WO2013117637A2

WO2013117637A2 - Method for drying a product treated with a substance containing a solvent

Info

Publication number: WO2013117637A2
Application number: PCT/EP2013/052423
Authority: WO
Inventors: Dimosthenis Trimis; Stephan ANGER; Isabel FRENZEL; Heiko Schneider
Original assignee: Technische Universität Bergakademie Freiberg; Gogas Goch Gmbh & Co. Kg
Priority date: 2012-02-11
Filing date: 2013-02-07
Publication date: 2013-08-15
Also published as: DE102012002865B4; DE102012002865A1; WO2013117637A3

Abstract

The invention relates to a method for drying a product (2) in a drying system, the product (2) having been treated with a substance containing an organic solvent, said method comprising the steps: a) heating the product (2) by means of infrared radiation (21) and evaporating the solvent, forming a solvent vapour; b) discharging the solvent vapour from the drying system; c) supplying at least a part of the solvent vapour to a burner (3) and burning the solvent vapour with the burner (3); and d) using the infrared radiation arising during the combustion of the solvent vapour (21) to heat the product (2) in step a).

Description

Process for drying a product treated with a solvent-containing substance

The invention relates to a method for drying a product in a drying plant, wherein the product is treated with a substance containing an organic solvent.

The process according to the invention can be used in all areas where products contacted with organic-solvent-containing and combustible substances have to be dried. The products should be geometrically designed in such a way that they can be supplied or removed via narrow inlet and outlet areas of the drying chamber, in order to prevent solvent vapor from escaping uncontrolled to the outside, or large quantities of air from the outside into the plant interior , Applications for the present invention include the coil coating and the drying of products that can be fed by means of conveyor belt through the narrow inlet and outlet areas of the drying chamber due to their low height. In general, drying is understood to mean the removal or reduction of the liquid content (moisture) in a solid by thermal treatment, wherein the liquid is converted into the vapor phase and mixed with a force-fed gas, eg. B. a foreign gas is transported away. Previous drying systems are operated in such a way that the composition of the solvent vapor / air mixture produced during drying in the drying chamber is kept below the lower explosion limit so that an ignitable mixture does not arise at any time in the entire system. EP 0 869 323 A2 describes a dryer for material webs with exhaust gas recirculation, in which heated gas is generated by a burner unit and distributed in the dryer with at least one fan in order to feed the material webs dry. A large part of the exhaust gas produced in the dryer is used as process gas for the burner unit (process air burner) and only the remainder is thermally post-combusted. The process air burner is shielded from the atmosphere in the dryer, and the combustion mixture is maintained at a sufficient temperature level for a sufficient residence time to completely combust volatile solvents before the resulting burned gas is returned to the interior of the dryer.

EP 0 582 077 A1 discloses a drying system for continuous product webs which release volatile solvents during the drying process. The drying air in the dryer must be kept at a temperature of about 200 ° C to allow the solvents to volatilize. Furthermore, the concentration of the solvent vapors in the dryer atmosphere must not exceed a predetermined value. Therefore, exhaust gas must be sucked out of the dryer and replaced with a corresponding amount of fresh air. The solvent-containing exhaust gas is thermally post-combusted and the resulting significant amount of heat is partially recovered by the resulting hot clean gas is partially returned to the dryer and / or used to heat a heat carrier.

A dryer with infrared heating is described in DE 10 2007 025 760 A1. The workpieces are first heated in the drying chamber by an infrared heater and then dried within the drying chamber with fan support. The infrared heater is heated by a burner whose flue gases flow into a flue gas channel communicating with at least one wall surface to heat the workpiece by infrared radiation.

EP 0 203 377 A1 describes a blast tunnel for drying painted workpieces. In the blast tunnel, a plurality of radiators (infrared radiators) and blowing nozzles are arranged on the tunnel walls and / or ceilings, which are connected to an air conveying system and supply air to the workpieces to be dried. Pieces blow. If varnishes with solvents which evaporate heavily are used for varnishing, then either a longer evaporation distance or a longer residence time in the blast tunnel is required compared to conventional varnishes. DE 696 04 31 1 T2 discloses a method for drying a moving material web which has a coating containing volatile substances. In this case, solvent-containing air is gradually heated, which recirculates in a drying housing. Also disclosed is a method for optimally controlling and directing the solvent-containing air. This effectively reduces or eliminates condensation and settling of moisture from solvent and solvent-based by-products. In addition to reducing the condensate, better and more uniform mixing of the atmosphere in the drying enclosure is achieved, thereby increasing safety and improving the drying process, as pockets of high concentration of solvent vapors are reduced.

DE 36 35 833 A1 relates to a continuous dryer for material webs, in which each supplied with gas and circulating air of the dryer heater and afterburner are combined into one unit and in which a closed combustion chamber is integrated in the dryer housing. It eliminates the risk of cracking of the volatile during drying and located in the dryer atmosphere solvent because the dryer atmosphere does not come directly with very hot elements, especially not with an open burning in the dryer flame in touch.

In conventional processes for drying products treated with organic-solvent-containing substances, the resulting solvent vapors are discharged with preheated air below the lower explosive limit, which can only be achieved by an enormously high air volume flow. The air volume flows to be applied require a significant amount of energy, which must be applied continuously via appropriate compressor capacities and heat input. The resulting from the drying process Solvent vapor-air-Gennisch can not be burned directly below the lower explosion limit, but must be implemented via a thermal afterburning, with an additional fuel gas entry is necessary. Heat, which is released here, must be returned to the system via heat transfer systems to save energy with a high constructional effort. A further disadvantage is the significantly high exhaust gas volume flow which sets due to the high air volume flow entered. Furthermore, only by suitable procedural measures local ignitable mixtures and thus a risk of explosion in the drying chamber can be avoided.

The object of the invention is to eliminate the disadvantages of the prior art. In particular, a drying process of products treated with organic-solvent-containing substances should be designed so that the solvent vapors can be discharged without high volume flows of drying air. The volume flows to be applied should be maintained with minimal energy expenditure and the formation of ignitable mixtures in the drying section should be avoided.

This object is solved by the features of claim 1. Advantageous embodiments of the invention will become apparent from the features of claims 2 to 1. 1

For drying a product treated with an organic solvent, a drying plant is advantageously operated above the upper explosion limit. A solvent vapor produced during the drying process is burned in a burner system. A generated infrared radiation is coupled into the drying process and used directly thermally. Mixtures of combustible organic solvents are explosive with the oxygen contained in air at certain mixing ratios. One area, which summarizes all explosive mixing ratios, becomes of two Explosion limits, the upper and lower explosion limit described. Below the lower explosion limit, the concentration of the flammable substance is so low that the mixture is not flammable. Above the upper explosive limit, the concentration of the flammable substance in the mixture is so high that the mixture is also non-flammable. The lower and upper explosive limits depend on the type of combustible substance and its concentration in the oxygen-containing gas, for example in air.

Exemplary embodiments of the invention will be explained in more detail below with reference to the drawings. Show it:

1 shows schematically a first process variant,

2 shows schematically a second variant of the method,

Fig. 3 shows schematically a first power current model and

Fig. 4 shows schematically a second Energiestrommodell. According to the invention, two preferred process variants are possible, which are shown in FIGS. 1 and 2. In both process variants, the removal of the solvent vapor takes place completely without the entry of dry air into a drying chamber 1 of the drying plant. In the first variant of the method shown in FIG. 1, the composition of the pure recirculating solvent vapor in the drying plant is far above the upper explosion limit. In continuous operation, the dry material 2 is continuously passed through the drying chamber 1 at the required operating temperature, the solvent completely evaporating. To ensure this, the operating temperature is kept above the evaporation temperature of the solvent. The solvent is removed via a Rezirkulationsstrang 10, which includes only solvent vapors. The transport takes place via a compressor module 4. The thermal energy for evaporating the solvent is generated by the burner system 3 Infrared radiation energy 21 coupled, wherein a combustion chamber and drying section or the drying chamber 1 by at least one infrared transparent glass module 6 are separated from each other. The drying chamber 1 can be subdivided into two zones, wherein a first zone alternately has infrared radiators and tuyeres and the second zone has only tuyeres.

The burner exhaust 13 is discharged separately and can continue to be used thermally. The necessary energy for the burner system 3 is obtained directly from the chemically bound energy of the solvent vapor, which is supplied via a first flow divider 15, mixed in a mixer 5 with the necessary combustion air 9 and immediately burned. The flow rate required in the drying chamber 1 for the drying process is set via the compressor module 4. In the continuously running process, the system is permanently deprived of an amount of solvent vapor via the burner device, which corresponds to the required burner power. Since usually more solvent is evaporated than is necessary for the operation of the burner device and to regulate the operating pressure of the system, excess solvent vapor is removed via a third flow divider device 17 from the drying plant. The third flow divider device 17 is used during the startup and shutdown processes in order to remove previously introduced inert gas from the drying plant. Excess solvent vapor can be thermally converted as required via a separate burner system (not shown here), the heat energy can be coupled out at a high temperature level at other heat sinks in the process (example: belt preheating). Alternatively, the excess solvent vapor can be condensed out and recovered. In order to avoid an uncontrolled escape of the solvent vapor from the system, the drying process is operated with a slight negative pressure (about 5 Pa below the ambient pressure). In this case, only a minimal proportion of oxygen with the ambient air enters the system, wherein the composition of the resulting solvent vapor mixture is still kept well above the upper ignition limit. To monitor the minimum O2 entry, an oxygen measurement in the system can be used. Age- natively, the system can be operated with a slight overpressure, in which case oxygen is not introduced at any point in the system, the consideration of the system then takes place beyond consideration of the explosion limits. At all inlets and outlets of the system corresponding exhaust devices are to be provided, through which small amounts of escaping solvent vapors are discharged through leaks. The compressor module 4 must then be positioned in the recirculation line in front of the drying chamber inlet 7 in order to realize a slight overpressure. One possible procedure for starting up the process line is the following: The recirculation line 10 is first closed and rendered inert at the drying chamber inlet 7 and at the drying chamber outlet 8. The inert gas (nitrogen, carbon dioxide or the like) is fed via the supply line 12 and the second flow divider 16 and circulated via the compressor module 4. The system is first premixed with conventional fuel (natural gas, propane gas) 14 and combustion air 9 in a mixer 5 and burned over the burner system 3. The thermal energy released in this process is entered directly in the process. The burner system 3 decouples a large part of the combustion energy in the form of IR radiation 21. This is introduced into the drying chamber 1 via an infrared-transmissive glass module 6. When the operating conditions are reached, the drying chamber 1 is opened. The dry material 2 is introduced via the dry chamber inlet 7, wherein the evaporation begins immediately. The removal of the inert gas-solvent vapor mixture via the burner system 3, which is thermally reacted in the starting process by the support fuel 14. The burner system 3 assumes thereby simultaneously the task of a thermal afterburning. During this process, the partial pressure of the inert gas in Rezirkulationsstrang 10 decreases. Depending on the still existing inert gas and the associated flammability of the mixture of externally supplied conventional fuel 14 is gradually substituted by the solvent vapor until the burner system 3 is operated completely self-sufficient with the solvent vapor mixture. The second process variant shown in FIG. 2 describes a modified form of the first process variant. Analogously to process variant 1, the drying process is operated in a further recirculation line 11. The difference to method variant 1 is that not only pure solvent vapors are moved in the further recirculation line 11, but also burner exhaust gases. These burner exhaust gases 13 are supplied to the recirculating solvent vapor via the distributing devices 19 and 20 of the burner system 3. The burner exhaust gases 13 contain by the combustion process only a small proportion of oxygen, the system is located above the upper explosion limit. The oxygen content in the recirculating solvent vapor-burner exhaust gas mixture can be monitored via an oxygen measurement. The aim of the burner exhaust gas supply is a targeted influencing of the process parameters in the other recirculation strand 1 1 for optimizing the drying. Parameters which can be influenced thereby include, among others, the partial pressure of the solvent vapors, with an influence on the condensation temperature and the combustion characteristics in the burner system 3. Analogously to process variant 1, the combustion chamber and drying chamber 1 are separated by an infrared-transmissive glass module 6. The starting process is analogous to process variant 1. For the two described variants of the method, the burner system 3 decouples a large part of the combustion energy, preferably in the form of infrared radiation. As a burner, an infrared radiant burner or pore burner can be used. In this case, a pore burner system is able to decouple a large part of the combustion energy in the form of IR radiation stably in a wide power modulation range. The pore burner is in particular able to convert low calorie combustion gases without supporting fuel. Especially in the combustion of solvent vapor, an exhaust gas quality is achieved in which no unburned hydrocarbons are detectable. For both process variants described above, the entire system is far above the upper explosion limit. Therefore, no ignitable mixture is formed throughout the system. The solvent vapor can be thermally reacted directly without thermal afterburning. The heat energy released in the process is used in the form of the infrared radiation generated by the burner system 3 via the glass module 6 directly in the process of drying.

EXEMPLARY EMBODIMENT The following consideration shows, on the basis of an energetic consideration, the potential savings of process variant 1 of the drying installation according to the invention compared with the conventional procedure of a drying installation. The basis for the comparison are typical plant data of a coil coating plant published by the European Coil Coating Association (ECCA), see Table 1.

Table 1: Characteristics of the coil coating process [www.ecca.de]

The following system data is assumed for the calculation:

• belt system: aluminum belt, belt width 1500 mm, belt thickness 1 mm, belt speed 100 m / min, density aluminum 2.7 g / cm ³ , average spec. Heat capacity 926 J / (kg K), system temperature 270 ° C. · Solvent: Commercially available solvent used in coil coating: Calorific value 34193 kJ / kg, layer thickness 160 μηη (solvent content 34 mass%), density (liquid Tu) 0.9 kg / l, solvent input 450 kg / h, density (gas., T _B = 270 ° C) 0 , 6 kg / m ³ , evaporation enthalpy 46.9 kJ / mol, molar mass of solvent 139 g / mol, lower explosion limit 52 g / m ³ (20 ° C, 1013 hPa).

• No loss of heat flows are considered.

FIG. 3 shows a simplified first power current model (without heat losses) of a conventional drying process. The coated strip 2 is fed with a chemically bound burn energy of 4274 kW. In the belt preheating 22, the belt is preheated to 270 ° C by the exhaust gases 13 of the thermal afterburning 24, this 1563 kW are required, the energy requirement of the solvent preheating can be neglected. Furthermore, it is assumed in a simplified manner that no solvent evaporates in the region of the strip preheating 22. In the drying chamber 1 only 42 kW are needed to evaporate the solvent. To determine the air volume flow 23, which is required for evaporation and removal of the solvent vapor from the drying chamber 1, it is assumed, taking into account DIN EN 1539, that the resulting solvent vapor-air mixture 26 in the drying chamber is 10% below the lower one Explosion limit is maintained. This corresponds to a required air volume flow of 43270 Nm ³ / h. The dried strip is then discharged without cooling from the drying chamber 1, the final solvent vapor-air mixture 26 is the thermal afterburning 24 supplied. A conventional thermal afterburning works under consideration of a sufficient residence time of the fuel-air mixture in the combustion chamber at about 900 ° C. This ensures a complete conversion of the hydrocarbons to CO2 and H ₂ O. Taking into account the chemically bound energy in the solvent vapor-air mixture 26 7102 kW must be supplied as additional fuel gas stream 25 of the thermal afterburning 24 to achieve this exhaust gas temperature. Here, no combustion chamber losses are taken into account. The enthalpy flow contained in the burner exhaust 13 is 15358 kW. 3982 kW are then connected via a heat exchanger system 27 to preheat the dry air 23 and transfer another 1563 kW for belt preheating 22. Per meter of belt to be dried in this conventional variant 1, 896 kJ entered into the system. The exhaust gases from the thermal afterburning 13 and the dried strip produce 1 1376 kW, which must be decoupled at other heat sinks.

FIG. 4 shows a simplified second energy current model of the drying installation according to the invention according to process variant 1. As in the conventional drying process, 4274 kW are supplied to the system as a chemically bound burn energy stream with the coated strip 2. 1563 kW are converted for the strip preheating 22 and 42 kW for the evaporation of the solvent in the drying chamber 1. As shown in the present invention description, the circulating solvent vapor is supplied directly to a burner system 3. In the present power model, the belt preheating 22 is thermally powered by the same burner system 3. In the embodiment of the drying system according to the invention, a pore burner system is to be used, which converts about 40% of the fuel energy in the form of infrared radiation. Taking into account the infrared component, 1 18 kW is decoupled for drying and 3908 kW for belt preheating. Per meter of belt to be dried in this process variant 0.712 kJ registered in the system. By the combustion exhaust gases 13 and the dried band 3984 kW are discharged, which must be decoupled at other heat sinks. In addition, 290 kW of chemically bound energy are discharged as unburned solvent vapor. This can then be condensed out and recovered.

In summary, it is found that with the process variants according to the invention compared to conventional drying systems, the fuel input through the backup firing is saved to 100%. The required thermal energy of the process can be completely covered by the chemically bound energy in the solvent. With the help of pore burner technology, a residue-free conversion of the solvent vapors is possible. LIST OF REFERENCE NUMBERS

1 drying chamber

2 dry goods

3 burner system

4 compressor module

5 mixers

6 Infrared transparent glass module

7 dry chamber entry

8 drying chamber outlet

9 combustion air

10 recirculation line

1 1 more recirculation strand

12 inert gas supply

13 burner exhaust gas

14 supporting fuel for starting processes

15 first flow divider device

16 second flow divider device

17 third flow divider device

18 fourth flow divider device

19 fifth flow divider device

20 sixth flow divider device

21 infrared radiation

22 tape preheating

23 dry air conventional procedures

24 thermal afterburning

25 Support fuel for thermal afterburning

26 Solvent-air mixture below the lower explosion limit

27 heat exchanger system

Claims

claims

1 . Process for drying a product (2) in a drying plant, the product (2) being treated with a substance containing an organic solvent, comprising the following steps: a) heating the product (2) by means of infrared radiation (21) and evaporating the product Solvent under formation of a solvent vapor, b) removal of the solvent vapor from the drying plant, c) feeding at least a portion of the solvent vapor to a burner (3) and burning the solvent vapor with the burner (3), and d) using the resulting when burning the solvent vapor Infrared radiation (21) for heating the product (2) in step a).

2. The method of claim 1, wherein in step a) a concentration of the solvent in a drying chamber (1) of the drying plant is kept above the upper explosive limit.

3. The method of claim 1 or 2, wherein a further portion of the solvent vapor in the drying chamber (1) is recirculated without foreign gas input.

4. The method of claim 3, wherein the removal of solvent vapor in step b) by supplying combustion exhaust gas (13) and / or the recirculation of the solvent vapor is supported.

5. The method according to any one of the preceding claims, wherein the drying plant is operated at negative pressure.

6. The method according to any one of claims 1 to 4, wherein the drying plant is operated at overpressure.

7. The method according to claim 6, wherein in the region of an inlet and outlet of the drying chamber (1) exiting solvent vapor is sucked by means of a suction device.

8. The method according to any one of the preceding claims, wherein as burner (3) a pore burner is used.

9. The method according to any one of the preceding claims, wherein the burner (3) is received in a burner chamber and the infrared radiation (21) by a the burner chamber from the drying room (1) separating infrared transparent glass module (6) is coupled into the drying chamber (1).

10. The method according to any one of the preceding claims, wherein for starting the drying plant, the drying chamber (1) is loaded with inert gas and then the inert gas is continuously substituted by solvent vapor.

1 1. Method according to one of the preceding claims, wherein an excess portion of the solvent vapor is discharged from the drying plant.