HEAT EXCHANGER ADAPTED FOR THE PRODUCTION OF CARBON BLACK

Abstract: A tube heat exchanger intended to be used for the production of carbon black, comprises a cylindrical chamber (16) which is enclosed by a cylindrically shaped outer shell wall (14') and two end walls (17, 18), and a plurality of tubes (13') which extend through the entire cylindrically shaped chamber (16), from one end wall to the other. To the heat exchanger are provided an inlet (1') and an outlet (25) for gas intended to flow on the outside of the tubes (13'). Moreover, the heat exchanger comprises a further shell wall (23), which is cylindrical and placed inside and concentrically with the outer shell wall (14'), so that a cylindrically shaped gap space (29), which is open at both ends, is formed between the two shell walls. The gas entering through the inlet (1') passes this gap space (29) before it comes into contact with the tubes (13'). The advantage of this is that the outer shell wall is cooled and that the amount of carbon precipitations within the tubes diminishes.

Description: HEATEXCHANGERADAPTEDFORTHEPRODUCTIONOFCARBONBLACK The present invention relates to a heat exchanger that may be used, e.g., for the production of carbon black. Further, the invention relates to a process for the production of carbon black and a plant including said heat exchanger for the realization of the process. Carbon black is the term used for the pulverized forms of carbon which are produced by incomplete combustion or thermic degradation of natural gas or mineral oil. Depending upon the method of production, different types of carbon black arise, namely so called channel black, furnace black and pyrolysis black (also called thermal black). Channel black is characterized by a lower pH, a higher content of volatile constituents and fewer chain-like structures between the particles. It has the smallest particle size of all materials produced in industry and its particles are within the colloidal size range. Its major field of use is as a reinforcement material in rubber, where it improves both the wear resistance and the oil resistance of the rubber. Thermal black consists of relatively coarser particles and is primarily used as a pigment. Furnace black, which has been produced of natural gas, has a medium size, while the furnace black produced of oil may occur within a broad range of controlled particle sizes and is particularly suitable for reinforcing synthetic rubber. Furnace black is by far the most important form of carbon black and is used to a considerably larger extent than the other two. Also the present invention relates to specifically this type of carbon black, which in the present application is referred to simply as just "carbon black". Carbon black is commercialized in the form of a powder, of pellets or of a paste. The powder is kept in multi-walled paper bags or in lined barrils. Carbon black is used as an additive in rubber tyres and other wear resistant rubber products. In plastics it is used as a reinforcing agent, as an opacifier, as a means for increasing the electrical conductivity and for absorbing ultraviolet light. Further, carbon black is used for instance in duplicating carbon, in ink ribbons for typewriters, in colour pigments and for influencing the weather. A plant for the production of carbon black (i.e., of the type furnace black) depicted in fig. 1 and 2 is conventional, except for the support base 15 in fig 2. Incoming combustion air flows through a tube conduit 1 into the upper part of a tube heat exchanger 2, in which it is preheated before the following combustion of oil in the burner 9 and the combustion reactor 3. The preheated air is passed into the combustion chamber via a conduit 5. Oil is added to said reactor via a tube conduit 4. The amount of air corresponds to about 50% of the stoichometric amount of oxygen gas required for a complete combustion ofthe oil, whereby carbon black is formed. It is also possible to add water into the reactor 3, which has an impact on the quality ofthe final product. The mixture of suspended carbon black in the consumed combustion air is led away from the top ofthe heat exchanger via a conduit 6, through a normally water-cooled cooler 7 to a filter arrangement 8, conventionally equipped with textile bag filters. In this filter arrangement the carbon black is completely filtered off from the gas flow, which is then passed out through a nonreturn valve 16 for further purification in a plant 11, before it is exhausted into the ambient air via a chimney 12. The construction ofthe heat exchanger 2 may be more clearly seen in figure 2. The heat exchanger is ofthe tube type, with a plurality of through, substantially vertical tubes 13. Inside these tubes rise the hot gases from the combustion, whereby they are cooled by the air that enters via the inlet 1 and passes outside the tubes 13 downwards towards the outlet 5, within a shell 14. The temperature at the inlet ofthe tubes 13 may be about 1000°C and the combustion air passing along the exterior ofthe tubes may be preheated to about 800°C. These conditions result in utmost severe stresses for the materials in the heat exchanger. The part of the heat exchanger that is submitted to the highest mechanical stress is the lower part of the shell, where the metal temperature may amount to about 900°C. A measured temperature profile in the shell 14 is shown in the diagram in figure 4. At a simultaneous inner pressure of about 1 bar and a shell diameter of about 2000 mm, and a tower height of about 13 m, this implies that very thick metal walls have to be used since the strength ofthe material at these temperatures only is in the order of magnitude 5 N/mm . Instability may easily arise in the shell at downtimes, whereby the risk of buckling is imminent. A number of shut-downs because of bucklings have also occurred, with considerable costs as a consequence. Thus, there are strong reasons for wanting to decrease the temperature of the shell, thereby working under conditions where the strength in the shell is sufficiently high to avoid problems relative to instability. An undesired process phenomenon that arises in the heat exchanger is that the tubes 13 may be fouled by carbon. Thus, the incoming air of ambient temperature or somewhat higher cools the tube wall and the burnt gases , which are in equilibrium at the inlets of the tubes 13 and will therefore be undercooled on the inside ofthe tube, whereby carbon may be formed according to the Boudoir reaction 2CO - • C + CO 2 . Together with the carbon particles in the gas, the thus formed carbon contributes to the fouling. One way of reducing the tendency of fouling would be to diminish the degree of undercooling ofthe gas, i.e., to operate with warmer tube walls. This could be achieved by preheating the air that flows into the conduit 1. However, then the lower part of the shell would become even hotter and the stress upon this part would thus be further accentuated. Hence, a primary object ofthe present invention is to reduce the thermal stress upon the shell of a tube heat exchanger, in particular at the production of carbon black. A second object ofthe present invention is to prevent foulings by carbon within the heat exchanger tubes. Further objects ofthe present invention is to extend the useful life ofthe heat exchanger during the production of carbon black and to minimize the number of running disturbances in said exchanger. Still another object ofthe present invention is to improve the efficiency of the heat exchanger. These and further objects have been attained by constructing the heat exchanger with double shell walls in accordance with the characterizing clauses of the independent claims. For illustrative but non-limiting purposes, the invention will now be further described with reference to the appended drawings. These are herewith briefly presented: Figure 1 shows schematically a conventional plant for the production of carbon black, such as already described above. Figure 2 shows a heat exchanger according to the state of the art (with the exception of support plate 15), as already described above. Figure 3 shows a heat exchanger according to the present invention. Figure 4 shows the shell temperature in the heat exchanger according to figure 2 and figure 3, respectively. The heat exchanger according to the invention occupies the same place in the basic scheme according to figure 1 as the previously known heat exchanger. At the bottom it has an inlet 1 ' for inflowing combustion air (heat exchange air). The temperature of this air is suitably between the ambient temperature and about 100°C. The heat exchanger comprises a shell 14' of a substantially circular cross-section, which encloses a chamber 16. The shell typically has considerable dimensions, such as a diameter of about 2 m and a height of between 10 and 15 m, and encloses between 50 and 150 tubes 13'. Equally to the other parts ofthe heat exchanger, the construction material is suitably a fire-resistant stainless steel. At the top and at the bottom, the heat exchanger is delimited by substantially circular end walls 17 and 18, respectively, of a suitable insulating material, such as, e.g., a ceramic substance. In accordance with a preferred embodiment ofthe present invention, the lower end wall 18 carries a support plate or manifold 15 with an inlet 19 and an outlet 20 for a cooling medium, suitably air, which is allowed to pass through the support manifold, in order to heat-insulate the heat exchanger as much as possible from the very hot evaporation chamber 10 (at about 1000°C). Heat exchanger tubes 13' extend substantially vertically and through the whole chamber 16. These tubes have for instance a diameter of between 5 and 10 cm. At the bottom, they are welded in sleeves 21 , while they at the top extend into tightening compensators or sleeves 22, which allow for a certain mobility in view of recurrent contractions and heat expansions, respectively, of the tubes. The gases that flow at the bottom into the tubes 13' have a temperature of between 900 and 1 100°C, while at the outlet of the tubes they have cooled to between 500 and 800°C, normally about 650°C. An essential feature of the present invention is that the heat exchanger has a second or inner shell wall 23 placed inside and concentrically with the outer shell wall 14', to form an annular space 29 between the walls 14', 23. This inner shell wall is suitably placed on the support plate 15, or if the latter is dispensed with, directly on the end wall 18. Further, wall 23 is solid, i.e., imperforate and impervious to gas flow. At its top, the inner wall 23 ends in the immediate proximity ofthe end wall 17, whereby a gap 24 is formed between the lower surface ofthe end wall 17 and the upper edge of the inner shell wall. In order to support and stabilize the inner shell wall, suitable rods may be placed between the two shell walls. These rods also make possible the obtainment of a substantially uniform gap between the two shell walls along their entire height. Mounted in the inner shell wall near the bottom thereof, is an outlet conduit 25 for the preheated combustion gases, which outlet conduit connects to the conduit 5. These have now a temperature of between 600 and 1000°C, usually around 800°C. In order to distribute the incoming combustion air around the whole annular space 29, there is provided a shield 26 at the height ofthe inlet 1 ', which shield is substantially cylindrical. Radially outside this shield, the outer shell 14' has an enlarged portion 27 for making space for the incoming gases through the inlet 1 '. The incoming air received through the inlet 1 ' is distributed around the outside of the shield 26 and then it flows through the annular opening 28 of the space 29, which space may suitably be between 3 and 10 cm wide. As the air flows upwardly within the space 29, it cools the two shell walls, and thus is heated by the inner shell wall. When the air reaches the gap 24, it may have a temperature of between 150 and 400°C, for instance around 250°C. At its top, the outer shell wall can be shaped with a widened part 30 for decreasing the pressure drop loss when the air is diverted into a downwardly directed flow. When the air now reaches the tubes 13', it is already to a certain extent preheated, whereby the gases within the tubes do not become undercooled to the same extent as with the prior art. Consequently, the problematic carbon foulings within the tubes are avoided. Furthermore, the surprising advantage is obtained that the heat transfer on the outside of the tubes is improved by the fact that the air there, as already mentioned, has been preheated and, thereby, increased considerably in volume, which in turn results in a higher flow velocity. Finally, the preheated air exits through the outlet tube 25, which extends through aligned holes provided in the shield 26 and the inner wall 23, respectively. In comparison with the heat exchanger according to figure 2, the exchanger according to the invention has made possible a considerably improved heat economy. Thus, with identical flows and temperatures, the temperature ofthe product gas in the conduit 6 has been decreased considerably. Moreover, the necessity of insulating the outside ofthe heat exchanger has been drastically reduced. Further, the thermal stresses on the loaded outer shell has been substantially reduced. In figure 4 is shown how, according to prior art (figure 2), the temperature ofthe shell at the bottom amounts to 900°C, and then decreases upwards to about 100°C at a height of 13 m. These temperatures have now been substantially reduced thanks to the unloaded inner shell, so that a maximal temperature of about 640°C is obtained adjacent to the bottom, see again figure 4.

Claims: CLAIMS 1. Tube heat exchanger, in particular intended to be used for the production of carbon black, comprising a substantially cylindrical chamber (16) enclosed by a substantially cylindrically shaped outer shell wall (14') and two end walls (17, 18), and a plurality of tubes (13') which extend through the entire substantially cylindrically shaped chamber (16), from the one end wall to the other, an inlet (1 ') and an outlet (25) being provided on the heat exchanger for gas to flow on the outside ofthe tubes (13'), characterized in that the heat exchanger comprises a further shell wall (23), which is substantially cylindrical and placed inside and substantially concentrically with the outer shell wall (14'), so that a substantially cylindrically shaped gap space (29), which is open at both ends, is formed between the two shell walls, the gas flowing in through the inlet (V), passing this gap space (29) before it comes into contact with the tubes (13'). 2. Heat exchanger according to claim 1, characterized in that at the same height as the gas inlet (1') is provided a shield (26) with the purpose of dispersing the incoming gas in the cylindrically shaped gap space (29). 3. Heat exchanger according to claim 1 or 2, characterized in that between the chamber (16) and the bottom end wall (18) is provided a support plate with an inlet (19) and an outlet (20) for passing a cooling medium. 4. Heat exchanger according to any ofthe preceding claims, characterized in that at the bottom the tubes (13') are clamped or welded into sleeve or casing tubes (21 ) and at the top the tubes run in compensators, sleeve or casing tubes (22), the latter allowing for a contraction and a heat expansion, respectively, of the tubes. 5. Plant for the production of carbon black comprising an oil burner (9), a combustion reactor (3) and a heat exchanger for preheating the incoming air to the combustion reactor by means ofthe product gas stream from said combustion reactor, characterized in that the combustion reactor is constructed in accordance with any of claims 1 to 4. 6. Process for the production of carbon black, according to which oil is burnt in a burner (9) and in a combustion reactor (3) with air that has been preheated by heat-exchanging it with the hot product gas stream from the combustion reactor in a tube heat exchanger, whereafter the product gas stream is passed from the heat exchanger through a conduit to a separation plant in which the formed carbon black is taken care of, characterized in that the air entering into the heat exchanger is first brought to cool the outer shell wall (14') of the heat exchanger, before it comes into contact with the heat exchanger (13'). 7. Process according to claim 6, wherein the air flowing into the heat exchanger first is brought to pass through a substantially cylindrically shaped gap space (29) that is open at both ends, which gap space is formed by the outer shell wall (14') and an inner shell wall (23) situated inside the former. 8. A tube heat exchanger, comprising: a cylindrical wall structure connected to first and second end walls for forming a chamber; a plurality of tubes extending longitudinally within the chamber from one ofthe end walls to the other end wall for conducting gases; and an air inlet and an air outlet communicating with the chamber for the conducting of heat exchanger air to and from the chamber, respectively, the heat exchange air flowing in heat exchanging relationship with the tubes within the chamber; the cylindrical wall structure being hollow and forming an air passage extending longitudinally from the inlet to the chamber so that the heat exchange air is conducted in heat exchanging relationship with a section of the wall structure before contacting the tubes. 9. The tube heat exchanger according to claim 8 wherein the inlet communicates with the air passage adjacent the first end wall of the chamber, and the air passage communicates with the chamber adjacent the second end wall. 10. The tube heat exchanger according to claim 9 wherein the tubes conduct gasses from the first end wall to the second end wall. 11. The tube heat exchanger according to claim 10 wherein the outlet is situated adjacent the first end wall. 12. The tube heat exchanger according to claim 8 wherein the wall structure includes radially spaced inner and outer wall sections forming the air passage therebetween. 13. The tube heat exchanger according to claim 12 further including a cylindrical shield extending coaxially with respect to the chamber and situated between the inlet and the air passages for dispersing the air incoming through the inlet. 14. The tube according to claim 12 wherein the inlet communicates with the air passage adjacent the first end wall, and the air passage communicates with the chamber adjacent the second end wall, a manifold disposed longitudinally outside of the first end wall for conducting a cooling medium across the ends ofthe tubes disposed adjacent the first end wall. 15. The heat exchanger according to claim 12 wherein ends ofthe tubes disposed adjacent the second end wall are loosely disposed in the sleeves to permit thermal expansion and contraction of the tubes. 16. Apparatus for the production of carbon black comprising: an oil burner having a first air inlet and an oil inlet; a combustion reactor communicating with the oil burner; and a heat exchanger comprising: a cylindrical wall structure connected to first and second end walls for forming a chamber, a plurality of tubes extending longitudinally within the chamber from the first end wall to the second end wall, the tubes communicating with an outlet ofthe combustion reactor for conducting hot gases received therefrom, a second air inlet and an air outlet communicating with the chamber for conducting heat exchanger air to and from the chamber, respectively, the heat exchange air flowing in heat exchange relationship with the tubes within the chamber, the outlet communicating with the first air inlet ofthe oil burner, the cylindrical wall structure forming an air passage extending longitudinally from the second air inlet to an end ofthe chamber adjacent the second end wall so that the heat exchange air is conducted in heat exchange relationship with a section ofthe wall structure to transfer heat from the first end ofthe wall structure toward the second end thereof. 17. The apparatus according to claim 16 wherein the structure includes radially spaced inner and outer wall sections forming the air passage therebetween. 18. A process for the production of carbon black, comprising the steps of: A. burning a mixture of air and oil or natural gases to generate combustion gases, B. conducting the combustion gases within tubes extending from a first end to a second end of a chamber of a heat exchanger, C. passing heat exchange air through the chamber in heat exchanging relationship with the tubes to preheat the air, D. conducting the heat exchange air from step C to be mixed with oil or natural gas in step A, and E. prior to step C, conducting the heat exchange air within a passage formed by a wall structure of the heat exchanger, the heat exchange air being introduced with the passage at a location adjacent the first end ofthe heat exchanger to transfer heat from the first end of the heat exchanger toward the second end thereof. 19. The process according to claim 18 wherein the heat exchanger is cylindrical, the wall sections forming a cylindrical space therebetween, step E comprising conducting the air longitudinally within the passage before entering the chamber, the air through the chamber in a direction opposite a direction in which the hot gases flow through the tubes.

Inventors: BERGLUND GOERAN, ERIKSSON ULF, BERGLUND, GOERAN, ERIKSSON, ULF
Applicants: EDMESTON AB
Application year: 1996
Main classification: F28F9
IPC codes: C09C1, F28D7, F28F9