High-purity Plants

High purity plants have specific solutions in design and construction, to be described here by the example of a nitrogen generator (Fig. 2.10). The apparatus is derived from the one-column nitrogen generator described in [2.24]. A central component is the pressure column (1). 

Theoretically the oxygen contamination of the nitrogen at the top of the column (a) can be reduced to any arbitrary small value, provided the column has a correspondingly high number of trays and the amount of withdrawn nitrogen product is sufficiently low. 

The oxygen concentration changes from one theoretical tray to the next one above by the separation factor S 

All components which are less volatile than oxygen do not accumulate significantly at the top of the pressure column and do not contaminate the product. This holds for all hydrocarbon compounds. 

Hydrogen can also be removed in the warm section by the catalytic reaction 

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The key issue in selecting materials of construction for high purity plants such as pharmaceutical and semiconductor plants is contamination. Selecting the most suitable materials — stable and cost-effective — for equipment and piping is a complex and critical issue, e.g. in the case of seawater desahnation plants. 

Concerning the constructive details of high-purity plants, numerous design rules have to be followed, in order to achieve the theoretically calctrlated product purities. For example ... 

PX—toluene splitter where high purity PX is taken as a bottoms product. Eight plants have been built worldwide. 

The Phillips process is a two-stage crystallisation process that uses a pulsed column in the second stage to purify the crystals (79,80). In the pulsed column, countercurrent contact of the high purity PX Hquid with cold crystals results in displacement of impurities. In the first stage, a rotary filter is used. In both stages, scraped surface chillers are used. This process was commercialized in 1957, but no plants in operation as of 1996 use this technology. 

Asahi Chemical Industry Company Ltd. was working to develop an adsorption process in the late 1970s and early 1980s that was to produce high purity EB as well as PX (100—103). In 1981 they reported that pilot plants results were being confirmed in larger equipment. However, this process does not appear to have been commercialized. 

The UOP Sarex process has been used since 1978 for the separation of high purity fmctose from a mixture of fmctose, glucose, and polysaccharides (87,88). The pilot-plant performance of fmctose—glucose separation is given in Table 6. 

Synthol coproducts include alcohols, ketones, and lower paraffins. They are used mainly as solvents in the paint and printing industries, although some alcohols are blended into fuels. In 1992 Sasol began producing 17,500 t/yr 1-butanol [71-36-3] from 5-07-acetaldehyde [75-07-0] and plaimed to start a plant to produce high purity ethanol [64-17-5] in 1993. Acetone [67-64-1] and methyl ethyl ketone [78-93-3] are two ketone coproducts sold as solvents. 

Zinc. The electrowinning of zinc on a commercial scale started in 1915. Most newer faciUties are electrolytic plants. The success of the process results from the abiUty to handle complex ores and to produce, after purification of the electrolyte, high purity zinc cathodes at an acceptable cost. Over the years, there have been only minor changes in the chemistry of the process to improve zinc recovery and solution purification. Improvements have been made in the areas of process instmmentation and control, automation, and prevention of water pollution. 

A few companies, eg, Enichem in Italy, Mitsubishi in Japan, and a plant under constmction at Eushun in China, separate the olefins from the paraffins to recover high purity (95—96%) linear internal olefins (LIO) for use in the production of oxo-alcohols and, in one case, in the production of polylinear internal olefins (PIO) for use in synthetic lubricants (syn lubes). In contrast, the UOP Olex process is used for the separation of olefins from paraffins in the Hquid phase over a wide carbon range. 

The first commercial plant to use CYANEX 272 became operational in 1985. An additional three plants were constmcted between 1985 and 1989. Of the four, one is in South America and three in Europe. An additional three plants have been built two in Europe (1994) and one in North America (1995). Approximately 50% of the Western world s cobalt is processed using CYANEX 272. Both high purity salts and electrolytic cobalt metal are recovered from solutions ranging in composition from 30 g/L each of cobalt and nickel to 0.2 g/L Co, 95 g/L Ni Operating companies usually regard use of CYANEX 272 as confidential for competitive reasons and identities cannot be disclosed. CYANEX 272 is being evaluated on the pilot-plant scale in many additional projects involving the recovery of cobalt and other metals. 

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. 

Pressure Swing Adsorption. Carbon dioxide can be removed by pressure adsorption on molecular sieves. However, the molecular sieves are not selective to CO2, and the gases must be further processed to achieve the high purity required for "over the fence" use as in the urea process. Use of pressure swing adsorption for CO2 removal appears most appHcable to small, stand-alone plants (29). 

Propylene has many commercial and potential uses. The actual utilisation of a particular propylene supply depends not only on the relative economics of the petrochemicals and the value of propylene in various uses, but also on the location of the supply and the form in which the propylene is available. Eor example, economics dictate that recovery of high purity propylene for polymerisation from a smaH-volume, dilute off-gas stream is not feasible, whereas polymer-grade propylene is routinely recovered from large refineries and olefins steam crackers. A synthetic fuels project located in the western United States might use propylene as fuel rather than recover it for petrochemical use a plant on the Gulf Coast would recover it (see Euels, synthetic). 

The ethylene feedstock used in most plants is of high purity and contains 200—2000 ppm of ethane as the only significant impurity. Ethane is inert in the reactor and is rejected from the plant in the vent gas for use as fuel. Dilute gas streams, such as treated fluid-catalytic cracking (FCC) off-gas from refineries with ethylene concentrations as low as 10%, have also been used as the ethylene feedstock. The refinery FCC off-gas, which is otherwise used as fuel, can be an attractive source of ethylene even with the added costs of the treatments needed to remove undesirable impurities such as acetylene and higher olefins. Its use for ethylbenzene production, however, is limited by the quantity available. Only large refineries are capable of deUvering sufficient FCC off-gas to support an ethylbenzene—styrene plant of an economical scale. 

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. 

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