GAS-TIGHT, HEAT-PERMEABLE MULTILAYER CERAMIC COMPOSITE TUBE

Abstract

Described herein is a gaslight multilayered composite tube having a heat transfer coefficient of >500 W/m.sup.2/K which in its construction over the cross section of the wall of the composite tube includes as an inner layer a nonporous monolithic oxide ceramic surrounded by an outer layer of oxidic fiber composite ceramic, where this outer layer has an open porosity of 5%<ε<50%, and which on the inner surface of the composite tube includes a plurality of depressions oriented towards the outer wall of the composite tube. Also described herein is a method of using the multilayered composite tube as a reaction tube for endothermic reactions, jet tubes, flame tubes or rotary tubes.

Claims

1. A multilayered composite tube having a heat transfer coefficient of >500 W/m.sup.2/K comprising at least two layers which in its construction over the cross section of the wall of the composite tube comprises as an inner layer a zero-open-porosity monolithic oxide ceramic surrounded by an outer layer of oxidic fiber composite ceramic, wherein this outer layer has an open porosity ε of 5%<ε<50%, and which on the inner surface of the composite tube comprises a plurality of depressions oriented towards the outer wall of the composite tube.

2. The composite tube according to claim 1, wherein the thermal shock resistance according to DIN EN 993-11 of the composite tube is greater than 50 K/h.

3. The composite tube according to claim 1, wherein the depressions have a depth of 0.5 mm to 2 mm.

4. The composite tube according to claim 1, wherein the depressions are uniformly distributed over the inner surface of the composite tube.

5. The composite tube according to claim 1, wherein the depressions are nonuniformly distributed over the inner surface of the composite tube.

6. The composite tube according to claim 1, wherein the inner surface of the composite tube is provided with depressions to an extent of 10% to 95% based on the total inner surface of the composite tube.

7. The composite tube according to claim 1, wherein the depressions are concave.

8. The composite tube according to claim 1, wherein the depressions have a construction that is circular in cross section and have a diameter of 2 mm to 30 mm.

9. The composite tube according to claim 1, wherein the total wall thickness of the composite tube is 0.5 mm to 50 mm.

10. The composite tube according to claim 1, wherein the tube internal diameter of the composite tube is 10 mm to 1000 mm.

11. The composite tube according to claim 1, wherein the employed oxidic fiber composite ceramic is SiC/Al.sub.2O.sub.3, SiC/mullite, C/Al.sub.2O.sub.3, C/mullite, Al.sub.2O.sub.3/Al.sub.2O.sub.3, Al.sub.2O.sub.3/mullite, mullite/Al.sub.2O.sub.3 and/or mullite/mullite.

12. The composite tube according to claim 1, wherein the composite tube contains two layers, including an inner layer and an outer layer, wherein the inner layer is constructed from nonporous monolithic oxide ceramic and the outer layer is constructed from oxidic fiber composite ceramic.

13. The composite tube according to claim 1, wherein the composite tube has a structure in which the nonporous monolithic oxide ceramic is covered by oxidic fiber composite ceramic.

14. The composite tube according to claim 1, wherein the inner layer has a minimum layer thickness of 0.5 mm to 45 mm.

15. A method of using the composite tube according to claim 1, the method comprising using the composite tube in the production of synthesis gas by reforming of hydrocarbons with steam and/or carbon dioxide, coproduction of hydrogen and pyrolysis carbon by pyrolysis of hydrocarbons, production of hydrocyanic acid from methane and ammonia or from propane and ammonia, production of olefins by steamcracking of hydrocarbons and/or coupling of methane to ethylene, acetylene and to benzene.

16. A method of using the composite tube according to claim 1, the method comprising using the composite tube as a reaction tube in reactors with axial temperature control, countercurrent reactors, membrane reactors, jet tubes, flame tubes and/or rotary tubes for rotary tube furnaces.

17. A process for producing the multilayered composite tube according to claim 1, the process comprising impressing the depressions by pressing processes.

18. The composite tube according to claim 1, wherein the outer layer has an open porosity ε of 10%<ε<30%.

Description

EXAMPLE 1: COMPARISON OF TEMPERATURE DISTRIBUTION ON AN INVENTIVE MULTILAYERED COMPOSITE TUBE WITH DEPRESSIONS AND A MULTILAYERED COMPOSITE TUBE WITHOUT DEPRESSIONS

[0151] The temperature distribution in a steam-conducting tube was determined by numerical simulation (CFD=computational fluid dynamics). In this example a 1 m-long multilayered ceramic composite tube of 0.047 m internal diameter and tube wall thicknesses of 4 mm for the monolithic ceramic and 1.5 mm for the fiber ceramic were simulated.

[0152] The following table 3 shows the properties of the tube materials employed here.

TABLE-US-00003 Fiber Metal Material data at 900° C. Al.sub.2O.sub.3 ceramic tube ρ (density, kg/m3) 2800 2900 7600 c.sub.p (specific heat capacity, J/kgK)  900  900  663 λ (thermal conductivity, W/mK) 706.1*T’.sup.(−0.672) 58.9*T’.sup.(−0.479)  24 T’ = local temperature in ° C.

[0153] In addition to a tube with inventive depressions a tube of identical structure without depressions was simulated. In the tube with depressions 8 depressions per circumferential segment with a radius of in each case 13.8 mm and a displaced arrangement in the axial direction with a distance of 12.5 mm between the centers of the depressions were modelled.

[0154] An entry temperature of the fluid of 750° C., a mass flow of 8 kg/s and a constant outer tube wall temperature of 950° C. were specified in the simulation.

[0155] The results of the simulation are shown in FIG. 1. The frequency distribution (number of surface elements discretized in the simulation) versus the tube wall internal temperature is plotted in the left-hand panel for the inventive tube with depressions and in the right hand panel for a tube of identical construction without depressions. It is apparent that the depressions altogether reduce the average tube wall temperature and thus coking compared to the tube without depressions while simultaneously the heat flow transferred to the fluid stream increases by 14% on account of the improved heat transfer. The example also shows that the distribution of the tube wall temperature becomes broader due to the locally improved heat transfer at the depressions. This is especially advantageous because this effect reduces coking in the interior of the depressions and the structure of the depressions and the effect of the improved heat transfer is thus retained even during the process of coking.

EXAMPLE 2: COMPARISON OF THE TEMPERATURE DISTRIBUTION ON AN INVENTIVE MULTILAYERED COMPOSITE TUBE WITH DEPRESSIONS AND A METALLIC TUBE (MATERIAL S+C CENTRALLOY® HT-E) WITH DEPRESSIONS

[0156] In a second example the above inventive multilayered composite tube with depressions was compared to a geometrically identical metallic tube with depressions. The results of the simulation are shown in FIG. 2. The frequency distribution (number of surface elements discretized in the simulation) versus the tube wall internal temperature is plotted in the left-hand panel for the tube according to the invention with depressions and in the right hand panel for a metallic tube of identical structure likewise with depressions of identical structure. As is shown in FIG. 2 the temperature distribution for the ceramic tube is broader. This mirrors a larger temperature difference between the depressions (low-temperature) and the remaining tube wall surface area (high-temperature) for the ceramic tube. It is thought that the poorer thermal conductivity in the ceramic tube results in this more marked temperature scattering. This result is surprising and shows that the depressions are more advantageous for a ceramic tube than for metallic tubes since for a ceramic tube coke formation is especially reduced at the depressions and the positive effect of the depressions is thus retained for longer. In the case of metallic tubes the depressions are rapidly filled by coke formation.