Patent Description:
In treatment vessels for cooking or impregnating paper pulp, cooking liquors need to be continuously supplied and withdrawn to optimize the treatment process. The withdrawal is performed through a plurality of screens arranged in a chamber wall of the treatment vessel, and withdrawal compartments are arranged to receive the withdrawn liquor and transport it through throttle holes into headers for removal and subsequent treatment. Throttle holes allow the withdrawal to be performed symmetrically around the circumference of the treatment vessel, allowing for a uniform cooking or impregnation process inside the treatment chamber.

<CIT> discloses a vertical pulp treatment vessel having a top and bottom, such as a continuous or batch digester, has at least a first substantially annular screen assembly disposed within the vessel between the top and bottom. A first substantially annular header is associated with the first screen assembly and first and second withdrawal conduits extend outwardly from the header and are in fluid communication with it. A barrier, such as a radially extending plate, is disposed in the header between the withdrawal conduits, the withdrawal conduits being adjacent each other (typically spaced from each other between about <NUM>-<NUM> DEG , e.g. between about <NUM>-<NUM> DEG ). A small, compared to the prior art, platform is provided for allowing access to the withdrawal conduits, and automatically controlled valves or like structures in or associated with the conduits, the platform having an arcuate extent of less than <NUM> DEG, typically less than about <NUM> DEG (e.g. about <NUM> DEG or less). Typically, a second annular screen assembly, like the first screen assembly, with third and fourth withdrawal conduits circumferentially spaced from each other between about <NUM>-<NUM> DEG , and have a centerline that is circumferentially spaced from the first and second withdrawal conduit centerline less than about <NUM> DEG (e.g. about <NUM>-<NUM> DEG , typically about <NUM> DEG ), so that on the single platform the four withdrawal conduits, and the automatically operated valves associated therewith, may be accessed.

<CIT> discloses a method and apparatus for selectively withdrawing liquor from digester cooking screens for controlling the temperature in continuous digesters. First and second pairs of screens, with cooperating headers, are provided, and first and second outlets spaced circumferentially around the digester are associated with each header. Each header provides a circumferential cavity free of dividing members. Within each pair of screens withdrawal is practiced so that each header is rested during each ultimate withdrawal step, and the side from which the liquor is drawn each successive withdrawal step that the same header is used is alternated. In practicing the invention, header blockage caused by deposited debris and scale is minimized and increased uniformity and improved product quality result.

<CIT> discloses a filtrate chamber back of a cylindrical screen girdle inserted in a cellulose digester that is divided by vertical partitions into compartments. An internal manifold or header located axially adjacent and coaxial to said filtrate chamber communicates with said compartments through narrow apertures. A second similar manifold may be provided on the other side of the screen girdle, the filtrate chamber compartments being alternately connected to the lower and the upper manifold.

One problem commonly associated with the withdrawal of treatment liquors is that withdrawal arrangements are blocked by different kinds of precipitations, making removal of liquors inefficient and requiring costly cleaning procedures during service time shut downs. Some cleaning can take place during operation by closing a header temporarily to allow the screens to self-clean according to known cleaning procedures, but throttle holes that have been filled with or blocked by matter preventing the passing of liquor can normally only be unblocked through a reverse action through the withdrawal system where liquor is pressed back into the withdrawal compartments, leaving the blocking matter inside the withdrawal compartment and risking a renewed blocking of the same throttle holes shortly after.

The closing of a header will generally result in a significant loss in efficiency of the impregnation or cooking process and will therefore also be costly since any slowing or hindering of the process will result in significant loss of revenue for the digester or impregnation vessel as a whole.

There is therefore clearly a need for improvements to the withdrawal of liquor from treatment vessels in continuous treatment vessel systems.

The object of the present invention is to eliminate or at least to minimize the problems mentioned above. This is achieved through an arrangement for withdrawing treatment liquors, a system comprising at least two such arrangements and a control system for controlling operation of such an arrangement according to the appended independent claims.

Thus, according to the invention an arrangement for withdrawing treatment liquors from a treatment vessel in a continuous treatment vessel system is provided, said treatment vessel comprising a chamber wall of a treatment chamber and the arrangement comprising a plurality of screens arranged in the chamber wall for withdrawing liquor from the treatment chamber, said screens being arranged in at least two screen rows, one above the other; at least one withdrawal compartment for each screen row for receiving liquor from the screens, and at least one header for receiving liquor from the withdrawal compartments via a throttle hole for each withdrawal compartment, wherein the header is connected to at least one withdrawal compartment on at least two screen rows. Thereby, the problems associated with blocking of the header can be solved and the cleaning and maintenance of the header and the arrangement as a whole be significantly improved.

According to the invention, the header is arranged at an angle to a vertical direction, said angle being smaller than <NUM>°, preferably smaller than <NUM>°, wherein the header more preferably is arranged essentially vertically or is arranged vertically. Thus, the angle to the vertical direction is such that <NUM>° ≤ angle < <NUM>°, and more preferably <NUM>° ≤ angle < <NUM>°. Thereby, the cleaning and maintenance of the header is improved and the need for interrupting the treatment process for cleaning is decreased.

According to another aspect of the invention, the header further comprises a hatch for cleaning, said hatch being arranged above the screen rows. Thereby a cleaning fluid can be supplied through the hatch and the header thoroughly cleaned in an improved and more time efficient way.

According to yet another aspect of the invention, the at least one withdrawal compartment is arranged to receive liquor from at least four screens. Also, the treatment vessel may comprise at least three screen rows and the header can be connected to at least one withdrawal compartment on each screen row. This also improves the withdrawing of liquors and allows the header to collect liquor from screens not immediately bordering on the header. In this way, subsections of the screen rows are created and serviced by one header each, so that one header collects treatment liquors from one sector of the circumference of the treatment vessel. By supplying at least two arrangements with headers in different sectors of the circumference of the treatment vessel, a system for withdrawing treatment liquors is created.

According to the invention a control system is also provided for controlling operation of an arrangement for withdrawing treatment liquors from a treatment vessel in a continuous treatment vessel system, said treatment vessel comprising a chamber wall of a treatment chamber. The control system comprises at least one sensor for detecting at least one operation parameter in a header, means for interrupting operation of the header, and a control unit configured to communicate with said at least one sensor and to receive a signal corresponding to said at least one operation parameter, and further also configured to control the means for interrupting operation of the header. Thereby, the arrangements can be monitored and the operation of each header be interrupted when a detected operation parameter signals that the arrangement is not functioning as desired, for instance due to blocking. The operation parameter is preferably a pressure or a flow inside the header.

According to an aspect of the invention, the control unit is configured to compare said at least one operation parameter detected by the sensor with another detected operation parameter or with a reference parameter and to control the means for interrupting operation of the header in response to said comparison. Thereby, the operation of two different arrangements around the circumference of the treatment vessel can be compared with each other and the operation of one of them interrupted if a pressure or flow inside the header is significantly lower than a corresponding pressure or flow inside the header of the other arrangement. Alternatively, the operation parameter of one arrangement can be compared with another operation parameter detected in the same arrangement or with a reference parameter stored by the control system so that operation of the arrangement can be interrupted if the two detected parameters in the arrangement differ from each other or if the detected parameter is similar to or different from the reference parameter in a way that indicates that the operation of the arrangement needs to be improved.

According to another aspect of the invention, the another detected operation parameter is detected by the same sensor, or wherein the another detected operation parameter is detected by a sensor arranged to detect an operation parameter in another header. Thereby, the operation parameters can be detected at different points in time to indicate a difference over time or the operation of two different headers can be compared.

According to yet another aspect of the invention, the control system further comprises input means and output means configured to transmit signals to and receive signals from the control unit, wherein the control system is preferably configured to communicate the detected at least one operation parameter to a user through the output means and wherein the control system is preferably configured to control the means for interrupting operation of the header in response to input given by a user. Thereby, a user of the system can receive information regarding the detected operation parameters and can also communicate with the control system in order to manually control the operation of the system, either in response to output given by the system itself or in response to other factors determined by the user.

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 a treatment vessel <NUM> that forms part of a continuous treatment vessel system, such as a digester or an impregnation vessel for use when treating wood chips in a pulping process. Typically, wood chips are fed to the treatment vessel <NUM> through a top <NUM>, cooked or impregnated inside the vessel <NUM> and removed through a bottom <NUM> after treatment and transferred to subsequent treatment in the pulping process. Treatment liquor is supplied through the top <NUM> and/or bottom or through separate liquor inlets (not shown).

Also provided on the treatment vessel <NUM> are a plurality of screens <NUM>, <NUM> through which treatment liquor can be removed. In <FIG> a first screen section <NUM> comprising a plurality of screen rows 2a, 2b, 3c, 2d, 2e is provided for withdrawal of treatment liquor at the middle of the cooking process and a second screen section <NUM> comprising a plurality of screen rows 3a, 3b, 3c, 3d, 3e, 3f is provided for withdrawal of treatment liquor at the end of the cooking process, according to a cooking profiling used. In other treatment vessels, only one screen section could be provided or alternatively more than two screen sections could be used. It is also to be noted that the number of screen rows in each screen section can differ from only one and up to <NUM>-<NUM> or even more screen rows. The configuration shown in <FIG> with two screen sections and five or six screen rows in each section thus merely serves as an example.

<FIG> disclose an arrangement for withdrawing treatment liquor according to a prior art technology, where a plurality of screen rows 2a-d are each provided with a header <NUM> for collecting treatment liquor from a plurality of screens <NUM> in a chamber wall <NUM>. Behind the screens <NUM> are withdrawal compartments <NUM> that are connected to the header <NUM> via throttle holes <NUM>, so that liquor withdrawn from the treatment vessel <NUM> can flow through one of the screens <NUM>, be collected in the withdrawal compartment <NUM> and reach the header <NUM> through the throttle hole <NUM>. The header <NUM> runs along a circumference of the treatment vessel <NUM> in an essentially horizontal orientation and collects treatment liquor from each withdrawal compartment <NUM> connected to one of the screen rows 2a-2d, before allowing the treatment liquor to flow from the header <NUM> through an outlet <NUM>. Due to the high pressure generally required inside the treatment vessel <NUM> for the cooking of the wood chips, the throttle holes <NUM> serve an important function in creating a pressure drop to the header <NUM>, so that the withdrawal of treatment liquor is evenly performed around the circumference of the treatment vessel <NUM> even though only one outlet <NUM> is typically provided. Without the throttle holes <NUM>, the withdrawal would be more efficient near the outlet <NUM> and inefficient at other points of the circumference, but there is also a problem associated with the prior art of throttle holes being blocked by substance withdrawn from the cooking inside the treatment vessel <NUM> and thereby preventing withdrawal from those screens that are connected with blocked throttle holes <NUM>.

<FIG> disclose an arrangement for withdrawing treatment liquor according to a preferred embodiment of the present invention, where components that are similar to the prior art solution of <FIG> are given the same reference numerals. The present invention differs from the prior art solutions through the configuration of the header <NUM> that is connected to at least one withdrawal compartment <NUM> on at least two different screen rows 2a, 2b, 2c, 2d. Thereby, the header has a substantially vertical orientation and is able to transport treatment liquors more efficiently towards an outlet <NUM>. The throttle holes <NUM> are preferably provided in a wall <NUM> of the withdrawal compartments <NUM> that borders on the header <NUM> and differ from the prior art in that the throttle holes <NUM> can be provided along the wall <NUM> on a distance from a bottom <NUM> of the withdrawal compartment <NUM>. This allows precipitations from the treatment chamber of the treatment vessel <NUM> to sink to the bottom <NUM> of the withdrawal compartment <NUM> and remain there while keeping the throttle hole <NUM> unblocked to allow treatment liquors to flow freely.

In this preferred embodiment the header <NUM> is arranged essentially vertically along the chamber wall <NUM> of the treatment vessel <NUM>, but in some embodiments the header <NUM> could also be arranged at an angle to a vertical direction, said angle preferably being smaller than <NUM>°, more preferably smaller than <NUM>°. The advantage of keeping the angle small is that the header <NUM> can be connected to withdrawal compartments <NUM> of a plurality of the screen rows 2a-e, 3a-f and that the treatment liquors can flow down towards the outlet <NUM> in an efficient way while minimizing the risk of liquor remaining inside the header <NUM> and blocking or clogging the header <NUM> so that the transportation of treatment liquor is prevented.

Thus, in this preferred embodiment, the header <NUM> extends in the vertical direction along the chamber wall <NUM> between screens and is connected via throttle holes <NUM> to withdrawal compartments <NUM> on either side of the header <NUM> for each screen row 2a-e so that treatment liquor from at least one screen <NUM> on each screen row 2a-2e in a screen section <NUM>, <NUM> can proceed through a withdrawal compartment <NUM> and a throttle hole <NUM> into the header <NUM> and proceed onwards to the outlet <NUM>. In this preferred embodiment, two screens <NUM> are connected to each withdrawal compartment <NUM> as shown in <FIG> but in other embodiments only one or more than two screens <NUM> could be connected to a withdrawal compartment <NUM>. It is advantageous to connect as many as four screens <NUM> to one withdrawal compartment <NUM>, in order to collect treatment liquor from a larger part of the circumference of the treatment vessel <NUM>.

Each header <NUM> has a hatch <NUM> provided in an upper part of the header <NUM> to facilitate cleaning. It is advantageous to provide the hatch <NUM> above the screen rows 2a-e, 3a-f so that cleaning fluid can be inserted through the hatch <NUM> and be allowed to flow along the header <NUM> towards the outlet <NUM> in order to remove any remaining treatment liquor or precipitations from the treatment vessel. The screens <NUM> may conventionally be cleaned through the supply of a cleaning fluid into the treatment chamber of the treatment vessel <NUM> after the cooking has been interrupted and any remaining contents of the treatment chamber removed; during such cleaning operations the cleaning fluid may flow through or around the screens and the adjoining withdrawal compartments and pass through the throttle holes <NUM> into the headers <NUM>. Thanks to the improvement relating to the location and orientation of the headers <NUM> and the throttle holes <NUM>, the cleaning of the screens can be performed without needing to remove the screens from the chamber wall <NUM>, which is a significant advantage compared to prior art cleaning methods and save considerable time and cost. When self-cleaning screens <NUM> are used, individual screens <NUM> can be cleaned without interrupting the cooking process simply through the interruption of operation of the header <NUM> connected to those screens <NUM>, as will be described in more detail further below.

As is explained above, it is advantageous to connect one header <NUM> to withdrawal compartments <NUM> on each screen row 2a-e, 3a-f, since this allows for one single header <NUM> to receive treatment liquor from an entire sector of the circumference of the treatment vessel <NUM>. In some embodiments, one header <NUM> could be arranged to receive treatment liquor from more than one screen section <NUM>, <NUM>.

Thus, an arrangement <NUM> for withdrawing treatment liquors from a treatment vessel <NUM> comprises a plurality of screens <NUM>, withdrawal compartments <NUM> connected to the screens <NUM> and throttle holes <NUM> connecting each withdrawal compartment <NUM> to a header <NUM> that serves to receive treatment liquor and transport it to an outlet <NUM>. If the header <NUM> is connected to each screen row 2a-e, 3a-f of a screen section <NUM>, <NUM> that header receives all treatment liquor withdrawn from a given circle sector of the circumference of the treatment vessel <NUM>. When more than one arrangement <NUM> is provided on the chamber wall <NUM> of the treatment vessel <NUM>, they can cooperate to form a system that each withdraw liquor from a circle sector of the treatment vessel <NUM>, and in a preferred embodiment arrangements <NUM> are provided around the entire circumference of a treatment vessel <NUM>.

The operation of an arrangement <NUM> according to the present invention is controlled by a control system <NUM> as disclosed schematically in <FIG>. The control system <NUM> comprises a control unit <NUM> that communicates with at least one sensor <NUM> in an arrangement <NUM> according to the present invention. The sensor <NUM> is preferably configured to detect a pressure of a flow in the header <NUM> and may be located inside the header <NUM> or at the outlet <NUM>, or at any other suitable location within the arrangement <NUM>. The sensor thus detects an operation parameter in the header, preferably a flow of treatment liquor or a pressure in the header <NUM>, and transmits that operation parameter to the control unit <NUM>. The control unit <NUM> is also configured to control operation of a means <NUM> for interruption operation of the header <NUM>, said means <NUM> preferably being a hatch or valve at the outlet <NUM> for stopping the flow through the header <NUM>.

The detected operation parameter will be received by the control unit <NUM> and a decision will be made by the control unit <NUM> as to whether operation of the header <NUM> should be interrupted. If the flow is too low or the pressure too high, this would indicate that at least some of the throttle holes <NUM> are blocked and need to be cleaned, and such a cleaning operation could be performed by interrupting operation of the header to allow blockage to sink to the bottom of the withdrawal compartments <NUM> so that the flow of treatment liquor through the throttle holes <NUM> can once again be performed in the desired manner. After a predetermined time, operation of the header <NUM> would be resumed by a renewed activation of the means <NUM> for interrupting operation, for instance by opening the hatch or valve at the outlet <NUM> to allow the flow of treatment liquor once again. The sensor <NUM> would once again detect the operation parameter and transmit the value to the control unit <NUM> that would decide based on the value of the operation parameter whether the operation in the header <NUM> has been improved or if further cleaning operations would need to be performed. The same cleaning operation could be repeated until the operation parameter has reached a predetermined value or interval that indicates that operation is sufficiently improved, or alternatively other cleaning measures such as reversing the flow of treatment liquor could be performed. If the operation is still not improved these and other cleaning procedures could be undertaken, or it could be decided by the control unit <NUM> that one of the arrangements <NUM> should be shut down permanently by the means <NUM> for interrupting operation or that the entire cooking process should be interrupted for cleaning.

Preferably, the control unit <NUM> is connected to input means <NUM> and output means <NUM> through which the system <NUM> can communicate with a user. The user could receive information regarding the operation of the system <NUM> and the detected operation parameter for each arrangement <NUM> connected to the system <NUM>, and could also control the operation of the system <NUM> by using the input means to manually instruct the system to interrupt operation of any given arrangement <NUM> or to give instructions for the operation of the system <NUM> or input or adjust reference parameters in the system <NUM>. Preferably, the control system <NUM> is connected to a plurality of arrangements <NUM> according to the present invention, but it would also be possible to be connected to one arrangement <NUM> only.

In one embodiment, the control system <NUM> would compare a detected operation parameter from a sensor <NUM> in one of the arrangements <NUM> with another detected operation parameter, either detected earlier by the same sensor <NUM> or detected simultaneously or earlier by a sensor <NUM> in another arrangement <NUM> connected to the control system <NUM>. Alternatively, the detected operation parameter could be compared to a reference parameter stored in the control unit <NUM> or given by a user as input through the input means <NUM>. Depending on a result of the comparison, i.e. whether the operation parameter of a sensor indicates that operation in that arrangement <NUM> is satisfactory or unsatisfactory compared to predetermined values or detected values, the control unit <NUM> decides whether a cleaning operation should take place. A number of operation criteria can be selected by a user and the control unit <NUM> configured so that the means <NUM> for interrupting operation of the header <NUM> is activated if the detected operation parameter does not meet at least one of the operation criteria.

The control unit <NUM> can also control the means <NUM> for interrupting operation of the header <NUM> in an arrangement <NUM> so that interruption takes place at predetermined times and can also control the arrangements <NUM> so that interruption of operation in one arrangement <NUM> does not coincide with interruption of operation in another arrangement <NUM> connected to the control system <NUM>.

Claim 1:
Arrangement for withdrawing treatment liquors from a treatment vessel in a continuous treatment vessel system, said treatment vessel comprising a chamber wall of a treatment chamber, wherein the arrangement comprises:
a plurality of screens (<NUM>) arranged in the chamber wall (<NUM>) for withdrawing liquor from the treatment chamber, said screens (<NUM>) being arranged in at least two screen rows (2a, 2b, 2c, 2d), one above the other;
at least one withdrawal compartment (<NUM>) for each screen row (2a, 2b, 2c, 2d) for receiving liquor from the screens (<NUM>); and
at least one header (<NUM>) for receiving liquor from the withdrawal compartments (<NUM>) via a throttle hole (<NUM>) for each withdrawal compartment (<NUM>), and wherein each header (<NUM>) is connected to at least one withdrawal compartment (<NUM>) on at least two screen rows, wherein the header is arranged at an angle to a vertical direction, said angle being smaller than <NUM>°, preferably smaller than <NUM>°, or wherein the header (<NUM>) more preferably is arranged vertically.