Tubular furnaces

Vitreous silica is used for gas-heated or electrically heated devices ia various shapes, eg, as a tube or muffle because of its electrical resistivity, impermeabihty, and low expansion. In its simplest form, an electric-resistance furnace consists of a vitreous siUca tube or pipe on which the resistance element is wound (see Furnaces, ELECTRIC). Because of its iadifference to temperature gradients, a tubular furnace of vitreous siUca maybe made to operate at different temperatures at various portions of the tube, either by arrangement of the heating elements or by cooling sections of the tube with water. Vitreous siUca pipes may be employed ia vacuum-iaduction and gas-fired furnaces (see Vacuum technology) (221). 

In addition to conventional thermal cracking in tubular furnaces, other thermal methods and catalytic methods to produce ethylene have been developed. None of these are as yet commercialized. 

FIG. 23-3 Temperature and composition profiles, a) Oxidation of SOp with intercooling and two cold shots, (h) Phosgene from GO and Gfi, activated carbon in 2-in tubes, water cooled, (c) Gumene from benzene and propylene, phosphoric acid on < uartz, with four quench zones, 260°G. (d) Mild thermal cracking of a heavy oil in a tubular furnace, hack pressure of 250 psig and sever heat fluxes, Btu/(fr-h), T in °F. (e) Vertical ammonia svi,ithesizer at 300 atm, with five cold shots and an internal exchanger. (/) Vertical methanol svi,ithesizer at 300 atm, Gr O -ZnO catalyst, with six cold shots totaling 10 to 20 percent of the fresh feed. To convert psi to kPa, multiply by 6.895 atm to kPa, multiply by 101.3. 

The tube is much longer than needed for the catalyst volume to provide a surface for preheating and to minimize temperature losses at the discharge end. The tube can be bent into a U shape and immersed in a fluidized sand bath, or it can be straight and placed inside a tubular furnace in a temperature-equalizing bronze block. Thermocouples are usually inserted... 

A test procedure which has proved very useful was first described by Hatfield. The samples are cylinders 32 x 12-5 mm in diameter with a standard abraded finish which are supported on open-ended refractory boats in a tubular furnace. In the original test the atmosphere, which was produced by burning towns gas with a 50% excess of air, was passed over the specimens at a standard velocity after first preheating to test temperature over refractory packing in a separate furnace chamber. More latterly, natural gas has been used with suitable modification of air gas ratio to give... 

A method has been developed for differentiating hexavalent from trivalent chromium [33]. The metal is electrodeposited with mercury on pyrolytic graphite-coated tubular furnaces in the temperature range 1000-3000 °C, using a flow-through assembly. Both the hexa- and trivalent forms are deposited as the metal at pH 4.7 and a potential at -1.8 V against the standard calomel electrode, while at pH 4.7, but at -0.3 V, the hexavalent form is selectively reduced to the trivalent form and accumulated by adsorption. This method was applied to the analysis of chromium species in samples of different salinity, in conjunction with atomic absorption spectrophotometry. The limit of detection was 0.05 xg/l chromium and relative standard deviation from replicate measurements of 0.4 xg chromium (VI) was 13%. Matrix interference was largely overcome in this procedure. 

The temperature dependence of the fabricated open cavity FP device was evaluated experimentally. The sensor was placed in a programmable electric tubular furnace. The temperature of the furnace was increased from room temperature to 1,100°C at a step of 50°C. The cavity length as a function of the temperature is plotted in Fig. 7.11, where it increased nearly linearly following the increase of temperature. The temperature sensitivity of the particular FP device under test was estimated to be 0.074 nm °C 1 based on the linear fit of the measurement data. The equivalent coefficient of thermal expansion (CTE) of the fiber FP device was 2.4x10 6oC. ... 

A few plants have been built to oxidize normal paraffins such as propane and butane. Air and paraffin are charged to a tubular furnace at a temperature of about 700 R Acetaldehyde yields from butane are about 30—35%. 

Five biomass samples (hazelnut shell, cotton cocoon shell, tea factory waste, olive husk and sprace wood) were pyrolyzed in a laboratory-scale apparatus designed for the purpose of pyrolysis (Demirbas, 2001, 2002a). Figure 6.4 shows the simple experimental setup of pyrolysis. The main element of the experimental device is a vertical cylindrical reactor of stainless steel, 127.0 nun in height, 17.0 nun iimer diameter and 25.0 mm outer diameter inserted vertically into an electrically heated tubular furnace and provided with an electrical heating system power source, with a heating rate of about 5 K/s. The biomass samples ground... 

The initial halogenated polymeric materials were obtained from the polyvinyl chloride-polyvinylidene chloride, PVC-PVDC (Rovil fiber) and chlorinated polyvinyl chloride, PVC. Dehydrochlorination was performed in the presence of a base solution in a polar organic solvent (dimethylsulfoxide, acetone or tetrahydro-furane). The products were filtered and extracted with water in a Soxhlet apparatus until all chloride ions were removed. Thermal treatment was performed in a tubular furnace in CO flow at 10 cm min". ... 

Displace all the air from the apparatus with a stream of ammonia. Connect the apparatus to cock 2 of the sublimation apparatus (Fig. 626). Fill the system with ammonia by opening cocks 2 and 1, and heat the part of the apparatus accommodating the ammonium iodide in a tubular furnace until sublimation begins (about 200 °C) use a thermocouple). Regulate the rate of ammonia flow and of furnace heating so that the ammonium iodide will decompose as little as... 

Preparation of Chromium(III) Nitride. Work in a fume cupboard Assemble an apparatus for preparing nitrides (see Fig. 84). Put 0.5-1 g of anhydrous chromium(III) chloride into a boat. Put the latter into a tubular furnace. Displace the air from the apparatus with a stream of dry ammonia and then heat the furnace to 600 °C. Continue the heating in an ammonia stream for one hour, next switch off the furnace and cool the apparatus without stopping the stream of gas. Extract the boat and weigh the product. Write the equation of the reaction. Calculate the yield in per cent. 

Preparation of Manganese(ll) Oxide. Dehydrate 1 g of manganese oxalate by heating in a porcelain bowl at not over 200 °C. Transfer the salt into a porcelain boat and roast it in a tubular furnace in a stream of dry hydrogen at 300-400 °C (see Fig. 115). Preliminarily check the tightness of the apparatus and the purity of the hydrogen ... 

Weigh about 0.5 g of iron oxide in a porcelain boat and place the latter into a tube for roasting in an inclined tubular furnace. Insert a thermocouple into the furnace and connect it to a pyrometer. Why must the furnace be inclined somewhat ... 

Figure 17.16. Basic types of tubular furnaces [Nelson, Petroleum Refinery Engineering, McGraw-HiU, 1958. Courtesy McGraw-Hill, New York],...

Figure 17.35. Temperature and conversion profiles of mild thermal cracking of a heavy oil in a tubular furnace with a back pressure of 250 pag and at several heat fluxes [Btu/hr(sqft)].

Significant amounts of CH4 and C2H2 are also formed but will be ignored for the purposes of this example. The ethane is diluted with steam and passed through a tubular furnace. Steam is used for reasons very similar to those in the case of ethylbenzene pyrolysis (Section 1.3.2., Example 1.1) in particular it reduces the amounts of undesired byproducts. The economic optimum proportion of steam is, however, rather less than in the case of ethylbenzene. We will suppose that the reaction is to be carried out in an isothermal tubular reactor which will be maintained at 900°C. Ethane will be supplied to the reactor at a rate of 20 tonne/h it will be diluted with steam in the ratio 0.3 mole steam 1 mole ethane. The required fractional conversion of ethane is 0.6 (the conversion per pass is relatively low to reduce byproduct formation unconverted ethane is separated and recycled). The operating pressure is 1.4 bar total, and will be assumed constant, i.e. the pressure drop through the reactor will be neglected. 

Fractional distillation is carried out in a tubular furnace with a fractionating column. Crude oil is heated in a furnace to 800°C and the resulting vapours are passed into the fractionating column. The vapours condense in the... 

The dried product was activated by heating in vacuo. The dried product was placed in a quartz tube and evacuated. The quartz tube was then heated in a tubular furnace using a ramp-and-soak method as follows ramped from room temperature to 220°C at l°C/min soaked at 220°C for 5 h, ramped from 220 to 500°C at l°C/min, soaked at 500°C for 5 h. About 18 h was required for the heat treatment of the sample. The sample was allowed to cool to room temperature and then stored under nitrogen. The product was very light chunky powder. However, these chunks were very fragile therefore, they could not be directly used for packed-bed reactors. 

In Chapter 2, we reviewed the concept of carrying out CVD processes at low pressure so that deposition becomes surface controlled. When the only thing controlling the uniformity of deposition is the temperature of the wafer surface, all we have to do is ensure that the wafer is in a uniform temperature furnace. Again, at low pressures, the diffusion coefficient is so large that we can stack wafers up next to each other so 50 to 100 can be placed in a long tubular furnace. 

Steam reforming refers to the endothermic, catalytic conversion of light hydrocarbons (methane to gasoline) in the presence of steam [see Eq. (5.1)]. The reforming reaction takes place across a nickel catalyst that is packed in tubes in an externally-fired, tubular furnace (the Primary Reformer). The lined chamber reactor is called the secondary reformer , and this is where hot process air is added to introduce nitrogen into the process. Typical reaction conditions in the Primary Reformer are 700°C to 830°C and 15 to 40 bar46. 

The Primary Reformer is a steam-hydrocarbon reforming tubular furnace that is typically externally fired at 25 to 35 bar and 780°C to 820°C on the process side. The reformer tubes function under an external heat flux of 75,000 W/m2 and are subject to carburization, oxidation, over-heating, stress-corrosion cracking (SCC), sulfidation and thermal cycling. Previously SS 304, SS 310 and SS 347 were used as tube materials. However these materials developed cracks that very frequently led to premature tube failures (see Table 5.10)88. 

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