WO1993025752A1

WO1993025752A1 - Screening of pulp

Info

Publication number: WO1993025752A1
Application number: PCT/SE1993/000384
Authority: WO
Inventors: Börje FREDRIKSSON; Göran Olsson
Original assignee: Sunds Defibrator Industries AB
Current assignee: Valmet AB
Priority date: 1992-06-05
Filing date: 1993-04-30
Publication date: 1993-12-23
Anticipated expiration: 1994-12-05
Also published as: SE9201735L; SE9201735D0; AU4030993A; SE470315B

Abstract

A method of controlling the screening process at the screening of cellulose-containing pulp suspensions in a screening device (1) where ingoing pulp suspension (2) is divided by means of a screening member into an accept fraction (3) and a reject fraction (4). The material concentration in the reject fraction (4) is measured continuously by a meter (5) and transferred to a signal (6, 7), which is utilized for controlling the ingoing pulp concentration and/or the energy supply to the screening process.

Description

Screening of pulp

This invention relates to the screening of cellulose-cont¬ aining pulp suspensions and, more precisely, to a method of controlling the screening process.

The object of the screening process is the fractionation of pulp suspensions or the separation of impurities in the form of coarse material, shives and the like from the pulp suspension. The efficiency of the screening, therefore, could be defined as the capacity of the screen to fraction¬ ate or separate impurities. In practice, however, also other factors have to be taken into consideration when deem¬ ing the efficiency of the screening. Such factors are, for example, the energy utilization and the energy costs for the screen and screening process, the capital investment utilization depending on installation costs and space re¬ quirement, the capacity, i.e. the flow and reliability of the screen and screening process, and the ability of low reject output and great flow variation, which is a pre¬ requisite for a flexible system.

It was found that variations in the concentration of the pulp suspension are of decisive importance for the screening process. A decrease in concentration implies an increase in the hydraulic load on the screening member, i.e. the flow rate through the apertures in the screening member increases. At concentrations below about 0.5% the capacity is unacceptably low. An increase in the concentration, on the other hand, implies an increase in the required energy intensity for breaking up the fiber network into individual fibers and render it flowing, so-called fluidization, which is a prerequisite for the screening process. The necessary energy intensity supplied to the rotating members of the screen starts increasing at about 1.5% and increases rapidly above 3%- The concentration, thus, restricts an effective utilization of the screen. At too high a concentration the screening member gets clogged.

Another problem in this connection is the thickening o^'f the reject fraction, which substantially restricts the capacity of conventional screens to operate at low reject output. When the pulp concentration is too high in the last portion of the screening zone, the screen gets plugged up. This restriction in the operation of the screen at low reject output also limits the impurity concentration. When the energy supply is not any longer sufficient for fluidizing the pulp, the local flow of the screening member is reduced, and only dewatering takes place whereby the concentration of impurities ceases. This is one of the reasons why screening often must be carried out in several steps in order to achieve a. sufficient concentrat¬ ion of impurities in the reject flow.

Optimum efficiency in a screening process (separation or fractionation) is obtained when a suitable balance is maintained between the strength of the fiber network, depending on the pulp concentration and type and quality of the pulp, and the energy supply, which is determined substantially by the speed of the screen rotor. It can be difficult to maintain this balance, because the strength of the fiber network cannot be measured in a simple way. Tnis balance is especially critical at screening with high pulp concentration, 3%_> boc∑.use in this pulp con¬ centration range even small variations ^'in the pulp con¬ centration bring about relatively great changes in the strength of the fiber network.

Conventionally the screening is carried out with great surplus energy in order to ensure the fluidization of the pulp and thereby the screening procedure. A known method of reducing to some extent this unnecessary energy consumption is to control the supply of pulp susp¬ ension by means of the pressure drop above the screening member. When this pressure drop decreases, the pulp supply can be increased, and vice versa. This control, however, is by far not sufficient enough for optimizing the energy input.

According to the present invention, the aforesaid problems are solved by continuously measuring the reject concen¬ tration and using it for controlling the ingoing pulp concentration and/or the energy supply to the screening process. The characterizing features of the invention are apparent from the attached claims.

The reject concentration being a measure of the thickening in the screen, this concentration can be used for adjust¬ ing the pulp concentration before 'the screen, for example by controlling the dilution water supply, so that the efficiency and work concentration of the screen are maxim¬ ized. Alternatively, the reject concentration can adjust the energy supply for the same reason, preferably by controlling the speed of the screen rotor. In this case, the energy demand of the screen at highest possible screening efficiency is minimized.

The method according to the invention results in savings in the energy consumption and in a more efficient utiliz¬ ation of the screening arrangement. In many cases the number of steps in a multi-step screening process can be reduced with maintained screening result. Investment costs and space requirements are thereby reduced. Further¬ more, both the capacity and the flexibility of the screen¬ ing process are improved. The invention is described in greater detail in the following, with reference to an embodiment thereof, which is illustrated by a flow sheet in the accompanying Figure .

A supply line 2 for ingoing pulp suspension, an outlet line 3 for the accept fraction and an outlet line 4- for the reject fraction are connected to a screening device 1, which preferably is a pressurized screen. The screening device 1 can be of conventional design, comprising a stationary screening member, and a separate rotor element with some form of pulsating member is rotatable in the screening device. Alternatively, the screening member can be a rotatable screening cylinder.

A meter 5 is provided for measuring the material concen¬ tration in the reject line 4-. Said meter 5 can be of conventional design, for example a shearing force meter or optical concentration meter.

From the meter 5 a signal 6 is sent for controlling a valve 9 in a supply line 8 for dilution water. This line 8 is connected to the pulp supply line 2 before the screening device 1. Alternatively, the meter 5 can send a signal 7 for controlling the speed of the rotor of the screening device 1.

It can- be imagined, of course, to use both these control systems with signals 6 and 7 simultaneously, but normally it is sufficient with one of them.

The pulp concentration preferably is controlled in such a way, that the reject concentration is maintained subst¬ antially constant at a predetermined value, for example in the range 4.8-5-2%, when the ingoing pulp concentration is about 3-2% and the accept concentration about 3%- For maintaining the reject concentration constant, for example at 5%, the ingoing pulp concentration may require adjustment within 3-0-3. %_> which can be achieved simply by controlling the supply of dilution water from the line 8 with the valve 9-

When, instead, the speed of a rotatable rotor element or a rotatable screening member is to be controlled, at the aforesaid material concentration a speed adjustment within - 5-10% may be required. For speeds within the range 4-00-600 rpm, for example, an adjustment by - 30 rpm can be required. This can be achieved with a conventional control equipment.

In addition to the aforesaid measures for controlling the screening process, it may be suitable also to maintain the reject flow constant on a predetermined level in a manner known per se.

The invention, of course, is not restricted to the embod¬ iment described above, but can be varied within the scope of the invention idea.

Claims

1. A method of controlling the screening process at the screening of cellulose-containing pulp suspensions in a screening device (1), where ingoing pulp suspension (2) is divided by means of a screening member into an accept fraction (3) and a reject fraction (4), c h a r a c t e r i z e d i n that the material con¬ centration in the reject fraction ( ) is measured cont¬ inuously by a meter (5) and transferred to a signal (6,7), which are utilized for controlling at least one of the process parameters ingoing pulp concentration and energy supply to the screening process.

2. A method as defined in claim 1, c h a r a c t e r i ¬ z e d i n that the controlling of the process param¬ eters is carried out so that the material concentration in the reject fraction is maintained substantially constant.

3. A method as defined in claim 1 or 2, c h a r a c t ¬ e r i z e d i n that the outgoing signal (6) of the meter (5) is utilized for controlling the ingoing pulp concentration by adjusting the dilution water supply (8) to the ingoing pulp suspension (2).

4.. A method as defined in any one of the claims 1-3, c h a r a c t e r i z e d i n that a separate rotor element is provided in the screening device (1), and that the outgoing signal (7) of the meter (5) is utilized for controlling the energy supply by adjusting the speed of the rotor element.

5- A method as defined in any one of the claims 1-3, c h a r a c t e r i z e d i n that the screening member of the screening device (1) is rotary, and that the outgoing signal (7) of the. meter (5) is utilized for controlling the energy supply by adjusting the speed of the screening member.

6. A method as defined in any one of the preceding claims, c h a r a c t e r i z e d i n that the flow of the reject fraction is maintained at a constant level.