Which refractory materials are used in thermal furnaces in petroleum and chemical industries?
The chemical and petroleum industries are essential fields of refractory applications. Due to many thermal furnaces and complex operating conditions, it is necessary to choose various refractory materials to suit their special application conditions.
Some prominent examples of special applications are selected for a brief explanation.
(1) Carbon black refractory materials for oil-fired furnaces. In producing carbon black, the reaction temperature must be increased to improve the production efficiency and degree. Refractory materials with high refractory properties are required to adapt to them. As another example, the reaction temperature must also be increased to produce hard carbon black, mainly to produce tires with long life and minor rolling wear. More complex carbon black is required, and the reaction temperature reaches 2000-2100 ° C, requiring more refractory materials to be compatible.
(2) Slagging coal gasification device. It is well known that refractory materials are widely used in the gas industry. Gasification equipment is lined with refractory materials under extremely harsh operating conditions, high operating temperatures, and pressure. A gasification slag is an acidic slag with strong erosive force, which has a solid ability to corrode refractory materials. At the same time, there are cases where highly aggressive gases are present. The refractory material with high acid slag corrosion resistance can be used to obtain high service life.
(3) Petroleum cracker. In the petroleum refining process, the operating conditions in the catalytic cracker are among the most severe operating conditions, and the process conditions include high temperature and high pressure. The refractory materials used for catalytic cracker lining must have two contradictory properties: high wear resistance and low thermal conductivity. Because high wear resistance generally requires high density, and low thermal conductivity generally requires low density, it is necessary to describe materials with both properties and the associated issues.
This paper mainly discusses and analyzes the above-mentioned refractory materials for carbon black reactors in the petrochemical industry and provides a basis for developing and applying related refractory materials.
Refractories for carbon black reactors
Carbon black is an aggregate of approximately spherical particles of fused or bonded elemental carbon. Today, the oil furnace process mainly produces carbon black of different varieties and particle sizes.
Brief introduction of the carbon black production process
Most of the production of carbon black uses oil-fired furnaces. Usually, the oil furnace process uses a cylindrical reactor similar to a large oil burner, which consists of 4 belts (or chambers), namely, the combustion zone (chamber), the throttle ring (chamber), and the reaction zone (chamber). And quench zone (chamber).
The added preheated air and burned raw materials in the combustion zone provide the high temperature required for the production process and are also the starting point for the passage of raw materials into the reaction zone.
The choke ring separates the combustion zone from the reaction zone while increasing the gas velocity entering the reaction zone.
The reaction zone forms carbon black, and the carbon black’s particle size, shape, and hardness are controlled by temperature, gas velocity, and different “seed” materials introduced with the feedstock. The quenching zone makes the temperature drop sharply by spraying water, thus ending the reaction in the furnace.
The operating temperature was varied throughout the reaction in the reactor. The warming operation begins at the front end of the combustion zone, and the temperature increases towards the throttle ring. At the throttle ring, the temperature is the highest, and the speed is the fastest. In the reaction zone, the temperature drops slightly, and the speed becomes relatively slow. The length of the reaction time is changed by spraying cooling water at different positions along the cooling zone. That is, the temperature is lowered by rapid water spraying.
In addition, when the carbon black produced changes, the temperature usually fluctuates sharply. When the position of the cooling zone changes, it will also cause a sharp temperature change. This drastic temperature change will cause the refractory material to peel off and be damaged.
The size of the black carbon particles is limited by the furnace size, structure, temperature, and time spent in the reaction zone before cooling. In contrast, the operating temperature and location of the first water spray depending on the carbon black grade.
Selection of refractories for carbon black reactors
The ideal carbon black reactor lining refractory material should have the advantages of high refractoriness, good thermal shock resistance, high density, low porosity, and high-temperature corrosion/erosion resistance.
Most carbon black reactor linings are built with Al2O3-SiO2 refractory materials, and the inner lining is made of high-alumina refractory materials or clay refractory materials. When the operating temperature range is different, choosing different grades of refractory materials for the lining is necessary.
The lining of reactors operating in the low-temperature range (1550~1750°C) is generally made of corundum bricks with mullite combined with Al2O3 (content greater than 90%). Sometimes, high-purity (Al2O3 content is 60%~70%) high-alumina bricks are used for masonry in the cooling zone.
Reactor linings operating at high temperatures (1750~1925°C) are usually built in partitions. On the hot surface of the combustion zone, 90%Al2O3-10%SiO2 refractory materials are generally used for lining; at high-temperature throttling, The ring and the hot surface of the reactor are built with 99% Al2O3 corundum bricks and 90% Al2O3-10% Cr2O3 bricks with good thermal shock resistance.
In the range of ultra-high temperature (greater than 1925°C, that is 2000-2100°C), Cr2O3-Al2O3 (70%Cr2O3) bricks are used for masonry.
1. SiO2-Al2O3 refractory material
In SiO2-Al2O3 series refractory materials, when w(Al2O3)>90%, it is called corundum refractory material.
When pure SiO2 and Al2O3 materials are used to produce corundum refractory materials, the temperature at which the liquid phase appears is 1840 ° C, as shown in Figure 5-1.
The corundum refractory material produced by the traditional process cannot achieve ideal physical and service performance, and its thermal shock resistance is poor. Now, through pure raw materials and in-situ forming bonding technology, corundum refractory materials’ physical properties, and performance have significantly progressed, and the variety has continued to increase. Among them, in-situ mullite with 90% Al2O3 corundum bricks and special Al2O3 combined with 94% Al2O3 and even 98% Al2O3 corundum bricks have been developed successfully.
Compared with the traditional corundum refractory products of the same material, these special ones have lower porosity, higher density, and more muscular strength, especially outstanding thermal shock resistance. The 50mm*50mm*76mm sample is kept at 1200°C for 10 minutes, water-cooled for 2 minutes, and placed in the air to dry for 8 minutes. The cycle is more than 40 times (see Table 5-1), while the traditional product is only 2~7 times, even the traditional Molai Stone corundum products are only 10~15 times.
Special mullite-bonded corundum bricks are mainly used in low-temperature carbon black reaction furnaces or the cooling zone of high-temperature carbon black reaction furnaces. The actual use results show that the service life of the special mullite combined with 90% A12O3 corundum brick is 50%~200% higher than that of the traditional mullite-corundum brick of the same material when used in combustion and restricted zones, which significantly prolongs the use of the furnace life. Special A12O3 bonded corundum bricks (95%~98%Al2O3) are mainly used in the reaction zone with harsh conditions in the carbon black reaction furnace, and the service temperature limit can reach 2000°C.
