Method for operating a plant for drying materials to be dried with superheated steam - Patent 7222247

Abstract

The apparatus for drying material with superheated steam comprises a downwardly opening chamber (10) having an inlet (61) for the material to be dried, an outlet (62) for the drying material, inlets (71a, 71b) for superheated steam, and outlets (73a, 73b) for steam. The apparatus further comprises a conveying system (60) for introducing the material into the chamber (10), transporting the material in the chamber (10) during drying, and discharging the dried material from the chamber (10). The apparatus also comprises a vapor compressor (40) for compressing a first portion of the steam recycled from the chamber (10), and a heat exchanger (30) for transferring heat from the compressed first portion by condensing a volumetric flow of the compressed first portion, which is fed to the heat exchanger. The apparatus is operated such that a steam atmosphere is formed in the upper region of the chamber (10) and that the steam atmosphere floats on ambient air in the lower region of the chamber (10), forming a transition layer (66) between the upper and lower regions. The height of the transition layer (66) is maintained within a specified range by determining the current height, and depending on the determined height, the volumetric flow rate of the compressed first portion supplied to the heat exchanger (30) or the volumetric flow rate of the steam generator (15) is controlled, the steam generator (15) being arranged and operated such that steam is supplied to and / or generated in the chamber (10).

Description

[Technical Field]

[0001] The present invention relates to a method for operating a plant for drying materials to be dried by means of superheated steam, and to a corresponding plant. [Background technology]

[0002] Industrial material drying consumes 12-25% of the total industrial energy demand in industrialized countries, making the drying process one of the most energy intensive. Drying is usually based on fossil energy sources, resulting in extremely high CO2 emissions. Therefore, reducing the energy demand required for drying is of great importance in terms of CO2 reduction.

[0003] In a method that is advantageous in terms of energy demand, superheated steam is used for drying instead of hot air. Patent document 1 (Heat-Win Limited) proposes a device for this purpose, in which moist material is continuously conveyed into a chamber through an opening, through the chamber, and out of the chamber through an opening. An atmosphere of superheated steam pervades the chamber, which atmosphere is caused by the moisture of the material to be dried and / or is supplied from the outside. In this case, a transition layer is formed in the openings and ventilation ducts between the steam atmosphere and the surroundings, which layer prevents vapor or steam from escaping the chamber, but at the same time allows the supply and removal of material.

[0004] The system therefore allows for simple material supply and removal. Excess steam is condensed. By keeping the material mass flow, residence time, superheated steam mass flow and its temperature constant, a constant dry matter content can be achieved in a simple manner. However, energy losses occur due to the condensation of excess steam and the corresponding return of the removed heat by reheating.

[0005] US Patent No. 5,629,999 (Epcon Evaporation Technology) describes a method using a closed chamber in which a mixing system is placed and in which a wet material is contacted with superheated steam. The excess steam is mechanically compressed and fed to a heat exchanger, allowing for energy recovery and therefore higher efficiency.

[0006] Process control is likewise guaranteed thanks to the closed chamber. However, here problems arise with the supply and removal of material. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] U.S. Patent No. 5,711,086 [Patent Document 2] International Publication No. 2012 / 140125 Summary of the Invention [Problem to be solved by the invention]

[0008] The object of the present invention is to provide a method belonging to the technical field mentioned at the beginning for operating a plant for drying materials to be dried with superheated steam, as well as a corresponding plant which allows for high energy efficiency through simple material supply and removal. [Means for solving the problem]

[0009] The objective solution is defined by the features of claim 1. According to the invention, a plant for drying a material to be dried is operated with superheated steam, the plant comprising: a) a downwardly opening chamber provided 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 steam; b) a conveying system for introducing the material to be dried into the chamber, transporting the material in the chamber during drying, and discharging the dried material from the chamber; c) a vapor compressor for compressing a first portion of the vapor recycled from the chamber; d) a heat exchanger for transferring heat from the compressed first portion by condensing a volumetric flow of the compressed first portion.

[0010] This plant is e) forming a steam atmosphere in the upper region of the chamber, the steam atmosphere floating above the ambient air in the lower region of the chamber, forming a transition layer (stratification) between the upper and lower regions; f) The height of the transition layer is kept within a predetermined range by determining the current height, and depending on the determined height: f1) the volume flow rate of the compressed first portion fed to the heat exchanger is adjusted, or f2) The volumetric flow rate of the steam generator is adjusted and the steam generator is arranged and operable so that steam is supplied to and / or generated in the chamber.

[0011] Correspondingly, a plant for drying materials to be dried by means of superheated steam according to the invention comprises: a) a downwardly opening chamber provided 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 steam; b) a conveying system for introducing the material to be dried into the chamber, transporting the material in the chamber during drying, and discharging the dried material from the chamber; c) a vapor compressor for compressing a first portion of the vapor recycled from the chamber; d) a heat exchanger for transferring heat from the compressed first portion by condensing a volumetric flow of the compressed first portion; e) a controller for acquiring and processing the measurements and for generating the control signals.

[0012] In this case, the controller may, in particular: f) forming an atmosphere of superheated steam in the upper region of the chamber, the atmosphere floating above ambient air located in the lower region of the chamber, forming a transition layer between the upper and lower regions; g) The height of the transition layer is kept within a predetermined range by determining the current height of the transition layer, and depending on the determined height: g1) The volumetric flow rate of the compressed first portion fed to the heat exchanger is adjusted, or g2) The volumetric flow rate of the steam generator is adjusted and the steam generator is arranged and operated so that steam is supplied to and / or generated within the chamber.

[0013] The materials to be dried are in particular bulk materials, such as generally products or by-products of the food or feed industry, combustibles or building materials, basic substances from the chemical or paper industry, and biomass. The technique can also be used in connection with the drying (including laundry) of textiles. In principle, the method is best suited to drying materials requiring drying temperatures of 100°C to 200°C and is not capable of vaporizing additional organic substances.

[0014] The steam is in particular water vapor, but other solvents, such as ethanol, can also be used. During drying, the absence of oxygen prevents oxidation of the material to be dried, so that fats in food or animal feed, for example, do not go rancid. Furthermore, the risk of fire and explosion is minimized.

[0015] Odor loss is minimized by the circulation process. No odor or dust emissions to the surroundings occur. Because temperatures are consistently above 100°C, the material being dried is pasteurized or even sterilized.

[0016] The chamber is closed at the top and completely or partially open at the bottom. The chamber may have one or more openings at its lower end, located at the main chamber boundary or in an inwardly or outwardly directed, e.g., tubular, extension. Correspondingly, the main volume of the chamber may be completely filled with steam, i.e., located entirely in the upper region of the chamber, and a transition layer may be formed in the tubular extension or in several tubular extensions. The opening or openings are particularly dimensioned to allow, on the one hand, pressure equalization and a certain gas flow between the chamber and the surroundings, and, on the other hand, to allow the introduction and discharge of the drying material through the opening or at least one of the openings. In particular, all the drying material is eventually introduced into the chamber and discharged from the chamber through one or more such openings. Therefore, the inlet for the material to be dried and the outlet for the drying material are formed by one or more of these openings. Therefore, the material can be introduced and discharged in a cost-effective design without significant amounts of air being transported into the steam during the process.

[0017] The steam atmosphere contains a residual proportion of air, which is preferably less than 4% by volume. The vapor is steam, the properties of which have been altered by interaction with the material to be dried and the atmosphere of the chamber relative to the superheated vapor introduced. Generally, the vapor has a higher saturation and a lower temperature than the superheated vapor introduced. Furthermore, the vapor may contain additional components, in particular small amounts of the material to be dried or dust as an aerosol, and also additional vapors from the material, such as odors.

[0018] The conveying system allows for particularly continuous operation of the plant. In a preferred embodiment, the conveying system comprises an ascending conveyor for introducing the material to be dried through an opening in the lower surface of the chamber, a belt conveyor for transporting the material to be dried within the chamber, and a gravity removal section through the same or preferably further openings in the lower surface of the chamber. In an alternative embodiment, the conveying system comprises one rotor or multiple rotors, and the plant is operated as a disk dryer. Additional conveying systems can be used within the scope of the present invention, for example, as a fluidized bed or paddle conveyor. The plant can also be used, for example, with a spray drying tower. The conveying system can be formed in part by an upstream plant component, for example, if the material to be dried is introduced directly into the steam atmosphere by this component, for example, an extruder, or is blown by a steam flow. The conveying system can comprise one or more conveyors (e.g., belts or gondolas). The chamber itself can be part of the conveying system, for example, in the case of a paddle mixer. Introduction, transport within the chamber, and / or discharge can in each case be achieved entirely or partially by gravity. Thus, for example, the dried material can fall into an opening located at the bottom.

[0019] Within the scope of the method according to the invention, the height, i.e. the vertical position, of the transition layer is now kept within a predetermined range, which corresponds to keeping the volume of the steam atmosphere in the chamber constant.

[0020] The current height can be determined directly or indirectly. Instead of the height, quantities dependent thereon can also serve as control variables for adjusting the volumetric flow rate of the compressed first part supplied to the heat exchanger or the steam generator, for example the temperature measured at one or more points in time and / or the content of any gases contained in the air at one or more points in time, for example O2 or N2.

[0021] In a first variant, the volumetric flow rate of the compressed first portion fed to the heat exchanger is preferably set by adjusting the volumetric flow rate of the vapor compressor. This can be done by adjusting the rotational speed of the vapor compressor and / or the opening of an adjustable orifice arranged upstream or downstream of the vapor compressor. Alternatively, an adjustable valve can be arranged downstream of the vapor compressor, so that a (proportional) volumetric flow rate of the compressed vapor is fed to the heat exchanger, while the (possible) remainder is recirculated to the chamber. This alternative variant can be used, for example, in connection with turbocompressors, which ideally operate at a certain volumetric flow rate. In principle, such a (bypass) valve can also be arranged downstream of the heat exchanger, in which case the entire compressed first portion is guided through the heat exchanger, but only a portion is condensed there, while the remaining remainder is recirculated to the chamber through the bypass valve. A combination of variants is also conceivable, in particular, in which the adjustment is effected up to a certain minimum volumetric flow rate by adjusting the vapor compressor, and the regulating valve is used only when the compressed portion fed to the heat exchanger needs to be further reduced.

[0022] In a first variant, in particular, adjustments are made so that when a drop in the transition layer is detected, the volumetric flow rate of the compressed first portion fed to the heat exchanger is increased, and when a rise in the transition layer is detected, the volumetric flow rate is decreased, so that the volumetric flow rate of the first portion is ultimately set for the portion discharged and therefore for the increase or decrease in the volume of the steam atmosphere in the chamber.

[0023] In a first variant, therefore, the adjustment of the volume flow rate of the compressed first portion fed to the heat exchanger is preferably brought about according to one of the following methods (which methods can also in principle be combined with each other):

[0024] 1. The height of the transition layer is adjusted by adjusting the rotation speed of the steam compressor. By increasing the rotation speed, the first portion of the steam recirculated from the chamber to be compressed is increased, i.e., a larger portion is compressed and then supplied as a volumetric flow to the heat exchanger.

[0025] 2. The height of the transition layer is adjusted by the volumetric flow rate returned to the chamber from the first compression section. For this purpose, the opening of the (bypass) valve downstream of the vapor compressor is adjusted so that the height of the transition layer remains in the desired range. If the (bypass) valve is opened more, the volumetric flow rate returned to the chamber increases and correspondingly less volumetric flow rate becomes available for condensation in the heat exchanger.

[0026] 3. The height of the transition layer is adjusted by setting the opening of the orifice upstream or downstream of the steam compressor. Thus, the first portion of the steam recirculated from the chamber to be compressed can be adjusted while the rotational speed of the steam compressor remains the same.

[0027] In a second variant, in particular, adjustments are made such that when a drop in the transition layer is detected, the volumetric flow rate of the steam generator is reduced, and when a rise in the transition layer is detected, the volumetric flow rate of the steam generator is increased, so that the volume of the steam atmosphere in the chamber is ultimately directly affected.

[0028] Returning the process heat by compression and condensation in a heat exchanger allows for higher efficiency, but leads to all relevant process parameters becoming interdependent, which leads to non-linear behavior of the system. This represents a challenge, especially when operating plants with open chambers, since it is necessary to ensure at all times that the transition layer between the steam atmosphere and the surroundings remains stable and is within the possible height range. Operation according to the invention makes it possible to achieve a stable process with a constant dry matter content even in the case of open chambers.

[0029] In particular, the vapor compressor can be operated by electrical energy, which easily allows operation with renewable energy, thereby significantly reducing CO2 emissions compared to conventional drying methods.

[0030] Preferably, the current height of the transition layer is determined based on measurements of at least one temperature sensor positioned within a height range corresponding to the predetermined range. At least one temperature sensor is preferably arranged in a pipe extending downwards, in particular vertically, from the main volume of the chamber and connecting the chamber to the surroundings. Alternatively or additionally, a temperature sensor may also be arranged at the opening for discharging the dried material to be dried.

[0031] The one or more temperature sensors function as steam fill level sensors and ultimately determine the height of the steam-air transition layer (or a parameter directly connected thereto). In a preferred embodiment, a drop in the transition layer is assumed when a particular temperature sensor provides a value above a first threshold, while a rise in the transition layer is assumed when a particular temperature sensor provides a value below a second threshold. The thresholds are selected in particular in the range of 90-100°C, preferably differing by 2-8°C. Particularly preferably, the first threshold is approximately 98°C, while the second threshold is approximately 96°C.

[0032] The temperature sensors can be located in the inlet and / or outlet area. A location below the chamber, especially in the outlet area, is preferred here, since the significant influence of the drying material on the temperature measurement is generally less there than that of the material to be dried at the inlet. The influence can be further reduced when the temperature sensors are located in a pipe away from or near the outlet. For this purpose, a pipe with a diameter of 1.5 to 6 cm has proven sufficient. Particularly preferably, temperature sensors are located both at the inlet and at the outlet. This allows for the best possible monitoring of the process and early detection of malfunctions.

[0033] The control of the system parameters, in particular the rotation speed of the steam compressor, is advantageously carried out by PID regulation based on measured temperature values, whereby the measured temperature values ​​of several temperature sensors arranged at different heights, and therefore in particular also in a temperature gradient, can be used. The controller can be integrated into or implemented by a conventional machine controller (PLC).

[0034] If a different solvent is used instead of water, different temperature values ​​will result. The prevailing air pressure also has an effect, especially due to the height above sea level, and this is taken into account when setting the temperature values. The above description relates to carrying out the drying method at sea level.

[0035] Instead of or in addition to one or more temperature sensors, other measurements may be used to determine the height of the transition layer, such as one or more lambda probes to determine the oxygen content, or chemical sensors to determine the nitrogen content or the content of another gas contained in the air.

[0036] Advantageously, within the scope of the method according to the invention, the drying temperature is kept within a predetermined range by comparing it with a set value, and depending on the comparison: g1) the steam generator may be arranged and operated such that steam may be supplied to or generated within the chamber, provided that the volumetric flow rate of the steam generator is regulated, wherein the height of the transition layer is kept within a predetermined range by adjusting the volumetric flow rate of the compressed first portion supplied to the heat exchanger, or g2) The heating power of the heating device is adjusted, or g3) The volumetric flow rate of the compressed first portion supplied to the heat exchanger is adjusted, provided that the height of the transition layer is kept within a predetermined range by adjusting the volumetric flow rate of the steam generator.

[0037] Therefore, the following variations are a result of adjusting the transition layer height and drying temperature.

[0038] [Table 1]

[0039] In variant 2A, in which the drying temperature is regulated by the heating power of the heating device and the transition layer is regulated by the steam generator, the vapor compressor serves to extract a first portion of vapor from the chamber, which is more than in variant 1B but less than in variant 2B. The vapor compressor therefore ensures that a higher temperature is achieved in the heat exchanger than in variant 1B, and therefore the heating power of the heating device can be reduced.

[0040] The drying temperature is in particular the temperature of the superheated steam introduced into the chamber or, in the case of indirect drying in particular, the temperature of the contact surface with the material to be dried. The corresponding setpoints depend in particular on the material and the desired dry matter content.

[0041] The dry matter content of the processed material to be dried can be determined in the chamber, for example, by measuring the temperature of the surface of the material to be dried. Infrared temperature sensors are also suitable for this purpose. Based on the measured surface temperature, the dry matter content can be estimated using a characteristic curve previously determined empirically. If this does not correspond to the specifications, the system parameters, in particular the setpoint drying temperature of the contact surface in the case of superheated steam or indirect drying, and / or the conveying speed of the conveying system (and therefore the residence time of the material to be dried in the chamber), are adapted.

[0042] In a preferred embodiment, the plant comprises a pipe system between the outlet for the steam and the inlet for the superheated steam, in which the following are arranged in the pipe system: g) steam compressors, h) Circulation fans, i) a heat exchanger for heating a second portion of the vapor recycled from the chamber by transferring heat from the compressed first portion by condensing a volumetric flow of the compressed first portion fed to the heat exchanger; and j) A heating device for steam, located between the heat exchanger and the inlet for superheated steam.

[0043] The heat transfer in the heat exchanger is effected in particular in counterflow, where the compressed steam flows from top to bottom. The circulation fan can be located upstream or downstream of the heat exchanger. It serves to maintain the steam flow in the circuit and thus compensate for the pressure drop incurred. It has been found that the required mass flow increases almost linearly with the evaporation rate. The mass flow to be delivered by the circulation fan should be at least 60 times the compressed mass fraction fed to the heat exchanger. Thus, in addition to the actual evaporation of liquid from the material to be dried, heat losses are also compensated, ensuring that the material to be dried, along with the contained water and its surface water, can be preheated. The mass flow is preferably set to more than 60:1, thereby providing a safety margin due to the anticipated losses due to the high mass flow entering the system internal heat and therefore contributing to the heating of the steam. Depending on the specific configuration of the plant, higher ratios of 100:1, 150:1, or even higher ratios may need to be set. The circulation fan therefore supports the heating device and, in some embodiments, can even replace it.

[0044] A vapor compressor is a mechanical compressor. It serves for heat recovery. The vapor is supplied from the chamber directly or through several sections of a pipe system. Vapor compressors can be of multi-stage design, i.e., several compressor stages are arranged in series.

[0045] The volumetric flow rate of the first portion of steam fed to the heat exchanger and compressed by the steam compressor is proportional to the amount of steam released into the circuit, particularly during drying of the material, so that a constant mass flow is achieved within the circuit. The first portion is determined by the pressure ratio and the rotational speed of the steam compressor according to the compressor characteristic map. Therefore, the first portion can be set by adjusting the rotational speed. The volumetric flow rate of the compressed first portion fed to the heat exchanger is generally between 1:30 and 1:160, in each case based on the circuit steam.

[0046] Further heat sources, such as waste heat or dedicated heating devices, are possible in the heat exchanger in addition to the condensation of compressed steam (and possibly steam from a steam generator). The heating device for the steam is independent of the heat exchanger. It is in particular an electric resistance heater. Alternatively, for example, a gas burner can also be used. As already mentioned, the heating device can be integrated into the circulation fan, in particular, where heating of the steam is caused by dissipation in the fan. If it is separate from the fan, it is arranged downstream of the fan in the circulation direction, preferably immediately upstream of the inlet to the chamber. The desired dry matter content can finally be set by adjusting the heating device. Instead of (or in addition to) a resistance heater, waste heat from the exhaust gases, for example from a gas engine, can be used in the heating device, which is released to the steam via an adjustable heat exchanger (for example a gas-gas heat exchanger for using hot gas waste heat).

[0047] The heating device may comprise one or more heating units, for example in the case of a belt dryer, each belt being assigned to a separate heating unit, so that the operation of the individual heating units can be regulated separately or centrally.

[0048] Ultimately, a constant dry matter content is desired. If the vapor compression ratio is changed to set the height of the transition layer, the condensation temperature of the condenser is changed in the medium term, so the evaporation rate of the material is also affected, which in turn leads to a change in the steam level. This effect can be compensated for by adjusting the heating power.

[0049] Although little air enters the system through the open lock, it is not possible to completely eliminate traces. A small percentage of the air in the system will concentrate in the condenser and, over time, block the useful heat transfer surface. To avoid this, the condenser must be continuously vented. Advantageously, therefore, the heat exchanger has a vent valve on the condenser side, the opening of which is adjusted based on the amount of air determined on the condenser side. The amount of air can be determined based on the condenser pressure and condensing temperature, based on the deviation from the saturation temperature, or directly by a lambda probe.

[0050] The vent valve, in particular a needle valve, is advantageously arranged above the condensate outlet. The latter allows excess condensed water to drain. Advantageously, it is regulated based on the measurements of one or more fill level sensors, which may be formed, for example, by capacitive limit switches. Finally, the water is recovered from the material to be dried, usually in a sterile and demineralized state.

[0051] Advantageously, the amount of air on the condenser side is adjusted to a value between 0 and 50%, preferably between 5 and 20%, particularly preferably between 7 and 12%. If a low setting is not achieved, significant steam losses occur. If the setting is too high, the efficiency of vapor compression is impaired.

[0052] If the air mass flow into the condenser is not constant but highly dependent on the operating conditions of the plant, the vent valve must be continually readjusted so that the amount of air in the condenser can be kept at the desired percentage.

[0053] In a preferred embodiment, the pipe leaving the condenser opens into a branch (for example a T-piece or a Y-piece). Therefore, at the outlet of the condenser, water and non-condensable gases are reliably separated, and steam loss to the environment is minimized. One leg of this branch leads to a horizontal or slightly upward discharge line equipped with a vent valve. The other leg leads downwards (especially vertically) to a pipe section with an enlarged cross section, where a water column is formed. This allows for a regulated and delayed discharge of condensed water.

[0054] Two (e.g. capacitive) fill level sensors are positioned along the pipe section containing the water column, and a shut-off valve is connected to the bottom, which opens and closes depending on the measurements of the fill level sensors, so that the level of the water column is always located between the fill level sensors.

[0055] A throttle (e.g., a needle valve) is placed downstream of the shut-off valve. This component ensures that the concentrated liquid flows out more slowly and that the gas / air mixture cannot escape downwards.

[0056] Due to the pressure drop across the shut-off valve due to the downstream throttle, steam is generated and the condensate outlet therefore forms a (further) steam generator, which can be returned via corresponding pipes to the steam circuit, the drying chamber and / or the material to be dried and used for further drying and / or preheating of the material.

[0057] In this preferred embodiment too, the vent valve is adjusted based on the amount of air determined on the condenser side, which, as already mentioned, can be determined directly by a lambda probe or indirectly based on the deviation of the static pressure from the steam pressure at the condensing temperature.

[0058] Advantageously, the inlet for the superheated steam is arranged in the chamber so that the superheated steam of the directional steam flow crosses the conveying path of the material to be dried in the chamber. This is preferably effected in cross-flow or counter-flow. The supply and removal of the superheated steam and the internal geometry of the chamber are in particular balanced with each other so that a circuit takes place through the steam atmosphere in the chamber.

[0059] In the case of an embodiment of the plant according to the invention as a belt dryer, it is advantageous for the inlet and outlet of the superheated steam to take place as close as possible to the material. Preferably, an element for homogenizing the steam flow is arranged on the chamber side of the inlet. In this case, the element forms 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 so that sufficient matching of the steam flow is achieved but unnecessary pressure losses, which would increase the power requirements of the circulation fan, are avoided. The element can be designed like a filter, i.e., from a material with fine holes. For example, a glass fiber mat is suitable. Upstream, a diffuser can be arranged that distributes the steam flow over a large cross section.

[0060] The circulation of superheated steam within the chamber can be controlled by such elements, and it has been found that this results in a stabilised transition layer. When the plant is started, the required steam atmosphere must first be formed in the upper region of the chamber. For this purpose, the plant is preferably equipped with a steam generator, and in particular the following steps are carried out:

[0061] - generating steam in a steam generator and introducing the generated steam into a chamber, whereby air present in the chamber is displaced downward from the chamber; During operation of the steam generator (after completion of accumulation of the steam atmosphere or displacement of air from the chamber) until the operating pressure is reached in the heat exchanger, - starting the circulation fan; - starting the heating device and / or the vapor compressor, - introducing the material to be dried by a conveying system, and -Step to start the steam compressor The generated steam is introduced, in particular from above, preferably at the highest point of the chamber and / or pipe system. The chamber is preferably preheated to 100° C. beforehand with air. Air is displaced by the introduced steam not only from the chamber but also, in particular, from the pipe system.

[0062] During the last phase, when both the steam generator and the steam compressor are active, the operating pressure increases in the heat exchanger operating as a condenser while maintaining the steam atmosphere. This is, depending on further machine and process parameters, in particular, 150 to 400 kPa (1.5 to 4 bar) above atmospheric pressure. Once the operating pressure is reached, the steam generator is switched off, so that a transition to nominal operation occurs.

[0063] In a further embodiment of the invention, the conveying system has a rotating hollow shaft, which is arranged in a chamber and has a plurality of disks and forms a heat exchanger, in which a cavity is arranged inside the hollow shaft, to which a volume flow of a compressed first portion of the vapor can be supplied by a vapor compressor to heat the disks. In this embodiment, the first portion therefore corresponds to the entire recycled vapor, while in the corresponding embodiment, a portion of it is returned to the chamber by a (bypass) valve arranged downstream of the vapor compressor. The hollow shaft therefore serves as a condenser for the recycled compressed vapor. The cavity can extend into the disks or be limited to the center of the hollow shaft.

[0064] The liquid material to be dried is supplied to the disc through the inlet, dried, and finally, after drying, removed from the disc, e.g., scraped, and discharged from the chamber through the material outlet. Thus, in this embodiment, drying is effected indirectly.

[0065] In embodiments with a rotating disk shaft, steam from a steam generator is preferably supplied to the chamber, this supply being particularly continuous (as well) during the drying process, after which the steam is compressed by the vapor compressor and fed to the hollow shaft, ultimately serving to dry the liquid material and heat the chamber and compensate for losses.

[0066] The steam generator is arranged and operated so that steam can be supplied to the chamber or generated within the chamber. The supply can be effected directly into the chamber or indirectly, for example via a pipe system. The steam generation can be effected, for example, by injecting water into an atmosphere of superheated steam. As a result, the steam generator can also be arranged directly within the chamber.

[0067] In one embodiment of the present invention, the volumetric flow rate of the compressed first portion fed to the heat exchanger is also adjusted based on the current height of the transition layer. In this case, the volumetric flow rate of the steam generator is preferably adjusted based on the measured condensation temperature in the hollow shaft cavity so that this condensation temperature remains within a predetermined interval. As a result, the dry matter content of the material to be dried is finally set. This corresponds to variant 1A shown above.

[0068] In a further adjustment method, which is also suitable for variants with a rotating disk shaft, the height of the transition layer is kept within a predetermined range not by adjusting the volumetric flow rate of the compressed first portion fed to the heat exchanger, but by adjusting the volumetric flow rate of the steam generator. In this alternative method, the volumetric flow rate of the compressed first portion fed to the heat exchanger is adjusted based in particular on the measured condensation temperature in the cavity so that this condensation temperature remains within a predetermined range. This corresponds to variant 2B shown above.

[0069] In all embodiments of the invention, the steam generator arranged in the plant can be operated with waste heat. In this case, the condensate from the heat exchanger can in particular serve as feed water. If the condensate is not sufficient for supply, additional water can be supplied, for example from a tank.

[0070] In a plant equipped with a steam circuit, steam from such a steam generator can be introduced into the circuit, thereby increasing the compressed first portion fed to the heat exchanger. This results in a higher condensation temperature and therefore an increase in the amount of heat output to the circuit flow via the heat exchanger. As a result, the power of the heating device can be reduced, which can increase process efficiency.

[0071] Further advantageous embodiments and combinations of the features of the invention emerge from the following detailed description and the entire claims. The drawings used to explain the exemplary embodiments show: [Brief explanation of the drawings]

[0072] [Figure 1A] 1 is a schematic block diagram of a plant according to the invention for drying a material to be dried by means of superheated steam according to a first embodiment; [Figure 1B] 3 shows a schematic block diagram of a plant according to the invention for drying a material to be dried by means of superheated steam according to a second embodiment; [Figure 2A] 1 is a schematic cross-sectional view of a plant according to a first embodiment; [Figure 2B] FIG. 2 is a detailed view of an advantageous embodiment of a condensate outlet for a plant according to the first embodiment. [Figure 3] FIG. 10 is a schematic cross-sectional view of a plant according to a third embodiment. [Figure 4A] 5 is a schematic cross-sectional view of a drying chamber of a plant according to the invention, according to a fourth embodiment; [Figure 4B] 5 is a schematic cross-sectional view of a drying chamber of a plant according to the invention, according to a fourth embodiment; [Figure 5A] 5 shows a schematic cross-sectional view of a drying chamber of a plant according to the invention, according to a fifth embodiment; [Figure 5B] 5 shows a schematic cross-sectional view of a drying chamber of a plant according to the invention, according to a fifth embodiment; [Figure 6] 1 is a diagram of actuators and control variables of a plant according to the invention; [Figure 7] 1 is a block diagram of a sensor system of a plant according to the present invention, according to a first embodiment; [Figure 8] Measured temperature profiles at three heights in the vertical pipe next to the outlet. [Figure 9] 1 shows the temperature and air volume profiles when a plant according to the invention is started up. DETAILED DESCRIPTION OF THE INVENTION

[0073] In principle, identical parts are provided with the same reference numbers in the figures. Methods of carrying out the invention 1A and 1B are schematic block diagrams of plants according to the present invention for drying material to be dried with superheated steam, according to a first embodiment and a second embodiment, respectively. FIG. 2A shows a schematic cross-sectional view of a plant according to the first embodiment. The first and second embodiments differ in the position of the circulation fan in the steam circuit. Furthermore, the second embodiment comprises an additional steam generator that can be operated with waste heat. When positioning the circulation fan as in the first embodiment, it should be taken into account that such a steam generator is also possible to use. Otherwise, all of the following statements apply to both the first and second embodiments.

[0074] The plant comprises a chamber 10 into which the wet material 1 can be introduced and the dry material 2 can be discharged by a conveying system 60. In the plant shown, the mass flow of the wet 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, which in the exemplary embodiment shown corresponds to an inlet 61 designed as a pipe extending obliquely upwards to the upper region of the side wall of the chamber 10, inside which the bucket conveyor 65.1 of the transport system 60 is located, which acts as an ascending conveyor, and whose cross section A1 perpendicular to the longitudinal axis of the pipe is approximately 0.10 m; 2 Entrance 61, an outlet 62 designed as an opening in the lower surface of the chamber 10, through which the drying material 2 is discharged under the action of gravity, the cross section A2 of which is approximately 0.02 mm 2 Exit 62, - a measuring tube 63 (see FIG. 2) arranged in the area of ​​the outlet 62 and equipped with a number of temperature sensors and opening downwards;

[0075] The bucket conveyor 65.1 is provided with trays for receiving the material to be dried, the trays being perforated so that air is not transported upwards on its passage into the chamber 10. During operation, the chamber 10 is filled with water vapor, which floats above the ambient air.

[0076] The feed line of the steam generator 15 opens into the upper side of the chamber 10 so that steam can be supplied directly to the chamber 10 as needed, particularly during start-up as described below. A steam exhaust valve can also be located above the chamber 10 to exhaust excess steam from the chamber 10 (not shown).

[0077] The bucket conveyor 65.1 slowly introduces the wet material 1 from the ambient air into the steam atmosphere from below at a speed of 10-30 mm / s, without any air being carried along. Two horizontal belt conveyors 65.2, 65.3 are arranged in the chamber 10, the first of these further belt conveyors 65.2 receiving the wet material from the bucket conveyor 65.1, conveying it through a first drying stage and discharging it onto the second of the belt conveyors 65.3, which conveys the material through a second drying stage. The area through which steam flows in the region of the belt conveyors 65.2, 65.3 is in each case approximately 0.45 m 2 From the second belt conveyor 65.3, the material falls out of the chamber 10 through the outlet 62 and through a steam-air transition layer.

[0078] The residence time of the material in the chamber 10 is set by the conveying speed of the conveying system 60. In the plant shown, this is typically around 20 to 30 minutes. A closed circuit steam duct is connected to the chamber 10. The circuit is driven by a circulation fan 20. In the plant shown, the volumetric flow rate in the circuit is 2,150 m 3 / h.

[0079] The treated superheated steam is split into two partial streams upon entering the chamber. Each partial stream first passes through a diffuser where it is distributed over a large cross-section and then passes through filter elements 72a, 72b. In the exemplary embodiment shown, these have a basis weight of 610 g / m 2 It is designed as a biaxially woven glass fiber mat, which produces a pressure loss coefficient ζ of 400 at a steam velocity of 1.3 m / s and a ζ value of 200 at steam velocities of 7 m / s or more.

[0080] They serve to homogenize the steam flow, which is then output 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 plant shown, the evaporation mass flow is approximately 16 kg / h (26.7 m of steam). 3 / h). The steam volume in the steam chamber is 0.85 m 3 where the air content is less than 4%.

[0081] Steam flows across the transport surface of the belt conveyors 65.2, 65.3 and is in each case similarly sucked out of the chamber 10 through the steam outlets 73a, 73b on the opposite side. The steam supplies heat to the material to be dried, resulting in evaporation of water. The dry matter content of the material is in this case determined by analysis of the material after it has escaped into the ambient air, and the steam temperature and residence time are readjusted accordingly. Alternatively, the dry matter content can also be checked by (optical) temperature measurement of the material surface in the steam; here, corresponding sensors can be arranged along the conveying path in the chamber to monitor the drying process.

[0082] To introduce heat into the circulation steam duct, the latter comprises a heat exchanger 30 followed by a heating device 50. The latter advantageously comprises a first heating unit 51a for the steam portion supplied to the first steam inlet 71a and a second heating unit 51b for the steam portion supplied to the second steam inlet 71b, which can be adjusted independently.

[0083] The heat exchanger 30 is a thin plate heat exchanger. 2 of outer heat exchanger area and approximately 2.3 m 2 10. The heat exchanger 30 has an internal heat exchanger area of ​​1000 psi. A filter may be placed upstream of the heat exchanger 30 to avoid its contamination with entrained materials. The heat exchanger 30 operates internally as a condenser by a vapor compressor 40 which compresses a portion of the steam drawn from the chamber 10 and supplies it to the condenser, where it condenses under elevated pressure, typically 250-400 kPa (2.5-4 bar) above atmospheric pressure, transferring in the process the enthalpy of evaporation to the circulating steam stream via the heat exchanger 30. In the exemplary embodiment shown, the vapor compressor 40 has an installed power of 3.7 kW. During operation, the power is typically around 1.1 kW.

[0084] A subsequent heating device 50 further superheats the circulating steam stream to the required drying temperature. To drain the water, the condenser of the heat exchanger 30 has a condensate discharge valve 31 that opens automatically depending on the water level. For this purpose, the water level is monitored by one or more volumetric fill level sensors, and the valve opens for a predefined time period when the water level exceeds a certain desired level. If two water level sensors are used, the upper sensor can serve to initiate the emptying process, while the response of the lower water level sensor during emptying shortens the predefined time interval. It is therefore ensured that the condensate column always remains in the height range between the lower sensor and the valve, with the result that the steam / air mixture cannot escape directly through the valve. In the plant shown, the condensate mass flow is typically around 7.5 kg / h (13.6 m 3 / h steam).

[0085] Furthermore, an air exhaust valve 32 is installed above the water outlet through which non-condensable gases are exhausted. The proportion of these gases in the steam is determined by temperature and pressure sensors at the condenser outlet.

[0086] Within the scope of the second embodiment, the condensate from the heat exchanger 30 is fed to the steam generator 17. Waste heat (for example at a temperature of approximately 170°C) is fed to said steam 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 branch of the feed pipe to the heat exchanger 30 and the steam compressor 40.

[0087] 2B shows a detailed view of an advantageous embodiment of the condensate outlet for a plant according to the first embodiment. The pipe 33 leaving the condenser of the heat exchanger 30 opens into a Y-branch. One leg of this branch leads to a horizontal or slightly upward-facing discharge line equipped with an air exhaust valve 32. The other leg leads vertically downward to a pipe section 34 with an enlarged cross section, in which the water column is formed. Two (e.g., capacitive) filling level sensors 35.1, 35.2 are arranged along this pipe section 34 with the water column. Connected to the bottom is a shut-off valve 36, which opens or closes depending on the measured values ​​of the filling level sensors 35.1, 35.2, so that the level of the water column is always located between the filling level sensors 35.1 and 35.2.

[0088] A further pipe section, and therefore a needle valve 37 as a throttle, is connected to the shut-off valve 36. Steam generated due to the pressure drop across the shut-off valve 36 and the needle valve 37 is finally returned via a pipe 38 arranged downstream of the needle valve 37 to the steam circuit, to the drying chamber and / or to the material to be dried.

[0089] The air exhaust valve 32 is then adjusted based on the amount of air on the condenser side, which is determined directly by the lambda probe 39 or indirectly based on the deviation of the static pressure from the steam pressure at the condensing temperature.

[0090] 3 shows a schematic cross-sectional view of a plant according to a third embodiment. The plant according to the third embodiment serves for indirect drying. It comprises a chamber 110, which is filled with steam and is closed at the top and partially open at the bottom. A conveying system 160 in the chamber 110 comprises a rotating hollow shaft 167 with a number of disks 168, which is hollow and serves to convey the material and to act as a heat exchanger, i.e., as a condenser internally. The plant further comprises a vapor compressor 40, a steam generator 115 and an inlet 161 for the liquid material to be dried.

[0091] The liquid material to be dried is fed through the inlet 161 to the outside of the hollow shaft 167 equipped with discs 168. Steam from the chamber 110 is fed to the steam compressor 40. For this purpose, the steam compressor 40 draws steam from the chamber and, after compression, sends at least a portion of it to the hollow disc condenser. There, the compressed steam condenses, thus heating the hollow shaft 167 equipped with discs 168, resulting in the drying of the material. In this case, the dry matter content of the material to be dried is set by the condensation temperature. The amount of air in the hollow shaft 167, which serves as a condenser, is regulated to a predetermined amount by a discharge valve. The dried material can be scraped from the discs 168 by a scraper that rubs against the hollow shaft 167 and discharged through the outlet 162 at the underside of the chamber 110. Steam is continuously drawn from the steam generator 115 to continuously heat the plant and to compensate for losses. In the plant according to the third embodiment, a steam blower is not required.

[0092] A plant according to the third embodiment can be regulated in two basic ways. According to a first method, the volumetric flow rate of the steam generator 115 is adjusted by measuring the condensation temperature of the hollow shaft so that this temperature remains within a predetermined interval. The volumetric flow rate of the compressed portion fed to the disc condenser is adjusted based on a temperature sensor in the area of ​​the steam / air separation layer so that this temperature remains within a predetermined range and thus the transition layer remains at a predetermined height.

[0093] According to a second method, the volumetric flow rate of the compressed portion fed to the disc condenser is adjusted by measuring the condensation temperature of the hollow shaft so that this temperature remains within a predetermined interval. The volumetric flow rate of the steam generator 115 is adjusted based on a temperature sensor in the area of ​​the steam / air separation layer so that this temperature remains within a predetermined range and thus the transition layer remains at a predetermined height.

[0094] 4A and 4B are schematic cross-sectional views of the drying chamber and the corresponding supply and removal of a plant according to the invention according to a fourth embodiment, where the rising conveyor, which is formed similarly to those 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 drawings. Fig. 4A shows a view in a vertical plane perpendicular to the rotation axis of the paddles, and Fig. 4B shows a view in a vertical plane extending through this rotation axis. Further components of the plant, in particular for the supply and removal and treatment of steam, for the material supply, for the sensor system and the controller, substantially correspond to those of one of the first three embodiments.

[0095] The chamber 210, which forms the conveying duct, has a substantially cylindrical shape. The paddles 267.1, 267.2 are mounted rotatably around the longitudinal axis of the chamber 210 and are at a fixed distance from the chamber wall. This distance should be selected to be short depending on the material being conveyed, so as to avoid clogging. Paddles 267.3, 267.4, 267.5 with a longer wall distance are mounted adjacent to paddles 267.1, 267.2 with a shorter wall distance, and the axial distance between paddles 267.1...5 is always the same. Here too, the gap dimensions should be selected to be long depending on the material being conveyed, so that relatively coarse pieces cannot become clogged and the transport of the material is facilitated. The paddles 267.1...5 in each case have an axial setting angle of, for example, 30° in the conveying direction. A different number of paddles can also be used.

[0096] Two vertical channels open into the chamber 210 at one end on the upper side, one serving as an inlet 261 for the material to be dried and the other as an outlet 273 for steam discharge. A lateral outlet 262 for discharging the dried material is located at the other end of the chamber 210 in the upper region. The height of the lower edge of the outlet 262, and therefore the filling height of the conveying duct, can be set by vertical adjustment of the weir 211. A filling level of more than 2 / 3 is desirable.

[0097] The paddles 267.1...5 rotate slowly at approximately 20-30 revolutions per minute. The paddles can rotate in both directions, the main direction of rotation (to convey the material towards the material outlet) is oriented so that the paddles 267.1...5 move downwards, where steam enters.

[0098] The dry material falls through outlet 262 into a conveying duct that includes a spiral structure 268 for controlled stagnation and controlled removal of the material. Once the material passes through spiral structure 268, it falls into a vertical removal duct where a transition layer 266 extends between the ambient and steam atmosphere. The controlled stagnation ensures that the transition layer 266 is stable.

[0099] Steam is supplied from above through the corresponding inlets 271 from the steam circuit, distributed over the length of the conveying duct and introduced laterally / horizontally via inlets 274.1...3 into the mixer / conveying trough in the lower region of chamber 210. In this case, the flow resistance of the material placed there is intended to generate a uniform inflow, thereby resulting in as homogeneous a drying process as possible. At the same time, this design of the steam supply prevents material from returning to the steam circuit.

[0100] In this case, the steam inflow is set so that there are no inflow openings at the axial positions of the paddles 267.1, 267.2 with small gap dimensions. The gap is in each case as wide as the paddle tip. In the positions where there are no or present short paddles 267.3...5 with large gap dimensions, steam is led to the chamber 211 via the inflow openings.

[0101] The lateral openings can be of different sizes, being smaller closer to the material inlet or steam outlet depending on the required steam distribution along the mixer axis (otherwise the steam would take the path of least resistance to flow, resulting in no or little flow in the majority of the mixer).

[0102] The steam temperature in the lateral inlet channels does not need to be uniform, but rather optimally increases along the conveying channel in the conveying direction. The drier the material is towards the end of the process, the hotter the introduced steam.

[0103] The steam coming in laterally flows through the loose material, first generally laterally and then countercurrent to the material flow direction. Finally, the steam next to the material inlet 261 is sucked upward through outlet 273, so that no particles are carried along with it.

[0104] 5A and 5B are schematic cross-sectional views of a drying chamber and corresponding feeds and removals of a plant according to the invention according to a fifth embodiment. Fig. 5A shows a view in a vertical plane perpendicular to the axis of rotation of the spiral structure, and Fig. 5B shows a view in a vertical plane extending along this axis. Further components of the plant, in particular for the feed and removal and treatment of steam, for material feed, for sensor systems and controllers, substantially correspond to those of one of the first three embodiments.

[0105] 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 structure is used as a mixing and conveying element within the chamber. The chamber 310 forming the conveying duct has a substantially cylindrical shape. The spiral structure 367 is rotatably mounted around the longitudinal axis of the chamber 310, with each turn being at a short distance from the chamber wall.

[0106] Two vertical channels open into the chamber 310 at one end on the upper side, one serving as an inlet 361 for the material to be dried and the other as an outlet 373 for steam discharge. A lateral outlet 362 for discharging the dried material is located at the other end of the chamber 310 in the upper region. The height of the lower edge of the outlet 362, and therefore the filling height of the conveying duct, can be set by vertical adjustment of the weir 311. A filling level of more than 2 / 3 is desirable.

[0107] The helix 367 rotates slowly at approximately 20-30 revolutions per minute. The helix can rotate in both directions, with the primary direction of rotation (to transport material towards the material outlet) being oriented so that the turns of the helix 367 move downward, where steam enters.

[0108] The dry material falls through outlet 362 into a conveying duct equipped with a helix or screw 368 for controlled stagnation and controlled removal of the material. Once the material passes through helix 368, it falls into a vertical removal duct where a transition layer 366 extends between the ambient and steam atmosphere. The controlled stagnation ensures that the transition layer 366 is stable.

[0109] Steam is supplied from above through the corresponding inlets 371 from the steam circuit, distributed over the length of the conveying duct, and introduced laterally / horizontally from the conveying duct via inlets 374 into a mixer / conveying trough in the lower region of chamber 310. The flow resistance of the material placed there is intended to create a uniform inflow, thereby resulting in as homogeneous a drying process as possible. At the same time, this design of the steam supply prevents material from returning to the steam circuit. The cross section of inlets 374 decreases against the material conveying direction. Since inlets 374 are divided into different temperature zones in the supply, the steam temperature increases along the conveying duct in the conveying direction: the drier the material is towards the end of the process, the hotter the introduced steam.

[0110] The steam coming in laterally flows through the loose material, first generally laterally and then countercurrent to the material flow direction. Finally, the steam next to the material inlet 361 is sucked upward through outlet 373, so that no particles are carried along it.

[0111] The operation of the plant according to the invention is described below with reference to the first two embodiments, however the corresponding description can easily be extended to the three further embodiments.

[0112] 6 is a diagram of the actuators and the control variables of a plant according to the invention, i.e., a plant according to the first embodiment when operated according to variant 1B, in which the volumetric flow rate of the compressed first portion fed to the heat exchanger 30 is set by adjusting the vapor compressor 40. The control variable 82 can be influenced by an actuator 81. The actuator 81 can comprise the circulation fan 20, whose rotational speed is adjusted in particular to set the circulation steam flow 82.3, the air discharge valve 32, which can be selectively opened or closed to set the ventilation mass flow 82.5, the vapor compressor 40, whose mass flow 82.4 can likewise be set by means of its rotational speed, the heating device 50, whose power can be set to adjust the steam temperature 82.2, and the conveying system 60, which makes it possible to set both the conveying speed and therefore the material throughput 82.1 and the residence time of the material to be dried in the chamber.

[0113] Variable material quantities 83 include the dry matter content at the inlet 83.1, the material consistency 83.2, the material morphology 83.3, and the material-dependent sorption isotherm 83.4. Control variables 84, primarily the dry matter content at the outlet 84.1, and the height 84.2 (or position) of the transition layer, are predetermined.

[0114] The condenser pressure 85.1 and the specific energy consumption 85.2 (kWh / kg water) result as quantities 85 resulting from the operating parameters.

[0115] 7 is a block diagram of the sensor system of the plant according to the invention. The following quantities are continuously measured and fed to the plant controller:

[0116] [Table 2]

[0117] FIG. 8 shows the profile of the measured temperature at three heights in the vertical pipe next to the outlet, measured by temperature sensors 91.8a, 91.8b, and 91.8c in the measuring tube 63 (see FIG. 7). The top temperature sensor 91.8a is located at a vertical distance of 50 mm from the chamber bottom. Adjacent sensors are located at a vertical distance of 50 mm from each other in each case. The top curve 95a represents the values ​​measured by the top temperature sensor 91.8a, the middle curve 95b represents the values ​​measured by the middle temperature sensor 91.8b, and the bottom curve 95c represents the values ​​measured by the lower temperature sensor 91.8c. The measurement series relates to a drying operation in which a state of equilibrium is desired by adjusting the control variable 82 described above. In this case, the temperature measured by the top temperature sensor 91.8a is used as the basis for adjusting the control variables 82, in particular the mass flow 82.4 of the vapor compressor 40, so that the transition layer is maintained at its height 84.2 by adjustment. The setpoint is 97.0°C. Alternatively, the middle temperature sensor 91.8b, or a quantity obtained from the measurements of several sensors, can be used. If the corresponding temperature or a quantity determined from the corresponding temperature leaves a predetermined range (e.g., the adjustment temperature ±1 K), the rotation speed of the vapor compressor 40 is adjusted up or down during operation according to variant 1A or 1B. Advantageously, a PID control known per se is used for the adjustment. Values ​​of P=1, I=10, and D=0 can be selected, for example, for adjusting the rotation speed of the vapor compressor 40.

[0118] The start-up of a plant according to the invention will now be explained with reference to Figure 9, which shows the profiles of temperature (at the top, in °C) and air content (at the bottom, in %) when a plant according to the invention is started. The start-up is divided into three phases: the heating phase with air (phase 1), the steam filling (phase 2), and finally the material filling (phase 3). The chamber temperature 96 in the upper region of the chamber, the temperature 97.2 measured by the temperature sensor 91.2 downstream of the heat exchanger 30, the temperature 97.7 measured by the temperature sensor 91.7 downstream of the steam compressor 40, as well as the temperatures 97.8a, 97.8b, 97.8c of the three temperature sensors 91.8a, 91.8b, 91.8c in the measuring tube 63 (the temperatures generally decrease downwards) are shown in the upper region. The air volume 98.1 in the chamber 10 measured by the lambda probe and the air volume 98.2 in the condenser determined indirectly based on the measured pressure and temperature of the steam at the condensate outlet downstream of the condenser are shown in the lower region. The steam charge can be accurately monitored by these measurements in combination with the temperature measurements.

[0119] Drying of the steam is carried out in a steam atmosphere at ambient pressure, where the air content in the steam atmosphere should be no more than 4%. Therefore, the plant chamber must first be preheated with air to a temperature of at least 100°C, and then a steam atmosphere must be created. This is achieved in three phases:

[0120] In the first phase, the plant is heated by hot air. For this purpose, air is circulated by the circulation fan 20 and heat is supplied to the process by the heating device 50. This phase begins at position A in FIG. 9 and lasts approximately one hour. Towards the end of this phase, the vapor compressor 40 is switched to idle (short-circuited) (position B), also preheating it and thus avoiding greater thermal stresses and condensation in the vapor compressor 40 during vapor charging. The phase ends when the chamber 10 reaches a temperature above 100°C. The temperature of the circulating air in the circulation duct 97.2°C is already above 100°C at this point due to the direct heating of the air.

[0121] After the chamber temperature of 100°C is reached, the steam filling begins (position C). For this purpose, the heating device 50, the steam compressor 40 and the circulation fan 20 are switched off and steam from the steam generator 15 is directed from above into the chamber 10. In this case, less dense air is displaced downwards from the chamber 10. This is evidenced by an increase in the temperature 97.8ac measured by the temperature sensor 91.8ac at the material outlet. Towards the end of this phase, the steam compressor 40 is switched on again to reach the operating temperature, resulting in a short-term sharp curve in the temperature 97.8ac (position D).

[0122] Regarding the air volume, the air volume 98.1 in the chamber initially drops sharply, followed by a slow increase in the temperatures 97.8a...c measured by the temperature sensors 91.8a...c as the hot air is displaced downward. When they reach 100°C, this indicates that the steam volume has reached the installation base. It is known that air can be displaced downward from the top of the chamber without any problems from the lighter steam. Finally, the steam floats above the cooler ambient air. In place of the opening at the underside of the chamber, a stable transition layer 66, or so-called stratification (see Figure 2), forms between the steam and the air. In the region of this layer, a temperature profile is established that extends from ambient temperature to over 100°C within approximately 50 cm. In the temperature gradient region from 100°C to 65°C, the temperature gradient is typically 0.13 to 0.26 K / mm. The air volume in the chamber 10 is reduced to less than 4% in this case.

[0123] As soon as the steam atmosphere is generated, the material can be led to the plant (position E). During this phase, steam still needs to be generated by the steam generator 15. This is necessary because steam condenses on the cold material and thus heats it. Since sufficient steam has not yet been generated by the drying process, it needs to be provided by the steam generator 15. During this process phase, the heating device 50 and the circulation fan 20 are again activated. As soon as the majority of the chamber 10's receiving capacity is filled with material, the steam compressor 40 is started again, thereby increasing the condenser pressure (position F). As a result, the condensation temperature in the condenser increases, and heat can again be output to the steam circuit (position G). When the chamber 10 is completely filled with material within its receiving capacity and a sufficient water evaporation rate has been reached, the steam generator 15 can be switched off and the normal drying process begins. The amount of air in the chamber 10 remains below 4% in this case. During operation according to one of variants 2A and 2B, the steam generator continues to operate (usually at reduced power) to adjust the height of the transition layer.

[0124] On the one hand, the target dry matter content of the material at the outlet depends on the relative pressure and therefore the steam temperature (provided the residence time is long enough), and on the other hand, since heat needs to be continuously supplied in a continuous process to preheat the material, the heat is supplied at a high temperature before the material is fed, while the preheating of the material on entry into the steam atmosphere is provided by steam at a lower temperature.

[0125] During this phase, the condenser needs to be further deaerated. The amount of air in the plant is very low, but residual air accumulates in the condenser and must be continuously vented (position I). In this case, the amount of air on the condenser side is regulated by the controller of the air vent valve 32 to a value below 15% by volume, in particular 7-10% by volume. The amount of air is determined based on the measurements of the temperature sensor 91.9 and the pressure sensor 92.9.

[0126] During the drying operation, moisture in the material to be dried is evaporated within the chamber 10 by supplying heat from the superheated steam. At the inlet of the chamber 10, the steam is superheated above its saturation temperature. As the material to be dried passes through, the heat energy of the steam is transferred to the material, causing additional water to evaporate.

[0127] At the outlet of the chamber 10, the steam mass flow increases due to the water evaporated from the material. The temperature then decreases depending on the dry matter content or sorption isotherm of the material and the degree of heat transfer to the material, so that the steam remains superheated.

[0128] The majority of the circuit steam then enters heat exchanger 30 and is re-superheated by condensing vapor-compressed steam at a higher temperature on the other side of heat exchanger 30 . After heating in the heat exchanger 30, heat losses are compensated by the heating device 50. As a result, the drying temperature, and therefore also the desired dry matter content at the outlet, can be set accurately and quickly. Generally, steam temperatures of 140-170°C are well suited for drying, although the material temperature is usually 105-130°C, depending on the sorption isotherm.

[0129] After the steam leaves the drying chamber, a portion of the additional steam is sucked from the circuit and compressed by a vapor compressor 40 to a pressure of approximately 250-500 kPa (2.5-5 bar) above atmospheric pressure. Depending on the pressure in the condenser, the steam condenses at its saturation temperature of 130-150°C. In the process, the enthalpy of vaporization released during condensation is returned to the steam circuit at an elevated temperature by a heat exchanger.

[0130] Demineralized sterile water above 100°C leaves the system through the condensate drain valve 31 and the circuit steam is retained. Finally, 100°C water can be used to preheat or in place of tap water.

[0131] During the drying process, the material to be dried or dried is continuously introduced or discharged, and in each case, it is guided through the stratification and discharged into the ambient air, where it is dried again due to the lower partial pressure of steam in the ambient air and the residual heat of the material to be dried. If the material flow increases, the compressed first portion fed to the heat exchanger must be increased accordingly. This works as long as the power of the circulation fan for returning the heat is sufficient. Within this framework, it has been found that the efficiency of the process can be further increased if the material flow is increased.

[0132] Within the scope of the third embodiment, when the plant is started up, the steam atmosphere is formed mainly by the following steps: 1. Air displacement by steam from a steam generator, 2. Introduction of the material to be dried; 3. Starting up the steam compressor (which starts the drying process), 4. After the operating pressure is reached in the heat exchanger, the steam generator continues to operate at a reduced volumetric flow rate (and is adjusted as described above).

[0133] The invention is not limited to the embodiments shown: in particular, the dimensioning of the respective plant and conveying system used can be adapted to the type and amount of material to be dried. The material can be introduced directly into the steam atmosphere from a previous process. Furthermore, the material can be preheated before being introduced into the plant, which in turn increases the amount of steam available for vapor compression. If waste heat, for example from an upstream or downstream process step, is available, the energy demand of the heating device can be reduced, as said waste heat can be easily supplied to the plant according to the invention.

[0134] In summary, it should be noted that the present invention provides a method for operating a plant for drying materials to be dried by superheated steam, as well as a corresponding plant that allows high energy efficiency through simple material supply and removal.

Claims

1. 1. A method for operating a plant for drying a material to be dried by means of superheated steam, said plant comprising: a) a downwardly opening chamber provided with an inlet for the material to be dried, an outlet for the drying material, an inlet for superheated steam, and an outlet for steam; b) a conveying system for introducing the material to be dried into the chamber, transporting the material to be dried in the chamber during drying, and discharging the dried material from the chamber; c) a vapor compressor for compressing a first portion of the vapor recycled from the chamber; d) a heat exchanger for transferring heat from the compressed first portion by condensing a volumetric flow of the compressed first portion; e) forming a steam atmosphere in an upper region of the chamber, the steam atmosphere floating above ambient air in a lower region of the chamber, forming a transition layer between the upper and lower regions; f) maintaining the height of the transition layer within a predetermined range by determining a current height, and depending on the determined height: f1) the volumetric flow rate of the compressed first portion fed to the heat exchanger is adjusted; or f2) A method wherein the volumetric flow rate of a steam generator is adjusted, said steam generator being positioned and operative such that steam can be supplied to said chamber, generated in said chamber, or both.

2. The method of claim 1 , wherein the current height of the transition layer is determined based on measurements of one or more temperature sensors positioned in a height range corresponding to the predetermined range.

3. The drying temperature is maintained within a predetermined range by comparing it with a set value of the drying temperature, and depending on the comparison, g1) the steam generator is arranged and operated such that steam can be supplied to or generated within the chamber, provided that the volumetric flow rate of the steam generator is adjusted and the height of the transition layer is kept within the predetermined range by adjusting the volumetric flow rate of the compressed first portion supplied to the heat exchanger, or g2) The heating power of the heating device is adjusted, or g3) The method according to claim 1 or 2, wherein the volumetric flow rate of the compressed first portion supplied to the heat exchanger is adjusted, provided that the height of the transition layer is kept within the predetermined range by adjusting the volumetric flow rate of the steam generator.

4. The plant comprises a piping system between the outlet for the steam and the inlet for the superheated steam, the piping system comprising: h) the vapor compressor; i) a circulation fan; j) a heat exchanger for heating a second portion of the vapor recycled from the chamber by transferring heat from the compressed first portion by condensing the volumetric flow of the compressed first portion fed to the heat exchanger; k) a heating device for the steam, arranged between the heat exchanger and the inlet for the superheated steam.

5. 4. The method according to claim 1, wherein the conveying system comprises a rotating hollow shaft, the rotating hollow shaft being arranged in the chamber and having a plurality of disks forming the heat exchanger, a cavity being arranged inside the hollow shaft, and the cavity can be supplied with the volumetric flow rate of the compressed first portion of the vapor by the vapor compressor to heat the disks.

6. 6. The method according to claim 1, wherein the heat exchanger has a vent valve on the condenser side, and the opening of the vent valve is adjusted based on the amount of air determined on the condenser side.

7. 7. The method according to claim 6, wherein the amount of air on the condenser side is adjusted to a value of 0 to 50%, preferably 5 to 20%, particularly preferably 7 to 12%.

8. 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, the air in the chamber being displaced downwards from the chamber; During operation of the steam generator, until an operating pressure is reached in the heat exchanger, - starting the circulation fan; - starting the heating device, the vapor compressor, or both; - introducing the material to be dried by means of said conveying system; - starting the vapor compressor.

9. 1. A plant for drying materials to be dried by means of superheated steam, comprising: a) a downwardly opening chamber provided 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 steam; b) a conveying system for introducing the material to be dried into the chamber, transporting the material to be dried in the chamber during drying, and discharging the dried material from the chamber; c) a vapor compressor for compressing a first portion of the vapor recycled from the chamber; d) a heat exchanger for transferring heat from the compressed first portion by condensing a volumetric flow of the compressed first portion; e) a controller for acquiring and processing measurements and for generating control signals; The controller f) forming an atmosphere of superheated steam in an upper region of the chamber, said atmosphere floating above ambient air in a lower region of the chamber, forming a transition layer between said upper and lower regions; g) maintaining the height of the transition layer within a predetermined range by determining a current height, and depending on the determined height: g1) the volumetric flow rate of the compressed first portion fed to the heat exchanger is adjusted; or g2) A plant operable such that the volumetric flow rate of a steam generator is regulated, said steam generator being arranged and operable such that steam is supplied to said chamber, generated in said chamber, or both.

10. The plant comprises a piping system between the outlet for the steam and the inlet for the superheated steam, the piping system comprising: h) the vapor compressor; i) a circulation fan; j) a heat exchanger for heating a second portion of the vapor recycled from the chamber by transferring heat from the compressed first portion by condensing the volumetric flow of the compressed first portion fed to the heat exchanger; k) a heating device for the steam, arranged between the heat exchanger and the inlet for the superheated steam.

11. 11. The plant of claim 10, wherein the inlet for the superheated steam is positioned in the chamber so that the superheated steam in a directional steam flow intersects the conveying path of the material to be dried in the chamber.

12. The plant of claim 11 , wherein an element for homogenizing the steam flow is located on the chamber side of the inlet.

13. 10. The plant of claim 9, wherein the conveying system comprises a rotating hollow shaft, the rotating hollow shaft being disposed in the chamber and having a plurality of disks forming the heat exchanger, a cavity being disposed inside the hollow shaft, the cavity being capable of being supplied by the vapor compressor with the volumetric flow rate of the compressed first portion of the vapor for heating the disks.

14. A plant according to any one of claims 9 to 13, wherein the steam generator is arranged and operable so that steam is supplied to the chamber, generated in the chamber, or both.

15. 15. The plant of claim 14, wherein the steam generator is connected to the heat exchanger such that it can be at least partially powered by condensate from the heat exchanger.

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