Monitoring system and belt dryer adapted for monitoring a superabsorbent polymer cake on a circulating conveyor belt in a housing of the belt dryer, monitoring method and process for producing water-absorbing polymer particles

Abstract

The invention relates to a monitoring system (1000) adapted for monitoring a superabsorbent poly- mer cake (1050) on a circulating conveyor belt (1400) in a housing (2100) of a belt dryer (2000), preferably forced-air belt dryer, wherein - the superabsorbent polymer cake (1050) during operation of the air belt dryer (2000) is placed on the circulating conveyor belt (1400) and the monitoring is based on an automated visual inspection of the surface of the superabsorbent polymer cake (1050) in order to monitor a variation in height of the superabsorbent polymer cake (1050) on the circulating conveyor belt (1400).

Description

[0001] Monitoring system and belt dryer adapted for monitoring a superabsorbent polymer cake on a circulating conveyor belt in a housing of the belt dryer, Monitoring method and process for producing water-absorbing polymer particles

[0002] The invention relates to a monitoring system according to the preamble part of claim 1, adapted for monitoring a superabsorbent polymer cake on a circulating conveyor belt in a housing of a belt dryer. The belt dryer preferably is a forced-air belt dryer. The invention also relates to a belt dryer having said monitoring system. The invention also relates to respective monitoring method and a process for producing water-absorbing polymer particles with said monitoring method.

[0003] For the monitoring system the superabsorbent polymer cake during operation of the air belt dryer is placed on the circulating conveyor belt and the monitoring is based on an automated visual inspection of the surface of the superabsorbent polymer cake in order to monitor a variation in height of the superabsorbent polymer cake on the circulating conveyor belt.

[0004] FIELD OF THE INVENTION

[0005] Water-absorbing polymer particles are used to produce diapers, tampons, sanitary napkins and other hygiene articles, but also as water-retaining agents in market gardening, agriculture or technical engineering or the like applications. The water-absorbing polymer particles are also referred to as "absorbent resins", "superabsorbents", "superabsorbent polymers", "absorbent polymers", "absorbent gelling materials", "hydrophilie polymers" or "hydrogels".A superabsorbing polymer in the state of an aqueous polymer gel is considered to be in a wet state; i.e. still having a considerable water content of the aqueous polymer gel before drying, in particular as outlined below. In this state preferably the aqueous polymer gel can be crosslinked; in particular, except of a residual part, practical totally crosslinked, in particular as outlined below.

[0006] A superabsorbing polymer in the state of water-absorbing polymer particles is considered to be in the state of after drying; i.e. having a lower residual moisture content of the water-absorbing polymer particles after drying of the aqueous polymer gel, in particular as outlined below. In this state preferably the water-absorbing polymer particles can be postcrosslinked; in particular, except of a residual part, practical totally postcrosslinked, in particular as outlined below.

[0007] The production of water-absorbing polymer particles is described in the monograph "Modern Superabsorbent Polymer Technology", F. L. Buchholz and A. T. Graham, Wiley-VCH, 1998, pages 71 to 103. The aqueous polymer gels obtained by polymerization are typically dried by means of a forced-air belt dryer. The use of air belt dryers, in particular multistage forced air belt dryers, is proposed, wherein the material being dried is newly distributed on the next conveyer belt in each case (see also in "Perry's Chemical Engineers' Handbook", 7th edition, McGraw-Hill, pages 12-48).

[0008] Belt dryers with conveyer belts in general are used for producing water-absorbing polymer particles like e.g. described with regard to specific relevant parameters of a conveyer belt surface roughness in US2011 / 0204288 A1 or a conveyer belt speed in US 2010 / 0041549A1 or a swivel belt in combination with a conveyer belt US 2012 / 0048973A1.

[0009] In a contemporary conveyer belt dryer, a plate conveyer belt design of contemporary art is described in WO 2015 / 074966 A1 for a process for producing water-absorbing polymer particles.

[0010] Thus, the production of superabsorbent polymer (SAP) usually contains the step of drying a SAP gel wherein the gel is fed on a belt dryer as indicated above. However, the distribution of the gel on the belt dryer can vary as a product distributor may distribute an aqueous polymer gel onto the circulating conveyor belt in a non-uniform manner. The aqueous polymer gel usually is provided in form of a wet hydrogel, preferably onto a plate conveyer belt.

[0011] Further, based on the recipe used for the SAP, the SAP's morphology can vary.The SAP gel forms, by being dried on the circulating conveyor belt, a mass which is also referred to as a drying cake. The thickness, which is characterized by the height of the SAP cake, is an important parameter since it impacts the quality and dry kinetics of the SAP. Usually the thickness of the cake is evaluated by a manual visual inspection of the operator in person.

[0012] An air belt dryer usually consists of a conveyor belt on which a SAP cake can be moved along in order to be dried. Further, an air belt dryer comprises fans and heating elements to generate hot air for drying. The conveyor belt, the fans and the heating elements are usually contained in a housing.

[0013] It has been shown that the evaluation of the cake thickness is dependent on the operator and is not consistent over different operators. Different operators seem to be evaluating the cake's thickness differently according to their subjective perception.

[0014] Due to a hazardous environment inside the belt dryer, for example hot and corrosive vapors, using measuring devices inside the belt dryer has shown to be difficult.

[0015] SUMMARY OF THE INVENTION

[0016] This is where the invention comes in, the object of which is to provide an apparatus and method to enable an automated, in particular accurate and consistent, monitoring of the height of the SAP cake on the belt dryer.

[0017] SUPPORT OF MAIN CLAIMS

[0018] The object of the invention as to the apparatus is achieved in a first aspect by a monitoring system according to claim 1.

[0019] The invention is directed to a monitoring system adapted for monitoring a superabsorbent polymer cake on a circulating conveyor belt in a housing of a belt dryer, preferably forced-air belt dryer, wherein

[0020] - the superabsorbent polymer cake during operation of the air belt dryer is placed on the circulating conveyor belt and the monitoring is based on an automated visual inspection of the surface of the superabsorbent polymer cake in order to monitor a variation in height of the superabsorbent polymer cake on the circulating conveyor belt.According to the invention the monitoring system comprises:

[0021] - at least one marking unit outside the housing, at least one image capturing device outside the housing, and an evaluation unit outside the housing, wherein

[0022] - the marking unit is adapted for generating a visible geometric light marking, and projecting at an projection angle the visible geometric light marking onto the surface of the superabsorbent polymer cake such that the visible geometric light marking on the surface undergoes a variation in height with the variation in height of the of the superabsorbent polymer cake on the circulating conveyor belt,

[0023] - the image capturing device is adapted for capturing an image of a section of the surface such that the visible geometric light marking projected onto the surface is visible in the image as an image of the geometric light marking.

[0024] Further according to the invention the evaluation unit is adapted for

[0025] - detecting at least one position of the image of the geometric light marking in the image, caused by the superabsorbent polymer cake when moving with the circulating conveyor belt along the capturing device and

[0026] - detecting a variation in the at least one position of the image of the geometric light marking within the image captured by the image capturing device, and

[0027] - assigning the variation in height of the visible geometric light marking on the surface to the variation in position of the image of the geometric light marking within the image,

[0028] - determining from a value of variation in position of the image of the geometric light marking within the image a corresponding value of variation in height of the superabsorbent polymer cake on the conveyor belt.

[0029] The invention is based on the realization that a visible geometric light marking will shift -in other words change its position- according to a height of an object passing the projection of the visible geometric light marking and the projection angle relative to a surface on which the object is moving. In this sense an object with varying height will cause the visible geometric light marking to change its position accordingly when said object is passing the projection of the visible geometric light marking, thereby creating a relation between the height of the object and the position of the visible geometric light marking.This change in position of the visible geometric light marking in accordance with the height of an object can be captured and analysed to accurately determine the height of an object passing the projection of the visible geometric light marking.

[0030] Further, the invention recognizes that using an image capturing device to capture the change of position of the visible geometric light marking caused by an object passing the projection of the visible geometric light marking can be used to analyse the change of position using an evaluating unit.

[0031] An image capturing device that can be used to track a change of position of the visible geometric light marking.

[0032] For example if the image capturing device is oriented such that it looks down on a surface of the superabsorbent polymer cake when moving with the circulating conveyor belt or the circulating conveyor belt as such; i.e. on a surface of an object on the circulating conveyor belt on which the object moves or the surface of the circulating conveyor belt as such. On the surface the visible geometric light marking is projected with a projection angle, the change in height or other variation in height or form of the visible geometric light marking will be captured by the image capturing device as a lateral distance deviation of the visible geometric light marking in terms of a position, respectively as a variation of position in the image. Hereinafter, a "change in height” of the visible geometric light marking is meant to describe a variation in height within the 3D space. In other words: a variation in height refers to the actual height of the visible geometric light marking on the conveyor belt or the SAP cake. A "change in position” is meant to describe a variation in position of an image of the visible geometric light marking within the image captured by the image capturing device. Thus, basically hereinafter the term "position” refers to the position of an image of the visible geometric light marking e.g. the image of the geometrical light marking in a two dimensional space of the image with the coordinates x and y of the image captured by the image capturing device.

[0033] An image of the light marking can be of a linear form (like e.g. in form of a line) in the image captured by the image capturing device, in particular the linear form can be perpendicular to the x-co-ordinate. In other words the image of the light marking in a linear form may have a constant x-coor-dinate over a y-coordinate. In this sense the position of the image of the marking in the image can be determined with only one constant x-coordinate in the case of a straight line without steepness. The image of a marking in the image however can in other cases also be of a non-linear form (like e.g. an arbitrary curve of varying steepness and slope). This would be the case when the visible geometric light marking itself on the surface of the superabsorbent polymer cake when moving withthe circulating conveyor belt or the circulating conveyor belt as such has a variation in height laterally across the surface of the superabsorbent polymer cake or circulating conveyor belt; so to say across the width of the circulating conveyor belt. In this case in terms of coordinates in the image the x-coordinate of the position in the image would vary over the y-coordinate in terms of an arbitrary function y=f(x). In this sense, a single x-coordinate preferably is to be supplemented to describe the position of the image of the marking in the image captured by the image capturing device. Preferably at every y-coordinate of the image of the marking a corresponding x-coordinate can be determined in order to define the position of the image of the marking in the image. In this case the position of the image of the marking in the image captured by the image capturing device can be described with multiple pairs of x-coordinate and corresponding y-coordinates.

[0034] The evaluation unit preferably contains a detection algorithm. The detection algorithm is adapted for analysing the image captured by the image capturing device in order to detect the position of the visible geometric light marking within the image. Based on the current position of the visible geometric light marking within the image, the detection algorithm is adapted for determining a corresponding height of a polymer cake passing the visible geometric light marking projected onto the surface of the conveyor belt.

[0035] The data input for the detection algorithm is the image taken by the image capturing device. The image shows a two dimensional view of a section of the surface of the conveyor belt on which the SAP cake is fed during operation of the air belt dryer, in which the visible geometric light marking is projected on.

[0036] From this view a change of position of the visible geometric light marking, caused by a SAP cake moving along the conveyor belt and passing the visible geometric light marking, is detectable as a lateral movement of the visible geometric light marking -in other word: as a change in position- inside the image, e.g. the visible geometric light marking moves from left to right within the image when the SAP cake passes the visible geometric light marking according to the height of the SAP cake. Using an algorithm to detect the position and the change of position of the visible geometric light marking within the image enables an automated, fast and precise way of determining a height value of the SAP cake, effectively enabling a real-time determination of the varying height of the SAP cake on the belt dryer as it passes the visible geometric light marking.

[0037] The detection algorithm preferably is trained to allow a precise allocation of a specific position of the visible geometric light marking within the image to a corresponding height value of the visiblegeometric light marking. The height value of the visible geometric light marking corresponds with the height of the SAP cake passing the visible geometric light marking.

[0038] The algorithm is trained with a reference position of the visible geometric light marking within the image that represents no superabsorbent polymer cake being at the position of the visible geometric light marking and therefore equals no detectible height. In other words: The reference position of the visible geometric light marking within the image equals a height value of the visible geometric light marking of zero. Further, the detection algorithm is trained with at least two objects of known height in order to generate an interpolation model to accurately determine a corresponding height value of the visible geometric light marking based on the position of the visible geometric light marking within the image.

[0039] The reference position is known to the detection algorithm and is the position of the visible geometric light marking when no object is at the positon of the visible geometric light marking. Therefore the reference point equals a height value of zero for the detection algorithm. Starting from this reference point any object that passes the visible geometric light marking will cause the visible geometric light marking to change its height which the detection algorithm detects as a lateral movement or as a change of position of the visible geometric light marking within the image captured by the image capturing device. The precision of the detection algorithm grows with an increasing number of objects used for training. Using two objects of known height for training is the minimum amount. Using a trained algorithm enables high accuracy in determining the height of SAP cakes.

[0040] The object of the invention as to the apparatus is achieved in a second aspect by a belt dryer according to claim 11.

[0041] The invention thus relates to a belt dryer for drying of superabsorbent polymer having the monitoring system of the invention. Therein the air belt dryer belt dryer having a housing, wherein the image capturing device and the marking unit are positioned at respective viewing glasses outside of the housing, wherein the viewing glasses allow for an automated visual inspection of the surface of the superabsorbent polymer cake in order to monitor a variation in height of the superabsorbent polymer cake on the circulating conveyor belt.

[0042] Placing the image capturing unit and the marking unit outside of the housing of the air belt dryers enables to avoid the hazardous environment inside the belt dryer where a monitoring system could not remain.In a third aspect of the invention the object of the invention is solved with regard to the method by a monitoring method of claim 12.

[0043] The monitoring method is adapted for monitoring a superabsorbent polymer cake on a circulating conveyor belt in a belt dryer according to the invention and / or using a monitoring system according to the invention.

[0044] Therein the monitoring method comprises the steps of:

[0045] - detecting at least one position of the image of the geometric light marking in the image, caused by the superabsorbent polymer cake when moving with the circulating conveyor belt along the capturing device and

[0046] - detecting a variation in the at least one position of the image of the geometric light marking within the image captured by the image capturing device, and

[0047] - assigning the variation in height of the visible geometric light marking on the surface to the variation in position of the image of the geometric light marking within the image,

[0048] - determining from a value of variation in position of the image of the geometric light marking within the image a corresponding value of variation in height of the superabsorbent polymer cake on the conveyor belt.

[0049] In a fourth aspect of the invention the object of the invention is achieved with regard to the method by a process of claim 15, i.e. a process for producing water-absorbing polymer particles.

[0050] The process for producing water-absorbing polymer particles, comprises the steps of:

[0051] processing an aqueous polymer gel from polymerization of a monomer solution or suspension,

[0052] drying the aqueous polymer gel in a conveyer dryer, in particular in a forced air conveyer dryer, wherein

[0053] the conveyer dryer has a circulating conveyer belt and the aqueous polymer gel is conveyed on the circulating conveyer belt as a superabsorbent polymer cake, in particular the circulating conveyer belt is formed as a circulating plate conveyer belt, comprising a number of belt plates separated by a hinge line of a hinge assembly and each belt plate having a surface for receiving the aqueous polymer gel.Further therein

[0054] - height values of the superabsorbent polymer cake are monitored by a monitoring method of the invention, wherein the method is adapted for monitoring a superabsorbent polymer cake on a circulating conveyor belt in a belt dryer of the invention and / or using a monitoring system according to the invention.

[0055] SUPPORT OF DEPENDENT CLAIMS

[0056] These and other aspects of the invention and further developments thereof are further outlined in the dependent claims which further develop the concept of the invention. Thereby the mentioned advantages of the proposed concept can be improved even more. For each feature of the dependent claims it is claimed independent protection, independent from all other features of the disclosure.

[0057] In particular the method in a development provides for

[0058] - setting up the marking unit outside the housing of the air belt dryer to generate a visible geometric light marking through a viewing glass onto the conveyor belt of the air belt dryer at an projection angle between 30° to 60° relative to the top surface of the conveyor belt of the air belt dryer, preferably at a projection angle of essentially 45° relative to the top surface of the conveyor belt of the air belt dryer, and / or

[0059] - setting up the image capturing device outside the housing of the air belt dryer at a viewing glass opposite of the marking unit so that the image capturing device captures the section of the conveyor belt where the visible geometric light marking is projected on, and / or

[0060] - setting up the evaluation unit with the pre-trained detection algorithm to determine a height of the superabsorbent polymer cake at the position of the visible geometric light marking in the image.

[0061] In a particular preferred development the method provides for

[0062] - having the image capturing unit capture an image of the section of the conveyor belt where the visible geometric light marking is projected on, while the air belt dryer is operated and a superabsorbent polymer cake is passing the visible geometric light marking and causing the visible geometric light marking to change its position within the image according to the corresponding height of the corresponding superabsorbent polymer cake passing the visible geometric light marking, and / or - having the evaluation unit determine a height of a superabsorbent polymer cake passing the visible geometric light marking, by detecting a current position of the visible geometric light markingwithin the image captured by the image capturing device. In a preferred development of the monitoring system, the evaluation unit is signal connected to a control unit and adapted to transmit to the control unit height values of the superabsorbent polymer cake. Preferably the control unit is adapted for controlling operation of the air belt dryer based on the height values of the superabsorbent polymer cake.

[0063] This allows for a fully automatic and precise control of the operation of the belt dryer based on the height of the SAP cake. The control unit is adapted for receiving height values of the SAP cake passing the visible geometric height marking on the conveyor belt of the air belt dryer and sending control signals to the air belt dryer accordingly to keep the operation of the air belt dryer within optimal parameters.

[0064] In a preferred development of the monitoring system, the image capturing device continuously detects an image of a section of the surface such that the visible geometric light marking projected onto the surface is visible in the image.

[0065] By continuously capturing an image of the visible geometric light marking projected onto the surface, a seamless observation of the thickness of the SAP cake on the conveyor belt is made possible. Therefore the production of SAP can be adjusted to small changes in the height of the SAP cake.

[0066] In a preferred development of the monitoring system, the image capturing device discontinuously detects an image of a section of the surface such that the visible geometric light marking projected onto the surface is visible in the image.

[0067] By discontinuously capturing an image of the visible geometric light marking projected onto the surface, the generated data that need to be analysed by the detection algorithm can be minimized. The image capturing device can be adapted for taking a picture after a fixed time interval. A nonlimiting example of such a fixed time interval can be every second, or every two seconds, or every four seconds. Also a time interval can be adjusted accordingly to a previous history of detected heights. For example, if the evaluation unit detects the height having only minor deviation in other words the height being essentially constant over time, the time interval can be prolonged. For example a time interval can be set from capturing an image every second to capturing an image every 5 seconds.Thus, an observation that can be adapted to the consistency of the thickness of the SAP cake on the conveyor belt is made possible. Therefore the production of SAP can be adjusted to small changes in the height of the SAP cake while minimizing generated data.

[0068] In a preferred development of the monitoring system, the evaluation unit is adapted for determining a perpendicular mean height over the width of the superabsorbent polymer cake by determining more than one height value at more than one position of the image of the geometric light marking in the image corresponding to multiple measuring points across the visible geometric light marking and is further adapted for determining a mean height over the length of the superabsorbent polymer cake, whereby the width of the superabsorbent polymer cake extends across a conveying direction of the conveyor belt and the length of the superabsorbent polymer cake extends longitudinal to the conveying direction.

[0069] If the SAP cake has a height that varies over its width -in other words: varies over a direction that extends across the conveyor belt— or that varies over its length - in other words: varies over a direction that extends along the conveyor belt-, determining only one height value will not be sufficient to accurately describe the perpendicular and longitudinal height of the SAP cake. For this case the evaluation unit is adapted for determining multiple height values corresponding to multiple measuring points across the visible geometric light marking that runs across the conveyor belt and covers the SAP cake over its entire width. Using the multiple height values the evaluation unit is further adapted for calculating a perpendicular mean height value of the SPA cake.

[0070] The evaluation unit can further be adapted for dismissing all measuring points across the visible geometric light marking whose position equals the reference position in order to not distort the mean height value of the SAP cake by using measuring points where no SAP cake is present on the conveyor belt.

[0071] Further, the evaluation unit is adapted for averaging the height of the SAP cake in the conveying direction, on other words: determining a longitudinal mean height along the length of the SAP cake. One way of doing so can be using time intervals after which the -within a given time interval- collected height values are averaged. For example every two, or every six, or every ten seconds a current longitudinal mean height value can be determined. In this regard, the evaluation unit can further be adapted for dismissing all measuring points along the length of the SAP cake whose position equals the reference position -e.g. when there is a short break in feeding the SAP cake in thebelt dryer- in order to not distort the longitudinal mean height value of the SAP cake by using measuring points where no SAP cake is present on the conveyor belt.

[0072] In general, averaging the height of the SAP cake over its width and / or its length can be done anytime or can be done when the height of the SAP cake over its width and / or its length is discovered to vary strongly. For determining if the height of the SAP cake should be averaged over the width and / or the length of the SAP cake, a threshold value can be defined that characterizes the degree in which the height of the SAP cake varies over its width and length.

[0073] In a preferred development of the monitoring system, the visible geometric light marking generated by the marking unit is a visible laser line running across the surface of the conveyor belt of the air belt dryer and the marking unit is a laser.

[0074] Using a laser line as a visible geometric light marking is preferred since the laser line can be easily projected across the belt dryer and therefore detects any object passing the visible geometric light marking over the complete width of the conveyor belt. In this sense a continuous laser line enables covering the complete width of the belt dryer and therefore leaving no spaces for objects to pass through the visible geometric light marking without being detected by the detection algorithm. A laser line is also simple to project onto the belt dryer and can be generated with standard lasers without the need of special components.

[0075] In another preferred development of the monitoring system the image capturing device is a camera.

[0076] The image capturing device can preferably be selected as device that allows a visible detection of the visible geometric light marking; e.g. the image capturing device is a camera, in particular a video camera. Preferably the image capturing device is adapted for tracking of the variation in position of the image of the geometric light marking within the image, caused by the SAP cake passing the visible geometric light marking. E.g. the image capturing device has a monitor or a read out interface for a digital image. Preferably the image capturing device can be connected via the interface to an evaluation unit to transfer the image to the evaluation unit.

[0077] In general there is no need to be able to "see” the conveyor belt or any of the environment in the image captured by the image capturing device. In an evaluation unit, for detecting a position of the image of the geometric light marking within the image an algorithm needs only a position of the im-age of the geometric light marking within the image to determine a corresponding height value assigned to the position. In this sense "visible” geometric light marking means "visible for the image capturing device”. For example the light used by a laser could be a light of a wavelength not visible for the human eye but detectable for the image capturing device.

[0078] Preferably the light source generates a light visible for the human eye. An advantage of using light visible for the human eye is an easy way of checking if the monitoring system works by just looking through a viewing glass and seeing the visible geometric light marking being projected onto the conveyor belt. Also a status indicator could be used to indicate the monitoring system works or does not work. For example a status LED could be used showing green light if the monitoring system works or red light if the monitoring system does not work.

[0079] In another preferred development of the monitoring system the marking unit is resided on a first side of the circulating conveyor belt and the image capturing device is resided on a second side of the circulating conveyor belt, such that the marking unit and the image capturing device are placed essentially opposite each other and / or transverse to the circulating conveyor belt on the opposite first and second side. This placement is preferred since it allows the image capturing device to be angled precisely to have a clear view of the conveyor belt while also enabling the marking unit to be calibrated to project the visible geometric light marking onto the belt dryer.

[0080] Preferably for projecting at a projection angle the visible geometric light marking onto the surface of the superabsorbent polymer cake, the marking unit and the image capturing device are placed at a monitor level above the circulating conveyor belt.

[0081] In another preferred development of the monitoring system, the projection angle with which the visible geometric light marking is projected onto the surface of the conveyor belt is between 30° and 60° relative to the surface of the conveyor belt.

[0082] The projection angle at which the visible geometric light marking is being projected onto the conveyor belt is of some importance, since the projection angle influences the degree in which the position of the visible geometric light marking in the image captured by the capturing device is being shifted when an a SAP cake passes the visible geometric light marking. It is advantageous when the deviation or other kind of variation in height of the visible geometric light marking is in 3D space is bigger instead of smaller when the SAP cake passes the visible geometric light marking. The bigger the deviation of the image of the geometric light marking's position within the image capturedby the image capturing device, the better the detection algorithm can detect the deviation, respectively said variation.

[0083] Preferably an interval of 30° to 60° enables to calibration of the marking unit in order to make sure the variation of the position of the visible geometric light marking is noticeably big in order for the detection algorithm to detect the variation of the visible geometric light marking.

[0084] In another preferred development if the monitoring system, the projection angle with which the visible geometric light marking is projected onto the surface of the conveyor belt is essentially 45° relative to the surface of the conveyor belt.

[0085] Calibrating the marking unit to project the visible geometric light marking at a projection angle of essentially 45° relative to the surface of the conveyor belt has shown to optimally facilitate the detection of height variations in objects passing the visible geometric light marking.

[0086] BRIEF DESCRIPTION OF THE DRAWINGS

[0087] Embodiments of the invention are now described below with reference to the drawing in comparison with the prior art, which is also shown in part. This is not necessarily intended to show the embodiments to scale, rather the drawing is, where useful for explanation, in schematized and / or slightly distorted form. Reference is made to the relevant state of the art with regard to additions to the teachings directly recognizable from the drawing. It should be borne in mind that various modifications and changes can be made to the shape and details of an embodiment without departing from the general idea of the invention. The features of the invention disclosed in the description, in the drawing and in the claims can be essential both individually and in any combination for the further development of the invention. In addition, all combinations of at least two of the features disclosed in the description, the drawing and / or the claims fall within the scope of the invention. The general idea of the invention is not limited to the exact form or detail of the preferred embodiment shown and described below, or limited to any subject matter that would be limited as compared to the subject matter claimed in the claims. In the case of specified measurement ranges, values lying within the stated limits are also to be disclosed as limit values and can be used and claimed as desired.

[0088] Further advantages, features and details of the invention are apparent from the following description of the preferred embodiments and from the drawing, which shows in:FIG. 1 a front view of a preferred embodiment of a monitoring system for a belt dryer with a conveyor belt as shown herein,

[0089] FIG. 2 in view (a) a side view of the preferred embodiment of the monitoring system for the belt dryer with the conveyor belt as shown herein,

[0090] in view (b) an image of a conveyor belt and a visible geometric light marking as an illustrating example for the preferred embodiment,

[0091] FIG. 3 in view (a) a side view of another preferred embodiment of the monitoring system for the belt dryer with the conveyor belt as shown herein,

[0092] in view (b) an image of a conveyor belt and a visible geometric light marking as an illustrating example for the preferred embodiment,

[0093] FIG. 4 in view (a) as an illustrating example a front view of a conveyor belt in a belt dryer, with a SAP cake on the conveyor belt,

[0094] in view (b) an image of the conveyor belt and the visible geometric light marking,

[0095] in view (c) an image of the conveyor belt with the visible geometric light marking and averaging points,

[0096] FIG. 5 a schematic diagram of the monitoring system,

[0097] FIG. 6 a schematic diagram of the monitoring system.

[0098] DETAILED DESCRIPTION OF THE DRAWINGS

[0099] Forced-air belt dryers suitable for the process and the method according to the invention are described for example, in the monograph "Modern Superabsorbent Polymer Technology", F. L. Buchholz and A. T. Graham, Wiley-VCH, 1998, pages 89 to 92. In a contemporary conveyer belt dryer, a plate conveyer belt design of contemporary art is described in WO 2015 / 074966 for a process for producing water-absorbing polymer particles. The content of WO 2015 / 074966 A1 is incorporated by reference herein.FIG. 1 shows a front view of a conveyor belt 1400 with a surface 1450 and an SAP cake 1050 in a schematic form.

[0100] At a first end a conveyer dryer in form of a belt dryer 2000 provides a receiving stage for the aqueous polymer gel and at a second end the conveyer dryer provides a discharge stage for the dry polymer. In the receiving stage a product distributor distributes an aqueous polymer gel, usually in form of a wet hydrogel, to a plate conveyer belt 1400. The product distributer is of the oscillating belt type but may also be of another design. The aqueous polymer gel then is conveyed on the surface 1450 of the conveyer belt 1400 from the receiving stage through a number of drying stages to the discharge stage. As a part of the discharge stage a product breaker adapted to break the dried polymer gel into particles of a certain size is provided at the exit.

[0101] The surfaced 1450 of the conveyor belt 1400 of FIG. 1 is the surface 1450 on which a SAP cake 1050 is fed on by the product distributor. The SAP cake 1050 is moved when the belt dryer 2000 is operated with a certain height of the polymer gel bed -hereinafter referred to as a polymer cake 1050- on the conveyer belt of the forced-air belt dryer depending of the feeding of the product distributor in the application zone. The conveying direction of the conveyor belt 1400 in FIG. 1 extends outwards of the picture plane, in other words: the conveyor belt 1400 moves the SAP cake 1050 in the direction of the viewer.

[0102] The length of the forced-air belt dryer 2000 from back to front is preferably from 10 to 80 m, more preferably from 20 to 60 m, most preferably from 30 to 50 m. Thus a circulating plate conveyer belt has a total length which is basically double or more than double the length of the forced-air belt dryer, i.e. upper length in conveying direction, lower length contra conveying direction and curved lengths at back and front. The total length of the circulating plate conveyer belt is preferably from 20 to 160 m, more preferably from 40 to 120 m, most preferably from 60 to 100 m.

[0103] The circulating plate conveyer belt speed of the forced-air belt dryer is preferably from 0.005 to 0.05 m / s, more preferably from 0.01 to 0.04 m / s, most preferably from 0.015 to 0.035 m / s.

[0104] The residence time on the forced-air belt dryer is preferably from 10 to 120 minutes. More preferably the residence time is from 20 to 90 minutes, most preferably from 30 to 60 minutes. Also preferably the residence time is from 10 to 60 minutes, most preferably from 12 to 30 minutes, in particular from 15 to 25 minutes.A water content of the aqueous polymer gel before drying in the forced-air belt dryer is preferably from 30 to 70 % by weight. In particular in the process a moisture content of the superabsorbent polymer after the drying in the forced-air belt dryer, i.e.in particular a moisture content of the superabsorbent, i.e. water-absorbing, polymer particles, is from 0.5 to 15 % by weight. The water content of the aqueous polymer gel, which is provided as a aqueous polymer gel bed, in the application zone, before drying is preferably from 30 to 70 % by weight, more preferably from 35 to 65 % by weight, most preferably from 40 to 60 % by weight. The mean gel particle size diameter of the aqueous polymer gel -that is to say the mean particle size diameter of the aqueous polymer gel particles- is preferably from 0.1 to 10 mm, more preferably from 0.5 to 5 mm, most preferably from 1 to 3 mm.

[0105] The gas inlet temperatures of the forced-air belt dryer are preferably from 150 to 220°C, more preferably from 160 to 210°C, most preferably from 170°C to 200°C; the temperature may depend and vary on the detailed layout and position in a drying stage of a belt dryer.

[0106] The gas stream used for drying may comprise water vapor. However, the water vapor content should not exceed a value that corresponds to a dew point of preferably at most 50°C, more preferably at most 40°C, most preferably at most 30°C.

[0107] The moisture content of the superabsorbent polymer after the drying on the forced-air belt dryer is preferably from 0.5 to 15 % by weight, more preferably from 1 to 10 % by weight, most preferably from 2 to 8 % by weight. For determining the above mentioned moisture content, the superabsorbent polymer after drying is analyzed and the moisture content is preferably determined according to EDANA test method WSP 230.2-05, title "Mass Loss Upon Heating”.

[0108] The conveyer dryer is of particular use in a specific process of polymer gel processing. A particularly advantageous drying process is described in WO 2006 / 100300 A1. The content of

[0109] WO 2006 / 100300 A1 is incorporated by reference herein.

[0110] The height of the polymer gel bed -hereinafter referred to as a polymer cake 1050- on the conveyer belt of the forced-air belt dryer in the application zone is preferably from 2 to 20 cm, more preferably from 5 to 15 cm, most preferably from 8 to 12 cm.

[0111] FIG. 1 shows a monitoring system 1000 adapted for automated visual based detection of a height of a superabsorbent polymer cake 1050 on a belt dryer 2000 in form of a forced-air belt dryer with asuperabsorbent polymer cake 1050 on a circulating conveyor belt 1400 in a housing 2100 of a belt dryer 2000. The superabsorbent polymer cake 1050 during operation of the air belt dryer 2000 is placed on the circulating conveyor belt 1400 and the monitoring is based on an automated visual inspection of the surface of the superabsorbent polymer cake 1050 in order to monitor a variation in height of the superabsorbent polymer cake 1050 on the circulating conveyor belt 1400.

[0112] The monitor system 1000 comprises at least one image capturing device 1100, at least one marking unit 1200 and an evaluation unit 1300 (further shown and explained with FIG. 5 and FIG. 6).

[0113] The marking unit 1200 outside the housing 2100 is adapted for generating a visible geometric light marking 1210 and projection of the visible geometric light marking 1210 on the surface 1450 of the SAP cake 1050; i.e. in direction of the conveyor belt 1400 on which the super absorbent polymer cake 1050 is placed during operation of the air belt dryer 2000.

[0114] FIG. 1 shows the image capturing device 1100, like a camera, and the marking unit 1200, like a laser e.g. a line laser, or a wide angle light source adapted to generate a light marking to the surface of the SAP cake 1050 with the conveyor belt 1400, being placed opposite each other with the conveyor belt 1400 in between. This arrangement allows the marking unit 1200 to project the visible geometric light marking 1210 onto the surface of the conveyor belt 1400 or -more precisely- the surface of the SAP cake 1050 on the conveyor belt 1400 without being obstructed by the image capturing device 1100.

[0115] Further this arrangement allows the image capturing device 1100 to get a clear image 1110 of the section of the surface 1450 of the superabsorbent polymer cake 1050 on the conveyor belt 1400 on which the visible geometric light marking 1210 is projected on.

[0116] Therefore, the image capturing device 1100 is able of capturing a SAP cake 1050 moving along on the conveyor belt 1400 and passing the visible geometric light marking 1210 and causing the visible geometric light marking 1210 necessarily to shift its height according to the height of the SAP cake 1050 as the visible geometric light marking 1210 is on the surface 1450 of the superabsorbent polymer cake 1050.

[0117] The visible geometric light marking 1210 onto the surface 1450 of the superabsorbent polymer cake 1050 is such that the visible geometric light marking 1210 on the surface 1450 undergoes avariation in height with the variation in height of the superabsorbent polymer cake 1050 on the circulating conveyor belt 1400.

[0118] The image capturing device 1100 is adapted for capturing an image 1110 of a section of the surface 1450 such that the visible geometric light marking 1210 projected onto the surface 1450 is visible in the image 1110 as an image of the geometric light marking 1210.

[0119] A marking unit 1200 in form of a laser and an image capturing device 1100 in form of a camera are preferred. The marking unit 1200 in form of the laser and an image capturing device 1100 in form of the camera are strategically positioned on opposite sides of the viewing glasses of a belt dryer 2000 as shown in FIG. 1. This setup of a monitoring system 1000 is designed to enhance the monitoring and measurement capabilities of objects moving along the belt as will be described in detail with FIG. 2 and FIG. 3.

[0120] A projection angle a (as further shown and described with FIG. 3 and FIG.4) can be set in an optimized way. The camera is angled precisely to ensure it captures a clear view of the belt 1400 of the dryer 2000, where the mounted laser projects a visible line across the surface 1450 as shown in FIG. 2. The view cone of the camera is depicted with dashed lines. The width of the laser, e.g. a line laser wide, or a wide angle light source adapted to generate a light marking to the surface of the SAP cake 1050 with the conveyor belt 1400 is depicted with dashed lines.

[0121] This line 1500 serves as a reference point for detecting the height of various objects that may traverse the belt, i.e. the SAP cake 1050 moving along on the conveyor belt 1400.

[0122] The marking unit 1200 in form of the laser is calibrated to emit its beam at approximately a 45-de-gree angle a relative to the belt's surface. This specific angle is optimized in so far, as it optimally facilitates the detection of height variations in objects passing by. The significance of this angle lies in the relationship between the height of an object and the degree to which the laser line shifts when an object is present as will become clear from the further explanation of the image 1110 of the surface 1450 of the belt 1400 respective of the polymer cake 1050 shown in FIG. 2 view (b) given the situation of the belt 1400, where the mounted laser projects a visible line across the surface 1450 of the belt 1400 respective of the polymer cake 1050 in FIG. 2 view (a).

[0123] The evaluation unit 1300 as will be further shown with FIG. 5 and FIG. 6 is adapted for detecting a position 15001, 15201, 15401, 15601 of the image of the geometric light marking 1210 in the image1110, caused by the superabsorbent polymer cake 1050 when moving with the circulating conveyor belt 1400 along the capturing device 1100.

[0124] Therefore, the evaluation unit 1300 is signal connected to the image capturing device 1100. The image 1110 captured by the image capturing device 1100 is an input signal for the evaluation unit 1300. The image data can be transmitted to the evaluation unit 1300 via wire or wireless connection.

[0125] Further, the evaluation unit 1300 is adapted for assigning the variation in height h, 1500, 1520, 1540, 1560 of the visible geometric light marking 1210 on the surface 1450 to the variation in position p, 1500i, 1520i, 1540i, 1560i of the image of the geometric light marking 1210 within the image 1120. From a value of variation in position p, 1500i, 1520i, 1540i, 1560i of the image of the geometric light marking 1210 within the image 1110 a corresponding value of variation in height h of the superabsorbent polymer cake on the conveyor belt 1400 is determined.

[0126] In the image 1110 the height value h assigned to the image of the geometric light marking 1210 (via the visible geometric light marking 1210) corresponds to a height value of the SAP cake 1050 passing the visible geometric light marking 1210.

[0127] In order to achieve the correlation between the variation in position p, 1500i, 1520i, 1540i, 1560i of the image of the geometric light marking 1210 within the image 1120 on the one hand and the variation in height h of the visible geometric light marking 1210 on the surface 1450 on the other hand, the evaluation unit 1300 provides a detection algorithm. The detection algorithm is pre-trained to allocate variations of positional values p of the image of the geometric light marking 1210 within the image 1110 to corresponding variations of height values h of the visible geometric light marking 1210 on the surface 1450; so to say corresponding with a variation in height of the SAP cake 1050 passing the visible geometric light marking 1210.

[0128] FIG. 2 shows in view (a) a side view of a conveyor belt 1400 with a projection of the visible geometric light marking 1210 with a projection angle a onto the surface 1450 of the conveyor belt 1400. The conveying direction of the conveyor belt 1400 is marked with the arrow. Three objects 1720, 1740, 1760 with different heights are positioned on the surface 1450 of the conveyor belt 1400 and are about to pass the visible geometric light marking 1210. The objects 1720, 1740 1760 have a constant height over their respective widths.Before the first object 1720 passes the projection of the visible geometric light marking 1210, the visible geometric light marking 1210 is being projected directly onto the surface 1450 of the conveyor belt. Thus, the visible geometric light marking 1210 is positioned at its reference position 1500.

[0129] In simple terms FIG. 2 view (a) depicts a scenario where an object 1720 is on the circulating conveyor belt 1400. Two lines 1500, 1520 are illustrated: line 1520 represents the location of the laser line on the object 1720 whilst line 1500 indicates where the laser line would be if no object were present (the reference case with laser line on circulating conveyor belt 1400 as explained hereinbefore).

[0130] The height distance h between these two lines 1500, 1520 can be utilized to estimate the object's 1720 height through an interpolation model, which can be trained using multiple calibration images.

[0131] FIG. 2 shows in view (b) in detail an image 1110 captured by the image capturing device 1100 of the section of the surface 1450 of the conveyor belt 1400, on which the visible geometric light marking 1210 is projected onto. The image 1110 shown in FIG. 2 in view (b) is a top down view from the position of the image capturing device positioned and angled as shown in FIG. 1.

[0132] In the image 1110 in FIG. 2 view (b) the reference positon of the visible geometric light marking 1210 within the image (15001) corresponds to the reference position 1500 of the visible geometric light marking 1210 on the conveyor belt 1400.

[0133] The detection algorithm knows the reference position 15001 of the visible geometric light marking 1210 within the image 1110 and therefore knows that when the visible geometric light marking 1210 is at its reference position 15001, that there is no object 1720, 1740, 1760 passing the visible geometric light marking 1210. The reference position 1500 in FIG. 2 view (a) which regards a height value h of the visible geometric light marking 1210, is transformed into a position of the visible geometric light marking 1210 within the two dimensional image 1110. The position p of the visible geometric light marking 1210 can only be determined by the x-coordinate of the visible geometric light marking 1210 within the image 1110. In other words: The height h in 3D space of the visible geometric light marking 1210 is transformed into a 2D image, wherein only one coordinate is needed to describe the positon p of the visible geometric light marking 1210 in the image 1110.Thus, the x-coordinate of the visible geometric light marking 1210 in the image 1110 equals a corresponding height value of the visible geometric light marking 1210 projected onto the conveyor belt 1400.

[0134] Regarding FIG. 2 view (a), when the first object 1720 passes the visible geometric light marking 1210, the visible geometric light marking 1210 changes its position according to the height of the object 1720. The change of height h of the visible geometric light marking 1210 shifts from the reference position 1500 to a second position 1520 as shown in FIG. 2 view (a). This shift of height h of the visible geometric light marking 1210 is visible in the image 1110 as a change of position p of the image of the visible geometric light marking 1210 in form of a lateral distance deviation from reference position 15001 to the first position 15201.

[0135] As marked in FIG. 2 view (b) with the little arrow, the lateral distance deviation of the image of the geometric light marking 1210 in the image 1110 is visible by the visible geometric light marking 1210 moving from left to right. In other words: only the x-coordinate of the image of the geometric light marking 1210 in the image 1110 changes according to the height of the second object 1720. The x-coordinate at position 15201 in the image 1110 corresponds to the height value of the visible geometric light marking 1210 being projected onto the first object 1720.

[0136] In other words - the absence of objects on the belt, the laser line remains fixed in the same position. However, when objects are present, the laser line shifts from the surface conveyor belt 1400 onto the first object 1720 whilst in the image 1110 the image of the laser line shifts from line 15001 to line 15201. The height h of the object correlates with the extent of this shift; taller objects cause a more significant movement of the laser line. This is the reason why the laser's angle with the belt is to some extent critical; a 45-degree angle facilitates the maximum shift in the laser line when objects are detected, thereby enhancing the system's sensitivity to height changes.

[0137] A detection algorithm has a known reference position 15001 of the visible geometric light marking 1210 within the image 1110. This reference position 15001 equals the visible geometric light marking 1210 being projected directly onto the surface 1450 of the conveyor belt 1400. In other words: The reference position 15001 of the visible geometric light marking 1210 within the image 1110 means that there is no object passing the visible geometric light marking 1210 and corresponds to a height value of zero.The detection algorithm is trained with at least to objects of known height. Preferably the detraction algorithm is trained with more than two objects of known height. In other words. The detection algorithm knows certain positions of the visible geometric light marking 1210 within the image 1110 and their corresponding height values.

[0138] By training the detection algorithm with objects of known height, an interpolation model can be created with which the detection algorithm is able to precisely allocate any position of the visible geometric light marking 1210 within the image 1110 to a corresponding height value.

[0139] For an effective calibration, a minimum of two images featuring known object heights is required. These images must accurately represent the possible object heights to ensure the model's reliability. By applying the detection algorithm, one can detect and measure the distance between the above mentioned illustrative reference line 15001 and the laser line 15201 in these calibration images 1110. The respective data serve as a foundation for constructing a predictive model.

[0140] Once established, this model allows for real-time height estimations of new objects that may be introduced onto the belt, even if their heights are unknown. While collecting two images is the minimum requirement, having additional images will enhance the accuracy of height estimations.

[0141] Thus, analogue to the first object 1720, when the second object 1740 passes the visible geometric light marking 1210, the height of the visible geometric light marking 1210 changes from the second height 1520 to a third height 1540 corresponding to the height of the second object 1740. Accordingly, the position of the visible geometric light marking 1210 in the image 1110 changes from position 15201 to position 15401, respectively the x-coordinate of the visible geometric light marking 1210 in the image 1110 changes.

[0142] Analogue to the second object 1740, when the third object 1760 passes the visible geometric light marking 1210, the height of the visible geometric light marking 1210 changes from the third position 1540 to a fourth position 1560 corresponding to the height of the third object 1760. Accordingly, the position of the visible geometric light marking 1210 in the image 1110 changes from third position 15401 to fourth position 15601, respectively the x-coordinate of the visible geometric light marking 1210 in the image 1110 changes.After the third object 1760 has passed the visible geometric light marking 1210 and no further object is passing the visible geometric light marking 1210, the visible geometric light marking 1210 is being projected onto the surface 1450 of the conveyor belt 1400. Thus, the visible geometric light marking 1210 falls back on its reference position 1500. The detection algorithm detects the visible geometric light marking 1210 being at its reference position 15001 in the image and therefore knows that no object is passing the visible geometric light marking 1210.

[0143] FIG. 3 in view (a) shows the same setup as in FIG. 2 view (a) in example wherein the SAP cake 1050 has a continuously varying height over its length along the conveying direction.

[0144] The height over the width -perpendicular to the conveying direction- of the SAP cake is constant. Analogue to FIG. 2 view (a) and FIG. 2 view (b), the visible geometric light marking 1210 first is at its reference position 1500 before the SAP cake 1050 passes the visible geometric light marking 1210. When the SAP cake 1050 is passing the visible geometric light marking 1210, the visible geometric light marking 1210 will shift its height according to the varying height of the SAP cake 1050.

[0145] FIG. 3 in view (b) shows the position of the visible geometric light marking 1210 in the image 1120 to shift from reference position 15001 to a second position 15201 and then continuously alternating back and forth between the second position 15201 and third position 15401 as marked with the little double arrow in FIG. 3 view (b).

[0146] FIG. 4 shows in view (a) a conveyor belt 1400 from a front view similar to FIG. 1. On the surface 1450 of the conveyor belt 1400 is a SAP cake 1050 with varying height across its width. The width of the SAP cake 1050 extends transversal across the conveyor belt 1400. The longitudinal conveying direction is shown with the conveyor belt 1400 in FIG. 3 as an arrow. When a SAP cake 1050 with a varying height across its width -as shown in plane in FIG. 4- passes the visible geometric light marking 1210, the position p of the image of the visible geometric light marking 1210 in the image 1110 is of a kind of a non-linear curve. tThus for example a single x-coordinate cannot really always be ascertained to the position p of the visible geometric line marking 1210 in the image 1110 by the evaluation unit and allocated to a specific height value h Since the exemplary SAP cake 1050 does not extend over the entire width of the conveyor belt 1400 the visible geometric light marking 1210 is at its reference position 1500 at both transverse edges of the conveyer belt 1400. The visible geometric light marking 1210 on the SAP cake 1050 takes a height 1510 that depends on a specific transverse height profile of the SAP cake 1050.FIG. 4 shows in view (b) the image 1110 with a corresponding visible geometric line marking 1210 caused by the SAP cake with varying height across its width as shown in plane in FIG. 4. The position 15101 of the image of the non-linear visible geometric light marking in the image can be seen in FIG. 4 view (b).

[0147] In other words and with reference to the coordinate system shown in Figure 4 view (b) so to say the position p of the visible geometric light marking 1210 in the image 1110 varies over the x-coordi-nate to reflect the height 1510 thereof in terms of the position 15101 of visible geometric light marking in the image 1110. In other words, for every x-coordinate of the image of the geometric light marking 1210 in the image 1110, a corresponding position 15101 of the image of the visible geometric light marking 1210 in the image 1110 can be ascertained in terms of a curved line of position P.

[0148] In view (b) one exemplary position p is marked from the curved line of positions p; therein the position 15101 of image of the geometric light marking in the image 1110 corresponds to a specific height h of the visible geometric light marking 1210 (shown in FIG. 4 view (a)) on the SAP cake 1050. The position p shown in FIG. 4 view (b) marks the deviation of the image of the visible geometric laser marking 1210 in the image 1110 at the x-coordinate of that specific position p in the image 1110 as compared to the reference position 15001 in the image 1110.

[0149] Fig. 4 view (c) shows an example where there are ten (n=10) measuring points 16501 distributed over the visible geometric light marking 1210. For every measuring point n, the detection algorithm detects the specific x-coordinate xnof the image of the image of geometric light marking 1210 in the image 1110, i.e. the respective positions 15101, p.

[0150] An evaluation unit 1300 may for each of the respective position 15101, p allocate a specific height value hn. In particular the evaluation unit 1300 can be adapted for determining a height value h of such a SAP cake 1050 by averaging the specific height values hnusing multiple measuring points 16501 across the width of the SAP cake 1050 in the image 1110. As an alternative multiple of the positions 15101, p can be used to specify an average value position. In the alternative the average value position can be used to determine the height value h of such a SAP cake 1050 in terms of an average height.FIG. 4 shows in view (c) the image 1110 with the visible geometric line marking 1210 and multiple measuring points 16501 distributed over the y coordinate of the image 1110, wherein the y-coordi-nate extends across the width of the conveyor belt 1400 respectively the SAP cake 1050. The number of measuring points 16501 can vary. The measuring points 16501 are distributed over the y-co-ordinate, which means that every measuring point 16501 has a specific y-coordinate. The detection algorithm is adapted for determining every x-coordinate of the visible geometric line marking 1210 at every y-coordinate of every measuring point 16501. Further, for every x-coordinate determined this way -whereby every x-coordinate means a specific position p of the image of the geometric light marking 1210 in the image 1110- a respective height value h of the visible geometric light marking 1210 on the SAP cake 1050 can be determined by the detection algorithm. The height values of the visible geometric line marking 1210 at every y-coordinate of the measuring points 16501 can then be used by the detection algorithm to calculate a perpendicular mean height h' for the SAP cake 1050 passing the visible geometric line marking 1210.

[0151] Further, it can be preferred that the detection algorithm dismisses those measuring points 16501 whose x-coordinates equal the reference positon of the visible geometric line marking 1210 in the image 1110, since using these x-coordinates for averaging the height of the SAP cake 1050, would distort the mean height of the SAP cake 1050.

[0152] FIG. 5 shows a schematic diagram of a preferred embodiment of a monitoring system 1000. The monitoring system 1000 is adapted for monitoring a superabsorbent polymer cake 1050 on a circulating conveyor belt 1400 in a housing 2100 of a belt dryer 2000, preferably forced-air belt dryer; said SAP cake 1050 is placed during operation of the air belt dryer 2000 on the circulating conveyor belt 1400 as has been described above. The monitoring is based on an automated visual inspection of the surface of the superabsorbent polymer cake 1050 in order to monitor a variation in height h of the superabsorbent polymer cake 1050 on the circulating conveyor belt 1400. The monitoring system 1000 comprises an image capturing device 1100, a marking unit 1200 and an evaluation unit 1300 outside the housing 2100 as has been described above.

[0153] As has been described above the marking unit 1200 is adapted for generating a visible geometric light marking 1210, and projecting at an projection angle a the visible geometric light marking 1210 onto the surface 1450 of the conveyor belt 1400 and therefore also onto the surface of the superabsorbent polymer cake 1050 such that the visible geometric light marking 1210 on the surface1450 undergoes a variation in height with the variation in height of the superabsorbent polymer cake 1050 on the circulating conveyor belt 1400.

[0154] As has been described above the image capturing device 1100 is adapted for capturing an image 1120 of a section of the surface 1450 such that the visible geometric light marking 1210 projected onto the surface 1450 is visible in the image 1120 as an image of the geometric light marking 1210.

[0155] The evaluation unit 1300 receives the image 1110 as input from the image capturing device 1100.

[0156] Further, the image 1110 comprises an image of the visible geometric light marking 1210.

[0157] The evaluation unit 1300 is adapted for detecting a variation in the at least one position p, 15001, 15201, 15401, 15601 of the image of the geometric light marking 1210 within the image 1120 captured by the image capturing device 1100. The evaluation unit 1300 comprises a detection algorithm that is adapted for detecting the position 15001, 15201, 15401, 15601 and a variation of position p of the image of visible geometric light marking 1210 in the image 1110.

[0158] The evaluation unit 1300 is adapted for assigning the variation in height h of the visible geometric light marking 1210 on the surface 1450 to the variation in position p, 15001, 15201, 15401, 15601 of the image of the geometric light marking 1210 within the image 1120.

[0159] The evaluation unit 1300 is adapted for determining from a value of variation in position p, 15001, 15201, 15401, 15601 of the image of the geometric light marking 1210 within the image 1120 a corresponding value of variation in height h of the superabsorbent polymer cake 1050 on the conveyor belt 1400.

[0160] The evaluation unit 1300 can be adapted for transferring a height value h to an output unit in order for a supervisor to inspect the height value of the SAP cake 1050.

[0161] FIG. 6 shows a schematic diagram of another monitoring system 1000 wherein the evaluating unit 1300 is further adapted for transferring a height value of the SAP cake 1050 on the conveyor belt 1400 to a control unit CU. The evaluation unit 1300 is signal connected to a control unit 1040 and adapted to transmit to the control unit 1040 height values of the superabsorbent polymer cake 1050.

[0162]

[0163] The control unit 1040 can be part of the monitoring system 1000 or part of the belt dryer 2000. The transfer of signals can be transmitted via wire or wireless connection.

[0164] The evaluation unit 1300 can be adapted for transferring a height value to an output unit in order for a supervisor to inspect the height value of the SAP cake 1050. More preferably the control unit 1040 is adapted for sending control signals to the air belt dryer 2000 in order to control the air belt dryer 2000 based on the height of the SAP cake 1050. As an example in the receiving stage a product distributor can be under control of the control unit 1040 to adapt distributing an aqueous polymer gel, usually in form of a wet hydrogel, to the plate conveyer belt. The product distributer can be controlled such to slow down according to first control signals of the control unit 1040 in the case the height values of the SAP cake 1050 are too high or above a first threshold value. The product distributer can be controlled such to speed up according to second control signals of the control unit 1040 in the case the height values of the SAP cake 1050 are too low or below a second threshold value. Preferably a preferred or requested thickness of the SAP cake 1050 can be set to a preferred value via support of the capturing device 1100, e.g. a camera system, evaluation unit 1300 and control unit 1040 according to the process described. For example, control unit 1040 is adapted to set control signals such that the process with the measurement of height values as explained to guarantee the same cake thickness on demand for a specified period of time.LIST OF REFERENCE SIGNS

[0165] 1000 monitoring system

[0166] 1040 control unit

[0167] 1050 superabsorbent polymer cake

[0168] 1100 image capturing device

[0169] 1110, 1120 image

[0170] 1200 marking unit

[0171] 1210 visible geometric light marking

[0172] 1300 evaluation unit

[0173] 1400 conveyor belt

[0174] 1450 surface of conveyor belt

[0175] 1500 position of visible geometric light marking

[0176] 1510 position of visible geometric light marking

[0177] 1520 position of visible geometric light marking

[0178] 1540 position of visible geometric light marking

[0179] 1560 position of visible geometric light marking

[0180] 15001 position of image of geometric light marking in the image 15101 position of image of the geometric light marking in the image

[0181]

[0182] 1520i position of image of geometric light marking in the image

[0183] 15401 position of image of geometric light marking in the image

[0184] 15601 position of image of geometric light marking in the image

[0185] 1600 detection algorithm

[0186] 16501 measuring points

[0187] 1700 object

[0188] 2000 air belt dryer

[0189] 2100 housing of belt dryer

[0190] 2200 viewing glass

[0191] 3000 monitoring method

[0192] h variation in height of visible geometric light marking

[0193] h' perpendicular mean height

[0194] h” longitudinal mean height

[0195] p variation in position of visible geometric light marking in the image

[0196] o projection angle

Claims

CLAIMS1. Monitoring system (1000) adapted for monitoring a superabsorbent polymer cake (1050) on a circulating conveyor belt (1400) in a housing (2100) of a belt dryer (2000), preferably forced-air belt dryer, wherein- the superabsorbent polymer cake (1050) during operation of the air belt dryer (2000) is placed on the circulating conveyor belt (1400) and the monitoring is based on an automated visual inspection of the surface of the superabsorbent polymer cake (1050) in order to monitor a variation in height (h) of the superabsorbent polymer cake (1050) on the circulating conveyor belt (1400),- wherein the monitoring system (1000) comprises:- at least one marking unit (1200) outside the housing (2100), at least one image capturing device (1100) outside the housing (2100), and an evaluation unit (1300) outside the housing (2100), wherein- the marking unit (1200) is adapted for generating a visible geometric light marking (1210), and projecting at an projection angle (a) the visible geometric light marking (1210) onto the surface (1450) of the superabsorbent polymer cake (1050) such that the visible geometric light marking (1210) on the surface (1450) undergoes a variation in height (h) with the variation in height of the superabsorbent polymer cake (1050) on the circulating conveyor belt (1400) ,- the image capturing device (1100) is adapted for capturing an image (1110, 1120) of a section of the surface (1450) such that the visible geometric light marking (1210) projected onto the surface (1450) is visible in the image (1110, 1120) as an image (1110, 1120) of the geometric light marking (1210), and- the evaluation unit (1300) is adapted for- detecting at least one position (15001, 15201, 15401, 15601) of the image of the geometric light marking (1210) in the image (1110, 1120) , caused by the superabsorbent polymer cake (1050) when moving with the circulating conveyor belt (1400) along the capturing device (1100) and- detecting a variation in the at least one position (p, 15001, 15201, 15401, 15601) of the image (1110, 1120) of the geometric light marking (1210) within the image (1110, 1120) captured by the image capturing device (1100), and- assigning the variation in height (h, 1500, 1520, 1540, 1560) of the visible geometric light marking (1210) on the surface (1450) to the variation in position (p, 15001, 15201, 15401, 15601) of the image (1110, 1120) of the geometric light marking (1210) within the image (1110, 1120),- determining from a value of variation in position (p, 15001, 15201, 15401, 15601) of the image of the geometric light marking (1210) within the image (1110, 1120) a corresponding value of variation in height (h, 1500, 1520, 1540, 1560) of the superabsorbent polymer cake (1050) on the conveyor belt (1400).

2. Monitoring system (1000) according to claim 1, wherein- the evaluation unit (1300) is signal connected to a control unit (1040) and adapted to transmit to the control unit (1040) height values (h) of the superabsorbent polymer cake (1050).

3. Monitoring system (1000) according to claim 1 or claim 2, wherein- the control unit (1040) is adapted for controlling operation of the air belt dryer (2000) based on the height values (h) of the superabsorbent polymer cake (1050).

4. Monitoring system (1000) according to any of the preceding claims, wherein the image capturing device (1100) continuously or discontinuously detects an image (1120) of a section of the surface (1450) such that the visible geometric light marking (1210) projected onto the surface (1450) is visible in the image (1120).

5. Monitoring system (1000) according to any of the preceding claims, wherein the evaluation unit (1300) is adapted for determining a perpendicular mean height (h') over the width of the superabsorbent polymer cake (1050) by determining more than one height value at more than one position of the image of the geometric light marking (1210) in the image (1110) corresponding to multiple measuring points (16501) across the visible geometric light marking (1210) and is further adapted for determining a longitudinal mean height (h”) over the length of the superabsorbent polymer cake (1050), whereby the width of the superabsorbent polymer cake (1050) extends across a conveying direction of the conveyor belt (1400) and the length of the superabsorbent polymer cake (1050) extends longitudinal to the conveying direction.

6. Monitoring system (1000) according to any of the preceding claims, wherein the visible geometric light marking (1210) generated by the marking unit (1200) is a visible laser line runningacross the surface (1450) of the conveyor belt (1400) of the air belt dryer (2000) and the marking unit (1200) is a laser.

7. Monitoring system (1000) according to any of the preceding claims, wherein the image capturing device (1100) is a camera, in particular a video camera.

8. Monitoring system (1000) according to any of the preceding claims, wherein the marking unit (1200) is resided on a first side of the circulating conveyor belt (1400) and the image capturing device (1100) is resided on a second side of the circulating conveyor belt (1400), such that the marking unit (1200) and the image capturing device (1100) are placed essentially opposite each other and / or transverse to the circulating conveyor belt (1400) on the opposite first and second side.

9. Monitoring system (1000) according to any of the preceding claims, wherein for projecting at an projection angle (a) the visible geometric light marking (1210) onto the surface (1450) of the superabsorbent polymer cake (1050), the marking unit (1200) and the image capturing device (1100) are placed at a monitor level above the circulating conveyor belt (1400).

10. Monitoring system (1000) according to any of the preceding claims, wherein the projection angle (a) is between 30° and 60° relative to the surface (1450) of the conveyor belt (1400), in particular essentially 45° relative to the surface (1450) of the conveyor belt (1400).

11. Belt dryer (2000) for drying of superabsorbent polymer having the monitoring system (1000) of claims 1 to 10, the belt dryer (2000) having a housing (2100), wherein the image capturing device (1100) and the marking unit (1200) are positioned at respective viewing glasses (2200) outside of the housing (2100), wherein the viewing glasses (2200) allow for an automated visual inspection of the surface of the superabsorbent polymer cake (1050) in order to monitor a variation in height (h) of the superabsorbent polymer cake (1050) on the circulating conveyor belt (1400).

12. Monitoring method (3000) adapted for monitoring a superabsorbent polymer cake (1050) on a circulating conveyor belt (1400) in a belt dryer (2000) according to claim 11 and / or using a monitoring system (1000) according to one of claims 1 to 10, the monitoring method comprising the steps of:- detecting at least one position (1500i, 1520i, 1540i, 1560i) of the image of the geometric light marking (1210) in the image (1120), caused by the superabsorbent polymer cake (1050) whenmoving with the circulating conveyor belt (1400) along the capturing device (1100) and- detecting a variation in the at least one position (p, 15001, 15201, 15401, 15601) of the image of the geometric light marking (1210) within the image (1120) captured by the image capturing device (1100), and- assigning the variation in height (h) of the visible geometric light marking (1210) on the surface (1450) to the variation in position (p, 15001, 15201, 15401, 15601) of the image of the geometric light marking (1210) within the image (1120),- determining from a value of variation in position (p, 15001, 15201, 15401, 15601) of the image of the geometric light marking (1210) within the image (1120) a corresponding value of variation in height (h, 1500, 1520, 1540, 1560) of the superabsorbent polymer cake (1050) on the conveyor belt (1400).

13. Monitoring method (3000) of claim 12, comprising the steps of:- setting up the marking unit (1200) outside the housing (2100) of the air belt dryer (2000) to generate a visible geometric light marking (1210) through a viewing glass (2200) onto the conveyor belt (1400) of the air belt dryer (2000) at an projection angle (a) between 30° to 60° relative to the top surface (1450) of the conveyor belt (1400) of the air belt dryer (2000), preferably at a projection angle (a) of essentially 45° relative to the top surface (1450) of the conveyor belt (1400) of the air belt dryer (2000), and / or- setting up the image capturing device (1100) outside the housing (2100) of the air belt dryer (2000) at a viewing glass (2200) opposite of the marking unit (1200) so that the image capturing device (1100) captures the section of the conveyor belt (1400) where the visible geometric light marking (1210) is projected on, and / or- setting up the evaluation unit (1300) with the pre-trained detection algorithm to determine a height (h) of the super absorbent polymer cake (1050) at the position (p) of the visible geometric light marking (1210) in the image (1120).

14. Monitoring method (3000) of claim 12 or 13, comprising the steps of:- having the image capturing unit (1100) capture an image (1110) of the section of the conveyor belt (1400) where the visible geometric light marking (1210) is projected on, while the air belt dryer (2000) is operated and a superabsorbent polymer cake (1050) is passing the visible geometric light marking (1210) and causing the visible geometric light marking (1210) to change its position (p)within the image (1120) according to the corresponding height (h) of the corresponding superabsorbent polymer cake (1050) passing the visible geometric light marking (1210), and / or- having the evaluation unit (1300) determine a height (h) of a superabsorbent polymer cake (1050) passing the visible geometric light marking (1210), by detecting a current position (p) of the visible geometric light marking (1210) within the image (1110) captured by the image capturing device (1100).

15. A process for producing water-absorbing polymer particles, comprising the steps of:processing an aqueous polymer gel from polymerization of a monomer solution or suspension,drying the aqueous polymer gel in a conveyer dryer, in particular in a forced air conveyer dryer, whereinthe conveyer dryer has a circulating conveyer belt and the aqueous polymer gel is conveyed on the circulating conveyer belt as a superabsorbent polymer cake (1050), in particular the circulating conveyer belt is formed as a circulating plate conveyer belt, comprising a number of belt plates separated by a hinge line of a hinge assembly and each belt plate having a surface for receiving the aqueous polymer gel, and wherein- height values (h) of the superabsorbent polymer cake (1050) are monitored by a monitoring method (3000) of one of claims 12 to14 adapted for monitoring a superabsorbent polymer cake (1050) on a circulating conveyor belt (1400) in a belt dryer (2000) according to claim 11 and / or using a monitoring system (1000) according to one of claims 1 to 10.

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