This is long, but it is the theory
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Advances in the Theory and Practice
of Hydrocyclone Technique
Neesse, Th., Donhauser, F.
University of Erlangen – Nuremberg, Germany and
AKW Apparate + Verfahren GmbH & Co. KG, Hirschau/Oberpfalz, Germany
Abstract
With numerical simulation of hydrocyclone separation based on the Navier-Stokes and mass transfer
equations, it is possible to perform hydrocyclone experiments on a computer. With the help of
computational engineering, the cost of conducting hydrocyclone experiments in the planning phase of
new hydrocyclone units can be cut drastically. The simulation models also enable a deeper insight into
the static and dynamic process behaviour in hydrocyclones. It is shown that the solids stored in the
hydrocyclone represents a sensitive process variable for separation in the hydrocyclone. On this basis,
a new control concept for a hydrocyclone battery has been developed. As the solids concentration in
the feed increases, the combined overflow of all the hydrocyclones in the battery is throttled. At the
same time, the feed pump speed is increased. In this way, while the total throughput remains constant,
the volume split changes to effect that more solid material is discharged in the hydrocyclone
underflow. This computer-based process control system is ideally suited for hydrocyclones in
separation plants for tunnel driving projects as well as for those integrated closed-circuit grinding
processes.
Keywords: hydrocyclone, process control, numerical simulation, computational engineering
Introduction
In recent years, alongside the traditional use of hydrocyclones in mineral processing,
new applications, particularly in the field of environmental engineering [1 – 4], have
opened up for these separators. Examples in this context include their use for gypsum
separation in wet flue gas desulphurization processes, in washing plants for
contaminated soils and in separation plants for tunnel driving projects. To ensure that
the potential of hydrocyclone engineering is fully utilized, the optimum hydrocyclone
geometry specific to the respective application, the correct combination of materials
and appropriate operating parameters are essential. Extensive experiments in pilot
scale are therefore required in the planning phase. The effort involved in this process
can be reduced substantially with the help of modern computational engineering. This
paper reports on hydrocyclone experiments performed with a PC, on the basis of
numerical simulation models. These simulation models enable a deeper understanding
of the static and dynamic processes in hydrocyclones. From the results of these
experiments, approaches for the development of a computer-based process control of
hydrocyclone plants can be derived. The paper describes a computer-controlled
battery of 150-mm hydrocyclones, to be integrated in separating plants with widely
varying feed conditions.
Computer-Based Hydrocyclone Experiments
The flow conditions in a hydrocyclone are characterized by a three-dimensional,
turbulent two-phase flow, which has still not been adequately understood and
documented. In the past, analytical models were elaborated based on highly
simplified conditions [5 - 7]. With these models, it is possible (allowing for
turbulence) to describe the separation efficiency in the hydrocyclone in the form of a
separation function. However, although this separation model may be correct on
principle, in individual cases the difference between the measured and calculated
values may be quite considerable. This discrepancy can be attributed to the simplified
model conditions as well as an imperfect understanding of the fundamentals of
hindered particle movement. The latter leads to difficulties particularly in the case of
dense flow separations (high solids concentration). New possibilities for such
applications are afforded by the numerical simulation of hydrocyclone separation on
the basis of the Navier-Stokes and mass transfer equations. The elements of
mathematical modelling in such simulations are the stable and exact discretization of
differential equations while maintaining the essential model properties, and the
elaboration of effective iterative algorithms for two-dimensional time-dependent
processes. With utilization of the facilities for computer visualization, such simulation
calculations can also be represented as hydrocyclone experiments on a PC. The
theoretical basis for numerical modelling of hydrocyclone separation will not be
discussed further in this paper as it has already be dealt with extensively in various
other publications [8 - 10]. For the calculations, the material data, the hydrocyclone
geometry and the operating parameters must be specified as the input values. As a
result of the simulation experiments a complete balance of the separation process is
obtained, i.e. the throughput, the volume split, the separation function as well as any
parameters derived from these results.
Moreover numerical simulation can allow a deeper insight into the static and dynamic
process behaviour of hydrocyclones and thus provide a basis for the development of
new automatic process control strategies. In particular, for every point in the
hydrocyclone, these models are able to yield the velocity of the fluid and the particles
and the particle concentrations for each size class. From the example shown in Fig. 1,
it can be seen that, as a function of the solids concentration and particle size
distribution of the feed, a specific spatial concentration distribution is established in
equilibrium conditions for each particle size class. At low feed concentrations (dilute
flow separation), the particle sizes examined are largely separated at the walls of the
hydrocyclones. With rising concentration of the feed, pronounced radial
concentration distributions develop, in which the particles are forced increasingly
towards the hydrocyclone axis and therefore in the direction of the overflow (Fig. 2).
If the path of the individual particles is traced accordingly, the diagram plotted in
Fig. 3 results. The figure shows that, as the feed concentration rises, the particles in
the cut-point range demonstrate an increasing tendency towards large-area
convection. Depending on their size, however, the particles may travel on these
vortex paths for considerable residence times.
Figure 1: Particles concentration of different fractions in the hydrocyclone
Inlet particles concentration 230 g/l
Figure 2: Particles concentration in the hydrocyclone of the fraction dp = 50 µm
Figure 3: Particles trajectories dp = 55 mm
Inlet solid concentration: a – 20 g/l, b –230 g/l, c– 370 g/l, d – 530 g/l
In response to feed conditions that vary over time, the hydrocyclone demonstrates a
very sensitive dynamic behaviour, i.e. in such cases, the time dependence of the
spatial concentration distributions must also be considered. In Fig. 4, the timedependent
development of the concentration distribution is plotted for a leap in the
concentration at the feed inlet from 0 to 200 g/l. These model calculations show that
the spatial solids distribution or the particle mass stored in the hydrocyclone
represents a characteristic process variable, which can be used for control purposes.
This concept is explored further in the following section.
Figure 4:Hydrodynamik und Separation im Hydrozyklon
t=0.125s t=0.625s t=1.25s t=1.875s t=2.5s
dp=3.1µm
Computer-Controlled Hydrocyclone Battery
An important function of a process control system for a hydrocyclone plant is the
stabilization of the cut-point or slurry density in the overflow respectively for feeds
with varying solid concentrations and particle size compositions. Appropriate
regulation of the volume split has so far been restricted to single hydrocyclones of
larger diameters in which adjustable underflow nozzles have been fitted. For smaller
hydrocyclones, interconnected in batteries, this concept is not feasible. Moreover, the
following two premises must be created:
- measurable process variables for the characterization of separation in the
hydrocyclone,
- regulated quantities for the process, without manipulation of the individual
hydrocyclone
These criteria are met by a new control concept [11], which is shown in the schematic
in Fig. 5. The process variables are simply determined at one measurement
hydrocyclone within the battery of parallel-connected single hydrocyclones. One
measurable value in this case is the mass of the solids stored in the hydrocyclone. The
solids mass is determined by means of a special fastening means of the cyclone with a
gravimetric measuring cell (1). Another measurable variable is the form of the
underflow discharge (rope or fan-shaped discharge). These two process values are
entered, together with the values for the power input (2) of the feed pump, pressure of
the feed into the hydrocyclone and the pressure drop (3) over the hydrocyclone, into
the process control computer. A throttle valve (4) for control of the combined
overflow of all hydrocyclones and the feed pump speed (5) serve as regulated
quantities. With increased throttling of the overflow, the volume split is changed to
the effect that the solids discharge in the underflow is intensified. To stabilize the
total throughput, at the same time the feed pump delivery rate is increased to the
extent that the pressure drop over the hydrocyclone ∆p remains approximately
constant. However, this causes the pressure inside the hydrocyclone to build up,
dp=54.8µm
which results in a further intensification of the (unthrottled) underflow discharge. The
control concept is oriented towards achieving the optimum operating state at the
transition from rope to fan-shaped discharge. For this purpose, the shape of the
discharge column of the underflow is monitored by means of a capacitive probe. The
shape of the discharge flow (i.e. rope or fan) is influenced by the concentration
distribution or the mass of the solids stored inside the cyclone, i.e. this signal of the
capacitive (6) probe corresponds to the mass of solids stored in the hydrocyclone.
Figure 5: Principle of the hydrocyclone regulation
With this control system implemented in a 150-mm hydrocyclone, it is possible to
stabilize separation for varying solid concentrations up to 500 g/l at a cut-point of
approximately 30 µm (Fig. 6) or a slurry density of 1.1 kg/m3 in the overflow.
Selected technological values of this 150-mm hydrocyclone separation process are
listed in Table 1. The table shows that, at a feed suspension of the particle size range
< 2 mm and feed concentrations of up to 500 g/l, a maximum solids recovery per
cyclone in the underflow of up to 79 % can be achieved. This represents such an
improvement in the discharge capacity of the 150-mm hydrocyclone that the
installation of a preliminary cyclone of a larger diameter is no longer necessary. With
such a computer-controlled process, it is also possible to replace a two-stage
hydrocyclone circuit with a single-stage plant. Other benefits are derived from the
possibility of a remote control of the hydrocyclone plant on the basis of remote data
transmission. The control concept was initially developed for separation plants for
tunnel driving projects, where the solids concentrations can vary over very wide
ranges between 50 and 500 g/l, with a corresponding solids discharge between
virtually 0 and 16 t/h per 150-mm hydrocyclone. The concept can be implemented for
hydrocyclone batteries or for large-sized single hydrocyclones if, for example,
especially high separation efficiency is required despite wide variations in the feed
parameters. In particular, this concept is to be further pursued with regard to
developing suitable control systems for hydrocyclones integrated in closed grinding
circuits.
Figure 6:Separation curves of the 150 mm-hydrocyclone with or without throttling (solid feed
concentration 420 g/l)
0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
1
1,00 10,00 100,00 1000,00
Particle diameter d [µm]
Separation function T [-]
Overflow throttled
Overflow open Throttling
Table 1: Control of a 150-mm hydrocyclone, Di = 50, DO = 72 mm, DU = 29 mm, with a solids
concentration of 550 g/l (< 2mm) in the feed
Feed pump speed
min-1
Counterpressure
in the overflow p1
bar
(throttling)
Split
V VO
/
%
Solids recovery in
the underflow
t/h
1080
1140
1200
1260
1310
0
0.10
0.25
0.30
0.45
79
78
74
73
68
54
54
65
70
79
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