Classifications

• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21B—FIBROUS RAW MATERIALS OR THEIR MECHANICAL TREATMENT
  • D21B1/00—Fibrous raw materials or their mechanical treatment
  • D21B1/04—Fibrous raw materials or their mechanical treatment by dividing raw materials into small particles, e.g. fibres
  • D21B1/12—Fibrous raw materials or their mechanical treatment by dividing raw materials into small particles, e.g. fibres by wet methods, by the use of steam
  • D21B1/14—Disintegrating in mills
  • D21B1/18—Disintegrating in mills in magazine-type machines
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21D—TREATMENT OF THE MATERIALS BEFORE PASSING TO THE PAPER-MAKING MACHINE
  • D21D1/00—Methods of beating or refining; Beaters of the Hollander type
  • D21D1/002—Control devices

Definitions

• the present invention relates to a method for the control of a groundwood pulping process in order to achieve an optimal value for both the drainability of the pulp and for another characteristic of the pulp, preferably for the tearing resistance of the pulp.
• one object is usually to have a constant drainability value or freeness (CF) of the pulp.
• the control is for instance made so that the wood supply pressure is kept constant, whereby the wood supply rate is allowed to vary.
• the wood supply rate can be kept constant and the supply pressure is allowed to vary.
• the CF value of the pulp When only the CF value of the pulp is used as the measured variable to control the process this of course has a disadvantage in that the CF value will not provide all information about the other quality properties of the pulp, which can be characterised by many measured quantities, such as tearing resistance and tensile strength, light- scattering and opacity.
• the Finnish patent FI 70438 proposes a method to control a groundwood pulp process with the aid of a new quantity, the plasticity of the wood, as the control parameter.
• a desired pulp property is obtained at a given (constant) peripheral speed of the grinding stone when the supply pressure and the wood supply rate is selected so that during otherwise constant operating conditions (constant wood quality, constant peripheral speed and sharpness of the grindstone) a plasticity value is obtained.
• the tensile strength (at a given freeness value) can be affected to a certain amount by the peripheral speed of the grinding stone, even if the effect is not as obvious as concerning the tearing resistance.
• the tensile strength increases about 35 % when the grinding is made at atmospheric pressure and the peripheral speed is reduced from 30 m/s to 10 m/s.
• the object of the present invention is to control the pulping process so that an optimal pulp quality is obtained, in other words so that optimal values are obtained both for the CF value of the pulp and for another quantity characterising the quality of the pulp, such as the tearing resistance, which usually is stated as the tear index (RI).
• RI tear index
• the method can be used in common stone pulping without overpressure (so called stone groundwood or SGW pulp) as well as in so called overpressure pulping (pressure groundwood or PGW) .
• the pulping process can be controlled by two control variables, i.a. the wood supply rate (or power) and the peripheral speed of the grinding stone.
• the supply rate can keep the CF value of the pulp at a desired level, and the peripheral speed of the stone can keep another variable at a desired level.
• the control can be effected with the aid of a multivariable control algorithm or with two SISO loops (single input, single output) .
• the CF value and the tear index of the pulp are kept on a desired level, and the sum of the deviations
• CF 0 freeness set point
• CF X measured freeness value
• RI 0 tear index set point
• RI X measured value of the tear index
• the multivariable control algorithm can also be made adaptive in order to compensate for changes in the grinding stone* s sharpness with time.
• V p peripheral speed m/s
• Tear tear index mmNNrm /g
• the method according to the invention also reduces the specific energy consumption (SER).
• SER specific energy consumption
• the controller is a generalisation of the multivariable control algorithm of Astr ⁇ m and Wittenmark (1973) .
• u is the input vector and y is the output vector
• ⁇ e(t) ⁇ is a sequence of independent evenly distributed random vectors with a mean value of zero and the covariance
• the dimension of all vectors u, y and e is p, and the dimension of all matrices A t , Bj. and C* . is pxp.
• the matrix B 0 is non-singular.
• the adaptive algorithm performs an identification based on the least squares method according to the model presented below.
• the algorithm estimates the parameters for the model
• k is selected as the dead time for the process (2), and A(z) and B(z) are pxp polynomial matrices according to
• B 0 is a matrix in the constant term of B(z) for the process (2) .
• ⁇ t [Ct j _! . . .v ⁇ j _p . ..OJ j _ j . . .Ot j _p
• the i th row in model (3) can be written as
• the criterion (6) can be written as
• V H ( ⁇ t ) 1/N ⁇ ⁇ yi (t) - Ui(t-k-l) - ⁇ (t-k-l) ⁇ i] 2
• the parameters for the controller are the same as the estimated parameters.
• the estimated parameter vector ⁇ L in (8) can be recursively calculated from
• ⁇ i (t) ⁇ i (t-l) + K(t-l)[y i (t)-u i (t-k-l)- ⁇ (t-k-l) ⁇ i (t-l)]
• P(t) is a normalised covariance matrix of the estimated parameters Q t .
• the covariance matrix P(t) is supplemented by adding to it a matrix R ⁇ instead of the division by ⁇ .
• P(t) P(t-l)-K(t-l)[l+ ⁇ (t-k-l)P(t-l) ⁇ T (t-k-l)] x K T (t-l) + R.
• the algorithm can be construed as a union of a plurality (here 2) of simple self-adjusting controllers.
• the controller 2 controls the output signal y 2 (t) by using the control variable u 2 (t).
• yj(t-i) and u ⁇ t-l-i) (i > 0) can be used as feedforward signals.
• the two simple self-adjusting controllers can operate in a cascade mode.
• Another possibility is an exclusively multivariable minimum variance control algorithm, which is not adaptive.
• the control of the tear index at lower peripheral speeds results in great advantages (40 % to 20 %).
• the multivariable control algorithm also is adaptive, changed sharpness is taken into account by increasing the peripheral speed. During this the freeness can be freely selected.
• the advantage is an improvement of about 10 % concerning the tear index, and the changes in sharpness can be controlled in the periods between sharpening actions. During these periods the freeness can be freely selected.
• the sharpening is not made with pressurised water or similar at regular intervals, then the sharpening is made at P- ⁇ at the maximum power consumption.

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• Engineering & Computer Science (AREA)
• Life Sciences & Earth Sciences (AREA)
• Wood Science & Technology (AREA)
• Mechanical Engineering (AREA)
• Paper (AREA)

Priority Applications (3)

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Publications (1)

Family

ID=8546280

Family Applications (1)

Country Status (5)

Cited By (2)

Citations (1)

• 1996
  • 1996-06-25 FI FI962626A patent/FI102975B1/sv active
• 1997
  • 1997-06-24 CA CA002294424A patent/CA2294424A1/en not_active Abandoned
  • 1997-06-24 EP EP97928285A patent/EP1015682B1/en not_active Expired - Lifetime
  • 1997-06-24 DE DE69718683T patent/DE69718683D1/de not_active Expired - Lifetime
  • 1997-06-24 WO PCT/FI1997/000406 patent/WO1997049857A1/en active IP Right Grant

Patent Citations (1)

Non-Patent Citations (2)

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