METHOD FOR DRYING SEWAGE SLUDGE AND DRYING PLANT

Description

[0001] The present invention relates to a method for drying moist material in the form of sewage sludge with the aid of a drying plant, wherein the moist material is fed via a feed line to a feeding unit of the drying plant, is fed onto a drying surface of the drying plant with the aid of the feeding unit and is dried there.

[0002] Furthermore, a drying plant for drying moist material in the form of sewage sludge is proposed, wherein the drying plant has at least one drying surface for receiving the moist material during its drying, wherein the drying plant has at least one feeding unit with the aid of which the moist material can be applied to the drying surface during operation of the drying plant, and wherein the drying plant comprises at least one supply line via which the moist material to be dried can be fed to the feeding unit.

[0003] In order to dry wet material, such as sewage sludge, for further use (e.g. its incineration), it has long been state of the art to apply the wet material to a drying surface and to expel the water it contains using appropriate heating devices.

[0004] Furthermore, it is known to select the drying capacity of the respective system or the amount or mass flow of the wet material to be applied to the drying surface depending on the water content of the wet material to be introduced into the drying system or the water content of the wet material already located at various positions on the drying surface (see, for example, DE 10 2011 004 788 A1). Another method for drying sewage sludge in which the water content is used to control the sludge addition is disclosed in JPS6434500A.

[0005] Even though the above-mentioned processes already achieve a relatively satisfactory drying of the wet material, there is still a certain need for improvement with regard to the energy efficiency of the drying system and the homogeneity of the dried wet material.

[0006] The object of the present invention is therefore to propose a process for drying moist material in the form of sewage sludge which is improved compared to the prior art, as well as a drying plant suitable for carrying out the process.

[0007] The problem is solved by a method and a drying system having the features of the independent patent claims.

[0008] According to the invention, a method for drying wet material in the form of sewage sludge using a drying system is proposed. For the purposes of the invention, the term "wet material" refers to sewage sludge that, in addition to a certain water content, also contains a certain amount of solids. A typical example of such a material is sewage sludge, i.e., sludge produced during wastewater treatment.

[0009] The method according to the invention provides that the moist material in the form of sewage sludge is fed via a feed line to a feeding unit of the drying plant, is fed onto a drying surface of the drying plant with the aid of the feeding unit, and there is dried to the desired residual water content. The feed line is preferably a pipe system consisting of rigid and / or flexible pipe sections through which the moist material can be drawn from a moist material storage container or other moist material collection point during operation of the drying plant and fed to the feeding unit. The feeding unit connected to the feed line finally serves to apply the moist material arriving via the feed line to the drying surface. An end piece with an opening at the end of the feed line through which the moist material can be discharged can serve as the feeding unit.However, it is also conceivable for the feed unit to divide the moist material present in the feed line as a continuous strand. For example, the feed unit can be designed as an extruder with one or more openings through which the moist material must pass, as well as a cutting unit movable relative to the openings to divide the moist material strand into a multitude of small pellet-shaped individual units, which ultimately fall side by side or one above the other onto the drying surface.

[0010] The drying surface is preferably a rotating, drive-driven drying belt through which, for example, a heated air stream flows to dry the moist material (the drying belt is preferably perforated for this purpose). In this case, the drying system is designed as a belt dryer.

[0011] The aim of the process is to dry the wet material in the form of sewage sludge applied to the drying surface in such a way that the final moisture content (i.e., the residual water content) of the dried wet material remains within certain and as narrow as possible limits, in order to enable targeted further utilization of the dried wet material. Furthermore, the drying capacity of the drying system should remain as constant as possible during the drying process and remain at a level at which the drying system's efficiency is at its highest.

[0012] To ensure this, the invention proposes that while the wet material, i.e. the sewage sludge, is being fed onto the drying surface and in the area of ​​the feed line and / or the feed unit, a parameter dependent on the water content of the wet material passing through the sensor is monitored with the aid of a sensor. The parameter can be the water content of the wet material or the measurement signal from a sensor that directly or indirectly measures the water content of the wet material. In general, any physically measurable parameter that depends on the water content of the wet material can be used within the scope of the invention. Furthermore, it should be noted that the sensor should be located either in or on the feed line or in or on the feed unit.

[0013] In any case, the invention provides that the quantity (volume or mass) of the moist material fed onto the drying surface per unit of time by means of the feed unit is continuously regulated, taking the characteristic variable into account. During the drying process, the moist material is therefore not fed onto the drying surface at a consistently constant mass or volume flow rate determined at the beginning of the drying process. Rather, the water content is taken into account during the feed, with the quantity of moist material fed in being selected to be smaller the higher the water content, so that the residual water content of the moist material leaving the drying system is always constant or at least fluctuates within very narrow limits. The drying capacity of the drying system therefore preferably remains at least virtually constant during the drying process.

[0014] In summary, the invention therefore provides for monitoring the water content of the wet material to be dried in the form of sewage sludge in an area located immediately before the point at which the wet material exits the feed unit and for regulating the amount of wet material exiting in dependence on the water content determined immediately beforehand, ie for continuously or continuously adapting it to the water content.

[0015] It is also advantageous if the magnitude of the parameter is determined at regular intervals rather than continuously (although the latter case is also conceivable). For example, it would be conceivable to determine the magnitude of the parameter between 1 and 100 times per hour and adjust the amount of moist material applied to the drying surface to the measurement result after each measurement of the parameter. In this case, the adjustment of the amount of moist material impinging on the drying surface is not continuous but nevertheless adjusted at regular intervals.

[0016] It is also advantageous if the value of the parameter is averaged over a defined period of time and the amount of moist material fed onto the drying surface per unit of time by the feed unit is regulated taking into account the respective averaged parameters. This can prevent the feed rate (= amount of moist material leaving the feed unit) from being changed due to a very short-term fluctuation in the water content of the moist material passing the sensor. Instead, the feed rate is adjusted depending on corresponding average values ​​obtained based on multiple measurements of the parameter and thus based on the water content of different sections of the moist material. The number of changes to the feed rate can thus be minimized.

[0017] According to the invention, a change in the quantity of moist material applied to the drying surface per unit of time only occurs if the value of the parameter changes by at least a certain minimum amount within a defined period of time or deviates by a certain minimum amount from a specified reference value. In this case, a change in the water content of the moist material passing the sensor does not necessarily result in a change in the discharge quantity. Rather, the change only occurs when certain limit values ​​are exceeded or undershot, or if there is a defined time-related change in the water content of the moist material passing the sensor.

[0018] It is also advantageous if the wet material is conveyed through the supply line to the feed unit using a conveying device, preferably a pump, with the conveying capacity of the conveying device being regulated taking the aforementioned parameter into account. For example, when drying sewage sludge, it would be conceivable to use a sludge pump and regulate its speed depending on the water content of the wet material passing the sensor. If a screw conveyor is also possible, its speed would be varied accordingly.

[0019] It is also advantageous if the parameter is determined without contact. In this case, the sensor does not come into direct contact with the moist material, thus preventing contamination or damage to the sensor by the moist material. For example, the sensor could be designed as a microwave sensor (also known as a microwave resonator). In this case, the sensor would be integrated into the conveyor line in such a way that the moist material passes through the resonator chamber of the microwave sensor. Depending on the water content, the resonance signal of the microwave sensor changes during the measurement, so that this parameter ultimately allows conclusions to be drawn about the water content of the moist material passing through the sensor and ultimately the regulation of the discharge quantity of the feed unit. The sensor is preferably arranged between the conveyor device and the feed unit (seen in the conveying direction of the moist material).

[0020] It is particularly advantageous if the amount of moist material fed onto the drying surface per unit of time is regulated taking into account the current drying capacity and / or other parameters of the drying system. In other words, drying system-specific parameters can also be taken into account when regulating the discharge rate of the feed unit. For example, with all other parameters remaining constant, the feed rate could be higher for a higher drying capacity of the drying system than for a lower drying capacity.

[0021] However, it is also conceivable that, in addition to the feed quantity, other parameters of the drying system or the drying process could be regulated depending on the water content of the wet material passing the sensor. For example, it would be possible to change the output of an extruder configured as part of the feed unit if the water content of the wet material passing the sensor changes. For example, the feed unit could comprise an extruder with a pelletizing device for pelletizing (i.e., fragmenting) the wet material. The pelletizing device would be operated taking the characteristic value into account, and its parameters would be changed depending on the water content of the wet material passing the sensor.

[0022] Furthermore, the invention relates to a drying system for drying moist material in the form of sewage sludge, wherein the drying system has at least one drying surface for receiving the moist material during its drying, wherein the drying system has at least one feed unit with the aid of which the moist material can be applied to the drying surface during operation of the drying system, and wherein the drying system comprises at least one supply line via which the moist material to be dried can be fed to the feed unit. With regard to possible physical features of the drying system or its components, reference is first made to the previous and subsequent descriptions, wherein all features can be implemented individually or in any combination, as long as this does not lead to technical contradictions.

[0023] In any case, the drying system (which is preferably designed as a belt dryer) comprises at least one sensor arranged in the region of the supply line or the feeding unit, with the aid of which sensor a parameter can be measured which depends on the water content of the moist material passing through the sensor. Furthermore, the drying system has means with the aid of which the quantity (i.e. the mass or volume flow) of the moist material applied to the drying surface by means of the feeding unit during operation of the drying system can be continuously regulated, taking the parameter into account. The means is, for example, a valve arranged in the supply line, with the aid of which the maximum moist material throughput in the supply line can be regulated.

[0024] According to a further aspect of the invention, said means comprises a conveying device, by means of which the moist material is fed to the feed unit via the supply line during operation of the drying system. Furthermore, the means comprises a control unit configured to regulate the conveying capacity of the conveying device, taking into account the characteristic value determined by the sensor.

[0025] In this context, it is also intended that the conveying device be designed as a pump (e.g., a high-density solids or sludge pump). In this case, the control unit evaluates the sensor's measured values ​​and compares them with corresponding reference values ​​stored in the control unit. Depending on the deviation of the current measured value (or an average of several measured values), the control system ultimately changes the pump's delivery rate in order to ultimately adapt the discharge volume of the wet material to the water content of the wet material passing the sensor and thus immediately before discharge.

[0026] Furthermore, it is advantageous if the sensor is designed to determine the parameter without contact. The sensor can be designed, for example, as an optical, capacitive, or microwave resonance sensor and is located, for example, behind a transparent or opaque wall of the conveyor line or the dispensing unit, so that the sensor itself does not come into direct contact with the moist material.

[0027] Further advantages of the invention are described in the following exemplary embodiment. It shows schematically: Figure 1 a perspective view of a drying system according to the invention. The single figure shows a perspective view of a drying plant for wet material 1 (sewage sludge) as an embodiment of a drying plant according to the invention, with which the method according to the invention can also be carried out.

[0028] The drying system comprises a drying surface 4 formed by a rotatingly driven drying belt 8, wherein the drying belt 8 is perforated in the example shown in order to allow the passage of drying air, which in turn is introduced by a heating arrangement 9 arranged, for example, below the drying surface 4.

[0029] In addition, the drying system has a feeding unit 3 which can be moved back and forth along a guide 7 and with the aid of a drive (not shown), with the aid of which the moist material 1 to be dried, which is fed to the feeding unit 3 via a partially flexible feed line 2 with the aid of a pump 6, can be applied to the drying surface 4 in a controlled manner.

[0030] The feeding unit 3 comprises, for example, an extruder unit with a die and a knife unit movable relative to the die, with the aid of which the moist material 1 pressed through the die is divided into individual pellet-shaped moist material units.

[0031] Of course, the moist material 1 can also exit the feed unit 3 in strand form (see figure) without being pelletized. In this case, the feed unit 3 does not have a knife unit.

[0032] In order to ensure that the moist material 1 is dried to a uniform degree of dryness regardless of its water content, the invention provides that a sensor 5 is assigned to the supply line 2 or the feed unit 3, with which the water content of the moist material 1 can be determined immediately before it is fed onto the drying surface 4. Furthermore, a control unit (not shown) is provided, which is designed to regulate the delivery rate of the pump 6 depending on the water content of the moist material 1 passing through the sensor 5. As a result, the feed quantity of the moist material 1 depends on the determined water content, whereby the feed can be reduced at a higher water content and increased at a lower water content.

[0033] An example of a thick spot 10 can be seen in the figure, which arose when the sensor 5 temporarily detected a lower water content and the feed rate was increased accordingly for a short time.

[0034] The present invention is not limited to the illustrated and described embodiments. The scope of protection is defined by the present claims. List of reference symbols

[0035] 1Wet material 2Feed line 3Feed unit 4Drying surface 5Sensor 6Pump 7Guide 8Drying belt 9Heating arrangement 10Thick spot

Claims

1. Method for drying moist material (1) in the form of sewage sludge with the aid of a drying plant, wherein the moist material (1) is fed via a supply pipe (2) of the drying plant to a feeding unit (3) of the drying plant, is fed with the aid of the feeding unit (3) onto a drying surface (4) of the drying plant and is dried there, wherein during the feed of the moist material (1) in the region of the supply pipe (2) and / or of the feeding unit (3) with the aid of a sensor (5) a characteristic variable dependent on the water content of the moist material (1) passing the sensor (5) is monitored, and wherein the quantity of the moist material (1) fed per unit of time with the aid of the feeding unit (3) onto the drying surface (4) is continuously regulated taking into account the characteristic variable, characterized in that the quantity of the moist material (1) fed per unit of time onto the drying surface (4) is selected to be smaller the greater the water content, wherein a change in the quantity of the moist material (1) fed per unit of time onto the drying surface (4) takes place only if the absolute value of the characteristic variable changes in a defined period of time by at least a certain minimum absolute value or deviates by a certain minimum absolute value from a fixed reference value or if a defined time-related change in the water content of the moist material (1) passing the sensor (5) is determined.

2. Method according to the preceding claim, characterized in that the absolute value of the characteristic variable is determined at regular time intervals.

3. Method according to one of the preceding claims, characterized in< / u> that the moist material (1) is conveyed with the aid of a conveying device, preferably with the aid of a pump (6), through the supply pipe (2) to the feeding unit (3), wherein the conveying capacity of the conveying device is regulated taking into account the said characteristic variable.

4. Method according to one of the preceding claims, characterized in< / u> that the absolute value of the characteristic variable is averaged over a defined period of time in each case and the quantity of the moist material (1) fed per unit of time with the aid of the feeding unit (3) onto the drying surface (4) is regulated taking into account the characteristic variables averaged in each case.

5. Method according to one of the preceding claims, characterized in< / u> that the characteristic variable is determined contactlessly, for example with the aid of a microwave sensor.

6. Method according to one of the preceding claims, characterized in that the regulation of the quantity of the moist material (1) fed per unit of time onto the drying surface (4) takes place taking into account the current drying capacity and / or further characteristic values of the drying plant.

7. Method according to one of the preceding claims, characterized in that the feeding unit (3) comprises an extruder with a pelletization device for making pellets of the moist material (1), wherein the pelletization device is preferably operated taking into account the characteristic variable.

8. Drying plant for drying moist material (1) in the form of sewage sludge with the aid of the method according to one or more of the preceding claims, - wherein the drying plant has at least one drying surface (4) for reception of the moist material (1) during the drying thereof, - wherein the drying plant has at least one feeding unit (3), with the aid of which the moist material (1) can be applied to the drying surface (4) during the operation of the drying plant, and - wherein the drying plant comprises at least one supply pipe (2), via which the moist material (1) to be dried can be fed to the feeding unit (3), - wherein the drying plant comprises at least one sensor (5) arranged in the region of the supply pipe (2) or of the feeding unit (3), with the aid of which a characteristic variable dependent on the water content of the moist material (1) passing the sensor (5) can be measured, - wherein the drying plant has means, with the aid of which the quantity of the moist material (1) applied to the drying surface (4) during the operation of the drying plant with the aid of the feeding unit (3) can be continuously regulated taking into account the characteristic variable, - wherein the means comprises a conveying device, with the aid of which the moist material (1) is fed during the operation of the drying plant via the supply pipe (2) to the feeding unit (3), - wherein the conveying device is designed as a pump (6), and - wherein the means comprises a control unit which is designed to regulate the conveying capacity of the conveying device taking into account the said characteristic variable, characterized in that the control unit is designed to select the quantity of the moist material (1) fed per unit of time onto the drying surface (4) to be smaller the greater the water content and to change the quantity of the moist material (1) fed per unit of time onto the drying surface (4) only if the absolute value of the characteristic variable changes in a defined period of time by at least a certain minimum absolute value or deviates by a certain minimum absolute value from a fixed reference value or if a defined time-related change in the water content of the moist material (1) passing the sensor (5) is determined.

9. Drying plant according to claim 8, characterized in that the sensor (5) is designed to determine the characteristic variable contactlessly.