Polyethylene Cracking

Although a wide variety of catalysts have been employed to crack PE, zeolites have proven particularly effective. For example, Garforth et al. reported that activation energies (£a) measured when PE was catalytically cracked by HZSM-5, HY, and MCM-41 were much lower than when no catalyst was present. [66] They concluded that HZSM-5 and HY have similar activities and that both of these zeolites were more effective than MCM-41. Manos and co-workers found that catalytic cracking of PE by HZSM-5 and HY was effective in producing gasoline size hydrocarbons in a laboratory semi-batch reactor [67, 68]. Mordi and co-workers reported that H-Theta-1 and H-Mordenite zeolites, which have pore diameters that are comparable to those for HZSM-5 and HY, were relatively ineffective in producing gasoline size hydrocarbons from PE cracking [69]. Clearly, catalyst pore size and acidity are important factors in polymer catalytic cracking.

Most PE catalytic cracking studies have been performed by heating reactor vessels containing catalyst and polymer and subsequently collecting and analyzing the products. This batch processing approach provides no information regarding the order in which products form. In addition, if sealed reaction vessels are employed, initial reaction products may react with catalyst to form secondary products. Recently, repetitive injection GC/MS has been used to characterize the volatile products generated by polymer cracking. This evolved gas analyzer facilitates real-time separation, identification, and quantification of volatiles generated by heating solid samples [70]. A diagram of this apparatus is shown in Figure 2.1. Thermal analyzer effluent (TA effluent) is the purge gas exiting from a thermogravimetric analyzer or a tube furnace in which the sample is heated. A ther-mogravimetric analyzer is employed when weight loss information is desired. Volatile polymer cracking products are separated by a small gas chromatograph which can be heated rapidly (300°C/min) and cooled rapidly (600°C/min) in order to facilitate rapid capillary gas chromatography separations employing column temperature ramps. Repetitive gas chromatographic injections are made by using an eight-port valve to divert effluent trapped in the injection loop onto the gas chromatographic column. The end of

Injection Sample loops

Injection Sample loops

Gas Sampling Valve Gas Chromatography
Figure 2.1 Apparatus used for repetitive injection gas chromatography analysis of volatile polymer decomposition products. (Reproduced from the Journal of Chromato-graphic Science by permission of Preston Publications, a Division of Preston Industries, Inc)

the fused silica capillary gas chromatography column is attached to the ion source of a quadrupole mass spectrometer for detection and analysis of eluting volatile products. By using this apparatus, product trapping is not required. Instead, volatile products are removed from catalysts with an inert purge gas and then analyzed on-line. This approach was used to study the primary reaction processes when PE is cracked by HZSM-5, HY, and MCM-41 aluminosilicate catalysts [71]. Because these three acid catalysts possess different acid strengths and pore structures, information regarding the effects of pore size and acid strength on cracking processes were obtained by comparing volatile product evolution profiles.

Figure 2.2 shows repetitive injection GC/MS chromatograms obtained while heating PE-catalyst samples at 2°C/min in a helium atmosphere. The tick marks on the x-axes in Figure 2.2 denote PE-catalyst sample temperatures at which evolved gases were injected into the gas chromatograph. Twenty-five successive gas chromatograms were obtained for each of the PE-catalyst samples. Purge gas effluent was analyzed at 5- min intervals, which corresponded to 10°C sample temperature increments. Figure 2.2 shows that the temperature range over which volatiles were produced depended on the choice of cracking catalyst. The maximum volatile product evolution rate for the PE-HY sample

250 350

250 350

3000

3000

250 350

6000

250 350

150 250 350

Temperature (°C)

Figure 2.2 Evolved gas chromatograms obtained by repetitive injection GC/MS for (a) PE-HZSM-5, (b) PE-HY, and (c) PE-MCM-41. (Reproduced by permission of John Wiley & Sons Ltd)

150 250 350

Temperature (°C)

Figure 2.2 Evolved gas chromatograms obtained by repetitive injection GC/MS for (a) PE-HZSM-5, (b) PE-HY, and (c) PE-MCM-41. (Reproduced by permission of John Wiley & Sons Ltd)

occurred at the lowest temperature (220°C), followed by the PE-MCM-41 (260°C), and PE-HZSM-5 (280°C) samples. After comparing the shapes of successive chromatograms in Figure 2.2(a), it is apparent that volatile product distributions changed significantly for the PE-HZSM-5 sample above 290°C. The dominant volatile species detected above 310°C were alkyl aromatics. Similar variations are apparent in the PE-HY chromatograms. Although volatile product slates for the PE-MCM-41 sample also changed with temperature, no alkyl aromatic species were detected in the repetitive injection chromatograms for this sample.

Chromatograms obtained while heating the three PE-catalyst samples show catalyst-dependent differences in volatile product distributions. Figure 2.3 shows the gas chro-matograms obtained at the temperatures corresponding to the maximum volatile product evolution rates for each PE-catalyst sample. Figure 2.3 clearly shows that relative hydrocarbon product yields depended on which catalyst was employed. For the PE-HZSM-5 sample, many isomeric hydrocarbons were detected, most of which were low molecular weight substances with short retention times. Volatile product diversity is less evident

4000 -i

6000 -i

6000

3000

3000

Retention time (min)

Figure 2.3 GC/MS chromatograms obtained at sample temperatures corresponding to maximum volatile product evolution for: (a) PE-HZSM-5 (280°C), (b) PE-HY (220°C); (c) PE-MCM-41 (260°C). (Reproduced by permission of John Wiley & Sons, Ltd)

Temperature (°C) (a)
Temperature (°C) (b)

Temperature (°C)

Figure 2.4 Species-specific evolution profiles for: (a) paraffin; (b) olefin; (c) alkyl aromatic products obtained when a PE-HZSM-5 sample was heated. (Reproduced by permission of John Wiley & Sons, Ltd)

Temperature (°C)

Figure 2.4 Species-specific evolution profiles for: (a) paraffin; (b) olefin; (c) alkyl aromatic products obtained when a PE-HZSM-5 sample was heated. (Reproduced by permission of John Wiley & Sons, Ltd)

in the PE-HY chromatogram. The volatile product slate generated by heating the PE-MCM-41 sample was similar to that obtained for the PE-HZSM-5 sample, except that low-molecular-weight products were not as abundant.

Figure 2.4 shows species-specific evolution profiles for paraffin, olefin, and alkyl aromatic volatile products formed by heating the PE-HZSM-5 sample. The numbers in parentheses denote the number of isomers detected. The volatile product slates for the PE-HZSM-5 sample reflect that C3-C5 hydrocarbons were the dominant species formed. The temperature corresponding to the maximum paraffin and olefin evolution rates was 280°C, whereas alkyl aromatic evolution maximized at 310°C. Below 200°C, volatile product mixtures were composed entirely of paraffins. Mass spectra for paraffin products were consistent with branched rather than straight-chain structures. The only C4 and C5 paraffins detected were isobutane and isopentane. Above 200°C, many different olefin isomers were detected in volatile mixtures. In addition to paraffin and olefin products, substantial quantities of alkyl aromatics were detected for the PE-HZSM-5 sample. Figure 2.4(c) shows that aromatics with C1-C4 alkyl groups were detected and that C2-substituted aromatics (xylenes and possibly ethyl benzene) were the dominant aromatic products.

Species-specific evolution profiles for the PE-HY sample are shown in Figure 2.5. Unlike the PE-HZSM-5 results, volatile mixtures were primarily composed of C4-C8 paraffin rather than olefin products. Evolution profiles for paraffins and olefins had similar shapes with maximum evolution rates occurring at 230-240°C. Like the paraffin evolution profiles for the PE-HZSM-5 sample, isobutane and isopentane were significant paraffin products and no straight-chain isomers were detected. Alkyl aromatic yields for the PE-HY sample were much lower than for the PE-HZSM-5 sample.

Species-specific evolution profiles for the PE-MCM-41 sample are shown in Figure 2.6. Like the PE-HZSM-5 sample, olefin yields were much greater than paraffin yields for the PE-MCM-41 sample. Paraffin evolution profiles in Figure 2.6(a) mostly represent single isomers. In contrast, many olefin isomeric species were detected. C4-C6 olefins comprised the largest fraction of volatile mixtures. Unlike the PE-HZSM-5 sample, propene was a minor volatile product. Alkyl aromatic products were not detected for this sample.

Volatile products derived from cracking PE with solid acid catalysts can be rationalized by carbenium ion mechanisms. Under steady-state conditions, hydrocarbon cracking processes that yield volatile products can be represented by initiation, disproportionation, P-scission, and termination reactions [72, 73]. Initiation involves the protolysis of PE with Bronsted acid sites (H+ S-) to yield paraffins and surface carbenium ions:

Propagation reactions involve disproportionation between feed molecules and surface carbenium ions to yield paraffins:

CnH2n+2 + CmH+2m+1S ^ Cm+xH2(m+x)+2 + Cn-xH+2(n-x) + 1S

When a surface carbenium ion undergoes P-scission to form olefin products, smaller carbenium ions are left on the catalyst surface:

When sufficiently small, olefins may desorb from catalyst surfaces. Surface olefins may also be protonated to form new carbenium ions. Termination reactions involve the destruction of surface carbenium ions. For example, surface carbenium ions may desorb to produce olefins and regenerate Bronsted acid sites:

200 300

3000 n

1000

500 -

  • C4 (2) C5 (1)
  • C6 (3)
  • C7 (3)
  • C8 (5)

200 300

"T

200 300

200 300

200 300 400

Temperature (°C)

200 300 400

Temperature (°C)

Figure 2.5 Species-specific evolution profiles for: (a) paraffin, (b) olefin; (c) alkyl aromatic products obtained when a PE-HY sample was heated. (Reproduced by permission of John Wiley & Sons, Ltd)

These chain reactions describe how paraffin and olefin cracking products are formed, but do not explain residue or aromatic product formation. Like the other reactions, aromatic-and coke-forming reactions involve surface carbenium ions. Carbenium ion thermal cracking can result in olefin ions, which may undergo dehydrogenation and cyclization reactions that are suspected to be the source of aromatic products from straight-chain paraffin feeds. When unsaturated ions are protonated, di-ions are produced. Doubly charged ions can also be formed by disproportionation reactions between adjacent surface carbenium ions. Multiply charged ions are strongly bound to surface conjugate base sites and are less likely to participate in reactions with feed than singly charged carbenium ions. Consequently,

200 300

30000 -i

20000

10000 -

200 300

20000

10000 -

Figure 2.6 Species-specific evolution profiles for: (a) paraffin; (b) olefin products obtained when a PE-MCM-41 sample was heated. (Reproduced by permission of John Wiley & Sons, Ltd)

catalyst sites occupied by polyions are unavailable for further reaction. Catalyst acidity and pore size dictate the relative rates of protolysis, disproportionation, P-scission, and termination reactions, which determine the abundance of volatile paraffin, olefin, aromatic, and nonvolatile hydrocarbon products.

Disproportionation reaction rates depend on carbenium ion reactivities, which are determined by catalyst site acid strength. Carbenium ions produced at strong acid sites are less likely to undergo P-scission or desorption. Compared with HY, the smaller pores in HZSM-5 inhibit bimolecular disproportionation reactions. In contrast, the low paraffin/olefin volatile product ratio for the PE-MCM-41 sample is likely due to the low acidity of the catalyst. Although the MCM-41 pore size is large enough to facilitate disproportionation, catalytic site acidity is too low for this reaction pathway to be dominant.

Aromatic products were detected at temperatures above those at which paraffin and olefin evolutions maximized. The shift in alkyl aromatic evolution profiles to higher temperatures relative to paraffins and olefins is consistent with a mechanism in which unsaturated surface ions are precursors for alkyl aromatic formation. Alkyl aromatic yields decrease in the order: PE-HZSM-5 > PE-HY > PE-MCM-41, which follows a trend in increasing pore size. Steric restrictions on reaction volume afforded by HY

and HZSM-5 promote aromatic ring formation from conjugated unsaturated polymer segments. The smaller pore HZSM-5 is significantly more effective at forming alkyl aromatics than HY.

Unsaturated residue formed during catalytic reactions that produced paraffins and olefins is the source of alkyl aromatics and nonvolatile residue. When HZSM-5 catalyst is employed, aromatic alkyl chain sizes are restricted to C4 or smaller. The pores of HZSM-5 are large enough to allow formation of small alkyl aromatics by cyclization and dehy-drogenation of surface species, but formation of fused unsaturated coke precursors are inhibited. Unlike HZSM-5, larger HY pores facilitate the formation of larger nonvolatile unsaturated coke precursors.

Trends in volatile paraffin/olefin ratios and alkyl aromatic yields observed when polyethylene is cracked by aluminosilicate catalysts cannot be correlated with catalyst acidity or pore size variations alone. Instead, product slate differences occur because relative rates of specific carbenium ion reactions are affected by the combined effects of catalyst acidity and pore size.

Trash To Cash

Trash To Cash

This book will surely change your life due to the fact that after reading this book and following through with the steps that are laid out for you in a clear and concise form you will be earning as much as several thousand extra dollars a month,  as you can see by the cover of the book we will be discussing how you can make cash for what is considered trash by many people, these are items that have value to many people that can be sold and help people who need these items most.

Get My Free Ebook


Responses

Post a comment