Impurities minimizing

Multiple reactions also can occur with impurities that enter with the feed and undergo reaction. Again, such reactions should be minimized, but the most efiective means of dealing with byproduct reactions caused by feed impurities is not to alter reactor conditions but to introduce feed purification. 

One further problem remains. Most of the n-butane impurity which enters with the feed enters the vapor phase in the first separator. Thus the n-butane builds up in the recycle unless a purge is provided (see Fig. 4.13a). Finally, the possibility of a nitrogen recycle should be considered to minimize the use of fresh nitrogen (see Fig. 4.136). 

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 ... 

Inclusions, occlusions, and surface adsorbates are called coprecipitates because they represent soluble species that are brought into solid form along with the desired precipitate. Another source of impurities occurs when other species in solution precipitate under the conditions of the analysis. Solution conditions necessary to minimize the solubility of a desired precipitate may lead to the formation of an additional precipitate that interferes in the analysis. For example, the precipitation of nickel dimethylgloxime requires a plT that is slightly basic. Under these conditions, however, any Fe + that might be present precipitates as Fe(01T)3. Finally, since most precipitants are not selective toward a single analyte, there is always a risk that the precipitant will react, sequentially, with more than one species. 

Faradaic currents due to impurities can usually be minimized by carefully preparing the sample. For example, one important impurity is dissolved O2, which is reduced first to H2O2 and then to H2O. Dissolved O2 is removed by bubbling an inert gas such as N2 through the sample before the analysis. 

As opposed to gaseous, pure formaldehyde, solutions of formaldehyde are unstable. Both formic acid (acidity) and paraformaldehyde (soHds) concentrations increase with time and depend on temperature. Formic acid concentration builds at a rate of 1.5—3 ppm/d at 35°C and 10—20 ppm/d at 65°C (17,18). Trace metallic impurities such as iron can boost the rate of formation of formic acid (121). Although low storage temperature minimizes acidity, it also increases the tendency to precipitate paraformaldehyde. 

In unalloyed steel containers formamide discolors slowly during shipment and storage. Both copper and brass are also subject to corrosion, particularly in the presence of water. Lead is less readily attacked. Aluminum and stainless steel are resistant to attack by formamide and should be used for shipping and storage containers where the color of the product is important or when metallic impurities must be minimized. Formamide attacks natural mbber but not neoprene. As a result of the solvent action of formamide, most protective paints and finishes are unsatisfactory when in contact with formamide. Therefore, formamide is best shipped in containers made of stainless steel or in dmms made of, or coated with, polyethylene. Formamide supphed by BASF is packed in Lupolen dmms (230 kg) or Lupolen canisters (60 kg) both in continental Europe and overseas. 

The cmde phthaUc anhydride is subjected to a thermal pretreatment or heat soak at atmospheric pressure to complete dehydration of traces of phthahc acid and to convert color bodies to higher boiling compounds that can be removed by distillation. The addition of chemicals during the heat soak promotes condensation reactions and shortens the time required for them. Use of potassium hydroxide and sodium nitrate, carbonate, bicarbonate, sulfate, or borate has been patented (30). Purification is by continuous vacuum distillation, as shown by two columns in Figure 1. The most troublesome impurity is phthahde (l(3)-isobenzofuranone), which is stmcturaHy similar to phthahc anhydride. Reactor and recovery conditions must be carefully chosen to minimize phthahde contamination (31). Phthahde [87-41-2] is also reduced by adding potassium hydroxide during the heat soak (30). 

Ethylbenzene Hydroperoxide Process. Figure 4 shows the process flow sheet for production of propylene oxide and styrene via the use of ethylbenzene hydroperoxide (EBHP). Liquid-phase oxidation of ethylbenzene with air or oxygen occurs at 206—275 kPa (30—40 psia) and 140—150°C, and 2—2.5 h are required for a 10—15% conversion to the hydroperoxide. Recycle of an inert gas, such as nitrogen, is used to control reactor temperature. Impurities ia the ethylbenzene, such as water, are controlled to minimize decomposition of the hydroperoxide product and are sometimes added to enhance product formation. Selectivity to by-products include 8—10% acetophenone, 5—7% 1-phenylethanol, and <1% organic acids. EBHP is concentrated to 30—35% by distillation. The overhead ethylbenzene is recycled back to the oxidation reactor (170—172). 

For once-through boilers, the treatment must be without soHd residues, so all-volatile treatment (AVT) is used. AVT, which is also used in some dmm boiler systems, rehes on the feedwater chemical additives, ammonia and hydrazine, to provide water appropriate to the boiler. Because the additives are volatile, they do not accumulate in the boiler and provide only minimal protection during contaminant ingress. Most plants using AVT have some form of condensate poHsher to remove impurities from the condensate. 

The zinc is normally melted in a gas, oU, or coal-fired reverberatory furnace with a capacity up to 100 tons or in a low frequency induction furnace with a capacity of a few tons. The more highly aUoyed compositions are more effectively melted and mixed in low frequency induction furnaces. The furnace must be refractory-lined to eliminate iron pickup by the molten metal. The metal temperature is maintained below 500°C to minimize loss by oxidation. A ladle is used to transfer the metal for casting into molds the pouring temperature is usuaUy ca 440°C. Zinc scrap is not generaUy suitable for remelting because it may contain undesirable impurities. 

Benzyl chloride is manufactured by the thermal or photochemical chlorination of toluene at 65—100°C (37). At lower temperatures the amount of ring-chlorinated by-products is increased. The chlorination is usually carried to no more than about 50% toluene conversion in order to minimize the amount of benzal chloride formed. Overall yield based on toluene is more than 90%. Various materials, including phosphoms pentachloride, have been reported to catalyze the side-chain chlorination. These compounds and others such as amides also reduce ring chlorination by complexing metallic impurities (38). 

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