Paper making and fiber liberation – Processes and products – With measuring – inspecting and/or testing
Reexamination Certificate
1998-07-16
2001-03-13
Silverman, Stanley S. (Department: 1731)
Paper making and fiber liberation
Processes and products
With measuring, inspecting and/or testing
C162S253000, C162S216000, C162S336000, C162S258000, C162S259000, C162SDIG004, C162S252000
Reexamination Certificate
active
06200421
ABSTRACT:
The present invention relates to an apparatus for feeding papermaking stock to a papermachine and to an improved process for producing paper. The present invention provides a method for compensating consistency variations when feeding stock to a papermaking machine. It also provides a method for the fast and exact changing of stock composition when changing paper grades or correcting product characteristics.
In a papermaking process of the prior art, in order to obtain a well controlled and uniform stock, the stock components are normally metered into a stock mixing tank. The mixed stock is further fed to a second tank, called the machine tank.
In the art of papermaking the flow of dry substance is traditionally controlled volumetrically, keeping the consistency or solids contents at a constant level by means of control loops including consistency sensors and dilution means. Due to the time lag inherent in the dilution process, the control cannot be very exact and is subject to variations. Therefore the dilution is made in multiple steps and the variations are compensated by large tanks. The tank volumes are dimensioned according to the machine production capacity, and typically have a capacity for about ten minutes of production.
The water balance of the short circulation of a papermaking process requires that excess water brought with the stock into the circulation has to be removed into a longer circulation. Thereby also material contained in the removed water is transferred to the long circulation, and is either lost or has to be recovered by fiber recovery units.
Constant stock feeding requires a good and uniform function of the stock feeding pump. A prerequisite herefore is a sufficient pressure on the suction side of the pump, which is obtained by a sufficiently high level of stock in the respective tank.
In commonly used conventional papermaking processes stock is fed to the suction side of a mixing pump, drawing dilution water from an open, constant-pressure backwater tank. The stock is commonly fed over a constant-level stock box over a so called basis weight valve controlling the flow of solid stock as a function of stock consistency, pressure difference and friction. The circulating backwater volume in conventional processes is huge, and reaching equilibrium after a change in stock composition takes several minutes. The same inventor, in WO 93/23612 (Fl Patent 89728) presents a solution which provides faster grade changes by reducing the volumes of circulating stock and water involved. The disclosure of WO 93/23612 is considered included herein by reference.
When changing the composition of the stock in prior art papermaking processes, for instance when changing paper grade, the stock fed to the mixing chest is changed, whereby the composition of the stock in the mixing chest gradually changes until it corresponds to the stock composition fed to the chest. Correspondingly, the composition of stock fed to the machine chest changes providing a further delay. This causes off grade paper to be produced during grade changes and also before a corrective stock change has reached the paper machine.
With conventional papermaking processes this does not cause a great problem, because the time needed for reaching an equilibrium is determined by the short circulation rather than by the stock feeding. However, the short circulation process of the above mentioned WO 93/23612 is fast and requires a process providing fast and exact control of the stock composition.
Recently new consistency sensors functioning on the principles of micro-wave technology have been developed. One example is a sensor named “Kajaani MCA”, marketed in Finland by Oy Valmet Fisher Rosemount Ab. The new micro-wave sensors have improved measurement of the consistency to an accuracy permitting a fast and accurate control of solid contents over a wider range of variable conditions, such as flow speed, pressure, temperature and stock composition, than previously used types of sensors.
In the industry these sensors are gaining foothold for a more accurate monitoring and control of stock consistency. The accurate monitoring of stock consistency has so far been used for providing a more accurate dilution response in the tanks preceding the stock feeding pump so as to achieve, as closely as possible, a constant consistency which is required for a proper functioning of the down-stream papermachine.
The present inventor has realized that it is not necessary for the stock being supplied to the papermachine to have a constant consistency. Other process parameters, and especially the flow of stock can be adjusted to compensate for any consistency variations as long as the amount of solid material being fed to the papermachine is kept at a constant value.
Numerous advantages can be gained from this invention not only in the art of making paper but also in the art of making boards of various kinds. The term “paper” as used in the present specification and claims is thus intended to include not only paper and board produced in conventional paper machines, but also any kinds of webs and sheets produced from a slurry by spreading the slurry onto a web-forming wire.
An object of the present invention is to improve known methods for feeding stock to a papermachine and to provide a fast and exact feeding of paper stock of a desired composition to the machine.
An object of the invention is also to provide a stock feeding arrangement with a fast reaction to changes in stock composition.
A special object of the invention is to provide a feeding arrangement, which combines a small stock volume with an efficient stock mixing and a stable and adequate pump feeding pressure.
An object is further to provide means for neat changes of paper grades, by separating the old stock and the new stock at the grade change.
A further object of the invention is to eliminate the disturbances caused by dilution in stock consistency control.
A further objective is to provide an arrangement for feeding stock to a paper machine at a relatively high consistency, without using dilution as a means for providing feed stock consistency control.
A particular object of the invention is to provide means for changing paper grade fast and exactly in connection with a compact papermaking process.
The apparatus aspect of the invention defines an apparatus for feeding papermaking stock to a papermachine, said apparatus comprising a vessel for receiving and mixing stock components and a pipe for feeding the resulting mixed stock to said papermachine. The feeding pipe has connected thereto stock flow metering means for continuously providing data on the volumetric flow (Q) of stock through said pipe; stock consistency sensing means for continuously providing data on the consistency (C) of said stock; and stock flow control means for adjusting said flow. All of said means are connected to a control system adapted for receiving said flow data and said consistency data. The control system is adapted for continuously computing a value for the product (Q×C) of said flow data and said consistency data, and for comparing said computed value to a target value of said product, and for providing adjustment of said flow control means in response to deviations in said computed value from said target value.
Contrary to prior art processes, the control system of the present invention is adapted for providing adjustment of the flow only, in response to deviations in the computed value caused by variations in the consistency. The system does not require a constant flow nor a constant consistency but makes sure of a constant feed of solid material by keeping the product of flow and consistency at a constant value.
In the preferred embodiment of the invention the apparatus includes a relatively small stock mixing vessel placed on top of a pressure pipe connected to said feeding pipe. The vessel is preferably provided with a stock mixer, a level sensor and individual inlets for stock components.
The process aspect of the invention defines a process for producing paper or the like
Browdy and Neimark
Halpern Mark
POM Technology Oy AB
Silverman Stanley S.
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