC with respectto liquid carry-over in the gas stream and gas carry-under in the liquidstream. Each fluid-flow path has its own particular set of calculations.The starting point for either calculation path is the global distributionof gas and liquid in the GLCC, namely, the equilibrium liquid level. Equilibrium Liquid Level. The equilibrium liquid level in theGLCC is determined by the pressure drop between the gas and liq-uid outlets. Because the frictional losses in the GLCC are low, theequilibrium liquid level is a reasonable indication of the amount ofliquid in the GLCC. The model is based on a pressure balance onthe gas and liquid legs. Ref. 2 gives details of this model. Vortex Shape and Location. The shape and location of the vortexare important for prediction of both liquid carry-over and gas carry-under. The vortex model assumes rigid-body rotation (i.e., a lineartangential-velocity profile in the radial direction).2Coupling the cal-culations for equilibrium liquid level and vortex shape makes deter-mination of the location of the vortex and the height of the vortexcrown 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 largelydependent on the flow pattern in the upper part of the GLCC. Floodingmay occur in the GLCC at high liquid levels and low gas rates, produc-ing bubbly flow. The unstable liquid oscillations, characteristic of churnflow at moderate gas rates, may splash liquid into the gas outlet. Liquidcan also be carried out in droplets at the onset of annular mist flow athigh gas rates. At very high gas rates, the centrifugal force of the swirlinggas pushes the liquid to the wall of the pipe, where it may form anupward-spiraling continuous ribbon of liquid.At present, the onset of liquid carry-over is predicted for low tomoderately high gas rates. The key to onset of liquid carry-over hasbeen 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 gasand liquid legs. Fig. 2 compares model predictions with experimen-tal results in plots of the maximum liquid holdup in the upper GLCCregion (i.e., zero-net-liquid-flow holdup, yL0, vs. the superficial gasvelocity, vgs, in the GLCC).2Additional data collected for a range ofliquid viscosities from 1 to 10 cp showed negligible effect on thezero-net-liquid-flow holdup.6Once the maximum liquid holdupFig. 2Zero-net-liquid-flow holdup in air/water system.2Fig. 3Operational envelope for liquid carry-over in a 3-in.GLCC operated with air and water.2vgs, ft/secyL0vgs, ft/secvLs,ft/sec JULY 1998 61allowed in the upper part of the GLCC is known for a given gas rate,the pressure-balance calculation is used to determine the liquid raterequired to achieve this holdup and initiate liquid carry-over.Fig. 3 compares the experimental and predicted operationalenvelopes for a 3-in. laboratory GLCC in a loop configuration,operated with air and water at low pressures.2The operationalenvelopes are presented in terms of superficial liquid velocity, vLs,vs. superficial gas velocity, vgs, in the GLCC. The agreement ofmodel predictions with the data is very good. Comparison withdata from Ref. 6 showed that the model seems to capture the phys-ical phenomena and predict well the reduction of the operationalenvelope with increasing liquid viscosity.Future improvements to liquid-carry-over modeling will includeexpansion to different operational conditions (e.g., high gas rates)as well as prediction of the quantity of liquid carry-over anddynamic 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) shallowbubble trajectories prevent small bubbles from escaping to the gas-corefilament, (2) rotational-flow instability causes helical whipping andbreaking of the gas-core filament near the liquid exit, and (3) liquid-rate surges can produce a concentrated cloud of bubbles that hindersbubble migration to the gas core. Currently, attempts to predict gascarry-under have focused only on the first mechanism, discussed next.Bubble-Tra