Trace component/impurity removal

The traditional application of adsorption in the process industries has been as a means of removing trace impurities from gas or liquid streams. Examples include the removal of H2S from hydrocarbon streams before processing, the drying and removal of C02 from natural gas, and the removal of organic compounds from waste water. In these examples the adsorbed component has little value and is generally not recovered. Such processes are generally referred to as purification processes, as distinct from bulk separations, in which a mixture is separated into two (or more) streams, each enriched in a valuable component, which is recovered. The application of adsorption to bulk separations is a more recent development that was stimulated to a significant extent by the rapid... 

The crude sample is injected or pumped onto the TE column. [Note Even a crude sample requires membrane filtration and/or centrifugation to prevent shortened column life of the trace enrichment column.] The TE column will attract the components of interest, depending on the nature bonded phase in the TE column, the solvent in which the sample is dissolved, and the solvent in which the TE column has been equilibrated. The TE column, most likely, will remove more than just the components of interest and, thus, might need an additional solvent wash sequence to remove other collected impurities. 

The analysis starts by ordering the components by composition, as given in Table 7.14. The first split suggested is the removal of the last three trace components hydrogen sulfide, benzene, and chloro-ethane. This split would correspond to the second heuristic in Table 7.12 remove troublesome trace impurities. The appropriate selector is of purification type. Four methods could be considered chemical absorption, catalytic conversion, molecular sieve adsorption and physical adsorption. 

The ability of porous solids to reversibly adsorb large volumes of vapor was recognized in the eighteenth century and early experiments were carried out by Scheele and Fontana but the practical application of this property to the large-scale separation and purification of industrial process streams is relatively recent. Perhaps the most familiar example of such a process is the use of an adsorbent column, packed with a suitable hydrophilic adsorbent, as a drier for the removal of traces of moisture from either gas or liquid streams. Similar processes are also in common use on a large scale for the removal of undesirable impurities such as HjS and mercaptans from natural gas and organic pollutants from water. Such processes are conveniently classified as purification processes since the components which are adsorbed are present only at low concentration, have little or no economic value, and are frequently not recovered. The economic benefit of the process is derived entirely from the increase in the purity and hence the value of the stream containing the major component. 

At the LC3 level and below, it may be necessary to specify high-purity metals, perhaps in the five 9 s range, for alloying components such as Ag and Cu, to remove traces of alpha-emitting impurities. There are two additional issues to achieve very low alpha emission levels. First, the detection of low-alpha emission levels is very difficult [87,98,99], with equipment limits at approximately 2-A a/khr cm [71]. Second, the measurement of low impurity levels in Sn can... 

Y Picoline. Commercially pure y-picoline contains )S-picoline and 2 6-lutidine and sometimes traces of non-basic impurities (aromatic hydrocarbons) which cannot be separated by fractionation. The non-basic impurities are removed by steam distillation of the base in dilute hydrochloric or sulphuric acid solution (for details, see under a Picoline). The impure y-picoline is converted into the zinc chloride complexes of the component bases the 2 6-lutidine - ZnClj complex is the least stable and upon steam distillation of the mixture of addition compounds suspended in water, 2 6-lutidine passes over flrst. The complete separation of the 2 6-lutidine may be detected by a determination of the density and the refractive index of the dry recovered base at varioiu stages of the steam distillation. The physical properties are —... 

Industrial examples of adsorbent separations shown above are examples of bulk separation into two products. The basic principles behind trace impurity removal or purification by liquid phase adsorption are similar to the principles of bulk liquid phase adsorption in that both systems involve the interaction between the adsorbate (removed species) and the adsorbent. However, the interaction for bulk liquid separation involves more physical adsorption, while the trace impurity removal often involves chemical adsorption. The formation and breakages of the bonds between the adsorbate and adsorbent in bulk liquid adsorption is weak and reversible. This is indicated by the heat of adsorption which is <2-3 times the latent heat of evaporahon. This allows desorption or recovery of the adsorbate from the adsorbent after the adsorption step. The adsorbent selectivity between the two adsorbates to be separated can be as low as 1.2 for bulk Uquid adsorptive separation. In contrast, with trace impurity removal, the formation and breakages of the bonds between the adsorbate and the adsorbent is strong and occasionally irreversible because the heat of adsorption is >2-3 times the latent heat of evaporation. The adsorbent selectivity between the impurities to be removed and the bulk components in the feed is usually several times higher than the adsorbent selectivity for bulk Uquid adsorptive separation. 

Among hybrid separations not involving membranes, adsorptive distillation (87) offers interesting advantages over conventional methods. In this technique a selective adsorbent is added to a distillation mixture. This increases separation ability and may present an attractive option in the separation of azeotropes or close-boiling components. Adsorptive distillation can be used, for instance, for the removal of trace impurities in the manufacturing of fine chemicals (it may allow for switching some fine chemical processes from batchwise to continuous operation). 

Poison in the Feed. Many petroleum feed stocks contain trace impurities such as sulfur, lead, and other components which are too costly to remove, yet poison the catalyst slowly over time. For the case of an impurity, P, in the feed-stream, such as sulfur, for example, in the reaction sequence... 

Induction periods and an accelerating stage in the polymerization may result from the presence of impurities which are slowly removed from the system by reaction with the catalyst components. In butadiene polymerization by the soluble catalyst from nickel salicylate/BF3Et2 0/LiBu there is a marked induction from traces of 1,2 butadiene (below 100 p.p.m.) in the monomer [67]. In the absence of 1,2 butadiene polymerization starts immediately. An induction period has been found with the similar catalyst, nickel naphthenate/BF3Et2 0/AlEt3 [68], but the origins of this were not identified. 

This method can be applied to the preparation of single catalysts or, by coprecipitation, to multi-component catalysts. Hydrous oxides, sulphides, carbonates and phosphates can be used. After precipitation, the catalyst must be washed free of impurities which might have an adverse effect on the final catalytic properties. The presence of these impurities can be minimised by the use of dilute solutions. An alternative is the mixing of a metal salt with ammonia or an ammonium salt. Any ammonium nitrate remaining in the precipitate is readily removed by washing because of its high water solubility, and the last traces can be eliminated by calcination. 

Removal of trace organic and inorganic impurities from a gas stream by an activated carbon is one of the oldest appH cations of adsorption technology. The other applications of Table 22.1 where the adsorbed components are present in dilute or bulk quantities in the feed gas mixture were developed during the last 30 years. 

When producing hydrogen as the final product, impurities such as CO, sulfur compounds, and other trace contaminants must be removed, particularly for application in fuel cells. Currently, pressure swing adsorption (PSA) is commonly used for the separation and purification of hydrogen from mixed gas streams. PSA systems are based on selective adsorbent beds. The gas mixture is introduced to the bed at an elevated pressure and the solid adsorbent selectively adsorbs certain components of the gas mixture, allowing the unadsorbed components, in this case hydrogen, to pass through the bed as purified gas. 

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