DE3145921A1 - Process and equipment for drying - Google Patents

Abstract

Process and equipment for drying solid materials, especially granular plastics (10), by means of a gaseous drying medium which is passed through a drier (32) having at least two drying agent chambers (34) with drying agent, wherein, during flow through one drying agent chamber, the other drying agent chamber is disconnected from the drying circulation for regeneration of the drying agent, while the other chamber is, for optimum utilisation of the drying agent, only reconnected to the drying circulation when the drying medium downstream of the drier has reached a certain moisture content; the moisture content is determined by means of a dew point sensor (56) in the line (20) between the drier (32) and a drying vessel (12) receiving the material (10) to be dried. <IMAGE>

Description

3U5921

description

The invention relates to a method for drying solid substances, in particular granular plastics, in which a gaseous drying medium is passed through the substance to be dried, the then dried in a desiccant, then heated and finally again through the drying agent Substance is conducted, whereby the desiccant is regenerated with increasing humidity and during the Regeneration the drying medium is dried in a desiccant that has been regenerated in the meantime.

The invention also relates to a device for drying solid substances, in particular granular ones Plastics by means of a gaseous drying medium passed through the substance to be dried whose drying at least two desiccant chambers are provided, which are used to divert the Drying medium have a reversing device such that the drying medium is one of the desiccant chambers crosses and flows on to the substance to be dried, while at the same time the other desiccant chamber to regenerate the desiccant from the drying cycle and switched off from hot air is flowed through.

In known adsorption dryers, it is common to switch the individual desiccant chambers over time, so that the desiccant regenerates in the desiccant chamber, which has been disconnected from the drying cycle can be. This time-dependent reversal has the significant disadvantage that it does not take into account the Degree of humidity and the achievement of the saturation limit

is taken so that the operation of the known Method and apparatus is uneconomical. The moisture content of a fabric to be dried is very different and depending on the initial moisture content, the consistency and the storage time of the material. The previously known dryers must therefore be designed for a maximum value for moisture absorption which has the disadvantage that the desiccant is rarely fully used.

The invention is based on the object of a method and to specify a device for drying solid substances in which the desiccant is optimally used will.

In a method of the type explained at the beginning, this object is achieved according to the invention in that can the drying medium flowing through the desiccant when a certain moisture content is reached which has been regenerated in the meantime by a stream of hot air Diverting desiccant.

This solution has the advantage over the previously customary, time-dependent reversals that the reversal only takes place when the desiccant has reached such a moisture content that the drying performance is insufficient. In this way, compared to the previously known methods, there is a considerable one Energy saving.

The invention is also achieved in a device of the type explained at the outset in that in the of the desiccant chambers lead to the fabric to be dried

running line a dew point sensor switched on which is connected to a control circuit which actuates the reversing device as soon as a certain moisture content is reached.

In this device, instead of a program switching mechanism, the reversing device is time-dependent actuated, a dew point sensor inserted, which actuates the reversing device only when the moisture content in the drying medium is below the drying chamber just switched on in the drying cycle has become so great that further drying beyond this point is insufficient or uneconomical will.

Advantageous developments result from the following description of an exemplary embodiment, that is shown in the drawing, and "from the subclaims.

Ls show:

Figure 1 schematically shows an apparatus for drying granular plastics according to the invention,

Figure 2 is a block diagram of the control circuit used

and
FIG. 3 diagrams of the voltage profile at the test points TP shown in FIG.

Ir

In Figure 1 is a device for drying granular plastics (granules 10), which are filled into a drying container 12 is shown. For filling in the to be dried Substance in the drying container 12 is an upper loading opening 14, while an emptying device at the bottom 16 is provided for removing the dried substance.

A line opens into the lower region of the drying container 12 via a drying gas heater 18 20 for supplying a gaseous drying medium, which flows out in the lower region of the drying container 12 through a sieve bottom 22 into the granulate 10. That heated drying medium, generally air, but also inert gas, nitrogen or the like, flows through the granules 10 from bottom to top, absorbing the moisture from the granules, after which it is the Drying container 12 leaves through an output line 24.

A thermostat 26 ensures that the temperature of the drying medium flowing into the drying container 12 always remains constant, and is in a known and not shown way with the dry gas heater 18 electrically connected.

The output line 24 leads through a filter 28, holding back the dust. A fan 30 behind the filter 28 presses the moist drying medium into an adsorption dryer 32.

_ C

The adsorption dryer 32 consists of two in parallel switched desiccant channels 34 and 34 '»the via an input line 36 or 36 'to the supply of the drying medium are connected to the fan 30. The drying medium leaves the respective desiccant chamber 34- or 34- 'through an output line 38 or 38 ', which opens into the line 20.

As Figure 1 also shows, is a regeneration fan 40 is provided, the 42 hot air via an air heater to regenerate the desiccant in the adsorption dryer 32, depending on the position a reversing device 44 either via a supply line 46 into the left desiccant chamber 34 or via a supply line 46 'into the right desiccant chamber 34 '. Via a drain line 48 or 48 ' the hot air flow reaches the outside after flowing through the drying chamber 34 or 34 '.

The reversing device operated by a control circuit 50 44 consists in the illustrated embodiment of an upper and a lower piston-cylinder unit 52 and 52 ', the piston rods of which with an upper slide 54 and a lower slide, respectively 54 'are connected. In the position of the reversing device shown 44 close the slide 54 and 54 ', the supply line 46 and the discharge line 48 for the hot air flow to the left desiccant chamber 34, while the corresponding lines 46 'and 48' of the right desiccant chamber 34 'are open. Simultaneously are the input line 36 and the output line 38 for the drying medium of the left drying

Middle chamber 34 open, while the corresponding lines 36 'and 38' of the right desiccant chamber 34- 'are closed. In the other, not shown position of the reversing device 44, the slides 54 and 54 'enable the supply of hot air to regenerate the left desiccant chamber 34, while the drying medium flows through ^{the right desiccant chamber 34- r.}

The input of the control circuit 50 is electrical connected to a dew point sensor 56, which is in the Line 20 is switched. When the drying medium flowing through the line 20 from the adsorption dryer 32 has reached a preselected degree of humidity which is set on the control circuit 50 and which generally corresponds to an optimal utilization of the desiccant, the dew point sensor 56 sends a electrical signal to the control circuit 50, which then the reversing device 44 is actuated in a manner still to be described.

In the position of the reversing device 44 shown in FIG. 1, the drying medium used for drying flows through it the desiccant in the left desiccant combex 34. It is brought to a moisture level, which corresponds to a dew point temperature of below -30 ° C. Via line 20, it then reaches the dry gas heater 18, where it is set to an adjustable and to drying good adjusted temperature is heated and held by means of the thermostat 26 at this temperature. The dry material (granulate 10) is then flowed through from below and dried intensively.

The is now moist via the output line 24 Desiccant through the fan 30 and the input

3U5921

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line 36 is printed back into the left desiccant chamber 34, where it is dried again »

During this circulation of the drying medium in the closed circuit, the desiccant is in the right desiccant chamber 34 'by means of the hot air stream which is supplied via the supply line 4-6', regenerated and then cooled. To control this process are in the drain lines 48 and 48 ' Thermostats 58 and 58 'connected electrically with a switching device 60 which actuates the air heater 42 are connected. Once the desiccant is in the desiccant chamber 34- 'is completely dry, the temperature of the hot air in the Ablaßleitüng 43 'rises sharply, because then the part of the air heater 42 generated heat that was previously required to evaporate the adsorbed water, in addition to Heating of the Eeißluftstrom is released. However, after The thermostat sends the complete drying of the desiccant, as mentioned, a cooling should take place 58 'when the mentioned temperature jump occurs, a signal is sent to the switching device 60, which then sends the Air heater 42 switches off.

Compared to the previously customary, time-dependent shutdown of the air heater 42, this method has the considerable advantage that this only remains switched on until the drying process has actually ended, whereby a considerable amount of energy is saved.

After switching off the air heater 42 from the regeneration fan 40 by the desiccant in the Desiccant chamber 34 'compressed air cools this then off.

As soon as the dew point sensor 56 reaches the Set moisture content is determined, the control circuit 50 does not operate Solenoid valves shown the reversing device 4Λ, so that the two slides 54 and 54 '- in the drawing to the right - and thereby the desiccant chamber 34 ·· in., which has been regenerated in the meantime switch on the drying cycle while the desiccant is in the left desiccant chamber 3 ^ is regenerated in the manner described.

The dew point sensor 56 used can be an absolute humidity measuring element as it is described and illustrated in DE-PS 16 98 096. This probe consists of an aluminum strip an oxide layer of a defined thickness is applied by an electrolytic process. One over this structure is a vapor-deposited gold coating and an aluminum strip form the two electrodes of a plate capacitor in which the amorphous aluminum oxide is used as Dielectric is used.

The thickness of the gold coating is chosen so that there is a cohesive, electrically conductive layer that is still permeable to water. This allows a balance between the moisture the medium surrounding the sensor and the number of water molecules absorbed by the aluminum oxide to adjust.

The electrical conductivity of the pore walls and the dielectric constant are proportional to the concentration of the water molecules in the dielectric. There

between a certain water content in the pores of the aluminum oxide and the corresponding capacity If there is a reproducible relationship with the value of the sensor, the electrical impedance of the sensor represents is a functional measure of the water vapor partial pressure.

The block diagram of the control circuit 50 shown in FIG. 2 shows the electronics for the evaluation the signals from the sensor, which works on the principle of pulse width modulation. A Oscillator 62, which, as the diagram at point TP1 in Figure 3 shows, square-wave pulses of the frequency 100 Hz (t = 0.01 second), consists of two NOR gates. This oscillator 62 is a differentiator 64 (high pass) connected downstream, its Voltage peaks are used to trigger a univibrator 65. The univibrator 66 is with the dew point probe 56 connected. The output signal of the univibrator 66 (point TP3, see FIG. 3) is rectified in a rectifier 68 (Point TP4-).

As the diagram at point TP3 in Fig. 3 shows, the pulse length t is a measure of the capacitance

Ji.

G of the dew point probe 56; the greater the pulse

length t, the greater the capacitance G. The DC voltage U in the rectifier 68 is

Jv JL.

given by

- 10 -

This means that U is proportional to t. The pulse length _ΐ χ in turn is on the capacitance C of the sensor is determined (t ~ R. C) and is therefore a radio "tion of the absolute humidity of the inert gas or the corresponding Taupunkttemperatür.

Between the magnitude of the direct voltage U and the absolute Humidity is therefore a reproducible relationship that is used for humidity measurement will.

The rectifier 68 is a voltage comparator 70 (Schmitt trigger) connected downstream, at whose reference input with the help of a coding switch (not shown) different setpoint values U for the voltage

have it adjusted. As soon as the measured value U exceeds the preselected setpoint U. When the dew point temperature rises, the voltage comparator 70 switches its output from V - (negative supply voltage) to V + (positive supply voltage) (compare TP5 in FIG. 5) This switching process in the positive direction is triggered via a differentiating element 72 ( High pass) a second univibrator 74- · whose pulse length t is approximately 0.6 seconds (cf.? P6 in Fig. 3). The square pulse of this univibrator 7 ^ - controls via an electronic switch 76, which consists of four NAIiD gates, and a JK flip-flop 78, the base of a transistor, not shown. At a base voltage of around 0 ₈ 7 V, this transistor switches through, which attracts a relay that switches the aforementioned solenoid valve of the reversing device 44 »

The control circuit 50 is designed so that one um the hysteresis shifted switching of the voltage comparator 70 (cf. point TP 5 in FIG. 3) in the negative Direction, d. H. from V + to Y -, has no effect on the subsequent circuit. So that will prevents that after the drop in the dew point temperature due to the flow through the meanwhile regenerated drying medium in the line 20, the reversing device 44 is switched over again immediately. The electronic switch 76 is only activated again when there is a renewed positive switchover pulse. on In this way, the switching state is only achieved with each positive switching pulse of the voltage comparator 70 of the relay operating the solenoid valves changed alternately.

As Figure 1 shows, the control circuit 50 is with the switching device 60 is electrically connected because at the beginning of a new regeneration of the desiccant it must be ensured that the air heater 42 is switched on again. A thermostat 80 provides that the hot air generated by the air heater 42 maintains the required temperature.

In the following, an example is to be used to illustrate which energy savings can be achieved with the help of of the invention over known drying processes is possible. It is assumed that a material with a maximum water content of 1% is dried should be, whereby 300 kg have to be dried every hour.

In a known dryer, the desiccant chambers of which must be regenerated every two hours the desiccant can therefore absorb 6 kg of water without deteriorating in the drying performance. For this will be requires the following amounts of energy:

12 χ 4 kWh per day for heating and evaporation the contained moisture

12 χ 16 kWh per day for heating the desiccant and the container.

This results in an energy requirement of 24-0 kWh per day. Has However, if the material to be dried only has a water content of less than 0.2%, only the following amounts of energy are required needed:

12 χ 0.8 kWh per day for heating and evaporation of the moisture contained in the drying medium y, 6 kWh

per day 1? Cycles 16 kWh each for heating the desiccant, um

to regenerate this 192.0 kWh

201.6 kWh

Although the water content of the goods to be dried is 80% lower, the energy saving is only around «. 16% lower!

With the method and the device according to the invention is at a water content of only 0.2% as

Mm

Input moisture a significantly lower energy requirement necessary because the desiccant in the desiccant chamber is always optimally used, before regeneration begins; one gets by with 2,4 regenerations per day. The energy requirement is calculated as follows:

2.4 χ 4 kWh per day for heating and evaporation the moisture contained in the drying medium

2.4 χ 16 kWh per day for heating the desiccant.

The energy requirement per day is 48 kWh, which is one corresponds to theoretical energy savings of over 75%. In other words, a conventional drying device requires 4 to 2 times more energy than the process and the device according to the invention.

Claims (7)

GERHARD GUSTORF PATENT ADVOCATE. 8300 LANDSHUT P 1 186 - HO Klaus Oschmann Mitterweg 9 8066 Bergkirchen Method and device for drying Patent claims 1. Process for drying solid materials, in particular of granular plastics in which the substance to be dried creates a gaseous drying medium passed through -ndrd, which is then put in a desiccant dried, then heated and finally passed through the fabric to be dried again, wherein the desiccant is regenerated with increasing humidity and the drying medium during the regeneration is dried in a desiccant that has been regenerated in the meantime, marked thereby net that one flows through the desiccant Drying medium when a certain moisture content is reached by the meanwhile by a Hot air flow diverts regenerated desiccant. • * 4 « 3H5921. I. 2. The method according to claim 1, characterized in that the temperature of the hot air stream after the desiccant has flowed through, the heating of the hot air flow is measured is interrupted as soon as this temperature rises by leaps and bounds. 3- Device for drying solid materials, in particular of granular plastics, by means of a through the fabric to be dried, gaseous drying medium, for the drying of which at least two desiccant chambers are provided, which have a reversing device for diverting the drying medium such that the drying medium crosses one of the desiccant chambers and continues to flow to the substance to be dried, while at the same time the other desiccant chamber to regenerate the desiccant from the drying circuit turned off and flowed through by hot air, thereby marked eichnet that in the of the desiccant chambers (34-, 34 ') to the to be dried Substance (10) leading line (20) a dew point sensor (56) is connected to a control circuit (50) is connected, which actuates the reversing device (44-) as soon as a certain moisture content is reached. 4-. Device according to claim 3, characterized in that ( '48, 4-8) for the hot air current of each desiccant! Chamber (34-, 34- ^1), a thermostat (58, 58 ¹⁾ is connected, the above in the drain conduits 3U5921 a switching device (60) is connected to an air heater (42) for generating the hot air flow ε in front of the desiccant ^{chambers (34, 34 1} ), the switching device (6o) in the event of a sudden rise in temperature in the thermostat (58, 58 ') the air heater ( 4-2) switches off. 5- An apparatus according to claim 3 or 4 _t characterized in that the control circuit (50) has a with the dew point sensor (56) univibrator connected (66), which is decisive for the time constant capacity by a capacitance change of the dew-point probe (56) is modified. 6. Apparatus according to claim 5 »characterized in that the univibrator (66) has a rectifier (68) and this a voltage comparator (70) is connected downstream, which is when an adjustable setpoint (U) for the voltage at the output is reached of the rectifier (68) triggers a second univibrator, which via an electronic switch (76) and a flip-flop (78) controls a relay which actuates the reversing device (44). 7. Apparatus according to claim 6, characterized in that the after falling below the Setpoint (U) with hysteresis switching voltage comparator (70) does not affect the second univibrator (74).