Modeling Dust Formation in Lime Kilns (original) (raw)

Modelling of particle mixing and segregation in the transverse plane of a rotary kiln

Chemical Engineering Science, 1996

Thorough mixing of particles in the transverse plane of a rotary kiln or drier is essential to the uniform heating or cooling of the charge and ultimately, to the generation of a homogeneous product. However, differences in particle size and density result in a de-mixing process whereby smaller or denser particles segregate to form an inner core or kidney of segregated material which may never reach the bed surface to be exposed to freeboard temperatures. A model has been developed to predict the preferential movement of particles in the shearing active layer. This model determines the extent of fine particle segregation and is based on the principle of percolation in the active layer, whereby fines sift through the matrix of the bed to form the segregated core. Incorporating results of a granular flow model developed for this purpose, it has been possible to establish the dimensions of the segregated core as well as fines (jetsam) concentration in the rest of the bed cross-section. This result is necessary in assessing the effect of segregation on bed temperature nonuniformities in rotary kilns.

Characterization of a dusting lime kiln û€“ A mill study OPEN ACCESS

Nordic Pulp and Paper Research Journal

The white liquor preparation is one of the main purposes of the recovery cycle in a kraft pulp mill and the lime kiln, being the single unit operation handling bulk solid materials, poses particular challenges in the recovery cycle. In this study, the influence of a dusting lime kiln on the preparation of white liquor was studied. The investigation showed that the dusting lime kiln produced a burned lime with a substantial fraction of fines at the expense of the fraction of granules, whereas a clear lime kiln had virtually no fines and more granules instead. Burned lime from both dusting and clear periods were collected and separated into fines and granules. The different fractions were slaked and causticized in a synthetic green liquor. The study found that the fines always delivered a lower effective alkali in the resulting white liquor, and that the particle sizes of the lime mud were always larger than those originating from the granules, implying that the fines were sintered harder. The industrial consequence is that a dusting lime kiln increases the load on the kiln because the yield of effective alkali is lower; it also introduces a greater dead load into the liquor cycle.

Diffusion effects in rotating rotary kilns

A novel approach to the modeling of mass transfer in rotary kilns has been described (Heydenrych et al, 2001). It considers the mass transfer to occur by the inclusion of gas in the interparticle voids in between the particles that move concentrically with the kiln. By doing so, the rate of mass transfer was found to be dependent on bed fill and the ratio of reaction rate constant to angular velocity (k/ ). The model was found to be valid at slow to medium fast reactions. For fast reactions it under-predicted mass transfer. Therefore in this paper, the model will be extended to include diffusion effects. An additional dimensionless number is necessary then to describe the system. This can either be a Peclet number (R2/De) or a Thiele modulus (kR2/De)1/2. The solution of the 2-dimensional partial differential equations that describe the extended model gives a handle on the effect of scale-up in rotary kilns. For industrial-scale kilns, the Peclet number is large, which means that dif...

A review on kiln system modeling

Renewable and Sustainable Energy Reviews, 2011

The purpose of this study is to evaluate performance cement of production and cement kiln. The design of energy efficient dryers employing heat pump systems and the dynamic response of the product to the kiln conditions must take into account. In this paper, the formulation of a dynamic and kiln-wide drying model is described. The model predictions have been verified by comparing them with the published experimental data. The model is then used to simulate performance of three industrial kilns. Numerical experiments are carried out to investigate influence of key operating and design parameters on energy consumption of kilns. The model is also used to explore the possibility of manipulating temperature profile within the kiln to reduce energy consumption per tonne of clinker. Cement kiln dust is a finegrained material produced during the manufacture of cement. At present reuse option is limited and the bulk of cement kiln dust that is not reused in the cement manufacturing process is sent to landfills or stored on-site. Due to the calcium oxide (CaO) content of cement kiln dust, it has the potential to be used as a replacement for lime in treating acidic wastewaters such as acid rock drainage. Slaking of two of the cement kiln dust samples with the highest free lime contents (e.g., 34% and 37% free of CaO) gave elevated pH values statistically comparable to those of the commercial quicklime sample that was characterized as having 87% of CaO. Acid neutralization trials indicate that cement kiln dust samples with low free lime contents could be effective at neutralizing acidic wastewaters.

Experimental investigation of the axial discharging velocity of particles from rotary kilns

Granular Matter, 2011

The axial discharging velocity of the particles was experimentally investigated at the discharge end of two rotary kilns with 400 and 250 mm internal diameter, respectively. Sand, glass beads and clinker were used as experimental materials. A physical method without using imaging technology was developed to measure the axial discharging velocity based on the discharge behavior. The mass flow and the rotational speed of kiln were changed in a wide range. The axial discharging velocity can be directly determined by the volume flow rate of solid bed divided by the whole cross section area of the bed at the kiln discharge end. The axial discharge velocity can be nominally expressed as the function of the axial transport velocity, if the kiln is fully loaded by the materials and the radial rotational velocity of the cylinder. Clinker among other two materials presents a higher difference of the extreme discharge velocities because of the wider particle size difference.

A pyrolysis rotary kiln dynamic model including bed flow properties variations

A global dynamic model of a pilot-scale rotary kiln has been developed. This model can be divided in three parts with coupling terms: a solid phase motion part, a thermal part and a chemistry part. The originality of the presented model consists in this sub-model. Generally, the solid motion models deal with a solid with constant properties all along the kiln. This assumption is not acceptable for thermal treatment units and particularly for pyrolysis units.

Lime Shaft Kilns: Modeling and Simulation

This study presents a mathematical model to predict the heat transfer in a lime kiln. We assume the reaction is not well-stirred. We prove the existence of unique solution of the timedependent problems. We also examine the properties of solution under certain conditions. The time-dependent temperature profiles are obtained through analytical method. It is discovered that to increase the furnace productivity depends on the parameters involved.

Wood chips flow in a rotary kiln: Experiments and modeling

Chemical Engineering Research and Design, 2015

Rotary kilns are well suited for processing woody biomass by torrefaction to produce bioenergy. One of the key issues for process design lies in the flow pattern modeling. The Saeman model is classically used to predict the mean residence time (MRT) and the bed depth profile of powder materials in rotary kilns. Its ability to describe wood chips flow arises. In the present study, residence time distribution (RTD) experiments are conducted with raw and torrefied wood chips. Effects of operating parameterskiln slope, rotational speed and inlet flow-rateon the average residence time, the variance and the mean solid holdup are discussed. A plug flow with small extent of dispersion is emphasized, even if some segregation phenomena are highlighted. Torrefaction did not evidence any significant influence on the flow pattern. With a discrepancy of 20 % between the measured and computed mean residence time, the predictive capacity of the classical Seaman model proved to be insufficient. The model is adapted to predict accurately the load profile and the mean residence time of particles with parallelepiped form. The discrepancy between experimental and calculated results is so reduced from 20 to 5 % for the MRT and from 25 to 5 % for the mean solid holdup .

Mass transfer in rolling rotary kilns: a novel approach

Chemical Engineering Science, 2002

A novel approach to modeling mass transfer in rotary kilns or rotating cylinders is explored. The movement of gas in the interparticle voids in the bed of the kiln is considered, where particles move concentrically with the geometry of the kiln and gas is entrained by these particles. The approach considers a di erential section along the length of a rotary kiln where the gas concentration in the freeboard is assumed to be uniform in that section. A reactor modelling approach has been used to derive e ectiveness factors for the bed as a function of bed ÿll, reaction kinetics and rotation speed. In many cases, the entrained gas becomes depleted within the bed, leading to a simpliÿed model for the bed e ectiveness factor. Experimental data conÿrms the validity of this model for slower rates. At faster rates, mass transfer can be much higher than the model predicts, indicating that other mechanisms, such as dispersion or di usion are also important in these conditions. ?