Light ends

Fig. 9. Xylenes separation via Mitsubishi Gas—Chemical Co. HF-BF extraction—isomerization process (107). A = extractor B = decomposer C = separator D = isomerization reactor E = heavy ends tower F = raffinate tower G = separator H = light ends fractionator ...

Carbon monoxide High-pressure Off-gas Light ends Catalyst recovery Dehydration... 

After polymeri2ation is carried out by blending mono- and difunctional chlorosilanes ia excess water, the siloxanes are separated from the water and neutraH2ed. Ratio of the mono-chain stopper to di-chain extender controls the length of the polymer. Once an equiHbrium mixture of chain lengths is catalyticaHy formed, volatile light ends are removed and the desired product results. 

The zinc oxide component of the catalyst serves to maintain the activity and surface area of the copper sites, and additionally helps to reduce light ends by-product formation. Selectivity is better than 99%, with typical impurities being ethers, esters, aldehydes, ketones, higher alcohols, and waxes. The alumina portion of the catalyst primarily serves as a support. 

Liquefied Petroleum Gas (LPG). Certain specific hydrocarbons, such as propane, butane, pentane, and their mixtures, exist in the gaseous state under atmospheric ambient conditions but can be converted to the Hquid state under conditions of moderate pressure at ambient temperature. This is termed Hquefied petroleum gas (LPG). Liquefied petroleum gas (qv) is a refinery product and the individual constituents, or light ends (Table 4), are produced during a variety of refining operations. 

Control Room. The control room location can be critical to the efficient operation of a faciHty. One prime concern is to locate it the maximum distance from the most ha2ardous units. These units are usually the units where LPG or other flammables, eg, hydrocarbons that are heavier than air, can be released and accumulate at grade level. Deadly explosions can occur if a pump seal on a light-ends system fails and the heavier-than-air hydrocarbons coUect and are ignited by a flammable source. Also, the sulfur recovery unit area should be kept at a healthy distance away as an upset can cause deadly fumes to accumulate. 

Propane and light ends are rejected by touting a portion of the compressor discharge to the depropanizer column. The reactor effluent is treated prior to debutanization to remove residual esters by means of acid and alkaline water washes. The deisobutanizer is designed to provide a high purity isobutane stream for recycle to the reactor, a sidecut normal butane stream, and a low vapor pressure alkylate product. 

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. 

Higher grade fatty acids with less than 2% rosin are obtained by further distiUation. Union Camp uses two columns to achieve this. The first column is used to separate light ends and the second column to separate a mixture of rosin and higher boiling fatty acids. This mixture with about 40% rosin is sold as DistiUed TaU Oil (DTO). Standard specifications for TOFA grades have been estabHshed by ASTM (25) as shown in Table 3. Also, the Pulp Chemicals Association provides specifications on TOFA having either more or less than 2% rosin. 

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... 

Steam Cracking. Steam cracking is a nonselective process that produces many products from a variety of feedstocks by free-radical reactions. An excellent treatise on the fundamentals of manufacturing ethylene has been given (44). Eeedstocks range from ethane on the light end to heavy vacuum gas oil on the heavy end. All produce the same product slate but in different amounts depending on the feedstock. 

These processes are aH characterized by low isobutane conversion to achieve high isobutylene selectivity. The catalytic processes operate at conversions of 45—55% for isobutane. The Coastal process also operates at 45—55% isobutane conversion to minimize the production of light ends. This results in significant raw material recycle rates and imposing product separation sections. 

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. 

At the Corpus Christi plant, liquid supplies of C4, C5, and Cg are delivered by truek and pipelines from nearby refineries. These liquids are proeessed through light end fraetionating units that remove the propane (C5 and Cg), and drop out isobutane and normal butane (whieh is reintrodueed into the stream for reproeessing.)... 

Isomerization Heater stack gas (CO, SO, NO, HCs and PM), HCI (potentially in light ends), vents and fugitive emissions (Hcs). 

Toxicity and Environmental Fate Information for Propylene CAS 115-07-1 Sourtes. Propylene (propene) is one of the light ends formed during catalytic and thermal cracking and coking operations, it is usually collected and used as a feedstock to the alkylation unit. Propylene is volatile and soluble in water making releases to both air and water significant. 

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. 

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