Product Distribution
The original polymer mass in the presence of a catalyst is generally converted into volatile products while a residue is left unconverted. This residue consists of a polymer solid residue of genuinely unconverted plastic, although usually not of the same composition and molar mass distribution as the original, and coke formed on the catalyst surface. From these two residue components, the first one can be eliminated by changing the reaction conditions; most obviously by increasing the polymer reaction temperature and time. The second one, coke, can not be avoided. The raw polymer material, being of hydrocarbon nature, undergoes coking reactions on acidic catalytic surfaces [29]. However, the coke amount can be minimized depending upon process conditions and characteristics, most importantly catalyst type.
The volatile (at reaction temperature) products can be grouped into liquids and gases, depending upon the state they are at ambient conditions. However, for practical reasons in all our experimental studies we used an ice bath (T = 273 K) to collect the liquid products. This way we avoided possible condensation problems of the collected gases as well as variations due to seasonal room temperature fluctuations. A diagrammatic overview of the process mass balance is shown in Figure 7.6.
Polymer
Catalyst
Liquid
Unreacted polymer" Coke
Catalyst
Figure 7.6 Diagrammatic overview of the transformations during catalytic polymer degradation
In the following the characteristics of product distribution over various microporous solid acidic catalysts are discussed. The catalysts tested were zeolites, commercial cracking catalysts, clays and their pillared analogues.
Zeolites are widely used in acidic processes and they are an obvious catalyst choice. The suitability of commercial cracking catalysts to degrade polymer waste is vital as one of the options of commercializing this polymer recycling method is to co-feed polymer waste to existing refinery crackers [13]. Furthermore, in the search for cheaper catalysts, clays and their pillared analogues are also introduced in the polymer catalytic degradation [16, 17].
6.1 CONVERSION, LIQUID YIELD, COKE CONTENT
First, the behaviour of these catalysts on the overall conversion, liquid selectivity/yield and coke formation is discussed. Overall conversion is the fraction of the original polymer mass that is converted into volatile products, liquids and gases. Liquid selectivity is the fraction of the volatile products that are in liquid form, while liquid yield is the fraction of the original polymer mass that is converted into liquid. Coke yield is the fraction of the original polymer mass that is converted to coke on the solid catalyst. As at most conditions applied, no unreacted solid polymer remnants were left, the sum of coke yield and conversion is equal to 1 (100%). Therefore overall conversion is not a good measure of the activity of the catalyst. It rather reflects coking tendency over the specific catalyst. Overall conversion levels over zeolites are lower than clay-based catalysts as more coke is formed over zeolites due to their stronger acidity. The same trend is followed by commercial FCC catalysts, where the lower amount of the active zeolitic ingredient leads to lower density of acid sites. Compared with levels of coke yield above 10% on USY, on commercial cracking catalysts coke yield is around 5% and well below that on clays/pillared clays [13].
An exception is ZSM-5 which has a coke-forming resistance due to its shape selectivity properties. The channels of ZSM-5 have a size comparable to that of many organic molecules. This small pore size hinders the formation of bulky coke precursors and coke molecules, decreasing the coke formation. Indeed, in polymer catalytic degradation experiments less than 1% coke is formed over ZSM-5, most probably on the external catalytic surface of the catalyst, compared with more than 10% over the large-pore and strongly acidic US-Y. Another characteristic of zeolite catalysts regarding coke formation and hence conversion is that over zeolites with cage and supercage structure, such as Y and USY, more coke was formed than over channel structure zeolites of similar acidity, such as P-zeolite, resulting in lower overall conversion [8].
Generally over clays and pillared clays higher liquid yield values are reached than over zeolites [13, 16, 17]. Over US-Y zeolite values around 45% are achieved compared with values around 70% over clay-based catalysts. The strong zeolitic acidity leads to overcracking forming products that are collected mainly in the gaseous fraction. Over clay based catalysts of weaker and/or lower acidity higher temperatures are needed for the polymer degradation to occur, but much less overcracking takes place leading to significantly higher liquid yields of above 70%.
6.2 CHARACTERIZATION OF GASEOUS/LIQUID PRODUCTS
The volatile products of catalytic degradation of polyolefins over zeolites, estimated using gas chromatography coupled with a mass spectrometer (GC/MS), lie in the range C3-C15 with distinctive patterns among various zeolitic structures [8]. Over large-pore zeolites, ultrastable Y, Y and P-alkanes are the main products with less alkenes and aromatics and only very small amounts of cycloalkanes and cycloalkenes. Over medium-pore zeolites, ZSM-5 and mordenite, lighter hydrocarbons are formed and significantly more olefins than over large-pore zeolites [8]. Both characteristics, length of the hydrocarbon chain formed and chemical character of the hydrocarbons formed, seem to depend upon the catalyst structure as well as strength/density of the catalytic acid sites. Over commercial cracking catalysts containing only a fraction of US-Y zeolite as well as clays/pillared clays with significantly weaker and less acid sites, significantly lower amounts of alkanes are formed than alkenes [6, 13, 17]. Alkenes as the primary products of a cracking process undergo secondary reactions the extent of which increases with the acidity. Furthermore secondary reactions, being bimolecular, are sterically hindered in the constrained internal structure of medium-pore zeolites [8].
For estimation of the relative amount of paraffins/olefins in order to avoid the laborious GC/MS method we also used solution H NMR. GC/MS needs painstaking search as the number of candidate molecules for a mass spectrum and their similarities increase steeply with the molecule chain length. GC/MS provides of course a full picture of the chemical identity of the components of a mixture. However, if specific information is looked for, other specific methods can shorten the analysis time. In H NMR, olefinic hydrogen atoms show separate distinctive peaks from paraffinic hydrogens. While over US-Y zeolite the ratio of olefinic hydrogens to aliphatic (olefinic plus paraffinic) hydrogens is below 1.5% over commercial cracking catalysts it is between 4.5 and 5.5% and over pillared clays above 10% [13]. Due to the milder acidity of pillared clays, secondary reactions have been limited with the result of a much higher presence of primary products alkenes in the sample. Cracking catalysts produced intermediate figures. Due to the presence of US-Y, secondary reactions occurred, but did not progress to the same degree as with pure US-Y. It should be emphasized that the ratio of olefinic hydrogens to aliphatic hydrogens is much lower than the actual molar ratio of alkenes to the sum of alkanes and alkenes, as only hydrogens of the double bond contribute to the olefinic NMR peak, not all hydrogen atoms of the alkene molecule, while the rest hydrogens do contribute to the paraffinic peak.
The chemical character of liquid products usually coincides with that of gaseous ones. The gaseous products over clay catalysts are predominantly alkenes compared with alkanes over US-Y [16, 17]. Similarly over ZSM-5 gaseous products are predominantly alkenes, although more aromatics are formed too [8].
6.3 BOILING POINT DISTRIBUTION OF LIQUID FRACTION
Liquid hydrocarbons are considered to be the most valuable products of a potential recycling process as they can be used as blends for motor engine fuels. In such a process short-chain hydrocarbons in the gas phase are also produced and they are crucial to provide the heat needed for an endothermic reaction such as polymer cracking, but their value is considered low due to their transportation cost.
- 0.8
0 200
300 400
Figure 7.7 Boiling point distribution of liquid fuel formed over US-Y zeolite, a commercial cracking catalyst, a pillared clay (polymer-to-catalyst ratio 2:1) and comparison with a commercial gasoline sample
Commercial gasoline
US-Y
Cracking catalyst Pillared clay
0 200
300 400
500 600 700
Figure 7.7 Boiling point distribution of liquid fuel formed over US-Y zeolite, a commercial cracking catalyst, a pillared clay (polymer-to-catalyst ratio 2:1) and comparison with a commercial gasoline sample
To characterize the quality of liquid fuel we use the boiling point distribution, which we estimate chromatographically using a 100 m nonpolar column that separates the components of a mixture according to their volatility/boiling point. Employing a calibration mixture consisting of normal alkanes, boiling points are allocated to retention times under the same chromatographic conditions as the ones used in the GC analysis.
Figure 7.7 compares the boiling point distribution of liquid products formed over US-Y, a commercial cracking catalyst and a pillared clay. In the same figure is plotted the boiling point distribution of a commercial gasoline sample.
The liquid hydrocarbon fraction formed over US-Y is similar to the commercial gasoline sample regarding volatility. Commercial gasoline contains slightly higher amounts of volatiles, around the boiling point range of heptane/octane. A considerable shift towards heavier components is obvious from US-Y to cracking catalyst and even more to clay-based catalysts. Stronger catalyst acidity enhances the extent of cracking reactions, leading to more volatile, shorter-chain hydrocarbons. Clays and generally not strongly acidic catalysts enhance the yield to liquid hydrocarbons. However this is accompanied by shifting the fuel quality towards diesel rather than gasoline.
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