EP3491312B1 - Contact dryer - Google Patents

Description

The invention relates to a contact dryer for drying moist material and a method for drying moist material.

Thermal dryer basics

The thermal drying of moist goods has always been an important technical process. The material and the liquid contained therein are surrounded by air or another gaseous medium into which the liquid can evaporate. The mixture of gaseous medium and steam is also known as vapor.

The basic principle of thermal drying is the introduction of heat into the goods to be dried. The goods to be dried and the liquid contained therein are heated here. The actual drying process begins as soon as the temperature of the product reaches the temperature that corresponds to the partial pressure of the vapor of the liquid to be dried in the surrounding atmosphere. Physically, this corresponds to an equilibrium between the vapor of the liquid and the liquid on the surface of the product, an equilibrium between condensation and evaporation. This state of equilibrium is known as the state of saturation. There is a saturation pressure for every temperature and a saturation temperature for every pressure.

If the vapor pressure is higher than the saturation pressure, the vapor condenses by forming droplets. If the vapor pressure is lower than the saturation pressure, the liquid evaporates. When condensing heat is released, when evaporating heat is required. Condensation and evaporation take place until a new state of equilibrium is reached through the exchange of substances and energy. This equilibrium temperature is also known as the dew point.

Convection dryer

A frequently used process of thermal drying is drying by convection. Here, a gas (mostly air) releases part of its thermal energy to the material to be dried and the liquid contained therein by cooling and in return absorbs part of the liquid contained in the material to be dried as vapor.

During this process it must be taken into account that the absorption of steam also increases the dew point of the gas. The gas cannot therefore be cooled down to the temperature which corresponds to the dew point at the beginning of the drying process, but only to the temperature which corresponds to the dew point at the end of the absorption of steam. This temperature at the end of the drying process is also known technically as the wet bulb temperature.

There are now a variety of liquids that are driven out of a good during drying processes; organic solvents may be mentioned, for example. The greatest technical importance, however, is the expulsion of water from goods that occur in nature and have a higher level of moisture there than is required or desired for their use.

• Als Verdampfungswärme, um die Flüssigkeit zu verdampfen
• Als fühlbare Wärme, um die Luft auf die Feuchtkugeltemperatur zu erhitzen
• Ein in der Regel kleiner Anteil wird zur Erhitzung des Gutes auf die Feuchtkugeltemperatur genutzt.

The expulsion of water is mostly done with the help of air. For this purpose, the air is first heated and then brought into contact with the product. The heat that is applied for heating is used for two main purposes:

• As heat of evaporation to evaporate the liquid
• As sensible warmth to heat the air to wet bulb temperature
• A usually small portion is used to heat the goods to the wet bulb temperature.

With the combination of air / water, the part of the energy that is used for evaporation is limited to approx. 70% of the energy used. The rest is needed to heat the air to wet-bulb temperature.

In order to increase the efficiency, the air was started to circulate several times, to recirculate it. This increases the overall load of the air with water vapor, but at the same time also increases the wet bulb temperature and thus the temperature of the drying process. The wet bulb temperature always corresponds to the temperature of the exiting air and the load with water vapor corresponds to the difference between the incoming fresh air and the exiting exhaust air.

Only a small part of the circulating, laden air is replaced by fresh air. The number of circulations results from the ratio of the amount of circulating air, which in turn results from the possible water absorption per circulation, and the amount of exhaust air that escapes.

The disadvantage of this process is that the load of the air with water decreases per circulation and thus significantly more air flows than is the case with a single load. Ultimately With this recirculation, a not inconsiderable part of the saved heat is compensated by an increased need for fan power in order to circulate the air.

Convection dryers basically have the property of carrying light particles of the goods with them. As a rule, you therefore need a separator for dust. In the case of many biogenic substances, not only water but also some organic compounds are evaporated. These organic compounds are mostly odorous, so that in addition to the dust separator, the dryer often also requires a so-called biofilter in order to separate these odorous organic substances from the exhaust air.

Contact dryer

In addition to the convection dryer, there is also what is known as contact drying as a drying process. With the contact dryer, the product itself is heated directly through contact with a solid surface.

The water to be evaporated is heated above the dew point temperature and evaporated. With a contact dryer, the dew point temperature is determined by the amount of air that flows into the contact dryer. By changing this amount of air, the dew point temperature can also be changed as required.

The surface temperature of the drying tube is always well above the dew point temperature. In the course of the drying process, less and less water is available on the surface of the goods, which can evaporate and absorb the heat transferred from the surface of the drying tube, so that the heat heats the product itself. In the case of a contact dryer, there is therefore a general tendency for the material to take on the temperature of the surface of the drying tube at the end of the process. Only the part of the surface on which the material to be dried and the drying tube touch (contact surface) is decisive. If this surface temperature could damage the product, appropriate measures are required to ensure that the material only comes into contact with the hot surface of the drying tube for a short time and then there is enough time to transport the heat supplied inside, where there is still water is present that must be expelled.

With the contact dryer there is no forced relationship between the expelled water and the air flowing in and out. A contact dryer is therefore not subject to the efficiency restrictions of convection dryers and theoretically allows efficiencies of almost 100%.

The performance of a contact dryer is determined by the amount of heat that can be transferred to the material to be dried via the solid surface of the drying tube. The effective contact areas (contact areas) are usually relatively small and only make up a fraction of the total surface of the drying tube. This results in a significant restriction for contact dryers, which so far have only been able to prevail over convection dryers in special cases.

Another problem with contact dryers is the supply of heat to the surface of the drying tube. Hot water, steam or thermal oil can be used as heat transfer media. This requires separate closed circuits that increase the complexity of the contact dryer and reduce its practicability; this is complex in terms of construction and process technology and is associated with additional costs.

In the case of smaller powers below approx. 1 MW, there is also the problem that no commercially available systems for providing the heating medium are available on the market that use solid fuels such as. B. Biomass can be fired; this is only possible if the system is dust tolerant, which is not available in this low power range. It is therefore necessary to provide the heat for drying using expensive fossil primary energy sources, provided that no waste heat is available in dust-free and soot-free flue gases.

Contact dryers that are designed as screw dryers are known. Both the screw helix and the screw jacket are heated. The heating of the screw spiral is - due to the rotary movement of the component - structurally very complex, but increases the area available for the heat input and is therefore usually an essential part of the structural solution. In the prior art, the screw jacket is designed in such a way that it consists of an inner and an outer wall with a small spacing, which as a rule only exists in the lower region of the drying tube. The space between the two walls (also referred to as the shell space) is flowed through by the heat transfer medium, whereby the drying tube is heated.

A main function of the screw in the prior art is to bring the material to be dried again and again into contact (contact) with the hot surface and also to limit the contact time through the continuous movement of the material. Further functions of the screw in the prior art are the conveyance, mixing and ventilation of the material to be dried. If the dryer is set at an angle, the conveying function can also be taken over by a weir at the outlet, which is used to set the filling level in the screw dryer. In this case the screw has a pure mixing function, not a conveying function. The disadvantage of this prior art method is always the direct coupling of the rotation the screw with the heat input, so that this design is suitable for relatively dry input materials, such as. B. mechanically pre-drained biomass, only suitable to a very limited extent.

A large number of individual contact dryers are required for higher capacities. This requires a considerable amount of connecting pipes and valves for the supply and discharge of the heating medium, for drainage, ventilation and for pressure protection.

State of the art

Many different dryers are known. In DE 87 09 563 U1 a drying device for bulk material is known. This drying device combines the contact drying process with a convection drying process in a device in the form of a rotating drum. After a wet feed, the bulk material to be dried is first brought into contact with a heating surface, which is heated in the form of pipes or pipe-like internals by means of hot water or steam. In the conveying direction through the drying device, a convection drying section follows the contact surfaces over the remaining length of the rotating drum. This is intended to combine the advantages of both processes. The disadvantage of this known rotary drum dryer is the continuous rotary movement of the drum, which is necessary in order to constantly move the material to be dried in order to achieve adequate and uniform drying while the material is in the dryer. The built-in components required in the dryer, in particular the heating pipes, represent an essential blockage of the interior, so that this type of construction tends to become clogged very easily. Pasty or non-pourable goods or goods that tend to stick together cannot be dried with this design. Another serious disadvantage of the drum dryer is the need to install a permanently tight connection for the supply and discharge of the heat transfer medium, which also allows the drum dryer to rotate.

In DE 39 11 716 A1 a method for drying sludge and a sludge drying system for carrying out the method are described. According to this known state of the art, pre-dewatered sludge is preheated in a heat exchanger to 70 to 80 ° C, then introduced into a twin-screw contact dryer and there with the supply of heat from the outside and by converting the mechanical energy introduced into the material to be dried by kneading into Post-dried with heat. This twin screw consists of two interlocking screws, which are arranged side by side in a common housing. The installation spaces of the individual screw spirals are connected. The goods to be dried can switch back and forth between the two screws at will, which is explicitly desired. The twin screw contact dryer is enclosed in a heating jacket embedded, which has to be heated to 120 ° C to 250 ° C, for which hot thermal oil or superheated steam is used. The interior and thus also the double-walled heating jacket are each designed in the form of a horizontal "8" (description, column 4, lines 23-30). The construction of the entire drying system, including the jacketed twin-screw contact dryer, is relatively complex, which disadvantageously increases the equipment costs for such a sludge drying system.

Furthermore, in DE 427 584 a method for drying coal on heating surfaces lying on top of one another is known. In the known dryer, heating surfaces are arranged one above the other, the coal to be dried being conveyed by means of endless scraper belts. The coal is initially heated indirectly by running steam-fed heating pipes through the coal and then passing the coal over surfaces heated with flue gas. In the last drying stage, the flue gases are passed from the flue gas duct, in which they have an indirect effect on the coal, directly into the drying room, which is indirectly heated with both steam and flue gases. By using steam and flue gas, an energetic improvement of the entire drying process should be achieved. Such a dryer is only useful where large amounts of flue gases occur. This known drying device is constructed in such a way that a steam dryer that only takes over the rest of the drying capacity is provided as the main dryer. The combination of steam and flue gas is intended to achieve the advantage that a highly overheated, relatively dry atmosphere is created in order to dry coal.

Furthermore, in DE 10 2014 113 307 A1 among other things, a reactor for generating a fuel gas from mechanically dewatered sludge is known. The pyrolysis reactor described consists of several double-tube heat exchangers, each of which has an inner tube with a hollow screw and an outer tube. The sludge to be dried is conveyed in the inner pipe by means of the hollow screw, whereby a heating gas is fed in countercurrent in the outer pipe. At least two double-tube heat exchangers of the same type are connected to one another in a series arrangement. This is to ensure that the water contained in the sludge is evaporated in the first double-tube heat exchanger by supplying heat, whereas in further double-tube heat exchangers the sludge is heated to approx. 550 ° C. Additional double-tube heat exchangers arranged afterwards are used to carbonize the organic components in the sludge to a solid pyrolysis residue in the absence of air and to generate a fuel gas, which is fed to energy recovery through a gas-solid countercurrent flow. The fuel gas is intended for use as heating gas or for generating energy in a CHP unit. The disadvantage of the known pyrolysis reactor is that the energy supply to the heating gas takes place outside the double-tube heat exchanger, which means that energetic losses cannot be ruled out. CH265602 A , EP0370144 A1 and JPS60138383 A disclose prior art contact dryers.

This invention covers not a dryer, but as a process a reactor for coupling several double-tube heat exchangers, which have different tasks in the process and thus different physical parameters such as e.g. B. have the temperature or the chemical composition of the expelled gases. The invention does not provide any structural coupling.

Object and subject of the invention

Compared to this known prior art, the object of the present invention is to dry moist material, in particular biomass, in an energetically favorable and energetically effective manner by means of a dryer with a simple structural design in a cost-effective manner. This object is achieved by a contact dryer with the features according to claim 1 and by a method which works with such a contact dryer with the features according to claim 6. Appropriate further developments are defined in the respective dependent claims.

Possible input materials

This type of contact dryer is particularly suitable for drying fibrous, fine-grained, pasty or dusty materials, such as. B. grass, leaves, algae, paper and sewage sludge, digestate, spent grains, food, sawdust, with or without mechanical pre-drying, pretreatment or processing.

Arrangement of several drying tubes in a common jacket

The contact dryer according to the invention is used to dry moist material, with biomass in particular being considered as the moist material. The basic structure of the contact dryer according to the invention has at least one drying tube in which the material to be dried can be conveyed through the drying tube by means of a conveyor device provided inside, which can preferably be a screw conveyor, with a heating medium being located on the outside of the drying tube and the The drying tube and the heating medium are surrounded by an envelope forming a jacket space.

The jacket space, which preferably has an axial extension, has at least one further drying tube in its interior, the material to be dried from at least two drying tubes not being able to mix in the jacket space at at least one point. The individual drying tubes of the contact dryer are preferably coaxial with one another and coaxial with the jacket. The jacket space is insulated from the outside to avoid heat loss. The construction and the outlay on equipment are accordingly simplified overall, and the manufacturing costs are correspondingly reduced.

The disadvantageous expense of connecting pipes and valves is avoided by using a common shell space according to the invention.

Furthermore, in this design, no movement of the shell space or the drying tubes is required, in particular no rotating movement. Sealing problems when supplying and removing heat media are therefore completely eliminated.

Design of the jacket pipe

The jacket space limits the spatial expansion area of the heating medium. The shell space is preferably designed as a cylindrical body which accommodates a plurality of drying tubes and encompasses them together. The cylindrical design is particularly suitable for absorbing the pressure that prevails in the heating medium (pressure vessel). With this design, heating pressures of up to approx. 40 bar can be achieved, which corresponds to a heating temperature of approx. 250 ° C. With normal drying temperatures for biomass of below 200 ° C, mostly even below 100 ° C and heating temperatures of a maximum of 250 ° C, mostly even below 160 ° C, this is more than sufficient and allows optimal operation in any case.

According to the invention, the shell space is tubular and each has an end region in the form of a flat bottom. This has the advantage that the degree of dryness to be achieved for the moist material to be dried can also be influenced or determined over the length of the contact dryer in addition to the heating power supplied, depending on the design selected. The tubular basic structure of the shell space also has the advantage that it forms the ideal shape of a pressure body. The drying tubes located in the drying area can easily pass through the jacket space at its respective end areas sealing lids are performed. This construction is particularly simple, since these covers (preferably flat bottoms) can simply be drilled after rolling or deep-drawing, at least in the areas in which drying tubes are located.

Arrangement of a heating area in the jacket pipe

The jacket space is also heated inside. It then has a heating area and a drying area in the interior, drying tubes being arranged by definition in the drying area and heating tubes being arranged by definition in the heating area. A jacket space can also comprise several heating and / or drying areas.

The heating tubes can be arranged coaxially to the drying tubes. This is always advantageous when the heating is carried out by a gaseous medium. If a liquid or a condensing gas is used for heating, the installation of serpentine heating pipes is recommended.

Furthermore, according to the invention, the jacket space is preferably provided in the lower area with tubes through which a medium for heating (heat supply medium) flows. These pipes are also referred to below as heating pipes. The heat supply medium is preferably hot flue gas from a combustion, but can in principle be any form of hot heat transfer medium, for example also thermal oil, liquid metal salts or liquid metals. The hot flue gas can also be, for example, the exhaust gas from a piston engine or a gas turbine. If the flue gas contains dust or soot components, the heating pipes would be provided according to the invention with a cleaning facility that can be used either during operation or also when not in use.

Heating area as an external heat exchanger

If the flue gas (heat supply medium) is not to flow through the heating pipes but around the heating pipes, these heating pipes can also be arranged directly below the jacket space and serve as a hot well (condensate collection space) of the jacket. In this case, the shell space would be connected to the heating pipes via one or more collectors, provided these are not connected individually and directly to the shell space.

According to the invention, this arrangement offers various advantages. The heat transfer on the air side (heat supply, heat supply medium) is always significantly lower than on the water side (heat absorption). If, as in this embodiment, the heating Air (heat supply medium) is located on the outside of the tubes, ribs can advantageously be applied to the tubes, which ribs significantly increase the effective heat transfer surface and thus the heat transfer coefficient. The size of the dryer can be further reduced in this way. This means that the entire jacket space is available for drying pipes, as there is no installation space for the heating pipes. Furthermore, there is no space between the heating and drying pipes, which is necessary to absorb the fluctuating water level between cold and warm conditions and the various outputs.

Heating medium

According to the invention, a heating medium is applied to the jacket space, which changes the phase from vapor to liquid when heat is given off and condenses on the outer surface of the drying tubes. The condensation temperature corresponds exactly to the dew point temperature at the vapor pressure with which the jacket space is exposed. This leads to a uniform heating temperature on the inside of the drying tubes, where the goods to be dried are located. The goods to be dried can in no case assume a higher temperature than the surface temperature of the drying tubes. The maximum temperature of the goods to be dried can be set very precisely by regulating the pressure of the steam in the jacket space. In the simplest case, the heating medium can be water vapor. Depending on the desired drying temperature, these can also be organic media, typical representatives for this are the commercially available refrigerants.

If the heating medium changes phase, the jacket space is only partially filled with the condensate. The remaining space inside the jacket is filled by the vapor phase. The vapor pressure here can be both above and below the ambient pressure. The vapor or gaseous phase of the heating medium has the advantage that even with a relatively dense and compact arrangement of the individual drying tubes within the drying area, the very good heat transfer during the phase change (here condensation) ensures sufficient heating and thus efficient drying. The shell space is then advantageously provided with a controlled ventilation or extraction system in order to avoid inert gases in the shell space of the dryer.

Execution of the heated dryer as a natural circulation steam generator with integrated condenser

The inventive arrangement of the heating area in the jacket space or directly below makes it possible to design the jacket space as a closed pressure body. The complete functionality of the heat transfer, e.g. B. by water and steam, from the heating in the heating pipes to drying in the drying pipes takes place internally, without external intervention is required, such as. B. the replenishment of liquid by a pump or the withdrawal of steam through a relief valve. If water is used as the medium, the jacket space works as a self-contained natural circulation steam generator with an integrated steam circuit. During operation, it does not require any further procedural connections with the environment and no further external units, in particular no connection to an external steam generator, no water treatment and no water treatment. It is considerably cheaper in terms of investment and operating costs. The same applies analogously to the use of commercially available refrigerants or other substances that change phase. All that is required is a safety valve that protects the jacket space against overpressure.

The arrangement of the heating in the lower part of the jacket and in the form of smoke pipes or an external heat exchanger makes it possible to use a solid fuel firing system for heating, even with low outputs, without the heat exchanger clogging up due to the dust in the flue gas (heat supply medium).

Mobility of the contact dryer

The contact dryer according to the invention is advantageously constructed in such a way that it can be transported in a 20 "or 40" container. It has only a few and simple connections to the outside world, so that it can easily be brought to different locations and used there - even for a short time - without this having a negative impact on economic profitability. The 20 "or 40" container is advantageously designed in such a way that it serves as a housing after the contact dryer has been set up and z. B. represents a weather protection; the same applies to possible sound insulation. A building that requires building permits can then be dispensed with, which makes use more flexible.

If a higher output is required than a single dryer can provide, several dryers in container construction are set up in a modular design and operated in series and / or in parallel.

This is particularly advantageous if the dryer is also heated in a container construction, as disclosed, for example, in the parallel application filed by the applicant on the same day.

This basic structure of a casing forming a jacket space with at least two drying tubes and possibly one or more heating tubes and with a heating medium inside the jacket space has the advantage that the material to be dried is physically separated from the heating medium, so that cleaning with the heating medium entrained parts of the goods to be dried, as is the case with the prior art, are omitted. This gives a simple structure. Since the heating medium can be supplied with heat energy in direct contact with the heating pipes and in the drying area this energy is transferred directly to the drying pipes in the form of heat and from there this heat energy is fed to the moist material to be dried conveyed inside the drying pipe for the purpose of drying can, results in an effective energetic balance of this two-part contact dryer, in which the drying area and the heating area are coupled to one another via the heating medium, so that the supply of heating medium from the heating area into the drying area and the return of the heating medium from the drying area into the heating area in can be implemented in the simplest manner, because this does not require any built-in components within the contact dryer.

It is of course just as possible to provide the jacket space of the dryer with a heating area, but to regulate the power control through external connections, for example maintaining the pressure by adding or removing steam and the water level by adding or removing condensate.

Reduction of the size, simplification of the structure

Due to the large number of drying tubes, which can preferably be installed in parallel, sufficient heating surface can be made available according to the invention so that the problematic heating of the screw helix can be dispensed with. As a result, the screw helix can also be replaced by a soulless helix or any other conveying device such as B. a piston, a Chain conveyors, single or double screws with the same or variable pitch and helix height can be replaced. With this measure, the costs for the manufacture of the dryer are considerably reduced. Furthermore, the disruptive coupling between the rotation of a heated screw and the resulting heating power is advantageously eliminated. The conveying device can thus be operated at any speed, as a result of which the dwell time of the goods to be dried in the dryer can be set as desired. Several contact dryers can also be connected in series or in parallel, provided that this is advantageous for drying the goods to be dried. In the same way, the heating by flue gas or another heat supply medium can be carried out sequentially or in parallel.

Instead of using an active conveyor element, according to the invention, the jacket can also be set up inclined or vertically, provided that the material to be dried can also be conveyed solely by gravity or, for example, by vibrating the entire dryer.

Vapors and treatment of the vapors

This type of drying avoids any heating of transport air for the vapors (mixture of air coming into contact with the material to be dried and escaping steam) and allows full use of the heat supplied to evaporate the water or the liquid that is to be expelled.

With the belt dryer, air is required to transport the heat to the goods to be dried and is in direct contact with the goods and the escaping steam. It mixes with the steam and thus represents a very large amount of gas laden with dust and odor-intensive substances. In the case of the contact dryer design according to the invention, no air is required. The amount of vapor can be reduced down to the amount of steam that evaporates during drying.

A suction device is preferably provided at at least one end area of the jacket when the drying tubes pass through the jacket, by means of which dust and expelled vapors entrained during drying, i.e. the expelled liquid as gas (e.g. as water vapor, also mixed with air and also for Partly containing foul-smelling organic components) can be sucked off and do not get into the environment.

If the expelled vapors are not too diluted with air or other intergases, the absorbed heat of evaporation can be cooled in a heat exchanger recover with condensation at a high temperature level. The vapors advantageously contain more than 20% steam, ideally more than 50%.

If the technically known vapor compression is used, the dryer can advantageously also be heated with the waste heat from the vapors by condensing the vapors at a higher temperature after the compression. In this case, you only need the mechanical energy to operate the compressor.

Treatment of odorous substances in the vapors

Many drying processes produce odorous vapors, such as B. in the drying of sewage sludge, fermentation residues, but also with a variety of other natural products such. B. when drying grass. In the prior art, the widespread use of dryers fails in almost all cases because of the complex and expensive biofilters that are required to separate the odor-intensive organic substances. Often there is a lack of space or free floor space for these large-volume filters. This is e.g. B. the case with numerous sewage treatment plants, which are now very limited in terms of space due to the advancing technology and the addition of a large number of new cleaning and treatment stages for wastewater and sewage sludge.

In these processes according to the invention, the vapors are not diluted with air or other inert gases. They can therefore easily be cleaned of the organic substances in small filters without voluminous filter systems. The organic substances can also be further concentrated before cleaning if vapor condensation takes place. This further reduces the size of the filter.

Combination with a furnace

Alternatively, it is also advisable according to the invention to use the vapors in a furnace connected to the dryer. This use is prohibited with many dryers, as the amount of air that dilutes the vapors is considerably larger than the amount of air required for combustion. It is then no longer possible to conduct a fire.

If the dryer according to the invention is heated by combustion, the vapors formed during drying can, according to the invention, be fed directly to the combustion. The organic components are burned directly through the combustion and converted into carbon dioxide and water vapor. Provided there is a dust separation after the firing is provided, the particles entrained during the drying process can ideally be collected and disposed of together with the ashes of the fuel without being released into the atmosphere as pollutants.

The utilization of the vapors in a furnace is a simple way of burning the unpleasant odorous organic substances and also the dust. This is usually forbidden in the case of firing with a combustion grate, since then the integration of the vapors into the air management of the combustion is no longer sensible, since the required combustion temperatures can no longer be guaranteed. The vapors are preferably used as secondary air in fluidized bed combustion, which basically consists of pre-combustion in the fluidized bed and post-combustion above the fluidized bed (in terms of process technology after the fluidized bed); the fluidized bed combustion works in this area without excess air, so that the integration can take place while maintaining the combustion temperatures. According to the invention, this combination with a fluidized bed furnace has the further advantage that the waste heat in the heat transfer medium (flue gas) can largely be used to preheat the combustion air after it has left the heating pipes of the dryer, which is not possible with grate firing. The efficiency is optimized with a combination of dryer and fluidized bed combustion. This combination not only optimizes the process efficiency. It also advantageously avoids the otherwise complex and expensive use of a biofilter. The heat released during fluidized bed combustion would be used advantageously to heat the dryer. In this way, optimal synergies would arise, especially when using sewage sludge. The same applies if a machine is used in front of the dryer, e.g. B. an ORC system, a steam cycle, a Stirling engine or an indirectly heated gas turbine.

If the furnace is designed as pre-combustion and post-combustion with interposed hot gas dedusting, the vapors would preferably be added in the pre-combustion.

Inherent explosion protection for dusty goods

It is particularly advantageous with this design that the goods to be dried and the dried goods do not have to come into contact with oxygen-containing air. If the product forms dust and there is a risk of explosion, this can be avoided simply by drying in an air-free atmosphere. The dryer is therefore particularly suitable for drying dusty and explosive goods.

Energy and efficiency

The heat introduced into the contact dryer for thermal drying is used, in addition to a small amount for heating the material to be dried to the drying temperature, exclusively for the evaporation of the liquid to be expelled. The efficiency of the dryer is defined as the ratio of the heat used for evaporation to the heat introduced. This ratio is significantly higher in the contact dryer according to the invention than in convection dryers and is very close to 100%. There is also an efficiency advantage compared to other contact dryers, since the surface exposed to the environment is smaller and thus the heat losses are reduced. In a contact dryer according to the invention with, for example, 35 drying tubes, the surface area relevant for heat losses drops from 35 x 0.2 m in circumference (traditional contact dryer according to the prior art) to the circumference of the jacket with a diameter of 2.2 m and thus with a length of 8 m, the total surface area from 176 m ² to 55 m ² . In this example, the 40 or so heating pipes are also integrated into the jacket; the integration of the heating tubes in a traditional contact dryer according to the state of the art is not possible; In the prior art, this consequently leads to additional surfaces with heat losses in the external steam generator and the connecting pipes.

In addition, it should be mentioned that 35 drives would also be required in the prior art, whereas a single drive is sufficient for the contact dryer according to the invention. This is more favorable in terms of the efficiency of the drive elements (electric motors, frequency converters). Significant investment costs can also be avoided.

Inerting the product to be dried

It can also be that the goods to be dried should be exposed to a minimum temperature for a certain period of time in order to achieve appropriate effects. This can advantageously be achieved in the contact dryer according to the invention by setting the drying temperature. The desired effect can be, for example, sterilization or pasteurization, but also targeted killing of all active bacteria.

Operation in mobile use

The dryer can also be operated in a mobile application. This is particularly useful when waste heat is available, for example from the drive motors of trucks or ships. In this case, only surge brakes have to be installed in the water phase in order to avoid sloshing of the condensate as a reaction to the rolling, yawing and pitching movements of the means of transport.

Control of the contact dryer

Inside the heating pipe, a heat supply medium is passed through which supplies heat to the heating medium located in the jacket space via the walls of the heating pipe, the heating medium in the jacket space surrounding the heating pipe on the outside. The advantage of such a construction of the contact dryer according to the invention is that a controllable amount of thermal energy can be supplied to the heating medium via the heat supply medium passed through the heating pipes, so that the degree of dryness of the moist material to be dried in the drying pipes can be influenced in a controllable manner. A plurality of drying tubes and also a plurality of heating tubes are preferably provided in the jacket space. In such a case, the drying tubes are arranged as tube batteries in the jacket space, i.e. in the drying area of the jacket space. This also applies equally to the arrangement of several heating pipes, which are preferably also provided in the form of a pipe battery in the heating area of the jacket space.

According to the invention, such a contact dryer can be regulated with the aid of regulating the steam pressure in the jacket space. If the energy input into the heating medium in the shell space takes place directly with flue gas, the output of the unit that provides the heat would be regulated in such a way that the pressure in the shell space is constant.

According to the invention, the regulation of the drying temperature can be regulated via the dew point temperature. The dew point temperature can easily be measured. To adjust, the drying tubes are exposed to the material to be dried with a higher or lower amount of air (from the environment). In this case, the vapors (mixture of the ambient air and the steam) would preferably be sucked off in order to work dust-free and odor-free on the outside. If the temperature is to be above ambient pressure, according to the invention no further amount of air is given up, but the outflow of the then almost pure steam is passed through a throttle. The pressure in the throttle is adjustable and reduced. Alternatively, this principle can also be applied to drying under negative pressure (with no or little ambient air), with the vapors or steam then having to be compressed after leaving the dryer. The thermodynamic efficiency of the dryer increases in this case. This effect has to be weighed against the additional electrical consumption of the compressor.

According to a further embodiment, pressure sensors are connected to the drying tubes, by means of which the pressures prevailing in them can be fed as pressure signals to a control device, on the basis of which heat energy corresponding to the pressure values can then be fed to the heating tubes in such a way that the drying temperature in the drying tubes can be regulated as a function of pressure. The degree of drying of the moist goods in the drying tubes or with the contact dryer is determined by the intended use of the dried goods. In some applications, a slightly higher residual moisture content will be important, while for certain applications a relatively high degree of dryness is aimed for.

According to a further aspect of the invention, a method for drying a moist material, in particular biomass, is provided, which is operated with a contact dryer of the type described above. The material to be dried is passed in the same direction through one or more drying pipes running in an axially extending jacket space coaxially to the jacket space, while a heating medium located in the jacket space in its heating area, which - if present - surrounds at least one heating pipe, via which heat energy is added to the heating medium is supplied, the heating medium being fed to the drying tubes in the drying area of the jacket space, inside which the moist material to be dried is dried. A preferred method step for the method according to the invention is the detection of the pressure inside the drying tubes by means of a pressure sensor, the pressure signal generated by the pressure sensor being fed to a control device on the basis of which the drying process is controlled or regulated. In this way, a temperature can also advantageously be set at which organically contaminated goods such as. B. sewage sludge to be sanitized; In the case of sewage sludge, the minimum temperature for sanitation is 70 ° C.

However, according to a further development of the invention, it is also possible for the pressure to be controlled or regulated on the basis of the heating medium temperature and / or the conveying speed of the material in the drying tubes. If the energetic expenditure required for drying is not to be increased, but a higher degree of dryness of the moist material to be dried is to be achieved, such a higher degree of drying can be achieved by reducing the conveying speed of the material to be dried through the drying tubes.

With the contact dryer - compared to a convection dryer - a large number of drives and control units can be omitted, such as B. the fan for the drying air, the belt control, the belt drive, etc.

Infeed and discharge of the goods to be dried by means of a soulless screw

The use of a soulless screw as a conveying element allows free-flowing goods to be easily fed into the drying tubes by pouring over the screws in the feed area. Overfilling the dryer can be avoided simply by changing the slope or placing a core pipe in the entry area. The screws are preferably offset and arranged in pairs. The entry in the individual pairs is preferably made in steps or steps from a common template. This template can consist of inclined walls to avoid bridging and / or be provided with a loosening device, as it is, for. B. represents a vibration drive, one or more compressed air nozzles or a spindle.

The soulless screws are preferably driven from the discharge side so that the screw spirals are subjected to tension, not pressure.

For pasty or sticky goods such as B. sewage sludge or digestate, the entry could also take place directly via pumps and throttles, if necessary with automatically controlled valves.

The conveying device in the interior of the drying tubes for transporting the goods to be dried is preferably designed as a soulless screw, which can preferably be subjected to tensile loads.

If the goods to be dried are to be conveyed through the dryer relatively quickly or the goods are unusually abrasive, the drying tubes can be provided with wear protection on the inside.

Modular construction

The preferred or optimal size fits well with a modular design in 20 or 40 ft containers, so that the systems can be used in a modular manner, in particular can also be used at different locations.

Embodiments

Fig. 1:

eine erste Ausbildung des erfindungsgemäßen Kontakttrockners,

Fig. 2:

eine zweite Ausführung des erfindungsgemäßen Kontakttrockners,

Fig. 3:

ein schematischer Aufbau eines Vorlagebehälters für einen erfindungsgemäßen Kontakttrockner,

Fig. 4:

eine Draufsicht des Vorlagebehälters aus Fig. 3,

Fig. 5:

eine Vorderansicht des Vorlagebehälters aus Fig. 3 und

Fig. 6:

eine schematische Darstellung einer als Schneckenwendel mit Kernrohr ausgebildeten Födereinrichtung

Further advantages and specific embodiments of the invention will now be shown in detail on the basis of an exemplary embodiment with reference to the drawing. It shows:

Fig. 1:

a first embodiment of the contact dryer according to the invention,

Fig. 2:

a second embodiment of the contact dryer according to the invention,

Fig. 3:

a schematic structure of a storage container for a contact dryer according to the invention,

Fig. 4:

a top view of the storage container Fig. 3 ,

Fig. 5:

a front view of the storage container Fig. 3 and

Fig. 6:

a schematic representation of a conveyor device designed as a screw helix with a core tube

Fig. 1 shows the first embodiment according to the invention of a contact dryer (1) as a natural circulation steam generator with an integrated condenser for the thermal drying of a material to be dried (2) with the aid of an evaporating heating medium (3).

The spatial extent of the contact dryer is defined by the jacket tube (6) and the two end regions (7, 8). The jacket pipe (6) is designed so that pressures of up to 40 bar (temperatures of approx. 250 ° C) can be achieved, the pressure being determined by the heating power introduced into the heating medium (3). The length of the jacket pipe (6) is variable and according to the invention can be structurally adapted to the heat requirement for the drying or the required area. The end areas (7, 8) can be easily constructed using perforated plates, at least in the drying area (4).

The drying area (4), which comprises the drying tubes (9), is located in the upper area of the jacket tube (6). In the latter are the conveying organs (10) - designed here as conveying spirals - as well as the material to be dried (2) conveyed by them. The drying tubes (9) run from the entry-side end area (7) to the discharge-side end area (8) coaxially to the jacket tube (6), are physically separated from one another and penetrate both end areas (7, 8). In them, the material (2) to be dried is conveyed through the contact dryer (1) by means of conveying elements (10) and dried by introducing heat. The dried material (11) leaves the contact dryer (1) at the discharge-side end area (8) with a defined residual water content. According to the invention, the residual water content can be regulated via the drying temperature and the conveying speed.

The heating area (5), which contains a defined number of heating tubes (12), is arranged in the lower area of the jacket tube (6). The heating pipes (12) run from the inlet end region (7) to the discharge end region (8) coaxially with the casing tube (6), penetrate both end regions (7, 8) and have a hot heat supply medium (13) flowing through them. The heat contained in the hot heat supply medium (13) is released during the flow to the heating pipes (12) and through them to the heating medium (3). The cooled heat supply medium (14) leaves the jacket tube on the discharge side.

The heat absorbed by the heating medium (3) leads to the evaporation of the heating medium (3) in the heating area (5) at constant temperature and constant pressure. The resulting steam rises from the heating area (5) to the drying area (4) and condenses there at the same pressure and temperature as during evaporation at the drying tubes (9), releasing its latent heat to the drying tubes (9) and the inside drying good (2). This ensures that the drying tubes (9) cannot under any circumstances assume a higher temperature than the vapor phase of the heating medium (3) and thus the material to be dried (2) cannot be exposed to a hotter temperature than the evaporation temperature at any time. The condensate runs down the drying pipes (9) and drips back into the heating area (5) where it is evaporated again. This results in a closed natural circulation, which ensures heat transport between the heating area (5) and drying area (4) without further units or connections with the environment.

The thermal drying process releases water in vapor form within the drying tubes (9), which mixes with any air that may be present to form vapors (15). These must be removed from the drying tubes (9) in order to ensure a high level of efficiency. This is implemented by a suction device (16) which is connected to the drying tubes (9) when the discharge-side end region (8) passes through. As a further advantage, the evacuation of the vapors (15) also results in the evacuation of dust that occurs as a result of the movement of the conveying elements (10) in the material (2) to be dried. With the help of the air supply (19) and the control valve (20), the proportion of air in the vapors can be controlled as required. This allows the drying temperature to be freely regulated.

If the drying takes place above atmospheric pressure, the control valve (20) would be arranged at the outlet and the fan (16) at the inlet of the drying tubes (9).

On the one hand, the conveying elements (10) take over the dosing of the goods to be dried (2) and thus regulate the filling level of the drying tubes (9), on the other hand they define the dwell time of the goods to be dried (2) within the drying area (4) via the conveying speed. . The conveying elements (10) are driven on the discharge side, so that the conveying elements (10) are subjected to tensile stress. Depending on the water content of the goods (2) to be dried and the desired residual water content in the dried goods (11), the residence time can be adapted according to the invention so that enough water can be removed from the goods (2) to be dried, depending on the drying temperature.

The dryer is supplied with material to be dried by filling the storage container (21) to above the level of the uppermost drying tubes (9). The metering itself takes place with the help of a variable speed of the conveying elements (10) and - associated with this - a different amount of conveyed good to be dried. In addition, the delivery rate can be regulated by changing the pitch of the screw helix of the conveyor element (10) or partially blocking the cross section of the screw helix in the storage container (21), for example by means of core tubes. For this purpose, the drying tubes (9) are offset and arranged in pairs, the pairs being arranged in steps or steps.

The structurally complex introduction of heat via the conveying elements (10) can be omitted due to the sufficient heating surface and the conveying elements (10) can be designed simply as a screw helix.

Fig. 2 shows a second embodiment of a contact dryer (1) according to the invention as a natural circulation steam generator with an external steam generator for thermal drying of an item (2) to be dried with the aid of an evaporating heating medium (3). While the drying area (4) is identical to that in Fig. 1 The illustrated embodiment of the contact dryer (1) and the heat transfer between the heating area (5) and drying area (4) also takes place in the same way, the difference in FIG Fig. 2 The illustrated embodiment of the contact dryer (1) in the design and positioning of the heating area (5).

The heating area (5) in Fig. 2 is in this case not contained in the casing pipe (6), but arranged below the casing pipe (6) and connected to the casing pipe by a riser pipe (17) and a downpipe (18). In this embodiment, the hot heat supply medium (13) flows around the heating pipes (12) and the heating medium (3) flows through it. The heat dissipation of the hot heat supply medium (13) leads to evaporation of the heating medium (3), which rises in vapor form in the riser pipe (17) and thus reaches the jacket pipe (6) and the drying area (4). In the drying area, the condensation of the heating medium (3) takes place with the release of latent heat to the drying tubes (9) and a reflux into the lower part of the jacket tube (6). From there the condensate returns to the heating area via the downpipe (18). Here, too, there is a closed natural cycle. If the downpipe (18) and the riser pipe (17) are separated, the heating pipes (12) are preferably slightly inclined, namely rising towards the riser pipe (17). Significant advantages result from the fact that the area available for the heat transfer on the side of the heat supply medium (13) can be increased considerably in a structurally simple manner by means of ribs. Furthermore, the overall size can be reduced by relocating the heating area (5) and / or, if necessary, the number of drying tubes (9) can be increased.

Regardless of the type of design, several contact dryers (1) can be connected in parallel or in series, provided that it is advantageous for drying.

The contact dryer (1) is regulated by regulating the steam pressure in the jacket tube (6) via the heat output introduced by the heat supply medium (13). The steam pressure determines the temperature at the drying tubes (9). The drying temperature can be regulated by drawing off the vapors (15) from or by supplying air (19) into the drying tubes (9) by means of the control valve (20).

The figures Fig. 3, Fig. 4 and Fig. 5 show a storage container of a contact dryer according to the invention from different perspectives. Fig. 6 a conveyor device or organ (10) designed as a screw helix with a core tube (22).

The material to be dried (2) is fed into the storage container (21) by means of an upstream conveyor system such as, for example, screw conveyors, conveyors, conveyors or solids pumps. The storage container (21) itself is preferably designed to be gas-tight and pressure-tight at operating pressures that differ from the ambient pressure, in order, for example, to be able to ensure the removal of vapors and odorous substances and to minimize the entry of false air. The walls of the storage container (21) are advantageously inclined and tapered upwards in order to counteract the formation of bridges between the materials used. In addition, pneumatic discharge aids (e.g. air / gas nozzles), oscillating discharge aids (e.g. vibrators, shakers, knockers, vibration grates), rotating discharge aids (e.g. clearing arms, rotating fixtures, paddle shafts) or discharge devices (e.g. B. discharge screws) in or on the storage container (21) to ensure that the storage container (21) is completely emptied. The material to be dried (2) is filled into the storage container (21) up to above the level of the uppermost drying tubes (9). With a corresponding, uniform and constant filling level, this also ensures that the interior of the drying tubes (9) is sealed against the environment and thus reduces the introduction of additional air that has to be heated into the drying area (4).

Inside the storage container (21), the conveying elements (10) of the contact dryer (1) - designed here as a screw helix - are open and are covered with the material (2) to be dried. On the one hand, the conveying elements (10) take over the dosing of the goods to be dried (2) and thus regulate the filling level of the drying tubes (9), on the other hand they define the dwell time of the goods to be dried (2) within the drying area (4) via the conveying speed. . The conveying elements (10) are driven on the discharge side, so that the conveying elements (10) are subjected to tensile stress. Depending on the water content of the material to be dried (2) and the desired residual water content in the dried material (11), the residence time can according to the invention be adapted so that enough water can be withdrawn from the item to be dried (2) depending on the drying temperature. The metering itself takes place with the aid of a variable speed of the conveying elements (10) and - associated therewith - a different conveyed amount of material to be dried. In addition, the delivery rate can be regulated by changing the pitch of the screw helix of the conveyor element (10) or partially blocking the cross section of the screw helix in the storage container (21), for example by means of core tubes (22). The core tube can either be firmly connected to the screw or attached to the rear wall or attached to an end plate attached to the drying tube. In the first case the core tube (22) would rotate with the screw, in the other two cases the screw would rotate around the core tube (22). The core tube (22) advantageously protrudes from the storage container (21) and into the inlet (24).

A limitation as a metering aid (23) can be arranged on the underside of the conveying elements (10). The dosing aid (23) can be designed, for example, as a bowl, mesh basket or rods. Fig. 4 ). Only in the case of free-flowing goods can the distance be selected to be larger, since the goods flow by themselves into the conveying elements (10) located below. In the case of a drying tube (9) with an outside diameter of, for example, 168.3 mm and a wall thickness of 4 mm, the result is a conveying cross-section (inside diameter) of 160.3 mm. If you now arrange two drying tubes (9) in the same plane with a center distance of 320 mm and another one in the middle below, the entire filling cross-section is discharged. This arrangement of several horizontal planes is called a step. Several stages of drying tubes (9) can be arranged in the dryer. If the item (2) to be dried is not free-flowing, trays are preferably arranged as metering aids (23). The entry areas of the various stages are arranged axially offset in this case.

If the material to be dried (2) is a non-free-flowing material (e.g. pasty goods such as sewage sludge), instead of metering via core tubes (22) and screw pitch, metering via displacement pumps (e.g. diaphragm pumps) can also be used , Piston pumps, eccentric screw pumps, rotary piston pumps or peristaltic pumps).

The material (2) to be dried is metered into the drying tube (9) via an inlet (24) and then dried in the contact dryer (1) as described above. At the discharge end (8) of the contact dryer, the conveyor elements (10) transfer the dried material (11) into an outlet. From this, the dried material (11) falls into a collecting container. Like the storage container (21), this is preferably gas-tight and pressure-tight, in order to be able to guarantee an extraction of the vapors and odorous substances and to work at operating pressures that differ from the ambient pressure.

The joint discharge of the dried material (11) from the collecting container takes place with the help of conveying technology such as, for example, conveying screws, conveying plates, conveying belts or solids pumps.

Reference list

1

Contact dryer

2

Good to be dried

3

Heating medium

4th

Drying area

5

Heating area

6th

Jacket pipe

7th

Entry-side end area

8th

Discharge-side end area

9

Drying tube

10

Funding body

11

dried goods

12th

Heating pipe

13th

Hot heat supply medium

14th

Cooled heat supply medium

15th

Vapors

16

Suction device

17th

Riser pipe

18th

Downpipe

19th

Supply air into the drying tubes

20th

Control valve for supply air to the drying tubes

21

Storage container

22nd

Core tube

23

Dosing aid

24

enema

Claims (13)

1. Contact dryer for drying moist material, consisting of at least one drying tube (9), in which the material (2) to be dried can be conveyed and on the outer side of which there is a heating medium (3) in a casing tube (6) at least partially surrounding the drying tube (9), wherein at least one further drying tube (9) is arranged in the casing tube (6), formed as a pressure vessel, and the at least two drying tubes (9) are formed in such a way that the material (2) to be dried cannot mix along at least one longitudinal section of the at least two drying tubes (9), wherein the casing tube (6) does not perform a rotating movement and the heating medium (3) is water vapour or an organic medium, which during operation changes to an at least partially gaseous or vaporous phase when heat is supplied and condenses back to the liquid phase when heat is removed, wherein in the casing tube (6) there is at least one heating tube (12), through which heat can be supplied to the heating medium (3), and the heating tube (12) is arranged in such a way that the heating medium (3) at least partially surrounds the heating tube (12) during operation or that the at least one heating tube (12) is outside the casing tube (6) and is connected to the casing tube in such a way that the heating region, in which the at least one heating tube (12) is arranged, and the drying region, in which the at least two drying tubes are arranged in the casing tube (6), represent a common pressure vessel, wherein the drying region is arranged above the heating region in such a way that the condensate from the drying region flows into the heating region, driven by gravitational force.
2. Contact dryer according to claim 1, characterized in that connected to the at least two drying tubes (9) is a suction device (16), by means of which vapours can be sucked away.
3. Contact dryer according to one of the preceding claims, characterized in that the at least two drying tubes (9) each have a conveying device (10), with the aid of which the material to be dried can be conveyed.
4. Contact dryer according to Claim 3, characterized in that the conveying device (10) is designed as a shaftless screw or a conveying plate.
5. Contact dryer according to Claim 4, characterized in that the shaftless screw is subjected to tensile loading during operation.
6. Method for drying a moist material, in particular biomass, with a contact dryer according to one of Claims 1 to 5, wherein the material (2) to be dried passes in the same direction through a number of drying tubes (9) running parallel to one another in the axially extending casing tube (6), and is dried inside said tubes, while a heating medium (3) located in a heating region is subjected to thermal energy, is introduced into the casing tube (6) surrounding the drying tubes, on the outer side of the drying tubes gives off heat to the drying tubes (9) and thereby at least partially changes phase, wherein the heat given off is used for drying the moist material in the drying tubes (9) and the casing tube (6) does not perform a rotating movement and the heating medium (3) is water vapour or an organic medium which during operation changes to an at least partially gaseous or vaporous phase when heat is supplied and condenses back to the liquid phase when heat is removed.
7. Method according to Claim 6, characterized in that in the casing tube (6) a pressure that is different from ambient pressure is imposed for the heating of the dryer.
8. Method according to Claim 6 or 7, characterized in that, by active control, a predefined pressure that differs from ambient pressure is imposed in the drying tubes (9) of the drying region for the drying.
9. Method according to Claim 8, characterized in that the predefined drying temperature is set by the pressure in the drying tubes (9).
10. Method according to Claim 8 or 9, characterized in that the drying tubes (9) of the drying region are subjected to different temperature levels.
11. Method according to one of Claims 6 to 10, characterized in that the vapours of the dryer are fed to the combustor, by which heat for the drying process is provided.
12. Method according to Claim 11, characterized in that part of the material to be dried and/or part of the dried material is fed to a combustor, and thereby provides heat for the drying.
13. Method according to one of Claims 6 to 12, characterized in that the dryer is heated by the compressed vapours.

Images