EP0029933B1 - Apparatus and method for the periodical cleaning of heat exchanger tubes of solid deposits and use of this apparatus - Google Patents

Description

The invention relates to a device and a method for the periodic cleaning of the tubes through which a process gas flows and of a heat exchanger used for heat dissipation from a hot, finely dispersed process gas stream containing deposits of the solid and the use of this device.

When separating finely dispersed solids, which are formed in a thermal process, such as the production of soot or the extraction of pyrogenic silicas, and which are transported by a gas stream - in these cases from the own process gas stream - the task is to add the heat carried by the transport gas withdraw in order to be able to separate the solid matter in bag filters or other devices and to be able to return the heat dissipated to the production process. The heat is usually extracted via heat exchangers, the exchange elements of which consist of tube bundles or individual tubes through which the transport or process gas is passed. Depending on the type of solid suspended matter contained in the gas stream, it deposits on the inner walls of the pipes over the course of the operating time, which on the one hand reduces the gas flow and on the other hand the heat transfer. This process can lead to the complete overgrowth of individual pipes, which can damage the heat exchanger. If, as is customary, the tube ends of tube-bundle heat exchangers are attached to or in the opposing end plates of the exchanger, the cooling of clogged tubes in the vicinity of hotter tubes due to increasing thermal insulation causes material stresses to occur. The added, shortened pipe can be torn off by the neighboring pipes stretching over it.

For processes in which a hot process gas or exhaust gas carries a finely divided solid product which has to be separated from the previously cooled gas stream, there has therefore been an urgent need to solve the batch problem in heat exchangers working with pipes without the Interrupting the operation of the heat exchanger or taking refuge in complex and repair-prone mechanical cleaning devices. The addition of heat exchangers in the manufacture of Russians and pyrogenic silicas, which tend to build up and grow particularly strongly due to their high surface activity, posed particular problems.

According to DE-A-1 501 477, a device for cleaning the pipes through which a process gas flows is known from a heat exchanger used to remove heat from a hot solid gas containing process gas from deposits of the solid. A line provided with at least one shut-off device for the periodic supply of a cleaning gas which is overpressure the process gas in connection with jet nozzles is provided centrally above the gas outlet openings of the heat exchanger tubes. The counterflow of process gas and cleaning gas is essential for a good cleaning effect.

The invention has for its object to provide a device for the same purpose, with which the risk of blockages of the blow nozzles or the process gas pipes in the blow nozzle area by the solid particles suspended in the process gas and the braking of the process gas flow by the purge gas jet can be avoided. In addition, the device should be structurally compact and insensitive to thermal stresses and should allow a simple cleaning of a limited number of tubes of the heat exchanger to avoid major pressure fluctuations in systems in which the heat exchanger is integrated.

It has now been found that the problems described can be solved simply and sustainably with a device which only needs to be installed in the process gas inflow pipe and uses jet nozzles which are aligned centrally over the mouths of the process gas pipes on one side of the heat exchanger and with one are provided with shut-off devices for the periodic supply of a cleaning gas which is overpressurized with respect to the process gas.

The device is characterized in that the jet nozzles above the gas inlet openings of the process gas pipes and the supply lines for the cleaning gas are each arranged on a line of pipes, at least two of these supply lines lying one above the other and the lower, shorter line on the outside and the upper line internal nozzles supplied and the length of the external and internal jet nozzles are dimensioned such that their outlet openings open in the same plane.

The jet nozzles are arranged at a distance above the gas inlet mouths of the heat exchanger tubes so that the gas jet, which is intended to clean off the solid deposits in the process of being prepared, attains its full cross section at the gas inlet mouth of each tube. Since the jet nozzles emit a fan-shaped gas jet, their outlet cross-section also has a distance-determining effect. In general, the cross section of the nozzle orifices is significantly smaller than the inlet cross section of the exchanger tubes and can e.g. have a ratio of 1: 9.4 in tube bundle heat exchangers used in furnace black systems. With such a design, nozzle spacings between 90 and 150 mm have proven to be favorable.

The shut-off devices switched on in the purge gas supply lines serve to specifically limit the supply of purge gas to certain purge gas supply lines. This means that the heat is cleaned off in series or in succession exchanger tubes enabled. As a result, the cleaning periods do not interfere with the continuous process in which the heat exchanger works.

The supply lines for the cleaning gas, which are led through the process gas inflow pipe on the heat exchanger, can be rigid with the inflow pipe, e.g. connected by welding, and the uppermost supply line can be divided into two line sections which are closed at the end and are at a short distance from one another. Each section of the uppermost line thus spans half of the relevant inflow pipe cross section and carries a shut-off device at its outer end. It is also advantageous if the closed ends of these two line sections are received in a sliding guide, because this avoids thermal stresses and the line arrangement becomes insensitive to vibration.

An advantageous variant of the invention, which can be applied to all embodiments of the device described so far, consists in that the individual supply lines for the cleaning gas are combined to form a main line and a pressure pulse generator for registering the cleaning periods is installed in the main line, the latter in the area of a or a venturi tube installed in the main line is connected.

Finally, since the cleaning effect is more intensive the higher the speed of the gas jet, a particularly preferred embodiment of the device according to the invention provides that the jet nozzles are designed as Laval nozzles with which gas inflow velocities above the speed of sound can be achieved.

With the device described, tubes of a heat exchanger charged with a hot, finely dispersed solid containing process gas stream can be easily and effectively cleaned from the inevitable deposits of the solid by periodically abruptly directing the tubes during the continuous process operation with a centrically directed into the gas inlet mouths of the tubes released and briefly maintained gas jet flushes at high speed.

The purge gas jet should suddenly, i.e. Unleash its full strength in less than 3 seconds.

The duration of the rinsing period is of course influenced by the type of solid to be cleaned, its tendency to adhere and aggregate. The cleaning effect increases with the speed of the flushing gas jet, which is why a variant of the method according to the invention provides that the gas jet has a supersonic speed. This can be achieved with Laval nozzles.

A preferred variant of the method provides for flushing the tubes of the heat exchanger in series during a cleaning cycle by means of the shut-off elements assigned to the cleaning gas in order to keep the effects on the parallel heat exchange and the effects on the overall process as low as possible.

The duration of the breaks between the cleaning periods is also of particular importance. It has proven to be useful. to choose an empirically determined time interval for the cleaning period, in which none of the heat exchanger tubes can overgrow with solids.

With tube bundle exchangers, one can proceed in such a way that the jacket temperature of the outer tubes, which has been found to be most at risk of clogging, is monitored and the flushing process is triggered when the empirically or mathematically determined threshold value is undershot. As an alternative to this, the temperature of the process gas flowing out of the external heat exchanger tubes can also be monitored and the purging process triggered if a predetermined value is exceeded.

The choice of the purge gas generally depends on the type of process gas that transports the solid particles. It is important, however, that the gas is dry, so that the separated solid does not stick or clump together. In the furnace black process, the use of hot steam at a temperature that is above the gas outlet temperature of the heat exchanger to be cleaned has proven itself.

Where the process delivering the so-called process gas flow permits, the purge gas can also be supplied to the jet nozzles in a pulsating manner. The intermittent gas jet pulses transmitted into the exchanger tubes amplify the detachment or degradation of the solid layers due to a kind of cavitation effect.

Finally, an object of the invention is the use of the cleaning device using the cleaning method for heat dissipation from the process exhaust gas of the production of carbon black or pyrogenic inorganic oxides, such as silicon dioxide or else titanium dioxide, aluminum oxide, Al-Si mixed oxides and oxide mixtures.

The structure and process function of the device according to the invention are explained in more detail below with reference to the accompanying drawing and an exemplary embodiment.

• Fig. 1 eine Seitenansicht der im Anströmrohr eines Rohrbündelwärmeaustauschers angeordneten Abreinigungsvorrichtung mit fixen Strahldüsen im Schnitt A-A von Fig. 2;
• Fig. 2 eine Draufsicht auf die Vorrichtung von Fig. 1.

The drawing shows:

• 1 shows a side view of the cleaning device arranged in the inflow pipe of a tube bundle heat exchanger with fixed jet nozzles in section AA of FIG. 2;
• FIG. 2 shows a top view of the device from FIG. 1.

1 and 2, the cleaning device is installed in the process exhaust gas inflow pipe 6 of a tube bundle heat exchanger which is switched on in the furnace black process in front of the filter system for soot separation.

The inflow pipe 6 is connected via flanges 12 to the jacket of the heat exchanger; however, it could also be part of the heat exchanger. Whose exchange tubes 2 with a 43.1 mm clear opening with their inlet openings for the soot particle-containing process exhaust gas in the bores of a holding and distribution plate 13 and are with this is connected by welding. Centrally above the gas inlet openings 1 and at a small distance from them (approx. 100 mm), jet nozzles 5 with a nozzle diameter of 14 mm open out. The nozzles 5 for the inner tubes are located on the upper feed lines 4 for the cleaning gas, the nozzles 5 'for the outer tubes on the lower purge gas feed lines 4'. The lines 4 ', each with their rear sections guided through the inflow tube 6 and rigidly connected to it, are firmly connected on their upper side to the underside of the lines 4 by welding. Opposing lines 4 span approximately half the cross-section of the inflow tube 6 and are accommodated at their closed end in a sliding sleeve 7, one line end each being firmly connected to the sleeve and the corresponding line end being slidably inserted into the sleeve.

In each of the purging gas supply lines, a blocking element 3 is provided outside the inflow pipe for controlling the purging gas flow. The individual supply lines for the cleaning gas are combined to form a main line 9, in which a pressure pulse generator 10 is connected in the area of a Venturi tube 11. The routing of the collecting and main line shown in FIG. 2 and curved around the inflow pipe not only reduces the space requirement, but also simplifies, above all, thermal protection measures against the formation of condensate within the pipe system.

1 and 2, 3 rinsing jets are let out of the nozzles 5, 5 'within a predetermined time interval or on the basis of a temperature signal taken from the jackets or gas outlet openings by suddenly opening shut-off elements. They generate a strong gas acceleration in the tubes 2, whereby the fine soot particles adhering to the tube walls are detached and discharged. Although basically possible, not all nozzles are put into operation at the same time, but in series or in succession. This ensures that a flushing process practically does not interfere with the operation of the heat exchanger and that the flushing gas supply lines can be economically dimensioned.

example

Teilchendurchmesser elektronenmikro-

Farbstärke

A powdered carbon black is produced in a plant for the production of furnace black with the following test data:

Particle diameter electron micro

Color strength

This soot is produced by generating a stream of hot combustion gases by converting air with fuel (e.g. fuel gas) and spraying a highly aromatic soot raw material into the hot combustion gases. After the soot formation, water is sprayed in and the soot-containing exhaust gas flow is first passed through a system of heat exchangers and then through filters which separate the soot from the exhaust gas.

In the present case, an exhaust gas flow of 6250 Nm ³ / h with a temperature of 780 ° C was sent into the first heat exchanger. This exhaust gas stream containing water vapor contained approximately 1050 kg / h of the soot defined above. The exhaust gas freed from water vapor has the following composition:

In the first heat exchanger, the soot exhaust gas mixture is cooled to 570 ° C by cooling with 3300 Nm ^l / h process air.

• Abgasführende Rohre: 57 Stück

43,1 mm Innendurchmesser 13060 mm LängeIn order to be able to supply the soot-containing exhaust gas to a filter equipped with glass filter bags, the temperature had to be reduced to at least 280 ° C. For this purpose, the exhaust gas was sent through a second heat exchanger, which had the following construction data:

• Exhaust pipes: 57 pieces

43.1 mm inner diameter 13060 mm length

This heat exchanger was charged with 12,000 Nm ³ / h of air at 70 ° C on the cooling air side. At the same time, the exhaust-gas-carrying pipes (mostly two pipes at a time) were flushed with 0.83 kg of steam at 310 ° G for 3 seconds, the steam having an exit speed of 960 m / sec. The rinse cycle was completed after 90 seconds, followed by a dead time of 7 minutes. With this method of operation, the heat exchanger cooled the soot-containing exhaust gas from 570 ° to 280 ° C. The pressure loss in the heat exchanger was 65 mbar. A water injection behind the heat exchanger to lower the temperature to the permissible temperature of the subsequent bag filter was not necessary.

For comparison, the same conditions were maintained in the soot production plant and in the heat exchanger / separator plant, but the steam purging was switched off. After only 60 minutes, the heat exchanger only cooled the soot-containing exhaust gas from 570 ° to 350 ° C and the pressure loss rose to 95 mbar.

• 1. Durch die Dampfspülung bleiben die Wärmeaustauscherrohre frei und bedingen einen geringen Druckabfall.
• 2. Durch den geringen Druckabfall können höhere Mengendurchsätze gefahren werden.
• 3. Durch die Dampfspülung wird dem russhaltigen Abgas mehr Wärme entzogen, so dass direkt in ein Schlauchfilter gefahren werden kann.
• 4. Ohne die Dampfspülung müsste hinter dem Wärmeaustauscher noch einmal Wasser eingespritzt werden, was durch Knötchenbildung zu einer Verschlechterung der Russqualität führt.

This experiment shows the following advantages for the working method according to the invention:

• 1. The steam flushing leaves the heat exchanger tubes free and causes a low pressure drop.
• 2. Due to the low pressure drop, higher flow rates can be achieved.
• 3. The steam flushing removes more heat from the soot-containing exhaust gas so that it can be driven directly into a bag filter.
• 4. Without the steam rinsing, water would have to be injected behind the heat exchanger again, which leads to deterioration of the soot quality due to the formation of nodules.

Claims (15)

1. An apparatus for the periodic cleaning of the pipes, through which a process gas flow, of a heat exchanger used for the removal of heat from a hot stream of process gas containing a finely-dispersed solid, from deposits of the solid, in which apparatus discharge nozzles (5) are adjusted centrally above the orifices of the process gas pipes (2) located on one side of the heat exchanger and are connected to a line (4) provided with stop members (3), for the periodic supply of a cleaning gas which is under excess pressure with respect to the process gas, characterised in that the discharge nozzles are positioned above the gas inlet orifices of the process gas pipes and the lines (4) to supply the cleaning gas are each positioned above a series of pipes positioned on a line, and at least two of these supply lines are superimposed in each case and the lower, shorter line (4') supplies external nozzles and the upper line supplies internal nozzles and the length of the external and internal discharge nozzles is calculated such that their outlet openings discharge in the same plane.

2. An apparatus according to claim 1, characterised in that the supply lines (4, 4') for the cleaning gas which are guided through the process gas inflow pipe (6) to the heat exchanger are rigidly connected to the inflow pipe, and the uppermost supply line is divided into two line sections which are closed at the end and are opposite each other in a small spacing.

3. An apparatus according to claim 2, characterised in that the ends of the two line sections are accommodated in a sliding guide (7).

4. An apparatus according to claims 1 to 3, characterised in that the individual supply lines for the cleaning gas are combined into a main line (9) and a pressure pulse generator (10) to register the cleaning periods is installed in the main line (9).

5. An apparatus according to claims 1 to 4, characterised in that the pressure pulse generator (10) is connected in the region of a screen installed in the main line or in the region of a Venturi tube (11) installed in the main line.

6. An apparatus according to claims 1 to 5, characterised in that the discharge nozzles (5) are designed as Laval nozzles.

7. A process for cleaning the pipes of a heat exchanger which are charged with a hot stream of process gas containing a finely-dispersed solid, from de- posists of the solid using the apparatus according to claims 1 to 6, characterised in that the pipes are periodically flushed during the continuous process operation with a gas stream of a high speed which is directed centrally into the gas inlet orifices of the pipes, is released suddenly and is maintained for a short time.

8. A process according to claim 7, characterised in that the gas stream has supersonic speed.

9. A process according to claim 7 or 8, characterised in that the pipes are flushed in series during a cleaning cycle by stop members allocated to the individual lines for the cleaning gas.

10. A process according to claims 7 to 9, characterised in that an empirically determined time interval is selected for the cleaning period, in which interval none of the heat exchanger pipes may become congested with solids.

11. A process according to claims 7 to 10, characterised in that the casing temperature of the external pipes is monitored and the flushing procedure is initiated when the temperature falls below an empirically- or computation-determined threshold value.

12. A process according to claims 7 to 11, characterised in that the temperature of the process gas flowing out of the external heat exchanger pipes is monitored and the flushing procedure is initiated when a pre-determined value is exceeded.

13. A process according to claims 7 to 12, characterised in that flushing is carried out using superheated steam or dry gases.

14. A process according to claims 7 to 13, characterised in that flushing is carried out using a pulsating stream of gas.

15. The use of the apparatus according to claims 1 to 6 for the removal of heat from the process waste gas of the production of carbon blacks or pyrogenic inorganic oxides, in particular silicon dioxide.

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