U.S. patent application number 11/691123 was filed with the patent office on 2008-10-02 for system and method for controlling a processor including a digester utilizing time-based assessments.
This patent application is currently assigned to Metso Automation USA Inc.. Invention is credited to Kari Juhani Lampela.
Application Number | 20080236771 11/691123 |
Document ID | / |
Family ID | 39792260 |
Filed Date | 2008-10-02 |
United States Patent
Application |
20080236771 |
Kind Code |
A1 |
Lampela; Kari Juhani |
October 2, 2008 |
SYSTEM AND METHOD FOR CONTROLLING A PROCESSOR INCLUDING A DIGESTER
UTILIZING TIME-BASED ASSESSMENTS
Abstract
A system for controlling a processor having at least one
sampling port connected to a stage of the processor in order to
sample a reactant product from the processor. The system includes a
controller configured to control a processing parameter of the
processor based on measurements of at least one property of the
reactant product such that changes to the processing parameter
maintain a target value for the at least one property of the
reactant product. The system further includes a dead time
compensator. The dead time compensator is configured, based upon a
prescribed dead time related to a time before at least one effect
of at least one change to the processing parameter is fully
realized, to evaluate the reactant product to determine if the
effect has been realized at a plurality of sequential times offset
from the dead time.
Inventors: |
Lampela; Kari Juhani;
(Duluth, GA) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
Metso Automation USA Inc.
Norcross
GA
|
Family ID: |
39792260 |
Appl. No.: |
11/691123 |
Filed: |
March 26, 2007 |
Current U.S.
Class: |
162/19 ;
162/253 |
Current CPC
Class: |
D21C 3/02 20130101; D21C
9/10 20130101; D21C 7/12 20130101; D21C 9/147 20130101 |
Class at
Publication: |
162/19 ;
162/253 |
International
Class: |
D21C 3/16 20060101
D21C003/16 |
Claims
1. A method for controlling a processor, comprising: analyzing a
reactant product from the processor; determining based on at least
one property of the reactant product a change to at least one
processing parameter of the processor; changing the at least one
processing parameter of the processor; and evaluating, following a
prescribed process dead time related to a time before at least one
effect of at least one of said change to the at least one
processing parameter is fully realized in said at least one
property of the reactant product, changes to the at least one
property for subsequent reactant products sampled at a multiplicity
of times offset about the prescribed process dead time.
2. The method of claim 1, wherein the evaluating comprises:
evaluating a reactor process susceptible to disturbances in plug
flow conditions.
3. The method of claim 2, wherein said evaluating a reactor process
susceptible to disturbances in plug flow conditions evaluates
disturbances due to channeling whereby said reactant product
prematurely flows into subsequent stages of the processor.
4. The method of claim 1, wherein the analyzing comprises:
analyzing the reactant product from any of a pulp digester, an
oxygen delignification processor, a bleaching processor, and a
causticizer.
5. The method of claim 1, wherein the analyzing comprises:
analyzing the reactant product of said processor from at least one
of several sequential stages of said processor.
6. The method of claim 5, wherein the analyzing comprises:
analyzing the reactant product of a pulp digester to determine at
least one of a Kappa number and a residual alkali
concentration.
7. The method of claim 1, wherein the determining comprises:
determining a prescribed change to the at least one processing
parameter based on a rule base.
8. The method of claim 7, wherein the determining comprises:
evaluating error deviations from at least two properties of the
reactor product; and changing at least one of said processing
parameters based on at least a bifurcated error state of the at
least two properties of the reactant product.
9. The method of claim 1, wherein the determining comprises:
determining a prescribed change to the at least one processing
parameter based on a model base.
10. The method of claim 1, wherein the changing comprises: changing
the at least one processing parameter in at least one of a
step-change and a ramped change.
11. The method of claim 1, wherein the changing comprises: changing
the at least one processing parameter in a time less than the
prescribed process dead time.
12. The method of claim 1, further comprising: revising at least
one of a rule-base or a model-base based on said changes to said at
least one property evaluated by said dead-time compensator prior to
repeating said determining step.
13. A computer program product, comprising: a computer storage
medium and a computer program code embedded in the computer storage
medium for causing a computer to control a processor, said computer
program code comprising, a first computer program component
configured to analyze a reactant product from the processor, a
second computer program component configured to determine based on
at least one property of the reactant product a change to at least
one processing parameter of the processor, a third computer code
program component configured to change the at least one processing
parameter of the processor, and a fourth computer code program
component configured to evaluate, following a prescribed process
dead time related to a time before at least one effect of at least
one of said change to the at least one processing parameter is
fully realized in said at least one property of the reactant
product, changes to the at least one property for subsequent
reactant products sampled at a multiplicity of times offset about
the prescribed process dead time.
14. A method for controlling a processor, comprising: analyzing a
reactant product from the processor; determining based on at least
two properties of the reactant product a change to at least one
processing parameter of the processor; and changing said at least
one processing parameter based on at least a bifurcated error state
of the at least two properties of the reactant product.
15. The method of claim 14, wherein the analyzing comprises:
analyzing the reactant product from any of a pulp digester, an
oxygen delignification processor, a bleaching processor, and a
causticizer.
16. The method of claim 14, wherein the analyzing comprises:
analyzing the reactant product of said processor from at least one
of several sequential stages of said processor.
17. The method of claim 16 wherein the analyzing comprises:
analyzing the reactant product of a pulp digester to determine at
least one of a Kappa number and a residual alkali
concentration.
18. The method of claim 14, wherein the determining comprises:
determining a prescribed change to the at least one processing
parameter based on a rule base.
19. The method of claim 14, wherein the determining comprises:
determining a prescribed change to the at least one processing
parameter based on a model base.
20. The method of claim 14, wherein the changing comprises:
changing the at least one processing parameter in at least one of a
step-change and a ramped change.
21. The method of claim 14, wherein the changing comprises:
changing the at least one processing parameter in a time less than
a prescribed process dead time.
22. The method of claim 15, wherein the changing comprises:
maintaining at least one of a Kappa number representative of a
cellulose fiber concentration and an alkalinity of the digested
pulp product within said target values.
23. The method of claim 22 wherein the changing comprises:
controlling an H-factor of the digester and at least one of an
input alkali dosage concentration and an alkali/wood-input ratio to
the digester, said H-factor derived from a time-integrated rate
constant for the pulp or paper product in the digester based on a
temperature and a throughput of the digester.
24. The method of claim 23, wherein said controlling comprises:
increasing at least one of said H-factor, said input alkali dosage
concentration, and said alkali/wood-input ratio when said Kappa
number is above a target value.
25. The method of claim 23, wherein said controlling comprises:
decreasing at least one of said H-factor, said input alkali dosage
concentration, and said alkali/wood-input ratio when said Kappa
number of the digested pulp product is below a target value.
26. The method of claim 23, wherein said controlling, when a
residual alkalinity of the digested pulp product is below a target
value, increases said input alkali dosage concentration or said
alkali/wood-input ratio or decreases said H-factor.
27. The method of claim 23, wherein said controlling, when a
residual alkalinity of the digested pulp product is above a target
value, decreases said input alkali dosage concentration or said
alkali/wood-input ratio or increases said H-factor.
28. The method of claim 15, wherein said changing controls said
pulp digester based on the following bifurcated relationship:
TABLE-US-00003 Blow Kappa Residual Alkali H-factor Alkali Dosage
Error Error Correction Correction High High ++ OK High OK + + High
Low OK ++ OK High + - OK OK OK OK OK Low - + Low High OK -- Low OK
- - Low Low -- OK
where blow Kappa error is representative of an error from an
expected cellulose fiber concentration in a discharge section of
the digester, residual alkali error is representative of an error
from an expected residual alkali concentration, H-factor correction
is based on a time-integrated rate constant for the pulp or paper
product in the digester based on a temperature and a throughput of
the digester, alkali dosage correction is based on a measured
addition of alkali to be added to the digester, "+" and "++"
indicate an increase and a stronger increase to the corrections,
"-" and "--" indicate a decrease and a stronger decrease to the
corrections, "OK" refers to an expected value of the residual
alkali concentration or the expected cellulose fiber concentration,
"High" refers to deviations above the expected values which are
predetermined to exceed process tolerances, and "Low" refers to
deviations below the expected values which are predetermined to be
below process tolerances.
29. A computer program product, comprising: a computer storage
medium and a computer program code embedded in the computer storage
medium for causing a computer to control a processor, said computer
program code comprising, a first computer program component
configured to analyze a reactant product from the processor; a
second computer program product configured to determine based on at
least two properties of the reactant product a change to at least
one processing parameter of the processor; and a third computer
program product configured to change said at least one processing
parameter based on at least a bifurcated error state of the at
least two properties of the reactant product.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is related to and claims priority under 35
U.S.C. .sctn. 121 to U.S. Ser. No. 10/639,509 filed Aug. 13, 2003,
the entire contents of which are incorporated herein by
reference.
DISCUSSION OF THE BACKGROUND
[0002] 1. Field of the Invention
[0003] This invention relates to the control of processes and
processing equipment, and in particular production
processes/equipment for use in the pulp and paper industry. The
invention is particularly advantageous for pulp digesters.
[0004] 2. Background of the Invention
[0005] Processors storing large quantities of chemically reactive
(and some cases reacting) fluids are utilized extensively
throughout the paper and pulp industry. Typically, the processes
and equipment are "in-line" with the output from one processor
providing the input to a subsequent downstream processor. Each
processor in the paper and pulp industry processes a large tonnage
of product. For example, a continuous digester, can process more
than 1,300 tons per day of digested pulp. Output from a pulp
digester becomes input to downstream processors such as for example
oxygen delignification processors, bleaching processors, and
causticizers.
[0006] FIG. 1 is a schematic diagram depicting a number of in-line
processing units processing raw pulp products toward a final output
product suitable for paper production. FIG. 1 shows specifically a
chemical pulp manufacturing process flowing from left to right with
output from a digester 2 flowing to an oxygen delignification
processor 4 and then to a bleaching processor 6. The spent liquid
effluent extracted from the digester 2 is feed "off-line" to a
causticizer 8 which restores the spent liquid effluent to a proper
alkali concentration before return to the digester 2. The
causticizer 8 itself represents another processor (e.g. a vat
processor containing large quantities of chemically reactive fluids
whose output product must be controlled to a desired standard in
the manufacturing line). With this type of arrangement or system,
deviations from target specifications of the resultant product from
any one stage can impact the downstream processors.
[0007] Continuous pulp digesters are very complex vertical reactors
(typically tubular) used in the pulp and paper industry to remove
lignin from wood chips. Usually, continuous digesters are separated
into multiple reaction and extraction zones. Optimal control of a
digester can be difficult due to long dead times in which changes
to input process variables are not immediately apparent. When a
process parameter is changed or a step commences, e.g., by the
addition of a material such as an alkali or by affecting a
temperature change, the end effect is not immediately apparent due,
e.g., the time required to realize the effect and the inertia of
the system. The time from when a change occurs to the point at
which the effect is realized, fully or partially, can be referred
to as "dead time."
[0008] In order to yield large or more optimum production
quantities of digested product and to be economical with a minimum
of chemicals and energy usage, the process must be controlled to
maintain optimum cooking conditions throughout the digester to
ensure selective delignification while simultaneously optimizing
pulp quality and production costs. To facilitate control, reliable
pulp quality measurements are often used to provide accurate
real-time information. Indeed, certain basic control and quality
measurements--Kappa, pulp strength, and chemical residuals--have
been made regularly for decades. In the past, analyses of these
properties were made off-line in the laboratory, but such analyses
were slow and error-prone. However, with recent advances in
measuring technologies, these analyses have been extensively
automated such that measurements can be made on-line. To maximize
the impact of automated measurements, there is a need for efficient
controls and/or control methods that are easy to modify, tune, and
configure, and yet can handle the complexity of continuous digester
processes.
[0009] As a consequence of the heterogeneities in the feedstock,
i.e. the wood pulp, a digester undergoes constant changes due to
the complicated structure and properties of the various wood pulps
being fed to the digester. Besides differences in the pulp
feedstock from one particular batch of wood chips to another, even
the moisture content of the chips being fed into the digester can
vary by as much 30% during a single day's production. Further, the
large amounts of wood pulp and chemicals contained in the digester
create a "chemical inertia" which makes instantaneous changes to
the digesting conditions, such as for example changes in alkali
concentration, cooking temperature, and white liquor concentration,
difficult if not impossible to rapidly adjust. As a consequence, it
is generally impossible to describe the dynamics of digester with
precise mathematical models. Furthermore, a typical retention time
for the pulp in a digester can in some cases exceed five hours. Due
to possible channeling (i.e., unexpected changes in plug flow in
the tubular reactor) or other unexpected disturbances, it is
impossible to estimate the retention time accurately for a
particular pulp product flowing through the digester.
[0010] As noted, a digester can process more than 1,300 tons per
day of digested pulp. Maximizing pulp production at a specified
Kappa number using a minimum input of chemicals and energy and a
minimal waste discharge is highly desirable in order to produce an
efficient pulp digesting process. In a digester, lignin is removed
from for example wood chips. Lignin is the naturally occurring
bonder in a wood product which bonds the wood fibers together. An
aqueous solution of the sodium hydroxide and hydrosulfide (i.e.,
white liquor) is used to react (i.e., to digest) the wood products
inside the digesters thereby dissolving the lignin from the wood
product.
[0011] Presently, a titration method is a known and commonly used
to measure a Kappa number of various pulps. This titration method
is described in Tappi Test Methods--T236 cm-85, Tappi Press, 1996,
the entire contents of which are incorporated herein by reference.
Using the titration method, a pulp Kappa number is calculated using
the difference between the initial volume of potassium
perrnanganate blank solution and the final volume of potassium
permanganate remaining after oxidation of lignin in the
pulp-permanganate solution. For example, the digestion of wood
chips in an alkali solution and the resulting pulp Kappa number
obtained using a permanganate solution are both described in
Bentvelzen et al. (U.S. Pat. No. 4,216,054), the entire contents of
which are incorporated herein by reference. Kappa number is not the
only one way to measure lignin, e.g. others like K-number, P-number
and others known in the art can be used.
[0012] Prior to entry into the digester, wood chips are typically
cooked and steamed (to remove air from the pores of the chips) and
fed into an impregnation vessel together with the white liquor.
While in the impregnation vessel, white liquor penetrates the
chips, and the chips are subsequently carried into a top section of
the digester where a mixture of the wood chips and the white liquor
is brought to a desired reaction temperature. In a top section of
the digester, the chips react with the white liquor to digest the
lignin, and spent liquor (i.e., that liquor which has been depleted
of its alkalinity by the chemical reaction with the lignin) is
extracted as the digested chips migrate into lower cooking
sections. Fresh white liquor is added to further continue the
delignification process. The blow Kappa number of the digested
(i.e., reacted) product can be assessed from a blow-line (i.e., an
exit line) in which the Kappa number provides a measure of how
effectively the lignin has been digested from the wood fiber.
[0013] As disclosed for example in Beller et al. (U.S. Pat. No.
5,032,977), the entire contents of which are incorporated herein by
reference, to address the complexities of controlling a wood
digester, "model" based control processes have been developed. In a
model-based control process, a model assumes the input properties
of the pulp product entering the digester, calculates expected
values for the resultant properties of the digested product, and
alters the process variables of the reactor (e.g., the pulp product
feed rate, the alkali input feed, and the digester temperature) to
affect the resultant properties. A model based approach is a
complex approach requiring complicated calculations if any kind of
reliable prediction of the reactor is to be made. Yet, for the
above-noted reasons, pulp digesters are not simple chemical fluid
beds conducive to model based predictions. Initial assumptions of
input properties and the resulting models of the digester are
susceptible to variations of the input properties and are
susceptible to unexpected changes in the product flow through the
large digester (i.e., the above-noted channeling). When unexpected
changes occur, model based controls have no way to recognize that
the unexpected changes may be spurious. The model based controls
consequently improperly compensate the input process variables,
thus producing control oscillations and instabilities in the output
properties of the digested wood product.
[0014] While model based controls, such as those described by
Beller et al. for example, can use adaptive control to learn and
refine the process control model, the learning process needs to be
based on at least a quasi-steady state condition maintained in the
reactor. Otherwise, what is learned is in error. Indeed, in those
models which use adaptive control, a disturbance to the steady
state operation can result in the models being temporarily skewed,
as the "learned" refinements are not representative of the process
when unexpected disturbances occur. As a result, when unexpected
disturbances occur, once again a series of oscillations in the
model-based control occurs, producing process control
instability.
[0015] The problems illustrated above for a pulp digester extend to
other paper mill processes listed above such as for example the
oxygen delignification processors, the bleaching processors, and
the causticizers, and in general are prevalent in any chemical
processor in which imhomogeneities in input feedstock, the chemical
inertia of the process reactor, and/or the fluid flow make
problematic the accurate prediction of future changes following
changes to input parameters.
SUMMARY OF THE INVENTION
[0016] Consequently, there exists a need for an improved system and
method for controlling processing equipment, particularly
processing equipment used in the pulp and pager industry.
Particularly needed is a system and method for minimizing or
avoiding instabilities which can result from disturbances or
changes to the processors or process conditions.
[0017] Thus, one object of the present invention is to provide a
control which reduces the impact of disturbances on the quality and
production of a processor.
[0018] Yet, another object of the present invention is not to
utilize model-based control in which process models or detailed
process knowledge are required for tuning and control. For example,
processors in the pulp and paper mill industry represent
applications where a complex predictive model, for example a neural
network based control, would not be an accepted practice as the
pulp and paper mill industry can not afford to risk the production
of more than 1,300 tons per day of digested pulp on complex
software installed on a processor controls which can not be
routinely upgraded, routinely monitored, and installed on site.
[0019] A further object of the present invention is to a provide a
control in which long-term disturbances on processors are
minimized.
[0020] Yet another object of the present invention is to provide a
control for pulp digesters and other paper mill processors such as
for example oxygen delignification processors, bleaching
processors, and causticizers.
[0021] Still, a further object of the present invention is to
provide a control in which exact knowledge of dead times (i.e.,
those times after a process change is implemented and before the
results are fully realized) are not needed for stable process
control. As such, in one aspect of the present invention, a tunable
time "window" is utilized to see if the processor in responding to
a process change matches the resultant change to an expected change
and consequently to a target value for an output property of the
reacted product.
[0022] These and other objects are accomplished, according to the
present invention. In accordance with an exemplary embodiment, a
system for controlling a processor is provided having at least one
sampling port connected to a stage of the processor to sample a
reactant product from the processor. The system includes a
controller configured to control a processing parameter of the
processor based on measurements of at least one property of the
reactant product such that changes to the processing parameter
maintain a target value for the at least one property of the
reactant product. The system further includes a dead time
compensator. The dead time compensator is configured, based upon a
prescribed dead time related to a time before at least one effect
of at least one change to the processing parameter is fully
realized, to evaluate the reactant product to determine if the
effect has been realized at a plurality of sequential times offset
from the dead time.
[0023] According to an exemplary method of the present invention, a
reactant product from the processor is analyzed to determine, based
on at least one property of the reactant product, a charge to at
least one processing parameter. The processing parameter(s) is/are
changed, and, following a prescribed process dead time, changes to
the at least one property of the reactant is evaluated at a number
of times/time intervals as the effects of the change(s) become
realized. By way of example, according to a preferred method, a
"dead time" can be estimated during which the effects of the
change(s) will not be expected to have been fully realized. After
this selected or predetermined dead time, one or more properties of
the reactant are evaluated at plural different times/time intervals
to determine the magnitude and timing of the effects of the process
parameter change(s).
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] A more complete appreciation of the present invention and
many of the attendant advantages thereof will be readily obtained
as the same becomes better understood by reference to the following
detailed description when considered in connection with the
accompanying drawings, wherein:
[0025] FIG. 1 is a schematic of a generic manufacturing line in a
pulp plant;
[0026] FIG. 2 is a schematic of a system for a processor including
a controller according to the present invention;
[0027] FIG. 3 is an illustrative schematic of a controller of the
present invention;
[0028] FIG. 4 is an illustrative schematic of a dead-time
compensator of the present invention executing control of a
digester H-factor;
[0029] FIG. 5 is an illustrative schematic of a dead-time
compensator of the present invention executing control of
alkalinity dosage;
[0030] FIG. 6 is a flowchart depicting one method of the present
invention; and
[0031] FIG. 7 is a systematic representation of a general purpose
computer configured to execute the computer program components of
the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0032] Referring now to the drawings, wherein like reference
numerals designate identical or corresponding parts throughout the
several views, and more particularly to FIG. 2 thereof, FIG. 2 is a
schematic diagram of a processor including a controller 80
according to the present invention. The processor in FIG. 2 is
schematically represented as a pulp digester 10, but could equally
well typify other pulp mill processes such as for example the
oxygen delignifcation processors, the bleaching processors, and the
causticizers mentioned above in which reactant products from the
processes are monitored to provide control.
[0033] As shown illustratively in FIG. 2, a pulp digester 10 of the
present invention includes sequential cooking sections illustrated
here by an upper cooking section 20, a mid-cooking section 30, an
extended-cooking section 40, and a discharge section 50. White
liquor, including for example the aforementioned sodium hydroxide
and hydrosulfide, is introduced into the pulp digester 10 by inlet
60 located at the top of the pulp digester 10. Also introduced by
way of the inlet 60 is an input pulp product which mixes with the
white liquor in the upper cooking section 20 of the pulp digester
10. Attached to the pulp digester 10 can be a number of sampling
loops 70 which permit extraction of chemically-spent white liquor
and the reacted pulp product from the pulp digester.
[0034] The extracted fluid can be used to measure alkali content or
a representative Kappa number of the digested pulp product at
whatever section 20, 30, 40, and/or 50 where the liquid is
extracted. A blow Kappa number and likewise a residual alkali
concentration taken from the discharge (i.e., blow) section 50 of
the pulp digester are pertinent production targets for control of
the digester.
[0035] In a digester, there are a number of ways to influence the
blow Kappa level. For example, either the temperature or the alkali
level can be changed. The temperature control is typically adjusted
through adjustments of an H-factor. The H-factor is a calculated
(i.e. integrated) factor which, as described in Beller et al., is a
time-integration of the delignification reaction rate constant k in
an Arrhenius rate equation. The H-factor thus captures numerically
a value indicative of a pulp product time and temperature as the
pulp product flows temporally through the entirety of the digester.
When an input alkali level is increased, the blow Kappa number
(representing the residual amount of lignin in the pulp product)
typically decreases, and at the same time the residual alkali
typically increases. On the other hand, when the digester
temperature is increased, the blow Kappa number is typically again
decreased, but the residual alkali also decreases due to faster
chemical reaction rates owing to the increased temperature. Thus,
the control of the present invention analyzes jointly the blow
Kappa number and the residual alkali to determine that the blow
Kappa number and the residual alkali are within targeted and/or
expected values. Upon recognizing an error, the control according
to the present invention selects, based on composite errors from a
target blow Kappa number and target residual alkali, appropriate
adjustments for example to the H-factor and the alkali dosage
concentrations accordingly. Alternatively, adjustments to an
alkali/wood-input ratio to the digester can be utilized instead of
a strict increase in the input alkali dosage concentration (e.g.,
the input pulp or paper product feed rate could be reduced while
maintaining the same input alkali feed rate).
[0036] Table 1 shown below depicts a generic rule base according to
the present invention used, for example by a pulp digester, to
select a response based on a composite error realized in the blow
Kappa number and the residual alkali. As can be seen from an
analysis of the generic rule base, the errors in blow Kappa and
residual alkali from target values are categorized into three
states ("high", "ok", and "low"). For two variables and three
states, there exist nine possible processor states to which rules
for each of these states are prescribed. For these nine states,
there are measured responses for changes to the input process
variables. From the measured responses, appropriate corrections
denoted as "++", "+", "--", and "-" are implemented, where blow
Kappa error is representative of an error from an expected
cellulose fiber concentration in a discharge section of the
digester, residual alkali error is representative of an error from
an expected residual alkali concentration, H-factor correction is
based on a time-integrated rate constant for the pulp or paper
product in the digester based on a temperature and a throughput of
the digester, alkali dosage correction is based on a measured
addition of alkali to be added to the digester, + and ++ indicate
an increase and a stronger increase to the corrections, and - and
-- indicate a decrease and a stronger decrease to the corrections.
"OK" refers to an expected value of the residual alkali
concentration or the expected cellulose fiber concentration. "High"
refers to deviations above the expected values which are
predetermined to exceed process tolerances and typically for pulp
processors is marked by a deviation of more than 0.05% above the
expected values. "Low" refers to deviations below the expected
values which are predetermined to be below process tolerances and
typically for pulp processors is marked by a deviation of more than
0.05% below the expected values.
[0037] When the alkali level is increased, the blow Kappa number
decreases and at the same time the residual alkali increases. On
the other hand, when the temperature is increased, the blow Kappa
is again decreased, but now the residual alkali decreases. The
control incorporates the residual alkali and blow Kappa to same
control algorithm to keep the blow Kappa and residual alkali level
both within respective targets.
TABLE-US-00001 TABLE 1 Blow Kappa Residual Alkali H-factor Alkali
Dosage Error Error Correction Correction High High ++ OK High OK +
+ High Low OK ++ OK High + - OK OK OK OK OK Low - + Low High OK --
Low OK - - Low Low -- OK
[0038] Some of these changes appear to contradict a simple linear
response, as might be used in a proportional control. For example,
the first row of Table 1 indicates the presence of a "High" error
for both the blow Kappa number and the output residual alkali.
Normally, in proportionate controls, one would correspondingly
adjust both the H-factor and the alkali dosage to compensate. Yet,
as illustrated here, the rules only require increasing only the
H-factor when both the blow Kappa number and the output residual
alkali are "High" to properly control the pulp digestion to
maintain digested pulp production without excessive use of alkali
The rule base recognizes that to perform both a H factor and an
alkali dosage correction would have resulted in the digester
depleting the alkali, generating incomplete digestion and forcing
another round of corrective actions.
[0039] Table 2, shown below, is an example of a specific rule base
according to the present invention, used by a pulp digester, to
select a response based on a composite error realized in the blow
Kappa number and the output residual alkali.
TABLE-US-00002 TABLE 2 Blow Alkali Dosage Kappa Residual Alkali
H-factor Correction Error Error (% of Na.sub.2O) Correction (%
Na.sub.2O) +2 0.05 100 0 +2 0 50 0.25 +2 -0.05 0 0.5 0 0.05 50
-0.25 0 0 0 0 0 -0.05 -50 0.25 -2 0.05 0 -0.5 -2 0 -50 -0.25 -2
-0.05 -100 0
In this exemplary table, consider a processor (e.g., a pulp
digester) operating with a caustic/wood weight percentage of 18%. A
correction in the processor Na.sub.2O concentration of 0.25, as
given for example in the second rule, would correspond to a change
in the processor percentage concentration of Na.sub.2O from 18% to
18.25%.
[0040] According to the present invention, values for H-factor and
alkali dosage concentrations are adjusted according to linear
interpolations of the H-factor and alkali dosage concentrations
based on respective proportionate errors in the blow Kappa and the
residual alkali. Thus, in one embodiment of the present invention,
the digester is controlled such that both the quality of the
digested pulp product exiting the pulp digester (e.g. a Kappa
value) and the residual alkali level are maintained within
acceptable target ranges by first determining the error state of
the processor and then making prescribed changes to the input
process variables depending on the bifurcated assessment of the
error states for the two reactant properties (e.g. the blow Kappa
and the residual alkali errors).
[0041] As shown in FIG. 2, a control 80 receiving error
measurements from target values of the resultant properties of the
processor and the measured values (i.e. the blow Kappa number and
the residual alkali) executes control of the digester by adjusting
the input H-factor and alkali dosage. In one embodiment of the
present invention, the control includes dead-time compensators
which assess the state of the processor about an estimated or
predetermined dead time in which an expected change to the output
properties, such as for example residual alkali and/or blow Kappa,
is anticipated to occur. According to the present invention, other
configurations and other control parameters can equally be used
according the present invention to permit control of the digester
and other processors. Regardless, the control of the present
invention utilizes a tunable time "window" to see if the system in
responding to control parameter changes are realized.
[0042] FIG. 3-FIG. 5 depict a controller of the present invention
as applied in an illustrative example to a digester. Control of the
digester is predicated on maintaining targeted levels of, for
example, the residual alkali and the blow Kappa level. Furthermore,
control of the digester is predicated on maintaining a requisite
production level. Thus, as shown by illustration in FIG. 3, inputs
to the controller 80 include a measured residual alkali
concentration sampled for example in the discharge section 50, a
residual alkali concentration target value, a requisite production
level, a Kappa level measured in for example the discharge section
50, and a target Kappa level. As shown for the purposes of
illustration, a compensator 302 sums through an arbitrary scale
factor a requisite production level B2 (adjusted by the arbitrary
scale factor G1) and the residual alkali concentration value A2,
and outputs the summation as B1. A comparator 304 computes an error
difference A1-B 1 where A1 is residual alkali concentration taken
for example from the discharge section 50. As shown in FIG. 3,
comparator 306 computes a difference (i.e., an error) between the
measured Kappa level A3 (taken for example from the blow line
section 50) and the target Kappa level B3 to output a difference as
either error differences B4 or B5.
[0043] Error differences from the comparators 304 and 305 are
provided to decoupling compensators 308 and 310. As shown in FIG.
3, gain factors G2, G3, and G4 are used in decoupling compensators
308 and 310 to provide weighted summations used to predict a
correction to the digester input. The weighted summations can use,
for example, linear interpolations of the rule base shown in Table
2 to produce an H-factor correction or an alkali dosage correction
scaled to engineering units. Other statistical processes known in
the art can be used in the weighted summations. The predicted
corrections (i.e., the H-factor correction and the alkali dosage
correction) are feed separately to dead-time compensators 312 and
314, respectively.
[0044] FIGS. 4 and 5, respectively, illustrate exemplary delay-time
compensators 312 and 314 of the present invention. As the details
of the dead-time compensators 312 and 314 are similar, for brevity,
only a detailed description of the dead-time compensator 312 will
be discussed. However, similar functions are performed by the
dead-time compensator 314, as illustrated by the similarities
between FIGS. 4 and 5.
[0045] As shown in FIG. 4, input from the decoupling comparator 308
is fed as one input into the dead-time compensator 312. This input
depicted here is an H-factor correction. Additionally, the
dead-time compensator 312 receives a control on/off signal and a
process delay signal. Due to the large chemical inertia of the pulp
digester and the variations in input pulp such as moisture content
and lignin concentration, a controller for a processor in one
embodiment of the present invention utilizes delay circuits 316 and
corresponding comparators 318 to determine if a change to an input
parameter, such as for example a H-factor correction or an alkaline
dosage concentration correction, have indeed resulted in at least
one of the resultant properties of the digested pulp product having
changed to acceptable levels. Without the delay circuits 316 and
corresponding comparators 318, a controller would at an estimated
and/or predetermined time evaluate the state of the digester, and
at that time would act on the measured value of the digested
product to re-adjust (i.e., control) the digester.
[0046] As discussed, measurements taken at that time could be
either premature as the expected change has not yet impacted the
digested products, or could be belated as the expected change
occurred and thereafter dissipated. Either way, a control response
without the delay circuits 316 and corresponding comparators 318 of
the present invention is non-optimum in that errors derived at the
determined dead time do not accurately depict the system response.
The delay circuits 316 and the corresponding comparators 318 of the
present invention avoid this problem by setting a time-offset (i.e.
a delay offset) about the expected dead time in which the "change"
should manifest itself. The controller utilizes output from the
corresponding comparators 318 to analyze if the change is occurring
or has occurred.
[0047] For example, as shown in FIG. 4, a process delay such as for
example 2.0 hr is input to a delay circuit 316. The delay circuit
316 generates a time offset of 30 min from the 2.0 hr process
delay. The time offset value is adjustable and set by the
controller. One comparator 318 begins analysis of the properties of
the digested pulp product based on the time offset value at 1.5 hr.
Another comparator 318 begins analysis of the properties of the
digested pulp product at 2.0 hr. Still another comparator 318
begins analysis of the properties of the digested pulp product at
2.5 hr. The comparators 318, as shown in FIG. 4, also receive an
input of the H-factor correction. However, as illustrated by
example in FIG. 4, the input of the H-factor first passes by a
conditional switch 320. The conditional switch 320 decides, based
on the value of the process delay and whether or not the digester
control has been activated, whether or not to pass the value of the
H-factor correction to the comparators 318. For example, if the
reactor has just started to warn-up, control may not yet have been
activated.
[0048] As shown in FIG. 4, outputs of the comparators 318 are
compared by a process evaluator 322 such that process evaluator 322
outputs, when all the comparators agree on a directional change for
the H-factor (i.e., all the comparators indicate that a positive or
a negative change is necessary), a minimum change to the H-factor.
Finally, in a preferred embodiment, output from the process
evaluator 322 is feed to a verifier 324 which makes sure that the
process control is still in an active state, and then to a limiter
326 which compares the output change for the H-factor to make sure
that the predicted change for the H-factor is within bounds for
prescribed changes to the H-factor.
[0049] Thus, the rule-base shown for example in Table 2 can be
utilized by the decoupling compensators 308 and 310 to determine
for example a scaled (i.e., proportionate) response to error
deviations between existing properties such as for example between
the blow Kappa number and a target Kappa number or between the
residual alkali concentration and a target residual alkali
concentration. In one embodiment, the rule-base prescribes an
H-factor response or an alkaline dosage response based on the
above-noted error states to meet these target values. In another
embodiment, the decoupling compensators utilize a model base
response such as described in Beller et al. Regardless, a response
to the digester, in a preferred embodiment of the present
invention, is qualified by evaluating at a multiplicity of
subsequent time intervals a response of the digester to a change in
H-factor or alkali dosage (i.e. a change in process parameters),
before further control (i.e., further adjustments of the H-factor
correction or the alkaline dosage concentration) is warranted.
[0050] Thus, unlike conventional controllers, a controller of the
present invention uses the aforementioned dead-time compensators to
assess resultant changes to a processor before taking subsequent
changes to the processing parameters. The evaluators in the
dead-time compensators of the present invention provide a mechanism
by which subsequent process changes (as for example might be
warranted in simple proportionate control), subsequent rule changes
(as for example might be warranted in an adaptive control) or
subsequent model changes (as for example might be warranted in a
model-based control) can be evaluated to ascertain if an expected
change has occurred.
[0051] Thus, in general, the present invention includes a system
and a method for control of a processor. The apparatus and methods
of the present invention can follow the illustrative steps depicted
in FIG. 6. At step 610, a reactant product from the processor is
analyzed. At step 620, at least one change to at least one
processing parameter of the processor is determined based on at
least one property of the reactant product. At step 630, the at
least one processing parameter of the processor is changed. At step
640, following a prescribed process dead time, changes to the at
least one property of subsequently sampled reactant products are
evaluated at a multiplicity of times about the prescribed process
dead time. Steps 620-640 and other similar process control steps
can be repeated during processor control.
[0052] Step 610 can evaluate a reactor process susceptible to
disturbances in plug flow conditions. Disturbances can be due to
channeling whereby a reactant product prematurely flows into
subsequent stages of the processor. Step 610 can analyze the
reactant product from any of one of a pulp digester and other paper
mill processors such as for example the above-noted oxygen
delignifcation processors, the bleaching processors, and the
causticizers. At step 610, analysis can be made on a reactant
product taken from for example different stages such as for example
the sequential cooking sections 10, 20, 30, and 40 and from the
discharge section 50 of the pulp digester. Analysis at step 610 can
determine a Kappa number, a residual alkali, or any other useful
metric of a reactant product. The analysis is preferably performed
automatically, but if need be, can be performed off-line and
subsequently entered.
[0053] Step 620 can determine a prescribed change to the at least
one processing parameter based on for example the rule base shown
in Table 2. The prescribed changes, however, can be determined from
a model base. Further, if the digester is in, for example a warm-up
or shut down stage, the determination of a prescribed change can be
nullified.
[0054] Step 630 can change the at least one processing parameter in
a step-change or by a ramped or progressive change to the
processing parameters. Preferably, the time to implement the change
should be small compared to the anticipated dead time. Step 630 can
change at least one processing parameter based on at least a
bifurcated error state of the at least two properties of the
reactant product.
[0055] Step 630 can for example maintain at least one of a Kappa
number representative of a cellulose fiber concentration and an
alkalinity of the digested pulp product within target values, and
can control an H-factor of the digester and at least one of an
input alkali dosage concentration and an alkali/wood-input ratio to
the digester. Step 630 cancan increase at least one of the
H-factor, the input alkali dosage concentration, and the
alkali/wood-input ratio when the Kappa number is above a target
value, and can decrease at least one of the H-factor, the input
alkali dosage concentration, and the alkali/wood-input ratio when
the Kappa number of the digested pulp product is below a target
value. Step 630 can for example, when a residual alkalinity of the
digested pulp product is below a target value, either increase the
input alkali dosage concentration or the alkali/wood-input ratio or
decrease the H-factor. Step 630 can for example, when a residual
alkalinity of the digested pulp product is above a target value,
either decrease the input alkali dosage concentration or the
alkali/wood-input ratio anor increase the H-factor.
[0056] Step 640 can evaluate the changes to the at least one
property at a multiplicity of times about an expected dead-time. In
an illustrative embodiment described herein, three times were
evaluated, but any other number of evaluation times such as for
example (2, 4, 5, . . . ) is possible. At step 640, measured values
of the at least one property of the reactant product, are compared.
By comparison, an assessment is made as to whether or not the
prescribed changes have occurred, have not occurred, or are
occurring.
[0057] FIG. 7 is a schematic of an illustrative computer 700 of the
present invention executing any of the above noted steps. Indeed,
the controller 80 of the present invention can include well-known
computers such for example a personal computer, a portable
computer, a computer workstation with sufficient memory and
processing capability, or any device configured to work like a
computer. The computer would include a central processing unit 704
(CPU) that communicates with a number of other devices by way of a
system bus 706. The computer 702 includes a random access memory
(RAM) 708 that stores temporary values used in implementing the
process control steps for the controller of the present
invention.
[0058] The central processing unit 704 can be configured for high
volume data transmission for performing a significant number of
mathematical calculations in controlling the mass spectrometer of
the present invention. A Pentium III microprocessor such as the 1
GHz Pentium III manufactured by Intel Inc. may be used for CPU 704.
The processor employs a 32-bit architecture. Other suitable
processors include but are not limited to the Motorola 500 MHZ
Power PC G4 processor and the Advanced Micro Devices 1 GHz AMD
Athlon processor. Multiple processors and workstations may be used
as well.
[0059] A ROM 710 is preferably included in a semiconductor form
although other read only memory forms including optical medium may
be used to host application software and temporary results. The ROM
710 connects to the system bus 706 for use by the CPU 704. The ROM
710 includes computer readable instructions that, when executed by
the CPU 704, perform different functions associated with
controlling the mass spectrometer of the present invention. An
input control 712 connects to the system bus 706 and provides an
interface with various peripheral equipment including a keyboard
714 and a pointing device such as a mouse 716 settles to permit
user interaction with graphical user interfaces. The input
controller 712 may include different ports such as a mouse port in
the form of a PS2 port or, for example, a universal serial bus
(USB) port. The keyboard port for the input controller 712 can be
in the form of a mini-DIN port although other connectors may be
used as well. The input controller 712 may also include serial
ports or parallel ports as well.
[0060] A disc controller 718 connects via driving cables to a
removal media drive 720 which may be implemented as a floppy disc
drive, as well as a hard disc drive 722 and a CD-ROM drive (not
shown). In addition, a PCI expansion slide is provided on a disc
controller 718, a motherboard that hosts the CPU 704. An enhanced
graphic port expansion slot is provided and provides 3-D graphics
with fast access to the main memory. The hard disc 722 may also
include a CD drive that may be readable as well as writable. A
communication controller 724 provides a connection to a network
728, which can be a local area network, wide area network, a
virtual private network (VPN), or an extranet. The communications
controller 724 can also provide a connection to a public switched
telephone network (PSIN) 726 for providing Internet access. In one
embodiment, the networks 728 and 726 and the communication
controller 724 are connected by way of a plurality of connections
including a cable-modem connection, digital subscriber line (DSL)
connection, fiber optic connection, dial-up modem connection, and
the like that connects to the communication controller 724.
[0061] An input/output controller 730 also provides connections to
the external components such as an external hard disc drive 732, a
printer 734, for example, by way of an RS 232 port and a bus line.
The input/output controller 730 can be connected to measurement
systems for determining for example the blow Kappa number and/or
the residual alkali concentration.
[0062] A display controller 736 interconnects the system bus 706 to
a display device, such as a cathode ray tube (CRT) 738. The CRT can
be used for display of the digester processing conditions as well
as providing information about the operational status of the
processor (e.g., digester temperatures at the sequential stages,
input pulp feed rate, input alkali rate, output production rate,
blow Kappa, and residual alkali.) While a CRT is shown, a variety
of display devices may be used such as an LCD (liquid crystal
display) 740, or plasma display device. Display device permits
displaying of graphical user interfaces.
[0063] The present invention thus also includes a computer-pro gram
product that may be hosted on a storage medium and include
instructions that can be used to program a computer to perform a
process in accordance with the present invention. This storage
medium can include, but is not limited to, any type of disk
including floppy disks, optical disks, CD-ROM, magneto-optical
disks, ROMs, RAMs, EPROMs, EEPROMs, Flash Memory, Magnetic or
Optical Cards, or any type of media suitable for storing electronic
instructions.
[0064] This invention may also be conveniently implemented using a
conventional general purpose digital computer programmed according
to the teachings of the present specification, as will be apparent
to those skilled in the computer art. Appropriate software coding
can readily be prepared by skilled programmers based on the
teachings of the present disclosure as will be apparent to those
skilled in the software art. In particular, the computer program
product controlling the operation of the processor of the present
invention can be written in a number of computer languages
including but not limited to C, C.sup.++, Fortran, and Basic, as
would be recognized by those of ordinary skill in the art. The
invention may also be implemented by the preparation of
applications specific integrated circuits or by interconnecting an
appropriate network of conventional component circuits, as will be
readily apparent to those skilled in the art.
[0065] As such, the present invention includes a computer program
product including a first computer program product component for
analyzing a reactant product from a processor, a second computer
program product component for determining based on at least one
property of the reactant product at least one change to at least
one processing parameter of the processor, a third computer program
product component for changing the at least one processing
parameter of the processor, a fourth computer program product
component for evaluating following a prescribed process dead time
changes to the at least one property of the reactant product, and a
fifth computer program product component for re-executing the first
through fourth computer program product components.
[0066] In addition, the present invention includes a computer
program product including a first computer program component for
analyzing a reactant product from the processor, a second computer
program product for determining based on at least two properties of
the reactant product a change to at least one processing parameter
of the processor, and a third computer program product for changing
the at least one processing parameter based on at least a
bifurcated error state of the at least two properties of the
reactant product.
[0067] Numerous modifications and variations of the present
invention are possible in light of the above teachings. It is
therefore to be understood that within the scope of the appended
claims, the invention may be practiced otherwise than as
specifically described herein.
* * * * *