Patent Application
20110303380
The present invention relates to washing and dewatering of cellulose pulp and in particular to a wash/dewatering apparatus with one, or preferably two co-operating, cylindrical press rolls.
Pulp washing is a key operation in the chemical pulping line. There are many different types of washing and dewatering apparatuses available, some of which are based on washing by pressing the pulp such that fluid is removed.
A well-known type of wash press has two co-operating cylindrical press rolls, arranged with their rotation center in the same horizontal plane. The outer surface of each press roll is perforated and, during operation, cellulose pulp is input to a restricted space between the perforated roll surface and a restriction member, such as a vat, whereby a pulp web is formed on the perforated roll surface. The press rolls are arranged to rotate in opposite directions so as to transport the respective pulp webs in the direction of rotation to be pressed in a so-called pinch or nip where the distance between the press rolls is smallest.
The fluid removed from the pulp (i.e. the filtrate) passes through the perforated roll surface in a radially inward direction and can for example be transported to the ends of the respective press roll by means of axial filtrate channels. There is normally a filtrate tank arranged in connection with the wash press to collect all filtrate resulting from the washing in the press. There is often a supply of washing liquid to the wash press and since the washing liquid displaces fluid in the pulp, the washing principle will in such a case be a combination of dewatering, displacement and pressing.
A wash press of the described general type is disclosed in U.S. Pat. No. 3,980,518, for example.
Another example of a wash press of the described general type is the wash press disclosed in EP 1 035 250. The objective of this wash press is to improve the total dewatering and this particular wash press has a vat design in which the vat is converging towards the outer surface of the press roll, in the direction of rotation of the press roll. The vat is arranged to enclose the outer surface of the press roll from a pulp inflow chamber placed in the region of the press roll's highest point and further round at least 230° of the outer surface's circumference, so that the pulp web formed is constrained to run between the outer surface and the vat round at least 230° of the circumference while being subjected to a converging vat before the fibrous web reaches the final pinch between the press rolls.
A problem associated with a wash press with a long converging enclosed area is the tendency of plugging of the pulp suspension in the confined area between the vat and the outer surface of the press roll. Plugging of pulp suspension leads to undesired stops in the operation of the wash press with time-consuming cleaning operations and loss of production as a result. Another problem associated with such a wash press is that the pressure profile associated with the converging space leads to a rather tough treatment of the pulp suspension confined in the space between the outer surface of the press roll and the surrounding vat. The pressure created by the converging geometry forces not only fluid from the fiber suspension, but also fibers, through the perforations of the outer surface of the press roll. This leads to losses of valuable fibers, which results in lower production of pulp coming out of the washing/dewatering operation. Moreover, fibers in the filtrate complicate the handling of the filtrate and may demand external equipment in order to recover the fibers from the filtrate. This is especially the case if the filtrate is to be purged to external treatment or to a recipient. Yet another problem with wash presses with converging vat profiles is the build up of friction forces acting on the vat structure, which calls for a strong load supporting structure for the vat structure, inducing increased costs for the load supporting structure.
Accordingly, there is a need for a wash press reducing the problems associated with a wash press of the kind described above.
A general object of the invention is to provide an improved arrangement for washing cellulose pulp. A specific object is to achieve improved runnability of a wash press with cylindrical press rolls where a rather large part of the circumference of the press rolls is enclosed by a vat. Another object is to reduce the fiber content in the output flow of filtrate from the wash press. Still other objects are to enable wash presses in which the fiber suspension treated is subjected to more lenient conditions, implicating less damage to the fibers in the suspension and to enable wash presses which are less sensitive to variations in terms of for example concentration and flow of the pulp suspension fed into the wash press.
These objects are achieved in accordance with the attached claims.
Briefly, the present invention is based on the recognition that the pressure profile created by a converging geometry is not needed to achieve the desired production capacity along with satisfactory washing and dewatering for a wash press where a major part of the roll circumference is enclosed by a vat. It was earlier believed that the pulp suspension had to be forced towards the outer surface of the press roll by a forcing geometry, e.g. a converging vat. However, due to recent findings it has been shown that the fiber suspension can be washed and dewatered in an efficient way by letting the fiber suspension be formed against the outer surface of the press roll in a more open geometry. The pressure in the confined space between the press roll and the vat is built up dependent on the drainage properties of the fiber suspension and the fiber suspension itself decides how and when it is to be dewatered. This also leads to a more lenient treatment of the fiber suspension with less fibers in the filtrate as a result. Runnability problems, e.g. due to plugging of the fiber suspension is avoided or reduced at the same time as high capacity and high washing efficiency is obtained. In this way, the negative effects associated with a wash press of the aforementioned kind is avoided or reduced.
Thus, in accordance with the invention, an arrangement for washing and dewatering cellulose pulp comprising: a press roll, which is arranged to rotate during operation and which has a perforated outer surface for dewatering the pulp; a stationary guide surface, arranged at a distance from the perforated outer surface of the press roll so as to enclose a portion of the press roll in the circumferential direction of at least around 225° of the press roll's circumference counting from the inlet to the pinch, the stationary guide surface forming a substantially closed vat over this portion; a pulp passage, being defined substantially between the perforated outer surface of the press roll and the stationary guide surface, such that during operation, wherein pulp that is fed into the pulp passage is transported in a direction of rotation and, at the end of the pulp passage, is pressed in a pinch, possibly between the press roll and a second press roll; at least one zone in the pulp passage where a wash liquid is added to the pulp passage and where a wash filtrate displaced by the wash liquid is subsequently drained via the perforated outer surface of the press roll. Further a radial distance D between the stationary guide surface and the associated press roll is non-converging over substantially the whole pulp passage enclosed by the stationary guide surface, towards the pinch.
The proposed washing arrangement leads to a number of advantages, including:
Also, by using a parallel non-converging vat in a conventional wash press, the risk of plugging is drastically reduced, since the dewatering of the pulp is not forced, which it would be in a converging vat. Nevertheless, the dewatering is found to be equally efficient in a parallel vat, and this is due to the force of the increased pressure in the vat that builds up as the pulp is transported from pulp inlet to the pinch. This improves the availability of the wash press and reduces its downtime to a minimum.
The capacity of a wash press is among other things determined by the dewatering capacity of the final pinch. One way to increase the capacity of a wash press is to increase the pulp consistency prior the pinch. In that way the dewatering work to be done in the pinch is reduced, and consequently, the capacity of the wash press is increased.
If the surrounding vat length is increased, the time for dewatering prior the final pinch also becomes longer. If the pulp, during this extra time, is subjected to a dewatering pressure, either by a converging geometry or vat pressure, the pulp consistency prior the pinch will be higher, which will result in a higher capacity.
In a converging vat, an increased vat length and an increased pulp consistency prior the pinch will imply a significantly higher risk of pulp plugging. This is due to the fact that the pulp consistency in the end of the converging vat will be higher and have difficulties to pass the narrow end of the vat. By using a parallel vat the risk of plugging is considerably reduced, since there are no restrictions in the vat that may cause pulp plugging.
The invention, together with further objects and advantages thereof, is best understood from the following description with references to the appended drawings, of which:
In the drawings, similar or corresponding elements are denoted by the same reference numbers.
During operation, pulp fed from pulp inlet 109 into the pulp passage 108 is guided by the guide surface 106 in direction of rotation and pressed in a so-called pinch or nip 110 between the press rolls 102. In the illustrated example, pulp is input at the upper portion of the respective press roll 102 to enter the pulp passage 108, where it forms a pulp web 112 on the perforated roll surface 104. Typically there is some form of distribution means (not illustrated in this figure) to distribute the pulp evenly along the length of the press roll 102. The press rolls 102 rotate in opposite directions so as to transport the respective pulp web 112 in the direction of rotation to be pressed in the pinch 110 where the distance between the press rolls 102 is smallest. In the circumferential direction, the pulp passage 108 extends from the position or area where pulp is introduced onto the outer surface 104 of the press roll 102 and to the pinch 110 between the press rolls. The illustrated press rolls 102 comprise axial filtrate channels 114 which receive the filtrate that passes through the perforated roll surface 104. Washing liquid is in this example supplied to the pulp web at two different points (lines 116) per press roll 102. Washing liquid could, if desired, be supplied at more points per press roll or only at one point. The term point should be understood to possibly involve a certain extension in the circumferential direction. Pulp is output by means of a screw arrangement 118 which transfers the pulp to a subsequent process stage (not shown), such as a standpipe or another unit where the pulp may be diluted and processed.
It has long been believed that the vat is a completely communicating space. Thus, the vat pressure has only been measured at one point, which has then been taken as a measure of the vat pressure for the entire vat. The conventional position for measuring the vat pressure has been at the bottom of the vat, i.e. in the vicinity of the lowest point of the press roll 102. Recently, during measurements of the pressure at different points of the vat, it was surprisingly found, that the vat pressure is not at all constant throughout the vat, but increases towards the nip. The fact that the pressure increases through the vat implies that the press rolls 102 drag the pulp forward in a direction towards the nip. This means that it is not the pump normally feeding the press that pushes the pulp forward in the press. The pulp becomes attached to the rolls 102 due to the pressure difference over the pulp web, and the rolls 102 subsequently drag the pulp forward in a direction towards the nip 110. The pressure close to the nip 110 seems to be mainly governed by production rate, while the pressure close to the pulp inlet 109 seems to be more dependent of the flow rate of pulp fed to the wash press 100.
The partial flow of pulp suspension 132 does not necessarily have the same fiber concentration as the main flow 130. Typically, the partial flow 132 will be of a lower concentration compared to the main flow 130, but it is also possible to have a partial flow with a higher concentration than the main flow. The partial flows 132 are typically smaller than the respective main flows 130.
By merging the two flows of pulp 130 from the pulp passages 108 into a wedge-formed volume before the nip, and immediately after the final end of the stationary guide surfaces 106, a gradual convergence is created in the merged wedge-formed pulp volume before the merged pulp flows enters the actual nip. This convergence is thus only due to the actual nip and not to any convergence of the guide surfaces 106. From the nip, a pressure build up is generated hydraulically in the pulp webs counter current to the flow of pulp, which pressure build up improves the pulp draining.
Based on this understanding, the present invention suggests an arrangement which is adapted to utilize the pressure profile created by the pulp suspension itself when allowed to flow more freely.
Thus, the pulp passage 108 has a constant gap size defined by the radial distance D. In other words the guide surface 106 is in a first embodiment substantially equidistant from the outer surface of the press roll 102 throughout mainly the entire area enclosed by the vat 107.
However, at some points throughout the pulp passage, for example at the zones where wash liquid is added or in the region of the pulp inlet 109, the distance between the outer guide surface 106 and the outer surface of the press roll 102 might be different from the radial distance D for the rest of the pulp passage (not illustrated). Such a differing radial distance is in the description denoted D1, but is not shown in the figures. Preferably, the radial distance between the press roll 102 and the guide surface 106 in those areas is somewhat larger or greater than the constant radial distance D for the main part of the enclosed area defined by the vat 107, such that flow impeding bulges or similar are still avoided.
Typically, the distance D1 deviates only 1-5 mm from the radial distance D of the main portion of the pulp passage. In connection to the addition of wash liquid there may be an expansion due to swelling of the pulp suspension and the pulp passage 108 could in those regions be somewhat wider to minimize the risk of plugging in such areas. According to one embodiment of the present invention, the radial distance D is constant over more than 80% of the pulp passage. The distance D may be in the range of 10-200 mm, preferably in the range of 20-60 mm, and even more preferably about 40 mm throughout the whole length of the vat.
In another advantageous embodiment of the invention the vat may even be arranged to be slightly diverging towards the nip, either throughout the whole length of the vat or only at specific parts. For instance the vat may be designed to diverge slightly over, or in the vicinity, of a point of addition of pulp liquid. In such an embodiment the radial distance would not only be different from the radial distance D for the rest of the pulp passage, but it would diverge in that area and then, in contrast to the embodiment described above, remain at that same level, or possibly diverge more, during the remainder of the vat. A main object of the invention is to avoid the effect that the vat converges such that the passage of the pulp is hindered. This object is generally fulfilled as long as the vat is not converging at any point. Another object is however not to worsen the dewatering of the pulp such that the pulp that exits the nip or pinch has been sufficiently dewatered. A strongly diverging vat may, which is obvious to the skilled person, compromise this object. However, a somewhat diverging, at parts or throughout the whole length of the vat, has been proven to have no negative effect on the dewatering of the pulp.
By avoiding the converging geometry, of course with the exception of the short portions described above where the radial distance D first diverges (D1) and then converges back to the initial radial distance D, pulp suspension is allowed to flow more freely and the pressure in the vat is created by a hydraulic pulse generated from the nip and backwards (in the direction against rotation of the press rolls). Wash liquid added to the wash press also contributes to the pressure profile.
In prior art the pulp suspension was forced towards outer surface of the press roll, whereas fibers were forced through the perforations of the press rolls along with the filtrate. In the arrangement according to the invention, the pulp suspension may, one might say, decide how and where it is to be dewatered, whereby the contents of fibers in the filtrate is minimized, i.e. the dewatering is not forced upon the pulp suspension, but progresses at a pace that is natural to the properties of the pulp suspension. This is accomplished as the pulp suspension is no longer forced by the geometry of the vat towards the outer surface of the press roll. The dewatering of the pulp suspension in a washing arrangement according to the invention is thus implies that the fibers in the pulp suspension will be less subject to fiber damages.
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In order to obtain the desired pressure profile it is advantageous that the vat, except for the dewatering openings in the press roll, is defined in a substantially closed space. Thus, in addition to the dewatering perforations of the press roll, there should preferably be no holes in or the like in the guide surfaces 106, which defines the outer limit of the vat 107. This is especially important close to the nip, as the pressure that builds up in the nip will be lost if there are opening or other “escape routes” for the filtrate in that area. Such a lost of pressure would in turn imply that the pressure may not be sufficiently so as to create a pressure gradient opposite the movement of the pulp towards the beginning of the vat, wherein an important aspect of the invention would be lost.
Another advantage of that the vat geometry is non-converging is the fact that the larger open space between the press roll 102 and the vat 107 minimizes the tendency of plugging of pulp suspension. Since the pulp is allowed to flow more freely, it always has the option of flowing backwards, i.e. in a direction countercurrent to the main flow in the direction of rotation of the press rolls 102. A minimized plugging tendency leads to increased availability, since undesired stopping of the press operation is avoided. The non-converging geometry also enables a higher vat pressure, measured in the bottom of the vat, to be used.
The pulp inlet is in the figure illustrated as being placed in the region of the press roll's highest point. However, it is equally possible to arrange the pulp inlet another point, such as at the lowest point. The pressure profile created by the aforementioned mechanisms is independent on the placement of the pulp inlet.
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 person skilled in the art. Thus, the scope of the invention is only limited by the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0701269-3 | May 2007 | SE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/SE08/50595 | 5/21/2008 | WO | 00 | 11/19/2009 |
