DD215381A1 - PROCESS FOR THE RE-BURNING AND TEMPERATURE OF CIRCULAR GAS FOR CONVECTION DRYERS - Google Patents

Abstract

The invention is preferably applicable to the drying of temperature-sensitive substances in which normally a high particulate matter content remains in the recycle gas. With the invention known Nachverbrennungsverfahren are to be improved so that on the one hand a safe afterburning is guaranteed, on the other hand, the permissible drying gas temperature must be realized by admixing cool, unpurified recycle gas. This is achieved by the entire cycle gas stream is post-combusted and mixed with flue gas and by heat transfer between unburned cycle gas and the highly heated cycle gas flue gas mixture, a temperature control of this mixture and a preheating of the unburned cycle gas.

Description

Process for the post-combustion and temperature control of circulation gas for Konvetetionstrockner ^N

Field of application of the invention

The invention relates to a process for the afterburning and temperature control of cycle gas for Conventionstrockner, in which the combustible components, such as. B. solvent vapors and / or particulate matter, containing recycle gas supplied to the burner chamber of a FriJchlufterhitzers, there heated to Verbrenhurigstemperatur the combustible components and tempered before entering the dryer to drying gas temperature.

The invention is preferably applicable for drying temperature-sensitive substances, such. B. Feed yeast *

Characteristic of the known technical solutions ».,

ι. , ·.

There are a number of methods for drying wet products are known in which a portion of the recirculated to the dryer inlet cycle gas is supplied to the burner of the fresh air heater to burn unwanted and combustible Bestandtei-Ie of the cycle gas. Such constituents may be solvent vapors (DE-AS 2653785; OE-PS 321820) or dusts (DE-OS 2437871).

The advantage of this method is that is lowered by the circulation mode, the Op content of the drying gas. However, a major disadvantage of these methods is that, in principle, only a portion of the recycle gas can be supplied to the afterburning and must be mixed after combustion with cool recycle gas, so that the permissible upper temperature limit of the drying gas does not exceed

"SUCTION 1983 * 108540

will be. However, this leads to the fact that in the drying of certain substances in which z. B. a relatively high proportion of fine dust is formed, the fine dust concentration in the cycle gas can rise above the permissible value for safe operation of the dryer.

Furthermore, it has proved in the application of these methods for temperature-sensitive substances as a disadvantage that the required operating temperature of the drying gas is achieved by mixing highly heated with cool recycle gas. In this procedure, thermal damage, in particular of the fine dust content, can not always be avoided.

The proportion of the post-combustion supplied recycle gas is in known methods below 50 % (DE-AS 2653785). When applying the method specified in OE-PS 321 820 to water-moist substances, this proportion is reduced to about 10 to 15%. Thus, the transported in the cycle fine grain is thermally damaged and the quality of the product reduced, sometimes so strong that the minimum quality standards can not be met. Although it is known a method which operates at a drying gas temperature of 300 ⁰ C to 600 ⁰ O and in which 55% to 75% of the cycle gas is supplied to the burner of Frischlufterhitζers (DE-OS 2843296). However, this temperature range is above the permissible drying temperature and below the combustion temperature for many substances. .

On the other hand, in DE-OS 2461415 a Brüdenrückführungs ^ ·. and combustion plant for directly heated drying plants, which is based on the fact that some of the exhaust gases or a multiple of this amount is returned to the drying circuit, the recycled gases are introduced into a specially designed combustion chamber through nozzles and the residues of fine dust and odors in brought the hot combustion chamber and burned, where by this-

-aAüG.1983 * iO854O

se arrangement of an oxygen-poor inert gas cycle is formed. Experiments in a laboratory apparatus, which fully corresponded to the Brüdenrückführungsund incineration plant, showed significant disadvantages: Despite optimally designed combustion chamber, the amount of recirculation can be increased up to the amount of flue gas, if the residues of dust and the odors to be burned, but at the same time occur much too high Temperatures for the drying of yeast, or as claimed in DE-OS 2461415, for feed, on. All products showed in this process, although working under inert conditions of the gas atmosphere, clear coking phenomena. It is therefore completely unsuitable for drying water-moist products.

Known methods, in which the entire cycle gas is supplied to the burner of the fresh air heater, thus show the following principal disadvantages: At low recirculation amounts, smaller than or equal to the amount of flue gas, the combustion of fine dust and odors is realized, but the temperature of the drying gas is too high for drying.

At high recirculation amounts, greater than 1.2 times the amount of flue gas, although the temperature of the drying gas begins to be bearable and decreases with increasing recirculation degree, at about 6-fold degree of recirculation, a temperature vorr about 300 ⁰ C is reached. In these conditions, however, the fine dust and odors can not be burned.

-HAUG. 1983 108540 *

Aim of invention

The object of the invention is to improve the efficiency of known afterburning processes for reducing the concentration of combustible constituents of the recycle gas.

Explanation of the essence of the invention

The invention has for its object to modify known methods for afterburning so that at recirculation quantities of the recirculation gas, which are substantially larger than the amount of flue gas, a safe afterburning is ensured without the permissible drying gas temperature must be realized by admixing cool, unpurified recycle gas.

This invention is inventively achieved in that the entire cycle gas stream is post-combusted and mixed with flue gas and that takes place by heat transfer between unburned cycle gas and hocherhitztem cycle gas flue gas mixture, a temperature of this mixture and a preheating of the unburned cycle gas. The cycle gas flue gas ratio is dimensioned so that the cycle gas flue gas mixture has the required drying temperature in the absence of heat transfer.

In a specific embodiment of the invention, the material flows are conducted during the heat transfer in a cocurrent or countercurrent.

The advantage of the invention is that a safe afterburning of the entire cycle gas is achieved by the use of a heat exchanger as a thermal feedback element with low energy expenditure. As a result, the reliability of the dryer is substantially increased. Another advantage is that the temperature does not take place by mixing with cool unburned cycle gas, so that a thermal damage to the contained in the recycle gas

1983 108540 *

new Peinstaubanteils is avoided. *

embodiment

The invention will be explained in more detail below by way of example. The accompanying drawing shows a schematic representation of a plant for carrying out the inventive Ben Ben process for the drying of yeasts with a high residual dust content in the recycle gas.

In a laboratory apparatus, a yeast-water suspension 1, '.' consisting of 20% by mass of dry yeast, 77% by mass of water and 3 ma- fed% of hydrocarbons in the diesel fuel boiling range of 40 ⁰ C above the suspension pressure line 2 to the drier 3, where it was atomized in conventional swirl.

The fuel 4 · was supplied via the fuel line 5 together with the air 6, which was supplied via air blower 7 and air line 8, the burner chamber 9 with burner. The fuel-air ratio was maintained in the range of the air ratio A = 1.08 to 1.15 and at these conditions with the from the hot-cycle gas line 11 at t .. = 975 ⁰ C via a nozzle system in the burner chamber 9 mixed, where at temperatures t _Q = 1050 ⁰ C, the Peinstaubanteile of 200 to 1000 mg / nr with the diesel fuel fraction of 15OO mg / m are completely burned and a dust and solvent-free gas through the burner chamber outlet conduit 12 is fed to the heat exchanger. In the heat exchanger 10, it is at t ^., = 260 ⁰ C, the drying gas temperature, cooled and introduced at this temperature in the dryer 3. Thus, the complete combustion of Peinstaubanteile is secured for the entire cycle gas.

In the dryer 3, the temperature of the drying gas decreases from t ₁₅ = 260 ⁰ C to t ₁₄ = 110 ⁰ C in the dryer-Austrittsgaslei ^

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tuhg. Via the dryer exit gas line 14, the seed deposit 15 and the dryer exit gas blower 16

promoted the dryer exit gas in the pressure line 17 to dryer exit gas, where a partial flow via line 18 for being schleustes exhaust gas to the exhaust chimney 19 was performed * The I £ rice run gas is completely with t _on = 110 ⁰ C and a residual dust content of 200 to 1000 mg / nr and 1.4 g DK / mr supplied via the line 20 to the heat exchanger 10, in which it is heated in countercurrent to the drying gas to t ** = 975 ⁰ G. With this temperature, it is supplied via the hot-cycle gas line 11 and the nozzles to the burner chamber 9 with 'burner. The dry yeast is conveyed via the dry yeast Leituag 21, in which also the fine-grain dry yeast line 22 opens, to the silo 23 for dry yeast.

1.1983 * 108540

Claims (2)

Claim. 1. A process for the afterburning and temperature control of circulating gas for Konvektiohstrockner in which the combustible components, such as. B, solvent vapors and / or JFein , dusts, containing cycle gas supplied to the burner chamber of a fresh air heater, heated there to combustion temperature of these components and tempered before entering the dryer to drying gas temperature, characterized in that the entire cycle gas stream is post-combusted and mixed with flue gas and that by heat transfer between the cold dust - And solvent-containing unburned cycle gas and the highly heated cycle gas flue gas mixture, a temperature of this mixture and a preheating of the unburned cycle gas, wherein the cycle gas flue gas ratio is such that the recycle gas flue gas mixture in the absence of heat transfer the required drying gas temperature between about 100 to about 500 ⁰ C. 2. The method according to item 1, characterized in that in the heat transfer, the streams are passed in a cocurrent or countercurrent. - See 1 page drawing -