Investigation of the pressure drop increase in a square free-vortex cyclonic separator operating at low particle concentration

Highlights

• A new cyclonic separator with a vortex limiter instead of a conical section is proposed.

• The fluid dynamics in single- and two-phase flows was evaluated experimentally and numerically.

• Injection of particles into the flow increased the pressure drop by approximately 10%.

• The effect of the particles on the pressure drop was also observed in numerical simulations.

• The increased pressure drop could lead to improved separation efficiencies.

Abstract

This paper reports experimental and numerical results obtained in a free-vortex gas-solid separator, in which the classic conical-section of the device is replaced by a larger square chamber, where the vortex is free to evolve. An adjustable vortex-limiting plate is introduced at the cross-section in order to allow the inversion of the coherent structure and guarantee a natural vortex length of the swirling flow according to the inlet velocity in the separator. Both single- phase and multiphase flows were analyzed. In the latter case, only very low particle concentrations, between 100 and 500 mg/m³, were considered. Compared to the classic cyclonic separators, the new geometry showed advantages in performance, such as a smaller pressure drop. Moreover, unexpected behavior was observed, since the pressure drop increased by an average of 10% when the particles were injected into the flow at the low concentrations considered.

Introduction

One of the main challenges in the engineering of complex systems for the chemical industry is the design of the single units that compose the rig. Among the numerous phenomena that need to be taken into account for a successful and efficient design, fluid dynamics undoubtedly plays a crucial role. In fact, an accurate study of the flow involved in the process can considerably improve the efficiency and performance of the entire system.

Gas-particle separators, cyclones being one of the most common types, are an example of how an appropriate fluid dynamics design can significantly improve the performance. These devices are commonly used in industry to separate small solid particles from the gas phase in two-phase flows. They provide robustness, consistency and high efficiency for a range of solid particle diameters from 20 to 200 μm [18]. Examples of their application include air purification (bringing environmental benefits), the clinker burning process of the cement industry and recovery of solid particles from chemical reactions, e.g. fluid catalytic cracking (FCC). In these swirling-flow separators, a tangential force generated by a bounded-vortex created artificially carries the solid phase toward the walls of the device, where the velocity of the flow is low [18]. Here, the gravitational force becomes higher than the centrifugal force and therefore particles fall toward the bottom of the separator, where they are collected. The vortex, ideally free of particles, bounces back at the bottom and exits the device through another outlet situated at the top. Due to the complex flow pattern that is created inside these systems, they need to be properly designed in order to guarantee high efficiency. Therefore, careful study and optimization of the flow inside the separator are essential. Several flow features affect these operating parameters and one of the most important is the so-called ‘natural vortex length’ (NVL). According to Hoffmann et al. [10] and Chen et al. [3], this length is strongly connected to the particle separation process and to the erosion that can be observed at the base of conventional separators. One of the main difficulties related to the NVL is keeping the vortex stable, as this parameter changes depending on the flow inlet velocity and the concentration, i.e., the density of the solid phase per cubic meter of the gas-solid mixture. Therefore, a device can be optimized for a particular working condition but when this condition is changed the separator no longer operates with optimal performance.

In general, two main operating parameters are considered in order to address the performance of these devices: the separation efficiency and the pressure drop. The former is associated with the capability of the separator to collect the solids from the gas phase effectively. In order to clarify the relation between the efficiency of the separator and solid particles of a specific dimension, the grade efficiency is often considered, which is expressed in terms of particle diameter. This involves an analysis of the characteristic of each particle diameter class of the distribution. The grade efficiency is associated with the loss of energy inherent to the fluid between the inlet and the outlet of the device due to wall friction and turbulent viscous dissipation.

Since cyclones were proposed as the first swirling flow gas-solid separators, they have significantly evolved in terms of geometry and thus in terms of performance. Several strategies have been tested to optimize their performance following two main directions: optimization of the geometric dimensions and addition of other devices to the original design [13]. Some of the cyclonic separator geometries commonly studied are shown in Fig. 1.

Stairmand cyclones (Fig. 1b) are designed to run under fixed operating conditions. Independently of the geometric proportions and parameters, these devices have a common limitation: the lack of operational flexibility. In fact, their optimal working point is limited to only one combination of mass flow rate and loading ratio. Moreover, if the conical region does not have exactly the vortex-length generated inside the main chamber, the vortex will become unstable and erosion of the walls may occur. Chen et al. [3] proposed the use of a limiting plate inside the conical region (Fig. 1c), to which the vortex is attached, thus stabilizing it and reducing the erosion at the walls. Avci and Karagoz [1] and Karagoz et al. [14] proposed a different geometry to improve the flexibility of cyclonic separators (Fig. 1d), where a cylindrical hopper replaces the conical region of the separator and an adjustable vortex- limiting plate is added to influence the behavior of the vortex. According to the authors, this geometry provides better results, mainly in terms of pressure drop. The increased distance between the vortex and bounding walls causes a slight drop in friction generation. It should be noted that, in conventional cyclones, the conical region that contains the vortex promotes the tangential acceleration of the flow, with a consequent increase in the pressure drop. This component of added velocity disappears in a geometry without the conical region, further contributing to the reduction in the total pressure drop decrease.

Cyclonic separators are relatively easy to build and they have proved to be operationally robust when appropriately designed. However, due to the complex flow pattern, it is possible to observe several phenomena in the flow field, such as coherent vortices, boundary layer detachment and recirculation regions, interaction between phases and intense turbulence [5,11,16]. All of these factors combined make the design and analysis of cyclone separators a challenging task. It has been demonstrated that the flow pattern inside swirling-flow separators significantly changes depending on the solids loading ratio. Yuu et al. [28] observed a significant decrease in the pressure drop for a particle concentration of around 5 g/m³, which then remains constant up to approximately 70 g/m³. Moreover, Hoffmann et al. [9] reported that a higher concentration promotes higher separation efficiency, and consequently the separation of phases in diluted flows is harder to achieve.

Considerable research effort has been focused on improving the efficiencyof gas-particle separators. Souza et al. [26] carried out a numerical study on the impact of post cyclone devices and overflow ducts on the grade efficiency. The authors found that the post cyclone device was able to reduce the cut-off diameter with negligible or no pressure drop increase. Similarly, Balestrin et al. [2] tested a secondary vortex finder diameter reduction, which caused a new downward swirling flow that increased the collection efficiency of particles smaller than 5 μm. Uniflow cyclones have also been analyzed in this way [20]. Naturally, the pressure drop also needs to be addressed. Noriler et al. [17] proposed a vortex breakdown device, arranged inside the finder, that reduces the pressure drop by at least 20%, which is the same amount achieved by Elsayed [7] through remodeling the Stairmand geometry. Demir [6] derived and adjusted a cyclone flow pressure drop correlation, presenting better predictions compared to classical models. However, the author stated that the clean flow pressure drop represents the maximum possible, i.e. the pressure drop decreases in the presence of solids. Given the geometry differences, we will show herein that the pressure drop may increase with very low solids concentrations, which was not noted by Ji et al. [12], who tested particle concentrations of around 5–2000 mg/m³.

The natural vortex length is another factor that affects the particle separation [19,22]. If the vortex is unstable, it may bend and attach to the separator wall, causing erosion and low efficiency [21]. It is easier to carry out NVL studies in separators without the conic section, as proposed by Karagoz et al. [14]. In this case, the moving vortex limiter allows the adjustment of the vortex anchor point, given that the vortex length varies with the gas phase inlet velocity. Sakin et al. [23] analyzed this flexibility numerically and observed reductionsin the pressure drop and collection efficiency as the distance of the vortex limiter from the cyclone inlet was increased, under the same operational conditions.

Considering the above-mentioned developments with regard to gas-solid separators, the aim of this study was to evaluate the pressure drop and solids separation efficiency for a cyclonic separator with a hybrid geometry. In this separator, the main vortex chamber has a square section instead of a conical or cylindrical one, with a length of around three-times the size of the cylindrical body. Therefore, in this design, the coherent structure is no longer bounded by the lateral walls, but it is free to develop and evolve. More specifically, the objectives of this study were: to explore new ways of adapting a single separator to various inlet velocities; and to analyze the separation performance and its effects on the pressure drop increase at very low solids concentrations, the latter being the main contribution of this paper.