Numerical investigation of two-phase flow in drum cyclone

Master's Thesis

Energy Conversion Engineering Trend

Abstract:                                          

In many processes in the industry, the combination of liquid droplets with gas flow will cause problems such as corrosion of downstream equipment and loss of expensive liquids. To solve these problems, the phases are separated from each other with the help of a separator. Cyclone separators, as a type of high efficiency and low volume separators, are widely used in the separation of liquid gas streams. One of the appropriate methods for investigating the performance of separators, including cyclone separators, is simulation using Computational Fluid Dynamics (CFD). In the effect studies, the best turbulent flow samples for the cyclone were obtained and the effect of the geometric parameters of the cyclone on the separation efficiency and the reduction of the unwanted phenomena of gas transport from the bottom and liquid transport from the top were investigated. Geometrical parameters have a significant effect on improving cyclone performance. For this reason, in this thesis, the performance of the cyclone separator and the effect of changing the geometrical parameters on it have been investigated with the help of computational fluid dynamics. After choosing the appropriate grid for the cyclone geometry, the effect of changing the geometrical parameters on the amount of liquid transport from above and the amount of gas transport from below was studied. In this design, the inlet cross-section, inlet angle, inlet height from the bottom of the cyclone, liquid outlet diameter and cyclone main diameter were simultaneously optimized. According to these simulations, the amount of gas transport from below decreases by decreasing the diameter of the liquid outlet or increasing the main diameter of the cyclone or increasing the inlet cross-section or decreasing the angle of the inlet, and there is an optimal point for changes in the gas transport from below by changing the height of the cyclone inlet. Also, an optimal point for carrying the liquid from above is obtained by changing the geometrical parameters such as the inlet cross-section, the height of the inlet from the bottom of the cyclone, the diameter of the liquid outlet and the main diameter of the cyclone. An increase in the inlet angle increases the liquid transport from above. Finally, the effect of using a cyclone in a boiler drum, which contains steam and water droplets at high temperature and pressure, was investigated.

-1 Introduction:

In order to separate the gas-liquid two-phase flow, oil and gas industries used tank separators in the past, which were large, heavy and expensive. The old separators worked in such a way that first the gas-liquid two-phase fluid entered the tank and passed through an inlet plate. At the inlet, the gas velocity decreased, and this decrease in velocity caused a change in the fluid momentum, and as a result, small and fine liquid particles collided and formed heavier particles that stuck to the inlet plate and the wall inside the separator. Then the gas at the outlet hit a plate where the final separation was formed and left the separator and the liquid particles were transferred to the lower part of the separator.

Old liquid gas separator [1] [

But the need for separators that can be used in different places in far away places and are portable as well as their use under the sea requires The separator with a smaller volume, which of course leads to a significant reduction in cost, has revealed more than before. Among the various alternatives that can exist to solve these problems, gas-liquid cylinder separators [1] presented by the University of Tulsa and Churn [2] and using centrifugal force in addition to gravity to separate the two phases have been selected for investigation. The separator of gas-liquid cylinders is a simple, low-volume and low-cost separator that can be a suitable replacement for old separators in a wide range of applications due to its low cost and portability. Although cyclones have been used for a long time as liquid-liquid, solid-liquid and gas-solid separators, they have been used for a limited time in gas-liquid separation, which, of course, is the main obstacle to the widespread use of gas-liquid separators, the lack of reliable facilities to predict their performance in order to know the proper performance of these separators..

GLCC consists of a vertical pipe with a sloping tangential inlet, which is located in the middle of the height of the pipe, and two outlets, one for the gas exit at the top and the other for the liquid exit at the bottom, and due to the presence of gravity, the lower part of the GLCC is occupied by liquid and the upper part by gas. The tangential force causes the fluid to rotate, which leads to the creation of centrifugal force and vortices inside the cyclone body. Therefore, in the lower part of the inlet, the gas bubbles trapped by the liquid are pushed radially towards the axis of the cylinder and form a gas strip that rejoins the vortex, and in the upper part, the liquid particles are thrown towards the separating wall and form a compact mass that cannot be carried by the gas and moves towards the liquid outlet. Figure (1-2) shows an example of a liquefied gas separator cyclone.

Liquid Gas Separator Cyclone (GLCC) [2] [

1-2- The need for research:

Considering the importance of separation in the industry and the need to design efficient separators for different processes, engineers decided to use tools Simulation with the help of software, identify the effective parameters in the design and according to the operating conditions, design suitable separators for the desired application. The use of simulation has significantly reduced the additional costs related to the construction of devices on a laboratory scale and conducting time-consuming and expensive tests, and it makes it possible to check many parameters without increasing the cost.

Today, liquid gas cylinder separators [3], since they have a great advantage over the old separators, are more and more interested in the oil and gas industries. The use of liquid gas separator cyclones in field applications requires a complete understanding of the hydrodynamics of the flow inside these separators [3]. In old separators, gravity was the only factor in separating phases, while in new liquid gas separators, centrifugal force along with gravity helps to separate phases. Various conditions such as cost and work pressure forced oil and gas industries to tend towards cheaper, smaller separators with better separation efficiency, separators that are compact and suitable for coastal and subsea applications [4].

In the separation of different phases in the separation of liquid gas cylinders, there may be problems that reduce the efficiency of these separators. One of these problems is that the liquid carries some gas with it when leaving the liquid outlet, which is called gas transport from the bottom [4]. Another problem that exists is the liquid transport by gas from the gas outlet, which is called liquid transport from above [5]. Mantilla et al. developed a bubble trajectory model to determine the time of gas transport from the bottom in liquid gas cylinder separators, to design these separators for field applications [5]. They provided examples to demonstrate how bubble trajectories can aid in the design of cyclone separators. Farhat Erdal et al presented numerical simulations of single-phase and two-phase flow in different cyclones to investigate the flow complexity field in liquid gas separators and predict phenomena related to these separators such as gas transport from the bottom [6]. They compared their work with experimental data that included plots of tangential velocity and tangential deceleration, and good agreement was obtained. They also developed an axisymmetric model that is computationally useful. Kirinos developed a mechanistic model to predict the percentage of liquid carried from above inside a cylindrical cyclone [7]. Also, an existing model for predicting liquid transport from above in high pressure conditions including advanced models for net zero flow of liquid [6] in the drop region and critical velocity has been developed by them. The aim of their work was to examine the data obtained for liquid transport from above and develop a mechanism model for high pressure conditions and predict the percentage of liquid transported from above. Gomes theoretically and experimentally studied the hydrodynamics of two-phase distributed rotating flow to predict gas transport from above and measure the performance of cyclone separators [8]. The aim of his study was two things, an experimental study of the hydrodynamics of the two-phase spread rotating flow in the lower part of the cyclone and the development of a mechanism model to characterize the complex behavior of this type of flow that makes it possible to predict the gas transport from the bottom in the liquid gas cyclone separator.