A novel centrifugal gas liquid pipe separator for high velocity wet gas separation

https://doi.org/10.1016/j.ijmultiphaseflow.2019.103190Get rights and content

Highlights

  • A novel centrifugal gas liquid pipe separator for high velocity wet gas separation is proposed.

  • The centrifugal device of four semi-elliptic guide vanes can achieve a stable and uniform swirling core-annular flow.

  • Nearly all swirling film can completely discharge from pipe relying on its own kinetic energy and inertial through NTC.

  • With the increase of gas velocity, the abnormal change rules of separation efficiency are studied.

  • The effect of structure parameters of NTC on separation efficiency is studied detailly.

Abstract

Cyclones are the most widely used separators at present, but it's still difficult to reduce their size to serve as pipe separators. A new type of gas liquid pipe separator for high velocity wet gas is proposed in this paper. The separator is actually a short section of pipe, with a centrifugal device in the center portion and 3 narrow tangential conduits (NTC) in the pipe wall. As gas liquid mixture flows through the centrifugal device downwardly, a strong rotation flow is created. Liquid in the mixture is pushed to the pipe wall by centrifugal force and forms a uniform liquid film with high tangential velocity. Then nearly all the liquid film can directly enter the NTC and be discharged from the pipe relying on its own kinetic energy and inertial, because of the little resistance characteristic of NTC to the liquid film, therefore the whole separation process can be completed within the pipe. The swirl characteristics in the pipe were studied by numerical method, and the simulation results indicated that the swirling liquid film is fairly uniform and the suitable installation position for NTC is about 2.5 pipe diameters downstream of the centrifugal device. A separation model for the pipe separator was established and experiments were carried out to verify the proposed model. The superficial velocity ranges of gas and liquid were 22–72 m/s and 0.07–0.54 m/s, respectively. The experimental results showed that the separation efficiency always increases with the increase of both gas and liquid superficial velocity, generally it is over 0.81, and the maximum attainable is 0.97. However, the separation efficiency will begin to drop if liquid superficial velocity exceeds the critical value UCSL, due to the onset of unstable liquid film waves. Experimental results also showed that UCSL increases with the increase of gas velocity.

Introduction

Separation technology of gas-liquid two-phase flow is very important to modern industries, it has been widely used in power, petroleum, chemical and nuclear engineering. Traditional separators mainly use gravity and centrifugal force to separate gas and liquid, such as cyclones which have been the most widely used separators, gas-liquid mixture is usually introduced into the cylinder of separator from tangential inlet, forming a swirling flow in the cylinder, under the action of gravitational and centrifugal forces, liquid phase is thrown to the wall and flows downwardly, while gas phase is concentrated to the center and exits from the top. To prevent the liquid film from being torn by the rising gas and leading to a serious entrainment, the axial gas velocity in the cylinder is limited in design guidelines, less than 0.4 m/s at high pressure and 1.1 m/s at low pressure (Li and Yang, 1986), so the diameter of the cyclone often is several times of that of the inlet pipe. For this reason, the traditional gas-liquid two-phase separators are usually very large, heavy and expensive, especially at the condition of high temperature and high pressure. They belong to a typical pressure vessel, for safety, inspection is required at regular intervals by government. In recent years, in order to reduce the cost, more and more compact separation devices have been studying, such as the gas-liquid cylindrical cyclone (GLCC) (Chirinos et al., 2000; Kouba and Shoham, 1996; Kouba et al., 1995; Kouba et al., 2006; Molina et al., 2008; Shoham and Kouba, 1998; Wang et al., 2003) and swirl-vane separators (Green and Hetsroni, 1995; Kataoka et al., 2008; Kataoka et al., 2009; Xiong et al., 2013; Xiong et al., 2014). GLCC is mainly composed of a tangential inlet and a vertical pipe. A gas-liquid mixture enters the GLCC through the tangential inlet nozzle, causing a swirling flow in the vertical pipe. Due to centrifugal and gravitational forces, liquid is pushed to the pipe wall and exported at the lower part, while gas is concentrated to the center and discharged from the upper part. Although GLCC has many advantages, it has not yet been widely used, mainly served as a pre-separator or a slug catcher or a partial separator (Kouba and Shoham, 1996; Kouba et al., 2006). The main reason for this is that with the increase of gas velocity, a large amount of liquid droplets are entrained into the gas stream, this is the so-called liquid carry-over (LCO) (Chirinos et al., 2000). LCO limits the upper limit of gas superficial velocity, usually it is less than 9.2 m/s for a regular GLCC (Wang et al., 2003). Wang et al., 2003 developed a modified GLCC by equipping with an Annular Film Extractor (AFE) at the upper section of GLCC. Liquid carried by the gas stream would be removed as they passing through AFE. Experimental results showed that this modified GLCC has higher operational gas velocity, the upper limit of gas superficial velocity is as high as 18.3 m/s at low pressure, but the highest liquid superficial velocity is still 0.15 m/s as the same as a regular GLCC. Besides, Wang et al., 2003 also recommended a gas velocity ratio USG/Uann (Uann is the minimum gas velocity required to initiate liquid carry-over) to analyze experimental results for both low-pressure and high-pressure conditions. Molina et al.’s (2008) field high pressure tests showed that the separation efficiency tendency with the increase of gas velocity ratio is similar to that at the low pressure, the gas velocity ratio is a useful similarity number. It means that improving the effective gas velocity range of a separator under low pressure can also improve that under high pressure.

Unlike cyclone separator, centrifugal force is generated as gas-liquid mixture flowing through swirl vanes in axial direction in swirl-vane separators, the liquid phase is thrown to the wall of cylinder, and the gas is gathered in the central of the cylinder. Gas core and liquid film are finally split at the end of the cylinder: gas core exits from a central tube and liquid film is discharged through the annular space between the cylinder wall and the central tube. Since the gas core has to have a sudden contraction and acceleration at the central tube, a portion of liquid is easily entrained by high velocity gas, so the operating range of gas velocity of the swirl vane separator is limited, for example, under normal temperature and atmospheric pressure, the gas superficial velocities were in the range of 12.0–24.1 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).

Perry and Graff (1975, 1979) invented a tubular type gas-liquid separator. The centrifugal device of this separator is directly placed in a tube and the tube is enclosed in a housing chamber. After liquid film is formed on the wall of the tube by the centrifugal effect, it is ejected from the tube with a portion of gas core through an annular ejection port. Although the liquid is directly removed from the tube, a portion of gas core has to be ejected from the annular ejection port at the same time, since the liquid film by itself may have not enough kinetic energy to overcome the resistance of the annular port. This part of gas and liquid are finally separated in the chamber by gravity, which makes the separator still like a vessel-type.

Hayes (1989, 1990) presented another method and device for wet gas separation. The device has a tube coaxially disposed in a shell, and a helical swirling device coaxially located within the tube. As wet gas passes through the helical device, liquid droplets are drawn off from the gas through holes in the tube wall and fall to the shell. It's more suitable for high quality wet gas, but moisture can't be separated from the holes as the wet gas velocity is over 4.9 m/s. Wen (2009) modified this type of separator by replacing the tube with a cone tube and installing a swirler at the entrance of the cone tube. Experimental results showed that the upper limit of gas superficial velocity extends to 30 m/s.

From the reviews above, one can see that it is not difficult to force the liquid in the gas-liquid mixture towards the wall and form a liquid film by any centrifugal means. However, it is really difficult to finally separate the liquid film from the gas core, because the liquid film is still strongly coupled with the gas core, especially at the gas-liquid interface. If the gas velocity is too high, the process of removing the liquid film by conventional means will give a significant disturbance to the gas flow and lead to serious entrainments or liquid carry-over (LCO). Hence it is still difficult to improve the gas velocity range of gas-liquid separator. In order to solve the problem, this paper proposes a new method and structure that can easily remove the liquid film from the gas core and induce little disturbance to the gas core, that is the narrow tangential conduit (NTC) in the pipe wall. Since NTC is just aligned with the direction of swirling liquid film flow, so it has little resistance to the liquid film and little disturbance to the gas flow during the separation process. Nearly all the liquid film may directly enter the NTC and be completely discharged from the pipe relying on its own kinetic energy and inertial. Hence the upper limit of gas velocity will be extended markedly. Compared with traditional gas-liquid separators, the operation range will be greatly improved, both in gas and liquid superficial velocity, and the separation process can be carried out totally in the pipe. Meanwhile, the swirling strength in the pipe separator will be also reinforced by the high velocity gas, which is extremely beneficial that more small droplets may easily deposit to the wall and form a liquid film and the liquid film will get more tangential kinetic energy to pass through the NTC.

The objective of this paper is to present the principle of this novel centrifugal pipe separator and establish a separation model of the pipe separator based on the liquid film flowing out of NTC. Experiments have been carried out in an air-water two-phase flow loop to verify the proposed model. In addition, the main parameters of the NTC and their effects on the separation efficiency were also studied experimentally. In normal pressure and temperature, the experiments have been conducted in a wide range of gas and liquid superficial velocity. Various phenomena in the separation process have been observed to verify the separation performance.

Section snippets

Theoretical analysis and separation model

A schematic diagram of the new gas-liquid pipe separator is given in Fig. 1. The structure of centrifugal device and NTC are shown in Fig. 2, Fig. 3 respectively. Along the flow direction, the new gas-liquid pipe separator consists of a centrifugal device, 3 evenly distributed NTCs along the circumference of the pipe, a liquid collection ring and a liquid discharge tube. As gas-liquid mixture flows through the centrifugal device downwardly, a strong rotation flow is created in the pipe. Liquid

The type of centrifugal device

The structure of centrifugal device used in the study is shown in Fig. 2. It is composed of four symmetrically arranged semi-elliptic guide vanes and the installation angle between each vane and cross section of the pipe is α. When the gas-liquid mixture flows through the vanes, centrally symmetrical vortex flow field is formed in the pipe. The swirl in the pipe is generated by the vanes, and the swirling performance is mainly affected by the number of guide vanes and installation angle α.

Test section

A schematic of test section is shown in Fig. 1. The diameter of the main pipe was 26 mm. The pipe separator was composed of a centrifugal device, 3 narrow tangential conduits (NTC), a liquid collection ring and a liquid discharge tube. The NTC was located at 65 mm (2.5 pipe diameters) downstream the outlet of centrifuge device. As shown in Fig. 2, the centrifugal device was consisted of four symmetrically arranged semi ellipses guide vanes, the angle between each guide vane and the cross

Experimental range

In this experiment, the inlet flow conditions were all annular flow. The inlet superficial velocity ranges of gas and liquid were 22–72 m/s and 0.07–0.54 m/s respectively. According to the flow pattern in the pipe separator, the experimental data could be divided into three parts: low performance, normal condition, and unstable condition as shown in Fig. 14, which are corresponding to the flow pattern shown in Fig. 15 (A), (B) and (C), respectively. The low performance, and the normal condition

Conclusions

A novel centrifugal gas-liquid pipe separator with NTC is proposed in this paper. Based on the numerical and experimental results, the following conclusions can be drawn.

  • (1)

    Gas-liquid mixture can be isolated as a uniform swirling liquid film with high tangential velocity near pipe wall and a gas core in the central of pipe by the centrifugal device. It is particularly suitable for NTC to remove the liquid film from pipe. The suitable NTC installation location is about 2.5 pipe diameters downstream

Declaration of Competing 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.

Acknowledgements

The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 51121092).

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