Light ends purification

Multitubular- Ethylene oxide Ethylene oxide Light ends Purification... 

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. 

EDC from the oxychlorination process is less pure than EDC from direct chlorination and requires purification by distillation. It is usually first washed with water and then with caustic solution to remove chloral and other water-extractable impurities (103). Subsequently, water and low boiling impurities are taken overhead in a first (light ends or heads) distillation column, and finally, pure, dry EDC is taken overhead in a second (heavy ends or product) column (see Fig. 2). 

The bulk of the effluent is run through a cooler (heat exchanger) and a condenser to remove the light ends that include traces of carbon monoxide and carbon dioxide and by-product water. The bottom stream is maleic acid, which is easily dehydrated, as in Figure 20—4, by vacuum distillation or azeotropic distillation with ortho-xylene. See Chapter 3 if youVe forgotten totally everything about azeotropic distillation.) The dehydrated maleic acid is maleic anhydride. Further purification is done by distillation.,... 

Converter exit gas containing methanol is cooled by heat exchange with cooling water. The condensed methanol and water mixture is then separated. This crude methanol is purified in a two or three column distillation. The first column separates light ends from methanol. The second column separates methanol from water and fusel oils. If very low ethanol content is required, a third column can be used for final purification. 

Part of the stream is washed countercurrently with a feed sidestream in the vent H2 absorber (9) for benzene recovery. The absorber overhead flows to the hydrogen purification unit (10) where hydrogen purity is increased to 90%+ so it can be recycled to the reactor. The stabilizer (11) removes light ends, mostly methane and ethane, from the flash drum liquid. The bottoms are sent to the benzene column (12) where high-purity benzene is produced overhead. The bottoms stream, containing unreacted toluene and heavier aromatics, is pumped to the recycle column (13). Toluene, C8 aromatics and diphenyl are distilled overhead and recycled to the reactor. A small purge stream prevents the heavy components from building up in the process. 

After the condensed crude methanol is recovered in the high-pressure separator, it is sent to a methanol purification column. Typically, methanol purification requires two columns, one to remove the light ends (mainly by-products generated in the methanol synthesis reactor such as dimethyl ether and dissolved gases) and another to separate methanol and water and any other by-products with a lower volatility than methanol. Specification-grade methanol (greater than 99.85 wl% methanol) is recovered as the overhead product of the heavy ends column and sent to storage. 

Figure 4. Acetone by direct oxidation of propene (a) reactor (oxygen), (b) separator, (c) reactor (propene), (d) flash column, (e) light ends distillation, (f) acetone purification.

This equipment may be supplemented upstream by the washing with cumene of the different liquid streams recovered (gas treatment, separation of solids), and downstream by the purification of tbe light ends, by cooling and condensation, and the removal of sodium carbonate and catalyst in the separators, particularly by washing with water. 

Several purification schemes are feasible. As a rule (Unisir version for example), the crude acetone is first separated at the top of a first distillation column (s55traysX followed by light ends separation (20 trays) of the light products (especially acetaldehyde)... 

Purification. The dehydrated methacrylic add is purified in a series of three vacuum distillation columns, with the separations the top in succession of acetic add by-product and light ends (40 to 50 trays), 99.9 per cent weight methacrylic add (10 to 50 trays) and residual amounts entrained in the heavy compounds. 

The subsequent purification of the product is carried out by a two-step distillation. At first (column k), the light ends, mainly carbon dioxide, methyl, and ethyl chloride, are removed overhead. In the second step (1), acetaldehyde goes overhead water, acetic acid, and some chlorinated products are removed as bottoms and chlorinated acetaldehydes as a side cut. 

Fluid-bed reactor 2 Catalyst separator 3 Liquid-PA condenser 4 Desublimation switch condenser 5 PA storage tank 6 Thermal pretreatment 7 Light-ends column 8 PA-col-umn 9 Gas purification... 

Fig. 22.18. One-stage process for direct oxidation of ethylene to acetaldehyde, (a) Reactor (b) separating vessel (c) crude aldehyde tank (d) scrubber (e) crude aldehyde tank (f) cycle-gas compressor (g) light-ends distillation (h) condensers (i) purification column (I) product cooler (m) regeneration. (Ullmann s Encyclopedia of Industrial Chemistry, W. Gerhartz, Ed. 5th erf., Vo . A1, p. 37, 1985. VCH Verlagsgesellschaft. Weinheim. Federal Republic of Germany. By permission.)...

By-products from EDC pyrolysis typically include acetjiene, ethylene, methyl chloride, ethyl chloride, 1,3-butadiene, vinylacetylene, benzene, chloroprene, vinyUdene chloride, 1,1-dichloroethane, chloroform, carbon tetrachloride, 1,1,1-trichloroethane [71-55-6] and other chlorinated hydrocarbons (78). Most of these impurities remain with the unconverted EDC, and are subsequendy removed in EDC purification as light and heavy ends. The lightest compounds, ethylene and acetylene, are taken off with the HCl and end up in the oxychlorination reactor feed. The acetylene can be selectively hydrogenated to ethylene. The compounds that have boiling points near that of vinyl chloride, ie, methyl chloride and 1,3-butadiene, will codistiU with the vinyl chloride product. Chlorine or carbon tetrachloride addition to the pyrolysis reactor feed has been used to suppress methyl chloride formation, whereas 1,3-butadiene, which interferes with PVC polymerization, can be removed by treatment with chlorine or HCl, or by selective hydrogenation. 

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