Optimization of the geometry of cyclone separators used in clinker burning process: A case study

    Optimization of the geometry of cyclone separators used in clinker burning process: A Case Study Marek Wasilewski, Lakhbir Singh Brar PII: DOI: Reference:

S0032-5910(17)30231-0 doi:10.1016/j.powtec.2017.03.025 PTEC 12430

To appear in:

Powder Technology

Received date: Revised date: Accepted date:

2 January 2017 5 March 2017 6 March 2017

Please cite this article as: Marek Wasilewski, Lakhbir Singh Brar, Optimization of the geometry of cyclone separators used in clinker burning process: A Case Study, Powder Technology (2017), doi:10.1016/j.powtec.2017.03.025

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ACCEPTED MANUSCRIPT Optimization of the geometry of cyclone separators used in clinker burning process: A Case Study

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Marek Wasilewski

Faculty of Production Engineering and Logistics, Opole University of Technology,

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76 Proszkowska St., 45-758 Opole, Poland, Tel. 48 693 28 62 61, E-mail: [email protected]

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Lakhbir Singh Brar

Mechanical Engineering Department, Birla Institute of Technology, Mesra, Ranchi 835215, India

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E-mail: [email protected]

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Abstract

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This study presents the possibilities for improving the process of clinker burning by

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optimizing the geometry of first-stage cyclones that form a part of a cyclone suspension

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preheater. Due to the high energy consumption and scale of the process involved in cement production, the aim is to improve the efficiency of the system. Since the exploitation data collected from real industrial installations frequently indicate significant discrepancies against

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the results obtained from experimental models, a case study of a particular industrial

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installation is, therefore, presented. Three highly efficient design models are proposed based on design guidelines to enhance the cyclone performance. In addition, a new method to

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clinker burning is also proposed.

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determine the basic characteristic dimension D of the first-stage cyclone separators used for

The multiphase flow inside the cyclones was analyzed using Computational Fluid

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Dynamics (CFD) analysis. As a closure model to the Reynolds-averaged Navier–Stokes (RANS) equation, the Reynolds Stress Model (RSM) was used, as this solves the transport

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equations for Reynolds stresses as well as the dissipation rate. For the discrete phase, one-way coupling was used, and the numerically predicted results were found to be in close agreement with the existing industrial installations. The results conclusively indicate that the proposed design guidelines, when applied to existing first-stage cyclones, result in better collection efficiency with a more than 43% reduction in the power consumption (with the third cyclone variant), which is highly significant.

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ACCEPTED MANUSCRIPT Key words: Cyclone separators, Collection efficiency, Pressure drop, Suspension preheater,

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Nomenclature

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Clinker burning

a - height of the gas inlet

Qi – inlet gas volumetric flow rate

b - width of the gas inlet

re - distance between the cyclone axis and inlet axis

B - diameter of the cyclone lower (dust) outlet

Re - Reynolds number

CD - drag coefficient

RANS - Reynolds average Navier–Stokes

CFD - computational fluid dynamics

RSM - Reynolds stress model

dp - diameter of a particle

s – the source term

D - cyclone body diameter

S - height of the outlet duct in the interior of the

De - diameter of the cyclone gas outlet

cyclone

De1 - upper diameter of the cyclone gas outlet

SIMPLE - semi-implicit method pressure-linked

De2 - lower diameter of the cyclone gas outlet

equations

DPM - discrete phase model

t - time

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ACCEPTED MANUSCRIPT u - gas velocity

F - area of cyclone inlet

ui (j, k) - gas velocity to direction i (j, k)

Fd - drag force

u’i (j, k) - fluctuating velocity to direction i (j, k)

g - acceleration of gravity

up - particle velocity

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Gi - inlet particle mass flow rate

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Dij - the stress diffusion term

αg - angle of the cyclone inlet head, degree

h - height of the cyclone cylindrical section

ΔP - pressure drop in a cyclone separator

H - height of the cyclone

δ - Kronecker factor

Hc - height of the cyclone conical section

εij - the dissipation term

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Gij - equals Buoyancy Production

k - turbulence kinetic energy

µ - viscosity of gas Πij - the pressure-strain correlation term

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P – pressure Pij – the shear production term

ρ – density of gas ρp – density of a particle

PRESTO - Pressure Staggering Option

τij - the Reynolds stress tensor

1. Introduction

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p' – dispersion pressure

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Issues related to rational exploitation of natural resources have always had a considerable impact on the rate and consistency of global industrial development. This is especially true for the cement industry, where fuel and energy exploitation have a wellestablished – and high – rank because the share of technological fuel alone contributes significantly to the overall cost of cement production. Depending on the production method, energy consumption in cement production ranges from 2.9 to 6.0 GJ/Mg of clinker [1, 2], which is quite high. The end of the 20th century and the beginning of the 21st century saw a dynamic development of new and energy-efficient methods of cement production. Such changes were needed to not only reduce the energy consumption of cement production, but also to reduce the harmful impact of the process on the environment. The high energy 4

ACCEPTED MANUSCRIPT consumption of cement production is related to an increase in CO2 and NOx emissions. Mikulcic and others [3] have addressed the subject of reducing these emissions in their

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studies. The rising prices of energy and increasingly stringent standards of Integrated

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Pollution Prevention Control (IPCC) [4] concerning the allowable gas and particulate matter

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emissions are major factors in this respect. The study by Worell and Galitsky [5] indicates that currently the most fundamental, effective and promising direction of development in clinker production using the dry method involves applying and improving a rotary kiln with a

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cyclone suspension preheater equipped with precalcinators.

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Significant energy and financial savings can be achieved by modernizing the designs of cyclone suspension preheaters, which currently show high hydraulic resistances and low efficiencies of solid particle separation. The modernization may be global (reconstruction of

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an entire suspension preheater) or local (optimization of the cyclone geometry at different stages) in nature.

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A cyclone suspension preheater operates in a manner similar to the traditional suspension heat exchangers. Modern installations consist of four to six stages. The design of

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cyclones at each stage of the suspension preheater are based on temperature conditions and their functions. Figure 1 shows an example configuration at different stages of cyclones that are currently used in the cement industry [6]. The first-stage cyclones have a significantly different design than conventional cyclones, as their role is to retain as much of the raw material within the suspension preheater as possible (i.e., to minimize the loss of raw materials). This results in more stringent requirements concerning separation efficiency, and better production capacity. Furthermore, the operating conditions (e.g., high temperature and a high share of the solid phase usually exceeding 500 g/m3) of these devices suggest that the traditional designs of cyclone separators, in general, are not suitable. The same has been confirmed by the data collected from industrial practice and product information obtained

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ACCEPTED MANUSCRIPT from manufacturers of cyclone separators dedicated to the cement industry. Analysis of the specialist literature reveals that only a few studies [e.g., 7–16] addressing this subject have

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described the flow inside cyclones as multiphase flows. Since limited information is available

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on this aspect, this further complicates our understanding of the fluid-particle interactions,

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which otherwise could have been useful in the optimization of performance parameters. As a result, no design guidelines were available for the developers of cyclones used for clinker burning to enhance its performance. These shortcomings were noticed and accounted for in

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the studies conducted by Wasilewski and Duda [17], and guidelines were proposed that could

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help to optimize the design of first-stage cyclones depending on the objective function. The existing cyclone models at the first stage are of a comparatively old structure that were once popularly used in the 1980s and 1990s. However, due to their low efficiency and

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high power consumption, their usage has gradually decreased and they have been almost discontinued. The first-stage of a suspension preheater comprises a two-cyclone system. To

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minimize the energy consumption while retaining the raw materials to a maximum possible extent, the authors propose three variants of cyclones. However, the crucial step before

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implementing a new design is to determine the optimum cyclone body diameter D, and based on this value all other characteristic geometrical dimensions are calculated. Therefore, the authors propose a model to determine the diameter D of a cyclone suspension preheater equipped with precalcinators. Next, three optimized design variants have been proposed with the objective of maximizing the separation efficiency, at the lowest possible pressure drop (first variant), to maximize collection efficiency (second variant), and lastly, to minimize pressure drop (third variant). The performance of all the proposed models are predicted by using numerical simulations and compared against the existing structure to elucidate the improvement. 2. Materials and methods

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ACCEPTED MANUSCRIPT Since the exploitation data collected from real industrial installations frequently indicate significant discrepancies compared to the results obtained from experimental models (design),

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the authors decided to present a case study of a particular industrial installation. Collaboration

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with several cement plants in Poland allowed for a balance of a particular installation to be

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drafted, which, in turn, was quite helpful to define the research conditions. The study uses first-stage cyclone models with designs that are common in several cement plants. This ensured that the obtained results are universal, rather than limited to a single plant to model

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the real working conditions of the installation. It is necessary to conduct the measurements

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concerning pressure drop, separation efficiency, gas volumetric flow rate and particle mass flow rate. In addition to defining the real parameters, several case studies are performed that, in turn, would help to validate the results. The most important parameters which constitute a

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reference point for the assessment of the new design are pressure drop and separation efficiency.

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2.1 Proposed model for cyclone body diameter Once the research conditions have been defined, new, highly efficient structures of first-

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stage cyclones can be proposed for the selected installation. In the first step, the basic geometrical parameter, i.e. the cyclone diameter D, is defined. Analysis of the subject literature indicates that most models describing this parameter require the gas volumetric flow rate, inlet velocity, and inlet dimensions to be defined [18-21]. As a result, the effect of temperature, which is a very important factor in cyclone separators [15, 22-27], has not been accounted. One of the models that incorporated the aforementioned factors was proposed by Kalen and Zenz [28]. They proposed calculating the diameter of cyclones as follows: a (1 − D) Q i ρ2 D = 𝑥1 [ ∗ x2 ] μg (ρp − ρ) a ( b ) D D

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𝑥3

(1)

ACCEPTED MANUSCRIPT However, this model used auxiliary coefficients (marked as x1, x2 and x3) that do not provide sufficiently precise results for the first-stage cyclones in a clinker burning installation.

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To overcome these shortcomings, the authors evaluated the coefficients in the aforementioned

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model so that the characteristics of first-stage cyclones in a clinker burning installation could

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be considered. The equation for cyclone body diameter D is now proposed as: 0.612

(2)

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a (1 − D) Q i 𝜌2 D = 0.11 [ ∗ 3] μg (ρp − ρ) a ( b ) D D

The present study makes use of this equation to evaluate the cyclone diameter and to

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determine the remaining characteristic geometrical dimensions. Although some mathematical models were also proposed earlier by Lapple [29], Stairmand [30] and Swift [31], they were

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dedicated to those cyclones with traditional designs rather than those used in cement

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industries.

2.2 The details of cyclone geometry The aim of this study is to present a practical application of the design guidelines

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proposed earlier by Wasilewski and Duda [17] for cyclones that form a part of a suspension preheater. Based on the cyclone body diameter D (calculated using Eq. (2)), three new cyclone designs are proposed over the existing structure to optimize the efficiency of production. The details of geometrical relationships are illustrated in Table 1 with the characteristic geometrical dimensions shown in Fig. 2. The dimensions of the three proposed models are elucidated in Table 2, corresponding to Figure. 3 that presents the 3D models of the proposed geometries. The proposed designs have some resemblances and some differences. Variants I and II have similar tapered vortex finder walls but different angles of the volute inlet head, whereas the inlet design of variants I and III are similar but the slope vortex finders are different. 8

ACCEPTED MANUSCRIPT 2.3 The governing equations of CFD Due to increasingly stringent economic and technical requirements, researchers are

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looking for new methods of research that could help to optimize the equipment used in

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chemical and process engineering. Among various techniques, computational fluid dynamics

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(CFD) is a widely accepted and promising tool to model the complex flows prevailing inside cyclone separators. Several studies have demonstrated the potential of CFD to predict the functionality and description of flow inside cyclones [10, 12, 13, 15-17, 32-41], and to

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optimize performance parameters [34, 35, 49].

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2.3.1 Continuous phase

For steady and incompressible fluid flow in cyclones separators, the Reynolds-averaged

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Navier–Stokes equations can be expressed as [15, 17, 32, 42]:

𝜕u

𝜕𝑃

𝜕u𝑖 𝜕𝑥𝑖

=0

𝜕

(3)

𝜕u

𝜕u

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𝜌𝑢𝑗 𝜕𝑥 𝑖 = − 𝜕𝑥 + 𝜕𝑥 + [𝜇 (𝜕𝑥 𝑖 + 𝜕𝑥𝑗 )] + 𝑗

𝑖

𝑗

𝑗

𝑖

𝜕𝜏𝑖𝑗 𝜕𝑥𝑗

(4)

where i, j = 1, 2, 3 indicate the components in the Cartesian coordinate system, and the

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Reynolds stress tensor is given as: ̅̅̅̅̅ ′ ′ 𝜏𝑖𝑗 = −𝜌𝑢 𝑖 𝑢𝑗

(5)

In this study, RSM is used as a closure model to solve the Reynolds stress tensor. Among all closure models for RANS, the Reynolds stress model (RSM) is currently the most popular and widely used turbulence model for flows involving high streamlined curvatures as in cyclone separators. The transport equations for RSM can be written as follows [15, 17, 33]: 𝜕 𝜕𝑡

𝜕 (𝜌𝑢̅𝑖′ 𝑢̅𝑗′ ) + 𝜕𝑥 (𝜌𝑢𝑘 𝑢̅𝑖′ 𝑢̅𝑗′ ) = 𝐷𝑖𝑗 + 𝑃𝑖𝑗 + Π𝑖𝑗 + 𝜀𝑖𝑗 + 𝑠 𝑘

𝜕 𝜕 ̅̅̅̅̅ ̅̅̅̅̅̅̅̅̅ ̅̅̅̅̅̅̅ ̅̅̅̅̅̅ ′ ′ ′ ′ ′ ′ ′ ′ ′ 𝐷𝑖𝑗 = − 𝜕𝑥 [𝜌𝑢 𝑖 𝑢𝑗 𝑢𝑘 + (𝑝 𝑢𝑗 )𝛿𝑖𝑘 + (𝑝 𝑢𝑖 )𝛿𝑗𝑘 − 𝜇 (𝜕𝑥 𝑢𝑖 𝑢𝑗 ) ] 𝑘

𝑘

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(6) (7)

ACCEPTED MANUSCRIPT ̅̅̅̅̅̅ ′ ′ 𝑃𝑖𝑗 = −𝜌 [𝑢 𝑖 𝑢𝑘

𝜕𝑢𝑗 𝜕𝑥𝑘

+ ̅̅̅̅̅̅ 𝑢𝑗′ 𝑢𝑘′

𝜕𝑢𝑖 𝜕𝑥𝑘

]

(8)

̅̅̅̅̅̅̅̅̅̅̅ 𝜕𝑢𝑗 𝜕𝑢 Π𝑖𝑗 = 𝑝′ (𝜕𝑥 𝑖 + 𝜕𝑥 ) 𝑗

(9)

𝑖

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̅̅̅′ 𝜕𝑢 ̅̅̅′ 𝜕𝑢

𝜀𝑖𝑗 = −2𝜇 𝜕𝑥 𝑖 𝜕𝑥 𝑖

𝑘

(10)

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𝑘

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In recent years, many studies have incorporated this model into their study [e.g., 34– 43].

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2.3.2 Discrete phase

In this study, solid particles are treated as a discrete phase and their tracking is done

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using a Lagrangian approach. Therefore, cyclonic flows are better modeled using the Euler– Lagrange (E–L) approach. This method treats the fluid as a continuous phase and the solid

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particles as a dispersed phase; the trajectory of each group of particles is predicted by adding

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up the forces that affect it. In the E-L approach, the equation for particle motion is given by [44]:

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𝑑𝑢𝑝 𝑑𝑡

= 𝐹𝑑 (𝑢 − 𝑢𝑝 ) +

𝑔(𝜌𝑝 −𝜌) 𝜌𝑝

(11)

The drag force per unit particle mass is given as [32, 44-47]:

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𝐹𝑑 =

18μ ρp d2p

∗

CD Rep 24

(12)

The drag coefficient is described by the Schiller–Naumann drag model, which assumes the following values depending on the value of Re [48]: 24

CD =

𝑓𝑜𝑟 𝑅𝑒 ≤ 1

𝑅𝑒 24(1+0.15 𝑅𝑒𝑝0.687 ) 𝑓𝑜𝑟 1<𝑅𝑒𝑝 ≤1000 𝑅𝑒

{

(13)

0.44 𝑓𝑜𝑟 𝑅𝑒 > 1000

The Reynolds number for a particle is given by the following equation [44, 49]: 𝑅𝑒 =

𝑑𝑝 𝜌|𝑢𝑝 −𝑢| 𝜇

2.4 Validation and methodology 10

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ACCEPTED MANUSCRIPT The present study makes use of the finite volume method based CFD code Fluent to model the flow field and to predict the performance parameters. Calculations were performed

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using the segregated (pressure-based) solver method. Differential equations were solved using

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the semi-implicit method for pressure-linked equations (SIMPLE) algorithms to correctly

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determine the conjugation of the pressure and velocity fields with respect to the momentum and continuity equations. Interpolations were performed using the second-order upwind method to select the representative values of the samples of the constituents on the surface of

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control volumes. In the near wall region, the fluid flow is modeled using the standard wall

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function available within the solver.

The fluid domain was discretized using a hexahedral mesh. Four different levels of meshes were accounted to check the dependency of results on mesh density. A comparison is

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also made between the numerically predicted results and those of the industrial installation. A complete overview of numerical predictions for different levels of mesh, along with error

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estimations, is presented in Table 3. Conclusive results show that a level 3 mesh consisting of 360,680 elements ensured a high consistency between the results, and required the least

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computational power. On the other hand, with a level 4 mesh, no significant difference (with regard to level 3 mesh) was noticed, although it required more computational power. Analyzes of the proposed models were conducted after validating the methodology and boundary conditions used in the numerical simulations against the data available from the industrial installations. This procedure involves comparing the measurement data obtained from the installation of the first-stage cyclones that the cement plant is currently using (Fig. 4) with the values obtained using CFD. To minimize computational errors, a single design was used while maintaining the division of flow parameters. A gas volumetric flow rate Qi of 16.3 m3/s, at a particle mass flow rate Gi of 5.75 kg/s, was used. The proposed three new

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ACCEPTED MANUSCRIPT design variants were also subjected to similar flow rates for both the continuous phase and discrete phase.

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2.4.1 Validation of the discrete phase

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Table 4 illustrates the comparison of the fractional efficiency and global efficiency of

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numerical predictions with the data of industrial installations. Numerically predicted separation efficiency amounted to 82.2%, which is quite close to the real value of 85.0% (based on the drafted balance and the performed measurements), with computational error

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amounting to only 3.3%. This deviation may be a result of agglomerations of small solid

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particles and collisions between the particles that take place in actual practice. These two phenomena are comparatively expensive (in the computational sense) to incorporate into such large scale models using CFD. Therefore, applying a two-way or four-way coupling may lead

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to excessive take-up of computational power. Consequently, only few studies [e.g., 50, 51] have addressed the effect of agglomerations on the efficiency of cyclone separators. Owing to

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a difference of as small as 3.3% in global collection efficiency, the choice of one way coupling model is appropriate to be applied on all the test models used in the present study.

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Figure 5 shows example trajectories and velocities of particles with small diameters (10 μm, 20 μm and 30 μm) within the current installation. Note that the solid particles are transported down along the cyclone walls following a spiral path composed of several turns (usually between two and four). The solid particles reach maximum velocity in the lower region of the vortex finder and minimum velocity near the walls of the cyclone. Also, in the lower conical region of the cyclone, the particle concentration is deemed to be high. 2.4.2 Pressure drops As a general practice, the pressure drop in cyclone separators is taken as the difference in the mean pressure values across the inlet and outlet. Conclusive results indicate that the pressure drop amounted to about 1378 Pa in the real installation and around 1290 Pa in the

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ACCEPTED MANUSCRIPT numerical simulations. Therefore, with the known issues concerning model flows involving high levels of anisotropy (as in cyclonic flows), a computational error that amounted to about

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6.4% may be considered to be within acceptable limits.

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2.4.3 Validation of continuous phase

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No data regarding the details of the flow field were available from industrial installations to validate the continuous phase simulation results. However, the authors were fortunate to have the actual data of the global collection efficiency and pressure drop for first-

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stage cyclones (that has already been discussed above). The accurate predictions of the

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collection efficiency and pressure drop are a consequence of accurate modeling of the continuous phase flow field. Based on this fact, such a small difference in the global efficiency and pressure drop signifies high accuracy in modeling the continuous phase. The

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obtained consistency between the results, in terms of both separation efficiency and pressure drop, is high and enables the conclusion that the applied procedures and numerical models,

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and the performed parameterization of computational conditions during research were correct. This type of validation may be especially effective in estimating the computational error for

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newly-designed structures of cyclones, which lack a real point of reference. Therefore, with known difficulties to modeling the cyclonic flows, the methodology (solver settings and boundary conditions) adopted may be used for all future simulations with confidence. 3. Results and discussion Once the potential range of the CFD computational error was estimated, the performance parameters of the three proposed structural variants were assessed. Each variant was developed for a different optimization objective function. The second variant was designed to maximize the separation efficiency, whereas the third variant was focused on minimizing the pressure drop. The first variant, on the other hand, takes into account both the

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ACCEPTED MANUSCRIPT key parameters, and as such provides the highest possible separation efficiency at the lowest possible pressure drop.

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3.1 For discrete phase

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Since the main goal of first-stage cyclones within a cyclone suspension preheater is to

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minimize the release (and thus increase in the production capacity of the installation), analysis of results began with the determination of separation efficiencies. Table 5 shows the obtained values of separation (i.e. the global as well as fractional) efficiency and pressure drop for each

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design variant. A comparison between the results obtained using CFD indicates that all new

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designs of cyclones (regardless of the assumed optimization objective function) provided a higher separation efficiency than the existing design that amounts to nearly 85%. According to the criteria based on the design guidelines, the highest separation efficiency was achieved

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with variant II, amounting to 88.6%, and this is as much as 6.4% higher (with regard to CFD results) than the reference variant. Considering the scale of cement production, this constitutes

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a very significant difference. The other two variants also provided a considerable improvement, with separation efficiencies of nearly 86.1% for variant I and 83.8% for variant

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III, respectively. The differences in the values for variants I and III from that for variant II stem primarily from the application of a greater angle of the inlet duct in variant II. Such an inlet configuration enables a greater amount of solid particles to be transported toward the wall of the cyclone, which in turn minimizes the uplift of particles by the rising gas flux in the lower part of the vortex finder. The application of a cylindrical vortex finder (variant III) led to lower separation efficiency. To complete the analysis of separation efficiency, grade efficiency curves for all cyclone structures were drafted, as shown in Figure 6. Emphasis was put on solid particles smaller than 60 μm, as these particles showed higher differences between each variant. For solid particles greater than 60µm, separation efficiency of 100% is achieved. However, for

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ACCEPTED MANUSCRIPT variants I and II, particles with a diameter of 45 μm exhibit nearly 100% collection efficiency, whereas with variant III this difference is quite significant. This finding may prove important

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for the management of clinker burning. When acting as separators, control over the

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granulometric distribution of solid particles is very limited. On the other hand, when used for

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technological processes (as for clinker burning), it is desired to optimize the raw material grinding curve (or to reduce the percentage share of small particles). 3.2 Pressure drop

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Analysis of pressure drop, a second parameter that characterizes energy consumption

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in cyclone separators, shows that the three new designs also proved more beneficial. Each of the three new design variants provides a lower pressure drop (regardless of the assumed optimization objective function) compared to the reference variant. The lowest pressure drop

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is achieved with variant III (778 Pa), the highest pressure drop with variant II (1015 Pa) and an intermediate value (828 Pa) with variant I. The combined effect of the contracting lip of

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the vortex finder followed by a greater inlet angle leads to high pressure drop in cyclone variant II. With the experimental value of 1378 Pa for the existing installation, a reduction of

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39.91%, 26.34% and 43.5% in pressure drop values was achieved with cyclone variants I, II and III, respectively. Cyclones are meant to operate continuously; hence, they extract a large fraction of power. Therefore, with any of the proposed design variants, the savings in power consumption is quite high, especially with the first and third cyclone variants. In any event, such a reduction in pressure drop followed by an increase in collection efficiency has great significance. 3.3 For continuous phase 3.3.1 The mean static pressure Figure 7 shows the static pressure distribution for the studied three variants of cyclone separators. A uniform scale was used for all structures to fully illustrate the differences

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ACCEPTED MANUSCRIPT between the obtained values. A similar pattern of pressure distribution is seen in all variants. The radial profiles of mean static pressure in the three proposed designs are presented in

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Figure 8. The wall pressure is highest for variant II and least for variant III, whereas variant I

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model bears an intermediate value. When moving down along the axis of a cyclone, the wall

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static pressure is nearly the same; however, slight variation in the core region is noticed. Near the geometrical axis, at Z = 3.17m, variant III possesses a lower pressure value than the other two variants, whereas at Z = 5.9m the values are nearly the same. In none of the cases does

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negative pressure (i.e. below the atmospheric pressure) develop in the core region; hence, the

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chances of entrapment of separated out particles is reduced. High wall static pressure is expected to yield higher pressure drop and vice-versa. Therefore, the second variant is likely to yield the highest pressure drop, the third variant the lowest, and the first variant an

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intermediate value. The same has already been confirmed in Sec. 3.2.

3.3.2 Mean tangential velocity

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Figure 9 and Figure 10 present the contour plots and radial profiles of mean tangential velocity, respectively. Tangential velocity is the most important velocity component, as it directly affects the pressure drop and separation capability of cyclone separators. Since pressure and velocity are directly coupled, those cyclone variants possessing higher wall static pressures are expected to set up a stronger tangential velocity field. The same can be depicted from Figures 9 and 10, where the variant II cyclone possesses the highest tangential velocity and variant III the lowest. A similar observation was also made for static pressure distribution. The structural differences between the proposed variants and the obtained results indicate that two of the introduced changes were crucial. The first change, that is, the application of a cylindrical vortex finder (variant III), led to a decrease in the values of tangential velocities

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ACCEPTED MANUSCRIPT near the walls of the cyclone, thereby decreasing both the pressure drop and collection efficiency. The second change was the increased angle of the inlet duct with a tapered vortex

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finder tube that increased both the pressure drop and collection efficiency. However, the third

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variant, with an inlet configuration similar to the first variant and vortex finder configuration

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similar to the second variant, yielded intermediate values for collection efficiency and pressure drop. 3.3.3 Mean axial velocity

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Figures 11 and 12 show the contour plots and radial profiles of mean axial velocities,

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respectively. No noticeable differences in the pattern and distribution of the mean axial velocity are observed, except in the core region where slight differences in the peak at Z = 5.9m are seen. However, the contour plot elucidates much higher values for axial velocity at

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the lower lip of the vortex finder, especially for Variants I and II. The local increase in axial velocity can be attributed to contraction of the vortex finder lip. Furthermore, the inside of the

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vortex finder tube forms a region of low axial velocity, and the same region is also associated with the lowest pressure (cf. Fig. 7).

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3.3.4 Mean velocity magnitude

The obtained values for pressure drop and the distribution of pressure fields are a direct result of velocity distribution inside the cyclone separators. Figure 13 shows the velocity distribution for the studied variants. The highest values occur near the lower part of the vortex finder (for conical finders), whereas in the separation space the profile is similar to that of the tangential velocity. Inside the vortex finder a region of low velocity magnitude is observed; the same has already been implicated for axial velocity and static pressure, and is a subject of further investigation. A clear picture of the velocity distribution inside the vortex finder is presented by the velocity vectors in Figure 14. The flow reversal takes place near the cyclone axis inside the

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ACCEPTED MANUSCRIPT vortex finder, and the fluid flowing downstream passing by that zone attains greater velocities. It is more prominent in case of the second variant followed by the third variant (in

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an increasing order of impact). The flow reversal is more likely be the vortex breakdown, a

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phenomenon that is usually observed in most of the swirl dominating flows encountering an

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abrupt change in cross-sectional areas downstream. Vortex breakdown is a complex phenomenon that is highly sensitive to the geometrical swirl number, flow conditions, and Reynolds number, and it may further lead to the formation of the flow instability in the core

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region of cyclone separators [52]. Such time-dependent instabilities of coherent structures

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(referred to as the precessing vortex core (PVC) phenomenon) develop at high Reynolds number that lead to increased levels of the fluctuations [53, 54]. Due to vortex breakdown, the surrounding fluid is forced toward the inner walls of the vortex finder. This results in higher

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flow velocities near the wall of vortex finder, and hence may lead to higher pressure drops. 4. Conclusions

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This study aimed to demonstrate the benefits of using new, highly efficient designs for cyclone separators for clinker burning. Clinker burning is the most energy-demanding stage

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of cement production, accounting for about 50% of overall energy consumption. Furthermore, considering the scale of cement production, reducing the usage of fuel per unit in clinker burning by even a few percent will have a significant impact. In addition, reducing release (i.e. raw material losses) will help to improve the efficiency of production lines. The application of a modern research method that was based on CFD allowed for analysis and description of multi-phase flow inside cyclone separators with three different designs. In order to implement the guidelines, a mathematical relationship was established that helped to correctly identify the basic design parameter for cyclone separators, that is, the diameter D. This diameter constituted a starting point for the selection of the remaining

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a) The pressure drop in variant I reduces by 39.91%

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c) The pressure drop in variant III reduces by 43.54%

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b) The pressure drop in variant II reduces by 26.34%

Such reductions in pressure drop values are highly significant. The newly-proposed designs of cyclone separators displayed a higher efficiency (i.e. a higher separation efficiency

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and a lower pressure drop) than the currently applied design, regardless of the assumed

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objective function of optimization. Furthermore, the obtained values for the assumed objective function of each design variant were consistent with the guidelines.

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References

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Lime and Magnesium Oxide 2013.

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emissions from the world cement industry, European Commission Joint Research Centre, Report EUR 20769 EN, June 2003. [3] H. Mikulcic, M. Vujanovic, D. K. Fidaros, P. Priesching, I. Minic, R. Tatschl, N. Duic, G. Stefanovic, The application of CFD modelling to support the reduction of CO2 emissions in cement industry, Energy 45 (2012) 464-473. [4] Directive of the European Parliament and of the Council: Integrated Pollution Prevention Control. [5] E. Worell, Ch. Galitsky, Energy efficiency improvement opportunities for cement making, Ernest Orlando Lawrence Berkeley National Laboratory, Report no LBNL 54036, January 2004.

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ACCEPTED MANUSCRIPT [6] J.I. Bhatty, Innovations in Portland Cement Manufacturing, Portland Cement Association 2004.

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[7] T. Santosh, R. Suresh, K.V. Sreenivas, U. S. Malliikrjun, CFD Analysis for design

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[22] M. Bohnet, T. Lorenz, Separation efficiency and pressure drop of cyclones at high temperatures, Gas Cleaning at High Temperatures, Springer, 1993 17–31. [23] M. Bohnet, Influence of the gas temperature on the separation efficiency of aerocyclones, Chem. Eng. Process. 34 (1995) 151–156. [24] P.A. Patterson, R.J. Munz, Cyclone efficiencies at very high temperatures, Can. J. Chem. Eng. 67 (1989) 321–328. [25] J. Gimbun, J. Chuah, T. Fakhru’l-Razi, A. Choong, The influence of temperature and inlet velocity on cyclone pressure drop: a CFD study, Chem. Eng. Process. 44 (2005) 7–12. [26] P.K. Gupta, Prediction of heat transfer coefficient in the cyclone separator of a CFB, Int. J. Energ. Res. 24 (2000) 1065–1079.

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Institution of Chemical Engineers 29 (1951) 356–383.

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performance parameters, J. Hazard. Mater. 182 (2010) 835–841. [33] B. Wang, D.L. Xu, K.W. Chu, A.B. Yu, Numerical study of gas–solid flow in a cyclone

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separator, App. Math. Model. 30 (2006) 1326-1342. [34] K. Elsayed, C. Lacor, Optimization of the cyclone separator geometry for minimum

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pressure drop using mathematical models and CFD simulations, Chem. Eng. Sci. 65 (2010) 6048–6058.

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[39] K. Chu, J. Chen, A. Yu, Applicability of a coarse-grained CFD–DEM model on dense

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medium cyclone, Miner. Eng. 91 (2016) 43-54.

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[41] J.J.H. Houben, S. Pirker, CFD Simulations of Pressure Drop and Velocity Field in a

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Cyclone Separator with Central Vortex Stabilization Rod, J. Appl. Fluid. Mech. 9 (2016) 487499.

[42] J.P. Wu, Y.H. Zhang, H.L. Wang, Numerical study on tangential velocity indicator of

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free vortex in the cyclone, Sep. Purif. Technol. 132 (2014) 541-551. [43] A. Wang, X. Yan, L. Wang, Y. Cao, J. Liu, Effect of cone angles on single-phase flow of

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a laboratory cyclonic-static micro-bubble flotation column: PIV measurement and CFD simulations, Sep. Purif. Technol. 194 (2015) 308-314.

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[44] K. Elsayed, C. Lacor, The effect of the dust outlet geometry on the performance and hydrodynamics of gas cyclones, Comput. Fluids 68 (2012) 134–147. [45] F.J. de Souza, R. de Vasconcelos Salvo, D. de Moro Martins, Simulation of the performance of small cyclone separators through the use of Post Cyclones (PoC) and annular overflow ducts, Sep. Purif. Technol. 142 (2015) 71-82. [46] F.J. de Souza, R. de Vasconcelos Salvo, D. de Moro Martins, Effects of the gas outlet duct length and shape on the performance of cyclone separators, Sep. Purif. Technol. 142 (2015) 90-100. [47] Y. Yang, Ch. Wen, CFD modeling of particle behavior in supersonic flows with strong swirls for gas separation, Sep. Purif. Technol. 174 (2017) 22–28.

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ACCEPTED MANUSCRIPT [48] ANSYS, ANSYS CFX14.0, Solver Theory Documentation, ANSYS, Inc., 2011. [49] H. Safikhani, Modeling and multi-objective Pareto optimization of new cyclone

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separators using CFD, ANNs and NSGA II algorithm, Adv. Powder Technol. 27 (2016)

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2277–2284.

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[50] J.J. H. Houben, Experimental investigations and CFD simulations on particle depositions in gas cyclone separators, PhD thesis, Montanuniversitaet Leoben, 2011. [51] G. Gronald, Experimentelle und numerische Untersuchungen zur Partikelagglomeration

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im Gaszyklon, PhD thesis, Graz University of Technology, 2005.

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[52] T. O’Doherty, A.J. Griffiths, N. Syred, P.J. Bowen, W. Fick, Experimental analysis of rotating instabilities in swirling and cyclonic flows, Dev. Chem. Eng. Mineral Process, 7(3/4) (1999) 245-267.

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[53] J.J. Derksen, H.E.A. Van den Akker, Simulation of vortex core precession in a reverseflow cyclone, AIChE Journal. 46 (2000) 1317–1331.

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[54] L.S. Brar, K. Elsayed, Analysis and optimization of multi-inlet gas cyclones using large

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eddy simulation and artificial neural network, Powder Technol. 311 (2017) 465–483.

List of Figures

Fig. 1. Schematic of different stages of the cyclone used in a suspension preheater [6]. Fig. 2. The characteristic geometrical dimensions of a cyclone. Fig. 3. 3D models of the proposed variants. Fig. 4. Geometry of the first-stage cyclone in the current installation. Fig. 5. Example trajectories of particles with a diameter of 10 μm (a), 20 μm (b) and 30 μm (c). Fig. 6. Comparison of the grade efficiency curves of all the cyclone variants. Fig. 7. Contours of static pressure for the studied variants of cyclone separators.

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Fig. 9. Contours of mean tangential velocity for the studied variants of cyclone separators.

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Fig. 10. Radial profiles of mean tangential velocity for the studied variants of cyclone

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separators at axial locations Z = 5.9m and 3.17m (from left to right), respectively. Fig. 11. Contours of mean axial velocity for the studied variants of cyclone separators. Fig. 12. Radial profiles of mean axial velocity for the studied variants of cyclone separators at

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locations Z = 5.9m and 3.17m (from left to right), respectively.

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Fig. 13. Contours of velocity magnitude for the studied variants of cyclone separators. Fig. 14. Velocity vectors colored with velocity magnitude for the three variants of cyclone

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separators.

List of Tables

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Table 1. Relationships between the geometrical dimensions of first-stage cyclones used for clinker burning according to project guidelines [17].

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Table 2. Dimensions of the characteristic parts of cyclones for variants I, II and III. Table 3. Characteristics of the CFD meshes.

Table 4. Separation efficiency of first-stage cyclones (results obtained based on CFD modeling with a level 3 mesh and measurements of the industrial installation). Table 5. Obtained values of separation efficiency and pressure drop for each design variant.

Vitae

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ACCEPTED MANUSCRIPT Marek Wasilewski After graduating from the Faculty of Mechanical Engineering at Opole University of

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Technology, Marek Wasilewski began work at the university and attained his PhD in 2015.

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His doctoral thesis concerned the optimization of cyclone separators. Currently, he is

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employed as an assistant within the Chair of Innovative Technological Processes. He is the editor of two monographs and the author of 20 papers. His research interests include the separation process, thermal engineering, heat transfer, renewable energy and optimization of

Vitae Lakhbir Singh Brar

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production processes, with a particular focus on the production of cement.

After post-graduation from B.I.T. Mesra in 2008, Lakhbir Singh Brar joined as a Lecturer at D.A.V. Institute of Engineering and Technology, Daltonganj. In 2010, he joined the Department of Mechanical Engineering, and later started his research work at B.I.T. Mesra. His doctoral thesis concerned the analysis and optimization of gas cyclone dust separators. He was awarded Ph.D. degree in 2016, and is continuing with the same Department. His area of research includes gas cyclones, swirling flows and optimization.

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ACCEPTED MANUSCRIPT Table 1. Relationships between the geometrical dimensions of first-stage cyclones used for clinker burning according to project guidelines [17].

1

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re

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αg

Minimisation of pressure drop

0.57 0.29 0.59 0.55 1.67 0.95 2.63 0.14 1.21

0.57 0.29 0.59 0.57 1.67 0.95 2.63 0.14 1

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a/D b/D De/D S/D h/D Hc/D H/D B/D De1 /De2

Maximisation of separation efficiency with the lowest possible pressure drop 0.57 0.29 0.59 0.56 1.67 0.95 2.63 0.14 1.21

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D [m] a [m] b [m] De1 [m] De2 [m] S [m] B [m] h [m] H [m] Hc [m] re [m] αg [⁰]

Variant I Maximisation of separation efficiency with the lowest possible pressure drop 3.35 1.9 0.97 1.98 1.64 1.875 0.46 5.6 8.8 3.17 1.65 180

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Mesh 4

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dp[μm] Variant optimization 45

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ΔP [Pa]

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ACCEPTED MANUSCRIPT Highlights The three highly efficient designs of cyclone separators were proposed

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A higher separation efficiency found for the newly-proposed designs of cyclone

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separators

A lower pressure drop found for the newly-proposed designs of cyclone separators

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The CFD results were similar to those obtained in an industrial installation

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A new method proposed to determine the body diameter D of the first-stage cyclone

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

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