Refractory design

Operating conditions in partial oxidation reactors and steam reformers vary considerably - depending on the feedstock that is being used. An operating temperature of 1400°C (i.e., the temperature of the inner surface of the lining) is not critical for many refractory materials. However factors other than temperature determine the refractory selection and the refractory design. 

This volume on ceramics and refractories is divided into three sections. The first section, "Ceramics and Refractories," is divided into seven chapters. Apart from an introductory section. Chapter 1 mainly details the applications of ceramics and refractories. Chapter 2 is on selection of materials and it describes the two stages in selection with a case study. Chapter 3 is on new developments in the ceramic and refractory fields. Chapter 4 describes the phase equilibriums in ceramic and refractory systems and outlines the three important systems, namely, unary, binary, and ternary. Corrosion of ceramics and refractories is the subject of Chapter 5. Chapter 6 discusses failures in ceramics and refractories. Design aspects are covered in Chapter 7. 

In conventional refractory designs, the lining is at least one brick thickness (>225 mm), and the lining typically features a safety lining for a total thickness of at least 450 mm. The slag freeze plane may be located in a zone of 40-75 mm behind the hot face. In some cases, slag may penetrate up to 150 mm behind the hot face. 

Refractory Linings. The refractory linings (2,3) for the hearth and lower wads of furnaces designed for melting ferrous materials may be acidic, basic, or neutral (see Refractories). Sdica has been widely used in the past, and is stid being used in a number of iron and steel foundries. Alumina, a neutral refractory, is normally used for furnace roofs and in the wads for iron foundries, but basic brick can also be used in roofs (4). 

Submerged-Arc Furnace. Furnaces used for smelting and for certain electrochemical operations are similar in general design to the open-arc furnace in that they are usually three-phase, have three vertical electrode columns and a shell to contain the charge, but dkect current may also be utilised They are used in the production of phosphoms, calcium carbide, ferroalloys, siUcon, other metals and compounds (17), and numerous types of high temperature refractories. 

Coreless furnaces derive their name from the fact that the coil encircles the metal charge but, in contrast to the channel inductor described later, the cod does not encircle a magnetic core. Figure 8 shows a cross section of a typical medium sized furnace. The cod provides support for the refractory that contains the metal being heated and, therefore, it must be designed to accept the mechanical loads as well as the conducted thermal power from the load. 

The refractory used to constmct the hearth can be in the form of bricks, preformed shapes, or monolithic. Often a furnace design utilizes all three. Openings or passageways through the walls are fashioned in the same manner as windows in a brick building. 

The steel shell that encloses the refractory is exposed to significant forces from the expansion of the refractory as well as the load from the refractory and the charge within the furnace. Similarly, the stmctures that support the furnace and the foundations must be designed to assure safe operation. A failure of any component can have serious consequences. 

Air-Atmosphere Furnaces. These furnaces are appHed to processes where the workload can tolerate the oxidation that occurs at elevated temperatures in air. In some special appHcations, the oxidation is not only tolerable but is desired. Some furnaces heat the work solely to promote oxidation. Furnaces designed for air operation are not completely gas-tight which results in somewhat lower constmction costs. There are no particular problems encountered in selecting the insulation systems because almost all refractory insulations are made up of oxides. Heating element materials are readily available for the common temperature ranges used with air atmospheres. 

At temperatures above 1150°C, alloys used for the hearth or material handling systems in low and medium temperature furnaces lose strength rapidly (2) and temperatures are reached where ceramic refractories are required to support the work. This results in less use of roUer-hearth and belt-type hearths and greater use of pushers or walking-beam designs for continuous furnaces. 

Fig. 10. (a) Wulff furnace design, (b) Checker detail of Wulff furnace refractory. 

The unit Kureha operated at Nakoso to process 120,000 metric tons per year of naphtha produces a mix of acetylene and ethylene at a 1 1 ratio. Kureha s development work was directed toward producing ethylene from cmde oil. Their work showed that at extreme operating conditions, 2000°C and short residence time, appreciable acetylene production was possible. In the process, cmde oil or naphtha is sprayed with superheated steam into the specially designed reactor. The steam is superheated to 2000°C in refractory lined, pebble bed regenerative-type heaters. A pair of the heaters are used with countercurrent flows of combustion gas and steam to alternately heat the refractory and produce the superheated steam. In addition to the acetylene and ethylene products, the process produces a variety of by-products including pitch, tars, and oils rich in naphthalene. One of the important attributes of this type of reactor is its abiUty to produce variable quantities of ethylene as a coproduct by dropping the reaction temperature (20—22). 

Any manufacturing process requiring refractories depends on proper selection and installation. When selecting refractories, environmental conditions are evaluated first, then the functions to be served, and finally the expected length of service. AH factors pertaining to the operation, service design, and constmction of equipment must be related to the physical and chemical properties of the various classes of refractories (35). 

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