EP0072836A1

EP0072836A1 - Method for decreasing the heat losses in a drying plant for solid bodies

Info

Publication number: EP0072836A1
Application number: EP82900634A
Authority: EP
Inventors: David Hodgett
Original assignee: Battelle Institut eV
Current assignee: Battelle Institut eV
Priority date: 1981-02-21
Filing date: 1982-02-17
Publication date: 1983-03-02
Also published as: WO1982002939A1; DE3106567A1

Abstract

Pour reduire les pertes de chaleur dans une installation de sechage de corps solides, dans laquelle la vapeur (1) vehicule la chaleur et qui degage de l'air d'evacuation humide (6) a des temperatures superieures a environ 65 C, l'air humide (6) et l'eau de condensation (4) sont conduits vers une pompe a chaleur a resorption. Cote resorption (7) de la pompe a chaleur, on soutire de la chaleur a l'air humide et on obtient de la vapeur cote resorption (8).To reduce heat loss in a solids drying installation, in which the vapor (1) conveys heat and which gives off humid exhaust air (6) at temperatures above about 65 C, the humid air (6) and condensed water (4) are led to a resorption heat pump. Resorption side (7) of the heat pump, heat is extracted from the humid air and steam is obtained on the resorption side (8).

Description

=================================================

Process for reducing heat loss in systems for drying solid bodies ======================================= ==========

The invention relates to a method for reducing heat losses in systems for drying solid bodies, in which steam serves as a heat carrier and moist air is emitted at temperatures of more than about 65 ° C, heat being recovered from the moist exhaust air using a heat pump.

In most industrial processes for drying solids, particularly in the textile, paper, ceramic, brick, clay and chemical industries, water is evaporated from the wet solids by introducing a stream of hot air. To generate the hot air stream, ambient air is usually heated by heat exchange with steam or another heat transfer medium or a fossil fuel is burned directly in the air stream. To support the drying process is also often Heat is transferred or radiated from hot surfaces to the material to be dried.

In such drying systems, large heat losses arise on the one hand from the moist exhaust air, which contains the latent heat of vaporization of the moisture evaporated in the dryer, which is greater than the sensible heat in the hot dry solids, and on the other hand from the dryer structure or its heat dissipation and radiation.

In many cases, the moist dryer exhaust air is partially returned to the air inlet opening of the dryer or brought to heat exchange with supply air. The former method has the disadvantage that the humidity of the supply air is increased and thus the drying rate is reduced. In the second case, only a small proportion of the exhaust heat can be recovered. After the heat exchange has taken place, the dryer supply air is heated to the desired final temperature, specifically by heat exchange with process steam, which is used to heat the surfaces. The moist exhaust air, which has lower temperatures after the heat exchange, can then be used to heat process water and / or space heating water before it is discharged outdoors.

Improved dryer performance is also achieved if the efficiency of the last drying stages is increased by additional microwave or high-frequency radiation. For the same purpose, the moist exhaust air can also be completely recirculated. Before that, however, it is first dehumidified using a heat pump, then reheated. Paper and cardboard are usually dried in direct contact with steam-heated rollers, with several such rollers on under a dryer hood are ordered. Hot air circulates under the hood and between the rollers, which supports the drying process and also removes the water vapor from the dryer. The moist dryer exhaust air is, as mentioned above, brought to heat exchange with the air supplied to the hood and thus preheats the supply air. Then it is used to heat process and / or space heating water. The supply air preheated in this way is brought to its final temperature with the help of process steam, which is used to heat the roller surfaces. With such a system, approx. 40% of the heat content of the exhaust air (above 0 ° C) is dissipated as waste heat in winter. In summer, when no room heating is required, the amount of heat dissipated is approx. 80%.

It is known that these heat losses can be reduced if the dryer supply air is not heated by steam but by means of a heat pump which uses the exhaust air after the heat exchange between exhaust air / supply air as a heat carrier ("Use of a heat pump in a paper mill", VDI Report, No. 345, 1979, pp. 95-104). This proposal has no significant effect on the energy balance of the drying press, since less than approx. 20% of the total energy requirement is required to heat the air in the dryer hood and half of it can already be applied by the exhaust air heat exchanger.

Furthermore, to reduce the heat losses, it has already been described to replace the complete dryer with a version in which steam serves as a direct heat transfer medium and the water vapor produced during the drying process is compressed and compressed, and its latent heat in a heat exchanger gives off to the main steam flow. This can drastically reduce the energy requirement, but a completely new design of the paper drying system is required.

The present invention is based on the object of reducing the heat losses in industrial dryers in which steam with a pressure of approximately 2-3 bar acts as a heat carrier and the moist air at a temperature of approximately 65-100 ° C. or higher and in particular to recover a larger proportion of heat than previously from the moist exhaust air. The process should be feasible with a minimum of design changes to conventional dryer types and additional primary energy.

It has now been shown that this object can be achieved with a method of the type mentioned at the outset, in which the moist exhaust air and the condensed water from the process steam lead to a resorption heat pump and the steam obtained on the resorber side of the resorption heat pump is returned to the drying process. Some advantageous embodiments and possibilities of the method according to the invention are explained in subclaims 2 to 17.

The solution according to the invention is based on the use of a resorption heat pump with a mechanical or thermal drive. Such heat pumps have already been described (E. Altenkirch, "The compression refrigeration machine with solvent circuit", Kältetechnik 2, 1950, Issue 10, pp. 251, 179, 310; "Absorption refrigeration machine", VEB-Verlag Technik, Berlin 1954), their applicability for However, the object underlying the invention was not recognized. In particular, it was not recognized that such a absorption heat pump made the process steam required for the drying process possible with a minimum of effort can be generated. Both exhaust air cooling and process steam generation are made possible with optimal adaptation to the process. The absorption heat pump can also be driven with any existing energy source and works at low pressure or pressure ratio, so that a higher performance figure can be achieved with low investment costs.

In the resorption cycle, the evaporator becomes one

Heat pump replaced by a desorption unit and the condenser by a resorption unit. A solvent circulates between the desorber and the resorber. The low-refrigerant solvent is pumped from the desorber to the resorber, while the refrigerant-rich solvent is expanded from the resorber to the desorber. The refrigerant vapor from the desorber is either compressed to the resorber pressure with the aid of a mechanical compressor or compressor (mechanical drive) or the compression is achieved by means of a further solvent circuit with absorber, pump, expeller and expansion device (conventional absorption heat pump) (thermal drive). A combination of the two drive types is also possible.

When using a resorption heat pump according to the invention between the exhaust air line and the condensate line from the process steam, heat is removed by the desorber from the moist dryer exhaust air, the exhaust air partially condensing. The desorber is effective over a wide temperature range, for example between 80 - 50 ºC. With the resorber, process steam for the dryer is generated again from the condensation water of the process steam. An electric motor, an engine, for example an internal combustion engine, heat engine with external combustion, or another drive unit can be used to mechanically drive the absorption heat pump. The heat released from the drive or exhaust gas cooler can also be used to generate steam or to heat process water. It is also possible to drive the compressor with a Rankine cycle, the evaporator of which can be used to heat process water.

Another alternative when using mechanical absorption heat pumps is that a system with multi-stage compression and intercooling or a type of expansion intercooler is used to reduce overheating heat in the gas leaving the compressor and to reduce the compression work. A blasting apparatus can also be used for compaction.

In thermally driven absorption systems, the heat given off by the absorption circuit absorber can also be used to generate steam or to heat the process water. The thermal absorption system can work with the same solution in the absorption circuit and absorption circuit, but the concentrations are selected so that the heat from the absorber is generated at the same temperature as that prevailing in the resorber or at the highest temperature required for steam generation and overheating, so that the

Maximum temperature in the resorber can be reduced. Different salts can also be used for the solutions in the two circuits, as a result of which optimal operating conditions can be set in both cases. The choice of the means suitable for the temperature range and the working principle, i.e. Refrigerant and solvent, in the absorption heat pump used according to the invention with mechanical compression or thermal mode of operation, is an important aspect in the system design. Ammonia as a refrigerant and water as a solvent have already been proposed for absorption heat pumps. (E. Altenkirch, "The compression refrigeration machine with solvent circuit" Kältetechnik 2, 1950, Issue 10, pp. 251, 179, 310). However, the critical temperature of ammonia is 133 ° C lower than the required absorber temperature, and at a temperature of approximately 140 ° C the pressure for normal concentrations of ammonia in water would be 100 bar. For this reason, the ammonia / water working medium combination is not suitable for the purpose according to the invention.

Suitable working material combinations contain water as the refrigerant and aqueous metal salt solutions as the solvent. Water has moderate pressures in the required temperature range, has good thermodynamic properties, is not chemically aggressive and is harmless. Metal salt solutions can work in this temperature range under atmospheric pressure, so that low construction costs are required. Halogenated hydrocarbons, e.g. Trifluoroethanol can also be used as a volatile component. The metal salts are e.g. Lithium salts, such as lithium bromide, lithium chloride and lithium thiocyanate or mixtures of lithium bromide and zinc bromide in question.

The advantages achieved by the present invention are essentially to be seen in the fact that the resorption machine achieves a high degree of efficiency over a wide temperature range, specifically as a result of the non-isothermal desorption and isothermal resorption processes and in Compared to conventional compression or absorption machines with a low pressure ratio of the machine. Compared to a multi-stage compression machine, which is normally preferred for applications with high pressure ratios, the use of a

Absorption machine, which works over a wide temperature range, has particular advantages from the point of view of steam generation.

The process according to the invention is suitable for a wide range of dryer types, in particular for the cylinder dryers customary in the paper industry, suction drum and belt dryers customary in the textile industry and rotary kilns customary in the fertilizer and chemical industry or for all dryers in which steam with a Pressure of 2 to 3 bar acts as a heat transfer medium and at which moist air with a temperature of more than 65 to 70 ° C is released.

The invention is explained in more detail with the aid of the attached drawings, which represent only one embodiment. In schematic simplification, they show:

Figure 1 shows an embodiment of the use of the absorption heat pump according to the invention in a

Plant for drying paper;

FIG. 2 shows a mechanically driven absorption heat pump;

FIG. 3 shows a thermally driven absorption heat pump; FIG. 4 use of a absorption heat pump driven by multi-stage compression;

Figure 5 isothermal absorption;

6 shows an embodiment in which the first section of the compression is carried out by a condenser, a pump and an evaporator and

Figure 7 shows another embodiment with connected absorption circuit and compressor.

In the system for drying paper shown in FIG. 1, process steam is fed into the steam cylinder 2 via line 1 and at the same time the space between the steam cylinder 2 and the hood 3 is coated with hot air. The condensate from the steam cylinder 2 is removed via line 4. Between the steam cylinder 2 and the hood 3, the paper 5 to be dried is in direct contact with the roller surface. The moist exhaust air is removed via line 6 at approx. 80 ° C. The exhaust air is dehumidified according to the usual methods and serves to preheat the ambient air, which is then returned to the hood as dry supply air at 100 ºC after the heat exchange with the process steam. The exhaust air is then used at a temperature of approx. 74 ºC for the treatment of process water and then at approx. 68 ºC for the treatment of space heating water for the paper mill and finally at a temperature of approx. 46 ºC for preheating the process water and then at a temperature of dissipated at approx. 43 ° C. In summer months, when space heating water is not necessary for the paper mill, the heat loss is also greater. According to the invention, the resorption machine is inserted between the exhaust air line 6 and the condensate line 4, so that the exhaust air is connected to the desorber 7 and the condensate 4 to the resorber 8. Condensation of the moist exhaust air in the desorber 7 generates steam on the resorber side 8 and leads it back into the cylinder via line 9. Part of the steam generated can also be used in the heat exchange at 10 with ambient air 11 to process the dryer supply air, which is returned to the hood 3 via line 12. The supply air 11 can be heated accordingly in the heat exchange with the condensate 4. After partial condensation, the moist exhaust air leaves the desorber 7 at a temperature of approximately 50 ° C. and can be used to process water 15 or space heating water 16 using conventional heat pumps 13 and 14.

Thereafter, their temperature is about 18 ° C and is discharged.

The absorption heat pump can be driven mechanically as well as thermally. A mechanically driven absorption heat pump is shown in FIG. It consists of a resorber 8 and a desorber 7, between which a solvent circulates. The solvent circuit is via lines 17, 18, 19 and 20 while the refrigerant circuit is via 21, 22, 17 and 18. The compression work is ensured by a compressor 23 in the refrigerant circuit. The compressor 23 can be operated by an electric motor 24, an engine or another drive unit. In the thermally driven absorption heat pump shown in FIG. 3, a resorber 8 and a desorber 7 with the same solvent circuit 17, 18, 19 and 20 are also provided. A conventional absorption heat pump can be used for the drive, which consists of an expeller 25, an absorber 26 with a solvent circuit 27, 28, 29 and 30. The refrigerant circuit is 31, 29, 30, 32, 17 and 18.

The systems shown in FIGS. 4 to 8 are explained using exemplary embodiments which relate to use in paper drying plants with the following boundary conditions:

Exhaust air temperatureϊ 80 ° C to 85 ° C

Absolute humidity of the exhaust air: 0.14 - 0.17 kg / kg Air Low pressure steam: 2 to 3 bar High pressure steam: 10 to 15 bar

Condensation temperature: 65 ºC to 95 ºC,

In the embodiment shown in Figure 4, a multi-stage compression at 33 and 34 and absorption at 35 and 36 is used. The desorber 7 is connected to the heat source exhaust air 6. A heat sink of steam of 2 to 3 bar is generated in the resorber 35 and 36. The resorber 35 works at a lower pressure than the resorber 36. The combination water / lithium bromide is selected as the working medium. Further process parameters are as follows:

Desorption: sliding 75 ° C to 50 ° C

Absorption: in two stages; sliding 110 ºC to 140 ºC

Compression: in two stages; 0.07 bar, 0.35 bar and 1 bar. The rich solution of the resorber 36 can be cooled in the heat exchanger 37 and process water or space heating water can be generated.

The absorption can also be carried out isothermally. The embodiment shown in FIG. 5 is proposed for this purpose. In this case, a single resorption container 38 is provided, within which the pressure increases or decreases in the vertical direction. The poor solution is supplied from the desorber via line 39. The rich solution is passed via line 40 to the desorber. This resorber 38 is designed such that the vapors conducted into the resorber 38 from various compression stages 41, 42, 43 have the pressure prevailing in the respective entry region in the resorber. The pressure prevailing in the resorber 38 increases due to the gravity of the solvent from top to bottom.

The first stage of compression can be omitted if, as shown in FIG. 6, a condenser 44, a pump 45 and an evaporator 46 are provided. The evaporator 46 is arranged in front of the desorber 7 in the exhaust air duct 6. The steam from desorber 7 is condensed at 44, for example against cooling towers or process or space heating water. The condensate is pumped to high pressure at 45 and evaporated in the evaporator 46 with exhaust air as the heat source. It is then absorbed in the resorber 47, which operates at a lower pressure than the resorber 48. The absorption can also be carried out as shown in FIG. 5. Furthermore, the rich solution from the resorber 48 can be used to generate process water or heating water. In an embodiment with water / lithium bromide, the process parameters are as follows:

Evaporation temperature: between 65ºC and 80ºC

Desorption: sliding 65 ° C to 40 ° C

Absorption: sliding 110 ° C to 140 ° C

Compression: 0.25 or 0.47 to 1 bar

Condensation with heat sink cooling tower and / or process or Space heating water: 15 to 30ºC.

The particular advantage of this embodiment lies in the reduced compression work, since only one compressor 49 is provided.

According to FIG. 7, an absorption circuit consisting of an expeller 50 and an absorber 51 is used. Furthermore, a compressor 52 is present between desorber 7 of the absorption circuit and absorber 51 of the absorption circuit. The desorption takes place between 75 ° C and 50 ° C, the resorber 53 and absorber 51 both produce process steam of 2 to 3 bar. The expeller 50 is heated with high pressure steam from 10 to 15 bar. The advantage of this embodiment is also the reduced compression work, since there is only one compressor 52. The high pressure steam can be expanded after being used to heat the expeller 50 and can further be used for low pressure steam. Here, too, the rich solution of resorber 53 and the poor solution of expeller 50 can be used to generate process water or heating water.

The compression by the compressor 52 can also take place, as shown in FIG. 6, by a condenser, a pump and an evaporator.

Claims

1. A method for reducing heat loss in systems for drying solid bodies, in which steam serves as a heat carrier and moist exhaust air is emitted at temperatures of more than about 65 ° C, heat being recovered from the moist exhaust air using a heat pump, characterized in that that the moist exhaust air and the condensate from the process steam are led to a absorption heat pump and the steam obtained on the resorber side of the heat pump is returned to the drying process.

2. The method according to claim 1, characterized in that the steam obtained is partly used to generate hot dry supply air for the drying plants.

3. The method according to claim 1 or 2, characterized in that the moist exhaust air obtained on the desorber side of the absorption heat pump is used in a manner known per se using conventional heat pumps for the treatment of process water and space heating water.

4. The method according to claim 1 or 2, characterized in that the steam expelled from the desorber is brought to the pressure level prevailing in the resorber in the absorption circuit by using a mechanically driven compressor.

5. The method according to claim 4, characterized in that the compression is carried out in several stages.

6. The method according to claim 4 or 5, characterized in that the absorption is carried out in several stages.

7. The method according to claim 6, characterized in that the multi-stage absorption is carried out isothermally in a single resorber, the pressure in the resorber rising due to the gravity of the solvent in the vertical direction downwards.

8. The method according to any one of claims 4 to 7, characterized in that a condenser, a pump and an evaporator are provided between the desorber and the compressor, with which the first portion of the compression takes place, the evaporator being used in front of the desorber in the moist exhaust air duct .

9. The method according to claim 1 or 2, characterized in that the absorption pump is connected to an absorption circuit, the absorber of which has the same temperature as the desorber and whose expeller has the same pressure as the absorber, and that the absorption circuit has a low concentration of refrigerant and that the heat is used by the resorber and absorber to process the process steam.

10. The method according to claim 9, characterized in that for the purpose of achieving the same temperatures, a compressor is provided between the desorber and absorber or a condenser, a pump and an evaporator are interposed, the evaporator being used before the desorber in the moist exhaust air duct .

11. The method according to claim 4 or 10, characterized in that the compressor by an electric motor, a

Motor or another drive unit is driven and that the waste heat from the drive unit is used to generate process water and / or heating water.

12. The method according to claim 8 or 10, characterized in that the heat from the condenser is used for the treatment of process water and / or heating water.

13. The method according to any one of claims 4 to 12, characterized in that the heat present in the rich solution in the absorption circuit is used for the treatment of process water and / or heating water.

14. The method according to any one of claims 9 to 13, characterized in that the same solutions are used in the absorption circuit and absorption circuit.

15. The method according to any one of claims 9 to 13, characterized in that different salts are used for the solutions in the absorption circuit and absorption circuit.

16. The method according to any one of claims 1 to 15, characterized in that water or halogenated hydrocarbons, e.g. Trifluoroethanol, as a volatile component (refrigerant) and metal salt solutions as solvents.

17. The method according to claim 16, characterized in that as metal salts lithium salts, e.g. Lithium bromide, lithium chloride and lithium thiocyanate, or a mixture of lithium bromide and zinc bromide can be used.