Patent Application
20070137806
This invention relates to pulp refining and papermaking and particularly to techniques for controlling the production and the quality of the pulp used in a papermaking process by employing a technique that efficiently and quickly removes latency from thermo-mechanical pulp (TMP) so that analysis of the pulp can be accomplished with minimum lag time to enable faster control of the TMP process.
Processes for making paper pulp consist in reducing the raw materials to separate fibers containing a greater or lesser amount of cellulose depending on the qualities which the pulp produced is required to have. The processes essentially consist of grinding operations, which are basically mechanical, which may be combined with more or less powerful delignification operations, which are basically chemical.
Depending on the relative importance of the two treatments, it is possible to distinguish five major types of pulp:
Refiner mechanical pulp (RMP) is produced by the mechanical reduction of wood chips (and sometimes sawdust) in a disc refiner. The process usually involves the use of two refining stages operating in series, i.e., two-stage refiring, and produces a longer-fibered pulp than conventional ground wood. As a result, it is stronger, freer, bulkier, but usually somewhat darker in color, than that of stone ground wood. Thermo-mechanical pulping (TMP) was the first major modification of RMP, and is still employed on a large scale to produce high-tear pulps for newsprint and board. This process involves steaming the raw material under pressure for a short period of time prior to and during refining. The steaming serves to soften the chips, with the result that the pulp produced has a greater percentage of long fibers and fewer shives than RMP. The refined pulp from the TMP process is stored in a latency chest that allows the beaten fibers to relax in hot water to remove latency. In the latency chest, pulp is agitated at a consistency of about 1.25% in a temperature range generally between about 70° C. to 90° C. for 20 to 30 minutes or more. A fiber quality monitor that collects samples and determines pulp quality parameters such as freeness and fiber length distribution, as measured by various standards, is typically positioned at the exit of the latency chest.
It is becoming increasingly important to produce TMP pulp that is both uniform and of a high quality. Papermakers desire to optimize paper machine operations, and in some instances to replace the expensive kraft furnish. Even though advanced process control has gained general acceptance in the pulp and paper industry, the therno-mechanical pulping process is still under manual control in most pulp mills. Reliance on manual control stems primarily from the complexity of the TMP process, which is highly interactive requiring control and variable inputs from many sections of the refining process. Additionally, control of the TMP process is further complicated as blow-line consistency in most cases is not measured using an online sensor. Pulp quality descriptive variables such as fiber length and freeness are also measured infrequently as operation of the latency chest requires a residence time of at least 30 minutes to ensure complete removal of the latency.
In order to produce high quality thermo-mechanical pulp, the refining process must be under tight control. Unfortunately, the response of the pulp quality variables is extremely slow, as the dynamics of the latency chest introduces a significant lag. The long latency chest dynamics limits the maximum achievable execution frequency of any closed loop control of the pulp quality variables. Any changes made to the refining system will have an immediate impact on the quality but due to the presence of the latency chest, the changes cannot be measured immediately. The industry is in need of a rapid sampling assembly that can quickly remove latency in the pulp for analysis.
The present invention is based in part on the development of a method and apparatus for latency removal which can be effected without introducing a significant lag time. As a result, immediate and accurate feedback is provided so that the refiner can be controlled precisely to produce mechanical pulp having the desired degree of refining. This allows the production of a maximum amount of pulp having desired properties and a minimum amount of pulp having undesired properties.
In one aspect, the invention is directed to process for analyzing a pulp material that includes the steps of:
With the present invention, the pulp samples can be removed, for instance, from the blow-line of a TMP refining process for analysis. Because steps (a) through (d) can be completed in significantly less time than can be achieved with current latency removal techniques, the pulp composition data is quickly available for feedback process control. In essence, the inventive rapid sampling assembly eliminates the delay associated with the latency chest.
A method of controlling a refiner which refines cellulosic fibrous material to produce mechanical pulp having latent properties that includes the steps of:
An apparatus for analyzing a pulp material that includes:
The present invention is directed to a pulp sampling assembly that can be employed in a continuous or batch mode. While the invention is particularly suited for use with thermo-mechanical pulp (TMP) processes, it is understood that the pulp sampling assembly can be employed with any pulping process to remove latency from pulp samples derived from various stages of the pulping process.
In a preferred embodiment of the inventive technique, a sample of high consistency pulp is diluted with hot or boiling water. After the latency is removed from the sample by hot disintegration, the sample can be analyzed with any pulp quality analyzer. Typically, the initial sample, which has a consistency of about 25% to 60%, is diluted to a consistency of about 0.1% to 5% and preferably about 4%. The term “consistency” describes the relative fiber content in a given stock quantity. Thus, an increase in consistency of the pulp stock indicates a relative increase in the dry, wood fiber constituent of a slurried, fiber-in-water suspension. The initial sample size typically ranges from about 1 to 2 liters in the batch mode or by continuous sampling. The hot disintegration step itself takes only about 1 to 5 minutes to complete so that the time required to accomplish the analysis of the pulp from obtaining the initial sampling to generating results from the pulp quality analyzer should be about 5 to 8 minutes.
In another embodiment of the invention, the signals from the pulp quality analyzer can be employed for feedback control of the refining process, e.g., TMP process, by which wood (or other fibrous raw material) is reduced to a fibrous mass which is used in papermaking. A TMP process can employ a single refiner line that empties into a latency chest or it may employ a plurality of parallel refiner lines that empty into a common latency chest. Each refiner line includes at least one refiner and preferably each line includes two or more refiners that are connected in series. Signals from the pulp quality analyzer can be applied to control any of the individual refiner lines in TMP process or any unit operation(s) thereof. Thus, the phrase the “entire” TMP process (or “entire” TMP refining process) is meant the process that is encompassed by a refiner line.
While the invention will be illustrated in connection with a single refiner line (with two refiners in the line) of a TMP process, it is understood that the present invention is applicable to any TMP process that includes at least one refiner line wherein each refiner line has at least one refiner. Moreover, while it is preferred that the refiner lines in a multiple refiner line TMP all have the same number of refiners, it is not necessary. Thus, as an example, the present invention is applicable to a TMP process that consists of a plurality of parallel refiner lines where each refiner line has a different number of refiners. Each refiner line can be controlled with the present invention.
TMP refining processes and refiner devices are known in the art and are described, for example, in U.S. Pat. No. 6,361,650 to Danielsson, et al., U.S. Pat. No. 5,016,824 to Pietinen et al., U.S. Pat. No. 4,231,842 to Ojala, U.S. Pat. No. 4,145,246 to Goheen et al., and U.S. Pat. No. 4,421,595 to Huusari, “Handbook for Pulp & Paper Technologists,” 2nd ed., G. A. Smook, 1992, Angus Wilde Publications, Inc., and Pulp and Paper Manufacture Vol III (Papermaking and Paperboard Making), R. MacDonald, ed. 1970, McGraw Hill, and Du, H., entitled, “Multivariable predictive control of a TMP plant,” Ph.D. dissertation, UBC, Vancouver, BC, Canada, 1998, which are all incorporated herein by reference.
Raw materials, e.g., chip feed, enter the presteamer 10 of the refiner line. The presteamer 10 preferably can use both mill steam and recycled process steam to increase the chip temperature, typically to 180° C. Presteaming removes entrained air from the wood chips and induces lignin softening. The screw speed determines the volumetric feed rate to the primary refiner 44. Following the primary refiner 44 via primary blow-line 34 is a steam separator 18 that separates the semi-refined pulp and steam from the primary blow-line. The steam from the separator 18 is vented to the atmosphere or recovered in a steam recovery system.
From the separator 18, the semi-refined pulp is then fed into the secondary refiner 46 for further fiber development. At the exit of the secondary refiner 46 via secondary blow-line 40 is the latency chest 26. The latency chest 26 allows the majority of the beaten fibers to relax in hot water to remove latency. The pulp from the latency chest will be subject to a wide range of processing steps, depending on their method of preparation, and their end use. A small portion of refined pulp from the secondary blow-line 40 is diverted into the rapid sample assembly 80 for latency removal and analysis.
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A preferred disintegrator for removing latency employs the hot disintegration technique (also referred to as the Domtar method) by rapidly recirculating the pulp through a centrifugal pump as the pulp is maintained at a temperature of about 90° to 95° C. The energy that is imparted on the pulp removes the latency. The time to liberate latent properties can be gauged by measuring the Canadian standard freeness of the pulp, in other words, the freeness of the pulp can be been taken as an indication of release of the latent properties. The Domtar method is described, for example, in U.S. Pat. No. 4,276,119 to Karnis et al. which is incorporated herein by reference. Suitable disintegrators are commercially available from Labtech Instruments, Inc. (Laval, Canada).
The rapid sampling assembly device of the present invention can operate continuously so that successive samples of pulp can be removed from the secondary blow-line 40 for analysis. Preferably, the storage cell 52 is cleaned before a sample is placed into it. This can be accomplished by flushing the storage cell 52 with water and cleaning fluid from water and cleaning fluid source 58. The disintegrator 54 can also be similarly cleaned prior to each disintegration procedure.
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TMP controller design can employ single-loop Proportional-integral-derivative (PID) based decentralized control architecture. PID based control strategy is acceptable for regulation of local control loops such as flow and pressure regulation, but a PID controller cannot handle complex multivariable dynamics. In addition, a PID controller can only control a single process output. However, to adequately control pulp quality at least two variables such as, for example, CSF and MFL must be controlled. Since these variables are physically linked, they cannot be independently controlled to arbitrary targets, instead these variables must be controlled within an operator defined quality window. The quality window is defined by setting the upper and lower limits on the pulp quality variables. In order to handle this control problem a multivariable controller is required that can also handle process constraints. Constrained model based predictive control (MPC) is a natural candidate in the process industry. MPC provides a unified framework to efficiently handle complex process interactions and constraints. MPC technology has also gained industrial acceptance and it can be easily integrated into existing mill distributed control system platforms.
U.S. Patent Application Publication 2005/0263259 entitled “System and Method for Controlling a Thermo-Mechanical Wood Pulp Refiner,” to Sidhu et al. and assigned to Honeywell International, Inc., which is incorporated herein by reference, describes a technique of extending decentralized control architecture and strategy extended into a centralized controller design framework. However, this strategy cannot be directly extended to a centralized framework by utilizing a single MPC controller. Since the process dynamics are spread over a wide frequency range, a single MPC, executing at a fixed frequency, would not be able to provide adequate control of both fast and slow dynamics. In order to mitigate this problem, a two-level control strategy was developed.
Specifically, method disclosed in the '336 patent application applied MPC technology in a two-level control strategy that can control the entire TMP refining line to increase throughput, reduce energy usage and improve pulp quality. The control strategy leverages the natural decoupling in the process dynamics. As a result, Model Predicative Range Control controllers can be designed to independently regulate the fast and slow dynamics of the process. The first level is the Stabilization Controller that preferably regulates the refiner line motor loads and the blow-line consistencies. The second level is the Quality Controller that preferably controls the slow dynamics associated with the pulp quality variables. The Quality Controller can directly manipulate the plate gap to control the final pulp quality. The direct manipulation of the plate gap removes the requirement to implement an internal specific energy loop. However a specific energy loop on each refiner can be included. In this control strategy the designer can independently select the execution frequency of the two levels. By operating the refiner lines at the maximum allowable motor loads the production is automatically maximized for a given pulp quality window. This modular approach can also handle multiple refiner lines that empty into a common latency chest. In order to integrate and coordinate the Stabilization and Quality Controllers, an Optimizer based on distributed quadratic programming was also developed. The Optimizer performs a global optimization of the process and improves the overall constraint handling of the control strategy. With the use of the inventive sampling assembly, the quality control layer can be directly incorporated into the stabilization layer since fast pulp quality measurements will be available and the issue of fast and slow dynamics is no longer a problem. The coordination of multiple refiner lines can still be accomplished using the Optimizer. This approach would be a subset of the methodology described in U.S. Patent Application Publication No. 2005/0263259.
The foregoing has described the principles, preferred embodiments and modes of operation of the present invention. However, the invention should not be construed as being limited to the particular embodiments discussed. Thus, the above-described embodiments should be regarded as illustrative rather than restrictive, and it should be appreciated that variations may be made in those embodiments by workers skilled in the art without departing from the scope of the present invention as defined by the following claims.
