Light-ends removing

Figure 6.4. Process scheme Monsanto a= depressurisation, b= light ends removal, c= acetic acid...

Figure 3.4 presents some common methods for light-ends removal ... 

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. 

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. 

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. 

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.)... 

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. 

Water and sulfur compounds are the principal non-hydrocarbon impurities present in light ends which frequently require removal. The sulfur compounds of concern are concerned with here are hydrogen sulfide and mercaptans, both of which have to be removed almost quantitatively from any light ends cut which is going to be marketed. There are two reasons for this First, they have an objectionable odor, even in minute concentrations. Second, they may cause corrosion either by themselves or through their combustion products. 

The removal of water from certain light ends streams may be necessitated by the subsequent processing step, such as isomerization or low temperature fractionation, or it may be required to meet product specifications, such as for LPG. 

Mercaptans are slightly acidic organic sulfur compounds. They can be removed from light ends mixtures by caustic washing in a Merox extraction... 

When both HjS and mercaptans are to be removed in the light ends treating plant, the amine scrubbing tower precedes the caustic washing step. If the order were reversed, the large quantity of HjS present would "fix" the caustic. Also, the caustic will act to remove any final traces of HjS when it follows the scrubbing tower. 

Figure 5-5. The ICI low-pressure process for producing methanol " (1) desulfurization, (2) saturator (for producing process steam), (3) synthesis loop circulator, (4) reactor, (5) heat exchanger and separator, (6) column for light ends recovery, (7) column for water removal.

The HPS liquid consists mostly of C3 s and heavier hydrocarbons however, it also contains small fractions of Cj s, H2S, and entrained water. The stripper removes these light ends. The liquid enters the stripper on the top tray. The heat for stripping is provided by an external reboiler, using steam or debutanizer bottoms as the heat medium. The vapor from the reboiler rises through the tower and strips the lighter fractions from the descending liquid. The rich overhead vapor flows to the HPS via the condenser and is fed to the primary absorber. The stripped naphtha leaves the tower bottoms and goes to the debutanizer. Usually, at least one draw is installed in the tower to remove the entrained water. 

A better method of increasing LCO yield is through better fractionation upstream. The removal of the fraction under 650°F from the feed requires better stripping. The total refinery yield of diesel will increase when the light ends are fractionated from the feed (Table 6-1). Some of the catalytic routes to maximize LCO yield are ... 

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.,... 

The reactor effluent is cooled and fed to the ethylene separator for recovery of unreacted gaseous ethylene. The liquid phase is filtered to remove small amounts of polymer and then treated with aqueous caustic to remove the catalyst. The dissolved light ends (C2 and C4 olefins) are separated by suitable fractionating towers in series. A portion of the ethylene is purged to remove methane and ethane, and the remaining ethylene is recycled to the compressor. The butene-1 is removed to storage. 

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