Chemical Purification Processes

Primary amines, MEA in particular, are chemically stronger than secondary ones, which in turn are stronger than tertiary amines. As their chemical strength decreases, the amines gain in selectivity for H2S over C02- This selectivity is particularly high in the case of those tertiary amines where the nitrogen has no hydrogen atom so that no carbamate can be formed by direct reaction with CO2. 

While CO2 reacts with primary and secondary amines to form carbamate according to the mechanism 

H2S reacts according to the same mechanism as in the case of primary and secondary amines 

This loop is suitable to reach a residual H2S content of about 0.6 ppm with some 100 kg of stripping steam per m of solvent, or - if steam saving is the primary objective - to reach a residual content of about 1 ppm with 75 kg of steam per m of amine solution. If the solvent were not split up into a partly regenerated and a completely regenerated slip stream, the same amount of steam would only lead to residual H2S contents of 2.5 to 5 ppm. 

Unlike all other amine solutions, which have hardly any physical effect, TEA solutions are physically effective to a noticeable extent This property has a favorable effect on the economics of this wash liquor for removing (X 2 insofar as the spent liquor is partially regenerated already by letting its pressure down. 

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As mentioned above, employment of MWCNT for field emitter will be one of the most important applications of MWCNT. For this purpose, MWCNT is prepared by the chemical purification process [30,38], in which graphite debris and nanoparticles are removed by oxidation with the aid of CuCl2 intercalation [38]. Purified MWCNT is obtained in the form of black and thin "mat" (a flake with thickness of ca. a few hundreds of [im). Figure 7 shows a typical transmission electron microscope (TEM) picture of MWCNT with an open end, which reveals that a cap is etched off and the central cavity is exposed. 

Chemical purification processes of petroleum products have been known for many years. The resulting products need to be colourless with no bad odours and chemically pure. The main contaminants are either those derived from chemicals present in the crude oil or chemicals added during processing. 

Several new BTX analogues have recently been identified. PbTx-13 and PbTx-14 are likefy to be artifacts of acid-catalysed methanol addition formed during the chemical purification process (see Fig. 2.2) (Baden et al. 2005 Bourdelais and Baden 2004). PbTx-11, PbTx-12, and PbTx-tbm (PbTx-2 lacking the side chain tail) are new toxins found both in cultmes and in nature. The toxicology of these... 

Concurrent with the solution trials were precipitation and decomposition experiments, for which Wollaston s numerical data again approached those predicted by modern stoichiometry. Thus Wollaston s innovations to the chemical purification process were principally of a quantitative nature, but they brought to the process a consistency that had previously been lacking, and yielded platinum much purer than had earlier been possible. His initial production runs, on 12-troy-oz quantities of crude platinum, began in February 1801 and yielded spongy platinum that was first consolidated and worked in April. The purification process was similar to Knight s published procedure, except for the details noted herein, and the analytical details of the reconstructed process are given in Table III. 

In slow-rate filtration biological, physical and chemical purification processes take place in the sandy filtration layer and thus, top quality water with preserved biological values is obtained. Therefore, this process is used for the treatment of drinking water. 

Another chemical purification process for CO removal is the methanation (Sabatier reaction). 

Gas chromatography, mass spectrometry, and UV spectroscopy are suitable analytical tools to verify the efficiency of the chemical purification process. 

In the treatment plants, the purification of the effluent is carried out mechani-cally-biologically in most cases. Less often the treatment also includes a third stage, for example via precipitation reactions. Specialised purification processes for particular effluent compositions, for example physical-chemical purification processes, such as have been listed by the Chemical Industries Association [49], are used only in industrial and commercial establishments. With the aid of such special processes, it might be possible in some cases to selectively remove the surfactants used for a special purpose from concentrated effluents of known composition. In most cases, however, the effluent is discharged into a sewer system and passed to central treatment plants. 

Oxygen. High purity oxygen for use in semiconductor device manufacture is produced in relatively small quantities compared to nitrogen. There are two different purification processes in general use for manufacturing the gas distillation and chemical conversion plus adsorption. 

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. 

As a result of the development of electronic applications for NF, higher purities of NF have been required, and considerable work has been done to improve the existing manufacturing and purification processes (29). N2F2 is removed by pyrolysis over heated metal (30) or metal fluoride (31). This purification step is carried out at temperatures between 200—300°C which is below the temperature at which NF is converted to N2F4. Moisture, N2O, and CO2 are removed by adsorption on 2eohtes (29,32). The removal of CF from NF, a particularly difficult separation owing to the similar physical and chemical properties of these two compounds, has been described (33,34). 

Aluminum. All primary aluminum as of 1995 is produced by molten salt electrolysis, which requires a feed of high purity alumina to the reduction cell. The Bayer process is a chemical purification of the bauxite ore by selective leaching of aluminum according to equation 35. Other oxide constituents of the ore, namely siUca, iron oxide, and titanium oxide remain in the residue, known as red mud. No solution purification is required and pure aluminum hydroxide is obtained by precipitation after reversing reaction 35 through a change in temperature or hydroxide concentration the precipitate is calcined to yield pure alumina. 

Electrorefining. Electrolytic refining is a purification process in which an impure metal anode is dissolved electrochemicaHy in a solution of a salt of the metal to be refined, and then recovered as a pure cathodic deposit. Electrorefining is a more efficient purification process than other chemical methods because of its selectivity. In particular, for metals such as copper, silver, gold, and lead, which exhibit Htfle irreversibHity, the operating electrode potential is close to the reversible potential, and a sharp separation can be accompHshed, both at the anode where more noble metals do not dissolve and at the cathode where more active metals do not deposit. 

Solvent Extraction Reagents. Solvent extraction is a solution purification process that is used extensively in the metallurgical and chemical industries. Both inorganic (34,35) and organic (36) solutes are recovered. The large commercial uses of phosphine derivatives in this area involve the separation of cobalt [7440-48-4] from nickel [7440-02-0] and the recovery of acetic acid [61-19-7] and uranium [7440-61-1]. 

Solvent extraction—purification of wet-process phosphoric acid is based on preferential extraction of H PO by an organic solvent vs the cationic impurities present in the acid. Because selectivity of acid over anionic impurities is usually not sufficient, precipitation or evaporation steps are included in the purification process for removal. Cmde wet-process acid is typically concentrated and clarified prior to extraction to remove post-precipitated sludge and improve partition of the acid into the solvent. Concentration also partially eliminates fluoride by evaporation of HF and/or SiF. Chemical precipitation of sulfate (as Ba or Ca salts), fluorosiUcates (as Na salt), and arsenic (as sulfides) may also be used as a prepurification step preceding solvent extraction. 

A.sahi Chemical EHD Processes. In the late 1960s, Asahi Chemical Industries in Japan developed an alternative electrolyte system for the electroreductive coupling of acrylonitrile. The catholyte in the Asahi divided cell process consisted of an emulsion of acrylonitrile and electrolysis products in a 10% aqueous solution of tetraethyl ammonium sulfate. The concentration of acrylonitrile in the aqueous phase for the original Monsanto process was 15—20 wt %, but the Asahi process uses only about 2 wt %. Asahi claims simpler separation and purification of the adiponitrile from the catholyte. A cation-exchange membrane is employed with dilute sulfuric acid in the anode compartment. The cathode is lead containing 6% antimony, and the anode is the same alloy but also contains 0.7% silver (45). The current efficiency is of 88—89%, with an adiponitrile selectivity of 91%. This process, started by Asahi in 1971, at Nobeoka City, Japan, is also operated by the RhcJ)ne Poulenc subsidiary, Rhodia, in Bra2il under Hcense from Asahi. 

Process validation should be extended to those steps determined to be critical to the quality and purity of the enantiopure drug. Establishing impurity profiles is an important aspect of process validation. One should consider chemical purity, enantiomeric excess by quantitative assays for impurity profiles, physical characteristics such as particle size, polymorphic forms, moisture and solvent content, and homogeneity. In principle, the SMB process validation should provide conclusive evidence that the levels of contaminants (chemical impurities, enantioenrichment of unwanted enantiomer) is reduced as processing proceeds during the purification process. 

In a chemical production process at least one of the unit operations (the chemical reactor) is the place in which chemical conversion takes place. However, the chemical upstream reactor is proceeded by a series of unit operations in which the new materials are downstream prepared (the upstream operations). After conversion has taken place, the products are operations subjected to a further series of unit operations (the downstream operations). These downstream operations include product recovery and purification steps. A typical example of a production process is illustrated in Figure 1.1. 

The cyclic oligomers are only slightly soluble in water and dilute solutions of caprolactam. They tend to separate out from die extracted waste during die process of concentration and chemical purification of die caprolactam. The cyclic oligomers tend to form on the walls of the equipment used in die process equipment. 6-Aminocaproic acid or sodium 6-aminocaproate may also be found in die oligomeric waste, especially if sodium hydroxide is used to initiate die caprolactam polymerization. 

Penicillin was discovered before the war, but could only be prepared in highly dilute, impure, and unstable solutions. Up to 1943, when chemical engineers first became involved with the project, industrial manufacturers used a batch purification process that destroyed or inactivated about two-thirds of the penicillin produced. Within 7 months of their involvement, chemical engineers at an oil company (Shell Development Company) had applied their... 

Chemical techniques for the isolation, purification and elucidation of the structure of toxins have evolved to the extent that it is frequently a routine procedure to identify the chemical nature of a newly discovered toxin once it has been purified, although difficulties arise when the toxin is a very large polypeptide, protein, or a very complex organic molecule. However, it is sometimes found that a toxin becomes progressively more labile and stabilizing contaminants are removed by the purification processes. An example of this is Cyanea toxic material which becomes increasingly labile with each purification step 111). 

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