DE3106567A1 - "METHOD FOR REDUCING HEAT LOSS IN FACILITIES FOR DRYING SOLIDS" - Google Patents

Abstract

To reduce the heat losses in a drying plant for solid bodies, wherein the steam (1) conveys the heat and releases wet evacuation air (6) at temperatures higher than approximately 65`C, the wet air (6) and the condensation water (4) are conveyed to a resorption heat pump. On the resorption side (7) of the heat pump, heat is tapped from the wet air and steam on the resorption side (8) is obtained.

Description

392-08 - 59/80 February 11, 1981

CASCH / UMA

TO BATTELLE - INSTITUT E.V., Frankfurt / Main

Process for reducing heat losses in systems for drying solids

¹ I 25 The invention relates to a method for reducing

j heat losses in systems for drying solids,

; in which steam is used as a heat transfer medium and moist air is used Temperatures of more than approx. 65 C is emitted, using a heat pump to remove heat from the damp

• 30 exhaust air is recovered.

F In most industrial processes for drying

r solids, especially in the textile, paper, ceramic,

■ /. ioqol, clay and chemical industries, water is made from dim

Vt wet solids by introducing a hot air stream

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pre-steamed. For egg generation of the hot air streami; will be honest Ambient air heated by heat exchange with steam or another heat transfer medium or a fossil fuel burned directly in the air stream. For support During the drying process, there is often also heat from hot surfaces onto the material to be dried conducted or radiated.

Large heat losses occur in such drying systems 'O on the one hand by the moist exhaust air, which the latent heat of vaporization the moisture content evaporated in the dryer which is greater than the sensible warmth in the hototi dry solids, and on the other hand through the dryer structure or its heat dissipation and radiation. ■ 5

In many cases, the humid dryer exhaust air is partially returned to the air inlet port of the dryer or to the Bred heat exchange with supply air. The former method has the disadvantage that the humidity of the supply air increases and ;: O so that the drying rate is reduced. In the second case, poor only a small proportion of the exhaust air heat can be recovered. After the heat exchange has taken place, the dryer is supplied with air heated to the desired final temperature by Heat exchange mLt process steam that is used to heat the upper ! 5 surfaces is used. 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 to the outside will.

Improved dryer performance is also achieved if the efficiency of the last drying stages is increased by additional microwave or high-frequency radiation. J'.u the same purpose, the moist air vollr.t -'An Ii 'i 5 ι c /. i rkul Ιοί t wordon. Before that, sir * jinloch urilnr Vm-wcinl-unM ι · i in · ι WM ι \ w ι »ιιiii | ><·'.'. UcI :! ι mi II ei n · Ιι I r · I,> I, mn ι · ι n <mi I .in I · ι «-In · ι.

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Paper and cardboard are usually dried in direct contact with steam-heated rollers, with one another Several such rollers are arranged in the dryer hood. Hot air circulates under the hood and between the rollers supports the drying process and also removes the water vapor from the dryer. The damp dryer exhaust air As mentioned above, the Moführlori r.uft qebrat-ht for heat exchange with the hood and thus heats the supply air. Then it is used to heat process and / or Kanmlii izungswasser utilized. The supply air preheated in this way is with the help of process steam that is used to heat the roll surfaces is used, brought to its final temperature. In such a system, approx. 40% of the heat content is used in winter the exhaust air (above 0 C) dissipated as waste heat. In summer, venn no space heating is required, is the proportion of algae? led heat approx. 80%.

I ·!:; i r; I have known that these heat losses have been reduced can, if the dryer air is not supplied by steam, aonden is heated with the help of a heat pump, which the exhaust air after the heat exchange between exhaust air / supply air as Heat transfer medium used ("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 process because it is used for heating the air in the dryer hood requires less than approx. 20% of the total energy requirement and half of it is ready; can be applied through the exhaust air heat exchanger.

Furthermore, it has already been described to reduce the heat losses to replace the complete dryer with a version that uses steam as a direct heat transfer medium serves and the water vapor generated during the drying process is compressed and compressed and thereby be ie 5 latent heat in a heat exchanger to the main steam flow

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gives away. As a result, the energy requirement can be drastically reduced, a completely new design of the However, paper drying equipment is required.

The present invention is based on the object Industrial dryers in which steam at a pressure of approx. 2-3 bar acts as a heat transfer medium and emits the moist air at a temperature of approx. 65 - 100 C or higher, to reduce the heat losses and in particular a to recover a greater proportion of heat than before from the humid exhaust air. The procedure should be with a minimum be feasible on constructive changes to the usual dryer types and additional primary energy.

E-; has now shown: that this task is with. pinnii Ancestors can solve the type mentioned at the beginning. to the the moist exhaust air and the condensation water from the process steam to a resorption heat pump with thermal or mechanical drive and the itc? the vapor obtained from the absorption heat pump is returned to the drying process. Some advantageous implementations? - forms and possibilities of the method according to the invention are explained in the subclaims 2 to 10.

; 5 DLe solution according to the invention is based on the use of ο iner Resorption heat pump with mechanical or thermal drive. Such heat pumps have already been described (E. Altenkirch, "The compression refrigeration machine with Lüsunqsmittelkreislauf", Refrigeration technology 2, 1950, issue 10, pp. 251, 179, 310; "Absorption chiller", VEB-Verlag Technik, Berlin 1954), the applicability of which for the task on which the invention is based was not recognized. Into the-

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in particular, it was not recognized that the process steam required for the drying process can be generated at the same time with a minimum of effort by such a resorption heat pump.
5

In the resorption cycle, the evaporator of a heat pump is replaced by a desorption unit and the condenser is replaced by a resorption unit. A solvent circulates between the desorber and the resorber. The Refrigerant] -MC) poor solvent is pumped from the desorber to the ^resorber,: while there. ¹ ; refrigerant-rich solvent is expanded from the resorber zi m = Dosorbor. The refrigerant vapor from the descr-'ber is either with the help of a mechanical compressor:

compressed to the absorber pressure (mechanical drive) M5 or the compression is carried out by a further solvent cycle with absorber, pump, expeller and expansion device (conventional absorption heat pump) j (thermal drive).

When using such a Resorptior heat pump according to the invention between the exhaust air line and the condensate line from the process steam is passed through the desorber

: extract heat from moist dryer exhaust air, whereby the exhaust air partially condensed. The desorber is about to ride

.; 25 Effective temperature range, for example between 80 - 50 ⁰ C. With the resorber, process steam for the dryer is generated again from the condensed water of the process steam.

For the mechanical drive of the absorption heat pump e .n Electric motor, a prime mover, e.g. an internal combustion engine, Heat engine with external combustion, or

another drive unit can be used. The freed-m- : de heat from the engine or exhaust gas cooler can also lead to ■ Steam generation or for heating process water serve i. 35 It is also possible to equip the compressor with a Rankine

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To drive the circuit, its evaporator to heat up can be used by process water.

Another alternative when using mechanical absorption heat pumps is that to reduce overheating in the compressor leaving the compressor Gas and, to reduce the compression work, a system with multi-stage compression and intermediate cooling or

a kind of expansion intercooler is used. A jet device can also be used for compression will.

In thermally driven absorption systems, the heat given off by the absorber of the absorption circuit can be used

'5 Steam generation or for heating the process water dinner :. The thermal absorption system can use the same solution work in the resorption cycle and the absorption cycle, but the concentrations are chosen so that that the heat from the absorber is generated at the same temperature as that in the absorber or that used to generate steam and overheating required the highest temperature so that the maximum temperature in the resorber can be reduced. In the two circuits, different salts can also be used for the solutions, which in both cases

:! 5 optimal operating conditions can be set.

The choice of the means suitable for the temperature range and the working principle, i.e. refrigerants and solvents, in the absorption heat pump used according to the invention with mechanical compression or thermal mode of operation, is an important aspect in system design. Ammonia has already been used as a refrigerant for absorption heat pumps and water is suggested as a solvent. (E. Altenkirch, "The compression refrigeration machine with solvent circuit" Refrigeration technology 2, 1950, runs 10, pp. 251, 179, 310). Dio kri-

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table temperature of ammonia is 133 C lower than the required resorber temperature, and at a temperature of approx. 140 C the pressure would be normal2 Concentrations of ammonia in water are 100 bar. For this reason, the work equipment combination

Ammonia / water is not suitable for the purpose according to the invention. For the same reason, most of the halogenated hydrocarbons proposed so far for absorption systems are e.g. difluorochloromethane, cannot be used. 0

Suitable combinations of working fluids contain water as a coolant and aqueous metal salt solutions as a solvent. ; Water has moderate temperatures in the required temperature range

Press, has good thermodynamic properties, is ! 5 chemically non-aggressive and harmless. Metal salt solubilizers can work in this temperature range under atmospheric pressure, so that low construction costs are required. ; that are. Metal salts include, for example, lithium salts,

such as lithium bromide, lithium chloride and lithium cyanate, infrege. ;:O

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, due to the non-thermal desorption and resorption processes and the r- in Ve [identical to conventional compression or absorption machines low pressure ratio of the machine. Compared to a multi-stage compression machine, which is normally used for areas of application with high pressure ratios! is preferred has the use of a resorption machine. no! which works over a wide temperature range, with particular advantages from the point of view of steam generation.

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The method 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 industries or for all dryers in which steam is mixed with a Pressure of 2-3 bar acts as a heat transfer medium and where moist air with a temperature of more than 65-70 C is emitted.
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The invention is explained in more detail with reference to the accompanying drawings, which show only one embodiment. It show in schematic simplification

5 Figure 1 flow diagram of a conventional paper drying process ;

FIG. 2 shows a mechanically driven absorption heat pump;

, _o Figure 3 is a thermally driven absorption heat pump and

FIG. 4 shows an embodiment of the insert according to the invention the absorption heat pump in a process for drying paper.

In the conventional paper drying process shown in FIG. 1, process steam is fed into the steam cylinder via line 1 2 and at the same time will see dpi: space between the steam cylinder 2 and the hood 3 with hot air

: .O sends. The condensate from the steam cylinder 2 is over Line 4 removed. Between the steam cylinder 2 and the Hood 3, the paper 5 to be dried is in direct contact with the roller surface. The humid exhaust air is over Line 6 taken at about 80 C. After dehumidification, the ambient air is preheated in the heat exchanger 7 and in the

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'iiii.'it, uiüfhi't B with "loin l'rcr / .oßdampf au :: l, ei I unq ") is or> returned to the hood 3 via line 10 as dry supply air at approx. 100 ° C. The condensate is drained off via line 11. The moist exhaust air is then used at a temperature of approx. 74 C for the treatment of process water and then subsequently at approx. 68 C for the preparation of space heating water 13 for the paper mill and finally at a temperature of approx. 46 C for preheating the process water 14 then dissipated at a temperature of " ⁿ approx. 4 3 C. In the summer months, when space heating water is not required for the paper mill, the heat input is consequently greater.

■ In order to reduce the heat losses, according to the invention the use of a mechanical or thermal absorption heat pump is proposed. In Figure 2, a mechanical driven absorption heat pump shown. It consists of a resorber 15 and a desorber 16, between which a solvent circulates. The solution

JO medium circuit goes through lines 17, 13, 19 and 20 during the refrigerant circuit runs through 21, 22, 17 and 18.

In the thermally driven Reso ptionswärmepumpe shown in Figure 3 is also a resorber 15 and a desorber 16 with the same solvent circuit 17, 18, 19 and 20 provided. A conventional absorption heat pump is used for the drive used, which from an expeller 2 to an absorber 24 with a solvent circuit 25, 26 ; -sO 27 and 28 exists. The refrigerant circuit is 29, 27, 28 30, 17 and 18. Between the resorber 15 and the expulsion ; A condenser can also be installed above 23 of the absorption machine

' be provided.

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In Figure 4, the use of a resorption machine in
a paper drying system shown. The Resorpt ic / i:; machine is inserted between the Atilur ": line 6 and the condensate line 4, so that the exhaust air with the desorber 16
and the condensate 4 is connected to the resorber 15.

As a result of the condensation of the moist exhaust air in the desorber 16, steam is generated on the resorber side 15 and via line 32
returned to cylinder 2. Part of the steam generated can also be exchanged with heat at 33 with urn

Ό ambient air 34 are used to prepare the dryer supply air, which is then returned to the hood 3 via line 35. The supply air 34 can also exchange heat with the condensate 4 are heated accordingly. After partial condensation, the moist exhaust air leaves with a temperature

'5 of about 50 C the desorber 16 and can using
conventional heat pumps 36, 37 for processing
Process water 38 or room heating 39 are used. In connection
at this their temperature is approx. 18 C and is dissipated,

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Claims (10)

3JI2-O8 - 59 / SO February 11, 1981 CJ. SCH / UMA ) BATTELLE - INSTITUT E.V., Frankfurt / Main 5 claims 2) 1. Method for reducing heat losses in systems for drying solids, in which steam is used as a heat carrier and moist exhaust air is emitted at temperatures of more than about 65 ° C., where υπίιτ Votwi.Mi-iim ·: a heat pump heat is zurUckqowonnen from humid exhaust air, characterized gekonnzeichnet that the wet exhaust air and the condensate from the Proze.ßdampf yu nor ei ^ sorption heat pump out with thermal or nipchan tronically drive and the at Kcsorbfrsci dor I c dor W'iriiu ·· - purnpo obtained steam to the drying proxus • / .un "k-kgt ⁱ f ^; ': hi < becomes. 2. The method according to claim 1, characterized in that the steam obtained is partially used to generate hot dry supply air for the drying:;. In I cHfc Ι »> πιι t; '.! wii'i. BAD ORIGINAL EPO COPY 3. The method according to claim 1 or 2, characterized gekennzeichne t that you on the desorber side of the resorption heat pump obtained moist exhaust air is used in a known manner using conventional heat pumps for processing water and room heating water. 4. The method according to any one of claims ¹ . 1 to 3, characterized in that the resorption by an electric motor, an engine or other drive; unit is driven and that the heat obtained from the drive is used to generate steam or to heat the process water. 5. The method according to claim 4, characterized in that a resorption heat pump with a multi-stage compressor and 'intermediate cooling or relaxation intermediate cooling is used. 6. The method according to any one of claims 1 to 3, characterized in that the absorption heat pump is driven durcli eini absorption circuit, the heat from the absorber being used to generate steam or to treat the process water. 7. The method according to claim 6, characterized in that the same solutions are used in the resorption circuit and absorption circuit, the concentrations of which are selected so that the heat from the absorber is obtained at the same temperature as that in the resorber or at the highest Steam generation and overheating required ι temperature. ^BAD ° *** ^ep ° ^C0PV 8. The method according to claim 6, characterized in that different salts are used for the solutions in the absorption circuit and absorption circuit. > 9. The method according to any one of claims 1 to 8, characterized in that the working fluid combination Wa.sser as a volatile component (refrigerant) and metal salt solutions are used as solvents. 1) 10. The method according to claim 9, characterized in that the metal salts used are lithium salts, for example lithium bromide, lithium chloride and lithium thicyanate. EPOCOPV g