Feed material

If the reaction involves more than one feed, it is not necessary to operate with the same low conversion on all the feeds. Using an excess of one of the feeds enables operation with a relatively high conversion of other feed material, and still inhibits series reactions. Consider again the series reaction system from Example 2.3 ... 

Recycling byproducts or contaminants which damage the reactor. When recycling unconverted feed material, it is possible that... 

If a vapor from the phase split is either predominantly product or predominantly byproduct, then it is removed from the process. If the vapor contains predominantly unconverted feed material, it is normally recycled to the reactor. In these cases, there is no need to carry out any separation on the vapor. 

If the vapor stream consists of a mixture of unconverted feed material, products, and byproducts, then some separation of the vapor may be needed. The vapor from the phase split is difficult to condense if the feed has been cooled to cooling water temperature. If separation of the vapor is needed, one of the following methods can be used ... 

Solution The reversible nature of the reaction means that neither of the feed materials can be forced to complete conversion. The reactor design in Fig. 

Now consider recycling unconverted feed material to the reactor. Figure 4.13a shows the recycles of unconverted feed material. The recycle from the... 

Losses in the reactor due to byproduct formation or unconverted feed material if recycling is not possible. 

Impurities in the feed materials can undergo reaction to produce waste byproducts. 

If the separation and recycle of unreacted feed material is not a problem, then we don t need to worry too much about trying to squeeze extra conversion from the reactor. 

Reducing waste from feed impurities which undergo reaction. If feed impurities undergo reaction, this causes waste of feed material, products, or both. Avoiding such waste is most readily achieved by purifying the feed. Thus increased feed purification costs are traded off against reduced raw materials, product separation, and waste disposal costs (Fig. 10.2). 

Heterogeneous catalysts are more common. However, they degrade and need replacement. If contaminants in the feed material or recycle shorten catalyst life, then extra separation to remove these contaminants before the feed enters the reactor might be justified. If the cataylst is sensitive to extreme conditions, such as high temperature, then some measures can help to avoid local hot spots and extend catalyst life ... 

Feed purification. Impurities that enter with the feed inevitably cause waste. If feed impurities undergo reaction, then this causes waste from the reactor, as already discussed. If the feed impurity does not undergo reaction, then it can be separated out from the process in a number of ways, as discussed in Sec. 4.1. The greatest source of waste occurs when we choose to use a purge. Impurity builds up in the recycle, and we would like it to build up to a high concentration to minimize waste of feed materials and product in the purge. However, two factors limit the extent to which the feed impurity can be allowed to build up ... 

Shaping method Type of feed material Type of shape... 

Essentially all the ammonium sulfate fertilizer used in the United States is by-product material. By-product from the acid scmbbing of coke oven gas is one source. A larger source is as by-product ammonium sulfate solution from the production of caprolactam (qv) and acrylonitrile, (qv) which are synthetic fiber intermediates. A third but lesser source is from the ammoniation of spent sulfuric acid from other processes. In the recovery of by-product crystals from each of these sources, the crystallization usually is carried out in steam-heated sa turator—crystallizers. Characteristically, crystallizer product is of a particle size about 90% finer than 16 mesh (ca 1 mm dia), which is too small for satisfactory dry blending with granular fertilizer materials. Crystals of this size are suitable, however, as a feed material to mixed fertilizer granulation plants, and this is the main fertilizer outlet for by-product ammonium sulfate. 

Plasticity, and hence granulation efficiency, varies considerably with the nature and proportion of feed materials. Pure salts, such as potassium chloride and ammonium sulfate, lend Httle or no plasticity and thus are difficult to granulate. Superphosphates provide good plasticity. The plasticity of ammonium phosphates depends chiefly on the impurity content of iron and aluminum. The higher the impurity the greater the plasticity. In some cases, binders such as clay are added to provide plasticity. 

Fig. 16. Steam granulation process for production of granular mixed fertilizers from dry, pulverized feed materials (7). A granulator producing 12 t/h would...

Fig. 18. TVA-type cogranulation process with preneutralizer, as used for production of granular mixed fertilizers. Feed materials such as ammonium sulfate, ammonium nitrate, urea, superphosphates, sulfuric acid, and potash are used.

A commercial design based on semicontinuous operation was developed for manufacture of silicate powders (27). A slurry, prepared containing the feed materials and water, is fed to the reactor tank and heated by circulating a heat-exchange fluid in channels located on the outside vessel wall. A six-bladed stirrer is operated at about 100 rpm in order to keep reagents well mixed. Once the slurry reaches the operating temperature, the vessel heat is maintained until reaction is complete. For most fine-particle products, this time is less than 1 hr. 

Burners and combustion air ports are located in the walls of the furnace to introduce either heat or air where needed. The air path is countercurrent to the sohds, flowing up from the bottom and across each hearth. The top hearth operates at 310—540°C and dries the feed material. The middle hearths, at 760—980°C, provide the combustion of the waste, whereas the bottom hearth cools the ash and preheats the air. If the gas leaving the top hearth is odorous or detrimental to the environment, afterburning is required. The moving parts in such a system are exposed to high temperatures. The hoUow central shaft is cooled by passing combustion air through it. 

Chemical Reaction Measurements. Experimental studies of incineration kinetics have been described (37—39), where the waste species is generally introduced as a gas in a large excess of oxidant so that the oxidant concentration is constant, and the heat of reaction is negligible compared to the heat flux required to maintain the reacting mixture at temperature. The reaction is conducted in an externally heated reactor so that the temperature can be controlled to a known value and both oxidant concentration and temperature can be easily varied. The experimental reactor is generally a long tube of small diameter so that the residence time is well defined and axial dispersion may be neglected as a source of variation. Off-gas analysis is used to track both the disappearance of the feed material and the appearance and disappearance of any products of incomplete combustion. 

Direct reduction (DR) is the process of converting iron ore (iron oxide) into metallic iron without melting. The metallic iron product, known as direct reduced iron (DRI), is used as a high quaUty feed material in steelmaking. 

DR Processes Under Development. The 1990s have seen continuous evolution of direct reduction technology. Short-term development work is focusing on direct reduction processes that can use lower cost iron oxide fines as a feed material. Use of fines can represent a 20 30/1 (20%) savings in DRI production cost compared to use of pehets or lump ore. Some examples of these processes include FASTMET, Iron Carbide, CIRCOFER, and an improved version of the EIOR process. 

The process is flexible and permits treatment of a wide variety of plant feed materials. Overall lead recovery is in the range of 96—98%. The operation is, however, cycHc which increases the cost of the sulfur fixation plant, and any 2inc contained in the concentrate is lost in the slag unless slag Aiming is added or already available at the site. 

Use of a blast furnace is preferred if a regular supply of a charge of coarse and consistent quaUty is available. However, the blast furnace is not suitable for treating finely divided feed material. 

The quantity of feed materials required are 1—1.05 kg of metallic reductant, 5.4 kg of dolime, and 0.35 kg of calcined bauxite or alumina to produce 1 kg of magnesium. The quantity of slag produced depends on the feed material composition and may vary from 5.2 to 5.9 kg/kg of magnesium. 

The most effective phosphoms production technology uses a submerged arc furnace. The submerged arc furnace performs three functions chemical reactor, heat-exchanger, and gas—soHd filter, respectively, each of which requires a significant amount of preparation for the soHd furnace feed materials. 

Transportation of Chemicals. Feed materials and finished products are frequendy transported by tank tmck and railroad tank cars. Design, constmction, and movement of these vehicles is regulated by the U.S. Department of Transportation (DOT) (97). The DOT regulations require placarding of material-transport vehicles to alert the pubHc and emergency personnel to the nature of their contents. 

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