High-purity products

In some cases the chemical manufacturer purifies a portion of this intermediate stream to make a high purity product. In other cases, the chemical manufacturer sells a low purity product to a gas company and the gas company purifies it to make a high purity product. In both bases, purification is done on a continuous basis, rather than cylinder by cylinder. The purification processes tend to utilize standard methods. 

Specifications. The use of tungsten hexafluoride in CVD appHcations in the manufacture of high density siUcon chips requires a high purity product, essentially free of all metallic contaminants. Several grades of WF are available. Table 2 shows the specifications for three grades of WF. ... 

Fra.ctiona.1 Crystallization. Fractional crystallization, used until the early part of the twentieth century, is uneconomical for processing large quantities of lanthanides. Many recrystallization steps are required to recover high purity products. Several salts and double salts have been used ... 

Nickel Fluoroborate. Fluoroboric acid and nickel carbonate form nickel fluoroborate [14708-14-6] Ni(BF 2 6H20. Upon crystallization, the high purity product is obtained (47). Nickel fluoroborate is used as the electrolyte ia specialty high speed nickel plating. It is available commercially as a concentrated solution. 

Most of the phosphoms produced as the element is later converted to high purity phosphoric acid and phosphate compounds the remainder is used in direct chemical synthesis to produce high purity products. In contrast, phosphoric acid produced by the wet process is used in lower purity apphcations, especially in fertiliser and to a lesser degree in animal feed (see Feeds AND FEED ADDITIVES). More recendy, a small portion of wet acid is purified in a second process and then also used in high purity acid and phosphate compound apphcations. 

Dialkylphenols are also produced in specialized plants. These plants combine complex batch reactors with vacuum distillation trains or other recovery systems. Alkenes with carbon numbers between 4 and 9 react with phenol to make an unrefined alkylphenol mixture, which is fed into the recovery section where very high purity product is isolated. The product is stored, handled, and shipped just as are the monoalkylphenols. 

Propylene oxide is a high purity product. Thus only the impurities are analy2ed and reported. Table 6 Hsts typical sales specifications (8). The sales specification may vary depending on the appHcation. 

High purity tetraalkyl titanates can be obtained by a process whereby the Hquid organic titanate esters are subjected to partial hydrolysis to form a sohd that can be separated from the reaction mass. The remaining Hquid is distilled to give a high purity product (4). For example, tetraisopropyl titanate purified in this manner contains ... 

Refining to a High Purity Product. The normal yeUowcake product of uranium milling operations is not generaUy pure enough for use ia most nuclear appHcations. Many additional methods have been used to refine the yeUowcake iato a product of sufficient purity for use ia the nuclear iadustry. The two most common methods for refining uranium to a high purity product are tributyl phosphate (TBP) extraction from HNO solutions, or distiUation of UF, siace this is the feedstock for uranium enrichment plants. 

Whereas there is extensive Hterature on design methods for azeotropic and extractive distillation, much less has been pubUshed on operabiUty and control. It is, however, widely recognized that azeotropic distillation columns are difficult to operate and control because these columns exhibit complex dynamic behavior and parametric sensitivity (2—11). In contrast, extractive distillations do not exhibit such complex behavior and even highly optimized columns are no more difficult to control than ordinary distillation columns producing high purity products (12). 

The DMR process has no aqueous effluent, gives high purity products, and is less expensive. However, if hydrogen is produced, it has to be removed carefully and should not reach explosive limits. Not all metals are sufftcientiy reactive to be suitable for the DMR process. 

Theoretical plots of ee (substrate) and eep (product) as a function of c are shown in Figure 2a and b. It can be seen that the ee increases with the extent of conversion. Consequently the enantiomeric purity of the substrate can be increased by sacrificing the yield and carrying out the reaction to higher degrees of conversion. Conversely, if high purity product is required the conversion should be terminated at early stages. 

Diafiltration If a batch process is run so that the permeate is replaced by an equal volume of fresh solvent, unretained solutes are flushed through the system more efficiently. A major use of UF is fractionation, where a solvent, a retained solute and an unretained solute are present. An example is whey, containing water, protein, and lactose. If the retention of protein is I and the retention of lactose is 0, the concentration of protein in the retentate rises during UF. The ratio of protein to lac tose rises, but the feed concentration of lactose is unchanged in retentate and permeate. Diafiltration dilutes the feed, and permits the concentration of lactose to be reduced. Diafiltration is used to produce high-purity products, and is used to fractionate high-value products. R is always 0 for eveiy component. 

Toluene is recovered as a high purity product by fractionating the mixed aromatics obtained from the extraction of catalytic reformate or powerformate. About 70 fractionation trays are required to produce toluene having a purity of 99.7 percent. Toluene is consumed principally as a feedstock for hydrodealkylation plants. Toluene is used in a number of solvent applications. 

The method is very effective for the separation of dissolved components and enables the production of high-purity products, in addition to its advantages of being a low-cost and simple method. Liquid—liquid extraction is performed continuously using simple equipment, which can be completely automated and does not require constant labor and service. 

A single-column distillation configuration called Flash Compact System has been proposed which is capable of delivering an equivalent high purity product. The key advantage lies in the lower capital and operating costs. The feed is heated and pre-flashed and then sent to a distillation column as two. separate vapour and liquid feeds. 

To produce a high purity product two distillation columns are operated in series. The overhead stream from the first column is the feed to the second column. The overhead from the second column is the purified product. Both columns are conventional distillation columns fitted with reboilers and total condensers. The bottom products are passed to other processing units, which do not form part of this problem. The feed to the first column passes through a preheater. The condensate from the second column is passed through a product cooler. The duty for each stream is summarised below ... 

The ability to produce high-purity products many of the alternatives to distillation only carry out a partial separation and cannot produce pure products. 

High purity products require the careful control of the column because of its dynamic state. 

Two of the main objectives in crystallization are to maximize the average crystal size and to minimize the coefficient of variation of crystal size. As pointed out in Chapter 10, large crystals are easier to filter and wash in order to produce a high purity product. 

The expense in preparation derives from the need for high purity products. The cost in fabricating three-dimensional shapes of SiC, SijN or BN arises from the need to sinter powders of these materials at high temperatures (>1500°C) and often under high pressure. Coatings of these materials are at present produced by chemical or... 

This biotechnology company has been a partner to researchers engaged in the field of DNA/RNA synthesis by starting as a supplier of high purity products and evolving to an active contract researcher. ChemGenes Corporation has developed its expertise in the field of DNA/RNA synthesis. Both, the reagent products and the specialized custom services are provided, either as services or as a research contractor. 

Recently, Dolbie2"21 reported a new, nonpyrolytic method for the synthesis of PA-F dimer [Eq. (4)]. This method allows easy laboratory preparation of gram quantities of PA-F dimer but still presents difficulties with regard to commercialization because of the high-dilution methodology that is required for a good yield of high-purity product. 

In practice, values of 110-150 per cent of the minimum reflux ratio are used although higher values are sometimes employed particularly in vacuum distillation. Where a high purity product is required, only limited improvements can be obtained by increasing the reflux ratio and since there is a very large increase in the number of trays required, an arrangement by which the minimum acceptable purity is achieved in the product is usually adopted. 

A discussion on the relative merits of batch and continuous distillation is given by Ellis(36), who shows that when a large number of plates is used and the reflux ratio approaches the minimum value, then continuous distillation has the lowest reflux requirement and hence operating costs. If a smaller number of plates is used and high purity product is not required, then batch distillation is probably more attractive. 

Peptides have many desirable properties as components of synthetic vectors. Peptide synthetic chemistry is well established, with the convenience of automated synthesis resulting in a well-defined, high-purity product of low toxicity and immunogenicity for in vivo use. Furthermore, even short peptides of 7 to 30 amino acids can accommodate enormous structural diversity, functionality, and combinations of properties. 

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