Separation of high velocity wet gas by phase-isolation and split-flow method

doi:10.1016/j.cherd.2020.05.027

Chemical Engineering Research and Design

Volume 160, August 2020, Pages 105-118

https://doi.org/10.1016/j.cherd.2020.05.027 Get rights and content

Highlights

•
A new method and separator for removing liquid in high velocity wet gas from pipeline are proposed.
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The swirler of four semi-elliptic guide vanes can achieve a stable and uniform swirling core-annular flow.
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Most part of gas core could be bypassed by split-flow method.
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With the increase of RLW height, the change rules of the splitting ratio KS and separation efficiency are studied.
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The effect of gas velocity on separation efficiency with different RLW height is studied.

Abstract

This paper proposes a new method for directly removing the liquid in wet gas from pipeline, especially for high velocity flow condition. The separation apparatus to be used is simply formed by enclosing a short section of the pipe with a compact cylinder and installing a swirler at the entrance. As gas–liquid mixture flows through the swirler downwardly, a strong swirl flow is created. Due to the centrifugal force, the two phases are isolated, forming a so-called swirling core-annular flow. Then a large portion of the gas core directly flows out of the separator, only the liquid film and a small portion of gas core (the conveying gas) enter the annular separation space through the upper portion of the ring-like window (RLW) in the pipe, where the conveying gas is separated from the liquid by centrifugal and gravitational forces and returns to the pipe through the lower part of the RLW. Numerical and experimental investigations were carried out in this study to determine the behavior of separation. The simulation and experimental results showed that the height of RLW has a strong effect on the behaviors of the separation, the conveying gas flowrate and the separation efficiency increase with the increase of RLW height, but the pressure drop also increases simultaneously. The separation efficiency even could approach 100% when the height of RLW is more than 1.5 pipe diameters with a 60% of conveying gas.

Introduction

Separation technology of gas–liquid two-phase flow is very important, widely used in petroleum, chemical, electric power and other fields. In order to prevent entrainments by high velocity gas and fully separate gas and liquid, the diameter of traditional separators is several times larger than that of the inlet pipes, those traditional separators are mainly vessel-type, large, heavy, expensive and maintenance inconvenience. In recent years, more and more compact separation devices have been studied to reduce the cost, such as the gas–liquid cylindrical cyclone (GLCC) (Kouba et al., 1995, Kouba and Shoham, 1996, Shoham and Kouba, 1998, Chirinos et al., 2000, Wang et al., 2001, Kouba et al., 2006, Molina et al., 2008) and swirl-vane separators (Green and Hetsroni, 1995, Kataoka et al., 2008, Kataoka et al., 2009, Xiong et al., 2013, Xiong et al., 2014). However, it is still difficult to remove liquid from gas stream, especially under high velocity gas condition, which results in seriously entrainments of liquid droplets at gas outlet.

GLCC is a compact gas–liquid separator. It consists of a vertical cylinder with a downward inclined tangential inlet. A gas–liquid mixture is introduced from the tangentially nozzle, causing a vortex in the cylinder. Liquid is forced towards the cylinder wall by centrifugal force and discharged from the bottom, while gas is driven to the center and discharged from the top. However, liquid will be re-entrained at the gas outlet by the high velocity rising gas. This phenomenon is called liquid carry-over (LCO) (Chirinos et al., 2000). Hence, the upper bound of gas superficial velocity is limited, generally less than 9.2 m/s in a regular GLCC (Wang et al., 2001). Wang et al. (2001) developed a modified GLCC which is added an Annual Film Extractor (AFE) on gas outlet to remove the LCO in the gas stream, the upper limit of gas superficial velocity was increased to 18.3 m/s at low pressure. Molina et al. (2008) studied the modified GLCC with an AFE at high pressure in field, and recommend a parameter, gas velocity ratio (U_SG/U_ann, U_ann is the minimum gas velocity required to initiate liquid carry-over), to conveniently study the performance of the modified GLCC at different pressures. Comparing the separation efficiencies under both high pressure and low-pressure conditions, they have a similar tendency with the increase of gas velocity ratio. Therefore, improving the upper limit of gas superficial velocity at low pressure can also improve that at high pressure. Nevertheless, the experimental results showed that the separation efficiency of modified GLCC will obviously drop as the gas superficial velocity exceeds the upper bound.

The swirl-vane separator has been widely used in steam generator for nuclear power station (Green and Hetsroni, 1995, Kataoka et al., 2008, Kataoka et al., 2009, Xiong et al., 2013, Xiong et al., 2014). A swirling flow is generated in the cylinder when the gas–liquid mixture axially flows through the swirl vanes. Liquid phase is pushed to the cylinder wall by the centrifugal force and finally removed from gas core by an orifice plate at the end of the cylinder. But the gas core will have a sudden contraction and acceleration at the orifice, the liquid film on the wall will be probably torn and easily entrained by high velocity gas, resulting in more entrainments in the gas stream and a greatly reduction in separation efficiency. So, this phenomenon limits the operating range of the gas velocity, for instances, under low-pressure conditions, the gas superficial velocity ranges were 12–24.1 m/s (Kataoka et al., 2009), 5.7–12.4 m/s (Xiong et al., 2013) and 5.8–12.3 m/s (Xiong et al., 2014) respectively.

Perry and Graff, 1975, Perry and Graff, 1979 presented a pipe gas–liquid separator. A housing chamber encloses a pipe that contains a swirl device for imparting a swirling motion to the gas–liquid mixture. The pipe is provided with one or two annular ejection ports at the downstream of the swirl device. Liquid droplets are driven towards the wall by the centrifugal force; gas moves to the center. The droplets and a portion of gas are ejected into the chamber through the annular ejection ports and finally separated in the housing chamber by gravity. However, it is still a vessel-type separator. Besides, the gas core will suddenly contract at the annular ejection ports, the liquid phase would be entrained by the high velocity gas.

Another method to remove liquid in gas–liquid mixture is the centrifugal method combined with holes in tube wall. Hayes, 1989, Hayes, 1990 proposed a moisture separation device. A gas–liquid mixture generates centrifugal motion by the helical swirling device coaxially located within the tube. The droplets are drawn off from gas through holes in the tube wall and fall to a liquid accumulator. However, the droplets cannot be removed as the gas velocity is over 4.9 m/s. Wen (2009) studied a similar swirl type tube separator experimentally. A cylinder contains a cone tube with holes in the wall, installing a swirler in the center of the cone tube. The experiment results showed that the separator has the best performance as the expansion angle of the cone tube is 6°. Nevertheless, the separation efficiency will sharply drop when the gas superficial velocity is over 30 m/s. So, this method still does not solve the problem of liquid separation from high velocity gas stream.

From the literature reviews above, one can see that it is not difficult to drive the liquid in the gas stream towards the tube wall and form a liquid film by any centrifugal method. However, it is really difficult to finally remove liquid film from gas core, because of the strong coupling between the liquid film and the gas core. Hence, the final step is always completed at a low gas velocity, or the size of separator is usually large. The liquid separation from high velocity wet gas flow is always a difficult problem. GLCC with AFE relatively has the highest axial gas velocity. Liquid phase (droplets and liquid film) may draw off from gas stream through AFE relying on its own kinetic energy under the action of centrifugal force. Experimental results showed that the AFE has a certain effect on removing LCO. The up limit of axial gas velocity in the GLCC with AFE is up to 18 m/s (Wang et al., 2001). Similar approaches like POR in swirl-vane separator, has an up-limit gas velocity about 24 m/s (Kataoka et al., 2008, Kataoka et al., 2009). Obviously, there is a certain distance from actual application, especially in high pressure conditions the separation performance is still poor. The problem of separation of high velocity wet gas flow is still unsolved. The reason for this can be explained as follows. Generally, there are two major factors that impact on the liquid flowing into AFE. The one is the resistance of AFE to liquid phase, the other is the pressure drag, i.e. the so-called pressure build-up that can be explained as follows. Since the annular trap of AFE has no passage for gas, hence a gas stagnation will be inevitable, resulting a pressure build-up there, so the liquid phase has to overcome this pressure drag between pipeline and the annular trap, as analyzed in Wei et al. (2020). As the increase of gas stream velocity, the relative resistance of AFE and the pressure drag are both getting great, thus the liquid is much easier carried downstream by the gas stream rather than collected in AFE. Hence, in view of the above, the separation of high velocity wet gas using phase-isolation and split-flow method is proposed in this paper. It uses the split-flow method to divide the all liquid and a small amount of gas (conveying gas) from the pipe to annular separation space after the phases have been isolated. Because of the action of conveying gas, all the liquid can easier enter the annular separation space than that in AFE or POR where gas does not carry the liquid but as well increase the resistance to the liquid flowing into the AFE by stagnation effect. On the other hand, gas–liquid two-phase flow has been isolated as gas core and annular liquid film, thus a large portion of gas core could be bypassed, only the liquid film and a part of gas (the conveying gas) enter the separation space, so it can greatly reduce the size of separator required and make the structure more compact.

The goal of this paper is to investigate the separation characteristics of this new compact swirl gas–liquid separator. The phase-isolation performance in the separator, the effect of structure parameters on the gas flow field and splitting ratio K_S (ratio of conveying gas flowrate to total gas flowrate) were studied by numerical method. Experiments were carried out in an air–water two-phase loop to verify the simulation results, study the influence of the gas velocity on the separation efficiency and observe various phenomena in the separation process.

Section snippets

The structure of the separator

The structure of the compact separator and parameters are illustrated in Fig. 1(a), and the simulation section is shown in Fig. 1(b). It is actually a pipe separator, which is formed simply by enclosing a short section of pipe with a compact cylinder and installing a swirler at the entrance in the pipe. The swirler is shown in Fig. 2, it is made up of four circumferentially equispaced vanes and the angle between each vane and cross section of pipe is 45°. There is a ring-like window (RLW) in

Geometry

The gas–liquid compact separator for numerical simulation is shown in Fig. 1(b). It is consisted of a pipe, a swirler, an RLW in the pipe and a compact cylinder. The length of compact cylinder and the outlet pipe are 260 mm and 50 mm respectively. The inner diameters of inlet pipe and compact cylinder are 26 mm and 40 mm respectively. In order to analyze the phase-isolation characteristic, we selected three different cross sections, S1, S2 and S3, as shown in Fig. 1(b). The swirler is shown in Fig.

Phase-isolation characteristic

For natural annular flow, there are considerable entrainments in the gas core, ring-like window (RLW) is especially suitable for removing liquid film. Hence, in order to make liquid phase from a liquid film as far as possible in the pipe, we adopted phase-isolation method (Wang et al., 2015, Yang et al., 2018, Niu et al., 2019). Phase-isolation method is a kind of method that can make each phase flow in a respective space in pipeline by applying various kinds of forces to two-phase mixture.

Experimental setup

The experiments were carried out in an air–water two-phase flow loop as shown in Fig. 9, the working fluids used were air and tap water. Air was supplied by a compressor to the accumulator, liquid was provided by a centrifugal pump. The total flow rate of air and the water were measured before they are mixed. Air was metered by YOKOGAWA vortex flowmeters (YF102) with an accuracy of ±1% in the full range of 0–2 × 10³ L/min, water was metered by a YOKOGAWA electromagnetic flowmeter (ADMAG AE) with

Experimental phenomena

From the simulation results, one can see that the most important factor in the separator is the RLW height H. The gas flow field in the cylinder changes obviously with the increase of the RLW height H, which particularly influences the splitting ratio K_S. Removing liquid depends on the conveying gas flow, so the H could determine the whole separation efficiency. Compared with the simulation results, the experiments also verify that the more liquid film will enter the annular separation space

Conclusion

Wet gas separation by phase-isolation and split-flow method has been studied by numerical and experimental method. It is especially suitable for high velocity flow conditions. Compared with AFE and POR, the conveying gas could ensure that all the liquid phase enters the annular separation space, especially for the liquid phase with small kinetic energy. The problem of entrainments by high velocity gas might have fundamentally been solved. This separator is formed simply by enclosing a short

Conflict of interest

We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.

Acknowledgments

This study was financially supported by the National Nature Science Foundation of China (Grant no. 51121092).

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View full text

Separation of high velocity wet gas by phase-isolation and split-flow method

Highlights

Abstract

Introduction

Section snippets

The structure of the separator

Geometry

Phase-isolation characteristic

Experimental setup

Experimental phenomena

Conclusion

Conflict of interest

Acknowledgments

Int. J. Multiphase Flow

Chem. Eng. Res. Des.

Flow Measur. Instrum.

Int. J. Multiphase Flow

Energy

Powder Technol.

Nuclear Eng. Des.

Annu. Nuclear Energy

Flow Measur. Instrum.

Liquid carry-over in gas–liquid cylindrical cyclone compact separators

SPE J.

Metering of wet steam

Chem. Process Eng.