DYNAMIC MODEL OF A DISC FILTER
Jouni Savolainen, Sami Tuuri and Kaj Juslin
VTT Automation, P.O. Box 1301, FIN-02044 VTT Finland
Jouni.Savolainen@vtt.fi
1. Abstract
The purpose of this paper is to describe the design and implementation of a dynamic
simulation model of a rotary disc filter to the APMS simulation environment. The
construction and operation of a disc filter is described. The APMS simulation platform
is introduced. The calculation principles and methods used in the model are described.
The test runs and their results as well as the conclusions are presented. The model was
found to respond in a qualitatively correct manner to feed flow and consistency
disturbances.
2. Introduction
The present work deals with the modeling and simulation of a rotary disc filter. The
model is based on empirical operating curves, the so called leaf-test curves, and
mechanistical balance equations. The model was built on the APMS simulation
software developed by The Technical Research Centre of Finland (VTT). The aim of
the work was to model the disc filter so that the relevant dynamic properties can be
simulated. This work concentrated in modeling such disc filters that are used in the pulp
and papermaking industry. /1/
3. The Disc Filter
A disc filter is a continuously operating rotary filter. The basic construction of a disc
filter includes the filtration discs, a horizontal vat, a center shaft and piping. The filter
discs are attached to the hollow center shaft and they rotate partly submerged in the vat.
Each disc is divided into several sectors. At one end of the center shaft there is motor
which turns the shaft and consequently the disc at a rate of usually less than 2 r/min. At
the other end of the filter the center shaft is connect to the filtrate output piping, which
usually are drop legs. The construction of a typical rotary disc filter according to
Wakeman et al. /4/ is depicted in Figure 1.
Figure 1 Typical construction of disc filter /3/
During the operation of the filter each sector in a disc is submerged in the liquid part of
the time. During this time the actual cake formation and filtration is carried out. The
cake is formed on the outside surface of the hollow discs. Usually there are two or three
filtrate flows depending on the construction of the device. These are named cloudy,
clear and super clear corresponding to their solids content. The division of the filtrates is
accomplished by a special valve fitting at the end of the center shaft. With this valve it
is also possible to alter the relative amounts of the filtrates.
The driving force for the filtration can come from different sources. The first and most
commonly used driving force is vacuum applied to the inside of the discs. This vacuum
can be attained for example by drop legs. The second possible driving force used is
overpressure on the outside of the discs ie. in the vat, which then is sealed from the
atmospheric pressure.
When a sector of a rotating disc rises above the liquid level the vacuum is turned off by
the filtrate valve and the filter cake is removed either by water showers or by scrapers.
After this the disc sector may washed and then it starts the cycle again.
In the papermaking industry disc filters are used mainly for two purposes. The first use
is as a saveall filter in the fiber recovery system of a paper machine. There its purpose is
to recover valuable fibers and other solids from the white water coming from the paper
machine's wire pit. The recovered fibers are then returned into the mixing chest in the
approach system of the machine as raw material for paper. The filtrates are lead to
different places according to their solids content. The cloudy filtrate is mainly recycled
straight back to the feed of the disc filter. Part of the clear filtrate is used as shower
water in the disc filter itself and part of it is pumped to the showers of the paper
machine and elsewhere in the mill. The second use of a disc filter is as a thickener in the
broke handling of the paper machine. The purpose of the broke handling system is to
treat the broke so that it can be reused in the paper making process without runnability
or paper quality problems at the paper machine. The broke from the pulpers is thickened
in the broke thickener (disc or drum filter) from about 2.5-3.5 % to 4.0-5.0 % before
being pumped to the mixing chest.
4. APMS Simulation Software
The APMS (Advanced Pulp and Paper Mill Simulator) is an extension to the APROS
(Advanced PROcess Simulator). APROS is a dynamic simulation software developed
jointly by VTT and Fortum Engineering Ltd /2/. With APROS it is possible to simulate
a variety full-scale industrial processes and their automation. Using different unit model
packages APROS can used to simulate nuclear power plants, combustion power plants,
pulp mills and paper mills. The last two constitute APMS. Currently APMS includes
models for most of the paper mill unit operations /3/. The pulp mill models are under
development. The models are configured with a graphical user interface by placing units
on the screen, connecting them and filling in properties with dialogue windows. With
APMS it possible to analyse both transient and steady state situations. The models can
be saved into a snapshot file which contains all the information concerning the model
and its current state. The snapshot files can be read back to simulation software in case
the user wishes to revert back to a previous situation.
5. Calculation principles and implementation of the disc filter model
The disc filter was modeled using empirical operational curves, so called leaf-test
curves. These curves describe the drainage capacity of the filter and the quality of a
filtrate as a function of the rotation speed and the consistency (percent of dry solids in
the suspension) in the filter vat. An example set of operational curves is shown in
Figure 2.
Figure 2 Operational curves of a disc filter
In the model the operational curves were approximated by a straight line through the
origin and a nominal operating point which is calculated from the values specified by
the user at the time of adding a new disc filter model into the simulation experiment.
The values the user must specify are the nominal flows and concentrations of the
filtrates as well as the feed. Also the nominal rotation speed must be defined. During the
simulation the filtrate mass flow is calculated using the linearized operational curves
plus a term for the pressure difference over the filter cake. The calculation of the
concentrations of the filtrate flows uses only the operational curves. The recovered fiber
mass flow and concentrations are then calculated from mass balances.
6. Test runs and results
6.1 The test model
The test model which is depicted in figure 3 included the disc filter, three filtrate tanks,
a recovered fiber tank and the pipe lines including pumps and control valves. The
process component connections in the model were made to resemble those of the fiber
recovery system of a paper machine.
There are three streams in the feed line which are mixed before the filter. The main flow
comes from the wire pit of the paper machine and is called white water. Part of the
recovered fiber is mixed, as the so called sweetener stock, with the white water. In real
life this is done in order to enhance the filtration properties of the stock in the disc filter.
The sweetener need not necessarily be the recovered fiber but other stocks will also do.
The third stream in the feed side of the filter is the recycle stream from the cloudy filrate
tank. There is also a third recycle stream in the process which recycles part of the clear
filtrate to the mat removal and disc washing showers of the filter.
The length of the filter vat was 8.0 m and it's diameter 4.0 m giving a total volume of
100m3 where as the filtrate tanks and the recovered fiber tank had each a diameter of
1.13 m and height of 5.0 m giving a volume of 5.0 m3. In the disc filter the filtrates
where divided in the ratio: 40% cloudy, 45% clear and 15% super clear. In addition to
the feed, filtrate and fiber recovery piping an overflow pipe was connected to the filter.
The height of the overflow from the filter bottom was set to 3.5 m. There were
overflows from all the other tanks as well, all of them set to 4.5 m.
In the model there were also two control loops. The first control loop was used to
control the liquid level of the disc filter by manipulating its rotation speed. The liquid
level setpoint was 3.0 m. The second control loop was used to control the flow of the
sweetener stock which is mixed with the white water. The sweetener flow setpoint was
10.0 kgs-1.
The model was tested by making disturbances to the white water feed flow and
consistency and by recording the responses in the filtrate mass flows and consistencies,
in the liquid level of the vat and in the rotation speed of the filter. In the test experiment
the flowing substances were water and one fiber component although APMS does
support multiple components in the flow network.
Figure 3 The test model
6.2 Feed flow increase
In the first test run the feed mass flow was abruptly increased from 178 kgs -1 to 326 kgs1
. This was accomplished by increasing the feed pressure from 420 kPa to 500 kPa. The
behaviour of the liquid level after the feed mass flow is depicted in the primary axis of
figure 4. In the figure the rotation speed is shown on the secondary axis.
Figure 4 Effect of feed mass flow increase on the liquid level
First the liquid level starts to increase but as its value deviates from the setpoint of 3.0
m the level controller starts to increase the rotation speed of the filter. In accordance to
the operational curves of figure 2 the filtrate mass flows increase in value as shown in
figure 5. This in turn returns the liquid level back to the setpoint value.
Figure 5 Filtrate mass flows
The effect of the feed mass flow on the consistencies in the disc filter vat and in the
cloudy filtrate flow are shown in figure 6.
Figure 6 Consistencies of the vat and cloudy filtrate
The behaviour of the consistency trends can be explained using the operational curves
of the filter. Because the level controller starts to increase the filter's rotation speed the
quality of the filtrates decreases ie. their consistencies increase. This is due to the fact
that the cake forming and filtration time decrease and the filtrate cake does not retain
the solids as effectively as with a lower rotation speed.
The increase in the vat consistency is due to the recycle stream from the recovered fiber
tank to the feed of the filter. The consistency of the vat does not exhibit such an abrupt
increase as the cloudy filtrate consistency because of the additional mixing in the fiber
tank.
6.3 Feed consistency increase
In the second test run the white water consistency was increased in step-like manner
from 0.4 % to 0.8 %. This caused a gradual increase in the consistency of the filter vat
which then affected both the filtrate flows and their consistencies. Also the consistency
of the recovered fiber increased. The effect of the disturbance on the vat and fiber
consistencies is shown in figure 7. The feed and vat consistencies are on the left vertical
axis and the recovered fiber consistency on the right vertical axis.
Figure 7 Feed and vat consistencies
First the feed consistency increases rapidly and then smooths down. After a while it
starts to increase gradually due to the increase in the consistency of the recycled fiber.
Also the filtrate consistencies increase which add to the feed consistency via recycle
streams.
The mass flows of the filtrates decrease when the consistency of the feed flow increases
as indicated by the operational curves of figure 2. This decrease in the flows causes the
liquid level in the vat to rise. The level is then returned back to its setpoint by the level
controller by increasing the rotation speed. The rotation speed evens out when the
setpoint is again reached causing the filtrate mass flows to stabilize. These effects are
shown in figures 8 and 9.
Figure 8 Filtrate mass flows
Figure 9 Liquid level and rotation speed
7. Conclusions
The unit model of a disc filter developed here was integrated as a part of the
APROS/APMS system and tested there. In the tests two step changes were made to the
input of the process and the dynamic behaviour of the system was monitored. From the
tests conducted it can be deduced that the system gives qualitatively correct responses in
all simulated cases. To get also quantitatively correct responses the model is possible to
tune to equipment specific industrial measurement.
8. References
1. Savolainen, J., Dynamic model of a disc filter, Master's Thesis, Department of
Chemical Technology, Helsinki University of Technology, 1999, Espoo, 76+24 p.
(In Finnish)
2. Silvennoinen, E., Juslin, K., Hänninen, M., Tiihonen, O., Kurki, J., Porkholm, K.,
The APROS software for process simulation and model development, VTT Research
Reports 618, Technical Research Centre of Finland, Espoo 1989, 106 p.
3. Niemenmaa, A., Lappalainen, J., Laukkanen, I., Juslin, K., An Advanced Platform
for Dynamic Simulation of Paper and Board Mills, SPCI '96.
4. Wakeman, R.J., Tarleton, E.S., Filtration Equipment Selection Modelling and
Process Simulation, Elsevier Advanced Technology, Oxford 1999, 446 p.