Light ends distillation

In light ends distillation, by contrast, the number of compounds present is usually less than 30 and may be as low as 3 or 4. Thus, the feed to a light ends distillation tower can be described in terms of the percentages of the various individual compounds present. 

Nutter, D. E., F. C. Silvey, and B. R Stober, Random Packing Performance in Light Ends Distillation," Inst, of Chem. Engrs. (England), publication date not given. 

Figure 2. Acetaldehyde from ethylene, single-stage process (a) reactor, (b) separating vessel, (c) cooler, (d) scrubber, (e) crude aldehyde tank, (f) light ends distillation,...

The majority of refinery light-ends distillation lowers are reboiled with steam. Likely, the use of steam reboilers predates the petroleum industry. 

In most large refineries, the principal light-ends distillation units are associated with a crude unit, cracking plant, coking unit, or naphtha reformer. The reason for this is energy conservation. 

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.)...

Feedstocks are light vacuum distillates and/or heavy ends from crude distillation or heavy vacuum distillates from other conversion processes visbreaking, coking, hydroconversion of atmospheric and vacuum residues, as well as deasphalted oils. 

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). 

Unreacted EDC recovered from the pyrolysis product stream contains a variety of cracking by-products. A number of these, eg, trichloroethylene, chloroprene, and benzene, are not easily removed by simple distillation and require additional treatment (78). Chloroprene can build up in the light ends... 

Distillation. Most fatty acids are distilled to produce high quaHty products having exceUent color and a low level of impurities. Distillation removes odor bodies and low boiling unsaponifiable material in a light ends or heads fraction, and higher boiling material such as polymerized material, triglycerides, color bodies, and heavy decomposition products are removed as a bottoms or pitch fraction. The middle fractions sometimes can be used as is, or they can be fractionated (separated) into relatively pure materials such as lauric, myristic, palmitic, and stearic acids. 

Bottoms and three side-cut strippers remove light ends from products and may utilize steam or reboilers. In Fig. 13-92 a reboiled stripper is utilized on the light distillate, which is the largest side cut withdrawn. Steam-stripping rates in side-cut strippers and at the bottom of the atmospheric column may vary from 0.45 to 4.5 kg (1 to 10 lb) of steam per barrel of stripped liquid, depending on the fraction of stripper feea hquid that is vaporizea. 

In light ends fractionation it is usually just as important to remove light material from the heavier cut as it is to keep heavy material out of the lighter cut sidestreams are seldom withdrawn. The desired purity (expressed as per cent of impurity) of the overhead and bottoms is determined by product specifications or by the requirements of subsequent processing units. To meet these purity requirements, higher reflux ratios and greater numbers of plates between cuts are required than in crude distillation units. 

Distillation and Light Ends Processing 75 raise the pH of this water and reduce corrosion. 

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