|
ABSTRACT
A lot of tests have been conducted to assess the influence of the length-to-diameter ratio of the screen basket.
This ratio has a great influence on the screening efficiency and on the fibre passage ratio. A short basket makes the passage of fibres easier while it reduces the screening efficiency. We have then tested various
means, which reactivates the pulp to be screened so as to be able to use a longer basket for a better screening efficiency with the fibre passage factor of a short basket.
The ID2 means increases the screen capacity by 50% at the same energy consumption and with a better screening
efficiency. This is obtained thanks to an optimal use of the whole screening area of the basket. Using this concept, we have developed: a new tailing screen and a new screen which is a screening system by itself
with ultra fine slots (0.15 to 0.08).
The paper includes the latest results in tissue mills like "Cartiera Lucchese" in Italy.
1. Introduction
After the Black Clawson Stock Preparation Division (now called KADANT BC-Lamort) acquisition by Thermo Fibretek
(now KADANT Inc), the decision was taken to develop a range of screens for the future.
The R&D program started at that time has leaded to the perfecting of a new screening technology.
2. Intermediate Defloculation Device
A very important point has been the choice for the screen/cylinder design of future screens. This design is quantifiable with the ratio of the length (L) compared to the diameter (D) or
'L/D ratio.
As we already had the rigid cage screen cylinder design RCR, which allows the mechanical
assembly of 20 mm high slotted rings in a rigid cage, it has been quite easy to modify the ratio L/D by replacing slotted rings with blind rings.
According the number of rings, we have tried to get out the maximum possible capacity by keeping the same reject rate in volume. The two curves (2,3 % and 3,3 % consistency) are
quite different and those differences lead to interesting conclusions.
The chart shows the increased capacity of the cylinder by the addition of slotted rings. Where only 2/3 of the ring is effective.
For the pulp at 2,3 % consistency, the added production per slotted ring is almost constant
from the beginning till approx. 2/3 of the screen cylinder, and then decreases drastically. Alternatively, for pulp at 3,3 % consistency, the added production is poor at the beginning,
then increases and finally drops off at the end of the screen cylinder.
From these trials, we can draw the following conclusions:
Pulp Defloculation
- Intensive defloculation is generated by high-speed difference between rotor and pulp.
- It is well known that higher consistency requires higher energy to achieve stock
defloculation. If we assume that the high consistency stock enters the screen more flocculated than stock at lower consistency, the defloculation action of the rotor can
therefore explain the production increase of the high consistency pulp after the third slotted ring.
Screen cylinder capacity
- Increasing the cylinder length does not increase the cylinder capacity after a certain
distance.
- A high inlet consistency leads surprisingly to a low capacity at the cylinder entrance
and to a higher capacity close to the end of the cylinder.
- Any device able to defloculate the stock during the screening should improve the
capacity at the end of the cylinder.
- The flow velocity of the stock through the slots all over the cylinder length is
certainly very variable, in spite of several authors' assessments. This is due to the rotor action (rotating speed and defloculation) and due to throughput rate of the
fibres (progressive thickening of the pulp along the cylinder length and flocculation).
Screening efficiency
- Increasing the cylinder length does not increase the screen capacity after a certain
point. But, in spite of an almost constant production, the efficiency increases.
- This identifies the need for long screen cylinders to be more efficient and the need
for devices to maximise capacity for the entire cylinder length.
However, we cannot assume an additional production per added ring as the passing velocity varies within the basket
The additional capacity in the middle zone of the cylinder identified in Figure four would not influence the first portion of the basket. But, as the screen reject rate by volume remains
constant, whatever degree of increase, of the screen overall production by the added slotted rings, increases the reject rate of all the slotted rings already installed. Which will
normally lead to an increase of production for the added rings.
This graph shows that the longer the cylinder the more difficulties the fibres have to pass the slots. This means a flow speed reduction as well as a throughput reduction. As a
consequence of this low fibre throughput, the reject consistency will gradually increase. If this reject consistency becomes too high, the screen will gradually (and even totally) plug.
Therefore for those two process parameters a short screen cylinder performs better than a long one.
The graph shows clearly that a long screen cylinder has greater efficiency than a short cylinder, both for the same flow speed or for the same accepts flow!
According to the mill's particular requirement, the screen cylinder will either be short to
operate with minimal fractionation and low rejects or long for a better efficiency.
The actual trend in the market is more oriented to have short screen cylinders in order to
improve the fibre throughput to the detriment of the efficiency, which can be maximised by using very fine slots (0,15 to 0,08 mm).
The solution was to achieve better efficiency by increasing the fibre throughput with a long
screen cylinder. To achieve this objective, is to use the potential energy of the stock for its own defloculation.
Putting obstacles (baffle bars) on the screen cylinder surface implies a compromise between the obstacle efficiency and the cleaning effect of the rotor. If the size of the
obstacle is increased to get a better defloculation effect, the rotor foil/cylinder clearance will also be increased, thus reducing the foil cleaning effect. Another disadvantage is that,
much higher energy is required across the total screen cylinder without being really necessary everywhere.
Due to these considerations, we have designed a defloculation device in a cavity located inside the screen cylinder. A deflector on the rotor directs into this cavity. This is called
the " Intermediate De-flocculation Device". or ID2.
The intermediate defloculation device is complimented by a 'closed' drum rotor, having a deflector located adjacent to the screen cylinder cavity and solid welded foils.
The actual results were better than expected:
- For the same production and same reject rate by volume, the thickening factor is drastically reduced. Even greater reduction when reject rate by weight is considered.
- Maximum capacity is doubled with the same efficiency.
- At the same rotor speed, power increases by 33 % when the maximum capacity is doubled.
- For equal power, rotor speed is reduced by 10 % when production increases by 50 %.
- For equal production, rotor speed is reduced by 20 % when power is reduced by 33 %
.
It appears a difficult statement to make that 'for doubled capacity' the efficiency remains the same. However this is due to a better use of the total screen cylinder surface, as well
as a reduced screen cylinder area being in contact with a thicker highly contaminated stock. In fact this figure showing the production evolution compared to the screen cylinder
length, we see the immediate impact of the intermediate defloculation device, which starts a new capacity (at same efficiency) in the last part of the screen cylinder.
This shows what happens to the flow speed along the screen cylinder length. We see that the average flow speed is increased with the INTERMEDIATE DEFLOCULATION DEVICE, and
also that the screen cylinder surface in contact with highly concentrated stock and with low capacity is reduced.
At same average flow speed average (or same capacity), we can see a big reduction of the flow velocity in the first screen cylinder section and always a reduced screen cylinder
surface in contact with highly concentrated 'thick' stock and low production. Those are the two important considerations for improvements to the efficiency.
3. Retrofit intermediate defloculation device
An intermediate defloculation device retrofit consists of a new rotor and screen cylinder
and to subsequently adjust the rotor speed accordingly to suit the particular application. This is possible on all our companies' centrifugal screens. It is also possible on centrifugal
screens from other OEM's, and payback is very attractive when these factors are considered:
- Energy savings
- Losses reduced
- Capacity increased
- Slot size or hole size reduction.
In addition to these, a retrofit can often replace the investment required for a new
screening line with all the inherent problems and costs:
- Installation, subsequent shut down
- Civil engineering, piping, electrical connections, control
- Longer start up
- Very high capital costs.
4. Intermediate Defloculation & Dilution Device
The agitation/Defloculation achieved by the pulp passing into the intermediate defloculation device can be further be utilised by the addition of dilution water into this zone.
Therefore due to this device, there are two separate accept sections created, which can
be independently adjusted. This then facilitates the fine-tuning of the reject rates of the first and the second section, and the concept is the basis of the FiberNET.
5. FiberNET
This screen has been developed as a reject screen for systems with holes or slots that operate with a continuous reject flow. The overall reject rate is greatly reduced due to the
intermediate dilution. (ie, the second section is working as a fibre recovery section), and the ID3 device utilises the total cylinder area.
Before commercial release the prototype was subjected to numerous trials with excellent results. As an average, reject rates of around 5% for white grades and 9% for brown
grades were achieved. Using fine slots results in very high efficiency, which makes it possible to take the screen, accepts forward to the stock preparation system.
Further to the trials a case study was carried out at "Everbal" mill with the main objective being to reduce fibre losses. A FiberNET size 5 has been installed to handle the 3rd stage
rejects of the slotted screening and accepts of the vibrating screen handling rejects of the holes screening.
The results are in accordance with those of the prototype. The reject rate by weight is
about 4,8 %, meaning a recovery of 95 % of the treated pulp. The white water consumption to clean the fibres is in a range of 22 cubic meters per ton. The power
consumption (14,7 kWh/t) is a little bit high compared to the prototype due to a constant speed (not variable) to be able for the screen to handle the worse operating conditions.
6. Incorporating primary secondary and tertiary screening into one: introducing the ScreenONE
The screen has been designed as a compact complete screening system with fine slots. In order to reduce the reject rate, we have inserted two intermediate defloculation and
dilution zones with a targeted overall reject rate of 2 % for the screen.
In order to reduce the development cycle, we have decided to build it by using the
mechanical base of the "Ultra V" screens, reputed world wide as very strong reliable screens.
The results of the trials made in our R&D facility are summarised in the previous chart.
The reject rate is very low for the white grades (0.5%), 2% for European OCC and a little higher for the American OCC (3.1%). The specific energy is low for white grades (10
.2kWh/t) and increases for brown grades (14kWh/t for AOCC).
A case study at "Papeteries de la Seine" belonging to the Smurfit – Socar Group produced the expected results with those made in our research centre. The specific energy and the
reject rate are slightly higher, probably due to higher consistency and maybe higher stickies and contaminants level.
Subsequent studies at "Papeteries du Limousin" belonging to the Smurfit – Socar Group have revealed results in their stock preparation line.
By "Cartiera Lucchese" in Italy, we have tested the ScreenONE prototype under severe operating conditions. The consistency is very high (over 5 %) and the flows are much
higher compared to those achieved in our research centre. This has been achieved due to the very low specific energy required on this pulp (between 6 and 7 kWh/t) and
demonstrated clearly the very high production potential of the intermediate defloculation device with and without the dilution stage.
Subsequent to this performance, the 3rd stage screening section was fitted with a 0.10mm wedge-wire screen cylinder. As a result the production, specific energy and the reject rate
have not been affected by this change. However the 3rd screen section cleanliness has been tremendously improved.
7. Conclusion
The first industrial applications for our RetroFIT INTERMEDIATE DEFLOCULATION DEVICE are
on going in KADANT equipment. This process is becoming very popular within Europe and is definitively the new screening process standard in North America and hopefully continued in Asia.
After numerous very satisfying industrial trials, we are now marketing the FiberNET as a:
- Last stage screen
- Unique screen for small production lines.
(Model size 3 and 5 are presently available)
Also due to interest with our ScreenOne we have started the commercial marketing and
production of the following four sizes.
- A : equivalent to the prototype
- B : 1,5 times the prototype
- C : 2,1 times the prototype
- D : 3,0 time the prototype
We maintain that the ID2/ID3 technology will allow the paper industry to improve globally
the stock screening and will enable lower grades and greater quantities of recycled waste to be utilised, and also reduce investment costs and power consumption.
BACK TO TOP
|
