The present invention relates to a feeding arrangement for feeding comminuted cellulose material to a treatment stage using a plug screw feeder.
Feeding of comminuted cellulose material to be treated in a pressurized treatment stage may be carried out in different ways and is dependent on a number of factors such as the characteristics of the material to be fed and the possible desired action on the material beside the actual feeding thereof. Plug screw feeders, in which a rotating screw transports the material forward by the continuous movement of the screw flights is a commonly used type of feeder. At the same time as the material is fed to the treatment stage, the screw and associated screw pipe or housing also exercises a volumetric compression function. Such plug screw feeders are very commonly used for feeding wood chips to a digester, an impregnator or a mechanical refiner, and especially to treatment stages operating at elevated pressures as the plug screw feeder provides for a pressure lock against back blow.
A known problem with such plug screw feeders is that the theoretical possible compression of the material in the plug screw feeder is often not obtained. A solution to such a problem may be to “force-feed” the plug screw feeder, i.e. making sure that material is fed evenly over the entire inlet of the plug screw feeder such that the feed screw flights are filled evenly before a compression is made in a subsequent compression zone of the plug screw feeder.
Several prior art solutions for “force-feed” has been disclosed in the patent literature. Already in DE 497,689 (1930) was disclosed a “force-feed” to a plug screw feeder using a smaller single force feed screw. In DE 566,267 (1932) was disclosed another version of a “force-feed” to a plug screw feeder using a larger single force feed screw.
A prior art screw feeder apparatus using the principle of force-feeding is also disclosed in U.S. Pat. No. 5,320,034(1994)/=EP 493,422. This patent is limited to how the speed of the single force feed screw as well as the feed screw of the plug screw feeder is controlled in order to increase the real compression ratio of the plug screw feeder. Similar speed control of the single force feed screw was also shown in JP 63299890 (1988).
However, when dealing with other types of materials, different considerations may apply. It has been discovered that the use “force-feed” with single feed screws feeding comminuted cellulose material to a plug screw feeder does not solve the problems with even initial filling of the screw feeders. Especially when feeding of non-wood plant material, such material having a much lower bulk density than wood chips, without modification.
The use of non-wood plants has recently become more and more important. In some cases the material is used as fuel for generating heat. The material may also be used for production of pulp for paper-making purposes or the like, replacing wood as the source material. In many such processes, the non-wood material is treated in a pressurized reactor, for example in prehydrolysis Bio treatment processes where both temperature and pressure are high, imposing special requirements on the feeding arrangement.
There is thus a need for an improved feeding arrangement for comminuted cellulosic material, and especially suitable for the feeding of bulky non-wood plant material, and still be able to establish a permanent plug in the plug screw feeder preventing back blow from the treatment stage while avoiding total stop of the feeding.
A common problem with the approach of using a single force feed screw feeding a plug screw feeder is that the initial filling of the plug screw feeder should be uniform over the inlet to the plug screw feeder as to the outflow from the single force feed screw. However, it has been found that the plug screw feeder does not “suck in” the flow of material to the feed screw evenly from the plug screw feeder inlet. This is caused by the fact that the flights of the feed screw on one half of the inlet, i.e. the half located on one side of the feed screw axis prevents filling, while the flights of the feed screw on the other half of the inlet, i.e. the half located on the opposite side of the feed screw axis, promotes filling. This effect occurs also with “starve-fed” solutions, and especially those not having “force-feed” means and having the inlet to the plug screw feeder from the side.
In order to avoid the above identified problems it has been found beneficial for plug screw feeder operation, i.e. as to improved formation of a compacted plug, as well as reduced wear in the plug screw feeder housing that the feed of comminuted material to the feed screw should be done with at least 2 separate chutes, and that the majority of the cross section area of the chute outlets are arranged on opposite sides of the feed screw axis.
These and other objects are achieved in accordance with the appended claims.
More specifically, the invention according to the most general definition is related to a feeding arrangement for feeding comminuted material to a treatment stage, the feeding arrangement comprising
a plug screw feeder for feeding the material to the treatment stage, the feeder comprising a feed screw with a conical part at its outlet end and driven by an independent first motor and rotating around a feed screw axis and a corresponding plug screw housing surrounding the feed screw,
a first feeding chute for feeding the comminuted cellulose material to the plug screw feeder, said first chute being arranged at an angle in relation to the first screw axis, said angle lying in the range 30-150 degrees, preferably 90 degrees,
the chute being arranged to provide the material to an inlet section of the plug screw feeder, said inlet section being arranged in the periphery of the feed screw,
According to a preferred embodiment of the invention could also each chute have force feeding means driven by its own independent motor. Hence, under the more general concept could also the chutes be void of any motor driven feeding means, and instead use chutes establishing variable static head of material in the chute using level control.
According to another preferred embodiment of the invention could each force feeding means comprise a force feeding screw driven by an independent motor for feeding the comminuted cellulose material to the plug screw feeder. Hence, under the more general concept could also other force feeding means be used in order to subject the feed of the material with a positive feeding action.
According to another preferred embodiment of the invention each force-feeding screw (20) is arranged essentially in the same horizontal plane as the feed screw (10), the force-feeding screw (20) feeding the material from the side. This embodiment is specifically exposed to uneven wear of the feed screw and poor establishment of a pressure blocking plug in the plug screw feeder, and the invention reduces these problems considerably. However, side fed plug screw feeders are often preferred in system where the building height is restricted or should be reduced or if other restrictions applies for routing feed pipes to the plug screw feeder.
According to another preferred embodiment of the invention each force-feeding screw is arranged essentially in the same vertical plane as the feed screw, the force-feeding screw feeding the material from above or alternatively from below to the feed screw. Vertical feed of material may be considered to improve pressure blocking plug formation in the plug screw feeder, but the problem with uneven wear may still also be a problem in vertical feed so a better and balanced formation of the plug is of interest.
According to another preferred embodiment of the invention a first pair of chutes are arranged in a staggered relation to a orthogonal plane to the first screw axis, such that the chutes are parallel to the flights of the feed screw sweeping over the chute outlets. This design of the relation of the chutes versus the flights of the feed screw has shown to be of particular effect.
According to another preferred embodiment of the invention a second pair of chutes are arranged in a staggered relation to a orthogonal plane to the first screw axis, such that the chutes are parallel to the flights of the feed screw sweeping over the chute outlets, thus adding up to at least 4 chutes feeding material to the feed screw. This design utilizes more of the ordinary rectangular conventional inlet to the plug screw feeder. The additional pair of chutes could functionally be operated as replenishment means if the first filling from the first pair of chutes collapses or changes during the forward transport of material in the screw.
According to another preferred embodiment of the invention the plug screw feeder housing surrounding the feed screw has an open inlet with a rectangular general shape covering the entire diameter of the feed screw, and that the chutes are connected to flow channels having similar cross section as the chutes said flow channels running through an insert body which otherwise is filling the entire rectangular open inlet, enabling the flow of material to be fed directly to the periphery of the feed screw via said flow channels in the insert body.
This design with an insert body is a perfect rebuild kit for a conventional screw feeder with a rectangular inlet. However, if the screw feeder is designed from scratch, then the chutes are fully integrated into the housing from start, and no functional insert body is needed. In following disclosed embodiments of the invention is used a conventional screw feeder with rectangular inlet, thus needing an insert body.
According to another preferred embodiment of the invention the independent first motor for the feed screw and the independent motors for the force feeding means located in the chutes, are connected to a control unit, such that each motor is adapted to be able to be driven at variable speed by the control unit in relation to preset threshold values stored in the control unit, such that the feed screw is rotated at a first speed, and each force feeding means located in the chute is driven at individual speed. The principles with independent motor controls for the force feed and the feed screw is all known prior art. However, the need to control the pair of force feeders differently, each force feeder working towards a half of the feed screw, is essential for this invention and used in a novel way.
The independent control of the force feeders may also be controlled in a feed-back manner dependent on the actual deflection on the screw. Said deflection may be measured in a number of ways using for example strain gauges on the screw per se, or using force sensors on the screw bearings. If the deflection is detected as an increased relative force in any direction could the independent force feeders be controlled incrementally until the deflection is compensated by different feed in the chutes.
For the purpose of this disclosure, the term comminuted cellulose material includes wood material as well as non-wood plant material. But the invention is especially suitable for bulky non-wood material from herbaceous plants, such as straw, bagasse, and wheat (bran and grain material). Peat material and empty fruit bunches are also encompassed by the term, In short, the term non-wood plant material is used for all kinds of plant/plant part containing material not being defined as wood. The particle dimensions of the comminuted cellulose material may range in the interval of 5-50 mm when feeding wood chips and in case of straw material the length of chopped straw may lie in the range up to 100-200 mm.
According to one embodiment, the force-feeding screw may be arranged essentially perpendicular to the teed screw, meaning that their respective rotational axes are essentially perpendicular to each other. In one embodiment the force-feeding screw is arranged in such a way that the material is fed to the plug screw feeder from the side, i.e. a transverse feed into the plug screw feeder in relation to its feeding direction. The invention do not exclude the option from arranging the force-feeding screws in pairs from opposite sides of the plug screw feeder, even if this would result in a large foot print area of the system, as this could further balance the forces working against the feed screw and hence reduce uneven wear.
The invention, together with further objects and advantages thereof, may best be understood by reference to the following description and appended drawings, in which:
In the drawings, similar or corresponding elements are denoted by the same reference numbers.
For the purpose of this disclosure, the term longitudinal is meant to be the direction along which a body or part has its greatest extension. When the term is used in connection with the axes of screws, the longitudinal axis corresponds to the rotational axis of the feed screw.
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The feeding arrangement may further comprise (not shown) a volumetric detection of material fed into the force-feeding screw 20. The feeding device sets a predetermined rate of material flow, i.e. the production flow. The rotational speed of the plug screw 10 is then set in relation to the predetermined rate of material flow in order to achieve a predetermined density increase of the material in the plug screw (preferably measured as the prevailing driving torque of the motor in amperes). If the production flow is increased, the rotational speed of the plug screw 10 has to be increased to maintain the same density in the plug screw feeder 40. If the rotational speed of the plug screw 10 is maintained while the production flow increases, the density will increase. By increasing the material density in the plug screw feeder an improved fluid-tight plug flow of the material is created through the plug screw feeder 40, but the plug density may not increase over a certain level where plugging occurs which level is specific for each kind of material being compressed.
The plug screw 10 and the force-feeding screws 20am-20dm are preferably arranged perpendicular to each other, i.e. having their respective rotational axes perpendicular to each other. In a preferred embodiment, they are arranged with their respective axes essentially in the same horizontal plane, so that the non-wood plant material is fed to the plug screw from the side. Preferably, the inlet section 12 is located close to a first end 13 of the plug screw 10, the first end being the one furthest away from the treatment stage 200.
Although the invention has been described with reference to specific illustrated embodiments, it is emphasized that it also covers equivalents to the disclosed features, as well as changes and variants obvious to a man skilled in the art, and the scope of the invention is only limited by the appended claims.
Number | Date | Country | Kind |
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1450611-7 | May 2014 | SE | national |
Filing Document | Filing Date | Country | Kind |
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PCT/SE2015/050552 | 5/19/2015 | WO | 00 |