Purification/distillation section

Olefin plants all have two main parts the pyrolysis or cracking section and the purification or distillation section. The ethane cracker in Figure 5—2 has a pyrolysis (from the Greek, pyros, fire) section that consists of a gas-fired furnace where the cracking takes place. The newer individual furnaces can each handle more than 400 million pounds per year of ethane feed. 

One can also recognize that application of sufficient pressure (above the equilibrium osmotic pressure n) to the right-hand chamber in (7.67) must cause the solvent flow to reverse, resulting in extrusion of pure solvent from solution. This is the phenomenon of reverse osmosis, an important industrial process for water desalination. Reverse osmosis is also used for other purification processes, such as removal of H20 from ethanol beyond the azeotropic limit of distillation (Section 7.3.4). Reverse osmosis also finds numerous applications in wastewater treatment, solvent recovery, and pollution control processes. 

Should the temperature rise steadily, instead of remaining virtually constant, it is then clear that this simple distillation procedure is unsuitable for the purification of the sample and some form of fractional distillation (Section 2.26) will have to be used. 

Most of the energy is consumed in the distillation section, namely for VAM recovery and purification. The reboiler duty for the azeotropic distillation of VAM is particularly high, of about 30 MW. It can be observed that this is due to the large recycle of VAM necessary to carry out the water formed by reaction (3 mole VAM per mol water). Thus, any measure is welcome that can reduce the water content in the crude VAM/acetic acid mixture. Figure 10.8 shows an ingenious method known as gas dehydration [1]. The reactor outlet, cooled up to the dew point,... 

When a product has been isolated from a reaction the next step is to purify it. The degree of purity required will depend on the use for which the sample is intended, a synthetic intermediate might only require rough purification, whereas a product for elemental analysis would require rigorous purification. This section describes the most important purification techniques, crystallization, distillation, sublimation, and chromatography. It is assumed that the reader is familiar with the basic principles of these methods, so the emphasis is on more demanding applications such as the purification of air-sensitive materials, and purifications on a micro-scale. 

At the distillation section, catalyst-free hydrocarbon portion of the reactor effluent proceeds to the first column where unconverted ethylene is recovered as a distillate and recycled to the dimerization reactor at an adequate pressure. The bottoms from the first column are fed to the butene-1 purification column where co-monomer grade butene-1 (99.7%) is distilled overhead as a final product. The purification column bottoms are mainly oligomers of C6. 

The reactor effluent leaving the air cooler is separated into hydrogen-rich recycle gas, a sour water stream, and a hydrocarbon liquid stream in the high-pressure separator. The sour water effluent stream is often sent to a plant for ammonia recovery and for purification so water can be recycled back to the hydrocracker. The hydrocarbon rich stream is pressure reduced and fed to the distillation section after light products are flashed off in a low-pressure separator. 

The above flowsheet can be simplified tremendously by catalytic distillation. Figure 7.32 depicts a conceptual configuration. The RD column consists of a reactive zone at the top, and a distillation section at the bottom. The reaction mixture is sent to a purification column, from which ethylbenzene is obtained as top distillate. A side-stream containing PEB is sent to transalkylation for EB recovery. Obviously, the feasibility of this process depends largely on the availability of an active and selective catalyst. For zeolites the optimal operating conditions are about pressure around 3 MPa, temperature less than 200 °C, and reaction rate capable to give a space-time of 5 h" for almost complete ethylene conversion. 

The reaction section effluent, containing unreacted methanol, DMC, water, and traces of catalyst and HCI, Is sent to the acid recovery section (4) where catalyst and HCI are separated and recycled back to the reaction section. The remaining effluent Is fed to the azeotropic distillation section (5). Methanol Is recycled back to the reaction section as a methanol/DMC azeotrope, while DMC with water is fed to the final purification section (6) to obtain DMC product. 

The liquid stream from the separator is pumped to the stripper to remove light hydrocarbons. The liquid stream from the stripper bottoms contains benzene, toluene, mixed xylenes and a small quantity of Cg-" aromatics. This liquid stream is sent to the product distillation section to obtain benzene product, toluene for recycle to the reactor, mixed xylenes to the PX recovery section and Cg-" aromatics. The PX in the mixed-xylenes stream has over 90% purity, which permits low-cost crystallization technology to be used for the PX 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. 

Ethylene Oxide Purification. The main impurities ia ethylene oxide are water, carbon dioxide, and both acetaldehyde and formaldehyde. Water and carbon dioxide are removed by distillation ia columns containing only rectifying or stripping sections. Aldehydes are separated from ethylene... 

The lack of significant vapor pressure prevents the purification of ionic liquids by distillation. The counterpoint to this is that any volatile impurity can, in principle, be separated from an ionic liquid by distillation. In general, however, it is better to remove as many impurities as possible from the starting materials, and where possible to use synthetic methods that either generate as few side products as possible, or allow their easy separation from the final ionic liquid product. This section first describes the methods employed to purify starting materials, and then moves on to methods used to remove specific impurities from the different classes of ionic liquids. 

The excess benzene is distilled over a column and used as recycled benzene in the alkylation. In the bottom of the stripping section of the column the raw alkylates, consisting of LAB, heavy alkylate, and excess paraffin, are separated. This mixture is fed to a second column in which the excess paraffin is separated off. The actual purification of the LAB follows in a third column. The bottom product, heavy alkylate, consisting mainly of dialkylbenzene is also separated. Heavy alkylates are used in various applications. Both the paraffin and the LAB column are operated under vacuum. 

These reactions are useful because they run under mild conditions, use inexpensive or easily recoverable starting materials, and have short reaction times. The major problem in purification is the separation of the sodium pyridone sulfonate from excess sodium sulfite, sodium bromide, and sodium bromoalkyl sulfonate. However, these latter compounds usually would not interfere with the use of the pyridone sulfonate as a water tracer. From a practical point of view, the pyridone sulfonates need not be purified, but can be used directly. A modified synthetic procedure involves the treatment of the pyridone sodium salt with a tenfold excess of a,iu-dibromoalkane in acetonitrile, followed by removal of the excess dibromide by vacuum distillation. The resulting product is treated with an excess of sodium sulfite in aqueous ethanol. Evaporation of the solvent yields a useful tracer. Procedures given in the experimental section were... 

At the present time, when gram to kilogram amounts of either Am isotope are available, the method of choice for the preparation of Am metal is the metallothermic reduction of Am02 with La (or Th) using a pressed pellet of the oxide and the reductant metal. An oxide reduction-metal distillation still system is shown schematically in Fig. 11. Yields of Am metal are typically >90% and purity levels equal or exceed 99.5 at %. Further purification of the product Am metal can be achieved by repeated sublimations under bigh vacuum in a Ta apparatus (Section III,B Fig. 4). A photograph of 2 g of Am metal distilled in a Ta apparatus is given in Fig. 12. 

Sodium was purified in the apparatus shown in Fig. 5.1(a), thus Ca. 12 g of sodium, cleaned as described in Section 4.4.3.1, was introduced into a tube A, which was then sealed to the apparatus at a this was evacuated for 8 h without heating the sodium, and then sealed off from the vacuum line at e. Tube A was then heated gently and the molten sodium poured swiftly into B, leaving its skin stuck to the tube, which was sealed off at b. This process was repeated by pouring the sodium successively into C and D and finally collecting the silvery metal in E, the sections being sealed off successively at c, d, and /. It is not known why this method of purification is more effective than distillation. The flask E was reattached to the vacuum line via the break-seal g (Fig. 5.1(i)) and then 300 ml of purified ethanol was distilled into E from the container F. 

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