Advantages of reactive

Industrial processes for recycling have been developed which take advantage of reactively separating hydrolysable polymers from nonhydrolysable waste plastics [651]. 

SAN can be modified, for example by grafting. For example, maleic anhydride (MA) can be grafted onto SAN. This process occurs preferably in a twin-screw extruder by reactive extrusion. An advantage of reactive extrusion is the absence of solvent as the reaction... 

Reactive Dyes. These dyes form a covalent bond with the fiber, usually cotton, although they are used to a small extent on wool and nylon. This class of dyes, first introduced commercially in 1956 by ICI, made it possible to achieve extremely high washfastness properties by relatively simple dyeing methods. A marked advantage of reactive dyes over direct dyes is that their chemical structures are much simpler, their absorption spectra show narrower absorption bands, and the dyeings are brighter. The principal chemical classes of reactive dyes are azo (including metallized azo), triphendioxazine, phthalocyanine, formazan, and anthraquinone (see Section 3.1). 

The advantages of reactive distillation as compared to a reaction plus a separate distillation process are ... 

Advantages of reactive distillation inclnde the elimination of complicated product recovery, and separation and recycling of nnconverted reactants, aU of which lead to savings in equipment and energy costs. Compared to the conventional approach (reactor followed by distillation eqnipment), reactive distillation may also improve other factors snch as selectivity and rates of mass transfer. 

An example that demonstrates the advantages of reactive distillation is the synthesis of methyl acetate by esterification of acetic acid with methanol. A chemical plant consisting of two reactors and nine distillation columns can be replaced by just one reactive distillation column. Fig. 3.2-8 shows the conventional process for... 

If heat limitations cause smaller scale extruders to be chosen, the investment costs and operating costs increase and the economic advantage of reactive extrusion disappears. In the example above a rather extreme case with eight extruders with screw diameters of 100 mm was used. It can be concluded that this leads to a price that is approximately 2.5% higher than in the classical batch solution polymerization. 

The calculations are based on a relatively low production capacity of 5000t/year. For highly exothermic reactions large capacities can only be achieved by parallel scale-up. For a continuously operated solution polymerization, however, scale-up can be achieved by an increase of size. This leads to the conclusion that above a certain production volume the economic advantage of reactive extrusion will decrease with increasing scale. 

In a reactive distillation column the chemical reaction and the separation by distillation take place simultaneously in a counter current column. This can offer advantages as for instance new product splits, reduced hardware requirement, increased conversion or selectivities, as well as energy savings which has been confirmed by many realized reactive distillation plants of industrial scale. Given these potential benefits, reactive distillation has been subject to a lot of research activities during the last years. However, a serious exploration of the advantages of reactive distillation requires optimization for a rigorous comparison of process alternatives. 

As mentioned in the Preface, a small number of industrial applications of reactive distillation have been around for many decades. One of the earhest was a DuPont process in which dimethyl terephthalate was reacted with ethylene glycol in a distillation column to produce methanol and ethylene terephthalate. The reactants were fed into the middle of the column where the reversible reaction occurred. The more volatile, low-boiling methanol product was removed from the top of the column, and the high-boiling ethylene terephthalate product was removed from the bottom. The removal of the products from the reaction zone drove the reversible reaction toward the product side. This is one of the fundamental advantages of reactive distillation. Low chemical equilibrium constants can be overcome and high conversions achieved by the removal of products from the location where the reaction is occurring. 

One important inherent advantage of reactive distillation is the feature of simultaneous production and removal of products. For reversible chemical reactions, the removal of the product components drives the reaction toward the product side. Thus, the chemical equilibrium constraint on conversion can be overcome and high conversions can be achieved, even in cases with small chemical equilibrium constants. Of course, the relative volatilities among the reactants and the products must be such that the products can be fairly easily removed from the region in the column where the reaction is occurring and reactants are not lost from this region. 

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