Process and device for mixing stock suspensions

Details Process and device for mixing stock suspensions Process and device for mixing stock suspensions

1999-06-28

2001-03-13

Silverman, Stanley S. (Department: 1731)

Paper making and fiber liberation

Processes of chemical liberation, recovery or purification...

With agitation or forced circulation

C162S056000, C162S023000, C162S028000, C162S052000, C162S063000, C162S261000, C162S343000, C162S344000, C162S190000, C162S264000, C073S061630, C366S307000, C366S175200, C008S156000

Reexamination Certificate

active

06200417

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
The present application claims priority under 35 U.S.C. § 119 of German Patent Application No. 198 28 998.7, filed on Jun. 29, 1998, and German Patent Application No. 198 59 770.3, filed on Dec. 23, 1998, the disclosures of which are expressly incorporated by reference herein in their entireties.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a process and a device for mixing and piping suspensions of different natures and/or compositions in the stable section of a paper machine.
2. Description of Background Information
A process and device for mixing suspensions is known from U.S. Pat. No. 4,477,313 to Andersson, issued Oct. 16, 1984. According to the Andersson patent, the backwater collected in the paper machine is passed into open backwater tanks, and is then fed back to the headbox via mixing pumps provided with a thick stock supply.
If a change in paper type is made on the paper machine, drainage conditions usually change and, thus, the concentration (e.g., solid content) of the backwater (in particular) usually changes. However, in the Andersson patent, due to the high residence time of the backwater in the backwater tank, the concentration in the backwater tank changes only slowly. This means that stable conditions are established very slowly in the backwater cycle. During this adjustment phase, production must often be slowed down to achieve the required paper quality. Consequently, production and quality losses occur.
Another process and device for mixing suspensions is known from the (Published) German Patent Application No. DE 195 09 522 A1, published Sep. 26, 1996. In this document, a stock suspension is fed to a headbox, that is sectioned over the width of the machine, through a plurality of lines feeding stock suspension. The lines feeding stock suspension are connected to a distributor. A portion of the backwater arriving in the drainage region of the paper machine is fed sectionally to the headbox, and is used for basis weight control according to the well-known dilution principle. The remainder of the backwater is passed into a backwater tank, and sent back from the backwater tank into the stock preparation system (“stable section”), although the stable section is not shown in DE 195 09 522 A1.
SUMMARY OF THE INVENTION
In view of the shortcomings of the prior art, an object of the invention is to provide a process for the mixing of suspensions of different nature and/or composition in the stable section of a paper machine, which provides an improvement in the quality as well as a reduction in production loss at the time of the changeover between types. It should be noted that in the context of the specification and claims, suspensions having different “characteristics” have differing natures and/or compositions.
A further object of the invention is to provide a mixing device and/or piping in the stable section of a paper or cardboard machine for the blending of suspensions with higher solid content into a first suspension with little or no solid content, which likewise effects a reduction in quality losses and production loss at the time of the changeover between types. It should be noted that in the context of the specification and claims, a “negligible” solid content means little or no solid content.
According to a first aspect of the present invention, a process for mixing suspensions having differing characteristics in the stable section of a paper machine includes piping of a first suspension in a mixing tube to form a main flow having a main flow direction in a longitudinal direction of the mixing tube, and injecting one or more additional suspensions into the mixing tube. The additional suspension(s) may have a different solid content than the first suspension.
By means of the process according to the invention, larger backwater tanks are avoided, thereby reducing the amount of water in circulation in the paper machine, and, thus, at the time of the type changeover in the paper machine, a more rapid change in the composition of the stock suspensions is possible. Based on this more rapid change, the quality losses, and therefore also the production losses, are reduced. The “backwater” indicates the total circulating backwater with which, along with the fresh stock, the concentration of the stock suspension required in the headbox is obtained, as depicted in FIG.
1
. The cycles in the stable section are described in detail in the literature.
Optionally, a solid content of an additional suspension injected downstream along the main flow direction is, in each case, higher than or equal to a solid content of another additional suspension injected upstream along the main flow direction.
Further optionally, the first suspension includes a suspension of a backwater of the paper machine, and an entire backwater volume stream flows through the mixing tube. In this case, the backwater volume stream may be reduced by a backwater substream sufficient, according to the dilution water principle, for weight basis control on a headbox of the paper machine.
In a particularly advantageous embodiment of the process, flow directions of each of the injected additional suspensions coincide with the main flow direction.
Another embodiment of the process according to the invention includes maintaining a flow rate of the main flow in the mixing tube at a substantially constant level despite added liquid in the injected additional suspension, the flow rate of the main flow in tile mixing tube increasing only in an end region ofthe mixing tube. For a rapid type changeover without losses with respect to paper quality, the residence time of the backwater in the system should be as short as possible. Consequently, the flow rate in the mixing tube is optionally greater than 0.2 m/s, and further optionally, greater than 0.45 m/s (e.g., the dimensions of the mixing device are arranged to maintain these numerical flow rates).
It is also advantageous if each additional suspension is injected concentrically in the main flow. If the recirculation from the headbox is not piped into the mixing tube, recirculation from a headbox may be passed via a line to a vertical separator second stage.
In one particular variation, the first suspension includes a backwater stream of the paper machine, and the injections of additional suspensions include, in order along the main flow, injection of recirculation from a headbox, followed by injection of accepted stock from a vertical separator second stage, followed by injection of recirculation of a first cleaner stage, followed by injection of accepted stock from a second cleaner stage, followed by injection of fresh stock. Although this sequencing of the insertions in the direction of flow is particularly advantageous, additional suspension streams may be injected between, before, or after the recited order, or the sequence may be adapted according to the concentration gradient in view of other conditions present, relative to the concentration of the suspension streams. Moreover, the language “followed by” is not intended to preclude preceding, intervening, or following process operations after any individual injection, group of injections, or all the injections—other process operations may be placed in such positions without departing from the spirit of the invention.
In another particular variation, the first suspension includes a backwater stream of the paper machine, and the injections of the additional suspensions include, in order along the main flow, injection of accepted stock from a vertical separator second stage, followed by injection of recirculation from a first cleaner stage, followed by injection of accepted stock from a second cleaner stage, followed by injection of fresh stock With this variation, pulsations originating from the headbox and changes in recirculation do not affect the stability of the stable section of the paper machine.
In still another variation, the first suspension includes a backwater stream of the paper machine, and the injections of the addition

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