Purification section

Solution processes often comprise an anionically catalysed polymerisation process. The catalyst is highly sensitive to the presence of polar impurities in the feedstream, particularly water. Therefore, it is essential that the solvent and monomers are free of such catalyst poisoning species. Even less sensitive catalysts, such as the Ziegler-Natta types, require a purification step on the feedstock prior to the reaction stage. 

Purification is generally operated in a continuous mode. Recycled and make-up solvent are passed through a bed containing molecular sieves. Alternatively, packed alumina columns or distillation columns may be used. 

Monomers are generally purified continuously to remove chain terminators such as water, oxygen, stabilisers like p-tert butyl catechol and polar compounds. This is generally done using alumina beds, or by using distillation columns. 

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After flashing the propylene, the aqueous solution from the separator is sent to the purification section where the catalyst is separated by a2eotropic distillation 88 wt % isopropyl alcohol is obtained overhead. The bottoms containing aqueous catalyst solution are recycled to the reactor, and the light ends are stripped of low boiling impurities, eg, diisopropyl ether and acetone. A2eotropic distillation yields dry isopropyl alcohol, and the final distillation column yields a product of more than 99.99% purity. 

The precipitated copper from this reaction is an important constituent of the slime that collects at the bottom of the electrolytic cells. The accumulation of copper as well as of impurities such as nickel, arsenic, antimony, and bismuth is controlled by periodic bleed-off and treatment in the electrolyte purification section. 

In many refineries, nickel is the principal impurity (up to 20 g/L) in the electrolyte. The nickel remains in the electrolyte as the copper is stripped out in the purification section and is recovered from the resulting acid solution by precipitation as the sulfate in evaporators. 

Etliylene production involves liigh temperatures (1500°F) in tlie pyrolysis section and cryogenic temperatures in tlie purification section. The feedstocks, products, and by-products of pyrolysis are flaimnable and pose severe fire liazards. Benzene, wliich is produced in small amounts as a byproduct, is a known carcinogen. Table 21.7.1 summarizes some of the properties of etliane (feedstock) and tlie product gases. Figure 21.7.1 shows a simplified schematic diagram of the pyrolysis and waste heat recovery section on an etliylene plant. 

Tlie cooled gaseous products are dried using an adsorbent such as molecular sieves and compressed to about 500 psig by a multistage compressor. The compressed gas is dien sent to an acetylene converter where acetylene is selectively hydrogenated to ediane. The gaseous mixture dien flows to die purification section of the plant where each component of die gas is recovered by means of cryogenic disdlladon. 

Figure 4.1. A process for producing hydrogen by steam reforming of hydrocarbons (1) reforming furnace (2,3) purification section, (4) shift converter, (5) pressure swing adsorption.

The dominant mechanism of purification for column crystallization of solid-solution systems is recrystallization. The rate of mass transfer resulting from recrystallization is related to the concentrations of the solid phase and free liquid which are in intimate contact. A model based on height-of-transfer-unit (HTU) concepts representing the composition profile in the purification section for the high-melting component of a binary solid-solution system has been reported by Powers et al. (in Zief and Wilcox, op. cit., p. 363) for total-reflux operation. Typical data for the purification of a solid-solution system, azobenzene-stilbene, are shown in Fig. 20-10. The column crystallizer was operated at total reflux. The solid line through the data was com-putecfby Powers et al. (op. cit., p. 364) by using an experimental HTU value of 3.3 cm. 

You can see that only 60% of the ethane has been cracked. Forty percent of the effluent stream is still uncracked ethane. So part of the purification section will be dedicated to separating the ethane so it can be fed back to the flir-... 

In the purification section of an ethane cracker, the gas can be handled in one of two ways. In order to fractionate the streams, they must be liquefied. Since they are all light gases, liquefaction can be done either by increasing the pressure in a compressor or by reducing the temperature to very low points in something called a cold box. The ethane cracker in Figure 5—2 shows the compressor option. (Even then, the streams have to be cooled to assure they liquefy.)... 

It may seem curious that an ethane cracker has propane and heavier included in the outturns. There are two reasons. The ethane used as feed is rarely pure. It generally has a couple percent of propane and heavier-in it that results in a small amount of heavier products. But why bother or go to the expense to get pure ethane feed In the first place, the olefins plant purification section can handle them. Secondly, some heavy hydrocarbons are actu-... 

The separation section of a gas oil cracker looks like a small refinery, as you can see in Figure 5 or in Figure 5—5. In addition to the fractionators and treaters used in the purification section of the simpler ethane cracker, there are facilities to separate the heavier coproducts. In the front end of the separator facilities in Figure 5-4, the cold box option for handling the liquefaction of the gases is shown. Temperatures as low as -220°F are achieved in this super-refrigerator. At those low temperatures, Freon wont do the job. Liquid air, methane, ethylene, or ammonia are often used as the refrigerant in much the same way Freon has been used in an air conditioner. 

The method of choice for the preparation of Th metal is reduction of the tetrachloride (Section II,B) by Mg (55), followed by refinement using electrotransport purification (Section III,D) (87, 88, 90). 

The total yield should be 50pmol per 100 pL reaction after silica purification (Section 8.3.1.5). 

To overcome these problems, a large recycle volume of unconverted ethylene usually is required. The process usually consists of a reaction section in which crude ethyl alcohol is formed, a purification section with a product of 95% (volume) ethyl alcohol, and a dehydration section, which produces high-purity ethyl alcohol free of water. For many industrial uses, the 95%-purity product from the purification section suffices. 

The nickel-based reforming catalysts which are commonly used in steam reforming are quite sensitive to sulphur, halogen and heavy metal poisons. Since these elements may all be found in natural gas, a feed gas purification section is normally required. Of the mentioned catalyst poisons, sulphur is by far the most important [6],... 

To facilitate working and increase output the nitrators are transported in wooden carts from the nitration section to a separate place, where the nitrator contents are allowed to cool prior to their transport to the purification section, where picric acid is separated from the spent acid and washed with water. 

Methanol synthesis plants utilizing the low-pressure process currently operate at capacities of 2 x 105 to 2 x 106 metric tons per year [15]. Such installations are composed of a synthesis gas production unit, the actual methanol synthesis reactor, and a separation and purification section. The production and purification of synthesis gas accounts for 50%-80% of the total cost of methanol production, with the remaining cost associated with the actual synthesis and purification of methanol [2, 8], Although a variety of carbonaceous feedstocks can be transformed into synthesis gas, the steam reforming of natural gas (Equation [4]) is by far the most common option, especially for large plants [2, 15-16] ... 

Another advantage is that a prereformer may be preferred in flowsheets for processing heavier feedstocks. The prereformer converts heavier hydrocarbons to methane before they are fed to the steam reformer. This allows the steam reformer to be designed for methane service. The prereforming catalysts are very sensitive to poisons. Therefore a good feed purification section is essential.70... 

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