Patent Description:
When treating cellulose-containing pulp in a pulping process, the pulp is subjected to a number of process stages and transported from one stage to the next, often by being pumped along a conduit or pipe to arrive at the next treatment stage. Especially at the bleaching stage the pulp is washed and subsequently dewatered a number of times until the chemical bleaching agents have been completely removed.

One problem associated with pumping pulp is that the high temperature of the pulp during treatment causes cavitation in the pump that prevents efficient pumping, and in order to avoid these problems the pulp is generally fed to a standpipe having the pump arranged at an outlet near the bottom. In the standpipe, pulp waiting to be transported generates a pressure at the bottom that prevents cavitation so that the pump can be used without risking damage or interruptions to the process. Thus, a number of standpipes are generally provided, one for each washing stage and also for some of the other process stages during the pulping process, and although they do provide a solution to the problems associated with pumping they are also cumbersome and space consuming, especially since they generally need to be <NUM>-<NUM> high. In order to feed the pulp to the standpipe, the pulp mill itself must be at least <NUM> high, so that the press arranged to feed pulp into the standpipe can be arranged at this height. Since the amount of pulp in the standpipe varies, the pressure at its bottom also varies so that it at times could be too low to achieve a reliable pumping of the pulp.

Examples of transporting arrangements and methods within this field are shown by <CIT>, <CIT> and <CIT>.

There is therefore a need for an improved arrangement and method for transporting pulp.

The object of the present invention is to provide an improved arrangement and method for transporting pulp from one process stage to a subsequent process stage where the problems relating to pumping pulp are resolved but the problems relating to using standpipes are simultaneously avoided. This is achieved through an arrangement and method according to the appended independent claims. Especially beneficial features of the present invention are set out in the dependent claims.

According to the invention as defined in claim <NUM>, the arrangement comprises a receiving portion for receiving pulp at a first dry solids content and a first pressure, a dilution portion for diluting pulp to a second solid content and for holding the pulp at a second pressure, a screw conveyor for transporting pulp from the receiving portion to the dilution portion, the screw conveyor comprising a screw, and the screw conveyor is arranged to transport the pulp and to also increase the pressure from the first pressure to the second pressure, and wherein the dilution portion has an outlet connected to a pump for pumping diluted pulp from the arrangement to a subsequent process stage, wherein the arrangement further comprises a pressure sensor arranged to detect the pressure inside the dilution portion, and wherein further the pump is arranged to control the pressure by operating in response to the detected pressure inside the dilution portion.

The corresponding method claim for transferring pulp from one process stage to a subsequent process stage is defined in the current claim <NUM>.

By pressurizing the pulp using the screw conveyor, the dilution portion can be kept at an elevated pressure and the pulp can be fed to a pump while still at this elevated pressure, thereby resolving the problems associated with pumping but also allowing for a space efficient solution that can be kept at a limited height and can be arranged in a more versatile way within a pulp mill. The pump is then operated to control the pressure in the dilution portion by altering the amount of pulp pumped in response to a detected pressure in the dilution portion.

According to an aspect of the invention, the pressure increase is achieved through providing a tapering screw portion through which the pulp is transported before entering the dilution portion. Thereby, a gradual pressure increase is achieved, independent of the rotation speed of the screw. In another embodiment, the screw conveyor is arranged to increase the pressure by providing a screw portion with smaller pitch than at least one other screw portion. Thereby, the pressure increase can be effected while maintaining a uniform shape of the screw and an outer wall of a transport portion between the receiving portion and the dilution portion, while still being independent of the rotation speed of the screw. In yet another embodiment, the screw conveyor is arranged to increase the pressure by controlling the turning speed of the screw, thereby facilitating controlling the pressure and altering the pressure depending on properties such as a pressure in the dilution portion at a given time or an operation of a pump connected to the dilution portion.

According to another aspect of the invention, the dilution portion further comprises a fluid inlet and agitation means for mixing the pulp with additional fluid to achieve a diluted pulp, and the agitation means preferably comprises an agitation screw. Thereby, the pulp can be diluted to a lower dry solids content as desired to make it suitable for pumping to a subsequent process stage and also to form part of a washing/dewatering cycle. By using an agitation screw, the mixing of the pulp with the dilution fluid is thorough and allows for a uniform mixture suitable for pumping.

In one embodiment, the screw and the agitation screw are arranged on a common axis. This allows for the use of one single screw conveyor that has a screw portion for transporting and pressurizing and another screw portion for the agitation screw for mixing the pulp with dilution fluid. Only one motor and controller are required in this embodiment, which also provides a space efficient and convenient arrangement for transporting, pressurizing and diluting pulp.

In another embodiment, the screw and the agitation screw are arranged on separate axes, said separate axes preferably being non-parallel. This allows for altering the rotational speed of each of the screws independent of the other, and by providing the agitation screw at an angle to the screw a more versatile arrangement is achieved.

According to one aspect of the invention, the dilution portion is arranged at a distance from the screw conveyor and the arrangement further comprises a connection between the screw conveyor and the dilution portion for transporting pulp, said connection preferably being a conduit. Thereby the dilution portion can be arranged separate from the receiving portion and screw conveyor, allowing for placement according to available space or other factors that may facilitate the transportation to the next process stage.

According to another aspect of the invention, the second pressure is at least <NUM> kPa, preferably at least <NUM> kPa, more preferably at least <NUM> kPa, in order to eliminate the risk of damage to the pump.

As stated above, the dilution portion has an outlet connected to a pump for pumping diluted pulp from the arrangement to a subsequent process stage, and wherein the arrangement preferably further comprises a pressure sensor arranged to detect the pressure inside the dilution portion. Thereby, the detected pressure can be used to control the operation of the pump and preferably also of the screw conveyor and the introduction of the additional fluid, so that the pressure in the dilution portion can be maintained at a suitable level and the mixing of the pulp with the additional fluid can be performed in an efficient manner.

Many additional benefits and advantages of the invention will become readily apparent to the person skilled in the art in view of the detailed description below.

The invention will now be described in more detail with reference to the appended drawings, wherein:.

<FIG> discloses an arrangement <NUM> for transferring pulp from one process stage to a subsequent process stage according to a preferred embodiment of the present invention. The arrangement <NUM> comprises a receiving portion <NUM> into which pulp is received in a direction shown by the arrow. From the receiving portion <NUM> the pulp is transported to a dilution portion <NUM> by means of a screw conveyor <NUM> having a screw <NUM> arranged around a first axle <NUM> that is journalled by a journal <NUM> and driven by a motor <NUM> to rotate about a first axis A. The screw <NUM> transports the pulp along a transport portion <NUM> into the dilution portion <NUM> where additional fluid is supplied to a dilution tank <NUM> through a fluid inlet <NUM> and mixed thoroughly with the pulp by means of an agitation screw <NUM> before being removed from the arrangement <NUM> through an outlet <NUM> that is connected to a pump <NUM> (see <FIG>).

The pulp received by the receiving portion <NUM> has typically undergone an earlier process stage and is generally dewatered to have a dry solids content of <NUM> %. Pulp at a bottom of the receiving portion <NUM> is at a first pressure, created by the weight of the pulp held in the receiving portion <NUM>. The first pressure will therefore vary depending on the amount and dry solids content of the pulp in the receiving portion <NUM> at any given moment. The screw conveyor <NUM> can comprise a second agitation screw <NUM> that serves to prevent formation of lumps of pulp that could prevent the efficient use of the arrangement <NUM>.

The screw conveyor <NUM> with the screw <NUM> thus acts to transport the pulp but simultaneously to pressurize the pulp so that the first pressure is gradually increased to a second pressure at the dilution portion <NUM>. In this preferred embodiment, the pressure increase is achieved by a tapering portion <NUM> of the transport portion <NUM> through which the pulp is transported. Thus, the pressure of the pulp is gradually increased along the tapering portion <NUM> to reach the second pressure before the pulp is being fed by the screw <NUM> into the dilution portion <NUM> where it is maintained at the second pressure until it is removed through the outlet <NUM>. Once en elevated pressure has been created in the dilution portion <NUM>, the pressure is controlled by the pump <NUM> acting in response to a detected pressure inside the dilution portion <NUM>. By slowing the operation of the pump, the pressure in the dilution portion <NUM> is increased and by increasing the operation of the pump a decrease of pressure is achieved. The screw <NUM> will generally operate in response to an amount of pulp introduced into the receiving portion <NUM> and strive to maintain a pulp level that is sufficient to generate the first pressure and prevent the screw from leakage, but it is generally advantageous to prevent excessive build-up of pulp in the receiving portion <NUM>. Due to the operation of the pump <NUM> in maintaining the pressure in the dilution portion <NUM>, the screw <NUM> can operate continuously to pressurize the pulp during transport through the transport portion <NUM> so that the operation of the arrangement <NUM> is efficient.

The additional fluid or dilution fluid is supplied in a suitable amount to dilute the pulp to a dry solids content suitable for pumping, preferably around <NUM> %. In some embodiments dilution fluid may also be supplied in the receiving portion <NUM> or in the transport portion <NUM> to make the pulp easier to transport, but the amount of dilution fluid in the arrangement <NUM> as a whole is determined in order to achieve the desired dry solids content in the dilution portion <NUM>.

The dimensions of the receiving portion <NUM>, transport portion <NUM> and dilution portion <NUM> are selected depending on the amount of pulp per time that is to be transferred by the arrangement <NUM>, and the dilution portion <NUM> is dimensioned especially to allow for a suitable staying time for the pulp during dilution and mixing so that a uniform mixture is achieved.

<FIG> and <FIG> disclose alternative embodiments of the invention and will now be described in more detail. Those features that these embodiments have in common with the embodiment of <FIG> are given the same reference numerals as in <FIG> and operate in the manner described herein with reference to the preferred embodiment.

In <FIG>, an arrangement <NUM>' for transferring pulp is disclosed according to a second embodiment of the invention. In this second embodiment, the pressure increase in the transport portion <NUM> is achieved through varying the rotational speed of the screw <NUM> so that a higher speed results in a pressure increase of the pulp. Also, in this embodiment the agitation screw in the dilution portion <NUM> is formed as a separate agitation screw arrangement <NUM> with an agitation screw axle <NUM> around which the agitation screw <NUM> is arranged. The agitation screw arrangement <NUM> is operated by a second motor <NUM> and the agitation screw axle <NUM> extends along a second axis B.

By the provision of a separate agitation screw arrangement <NUM>, the mixing of the pulp and the dilution fluid can be performed independently of the transporting and pressurizing of the pulp performed by the screw conveyor <NUM>. By arranging the agitation screw arrangement <NUM>, so that the second axis B is non-parallel to the first axis A, i.e. with the second axis B at an angle with respect to the first axis A, a greater versatility is also achieved.

<FIG> discloses a third embodiment of an arrangement <NUM>" that differs from the second embodiment of <FIG> by the arrangement of a connection <NUM> between the transporting portion <NUM> and the dilution portion <NUM> to allow the dilution portion <NUM> to be separated from the receiving portion <NUM> and transporting portion <NUM>. Preferably, the connection <NUM> is in the form of a conduit <NUM>. The pressure increases along the transporting portion <NUM> is achieved through a screw portion <NUM> with smaller pitch than the pitch of the screw <NUM> in the receiving portion <NUM>, or at least with smaller pitch than at least one other portion of the screw <NUM>.

<FIG> discloses a fourth embodiment of the arrangement <NUM>‴ in a view from above, differing from the previously described embodiments through the arrangement of the screw conveyor <NUM> and the agitation screw <NUM> in parallel. Pulp entering the receiving portion <NUM> is transported by the screw <NUM> through the transport portion <NUM> and is allowed to flow through a passage <NUM> into the dilution portion <NUM>. By supplying dilution fluid through two separate inlets <NUM>, <NUM>' any clogging or blocking of the screw <NUM> is prevented and the movement through the arrangement <NUM>‴ facilitated. The agitation screw <NUM> is in this embodiment journalled at an agitation screw journal <NUM>, but it is to be noted that other kinds of screw arrangements may also be used.

The operation of the arrangements <NUM>, <NUM>', <NUM>", <NUM>‴ will now be described in more detail to disclose the method for transferring pulp according to the present invention.

Pulp is received in the receiving portion <NUM>, said pulp arriving from an earlier process stage that is typically a washing stage where the pulp has been diluted and then dewatered to remove bleaching chemicals or the like. Upon arrival in the receiving portion <NUM>, the pulp typically has a dry solids content of about <NUM> %, but this can be varied depending on properties of the process where the arrangement <NUM>, <NUM>', <NUM>", <NUM>‴ is placed.

The pulp will remain in the receiving portion <NUM>, which also acts as a storage for pulp, and during its time in the receiving portion <NUM> the pulp will steadily move towards the bottom until it is engaged by the screw <NUM> of the screw conveyor <NUM> that moves the pulp along towards the transporting portion <NUM>. Before reaching the transporting portion <NUM>, the pulp is held at a first pressure that is determined by the amount of pulp in the receiving portion <NUM> pressing down towards the bottom and thereby applying a pressure to the pulp at the bottom. Through the movement caused by the screw <NUM>, the pulp is pressurized in the transporting portion <NUM> in a suitable way to gradually increase the pressure until a predetermined second pressure is reached. The increase is in the preferred embodiment described above achieved through passing the pulp through a tapering portion <NUM> of the transporting portion <NUM>, but other ways such as a smaller pitch of the screw or an increased rotational speed of the screw <NUM> can also be used as is described with reference to the third and second embodiments, respectively.

The pulp is delivered at the second pressure into the dilution portion <NUM>, where additional fluid is provided through the inlet <NUM> and the agitation screw <NUM> mixes the pulp with the dilution fluid until a pulp with a uniform dry solids content of about <NUM>% is achieved. A desired dilution time is preferably determined and operation of the arrangement <NUM>, <NUM>', <NUM>", <NUM>‴ adjusted to allow the pulp to remain in the dilution portion <NUM> for at least the desired dilution time, so that a thorough mixing can be performed. Preferably, the desired dilution time is at least <NUM>-<NUM> seconds but other dilution times may also be used if suitable for a specific embodiment of the present invention. If desired, the pulp can be diluted already in the receiving portion <NUM> or in the transporting portion <NUM> to make it easier to handle and transport the pulp and to prevent clogging of the screw <NUM>.

After the pulp and the dilution fluid have been thoroughly mixed, the resulting pulp with a lower dry solids content is removed through the outlet <NUM> and transported to the pump <NUM> that pumps the pulp to a subsequent process stage. Typically, the arrangement and method of the present invention are used after a dewatering stage and acts as a washing stage so that the diluted pulp is pumped to another dewatering stage before being subjected to a repeated wash but other placements and uses during the pulping process are also possible. The arrangement <NUM>, <NUM>', <NUM>", <NUM>‴ is preferably operated to maintain the second pressure in the dilution portion <NUM>, or at least to maintain the pressure within a pressure range centered on the second pressure. A pressure sensor <NUM> is preferably provided for this purpose in the dilution portion <NUM>, and by controlling the operation of the pump <NUM> and the screw conveyor <NUM> the pressure can be increased or decreased in order to adjust the pressure to remain at the second pressure or within a pressure range to which the second pressure belongs. An excessive pressure inside the dilution portion <NUM> can lead to damages to the arrangement <NUM>, <NUM>', <NUM>", <NUM>"', but if the pressure were to sink below a predetermined lower threshold the operation of the pump <NUM> would be less efficient or hindered altogether; it is therefore a significant advantage to be able to control the pressure of the pulp in the dilution portion <NUM>.

Another way of controlling the operation of the arrangement <NUM>, <NUM>', <NUM>", <NUM>‴ is to provide a level sensor, pressure sensor or flow sensor (not shown) to the receiving portion <NUM> and to control the operation of the screw <NUM> depending on the amount of pulp received. However, since there is a risk of damages to the arrangement <NUM>, <NUM>', <NUM>", <NUM>" due to the pressure in the dilution portion <NUM> being outside a preferred range or deviating from the second pressure, it is important to control the operation of the arrangement <NUM>, <NUM>', <NUM>", <NUM>‴, so that a steady supply of pressurized pulp to the dilution portion <NUM> can be balanced by a removal via the outlet <NUM> and pump <NUM> of the diluted pulp.

Claim 1:
Arrangement (<NUM>, <NUM>', <NUM>", <NUM>‴) for transferring pulp from one process stage to a subsequent process stage, the arrangement comprising:
a receiving portion (<NUM>) for receiving pulp at a first dry solids content and a first pressure,
a dilution portion (<NUM>) for diluting pulp to a second solid content and for holding the pulp at a second pressure, and
a screw conveyor (<NUM>) for transporting pulp from the receiving portion (<NUM>) to the dilution portion (<NUM>), the screw conveyor (<NUM>) comprising a screw (<NUM>),
wherein the screw conveyor (<NUM>) is arranged to transport the pulp and to also increase the pressure from the first pressure to the second pressure,
wherein the dilution portion (<NUM>) has an outlet (<NUM>) connected to a pump (<NUM>) for pumping diluted pulp from the arrangement to a subsequent process stage, wherein the arrangement further comprises a pressure sensor (<NUM>) arranged to detect the pressure inside the dilution portion (<NUM>),
and wherein further the pump (<NUM>) is arranged to control the pressure by operating in response to the detected pressure inside the dilution portion (<NUM>).