Refractory Materials Used in Reactors, Reformers and Boilers used in Refineries, Petrochemical and Power-Generation Plants
Varieties of polymer, glass and refractory materials are used for low-, medium- and high- temperature insulation of reactors, reformers, regenerators, heaters, high- temperature and pressure vessels as well as boilers and pipes used in petroleum refining and also petrochemical- and chemical- Industries as follows:
Reactors and reformers used in petroleum refining
Petroleum or crude-oil refining processes may reach temperatures up to
1000°C and may entail some aggressive chemicals, where glass and bare steel linings are not recommended. These aggressive chemicals include: NaCl, organic compounds of oxygen, nitrogen, sulfur and metals, such as vanadium, nickel, and others. Petroleum refining processes largely includes the following processes:
Stills and vacuum stills used for fluid catalytic cracking (FCC) and fractional distillation are for the most part made of steel. Since the residues from petroleum refining, e.g. tar, pitch, asphalt, and coke contain concentrated corrosive nonvolatile impurities, which can lead to corrode the outer steel shell. Hence, such hot-chemical treatment requires for refractory materials with high resistance to attack by acids and salts. Accordingly, Varieties of shaped and unshaped as well as dense and lightweight alumino-silicate refractories are used for lining the reactors, regenerators, heaters, high- temperature and pressure vessels as well as boilers and pipes of the FCC units.
Figure 1 exhibits a sketch diagram of a fluid catalytic cracking (FCC) unit. It shows a CO-gas boiler connected with a chimney-stack through a dust collector, on the left side. On the right side, there is a riser-pipe that transferring the boiler-steam in a steel pipe through the top of a regenerator with simultaneous feeding of crude-oil through its hearth. This enables in preheating the crude-oil up to 500oC, before its pushing into the fractional distillation tower. Accordingly, light to heavy hydrocarbon fraction-products are obtained from the tower at different levels going downwards; namely, gas, gasoline as well as light- and heavy- cycle gas-oil products.
Figure 1: Sketch Diagram of a Fluid Catalytic Cracking (FCC) Unit
Figure 2 shows the refractory-lining sections of the reactor and riser- pipe (A), which made of dense and/or lightweight fireclay castables, supported by steel hexagonal-mesh and/or V-shape metallic anchors. Also shown are the inner (A) and outer (B) lightweight-castable linings fixed with V-shaped anchors. The linings of the FCC reactor are subjected to high-pressure during operation and hence these linings should provide gas-sealing and good thermo- mechanical properties. Figure 3 also shows a cross-section view of a typical petroleum reactor vessel, internally lined with dense and/or lightweight castables, supported by hexagonal steel grating (mesh) and/or Y-shape alloy- steel anchors (or clips). In addition, the standing steel-supports are covered with a lightweight-castable to protect it in case of fire, i.e. fire-proof insulating castables.
Figure 2: A Regenerator and A Reactor of the FCC Unit
Figure 3: Typical Petroleum Reactor Vessel
The castable linings of the high- temperature and pressure reactors (vessels), also used in petroleum refining, as shown in Figure 4, should be durable under such conditions. Therefore, these linings are directly applied to the vessel's steel-shell by means of pouring or gunning methods and also should be rigidly attached with anchors. This prohibits penetration of the high- temperature gases into the gaps between the refractory lining and the vessel's shell. Figure 4 also shows that linings for heater parts of lower service temperatures (above and below) often consist of a single-layer made of medium- weight castable (A). On the other hand, two-layer linings, consisting of a medium-weight face layer, under-lied with a lightweight back one (B) is applied for the middle part
Figure 4: A high- temperature and pressure reactor and its dense and lightweight refractory castable linings
Various types of heaters (or furnaces), e.g. vertical cylindrical or box types, as shown in Figure 5, are also applied in petroleum refinery and chemical industries. It should be applicable for heating and decomposing process-fluids. The type of castable-linings applied in the heaters differs according to service temperatures of the furnace-lining's hot- and cold- faces. Linings for heater parts of lower service temperatures often consist of a single- layer made of medium-weight castable (A). Whereas, two-layer linings (B, C, D, E and F), consisting of a dense or medium-weight face layer, under-lied with a lightweight back one are used for parts with higher service temperatures as shown in Figure 6.
Figure 5: A box-type heater and a transfer-pipe in the FCC unit
Figure 6: Sections of the designed one-layer (A) as well as two-layers (B, C, D, E and F) linings for the different parts of a box-type heater and a transfer-pipe in the FCC unit
Catalytic chemical treatments
These processes are designed and carried out to crack, polymerize or reform oil into various hydrocarbon fractions, as summarized in the following:
- High-pressure H2 is used in hydro-cracking and desulphurization of oil together with circulating solid metallic and abrasive catalysts. Therefore, refractory linings for such reactors should provide high-resistance to abrasion and reduction-oxidation reactions (Redox reactions) as well as good thermal insulation to reduce the outer steel-shell temperatures, so as to impede hydrogen embitterment.
- Reforming of the liquid-oil or its derivatives, e.g. naphtha is an acid- catalyzed process, calling for refractory linings and/or coatings with high resistance to acids and abrasion.
According to the above service conditions, the acidic dense fireclay bricks and/or castables are adequate as one-layer face-lining for both of the oil- desulphurization and oil-reforming- reactors. Also, two-layer fireclay lining compose of a face dense-layer and a back lightweight-layer are recommended. The thickness of the two layers should be calculated according to the thermal conductivity coefficient (K) of its type in order to minimize temperature of the outer steel-shell down to <50oC. In addition, the face-dense layer should have minimum apparent porosity (<15%), pore-sizes (<2 μm) and iron content (<1.0% Fe2O3).
The following Figure 7 exhibits a schematic diagram for liquid- dehexanized naphtha- reforming process. It includes three successive units, each consists of a fired-heater and a catalyst fixed-bed reactor for preheating and cracking of the liquid naphtha by hydrogen gas at 495-525oC, respectively. These units are followed by a cooled gas-separating unit at 38oC and then a stabilizer connected with a steam-boiler as well as a condenser and a reflux-drum in order to separate the liquid and gas reformed naphtha products.
Figure 7: Schematic diagram of a typical liquid-naphtha catalytic reforming process in a petroleum refinery
Reactors used in petrochemical industries
Production of polyethylene
Ethylene is produced worldwide by a non-catalytic process called "steam cracking", in which cracking of large hydrocarbon molecules into smaller ones occurs. In the year 2003, about 97 million tons of ethylene was used to produce polyethylene and other petrochemicals by the steam-cracking of various
hydrocarbons, e.g. methane, ethane, naphtha, and fuel oils.
Most petrochemical processes, including polymer manufacture, have low working temperatures of 100-400°C and are not very aggressive. Hence, their reactors don’t need much usage of refractory materials. Meanwhile, the following materials are in common use up to about 300°C:
- Glass-lined steel vessels, even up to moderate pressures.
- Stainless steel as well as steel- clad or unclad plastic vessels generally made of thermo-set or cross-linked polymers).
- Impervious carbon equipment finds limited use over the whole temperature range (100-400°C).
Reactors used in production of nitrogen (N) fertilizers
Figure 8 summarizes the various steps involved in the manufacture of fertilizers, from raw materials through intermediate products as well as the different types of fertilizer products, either straight- or multi- nutrient. This figure also summarizes the source of starting materials as well as the intermediate and final products of the straight- and multi- nutrient mineral-fertilizers.
Figure 8: Source of the starting materials as well as the intermediate and final products of the straight- and multi- nutrient mineral-fertilizers
From Figure 8, it is also evident that the intermediate product in nitrogen (N) fertilizers is ammonia (NH3), which is produced by combining nitrogen extracted from the air with hydrogen from hydrocarbons, such as naphtha or natural gas by means of the Steam Reforming Process (SRP). Approximately 85% of the anhydrous ammonia plants in Europe use natural gas for the production of hydrogen gas.
Steam reforming of natural gas
Due to the least expenses of the natural-gas-steam reforming process, which sometimes referred to as steam-methane reforming (SMR) process, it becomes the most common method used worldwide in producing about 80% of hydrogen gas, especially used in the production of ammonia. In the year 2004, the amount of worldwide commercial ammonia production, using SMR method was about 109 million tons.
Figure 9 exhibits a lay-out of an industrial line for the production of commercial ammonia, including feed-gas heater, followed by a de-sulphurizing unit as well as successive primary- and secondary- steam-reforming units connected at the end with a waste-heat boiler.
Figure 9: A lay-out for a commercial ammonia production line, including gas heater, de-sulphurizing unit, primary- and secondary- reformers and a waste-heat boiler
Figure 10 also shows a schematic diagram for the main four successive steps applied in steam-natural gas (methane) reforming process (SMRP), which briefly described in the following:
- First, desulphurization, i.e. removal of sulfur compounds from the natural gas by catalytic-treatment with some of the produced hydrogen gas.
- The pure natural-gas (methane) is then mixed with steam and reformed by passing both over a nickel-catalyst loaded on an alumina substrate, with formation of CO - hydrogen gas-mixture according to the following reaction:
CH4 + H2O → CO + 3H2 ……………………………. .. (1)
This reaction requires a large amount of heat (206 kJ/mol methane). In current commercial practice of this step, this amount of heat is added using furnaces, containing tubular reactors filled with the Ni/Al2O3 catalyst, at 700-
- This is accompanied by a catalytic treatment (shift) of the CO-gas by steam to convert it to CO2 – hydrogen gas-mixture according the following reaction:
CO + H2O → CO2 + H2 (Steam-gas- shift reaction) ….. (2)
- The overall reaction (No. 3) is obtained by summation of the preceding equations Nos. 1 and 2 as follows:
CH4 + 2 H2O → CO2 + 4H2 …………………………… (3)
- Finally, the obtained hydrogen gas is purified by pressure-swing adsorption (PSA) method. The hot CO2 – gas, by-produced in this method, forms a portion of the large heat-energy required for use in the reformer, before discharging with the furnace flue gases. If there is need for CO2 gas, a separation process unit would be then added for its utilization.
Figure 10: The four basic steps for steam-methane reforming processes
This steam-natural-gas (methane) reforming (SMR) process is quite different from and not to be confused with the catalytic-reforming process of naphtha by steam, which is one of the petroleum-oil refinery processes, that also produces significant amounts of hydrogen gas, along with high octane gasoline.
According to the above data, there is relatively high- pressures ( 3-5 atm) and temperatures (700-1100oC), together with acidic and reducing character of the evolved gases, including: H2S, SO3 and CO, H2 as well as the abrasive nature of the hard Ni/Al2O3 catalyst are available in the reactors used for desulphurization, SRP, shift and PSA processes carried out for the pure hydrogen gas production. Therefore, varieties of the acidic fireclay and/or the neutral high-alumina bricks or castables are applied as one-layer dense face- lining or two-layer lining of a face dense-layer and a back lightweight-layer. In order to minimize temperature of the reactor's outer steel-shell, thickness of these layer-linings should be calculated according to its thermal conductivity (K). The dense-face layer should also have minimum apparent porosity (<15%), pore-sizes (<2 μm) and iron content (<1.0% Fe2O3).
Steam generating boilers
Economically, steam generation is one of the most important applications, in which heat is put. The furnaces used for boiling water are usually called boilers, which have three main types as follows:
Small shell boilers:
These boilers are small and simple units usually used for central heating of buildings. The combustion-space above the grate of water-pipes and the flue of the waste-gases to the chimney are surrounded by a water-pipe jacket, in the top of which the wet steam is collected. These are cheap, easily operated and maintained but not very efficient.
The internally-fired tube boilers:
These boilers are used, when larger quantities of steam are required under moderate pressures. The combustion gases pass through a number of parallel tubes, running through the space available for water. For instance, the Lancshire boiler, as seen in Figure 11, consists of two wide flues passing along the length of a cylindrical boiler-drum, back along the bottom of the drum and then in two-side flues again to the rear, where super-heaters are fitted, and finally to the chimney. The heat transfer in this type of boilers is mainly taken place by convection mechanism.
Figure 11: A simple Lancashire, fire-tube Boiler
The modern water-tube boilers:
This type of boilers is stronger than the two preceding types and used under higher-pressure operation conditions. As shown in Figures 12 and
7.3.13, the boiler drums are aside from the path of the hot gases, which pass through one or more banks of tubes, in which water is circulated by a pump from the drums. There is another similar bank of tubing constitutes the super- heater, in which wet steam is raised to its working temperature. The boiler hot- flue gases are also passed through a generator, in which feed water is preheated before entering the boiler drum and finally through an air-heater to preheat the combustion air. Also here, the main heat-transfer in these boilers is almost entirely occurred by convection mechanism.
Figure 12: A modern water-tube boiler
According to the moderate pressures (2-3 atm.) and temperatures (up to 1000oC) as well as acidic nature of the burner's flue-gases, e.g. SO3 inside the different types of boilers, varieties of the acidic fireclay bricks and/or castables are applied as one-layer dense face-lining. In order to minimize temperature of the boiler's steel-shell, the optimum thickness of the lining should be calculated according to its thermal conductivity (K).
Figure 13: A modern water-tube boiler