Gas-liquid Separation Process Research Articles

Application of a Bubble Tracking Technique for Estimating Downhole Natural Separation Efficiency

Abstract Liquid velocity and pump intake pressure are known to have strong effects on the separation efficiency of gas and liquid which may impact the flow efficiency of downhole pumps. This paper describes a new model to evaluate the effectiveness of this separation. The model correctly predicts the decline in separation efficiency with higher liquid flowrates, which is shown to be in agreement with presented experimental data. A calibration procedure is conducted which shows a descending trend of bubble size when the pump intake pressure is increased. The new model shows good agreement with the experimental data in the bubble flow regime, since it handles the two-phase natural separation problem by combining the single-phase flow field solution with a bubble tracking method. It is the first CFD-based simplified model which incorporates the multiple effects that govern the natural gas-liquid separation process. If a correlation for bubble size is properly obtained, the model will cover the omprehensive effects of operational and geometric conditions and provide an accurate prediction of gas-liquid separation efficiency over the full spectrum of normal bottomhole pump operating conditions. Finally, the single-phase flow field is solved in a 2D Cartesian coordinate system. The acceleration of the liquid phase, a nature of fluid behaviour in a cylindrical system, is not considered. This is a necessary improvement in the model to properly calculate the slip velocity in the radial direction. Introduction In pumping wells, the presence of free gas can cause several operational problems. The gas ingested by the pump can lead to poor pump performance and low overall efficiency of the entire pumping system. For any pumping lift system, estimating natural separation efficiency is a very important task whether the well is equipped with a bottomhole gas separator or not. A sketch of the natural separation process is shown in Figure 1. The movement of the free gas in the annulus space in front of the pump intake is the result of gravitational, inertial, and interfacial effects. The gas phase being lighter than the liquid tends to continue flowing vertically due to buoyancy effects. On the other hand, the gas phase is also being dragged due to interfacial forces and pressure gradient effects toward the pump intake by the liquid phase. As a consequence, part of the gas stream will eventually follow the liquid hase entering the pump and the rest of the gas will continue its vertical movement. The fraction of gas that is vented through the annulus is known as natural separation efficiency. FIGURE 1: Sketch of the natural separation process. Available in Full Paper. Lea(1) was the first investigator to conduct test programs to evaluate the effects of free gas on the performance of submersible pumps; his results included a graph of natural separation efficiency as a function of liquid flow rate and gas fraction at the pump intake. Alhanati(2) derived the first mechanistic model to estimate the natural separation efficiency for vertical wells.

Diagnostic reasoning under conflicting data: the application of the transferable belief model

One of the issues in diagnostic reasoning is inferring about the location of a fault in cases where process data carry inconsistent or even conflicting evidence. This problem is treated in a systematic way by making use of the transferable belief model (TBM), which represents an approximate reasoning scheme derived from the Dempster–Shafer theory of evidence. The key novelty of TBM concerns the paradigm of the open world, which turns out to lead to a new means of assigning beliefs to anticipated fault candidates. Thus, instead of being ignored, inconsistency of data is displayed in a portion of belief that cannot be allocated to any of the suspected faults but rather to an unknown origin. This item of belief is referred to as the strength of conflict (SC). It is shown in this paper that SC can be interpreted as a degree of confidence in the diagnostic results, which seems to bring a new feature to diagnostic practice. The basics of TBM are reviewed in the paper and the implementation of the underlying ideas in the diagnostic reasoning context is presented. An important contribution concerns the extension of basic TBM reasoning from single observations to a batch of observations by employing the idea of discounting of evidence. The application of TBM to fault isolation in a gas–liquid separation process clearly shows that extended TBM significantly improves the performance of the diagnostic system compared to ordinary TBM as well as classical Boolean framework, especially as regards diagnostic stability and reliability.

Carbon dioxide degassing from the Albani Hills volcanic region, Central Italy

The Albani Hills are affected by strong CO 2 degassing processes. The amount of CO 2, which rises from the depths and subsequently dissolves into shallow groundwater, has been estimated to be more than 4.2×10 9 mol year −1. Most of the gas is released in localised anomalous spots (Lavinio, Solforata, and Ciampino–Albano–Nemi areas), generated by fluid leakage processes from buried pressurized reservoirs hosted by structural highs of the calcareous–siliceous marly basement. The groundwater circulating in the anomalous areas becomes CO 2 oversaturated, generating observable gas manifestations whose composition is mainly controlled by gas–liquid separation processes. A gas flow rate of 6.1×10 8 mol year −1 has been measured from the two larger manifestations using the accumulation chamber technique. The presence at shallow level of CO 2 oversaturated groundwater can explain several episodes of sudden release of gas which was documented by historical chronicles from Roman times, until now.

Parallelism and differences of pervaporation and vacuum membrane distillation in the removal of VOCs from aqueous streams

In this work two gas–liquid separation processes, pervaporation (PV) and vacuum membrane distillation (VMD), have been compared in their application to the separation of chloroform–water mixtures. After selection of the adequate separation membrane the comparison of the PV and VMD should be based on the kinetics and selectivity towards the desired compound. The kinetic models and parameters previously reported by the authors (A.M. Urtiaga, E.D. Gorri, J.K. Beasley, I. Ortiz, J Membr Sci 156 (1999) 275–291 and A.M. Urtiaga, G. Ruiz, I. Ortiz, J Membr Sci 165 (2000) 99–110) for the separation of chloroform from aqueous solutions in the range of concentrations 200<C<2000 mg/l, temperature 5<T<44°C and flowrate 140<Re<1400 have been used to compare the mass transfer flux and selectivity in both separation processes. The use of hollow fiber membranes of polydimethylsiloxane in the PV system and microporous polypropylene in the VMD process resulted in the same rate of removal in both systems under the simulated conditions. Particularly, the PV system offered the uppermost selectivity, providing a high concentration of chloroform in the permeate.

Effects of moisture on the cold vapour determination of mercury and its removal by use of membrane dryer tubes

Continuous-flow vapour generation is now a well-established sample-introduction technique for mercury and the hydride-forming elements. One of the problems associated with this technique is moisture carryover, which originates during the gas–liquid separation process. Excessive moisture carryover causes gradual loss in sensitivity and baseline drift for atomic absorption and atomic fluorescence spectrometry and inductively coupled plasma detection system. Chemical desiccants and physical moisture traps can be used to reduce moisture for short periods of time, although these may give rise to contamination and analyte losses. A more effective way to remove moisture carryover has been found by using a semi-permeable Nafion membrane dryer tube, which continuously desolvates the wet gaseous stream. This device improved the long-term stability and enhanced sensitivity. A relative standard deviation of 2% was achieved for 90 runs of a 1 µg l–1 standard, obtained over a period of 3 h.