Patent Application
20090242155
The present invention relates to a method and apparatus for mixing, and more particularly to a method and apparatus for mixing liquid and paper pulp, especially for improving outlet consistency of and removing gas from paper stock prior to feeding the paper pulp mixture to a paper making machine.
In the process of making paper stock, ingredients such as paper pulp and water are fed at a controlled rate into a stock mixing tank, often called a “machine chest.” After initial mixing in the unrefined stock chest, the paper pulp and water mixture is fed into a second vessel called a “primary machine chest.” A higher consistency of paper pulp output from the primary machine chest (by weight percent) tends to lead to a more efficient papermaking process (e.g., less paper stock required for a given minimum paper thickness).
Consistency of paper pulp weight percentage in paper stock has traditionally been achieved by using large mixing vessels and adding water injection control loops and water bypass loops into the paper pulp and water mixing system. Also, in many typical paper pulp mixing systems, a single impeller is located at the vessel bottom, near the outlet, such that there is only a single mixing zone. This configuration may be prone to channeling or insufficient mixing.
Some paper pulp mixing systems use multiple impellers. However, the inventors theorized that when impellers are spaced too close to each other or are operated at a high speed, they may create a single large mixing zone. This results in a “short circuiting” tank dynamic behavior, where fresh material added to the top of the stock chest is quickly pulled downward by the impellers towards the outlet, without much mixing. As in channeling, the short circuiting problem has the disadvantages of a limited mixing zone (some of the paper pulp towards the vessel sidewalls is not being mixed, producing “stagnant zones”) and inconsistency of paper pulp weight percentage at the outlet (because the newly-added paper pulp is forced towards the outlet before sufficient mixing can be achieved). This system dynamic can only have limited improvement by using a control loop feedback system, because there is too short of a time delay between the inlet disturbance and the outlet signal.
The inventors theorize that in order to prevent short circuiting behavior, some paper pulp mixing systems use multiple impellers that are spaced too far apart from each other or are operated at too low of a speed, which may create separate mixing and dynamic behavior that forms “caverns” and stagnant zones. The caverns are the separate mixing zones that mix a relatively small portion of the paper pulp in the stock chest. This leads to inadequate mixing of the paper pulp and water, and it results in poor paper pulp weight percentage consistency at the outlet.
Paper pulp often contains more than 10% air (by volume), which is bound in the fiber network, primarily in the form of small bubbles. Excessive entrapped air in the paper pulp is undesirable in the paper-making process.
This description of the background summarizes some observations of the prior art. However, the disclosure identified as the theorizing of the inventors is not intended to be an admission that the observations are part of the prior art. Further, the present invention is not limited to possessing all of these characteristics that constitute an advance over the prior art nor is the present invention limited to possessing all the solutions to the problems of the prior art.
A method and apparatus for mixing liquid and paper pulp includes one or more of the following attributes, diminished: paper stock consistency fluctuation at the outlet of the unrefined stock chest or primary machine chest; channeling; short circuiting; creation of caverns and stagnant zones; and entrapment of excess gas.
A method of mixing of paper stock, having improved outlet consistency, includes: (a) feeding liquid and feeding paper pulp into a vessel to form a mixture; (b) providing at least one counterflow impeller that is submerged in the mixture, the counterflow impeller being capable of simultaneously inducing both upward flow and downward flow; (c) rotating the counterflow impeller such that downward flow from the impeller is partially recirculated by upward flow from the impeller to form a mixing zone; (d) sensing a parameter of the mixture that is discharged from the vessel; and (e) controlling the feed rate of the liquid and the feed rate of the paper pulp in the feeding step (a) based on the sensing step (d).
An apparatus for mixing of paper stock, having improved outlet consistency, includes a vessel for containing liquid and paper pulp, at least one counterflow impeller, and a feedback system for controlling outlet consistency. The counterflow impeller is adapted for submerging below the liquid and paper pulp surface and adapted for simultaneously inducing both upward flow and downward flow. The feedback system for controlling outlet consistency includes a sensor capable of determining a parameter of the mixture that is discharged from the vessel and a controller capable of adjusting the feed rate of the liquid and the feed rate of the paper pulp that enters the vessel.
The method and apparatus of mixing paper stock, having improved outlet consistency, may also include providing at least one additional impeller, spaced apart from the counterflow impeller such that each impeller can produce a substantially separate mixing zone. The parameter that is sensed in the method and apparatus of mixing paper stock, having improved outlet consistency, may also be percent paper pulp by weight or a proxy for percent paper pulp by weight. Proxies for the percent paper pulp by weight parameter will be understood by persons familiar with paper pulp technology and its instrumentation.
The paper stock produced at the outlet in the method and apparatus of mixing paper stock, having improved outlet consistency, may also contain up to approximately seven (7) percent paper pulp by weight.
At least one of the counterflow impellers that are provided in the method and apparatus of mixing paper stock, having improved outlet consistency, may also have a diameter that is between approximately seventy (70) percent and approximately ninety (90) percent of the vessel diameter. One of the counterflow impellers may also be adapted to be adjacent to the mixture surface. Each of the counterflow impellers may also have a tip speed that does not exceed approximately three (3) meters per second. The vessel may also include at least one water injection port. The vessel preferably is a primary machine chest or an unrefined stock chest having a vertical orientation, a top end, and a bottom end, and the paper pulp enters the vessel near the top end and exits near the bottom end. At least one of the counterflow impellers preferably has an axis of rotation that is substantially perpendicular to the vertical orientation of the vessel.
These and various other advantages and features are pointed out with particularity in the claims annexed hereto and forming a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to the accompanying descriptive matter, in which there are illustrated and described preferred embodiments of the invention.
Referring to
Typically, because of the specific rheology of paper pulp at consistency levels up to seven percent (7%) paper pulp by weight, the active mixing zone 162 occupies only part of vessel 110, as illustrated in
Referring to
In an exemplary embodiment, a paper stock mixing system 10 allows mixing of paper pulp mixture 60 up to approximately seven percent (7%) paper pulp by weight, with the remainder of the mixture comprising water.
Counterflow impellers 24a, 24b, and 24c are used to mix the paper pulp mixture 60 in vessel 30. System 10 preferably includes two or three counterflow impellers 24, as shown in
Using multiple mixing impellers 24, in embodiments having multiple impellers 24, helps create separate mixing zones Z1, Z2, and Z3 in the portions of the vessel surrounding each impeller, which helps to prevent the problem of channeling. In some examples of channeling, there is only a single active mixing zone at the vessel bottom, so newly added paper pulp is forced down to the active mixing zone in a “channel” through the “stagnant zone” that occupies the upper portion of the vessel.
To maintain separate active mixing zones Z1, Z2, and Z3, the impellers 24 preferably are spaced far enough apart and be operated at a slow enough rotational velocity such that mixing is complete within each mixing zone (i.e., the paper pulp mixture that enters an active mixing zone reaches approximately the same percentage of paper pulp by weight as the paper pulp mixture already in the particular zone), and such that the mixing zones don't combine together into a single mixing zone. In a preferred embodiment, in order to create separate mixing zones between the impellers, the impellers are spaced at least 33% of the impeller diameter apart from each other. The optimal spacing between the impellers depends on the dimensions of vessel 30, impeller rotational speed, parameters of mixture, and the like, as will be understood by persons familiar with mixing technology in view of the present disclosures.
To prevent active mixing zones from being too far apart, the impellers 24 preferably are spaced close enough together and operated at a high enough rotational speed such that newly added paper pulp spends some time being mixed in each active mixing zone Z1, Z2, and Z3, without spending substantial time in a stagnant zone between mixing zones.
Using multiple impellers 24 of a counterflow design, such as is shown in the Figures, at a predetermined distance from each other helps in many circumstances to achieve a balance between two undesirable conditions: (1) short circuiting behavior, where active mixing zones are so close together that they effectively combine into a single mixing zone, so newly added paper pulp quickly reaches the vessel bottom and exits via the paper pulp outlet before complete mixing can be achieved, and (2) caverns with stagnant zones, where active mixing zones are so far apart (creating “caverns”) that a substantial portion of the tank is not effectively mixed (“stagnant zones”). Impellers 24 preferably are configured such that the inner part pumps fluid in the opposite direction from the outer part. The spacing between impellers 24 depends on tank dimensions, impeller speed and configuration, desired paper pulp weight percentage in mixture 60, and the like, as will be understood by persons familiar with mixing technology.
Using multiple counterflow impellers 24 allows, in many circumstances, each of the active mixing zones Z1, Z2, and Z3 to slightly overlap at the outer boundary. For example, the lower boundary of mixing zone Z1 slightly overlaps the upper boundary of mixing zone Z2. Each counterflow impeller 24, as shown in
Using multiple counterflow impellers 24 provides multiple areas of “zonal mixing” to maximize the time paper pulp mixture 60 spends being actively mixed, while minimizing the short circuiting potential from new stock flow. Also, using multiple counterflow impellers 24 allows for complete mixing at a slower impeller rotation speed than using multiple conventional impellers, resulting in an energy cost savings, minimization of potential fiber shear damage, and an increase in the mean time between failure of the mixing system 10 components. Conventional impeller systems require significantly greater power and produce less flow than counterflow impeller systems. By pumping simultaneously upwards and downwards, counterflow impellers can mix paper pulp stock more evenly throughout all of vessel 30, with minimal stagnant zones.
Using multiple counterflow impellers 24 enhances the capability of paper stock mixing system 10 to achieve a higher pumping rate (faster production of completely mixed paper stock out of outlet 38) than using conventional impellers because the system can efficiently operate at much higher impeller-to-tank diameter ratios than a system using conventional impellers. The counterflow impellers may have any diameter relative to the diameter of vessel 30, depending on the desired process parameters. In a preferred embodiment, counterflow impellers 24a, 24b, and 24c have a diameter that ranges from seventy (70) to ninety (90) percent of the diameter of vessel 30. Paper stock at up to 7% consistency typically has a very high yield stress and is difficult to mix. Using multiple counterflow impellers with a diameter between 70% and 90% of the diameter of vessel 30 helps circulate the portions of paper pulp mixture 60 that are close to vessel sidewall 32, thereby minimizing stagnant zones at the vessel sidewall.
Water inlet control system 50, which preferably is conventional, uses the signals from outlet water sensor 51 to adjust the water flow volume and rate of entry into vessel 30. Control system 50 keeps consistency fluctuations (percentage paper pulp by weight) at target values by adjusting the flow volume and rate of water that enters vessel 30. Within paper pulp outlet 38, outlet water sensor 51 measures paper pulp mixture 60 flow speed and consistency (percentage paper pulp by weight). The parameters sensed by outlet water sensor 51 of the flow of mixture 60 that is discharged from vessel 30 may include, but is not limited to, flow speed, percentage paper pulp by weight, moisture content, viscosity, or any other parameter that is a proxy for the listed parameters. Other parameters may be sensed by outlet sensor 51, the control of which would help improve the consistency of paper pulp mixture 60 that is discharged from vessel 30.
The water inlet control is accomplished by sending the signals from outlet water sensor 51 to inlet water controller 52, which controls the rate and volume of water flow through upper water valve 53, lower water valve 54, and pulp drainage valve 55. These three valves 53, 54, and 55 control the rate and volume of water flow into vessel 30 via upper water inlet 56, lower water inlet 57, and pulp drainage inlet 58, respectively.
Using multiple counterflow impellers 24 increases the time constant of water inlet control system 50 (i.e., the newly added paper pulp mixture 60 entering through paper pulp inlet 36 spends a longer time being mixed before it reaches paper pulp outlet 38). This longer time constant decreases the fluctuation of paper pulp consistency (percentage paper pulp by weight) at paper pulp outlet 38 that results from introduction of new paper pulp with a different percentage paper pulp by weight than the paper pulp mixture 60 already in vessel 30.
Mechanical drive 26 may be any mechanical drive known in the pertinent art that may be adapted to rotate shaft 22 and blade assemblies 40 to the desired speed, such as a gear box, a belt drive, and the like. Mechanical drive 26 is coupled to the upper end of shaft 22. In a preferred embodiment, a reinforced gear drive is used, which is specifically designed to handle the high level of torque required for paper pulp mixing applications. The reinforced gear drive includes a heavy-duty stress relieved housing with additional gusseting to resist “racking.”
Another embodiment of the invention is shown in
In
Referring to
Preferably, inner blade 42 has a length of approximately seventy (70) percent of the radius of counterflow impeller 24. The distal end of inner blade 42 is connected to vertical flow divider 44. The proximal end of outer blade 46 is connected to vertical flow divider 44, on the opposite side from inner blade 42.
Hub 48 attaches each counterflow impeller 24 to the shaft 22 (shown in
Referring to
The present invention contemplates any counterflow impeller 24, any number of blade assemblies 40, and blade assemblies 40 of any length and configuration. The length of impeller blade assemblies 40, the length of inner blade 42 and outer blade 46, and the pitch angles A1 and A2 shown in
Paper pulp that enters the paper stock mixing chest also tends to be degassified, as can be best seen in
In a paper stock mixing system that uses a conventional axial impeller, there may be an opposite effect of gas entrapment, where additional gas is incorporated into mixture 60. In a conventional system, the axial impeller may generate a vortex that incorporates gas from above surface 61 into mixture 60 after freshly added material enters via inlet 36.
Channeling system 84 is a system, for example, where a single impeller is located at the vessel bottom, near the outlet, and there is only a single mixing zone. This results in a “channeling” tank dynamic behavior, where the new paper pulp added to the top of the stock chest is pulled downward towards the outlet, through the old paper pulp already in the chest, creating a narrow “channel.” In graph 80, channeling system 84 quickly (low time constant) transmits a high portion of the magnitude of the inlet step function disturbance 83 to the outlet. This behavior means that when new paper pulp is added to channeling system 84 that has a different paper pulp percentage by weight than the paper pulp mixture already in the vessel, this disturbance quickly affects the paper pulp percentage at the outlet. This low degree of dampening of an input disturbance is not ideal for the output of paper pulp mixing system.
Short circuiting system 85 is a system, for example, where multiple impellers are spaced too close to each other or are operated at a high speed, so a single large mixing zone is created. This results in a “short circuiting” tank dynamic behavior, where fresh material added to the top of the stock chest is quickly pulled downward by the impellers towards the outlet, without much mixing. In graph 80, short circuiting system 85 quickly (low time constant) transmits a high portion of the magnitude of the inlet step function disturbance 83 to the outlet. This system achieves better mixing than channeling system 84, because the paper pulp percentage at the outlet experiences less of a disturbance due to the step function input. However, the time constant here is lower than in channeling system 84, meaning that the input disturbance reaches the outlet faster, so it is harder for a water inlet feedback control system to improve the outlet paper pulp consistency.
Counterflow impeller system 86 is a system like that described in
The foregoing description is provided for the purpose of explanation and is not to be construed as limiting the invention. While the invention has been described with reference to preferred embodiments or preferred methods, it is understood that the words which have been used herein are words of description and illustration, rather than words of limitation. Furthermore, although the invention has been described herein with reference to particular structure, methods, and embodiments, the invention is not intended to be limited to the particulars disclosed herein, as the invention extends to all structures, methods and uses that are within the scope of the appended claims. Further, several advantages have been described that flow from the structure and methods; the present invention is not limited to structure and methods that encompass any or all of these advantages. Those skilled in the relevant art, having the benefit of the teachings of this specification, may effect numerous modifications to the invention as described herein, and changes may be made without departing from the scope and spirit of the invention as defined by the appended claims.
