Method for operating an installation for drying material by means of superheated steam

Abstract

An installation for drying material by means of superheated steam comprises a downwardly open chamber (10) with an inlet (61) for the material to be dried, an outlet (62) for dried material, an inlet (71a, 71b) for superheated steam and an outlet (73a, 73b) for a vapour. It also comprises a conveying system (60) for introducing the material into the chamber (10), transporting the material in the chamber (10) during the drying, and discharging the dried material out of the chamber (10), and also a vapour compressor (40) for compressing a first fraction of the vapour returned from the chamber (10), as well as a heat exchanger (30) for transferring heat from the compressed first fraction by condensing a volume flow of the compressed first fraction fed to the heat exchanger. The installation is operated in such a way that a steam atmosphere is formed in an upper region of the chamber (10) and floats on ambient air in a lower region of the chamber (10), wherein a transitional layer (66) is formed between the upper region and the lower region. A height of the transitional layer (66) is kept in a specified range by determining a current height and, depending on the height determined, controlling a volume flow of the compressed first fraction fed to the heat exchanger (30) or a volume flow of a steam generator (15), wherein the steam generator (15) is arranged and operated in such a way that steam is fed to the chamber (10) and / or is generated in the chamber (10).

Description

[0001] Method for operating a plant for drying material by means of superheated steam

[0002] The invention relates to a method for operating a plant for drying material by means of superheated steam and a corresponding plant.

[0003] State of the art

[0004] Industrial material drying consumes between 12 and 25% of total industrial energy consumption in industrialized countries, making it one of the most energy-intensive processes. Since drying is largely based on fossil energy sources, CO2 emissions are extremely high. Reducing the energy required for drying is therefore essential for CO2 reduction.

[0005] In processes that are advantageous in terms of energy consumption, superheated steam is used for drying instead of hot air. In US Pat. No. 5,711,086 (Heat-Win Limited), a device was proposed in which moist material is continuously conveyed through an opening into a chamber, through the chamber, and out of the chamber through an opening. The chamber contains an atmosphere of superheated steam, which is generated from the moisture in the material to be dried and / or is supplied from outside. A transition layer forms in the openings and in a vent duct between the steam atmosphere and the ambient air, preventing steam from escaping from the chamber but simultaneously allowing material to be fed in and out.

[0006] The system thus enables easy material supply and removal. The excess steam is condensed. By maintaining constant material mass flow, residence time, superheated steam mass flow, and temperature, a constant dry matter content can be easily achieved. However, the condensation of the excess steam and the recovery of the dissipated heat through reheating result in energy loss. WO 2012 / 140125 A1 (Epcon Evaporation Technology) describes a process with a closed chamber containing a mixing system in which the moist material is brought into contact with superheated steam. The excess steam is mechanically compressed and fed to a heat exchanger, enabling energy recovery and thus greater efficiency.

[0007] Process control is also ensured thanks to the closed chamber. However, this poses challenges with material supply and removal.

[0008] Description of the invention

[0009] The object of the invention is to provide a method for operating a plant for drying material by means of superheated steam, belonging to the technical field mentioned at the outset, and a corresponding plant which enables high energy efficiency with simple material supply and removal.

[0010] The solution to the problem is defined by the features of claim 1. According to the invention, a system is operated for drying material to be dried with superheated steam, which system comprises the following: a) a downwardly open chamber with an inlet for the material to be dried, an outlet for the dried material to be dried, an inlet for superheated steam, and an outlet for a vapor; b) a conveyor system for introducing the material to be dried into the chamber, transporting the material to be dried in the chamber during drying, and removing the dried material from the chamber; c) a vapor compressor for compressing a first portion of the vapor returned from the chamber; d) a heat exchanger for transferring heat from the compressed first portion by condensing a volume flow of the compressed first portion.

[0011] This system is operated in such a way that e) a steam atmosphere is formed in an upper region of the chamber, which steam floats on ambient air located in a lower region of the chamber, wherein a transition layer (stratification layer) is formed between the upper region and the lower region, and f) a height of the transition layer is maintained in a predetermined range by determining a current height and, as a function of the determined height f 1) the volume flow of the compressed first portion supplied to the heat exchanger is regulated; or f2) a volume flow of a steam generator is regulated, wherein the steam generator is arranged and operated in such a way that steam is supplied to the chamber and / or generated in the chamber.

[0012] Accordingly, a system according to the invention for drying material to be dried using superheated steam comprises: a) a downwardly open chamber with an inlet for the material to be dried, an outlet for the dried material to be dried, an inlet for superheated steam, and an outlet for a vapor; b) a conveying system for introducing the material to be dried into the chamber, transporting the material to be dried within the chamber during drying, and removing the dried material from the chamber; c) a vapor compressor for compressing a first portion of the vapor returned from the chamber; d) a heat exchanger for transferring heat from the compressed first portion by condensing a volume flow of the compressed first portion; and e) a controller for recording and processing measured values ​​and for generating control signals.

[0013] In particular, the control is operated in such a way that f) an atmosphere of superheated steam is formed in an upper region of the chamber, which atmosphere floats on ambient air located in a lower region of the chamber, wherein a transition layer is formed between the upper region and the lower region, and g) a height of the transition layer is maintained in a predetermined range by determining a current height of the transition layer and, as a function of the determined height g1), the volume flow of the compressed first portion supplied to the heat exchanger is regulated; or g2) a volume flow of a steam generator is regulated, wherein the steam generator is arranged and operated in such a way that steam is supplied to the chamber and / or generated in the chamber.

[0014] The materials being dried are primarily bulk materials, e.g., products or by-products from the food or feed industry, fuels or building materials, raw materials for the chemical or paper industry, and biomass in general. The technology can also be used in connection with the drying of textiles (including laundry). In principle, the process is best suited for drying materials that require a drying temperature between 100°C and 200°C and do not allow additional organic substances to be gasified.

[0015] The steam is primarily water vapor, but other solvents can also be used, such as ethanol.

[0016] The absence of oxygen during drying prevents oxidation of the dried product. This prevents fats in food or animal feed, for example, from becoming rancid. Furthermore, the risk of fire and explosion is minimized.

[0017] The recirculating process minimizes flavor loss and eliminates odor and dust emissions into the environment. Because the temperature is consistently above 100°C, the material to be dried is pasteurized or even sterilized.

[0018] The chamber is closed at the top, while it is completely or partially open at the bottom. It can have one or more openings at the lower end, which are arranged at the main chamber boundary or on an inward- or outward-facing, e.g. tubular, extension. Accordingly, the main volume of the chamber can be completely filled with the steam atmosphere, i.e. lie entirely in the upper region of the chamber, and the transition layer can form in the tubular extension or in several tubular extensions. The opening or openings are dimensioned in particular so that, on the one hand, pressure equalization and a certain gas flow between the chamber and the environment is possible and, on the other hand, that the dried material can be passed in and out through the opening or at least one of the openings.In particular, all dried material is ultimately transferred into and out of the chamber through one or more of these openings. The inlet and outlet for the material to be dried are thus formed by one or more of these openings. This allows material to be transferred in and out in a cost-effective manner without introducing significant amounts of air into the steam.

[0019] The steam atmosphere contains a residual proportion of air, preferably less than 4 vol.%.

[0020] The vapor is steam whose properties have been altered by its interaction with the material being dried and the chamber atmosphere compared to the superheated steam introduced. Typically, the vapor will have a higher saturation and a lower temperature than the superheated steam introduced. The vapor may also contain other components, particularly small amounts of the material being dried or dust, as an aerosol, as well as other vapors from the material, such as aromas.

[0021] The conveyor system enables, in particular, continuous operation of the plant. In a preferred embodiment, it comprises an ascending conveyor for introducing the material to be dried through an opening on the underside of the chamber, a belt conveyor for transporting the material to be dried in the chamber, and a gravitational discharge through the same or preferably another opening in the underside of the chamber. In an alternative embodiment, the conveyor system comprises one or more rotors, and the plant is operated as a disc dryer. Other conveyor systems can be used within the scope of the invention, e.g., a fluidized bed or a paddle conveyor. The plant can also be used, for example, together with a spray drying tower. The conveyor system can be partially formed by an upstream plant component, e.g., if the material to be dried is fed directly from this component, e.g.,an extruder, into the steam atmosphere, or blown in via a steam stream. The conveying system can comprise one or more conveyors (e.g., belts or gondolas). The chamber itself can be part of the conveying system—e.g., in a paddle mixer. The introduction, transport within the chamber, and / or discharge can all be entirely or partially due to gravity. For example, the dried material can fall out through an opening at the bottom.

[0022] Within the scope of the inventive method, the height of the transition layer, i.e., its vertical position, is maintained within a predetermined range. This corresponds to keeping the volume of the vapor atmosphere in the chamber constant.

[0023] The current altitude can be determined directly or indirectly. Instead of altitude, a dependent variable can also serve as the control variable for regulating the volume flow of the compressed first portion supplied to the heat exchanger or the steam generator, e.g., a temperature measured at a specific location or several locations and / or the concentration of a gas contained in the air, e.g., O2 or N2, at a specific location or several locations.

[0024] In a first variant, the volume flow of the compressed first portion fed to the heat exchanger is preferably adjusted by regulating the volume flow of the vapor compressor. This can be achieved by adjusting the speed of the vapor compressor and / or the opening of an adjustable orifice arranged upstream or downstream of the vapor compressor. Alternatively, a controllable valve can be arranged downstream of the vapor compressor, whereby a (proportional) volume flow of the compressed vapor is fed to the heat exchanger, while the (possible) remainder is returned to the chamber. This alternative variant can be used, for example, in conjunction with turbocompressors, which are ideally operated at a certain constant volume flow.In principle, such a (bypass) valve can also be arranged downstream of the heat exchanger, in which case the entire compressed first portion is passed through the heat exchanger, but only a portion is condensed there, while the remainder is returned to the chamber through the bypass valve. A combination of the variants is also conceivable, with control up to a certain minimum volume flow being achieved by controlling the vapor compressor, and the control valve only being used when the compressed portion supplied to the heat exchanger needs to be further reduced.

[0025] In the first variant, control is carried out in particular in such a way that the volume flow of the compressed first portion supplied to the heat exchanger is increased when a falling transition layer is detected, and that the volume flow is reduced when a rising transition layer is detected. This ultimately adjusts the volume flow of the first portion to increase or decrease the discharged portion, and thus the volume of the steam atmosphere in the chamber.

[0026] In the first variant, the control of the volume flow of the compressed first portion supplied to the heat exchanger is preferably carried out according to one of the following methods (although these methods can also be combined with each other):

[0027] 1. The height of the transition layer is controlled by adjusting the speed of the vapor compressor. Increasing the speed increases the initial portion of the vapor recirculated from the chamber that is to be compressed, thus increasing the volume of vapor compressed and subsequently fed to the heat exchanger as a volume flow.

[0028] 2. The height of the transition layer is controlled by a volume flow from the first compressed portion returned to the chamber. To achieve this, the opening of a (bypass) valve downstream of the vapor compressor is controlled such that the height of the transition layer remains within the target range. If the (bypass) valve is opened more, the volume flow returned to the chamber increases, and a correspondingly lower volume flow is available for condensation in the heat exchanger.

[0029] 3. The height of the transition layer is controlled by adjusting the opening of a baffle before or after the vapor compressor. This allows the first portion of the vapor recirculated from the chamber to be compressed to be controlled while maintaining a constant speed of the vapor compressor. In a second variant, the control is carried out in such a way that the volume flow of the steam generator is reduced when a falling transition layer is detected, and that the volume flow of the steam generator is increased when a rising transition layer is detected. This ultimately directly influences the volume of the steam atmosphere in the chamber.

[0030] The recycling of process heat through compression and condensation in a heat exchanger enables greater efficiency, but results in all relevant process parameters becoming interdependent, resulting in nonlinear system behavior. This poses a particular challenge when operating a plant with an open chamber, as it must always be ensured that the transition layer between the steam atmosphere and the ambient temperature remains stable and within a permissible height range. The inventive operation makes it possible to achieve a stable process with a constant dry matter content even in an open chamber.

[0031] In particular, the vapor compressor can be operated using electrical energy, which allows it to be operated with renewable energy and thus enables a massive reduction in CO2 emissions compared to conventional drying processes.

[0032] Preferably, the current height of the transition layer is determined based on measured values ​​of at least one temperature sensor arranged in a height range corresponding to the specified range.

[0033] The at least one temperature sensor is preferably arranged in a tube that extends downwards from the main volume of the chamber, particularly vertically, and connects the chamber to the environment. Alternatively or additionally, the temperature sensor can also be arranged in the opening for discharging the dried material.

[0034] The temperature sensor(s) function as vapor level sensors and ultimately determine the height of the vapor-air transition layer (or a directly related parameter). In a preferred embodiment, the transition layer is assumed to be falling if a specific temperature sensor delivers a value that exceeds a first threshold, while the transition layer is assumed to be rising if the specific temperature sensor delivers a value that falls below a second threshold. The threshold values ​​are selected, in particular, in the range of 90-100°C and preferably differ by 2-8°C. Particularly preferably, the first threshold is approximately 98°C, while the second threshold is approximately 96°C.

[0035] Temperature sensors can be located near the inlet and / or outlet. A location below the chamber, particularly near the outlet, is preferred because disruptive influences from the dried material on the temperature measurement are generally smaller there than disruptive influences from the material to be dried at the inlet. These influences can be further reduced if the temperature sensors are located in a separate pipe near the outlet. A pipe with a diameter of 1.5–6 cm has been shown to be sufficient for this purpose. Temperature sensors are particularly preferably located at both the inlet and outlet. This allows for the best possible monitoring of the process and early detection of malfunctions.

[0036] The control of the system parameters, in particular the speed of the vapor compressor, based on the temperature measurements is advantageously carried out using a PID controller, whereby the temperature measurements from several temperature sensors arranged at different heights, and thus also temperature gradients, can be used. The control can be integrated into a standard machine control system (PLC) or be processed by it.

[0037] If a different solvent is used instead of water, different temperature values ​​will result. The prevailing air pressure also has an influence, particularly due to the altitude above sea level, which must be taken into account when setting the temperature values. The above information refers to the drying process being carried out at sea level.

[0038] Instead of the temperature sensor(s) or in addition to them, other measured values ​​can be used to determine the height of the transition layer, e.g. one or more lambda sensors to determine the oxygen content or a chemical sensor to determine the nitrogen content or the content of another gas present in the air.

[0039] Advantageously, within the scope of the method according to the invention, a drying temperature is maintained within a predetermined range by comparing it with a target value and, depending on the comparison: g1) a volume flow of a steam generator is regulated, wherein the steam generator is arranged and operable such that steam can be supplied to the chamber or generated in the chamber, provided that the height of the transition layer is maintained within the predetermined range by regulating the volume flow of the compressed first portion supplied to the heat exchanger; or g2) a heating output of a heating device is regulated; or g3) the volume flow of the compressed first portion supplied to the heat exchanger is regulated, provided that the height of the transition layer is maintained within the predetermined range by regulating the volume flow of the steam generator.

[0040] The following variants result for controlling the height of the transition layer and the drying temperature:

[0041] In variant 2A, according to which the drying temperature is controlled by the heating capacity of the heater and the transition layer by the steam generator, the vapor compressor is responsible for extracting an initial portion of vapor from the chamber, which is greater than in variant 1B, but less than in variant 2B. The vapor compressor thus ensures that a higher temperature is achieved in the heat exchanger than in variant 1B, thus reducing the heating capacity of the heater.

[0042] The drying temperature is, in particular, the temperature of the superheated steam introduced into the chamber or—especially in the case of indirect drying—the temperature of a contact surface with the material being dried. The corresponding target value depends particularly on the material and the desired dry matter content.

[0043] The dry matter content of the processed drying material can be determined in the chamber, e.g., by measuring the temperature of the surface of the drying material. Infrared temperature sensors are well suited for this purpose. Based on the measured surface temperature, the dry matter content can be determined using a previously empirically determined characteristic curve. If this does not meet the specifications, system parameters are adjusted, particularly the target drying temperature of the superheated steam or the contact surface in indirect drying and / or the conveying speed of the conveyor system (and thus the residence time of the drying material in the chamber).

[0044] In a preferred embodiment, the plant comprises a piping system between the vapor outlet and the superheated steam inlet, the following being arranged in the piping system: g) the vapor compressor; h) a circulation fan; i) the heat exchanger for heating a second portion of the vapor recirculated from the chamber by transferring heat from the compressed first portion by condensing the volume flow of the compressed first portion fed to the heat exchanger; and j) a heating device for the steam, arranged between the heat exchanger and the superheated steam inlet. Heat transfer in the heat exchanger takes place in particular in countercurrent, with the compressed steam flow running from top to bottom.

[0045] The circulation fan can be located before or after the heat exchanger. It serves to maintain the steam flow in the circuit, thus compensating for the pressure drop experienced. It has been shown that the required mass flow increases approximately linearly with the evaporation rate. The mass flow delivered by the circulation fan should be at least 60 times the compressed mass fraction supplied to the heat exchanger. This ensures that, in addition to the actual evaporation of the liquid from the material to be dried, heat losses are also compensated and the material to be dried, including the water it contains and any surface water on it, can be preheated. The mass flow is preferably set higher than 60:1 to provide a safety margin, as the expected dissipation due to the high mass flow is converted into heat within the system and thus contributes to heating the steam.Depending on the specific design of the system, higher ratios of 100:1, 150:1, or even higher can be set. The circulation fan thus supports the heating system and can even replace it in certain versions.

[0046] The vapor compressor is a mechanical compressor used for heat recovery. The vapor is supplied via parts of the piping system or directly from the chamber. The vapor compressor can be multi-stage, i.e., with several compressor stages arranged in series.

[0047] The volume flow of the first portion of the vapor fed to the heat exchanger and compressed by the vapor compressor is proportional to the amount of steam released during drying of the material, resulting in a constant mass flow in the circuit. The first portion is determined by the pressure ratio and the speed of the vapor compressor according to the compressor map. The first portion can therefore be adjusted by controlling the speed. The volume flow of the compressed first portion fed to the heat exchanger is generally between 1:30 and 1:160, based on the circulating steam. In addition to the condensation of the compressed vapor (and possibly steam from the steam generator), other heat sources are possible in the heat exchanger, e.g. waste heat or dedicated heating devices.

[0048] The heating device for the steam is independent of the heat exchanger. It is, in particular, an electric resistance heater. Alternatively, a gas burner, for example, can also be used. As mentioned, the heating device can be integrated into the circulation fan; in particular, the steam is heated due to dissipation in the fan. If it is separate from the fan, it is arranged downstream of the fan in the circulation direction, preferably immediately before the inlet to the chamber. The desired dry matter content can ultimately be adjusted by controlling the heating device. Instead of a resistance heater (or as a supplement to it), waste heat can be used in the heating device, e.g., from exhaust gases from gas engines, whereby this heat is transferred to the steam via a controllable heat exchanger (e.g., a gas-gas heat exchanger for utilizing hot gas waste heat).

[0049] The heating system can comprise one or more heating units. For example, in a belt dryer, each belt is assigned a separate heating unit. Accordingly, the operation of individual heating units can be controlled separately or centrally.

[0050] The ultimate goal is a constant dry matter content. Changing the vapor compression rate to adjust the height of the transition layer will, in the medium term, change the condensation temperature in the condenser and thus also influence the evaporation rate in the material, which in turn leads to a change in the vapor level. This effect can be compensated for by regulating the heating power.

[0051] Although hardly any air enters the system through the open locks, trace amounts can never be completely eliminated. The small amounts of air in the system concentrate in the condenser and, over time, block valuable heat transfer surface. To prevent this, the condenser must be continuously vented. Therefore, it is advantageous for the heat exchanger to have a vent valve on the condenser side, and the opening of the vent valve is controlled based on a specific air fraction on the condenser side. The air fraction can be determined based on the condenser pressure and condensation temperature, based on the deviation from the saturation temperature, or directly using a lambda probe.

[0052] The vent valve is typically arranged as a needle valve, preferably above the condensate drain. The latter allows excess condensed water to be drained away. It is preferably controlled based on the measured values ​​of one or more level sensors, which can be formed, for example, by capacitive limit switches. Ultimately, the water is recovered from the dried material, usually in a sterile and demineralized state.

[0053] It is advantageous to regulate the condenser-side air fraction to a value of 0-50%, preferably 5-20%, and particularly preferably 7-12%. If the air fraction falls below a lower setpoint, a substantial vapor loss results. If the value is too high, the efficiency of vapor compression suffers.

[0054] Since the air mass flow into the condenser is not constant and depends heavily on the operating state of the system, the vent valve must be continuously adjusted to maintain the air content in the condenser at the desired percentage.

[0055] In a preferred embodiment, a pipe leading from the condenser ends in a branch (e.g. a T-piece or Y-piece).

[0056] This reliably separates water and non-condensable gases at the condenser outlet, ensuring minimal vapor loss to the atmosphere. One branch of this branch leads to a horizontal or slightly upward drain equipped with a vent valve. The other branch (mainly vertical) leads downward into a section of pipe with an enlarged cross-section, where a water column forms. This allows for controlled and delayed drainage of the condensed water.

[0057] Two (e.g., capacitive) level sensors are arranged along the pipe section containing the water column. A shut-off valve is connected at the bottom, which opens or closes depending on the measured values ​​of the level sensors so that the water column level is always between the level sensors.

[0058] A throttle (e.g., a needle valve) is arranged downstream of the shut-off valve. This component ensures that the condensed liquid drains more slowly and prevents the gas-air mixture from escaping downward.

[0059] Due to the pressure drop across the shut-off valve with its downstream throttle, steam is generated, thus forming the condensate drain into a (further) steam generator. This steam can be fed back into the steam circuit, the drying chamber, and / or the material being dried via the appropriate line and used for further drying and / or preheating of the material.

[0060] In this preferred embodiment, the vent valve is also controlled based on the specific condenser-side air fraction, which, as mentioned, can be determined directly, by means of a lambda probe, or indirectly based on the deviation of the static pressure from the vapor pressure of the condensation temperature.

[0061] Advantageously, the inlet for the superheated steam is arranged on the chamber in such a way that the superheated steam intersects a conveying path of the material being dried in the chamber in a directed steam flow. This preferably occurs in cross-current or countercurrent flow. The supply and discharge of the superheated steam and the internal geometry of the chamber are, in particular, coordinated in such a way that a circulation takes place through the steam atmosphere in the chamber.

[0062] When the system according to the invention is designed as a belt dryer, it is advantageous if the inflow of the superheated steam and the extraction take place as close as possible to the material.

[0063] Preferably, an element for homogenizing the steam flow is arranged on the chamber side of the inlet. The element forms, in particular, a flow resistance that calms the steam flow, i.e., in particular, eliminates large-scale vortices or secondary flows, and standardizes the flow profile. The resistance is dimensioned such that sufficient harmonization of the steam flow is achieved, while avoiding unnecessary pressure loss and the associated increase in the required power of the circulation fan. The element can be designed in the form of a filter or made of finely perforated material. Suitable materials include fiberglass mats. A diffuser can be arranged upstream to distribute the steam flow over a larger cross-section.

[0064] Such an element allows the circulation of superheated steam in the chamber to be controlled. It has also been shown to stabilize the transition layer.

[0065] When commissioning the system, the required steam atmosphere must first be created in the upper part of the chamber. For this purpose, the system preferably includes a steam generator, and the following steps are carried out in particular:

[0066] Generating steam in a steam generator and introducing the generated steam into the chamber, whereby air in the chamber is displaced downwards from the chamber; during operation of the steam generator (after completion of the build-up of the steam atmosphere or the displacing of the air from the chamber), until an operating pressure is reached in the heat exchanger:

[0067] Activating the circulation fan,

[0068] Activating the heating device and / or the vapor compressor,

[0069] Introducing material to be dried using the conveyor system and

[0070] Activating the vapor compressor.

[0071] The generated steam is introduced primarily from above, preferably at the highest point in the chamber and / or the piping system. The chamber is preferably preheated with air to 100°C beforehand. The air is displaced by the introduced steam not only from the chamber but also, in particular, from the piping system. During the final phase, in which both the steam generator and the vapor compressor are active, the operating pressure is built up in the heat exchanger, which operates as a condenser, while maintaining the steam atmosphere. This pressure is typically 1.5–4 barg, depending on other machine and process parameters. When the operating pressure is reached, the steam generator is switched off, thus transitioning to nominal operation.

[0072] In a further embodiment of the invention, the conveying system comprises a rotating hollow shaft arranged in the chamber with a plurality of disks, which forms the heat exchanger, wherein a cavity is arranged in the interior of the hollow shaft, to which the volume flow of the compressed first portion of the vapor can be fed from the vapor compressor to heat the disks. In this embodiment, the first portion therefore corresponds to the entirety of the recirculated vapor, although in a corresponding embodiment, a portion thereof can be returned to the chamber through a (bypass) valve arranged downstream of the vapor compressor. The hollow shaft thus acts as a condenser for the recirculated, compressed vapor. The cavity can extend into the disks or be limited to the central part of the hollow shaft.

[0073] The liquid material to be dried is fed to the discs through an inlet, dried, and finally, after drying, removed from the discs, e.g., scraped off, and discharged from the chamber through a material outlet. In this embodiment, drying thus occurs indirectly.

[0074] In the embodiment with the rotating disk shaft, steam is preferably supplied to the chamber from a steam generator. This supply occurs (also) during the drying process, in particular continuously, and, after the steam has been compressed by the vapor compressor and fed into the hollow shaft, ultimately serves to dry the liquid material, heat the chamber, and compensate for losses.

[0075] The steam generator is arranged and operated in such a way that steam can be supplied to the chamber or generated within the chamber. The supply can be made directly into the chamber or indirectly, e.g., via a piping system. Steam is generated, for example, by injecting water into an atmosphere of superheated steam. Accordingly, the steam generator can also be arranged directly within the chamber.

[0076] In one embodiment of the invention, the volume flow of the compressed first portion fed to the heat exchanger is again controlled based on the current height of the transition layer. The volume flow of the steam generator is preferably controlled based on a measured condensation temperature in the hollow shaft cavity such that this condensation temperature remains within a predetermined range. This ultimately adjusts the dry matter content of the material being dried. This corresponds to variant 1A shown above.

[0077] In another control method, which is also suitable for the variant with the rotating disk shaft, the height of the transition layer is maintained within the specified range not by controlling the volume flow of the compressed first portion supplied to the heat exchanger, but by controlling the volume flow of the steam generator. In this alternative method, the volume flow of the compressed first portion supplied to the heat exchanger is controlled, in particular based on the measured condensation temperature in the cavity, such that this condensation temperature remains within a specified range. This corresponds to variant 2B shown above.

[0078] In all embodiments of the invention, a steam generator located in the system can be powered by waste heat. Condensate from the heat exchanger, in particular, can serve as feed water. If the condensate is insufficient for feed, an additional water supply, e.g., from a tank, can be provided.

[0079] In systems with a steam cycle, steam from such a steam generator can be introduced into the cycle, increasing the compressed first portion supplied to the heat exchanger. This results in a higher condensation temperature and thus a greater amount of heat transferred to the cycle stream via the heat exchanger. Accordingly, the power of the heating device can be reduced, which can lead to increased process efficiency. Further advantageous embodiments and combinations of features of the invention emerge from the following detailed description and the entirety of the patent claims.

[0080] Short description of the drawings

[0081] The drawings used to explain the embodiment show:

[0082] Fig. 1A, B Schematic block diagrams of an inventive system for drying material by means of superheated steam according to a first embodiment and a second embodiment, respectively;

[0083] Fig. 2A is a schematic sectional view of the system according to the first

[0084] embodiment;

[0085] Fig. 2B is a detailed view of an advantageous embodiment of the

[0086] Condensate drain for the system according to the first embodiment;

[0087] Fig. 3 is a schematic sectional view of the system according to a third

[0088] embodiment;

[0089] Fig. 4A, B schematic sectional views of a drying chamber of a plant according to the invention according to a fourth embodiment;

[0090] Fig. 5A, B schematic sectional views of a drying chamber of a plant according to the invention according to a fifth embodiment.

[0091] Fig. 6 shows a representation of the actuators and control variables of the inventive

[0092] Attachment;

[0093] Fig. 7 is a block diagram of the sensor system of the inventive system according to the first embodiment;

[0094] Fig. 8 Temperature curves measured at three heights in a vertical pipe next to the outlet; and

[0095] Fig. 9 Temperature and air content curves during commissioning of the system according to the invention; In principle, identical parts in the figures are provided with identical reference numerals.

[0096] Ways to implement the invention

[0097] Figures 1A and B are schematic block diagrams of a system according to the invention for drying material using superheated steam, according to a first embodiment and a second embodiment, respectively. Figure 2A shows a schematic sectional view of the system according to the first embodiment. The first and second embodiments differ in the positioning of the circulation fan in the steam circuit. In addition, the second embodiment comprises an additional steam generator that can be operated using waste heat. It should be noted that the use of such a steam generator is also possible with a circulation fan positioned as in the first embodiment. Otherwise, all of the following statements apply to both the first and second embodiments.

[0098] The system comprises a chamber 10, into which moist material 1 can be introduced and dried material 2 can be discharged by means of a conveyor system 60. In the system shown, the mass flow of the moist material 1 (dry matter content 50%, temperature 50-70 °C) is 36 kg / h. The chamber 10 is closed at the top and sides and open at the bottom; in the illustrated embodiments, it accordingly comprises an inlet 61, which is designed as a pipe extending obliquely upwards to an upper region of a side wall of the chamber 10, with a bucket conveyor 65.1 of the conveyor system 60 arranged therein, acting as an ascending conveyor; the cross-section A 1 perpendicular to the longitudinal axis of the pipe is approximately 0.10 m 2 ; an outlet 62, which is formed as an opening at the bottom of the chamber 10, through which the dried material 2 is discharged under the effect of gravity; the cross-section A2 of the opening is approximately 0.02 m 2; a downwardly open measuring tube 63 (see Fig. 2), which is arranged in the region of the outlet 62 and contains several temperature sensors. The bucket conveyor 65.1 comprises trays for receiving the material to be dried, which are perforated so that air is not transported upwards during the transition into the chamber 10.

[0099] During operation, chamber 10 is filled with water vapor that floats above the ambient air.

[0100] A supply line of a steam generator 15 opens into the top of the chamber 10, so that steam can be directly supplied to the chamber 10 when needed—particularly during commissioning as described later. Steam release valves can also be arranged on the top of the chamber 10 to release excess steam from the chamber 10 (not shown).

[0101] The bucket conveyor 65. 1 feeds the moist material 1 from below from the ambient air slowly, at a speed of 10-30 mm / s, without entraining air into the steam atmosphere.

[0102] In chamber 10, two horizontal belt conveyors 65.2, 65.3 are arranged such that the first of these additional belt conveyors 65.2 receives the moist material from the bucket conveyor 65.1, conveys it through a first drying stage, and delivers it to the second belt conveyor 65.3, which conveys the material through a second drying stage. The area through which the steam flows in the area of ​​the belt conveyors 65.2, 65.3 is approximately 0.45 m each. 2 From the second belt conveyor 65.3, the material falls out of the chamber 10 through the outlet 62, through the vapor-air transition layer.

[0103] The residence time of the material in chamber 10 is adjusted by the conveying speed of the conveyor system 60. In the system shown, it is typically approximately 20-30 minutes.

[0104] A closed-circuit steam duct is connected to chamber 10. The circuit is driven by a circuit fan 20. In the system shown, the volume flow in the circuit is 2'150 m 3 / h. The treated, superheated steam is split into two partial streams upon entering the chamber. Each of the partial streams first passes through a diffuser, where it is distributed over a larger cross-section, and then through a filter element 72a, 72b. In the illustrated embodiment, these are biaxially woven glass fiber mats with a basis weight of 610 g / m 2This results in a pressure loss coefficient of 400 at a steam flow velocity of 1.3 m / s and a value of 200 at a steam flow velocity of 7 m / s or more.

[0105] They serve to homogenize the steam flow. The steam flows are then discharged in divided form through a first steam inlet 71a adjacent to the first belt conveyor 65.2 and through a second steam inlet 71b adjacent to the second belt conveyor 65.3. In the system shown, the evaporation mass flow is approximately 16 kg / h (corresponding to 26.7 m 3 / h steam). The steam volume in the steam chamber is 0.85 m 3 , where the air content is lower than 4%.

[0106] The steam streams cross the transport surfaces of the belt conveyors 65.2, 65.3 and are extracted from the chamber 10 through a steam outlet 73a, 73b on the opposite side. The steam adds heat to the material to be dried, causing water to evaporate. The dry matter content of the material is determined by analyzing the material after it has escaped into the ambient air, based on which the steam temperature and residence time are adjusted. Alternatively, the dry matter content can also be checked by (optical) temperature measurement of the material surface in the steam. Several corresponding sensors can be arranged along the conveying path in the chamber to monitor the drying process.

[0107] To introduce heat into the circulating steam channel, it includes a heat exchanger 30 and a subsequent heating device 50. The latter advantageously comprises a first heating unit 51a for the steam portion supplied to the first steam inlet 71a and an independently controllable second heating unit 51b for the steam portion supplied to the second steam inlet 71b. The heat exchanger 30 is a finned heat exchanger. It has an external heat exchange surface of approximately 95 m 2 and an internal heat exchanger surface of approx. 2.3 m 2A filter can be arranged upstream of the heat exchanger 30 to prevent contamination by entrained material particles. The heat exchanger 30 functions internally as a condenser, with a vapor compressor 40 compressing a portion of the vapor extracted from the chamber 10 and feeding it to the condenser, where it condenses under increased pressure, typically 2.5-4 barA, while transferring the evaporation enthalpy to the circulating steam stream via the heat exchanger 30. In the illustrated embodiment, the vapor compressor 40 has an installed power of 3.7 kW. During operation, the power is typically approximately 1.1 kW.

[0108] The subsequent heating device 50 further superheats the circulating steam flow to the required drying temperature.

[0109] To discharge the water, the condenser of the heat exchanger 30 has a condensate drain valve 31, which opens automatically depending on the water level. For this purpose, the water level is monitored with one or more capacitive level sensors, and the valve is opened for a predetermined period of time when the water level exceeds a certain target level. If two water level sensors are used, the upper sensor can be used to initiate the draining process, while the predetermined time interval is shortened if the lower water level sensor is triggered during draining. This ensures that a column of condensate always remains in the height range between the lower sensor and the valve, so that the vapor-air mixture cannot escape directly through the valve. In the system shown, the mass flow of the condensate is typically approximately 7.5 kg / h (corresponding to 13.6 m 3 / h steam).

[0110] In addition, an air release valve 32 is mounted above the water outlet, through which non-condensable gases are released. The proportion of these gases in the steam is determined by temperature and pressure sensors at the condenser outlet.

[0111] In the second embodiment, the condensate from heat exchanger 30 is fed to a steam generator 17. Waste heat (e.g., at a temperature of approximately 170°C) is supplied to this generator to evaporate the condensate. The generated steam is then fed to the steam circuit, downstream of the circulation fan 20 and upstream of the branching of the supply lines to heat exchanger 30 and vapor compressor 40.

[0112] Figure 2B shows a detailed view of an advantageous embodiment of the condensate drain for the system according to the first embodiment. A pipe 33 leading from the condenser of the heat exchanger 30 opens into a Y-branch. One branch of this branch leads into a horizontal or slightly upwardly directed drain, which is provided with the air release valve 32. The other branch leads vertically downward into a line section 34 with an enlarged cross-section, in which a water column forms. Two (e.g., capacitive) level sensors 35.1, 35.2 are arranged along this line section 34 with the water column. A shut-off valve 36 is connected at the bottom, which is opened or closed depending on the measured values ​​of the level sensors 35.1, 35.2 so that the level of the water column is always between the level sensors 35.1, 35.2.

[0113] A further pipe section and then a needle valve 37 are connected to the shut-off valve 36 as a throttle. The steam generated due to the pressure drop at the shut-off valve 36 and the needle valve 37 is ultimately fed back into the steam circuit, into the drying chamber, and / or to the material being dried via a line 38 downstream of the needle valve 37.

[0114] Again, the air discharge valve 32 is controlled directly based on the condenser-side air portion determined by means of a lambda probe 39 or indirectly based on the deviation of the static pressure from the vapor pressure of the condensation temperature.

[0115] Figure 3 shows a schematic sectional view of the system according to a third embodiment. The system according to the third embodiment is used for indirect drying. It comprises a steam-filled chamber 110 that is closed at the top and partially open at the bottom. The conveying system 160 in the chamber 110 has a rotating hollow shaft 167 with several disks 168, which is hollow and functions both for conveying the material and as a heat exchanger, thus acting as a condenser internally. The system further comprises a vapor compressor 40, a

[0116] Steam generator 1 15 and an inlet 161 for liquid material to be dried.

[0117] The liquid material to be dried is fed through inlet 161 to the outside of hollow shaft 167 with discs 168. Vapors from chamber 110 are fed to vapor compressor 40. For this purpose, vapor compressor 40 sucks the vapors from the chamber and, after compression, feeds at least a portion into the hollow disc condenser. There, the compressed vapor condenses and heats hollow shaft 167 with discs 168, thereby drying the material. The dry matter content of the material to be dried is adjusted via the condensation temperature. The air content in hollow shaft 167, which acts as a condenser, is regulated to a specific proportion by a discharge valve. The dried material is scraped off the discs 168 by a scraper grinding on hollow shaft 167 and discharged through an outlet 162 on the underside of chamber 110.To continuously heat the system and compensate for losses, steam is continuously drawn from the steam generator 115. A steam blower is not required in the system according to the third embodiment.

[0118] The system according to the third embodiment can be controlled in two basic ways:

[0119] According to a first method, the volume flow of the steam generator 115 is controlled by measuring the condensation temperature of the hollow shaft such that this temperature remains within a predetermined range. The volume flow of the compressed portion supplied to the disc condenser is controlled based on a temperature sensor in the area of ​​the steam-air separation layer such that this temperature remains within a predetermined range, thus maintaining the transition layer at a predetermined height.

[0120] According to a second method, the volume flow of the compressed portion supplied to the disc condenser is controlled by measuring the condensation temperature of the hollow shaft such that this temperature remains within a predetermined range. The volume flow of the steam generator 115 is controlled based on a temperature sensor in the area of ​​the steam-air separation layer such that this temperature remains within a predetermined range, thus maintaining the transition layer at a predetermined height.

[0121] Figures 4A and 4B are schematic sectional views of a drying chamber of a plant according to the invention according to a fourth embodiment, as well as the corresponding supply and discharge systems. An ascending conveyor, which is designed analogously to that according to the first three embodiments and serves to introduce the material to be dried into the steam atmosphere of the chamber, is not shown in the figures. Figure 4A shows a view in a vertical plane perpendicular to the axis of rotation of the paddles, while Figure 4B shows a view in a vertical plane passing through this axis of rotation. The other components of the plant, in particular the steam supply and discharge and preparation, the material supply, the sensors, and the control system, essentially correspond to those of one of the first three embodiments.

[0122] The chamber 210 forming a conveying channel has an essentially circular-cylindrical shape. The paddles 267.1, 267.2 are mounted to rotate about the longitudinal axis of the chamber 210 and are at a constant distance from the chamber wall. Depending on the material being conveyed, this distance should be selected to be small enough to prevent the material from becoming trapped. Adjacent to the paddles 267.1, 267.2 with a small wall distance, paddles 267.3, 267.4, and 267.5 with a larger wall distance are mounted, whereby the mutual axial distance of the paddles 267.1...5 is always the same. Here, too, the gap size should be selected to be large enough, depending on the material being conveyed, to prevent larger pieces from becoming trapped, yet facilitate material transport. The paddles 267.1...5 each have an axial angle of attack in the conveying direction of, for example, 30°. A different number of paddles can also be used.

[0123] Two vertical channels open into the chamber 210 at one end on the upper side. One serves as an inlet 261 for the material to be dried, and the other as an outlet 273 for steam removal. At the other end of the chamber 210, a lateral outlet 262 for discharging the dried material is located in the upper area. The height of the lower edge of the outlet 262, and thus the fill level of the conveying channel, can be adjusted by vertically adjusting a weir 211. A fill level of 2 / 3 or more is desired.

[0124] The paddles 267.1 ... 5 rotate slowly, at approximately 20-30 revolutions per minute. They can rotate in both directions, with the main direction of rotation (for conveying the material toward the material outlet) pointing so that the paddles 267.1 ... 5 move downward when steam enters.

[0125] The dried material falls through outlet 262 into a conveyor channel with a spiral 268 for controlled backflow and controlled removal of the material. Once the material has passed through spiral 268, it falls into a vertical discharge channel, where the transition layer 266 extends between the ambient and vapor atmosphere. The controlled backflow ensures that the transition layer 266 is stable.

[0126] Steam is supplied from the top of the steam circuit through corresponding inlets 271 and introduced into the mixer / conveyor trough in the lower area of ​​chamber 210, distributed laterally / horizontally along the length of the conveying channel via inlets 274.1...3. The flow resistance of the material lying on top is utilized to generate a uniform flow, thus ensuring the most homogeneous drying process possible. At the same time, this steam supply design prevents material from falling back into the steam circuit.

[0127] The steam inflow is adjusted so that there is no inflow opening at the axial positions of the paddles 267.1 and 267.2 with a small gap. The gap is as wide as the paddle tip in each case. At the locations without or with a shortened paddle 267.3...5 with a large gap, the steam is directed into the chamber 21.1 through an inflow opening.

[0128] The side openings can vary in size. Depending on the required steam distribution along the mixer axis, they become smaller the closer they are to the material inlet or steam outlet. (Otherwise, the steam would choose the path of least flow resistance, meaning that a large portion of the mixer would be left unattended or barely flowed through.) The steam temperature of the side inlet channels does not need to be uniform, but ideally increases along the conveying channel in the conveying direction. The drier the material becomes toward the end of the process, the hotter the introduced steam becomes.

[0129] The steam entering from the side initially flows through the loosened material almost transversely and then countercurrently to the direction of material flow. Finally, the steam is extracted upwards through outlet 273 next to the material inlet 261, preventing any particles from being entrained.

[0130] Figures 5A and 5B are schematic sectional views of a drying chamber of a system according to the invention according to a fifth embodiment, as well as the corresponding supply and discharge lines. Figure 5A shows a view in a vertical plane perpendicular to the rotational axis of the spiral, while Figure 5B shows a view in a vertical plane passing through this rotational axis. The other components of the system, in particular the steam supply and discharge and processing, the material feed, the sensors, and the control system, essentially correspond to those of one of the first three embodiments.

[0131] The drying chamber according to the fifth embodiment has many similarities to that of the fourth embodiment. The main difference is that instead of paddles, a spiral is used as the mixing and conveying element in the chamber. The chamber 310, which forms a conveying channel, has a substantially circular-cylindrical shape. The spiral 367 is mounted for rotation about the longitudinal axis of the chamber 310, and the individual windings are spaced a short distance from the chamber wall.

[0132] Two vertical channels open into the chamber 310 at one end on the top side, one serving as inlet 361 for material to be dried and the other as outlet 373 for steam removal. At the other end of the chamber 310, in the upper area, there is a lateral outlet 362 for discharging the dried material. The height of the lower edge of outlet 362 and thus the fill level of the conveying channel can be adjusted by vertically adjusting a weir 311. The aim is to achieve a fill level of 2 / 3 or more. The spiral 367 rotates slowly, at approximately 20-30 revolutions per minute. It can rotate in both directions, with the main direction of rotation (to convey the material towards the material outlet) pointing such that the turns of the spiral 367 move downward when steam enters.

[0133] The dried material falls through outlet 362 into a conveyor channel with a spiral or screw conveyor 368 for controlled backflow and controlled removal of the material. Once the material has passed through the spiral 368, it falls into a vertical discharge channel, where the transition layer 366 runs between the ambient and vapor atmosphere. The controlled backflow ensures that the transition layer 366 is stable.

[0134] Steam is fed from the top of the steam circuit through corresponding inlets 371 and introduced into the mixer / conveyor trough in the lower area of ​​the chamber 310, distributed laterally / horizontally over the length of the conveying channel via an inlet 374. The flow resistance of the material lying on top is used to generate a uniform flow, which should produce the most homogeneous drying process possible. At the same time, the design of the steam supply prevents material from falling back into the steam circuit. The cross-section of the inlet 374 decreases opposite to the material conveying direction. The inlet 374 is divided into different temperature zones in the feed line, so that the steam temperature increases along the conveying channel in the conveying direction: the drier the material becomes towards the end of the process, the hotter the introduced steam.

[0135] The steam entering from the side initially flows through the loosened material almost transversely and then countercurrently to the direction of material flow. Finally, the steam is extracted upwards through outlet 373 next to the material inlet 361, preventing any particles from being entrained.

[0136] The operation of the system according to the invention is described below using the first two embodiments. However, the corresponding information can also be readily applied to the three other embodiments.

[0137] Figure 6 is a representation of the actuators and controlled variables of the system according to the invention, specifically the system according to the first embodiment, when operated according to variant 1B, wherein the volume flow of the compressed first portion supplied to the heat exchanger 30 is adjusted by controlling the vapor compressor 40. The actuators 81 can influence the controlled variables 82. The actuators 81 comprise the circulation fan 20, which is controllable in particular via its speed in order to adjust the circulation vapor flow 82.3, the air discharge valve 32, which can be selectively opened or closed to adjust the venting mass flow 82.5, the vapor compressor 40, whose mass flow 82.4 can also be adjusted via the speed, and the heating device 50, whose output is used to regulate the steam temperature 82.2 can be adjusted, as well as the conveyor system 60, which allows adjustment of the conveyor speed and thus both of the material throughput 82.1 and of the residence time of the material to be dried in the chamber.

[0138] The variable material variables 83 include the dry matter content 83.1 at the inlet, the material consistency 83.2, the material shape 83.3, and the material-dependent sorption isotherm 83.4. The control variables 84 are primarily the dry matter content 84.1 at the outlet and the height 84.2 (or position) of the transition layer.

[0139] The resulting values ​​85 from the operating parameters are the condenser pressure 85.1 and the specific energy consumption 85.2 (in kWh / kg water).

[0140] Figure 7 is a block diagram of the sensor system of the system according to the invention. The following variables are continuously measured and fed into the system control system:

[0141] Figure 8 shows temperature curves at three heights in a vertical pipe next to the outlet, measured by temperature sensors 91.8a, 91.8b, and 91.8c in the measuring pipe 63 (cf. Figure 7). The uppermost temperature sensor 91.8a is arranged at a vertical distance of 50 mm from the chamber floor. Neighboring sensors are arranged at a vertical distance of 50 mm from each other. The uppermost curve 95a represents the values ​​measured by the uppermost temperature sensor 91.8a, the middle curve 95b represents the values ​​measured by the middle temperature sensor 91.8b, and the lowest curve 95c represents the values ​​measured by the lower temperature sensor 91.8c. The series of measurements relate to drying operation, in which an equilibrium state is sought by controlling the aforementioned manipulated variables 82. In the present case, the temperature measured by the uppermost temperature sensor 91.The temperature measured at 8a is used as the basis for controlling the manipulated variables 82, in particular the mass flow 82.4 of the vapor compressor 40, so that the transition layer is maintained at its level 84.2 by control. The setpoint is 97.0 °C. Alternatively, the average temperature sensor 91.8b can be used, or a variable derived from the measured values ​​of several sensors. If the corresponding temperature or a variable determined from the corresponding temperatures leaves a specified range (e.g., control temperature ±1 K), the speed of the vapor compressor 40 is increased or decreased during operation according to variant 1 A or 1 B. A known PID controller is advantageously used for control. For the speed control of the vapor compressor 40, values ​​of P = 1, I = 10, and D = 0, for example, can be selected.

[0142] The commissioning of the system according to the invention is described using Figure 9, which shows the temperature (top, in °C) and air content (bottom, in %) curves during commissioning of the system according to the invention. Commissioning is divided into three phases: a heating phase with air (phase 1), steam filling (phase 2), and finally material filling (phase 3). Shown in the upper area are the chamber temperature 96 in the upper area of ​​the chamber, the temperature 97.2 measured by temperature sensor 91.2 downstream of heat exchanger 30, the temperature 97.7 measured by temperature sensor 91.7 downstream of vapor compressor 40, and the temperatures 97.8a, 97.8b, 97.8c of the three temperature sensors 91.8a, 91.8b, 91.8c in measuring tube 63 (with the temperature generally decreasing downwards). In the lower range, the air content measured by a lambda sensor is 98.1 in chamber 10 and the air fraction 98.2 in the condenser, determined indirectly based on the measured pressure and temperature of the steam at the condensate outlet after the condenser. These measured values, in combination with the temperature measurements, allow the steam charge to be precisely monitored.

[0143] Steam drying takes place in a steam atmosphere at ambient pressure, with the air content in the steam atmosphere not exceeding 4%. Therefore, the system's chamber must first be preheated with air to a temperature of at least 100 °C, and a steam atmosphere must be created within it. This is achieved in three phases.

[0144] In a first phase, the system is heated with hot air. For this purpose, air is circulated by the circulation fan 20, while heat is supplied via the heating device 50. This phase begins at position A in Figure 9 and lasts approximately one hour. Towards the end of this phase, the vapor compressor 40 is switched on (short-circuited) at idle (position B) to preheat it as well, thereby preventing significant thermal stresses and condensation in the vapor compressor 40 during steam filling. The phase is completed when the chamber 10 reaches a temperature of over 100°C. The temperature 97.2 of the circulating air in the circulation channel has already far exceeded 100°C at this point, since the air is heated directly.

[0145] Once the chamber temperature of 100 °C is reached, steam filling begins (item C). For this purpose, the heater 50, the vapor compressor 40, and the circulation fan 20 are switched off, and steam from the steam generator 15 is fed into the chamber 10 from above. The air, which has a lower density, is displaced downwards from the chamber 10. This is evident by the increase in the temperatures 97.8a...c measured by the temperature sensors 91.8a...c at the material outlet. Towards the end of this phase, the vapor compressor 40 is switched on again to reach operating temperature, causing the temperature 97.7 to briefly drop (item D).

[0146] With regard to the air content, the air content 98.1 in the chamber initially decreases suddenly, then the temperatures 97.8a...c measured by the temperature sensors 91.8a...c slowly rise as the hot air is displaced downwards. When these reach 100°C, this means that the steam volume has reached the bottom of the system. It has been shown that the air can be displaced from top to bottom out of the chamber without any problems by the lighter steam. Eventually, the steam floats above the cold ambient air. Despite the openings on the underside of the chamber, a stable transition layer 66 forms between steam and air, the so-called stratification layer (see Fig. 2). In the area of ​​this layer, a temperature profile is established that extends from ambient temperature to over 100°C within approximately 50 cm. In the range of temperature gradient from 100 °C to 65 °C, the temperature gradient is typically 0.13-0.26 K / mm.The air content in chamber 10 drops to less than 4%.

[0147] Once the steam atmosphere has been generated, material can be fed into the system (Pos. E). During this phase, steam must still be generated using the steam generator 15. This is necessary because steam condenses on the cold material and thus heats it up. Since insufficient steam is generated by the drying process, it must be provided by the steam generator 15. During this process phase, the heating device 50 and the circulation fan 20 are restarted. As soon as a large part of the holding capacity of the chamber 10 is filled with material, the vapor compressor 40 is ramped up further, which increases the condenser pressure (Pos. F). This increases the condensation temperature in the condenser, allowing heat to be released back into the steam circuit (Pos. G).Once chamber 10 is completely filled with material within its capacity and a sufficient water evaporation rate has been reached, steam generator 15 can be shut down and the regular drying process begins. The air content in chamber 10 remains at less than 4%. When operating according to one of variants 2A and 2B, the steam generator continues to run (usually at reduced power) to regulate the height of the transition layer.

[0148] Since, on the one hand, the target dry matter content of the material at the outlet depends on the relative pressure and thus on the steam temperature (provided the residence time is sufficiently long) and, on the other hand, heat must be continuously supplied to the continuous process for material preheating, the heat is supplied at a high temperature before the material is fed in, while the preheating of the material takes place at a lower temperature by the vapor upon entry into the steam atmosphere.

[0149] During this phase, the condenser must continue to be vented. Although the air content in the system is very low, the residual air accumulates in the condenser and must be continuously vented (Item I). The air content on the condenser side is regulated by the air vent valve 32 to a value of less than 15 vol.%, particularly 7-10 vol.%. The air content is determined based on the measured values ​​of temperature sensor 91.9 and pressure sensor 92.9.

[0150] During drying, the moisture in the material to be dried evaporates in chamber 10 due to heat input from the superheated steam. At the inlet to chamber 10, the steam is superheated above its saturation temperature. As it passes through the material to be dried, the thermal energy of the steam is transferred to the material, and additional water evaporates. At the outlet of chamber 10, the steam mass flow is increased by the water evaporated from the material. The temperature is reduced depending on the dry matter content of the material, the state of the sorption isotherm, and the degree of heat transfer to the material, so that the steam remains superheated.

[0151] The majority of the cycle steam then enters the heat exchanger 30 and is superheated again by the condensation of the vapor compression steam at a higher temperature on the other side of the heat exchanger 30.

[0152] After superheating in heat exchanger 30, heat losses are compensated by the heating device 50. This also allows the drying temperature and thus the desired dry matter content at the outlet to be adjusted precisely and quickly. Steam temperatures of 140–170 °C are generally well suited for drying, while the material temperature, depending on the sorption isotherm, is usually between 105–130 °C.

[0153] After the steam leaves the drying chamber, a portion of the additional steam is extracted from the circuit and compressed by the vapor compressor 40 to a pressure of approximately 2.5 to 5 barA. Depending on the pressure in the condenser, the steam condenses at its saturation temperature between 130°C and 150°C. The evaporation enthalpy released during condensation is returned to the steam circuit at an elevated temperature through the heat exchanger.

[0154] Demineralized, sterile water at temperatures above 100°C leaves the system through the condensate drain valve 31, preserving the circulating steam. Finally, the 100°C water can be used for preheating or instead of tap water.

[0155] During the drying process, the material to be dried or dried is continuously fed in and out, passing through the stratification layer. Upon discharge into the ambient air, it undergoes a secondary drying process due to the lower partial pressure of the vapor in the ambient air and the residual heat in the material being dried. If the material flow is increased, the compressed first portion fed to the heat exchanger must be increased accordingly. This works as long as the power of the circulation fan is sufficient to return the heat. It has been shown that, within this framework, the efficiency of the process actually increases when the material flow is increased.

[0156] In the third embodiment, the steam atmosphere is formed during commissioning mainly through the following steps:

[0157] 1. Air displacement by steam from steam generator;

[0158] 2. Introduction of material to be dried;

[0159] 3. Activating the vapor compressor (which starts the drying process);

[0160] 4. After the operating pressure in the heat exchanger has been reached, the steam generator continues to operate with a reduced flow rate (controlled as described above).

[0161] The invention is not limited to the illustrated embodiments. In particular, the dimensions of the respective systems and the conveyor systems used can be adapted to the type and quantity of material to be dried.

[0162] The material can be introduced into the steam atmosphere directly from a preceding process. The material can also be preheated before being introduced into the system. This increases, in particular, the amount of steam available for vapor compression. If waste heat is available, e.g., from an upstream or downstream process step, it can be easily fed into the system according to the invention, thus reducing the energy requirements of the heating device.

[0163] In summary, the invention provides a method for operating a plant for drying material by means of superheated steam and a corresponding plant, which enables high energy efficiency with simple material supply and removal

Claims

Patent claims 1. A method for operating a plant for drying material to be dried using superheated steam, the plant comprising the following: a) a downwardly open chamber with an inlet for the material to be dried, an outlet for the dried material to be dried, an inlet for superheated steam, and an outlet for a vapor; b) a conveyor system for introducing the material to be dried into the chamber, transporting the material to be dried in the chamber during drying, and removing the dried material from the chamber; c) a vapor compressor for compressing a first portion of the vapor returned from the chamber; and d) a heat exchanger for transferring heat from the compressed first portion by condensing a volume flow of the compressed first portion;wherein the system is operated in such a way that e) a steam atmosphere is formed in an upper region of the chamber, which steam atmosphere floats on ambient air located in a lower region of the chamber, wherein a transition layer is formed between the upper region and the lower region, and f) a height of the transition layer is maintained in a predetermined range by determining a current height and, as a function of the determined height f 1) regulating the volume flow of the compressed first portion supplied to the heat exchanger; or f2) regulating a volume flow of a steam generator, wherein the steam generator is arranged and operable in such a way that steam can be supplied to the chamber and / or generated in the chamber.

2. Method according to claim 1, characterized in that the current height of the transition layer is determined based on measured values ​​of at least one temperature sensor which is arranged in a height range corresponding to the predetermined range. Method according to claim 1 or 2, characterized in that a drying temperature is maintained in a predetermined range by comparing it with a target value and, depending on the comparison, g1) a volume flow of a steam generator is regulated, wherein the steam generator is arranged and operable such that steam can be supplied to the chamber or generated in the chamber, provided that the height of the transition layer is kept in the predetermined range by regulating the volume flow of the compressed first portion supplied to the heat exchanger; or g2) a heating output of a heating device is regulated; or g3) the volume flow of the compressed first portion supplied to the heat exchanger is regulated, provided that the height of the transition layer is kept in the predetermined range by regulating the volume flow of the steam generator.Method according to one of claims 1 to 3, characterized in that the plant comprises a piping system between the outlet for the vapor and the inlet for the superheated steam, the following being arranged in the piping system: h) the vapor compressor; i) a circulation fan; j) the heat exchanger, for heating a second portion of the vapor recirculated from the chamber by transferring heat from the compressed first portion by condensing the volume flow of the compressed first portion supplied to the heat exchanger; and k) a heating device for the steam, arranged between the heat exchanger and the inlet for the superheated steam.

5. Method according to one of claims 1 to 3, characterized in that the conveying system has a rotating hollow shaft arranged in the chamber with a plurality of discs, which forms the heat exchanger, wherein a cavity is arranged in an interior of the hollow shaft, to which the volume flow of the compressed first portion of the vapor can be fed from the vapor compressor for heating the discs.

6. Method according to one of claims 1 to 5, characterized in that the heat exchanger has a vent valve on the condenser side and that an opening of the vent valve is regulated on the basis of a certain air proportion on the condenser side.

7. Method according to claim 6, characterized in that the condenser-side air content is regulated to a value of 0-50%, preferably 5-20%, particularly preferably 7-12%.

8. A method according to claim 4, characterized in that the following steps are carried out to form the steam atmosphere in the upper region of the chamber: Generating steam in a steam generator and introducing the generated steam into the chamber, whereby air in the chamber is displaced downwards from the chamber; during operation of the steam generator, until an operating pressure is reached in the heat exchanger: Activating the circulation fan; Activating the heating device and / or the vapor compressor, Introducing material to be dried using the conveyor system and Activating the vapor compressor.

9. Installation for drying material by means of superheated steam, comprising: a) a downwardly open chamber with an inlet for the material to be dried, an outlet for dried material, an inlet for superheated steam and an outlet for a vapor; b) a conveyor system for introducing the material to be dried into the chamber, transporting the material to be dried in the chamber during drying, and removing the dried material from the chamber; c) a vapor compressor for compressing a first portion of the vapor returned from the chamber; d) a heat exchanger for transferring heat from the compressed first portion by condensing a volume flow of the compressed first portion; and e) a controller for acquiring and processing measured values ​​and for generating control signals;wherein the control is operable in such a way that f) an atmosphere of superheated steam is formed in an upper region of the chamber, which atmosphere floats on ambient air located in a lower region of the chamber, wherein a transition layer is formed between the upper region and the lower region, and g) a height of the transition layer is maintained in a predetermined range by determining a current height and, depending on the determined height g1), the volume flow of the compressed first portion supplied to the heat exchanger is regulated;or g2) a volume flow of a steam generator is regulated, wherein the steam generator is arranged and operated such that steam is supplied to the chamber and / or generated in the chamber. Plant according to claim 9, characterized in that the plant comprises a piping system between the vapor outlet and the superheated steam inlet, wherein the following is arranged in the piping system: h) the vapor compressor; i) a circulation fan; j) the heat exchanger for heating a second portion of the vapor returned from the chamber by transferring heat from the compressed first portion by condensing the volume flow of the compressed first portion fed to the heat exchanger; and k) a heating device for the steam, arranged between the heat exchanger and the inlet for the superheated steam.

1. Plant according to claim 10, characterized in that the inlet for the superheated steam is arranged on the chamber such that the superheated steam intersects a conveying path of the material to be dried in the chamber in a directed steam flow. . Plant according to claim 11, characterized in that an element for homogenizing the steam flow is arranged on the chamber side of the inlet. .Plant according to claim 9, characterized in that the conveying system has a rotating hollow shaft arranged in the chamber with a plurality of disks, which forms the heat exchanger, wherein a cavity is arranged in an interior of the hollow shaft, to which the volume flow of the compressed first portion of the vapor can be fed from the vapor compressor for heating the disks. Plant according to one of claims 9 to 13, characterized by a steam generator which is arranged and operable such that steam is supplied to the chamber and / or generated in the chamber. Plant according to claim 14, characterized in that the steam generator is connected to the heat exchanger such that it can be operated at least partially with condensate from the heat exchanger.

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